Simple Chemical Methods for Thin Film Deposition: Synthesis and Applications 9819909600, 9789819909605

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Babasaheb R. Sankapal · Ahmed Ennaoui · Ram B. Gupta · Chandrakant D. Lokhande   Editors

Simple Chemical Methods for Thin Film Deposition Synthesis and Applications

Simple Chemical Methods for Thin Film Deposition

Babasaheb R. Sankapal · Ahmed Ennaoui · Ram B. Gupta · Chandrakant D. Lokhande Editors

Simple Chemical Methods for Thin Film Deposition Synthesis and Applications

Editors Babasaheb R. Sankapal Department of Physics Visvesvaraya National Institute of Technology Nagpur, Maharashtra, India Ram B. Gupta College of Engineering Virginia Commonwealth University Richmond, VA, USA

Ahmed Ennaoui President of Scientific Council of IRESEN Research Institute for Solar Energy and New Energies Rabat, Morocco Department of Heterogeneous Material Research Group Berlin, Germany Chandrakant D. Lokhande Centre for Interdisciplinary Research D.Y. Patil Education Society Kolhapur, Maharashtra, India Department of Physics Shivaji University Kolhapur, Maharashtra, India

ISBN 978-981-99-0960-5 ISBN 978-981-99-0961-2 (eBook) https://doi.org/10.1007/978-981-99-0961-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Thin film research is an important field playing a vital role in miniaturization, which not only required to reduce the cost of material and hence, device but also served to fulfill the requirement with the desired properties. Thin films have a wide range of applications from small-scale to large-scale integration for advanced domains like energy, environment, and health. In this regard, two approaches have been evolved to get a material in a thin-film form as “top-down” and “bottom-up.” The topdown approach basically is a mechanical system that includs crushing, grinding, and ball milling in order to get a material from bulk to a thin-film form, whereas the bottom-up approach involves growing the material into a thin-film form by using the concept of nucleation and growth involving constituents like atoms, molecules, or ions, and hence, possible to control the surface architecture embedded in thin-film form at low dimensions. Everything from design to manufacturing to integration affects by materials and materials properties. Unlike top-down techniques, which are limited to going past critical thickness, bottom-up approaches may regulate and hence, accomplish the desired qualities. Physical methods can be used to grow thin films, but they are expensive, need a vacuum at every stage of deposition, and have limitations in terms of large-scale integration. Contrary, the “chemical method” offers low cost, where simplicity and diverse nanostructured morphologies possibly help to get in a thinfilm form; possible to integrate from small to large scale and hence, easily scalable to technology. The main purpose of this book is to present and study all chemical methods at one platform for the deposition of thin films with diverse nanostructured morphologies and applications in the field of energy conversion (solar cells), energy storage (supercapacitors), gas sensors, and transistors, etc. The book primarily focuses on innovative simple, low cost, and easy thin-film chemical methods like chemical bath deposition, successive ionic layer adsorption and reaction, ion exchange, electroless deposition, electrodeposition, hydrothermal, spray pyrolysis, spin coating, dip coating, doctor blade, screen printing, and solgel. Since it is indeed crucial to pick up the correct deposition method for material

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synthesis in thin-film form depending upon specific targeted device grade applications and their requirements, knowledge about all chemical methods is essential so one can choose a specific method depending upon the type of material, its desired surface architecture and properties as one device cannot be accomplished with the use of single material or method. The roles of preparative factors are equally important for thin-film alterations in terms of nanostructured surface morphologies which have been extensively discussed along with the underlying science of film synthesis. Henceforth, this book provides a comprehensive overview of the important field of chemical methods for thin-film synthesis, optimization to applications along with case studies in the field of energy conversion (solar cells), energy storage (supercapacitors), gas sensors, and transistors. The coverage of principles allows the reader to appreciate the topic of thin-film deposition. As a result, concentrated thin-film synthesis routes will be of great interest to university/college professors, students, and new engineers as well as postdocs and scientists in the advanced functional materials science area. Nagpur, India Berlin, Germany Richmond, USA Kolhapur, India

Babasaheb R. Sankapal Ahmed Ennaoui Ram B. Gupta Chandrakant D. Lokhande

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Akanksha Agarwal and Babasaheb R. Sankapal

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Chemical Bath Deposition: Thin Films with Assorted Morphologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prashant K. Baviskar and Swapnil S. Karade

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Well-Controlled Nanostructured Growth: Successive Ionic Layer Adsorption And Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bidhan Pandit, Pratibha Nikam, and Mohd Ubaidullah

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Ion-Exchange Method: Nanostructured Thin Films . . . . . . . . . . . . . . 159 Sutripto Majumder and Ki Hyeon Kim

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Electroless Assisted Nanostructured Morphologies . . . . . . . . . . . . . . . 211 Akanksha Agarwal and Tetsuo Soga

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Electrochemical Deposition Toward Thin Films . . . . . . . . . . . . . . . . . . 245 Bidhan Pandit, Emad. S. Goda, and Shoyebmohamad F. Shaikh

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Nanostructured Thin Films by Hydrothermal Method . . . . . . . . . . . . 305 Sutripto Majumder

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Spray Pyrolysis: Thin Film Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347 Tushar Deshmukh and Nelson Yaw Dzade

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Spin Coating: Easy Technique for Thin Films . . . . . . . . . . . . . . . . . . . . 387 Avinash C. Mendhe

10 Dip Coating: Simple Way of Coating Thin Films . . . . . . . . . . . . . . . . . 425 Savita L. Patil, Suraj R. Sankapal, and Faizal M. A. Almuntaser

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Contents

11 Screen Printing: An Ease Thin Film Technique . . . . . . . . . . . . . . . . . . 449 Lakshmana Kumar Bommineedi, Nakul Upadhyay, and Rafael Minnes 12 Doctor Blade: A Promising Technique for Thin Film Coating . . . . . . 509 Ganesh C. Patil 13 Sol–Gel Derived Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531 Nikila Nair

Editors and Contributors

About the Editors Babasaheb R. Sankapal is Professor and formerly Head, Department of Physics, Visvesvaraya National Institute of Technology (VNIT), Nagpur, Maharashtra, India [https://phy.vnit.ac.in/people/brsankapal/]. Dr. Sankapal served as Chairman, Board of Studies for the Department of Physics and Associate Dean (Exam) for VNIT. He received his Ph.D. degree from Shivaji University Kolhapur, Maharashtra, India, in 2001. Earlier, he worked as Scientist at Helmholtz Centre, Berlin, Germany; JSPSPostdoctoral Fellow at Gifu University, Japan; and Research Associate at the University of Wisconsin, Milwaukee, USA. Dr. Sankapal has research expertise in synthesis of nanomaterials, quantum dots, nanowires and nanotubes of inorganic semiconducting materials by using simple and low-cost chemical routes towards solar cell, sensors, and supercapacitors device grade developments. With more than 25 years of research experience, Dr. Sankapal was nominated for the Shanti Swaroop Bhatnagar Award in 2018 by the Director, VNIT. He was recipient of the Young Scientist-Outstanding work presentation award by Material Research Society, Japan (MRS-J), in 2005 and by the Department of Science and Technology (DST), the Government of India, to participate in the meeting of Nobel Laureates in Lindau, Germany, in 2001. Dr. Sankapal has supervised 16 Ph.D. and 20 graduate students. He has 165 publications to his credit with average impact factor above 5 with Google Scholar h-Index 51 with citations above 6400. His name is listed in the World’s Top 2% Scientists 2020 and 2021 as per the Stanford’s University data. Dr. Sankapal completed 9 major research projects from DST, SERB, DAE-BRNS and UGC funding agencies. He is Fellow of the Maharashtra Academy of Sciences, Member of the Institute of Physics, UK, The Indian Science Congress Association, Indian Association of Physics Teachers, and Material Research Society of India. Ahmed Ennaoui is former Head of Research Group at the Heterogeneous Material Department, Helmholtz–Zentrum Berlin (HZB), Berlin, Germany, and President of the Scientific Council of IRESEN (Research Institute for Solar Energy and

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New Energies), Morocco. Earlier, he worked as Associate Professor at Université Mohammed V (UM5), Morocco, before joining HZB, in 1983. During his career at HZB, he served as visiting professor at Osaka University, Japan, and taught graduate students advanced courses on thin film chalcogenide materials for PV application. He worked as Research Director of Solar Energy group at Qatar Environmental and Energy Research Institute (QEERI), from 2015–2017; and Professor at Hamad Bin Khalifa University (HBKU), Qatar. He is a permanent member of the editorial board of Solar Energy Materials and Solar Cells and an IEEE senior member. A material scientist, Prof. Ennaoui has graduated (magna cum laude) from the University de Bourgogne, France, and earned his Doctoral thesis (summa cum laude) in solid state electronics, in 1979. He conducted research at Hahn–Meitner–Institut Berlin (HMI), and obtained his habilitation (Doctorat d´état, summa cum laude), in 1987. Ennaoui worked as Scientist at HZB before he was promoted to head a research group pursuing research on several European Union’s Framework programs for PV Technological Development, in 2000. Ennaoui’s research interests are primarily focused on low-cost emerging thin film technologies and recently worked on ink formulation and D-O-D inkjet printing. He also explored 2D-layered, superhydrophobic materials coating and photoactive electrodes and photoelectrochemical solar cells (PEC). With more than 300 journal articles and h-index 51, he has supervised more than 40 Ph.D. thesis and more than 15 post-doctoral researchers. In 2019, he founded Virtuasl–Learnin University committed to sciences and techniques and online-opening lectures. Recently, he is listed by Stanford University as one of the top 2% of researchers in the world out of 224,856 researchers in applied physics ranked by the prestigious American university. Per the AD Scientific Index, his Morocco Country rank is among the top 2%. Professor Ennaoui has contributed to the development of renewable energies by following the completion of IRESEN’s research and development projects, by organising international conferences in Morocco on green energies and through the virtual university of which he is the founder. Ram B. Gupta is Professor and Associate Dean for Research and Graduate Affairs at the College of Engineering, Virginia Commonwealth University, Richmond, USA. After joining the College of Engineering, in 2014, he significantly enhanced its national reputation as an engine of education and research and made important contributions to the success of student, staff and faculty. At the College of Engineering, he has been responsible for sponsored research programs, graduate education, facilities, centers and institutes. Widely regarded as one of the nation’s leading researchers on sustainable energy, materials and technologies. Earlier, Prof. Gupta served the U.S. National Science Foundation (NSF) as the Director of the Energy for Sustainability Program. Presently, he extends his expert help to NSF in its Engineering Research Centers program. Professor Gupta began his career as Teacher of engineering at Auburn University, USA, risen through the ranks and served as the Chair of the Graduate Program at the Department of Chemical Engineering. For his academic excellence, he has been given many notable awards, endowed professorships, and 54 research grants. He was

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ranked among top 2% most cited researchers in his field in 2021. He is Fellow of the American Institute of Chemical Engineers and of the Alabama Academy of Science. He received Bachelor of Engineering from the Indian Institute of Technology, masters from the University of Calgary, and Ph.D. from the University of Texas at Austin, USA, all in Chemical Engineering. After that, he completed postdoctoral work at the University of California, Berkeley. Recently, he completed Management Development Program at Harvard University, USA. Professor Gupta has published several books on sustainable development goal (SDG) topics: Nanoparticle Technology for Drug Delivery; Solubility in Supercritical Carbon Dioxide; Hydrogen Fuel: Production, Transport, and Storage; Gasoline, Diesel and Ethanol Biofuels from Grasses and Plants; and Compendium of Hydrogen Energy. A frequent keynote speaker and writer on the new developments and innovations, his articles have been published by several leading media houses: such as Virginia Business, Reuters, London Daily Mail, Japan News, Science Newsline, Chemistry News, and Agriculture Markets. Chandrakant D. Lokhande is Dean of the Centre for Interdisciplinary Studies, and Research Director of D.Y. Patil Education Society, Kolhapur, Maharashtra, India. Earlier, he was Professor and Head of the Department of Physics, and Director of the International Affairs Cell, Shivaji University, Kolhapur, India. He received his Ph.D. from Shivaji University, Kolhapur, in 1984, Immediately after accomplishing his first postdoctoral stay at the Weizmann Institute of Science, Israel, he joined Shivaji University as Assistant Professor of Physics in 1987, where he later became Professor and Head. He was appointed as Fellow of Institute of Physics, London, in 1990; was visiting scientist in the Indo-Polish CEP scheme, in 1991; was INSA Visiting Fellow, in 1993; is the first recipient from Shivaji University of the prestigious Alexander von Humboldt Fellowship, Germany, in 1996; and Brain Pool fellowship of South Korea, in 2003; was participant in the Noble Laureates Meeting, Lindau, Germany, in 2001; was Visiting Professor at Hanyang University, South Korea, in 2006; was awarded a Rajya Shishak Purshakar, Government of Maharashtra, in 2009; and the Best Teacher Award from Shivaji University, in 2010. His areas of research interest are thin film technology, ranging from chemical synthesis of thin films to their applications in solar cells, gas sensors, and supercapacitors. He made a great contribution in designing several prototype devices such as supercapacitors and heterojunction-based room temperature gas sensors. He is on the editorial board of the Electrochemical Energy Technology journal; Fellow of the Maharashtra Academy of Sciences, from 2012; an expert member and a distinguished Visiting Professor in polymer chemistry of the Institute of Chemical Technology, Mumbai, from 2012. An author of more than 600 research papers published in several international journals with h-index 100 and more than 37200 citations, Prof. Lokhande has edited 11 books, filed more than 65 patents, and guides more than 66 Ph.D. students. He has been listed at the first position in top 2% scientists in the subject of Applied Physics in India as per the Stanford University Survey.

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Editors and Contributors

Contributors Akanksha Agarwal Department of Physics, Visvesvaraya National Institute of Technology, Nagpur, Maharastra, India Faizal M. A. Almuntaser Department of Physics, Radfan College, University of Aden, Aden, Yemen Prashant K. Baviskar Department of Physics, Sangamner Nagarpalika Arts, D. J. Malpani Commerce and B. N. Sarda Science College (Autonomous), Sangamner, M.S., India Lakshmana Kumar Bommineedi Department of Physics, Ariel University, Ariel, Israel Tushar Deshmukh Department of Physics, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra, India Nelson Yaw Dzade John and Willie Leone Family Department of Energy and Mineral Engineering, The Pennsylvania State University, University Park, PA, USA Emad. S. Goda Organic Nanomaterials Lab, Department of Chemistry, Hannam University, Daejeon, Republic of Korea; Gas Analysis and Fire Safety Laboratory, Chemistry Division, National Institute of Standards, Giza, Egypt Swapnil S. Karade Department of Green Technology, University of Southern Denmark, Odense M, Denmark Ki Hyeon Kim Department of Physics, School of Natural Science, Yeungnam University, Gyeongsan, Republic of Korea Sutripto Majumder Department of Physics, School of Natural Science, Yeungnam University, Gyeongsan, Republic of Korea Avinash C. Mendhe Department of Civil and Environmental Engineering, Hanyang University ERICA, Ansan, South Korea Rafael Minnes Department of Physics, Ariel University, Ariel, Israel Nikila Nair Department of Mechanical Engineering, Indian Institute of Science, Bengaluru, India Pratibha Nikam School of Physical Sciences, (Autonomous), Jalgaon, Maharashtra, India

Moolji

Jaitha

College

Bidhan Pandit Department of Materials Science and Engineering and Chemical Engineering, Universidad Carlos III de Madrid, Leganés, Madrid, Spain Ganesh C. Patil Center for VLSI and Nanotechnology, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra, India

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Savita L. Patil DDSP Arts, Commerce and Science College, Erandol, Maharashtra, India Babasaheb R. Sankapal Department of Physics, Visvesvaraya National Institute of Technology, Nagpur, Maharastra, India Suraj R. Sankapal Centre for Interdisciplinary Research, D. Y. Patil Education Society, Kolhapur, Maharashtra, India Shoyebmohamad F. Shaikh Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia Tetsuo Soga Department of Electrical and Mechanical Engineering, Nagoya Institute of Technology, Nagoya, Aichi, Japan Mohd Ubaidullah Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia Nakul Upadhyay Department of Physics, Visvesvaraya National Institute of Technology, Nagpur, M.S., India

Chapter 1

Introduction Akanksha Agarwal and Babasaheb R. Sankapal

List of Annotations CBD CVD IE IR Ksp LPG PCB SILAR SMD SP

Chemical bath deposition Chemical vapor deposition Ion exchange Infrared Solubility product Liquified petroleum gas Printed circuit board Successive ionic layer adsorption and reaction Surface mountain devices Spray pyrolysis

1 General Introduction Thin films technology, which was primarily developed for the integrated circuit industry, has now gained worldwide attention because of its effective features [1–3]. The idea to use thin film instead of using bulk material if the purpose can be solved by coating a thin layer causing a drastic reduction in cost issue of the final product [4]. Thin films have played a vital role in reducing the size and processing cost of devices A. Agarwal (B) · B. R. Sankapal Department of Physics, Visvesvaraya National Institute of Technology, South Ambazari Road, Nagpur, Maharastra 440010, India e-mail: [email protected] B. R. Sankapal e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 B. R. Sankapal et al. (eds.), Simple Chemical Methods for Thin Film Deposition, https://doi.org/10.1007/978-981-99-0961-2_1

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in a variety of technological applications [5]. Because of their thinness, they have various unique features that lead them to have a higher surface area to volume ratio due to nanostructure surface architectures and can have diverse structural, electrical, magnetic, and optical properties than bulk materials [6–8]. Thin film is a layer of material with a thickness ranging from fractions of a nanometre (monolayer) to a few micrometers which is substantially thinner than the other two dimensions. The practicability of thin films as well as scientific curiosity about twodimensional solids has sparked a burgeoning interest in thin film science and technology. A wide range of uses ranging from micrometer dots in microelectronics to multi-square-meter window coatings the thin films have [9]. The key applications that profit from thin film manufacturing are electronic semiconductor devices and optical coatings [10, 11]. Dielectric micro-capacitors, pyro-electric microsensors, and photovoltaic devices are additional well-known thin film-based technologies [9–15]. Figure 1 shows the applications of thin films in various devices. The topic of thin films has grown rejuvenated in recent years because of the potential of two-dimensional solids, technical values, and scientific curiosity in the characteristics. In the current era, there is a huge requirement for these devices which are low cost, easy to manufacture, environment friendly, energy generating, and storing to meet the various leading challenges in this modern world. Thus, the thin film is a very important field of research due to its widespread applications in miniaturized, compact, and integrated devices which lower down the cost issue [16]. As the need for small-scale devices grows, it’s not unexpected that thin film-based transducers, energy harvesters, and storage devices have seen significant progress in recent years Fig. 1 Applications of thick, thin, and ultrathin films in various devices [9–15]

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[17, 18]. The paramount importance of thin film lies in the size effect phenomena associated with film; which can be achieved by controlling the preparative parameters in a proper way. The size effect dependence film properties may be established by comparing the thickness of the film to the mean free path of the charge carriers [16, 18]. As a consequence, the growth process, which is then followed by a particular deposition method, has a substantial influence on the structural behavior and features of films. Consequently, with a help of controlled and selective deposition for these films, one can have specific physical properties which can be used in different areas. Several challenges related to materials design, processing, integration, and characterization emerged as thin film-based devices have progressed, impacting the entire materials optimization process.

2 General Difference Between Bulk, 3D, 2D, 1D, 0D There is a size range between atomic-scale dimensions and the conventional bulk material dimensions where condensed matter exhibits distinct physical characteristics that vary significantly from bulk material physical qualities. Thin film (2D) materials vary from their bulk (3D) counterparts in this respect due to exceptional properties such as high surface-to-volume ratio and spatial confinement [19]. This is due to the fact that the electrical configurations of nanomaterials differ greatly from those of their bulk counterparts. Regular transitions within the density of electronic energy levels in response to size will now cause alterations, leading to considerable variances in optical and electrical properties [20]. Interestingly, the size, shape, and amount of aggregation may all be changed to modify the attributes of nanostructured materials. When the size of a nanocrystal (single crystalline nanoparticle) is smaller than the de Broglie wavelength, electrons and holes are spatially confined and electric dipoles are formed [21]. A decrease in characteristic size restriction under a threshold value, i.e., the electron de Broglie wavelength causes an electrical structural shift, resulting in a larger and more prominent band gap, as depicted in Fig. 2. The energy difference between consecutive layers widens as particle size decreases, analogous to the case of a particle confined in a box. As a consequence of such a change, electrical conductivity would be reduced. This is due in part to quantum size effects, which results in higher electronic energy states as the critical sizes of materials are reduced to a certain limit [21].

3 General Route for Thin Film Fabrication The synthesis of nanostructured thin films can be done in one of two ways (a) Top down approach and (b) bottom up approach (Fig. 3).

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Fig. 2 a Density of states (DOS) against energy as a function of size, b The energy separation between consecutive layers rises with decreasing size [21]

Fig. 3 Nanostructured synthesis approaches [19, 20]

3.1 Top Down Approach This method entails slicing or cutting a bulk substance into nano-sized particles in a series of steps. These methods employ bigger (macroscopic) starting structures that

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may be manipulated externally during nanostructure processing. Etching through the mask, ball milling, and extreme plastic deformation are only a few examples. But controlled dimensions to a nano or micro scale with proper reproducibility are quite difficult.

3.2 Bottom Up Approach This approach involves building a material atom by atom, molecule by molecule, or cluster by cluster from the bottom up. Miniaturization of material constituents (down to the atomic level) is paired with a self-assembly process to produce nanostructures. During self-assembly, physical forces at the nanoscale are employed to organize fundamental units into bigger and steadier structures. There are two examples which are (i) quantum dot production during epitaxial growth, and (ii) nanoparticles development from colloidal suspension. Both approaches are critical in modern business, and nanotechnology is likely to be no exception. Both systems have advantages and downsides. Advantages of top down approach (i) Top down approaches are good for producing structures with long-range order. (ii) Well-established techniques are dominant in macro fabrication. Disadvantages of top down approach (i) (ii) (iii) (iv)

Limited for nanofabrication as tolls available is big. Slow and inefficient for large-scale manufacturing. Internal stress is produced along with surface defects and contaminations. Limitation comes from the wavelength of light or tool.

Advantages of bottom up approach (i)

Bottom up approaches are best suited for assembly and establishing short-range order at nanoscale dimensions. (ii) Suitable for mass manufacturing. (iii) Fabrication is much less expensive. (iv) It promises a higher probability of obtaining nanostructures inclusive of minimal defects and added homogenous chemical composition. Disadvantages of bottom up approach (i) Needs compatible surfaces and molecules. (ii) Fewer tools to manipulate molecules and atoms.

4 Thin Film Deposition Methods Physical and chemical thin film deposition processes can be broadly categorized. Following schematic flow chart represents thin film deposition methods (Fig. 4).

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Fig. 4 Schematic flow chart represents thin film deposition methods [10]

Vacuum evaporation and sputtering are physical processes in which the thin layer to be grown is converted to a gaseous form through evaporation or collision, and then deposited over the substrate surface. However, physical methods are plugged with certain difficulties such as small area of deposition, time-consuming, typically high temperature processes requiring the use of sophisticated and high working cost instruments, wastage of depositing material, vacuum necessity, substrate restricted deposition, pure source materials requirement, and cleaning after each deposition. Furthermore, problems with thermal expansion mismatch, impurity, interdiffusion, and grain boundary effects are found to be significant. Chemical procedures, on the other hand, are classified into two categories: gas phase and liquid phase. Chemical vapor deposition (CVD) in the gas phase includes conventional CVD, photo CVD, laser CVD, plasma accelerated CVD, and metalorgano CVD. Chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR), electrodeposition, spray pyrolysis (SP), sol–gel, anodization, liquid phase epitaxy, and others are liquid phase chemical processes. Chemical processes in the liquid phase provide a number of benefits over physical approaches, including being easy, cost-effective, and suitable for large-scale deposition. Chemical procedures feature easily changeable preparative parameters including concentration, complexing agent type, temperature, pH, and so on. Hence, emphasis has

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been given to chemical methods as they can be considered a hotspot of the interdisciplinary research. It is nowadays generally accepted that thin films allow for the production of highly competitive structures in a complex design by simple and reliable chemical methods, with minimum consumption of materials and very low production costs. On the other hand, it was demonstrated that thin films should not only just reproduce the composition of the base material, but present a preferential structure and even morphology, in any case, guaranteeing the desired functionality.

5 Film Formation Mechanism The thermodynamic equilibrium approach and the kinetic approach may be used to categorize various synthesis procedures or deposition methods [10]. a. Kinetic approach to nanostructure development in which the amount of precursor accessible for growth is restricted or the process is contained in a small space. b. The production of supersaturation, nucleation, and subsequent growth are all steps in the thermodynamic equilibrium approach to synthesis.

5.1 First Principles of Nucleation and Growth: Role of Temperature and Concentration Grain size, shape, and orientation with other characteristics of particle deposits and thin films on the substrate are mostly defined during the early phases of nucleation and development which can be modified by deposition circumstances [22]. As a result, having a thorough understanding of such connections is beneficial for improving thin film production processes in terms of practical applications. A supersaturation of growth species is required for nucleation to occur in the development of nanostructured films. Supersaturation occurs when the temperature of an equilibrium mixture or saturated solution is decreased. A new phase is formed either when the content of solute within a solvent surpasses its equilibrium solubility or when the temperature descends the phase transition threshold. Through the creation of a solid phase while separating the solute from the solution and sustaining an equilibrium concentration in the solution, the energy of the entire system would be lowered. The reduction of Gibbs free energy fuels nucleation and subsequent growth. The Gibbs free energy change per unit volume for homogeneous nucleation of a solid phase from a supersaturated solution is given by the following equation [23]; /\G = −

C kT ln u Co

(1)

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where C represents the solute concentration, Co represents the equilibrium concentration or solubility, k represents the Boltzmann constant, u is the atomic volume, and T is the temperature.

5.2 Nucleation and Growth Nucleation is a creation of stable small particles of the product phase, upon which growth can occur. In this process, a succession of atoms or molecules from the reactant phase reorganize into nuclei, which are massive clusters of the product phase. Nucleation can be homogeneous and heterogeneous nucleation.

5.2.1

Homogeneous Nucleation

Homogeneous nucleation is the process in which solid nuclei emerge spontaneously inside the process. It happens when a phase has no unique items that might trigger nucleation or no foreign particle exists. In other words, homogenous nucleation occurs when there are no special objects inside a phase that can cause nucleation. Pure homogeneous nucleation is uncommon, yet it is simple to model because of the lack of intricate heterogonous sites [24]. As an example, consider the homogeneous nucleation of a product phase which is represented by a spherical particle of radius r . . On the transition of a system from gas to a solid phase, its chemical free energy diminishes (Fig. 5a). Since the amount of free energy liberated on transformation is directly related to the nucleus volume, the change in chemical free energy per unit volume of the nucleus may be calculated as follows [24]: 4 3 πr /\G V 3

Fig. 5 The energy involved in homogeneous nucleation [24]

(2)

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Subsequently, new surfaces as well as interfaces are evolved which in turn increases the system’s surface free energy. The amount of free energy consumed for the formation of the interface is directly proportionate to the particle surface area and its interfacial energy, as follows: 4πr 2 γ

(3)

here, γ symbolizes solid–vapor interfacial energy per unit area. Henceforth, the total change in free energy occurred on nucleus formation as follows: /\G =

4 3 πr /\G V + 4πr 2 γ 3

(4)

Furthermore, the aforementioned equation when differentiated yields a stationary point, the point where rate of change in free energy becomes negative (Fig. 5b), which is called as critical radius (r ∗ ) at which the nucleating particle gets stabilized and the corresponding free energy is identified as critical free energy (/\G ∗ ); 4 d/\G = 0 = πr 3 /\G V + 4πr 2 γ dr 3

(5)

0 = 4πr 2 /\G V + 8πr γ

(6)

2γ /\G V

(7)

16π γ 3 3(/\G V )2

(8)

r∗ = − /\G ∗ =

This /\G ∗ serves as the effective energy barrier for nucleation.

5.2.2

Heterogeneous Nucleation

The process by which the product phase is visible on the container walls and/or impurity/foreign particles is known as heterogeneous nucleation [25]. When there are particular items within a phase that might trigger nucleation, this is referred to as heterogeneous nucleation (Fig. 6). Understanding the heterogeneous nucleation using the capillarity model The capillarity theory of heterogeneous nucleation is based on the following key considerations. a. Nucleation on a substrate. b. Nuclei are spherical caps.

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Fig. 6 Schematic presentation of heterogenous nucleation [25]

Assume that in the vapor phase, film-forming molecules or atoms collide with the substrate, creating nuclei with a mean dimension of r (Fig. 6). The following equation computes the free energy shift involved in the creation of such aggregation [23, 25] /\G = a3 r 3 /\G V + a1 r 2 γv f + a2 r 2 γ f s − a2 r 2 γsv

(9)

where a1 = 2π [1 − cos(θ )], a2 = π sin 2 θ, a3 = π [2 − 3cos(θ ) − cos 3 (θ )], γv f represents surface free energy of aggregates, γs f implies to the interfacial free energy of aggregates, and γsv is the surface energy of the substrate. While the terms, a3r 3 /\G V , a1 r 2 γv f , and a2 r 2 γ f s relates to the volume of cap, cap surface area, and projected surface area, respectively. Thus, the critical radius (r ∗ ) for heterogeneous nucleation can be given by d/\G =0 dr −2(a1 γv f + a2 γ f s − a1 γsv ) 3a3 /\G V )3 ( 4 a1 γv f + a2 γ f s − a1 γsv ∗ /\G = 27a3 2 /\G V 2 r∗ =

(10) (11)

(12)

or, /\G ∗ = ( Taking

2−3cosθ +cos 3 θ 4

)

( ) 16π γv f 3 2 − 3cosθ + cos 3 θ 4 3/\G V 2

(13)

as f (θ ), Eq. (13) becomes /\G ∗ =

16π γv f 3 f (θ ) 3/\G V 2

(14)

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Case 1: when, θ = 0◦ f (θ ) = 0 /\G ∗ = 0, which implies complete wetting Case 2: when, θ = 180◦ f (θ ) = 1 16πγ 3 /\G ∗ = 3/\Gv f 2 , which implies no wetting. That is, the foreign surface is inactive V in the nucleation process. In general, equation can be expressed as follows: /\G het (r ) = /\G hom (r ). f (θ )

(15)

5.3 Effect of Temperature The dynamics of heterogeneous nucleation and film growth are greatly influenced by temperature [26]. Figure 7 depicts the change in Gibbs free energy of a system when the temperature is changed (/\G ∗ ). The particles formed at two different temperatures T1 and T2 (T1 >T2 ) which are less than melting point (Tm ) with a radius of r1∗ and r2∗ . The change in Gibbs free energy of particles at a critical radius r1∗ is /\G ∗1 and for r2∗ is /\G ∗2 . According to Fig. 7, greater temperatures often result in bigger r ∗ and, as a result, larger /\G ∗ . This means that the barrier to nucleation is greater at higher temperatures, resulting in a reduction in the number of critical nuclei (i.e., larger fewer grains). Low nucleation centers result in larger and more perfect single crystals. In other words, greater undercooling results in smaller r ∗ and /\G ∗ , and thus easier nucleation. Fig. 7 The change in Gibbs free energy of a system with change in temperature [24]

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Thus, the temperature had a marked influence on particle size, with lower temperatures resulting in more and thinner particles with a greater surface area than those produced at higher temperatures.

6 Device Grade Design and Development For device grade applications, knowledge about one method is not the sufficient, and single method is not sufficient to complete the device architecture. Also, each method has its own advantages and disadvantages. Hence, in order to develop any device, the idea has to be passed through the three major points: (i) materials (ii) process, and (iii) properties of materials. Materials may be nanocrystalline, crystalline, polycrystalline, amorphous, or even epitaxial embedded in thin film form. Thin film with nanostructured surface architecture may inclusive of nanoparticles, nanowires, nanotubes, nanoparticles, or quantum dots. But whatever process or method adopted to synthesize these materials in thin film form with desired properties should be at low cost, simple, and easily accessible, along with raw materials should be with high purity and easily available. Most importantly, the process should be compatible with small to large-scale integration for industrial mass production. Furthermore, the method or process should produce desired surface architecture with the required properties. As specific device required specific materials which can have different properties like thermal, electrical, optical, mechanical, or magnetic that can be possible to tune by adopting specific materials and processes to meet the demand (Fig. 8). For this, one has to need a minimum basic background in physics and chemistry to synthesize and characterize material in thin film form with desired properties along with engineering and technology in order to commercialize. Hence, knowledge of most deposition methods is essential to choose the appropriate method.

7 Nanostructures Cubes, polyhedrons, rods, rings, wires, flowers, dots, and a range of other unique forms are all possible morphologies for nanostructures [27–31]. Surface atoms per unit volume are abundant in nanostructures and nanomaterials. The substantial fall in the fraction of core atoms to surface atoms in nanostructured materials may elucidate the occurrence of physicochemical changes in the properties of nanomaterials with variations in size/dimension. Total surface energy rises as the total surface area expands, a factor that is greatly impacted by material critical size. It is worth mentioning that surface energy and surface area are enhanced by an order of seven in magnitude when the size of the particle is reduced to nanoscale from centimeter. All nanostructured materials have a high surface energy due to their huge surface area, therefore assembling nanoparticles into highly ordered architecture requires optimal

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Fig. 8 Materials, process, and properties required for diverse application of thin films [5–8]

processing processes that are low cost, tuneable, and scalable to large area deposition. Chemical methods for preparing high-quality thin films of good uniformity are generally identified as liquid phase synthesis, which includes the following.

8 Brief Regarding Chemical Methods 8.1 Chemical Bath Deposition (CBD) The chemical bath deposition (CBD) approach entails the controlled precipitation of a chemical from a solution on a suitable substrate [32]. The surface of the substrate is subjected to a number of reaction interval phases in order to create a thin coating through CBD. Typically, nucleation occurs first, followed by the adsorption of more ions from the solution, with subsequent growth accompanied by either an ion-byion or cluster-by-cluster mechanism. The ion-by-ion growth mechanism involves the diffusion of cations and anions to the substrate followed by nucleation assisted by the substrate. In the cluster mechanism, cations and anions react first exclusively inside the solution to form clusters, before diffusing onto the substrate to form nucleation centers. The solubility product of the precursor greatly contributes to the product’s homogeneity and stoichiometry. When an ionic product is greater than the solubility product, supersaturation happens, precipitation occurs, and ions assemble from the solution on the substrate to form nuclei. The CBD method enables a precise control over the deposition rate, in turn, film thickness through fine tuning of synthesis parameters like concentration, temperature, and pH. Optimal thickness and deposition rate are determined by variables like stirring rate, number of nucleation centers,

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and solution supersaturation. During the deposition process, the growth kinetics is also reliant on the ion concentration, their velocities, nucleation, and growth developments on the dipping substrates. The CBD approach has been used to synthesize a wide number of compounds from almost every class, including oxides, chalcogenides, phosphates, and many more. The major advantages of this method are costeffectiveness, simplicity, scalability, versatility, and capability to coat a variety of nanostructured morphologies. A key downside of this method is its incompetence in regard to precursor usage and transformation to a thin film. Furthermore, metallic coatings cannot be deposited with CBD.

8.2 Successive Ionic Layer Adsorption and Reaction (SILAR) As aforementioned, when the ionic product exceeds the solubility product in the CBD method, a film is formed. As a result, a large amount of precipitate may accumulate in the bath, and this precipitate cannot be eliminated. In order to minimize such excessive bulk precipitation, the CBD method has been modified and renamed the successive ionic layer and adsorption and reaction (SILAR) method. The SILAR approach is based upon the sequential adsorption and reaction of ions from the precursors, as well as washing with deionized water in between every immersion for the occurrence of ensured uniform precipitation [33]. Pre-adsorbed (cations) and newly adsorbed (anions) materials react to generate thin films of the desired substance [34]. By adopting the SILAR method, the best characteristics of film creation such as homogeneity, adhesion, and thickness, are achievable owing to its efficacy. Adsorption is a crucial step in the SILAR process, as it plays a decisive role in the layer-by-layer coating of the desired film. The adsorption process is influenced by many preparative factors such as cation and anion concentration, solution temperature, substrate nature, substrate area, complexing agent, pH, rinsing, adsorption, and reaction time. This method has been used to deposit oxides, sulfides, and selenides of many metals. Without the need for any specialized instrumentation, it enables the selection of substrate materials that may have any melting point or conductivity properties. However, only part of the precursor amount utilizes to form a thin film in SILAR and do not valid for metal deposition.

8.3 Ion Exchange (IE) Ion substitution processes induce a variety of re-equilibrium mechanisms in rocks and metal oxide displacement reactions. In this context, the ion exchange method has long been used to change the composition and properties of crystalline materials by substituting enlarged solidified components with those in solution. The ion exchange (IE) method is substantially quicker because nanoparticles have a high surface-tovolume ratio. After full conversion, the exchange mechanism is almost topotactic; it

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maintains the structure with very minor strain changes [35, 36]. Depending on the kind of components required, the IE approach permits the fabrication of a wide range of nanostructured thin films made up of metal oxide, chalcogenides, and perovskite structures with intricate designs that include nanoparticles, alloys, and core-shells. The IE may be cationic or anionic, depending on the sort of needed components. The anionic type of exchange, on the other hand, is troublesome owing to the vast size and limited permeability of the lattice anions, which results in the collapse of the host structure. This approach is profoundly influenced by the initiation barrier and thermodynamics. The spontaneity of the exchange process may be attributed to the reduced activation barrier for atom transport in nanocrystalline materials compared to the bulk state. Initialization and development of the cation-exchange process are dependent on the crystalline structure and shape of the component nanocrystal. In addition, surfaces with higher curvature and a lower coordination number are the most favorable sites for the initiation of such a reaction. Once the parent nanostructure is placed into the cationic precursor, the compound creates a substantial number of non-uniform islands on the parent nanostructured surface over time. The discrepancy in solubility product (Ksp ) between the starting material and the resultant material causes the formation of these islands. The lower the Ksp , the more likely it is that the parent material will be transformed into the final material with minimal structural changes. At the contact, the island formed on the surface of the parent structure generates a strain field. By increasing the cationic concentration or lengthening the time the parent structure is submerged in the initial cationic precursor concentration, a homogenous portion of the final material is formed on the surface of the nanostructure. These segments are generated by an increase in cationic concentration or the absorption of small islands into bigger ones, a phenomenon known as “Ostwald ripening.” The islands eventually cover the whole surface of the parent structure, generating a case-like covering on the original morphology. This coating represents the shell of a core–shell heterostructure that converts the whole reactant to the final product without altering the morphology of the beginning reactant.

8.4 Electroless Electroless deposition is a simple and adaptable chemical deposition for the creation of thin films of different metal nanostructures without the need for external electricity. The basic principle of this method involves metal ions being reduced from the solution by spontaneous reactivity, followed by nucleation and development on the substrate without the need for an electric source. The method of electroless coating is demonstrated to be a solely electrochemical governed system, which involves both oxidation and reduction reactions accompanied by electron transfer between reacting chemical species. Further, both the processes (cathodic and anodic) occur simultaneously at the catalytically active surface of the substrate immersed in a bath comprising metallic salt along with a reducing agent.

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The electroless method may coat thin coatings of metals, alloys, and even compounds. Furthermore, the deposition of compounds like ceramics or polymers is also being recently accomplished via an electroless process, which is absolutely desirable in the field of nanotechnology. Electroless plating has various advantages such as being simple, low cost, scalable, and capable of coating any type of substrate due to its self-driven reduction reaction process. Additionally, no electric current requirement of the electroless process enables flexibility in choosing a wide range of shape without limitations of size and shape for deposition. Being an easy and uncomplicated method, electroless is prone to offer good control over the deposition rate, in turn, morphology through proper tuning of bath parameters like immersion time, temperature, concentration of metal ions, nature of reducing agent, etc. Benefitting from these technical values, an ample of reports have been published on the electroless method. Furthermore, there are a few drawbacks to adopting electroless plating for thin film fabrication, such as the short bath life, environmental concerns, and surface pre-treatment needs [37].

8.5 Electrodeposition Electrodeposition is a versatile and inexpensive approach for fabricating a broad variety of 2D and 3D nanostructured materials, such as coatings and films. The electrodeposition process is based on electrochemical principles governing the reduction or deposition of electroactive and related species on the cathode surface. In the electrodeposition process, a layer of one metal is simply coated on the other metal only by exchanging electrons with ions (oxidation–reduction reaction) in the electrolyte. When the cathode, anode, and reference electrode are immersed in an electrolyte that contains metal ions, the positively charged metal ions are drawn to the cathode and deposited by being electrically neutralized. It is possible to organize the deposition process by managing the charge quantity and charge rate that pass through the electrolyte. Electrical energy causes chemical changes that can be seen on the substrate or in the electrolyte. This would increase the controllability of the electrodeposition process when considering the electrochemical principles applicable to the intended objectives and applications. This method is widely employed for both soft and hard coatings for ornamental and protective reasons, and as a result, it is being investigated for new emergent uses in the electronics sector. Many different features, such as the conductivity of a semiconductor (whether it be n-type or p-type), the change in band gap, control over stoichiometry, and doping may be readily and accurately regulated by using this method. Electrodeposition has been used to deposit many materials in the form of thin films for a variety of purposes. These include ceramics, metals, semiconductors, superconductors, oxides, and hydroxides. The preparative parameters have a direct influence on the qualities of the material, including its structural, optical, and morphological characteristics. Controlling preparative factors such as the nature of the substrate,

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the applied electric field, bath temperature, complexing agents, and pH of the bath, result in a coating of smooth, uniform, and stoichiometric in composition [38].

8.6 Hydrothermal Hydrothermal synthesis is a single-crystal synthesis process that relies on the solubility of minerals in hot water under high pressure. The crystal growth is carried out in an autoclave, which is a pressure vessel made of steel. Between the two ends of the growing chamber, a temperature differential is maintained. The solute dissolves at the hotter end, while it is deposited on a seed crystal at the cooler end, forming the desired crystal. The capacity to form crystalline phases that are not stable at the melting point is one of the possible benefits of the hydrothermal approach over other kinds of crystal development. The hydrothermal process may also be used to grow materials with a high vapor pressure around their melting temperatures. The approach is also particularly well suited to the formation of big, high-quality crystals with precise composition control. The drawbacks include the requirement for costly autoclaves and the inability to observe the crystal as it develops. Under hydrothermal circumstances, a significant number of compounds from nearly every class have been synthesized, including elements, simple and complicated oxides, tungstates, molybdates, carbonates, silicates, and germanates, among others. Furthermore, hydrothermal synthesis is often used to generate commercially valuable single crystals such as synthetic quartz, diamonds, and other single crystals. Both in the search for novel compounds with specified physical characteristics and in the systematic physicochemical analysis of complicated multicomponent systems at increased temperatures and pressures, the approach has proven to be remarkably efficient [39].

8.7 Spray Pyrolysis (SP) Spray pyrolysis (SP) is a thin-film deposition involving the spraying of a liquid solution over a heated substrate. Due to the high temperature of the substrate, pyrolytic reactions occur in which solvent is evaporated and salt is deposited with strong adhesion on the substrate. The SP does not need a vacuum or exceptionally pure starting material. Researchers and businesses are attracted to SP because of its simplicity and reproducibility when dealing with thin films of metal oxides, metal chalcogenides, and other substances. Thin films of metal chalcogenide, simple metal oxides, mixed oxides, metallic spinel-type oxides, chalcogenides (groups I-VI, II-VI, III-VI, V-VI, and VIII-VI), and ternary chalcogenides (groups I-III-VI, II-III-VI, II-VI-VI, and V-II-VI) may be produced using the SP. Superconducting oxide films have also been successfully produced using the SP. The SP synthesized metal oxides and metallic spinel oxide materials provide characteristics appropriate for a variety of applications. Single- or multi-metal films may be deposited by SP and provide benefits in a

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number of applications. SP synthesized thin films are primarily used in the solar cell, fuel cell, battery, and supercapacitor industries in addition to gas sensors and antifogging. It also deposits photosensitive materials suited for optical recording and laser diodes. Different kinds and forms of substrates may be employed, but they must be able to withstand high temperatures and maintain their structural integrity. However, SP has disadvantages, including toxic gas emission, limited yield, trouble forming non-oxide coatings, and difficulties evenly coating a rough substrate surface [40].

8.8 Spin Coating It is defined as a method of spreading a solution equally across a surface utilizing centripetal force to coat uniform and homogeneous thin film on to substrate surface through prepared precursor or gel. When a gel of precursor inclusive of a specific solute and solvent is rotated with high rotation speeds, the centrifugal force as well as the surface tension of the precursor solution results in the release and vaporization of the solvent toward the formation of a solid film producing a uniform covering. The centripetal force, which is responsible for the spread of liquid across the wafer, is the fundamental mechanism of spin coating [41]. Spin coating is utilized in an extensive widespread application of materials through productions and technology segments including energy storage and conversion, field emission transistors, sensors, supercapacitors, and many others applications. In the microelectronics sector, it is a common method of producing thin films. The primary advantage of spin coating process over other techniques is its capability to quick and ease of deposition a very uniform and variety of nanostructured films. Spin coating allows a highly homogeneous layer to be applied for planar substrate across wide region (up to 30 cm) by a highly tunable as well as reproducible thickness. Inorganic, organic, and inorganic/organic solution mixes can all be coated. It is been used in the thin film synthesis of many oxides, including ZnO, Bi2 O3 , Fe2 O3 , CdO, CuO, TiO2 , RuO2 , and WO3 [42–49]. Notably, any material that can form gel or viscous can be possible to coat via on substrate to form a thin film via spin coating. The only degrees to modify in this approach are spin speed and fluid viscosity, making spin coating fairly durable. Changing the spin speed or switching to a different viscosity solvent may simply vary the film thickness. Disadvantages of the method include the cost increment and lesser throughput as substrate size is increased [41]. Additionally, lack of material efficiency is another drawback of spin coating. Typically, only 2–5% of the material poured onto the substrate is used in spin coating operations, with the remainder 95–98% tossed off into the coating bowl and discarded [47, 48].

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8.9 Dip Coating Dip coating is a simple and cost-effective process for manufacturing thin films with controlled thickness compounds at room temperature onto a variety of substrates such as metallic, ceramic, polymer films, and fibrous materials. The underlying principle of this method is the application of aqueous-based liquid solutions to the substrate surface for coating. Typically, target materials are dissolved in solutions that are directly deposited on the surface of the substrate, followed by the evaporation of the sedimentary wet coating using an infrared (IR) light to produce a dry film. It is possible to create superconducting, semiconducting, conducting, or insulating thin films. Various nanoscale morphologies, including nanoparticles and nanowires, may be deposited with homogeneity and the required thickness. In addition, films may be coated in any form, including cylindrical, spherical, and irregular ones that can be integrated across a vast surface area. However, the seemingly simple process of dip-dry coating method requires a complicated set of chemical and physical multivariable factors. Many parameters, including immersion duration, withdrawal speed, dip coating cycles, density and viscosity, surface tension, substrate surface, and coating solution evaporation conditions, influence the thickness and structure of thin films formed during dip-dry method. Furthermore, as adsorption is a key element of the coating process, it is important to note that the hydrophobic and hydrophilic properties of the substrate play a significant influence. In certain instances, coating requires surface passivation, activation, and nucleation center creation [50].

8.10 Doctor Blade One of the most extensively used solution processing for large area thin film manufacturing at a cheap cost is doctor blade coating (tape casting). The phrase “doctor blade” was first created in 1940, and it was used to describe thin sheets of piezoelectric materials and capacitors. The primary operating idea of this technology is that the blade and the substrate are constantly moving relative to one another, either by passing the blade over the substrate or by sliding the substrate below the blade. When a relative motion between the blade and the carrier surface is formed in this method, a well-mixed coating solution is deposited on a substrate beyond the blade, and the prepared slurry efficiently spreads across the substrate surface, generating films of varying thicknesses upon evaporation. The geometry of the doctor blade machine and the viscoelastic qualities of the coating solution, which must be carefully managed, have a significant impact on the ultimate thickness, quality, and uniformity of the resultant film [51].

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8.11 Screen Printing Screen printing is a pattern-making process that involves transferring ink or metal to a surface through the use of a fine-mesh screen. The basic premise of screen printing is to apply ink to a substrate with a mask and a Squeegee to create a pattern on the surface. Screen printing is an effortless process to perform at a low cost. Depending on the intended usage, the substrate and screen mesh might be made of cloth, plastic, metal, or any other material. The factors that influence the thickness, homogeneity, microstructure, and shape of films include squeegee form, screen frame, mesh count, ink properties, and sintering temperature and time. Owing to its simplicity, industrial scalability, and versatility, screen printing technology has been widely used for massproducing thin films for a number of applications [52–54]. The creation of patterns for apparel and textiles, greeting cards and paper printing, displays and signboards, and the creation of patterns and logos for the automotive sector all include the usage of screen printing for the development of electrical circuitry and thick films screen printing is most crucial. It is often used to create printed circuit board (PCBs) and also used to produce surface mountain devices (SMD) specifically on PCB. Additionally, it is used to create thick film alkaline batteries together with solar cell fabrication.

8.12 Sol–gel This method is regarded as the versatile and cost-effective ways to make thin films. The procedure starts with the creation of a sol, which is followed by successive gelation and subsequent solvent withdrawal to yield a gel, which is then spread across the substrate surface using different methods to produce the thin film. Ceramics, porous, and glass-like materials along with nanoparticles and nanocomposites can be made using the sol–gel process depending on the bath components and condition. The preparative parameters of sol–gel method are temperature, precursor nature, water: alkoxide molar ratio, catalyst concentration, pH, and other additives. Sol–gelderived materials have critical applications in optics, chemistry, biology, electronics, and biomaterials [55]. Henceforth, it is inferred that the sol–gel deposition process has a significant impact on structural and hence, physical qualities.

9 Applications Thin films have tremendous applications ranging from small-scale microelectronics to large-scale industrial mass production even in large area deposition. Applications are in bio-medical field, energy sector through photovoltaic and energy storage,

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sensors, electronic industry, optical devices, ceramic coatings, environmental analysis, decorative coatings, abrasive materials due to simple fabrication, high reliability, superior performance, reduction in size, weight, and low power consumption. In this section, some applications like photocatalysis, solar cells, supercapacitor, and sensors have been elaborated with incorporated case studies.

9.1 Photocatalysis To improve global water quality and sustain a healthy environment, water pollution is a life-threatening problem that must be tackled urgently. Despite the fact that the majority of research is currently focused on organic pollutants, dye pollution is a severe issue due to their large worldwide output. A rigorous treatment method is required due to the evident toxicity of both organic and dye pollutants. In this regard, photocatalysis is considered as a promising method. Photocatalysis is the process of accelerating a photoreaction in the presence of a catalyst. A photocatalyst is a substrate that absorbs light and functions as a catalyst for chemical processes. Photocatalysts are all made out of semiconductors. One of the main variables that determine photocatalytic activity is the hierarchical heterostructure, which includes shape, particular composition, and functions [56]. High crystallinity, nano-size structure, a substantial number of surface hydroxyl species, and decreased band gap are all associated with increased efficiency.

9.2 Gas Sensors Gas sensors are devices that detect and respond to chemical species that have been adsorbed and can change electrical properties. These sensors rely on the adsorption– desorption theory as well as a heterocontact type sensing mechanism. The porous nature of the gadget allows liquified petroleum gas (LPG) species to reach the interface when it is exposed to LPG. Because of the abrupt change in electrical conductivity in the gas environment, the design and development of gas sensors necessitate a diverse range of materials with numerous surface architectures in nano forms with nanoscale sizes, such as quantum dots, nanobelts, nanotubes, nanoparticles, nanowires, and nanorods [57]. When exposed to the gas, high surface-to-volume ratios and a large number of active sites associated with nanostructured design assist in achieving maximum sensitivity.

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9.3 Solar Cells Solar cells are devices that directly convert light energy into electricity via the photovoltaic effect, resulting in electrical changes that can readily flow through semiconductors. In the realm of solar cells, the fabrication of nanostructured thin films involving inorganic, organic, hybrids, and even polymers have a significant influence. The structural qualities of a powerful photoelectrode include (a) a large surface area, (b) a scattering effect, and (c) efficient electron transport [58]. Nanorod and nanotube morphologies, in particular, are excellent for use in solar cells due to their lowest electron transfer resistance.

9.4 Supercapacitors Supercapacitors are devices that can store a greater amount of energy than batteries. Furthermore, supercapacitors deliver hundreds to thousands of times more power in the same volume as batteries, but they are unable to store the same amount of charge. Supercapacitors, in contrast to capacitors, have electrochemical characteristics such as exceptionally high energy density, high capacitance, and extended life. Generally, for high performance supercapacitors, nanostructured electrode materials are required that are low cost, non-toxic, and morphologically tuneable. This is because nanomaterials possess three fundamental characteristics: nanoscale dimension, high surface-to-volume ratio, and improved porosities, which are beneficial for supercapacitor application [17, 59].

9.5 Transistors Thin film transistors are field-effect transistors made by depositing a thin film over the supporting substrate or dielectric layer to generate the active semiconductor layer. Thin film transistors are constructed from common semiconductors such as silicon, cadmium selenide, or metal oxide [60, 61]. Thin film transistors have been used in the production and further reduction of integrated circuits. Conductive films, transparent and conductive films, luminous or fluorescent films, as well as dielectric and insulating layers, are required for the manufacturing of these tiny devices.

10 Summary • In summary, this book will concentrate on cutting-edge methods for the synthesis of thin films, including chemical bath deposition (CBD), successive ionic layer

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• • •

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adsorption and reaction (SILAR), ion exchange (IE), electroless deposition, electrodeposition, hydrothermal, spray pyrolysis (SP), spin coating, dip coating, doctor blade, screen printing, and sol–gel. In-depth discussion has been given to the role of the preparative factors needed for modifying thin films in terms of structure, surface morphology, electrical conductivity, and optical band gap, as well as sophisticated complementary characterization utilizing several specialized techniques. In a single volume, this book offers an up-to-date treatment of the crucial subject of chemical methods for thin film applications. A discussion of the underlying science of the films has been conducted, with an emphasis on nucleation, film structures, growth mechanisms, phase diagrams, film behavior, and, most importantly, applications. The advancement of thin film synthesis, novel materials in thin film form (such as oxides, sulfides, and selenides), characterizations, instrumentation, and industrial application are also covered in the book.

References 1. J.E. Greene, Tracing the 5000-year recorded history of inorganic thin films from 3000 BC to the early 1900s AD. Appl. Phys. Rev. 1, 041302 (2014) 2. J.E. Greene, Tracing the 4000-year recorded history of organic thin films: from monolayers on liquids to multilayers on solids. Appl. Phys. Rev. 2, 011101 (2015) 3. J.E. Greene, Organic thin films: from monolayers on liquids to multilayers on solids. Phys. Today 67, 43 (2014) 4. B.K. Kuanr, S. Maat, S. Chandrashekariaih, V. Veerakumar, R.E. Camley, Z. Celinski, Determination of exchange and rotational anisotropies in Ir Mn/Fe (t)/Ir Mn exchange coupled structures using dynamic and static techniques: application to microwave devices. J. Appl. Phys. 103, 07C107 (2008) 5. G. Hodes, Chemical Solution Deposition of Semiconductor Films (Marcel Dekker Inc., New York, 2001) 6. Q. Ma, G. Ren, K. Xu, J.Z. Ou, Tunable optical properties of 2D materials and their applications. Adv. Opt. Mater. 9, 2001313 (2021) 7. J. Lee, T.Y. Ko, J.H. Kim, H. Bark, B. Kang, S.-G. Jung, T. Park, Z. Lee, S. Ryu, C. Lee, Structural and optical properties of single- and few-layer magnetic semiconductor CrPS4. ACS Nano 11, 10935–10944 (2017) 8. S. Chowdhury, D. Jana, A theoretical review on electronic, magnetic and optical properties of silicene. Rep. Prog. Phys. 79, 126501 (2016) 9. M.J. Cordill, P. Kreiml, C. Mitterer, Materials engineering for flexible metallic thin film applications. Materials 15, 926 (2022) 10. C. Glynn, C. O’Dwyer, Solution processable metal oxide thin film deposition and material growth for electronic and photonic devices. Adv. Mater. Interf. 4, 1600610 (2017) 11. T. Schneller, R. Waser, M. Kosec, D. Payne, Chemical Solution Deposition of Functional Oxide Thin Films (Springer, London, 2013), p. 796 12. E. Fortunato, P. Barquinha, R. Martins, Oxide semiconductor thin-film transistors: a review of recent advances. Adv. Mater. 24, 2945 (2012) 13. M.S. Bhuiyan, M. Paranthaman, K. Salama, Solution-derived textured oxide thin films—a review. Supercond. Sci. Technol. 19, R1 (2006)

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38. R. Manivannan, S.N. Victoria, Preparation of chalcogenide thin films using electrodeposition method for solar cell applications–A review. Sol. Energy 173, 1144–1157 (2018) 39. M. Shandilya, R. Rai, J. Singh, Hydrothermal technology for smart materials. Adv. Appl. Ceram. 115, 354 (2016) 40. D. Perednis, L.J. Gauckler, Thin film deposition using spray pyrolysis. J. Electroceram. 14, 103 (2005) 41. H.A. Mustafa, D.A. Jameel, Modeling and the main stages of spin coating process: a review. J. Appl. Sci. Technol. Trends 2, 91–95 (2021) 42. M. Smirnov, C. Baban, G.I. Rusu, Structural and optical characteristics of spin-coated ZnO thin films. Appl. Surf. Sci. 256, 2405–2408 (2010) 43. P.H.E. Falsetti, F.C. Soares, G.N. Rodrigues, D.M.S. Del Duque, W.R. de Oliveira, B.F. Gianelli, V.R. de Mendonça, Synthesis and photocatalytic performance of Bi2O3 thin films obtained in a homemade spin coater. Mater. Today Commun. 27, 102214 (2021) 44. R.K. Sonker, B.C. Yadav, Low temperature study of nanostructured Fe2O3 thin films as NO2 sensor. Mater. Today: Proc. 3, 2315–2320 (2016) 45. S. Duman, G. Turgut, F.S¸ Özçelik, B. Gurbulak, The synthesis and characterization of sol–gel spin coated CdO thin films: as a function of solution molarity. Mater. Lett. 126, 232–235 (2014) 46. M. Dhaouadi, M. Jlassi, I. Sta, I.B. Miled, G. Mousdis, M. Kompitsas, W. Dimassi, Physical properties of copper oxide thin films prepared by sol–gel spin–coating method. Am. J. Phys. Appl 6, 43–50 (2018) 47. I. Sta, M. Jlassi, M. Hajji, M.F. Boujmil, R. Jerbi, M. Kandyla, M. Kompitsas, H. Ezzaouia, Structural and optical properties of TiO2 thin films prepared by spin coating. J. sol-gel Sci. Technol. 72, 421–427 (2014) 48. E: Petrucci, M. Orsini, F. Porcelli, S. De Santis, G. Sotgiu. Effect of spin coating parameters on the electrochemical properties of ruthenium oxide thin films. Electrochemicals 2, 83–94 (2021) 49. Y.G. Choi, G. Sakai, K. Shimanoe, N. Yamazoe Wet process-based fabrication of WO3 thin film for NO2 detection. Sens Actuat B: Chem. 101, 107–111 (2004) 50. D. Dastan, S.L. Panahi, N.B. Chaure, Characterization of titania thin films grown by dip-coating technique. J. Mater. Sci.: Mater. Electron. 27, 12291–12296 (2016) 51. A. Berni, M. Mennig, H. Schmidt, Doctor blade Sol-gel technologies for glass producers and users (Springer, Boston, MA, 2004), pp.89–92 52. M.R. Somalu, A. Muchtar, W.R.W. Daud, N.P. Brandon, Screen-printing inks for the fabrication of solid oxide fuel cell films: a review. Renew. Sustain. Energy Rev. 75, 426–439 (2017) 53. N. Zavanelli, W. Yeo, Advances in screen printing of conductive nanomaterials for stretchable electronics. ACS Omega 6, 9344–9351 (2021) 54. M. Tudorache, C. Bala, Biosensors based on screen-printing technology, and their applications in environmental and food analysis. Anal. Bioanal. Chem. 388, 565–578 (2007) 55. H.J. Jeon, S.C. Yi, S.G. Oh, Preparation and antibacterial effects of Ag–SiO2 thin films by sol–gel method. Biomaterials 24, 4921–4928 (2003) 56. A. Di Mauro, M.E. Fragala, V. Privitera, G. Impellizzeri, ZnO for application in photocatalysis: from thin films to nanostructures. Mater. Sci. Semicond. Process. 69, 44–51 (2017) 57. P.P. Sahay, R.K. Nath, Al-doped zinc oxide thin films for liquid petroleum gas (LPG) sensors. Sens. Actuators, B Chem. 133, 222–227 (2008) 58. A.V. Shah, R. Platz, H. Keppner, Thin-film silicon solar cells: a review and selected trends. Sol. Energy Mater. Sol. Cells 38, 501–520 (1995) 59. M. Yu, X. Feng, Thin-film electrode-based supercapacitors. Joule 3, 338–360 (2019) 60. G.C. Patil, V. Kristaparapu, S.T. Ingle, S. Majumder, B.R. Sankapal, Approach for fabricating JLT using chemically deposited cadmium sulphide and titanium dioxide. Micro Nano Lett. 14, 1060–1063 (2019) 61. C.D. Jadhav, S.S. Karade, B.R. Sankapal, G.P. Patil, P.G. Chavan, Reduced turn-on field through solution processed MoS2 nanoflakes anchored MWCNTs. Chem. Phys. Lett. 723, 146–150 (2019)

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Dr. Akanksha Agarwal recently awarded a doctorate in the Department of Physics, Visvesvaraya National Institute of Technology (VNIT), Nagpur (India) under DST-Inspire scheme, Government of India. She has awarded with one of the most prestigious fellowships named DST INSPIRE FELLOWSHIP for pursuing her Ph.D. degree. Before that, she received her Masters’s degree in Physics (2018) from VNIT (Nagpur), India and received the Gold Medal for securing the highest CGPA in the tenure of degree. Her doctoral work centered around the design and development of hybrid nanostructured organic and inorganic materials for device grade application of supercapacitors. She has published 9 articles in the reputed international journals including reviews and research papers with more than 60 citation index and h index of 4.

Chapter 2

Chemical Bath Deposition: Thin Films with Assorted Morphologies Prashant K. Baviskar and Swapnil S. Karade

Abstract From ancient times till date, chemical bath deposition (CBD) is the most extensively used method for the synthesis of the thin film. This method was reported in 1884 by Reynolds et al. for the synthesis of PbS and described in detail by Bruckman in 1933. The CBD involves the deposition of film in a precursor involving sparingly soluble metal salts and solute of the desired deposits. The present chapter is mainly focused on the deposition of various types of metal oxides/hydroxides/chalcogenides thin films synthesized using CBD method. The chapter explores the experimental setup for the synthesis of thin film, the reaction mechanism, and the effect of preparative parameters on the growth of desired deposits. Advantages and disadvantages have been explored along with applications of chemical bath deposited thin films in thrust areas such as photovoltaic for energy conversion and energy storage through supercapacitor, transistor, and gas sensor.

Annotations C343 CBD CE CIGS CIS CV DEA DSSCs EDTA

Coumarin 343 Chemical bath deposition Counter electrode Copper indium gallium selenide Copper indium selenide Cyclic voltammetry Diethanolamine Dye sensitized solar cells Ethylenediaminetetraacetic acid

P. K. Baviskar (B) Department of Physics, Sangamner Nagarpalika Arts, D. J. Malpani Commerce and B. N. Sarda Science College (Autonomous), Sangamner, M.S. 422605, India e-mail: [email protected]; [email protected] S. S. Karade (B) Department of Green Technology, University of Southern Denmark, 5230 Odense M, Denmark e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 B. R. Sankapal et al. (eds.), Simple Chemical Methods for Thin Film Deposition, https://doi.org/10.1007/978-981-99-0961-2_2

27

28

EDX/EDS/EDAX EIS EQE EY FE-SEM FSS-SSC FTO HMTA HR-TEM IP ITO I-V J-E JLT J-V LED LPG MWCNTs NH3 NW OLED PEC pH PL PVA QD QDSSC SAED SC SEM SP SS TCO TEA TEM TFT UV XRD

P. K. Baviskar and S. S. Karade

Energy dispersive X-ray Electrochemical impedance spectroscopy External quantum efficiency Eosin-Y Field emission scanning electron microscopy Flexible solid-state symmetric supercapacitor Fluorine-doped tin oxide Hexamethylenetetramine High-resolution transmission electron microscopy Ionic product Indium tin oxide Current-voltage Current density-applied field Junctionless transistor Current density–voltage Light emitting diode Liquefied petroleum gas Multi-walled carbon nanotubes Ammonia Nanowire Organic light-emitting diode Photoelectrochemical Potential of Hydrogen Photoluminescence Polyvinyl alcohol Quantum dot Quantum dots-sensitized solar cell Selected area electron diffraction Supercapacitor Scanning electron microscopy Solubility product Stainless steel Transparent conducting oxides Triethanolamine Transmission electron microscopy Thin film transistor Ultraviolet X-ray diffraction

2 Chemical Bath Deposition: Thin Films with Assorted Morphologies

29

1 Introduction The selection of a specific deposition method with the desired properties of the thin film strongly depends on the availability of resources, a particular application, and production cost [1, 2]. The chemical bath deposition (CBD) method has many advantages including a simple experimental set-up, easily controlled over preparative parameters, cost effective, low-temperature process, and suitable for small to large area deposition [3, 4]. This method does not use any toxic volatile constituents and could produce binary, ternary, quaternary, and penternary films of metal oxides and/or chalcogenides [5]. The film properties along with nanostructured surface architecture can be controlled by preparative parameters like pH, complexing agent, bath temperature, deposition time, solution concentration, and substrate [6, 7]. The film prepared at a low concentration of solution showed very thin and non-uniform deposits due to an insufficient supply of ionic species during the synthesis process. In terms of deposition temperature, the films prepared at higher temperature yields larger granule growth; whereas, lower bath temperature yields nano or quantum size particles or some time suffers through sluggish deposits or the re-dissolution stage. Group II–VI materials possess direct band gap semiconducting behavior having pronounced potential in optoelectronics such as photovoltaic devices. In order to make the end product more affordable, we require a cost-effective methodology. CBD is a low-cost process for the production of superior quality thin films of semiconducting material at comparatively lower temperature over a large area towards industrialization. Deposition of II–VI compound films using CBD method results from the heterogeneous reaction in a basic aqueous solution. Since bulk II–VI compounds have very low solubility products, chemical reactions lead to the formation of precipitation in the reaction chamber which leads to poor film quality and non-uniform thickness. The crystallinity of chemical bath deposited semiconductor thin films can be simply tuned by adjusting the process temperature. Generally, the CBD method can be used to prepare any compound satisfying four basic conditions such as: (i) reaction between the used cation and anion should be spontaneous through simple precipitation for the material to be synthesized, (ii) the used precursor should have larger solubility releasing free ions to reactions to proceed, (iii) final obtained thin film should not re-dissolve in same precursor, and (iv) the rate of reaction is necessary to be slow down to get the desired deposit in thin film form [8]. The CBD is an appropriate choice of thin film deposition method for smallscale to large-scale area growth towards numerous applications such as gas sensors, solar cells, supercapacitor, field emission, and photocatalysis. Using CBD method, semiconductor thin films can be grown-up over the surface substrates which are dipped in dilute precursor having metal ions for oxide, hydroxide, sulfide, selenide, or telluride. In 1884, the first report for the deposition of PbS using CBD method was by Reynolds et al. [9]. The basic principles underlying the growth of semiconducting materials by CBD were reported in 1982 [10] and much work has been explored by many research groups [11–13]. The CBD has better profitable than physical methods

30

P. K. Baviskar and S. S. Karade

for the growth of a variety of films through wide spread morphologies from smallto large-scale deposition [14].

2 Experimental Set-Up Among several diverse methods available for the deposition of semiconductor films, CBD is probably the simple and less expensive method. It simply requires a reaction bath that contains a solvent which can be aqueous or non-aqueous, soluble metal salts, and the substrate on which thin layers are expected. The crystals grown (asdeposited) through chemical bath deposited films are often very tiny; this is the foremost reason for interest in the CBD method. Taking into account the present interest in nanostructures, CBD is an admirable route toward the synthesis of a thin film with nanostructured constituents. Figure 1 illustrates the experimental setup of the CBD method [12]. In this method, the magnetic stirrer is used to stir the solution where the substrates are kept stationary. Some cases involve stationary or stirring precursor, or the stirring of the substrate before or after the reaction starts. The chemical bath is kept in a vessel containing water or paraffin oil to maintain the bath temperature constant throughout the reaction. In addition to these different obstacles like the stirring mechanism, a reaction bath required constant bath temperature. One can control the generation rate of ions by optimizing a variety of preparative parameters, for example, precursor concentration, solution pH, bath temperature, and complexing agent.

Fig. 1 Experimental set up of chemical bath deposition method [12]

2 Chemical Bath Deposition: Thin Films with Assorted Morphologies

31

2.1 Basic Requirements The major requirement of thin film deposition by using CBD is the spontaneous reaction involved which is occurred through controlled precipitation; also known as simply chemical deposition or solution growth, or controlled precipitation method. From the literature survey, many articles were available which were discussing the current status of CBD. There are several review articles summering the deposition of many oxides, hydroxides, sulfides, selenides, and telluride material films using the CBD method [5, 8, 15–17]. Mane et al. [18] and Pawar et al. [12] explored the particulars of CBD and the different kinetics of film growth. The starting chemicals are usually available at a low cost. Using the CBD method, many substrates can be coated in a single run. It is not necessary that the substrate has to be electrically conducting. As ions are the building blocks instead of atoms in the chemical deposition method, it results in uniform and pin holes free growth. It is easy to control the preparative parameters and obtain better grain orientation and structure. Growth of the film occurs only after the ionic product surpasses the solubility product.

2.2 Solubility and Ionic Products Solubility is defined as the maximal quantity of solute that can dissolve in solvent leading to a saturated solution. Any additional quantity will precipitate and result in a suspension of particles. Solubility is expressed in terms of concentration units for a given pressure and temperature [19]. Sparingly dissolvable salt MN results in a saturated precursor after placing in aqueous media. It includes M+ and N− ions with non-dissolved solid MN. Between solid-phase and ions, equilibrium is established as: MN (S) = M+ + N−

(1)

At equilibrium, using law of mass action − K = C+ M CN / CMN (S)

(2)

where CM + , CN − and CMN represents concentrations of M+ , N− and MN in the precursor, respectively. Formation of solid-phase concentration yields a constant number, i.e., CMN (S) = constant = K’ − ' K = C+ M CN / K

(3)

32

P. K. Baviskar and S. S. Karade − orKK ' = C+ M CN

(4)

Since K and K’ are constant, the product KK’ is also constant, say Ks, therefore, equation becomes − KS = C+ M CN ,

(5)

where KS is termed as the solubility product (SP) and the ionic product (IP) is the product of CM + and CN − (IP = CM + CN − ). When equilibrium is established between the ionic and solubility products precursor becomes saturated. But the solution becomes supersaturated when IP go above the SP, i.e., IP/SP = S > 1. In this case, a combination of ions yields the precipitation over the substrate as well as in the precursor to create nuclei. Ambient temperature, type of the solvent employed, and particle size will affect the solubility product.

2.3 Basic Study of Chemical Bath Deposition Method The CBD also denotes as a process of producing a solid film with controlled kinetics of its formation in single immersion. Usually, chemical reaction bath contains one or many metal salts Mn+ , chalcogenide (X) source (X = O, OH, S, Se, Te, etc.), and generally a complexing agent (e.g., NH4 OH, KOH, NaOH, EDTA, etc.), in an aqueous media. The synthesis of metal chalcogenide by CBD occurs via four steps as follows [12]: (1) Equilibrium among the water and complexing agent. (2) Hydrolysis of oxide or chalcogenide source. (3) Creation or dissociation of complexes of ionic metal ligand [M(L)i ]n −ik , where Lk− represents one or many ligands. (4) Solid formation. As applied to the chemical deposition of oxide films from a metal cation Mn+ complexed by i ligands Lk− , these steps can be formulated as follows [16]: (1) Complexant—H2 O equilibrium: 2Lk− + 2H2 O _ 2LH −k+1 + 2OH −

(6)

(2) Dissociation of water: nH2 O _ nOH − + nH +

(7)

(3) Displacement of ligands: nOH− + M(L)i (n−ik)

+

→ M(OH)n (s) + i Lk−

(8)

2 Chemical Bath Deposition: Thin Films with Assorted Morphologies

33

(4) De-protonation to form oxide: M (OH)n (s) → MOn/2 (s) + (n/2) H2 O

(9)

The final overall kinetics of oxide formation can be well understood as M (L)i (n-ik)+ + (n/2) H2 O → MOn/2 (s) + nH + + iLk−

(10)

When a complexant is used, it is typically chosen based on its ligands affinity for the metal. This could make rate-determining step (step 8), thus decelerating the degree of solid-phase creation and hence, establishing quantification over the control. The hydrolytic route shown in Eqs. (9) and (10) is referred to as “forced hydrolysis” [20]. The addition of base is not necessary, even if the hydroxyl ion appears in Eqs. (8) on the left side. It can be speed up just by increasing the solution temperature, which promotes dehydrogenation (deprotonation) of the hydrated metal species as represented in Eq. (9). When the cation of metal is simply hydrolyzable as Al3+ , Fe3+ , Ti4+ , and Zr4+ then the hydrolysis can take place even in acidic solutions. On the contrary, chemical synthesis of non-oxides can be performed in alkaline precursors. The CBD entails precursor to be supersaturated when IP (product of metal and chalcogenide concentrations) should go beyond the solubility product of the required solid. When the reactants have been drained below this point, growth is not possible in a closed system. Thus, the supreme attainable film thickness is restricted by the source of reacting species. Hence, as the degree of supersaturation enhances (using upgradation of pH value, especially for oxides) the growth level of layer formation increases [21–24]. During this process, some of the solid powdery particles or comparatively larger size particles can form; those can be loosely attached to the film. This may result in lesser thickness than the expectation as appeared surface. Lightly supersaturated precursors yield moderate growth leading to films with thicker values; so-called “tortoise and hare” effect. The solubility product is mainly affected by three major factors viz. solvent, particle size, and temperature [25]. The solubility changes its path with respect to temperature, as stress increases with an increase in temperature. The balance among the precipitate and its ions in precursor will alter depending on whether the precursor’s reaction is exothermic or endothermic. It is found that the solubility constant is medium, temperature, and process of measurements dependent and hence, several orders of a degree difference in materials solubility constant [26–28]. The CBD for the growth of metal oxides has been fascinated because of easiness, inexpensive, and comparatively lesser processing temperature. The solution chemistry approach for the synthesis of metal oxides is established on the annealing of metal salt in an alkaline solution [29]. For example, in order to prepare an alkaline bath, ammonium hydroxide (NH4 OH) is used as a complexing agent. It is observed that the ammonia concentration decreases gradually if the ammonia solution is heated resulting in a gradual increase in the free cation concentration. It also increases as a function of temperature, the reduction in the stability of complexes is observed with a

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P. K. Baviskar and S. S. Karade

rise in temperature. Hence, the synthesis of metal oxide films is accomplished through metal salts-hydroxide ions reactions. Nevertheless, in several cases, the formation of metal hydroxide along with oxide is because of the use of an aqueous alkaline reaction bath [30, 31].

2.4 Literature Review The CBD is beneficial for the synthesis of a variety of thin films via metal oxides/hydroxide/chalcoginide (sulfide, selenide, and telluride)/polymer from groups II–V, II–VI, and V–VI [18] with assorted morphologies like nanoparticles [32], nanobeads [33], nanodots [34], quantum dots [35], nanotubes [36], nanorods [37], nanowires [38], nanonecklace [39], nanocoins [40], nanosheets [41], nanoflakes [42], nanoplates [43], etc.

3 Growth/Reaction Mechanisms During Chemical Deposition Establishing the relationship between synthesis parameters with resulting morphology of deposited thin films is extremely complex due to the interaction of various factors, which affect the nucleation as well as growth stage. Regardless of the wide range of variability, there are some typical morphological features that are found common in a wide range of thin films. Along with the gentle creation of the reacting species, we should now examine how the CBD compounds are formed by diverse mechanisms. The reactions take place in chemical bath deposition via the following four probable mechanisms toward the formation of a compound. The choice of which depends on the reaction parameters and specific process [12, 29, 91]. (1) (2) (3) (4)

Simple ion-by-ion mechanism, Simple cluster (hydroxide) mechanism, Complex decomposition ion-by-ion mechanism, and Complex decomposition cluster mechanism.

3.1 Simple Ion-By-Ion Mechanism Ion-by-ion mechanism is theoretically the simplest one which occurred in major CBD, frequently assumed to be the functioning one in general. The low activation energy is required for ion-by-ion mechanism [92] and it happens with ionic reactions by a sequential manner. In general, the reaction mechanism for the same is M n+ + X m− → Mm Xn

(11)

Ni–Fe hydroxide [49]

Metal sulfides [53]

Metal hydroxide

Metal chalcogenide

Polyaniline–PolyAMPS [87]

CdS/ZnO and CdS/ZnS [86]

Sb2 S3 and CuSbS2 [88]

Sb2 O3 /Sb2 S3 [83]

Cobalt–nickel [82]

Titania/vanadia [81]

Composite

SnTe [70]

Poly (3-methyl thiophene) [75] Polythiophene [76]

Polyaniline–polyacrylic acid [74]

Sm2 Te3 [69]

FeSe [68]

PbSe [67]

MnS[60] CdSe [63]

CdS [59]

Cu2 Se [62]

Bi2 S3 [55, 56]

Cadmium hydroxide [51]

TiO2 [46]

Bi2 S3 , Cux S, and PbS [54]

Copper hydroxide [50]

TiO2 -SiO2 [45]

Polymer

Metal telluride

Metal selenides [61]

ZnO [44]

Metal oxide

Chemical Bath Deposition (CBD)

CdS/PbS [89]

Bi2 Se3 –Sb2 Se3 [84]

Polypyrrole [77, 78]

CdTe [71, 72]

Ag2 Se [64, 65]

PbS [57]

Nickel hydroxide [52]

Bi2 O3 [47]

ZnS-PVA [90]

ZnO-SiO2 [85]

Polyaniline [79, 80]

ZnTe [73]

CoSe [66]

FeS, ZnS [58]

Fe2 O3 [48]

Table 1 Summarizes the list of metal oxides/hydroxide/chalcoginide (sulfide, selenide, and telluride)/polymer/composite films deposited using the CBD method

2 Chemical Bath Deposition: Thin Films with Assorted Morphologies 35

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P. K. Baviskar and S. S. Karade

The solid creation Mm Xn is built on the foundation at [Mn+ ][Xm- ] (IP), surpasses the Ksp , of Mm Xn (SP), then Mm Xn which can result in a solid-phase [17]; though a higher IP may be essential to get supersaturation. If the IP does not go beyond SP may end with up unsuccessful formation of the solid-phase. Except for possible indigenous fluctuations in the precursor, the tiny solid nuclei will re-dissolve prior to growing to reach critical size nuclei. Hence, the development of precipitation is presented as an equilibrium alternative to a one-way reaction. In such circumstances, heterogeneous nucleation tends to the direct growth of nuclei (inorganic phase) over the substrate, resulting in the film formation with comparatively large grain size [93]. The size of the nanoparticles is controlled by tuning the preparative parameters which are most favorably based on the ion-by-ion mechanism [32]. A polycrystalline film formation takes place with random crystal orientation forms due to the involvement of nucleation in the solution [16].

3.2 Simple Cluster (Hydroxide) Mechanism In CBDs, the synthesis parameters have to be preferred to deplete the occurrence of metal hydroxide. Furthermore, most of the CBD’s reactions are performed in an alkaline medium where the formation of metal hydroxide is more probable. In the beginning of the CBD, it might appear that the formation of a metal hydroxide [M(OH)n ] precipitate. Rather than a precipitate, metal hydroxide forms either as a colloidal form or as an adsorbate on the surface of the substrate but not inside the bulk precursor. It is noticed that numerous metal hydroxides, viz. Cd(OH)2 , Cu(OH)2 , Ni(OH)2 , Zn(OH)2 , etc. possess colloidal behavior and colorless hence not noticeable due to limited light scattering. Presently, metal chalcogenide (Mm Xn ) is created by the reaction of Xm− ion with M(OH)n . M n+ + nOH − → M (OH )n

(12)

M (OH )n X m− → Mm Xn + nOH −

(13)

Followed by

In general, ion-by-ion grown mechanism leads to monocrystalline film with bulklike structure and whereas the cluster mechanism results in nanocrystalline film with hydroxyl content [94, 95]. Hydroxide form can be stabilized to MX form relying on the solubility product. As the value of solubility product (Ksp = 2 × 10−14 ) for Cd(OH)2 is higher than the value of solubility product (Ksp = 8 × 10−28 ) for CdS leading to the replacement of OH− with S2− ions resulting in the formation of CdS without altering the morphology. In another way, we can say that the free energy of formation for Cd(OH)2 is more positive than that of CdS.

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3.3 Complex Decomposition Ion-By-Ion Mechanism The complex decomposition mechanism can be further divided into ion-by-ion and cluster pathways. These studies have focused more on cluster pathways, which are being investigated through ion-by-ion mechanisms using CdS thin films. In this case, the formation of free metal cations (Mn+ ) complex using thiourea takes place to give an M-thiourea complex ion. Understanding of this mechanism can be made simple with CdS thin film formation by CBD method as follows: Cd2+ + (NH2 )2 CS = [(NH2 )2 CS − Cd]2+

(14)

This ion hydrolyzes by breaking the bond between S and C to form CdS [(NH2 )2 CS − Cd]2+ + 2OH− → CdS + CN2 H2 + 2H2 O

(15)

This results in the growth of CdS within the precursor. If Cd2+ with a hydroxide linkage is adsorbed directly/indirectly over the substrate surface; the above reaction ends up with the formation of CdS through the growth assisted by the ion-byion mechanism. This kinetics may also contribute to thioacetamide decomposition through an acidic medium; particularly for weakly acidic solutions (pH ≥ 2) at intermediate pH values via a thiosulphate complex instead of the intermediate formation of sulfide. Depositions through thiosulfate act as a sulfur source and serve as one of the examples of this type of mechanism. In addition to this, depositions of thiosulphate under some acidic conditions suggest more possibility for film formation through an ion-by-ion mechanism only. This can be understood by the formation of silver sulfide through the hydrolysis of silver thiosulphate [29]. Ag2 S2 O3 + H2 O → Ag2 S + H2 SO4

(16)

In the present case, the acidic decomposition of thiosulphate resulted in the reduction of S element toward the formation of the sulfide ions.

3.4 Complex Decomposition Cluster Mechanism The creation of a solid-phase rather than responding directly with free anions is grounded on the complex decomposition cluster mechanism through the creation of a midway complex with the anion forming reagent. Progressing with the deposition of CdS using a thiourea precursor can be accessed through the following reaction kinetics: Cd (OH)2 + (NH2 )2 CS → Cd(OH)2 · SC(NH2 )2

(17)

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where the Cd(OH)2 molecule is in a solid-phase cluster. This complex or a similar one having ammine ligands subsequently decomposes to CdS; Cd(OH)2 · SC(NH2 )2 → CdS + CN2 H2 + 2H2 O

(18)

That is the S-C bond of the thiourea breaks, separating the S bound to Cd. It is also suggested that initially the formation of Cd(OH)2 on the surface of the substrate catalyzes the decomposition of thiourea. It is possible that the catalytic effect of the solid surface may be to decompose thiourea into sulfide ions instead of catalyzing the complex decomposition mechanism. It must be noted that Cd(OH)2 is spontaneously transformed to CdS in the presence of thiourea. Whereas such kinds of reactions are limited in some cases like the conversion of Zn(OH)2 to ZnS using CBD [96].

4 Effect of Preparative Parameters The discovery of nanomaterials and nanostructures fascinated enormous attention as they offer large surface area and are helpful for many device grade applications. A range of precursors are available for the growth of thin films using CBD. Depending on the type of precursors, synthesizing parameters for the deposition of the film play a crucial role and produce diverse morphologies. The deposition rate and terminal thickness depend on the supersaturation of the solution (i.e., IP/SP), the number of nucleation centers, and the stirring speed of the solution. Furthermore, the concentration and mobility of ions also affect the nucleation and growth processes on the surface of substrates immersed in the solution. By tuning the reaction period and the precursor concentration, ZnO thin films with porous surfaces were prepared using CBD. It is reported that citric acid had a major influence on the particle size and morphology of ZnO mediated through the nuclei and crystal growth process. The solvent also influenced the size and morphology of ZnO for the different polarity and dielectric constant [97]. We have reported the effect of various preparative parameters via type of precursor, concentration, pH, deposition time, bath temperature, etc. to get assorted morphologies of chemically bath deposited ZnO such as 3D fibrous nanoflakes, 2D cactus, highly crystalline 1D nanoneedles & hexagonal nanorods along with 0D nanobeads, nanoparticles [98] as shown in Fig. 2. Cao et al. [99] reported the influence of cadmium salts on the optical, structure with morphological behavior of CdS thin films synthesized through acidic media using CdCl2 ·2.5H2 O, 3CdSO4 · 8H2 O and Cd(CH3 COO)2 · 2H2 O precursors with thiourea as sulfide source and observed variation in surface morphology as shown in Fig. 3. The film prepared using Cd(CH3 COO)2 · 2H2 O precursor have a higher deposition rate than the other films due to the easier releasing rate of Cd(CH3 COO)2 in the reaction solution. Shinde et al. [100] observed the influence of ionic liquids toward the functionalization of CuO surface for supercapacitive application and observed the change in

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Fig. 2 Chemical bath deposited nanostructures of ZnO with assorted morphologies viz. fibrous nanoflakes (a–c), nanobeads (d–f), cactus (j–l) and highly crystalline 1D nanoneedles (m–o) & hexagonal nanorods (p–r) [98]

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Fig. 3 Morphology images of as-synthesized and annealed CdS thin films deposited from different cadmium salts: a1 CdCl2 · 2.5H2 O, b1 3CdSO4 · 8H2 O and c1 Cd(CH3 COO)2 · 2H2 O [99]

morphology and porosity of the surface with respect to the ionic liquids depicted in Fig. 4. The effects of different preparative parameters during the deposition of films by CBD are discussed below.

4.1 Reaction Bath In CBD, a reaction bath means the precursor containing solvent and solute with appropriate proportions in terms of concentrations. Solute: In CBD, solute has to be chosen which is sparingly soluble in the desired solvent. It can be salts of metal with acetate, sulfate, nitrate, or chlorides. We have reported the deposition of ZnO in acetate and nitrate baths where different morphologies are possible to achieve as shown in Fig. 2 [98]. For CBD while choosing solute, it must be of high purity otherwise the impurities can be easily added to the deposits which can alter the film properties. Solvent: Two types of solvents can be used in CBD, one is aqueous and another is non-aqueous. Aqueous solvent: Water is a low-cost solvent that is a natural gift and can be chosen in the form of deionized or double distilled water as a solvent. Hence, many researchers chose an aqueous solvent for most of the chemical reactions as it favors sparingly solubility for most of the salts. The main disadvantage is being inclusive of OH− as hydroxyl ions result in the formation of hydroxide rather than metal chalcogenide as intermediate and inclusion of small content of hydroxide group in deposits which cannot be excluded entirely. Hence, annealing is required to remove the hydroxyl group which may change the film properties. Another disadvantage being it has a limited temperature range (4–90 °C). Below 4 °C, condensation of water takes place, hence, it is not possible to get desired films whereas above 90 °C minute boiling of water starts that can result into bubble formation which is responsible for the formation of the porous film. Non-aqueous solvent: Widespread non-aqueous solvents are available but are more expensive than the aqueous. These include ethanol, methanol, acetonitrile, etc.

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Fig. 4 Morphology images of the a–b pure, and ionic liquid functionalized with c–d HPDMIM (C1), e–f DHPMIM (C1), g–h MOCPP (C1) with CuO thin films [100]

The non-aqueous solvent can be chosen (i) to widen the deposition temperature range, i.e., to get the deposition below 4 °C or above 100 °C, (ii) to avoid hydroxide content in the deposits, and (iii) some solutes are not dissolved in aqueous media that are dissolved in non-aqueous, hence, non-aqueous solvent can be chosen. In some cases, a combination of both aqueous and non-aqueous solvents can also be chosen.

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4.2 pH Films prepared at varied pH values realized that at the lesser value of pH, the ion-by-ion and complex decomposition mechanisms were pronounced; whereas with increased pH the hydroxide aggregation mechanism was dominant [101]. The powdery, non-uniform, and poorly adherent films were grown for pH value ≤ 11. However, at the higher alkaline (pH ≥ 12) range, structured and well-adherent film formation takes place [102]. The rate of reaction is governed by the supersaturation condition. At minimal supersaturation resulted in to the dawdling formation of MX (where M: metal ions and X: chalcogenide ions). With higher OH− ion concentration in the precursor results in a lower down concentration of M ion yielding a decrease in reaction rate. However, with increasing pH value, the M ion concentration diminished responsible for decreases in the rate of film formation. The film formation will not be possible if the concentration of M ion dropped down below where the solubility of MX becomes more advanced than the ionic product of M and X at a specific value of pH. Makhova et al. [103] reported CBD synthesized ZnS thin films through an acidic and basic bath. Films prepared in the acidic media exhibited a lesser concentration of Zn–O; much nearer to ZnS stoichiometry. The process of CBD is usually realized in an alkaline (ammonia) bath, the hydrolyzed species formation in the precursor and creation of –Zn–O— bonds in the structure of the ZnS layer is unavoidable in neutral and basic medium. The impact of pH on optical, structure, and surface morphologies of CBD-grown ZnS film has been seen. The pH distinctly changes the growing kinetics and thus, the structure of films. It is predominantly witnessed that the finest crystallinity thin film of ZnS is acquired at pH of 10. Diminishing value of pH from 10.99 to 10 is associated with the increase of crystalline orientation along (111) direction. It is also observed that the coefficient of optical transmission increases with pH value from 10 to 11.5. This may lead to the decline in layer thickness [104]. For CdS thin film deposited by CBD, blue shift is observed with band gap enhancement from 2.29 to 2.40 eV and the grain size decreases with increasing pH values [105]. Castillo et al. [106] observed that PbS film thickness increase with increasing in pH as well as deposition time. The effect of pH on SnS film thickness and particle size was credited to the increase in the concentration of OH− , promoting a greater reaction of Sn2+ ions with complexes results in a higher amount of SnS material deposited on the substrate, where thioacetamide was used as sulfide source. This indicates that the growth kinetics is conducted via cluster-by-cluster mechanism, which is more prominent at high pH values [107]. Mohammed et al. [108] observed that the surface of chemical bath deposited CdS is subject to change when the pH value is controlled by adding ammonia to the reaction solution. From reported SEM images it is observed that nanosheets morphology having size up to a micrometer appears and also appears as petals, where

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this formation occurred through the gathering of a large number of nanoparticles with high dense growth on the substrate, resulting in the nanosheets formation. By reducing the amount of pH, the petals expanded with some twists that became comparable to cabbage. The surface shape is subject to change when the pH value is controlled by adding ammonia to the reaction solution, and as the pH of the solution continues to decrease, leads to the expansion of the petals. The increase in grain size along with the smoothness with decreasing the solution pH value was also reported.

4.3 Complexing Agent Concentration enhancement in the complexing agent decreases the number of free releases of metal ions. As a result, the reaction rate and hence, the precipitation tends to condense resulting in higher terminal film thickness. The presence of a complexing agent during the formation of films will diminish the growth rate. The main complexing agent includes NaOH [109], ammonia (NH3 ) [39], sodium tartrate [110], tartaric acid [111], disodium ethylene diamine tetraacetate (Na2 EDTA) [112], diethanolamine [113] triethanolamine [114], oxalic acid [115], acetic acid [116], hydrazine hydrate [117], ammonium sulfate [118], hexamethylenetetramine (HMTA) [119], nitrilotriacetic acid [120], and ethylenediamine tetraacetic acid (EDTA) [121]. Sengupta et al. [17] reviewed the role of complexing agents over metal chalcogenide thin films prepared by CBD. Khallaf et al. [122] investigated the role of six dissimilar complexing agents, viz. hydrazine, ammonia, methylamine, ethanolamine, dimethylamine, and triethanolamine on chemical bath deposited ZnO. The presence of complexing agents make sure of the active complexation of the metal cation in the precursor, preventing the bulk precipitation due to its slow release in the solution. Khallaf et al. [123] reported the consequence of three dissimilar complexing agents on chemical bath deposited CdO films and detected that the optical energy band gap and phase of CdO changes according to change in complexing agents. The effect of the complexing agent on surface morphology, optical properties, and crystalline phase of CdS films grown by CBD was reported by Zhang et al. It is also noted that clusters-by-clusters deposition is dominant when ammonia (NH3 ) served as a complexing agent; resulting in rough morphology, while the ion-by-ion deposition mechanism assists the growth of smooth morphology films when EDTA was employed as the complexing agent [124]. PbS thin films deposited by using CBD route in the existence of diverse complexing agents inclusive of diethanolamine (DEA), triethanolamine (TEA), and hexamine as reported by Preetha et al. It is observed that the different complexing agents affect the structural (crystalline), optical (band gap energy), and morphological (grains) behaviors of PbS films [113]. Gopinath et al. reported the use of acetic acid as a complexing agent for the preparation of In2 S3 films using the CBD method for solar cell application. It is noted that the increasing acetic acid concentration results in decreased granular density and increased in the grain size. In addition, the variation in optical band gap values has been observed with a change in the

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complexing agent concentration [125]. A chemical bath deposited ZnS thin layer by adopting hydrazine hydrate or tri-sodium citrate as the complexing agent. The nonuniform film with particles diameter of ~100–200 nm whereas uniform and dense film with particles diameter of 20–30 nm were observed for ZnS thin films prepared to expend tri-sodium citrate and hydrazine hydrate as complexing agents, respectively. The optical band gap increases as a function of complexing agent concentration. The ion-by-ion mechanism is predominant during the growth of ZnS thin films [126]. Gayathri et al. reported the impact of complexing agents on CBD deposited FeZnS2 films toward the photocatalytic degradation of dye molecules. It is also observed that with the EDTA and Leishman stain as a complexing agent for FeZnS2 layer, the crystallite size was observed as 5.58 nm and 6.25 nm, respectively. The surface morphology and optical band gap also show the variation with change in complexing agent [127]. Arepalli et al. [128] studied the role of complexing agents on Cu2 ZnSnS4 thin films prepared by CBD method and observed that thickness of the film continuously increased with increased concentration of complexing agent (NH3 ), because of enhanced heterogeneous growth rate rather than homogeneous precipitation. Surface morphology and cross-sectional images of the nanoflake NiO films synthesized using the varied quantity of aqueous ammonia are illustrated in Fig. 5. The as-prepared films with different amounts of aqueous ammonia (10, 15, 20, 25, 30, and 40 ml) were denoted as NiO−10, NiO−15, NiO−20, NiO−25, NiO−30, and NiO−40, respectively. It is well-established that the microstructure of NiO is drastically affected by the concentration of aqueous ammonia from a general flat surface to 2-dimensionally networked uniform nanoflake array. The NiO−10 film shows a general flat surface that was formed by packing closely stacked 70 nm thick nanoflakes. However, with an increase in the amount of aqueous ammonia until 30 ml, the thickness of the film becomes gradually increases up to 735 nm and 2-dimensionally networked tiny flake array with nearly vertical directions was formed. Interestingly, it was found that a uniform nanoflake array cannot form with further addition of aqueous ammonia over 30 ml. For NiO−40 film, 2-dimensionally networked structure became randomly oriented flakes with a reduced thickness of 285 nm [129].

Fig. 5 Top and cross-section SEM images of NiO films: a and g NiO−10, b and h NiO−15, c and i NiO−20, d and j NiO−25, e and k NiO−30 and f and l NiO−40 [129]

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Strano et al. explored the twin role of hexamethylenetetramine (HMTA) in the growth of ZnO nanorods using CBD. The pH buffering activity of HMTA is complemented by a sturdy steric hindrance influence that ensures vertical growth along the c-axis [130]. The effect of complexing agents on crystallinity and optical band gap along with microstructural changes were observed in CdS hierarchical nanoflakes. It can be noticed that the density of the nanoflakes changes with complexing agents (TEA and EDTA) along with NH3 , the increase in substrate coverage area can be easily noticed [131].

4.4 Precursor Concentration A quantifying value of precursor concentration exhibits a key parameter in tuning the film morphology. A decrease in grain size with decreasing concentration of iron (Fe) has been seen in FeS2 film by Vedavathi et al. [132]. Urgessa et al. [133] observed that the ZnO nanostructures change from particles to rods with an increase in the concentration of zinc nitrate hexahydrate. Jayasree et al. [134] explored the influence of solution concentration on the growth of tin sulfide films and observed the decline in the optical band gap energy along with a change in the phase of tin sulfide with enhancement in tin salt concentration. When Mn and S salt precursor’s concentration decreased, films crystallinity and energy band gap decreased with the increasing precursor’s concentration [135]. The optical transmittance of ZnIn2 Se4 (ZIS) films increases with increasing of precursor concentration [136]. Molefe et al. reported the increase in average crystallite size and diminished luminescence (PL) intensities with an enhancement in zinc acetate molar concentration. The shift of absorption band edge to the higher wavelength leads to the reduction in band gap energy of ZnO with enhancement in zinc acetate molar concentration has been observed. It is also noted that the surface morphology relies on zinc acetate concentration in solution [137]. For ZnO nanowires, it is observed that the average diameter increases and length of ZnO NWs decreases with increasing the concentration of the reactants. Figure 6 exhibits the FE-SEM top views of ZnO NW arrays with different concentrations of the reactants [138].

4.5 Deposition Temperature Temperature affects the dissociation of the complex and the anion of the compound (X compound). The dissociation of the compound enhances with the increase in temperature and leads to the larger concentrations of ions (M and X) which enhances the rates of deposition. The thickness of the deposited films depends on the temperature of the bath and also based on the situation through which films being deposited.

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Fig. 6 FE-SEM images of the top view of ZnO NW (a–e) arrays and variation in diameter and length of ZnO NW (f) as a function of reactants concentration [138]

Drici et al. [139] reported the temperature effect during the deposition of ZnO film with no film formation taking place below 323 K. Ke et al. [140] state that the higher temperature is favorable for the formation of relatively high stoichiometric film by CBD. It was noted that the crystallization of films improves and the value of grain size increases along with an increase in energy band gap as the deposition temperature increases for ZnS [141]. The film crystallinity increases and band gap energy decreases with increasing the bath temperature for chemically grown MnS thin film[135]. The rate of reaction and hence the growth of CdS thin film improved prominently with the enhancement in bath temperature and film densification [142]. CdS film exhibited an increase in crystallite size with increasing bath temperature [143]. The film thickness enhanced with a rise in bath temperature due to the increase in supersaturation of the reaction bath [134]. Rondiya et al. reported a decrease in

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band gap energy and an increase in crystallite size with enhancement in precursor temperature for CBD-grown CdS films [144]. The transition from compact-to-platelet morphology as well as from Cub-to-ORcrystalline structure is observed with an increase in film thickness as a function of deposition temperature for SnS thin films [145]. PbSe thin films show the result of precursor temperature over the growth of PbSe film. The study discovered that the optical band gap decreased with an increase in the bath temperature [146]. The increase in deposition temperature caused the increase in grain size, and the optical band gap falls with the enhancement in deposition temperature for PbS thin films [147]. CdS nanostructures were grown at different deposition temperatures. As deposition temperature increases, the thickness, particle size, and lattice constant increase whereas the optical band gap decreases [148]. Figure 7 shows the surface morphologies of CdS nanostructures deposited at different temperatures. Figure 8 shows the top view and cross-sectional view SEM images of Sb2 S3 thin films at varied precursor’s temperature. The increase in film thickness and particulate size with an enhancement in bath temperature is due to the increase in thermal energy, which slightly accelerates the dissociation of Sb ions from the complex and assists the film growth [149].

4.6 Deposition Time Deposition time plays a vital role that influences the numerous properties of the films [150]. It is witnessed that the terminal layer thickness of the film enhances with synthesis time and attained a maximum value and then decreases with a further rise in deposition time; may be due to the loss of an appreciable amount of solute concentration [84]. It is reported that with an increase in deposition time, the particle size along with film thickness increases for SnS film [107]. The PbS films show that the grain size and film thickness increase and optical bad gap decreases depending on deposition time [151]. The role of growth time on the performance of FeS2 thin films was described by Anuar et al. He found that the enhancement in crystallinity and grain sizes along with diminishing the band gap energy with an increase in the synthesis time [152]. As2 S3 films show band gap energy and electrical resistivity decrease with an increase in thickness corresponding to the deposition time [112]. Variation of the terminal thickness of Bi2 Se3 –Sb2 Se3 composite thin film with deposition time was reported earlier. Hannachi et al. reported that the stoichiometric ratio and energy band gap of MnS films decreased as a function of deposition time [135]. Perumal et al. prepared CdO layer by CBD and found an increase in crystallinity and grain size with enhancement in the deposition time [153]. As the synthesis period enhances, the crystalline nature increases for chemically synthesized zinc oxide thin films [154]. With enlarging the reaction time the cubic ZnO structure is getting transformed into the well-known hexagonal wurtzite structure. Optical absorption spectra exhibited a moderate enhancement in the reflectance value and the red shift in absorption

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Fig. 7 Shows the FE-SEM images of CdS nanostructures deposited at different temperatures, a 65 °C, b 75 °C, c 85 °C and d 95 °C [148]

edges with a rise in synthesis time results in a decrease in band gap energy with increasing reaction time. The photoluminescent intensity also decreased as reaction time increases [155]. Koao et al. reported the average grain size of flower-like ZnO nanostructures synthesized using the CBD rises with an enhancement in the synthesis time which is due to the “Ostwald ripening process.” The optical absorption edge moved to a longer wavelength yielding a decrease in band gap energy while the reflectance intensity also diminished slightly with an enhancement in the reaction time [156]. The effect of deposition time on chemically synthesized MnO2 film shows the spheres, flower-like nanostructures, which consist of nano whiskers, nano wires and sea-urchinlike morphologies [157]. Chemical bath deposited CdS spherical nano-grains show that with enhancement in synthesis time the grain diameter and crystallinity were decreases and the optical properties shows that band gap energy of the films was gradually increases [158].

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Fig. 8 Surface morphology and cross-sections of Sb2 S3 films prepared at bath temperatures: a 30 °C, b 40 °C, c 50 °C, and d 60 °C [149]

The SEM images of ZnO nanostructured thin films synthesized at varied time intervals are depicted in Fig. 9. The nanostructures of ZnO undergo a gradual morphological evolution can be seen along with enhancement in the film thickness with synthesis time [159]. Anwar et al. reported the chemical synthesis of PbSe films and observed that the dependency of morphology on deposition time. It is noted that at low deposition time the growth was spherical and cauliflower type structure was observed at higher deposition time as shown in Fig. 10. The agglomeration process becomes more and more prominent as the deposition time increases. The XRD results confirmed the polycrystalline nature with the cubic structure of PbSe thin films. From the XRD data, with enhancement in deposition time, the enhancement in average crystallite size is visualized [160]. Well-controlled film thickness through deposition time plays not only to alter the morphology but different properties can be changed depending on the quantum size effect. This is well studied for As2 S3 thin films synthesized at 6 °C bath temperature for various time intervals. Properties mainly focused inside these studies are grain size, band gap, resistivity, and activation energy [112] (Table 2).

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Fig. 9 ZnO synthesized at time period of a 2 h, b 4 h, c 6 h, d 8 h, e 10 h and f 12 h. Inset exhibits magnified and cross-section images [159]

4.7 Substrate Substrate cleaning plays a vital role in CBD. The nature of the substrate surface whether it is hydrophilic or hydrophobic for the used precursor has a major influence. Synthesis of the film with desired properties takes place only at certain optimum preparative conditions, i.e., the formation of MX or the formation of single crystal films if the substrates have special properties. The nucleation starts only if there is a small change in free energy and this can be achieved with special properties of the substrate that favors the film formation. In this case, the special properties of the substrate and lattice of the deposited material matches well with each other. One of the advantages of CBD is that thin films can be deposited on any surface. Substrates of any shape, size, or different natures can be coated uniformly. Moreover, it is not essential that the substrate must have electrical conductors. In principle, most clean surfaces can be used as substrates, although the degree of adhesion can vary greatly from material to material. Glass is the frequently used substrate in CBD. The lowtemperature deposition can avoid the corrosion and oxidation of metallic substrates and hence, metallic substrates can be used in CBD.

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Fig. 10 SEM images of PbSe films grown at different deposition time. Inset shows the SEM images at higher magnification [160]

Table 2 Role of thickness on As2 S3 thin films [112] Sr. No

Deposition time (h)

Film thickness (nm)

Grain size (±2 nm)

Band gap energy (eV)

1

4 (A)

185

11

2.58

2

8 (B)

380

19

2.44

3

12 (C)

440

29

2.36

4

16 (D)

505

33

2.24

5

20 (E)

520

38

2.20

A variation in morphologies over the various substrates for ZnS thin films was observed. The thin films deposited on ITO substrate showed pinholes free compact and uniform and growth, whereas poor growth is observed on HfO2 and SiO2 substrates. [161]. Ulutas et al. reported that the gamma-MnS thin films on glass and indium tin oxide (ITO) show the variations in grain size, energy band gap, refractive index, and resistivity values [162]. For CdS thin films deposited over different

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Fig. 11 The SEM micrographs of Cd(OH)2 nano-nest synthesized on a glass, b FTO coated glass, and c stainless steel substrates [171]

substrates such as glass, Si, GaAs, Al2 O3 , it is observed that the variation in roughness of CdS layers, measured using AFM depends on the substrate chemical nature [163]. Suitable substrates toward the thin layer synthesis using CBD [136, 164–169] can be used as metal, semiconductor, and insulators like routine soda lime glass, microscope glass slide, corning glass, fluorine doped tin oxide (FTO) coated glass, indium tin oxide (ITO) coated glass, stainless steel (SS), titanium, mica, and Si. Thin films of BiSe have been prepared on various substrates (glass, ITO, PMMA, and Si wafer) using CBD method. The thickness, grain sizes, contact angle, and optical band gap of BiSe thin films vary with the substrate [170]. The 2D surface morphologies of chemically grown Cd(OH)2 films on a variety of substrates such as (a) glass, (b) FTO, and (c) stainless steel are shown in Fig. 11. The nest-like growth of Cd(OH)2 over all the substrates is observed. On careful examinations of the nest-like morphology, it is observed that the porous nature with interconnected wires in a complex way. The nest-like morphology develops denser over FTO-coated glass and stainless steel substrate in comparison with the glass substrate [171]. Fortunato et al. explored the growth of ZnO nanostructured thin films over two different substrates and observed the variation in morphology as vertically oriented nanorods arrays on ITO-coated glass substrate and nanowalls over aluminum substrate [172].

4.8 Substrate Alignment and Solution Stirring There are two possible reaction channels in typical CBD routes that yield the growth of solid materials. The formation within the solution is due to homogeneous precipitation and the surface of substrate or reaction vessel is due to heterogeneous precipitation. Li et al. reported the deposition of Zn(O,S) using a chemical bath for different substrate alignment and the solution movements, viz., a combination of substrate alignment and solution movement as “Vertical”, “Horizontal”, “Inclined”, “Vertical + Stir”, “Horizontal + Stir”, “Inclined + Stir”, and “Horizontal + Rocking” and observed variation in optical, electrical and morphological properties [173].

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Fig. 12 a Experimental setup showing substrate alignment. SEM images of ZnO nanostructures: b hexagonal shaped nanorods, Sample 1, c narrower rod-like structures, Sample 2 and d agglomerated small nanoparticles, Sample 3 [176]

It is also reported that the alignment of the substrate during the chemical synthesis of the CdS films has a strong impact on the stoichiometric, optical, and morphological properties [174]. Zhang et al. reported that for heterogeneous process, stirring speeds do not affect the film thickness, but for the homogeneous reaction, enhancement in thickness was observed with enhancement in the stirring speeds and/or deposition time for ZnS thin film [175]. In order to get rid of powdery particles attached to the surface during deposition; stirring can be used to get a smoother surface. Mwankemwa et al. [176] explored the effect of a substrate positioned in a chemical bath and observed that the variation in optical, electrical, and surface morphological properties ZnO nanostructures with respect to the substrate positioned in the growth solution as shown in Fig. 12(a). FE-SEM images revealed three different morphologies of nanostructures (Fig. 12): (b) vertically aligned hexagonal nanorods having flat tops, (c) narrower nanorods lying parallel to the substrate, and (d) randomly oriented agglomerated nanoparticles and these resulted when the seeded substrate was placed facing down, vertically and up in the beaker, respectively.

4.9 Doping It has been reported that doping does not only affect the optical properties but also the physical properties of the thin film. Many research groups showed a great interest to study the doping effect on various physicochemical properties of various thin films

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using a range of metal elements such as B, Cd, Co, Cr, Cu, Fe, Ga, Hg, K, Mn, Ni, Sm, Sn, Zn, and Al [177–190] in order to tune the properties for practical applications. Ahmed et al. [180] investigated the influence of Cr incorporation in PbS films. The bath for the deposition of PbS films with three different levels of doping Cr was obtained by the addition of 2, 4, and 6 ml of 1 mM CrO3 into sodium hydroxide in the above bath. It is observed that the average crystallite size and average sizes of nanoclusters decreased whereas the strain, dislocation density, and optical band gap enhanced with increasing Cr doping. They have also reported that the concentration and nature of the dopant are responsible to enhance the physical properties along with the efficiency of PbS semiconductor-based devices. The optical, structural, magnetic, and half-metallic properties of Co-doped ZnS thin films were reported by Akhtar et al. The value of lattice parameter ‘a’ decreases and the shift of the diffraction peak corresponding to the (111) plane toward a slightly higher value of 2θ with an increase in Co concentration. It is also observed that with an increase in doping concentration, the band gap increases along with the significant increase in the PL intensity at 510 nm corresponds to green emission [191]. Abbas et al. reported the increase in average crystallite size as Cu concentration increases. Besides, the values of dielectric constant, refractive index, and extinction coefficient of SnO2 films were progressively enhanced with respect to the increase in Cu concentration. In contrast, it is noted that the values of optical band gap energies were decreased with enhancement in Cu concentration [192]. FE-SEM images of Ni-CuO with different Ni doping concentrations from top and cross-sectional views are shown in Fig. 13. It is observed that the vertically grown nanorod structure at a low concentration (0–2 at.%) of Ni. With further increases in Ni doping concentrations to 5 at.%, the nanorod structure became thicker and tilted. Finally, a network structure was observed when the Ni doping concentration increased to 20 at.%. These variations in morphology can be explained by the etching effect. In this case, the NiCl2 source is used as doping material with H2 O as a solvent which leads to the formation of hydrochloric acid (HCl). Therefore, increasing the amount of NiCl2 results into formation of more HCl and leads to increases in the etching process. In addition, it is noted that the film thicknesses measured using the cross-sectional FE-SEM images were reduced significantly for Ni concentrations greater than 5 at.%, which might be due to the etching effect, as shown in Fig. 13 (inset) [193].

5 Composite Formation TiO2 has been successively synthesized at room temperature using TiOSO4 and H2 O2 aqueous precursor in acidic media followed by annealing at higher temperature to convert the hydroxide phase to get the anatase phase [194]. Remarkably, very uniform and ultrathin film covered on the substrate surface is observed. Interestingly, TiO2 -SiO2 composite has been prepared very easily just by adding commercially available SiO2 gel during the synthesis in precursors solution. The

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Fig. 13 FESEM images of Ni−CuO structures from top view and cross-sectional view (inset) with various Ni doping concentrations: a 0 at.%, b 0.5 at.%, c 1 at.%, d 2 at.%, e 5 at.%, f 10 at.% and g 20 at.% [193]

Fig. 14 CBD deposited TiO2 film (LHS) and inset shows compact nature of TiO2 . RHS image shows TiO2 –SiO2 composite film [45]

spherical surface architecture of SiO2 is seen clearly embedded in the deposited film as shown in Fig. 14 (RHS) [45].

6 Case Study Various synthesis routes have been explored to grow metal oxides, hydroxides, chalcogenides, and polymer films by using the CBD method for diverse applications. Nair et al. reported the CBD route for the synthesis of semiconductor thin films for

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solar energy related applications [195]. Table 3 summarizes the list of chemical bath deposited films of various metal oxides, metal hydroxides, metal chalcogenides, and polymers for diverse applications. Here through the literature survey, we have explored the state of the art for the use of chemical bath deposition method for some applications such as energy conversion, storage, sensing, and field effect transistor.

6.1 Dye Sensitized Solar Cell Dye sensitized solar cell (DSSC) technology is also called as artificial photosynthesis as it mimics the natural absorption of photon energy (Sunlight). DSSC provides a technically simpler and economically reliable alternative to traditional silicon solar cells. The dye (sensitizer) is anchored over the wide band gap high surface area semiconductor. At the interface, the charge separation takes place through the injection of a photoinduced electron from the sensitizer into the semiconductors conduction band followed by the transport of charges from the semiconductor conduction band to the charge collector.

6.1.1

Device Construction

The construction of the device consists of wide band gap n-type semiconductor metal oxide (TiO2 or ZnO) usually deposited on the transparent conducting oxide (TCO) substrate. This semiconductor film consisted of nanostructures having a high surface area with interconnected particles for adsorption dye monolayer. The photoanode sensitized with dye and counter electrodes usually a platinum coated TCO were sandwiched together using spacer over dye loaded metal oxide semiconductor film. The iodide/tri-iodide redox liquid electrolyte was incorporated at a junction between the sensitizer (dye) and counter electrode (cathode) by capillary action. Figure 15(a) Schematic represents device grade DSSC and energy band diagram for nanostructured ZnO sensitized with Eosin-Y dye. Figure 15(b) shows the graph of efficiency (− • − ) and current density (− * − ) as a function of various preparative parameters.

6.1.2

Effect of Preparative Parameters on the Performance of DSSC

The influence of different optimization conditions on the performance of DSSCs fabricated using chemically synthesized ZnO films was systematically examined. It has been observed that these preparative parameters (deposition time and solute concentration) play a crucial role in improvement of device performance. In short, the growth of nanoporous zinc oxide films at room temperature (27 °C) was carried out for discrete intervals (10–30 h) in an aqueous chemical bath containing a mixture of HMTA (concentration: 0.02 M) and zinc acetate (concentration: 0.1–0.4 M). It

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Table 3 List of chemical bath deposited metal oxides, hydroxides, chalcogenides, and polymers materials and their role in diverse applications Material TiO2

ZnO

Application

Role of Material

References

Dielectric

Electrode

[196]

Junctionless transistor (JLT)

Electrode

[197]

Photoelectrochemical cell

Electrode

[198]

LPG sensor

Electrode

[199]

Perovskite solar cells

Electron transport layer

[200]

Perovskite solar cells

Hole-blocking layer

[201]

Electro/photochromic properties

Electrode

[202]

UV photodetectors

[203]

UV-OLEDs

[204]

Dye sensitized solar cell Photoanode

[205–209]

Perovskite solar cells

Electron transport layer

[210]

Photocatalytic

Electrode

[211]

Field emission

Cathode

[212]

Inverted organic solar cell

Electron transport layer

[213]

Semiconductor sensitized solar cell

Photoanode

[214, 215]

LPG sensor

[216]

SnO2

Perovskite solar cells

Electron transport layer

[217–219]

WO3

Gas sensor

Electrode

[220]

CdO

LPG sensor

Electrode

[221]

Photo-electrochemical cell

Photoanode

[222]

CuO

Electronic and optical applications

[223]

Solar cell

[224]

PbO

Photo catalysis

CeO2

Supercapacitor

Supercapacitor electrode

[226]

Supercapacitor

Supercapacitor electrode

[227]

NiO

[225]

Supercapacitor

Supercapacitor electrode

[228]

Perovskite solar cells

Hole transporting layers

[229]

α-Fe2 O3

Photoelectrochemical water splitting

γ-Fe2 O3

Supercapacitor

Supercapacitor electrode

[48]

V2 O5

Supercapacitor

Supercapacitor electrode

[231–233]

[230]

(continued)

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Table 3 (continued) Material

Application

Role of Material

References

Field emission

Cathode

[234]

Mn3 O4

Supercapacitor

Supercapacitor electrode

[235]

RuO2

Supercapacitor

Supercapacitor electrode

[236]

IrO2

Supercapacitor

Supercapacitor electrode

[237]

Zn2 SnO4

Perovskite solar cell

Electron transport layer

[238]

NiCo2 O4

Supercapacitor

Supercapacitor electrode

[239]

Cd(OH)2

Field emission

[40]

Supercapacitor

Supercapacitor electrode

Cu(OH)2

Supercapacitor

Supercapacitor electrode

[240] [241]

Co(OH)2

Supercapacitor

Supercapacitor electrode

[242]

Ni(OH)2

Glucose sensor

β-Ni(OH)2

Supercapacitor

Supercapacitor electrode

[244]

Mg(OH)2

Solar cells

Buffer layer

[245]

[243]

Mg(O, OH)

Solar cells

Buffer layer

[246]

Zn(O,S)

Solar cell

Buffer layer

[173]

Sb2 O3 /Sb2 S3

Photoelectrochemical water splitting

[83]

CdS

Light emitting diode (LED)

[247]

Dye sensitized solar cells

Photoanode

Nanoheterojunction Solar cell

PbS

ZnS

[248] [38]

Perovskite solar cell

Electron transport layer

[249]

Supercapacitor

Electrode

[250]

Photoelectrochemical (PEC)

Electrode

[251]

Gas sensor

Electrode

[252]

Q-dot sensitized solar cells

Counter electrode

[253]

LPG sensor

Electrode

[254]

Buffer layer

[256]

Solar cells CIS Solar cell

[255]

SnS

Photocatalytic activity

[257]

HgS

Photoelectrochemical (PEC)

[258]

CuS

Q-dot sensitized solar cells

Counter electrode

[259] (continued)

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Table 3 (continued) Material Cu2 S

Bi2 S3

Sb2 S3

Application

References [260]

Photoelectrochemical (PEC)

[261]

Photodetector

[262]

Solar cell

Sensitizer

[263]

Photoelectrochemical (PEC)

Photoanode

[264]

Supercapacitor electrode

[266]

Solar cell Supercapacitor

MoS2

Role of Material

Dye sensitized solar cell Counter electrode

[265]

Supercapacitor

Supercapacitor electrode

[42, 267]

Field emission

Cathode

[268]

Dye sensitized solar cells

Counter electrode

[269]

α-Ce2 S3

Supercapacitor

Supercapacitor electrode

[270]

In2 S3

Perovskite solar cells

Electron transport layer

[271]

Supercapacitor electrode

Yb2 S3

Supercapacitor

ZnMgS

Photosensor

[273]

CdSe

Photoelectrochemical

[274]

[272]

Q-dot sensitized solar cells

Photosensitizer

[275]

Q-dot sensitized solar cells

Photosensitizer

[35]

Photoelectrochemical

Photosensitizers

[276]

Buffer layers

[278]

ZnSe

Photoelectrochemical

Zn(X, OH) (X = S, Se)

Solar cell

Bi2 Se3

Photoelectrochemical (PEC)

MoSe2

Supercapacitor

CdTe

Q-dot sensitized solar cells

[281]

ZnTe

Photovoltaic

[282]

Polyaniline (PANI) Polypyrrole

Polythiophene PbS/polyvinyl alcohol

[277] [279] Supercapacitor electrode

[280]

Solar Cell

Hole transport layer

[283]

Supercapacitor

Supercapacitor electrode

[284]

Solar Cell

Hole transport layer

[77]

Gas sensor

Electrode

[285]

Supercapacitor

Supercapacitor electrode

[286]

Gas Sensor

Electrode

[287]

Supercapacitor

Supercapacitor electrode

[288]

Dielectric

Electrode

[289]

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Fig. 15 (a) Schematic illustration of device grade DSSC with energy level diagram for Eosin-Y dye sensitized nanostructured ZnO. (b) Graph for efficiency (red line with stars) and current density (blue line with circles) for used preparative parameters [7]

is observed that initially the device performance increases with deposition time and then diminishes abruptly with further increase in deposition time which leads to enhancement in electrode thickness with interval (> the light penetration depth), and therefore, Jsc and hence, the efficiency of DSSC cannot be increased any more. As the precursor concentration (zinc acetate) increases above the optimum value, it was witnessed that the dramatic growth in average crystallite size due to the nanoparticles clustering, which leads to the decrease in porosity and surface area for the amount of dye adsorption results in the reduction in performance of DSSC [7]. Chemically synthesized ZnO showing cactus-like morphology with a network of interconnected nanoparticles (Fig. 16a) sensitized with two different dyes via. metal-free indoline D149 and Ru-metal based N719 for the construction of device grade DSSC and reported more than 3% efficiency for both the devices. Quantum efficiency curves show incident photon to current conversion from 350 to 700 nm wavelength and observed two intense peaks around 400 and 540 nm for both the dyes which are more or less similar to that of the optical absorption spectrum of respective dyes (Fig. 16b) [206]. In addition to this, the same cactus-like morphology of chemically synthesized ZnO was used to boost the performance of DSSC using sensitization of blended dye on mimicking the “Basic idea, advance approach” and reported the enhancement in device performance with maximum efficiency of 2.45% for Coumarin 343 (C343) and Eosin-Y (EY) mixed dye which was prepared in ethanolic solution, Fig. 17a shows the images of ZnO photoanodes sensitized with individual metal-free organic dye and molecular structures. The probable schematic of blending of C343—yellow

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Fig. 16 a Cactus like surface morphology of ZnO (top and cross-sectional view). b Optical absorption (solid) and quantum efficiency (hollow) spectra wrt wavelength for D149 dye (stars) and Ru-metal N719 dye (circle) [206]

and EY—red to get orange color dye and its sensitized ZnO photoelectrodes of individual and blended dye is shown in Fig. 17b [205]. Apart from metal oxides, metal chalcogenides were also successfully used as a photoanode in the construction of device grade DSSC. We have reported the chemically grown CdS 1D nanowires sensitized with metalfree Rhodamine B and Rose Bengal dyes for DSSC application. Figure 18 shows the graphic of the constructed device with dye loaded CdS for DSSC application [290]. Furthermore for DSSC application, cost-effective, room temperature CBD deposited MoS2 nanoflake grown on the dip and dry coated MWCNTs has been successfully employed as a counter electrode where ZnO sensitized with eosin Y was used as a photonode [269]. Figure 19 shows the (a and b) J–V and (c) external quantum efficiency (EQE) spectra for DSSCs for MoS2 /MWCNTs thin film as CE w.r.t. MWCNTs with corresponding (d) energy band diagram.

Fig. 17 a ZnO sensitized with different metal free dyes and their molecular structure. b Schematic representation of C343 with EY bleded dyes and their ethanolic and ZnO adsorbed colours [205]

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Fig. 18 Schematic dye loaded CdS NWs towards DSSC application [290]

Fig. 19 For DSSC assembled using bare MoS2 and MoS2 /MWCNTs as CE, a, b dark and illuminated J–V characteristic curve. c EQE, and d Schematic energy band diagram [269]

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6.2 Quantum Dot Sensitized Solar Cel Influence of deposition time on the behavior of quantum dot sensitized solar cell (QDSSC) for linker free novel chemical bath deposited CdSe quantum dots (QD’s) sensitized TiO2 photoanode. The deposition of CdSe QD’s was carryout at room temperature for different deposition times as 45, 90, 120, and 150 min to tune CdSe QD’s size through nucleation and growth process. The successive growth of CdSe QD’s over the TiO2 surface was accompanied by a series of color variation (size quantization). Schematics for the growth of CdSe by CBD on TiO2 nanoparticles with photo images at varied reaction times are shown in Fig. 20. With increasing deposition time the particle size as well as the grain size of CdSe QD’s enhances, which results in a reduction of band gap energy. The enhancement in performance of QDSSC clearly indicates the coating of CdSe QD’s over mesoporous TiO2 film which is noticeably observed in J-V characteristics of the device (Fig. 21) [35]. CdS nanoparticles anchoring on CBD deposited ZnO was used for the construction of nanoparticle sensitized solar cell. Figure 22 shows the surface morphology of ZnO nanorods array without and with sensitization of CdS nanoparticles and crosssectional FE-SEM image of ZnO nanorods. The EDX spectra confirm the formation of CdS/ZnO heterostructure.

Fig. 20 Scheme of CdSe nanoparticles growth on TiO2 and corresponding SEM image, Inset exhibits CdSe Q-dots grown on TiO2 wrt to varied deposition period [35]

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Fig. 21 J-V curved for CdSe sensitized TiO2 under a dark and illumination for CdSe with b 45 min, c 90 min, d 120 min, and e 150 min. deposition times [35]

Fig. 22 SEM image of a ZnO nanorods, CdS nanoparticles coated ZnO, b surface and c crosssectional viewm, d corsponding EDX spectra of CdS/ZnO [214]

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Fig. 23 a Schematic of ZnO/CdS Q-dots solar cell. b Schematic of energy band diagram. c J–V curves under dark and under illumination conditions [214]

Device having the structure FTO/ZnO/CdS/electrolyte/platinum, energy band, and J-V characteristic are shown in Fig. 23 [214]. The device exhibited 0.12% efficiency under 100 mW/cm2 illumination of simulated sunlight.

6.3 Photoelectrochemical Cell Initially, Cd(OH)2 nanowires were grown by using CBD as a template followed by its conversion to CdS NWs based on solubility product. Effect of postannealing was performed on photoelectrochemical (PEC) devices fabricated using 3D-CdS nanonetwork. Figure 24 exhibits surface morphologies of as-synthesized and

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Fig. 24 Surface morphologies of CdS NWs, a as synthesized and annealed at, b 150 °C, c 200 °C and d 250 °C [59]

annealed CdS NWs at different temperatures where the straightening of nanowires has been observed. Figure 25 exhibits PEC device, J-V cures of the as-deposited and annealed CdS NWs with energy band diagram, EQE curve, and Mott-Schottky plots along with the obtained values of the flat band potential with respect to annealing temperature [59]. The enhancement in crystallite size by annealing was correlated with the PV performance of CdS based device and reported the enhancement in efficiency by 2.6 times that for the annealed sample in comparison with as-prepared sample. Report is available for the growth of Bi2 S3 thin film from non-aqueous media containing acetic acid and formaldehyde. Film shows photovoltaic activity in polysulfide electrolyte through PEC assembly under the illumination of light with an intensity of 70 mW/cm2 and shown in Fig. 26 [264] which exhibited a very low value of efficiency. Chemical bath deposition of wide band gap CdS [291] and Cd-free materials [278, 292] for solar cell application was reported and some of them were used as buffer layers in the solar cell. Figure 27 summarized the used deposition conditions and achieved remarkable results. Interestingly, chemically grown ZnSe yields 4.8% with CIS, ZnS, and Zn(Se, OH) giving more than 14% device efficiency as wide band gap Cd-free less toxic materials [293, 294].

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Fig. 25 a Exhibits designed PEC device, b J–V characteristics. c EQE and d Mott-Schottky plot of as deposited and annealed CdS NW samples. Inset b shows energy band diagram and inset d exhibits variation in the flat band potential w.r.t. to annealing temperature [59]

The highly efficient CIGS solar cells usually have CdS as a buffer layer deposited by using CBD method. Lee et al. [291] and Cho et al. [295] reported the efficiency of CIGS solar cell was affected by the thickness of CdS buffer layer. The thickness of CdS buffer layer film was controlled by various preparative parameters of CBD method such as the reaction time, reaction temperature, and bath pH. [296] One of the main factors for its success has been related to positive conduction band offset, which reduces the effective density of holes at the interface and thereby the recombination. It would be preferable to use the wet chemical CBD-CdS process [297] as with CBDCdS buffers; high and reproducible efficiencies are obtained whatever the absorber used [298].

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Fig. 26 Photovoltaic output characteristics for n-Bi2 S3 /polysulphide PEC cell (illumination intensity 70 mW/cm2 ) [264]

Fig. 27 Chemically grown wide band gap materials synthesis parameters and their use as a heterojunction partner along with the cell efficiencies [278]

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Fig. 28 SEM images of ZnO films prepared at a 60 min, b 80 min, and c 100 min reaction time [299]

6.4 Gas Sensor The influence of deposition time for chemically grown ZnO film on surface morphology (Fig. 28) and gas sensing properties was reported by Shinde et al. [299]. It is observed that the smooth surface with well-defined sub-micron hexagonal nanorods was observed for the deposition period of 60, 80, and 100 min. It is also noticed that for the interaction between the sensor surface and adsorbed gas molecules is stronger for materials having a large surface area (high gas sensitivity). With the enhancement in the diameter of ZnO rods in a particular film area, the effective surface area decreased. As a result, as deposition time increases the sensitivity decreases.

6.5 Supercapacitor The energy storage devices, namely, supercapacitors (SCs) have the properties in between the batteries and conventional capacitors which are used to store energy in a short time and deliver the same in a short time. A simple chemical route was employed for the synthesis of V2 O5 /MWCNTs electrode towards the fabrication of a flexible solid-state symmetric supercapacitor (FSS-SSC) device [232]. Figure 29 shows the schematic for the chemical bath deposition of V2 O5 flakes over multiwalled carbon nanotubes (MWCNTs). Using PVA-LiClO4 gel electrolyte, an FSSSSC was constructed in order to demonstrate the highly reversible electrochemical redox processes of V2 O5 electrodes and the fast formation of electric double layers over MWCNT. Electrochemical investigations were performed in ambient conditions. During the manufacturing of symmetric devices, the polymer gel served both as an electrolyte and a separator. These two electrodes were sandwiched together using a layer of polymer which leads to the FSS-SSC device formation. The constructed device yields a potential window of 1.8 V with an energy density of 72 Wh/kg and specific power of 2.3 kW/kg @ 1 A/g along with 96% stability at 4000 cycles with 90% mechanical stability even at 175 degrees of mechanical bending. As a practical demonstration, a flexible device used to glow the VNIT panel consisting of 21 red LEDs is shown in Fig. 30.

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Fig. 29 Schematic diagram for the deposition of V2 O5 flakes over MWCNTs and representation of formed symmetric V2 O5 /MWCNTs based flexible solid-state device mediated through PVA-LiClO4 gel [232]

Fig. 30 a Mechanical stability of formed device through bending. b Stability studies of formed device for one week. Discharging of formed device through VNIT panel consisting of 21 red LEDs. c–f till 6 min [232]

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Synthesis of CeO2 nanostructures with the high surface area has been reported onto stainless steel (SS) as well as SS-coated MWCNTs [226, 227] as an active electrode material for supercapacitor application. Nanostructured CeO2 with solidstate device grown on SS (Fig. 31) shows energy (61.4 Wh/kg), power densities (6.9 kW/kg) along with 90.1% capacitive retention at 5000 cycles, and extraordinary 96.1% mechanical stability @5000 cycles [227]. Room temperature CBD method has been used to deposit cadmium hydroxide nanowires [240] with high surface area at room temperature on the stainless steel substrate. Use of cadmium chloride along with ammonia in aqueous media leads to the formation of cadmium hydroxide nano bundles as shown in Fig. 32. Formed nanowires employed as electrode material for supercapacitor application and reached a specific capacitance of 267 F/g with a scan rate of 5 mV/s in 1 M NaOH

Fig. 31 Designed CeO2 electrode on SS along with its mechanical flexibility and its surface morphology by SEM [227]

Fig. 32 Schematic diagram for growth of cadmium hydroxide nanowire along with surface morphology of obtained film [240]

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electrolyte. Interestingly, a symmetric supercapacitor achieved an extreme energy density of 11.09 Wh/kg with a power density of 799 W/kg at a current density of 0.84 A/g. Also, the stability studies for the formed symmetric device show enhancement in the capacitance value of 17% @1000 cycles (Fig. 33). Furthermore, these grown cadmium hydroxide nanowire bundles have been used as a template to achieve Fe(OH)2 @Cd(OH)2 core–shell surface architecture based on Gibbs free energy of formation by dipping template in iron precursor where Cd(OH)2 serve as core and Fe(OH)2 acts as a shell using low-temperature chemical route [51]. The surface morphologies of Cd(OH)2 and Fe(OH)2 @Cd(OH)2 core– shell nanostructure thin films exhibited specific capacitance as 331 F/g (@ scan rate of 5 mV/s). Not only the metal oxide and hydroxide materials were used as the electrode materials, but also metal chalcogenides are known to be electrochemically active

Fig. 33 Ragon plot of symmetric device using Cd(OH)2 nanowires and corresponding stability studies for 1000 cycles [240]

Fig. 34 a TEM images of MSMC sample. b HR-TEM image of MSMC along with corresponding enlarged views of lattice fringes (i and ii) and c SAED pattern [280]

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materials for supercapacitor applications. Chemical bath deposited MoS2 [42, 267] and MoSe2 [280] was successfully employed towards the fabrication of the supercapacitor. Figure 34 shows TEM, HR-TEM, and SAED images of MoSe2 deposited over MWCNTs. MoSe2 grown on MWCNTs exhibited cryptomelane-like growth yielding particle agglomeration resulting in well cover membrane over MWCNTs and facilitate tight interfacial conjugation. Figure 35 shows the Ragone, EIS, and CV plots along with the demonstration of the device by lighting up LED. Designed symmetric supercapacitor solid-state device based on MoSe2 /MWCNT exhibited extreme energy and power densities of 7.41 Wh/kg and 681 W/kg, respectively along with cyclic stability of 95% @1000 cycles. In addition to that, Sb2 S3 thin films have been grown through CBD using nonaqueous bath and employed as an active material for supercapacitor application [266]. Figure 36 shows the structural, morphological, and elemental results of CBD-grown Sb2 S3 film on stainless steel (SS) substrate. The variation in specific capacitance as a function of scan rate and discharge current densities is shown in Fig. 37. Sb2 S3 coated on SS exhibited 248 F/g as an extreme specific capacitance with the current value of 0.5 A/g in 1 M Na2 SO4 electrolyte.

Fig. 35 a Ragone plot, b EIS plot, c cyclic stability of 95% @1000 cycles and d Lightening LED using formed device as demo [280]

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Fig. 36 a XRD, b & c SEM, and d EDS of Sb2 S3 grown on SS substrate [266]

Fig. 37 a Effect of scan rate of specific capacitance (5–100 mV/s), b Effect of current densities on charge–discharge curves in 1 M Na2 SO4 electrolyte [266]

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6.6 Field Emission The field emission is based on a pure quantum mechanical tunneling process. Under the influence of an extremely strong electrostatic field, electrons are extracted from solid surfaces into the vacuum. The modification in the field emission properties can be done through doping or surface topography modification of the emitter. Chemical synthesis route is a relatively simple way to modify the surface topography. The formation of nanoprotrusions’ on the surface of the emitter plays a key role as the field emission process is geometry dependent. Many chemical bath deposited metal oxide semiconductor nanomaterials such as ZnO [212], V2 O5 [234], and CdO [300, 301] with surface modification have been investigated for enhanced field emission applications. The diode assembly for the study of field emission properties consists of vacuumcompatible conducting silver paste as cathode over a copper rod and the same was kept at a distance of ∼1 mm in front of the anode screen. The measurements such as field emission current density–applied field (J–E) and current–time (I–t) were carried out once the pressure attained the value of ~1 × 10−8 mbar. The turn-on field defined as the field required to draw an emission current density. Interestingly, V2 O5 /MWCNTs exhibited 2.5-fold decline value of the turn-on field as compared to bare V2 O5 [234]. The synthesis of Cd(OH)2 nanosheets followed by thermal annealing in order to convert them into porous aligned nanosheets of CdO and planar diode configuration is used to study field emission property. In addition to this chemical bath deposition method were successfully employed for the construction of junctionless transistor (JLT) using cadmium sulfide [164] and titanium dioxide [197] and also field effect transistors with lead sulfide and MWCNTs and TiO2 [302].

7 Advantages of CBD [164] • The CBD is a simple, convenient, and cost-effective method for small to large area deposition. • The method does not rely on a vacuum and clean room environment or much sophisticated instruments. • The deposition can be carried out in an open atmosphere even at relatively low temperatures; hence, range of substrates, viz., insulator, semiconductor, metal, and polymer of any shape can be coated. • Due to low-temperature process, corrosion can be limited for the metallic substrates.

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• Conducting substrate is not an essential criterion. • It is ideally suitable for the substrate of accessible and non-accessible nature for large and porous area deposition. • As the basic building blocks are ions and not atoms, the stoichiometry of the deposit can be maintained without difficulty. • Mixed and doped film structures could be obtained by simply adding the mixing/dopant into the reaction bath directly. • The deposition process is not line-of-sight, unlike many physical deposition methods such as evaporation or sputtering. • There is no involvement of organometallic solvents, no toxic or pyrolyzed gases. • A slow processing at ambient temperature facilitates better crystal orientation with an better grain structure. • The solution always remains in touch with the substrates leading to the formation of pinhole free and uniform deposits.

8 Disadvantages of CBD • The CBD process has the major disadvantage of being inefficient with respect to its utilization of starting materials and converting them into thin films. • Two major factors limit the extent of the heterogeneous reaction on the substrate surface: the competing homogeneous reaction in solution (which causes vast precipitation in solution) and the deposition of material on the walls of the reactor (chemical bath). • Wastage of material is very high (~99.9%). • Clean substrate is required and impurities embedded in the sources precursor used can be embedded in the film which can alter the film properties. • Metallic films are not possible. • Film formation only possible when the reaction between cation and anion occurs spontaneously.

9 Summary In the present chapter, the attempt has been made to highlight the basic principle of chemical bath deposition (CBD) method and factors affecting on the physicochemical prosperities of the synthesized thin films. The deposition or preparative parameters are easily controlled and optimized systematically. Usually, the heterogeneous nucleation (adsorption or deposition of aqueous ions over a surface of solid substrate) is responsible for the thin films formation of various metal oxides, hydroxide, chalcogenides (sulfides, selenides, and tellurides) polymer, and composites materials. The structural, optical, and morphological properties of the film depend on different

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preparative parameters such as pH, complexing agent, precursor concentration, deposition temperature, deposition time, type of substrate, substrate alignment, solution stirring, and doping. The CBD method is useful for the synthesis of nanostructured materials, as the basic building blocks are ions and not the atoms with assorted morphologies. The method has industrial applications because it requires minimal infrastructure, simple process, low-temperature ( Pd > Co > Pt. HCHO: Cu > Au > Ag > Pt > Pd > Ni > Co. NaBH4 : Ni > Co > Pd > Pt > Au > Ag > Cu. DMAB: Ag < Pt < Au < Pd < Co < Ni. NH2 NH2 : Au < Ag < Cu < Pd < Pt < Ni < Co. It should be mentioned that the sequences listed above subjects to particular experimental conditions like bath composition, temperature, and pH. Nevertheless, this trend of catalytic activity series may be useful in choosing the most suitable reductant for the metal to be plated.

2.1.3

Mechanistic Overview

Depending on the nature of reductant and metal, polarization profile for anodization of reductants exhibits different shapes and thus, is inevitably more complex to realize solely by means of simple electrochemical reactions. Therefore, a number of fundamentally different mechanisms were explored to illustrate the electroless

5 Electroless Assisted Nanostructured Morphologies

219

kinetics. These classical mechanisms mainly include atomic hydrogen reduction, hydride ion reduction, metal hydroxide mechanism, and stabilization mechanism. Appreciably, researchers realized that each proposed mechanism commonly defines many electroless deposition systems despite of having a uniquely distinct feature to it. Taking into account the following general observations: (a) hydrogen gas evolution, (b) that metals to be coated are generally found to be efficient hydrogenationdehydrogenation catalysts, (c) sulfur containing poisons for above stated class of catalyst (thiourea, mercaptobenzothiazole etc.,) act as stabilizers, and (d) that rate of deposition depicts an increment with increasing pH count of the solution, Van den Meerakar [20] hypothesised a comprehensive mechanism that involves reductant adsorption with subsequent dissociation, and is viable for all electroless systems. The postulated kinetics for the anodization of reductant (RH) begins with its dissociation or dehydrogenation upon adsorption at the catalytic deposited metal surface to yield atomic hydrogen (H) and radical R: RH → Rads + Hads

(4)

The electrons are derived from the oxidation of adsorbed R and/or H atoms for cathodic reduction of metal ions: Rads + OH− → Rads OH + e−

(5)

Hads + OH− → H2 O + e−

(6)

The recombination of two atomic hydrogen atoms attributes to the evolution of hydrogen gas: 2Hads → H2

(7)

Electrochemical ionisation of water at a catalytic metal surface may also result in the cathodic evolution of gaseous hydrogen: H2 O + 2e− → H2 + 2OH−

(8)

Finally, cathodic reactions utilize the electrons generated in Eqs. (5) and (6) to reduce metal M for deposition: Mn+ + ne− → M

(9)

Although the Van den mechanism, which is illustrated in Eqs. (4)–(9), is thought to be an effective way to describe the primary reactions that occur when a reducing agent is oxidised in a conventional electroless deposition system, one or more additional reactions must also be taken into consideration. When P and B containing reducing agents like hypophosphite, DMAB, and borohydride are used, co-deposition of P

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and B occurs along with the desired metal deposit as in the case of reduction using hypophosphite in acidic bath as shown below: + − H2 PO− 2ads + 2H3 O + e → P ↓ +4H2 O

(10)

The deposits obtained from electroless systems using P or B containing reductants are usually P or B based alloys depending on the solution bath. Remarkably, the deposition kinetics of P and B is greatly influenced by the operating conditions and composition of solution bath. Though inevitable side products of the electroless deposits, P and B confer unique physico-chemical properties to electroless deposits like the example of good corrosion resistance in Ni–P alloy [21, 22].

2.1.4

Anodic Oxidation

It is worth noting that in addition to the electroless plating of metals and alloys, coating of compounds without the assistance of outer voltage supply is also possible. As described earlier, reduction of metal ion in the presence of reducing agent results in metal electroless deposition and is usually a cathodic reaction. While electroless deposition of compounds is the anodization mechanism prompting from the metal oxidation in presence of oxidizing agents. This kind of deposition can be elucidated viz. chemical oxidation of aluminium in chromic acid contained solution bath [22]. The anodic reaction includes the oxidation of Al metal, presented as: 2Al + 3H2 O → Al2 O3 + 6H+ + 6e−

(11)

On the other hand, reduction of Cr(4+) to Cr(3+) with evolution of hydrogen gas is outlined in two cathodic stages: Cr 2 O7 2− + 8H+ + 6e− → Cr 2 O3 + 4H2 O

(12)

2H+ + 2e− → H2 ↑

(13)

Equations (11) to (13) can be summed up to give overall oxidation process of aluminium in chromic acid solutions, producing a blend of Cr2 O3 and Al2 O3 in thin film form: 4Al + 8H+ + Cr 2 O7 2− → Al2 O3 + 2Al3+ + Cr2 O3 + 3H2 + H2 O

(14)

5 Electroless Assisted Nanostructured Morphologies

2.1.5

221

Thermodynamic View

Depositions that are possible without any external current input need to be spontaneous from thermodynamic consideration. The spontaneity of a process is determined from the change in Gibbs free energy (ΔGo ) and is negative for a spontaneous process, which is given by [23]; ΔGo < 0

(15)

ΔGo = −z F E o

(16)

ΔGo = −CTlnK

(17)

And,

Or,

here, K symbolises equilibrium reaction constant. The equilibrium law for a general reaction is given by: A + B → AB K=

[AB] [A][B]

(18) (19)

where, C stands for universal gas constant, z implies the number of electrons exchanged, T denotes temperature, F is the Faraday’s constant, A and B are the reactants which electrochemically reacts to yield product AB, and Eo implies standard electrode potential. From Eqs. (16) and (17), it is inferred that Eo must be greater than 0 for electroless plating to take place.

3 Components of Electroless Bath Chemical as well as physical properties of the film resulting from electroless deposition are extremely sensitive to the composition of the bath, which is greatly influenced by its preparation and operative preparative parameters (see Fig. 6). Steps Involved in Electroless Deposition. The schematic below depicts the stages of electroless deposition (Fig. 7) employing the optimal combination of reductant, bath additives, and metallic salt, as well as plating solution requirements such as pH and temperature, as detailed below [24];

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Fig. 6 Schematic of experimental set-up for electroless deposition

Fig. 7 Steps involved during electroless plating [24]

1. First and foremost, substrate cleaning with an additional step of activation or sensitization in case of non-metallic surfaces. 2. Dipping of the substrate in the plating bath where adsorption of constituents of bath occurs onto the surface. 3. Then, a series of intermediate reactions happen, such as the reduction of a metal complex and the oxidation of an adsorbed reducing agent, which lead to the deposition of the desired metal film. 4. Desorption and transport of unreacted species, intermediates, and by-products away from the substrate. 5. Once the film of desired thickness is achieved, the substrate is removed from the plating bath.

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4 Role of Electroless Components In order to fabricate a reproducible electroless coating, it is utmost important to study each constituent of an electroless system in detail to understand their fundamental role in the deposition process (Fig. 8).

4.1 Metal Salts Metal ions may be obtained from any soluble salt in water, like chlorides, sulphates, cyanides, nitrates and many more can serve as source of metal ions. The salt is usually chosen depending on the solution bath sustainability, desired film characteristics, and on the environmental concern. Moreover, the deposition kinetics and quality of the deposited film on the substrate surface greatly depend upon the availability of metal ions in the solution bath, which in turn, depends on the type of metal salt used for deposition. The change in concentration of the concerned metal salt can substantially alter the autocatalytic processes of heterogeneous nucleation performed via reducing agents and ultimately the growth of desired plating. Generally, the films obtained in

Fig. 8 Flow chart demonstrating the bath components and their role in deposition process

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low concentrated metal ion solution are too thin, non-uniform and insufficiently homogeneous. This can be related to the lack of required number of ionic species. On contrary, the electroless solution bath containing relatively higher concentration of metal ions usually favours the better quality film of extreme uniformity. However, when the concentration of metal ions is increased beyond a certain limit which is identified as a point of ‘solution instability’, may result in powdery particle rather than thin film growth on the substrate surface [25].

4.2 Reducing Agents In electroless deposition process, film formation over substrate surface is accomplished by an autocatalytic reduction of metal ions from an aqueous precursor comprising of reducing agent. Electrons required for the reduction of metal ions are supplied by a chemical reductant in homogeneous solutions having an aqueous phase. Reductants with good solubility and reasonable stability are usually preferred in the making of plating solution bath. Most commonly employed reducing agents for electroless deposition are NaH2 PO2 , HCHO, NH2 NH2 , borohydrides, amine boranes, and some of their derivatives [26]. The selection of a reducing agent for particular metal depends on the current–potential response of the metal to be deposited and on the characteristics of end product. With metal ions, the concentration of the reducing medium exhibits a decisive part in determining the deposition rate and stability of electroless plating. The deposition rate exhibits a linear increment trend with increasing reductant concentration until a point so called ‘solution instability’ is achieved and then a significant decrement in the growth rate can be observed.

4.3 Complexants In general terms, the vital role of complexants in any electroless system is to enhance its solution stability by controlling free metal ions activity. The type of complexants used in deposition relies on metal ions, degree of acidity or basicity, temperature and other operating conditions [27]. For acid-based solutions, glycolic or lactic acids are usually preferred as complexants while in case of alkaline solutions, ammonium and pyrophosphate ions are employed. Briefly, when complexants are added to the bath, they replace water molecules attached to hydrated (free) metal ions with an ion or molecule provided by them, producing a complex metal ion source. As a result of complexation, the process of hydrolysis of metal ions gets restricted otherwise bulk precipitation of metal may occur in the solution bath. Lower concentrations of complexing agent typically led to rapid decomposition of solution while higher concentration results in thinner film due to less availability of metal ions. Also, complexants exert buffering actions and help to inhibit the hydroxide formation tendency of metal ions. Furthermore, the deposition rate and pH of the solution can

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be controlled through proper choice of complexing agents, thus providing an ease in bath management.

4.4 Stabilizers Addition of stabilizers (sometimes referred to as inhibitors) to the plating solution is necessary owing to the spontaneous nature of electroless deposition process. Being an autocatalytic process, electroless decomposition of solution can initiate at other active sites of the bath including walls of the plating tank as well as bulk of the solution, which needs to be retarded. When strongly adsorbing solution stabilizers are added to the bath in few ppm, they get adsorbed on such sites and impede the reductant oxidation, and hence deposition [28]. To date, a number of compounds are being employed as stabilizers which can render bath stability or at least inhibit extraneous precipitation. These includes group IV elements (selenium, tellurium etc.) compounds, unsaturated organic acids (e.g. maleic acid), heavy metal cations (e.g. Pb2+ , and Sn2+ ), and oxygen-containing compounds like molybdates or iodates. Stabilizer’s concentration plays a very crucial role in final product film. This is because, too high concentration of stabilizer in solution can either prevent deposition from initiating or can stop the plating process completely. The catalytic activity of substrate can be appreciably altered even with the addition of a trace amount of stabilizers. Further, not all stabilizers work as inhibitors or catalytic poisons in electroless solution system. Some additives actually function to enhance the activity and hence, increase the rate of deposition, which are tend to be known as accelerators. Thiourea at lower concentrations (less than a millimole), formates, and glycine are some examples of accelerators. This increase in rate feature of accelerators is endorsed to the facile electrons transfer from reductant to metal ion source [29]. Notably, there are some instances where adsorbed additives become incorporated as impurities into deposits because of their tendency to undergo reduction during the electroless process in a manner similar to that of ternary alloy deposition [30].

4.5 Buffers It is well established that anodic oxidation of reducing agents employed in electroless plating inevitably involves formation of hydrogen or hydroxide (H+ /OH− ) ions, which cause an abrupt change in the pH of the plating solution and thus, affect the deposition rate and physico-chemical properties of the deposited film. A high pH value of the plating bath can result in extraordinary enhancement in deposition rates, which in turn, may lead to powdery and weakly adherent film. Also, inadequately low pH slows down the deposition rates and ultimately gives rise to dull or non-reflecting deposits. Henceforth, in order to obtain a film with the required

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physico-chemical or morphological characteristics, the pH of the solution should be constantly maintained throughout the experimental process. With this consideration, buffers as another important class of additives are being commonly used in electroless deposition process for stabilizing pH of the precursor. In simple words, buffers are the substances or amalgam of substances having the capability of neutralizing both acids and bases without significantly changing the native pH value of the solution. The quality of a buffer is measured in terms of its capacity which is determined by the amount of say, acid required to change the solution’s pH by a particular amount [31]. Obviously, the greater that amount, the better is the buffer. Carboxylic acids and its salt in acidic bath whereas organic amines for alkaline bath are commonly served as buffering agents in electroless plating. Even while using buffering agents, a slow decline in pH can be observed as deposition occurs, which is corrected by further chemical additions, distinctly known as pH regulators.

4.6 Bath Temperature The temperature of the electroless solution bath at which deposition takes place is the most dominant variable affecting reaction rate as it gives a measure of energy (in the form of heat) required for reactions to proceed. As the temperature is raised, the plating rate increases exponentially [32]. It must be noted that for the growth of good quality films, temperature of the solution must be closely monitored. This is because metal ions are present in complex-bound state within the solution bath, there may not be free ions existing for film formation at room temperature. When temperature of the bath is slowly increased, sufficient thermal energy is available to make metals free from bound complex state which increases the rate of reaction. However, the bath temperature should rarely be raised too high otherwise solution plate out or bath decomposition then becomes a real possibility rather than obtaining a film.

5 Pros and Cons—Electroless Plating Because of its self-driven reduction reaction mechanism, electroless plating owns distinct benefits over conventional thin film deposition counterparts, as noted below (Fig. 9); i.

Possible to coat metal, alloy and subsequent conversion to metal oxides in thin film form. ii. Simple, low-cost and offers scalability to large area deposition. iii. Requires no electricity, giving the process a wide range of substrate material in any shape and size options. As a result, any kind of substrate, both conductive and non-conductive, can be plated.

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Fig. 9 Advantages and disadvantages of electroless deposition method

iv. Results in uniform deposits even on irregularly sized and shaped substrates. v. Since this method does not face current density uniformity issue, it can reach the concealed surfaces of complex parts. vi. Due to the consistency in plating, it produces high quality deposits with exceptional anti-corrosion properties. Besides, there also exist a few downsides to using an electroless plating method for thin film production, including the following; i.

Only metallic elements are possible to reduce and formation to oxides in limited range ii. Limited bath life: Consistent monitoring of solution bath and replenishment must be required to ensure the optimum concentration of metal ions throughout the deposition. iii. Environmental concern: Alkaline and acidic electroless baths suffer environmental disposal issues. iv. Requires surface pre-treatment steps for good quality deposition for some substrates.

6 Research Developments: A Brief Review Right through the discovery made by Brenner and Riddle, the method of electroless deposition is being progressively explored by scientists all over the world towards coating of various materials for diverse range of applications including electronics, energy storage and conversion technologies, anti-corrosion coatings, nanoscience etc. (Fig. 10).

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Fig. 10 Applications of electroless deposition

To date, there is wealth of metals or their alloys, and compounds which have been coated through electroless deposition in thin film form from various bath formulations (see Table 1). In particular, most intensely investigated electroless systems include Au, Ag, Cu, Ni, Co, Ru etc., and their related alloys. These researchers exclusively demonstrated a diversity in morphological and physico-chemical properties of end product film by tuning their bath composition and operating conditions, and thus accordingly its potent utility. In addition to metals and/or alloys, the method is extended lately for thin film coating of oxides. There are many examples of metals available in the literature that have been electroless deposited over a range of substrates from metallic to non-metallic surfaces due to their promising applications in the modern technology. For instance, Pandit et al. [33] proposed a room temperature, hydrazine hydrate based electroless deposition of Ag nanoparticles on stainless substrate (SS) without the addition of any further additives for thin film supercapacitor applications [33]. This Ag deposit thin film electrode depicted an extremely porous, uniform, and well-adherent nanostructure. As another example, hierarchical nanospike Ni arrays were electrodeposited over Cu substrate from a solution bath of Ni ions, NH2 NH2 , glycine, and H3 BO3 at pH 12 [34]. A considerable enhancement in hydrogen evolution reaction (HER) activity of Ni was realized by the electroless deposited nanospike arrays in comparison to electrodeposited Ni as well as commercial Ni plate, due to the good electrochemically

SS

Cu GCE

Nanoparticles

Nanospikes

–

Granular particles

Granular particles

Granular particles

–

–

Crystals

Rice-like granules

Nanoparticles

Nanoparticles

Cauliflower type structure

Nanocrystallines

Hexagonal prism

Porous structure

Ag

Ni

Co

Cu

Ag

Ru

Ni-Co

Ni-Co-P

CoWB

Co-P

ERGO-Au

MWCNTs/Ag

Ni–B-TiO2

Si NWs/Cu2 O

ZnO

Vanadium oxide

VOSO4

Zn(NO3 )2 .6H2 O

CuSO4 .5H2 O

NiCl2 .6H2 O TiO2

AgNO3

HAuCl4

CoSO4 .7H2 O

CoSO4 .7H2 O Na2 WO4

NiSO4 .6H2 O CoSO4 .7H2 O

NiSO4 .6H2 O CoSO4 .6H2 O

K2 RuCl5 .xH2 O

AgNO3

CuSO4 .5H2 O

CoSO4 .7H2 O

NiCl2 .6H2 O

AgNO3

Metal Ion source

NaOH

(CH3 )2 NHBH3

HCHO

NaBH4

Hydrazine hydrate

NH2 OH

NaH2 PO2 .H2 O

Morpholine borane

NaH2 PO2

Dimethylborane

NaNO2

Hydrazine hydrate

Formalin

DMAB

N2 H4

Hydrazine hydrate

Reducing agent

ES

–

–

–

ES

–

Magnetic applications

–

ULSI

HER and OER

–

ULSI

ULSI

–

HER

ES

Application

[47]

[46]

[45]

[44]

[43]

[42]

[41]

[40]

[39]

[38]

[37]

[36]

[36]

[35]

[34]

[33]

Refs.

Abbreviations: DMAB—Dimethyl amine borane, ERGO—Electrochemically reduced graphene oxide, ES—Electrochemical supercapacitor, ESM—Egg shell membrane, GCE—Glassy carbon electrode, HER—Hydrogen evolution reaction, MWCNTs—Multiwalled carbon nanotubes, NF—Nickel foam, NWs— Nanowires, OER—Oxygen evolution reaction, SS—Stainless steel, ULSI—Ultra large scale integration

Ti

ESM

Si

Steel

SS

Cu

Corning glass

NF

Si

Glass

Glass

Cu

Cu

Substrate

Morphology

Material

Table 1 Literature overview of electroless deposited metals/alloys/compounds for different applications

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active surface area. In addition, a new type of electrochemical quartz crystal microgravimetry assisted analysis of electroless deposition of Co layers in the presence of different amines was performed recently [35]. They concluded that induction period of deposition of Co is strongly influenced by solution composition, temperature, and presence of different amines. Electroless deposition of Cu and Ag thin films on glass substrate which can be used as interconnects in ultra large scale integration (ULSI) metallization technology was analysed by Sabayev et al. [36]. Prior to metallization, surface activation by Pd nanoparticles was performed which resulted in finely tuned electrical properties, surface morphology, and homogeneous coverage of these deposits. Apart from traditionally studied noble metals based electroless plating, thin film coating of Ru via electroless method has gained growing interest as an efficient diffusion barrier in semiconductor devices and as an electrocatalyst in energy conversion systems. So far, both conducting as well as non-conducting platforms such as Cu, C, Pd-InGaAs, Si, and acrylonitrile butadiene styrene have been autocatalytically plated with Ru deposit. Nevertheless, a detailed discussion concerning surface pretreatment is neglected which is rather necessary to ensure uniform deposit in case of non-conducting substrates. In this regard, Chen et al. [37] thoroughly studied the effect of surface pre-treatments like HF etching, sensitization, activation and their combination, carried out on Si surface for the formation of continuous Ru film via an electroless approach [37]. The contact angle measurement inevitably depicted a remarkably hydrophilic surface after sensitization and activation, which favoured uniform deposition of Ru film at reasonable growth rates in comparison to HF-etched sample. With an aim to wholly accomplish the commercial applicability of electroless deposited systems, current research is exclusively focused on the development of metal binary alloy systems where any metallic and/or non-metallic element is incorporated with the active material, synergistically resulting in ameliorated physicochemical properties of the final product. With this anticipation, Perez-Alonso et al. [38] has investigated a set of Ni, Co, and bimetallic Ni/Co electroless coatings of various compositions onto nickel foam (NF) to evaluate the influence of the Ni/Co ratio in hydrogen and oxygen evolution reaction [38]. This work showed that the composition of alloy deposits strongly depends on electroless bath composition and other preparative parameters. Kumar et al. [39] proposed the transition of Ni-Co-P film coated on corning glass from amorphous to crystallised phases of Ni, NiP, and Ni3 P with a grain size lying in 20 to 40 nm range at a post-annealing temperature of 600 °C and observed a decrease in the sheet resistance values when post-annealing temperature was reached to 400 °C. Ternary alloy CoWB was electrodeposited from cobalt-glycine plating bath using morpholine borane as a reducing agent as well as a B source and was first reported by Sukackiene and his group [40]. It is noteworthy to mention that agitation during electroless deposition plays a decisive role in determining the properties of plated film. In this approach, Lin et al. [41] reported that ultrasonic agitation efficiently lowered the operating temperature of electroless deposited Co-P having tuned nanocrystalline rice-granular like morphology resulting in retained strong mechanical and magnetic properties. This is because ultrasonic

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agitation enables better mixing and dispersion of bath constituents, facilitates the transit of reactants, and reduces the nucleation activation energy. Past few years, an increasing interest has been triggered in the synthesis of nanostructured multifunctional materials consisting of electron rich metal nanoparticles (for e.g. Au, Ag etc.) and carbon derivatives. For instance, Raj et al. [42] have constructed a composite film of electrochemically reduced graphene oxide-gold nanoparticles (ERGO-Au) onto glassy carbon electrode (GCE), where ERGO modified GCE was covered with Au nanoparticles through electroless deposition from AuCl4− solution bath and NH2 OH as a mild reducing agent. The particle coverage was found to be highest for Au plated ERGO in comparison to that of GO film and bare GCE, which was ascribed to the capability of ERGO to spontaneously reduce Au(3+) ions to Au(0) together with NH2 OH assisted reduction. Pandit et al. [43] uniformly anchored Ag nanoparticles over dip-dry coated multi-walled carbon nanotubes (MWCNTs) network via electroless deposition at room temperature to design a hybrid electrode and test its electrochemical performance in supercapacitor application. Furthermore, a novel spontaneous electrodeposition of Ni–B-TiO2 over mild steel was developed for improving the corrosion resistance of Ni–B coatings. In their report, TiO2 particles were dispersed in bath suspension by ultrasonic irradiation to ensure fine incorporation of TiO2 in Ni–B matrix [44]. Besides metals and/or alloys, studies and practices of electroless deposition method are further extended for plating of different types of compounds, such as salts and oxides over non-metallic as well as metallic surfaces. Xiong et al. [45] studied the correlation of morphology of Cu2 O nanocrystallines grown on the surface of Si nanowires (NWs) with deposition time and pH value of the bath solution. In this report, the deposition time of 10 min and the pH value of 12.5 are directed to the evolution of cube-like morphology of Cu2 O nanocrystallines over Si NWs to form a radial nanoheterostructure for energy storage and conversion devices. Preda et al. [46] found that electroless ZnO nanostructures could be fabricated on egg shell membranes (ESM), after coating ESM with Au catalytic layer. This rationally designed organic/inorganic hybrid network from bio-waste through a simple and effective electroless deposition method can find potent applications in various fields. In the year 2014, a report about the preparation of highly porous vanadium oxide film on Ti surface via electroless deposition affirmed its potential for electrochemical supercapacitor [47].

7 Electroless Deposits: Morphology and Applications 7.1 Electroless Ag Nanoparticles: Supercapacitor Usefulness of electroless deposition approach in the development of nanostructured metallic electrodes has gained sporadic interest in the field of industrial applications, energy storage and conversion devices, and many more. A detailed study of one

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such electroless deposition system adopted for production of thin film consisting of Ag nanoparticles as a supercapacitor electrode is presented in this section [33]. The report exclusively focussed on morphological and structural properties of assynthesized Ag thin film in correlation with its electrochemical characteristics for supercapacitors. (a) Film Fabrication In their study, Ag+ ions from silver nitrate solution were chemically reduced using hydrazine hydrate as a reducing agent to produce a thin film of Ag nanoparticles on SS. It is noteworthy to mention that the proposed state-of-art via electroless approach was carried out at room temperature with no further additives and no additional substrate surface activation processes. The process involves the adsorption of Ag+ ions onto the SS surface followed by reduction of adsorbed Ag+ ions by hydrazine hydrate with the by-products expulsion from substrate’s surface (Fig. 11a). The overall mechanism for the film formation is purely electrochemical in nature involving oxidation and reduction reactions as a result of electron transfer between chemical species, which was described in reaction given below;

Fig. 11 a Schematic illustration for the deposition of Ag nanoparticles; b XRDs of bare SS, SS/Ag, and G/Ag; c FESEM image of Ag thin film; d the cyclic voltammetry (CV) curves at different scan rates ranging from 2 to 100 mV s−1 ; e the charge–discharge curves at different current densities ranging from 2 to 5 mA cm−2 [33]

5 Electroless Assisted Nanostructured Morphologies

4Ag+ + N2 H4 → 4Ag ↓ +N2 ↑ +4H+

233

(20)

(b) Structure The as-deposited electroless Ag thin film exhibited a face-centred cubic structure with major (110) and (220) orientations. However, there was some unwanted silver oxide phase obtained in deposits, which may be due to the interference of atmospheric oxygen as inferred from X-ray diffraction (XRD) patterns (Fig. 11b). (c) Morphology The Ag deposits on SS substrate using electroless method were found to be well-adherent, compact and extremely homogeneous as manifested from fieldemission scanning electron microscopy (FESEM) analysis (Fig. 11c). The film morphology typically displayed a nodular structure consisting of closely bound Ag nanoparticles having a diameter in the range of 20–40 nm. The nanoparticles featured Ag thin film contained an ample free space in between the particles, thus exhibiting high porosity. Such nano-sized assembly of Ag particles projects enormous potential for applications in electrochemical energy storage devices where highly conductive as well as porous thin film electrodes are required. Notably, agglomeration of few Ag nanoparticles was also observed over the region of substrate’s surface, which may be due to high catalytic activity of Ag nanoparticles or the presence of defect sites at the surface. (d) Supercapacitor Studies The Ag nanoparticles thin film electrode displayed outstanding electrochemical performance with a capacitance value of 452 F g−1 in 0.5 M NaOH owing to its highly pseudocapacitive charge storage behaviour resulting from Ag/Ag+ redox couple (Fig. 11d and e). The Ag film assembled from nanoparticles gave rise to facile and fast charge transport of electrolyte ions through active material. Subsequently, the electroless assisted Ag film exhibited a maximum energy density of 27.8 Wh kg−1 and delivered a high power density of 10.2 kW kg−1 . The outcomes distinctly display that the development of Ag nanoparticles by adopting electroless deposition method is a propitious strategy in the present high power andenergy-efficient storage automations.

7.2 Nickel Nanospike Arrays: Hydrogen Evolution Reaction (HER) Hydrogen has the potential to be a viable alternative to fossil fuels like natural gas and oil. One of the major benefits of using hydrogen fuel is its environmental cleanliness. At the burning of hydrogen, only water is created, so no greenhouse gases or toxic compounds are released. Currently, hydrogen generation procedures rely mostly upon hydrocarbons partial anodization and steam generation, both of which create common greenhouse gas in considerable amounts i.e. carbon dioxide (CO2 ) as a by-product. Water electrolysis is an alternative method for producing hydrogen [34].

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(a) Film Synthesis In this study, the bath containing NiCl2 , NH2 NH2 , glycine, H3 BO4 and KOH was used as a Ni2+ ions source, reducing agent, complexing agent, buffer and pH adjusting alkaline solution, respectively. The Cu plate was used as a substrate, which was pre-cleaned with metal polish reagent followed by ultrasonication in acetone. The electroless deposition of Ni nanospikes array was performed at 80 °C temperature for different deposition times of 2, 5, 20 and 60 min. For a comparison, Ni platting was also performed using electrodeposition as a standard benchmark. The possible reaction kinetics for the above reaction is given below: 2Ni2+ + N2 H4 + 4OH− → 2Ni ↓ +N2 ↑ +4H2 O

(21)

(b) Structure The as-deposited nickel nanospikes array exhibits a cubic metallic phase of Ni grown in the direction of (111) plane which is a strong evidence of forming nanospikes array. The reaction time did not affect the growth process, all samples show preferred growth orientation in (111) direction (Fig. 12a). (c) Morphology The SEM images of Ni deposited with the aid of electroless process on the Cu plate is shown in the Fig. 12b, which reveals a spikey structure. The sizes of the vertically grown nanospikes are shown in the range of few tens to many hundred nanometres scale as NH2 NH2 was utilized as Ni2+ reductant. NH2 NH2 was adsorbed on Ni crystal planes oriented in specific directions, causing anisotropic development of nanospikes over substrate surface. (d) HER activity The HER activity was performed in 1 M KOH and evaluated using linear sweep voltammetry. The comparative HER activity was recorded for Ni nanospikes arrays and electrodeposited Ni. The HER activities of Ni produced from electrodeposition and Ni plate are nearly identical in Fig. 12c, however, the nanospikes array has a substantial higher HER activity than both. Electroless deposited Ni nanospikes array, Ni from electrodeposition, and Ni plate had onset potentials of −1.20, −1.23, and −1.27 V, respectively. The results show Ni nanospikes array deposited using electroless method exhibited efficient HER.

7.3 Ni-Co Electroless Deposits: Oxygen Evolution Reaction (OER) The oxygen evolution reaction (OER) is essential for renewable energy conversion and storage devices such as Li-air batteries, fuel cells, and water electrolyser [38].

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Fig. 12 a XRD profile: (i) Ni plate, (ii) Ni from electrodeposition, and electroless deposited Ni nanospike arrays at different reaction times ((iii) 5, (iv) 20, and (v) 60 min); b FESEM images of Ni nanospike arrays for electroless deposition time of 60 min; c linear sweep voltammetry curves for Ni plate, Ni from electrodeposition, and electroless deposited Ni nanospike array in 1 M KOH at 10 mV s−1 [34]

(a) Synthesis The Ni-Co electroless platting was performed in the solution bath containing optimised amount of CuSO4 and NiSO4 complexed with sodium citrate and DMAB as a reducing agent. The NF was used as a substrate. The reaction was performed for 1 h at 75–80 °C. The Ni/Co atomic ratio was varied in order to optimize the best OER activity. (b) Film properties The morphology of Ni/Co samples was identical only the change occurred in the thickness of the plating (Fig. 13a–f). The samples of electroless deposited pure Ni (Ni-ELP) and Co (Co-ELP) reveals the thickness of 2.6 and 2.3 µm, respectively. Further, the thickness was increased with decreasing Co concentration in the bimetallic electrodes. The thickness of the bimetallic layer was revealed

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Fig. 13 SEM micrographs: a Ni-foam, b Ni85Co15-ELP, c Ni70Co30-ELP, d Ni50Co50-ELP, e Ni-ELP and f Co-ELP; and g corresponding quasi steady-state anodic polarization traces [38]

as 0.5, 1.1, and 1.8 µm for the Ni/Co atomic ratio of 50/50 (Ni50Co50-ELP), 70/30 (Ni70Co30-ELP), and 85/15 (Ni8Co15-ELP). (c) OER Anodic polarization curves of Ni-Co bimetallic electrodes were recorded in 30% KOH using CV from open circuit potential to 2.2 V with reference to a reversible hydrogen electrode (RHE) at the scan rate of 1 mV s−1 (Fig. 13g). The best OER activity observed for the Ni/Co bimetallic alloys containing the ratio of 50/50 and 70/30 than that of the bare electroless deposited Ni electrode.

7.4 Ni-Co-P Alloy Thin Film: Ultra Large Scale Integration (ULSI) Application Electroless derived metals/alloys thin films have also been studied as ohmic contacts as well as interconnects for ULSI technology in microelectronic circuits [39]. (a) Synthesis Corning glass was used as a substrate for the thin film coating of Ni-Co-P alloy from an electroless bath consisting of CoSO4 .7H2 O and NiSO4 .6H2 O as respective sources for cobalt and nickel ions. While, the role of a reducing agent as well as the phosphorous source was served by NaH2 PO2 .H2 O. Moreover, to limit the rate of free metal ions in the solution during reduction mechanism, complexing agent Na3 C6 H5 O7 was employed. Within the reacting species solution, ammonium chloride was added as a buffer and the pH of the deposition bath was regulated with the help of sodium hydroxide (see Fig. 14a).

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Fig. 14 Electroless Ni-Co-P films: a bath composition and plating parameters, b XRD patterns, c atomic force micrograph and d variation of sheet resistance with annealing temperature [39]

(b) Phase structure The XRD traces of Ni-Co-P amorphous thin film coating and the films at annealing temperatures of 400, 500, and 600 °C for 30 min (Fig. 14b), manifesting that amorphous phase of alloy film was converted to crystalline with post-deposition annealing. (c) Morphology The deposited films having a granular structure with grain size ranging from 20 to 40 nm were observed and showcased in surface topographical images of Ni-Co-P thin film at conditions with deposition time of 2 min and pH ~ 8.5 (Fig. 14c). (d) ULSI application The variation in sheet resistance of alloy film with respect to bath temperature is shown in Fig. 14d which records the least sheet resistance of 3.548 Ω square−1 .

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7.5 Bimetallic Phosphide (Co–P): Magnetic Application The electroless films have a promising application for microelectromechanical systems and magnetic sensors [41]. (a) Synthesis Optimized concentrations of CuSO4 , NaHPO2 , C4 H4 KNaO6 , and H3 BO3 were prepared in a 250 mL volume of solution and served as a medium of Cu ions, phosphide, complexing agent and buffer agent. The Cu plate was used as a substrate for the deposition of Co-P. The reaction condition was maintained at 40–60 °C for different periods (20, 30 and 40 min). (b) Film Properties From the XRD spectra (Fig. 15a), it is revealed that the electroless deposited CoP film shows hexagonal close-packed (HCP) structure for all samples prepared at different temperatures. (c) Morphology Even at varied temperatures, no considerable variation in the morphologies of the as-deposited Co-P was found. Almost all samples show morphology like rice granules in all different conditions (Fig. 15b–e). (d) Magnetic Application The increased squareness in loop resulting from the occurrence of the unique HCP microstructural design with (002) plane orientation, may explain such an ameliorated magnetization of CoP film processed at 60 °C. Correspondingly, in-plane characteristics for Co-P showed maximum values of 860 emu g−1 and 99 Oe for Mr and Hci , respectively, while out-of-plane properties have maximum values of 1640 Oe and 26 emu g−1 (Fig. 15f–i).

7.6 Electroless Copper Plating on Non-conducting Substrates for Other Applications Electroless process does not require any electric current as it is self-driven by chemical redox reactions. Consequently, it is considered to be an ideal thin film deposition method for coating non-conductive substrates for a multitude of applications ranging from fabrication of printed wiring boards to decorative plating on plastics. Variety of the base materials of the circuit board including paper-phenolic, epoxyglass, polyethylene terephthalate (PET) or other dielectric, has been electroless deposited in thin film form for printed wiring board applications. For decorative electroless plating on plastics, substrate materials comprise acrylonitrile–butadiene–styrene, polycarbonate, nylon, polyester, and so on. Note that, electroless coating on surfaces that do not hold electrical continuity, pre-treatments like substrate surface sensitization or activation steps are required (Fig. 16).

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Fig. 15 a XRD spectra for Co-P films coated by electroless with different process temperatures; surface morphological analysis of Co-P coated via ultrasound aided-electroless method: b, c 60 °C, d 52 °C, and e 44 °C of bath temperatures; corresponding magnetic measurement findings: f and g in-plane and out-of-plane normalised hysteresis curves, h and i in-plane and out-of-plane M-H traces [41]

8 Summary The method of electroless deposition is increasing world-wide interest as an allround chemical deposition method in fabrication of thin films of various nanostructured materials for applications ranging from anti-corrosion coatings to energy storage technologies owing to its simplicity, cost effectiveness and scalability. This

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Fig. 16 a Copper patterning on sandpaper for practical circuit application, b stretched PET substrate with flexible Cu pattern, c customised printed circuit board, and d decorative purposes [12].

facile method produces exceptionally continuous and homogeneous coatings that are usually challenging to attain using traditional thin film deposition methods. Additionally, no electric current requirement of electroless process enables flexibility in choosing wide range of shapes without limitations of size and shape for deposition. Being an easy and uncomplicated method, electroless is prone to offer good control over the deposition rate, in turn, morphology through proper tuning of bath parameters like immersion time, temperature, concentration of metal ions, nature of reducing agent etc. Benefitting from these technical values, an ample of reports have been published on electroless method. Interestingly, many electroless investigations have been executed in detail to coat films of metals, alloys, and even compounds through fine variation of synthesis parameters. It is important to note that electroless method can produce nearly all alloys/metals that can be deposited using electrodeposition, under optimized conditions and proper reducing agents. Furthermore, deposition of compounds like ceramics or polymers is also being recently accomplished via electroless process, which is absolutely desirable in the field of nanotechnology. However, there exists scope for further exploration and applied investigations to be performed toward ensuring the correct electroless plating bath that would be able to achieve the targeted characteristics of product thin film.

9 Future Outlook The electroless deposition method is a simple, controllable, large scale and industrially scalable method and have great potential to explore the development of new materials to extend the application field.

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i.

The electroless deposition method has been mostly used to deposit metallic nanoparticles even though there is large scope not only to deposit d-block transition metal nanoparticles but also f-block rare earth elements. ii. The present method can be possibly used for the recovery of noble metal nanoparticles such as Ag, Au and Pt from the waste. iii. Also, using the electroless reduction process, there is possibility to recover the nanoparticles from dead batteries, fuel cells, electronic equipment etc.

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38. F.J. Pérez-Alonso, C. Adán, S. Rojas, M.A. Peña, J.L.G. Fierro, Ni-Co electrodes prepared by electroless-plating deposition. A study of their electrocatalytic activity for the hydrogen and oxygen evolution reactions. Int. J. Hydrogen Energy 40(1), 51–61 (2015). https://doi.org/10. 1016/j.ijhydene.2014.11.015 39. A. Kumar, A.K. Suhag, A. Singh, S.K. Sharma, M. Kumar, D. Kumar, Deposition and characterization of amorphous electroless Ni-Co-P alloy thin film for ULSI application. Mater. Res. Express 1(3) (2014). https://doi.org/10.1088/2053-1591/1/3/035007 40. Z. Sukackien˙e, L. Tamašauskait˙e-Tamaši¯unait˙e, V. Jasulaitien˙e, A. Balˇci¯unait˙e, A. Naujokaitis, E. Norkus, Electroless deposition of CoBW coatings using morpholine borane as a reducing agent. Thin Solid Films 636, 425–430 (2017). https://doi.org/10.1016/j.tsf.2017.06.034 41. C.C. Lin et al., Ultrasound-assisted electroless deposition of Co-P hard magnetic films. Surf. Coatings Technol. 388, 125577 (2020). https://doi.org/10.1016/j.surfcoat.2020.125577 42. M.A. Raj, S.A. John, Assembly of gold nanoparticles on graphene film via electroless deposition: spontaneous reduction of Au3+ ions by graphene film. RSC Adv. 5(7), 4964–4971 (2015). https://doi.org/10.1039/c4ra11848k 43. B. Pandit, B.R. Sankapal, Highly conductive energy efficient electroless anchored silver nanoparticles on MWCNTs as a supercapacitive electrode. New J. Chem. 41(19), 10808–10814 (2017). https://doi.org/10.1039/c7nj01792h 44. V. Niksefat, M. Ghorbani, Mechanical and electrochemical properties of ultrasonic-assisted electroless deposition of Ni-B-TiO2 composite coatings. J. Alloys Compd. 633, 127–136 (2015). https://doi.org/10.1016/j.jallcom.2015.01.250 45. Z. Xiong, M. Zheng, H. Li, L. Ma, W. Shen, Fabrication and optical properties of silicon nanowire/Cu2O nano-heterojunctions by electroless deposition technique. Mater. Lett. 112, 211–214 (2013). https://doi.org/10.1016/j.matlet.2013.09.007 46. N. Preda, A. Costas, M. Beregoi, I. Enculescu, A straightforward route to obtain organic/inorganic hybrid network from bio-waste: Electroless deposition of ZnO nanostructures on eggshell membranes. Chem. Phys. Lett. 706, 24–30 (2018). https://doi.org/10.1016/j. cplett.2018.05.073 47. H. Wu, K. Lian, Vanadium oxide electrode synthesized by electroless deposition for electrochemical capacitors. J. Power Sources 271, 534–537 (2014). https://doi.org/10.1016/j.jpo wsour.2014.08.034

Dr. Akanksha Agarwal recently awarded a doctorate in the Department of Physics, Visvesvaraya National Institute of Technology (VNIT), Nagpur (India) under DST-Inspire scheme, Government of India. She has awarded with one of the most prestigious fellowships named DST INSPIRE FELLOWSHIP for pursuing her Ph.D. degree. Before that, she received her Master’s degree in Physics (2018) from VNIT (Nagpur), India and received the Gold Medal for securing the highest CGPA in the tenure of degree. Her doctoral work centered around the design and development of hybrid nanostructured organic and inorganic materials for device grade application of supercapacitors. She has published 9 articles in the reputed international journals including reviews and research papers with more than 60 citation index and h index of 4. Dr. Tetsuo Soga received M. Sc. and Ph. D. degrees from Nagoya Institute of Technology and Nagoya University, respectively. He was a research associate from 1987 to 1992 and an associate professor from 1992 to 2005 at Nagoya Institute of Technology. Since 2005 he has been a professor at Nagoya Institute of Technology. Currently, he belongs to the Department of Electrical and Mechanical Engineering. He has published more than 400 peer-reviewed publications and has h-index of 45 (Scopus). He has supervised about 70 M.Sc. students and 18 Ph.D. students. He has the expertise in the field of nanostructured materials and solar cell application. Especially, he has an experience in AlGaAs/Si tandem solar cell, flexible dye-sensitized solar cell, carbon solar cell, polymer solar cell, bismuth-based perovskite solar cell, etc. He has conducted several

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national projects on solar cells as a principal investigator and participated in a lot of research as a research collaborator. His current research deals with lead-free perovskite materials for energy conversion device.

Chapter 6

Electrochemical Deposition Toward Thin Films Bidhan Pandit, Emad. S. Goda, and Shoyebmohamad F. Shaikh

Abstract Electrochemical deposition or Electrodeposition (ED) is a well-known method to acquire metallic coatings by the accomplishment of electric current on any conductive material immersed in a precursor solution of specific salt of associated metal to be deposited. This bottom-up fabrication method is useful and can be functional to a widespread potential application. Electrodeposition is gaining popularity in current years because of its ability in engineering three-, two-, and onedimensional (1D) nanostructures inclusive of nanotubes, nanowires, and nanorods. This chapter explores a wide discussion on the benefits of electrodeposition procedure in synthesizing various nanomaterials that display improved properties in comparison with materials prepared by traditional strategies. The properties of different nanostructured materials prepared by electrodeposition method are additionally represented. The significance of nanostructured coating by electrodeposition for diverse applications associated with the advancements of future technology is elaborately discussed.

Abbreviations CE CED

Counter electrode Cathodic electrodeposition

B. Pandit (B) Department of Materials Science and Engineering and Chemical Engineering, Universidad Carlos III de Madrid, Avenida de la Universidad 30, 28911 Leganés, Madrid, Spain e-mail: [email protected]; [email protected] Emad. S. Goda Organic Nanomaterials Lab, Department of Chemistry, Hannam University, Daejeon 34054, Republic of Korea Gas Analysis and Fire Safety Laboratory, Chemistry Division, National Institute of Standards, 136, Giza 12211, Egypt S. F. Shaikh Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 B. R. Sankapal et al. (eds.), Simple Chemical Methods for Thin Film Deposition, https://doi.org/10.1007/978-981-99-0961-2_6

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C-reactive protein Copper calcined layered double hydroxide Cu2 ZnSnS4 Direct current Dye-sensitized solar cell Electrodeposition Electrochemically reduced graphene oxide Fluorine-doped tin oxide Graphene Graphene hydrogel Light-emitting diode Normal hydrogen electrode Nickel oxide Nanoparticles Polyaniline Reference electrode Simulated body fluid Supercapacitor Saturated calomel electrode Ultraviolet Working electrode Zinc oxide

1 Introduction In association with the small particle size and high surface-to-volume ratio, nanomaterials exhibit excellent magnetic, optic, mechanical, and electronic properties [1–5] and materials may exhibit a couple of changes in typical characteristics when shifting from a microstructure toward nanostructure [6–10]. The increase in surface/volume rate results in dominant activities of surface atoms compared to inner ones [11–15]. This part impacts both the particle’s characteristics and its activities with various materials. Much higher surface area of the nanoparticle features in advancement in thermal, physio-chemical, and mechanical characteristics [16–18]. In nanostructures, all size-accompanied properties can be consolidated by tuning the size of the related material. For case, nanostructured ceramic and metals can have better mechanical characteristics compared to conventional bulk materials [19–21]. Moreover, nanostructured materials are accomplished for sintering process at less temperatures than conventional material, engaging the complete compactness at lesser temperatures [22, 23]. The semiconductor nanostructure materials in optoelectronic cells such as quantum dot and photodiode semiconductors are associated with size optimization of materials, mainly quantum-sized-related properties are achieved by the spatial constraint of delocalized electrons in confined grain sizes [24–28]. There is

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huge application of nanostructured materials in supercapacitors [29–34], and Li/Nabased ion batteries [35–40]. Nanostructure materials are also capable of exhibiting some diverse characteristics such as advancement of magnetic refrigerator, excellent magnetic properties with magnetoresistance effect, and scrutinize computer hard drive data by magnetic heads [41–47]. Nanostructured materials are also engaged for exhibiting excellent catalytic activities [48–53]. Nanomaterials are prepared by two essential particular methodologies: “bottomup” and “top-down” methodology [54–58]. In the bottom-up methodology, the material is made from the base (molecule by molecule or atom by atom). Colloidal dispersion is obtained by this methodology. The top-down methodology initiates through bulk material and advances toward the required state by its designing/abrasion which electron beam lithography. In nanotechnology, top-down and bottom-up methodologies are critical part. Main benefit of the two methodologies is foundation of small structure in pure state. Nevertheless, using bottom-up method produces homogeneous and defect-free nanostructures as the decrease in Gibbs free energy is associated with thermal stability. Nevertheless, in the top-down methodology, generally the material encounters surface defect, since the material is subjected to inside stress. Among the deposition methodologies, electrodeposition (ED) is remarkably versatile because of the important applications as shown in Fig. 1 [59–64]. This procedure is significant as a result of cost-effective and simple course of coating. Furthermore, the ED can make a wide extent of nanostructured materials. While ED continues to be for the decorative or protective coatings, testing new applications have been found in the electronics application, particularly hard disk recording heads, and replacement of aluminum and associated alloys by electrodeposited copper in large-scale integrated circuits. ED has become attractive as a result of its advantages over the other chemical and physical methods [65]. The semiconductor properties like stoichiometry, band gap, n- or p-type conductivity, doping, etc. can be controlled with a precision [66, 67]. Various materials (conducting polymers, superconductors, crystals, semiconductors, metals, etc.) in thin-film structure have been prepared by ED. Synthesis of thin film using ED has following excellent features [68–71]: (1) Alloys and compounds with different composition and structures can be deposited, which is not possible using conventional deposition procedures. (2) Because deposition can commonly be done at room temperature, semiconductor junctions can be developed without interdiffusion. (3) Possibility of depositing on complex shapes. (4) No need for toxic gaseous precursors (unlike gas-phase methods). (5) Deposition occurs more quickly. (6) The necessary equipment is inexpensive and does not require complex instrumentation or vacuum. Alessandro Volta in 1800 developed an electrochemical cell in order to process electrolytic deposition, but electroless deposition, in which there is no flow of external current, existed for centuries before [72]. The attractive gold coating is produced by replacement plating, which happens when a copper surface is exposed to gold ions

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Fig. 1 Electrochemically grown thin films for widespread applications

since the former is the more reactive metal. A procedure that is closely related to this one which was developed in the 1940s by Grace Riddell and Abner Brenner. In this, a chemical reducing agent provides the electrons necessary for the electrochemical reaction. Today, this procedure is essential for applying protective coatings to metal parts used in a range of sectors, from mining to aerospace. Research and development are ongoing in the field of electroless deposition. Electrolytic deposition really took off about 1840, when effective electrolytes for the deposition of silver and gold were created. Along with developing new techniques, a lot of effort has gone into studying the fundamentals of electrodeposition. In 1905, Tafel discovered a linear relationship between overpotential and the deposition current’s logarithm, and statistical thermodynamics was employed to explain it. Kaischew, Budevski, and colleagues carried out some incredibly elegant experiments on perfect silver crystals in Bulgaria in the 1960s that greatly improved our understanding of the role of defects, particularly screw dislocations, in the growth of electrodeposited metals; however, atomic-level experimental studies were restricted for a long time since it was not possible to use in electrolyte; the electron scattering-based experimental techniques that had been developed so effectively for analyzing surfaces in vacuum. Why does electrochemical deposition, also known as electrodeposition, still enthuse researchers and professionals in the field? It has been around for quite a while. There are several choices. First of all, electrodeposition is a remarkable phenomenon. Experts were always shocked by research into the technique at the atomic level. It is incredible that by just donating electrons to ions in a solution, one may cover one metal with a brilliant layer of another metal. Numerous useful applications for electrodeposition are constantly being developed. Even though this

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chapter provides a complete and brief summary, we still need to explore more in near future. Metallic coatings can be electroplated to produce metallic mirrors and surfaces that are resistant to corrosion. It offers a flexible option in industrial applications where big areas and unusually random-shaped surfaces need to be covered. Michael Faraday reported in his research on how to decompose materials using the voltaic column and electric current in the 1830s. The quantitative correlations between the quantity of materials accumulated at the electrodes and the quantity of electric charge (number of electrons) which flowed through an electrolytic cell were originally analyzed by Faraday. These are known as Faraday’s laws of Electrolysis as a whole [73]: (a) Total sum of chemical change produced by an electric current is proportional to the entire charge passing through the electrolyte. (b) Masses of the dissimilar substances liberated in the electrolysis are proportional to their chemical equivalent weights. This relationship between the mass “/\m” deposited over a unit area and the current density “j” flowing for a duration “t” may be stated mathematically as /\m = constant × jt = Ce jt

(1)

where Ce = electrochemical equivalent. The electrodeposits’ mass to their gram-equivalent weight ratio is a constant and equals to 1 F, in accordance with Faraday’s second law. The following equation describes how many electrodeposits grow in a given unit area: d/\m = Ce j

(2)

If “z” is the ionic charge, then j/zF gives the growth rate of the electrodeposit in gram molecules of the substance. The average deposit thickness (x) can be estimated by Faraday’s Laws of Electrolysis which is as follows: x = (MJt/ρzF)

(3)

where M specifies molar mass of metal, J signifies current density (current per unit area), t is the required time, ρ is the density of the material, z is the number of electrons, and F is Faraday constant. The thickness of plate depends on (i) the applied current, (ii) the time, (iii) the exposed area of sample, and (iv) the constant (M/ρAzF) which is reliant on the bath and metal. In electrochemistry, the Nernst condition is an engineered thermodynamical relationship that permits the calculation of the diminishing ability of a reaction (half or full cell) from the standard terminal potential, by and large temperature, the number of

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electrons related with the oxydo-reduction reaction, and activities (routinely approximated by groupings) of the compound species going through reduction and oxidation separately. It was named after Walther Nernst, a German physicist who proposed the equation [74]: E = E0 −

RT ln Q nF

(4)

where E is reduction potential, Eo is the standard potential, R is gas constant, T is temperature, n is ion charge (moles of electrons), F is Faraday constant equal to 96,485 C/mol, and Q is reaction quotient related to oxidation/reduction. Engineers use controlled electrolysis to operate the most effective metal covering from associate degree—anode (electrode containing the metal which will be used for the plating) to a cathode (to be plated) [75–77]. The associate anode and cathode are placed in an electrolyte-associated bath and introduced to a determined electrical charge (Fig. 2). Power causes negatively charged particles (anions) to move to the anode and positively charged particles (cations) to move to the cathode, coating or plating the associated metal. Electroplating takes a substrate material (oftentimes a lighter and additionally less costly material) and characterizes the substrate in an exceedingly additive coating of metal, the same as nickel or copper. It can be electroplated one thin layer of metal onto a different metal. Various manufacturers favor to coat metals, the same as copper and nickel, to expand strength and physical phenomenon. The materials used in electroplating are brass, cadmium, iron, copper, gold, titanium, zinc, including plastic, metals, and stainless steel. It’s crucial to remember that before being electroplated non-conductive substrates like plastic, wood, or glass must

Fig. 2 Electroplating process

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first be converted into conductive substrate. For this, conductive paint or spray-coated conducting layer can be applied to a non-conductive substrate. Increasing the conductivity, durability, and longevity of components are only a few advantages of electroplating. The manner in which engineers, producers, and artists employ these advantages varies. Engineers regularly use electroplating to improve the durability and tensile strength of different designs. The metallic coating on chemical compound elements can prevent corrosion. For instance, coating 3D printed things with copper or nickel may boost their tensile strength by 400% or more. Polymer parts can be made more resistant to external elements like chemical exposure and ultraviolet (UV) radiation by covering them with a metallic coating. Artists sometimes utilize electroplating to conserve naturally degrading materials, such as leaves, and transform them into more robust pieces of art. Electroplating is utilized in the medical industry to provide corrosion-resistant, appropriately sterilizable medical implants. In order to produce lighter components that are less expensive to ship and export, manufacturers frequently electroplate a substrate. Despite the fact that electroplating has many advantages, it is constrained by difficult and toxic nature of substances associated to the process. Researchers performing electroplating can experience the dangerous effects of hexavalent chromium if they don’t avoid potential risk. For them, a well-ventilated workspace is essential. The Occupational Safety and Health Administration of the United States Department of Labor has published several materials emphasizing the risks connected with electroplating. Due to the expertise required and the hazards involved, many engineers and designers choose to collaborate with a third-party electroplating manufacturer who specializes in this process. The electrolysis of water initiates electrodeposition coatings by enabling the power stream and initiating association [78]. The cathode causes oxidation of water, while the anode is responsible for reduction of water in the system. Cathodic case: 2H2 O(l) + 2e− ↔ H2 (g) + 2OH− (aq)

(5)

2H2 O(l) ↔ O2 (g) + 4H+ (aq) + 4e−

(6)

Anodic case:

2 Experimental Setup The fundamental electrochemical setup is as shown in Fig. 3. It consists of three major parts: (i) power supply, (ii) electrodes, and (iii) electrolyte.

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Fig. 3 Basic experimental set-up of electrodeposition

2.1 Power Supply Power supply is the major unit in electrochemical setup. Its purpose is to either supply potential (voltage) and measure current or supply current and measure voltage. There is no spontaneous electrochemical reaction in an electrolytic cell. Therefore, a driving force must be provided by an external source of current or voltage in order to conduct a reaction. In contrast to the oxidation reaction, which includes the dissolution of metal or the evolution of gas, the reduction reaction involves the deposition of metal or the development of gas and occurs on the cathode. A solid thin film called an electrodeposit develops on the cathode during the regulated electrolysis. For research and development purpose, sophisticated instrument like potentiostat/galvanostat can be used for design and development [79, 80]. The power supply can be galvanostatic or potentiostatic [81]. (a) Galvanostatic (constant current): This can be chosen if deposition potential of element is not known or in some cases purposefully required at constant current [82–84]. In order to find out the reduction potential, this method is also helpful but required multiple experiments to know the values. Here, graph of voltage versus time can be obtained for specific constant current value (Fig. 4a). (b) Potentiostatic (constant voltage): In order for electroplating of a specific material, when a reduction potential is known, then potentiostatic deposition is preferred. This constant potential (e.g., reduction potential) is given to the electrode to reduce the material on it and

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Fig. 4 Schematic graph of a Galvanostatic and b potentiostatic deposition w.r.t. time

deposition is performed at specific interval of time depending upon amount of thickness required. Here, you can get the plot of current versus time at a constant potential (Fig. 4b). If more than one material is aimed to deposit (e.g., alloy), usually the deposition potential is chosen for the element whose reduction potential is higher and some complexing agent is used for elements for the lower deposition potential in order to reduce its rate of deposition. Depending on stoichiometry requirement (e.g., means proportion of each element in alloy), complexing agent as well as concentration of metal salts can be chosen. (c) Voltage/current waveform (C-V curves) This is most common process used by electrochemist for research and development process [85–87]. Here both voltage and current are varying where current is taken as Y-axis and voltage is taken as X-axis in four-quadrant system and curve is analogous to hysteresis-like behavior. From this, it is possible to evaluate oxidation or reduction potential of a specific material. (d) Voltage/current pulse This process includes providing pulse of constant potential or constant current for short time and then cease for some interval for providing some time for adatoms arriving on the substrate surface to nucleate. In order to fill thin film with certain thickness having porous or nanoporous area with another material, this method suits well.

2.2 Electrode Electrodeposition process required minimum two electrodes, cathode and anode, but in order to have precise measurements, three-electrode system is the prime requisite as (a) Working electrode (WE) (either cathode or anode), (b) Counter electrode (CE) (either anode or cathode), and (c) Reference electrode (RE)

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To use as a cathode on which deposition is accomplished is called cathodic deposition, otherwise termed as anodic deposition. Cathode or anode can be active electrode (or simply substrate) which is discussed as follows.

2.2.1

Substrate

Electrode on which the deposition has occurred is referred to as substrate. It may be cathode or anode depending on type of deposition. Substrate in electrodeposition is playing very important role, it can affect the properties like optical or electronic along with providing the mechanical support to the electrodeposits. Criteria to choose the substrate: (1) Conductivity is the prime requirement of the substrate to have easy access of charges and to obtain ideal charge collection efficiency. (2) Substrate should be stable, and it should not react with electrolyte used or it should not corrode. (3) Good mechanical strength with lower cost. (4) Matching of thermal expansion coefficient between the substrate and deposit is needed to have requite annealing step of obtained deposit after deposition. (5) Smooth substrate is required but not necessarily in all the cases. (6) While using semiconductor as substrate, good ohmic contact is required to avoid contact resistance. 2.2.2

Counter Electrode (CE)

The counter electrode, also known as the auxiliary electrode, provides electrons in the electrochemical circuit including working electrode. With this third electrode added to the potentiostat, current can now pass through the analytical solution without entering or leaving the reference electrode. As a consequence, electrode polarization or iR drop-induced reference voltage changes are eliminated. The characteristics of the counter electrode should, in principle, have minimal to no impact on an electroanalytical measurement. It is recommended to keep the counter electrode’s surface area at a high level. The counter electrode should be made up of a comparable electrochemically inert material, such as graphite or platinum [88–90]. For circular object on which deposition is expected for industrial use, circular type of pot usually made of graphite is used to keep active electrode substrate exposed entirely.

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Reference Electrode (RE)

In order to detect the relative changes in the interfacial potential, the reference electrode’s function is to serve as a stable reference to the redox process taking place at the working electrode. Charges may readily pass through the R.E./electrolyte interface’s surface, rendering it non-polarizable and limiting the ability of interfacial drops to vary in response to applied potential. Most applied reference electrodes are Ag/AgCl, the saturated calomel electrode (SCE), and the normal hydrogen electrode (NHE). The most used electrode for aqueous electroanalytical chemistry is without a doubt the saturated calomel electrode (SCE). The highlights of SCE are (i) easy to prepare, (ii) easy to maintain, (iii) 0.2444 V at 25 °C, (iv) dependent on temperature, and (v) toxic. Hg2 Cl2 (s) + 2e− ⇔ 2Hg(I) + 2Cl− (aq)

(7)

It has a low-temperature coefficient and may be constructed compactly. However, neither is helpful when metal ions could precipitate with chloride or when chloride ion leakage might affect the analyte. Both electrodes may also encounter issues when employed with non-aqueous analyte solutions, particularly when it’s critical to prevent water contamination. The Ag/AgNO3 (CH3 CN) electrode would be an excellent option under these circumstances. Utilizing a pseudo-reference electrode is an additional option. There are available reference electrodes that are the simplest and most portable, but they lack stability and mechanism. In both aqueous and nonaqueous situations, a cheap silver wire can act as a pseudo-reference electrode. To calibrate the electrodes, a little quantity of a well-defined reversible redox species, such as ferrocene (for non-aqueous solutions) or ferricyanide solution (for aqueous solutions), should be added (Fig. 5).

Fig. 5 Reference electrodes: a silver-silver chloride electrode and b saturated-calomel electrode. Reproduced with permission [91]

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Blocking of porous plug is a major issue due to electrolytic ion condensation causing inactive, unsteady electrical response [91]. This can be overcome by the use of a free-flowing capillary instead of routine porous plug. So, potential for the solution saturated with KCl: AgCl(s) + e− ↔ Ag(s) + Cl− Eo = −0.197 V

(8)

Hg (mercury) electrode in association with saturated KCl is termed as a saturated calomel electrode, abbreviated as S.C.E. The advantage in using saturated KCl is that [Cl− ] does not change if some liquid evaporates [92]. ECalomel (Saturated with KCl) = 0.241 V

(9)

A positive voltage associates with spontaneous oxidation–reduction reaction with oxidation at the anode and reduction at the cathode. On the other hand, if the potential values of the half-cells are known, it is attainable to guess whether the redox reaction is spontaneous or not. A negative voltage shows that the contrary reaction is spontaneous (for instance, reduction at the anode and oxidation at the cathode). The following series of metals records the metals in diminishing order of their overall ease of oxidation (Fig. 6) [93]. For example, copper oxidation full reaction is Fig. 6 Series of metals associated with oxidation–reduction [93]

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Cu(s) + 2H+ (aq) → Cu2+ (aq) + H2 (g)

(10)

Actually, reaction kinetics involved two half-cell reactions: Oxidation : Cu(s) → Cu2+ (aq) + 2e−

(11)

Reduction : 2H+ (aq) + 2e− → H2 (g)

(12)

Every half-reaction is associated with standard reduction potential. The reduction half-reaction refers to the hydrogen ion reduction as shown by the following equation: 2H+ (aq) + 2e− → H2 (g) E0red = 0 V

(13)

The standard reduction potential for copper yield copper metal is Cu2+ (aq) + 2e− → Cu(s) E0red = +0.34 V

(14)

The copper metal takes part in oxidation reaction to yield copper ion. So, E0 = E0red (reduction process) − E0red (oxidation process)

(15)

The standard reduction potential for the above process is E0 = 0 V − 0.34 = −0.34 V

(16)

This redox reaction is not spontaneous as standard reduction potential is negative. Therefore, the origin of the abovementioned activity series depends on the relative reduction potential. A table is given below with standard reduction potentials with different elements (Table 1). If the reduction potential is strong, it’s hard to oxidize the element, but in case of weaker potential, it’s very easier indeed. Table 1 Standard reduction potential series [94–98]

Standard reduction potential (E 0 red ) in volts

Reduction half-reaction

0.80

Ag+ (aq) + e− → Ag(s)

0.34

Cu2+ (aq) + 2e− → Cu(s)

0

2H+ (aq) + 2e− → H2 (g)

−0.28

Ni2+ (aq) + 2e− → Ni(s)

−0.76

Zn2+ (aq) + 2e− → Zn(s)

−3.05

Li+ (aq) + e− → Li(s)

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2.3 Electrolyte Electrolyte is source of ions to be deposited on the substrate. It should be conductive in nature. Electrolyte is associated with soluble metal salt which can provide metal ions. Facile soluble salts can serve as solute, and hence selection of proper solvent is necessary. Depending upon deposition potential, solvent can be chosen as aqueous or non-aqueous, as aqueous solvent has limited range in terms of potential as well as temperature [99]. Contrarily, cost issue has to be taken into consideration while selecting solvent, as it should be at lower cost. In order to control the rate of reaction, to change conductivity of solvent, and to change the properties of electrodeposits, some additional precursors are incorporated into electrolyte like complexing agent, additives, etc. An ideal additive should not be incorporated in the film but should lead toward the improvement of its adhesion, surface homogeneity, brightness, etc. Ionization allows salts, acids, and bases to conduct electricity in their aqueous solutions. These substances are known as electrolytes. The ions in the electrolyte provide conduction, which causes current or free electrons to move across the wires. When a voltage is applied across the cell and under the influence of the potential difference, the positively charged ions migrate to the cathode and the negatively charged ions migrate to the anode. The ions required for electrodeposition are provided by the electrolyte or bath. It must have the right metal salts and be electrically conductive. It might be molten, solid, or liquid. On rare occasions. Aqueous bath: Water is employed in practically all electroplating techniques since it is the best solvent that nature offers. It works well with a variety of salts, complexing agents, and other chemicals. Despite the fact that aqueous solutions always include H+ and OH− ions, which complicate the electrodeposition process by causing the production of H2 and/or O2 at the electrode, water is typically a non-reactive solvent. Sometimes, this might lead to electrodeposits that are weak, asymmetrical, and less sticky. Non-aqueous bath: Due to their enhanced versatility in terms of solute, dopant, complexant, temperature range, and working electrode potentials, non-aqueous solvents have lately received a lot of attention. Preparation of the electrolyte bath 1. Selection of the solvent [100]: (a) It should be electrically conductive and should contain appropriate metal salt ions. (b) ED solvents should be stable in deposition potential range. (c) The solvents should give more noteworthy adaptability to wide temperature range. (d) High vapor pressure solvents are used since the electrolyte’s concentration is constant.

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(e) Low dielectric constant corresponds to the formation of ions pairs, results in poor conductivities and ionization in the solution. The dielectric constant must be greater than 10. (f) Since lower viscosity gives greater conductivity and dispersion, it is frequently preferred. Mass transport is another barrier to many irreversible electrochemical processes. 2. Selection of the supporting electrolyte [101]: The important requirements are (a) increasing the conductivity of the electrolyte, (b) reduction of the electrode double layer thickness, and (c) removal of the effect of migration in mass transport. When selecting the supporting electrolyte, two factors need to be taken into account: 1. Solubility in the solvent and high dissociation constant to provide sufficient conductivity. 2. Oxidation and reduction via electrochemistry at higher anodic or cathodic potentials. Ammonium salts and lithium perchlorate have been employed in non-aqueous media, although KCl and HCl are often used in aqueous media.

2.4 Additives Typically, the electrodeposition process is connected to nucleation and formation. The nucleation rate increases with decreasing crystal grain size. The nucleation and growth process have an influence on the properties, microstructure, and surface quality of the deposits. Additives like saccharin and thiourea are mostly added to the deposition electrolytes in this process because of their high adsorption ability and consequent effect on the interfacial characteristics. This integrates them into metal deposits easily. Numerous organic and inorganic additive types have been exhaustively investigated in order to provide long-lasting, homogeneous, and compact metal coatings that resist corrosion. These additives frequently lower electrodeposition currents or, alternatively, increase the potential required for a certain current when the voltage is constant. The significance of additives in metal electrodeposition as well as the crucial experimental safety measures must thus be clear to researchers. Additives (surfactant, brightening agent, complexant, etc.) are typically added to the plating bath in order to give a smoother and brighter deposit, a predictable reaction rate, higher adherence, and improved texture. The main purpose of the additive is to control electrodeposition rate or to modify deposit structure and morphology.

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The adsorbed additives affect the rate of electrodeposition by altering the Helmholtz layer potential, functioning as a bridge to mediate the electron transfer process between the electrode and the discharge species, and preparing complexes with the ionic species that will be plated. Ion pairing: Ion pairing between an additive species and an ionic species affects the kinetics of an electrode reaction. By creating ion coupling between the negative ions and the quaternary salts, this goal is often achieved [102]. Interfacial Tension and filming of the electrode: Surfactants are known to change the interfacial tension by eliminating the species like hydrogen that may hinder the electrodeposition process. Furthermore, it controls the coating on the electrode surface by affecting the distribution and rate of electrodeposit reduction [103]. Complex formation: Complexing compounds are frequently used to change the deposition potential. They have the power to affect the ionic activity of the species that will be electrodeposited when present in large amounts. Many complexing chemicals, including CN− , Cl− , I− , and SCN− , have the ability to cause adsorption at low concentrations, which is known to speed up chemical reactions [104]. Briefly said, improvements in texture, uniform distribution, and greater brightness are the desired results of additions to the properties of electrodeposits, whereas additive association to the electrodeposits and growing dendrites and whiskers are not undesirable outcomes.

3 Classification When a positively charged polymer is drawn to a negatively charged area during cathodic electrocoating, the process is known as electrophoresis. In the anodic case, similar things happen, but the component and polymer are positively and negatively charged. The electrodeposition coating process commences on the component after the oxidation or reduction of the water there. Cathodic electrocoat produces hydroxide locally to neutralize the positively charged polymer’s salting acid prior to coating the component with the polymer. When a positively charged component is drawn to a negatively charged area during cathodic electrocoating, the process is known as electrophoresis. In the anodic case, similar things happen in reverse. The electrodeposition coating process commences on the component after the oxidation or reduction of the water. Cathodic electrocoat produces hydroxide locally to neutralize the positive charge prior to coating the component. It is important to note that during this electrodeposition process, the coating is neither reduced nor oxidized; instead, a change in the coating’s solubility makes it possible for the coating to be deposited onto the component. Electrolytic deposition associates with the following reactions to create alkaline nature at an electrode surface:

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2H2 O + 2e− ⇔ H2 + 2OH−

(17)

− − − NO− 3 + H2 O + 2e ⇔ NO2 + 2OH

(18)

O2 + 2H2 O + 4e− ⇔ 4OH−

(19)

Consequently, the negative charge of associated colloidal ions is created nearby the electrode surface: M − OH + OH− ⇔ M − O− + H2 O

(20)

Rahimi et al. prepared Co(OH)2 -reduced graphene oxide composite on the stainless steel plate using cathodic electrodeposition (CED) method at a constant current in two-electrode system, and then applied as a electroactive material [105]. Liang et al. prepared α-Fe2 O3 film using same cathodic electrodeposition which showed photocurrent density around 257.08 μA/cm2 at 1.23 V with respect to reversible hydrogen electrode [106]. Following the declaration of the material coatings, water flows from the component (a process known as electroendosmosis), protecting the part and completing articulation. The concept is used to restrict the film thickness of the final coating. Because oxidation occurs at the portion in anodic deposition, several metals may be oxidized in this cycle [107, 108]. The electrolysis of water is widely recognized as the principal electrochemical reaction in anodic electrodeposition of water-soluble materials [109, 110]. H2 O → 2H+ + 1/2O2 + 2e−

(21)

The anodic potential should be more positive than the negative electrode for oxygen evolution. In any case, most metals become thermodynamically oxidizing before this happens. Thus, there is a relation between oxygen evolution and metal anodic dissolution. Me0 → Me + n + ne−

(22)

Jia et al. developed 2D conducting metal oxide framework (MOF) on nickel foam by using anodic electrodeposition as self-standing and binder-less supercapacitor (SC) electrode [111]. Similarly, Girolamo et al. prepared the perovskite solar cells associating with nanostructured NiO using potentiostatic anodic electrodeposition of NiOOH [112]. Cathodic deposition is commonly utilized in automotive applications, when corrosion resistance is crucial. If aluminum and steel are utilized in a comparative cathodic bath, special pre-treatment may be required. Cathodic or anodic electrodeposition procedures can be used to deposit various materials. In any case, the value of anodic

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electrodeposition is limited in terms of prospective materials to be maintained by this approach and substrates used for testing. It has several advantages in current applications [113].

4 Principle: Thermodynamic and Kinetics of Electrodeposition ED suggests a film advancement technique that associates with metallic covering over a substrate with the aid of reduction of associated metal ions from a specific precursor, also referred to as electroplating. The arrangement contains negative and positive ions, and works as ionic conductor. For serving as an electrolyte, the necessary metal in a chemical species is dissolved in solvent (usually dissolved in water) to produce a molten salt. Various organic and ionic liquids are now employed for specific electrocoating procedures. To start the process, the working electrode is immersed inside the vessel (cell) which also contains counter electrode. The second terminal is now connected with a battery or another power supply for the current flow inside the circuit. The cathode connected to negative and anode associated to positive terminal of the battery, resulting in the reduction of metal ions toward metal atoms as a deposit. This sort of circuit strategy drives electrons into the prepared bath from the electric supply to the cathode. Inside the precursor, the current is driven by positive ions from anode to the cathode which is negatively charged enabling the metal ions inside the precursor to diffuse toward the active substrate where more electrons are present at or near the cathode. As a result, the metal ions are detached from precursor to deposit on the substrate. Layer thickness of the electrodeposits is subject to the plating duration. The longer time association results in thicker deposits on the surface. Electrodeposition has lately acquired popularity and is commonly used by researchers across a wide range of subjects. A conductive plate is dipped in an electrolyte associating ions to be deposited and a voltage is applied across this solid/electrolyte interface, responsible for a charge donation resulting in film formation. The applied voltage can be simply and accurately changed down to the mV and during time interval as short as ns. This feature enables a great level of tuning of the material production process, microstructure, and characteristics. Today’s difficulty in constructing electrodeposition procedure is not the growth of a predetermined material, but rather finding a balance between the optimal parameters employed to grow the material and the process’s practicality. Other often claimed benefits of electrodeposition are high raw material utilization, low energy utilization, minimal material waste, minimal capital investment, and ease of application. To control all the parameters of the process, it’s very important to understand the proper and deep mechanism of it.

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5 Mechanism of Electrochemical Deposition The electrodeposition bath consists of a counter electrode, an electrode or substrate on which the desired deposition will occur, and an electrolyte containing metal ions (for example, CuSO4 solution for the deposition of Cu). The cations and anions travel in the directions of the cathode and anode, respectively, when a current is passed through the electrolyte. After a charge transfer process, they might deposit on the electrodes. If copper is to be electrodeposited, the bath could contain some Cu salt, for example, copper sulfate. The Cu2+ ions accumulate on the substrate surface after acquiring the charges. The following are steps leading to ED: To make a clear understanding, we are discussing an example case study of cathodic electrodeposition involving MZ+ ions. On the utilization of electric field, M2+ would move to the cathode and the electrodeposition cycle can be presented as M2+ + 2e → M

(23)

If electrolyte contains more than one species then M+ + N+ + 2e → MN

(24)

Thus, a compound or an alloy of a multi-component system can be deposited using this method. The ions can be 1. In a hydrated form, depositing with the overall cell reaction: Mz + .nH2 O + ze → M + nH2 O

(25)

2. Complexed form electrodepositing with the overall cell reaction: Mz + .Az−x + ze → M + xA− x

(26)

There are two important processes during ED which are given below [114]: 1. Processes that occur near the electrode but within the electrolyte • Ions are typically encircled by a hydration sheath. • They move together as one element and show up close to the terminal surface where the ion and ligand framework either admits electrons from the cathode or offers electrons to the anode. • This ion release response happens in the electrolyte somewhere in the range of 10–1000 Å from the electrode. Then all the processes occur in the electrolyte bulk, and can be expressed by Nernst–Planck equation:

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( j = zF

dc Dc d∅ +D + cv RT d x dx

) (27)

where F is Faraday’s constant, v is velocity of electrolyte, R is gas constant, T is the temperature, D is diffusion coefficient, dφ/dx is potential gradient, and dc/dx is concentration gradient. 2. Processes that occur on the electrode surface • The associated ions appear near the electrode where in a step-by-step manner, they lead to the growth of new solid stage or the advancement of an electrodeposited film. • The ions together will generally process either an organized clear phase or an indistinct phase. These occur in the following successive steps: (i) transport of ions, (ii) discharge process, and (iii) adatom incorporation on the substrate surface followed by nucleation and growth (Fig. 7). The whole process for the development can be presented in the following steps: 1. Ion transport in the electrolyte toward the point of interaction. 2. Release of charge at the electrode, starting association of adatoms. 3. Nucleation and growth. Film formation further takes place by following step-wise development (Fig. 8): i. Growth helped by surface diffusion. ii. Growth helped by arrangement of clusters and nucleation. iii. Arrangement of monolayer and lastly the development of electrodeposit. Surface defects are also important for this step.

Fig. 7 Electrodeposition mechanism

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Fig. 8 Schematic representation of steps involved in electrodeposition

6 Influencing Factors Various factors can influence the electrodeposits which can be summarized in Fig. 9.

Fig. 9 Factors affecting during electrodeposition

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6.1 Current Density The growth rate slows as the crystallinity increases when the ion discharge occurs slowly at low current densities (or overpotential), producing densely packed forms. With an increase in current density, the pace of nuclei formation will quicken, resulting in a deposit of finer grains. Higher current densities promote the growth perpendicular to the substrate surface because there are more ions close to the cathode and the rate of ion discharge accelerates in comparison to the rate of ion supply. Under these conditions, spongy dendritic growth is often observed. Second, hydrogen evolution proceeds more quickly at very high current densities, inhibiting crystallization and raising the likelihood of spongy, porous deposits. This may also promote the development of hydrous oxides or basic salts since the local pH is being elevated.

6.2 Nature of Ions (Anions/Cations) in Solution Control of the cation/anion of the bath is required to run a plating bath as effectively as feasible and to preserve the right physical characteristics of the deposit. In addition, a pH that is too low may lead to the cathode accumulating hydroxide ions, which may subsequently precipitate basic salts that might be incorporated into electrodeposition and affect the deposit’s characteristics. It is actually probable that hydrogen gas will emerge at the cathode in every deposit from an aqueous bath since H+ ions are present in all aqueous solutions. As a result, the effectiveness of metal deposition is reduced. The bath permits the use of a high current density to develop a good deposit with a relatively high efficiency at low pH since both efficiency and hydrogen discharge potential are partly dependent on hydrogen ion concentration. On the other hand, it may be assumed that bringing up the pH of the bath caused a reduction in the number of ions in the coating. Additionally, lowering pH is suggested to relax internal stress.

6.3 Bath Composition The metal complex that has to be plated is always present in the plating bath, an aqueous solution. Higher metal component concentrations in the bath solution are frequently beneficial. High metal-bearing baths can be employed with high current densities. Under some circumstances, an increase in metal concentration can lead to a decrease in cathode polarization and an increase in crystallite size. The kind of ions affects how the bath’s composition affects ion integration. It is well known that if there are more ions in the solution, there will inevitably be more ions in the deposit. The kind of ions is still a crucial consideration, though. The benefits of conductive and non-conductive ions are similar. The attraction of conducting ions to the cathode (such as graphite) and their role as depositing sites led to dendritic development.

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Even though conducting ions were readily pulled to the cathode, selective deposition on the conducting areas increased the surface roughness. Less-conductive ions, on the other hand, lead to deposits with reduced porosity and smoother surfaces. The type of embedded ions can alter the mechanical characteristics. During composite electroplating, smaller sizes are more readily stirred, increasing the ion concentration in the deposit.

6.4 Temperature The increase in bath conductivity is due to the rise in bath temperature, which accelerates the rate of diffusion and raises ionic mobility. The crystals develop more quickly as the temperature rises, preferring the coarse deposits. The decrease in polarization causes increase in the crystal size. Temperature-dependent current densities speed up the nucleation process and allow for the formation of smooth, fine-grained deposits. An increase in bath temperature promotes gas evolution and the precipitation of basic salts by lowering the hydrogen overvoltage. The ideal bath temperature is difficult to optimize because of the conflicting effects, however actual tests can be useful.

6.5 Concentration of Solution The metal complex that has to be plated is always present in the plating bath, an aqueous solution. Higher metal ion concentrations in the bath solution are frequently beneficial. Under some circumstances, an increase in metal concentration can lead to a decrease in cathode polarization and an increase in crystallite size. Up to a certain point, increasing the ion concentration in the bath frequently results in an increase in the weight percentage of the ions in the deposit. One may relate that position to the saturation point. In locations with low ion concentration, the number of ions increases quickly, but only slightly in areas with high ion concentration. Because there are fewer collisions between the ion and the cathode in the high concentration zone, there is a slight increase or decrease in the quantity of ions in the developing deposit.

6.6 Current Waveform of Power Supply These advantages make electrodeposition a practical choice for the investigation and advancement of PV solar cells. The two main electrodeposition techniques are direct current (DC) plating and pulse plating, which also has pulse and pulse-reverse features. Because they offer more process control variables, these features make electrodeposition a suitable approach for the deposition of semiconductors.

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A deposit with finer grain size is produced by DC electrodeposition when the period between pulses during the process of a coating by pulsed electrodeposition controls the deposit’s development trend and restricts the expansion of the coating grains. For pulsed electrodeposition, pulsed power currents can have a variety of waveforms, such as rectangular, sinusoidal half-wave, and saw tooth. Metal coatings are typically produced using a double pulse and rectangular pulses. The pulse current density, the pulse on time, and the pulse off time are three additional parameters for the pulsed electrodeposition preparation procedure. The features of the generated deposits are adjusted when the three key parameters of the pulsed electrodeposition process are adjusted. Double-pulse electrodeposition is the introduction of a reverse pulse current after the output of a forward pulse current. The metal ions in the plating solution are deposited onto the base metal during the forward pulse operation of pulsed electrodeposition, and the metal that has already been plated on the base metal is dissolved during the reverse pulse operation. This makes it possible to decrease the thickness of the plating and to rectify its uneven thickness distribution. As a result, in pulsed electrodeposition, the forward pulse’s working duration is longer than the reverse pulse’s working period.

6.7 Presence of Impurities It has been shown that almost all metal electrodepositions frequently produce deposits that are smooth, fine-grained, and nanocrystalline structure when specific compounds are added in modest amounts. They are referred to as additional agents. To generate smoother, brighter deposits, predictable reaction times, increased adhesion, and superior texture, brightening agents, surfactants, complexants, and other addition agents are frequently added to the bath. Adsorbed additives interact with the ionic species that will be plated to affect the rate of deposition, the Helmholtz layer potential, and serve as a bridge for the process of electron transfer between the electrode and discharging species. Also, impurities present inside solute plays important role. If reduction potential of impurity is lesser than the deposits, it will be deposited first rather than actual deposits, and hence hinder the properties of deposits and change complete scenario. Hence, care must be taken about content of impurity and its reduction potentials.

6.8 Nature of Substrate Surface (Physical/Chemical) For electrodeposition to be successful, substrates are essential. These factors affect the morphological, electrical, and optical characteristics of the growing layer while mechanically advancing the electrodeposit. The following points [115] must be considered while choosing an appropriate substrate: (a) it must have a high conductivity since it is one of the electrodes used in electrodeposition, and the efficiency

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of carrier collection is aided by a good conductivity; (b) it must match the thermal expansion coefficient of the electrodeposit and a mismatch might cause the film to peel or crack; (c) it must have high mechanical integrity; (d) it must keep its stability while submerged in an electrolyte bath; and (e) it has to be smooth. Voids, pores, and other abnormalities have an impact on the current distribution. Metals have been used as substrates a lot due to their excellent conductivity, simplicity of availability, low cost, and generally simple handling.

6.9 PH and Pourbaix Diagram A Pourbaix diagram, also referred to as a potential/pH diagram, is a graph that is used in electrochemistry and solution chemistry more generally to show potential thermodynamically stable phases of an aqueous electrochemical system, i.e., those in chemical equilibrium. The principal chemical species (solid phases or aqueous ions in solution) phase boundaries are depicted as lines with a 50%/50% ratio. Pourbaix diagram may be considered as a typical phase diagram. It doesn’t consider reaction rate or kinetic effects, much like phase diagrams do. Potential, pH as well as other elements like temperature, pressure, and concentration have an impact on equilibrium concentrations. Pourbaix diagrams are commonly produced at molar concentrations of 10−6 , ambient temperature, and atmospheric pressure; changing any one of these variables will provide a different outcome [116].

7 Controlled Morphology Using Electrodeposition Electrodeposition provides an easy synthesis route for wide range of materials including metal oxide and chalcogenides (sulfides, selenides, and tellurides) with different morphologies intending toward various applications [117–120]. Not only that, electrodeposition presents a wide variety of morphology of materials. Thin morphology of ZnO was electrodeposited in 50 mM Zn(NO3 )2 as reported by Izaki and Omi [121]. To control the morphology, a couple of amine and inorganic salts like KCl, NH4 F, CH3 COONH4 , and ethylenediamine were added to the zinc nitrate baths on indium-doped tin oxide (ITO) substrate at a potential of −1.10 V versus SCE [122]. ZnO films electrodeposited from 50 mM Zn(NO3 )2 yield single-glass-like hexagonal particles (Fig. 10). Whenever a small amount of NH4 F was added, the hexagonal particles turned out to be more thin and greater toward < 0001 > direction but more NH4 F addition corresponds to ZnO nanorods with needle-like branches (length 10 μm with sharp tip). ZnO rods (widths 0.2–1 μm) with the rhombohedral morphology are procured at significantly higher NH4 F. A further increase of the NH4 F causes the reduction of length-to-width ratio with same rhombohedral morphology. When ethylenediamine (EDA) was used, homogeneous taper-like

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Fig. 10 Morphologies of electrodeposited ZnO nanostructures using a 0.05 M Zn(NO3 )2 and b, c 0.05 M Zn(NO3 )2 + 0.06 M KCl And using 0.05 M Zn(NO3 )2 and NH4 F with concentrations c 9 mM, d 0.02 M, and e 0.2 M. g–i SEM and j\ TEM of the ZnO obtained 0.05 M Zn(NO3 )2 and 0.013 M EDA. Reproduced with permission [122]

nanostructures with area of a couple of square centimeters are obtained (height 2 μm and base diameter 100–500 nm). Additionally, the control test, from solution containing 0.05 M Zn(NO3 )2 , 0.06 M KCl, and 10 mM EDA, yielded vertical changed hexagonal ZnO rods (length 2 μm, width 100–300 nm). There are similar types of studies available in literature describing the morphology variation by small changes in parameters in ED [123, 124]. Yoshida et al. [125] described a complete review on the hybrid structures obtained by ED. He demonstrated a sequence of SEM pictures demonstrating the modification of film structure. The pure ZnO film electrodeposited from a dye-free bath was composed of hexagonal columnar ZnO particles on average. The particles’ exterior layer was smooth, and each molecule had all the properties of being monolithic and compact. The ZnO/eosinY composite film acquired with modest eosinY showed cauliflower-like nanosurface. Its cross section clearly demonstrated a spherical top surface and an internal nanostructure oriented toward film formation. Because the film included a lot of eosinY, obtaining clear SEM images at high magnification is difficult. This problem did not exist in the dye-desorbed experiments, and clear images were produced. Despite the fact that the overall shape was similar to that of the previous film, the development of tiny holes inside the spherical deposit is visible. The cross section revealed an outstanding linked “nanowire” ZnO structure as well as near to vertical pores formed inside the grain. Epitaxial Growth Lattice matching between the deposits with that of the substrate favors the epitaxial growth. Nanopillers of ZnO with hexagonal structure have been successfully grown toward achieving epitaxy on the single crystal gold substrate oriented in different directions, viz., (100), (110), and(100) as shown in Fig. 11 [126]. Wurtzite structure

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Fig. 11 Morphology of 500 nm thick ZnO on a Au(111), b Au(110), and c Au(100). The hexagonal pillars have an average size of about 85 nm aligned with each other on the surface of the film on Au(111). Reproduced with permission [126]

of zinc oxide possesses P63mc space group with the lattice parameters with similar values of a and b as 0.3250 nm along with different values of lattice parameter (c) as 0.5207 nm. Interestingly, it ends up with lattice mismatch of −20.3% quantified through the relation (aZnO-aAu)/aAu since gold metal possesses cubic close-packed crystal structure having space group Fm3m. An average size of aligned hexagonal pillers of ZnO is about 85 nm. Well alignment with each other has been seen for ZnO grown on Au(111). Two types of ZnO hexagonal columns with a right angle rotation have been detected on Au(110) and Au(100).

8 Review of Electrodeposited Nanostructures for Applications Table 2 summarizes all the aspects of synthesized materials through ED. Nanostructures developed using electrochemical route have been employed in widespread application. Following discussion highlights some applications. Recently, new materials explored to join in hardware assortment to permit new functionalities in MEMS, lab-on-a-chip, and microfluidic devices. Polymer and biomaterial region units are electrodeposited for medication applications and metal oxide and compound semiconductor region units grew electrochemically for electronic or optoelectronic applications. Store in new electrolytic media, for example, ionic liquids or basic fluids is by and gigantic without equivocalness needed with some activity. Store expects a significant half inside the headway of rectifiable energy change developments, each at the helpful and on an overall scale.

Metal selenide

Metal sulfide

Metal oxide

Material

Sheets Nanometer-sized grains Cauliflower structure

1.8 M Ni(NO3 )2 + 3.0 M KOH Co(NO3 )2 ·6H2 O + 1.0 M KOH 150 mM SnCl2 + 0.1 M NaOH 0.05 M CuSO4 + 1.0 M Na2 SO4 8.5 g/L InCl3 + 25 g/L Na3 C6 H5 O7 ‚2H2 O + 1.0 M Na2 SO3 50 mM FeCl3 .6H2 O + 150 mM Na2 S2 O3 .5H2 O 5 mM MnSO4 + 0.75 M thiourea 5 mM CoCl2 .6H2 O + 0.5 M thiourea 0.025 mol SnCl2 ·2H2 O + 0.025 mol Na2 S2 O3 ·5H2 O + 0.01–0.06 mol C6 H5 Na3 O7 ·2H2 O 0.44 g Ni(NO3 )2 ·6H2 O + 2.28 g CS(NH2 )2 10 mM CoCl2 ·6H2 O + 20 mM SeO2 0.03 M FeCl2 ·4H2 O + 0.015 M SeO2 + 0.1 M Na2 SO4

−0.70 V verus SCE

−1.0 V verus Ag/AgCl

current 19 mA/cm2 for 10 min

1.05 V/SCE

−0.6 to −1.3 V verus Ag/AgCl

−1.65 V verus SCE

−1.2 to 0.2 V vs Ag/AgCl

−1.2 and 0.3 V at a scan rate of 10 mV/s

−11.1 V verus Ag/AgCl

−0.9 V and 0.7 V verus Ag/AgCl

0 to − 0.8 V verus Ag/AgCl

−0.7 to −1.3 V Ag/AgCl

Ni(OH)2

α-Co(OH)2

SnO2

Copper oxide

In2 O3

FeS2

MnS@rGO

CoS

SnS

Ni3 S4

CoSe2

FeSe

-

Nanostructure

Nanoparticles

Nanorods

Granular morphology

Sheet like

Cauliflower like

Nanorods

(continued)

[140]

[139]

[138]

[137]

[136]

[135]

[134]

[133]

[132]

[131]

[130]

[129]

[128]

0.1–0.5 M Ni(NO3 )2 + 1.0 M KOH Porous

0.5 V verus SCE

Nickel oxide (NiO) Hexagonal nanostructure

[127]

Porous

RuCl3 :xH2 O + 0.5 M H2 SO4

−0.45 V/SCE

References

Morphology

Bath composition

Deposition parameter

Ruthenium oxide

Table 2 Metal oxide and chalcogenides synthesized by electrodeposition

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Metal telluride

Material

Nanorods Fern shaped

0.1 M NaClO4 + 50 mM K2 MoO4 0.05 M Bi(NO3 )3 ·5H2 O + 0.05 M Na2 TeO3

− 0.8 V Vs Ag/AgCl

0.170 V/SCE

MoTe2

Bi2 Te3

Spherically shaped grains

0.01 M CdSO4 + 0.01 M SeO2 + TEA

0 to − 750 mV vs SCE

Morphology

Bath composition

Deposition parameter

CdSe

Table 2 (continued)

[143]

[142]

[141]

References

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8.1 Solar Cells Optoelectronic materials with proper crystal engineering is gaining much interest because of their excellent physicochemical properties and real-world applications [144–147]. Solar cells are the important component considering vast energy-related requirements in today’s society. Electrodeposition of different nanostructures is the most generally contemplated as a result of their promising properties for solar cell applications [148]. SnO2 has been successfully sensitized by erythrosine dye by ED [149] and used in a dye-sensitized solar cell (DSSC). It exhibits short-circuit photocurrent of 760 μA/cm2 with photovoltage of 0.21 V, and overall conversion efficiency of 0.06% under direct sunlight. Cobalt-based nanomaterials are well studied for various applications due to their unique properties [150–153]. The hierarchical cobalt sulfide (Co-S) nanobowl array for the counter electrode with a light scattering effect was constructed using a voltage reversal electrodeposition approach [154]. The dye-sensitized solar cell (DSSC) assembled with Co-S nanobowl array exhibits remarkable photovoltaic performance with a champion power conversion efficiency as high as 7.78%, in contrast to DSSC built with Pt (6.51%) and Co-S (7.14%) counter electrodes. Hierarchical zinc oxide (ZnO) nanorod arrays on stainless steel meshes were used to make flexible DSSC photoanodes using a two-step electrodeposition approach [155]. After adjusting the electrodeposition parameters, the DSSC produced the best photovoltaic results, with a power conversion efficiency of 1.81% as shown in Fig. 12a–h. The AgCuO2 nanocrystalline films on conductive substrates were produced by electrochemical deposition for perovskite solar cell applications (Fig. 12i–p) [156]. The AgCuO2 films that were electrodeposited possessed morphologies that were associated with smooth, hole-free, good conductivity, and high transmittance. Finally, the perovskite solar cells based on AgCuO2 attain a power conversion efficiency of 10.24%. Jiang et al. generated nickel cobalt selenide (Nix Coy Se) films in situ on fluorinedoped tin oxide (FTO) glasses using the potential reversal electrodeposition method [157]. It has been demonstrated that the DSSC produced by Nix Coy Se CE has a power conversion efficiency of over 7.40%, which is higher than that of a platinum (Pt)-based device, which has a PCE of 6.32%. The polystyrene array is also used to create the Nix Coy Se array CE template. The PCE of the DSSC with Nix Coy Se array CE reaches a maximum value of 7.64% and 20.9% compared to a Pt-based device. Energy conversion-related issues are still a point of discussion. In this regard, European Union has started to work on CISLINE project (https://cordis.europa.eu/ project/id/ENK6-CT-2001-00519/pl). CISCuT-based solar cells, for instance, CIS set up solar cells as for a versatile and humble copper tape, and have the potential for photovoltaic negligible cost creation. CISLINE project is based on the development of flexible solar cell based on copper tape. Inside this, copper and indium were electrodeposited followed by sulfurization and etching which resulted interestingly

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Fig. 12 SEM images of primary (a–c) and hierarchical ZNRAs d–f fabricated with first-step. J-V curves of DSSCs assembled with the primary (g) and hierarchical ZNRAs h photoanodes fabricated with first-step electrodeposition durations of 900, 1800 and 3600 s. Reproduced with permission [155]. SEM images of AgCuO2 films obtained under different deposition time: i 15 s, j 30 s, k 45 s, and k 60 s. m UV–vis absorbance spectrum and n the corresponding Tauc plot of the AgCuO2 film with deposition time of 30 s, and the inset shows the digital photograph. o Transmission spectra of blank FTO and FTO/AgCuO2 substrates under different deposition time. p UPS spectrum of the AgCuO2 film. Reproduced with permission [156]

in n-CuInS2 . Spray-deposited p-CuI was used as heterojunction partner followed by device completion and finally yields 7% efficiency. These cells are stacked together to form flexible device module [158]. Figure 13 displays how roll-to-roll process is utilized to develop solar module, whereas Fig. 14 illustrates energy band alignment of complete device. This represents state of the art of electrodeposition which can be directly applied to roll-to-roll technology.

8.2 Electrochemical Supercapacitor Nowadays, nanostructured materials related to metal oxides and chalcogenides are important aspects for energy storage applications [159–165]. Zheng et al. employed functionalized graphene (G) for anchoring MnO2 nanoflower (NF) through

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Fig. 13 Scheme of the base-line for CISCuT tape cell production. Reproduced with permission [158]

Fig. 14 a Band diagram of CISCuT solar cell b I–V curve of a CISCuT module of an area of 5 × 5 cm2 with efficiency = 7.1%. Reproduced with permission [158]

controlled electrodeposition toward supercapacitors [166]. The G/MnO2 displays excellent electrochemical activity with a high capacitance of 320.59 F/g at a current density of 0.5 A/g and phenomenal cycling stability with 95.5% capacitance conservation further than 3000 cycles. The symmetric cell provides promising attributes with a capacitance of 55.37 F/g at scan rate of 5 mV/s, with energy density of 5.67 Wh/kg and power density of 5.11 kW/kg. Zhao et al. prepared new NiCo2 O4 @Ni4.5 Co4.5 S8 composite on nickel head which is incorporated by two-step electrodeposition process and results are illustrated in Fig. 15a–h [167]. As a supercapacitor material, NiCo2 O4 @Ni4.5 Co4.5 S8 exhibits a high capacitance of 369 mAh/g at 1 A/g, an excellent rate capability of showing 258 mAh/g at 20 A/g and an optimal cycling retention 4.8% after 5000 cycles. Fabricated NiCo2 O4 @Ni4.5 Co4.5 S8 -4//activated carbon asymmetric supercapacitor displays an excellent energy density (124.77 Wh/kg at 1.08 kW/kg) and power (39.29 Wh/kg at 15.21 kW/kg). Till date, two types of nickel hydroxide (α-and β-Ni(OH)2 ) with different morphologies have been prepared by CED and their SC activities are examined. As a case, nanoparticles of nickel hydroxide have been prepared using CED from

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Fig. 15 Low- and high-magnification FE-SEM images of the a, b NiCo2 O4 nanosheets; c, d NiCo2 O4 @Ni4.5 Co4.5 S8 -1 composite and (e, f) [email protected] Co4.5 S8 -4 composite. Electrochemical characteristic of the as-fabricated electrodes at a three-electrode system: g Comparison CV curves for NiCo2 O4 , Ni4.5 Co4.5 S8 and NiCo2 O4 @Ni4.5 Co4.5 S8 -4 electrodes at 8 mV/s; h Comparison GCD curves for NiCo2 O4 , Ni4.5 Co4.5 S8 and NiCo2 O4 @Ni4.5 Co4.5 S8 -4 electrodes at 1 A/g. Reproduced with permission [167]. TEM images of i non-porous RuO2 film, j RuO2 film electrodeposited with SDS, and k RuO2 film electrodeposited using a mixture of SDS and CTAB as a templating agent. l Cyclic voltammograms for electrochemical performance of as-synthesized RuO2 electrodes at a scan rate of 10 mV/s and m Charge–discharge curves at 1 A/g of RuO2 electrodes; non-porous RuO2 electrode (dashed line), RuO2 electrode electrodeposited with SDS (thin solid line), and RuO2 electrode electrodeposited with a mixture of SDS and CTAB (thick solid line). Reproduced with permission [170]

5 mM NiCl2 at temperature of 10 °C. It showed a capacitance of 740 F/g in the working voltage window of 0–0.55 V versus Ag/AgCl [168]. Also, pulse current deposition of nanoporous Co(OH)2 flicks onto Ni head was reckoned by Chen et al. [169] and the manufactured electrode displayed a capacitance worth of 1681 F/g at a current mound of 2 A/g in an anticipated window of -0.1 to 0.4 V. The size and density of the film could be optimized and tuned through parameters of the applied pulse. Kim et al. [170] reported the electrochemical deposition of RuO2 onto ITOcovered glass substrate from an aqueous RuCl3 .nH2 O bath in the presence of templating agents of SDS and CTAB. They observed that this RuO2 film prepared within the combination of SDS and CTAB shows an exceptionally high capacitance (503 F/g), which was 2.6 times greater than non-porous RuO2 (192 F/g). The results are summarized in Fig. 15i-m. To acquire better electrochemical prosecution, different Co3 O4 nanostructures, including nanorods [171, 172], nanoparticles [173, 174], nanowires [175], nanoflowers [176] and so on, have been effectively prepared using different preparation strategies. Hierarchical Co3 O4 film was also using CED

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by Yuan et al. [177]. The film showed a morphology of connected nanoflakes of 15–20 nm thickness, and a capacitance of 443 F/g (at 2 A/g) and 334 F/g (at 40 A/g). The energy storage capacity of magnetite electrodes has been studied by Wu et al. [178]. They studied the electrochemical behaviors in different aqueous electrolytes of 1 M Na2 SO3 , Na2 SO4 , NaCl, and KOH and saturated Na3 PO4 , and showed that Fe3 O4 electrode (in a Na2 SO3 electrolyte) provided a high capacitance of 510 F/g as compared to other electrolytes. Nickel oxide (NiO) is particularly important as the electrode material for its high theoretical capacitance of 2573 F/g [179], non-toxic, and good chemical stability [180]. Liu et al. [181] electrodeposited NiO nanosheets onto a 3D graphite film (UGF/CNT) with a capacitance of 575.6 F/g at 10 A/g and 100% capacitive retention at 3000 cycles at 1 A/g. Yang et al. [182] published the coating of Mn(OH)2 nanosheet on Au-covered polyethylene terephthalate substrate by CED. The electrochemical studies by GCD showed the capacitance value as high as 240.2 F/g at 1 A/g. Gao et al. [183] manufactured a asymmetric supercapacitor with a graphene hydrogel (GH) as a negative and electrodeposited MnO2 nanoplates onto Ni head (MnO2 -NF) as the positive electrodes in aqueous Na2 SO4 electrolyte. They showed that the device showed reversible cycling over a wide voltage window of 0–2 V, an energy density of 23.2 Wh/kg, a power density of 1.0 kW/kg, and a 83.4% capacitance retention after 5000 cycles, which were upgraded in examination with those of symmetric supercapacitors associated with GH (5.5 Wh/kg) and MnO2 -NF (6.7 Wh/kg) reckoned for [183].

8.3 Sensors Metal nanoparticles (NPs) are the ideal option for use as various sensors because of their distinctive chemical and physical characteristics [183–185]. A sensitive technique for detecting Hg(II) electrochemically in actual environment has been revealed [186]. It makes use of a nanocomposite produced by electrochemically depositing Au-NPs on a GCE modified by reduced graphene oxide. Using cysteamine, thymine-1-acetic acid, which has a high affinity for Hg2+ , was also covalently linked to Au-NPs. The sensor demonstrated very high selectivity for Hg(II) when compared to a variety of other heavy metal ions, and it was capable of detecting mercury in the range of 10 ng/L–1.0 mg/L. On a Ni foam, dealloying was used to prepare 3D nanoporous Au sheets with linked nanopores and filaments morphology and applied for sensor performance (Fig. 16) [187]. The resultant electrodes were stable over time and showed increased electroreduction activity for H2 O2 in an acidic solution. For instance, the currents remained constant when the same electrode was continuously used for chronoamperometric responses for 6400 s while it was submerged in 0.5 M H2 SO4 and 1.5 mM H2 O2 .

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Fig. 16 SEM images of the Ni foam at different magnifications (a, b, c), SEM images with different magnification of the Ni foam surfaces after Au–Sn alloy film was electrodeposited (d, e, f) and SEM images with different magnifications of the Ni foam surfaces after chemical dealloying of electrodeposited Au–Sn alloy film g, h, i. High resolution SEM image j of the sample in shown in i. k Linear scan voltammetric curves for H2 O2 electroreduction at various H2 O2 concentrations on the NPG/Ni foam hybrid electrode. Electrolyte: 0.5 mol/L + H2 O2 . Scan rate: 5 mV/s. Inset: CV for the NPG/Ni foam hybrid electrode in a 0.5 mol/L H2 SO4 solution at a scan rate of 50 mV/s. Reproduced with permission [187]

Due to the mesoporous nature and the presence of Au, the Pt-Au alloy films with a controlled composition prepared by using a square-wave potential program in the presence of surfactant showed excellent electrocatalytic activity for oxidation of glucose [188]. Pt51 Au49 alloy film delivered the best performance out of all compositions. The detection limit for this material’s reaction to glucose was 6.0 M, and the response was linear up to 11 mM.

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After Au-NPs were precisely electrodeposited on nanoscale carbon inter-digitized electrodes by adjusting the step-potential and time duration, cholesterol oxidase was immobilized by the electrochemical reduction of the diazonium cation [189]. The effective redox mediators, ferricyanide and ferrocyanide, were used to provide the biosensor its excellent sensitivity and selectivity for cholesterol. For instance, the LOD was 1.28 M and the detection range was large (0.005–10 mM). 3D ordered macroporous Au film were used to build a new label-free immunosensor for the detection of C-reactive protein (CRP) [190]. The film is associated with nanoparticle morphology, which had a significant surface area (almost 15 times that of the flat Au electrode) and linearly rising impedance values with CRP concentration immobilized the protein (0.1 to 20 ng/mL). Additionally, phenolic, gallic, and caffeic acids were detected using GCEs covered with Zn/Al LDH prepared by electrodeposition [191]. The outcomes showed that the LDH’s preconcentration operation caused the oxidation currents to significantly rise following electrode modification. The greatest electrocatalytic activity for H2 O2 oxidation was prepared by Xu et al. using a nanocomposite of Au-NPs and Co/Mn LDH on an ITO electrode to fabricate a hydrogen peroxide sensor with excellent stability [192]. Multiple synergistic interactions between Au-NPs and the Co/Mn LDH support, which lower the size of the nanoparticles and increase the conductivity of the composite material, are responsible for the sensor’s exceptional performance [40]. Ag dendrites and LDH were combined to create a composite material that was then electrodeposited onto a GCE that had previously been coated with a Mg/Al LDH to form a sensor for H2 O2 [193]. Based on the electrodeposition of Au-NPs on a GCE modified with copper calcined layered double hydroxide (Cu-CLDH), Cui et al. prepared a nitrite sensor [194]. Due to the synergistic interaction between Cu-CLDH and Au-NPs, electrochemical studies demonstrated that the Au-NPs/CLDH composite film displayed outstanding electrocatalytic activity for nitrite oxidation. The higher surface area, lower diffusion resistance, and improved electron transport of the Cu-CLDH and Au-NPs composite film were the main contributors to the enhanced electrocatalytic reaction to nitrite. Increased performance for the detection of glucose and ethanol was also observed with GC or graphite electrodes modified by electrodeposition of Pt-NPs on which a Ni/Al LDH was electrochemically deposited [195]. In particular, the presence of PtNPs allowed for the formation of a larger linearity range. The highest concentration of ethanol that could be detected using GC as the support was 65 mM. An illustration is the non-enzymatic glucose sensor that Yang et al. created using a composite Cu2 O/TiO2 [196]. First, helical TiO2 nanotube arrays with a diameter of around 105 nm were electrodeposited by anodic oxidation as part of the electrode preparation process. A layer of copper and copper oxide was then deposited, with a thickness of several hundred nanometers. With a sensitivity of 4895 A/cm2 -mM, a linear relationship between the response current and glucose concentration was established across the concentration range of 0.1–2.5 mM.

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Nickel oxide nanoparticles and electrochemically reduced graphene oxide (ERGO) were electrodeposited onto GC electrodes in order to detect acetaminophen, according to Liu et al. [197]. As 2D layered material, graphene and associated nanocomposites have been studied in multifunctional applications due to its high mechanical, thermal stability, and electrical properties [198–200]. Wu et al. [201] conducted similar studies on the electrochemical polymerization of aniline in solutions containing different macromolecules, resulting in different nanostructures. The electrode has been utilized in order to detect hydrogen peroxide.

8.4 Photocatalysis Photocatalysis application is the most promising approach nowadays keeping environmental aspect in mind [202–205]. The size and morphology of Cu2 O nanoparticles on the TNA have been effectively constrained by changing electrodeposition eventuality [206]. Under light irradiation (visible), the photocatalytic rate for hydrogen development on Cu2 O/TNA-0.8 was around 42.4 occasions on the unalloyed TNA. Novel ED of ZnO on graphitic (GO plate) substrate for photocatalytic operation [207] was examined as an n-type semiconductor with a wide band of Efb = 0.37 V. An effective degradation rate of around 91% was fulfilled at 6 h when examined in a test for cefixime degradation in the reactor. Bi2 WO6 and Bi2 WO6 /graphene particles were prepared by electrodeposition [208]. The photocatalytic degradation of methylene blue in aqueous solution under visible light was used to test the photocatalytic activity of both Bi2 WO6 and Bi2 WO6 /graphene particles. The results demonstrated that the highest photocatalytic activity occurred at medium concentrations of graphene, which was likely due to the improved electron/hole separation during photocatalysis. Because graphene prevented photons from reaching Bi2 WO6 ’s surface, an increase in graphene concentration reduced the photocatalytic effectiveness of Bi2 WO6 /graphene. With variable Cu2 ZnSnS4 (CZTS) deposition times, Cu2 ZnSnS4 /TiO2 (NRs) heterojunction was produced [209] by combining hydrothermal and electrodeposition procedures for photocatalyst application (Fig. 17a–c). A 5-min deposition period resulted in the creation of the highest performance CZTS/TiO2 heterojunction, demonstrating that recombination suppression and carrier lifespan extension were accomplished at the deposition time. The breakdown rate of Methylene Blue (MB) may reach 46.83% in several photocatalytic tests employing aqueous organic dye solutions and visible light radiation, with deposition times as long as 10 min. In order to evaluate the efficiency of photoelectrochemical water splitting, bismuth vanadate (BiVO4 ) thin films were produced on fluorine-doped tin oxide glass using a simple and inexpensive ED method [210]. Various times of the electrochemical deposition were conducted using vanadyl sulfate and bismuth (III) nitrate solutions. Analyses of the photoelectrochemical characteristics of the films were carried out by linear sweep voltammetry, chronoamperometry, and open-circuit potential. After being deposited

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Fig. 17 a SEM images of TiO2 nanorods, b Absorption spectrum of MB solution obtained after 5 h in the contact of without any sample (MB), CZTS, TiO2 nanorods and CZTS/TiO2 (NRs) with different CZTS deposition times, c Dye removal efficiency for TiO2 nanorods, CZTS and CZTS/TiO2 (NRs) (H10). Reproduced with permission [209]. Top view and cross sections SEM images of d ZnO nanowires and e MoS2 /ZnO heterostructures electrodeposited on SSM substrate. f Photoconversion efficiency of ZnO nanowires and MoS2 /ZnO heterostructure. Reproduced with permission [211]. SEM images of g BiVO4 on WO3 after heat treatment with NH4 VO3 and dissolution of the excess V2 O5 with 1.0 mol/L NaOH and h cross-section of the photoanode showing the FTO, WO3 and BiVO4 layers. The colors in Fig. g and h are an artifice for a better understanding of the layers of the compounds. Blue represents the BiVO4 deposit, WO3 magenta, and green the FTO substrate. i Linear voltammograms at 5 mV/s obtained with heterojunctions produced by Bi electrodeposition, spin-coating and by drop-casting using different solution compositions. The experiments were performed in Na2 SO4 0.5 mol/L + Na2 HPO4 0.1 mol/L pH 7.0 under solar simulated illumination. Reproduced with permission [215]

for 30 min, the film had a sizable photocurrent density of 0.068 mA/cm2 at 1 V (vs. Ag/AgCl). By using a novel two-step ED method, a new MoS2 -coated ZnO heterostructure (Fig. 17d, e) was effectively grown onto a stainless steel substrate [211]. PEC studies show that the resulting heterostructure has the maximum photocurrent density at a bias of 0 V with improved stability and the best solar-to-hydrogen energy efficiency

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when compared to bare ZnO (Fig. 17f). As a result, the MoS2 acts as a co-catalyst to stop the recombination of electron–hole pairs and avoid photocorrosion of ZnO, referring to a potential transfer mechanism inside the MoS2 /ZnO heterostructure. The deposition of nanocrystalline titanium dioxide coatings on a variety of carbon substrates has been accomplished using a one-step ED method [212]. The titanium dioxide films outperformed the benchmark P25 titanium dioxide films in terms of photoelectrochemical studies. By electrodepositing and then annealing the material, nanostructured zinc oxide (ZnO) thin films were prepared [213]. The greatest photocatalytic activity and a high surface area are both found in the ZnO film produced at 500 °C. LaFeO3 /TiO2 was prepared by electrochemically soaking perovskite LaFeO3 nanoparticles onto TiO2 nanotube arrays [214]. When compared to individual TiO2 and LaFeO3 , the LaFeO3 nanoparticle/TiO2 heterostructure showed higher activity and outstanding photochemical stability for the visible light degradation of methylene blue (MB). The broad visible light harvesting and easier electron–hole separation revealed by the LaFeO3 nanoparticle-modified TiO2 helped to enhance the high photocatalytic degradation rate of MB. According to a recent research, the process by which Bi forms BiVO4 with the addition of NH4 VO3 and heat treatment is known as electrodeposition of Bi on WO3 film (Fig. 17g, h) [215]. By using this technique, a photocurrent of 2.1 ± 0.3 mA/cm2 is produced by the photoanode at 1.23 V versus RHE, which is greater than pure BiVO4 and WO3 . Studies using transient absorption spectroscopy show an increase in time constants for charge carriers’ recombination at the WO3 /BiVO4 heterojunction (Fig. 17i).

8.5 Light-Emitting Diode Electrochemical route has been successively employed to deposit ZnO in the form of nanowire arrays over GaN (0001) single crystalline thin films with p-type conductivity toward the formation of heterojunction [216]. The room temperature formed heterojunction between electrodeposited n-ZnO with p-GaN which demonstrates its ability as a complete solid-state light-emitting diode. Under the forward-bias condition, this diode exhibited a rectifying behavior with the current onset at 3 V. Formed device radiated 397 nm of wavelength as a unique UV light with threshold emission value of voltage as 4.4 V for as-prepared or annealed samples. Under the visibility for the naked eye, clear-cut emission for the violet visible tail has been seen above 5–6 V. This research explores state of the art of epitaxial-grown electrodeposited n-ZnO in nanowire form with p-GaN toward device-grade solid-state UV light-emitting diode (LED) structure. Figure 18 exhibits designed LED device and spotted light along with room temperature luminescence spectra.

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Fig. 18 a Schematic of our ITO/ZnO-NWs/p-GaN/In—Ga heterojunction light emitting diode. b Image of the light spot emitted at room temperature under a forward bias of 6.5 V by a LED structure fabricated with an annealed sample B. c RT-EL spectra of the LED under 6.5 V bias for as-grown and thermal annealed samples B. d Same as c for sample C. (S devices 0.07 cm2 ). Reproduced with permission [216]

8.6 Other Applications To prepare a nanocrystalline hydroxyapatite coating with a high interfacial bond strength on a titanium substrate for implant applications, ED method was established [217]. This anodization method generated vertically aligned TiO2 nanotubular arrays that acted as templates and anchors the subsequent pulsed electrodeposition operated at 80 °C which created the hydroxyl apatite nanocrystals (Fig. 19a–c). Bond strength of 40 MPa was reached following a 30-min heat treatment at 600 °C in an argon environment. Optoelectronic, microelectronic, and microsystem devices are frequently made using the electrodeposition of gold [218]. Hydroxyapatite has been electrodeposited on magnesium for biological reasons. The corrosion resistance of commercially pure magnesium (CP-Mg) in simulated body fluid (SBF) is explored to show the effectiveness of hydroxyapatite (HA) coating

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Fig. 19 Morphology of TiO2 nanotubes at different pH conditions. a, b: Top and side views of nanotubes formed in pH 2 respectively. c TiO2 nanotubes formed in pH 5. Reproduced with permission [217]. d FTIR spectrum and e SEM of electrodeposited PANI film from 1 M tosylic acid containing 0.16 M aniline after the cathodic pre-treatment. Comparison of imax and itot of f PANI coated AA1100 with g bare AA1100. Reproduced with permission [221]. h SEM image of the film plated from the electrolyte 1 after annealing at 350 °C for 2 h. i M s and H c of the films plated from electrolyte 1–4 after annealing at 450 °C for 2 h. Reproduced with permission [222]

via ED method [219]. Due to its capacity to resist corrosion and the bioactive characteristics of the HA coating, ED of HA is a suitable way to change the surface of CP-Mg when it comes to the creation of Mg-based biodegradable implant materials. Intriguing biological characteristics (such as increasing Osseo integration, cell differentiation, and cell adhesion) have been seen in orthopedic implants treated with nanotubes [220]. The adhesion of HA, which was electrodeposited on a TiO2 nanotube, was tested using an adhesive tape. TiO2 nanotubes with HA coatings had higher cell densities, live cell counts, and more MC3T3-E1 cell spreading than titanium surface. Polyaniline (PANI) films have been effectively electrodeposited as corrosionresistant coatings on aluminum AA1100 electrode surface in acidic solutions (Fig. 13d–g) [221]. However, it has been demonstrated that a non-uniform PANI coating accelerates the AA1100’s general corrosion as well.

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By electrodepositing from a sulfate bath at room temperature (25 °C), Ni-Co films on ITO glasses with a variety of microstructures and compositions were formed for the specific application (Fig. 19h, i) [222]. The saturation magnetization rises as the heat treatment temperature rises from 25 °C to 450 °C, whereas the coercivity falls from 15.27 kA/m to 7.27 kA/m. The saturation magnetization and coercivity increase after annealing at 450 °C from 340.97 kA/m and 7.98 kA/m, respectively, to 971.58 kA/m and 18.62 kA/m, while the Co content increases from 22.42% to 56.09%. Electronic waste (e-waste) can include precious metals that can be recovered, which has major economic and environmental benefits [223]. Bulk of current technologies either pollutes the environment, has low recovery efficiency, or required high energy consumption. It was suggested that the targeted recovery of Ag-Pd alloy could be carried out using the green ED strategy from a polymetallic e-waste leaching solution containing Ni, Cu, Ag, Pd, and Bi. The recovery efficiencies of Ag and Pd after 5 h in 0.5 mol/L HNO3 with an applied potential of 0.35 V were 97.72% and 98.05%, respectively, according to linear sweep voltammetry. Films were formed by electrodepositing bismuth from perchlorate electrolyte [223]. These films have remarkable electrical and structural characteristics. A cathode current density of 0.18 to 70.0 mA/cm2 and an acidic perchlorate electrolyte were used to create polycrystalline samples, which were then annealed at 265 °C in an environment of He gas. Under magnetic fields of up to 8 T, investigations of magnetoresistance, electrical resistance, and Hall coefficient were conducted in the temperature range of 5–300 K. Samples produced at current densities as low as 0.18 mA/cm2 after annealing showed electrical characteristics, such as magnetoresistance, that were comparable to those of single crystals. Recycling waste multilayer ceramic capacitors for palladium has recently attracted a lot of attention. An effective way to extract and purify palladium from used capacitors utilizing ED technology was suggested in a recent research based on the individual metal ions’ varied reduction potentials in HNO3 solution [224]. According to studies on the electrochemical behavior of palladium at a titanium electrode, higher HNO3 concentrations reduced the peak current of palladium and caused a negative shift in the corresponding potential. It was also discovered that 0.25 V of applied voltage, 240 rpm of agitation, and 0.5 M of HNO3 concentration made for the ideal electrodeposition conditions. A high purity (>99%) palladium with a 99.02% recovery rate was obtained under these circumstances. Morphological studies of the sample revealed that palladium was electrodeposited as nanoparticles. Metal implants may contain bacteria that cause infections and other issues. The aforementioned issues can be resolved using calcium phosphate coatings as a drug delivery technique. According to the current study [225], an antimicrobial agent should be added during a pulsed and reverse-pulsed electrodeposition. Using a range of pulse waveforms (unipolar–bipolar), current densities (2–5 mA/cm2 ), and temperatures (40–60 °C), calcium phosphate coatings were formed in 30 min. In order to choose the best electrodeposition behavior, the mechanical stability of the coated surfaces was examined. The findings of this study indicate that a pulsed codeposition strategy has enormous promise for controlling local antibiotic therapy,

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such as chlorhexidine digluconate, which may stop the associated infections in case of metallic implants. Recent research investigated on the dry sliding wear conduct of electrodeposited Ni-W/SiC nanocomposite coatings following heat treatment [226]. In particular, pulsed current ED was used to deposit Ni-14 (at%) W/SiC composite coatings, which were later heated to 500 °C in vacuum. It has been demonstrated that the mechanical characteristics of nanocomposite coatings adhere to the rule of mixtures (ROM). It has been proven that the inter-particle spacing of the SiC particles implanted in the Ni-W matrix has a linear relation with the wear rate of the nanocomposite system.

9 Advantages and Disadvantages 9.1 Advantages 1.

ED is driven by power (electricity), and hence structure and morphologies can be well controlled through optimized preparative parameters. 2. ED is a low-temperature process, and hence corrosion and thermal stresses can be minimized. 3. Small-to-large areas are possible to coat in a single run, and hence widely used in industry. 4. Nanocrystalline, polycrystalline, amorphous, single crystalline, epitaxial film, and even quantum dots are possible to form. 5. Single, binary, ternary, and quaternary films are possible to coat. 6. Metal, metal oxide, metal chalcogenides, polymers, and composites are possible to deposit. 7. Rough surface, shape, size, and multiple substrates in a single run can be possible to coat. 8. High-quality deposition is possible and it is possible to reuse precursor for many times after deposition with no wastage. 9. Greater extent of purity along with well-controlled thickness is possible to achieve. 10. Doping, composite, multilayer, and alloy are possible through ED.

9.2 Disadvantages 1. ED is driven by power (electricity), and hence electricity is essential and conductivity of substrate and electrolytes is prime requirement. 2. Impurity governed by source impurity has lower deposition potential which can be easily added and hence can change the film properties. 3. Deposition potential is requisite and hence difficult to control lower deposition potential material while coating higher deposition potential materials.

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4. Deposition on pre-grown layer with lesser conductivity is not easy. 5. Preparative parameters are very sensitive to temperature and pH. 6. Multicomponent deposition is complex.

10 Limitations and Future Prospects To carry out a variety of applications, high surface area, porous, and multi-layered nanostructures should be fabricated. As a result, several attempts to produce metal oxide/hydroxide nanomaterials by electrodeposition have been made, with notable academic success. There are a few challenges that must be further considered when employing ED to make nanostructures and their composites. First, pulse and reverse pulse electrodeposition through base generation have only sometimes been employed in the synthesis of nanomaterials, despite how simple it is to vary the size and morphology of the deposited materials utilizing these modes. Second, although EDderived materials have poor electric conductivity, they may have enhanced conductivity and capacitive properties when combined with other carbon materials. Some examples of these materials are graphene, reduced graphene oxide, graphitic carbon nitride, mesoporous carbon, functionalized carbon nanotubes, and combinations of these materials. However, among these carbonous materials, one-step electrodeposition of graphene/metal oxides(hydroxides) composites has been the subject of multiple studies. Therefore, it is crucial to consider and investigate ED of alternate carbon-based materials. Regardless of the features of chemical modifiers used, the main advantage of using nanomaterials is the substantial increase in the electrochemically active area and the improved accessibility of the electrode surface. This is shown by the improvement in both sensitivity and limit of detection. It is ideal to immobilize biomolecules over a larger surface area since doing so would enable the development of biosensors. The selectivity of the sensors, which is normally their primary drawback as compared to electrochemical ones, may also be improved using the properties of the nanostructure. Due to the different standard potentials of the redox active metals, it may be possible to distinguish between oxidizable molecules with one or more hydroxyl groups in the case of ED. Research on metal nanoparticles may focus largely on different metal composites to promote stability since electrochemical reactivity differs. The potential increase in electrical conductivity and accessible area that may be attained with the use of carbonbased nanomaterials should be considered while developing novel electrochemical processes for the one-step deposition of the composites. These conclusions suggest that throughout the next years, ED will probably be used more frequently in combination with nanotechnology. Although electrochemical syntheses have fascinating properties, their general usage is constrained by their complexity when compared to alternative methods like inkjet printing, spin coating, or roll-to-roll production. Research should scale up the electrochemical production of such devices, which offer wide application that cannot be obtained with traditional methods.

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The aerospace, transportation, and heavy engineering sectors have so far paid the most attention to electrodeposited composite coatings. Due to the availability of more novel nanostructured materials during the past 10 years, significant progress has been achieved. Currently, research is being progressed on both coatings with mixed functional nanomaterials for specific engineering applications and superhydrophobic surfaces with effective tribology and increased corrosion resistance. The highlight points are 1.

Carefully observing the amount of ion dispersion in the bath, paying particular attention to settling times, optical scattering, ion size distribution, zeta potential, and ion charge. 2. The usage of nanoparticles more often than submicron ones, which makes it easier for particles to disperse in baths and results in coatings that are more corrosion and wear resistant. 3. Enhanced electrolyte agitation using techniques including flow cells, jet impingement, submerged jet eductors, and ultrasound. 4. The tribological characteristics and hardness have been improved by the development and widespread usage of ceramic particles. 5. Coatings that are self-lubricating and have a lower COF than steel and stainless steel counterbodies as a consequence of enhanced inclusion of conventional particles like graphite and PTFE or the addition of more contemporary 2D layered particles. 6. Surfaces made from textured coatings, silicones, fatty acids, or PTFE that are highly water repellent or self-cleaning. 7. Making use of pulsed current to reduce grain sizes, improve engineering characteristics, and make it possible to employ larger currents to quicken electrodeposition in order to improve electrocrystallization and morphology, and pulsed current is also utilized. 8. Specific in situ interactions with the cathode surface, including cleaning a deposit as it occurs with an ultrasonic brush to enhance deposit morphology and regional flow. 9. Mixtures of particles with different characteristics, such as PTFE and SiC or WC and MoS2 . 10. Making use of metal alloy matrices like Ni–P, which may then undergo further heat treatment to boost its durability, or Ni-Co (which can provide increased corrosion and thermal oxidation resistance). 11. More investigation into advanced, specialized methods including electroless deposition. 12. Considering coatings with decreased through-porosity, uniform surface coverage, and less permeable barrier layers, which offer greater corrosion protection. To progress the field of electrodeposited composite coatings of ions contained in a metal matrix, more advancements in the following areas are required: 1. The ion dispersion of the electrolyte is characterized in terms of size distribution, electrical charge, species adsorption, and zeta potential. Adsorption isotherms

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should be used in controlled electrolyte compositions to assess the surface coverage of adsorbed surfactants on the surface. Novel, multifunctional coatings may be made by combining different particle types and sizes. To create a reaction environment that is efficient and uniform in terms of the distributions of current, concentration, and voltage at the cathode, the electrode shape should be changed. Different operational parameters, flow scenarios, and rate control types must be considered while altering the electrodeposition of composite layers. To anticipate deposit particle composition and characteristics from knowledge of bath composition and operating parameters, more accurate computer modeling is required. Reducing the use of persistent surfactants, understanding bath and deposit recycling, and proper product disposal at the end of its useful life are all necessary in order to address environmental problems. High-shear mixing or ultrasonics may be required for the proper production of a stable ion-bath suspension. Laboratory research should have a well-defined flow and proper experimental design to evaluate the influence of operational factors rather than focusing on realistic but unwanted reaction circumstances, such as vertical plate electrodes in a magnetically stirred beaker. Characterizing surface charge and ion dispersion in the electrolyte has been sluggish, despite how crucial they are to deposit quality.

11 Summary Electrodeposition is a simple, inexpensive, and highly productive approach for combining nanostructured metal hydroxides/oxides and their composites with carbon-based materials. Through one-shot or step-wise cathodic ED, distinct nanostructures (1D, 2D, and 3D), including nanoplates, nanotubes, nanorods, nanocapsules, nanoflakes, nanowires, and multi-structural complex nanomaterials with high surface area, may be effortlessly synthesized. Furthermore, this process provides numerous advantages: (a) the principal straightforward bath containing metal chloride/nitrate is required; (b) possible synthesis in normal pressure/temperature; (b) facile/easy bath conditions; (d) control the morphology, size, structure, and association of ions through easy factors like voltage and current; (f) provide pure material; and (e) applicable with various modes, which provides its own benefits. The featured highlights are 1. It’s predicated that whole process occurs as per Faraday’s laws of electrolysis. The ion is transported to the electrode by a combination of convective and electrophoresis diffusion. 2. The electrodeposition of polymeric, metallic, or ceramic films deposited on different metals has turned into an efficient method.

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3. The enormous number of functional factors effectively affect the quality and composition of the as-deposited film. 4. The area has broadened with the applications toward electrocatalytic coatings; electronic materials; and obviously for batteries, supercapacitors, and fuel cells. 5. In the course of the last 10 years, technological applications have also included superhydrophobic or biomimetic applications with other practical applications of materials and its composites. At this point, the electrodeposition is efficient coating method superior to other classical methods, and the nanostructures are easily deposited without the application of substrate or binders that can slow down performing operations. On the other hand, it produces a low amount of material making the characterization process more complicated. For case, the information about the particle size distribution for synthesized NPs can be painlessly acquired with dynamic light scattering, a procedure that cannot be employed for coating studies. Also, the elemental and structural analysis is hard to achieve except if the ED cycle is repeated many times. In case of electrodeposited polymers, it’s hard to get information of molecular weight property. So, more research should be dedicated to the advancement of characterization tools, to corelate chemical/structural property with the specific performance. Acknowledgements Bidhan Pandit acknowledges support from the CONEX-Plus programme funded by Universidad Carlos III de Madrid and the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 801538.

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213. T. Wanotayan, J. Panpranot, J. Qin, Y. Boonyongmaneerat, Microstructures and photocatalytic properties of ZnO films fabricated by Zn electrodeposition and heat treatment. Mater. Sci. Semicond. Process 74, 232–237 (2018). https://doi.org/10.1016/j.mssp.2017.10.025 214. C. Gong, Z. Zhang, S. Lin et al., Electrochemical synthesis of perovskite LaFeO3 nanoparticlemodified TiO2 nanotube arrays for enhanced visible-light photocatalytic activity. New J. Chem. 43, 16506–16514 (2019). https://doi.org/10.1039/c9nj03908b 215. D. Coelho, J.P.R.S. Gaudêncio, S.A. Carminati et al., Bi electrodeposition on WO3 photoanode to improve the photoactivity of the WO3/BiVO4 heterostructure to water splitting. Chem. Eng. J. 399, 125836 (2020). https://doi.org/10.1016/j.cej.2020.125836 216. O. Lupan, T. Pauporté, B. Viana et al., Epitaxial electrodeposition of ZnO nanowire arrays on p-GaN for efficient UV-light-emitting diode fabrication. ACS Appl. Mater Interfaces 2, 2083–2090 (2010). https://doi.org/10.1021/am100334c 217. A. Kar, K.S. Raja, M. Misra, Electrodeposition of hydroxyapatite onto nanotubular TiO2 for implant applications. Surf. Coatings Technol. 201, 3723–3731 (2006). https://doi.org/10. 1016/j.surfcoat.2006.09.008 218. T.A. Green, Gold electrodeposition for microelectronic, optoelectronic and microsystem applications. Gold Bull. 40, 105–114 (2007). https://doi.org/10.1007/BF03215566 219. M. Jamesh, S. Kumar, T.S.N.S. Narayanan, Electrodeposition of hydroxyapatite coating on magnesium for biomedical applications. J. Coatings Technol. Res. 9, 495–502 (2012). https:// doi.org/10.1007/s11998-011-9382-6 220. Y. Parcharoen, P. Kajitvichyanukul, S. Sirivisoot, P. Termsuksawad, Hydroxyapatite electrodeposition on anodized titanium nanotubes for orthopedic applications. Appl. Surf. Sci. 311, 54–61 (2014). https://doi.org/10.1016/j.apsusc.2014.04.207 221. T. Wang, Y.J. Tan, Understanding electrodeposition of polyaniline coatings for corrosion prevention applications using the wire beam electrode method. Corros. Sci. 48, 2274–2290 (2006). https://doi.org/10.1016/j.corsci.2005.07.012 222. L. Tian, J. Xu, C. Qiang, The electrodeposition behaviors and magnetic properties of Ni-Co films. Appl. Surf. Sci. 257, 4689–4694 (2011). https://doi.org/10.1016/j.apsusc.2010.12.123 223. Liu Y, Song Q, Zhang L, Xu Z (2022) Targeted recovery of Ag-Pd alloy from polymetallic electronic waste leaching solution via green electrodeposition technology and its mechanism. Sep. Purif. Technol. 280. https://doi.org/10.1016/j.seppur.2021.118944 224. Y. Liu, L. Zhang, Q. Song, Z. Xu, Recovery of palladium as nanoparticles from waste multilayer ceramic capacitors by potential-controlled electrodeposition. J. Clean Prod. 257 (2020). https://doi.org/10.1016/j.jclepro.2020.120370 225. E. Vidal, J. Buxadera-Palomero, C. Pierre et al., Single-step pulsed electrodeposition of calcium phosphate coatings on titanium for drug delivery. Surf. Coatings Technol. 358, 266–275 (2019). https://doi.org/10.1016/j.surfcoat.2018.11.037 226. N.P. Wasekar, L. Bathini, L. Ramakrishna L et al. Pulsed electrodeposition, mechanical properties and wear mechanism in Ni-W/SiC nanocomposite coatings used for automotive applications. Appl. Surf. Sci. 527 (2020). https://doi.org/10.1016/j.apsusc.2020.146896

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B. Pandit et al. Bidhan Pandit is now a Marie Curie CONEX-Plus researcher at University Carlos III de Madrid (UC3M), Madrid, Spain. He received his Ph.D. degree (2019) in Physics from Visvesvaraya National Institute of Technology (India) and joined as CNRS Postdoctoral Research Fellow at the Institut Charles Gerhardt Montpellier (ICGM), Université de Montpellier (France). His previous scientific interests focus on the synthesis of nanostructures and fabrication of flexible devices for supercapacitor applications. His current research focus includes the synthesis of cathode materials for lithium, sodium and potassium-ion batteries, as well as the in situ/operando X-ray based characterizations for the understanding of battery mechanisms.

Dr. Emad. S. Goda received his M.Sc degree in chemistry, Cairo University, Egypt in 2016, and Ph.D. from Hannam University, South Korea. He is also a researcher at the national institute of standards, Egypt. His research interests focus on energy storage devices, antibacterial membranes, polymer nanocomposites, sensors, and biomedical applications including MOFs, PB, 3D graphene, hierarchical hybrids, and conducting polymers. Dr. Shoyebmohamad F. Shaikh received his M.Sc. (2009) and Ph.D. (2015) in Clean Energy and Chemical Engineering. from at University of Science and Technology, South Korea. Presently, He is an assistant professor at King Saud University, Riyadh, Saudi Arabia. His present research interest includes ambient temperature chemical synthesis and characterization of metal oxide/chalcogenide nanostructured thin films for energy conversion and storage/harvesting applications.

Chapter 7

Nanostructured Thin Films by Hydrothermal Method Sutripto Majumder

Abstract A facile, scalable and binder free chemical method needs to be developed for the deposition of nanomaterials in thin film form for wide spread application in advanced optoelectronics, energy conversion, energy storage, memory and biomedical devices. Hydrothermal method is one of the superior chemical route for the growth of a uniform and well adherent thin film with various nanostructured morphologies. Hence, this chapter deals with the basics about the hydrothermal method along with film formation mechanism and the factors affecting the thin film formation. The low temperature (below 180

ITO

Photovoltaic devices

[32]

Metal selenide

1,2-ethanedithiol

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[33]

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Poly 3-hexylthiophene

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[34]

Silicon dioxide

Silicon dioxide (SiO2 ) particles

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[35]

dip coating method having thickness of 273 nm, a specific resistivity of 10–3 Ω cm, and a transparency of 95% sintered at 518 °C. Using the dip coating process with PEDOT:PSS, the performance of carbon nanofibers electrode is increased, yielding an electrode material suitable for energy storage devices [38]. Many examples of the dip coated across a variety of substrates from metallic to non-metallic are available in literature due to their prospective uses in technological applications. ZnO thin film synthesis allows large-area deposition at low temperature while maintaining an acceptable film quality [30]. CuS thin films were made at room temperature using chemical bath deposition and dip coating methods. The obtained CuS thin film possesses a semiconducting nature [31]. Considerable attention has been made on the basic and fundamental features such as optical properties, structural

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properties and photoconductive response of the cadmium telluride thin film formed by using the dip coating process and used for the photovoltaic applications [32]. Layer by layer dip coating of PbSe nanocrystal thin film yields a homogeneous and conductive film with great control of film thickness across broad areas having application towards optoelectronics and solar energy conversion [33]. In the presence of a solvent additive, a unique water based biphasic dip coating process creates a homogeneous, smooth and non-amorphous linked polymer thin film. The results points the way towards developing a dependable and promising organic thin film transistors [34]. The use of the dip coating method to prepare a selfcleaning super hydrophobic silica coating is described as a simple and time saving method which has a lot of potential in the industrial world [35]. Dip coating method can be applied to coat graphene oxide having applications in electronics [39]. Coating of metal oxide nanoparticles, CNT and quantum dots in photovoltaic devices, screens and sensing devices have been reported [40, 41]. Coating of polymers and nanocomposites for the possible application in nanolithographic patterning have been explored [42]. Interestingly, dip coating is used for depositing organic semiconductors over a broad surface area on variety of substrates [43]. For field effect transistor application, nanopolymer films have been synthesized by using the fast dip coating method [44]. Microgel deposition on the optical fibre tip using dip coating have industrial applications [8].

7 Dip Coating: Case Studies 7.1 Dip Coated MWCNTs as Electrode in Supercapacitor Application The excellent form of carbon is the multi-walled carbon nanotubes (MWCNTs), which have more application in energy storage devices. A detailed study of deposition of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) over a MWNCTs thin film by using the dip and dry method for electrochemical supercapacitors is presented in this section. The report completely focused on morphological and structural properties in correlation with its electrochemical characteristics for supercapacitors [9]. Film Formation In this study, MWCNTs are coated on a stainless steel (SS) substrate and then a PEDOT:PSS shell is encapsulated on the MWCNTs using a dip and dry coating method to improve electrochemical performance. The synthesis process comprises (a) MWCNT’s synthesized on SS substrate then (b) PEDOT:PSS shell coating on MWCNT’s by using dip and dry method to serve as an active material for the electrode towards supercapacitor application. Figure 8 depicts a diagram of this process [9].

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Fig. 9 a Schematic representation of wrapped PEDOT:PSS on MWCNT, b FESEM image of PEDOT:PSS/MWCNT, c XRD pattern of bare SS, MWCNT and PEDOT:PSS, d curves of charge discharge at various current densities, and e cyclic voltammetry (CV) at various scan rates ranging from 5 to 200 mV/s. Inset shows variation in specific capacitances [9]

Structure The structural study of MWCNTs and PEDOT:PSS/MWCNT shows (002) the graphite peak of MWCNT. No peak was observed for PEDOT:PSS hence it is polymeric and amorphous in nature. However, because of the remaining oxygen having functional groups in CNT, a small peak appeared at 21.38° in MWCNT and the composite film. The stainless steel substrate is input of some peaks at 74.34, 50.40 and 74.34° (Fig. 9c). Morphology With a size bar of 300 nm, the surface morphology demonstrates a homogeneous coverage of PEDOT:PSS on the exterior surface of MWCNTs (Fig. 9b). The wrapped PEDOT:PSS over MWCNT shows a mesoporous structure of the nanotubes which is beneficial for better interaction between the electrolyte and electrode. Supercapacitor Studies The wrapped PEDOT:PSS over the MWCNT mesoporous structure provides the maximum specific capacitance 235 F/g at the scan rate 5 mV/s in 1 M H2 SO4 . Moreover, the symmetric device attains the highest energy density of 12.18 Wh/kg with power density of 1100 kW/kg. Therefore the outcome of this composite system forms a way to design a supercapacitor electrode with high performance.

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7.2 Dip Coated MWCNT as Counter Electrode (CE) in Dye-Sensitized Solar Cell MWCNT’s deposited at ambient temperatures onto fluorine doped tin oxide (FTO) coated glass substrate with varying number of coating cycles to achieve the desired thickness utilizing the simple and cost effective dip and dry coating method [10]. Film Formation and Device Fabrication Using the dip and dry coating process, MWCNTs were coated over the FTO substrate. 0.125 g functionalized MWCNTs were added to 25 ml of double distilled water (DDW) along with 0.5 ml TritonX-100 to make MWCNTs solution. A clean FTO substrate was dipped vertically in the solution for 10 s to deposit MWCNTs thin film, then dried under an IR lamp until the deposited layer dried. This dip and dry process combined to deposit MWCNTs uniformly over a FTO coated glass sample in a single cycle. Different thicknesses were obtained by varying dipping cycles. The Eosin-Y coated ZnO layer and MWCNTs were kept above each other using tixo-tape as a barrier and fastened together using two binder clips at two ends. To complete the DSSC device iodide-triiodide electrolyte was pumped between the two electrodes by capillarity (Fig. 10a).

Fig. 10 a Schematic structure of DSSC assembled device, b structural study, c stability study, d J–V study, e FESEM scan of thin films of dip and dry coated MWCNT’s for 10, 15, 18, and 20 cycles, and f EDAX study for MWCNTs [10]

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Structure Structural study for FTO and FTO/MWCNTs thin film is shown in Fig. 10b. The high intensity peak of SnO2 confirms the FTO finding. For MWCNTs, additional peaks at 26.43, 42.60 and 54.52° (Fig. 10b) were observed. Morphology The FESEM image of dip and dry coating MWCNTs on the FTO substrate for 10, 15, 18 and 20 cycles indicate a fibre like structure (Fig. 10e (a–d)) that gives a wide surface area that is conductive to increase electrocatalytic activity. This opens the door to using the dip and dry coated MWCNTs as a counter electrode in the dye-sensitized solar cell instead of expensive platinum. The energy dispersive X-Ray spectroscopy (EDAX) spectra for 18 cycled MWCNT thin films confirms the presence of oxygen and carbon (Fig. 10f). Stability Study of CE Electrode The 50 cycled cyclic-voltametry (CV) curve for 18 cycled dip coated MWCNTs is shown in Fig. 10c. Inset of Fig. 10c shows the stability of CE for 50 cycles. It shows 91% retention at 50 cycles, demonstrating the produced CE’s high stability. Current Density–Voltage (J–V) Study Figure 10d shows the J–V curves for of platinum in both in dark and under light conditions, as well as the J–V curves of MWCNT thin films with 10, 15, 18 and 20 deposition cycles under light conditions which exhibits optimum coating requirement for best performance.

7.3 Dip Coated PEDOT:PSS Shell on CdS Nanowires Towards LPG Gas Sensor Many attempts have been explored to extend durable and extremely effective LPG sensors with excellent sensing. An effort has been made to form a uniform shell of p-PEDOT:PSS on n-CdS nanowires in order to form the nanohetero junction that will form the device to sense LPG [45]. Film Formation For the deposition of CdS nanowires, the Cd(OH)2 film was deposited on the FTO substrate by using the chemical bath deposition method at room temperature (27 °C). In order to form CdS, the Cd(OH)2 thin film covered with the FTO substrate was immersed in the beaker containing 0.01 M sodium sulphide (Na2 S) solution and kept for 2 h [46]. The shell formation of PEDOT:PSS on CdS nanowires was performed by using the simple dip coating method. The shell was created using a commercially available PEDOT:PSS solution. After coating, the dip coated film of FTO/CdS/PEDOT:PSS

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was heated in air for 10 min at 100 °C. The PEDOT:PSS::H2 O ratio, ultrasonication, dipping, and heating times were all optimized in order to achieve a uniform and thin layer coating of PEDOT:PSS on CdS nanowires for improved gas sensing performance [45]. Structure When the elemental spectrum of bare CdS (Fig. 10a) is compared to the elemental spectrum of PEDOT:PSS shell on CdS nanowires (Fig. 10b), an increase in ‘S’ peak intensity is observed, which is due to the formation of the PEDOT:PSS shell on CdS nanowires. The carbon content of the PEDOT:PSS shell coated CdS nanowires provides preliminary confirmation of the presence of a PEDOT:PSS layer on the CdS nanowire. The presence of Sn in CdS is due to the FTO-coated glass substrate. In PEDOT:PSS coated CdS nanowires, the intensity of the Sn peak decreased (Fig. 11). Morphology Surface morphology and compositional study Fig. 12a, b show SEM images of CdS nanowires at magnifications of 500 nm and 5 µm, respectively, while Fig. 12c, d show PEDOT:PSS coated CdS nanowires at magnifications of 500 nm and 5 µm, respectively. Stability Study LPG Detection at CdS/PEDOT:PSSnanoheterojunction Figure 13a shows the forward biased current density–voltage (J–V) characteristics of the FTO/n-CdS/p-PEDOT:PSSnanoheterojunction in air and under various concentrations of LPG (300–1500 ppm). Inset of Fig. 13 shows the gas response of the p-PEDOT:PSS/n-CdSnanoheterojunction at various LPG concentrations at 1.5 V applied voltage. Figure 13b depicts the response transient curves for the first two LPG exposures at 1.5 V fixed voltage and 900 ppm LPG. The p-PEDOT:PSS/nCdSnanoheterojunction is reversible to LPG, with a maximum response of 58.9%. Figure 13c depicts the variation of gas response as a function of time. It is clear that the gas response drops to 55.3% and remains stable after 10 days.

Fig. 11 Elemental analysis of a FTO/CdS, and b FTO/CdS/PEDOT:PSS layers [45]

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Fig. 12 The scanning electron micrographs of n-CdS film on FTO coated glass substrate with scale bar of 500 nm (a), and 5 µm (b), respectively, and p-PEDOT:PSS layer coated CdS nanowires with scale bar of 500 nm (c), and 5 µm (d), respectively [45]

8 Advantages and Disadvantages • Advantages 1. 2. 3. 4.

Dip coating is highly efficient with minimal wastage. It is simple and inexpensive technique. Specific thickness can be obtained by controlling few preparative parameters. Coating on flat and uneven surfaces as well as tubes is possible. Both side coating on flat surface also possible at a time. 5. Uniform coating with roughness in nanometres can be possible. 6. By varying withdrawal speed gradient coating also possible. 7. Fairly good adhesion • Disadvantages 1. During the drying phase, wet film is affected by environmental condition such as turbulent air flow. To avoid this process can be done in a clean room. 2. Curved or flexible substrates difficult to coat. 3. When changing from liquid to solid layer, material shrinkage can lead to cracking in the film. 4. Light parts can float or fall from carrier and thickness of film can change from top to bottom.

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Fig. 13 a Forward biased I–V characteristics of FTO/n-CdS/p-PEDOT:PSSnanoheterojunction in air and at various concentrations of LPG as 0 (air), 300, 600, 900, 1200, and 1500 ppm. b Variation in gas response (%) versus LPG concentration (ppm) of n-CdS/p-PEDOT:PSS nanoheterojunction at 1.5 V [45]

9 Summary and Conclusions Dip coating is one of the oldest and simples film deposition method. By increasing the technological development the controlled synthesis of various thin film microstructures to nanostructure made is possible. Evaporation is an important stage to determine the properties of the film. Film thickness can be determined by struggling between the factors like capillary force, effect of gravity and viscosity. In many systems, withdrawal speed is proportional to film thickness but in the capillary regime region low withdrawal speeds thicken the film because evaporation rate is faster than the drying line movement. For different shapes the dip coating method is technically modified into dip-drain coating and angle dependent dip coating. Numerous variations lead to nanostructure thin films. Such nanostructures and interest for MWCNTs have various application including electrochemical supercapacitor and counter electrode in the dye sensitized solar cell.

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10 Future Scope • Dip Coating method is a simple, low cost, large scale production with industrial applications. Much research is expected to avoid cracking of the film due to shrinkage. • Future research should examine the effects of various temperatures, lower concentrations and humidity. • Evaporation induced self-assembly variation leads to thin film nanostructures having application in Dye-sensitized Solar Cell (DSSC), energy storage like supercapacitor and other applications. • Moreover the cost effective and facile production of nanomaterials should be realized to their large scale device fabrication and their application.

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S. L. Patil et al. halogenated solvents for transistor applications. RSC Adv. 9(11), 6356–6362 (2019). https:// doi.org/10.1039/c8ra09231a A.B. Gurav et al., Superhydrophobic coatings prepared from methyl-modified silica particles using simple dip-coating method. Ceram. Int. 41(2), 3017–3023 (2015). https://doi.org/10. 1016/j.ceramint.2014.10.137 T. Furusaki, J. Takahashi, K. Kodaira, Preparation of ITO thin films by sol-gel method. J. Ceram. Soc. Jpn. Int. Ed. 102(2), 202–207 (1994). https://doi.org/10.2109/jcersj.102.200 D. Gallagher, S. Francis, R. Houriet, H.J. Mathieu, T.A. Ring, Indium-tin oxide thin films by metal-organic decomposition. J. Mater. Res. 8(12), 3135–3144 (1993). https://doi.org/10.1557/ JMR.1993.3135 Y. Altin, A.C. Bedeloglu, Poly (3, 4-ethylenedioxythiophene): polystyrene sulfonate-coated carbon nanofiber electrodes via dip-coating method for supercapacitor applications. J. Mater. Sci. Mater. Electron. 32, 28234–28244 (2021) Q. Zheng et al., Supporting information transparent conductive films consisting of ultra-large graphene sheets produced by Langmuir-Blodgett assembly. ACS Nano 5(7), 6039–6051 (2011) G. Giancane et al., Aligning single-walled carbon nanotubes by means of Langmuir-Blodgett film deposition: optical, morphological, and photo-electrochemical studies. Adv. Funct. Mater. 20(15), 2481–2488 (2010). https://doi.org/10.1002/adfm.201000290 K. Lambert et al., Langmuir-Schaefer deposition of quantum dot multilayers. Langmuir 26(11), 7732–7736 (2010). https://doi.org/10.1021/la904474h I.I. Perepichka, A. Badia, C.G. Bazuin, Nanostrand formation of block copolymers at the air/water interface. ACS Nano 4(11), 6825–6835 (2010) X. Liu, Y. Zhang, X. Zhang, R. Li, W. Hu, Continuous and highly ordered organic semiconductor thin films via dip-coating: the critical role of meniscus angle. Sci. China Mater. 63(7), 1257– 1264 (2020). https://doi.org/10.1007/s40843-020-1297-7 S. Sathish, B.C. Shekar, R. Sathyamoorthy, Nano polymer films by fast dip coating method for field effect transistor applications. Phys. Procedia 49, 166–176 (2013). https://doi.org/10. 1016/j.phpro.2013.10.023 N.B. Sonawane, R.R. Ahire, K.V. Gurav, J.H. Kim, B.R. Sankapal, PEDOT:PSS shell on CdS nanowires: room temperature LPG sensor. J. Alloys Compd. 592, 1–5 (2014). https://doi.org/ 10.1016/j.jallcom.2013.12.090 V.R. Shinde, H.S. Shim, T.P. Gujar, H.J. Kim, W.B. Kim, A solution chemistry approach for the selective formation of ultralong nanowire bundles of crystalline Cd(OH)2 on substrates. Adv. Mater. 20(5), 1008–1012 (2008). https://doi.org/10.1002/adma.200701828

Dr. Savita L. Patil is currently employed as an assistant professor for 2 years in the Department of Physics, DDSP Arts, Commerce and Science College, Erandol, Dist Jalgaon, Maharashtra (India). She has completed her M.Sc. and Doctorate degree in Physics from KBC North Maharashtra University Jalgaon under the guidance of Dr. R. B. gore and co-guidance of Prof. B. R. Shankapal. Her doctoral work focuses on studies and the development of metal hydroxide or oxide nanoforms toward supercapacitor application. She received “Shri. G. H. Raisoni doctoral fellowship” During her Ph.D. work in the year 2015. She has been authored and co-authored six research papers in nationals and internationals peer-reviewed journals. Among them one research paper is published in the Journal of Materials Science: Materials in Electronics (2021) with an impact factor of 2.195, and another in the journal RSC Adv (2020), with an impact factor of 3.36. She has 2 years’ work experience in teaching and 6 years of research experience. Recently she is a member of the syllabus framing committee of F.Y.BSc. Physics. Her research interest focused on employing a wet chemical route to synthesize and characterize thin films toward supercapacitor applications.

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Mr. Suraj R. Sankapal has completed his bachelor degree from Shivaji university, Kolhapur and Master degree in Physics from KBC North Maharashtra University, Jalgaon. Currently, he is pursuing his doctoral degree from D Y Patil Education Society (Deemed University) Kolhapur, (M S). Dr. Faizal M. A. Almuntaser is currently employed as an assistant professor in University of Aden, Yemen. He has completed his Ph.D. from KBC North Maharashtra University Jalgan in 2017 and M.Sc. (Physics) from University of Pune. He has four international journal publications on his credits.

Chapter 11

Screen Printing: An Ease Thin Film Technique Lakshmana Kumar Bommineedi, Nakul Upadhyay, and Rafael Minnes

Abstract Screen printing is a process of pushing ink of material to be deposited onto a surface through a fine-mesh screen to create a desired pattern. Printing on a variety of surfaces is feasible with screen printing. Screen printing permits printing on a variety of shapes and sizes of surfaces. It is thought that screen printing was created in Asia about the year 500 A.D. Silk-screen printing had progressed to a high level in China and Japan by the 1600 s. It made its way over Europe gradually. The first patent for silk-screen stencil printing was granted to Samuel Simon of England in 1907. The basic premise of screen printing is to apply ink to the substrate using a specified mask and a squeegee to create a pattern on the substrate. Specifying the intended purpose, the substrate and screen mesh might be made of cloth, plastic, metal, or any other material. Film properties such as thickness, uniformity, microstructure and morphology are governed by several parameters including the geometry of the squeegee, screen frame, mesh count, ink properties, the sintering temperature, and sintering time. This chapter examines two commonly used screenprinting processes, as well as their structure, operation, strengths and drawbacks, and current research status. This chapter discusses the complex multi-step, multimaterial, composite and multi-layer process of screen printing. This chapter covers various applications of screen-printed electrodes, including printed electrodes, transistors, solar cells, solid oxide fuel cells (SOFC), supercapacitors, electrochromic display (ECD), piezoelectric display, anti-reflection coating (ARC) and batteries to provide a clear understanding of the method in terms of its versatility, scalability, and simplicity for future research opportunities.

L. K. Bommineedi (B) · R. Minnes Department of Physics, Ariel University, Ariel, Israel e-mail: [email protected]; [email protected] N. Upadhyay Department of Physics, Visvesvaraya National Institute of Technology, South Ambazari Road, Nagpur, M.S. 440010, India

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 B. R. Sankapal et al. (eds.), Simple Chemical Methods for Thin Film Deposition, https://doi.org/10.1007/978-981-99-0961-2_11

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Abbreviations AC AFM AgNWs ARC C CAB CNTs CV DMPU-N, N0 DNA DS DSSC EC ECD EDX EIS FE-SEM FTO GCD GO MH Misc MWCNTs NADH NMP OLED PB PEDOT:PSS PEI PET P PQQ Pt PVC PVDF PVDFTrFE rGO RNA SEM Si SOFC SP

Activated carbon Atomic force microscopy Silver nanowires Anti-reflection coating Carbon Cellulose acetate butyrate Carbon nanotubes Cyclic voltammetry Dimethylpropyleneurea Deoxyribonucleic acid Drop shape analysis Dye sensitised solar cell Ethylcellulose Electrochromic display Energy Dispersive X-Ray Analysis Electrochemical impedance spectroscopy Field-Emission Scanning Electron Microscopy Fluorine-doped tin oxide Galvanostatic charge–discharge Graphene oxide Metal hydride Micro-supercapacitors Multiwall carbon nanotubes Nicotinamide Adenine Dinucleotide N-Methyl 2 pyrrolidone Organic light-emitting diode Prussian blue Poly(3,4-ethylene dioxythiophene) polystyrene sulfonate Polyethyleneimine Polyethylene terephthalate Phosphors Pyrroloquinoline quinine Platinum Polyvinyl chloride Polyvinylidene fluoride Polyvinylidene fluoride co-trifluoro ethylene Reduced graphene oxide Ribonucleic acid Scanning electron microscope Silicon Solid oxide fuel cell Screen printing

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Stainless steel Stress sweep step Single wall carbon nanotubes Thin film transistor Ultra-violet radiation Yttria-stabilized zirconia

1 Introduction A screen-printing (SP) is most correctly defined as the method of formation of a thin layer on objects with desired two-dimensional geometry by using a mesh screen on which the ink is spread with the help of an instrument called a squeegee. Screen printing is an effortless process to perform, it is also low-cost; hence, many industries are using this process of screen printing [1]. Depending on different industries, the name of screen printing is altered as serigraphy or silk screen printing. The historical record traced the origin of screen printing to the Song dynasty of China. Chinese people acclimated to creating different patterns on clothes and paper [2]. Japan also contributed further and started the use of silk as a mesh. In the seventeenth century, this process found its way into Europe. Until this time stiff brushes were used to spread the ink. One of the most important developments is patented in 1877 in which rubber squeegee is used. The success of any deposition method in an industry depends upon its simplicity and cost-effectiveness. These qualities are very well possessed by SP. The electronic circuit sector uses the SP since 1940 for the mass production of electronic circuitry [1]. Thick film screen printing is used for the metallization and to print soldering paste for the production of surface-mounted devices (SMD) and printed circuit boards (PCB). Dielectric components are also deposited with ease. Presentday screen printing takes over many manufacturing industries. Starting from the visiting cards, and graphic art industry to the high-sensitivity sensors, the SP proves a simple approach to production. Different types of screen printing machinery are shown in Fig. 1 [3, 4]. Manual screen printing requires a screen that consists of a pattern, the screen is held by a frame, and with the help of a squeegee, the paste or ink is spread on a substrate. In industry, SP is performed with automatic mechanised robots that significantly boost the process. Screen printing is extensively employed in pattern formation in the clothes and textile industry, for the design sketches and drawing on greeting cards and paper printing industry, to form displays and signboards, patterns and logo formation in the automobile industry. The most important use of screen printing is in the development of electronic circuitry and thick film. Film formation is the primary method used in the semiconductor industry for the manufacturing of different devices. It is widely used for the construction of PCBs. SMD are specially formed on PCB by this

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Fig. 1 Different types Screen printing machinery [3, 4]

method [5]. It is also utilized for the formation of thick film alkaline batteries [6]. One more but not least application of screen printing in the current era is the formation of a solar cell [7]. For decades, this method has demonstrated its usefulness in the production of solar cells [8]. For layer-by-layer film deposition, the SP is used. It is feasible to deposit all layers using the SP approach or perhaps a few layers using other procedures on the same films. Because of its adaptability, it is a necessary approach for the use of solar cells; as a result, Mariani and team [10] created a Dye sensitized solar cell (DSSC) module using the SP. Transistors have a complicated structure; thus, their production process must be accurate. However, the SP has been successfully employed for the fabrication of transistors [9]. With the use of SP, a wide range of substrates may readily be coated with a wide variety of materials, including polymers, organic compounds, carbon-based inks, metal oxides, metal chalcogenides, etc. This is why it is employed in the industries for manufacturing materials for a variety of applications. In this chapter, we will cover a cutting-edge screen-printed electrochromic display created by Linderhed et al. [10]. Only screen-printing was used to construct these displays from the ground up. For the use of energy storage devices like supercapacitors, fuel cells, and batteries, a variety of electrodes can be screen printed; those are also extensively covered in this chapter.

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2 Screen Printing Process Generally, the production process includes the deposition of ink or paste on the substrate with controlled parameters. In the method of screen printing, there is a substrate on which paste or ink is pushed through the screen, which consists of a specific pattern with the help of a squeegee. The pressure, speed and movement of the squeegee may alter the property of the formed film. The screen is consisting of a frame with sufficient strength to stand the ruinous effect of the process and the stretched stencil, which is made up of mesh. The emulsion is attached to the mesh with the negative of the required pattern. The substrate should be even and the paste or ink should stick to it with controlled thickness parameters. The squeegee is used to spread and press the ink or paste through the screen on the substrate. The most important parameter for a squeegee that decidse the quality of a printed image is a flexible blade, and it also requires a resilient blade that withstands the pressure and forces acting on it at the time of the screen-printing process. The initial step of screen printing is to hold the mesh above the substrate at a certain specified distance. If the distance is not adjusted properly then it will affect the quality of the printed pattern. After that, the suitable ink or paste is pushed down on the mesh. With the help of a squeegee, the ink should spread evenly on the screen, and then the second stroke is made in the opposite direction so that the excess ink is removed and a suitable thickness is achieved. Then the ink or paste is transferred through the open area of the mesh to create a required pattern on the substrate. The handling of the squeegee is very critical during the process of screen printing. Even a very small error for a very short time can completely alter the properties of printed film. The last step is to clean the screen to avoid the formation of ghost and haze images (since the ink is trapped in the mesh) in another turn of the process. Manual printing is time-consuming due to all these steps, and hence, the industries are using fast and efficient automatic machinery for the SP process [11].

2.1 Types of Screen Printing The screen-printing process has two types: contact and off-contact [12] (Fig. 2, [1]). The gap between the mesh and substrate at the time of the process creates a difference.

2.1.1

Contact Printing

In the contact printing process, the mesh remains in contact with the substrate throughout the stages of filling and metering. One of the most important disadvantages of this process is that during the time of a screen lift-off after performing the screen printing; there are appearances of cracks and deformities at the edges of the printed pattern, and the quality of the print is greatly reduced.

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Fig. 2 Screen printing schematic

2.1.2

Off-Contact

In the off-contact printing process, there is a distance between the mesh and substrate. The mesh comes in contact with the substrate at the time of a stroke of a squeegee. This contact quickly breaks after delivering the ink onto the substrate [13]. Offcontact printing has an advantage over contact printing in such a way that it greatly reduces the damages that occur due to contact between the screen and the substrate. The distance between the screen and the substrate should be adjusted with proper care [14]. If the distance is large, then the printed image may be incomplete because of the breaking of a contact between the substrate and the screen in the middle of the process. The fine adjustments and the thickness of the film cannot be controlled. The screen would be elongated permanently or damaged due to the use of high pressure in the screen-printing process at a large distance. If the distance between the screen and the substrate is small, the screen will stick for a longer time to the substrate due to the sticking effect of the paste and the low restoring force of the screen. The sticking-down effect introduces defects in the screen printing. In the field of printed electronics, screen printing is a frequently used technology. Screen printing may be categorized into two kinds: Flat-bed and rotary processes. Multi-layer printing may be done with precision using the flat-bed screen approach which also has a lot of promise for large-scale printing because of its minimal price, the versatility of substrates, the malleability of the designed screens, and ease of use.

3 Essential Constituents of Screen Printing The essential constituents of a screen-printing machine are squeeze, mesh, screen frame, ink or past and substrate on which the design printed. A detailed discussion is added below about the constituents mentioned above.

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3.1 Squeegee When it comes to fine-line printing, squeegee selection is critical. Chemically, both the ink and the cleaning solvents must be robust to the squeegee material. Polyurethane and silicone are two of the most popular squeegee materials. Squeegee hardness must also be determined by the printing substrate and design. For printed electronics, the usual squeegee durometer is between 60 and 90 shore A hardness. Fine-line printing (line width down to 50–100 μm) or any other precise printing that requires a high mesh number screen is generally done using squeegees with a higher durometer (over 70 shore A). For smooth substrate surface and high-speed printing, higher durometer squeegees are also suggested. Rough and non-planar surfaces, as well as low mesh count printing, can benefit from softer squeegees with durometer of 60–70 shore A. Soft squeegees can also be used for thicker layer ink application while printing at a slower rhythm and with less pressure. The choice of squeegee durometer should be based on the specific printing application and substrate surface. Different squeegee profiles such as square edge, round edge, and beveled edge can also be used depending on particular printing application. The round edge profile is designed for printing very thick, non-precision textiles. The beveled edge profile is designed for printing curved surfaces in full resolution and it is an excellent choice for high-speed printing. The printed batteries should use the typical square edge profile since it provides a good compromise of printed film thickness, adherence, and perfection [15]. The squeegee is the cornerstone of screen printing. To achieve high-quality screen printing patterns, there should be ideal management of the hardness, edge shape and profile of the squeegee [16]. The three important steps that decide many parameters of the screen printing performed through a squeegee are. (1) Spreading of the paste on the screen. (2) Pressing of paste through the mesh. (3) Removing the excess paste. To improve the quality of the squeegee, combinations of layers of different materials with different flexibility and hardness are added. The type of mesh, the abrasion of the paste and the applied pressure on the squeegee decides the durability of the squeegee. Weariness causes roundness of the edges, because of which the scraping property of the squeegee gets reduced significantly. A round edge squeegee cannot remove the paste or ink effectively, which leads to bad print quality. The beveled profile provides more control over the squeegee parameters than the trailing one. Depending upon the pressure applied on the trailing profile squeegee, the effective angle of the trailing profile significantly changes. The beveled profiles are useful when thinner films are required because they scrape excess ink effectively. The effective angle of the squeegee can have a significant impact on the printing process. The small effective angle results in a decrease in effective pressure, which decreases the paste or ink removal property of the squeegee. On other hand, if the

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Fig. 3 Different geometries of squeegee

Fig. 4 Roller squeegee screen printing schematic

effective angle is larger, there is a decrease in filling time. Due to these reasons, the effective angle should be maintained with proper care. Normally, effective angle of the squeegee should be 45°, as this angle is found advantageous in many processes. Hard squeegee provides a large amount of force to the paste on the screen, while the soft squeegees have a higher degree of adaptability to the surface of a substrate. The squeegee’s edge form determines the ink delivery criteria to the substrate (Fig. 3). The sharper the edge of the squeegee, the less ink will be delivered to the substrate. To achieve high speed as well as uniformity of thickness of the film, a wonderful advancement is achieved in the form of a roller squeegee. With the help of a roller squeegee, the thickness of the film can be controlled [16]. Roller squeegees (Fig. 4) are effective in maintaining a uniform layer of film.

3.2 Mesh of the Screen In addition to squeegees, choosing the right screen mesh is crucial for better printing yield. Polyester, nylon, and stainless steel are the three common forms of screen mesh. Polyester has the benefits of being solvent, and resistant to water, as well as being able to tolerance to extreme temperatures, minimal stretchability, and being

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used for high-precision printing. However, when compared to nylon, it has a limitation wear resistance. Nylon, on the other hand, has good strength, great wear resistance, and good solvent and water resistance. However, the nylon has the problem of being quickly stretched with great stress, which makes it inappropriate for fineline printing. The best of the three is stainless steel mesh, which has high strength, minimal stretching ability, and abrasion resistance. These characteristics make stainless steel mesh extremely long-lasting and perfect for fine-line printing, particularly over longer timeframes. The cost, deformation, stretching are disadvantages of stainless-steel mesh. Polyester, which is less robust than stainless steel but still useful when long-term fine-line printing is not necessary, is a much more budget-friendly alternative [15]. The most common material used to create the mesh is polyester and metal. Metal mesh is generally more durable due to its strength and resilient properties. An additional important advantage of the metal mesh is its ability to handle ink or paste having high temperatures. On the contrary, the advantage of polyester mesh is the production at a lower cost [17]. The mesh is stretched on a frame in such a way that it should distribute the forces acting on a frame to increase the durability of the screen. The thread of the filament can be distinguished into two types that is multifilament and monofilament. Monofilament threads forms a thin film with even edges, but the disadvantage of this filament is that it is very expensive.

3.3 Screen Frame The screen frame decides the structural integrity of the screen and should withstand and endure all the forces working on the screen without breaking or twisting. To handle the screen conveniently, it must be lightweight. Frames are designed in such a way that they can perform for a long period of time without breaking. The materials that can be chosen to build screen frames are wood, steel, aluminum and some cases magnesium.

3.4 The Ink The most difficult problem in the printed electronics industry is selecting a suitable ink and perfecting it. The properties and type of substrate mainly decide the nature of ink employed during the screen-printing [18]. The filler, solvent, and binder in printing ink are blended to tune the required viscosity. The viscosity of the ink is greatly affected by the binder. Binder has a vital role in deciding not only the binding properties of ink to the substrate but also the rheological properties of the ink. Because of environmental concerns, organic and water-based inks have been extensively researched. To improve the electrical conductivity property of organic or water-based inks, graphene, nanotubes, and carbon black are used. Because graphene

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has excellent conducting properties and a two-dimensional structure, it is increasingly investigated. But the problem with graphene sheets is that they get restacked together and aggregate. This problem is effectively solved by using cellulose acetate butyrate (CAB) as a binder with graphene. CAB convincingly stabilizes the ink because of electrostatic repulsion between graphene and CAB molecules [19].

4 Preparative Parameters 4.1 Squeegee Pressure The thickness and uniformity of the films are greatly influenced by the effective management of squeegee pressure. The pressure applied by a squeegee must be uniform so that the paste or ink spread consistently on the screen. It should also be sufficient to press the paste through the mesh opening. However, when too much force is applied to the squeegee, it might shatter or distort the squeegee or the screen. It is very difficult to predict the optimum pressure required in the screen-printing process by employing some sort of theory due to the fact that there are too many variables involved in the process. It is better to use the trial-and-error method to find the optimal point of squeegee pressure. The applied pressure should remain constant throughout the process; other wise, the printed layer will bulge in the middle, resulting in a nonuniform layer. The flexibility of the squeegee is also a very important parameter for deciding the uniformity of the printed film.

4.2 Squeegee Speed During the printing process, the velocity of the squeegee has an impact on the hydrodynamic pressure that drives the ink through the screen. As the velocity of the squeegee increases, so the pressure on the ink also increases, which favoring the printing process. If the squeegee velocity is too high, the contact time between the screen and the substrate may not be long enough to allow all of the ink to flow through completely. When using a higher squeegee velocity on a dense and highly viscous ink, the squeegee may slip over the ink, detaching the squeegee from the screen and causing unwanted ink deposition [20]. A squeegee’s high-speed movement generates unevenness in the finished film. If we perform the movement of the squeegee slowly then the time of screen printing will increase. This is the reason for achieving a favorable speed for the process. A slow squeegee action is usually necessary to print finer details.

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4.3 Prior Printing Parameters: Ink-Rheology Manufacture aspects, inclusive of screen printer settings and printing process, as well as ink rheology, have a substantial influence on film quality. Variables such as solid loading, solvent, and binder must be properly selected before the ink creation procedure to make high-quality ink. Picking the optimal quantities of the aforementioned factors not only ensures ink quality, but also enhance the printed electrode film’s properties through physical, mechanical, electrical, and electrochemical capabilities and stability.

4.4 Ink Preparation Composite powder, a solvent, a dispersant, and a binder make up the inks used to screen print. In screen printing, elements such as solid content, solvent, binder, and dispersant weightage or volume content influence ink rheology. The extreme quantity required to make the paste or ink for solid contents is determined by the size of particles and the mix of powders (solid content) with the solvent and binder. In contrast, if the binder concentration is too low, the film will delaminate when exposed to heat even during the drying or sintering step. The microstructure and electrical properties of the electrode are also influenced by the binder. The electrical properties and microstructure of the films are impacted by the binder used as per Piao et al. research [21]. The solvent, in addition to the binder, plays a vital function in ink preparation. The solvent has an impact on the optimum ink content for screen printing. Terpineol (a solvent composed of 94 wt.% terpineol with EC binder) and texanol are two of the most frequent solvents used in screen printing. In screen printing inks, tremendously large or very low binder solid loadings harm ink output. Due to the evaporation of binder during the sintering stage, the use of much higher binder concentration results in thick films with low density (increases porosity). However, due to the lower network strength of the inks, a very tiny quantity of binder might induce film breaking. Throughout screen printing, the thickness of the screen-printing layer is determined by the printing number. According to studies, the layer thickness of a film rises linearly with the printing time. For grain creation, green electrode layers are usually passed through the treatment of heat. The green electrode layer refers to the film that forms following screen printing. The relationship between the electrode particle’s active surface area and the optimal screen-printed layer numbers. The basic idea is that a few printing layers are sufficient for a film with a large surface-active area. The electrochemical performance of each electrode material necessitates a variable number of screen-printing layers.

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The eminence of the paste or ink used before screen printing, and the optimum number of layers are employed during the screen-printing process followed by annealing; all have a role in shaping the feature of the layers deposited by screen printing. For screen printing inks, there are a few suitable characteristics. In general, liquids with a high viscosity, shear thinning, high degree of structuration, and a fast thixotropic recovery rate are something that screen-printing inks comprise. While that may seem like a handful, each of those elements is essential when making a screen-printing ink or paste since they are all observable, quantifiable, and important.

4.5 Ink Viscosity Managing the viscosity of the ink can be difficult, since the viscosity fluctuates from batch to batch, even when the same quantity of binder solution is used. It’s best to add the solvent slowly, so that the viscosity doesn’t drop far below an acceptable screen-printable viscosity too quickly. Increasing the viscosity of ink so much is more difficult, since the solvent must be drained from the ink, which might compromise the ink’s quality. Thixotropy-inducing chemicals are frequently utilized in screen printing. When the ink is sheared through the screen by the squeegee, these additives cause the viscosity of the ink to decrease. Whenever the ink is not sheared, the viscosity rises to prevent the ink from running over the screen while another layer printing process is being completed. Conventional silver inks contain viscosities ranging from 10,000 to 100,000 cPs at 20 rpm, which are deemed adequate for screen printing [22]. Ink can transform its physical morphology when subjected to high shear stresses during mesh squeezing. Shear thinning occurs once the viscosity of the ink decreases when force is applied to it. This might imply that the ink you remove from the pot is different from the ink that contacts your substrate. Too much shear thinning might cause your ink to spread or creep after it has been applied, so a modest quantity is optimal. Viscosity may be determined at very low shear rates, lower than those obtained on typical viscometers, by gently probing the ink. A crucial criterion to evaluate the durability of the ink is “zero shear viscosity” [23].

4.6 Yield Stress The amount of pressure necessary for a material to flow significantly is referred to as yield stress. When it comes to printing inks, the yield value might provide important information for determining squeegee pressure. The patterned screen-printing ink’s yield value must be surpassed by a squeegee to push it through the mesh. Extra ink may be spread onto the surface if excessive squeegee pressure is used beyond the

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Fig. 5 Effect of Squeegee pressure and Yield stress on pattern ink deposition [23]

maximum yield value. Figure 5 shows the effect of Squeegee pressure and Yield stress on pattern ink deposition [23].

4.7 Thixotropic Recovery Rate [23] When a liquid ink is subjected to shear stress, for instance, when being forced through a screen-printing mesh, its chemical composition may be altered. The thixotropic recovery rate is a measure of how quickly a liquid returns to its structure while at rest. The ideal thixotropic recovery rate for screen-printing ink is extremely rapid— almost instantaneous. This implies that once deposited, the ink stays intact after flowing smoothly through the mesh. Even a lag time in the restoration of the coating can result in misprints, which can result in short circuits, reproductions, and the wastage of resources, particularly in electronic applications in which the distances among coated regions might be in the range of microns.

4.8 Thermo-Rheology The aforementioned characteristics can all be altered in response to changes in temperature. To get beneficial performance from ink, adjust the system’s operating temperature. Things heat up when the equipment gets going and the squeegee starts adding friction to the action. To evaluate the characteristics of screen-printing paste at a variety of temperatures, rheological testing can be done while changing the temperature [23].

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Fig. 6 The ink’s wetting and spreading behavior over the time [23]

4.9 Interfacial Rheology The thickness of a screen-printed coating after it has been put on the substrate might range from about 30–40 microns. The comprehension of the ink’s wetting and spreading behavior depends heavily on surface tension and surface energy. The behavior of extremely thin coating films once applied to the substrate may be predicted by monitoring the advancing contact angle, retreating contact angle (Fig. 6), and surface energy [23]. Rheological measures can be important for obtaining rapid print rates while preserving precision, whether when creating a screen-printing ink or putting up a screen print. An ink that goes through a mesh leaves a consistent coating of a specific thickness, and prevents spreading after being applied. This can be determined by measuring its viscosity, yield stress, and surface tension. The ink’s thixotropy and thermo-rheological characteristics can be used to better the squeegee’s speed, pressure, and angle to work effectively with the ink.

4.10 Counting Mesh and Repeated Printing Time The actual number of wires per linear inch is indeed the mesh count of the screen, whereas the repeated printing time determines the number of layers printed on the target; this process is dependent on the materials used in the paste or ink production. The eminence of the synthesized ink, which is connected to the ink’s rheological qualities, requires specific consideration. This is significant because the quality of the ink has a solid influence on the quality of the film. The thickness of a screen print layer can be modified via screen printing, which aids to improve Solid oxide fuel cell (SOFC) performance by lowering polarization resistance. Screen printing can create films with a layer thickness ranging from 10 to 100 μm. Yttria-stabilized zirconia (YSZ), gadolinium doped ceria, lanthanum strontium magnetite (LSM), and NiO/YSZ are some of the oxide films, ferroelectrics, photoconductors, and thick SOFC films that screen printing is often employed for [24, 25].

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As the squeegee forces, the ink or paste through the mesh and spreads uniformly over the surface of the target or substrate to generate a film with a thickness ranging from 10 to 200 μm, the ink or paste is deposited on the mesh’s edge. Screen-printed electrodes support an excellent performance for low polarization resistance. Sintering is usually done after the screen-printing procedures to solidify the deposited slurry layer.

5 Mask Design The emulsion forms an impenetrable barrier that prevents ink from passing through it via mesh, and conversely, empty spots without emulsion enable ink to flow via the mesh and onto the substrate. The emulsion layer is prepared by coating the screen with photosensitive emulsion fluid. The required mask design is then printed on a transparent film and put on top of the screen using a laser printer. After that, the design is irradiated with UV radiation and transmitted onto the screen. Upon UV exposure, the emulsion layer solidifies, and the uncovered emulsion areas peel away while washing, producing the specific design [26, 27]. Using a squeegee, ink is first ‘flooded’ or covered, over the complete mesh area of the screen before printing begins. The squeegee’s second stroke exerts extra pressure on the substrate, transferring the ink. Depending on the viscosity and crudeness of the ink, the gap (distance) between the mesh and the substrate must be adjusted. To achieve high printing quality with viscous inks, greater printing intervals are required. The substrate is dried in the oven for a fixed period of time after printing is completed to dry the ink.

6 Mechanism of Ink Transfer Both Riemer [28–31] and Messerschmitt [32] proposed historical analytic models in the 1980s that attempted to explain the physical process where ink is transported from the mesh to the substrate and then detached. According to Riemer [29], the screen mesh works like a piston in a syringe, propelling ink into the substrate as a column of ink survives a tube. The mesh is withdrawn by its tension after each print stroke, but the ink stays on the substrate owing to adhesion forces between the ink and the substrate. After being liberated from the mesh, the ink would fall and form the print. Messerschmitt [32] claimed that the divergence forces would not be capable of overcoming the ink-mesh adhesion in this fashion. This would cause a flow by combining shear and extensional forces, causing the ink to divide between the mesh and the substrate. Adhesion, extension, flow, and separation were the four main steps in this process [33].

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The scientists used high-speed imaging investigation, using a camera mounted to the side of screen-printing equipment, to see if either of these models was adequate. This allowed them to observe these separation processes in real-time. Throughout the screen-printing of a carbon-based ink, the experiment visualized the implications of a snap of distance between substrate mesh and print speed at the ink spreading mechanism and was able to recognize resemblances between the flow processes observed and Messerschmitt’s four key stages of ink transfer [32]. These have been divided into two definable regions: adhesion to extension, in which the ink stays in constant communication with the mesh and substrate, and flow to separation, in which the major part of the ink divides into filaments that finally separate (Fig. 7 [34]). The dimensions of these different zones varied quantitatively with line width, snap-off distance, and print speed, all are connected to the amount of ink dropped and the print’s homogeneity [35]. From the printed side of the substrates, estimation of roughness and contact angles can be done. The ink transfer was discovered to be influenced by the surface roughness of the substrate having rougher surfaces absorbing additional ink, but if the surface roughness becomes too high, the ink–substrate contact area diminishes, and the ink transfer minimizes [36]. The roughness of paper-based substrates is much higher than that of PET substrates, indicating variations in ink transfer, printability, and conductor line enactment between the two substrate types. Ink transfer and the development of highly conductive layers have been discovered to benefit from certain layer roughness and porosity [37].

Fig. 7 Various stages of ink deposition and screen printing [34]

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7 Electrode Printing Printing electrodes with ink involves the process of transferring the electrode ink onto a current collector. The process of transferring ink over the current collector is known as electrode printing. Filling or spreading the ink equally with a squeegee on the top of the screen mesh without exerting any pressure is indeed the initial stage in screen printing. The squeegee is then stroked back toward the starting point while putting pressure, evenly transferring the ink from the mesh to the substrate’s surface. Squeegee, screen mesh, emulsion, ink, and substrate are the main components needed for screen printing. While printing an electrode, a few measures must be made, including selecting a squeegee, screen mesh, emulsion, printing equipment, printing variables, and printing procedure [15].

8 The Thickness of the Film Film thickness plays a vital role in many applications and can be tuned through optimized parameters during the screen-printing process. When a film’s thickness approaches the nanoscale level, the quantum confinement effect of electrons has an impact on its properties. If the thickness of the film is not consistent, it will have a significant consequence on the material’s qualities. The thickness of the film is inversely proportional to the resistance and can also affect the strength of the electromagnetic field. The electrical and magnetic characteristics of a screen-printed film can be used to make electronic devices. Hence, film thickness is an area of interest in the electrical circuit industry [38]. The thickness of the film created by screen printing might range from 8 to 30 μm depending on the appropriate experimental setting [39]. Thickness can be reduced by polishing the surface using various methods. With the help of polishing methods, a two micrometer thickness can be achieved [40]. In certain experiments, ultra-thin layers of roughly 15 nm in thickness have been obtained [41].

9 Drying of Ink In the physical drying method, the evaporation phenomenon is utilized to form a dry ink layer on the substrate. Due to its simplicity, many inks can be dried using this method. The chemical method is somewhat difficult. In the chemical method, a solvent is not eliminated but changes its form with time. In the case of electrically conductive inks, the evaporation process enables the creation of percolation networks to be controlled. Fast evaporation creates stress in the film but it also significantly increases the electric conductivity among the ink particles. A balance should be

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established between these two parameters. Larger mechanical stress causes lower adhesion [42].

10 Sintering (Firing) Temperature The screen-printing process is used in many industries for different applications. An important application of this process is to form electrodes for energy-storing devices. The screen-printed electrodes required heat treatment to improve their properties. The temperature has a major influence on the formation of grain. The sintering process is mostly used for the formation of electrode layers. Sintering causes connections among the printed layer particles. This connection among the particles results in an increasing density of the film and a reduction in its porosity. If a screen-printed film is used as an electrode film for the application of supercapacitors, batteries, fuel cells, etc., then the film’s porosity also plays an important role, and the sintering temperature should be selected with great care so that the required density and porosity of the film are obtained. Insufficient heat treatment causes poor adhesion between the screenprinted layer and the substrate [24].

11 Ink Substrate Interaction Understanding the interaction between ink and substrate is critical in printing technology because the quality of printing depends on the right choice of materials. It is crucial to assess the wettability of the substrate by the ink when using a variety of ink formulations and substrates. Ink wetting refers to the strength of adherence of the ink to the substrate. An indication of the quality of the wetting can be obtained by measuring the contact angle, for example. The sessile drop method is a common process for determining the contact angle of a known liquid on an unknown substrate. Using drop shape analysis, a drop of fixed volume is applied to the surface to be investigated and assessed (DSA). A camera system is used to detect the contact angle, which creates a monochrome picture for curve assessment on a computer. The pendant drop method, which enables the measurement of the liquid’s surface tension or interfacial tension, also uses DSA [42, 44].

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12 Factors Affecting the Morphology of Screen-Printed Thin Film Screen-printed thin films may have different morphologies, microstructures, and densifications depending on the different parameters involved in the process of screen printing. They are mesh number (count), film thickness, binder, sintering temperature, and sintering time.

12.1 Mesh Number (Count) Chen et al. [43] varied the mesh number from 200 to 400, affecting the surface architecture of the screen-printed fluorescent layer. Although the fluorescent powder particles are homogeneously dispersed in the created films, the film surface becomes uneven with a decreasing mesh number due to the penetration of larger particles into the mesh. With a 200 mesh density, the fluorescent film presents air bubbles and a rough surface [43]. When employing the screen-printing procedure to manufacture porous film, the mesh screen size number is crucial. The number of mesh screens used can have a big impact on the microstructure of the film. Films created using a 100- and a 140-mesh screen exhibited less homogeneity of the pores than those prepared with a 120-mesh screen [21].

12.2 Film Thickness A polyethylene terephthalate (PET) substrate was employed by Ali et al. [27] to coat a silver (Ag) thin layer and examine the surface morphology and microstructural behavior as depicted in Fig. 8. The images labelled a, b, and c belong to 200, 400, and 800 nm films, correspondingly. The morphologies revealed rougher surfaces and exhibited nanostructured granules with a size distribution of 200–300 nm. Owing to the printing mechanism, sample (b2) is flatter than sample (c3) as increasing thickness yields less pressure on the squeegee and a low angle (θ) between both the squeegee and the screen [44], and surface roughness rises as pressure and angle (θ) diminish [27].

12.3 Binder The specified route factors had a significant impact on the structure of the film, according to SEM findings [21]. The microstructure of the film would be affected

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Fig. 8 Micrographs of Ag thin films on PET substrates at different film thicknesses (a, a1) 200 nm, (b, b2) 400 nm, and (c, c3) 800 nm [27]

by the binders chosen. Ethyl cellulose, polyvinyl-butyral, and binders for screen printing films were tested. In Table 1, the parameters of the manufacturing process were presented (sample A for ethyl cellulose as a binder and sample B for polyvinylbutyral as a binder). Surface morphology studies were employed to investigate the microstructures of electrode films (Fig. 9). When ethyl cellulose was used as the binder, the electrode film showed a uniform pore size distribution (Fig. 9a). The sample utilizing polyvinyl-butyral as the binder revealed a thick layer with only around 10% porosity (Fig. 9b). The obtained layer seemed to have a homogenous pore size distribution with 30–40% porosity (measured from SEM), and the binder was ethyl cellulose (Fig. 9a). The film was thick and had only around 10% porosity when polyvinyl-butyral was used as the binder (Fig. 9b). To achieve fine and uniform microstructures, ethyl cellulose was shown to be the best binder due to its linear structure [21].

12.4 Sintering Temperature and Sintering Time The microstructure of the film formed is greatly influenced by the sintering temperature. Figure 10 depicts micrographs of films sintered at different temperatures (1150– 1300 °C) (sample A photo in Fig. 10a). When the sintering temperature was reduced, finer pores and tiny particles appeared in the deposited layer. Sample B showed a low porosity surface (Fig. 10b; sintered at 1250 °C) (99.7%, Sigma–Aldrich) was used to dissolve 2–3 wt. percent poly(styrene sulfonic acid) sodium salt. To thoroughly dissolve the solute, the solution was placed in an ultrasonicator at ambient temperature for an hour. To improve the viscosity, 0.1% hydroxypropyl cellulose was added, and the gel was Table 2 Screen printing processing, ink drying and sintering parameters at (squeegee angle 45°) Parameters

Values

Ink material

Drying Time (min) Heat treatment temperature (°C) temperature (°C)

Flood speed

70 mm/min Carbon

120

5

130

Print speed

70 mm/min Ag

120

5

130

Snap off

2 mm

PEDOT: PSS 100

5

130

PEI

10

90

Squeegee load 10 kg

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then spread on a flat surface and dried to create a film. The electrodes were covered with an electrolyte layer (sodium styrene sulfonate—PSSNa) in the form of a gel or a pre-cast film with a thickness of about 3 μm. The battery was kept together with a clip after the cathode and anode were printed, and the film electrolyte was inserted between these two electrodes. After that, the battery was ready for testing in a stack arrangement. The capacity of the batteries was investigated during a number of charge–discharge cycles, with the findings displayed in Fig. 21d. There has been created a screen-printed rechargeable, flexible, all-polymer battery with an essentially constant open-circuit voltage and an active area of 400 mm2 that can charge and discharge with a realistic specific capacity of 5.5 mAh/g and with an average thickness of roughly 440 μm. A screen-printed nickel metal hydride (Ni-MH) rechargeable battery with a capacity of 32 mAh has been created using thin, flexible roll-fed plastic materials [100]. Screen-printing a zinc/carbon/polymer composite anode and a vapor polymerized PEDOT cathode onto two sides of photo-grade paper created a newly invented flexible zinc-air battery [101].

16.7 Supercapacitor 16.7.1

Activated Carbon/Silver Nanocomposite Supercapacitor Electrode

Asrar et al. [54] explored screen-printed activated carbon (AC) with silver nanopowder (Ag) as a supercapacitor active electrode material. Figure 22 shows a strategy for making an AC/Ag composite supercapacitor using a screen-printing procedure. To make the AC/Ag ink, AC (80%) and Ag nanopowder (15%), PVDF (5%), and N-methyl-2-pyrrolidinone (NMP) were employed as active materials, binder, and solvent, sequentially. The components were appropriately combined in a mortar and pestle to obtained a uniformly blended slurry ink (Fig. 22 a). The produced slurry ink was screen printed on the stainless-steel (SS) substrate with the help of mesh (Fig. 22b, c) and baked at 120 °C for 2 h. The prepared screen-printed AC/Ag electrode was put through different characterizations such as SEM, AFM, and contact angle studies for morphological, surface roughness (uniformity), and surface wettability analysis, as they are very important to understand supercapacitor electrode application. The active composite electrode material has a spherical particle-type morphology, as seen in the FE-SEM pictures (Fig. 23a). EDX examination of the AC/Ag composite ink reveals the presence of C and Ag (Fig. 23b). At the interface between electrode and electrolyte, surface wettability is critical. The non-uniformly dispersed gritty particles of the printed electrode material are seen (Fig. 23c) in this FIB picture of the 3D structure of the screen-printed AC/Ag. Figure 23d shows an AFM image of the AC/Ag composite, which reveals tightly

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Fig. 22 Shows a schematic of synthesis of AC/Ag composite a Ink preparation, Screen printing b process, c of SS/AC/Ag substrate, and d CV characteristic outcome of supercapacitor [54]

Fig. 23 a FE-SEM, b EDX, c FIB, d AFM, and e and f contact angle and surface energy of AC and AC/Ag electrodes [54]

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packed asperities. The average roughness and surface root mean square roughness were determined to be 56 and 70 nm, respectively. Contact angle parameters were measured with deionized water to examine electrode-electrolyte interactions. For AC/Ag and AC, the contact angles were determined to be 43 and 76°, respectively (Fig. 23e–f). Furthermore, surface energies estimated using the Owens–Wendt Approach indicated that AC/Ag and AC have surface energies of 63.70 and 50.60 mJ m−2 , respectively. The inclusion of Ag nanopowder in the AC/Ag electrode enhanced the depth of electrolyte ion intercalation, as evidenced by a decrease in contact angle and a lower surface energy value of the AC/Ag electrode when compared to the AC, both of which are advantageous to the AC/Ag composite’s supercapacitive performance [102]. Figure 24a depicts the comparative CV curves of the AC and AC/Ag electrodes; Fig. 24b depicts scan rate variance; Fig. 24c, d depict GCD plots; and Fig. 24e depicts the AC/Ag electrode’s EIS Nyquist plot. At a significant current density of 1 A/g, the screen-printed electrode had a high specific capacitance of 194 F/g. Eventually, the screen-printed AC/Ag composite electrode’s long-term sustainability was tested for 1000 GCD cycles at 1 A/g (Fig. 24f), and it preserved 89% of its start capacitance value.

16.7.2

Micro-Supercapacitors

Li et al. [56] have demonstrated an overall and repeatable screen-printing method for the one-step development of the increased performance of printed microsupercapacitors (MSCs), using a thixotropic blended ink of hydrous ruthenium oxide (RuO2 ) nanomaterials, silver nanowire (AgNW), and graphene oxide (GO) as interdigital in-plane microelectrodes. Printable inks produced from high-conductivity graphene nanomaterials, including reduced graphene oxide (rGO) and liquidexfoliated graphene, have in recent times been used to make fully printed MSCs. Li et al. [56] created a unique thixotropic ternary nanocomposite-based electrode ink that has all of the necessary features to address the two basic challenges of restricted electrochemical functionality and reduced printing quality for completely printed MSCs. Silver nanowires (AgNWs) in combination with graphene oxide (GO) nanosheets and hydrous ruthenium oxide (RuO2 .xH2 O) nanomaterials not only provide high viscosity and suitable rheological properties for the electrode ink deprived of any non-active additives but also create a nano-porous structure that increases the ion-accessible surface and ion-diffusion channel. Furthermore, the uninterrupted AgNW network can produce a strongly conducting electron channel to enhance charge transfer, rGO can ensure efficient rate capability and strong cyclic stability, and RuO2 may increase the printed electrode’s volumetric capacitance. With these advantages, consistent designs with the least pores size of 50 μm and electrical conductivity as good as 5,000 S cm−1 may be achieved without even any additional post-treatment by screen-printing the thixotropic ternary electrode ink. Figure 25a shows a representation of the screen-printing procedure used to create fully-printed wearable MSCs on a paper substrate. AgNW was chosen because of its

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Fig. 24 a CV curves of AC and AC/Ag at 25 mv/s, b Scan rate variation of AC/Ag, c GCD curves, d current density vs. specific capacitance, e EIS nyquist plot and stability studies of AC/Ag electrode in 1 M Li2 SO4 electrolyte [54]

good conductivity, nanonetwork structure, and potential to create a thixotropic solution without the need for any chemicals when coupled with GO. GO nanosheets are used as a dispersant, thickener, and stabilizer. Hydrous RuO2 with pseudocapacitive and rGO EDLC behaviors exhibited a decent rate and cyclability. In simple terms, three main electrode ink production stages are followed: homogeneous mixtures of Ru(OH)3 nanoparticles, AgNWs, and GO; vacuum filtration and well-controlled ternary nanocomponent gel formation. At the fixed shear rate, the inks with increased Ru(OH)3 concentration have higher viscosity; yet, all composite inks exhibit effective shear-thinning behavior. Whenever the shear rate is increased from 0.1 to 100 s−1 , the viscosity of the electrode

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Fig. 25 a Schematic of screen-printed flexible MSCs devices production all-through functional nano components b Ink-4 viscosity Vs shear rate. c The ink’s rheological behavior during the screenprinting procedure. d Variation of storage (G' ) and loss (G'' ) moduli as a function of oscillation strain [56]

ink drops rapidly from 170.55 to 0.68 Pas (Fig. 25b). After shearing for 30 s at a shear rate of 100 s−1 , the ink’s timely lowered viscosity could swiftly recover to its original value in a few seconds as the shear rate went up and recovered to 0.1 s−1 (Fig. 25c). Furthermore, oscillatory rheological observations for the electrode inks were performed in a stress sweep step (SSS) test to better define their viscoelastic impact (Fig. 25d). Using fully printed electrodes on paper without metal current collectors or polymer binder, as exhibited in Fig. 26 a and b, is a reliable way to produce flexible MSCs on paper since the electrode inks and the printed electrodes are both extremely printable as well as extremely flexible and conductive. Optical images in Fig. 26c and optical microscopy images in Fig. 26d show that the interdigital electrodes in RArG–4 MSCs can be printed with various widths (RArG–4–200, RArG–4–150, RArG–4–100, and RArG–4–50, where the number denotes the width in microns) [56]. Furthermore, the polymer gel electrolyte (KOH + PVA) was poured over the interdigital pattern region of the MSCs, wetting the electrode thoroughly. RuO2 – AgNWs–rGO–(RArG)-based flexible MSCs were generated when the surplus water was evaporated under atmospheric circumstances and the electrolyte dried into a gel. RArG–4 MSCs may be produced in a variety of sizes (RArG–4–200, RArG4–150, RArG–4–100, and RArG–4–50, with the number representing the widths in microns). At a current density of 0.08 mA cm−2 , the areal and volumetric specific capacitances of the completely printed devices had been measured to be 10.4 mF cm−2 and 135.6 F cm−3 respectively, based on GCD measurements (Fig. 27a). At a higher current density of 6.3 mA cm−2 (Fig. 27b), the fully-printed MSCs can withstand higher areal and volumetric specific capacitances of 2.5 mF cm−2 and 32.7 F cm−3 ,

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Fig. 26 a-d: Photographs of a flexible interdigitated RArG–4 MSCs array printed on filter paper substrates in two states: flat and bent and a single MSC with various electrode widths. In (a) and (b), the scale bar is 1 cm, and in (d), the scale bar is 500 μm [56]

respectively, indicating the extraordinary rate capability of the completely RArG– 4–200 MSCs. Figure 27c shows the comparison of volumetric capacitance with preexisting literature data. RArG–4–200 MSCs exhibited stability of 91.6% even after 8000 CV cycles (Fig. 27d). Figure 27e shows the Ragone plot of the RArG–4200 MSC device in comparison with pre-existing literature data. The electrochemical behavior of RArG–4–200 MSC was examined at varied bending strains to highlight its potential uses in wearable energy storage devices. Even with a considerable bending strain of 0.6%, the capacitance maintains 80% of its initial condition (Fig. 27f). The mechanical resilience of our fully printed MSC devices is demonstrated by the fact that the RArG–4–200 MSC can preserve 88.6% of its original capacitance after 2,000 bending cycles at a bending strain of 0.6% [56].

16.8 Wearable Supercapacitors The future belongs to technologies that can be worn. This kind of technology is becoming more and more necessary for wearable screens [103]. Using a screenprinting process with graphene oxide as an ink, Abdelkader [104] and colleagues presented a solid-state flexible supercapacitor device printed on a cloth. Due to its flexibility, biocompatibility, stability, and durability, textile is a great substrate for

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Fig. 27 GCD curves at current densities a 0.08 to 0.84 mA cm2 and b 1.26 to 6.30 mA cm2 . c Volumetric specific capacitance, d CV stability at 100 mV/s, e Ragone plot of the RArG–4– 200 MSC device compared to other MSCs by printing techniques. f Capacitance retention of the RArG–4–200 MSC through bending cycles with a 0.6% bending strain [56]

wearable devices. The association between the cotton fabric and the hydroxyl of graphene allowed the imprinting of graphene onto the fabric with highly porous properties and mechanical stability using the screen-printing method. Graphene oxide (GO) ink was created by blending graphene oxide, water, and acrylate as a thickener. The cotton cloth with the ink screen printed on it was allowed to dry for five minutes at 100 °C. By employing an electrochemical process, graphene oxide was reduced from GO (rGO). The procedure used to make rGO screen printing on cotton cloth is shown schematically in Fig. 28. The consistent coating of rGO over cotton fibers can be seen in the SEM picture in Fig. 29a. A symmetric device made of two graphene cotton electrodes and an H2 SO4 polyvinyl alcohol gel electrolyte was evaluated for supercapacitor performance. The electrochemical double-layer capacitance behavior of the device was confirmed by cyclic voltammetry and galvanostatic charge–discharge tests (Fig. 29b, c) [104].

16.9 Anti-reflection Coating (ARC) The problem with solar cells is often that reflection causes direct incident radiation intensity to diminish by more than 30% on the glossy silicon wafers [105]. The penetration of incoming light into solar cells is increased with an anti-reflection coating (ARC). Such ARC coatings can be applied using a straightforward screenprinting deposition method. Figure 30 shows the ARC application in solar cells. A thin layer of titanium oxide with a quarter-wavelength anti-reflective coating was created by the Boukennous et al. [8] team using a screen-printing process on

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Fig. 28 Schematic of screen printed werable supercapacitor process [104]

a silicon substrate. The qualities of the films of TiO2 and Ta2 O, which exhibit the lowest reflectivity on the polished C–Si surface, make them acceptable for ARC application. Using terpineol as the solvent and octyphenoxypolythoxy as the carrier, titanium ethoxide was utilized as an organometallic compound in the ink. Their core philosophy was always to regulate the viscosity of the ink and other variables to evaluate the impact on the imprinted layer. The film thickness was controlled by varying the ink viscosity, heat treatment, and parameters of the screen-printing process. Boukennous et al. [8] examined numerous combinations of screen-printing parameters, including squeegee pressure, squeegee speed, printing gap, viscosity, and firing temperature. At a squeegee speed ranging from 25 to 75 mm/s, the thin layer of 700 Å was attained. TiO2 films were fired at 550 °C to create the polycrystalline phase. The firing temperature was increased from 550 to 700 °C, which caused a rise in the refractive index. The refractive index was determined to be 2.2 at 550 °C and 2.3 at 700 °C. Using a firing temperature of 550 °C, a minimal reflection coefficient of 2% was attained at 600 nm and a minimal reflection coefficient of 3% at 639 nm at a temperature of 700 °C. A firing temperature of 700 °C is advantageous since it can also be utilized for metallization.

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Fig. 29 a CV of a device using different electrolytes b GCD curves of a device using different electrolytes c SEM image of an rGO coated cotton fiber [104]

Fig. 30 Anti-reflection coating for solar cell applications [113]

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Electrochromic Display (ECD)

Wearable electronics are gaining popularity due to their adaptability. Some advantages of wearable electronics are their light weight, simplicity of usage, range of functions, etc. In general, information-visualizing displays are needed for electronic devices and the internet of things. Stretchable displays are a major challenge for wearable electronics. Wearable displays ought to be secure for the wearer, flexible, strong, and comfortable. Stretching and bending shouldn’t have an impact on its performance. A lot of research is being done by many researchers on a variety of display technologies. Appropriate display technology should be selected based on the required output specifications. Electrochromic displays are advantageous due to their low manufacturing costs and low energy consumption, allowing them to be employed in a variety of applications. ECDs operate in a very simple manner; the materials utilized to produce them undergo a reversible electrochemical reaction that causes them to change colour when voltage is applied. Utilizing electrochromic PEDOT: PSS inks, Linderhed [10] and the team created a stretchy ECD display with screen printing. PEDOT: PSS films exhibit a translucent shade in an oxidized state and a vivid blue colouring in the reduced state. These behaviors aid in the development of high-contrast, backlit devices. Screen printing is appropriate for this type of application because the pixels in ECDs range in size from millimeters to decimeters [106]. As a result, Linderhed et al. [10] employed a screenprinting approach with widely viable PEDOT: PSS inks to create seven-segment cell ECD displays. They created two-digit, seven-segment screens. As illustrated in (Fig. 31a, b, c), these displays were shaped into spherical surfaces and distorted with elastic fabric. Linderhed et al. [10] studied different strain-induced effects on the ECD display devices by applying a range of strains 10, 20 and, 30% at least 500 times in a uniaxial direction. Different countable parameters were recorded (switching time, contrast values) and compared those parameters of ECD displays with no strain and different strain applied conditions in uniaxial direction at least 500 times, those results are depicted in Fig. 32a–d. Maximum colour for cycled and uncycled samples did not differ noticeably (Fig. 32b), and the retention time of the pixels did not differ either (Fig. 32c).

16.9.2

Piezoelectric Touchscreen

As technology advances quickly, new, inexpensive, straightforward, and simple-toproduce materials and gadgets are needed. The piezoelectric phenomenon expands the scope of research by enabling a variety of applications based on force, pressure, and electricity. They are being researched for actuator and sensor applications as a result [107]. A piezoelectric effect can be seen in some semicrystalline polymers. Polyvinylidene fluoride (PVDF) and its copolymers are the most suitable polymers for use in piezoelectric applications owing to their high piezoelectric d33 coefficient, which ranges from −18 to −32 pC/N [108]. By using screen

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Fig. 31 a Flexible 7 segment display b Display conform to the spherical surface in off condition, c Display in on condition conform to the spherical surface [10]

Fig. 32 a Switching time for display after 500 cycles b For cycled sample maximum color contrast, c Times for color contrast retention, d Uniaxial stretched setup [10]

printing, doctor blade printing, and spray printing with this ink, Goncalves et al. [60] produced piezoelectric films. They investigated the use of these films with actuators, sensors, and touch screens. The highest piezoelectric coefficient was found in screen-printed films, which was |d33 | = 19 pC/N. This coefficient held steady for more than six weeks. Utilizing screen-printed films on screen-printed silver electrodes, a variety of touchscreen buttons were created. These touchscreen button arrays were customizable.

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Fig. 33 a Dielectric constant of the films deposited with different methods b tan δ as a function of frequency, c 2 × 3 touch screen sensor matrix, d d33 coefficient for films with time [60]

As a result of their sensitivity and accuracy, piezoelectric polymer touchscreens exhibit superior qualities to capacitive touch sensors. A multi-touch 2 × 3 sensor matrix was constructed by Goncalves et al. [60] Water-based silver nanoparticle ink was used to screen print the electrodes. Then, using the screen-printing process, a 30 μm thick PVDF–TrFE film with a weight of 23 wt.% was deposited. The film was fired to 80 °C for an hour to dry the ink. A thin, elastic touch screen was made by covering an insulating adhesive with a protective coating. The fabricated gadget is shown in Fig. 33c. The dielectric and piezoelectric behavior of the screen-printed films with wt.% of 17 and 23, doctor blade films, and spray printing films were compared. Figure 33a show dielectric behavior of the films, while Fig. 33b represents tan δ as a function of frequency. The higher d33 coefficient was obtained for screen-printed films among all the produced films. (Fig. 33d) shows the d33 coefficient and its stability with time for films deposited with different methods.

17 Conclusions To summarize, screen printing is a straightforward procedure that demands expertise, knowledge, and the use of the appropriate equipment to produce high-quality results. Screen printing has long been a widespread process for swiftly and simply

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producing various patterns. Several aspects of the screen-printing process, such as mesh type, squeegee, ink viscosity, raster spacing, and so on, influence the quality of the final product. The material to be printed is just as crucial as the screen-printing process’s unique characteristics. The quality of screen-printed materials is influenced by the composition, structure, and characteristics of the printed goods, as well as the composition, viscosity, and other ink qualities (rheology). Screen printing is, the most adaptable of all printing methods. It can print on a broad range of materials, like paper, chipboard, plastics, glass, metals, textiles, ceramics, and many more. Screen printing is now faster, less expensive, creates higher-quality prints, and has a variety of uses. Screen printing appears to have come a far towards quality. With today’s automated precise machinery, it’s feasible to achieve results that would have been unthinkable only a few years ago. Screen printing is the preferred method for printing electronic components, printed electrodes, sensors, battery electrodes, supercapacitor electrodes, and fuel cells; the properties of screen printing that make it so popular are strongly linked to the rheology of screen-printing inks and the processing parameters discussed in this chapter. Throughout this chapter, we explored how different screen-printing parameters affected the morphology of the electrodes.

18 Future Perspective The development of portable energy conversion devices, the internet of things, electronic gadgets, and energy storing devices forced the industry to miniaturize of these devices. Hence, screen-printed electronic devices are produced using ultrathin film nanomaterials. Lithography, template filtration, spray masking, and laser scribing are used in the electronics sector. These processes are complicated and produce large amounts of waste. Screen printing can address all these problems effectively. Therefore, screen printing plays an important role in the production of electronic devices [109]. Screen-printed films can be flexible, so they are researched for different flexible electronics. Flexible electronics completely revolutionize robotics, the medical sector, the fabric industry, digital screens, etc. [34]. Besides many admirable properties, there is a requisite to improve the quality of the printed pattern. The quality of the pattern can be significantly improved by developing inks with good rheology and wetting properties [110]. Due to this, sediment inks have attracted the focus of researchers [111]. In recent times, liquid metals have also been researched extensively and can be used for the formation of printed thin films [112]. In the right circumstances and with the right inks, screen printing can be an incredible technology used in many industries, for a wide variety of old, new, and yet unimagined purposes—both on flat and non-flat surfaces. One issue with screen printing is the printing resolution, which is affected by several factors, as follows:

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(i) Screen mask characteristics are critical, including metal mesh mechanical strength, emulsion viscosity, mesh size, and aperture ratio. By raising the efficiency of the screen masks, 6 μm resolution screen printing has recently been reported. (ii) Printing resolution is influenced by the ink and substrate used. Nanoparticles, binders, and solvents make up the majority of screen-printing inks. The printing resolution is influenced by the diameter of the nanoparticles and the affinity of the ink to the substrate. Overall, screen printing offers a lot of potential for future high-resolution printing.

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90. S. Ito et al., Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%. Thin Solid Films 516(14), 4613–4619 (2008). https://doi. org/10.1016/J.TSF.2007.05.090 91. L. Vesce, R. Riccitelli, Processing and characterization of a TiO2 paste based on small particle size powders for dye-sensitized solar cell semi-transparent photo-electrodes. Prog. Photovoltaics Res. Appl. 20(8), 960–966 (2012). https://doi.org/10.1002/PIP.1166 92. M.R. Somalu, N.P. Brandon, Rheological Studies of Nickel/Scandia-Stabilized-Zirconia Screen Printing Inks for Solid Oxide Fuel Cell Anode Fabrication. J. Am. Ceram. Soc. 95(4), 1220–1228 (2012). https://doi.org/10.1111/J.1551-2916.2011.05014.X 93. J.Z. Jeffrey Fergus, R. Hui, X. Li, D.P. Wilkinson (eds.), Solid Oxide Fuel Cells: Material Properties and Performance—Anodes. Routledge Taylor and Francise Group (2009). Accessed 19 Mar 2022. [Online]. Available: https://www.routledge.com/Solid-Oxide-Fuel-Cells-Materi als-Properties-and-Performance/Fergus-Hui-Li-Wilkinson-Zhang/p/book/9780367386436 94. K.H. Tan, H.A. Rahman, H. Taib, Coating layer and influence of transition metal for ferritic stainless steel interconnector solid oxide fuel cell: a review. Int. J. Hydrogen Energy 44(58), 30591–30605 (2019). https://doi.org/10.1016/J.IJHYDENE.2019.06.155 95. Y. Zhang et al., A screen-printed Ce0.8Sm0.2O1.9 film solid oxide fuel cell with a Ba0.5Sr0.5Co0.8Fe0.2O3-δ cathode. J. Power Sources 160(2), 1217–1220. SPEC. ISS (2006). https://doi.org/10.1016/j.jpowsour.2006.02.048 96. J.F.M. Oudenhoven, R.J.M. Vullers, R. van Schaijk, A review of the present situation and future developments of micro-batteries for wireless autonomous sensor systems. Int. J. Energy Res. 36(12), 1139–1150 (2012). https://doi.org/10.1002/ER.2949 97. J.B. Bates, N.J. Dudney, B. Neudecker, A. Ueda, C.D. Evans, Thin-film lithium and lithiumion batteries. Solid State Ionics 135(1–4), 33–45 (. 2000). https://doi.org/10.1016/S0167-273 8(00)00327-1 98. Ultrathin lithium rechargeable battery|Printed Electronics World. http://www.printedelectron icsworld.com/articles/1219/ultrathin-lithium-rechargeable-battery. Accessed 14 Mar 2022 99. Thin Film Batteries. https://www.slideshare.net/HarshadKarmarkar/thin-film-batteries. Accessed 14 Mar 2022 100. M. Wendler, G. Hübner, I.M. Krebs, Development of printed thin and flexible batteries. Int. Circ. Graph. Ed. Res 4, 32–41 (2011). [Online]. Available: https://www.internationalcircle. net/circular/issues/11_01/ICJ_04_32_wendler_huebner_krebs.pdf 101 M. Hilder, B. Winther-Jensen, N.B. Clark, Paper-based, printed zinc–air battery. J. Power Sour. 194(2), 1135–1141 (2009). https://doi.org/10.1016/J.JPOWSOUR.2009.06.054 102. M.M. Vadiyar et al., Contact angle measurements: a preliminary diagnostic tool for evaluating the performance of ZnFe2 O4 nano-flake based supercapacitors. Chem. Commun. 52(12), 2557–2560 (Feb.2016). https://doi.org/10.1039/C5CC08373G 103. D.H. Kim, J.A. Rogers, Stretchable electronics: materials strategies and devices. Adv. Mater. 20(24), 4887–4892 (2008). https://doi.org/10.1002/ADMA.200801788 104. A.M. Abdelkader, N. Karim, C. Vallés, S. Afroj, K.S. Novoselov, S.G. Yeates, Ultraflexible and robust graphene supercapacitors printed on textiles for wearable electronics applications. 2D Mater. 4(3), 035016 (2017). https://doi.org/10.1088/2053-1583/AA7D71 105. G.E. Jellison, R.F. Wood, Antireflection coatings for planar silicon solar cells. Sol. Cells 18(2), 93–114 (1986). https://doi.org/10.1016/0379-6787(86)90029-3 106. S. Santiago-Malagón, D. Río-Colín, H. Azizkhani, M. Aller-Pellitero, G. Guirado, F.J. del Campo, A self-powered skin-patch electrochromic biosensor. Biosens. Bioelectron. 175, 1–9 (2021). https://doi.org/10.1016/j.bios.2020.112879 107. S. Tuukkanen, S. Rajala, A survey of printable piezoelectric sensors, in 2015 IEEE Sensors– Proceedings (2015). https://doi.org/10.1109/ICSENS.2015.7370542 108. P. Martins, A.C. Lopes, S. Lanceros-Mendez, Electroactive phases of poly(vinylidene fluoride): determination, processing and applications. Prog. Polym. Sci. 39(4), 683–706 (2014). https://doi.org/10.1016/J.PROGPOLYMSCI.2013.07.006 109. G. Hu et al., Functional inks and printing of two-dimensional materials. Chem. Soc. Rev. 47(9), 3265–3300 (May2018). https://doi.org/10.1039/C8CS00084K

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110. P.K. Chakraborty et al., Advancements in therapeutics via 3D printed multifunctional architectures from dispersed 2D nanomaterial inks. Small 16(49), 2004900 (2020). https://doi.org/ 10.1002/SMLL.202004900 111. Y. Wang, Y.Z. Zhang, D. Dubbink, J.E. ten Elshof, Inkjet printing of δ-MnO2 nanosheets for flexible solid-state micro-supercapacitor. Nano Energy 49, 481–488 (Jul.2018). https://doi. org/10.1016/J.NANOEN.2018.05.002 112. K. Kalantar-Zadeh et al., Emergence of liquid metals in nanotechnology. ACS Nano 13(7), 7388–7395 (2019). https://doi.org/10.1021/ACSNANO.9B04843/ASSET/IMAGES/ ACSNANO.9B04843.SOCIAL.JPEG_V03 113. X. Li, J. He, W. Liu, Broadband anti-reflective and water-repellent coatings on glass substrates for self-cleaning photovoltaic cells. Mater. Res. Bull. 48(7), 2522–2528 (2013). https://doi. org/10.1016/J.MATERRESBULL.2013.03.017

Dr. Lakshmana Kumar Bommineedi serving as a postdoctoral Researcher at Ariel University, Israel under supervision of Dr. Rafael Minnes. He completed his Ph.D. from the Department of Physics, Visvesvaraya National Institute of Technology (VNIT), Nagpur (India); Institute of National Importance, under the supervision of Dr. B.R. Sankapal. He receives his master’s degree in physics from Acharya Nagarjuna University, Namburu, Andhra Pradesh (India) in 2014. He worked in a DST-TMD project entitled “Flexible solid-state supercapacitor device” in VNITNagpur campus in collaboration with an industry SPEL technologies, India for 3 years as a research fellow (Physics department) and successfully completed. He has 9 research articles in reputed international SCI/SCIE journals, Journal of Energy Chemistry, Sustainable Energy & Fuels, International Journal of Hydrogen Energy, Ceramics International, Journal of Alloys and Compounds, Synthetic Metals, and Inorganic Chemistry Communications. I have participated in Second International Meeting on Clean Energy Materials Innovation Challenge (IC6)-2019 conducted by DST, Govt. of India. He had participated and presented a poster in Proceedings of the international meeting on energy storage devices (IMESD-2018) and industry-academia conclave, conducted by IIT-Roorkee, India. He has research expertise in the synthesis of nanomaterials, thin films, and carbon composite films of different semiconductors like metal oxides, and metal chalcogenides by using simple and low-cost chemical methods for supercapacitor and solar cell applications. He has expertise in operating instruments electrochemical workstation (PARSTAT, AUTOLAB, and GAMRY), UV-VIS/NIR spectrophotometer (JASCO v770), and magnetron sputtering. Mr. Nakul Upadhyay is a Ph.D. student in the Department of Physics, Visvesvaraya National Institute of Technology (VNIT), Nagpur, India. His area of interest is advanced and green energy storage devices. He is trying to improve electrode materials for supercapacitors in his doctoral research using chemical methods. He completed his master’s degree from the University of Nagpur. In his master’s project, he developed hybrid supercapacitor electrode material. He succeeded in qualifying National Eligibility Test for lectureship (NET-LS-2020), Graduate Aptitude Test in Engineering 2021 (GATE-2021), and State Eligibility Test For Assistant Professor (Maharashtra). He also worked as an adjunct professor at Anand Niketan Science College, Warora from October 2021 to December 2021. Dr. Rafael Minnes is a Senior Lecturer in the Department of Physics, Ariel University, Israel. He receives his Ph.D. degree from Bar Ilan University (Israel) in 2008. He worked abroad for more than 4 years as a HFSP-Postdoctoral Fellow at the University of Pennsylvania, USA. Dr. Minnes has 10 years of teaching and 19 years of research experience. In Ariel University, Dr. Minnes serves as the Head of the Bio-electromagnetism Laboratory and the Head of the Physics Teaching Certificate Program. As a faculty at Ariel University, he successfully received grants of more than $ 400K. Dr. Minnes was elected as an outstanding young scientist to participate in the

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60th Meeting of “The Lindau Nobel Laureate” in Lindau, Germany-2010. Dr. Minnes is supervising 4 Ph.D. and a post-doctoral researcher. On his credit, he has 16 publications with an average impact factor of 3.2 with Google Scholar h-Index 8 with above 200 citations. Dr. Minnes has research expertise in spectroscopy and microscopy, medical physics, synthesis of nanomaterials, solar cells, and supercapacitors.

Chapter 12

Doctor Blade: A Promising Technique for Thin Film Coating Ganesh C. Patil

Abstract Doctor blade coating (tape casting) is the most widely employed solution processing technique for large-area thin film fabrication at low cost. The term ‘doctor blade’ coating was originally coined in 1940 and was particularly known to form thin sheets of piezoelectric materials and capacitors. The basic working principle of this process relies on continuous relative movement between the blade and the substrate either through running a blade over the substrate or moving the substrate underneath the blade. In this process, a well-mixed coating solution is positioned over the substrate in front of the blade and when a relative motion amongst blade and carrier surface is established, the prepared slurry effectively spreads over the substrate surface creating a film of wide-ranged thicknesses upon evaporation. The final thickness uniformity and homogeneity is greatly influenced by the design of the doctor blade unit and the viscoelastic behavior of the coating precursor, which needs to be carefully monitored. This chapter deals with two different generally used blade coating techniques, their constructions, working, strengths, and weaknesses as well as current research status. In particular, the chapter presents a detailed study of doctor blade-coated films in solar cell application with an aim to provide a clear understanding of the process in regards to its versatility, scalability, and simplicity for future research-oriented opportunities.

Abbreviations ρt ρg ρ solute Φconc θE hblade

Solute concentration of the solids Density of the material Density of ink paste Concentration of the ink paste Surface Contact angle Blade height

G. C. Patil (B) Center for VLSI and Nanotechnology, Visvesvaraya National Institute of Technology, South Ambazari Road, Nagpur, Maharashtra 440010, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 B. R. Sankapal et al. (eds.), Simple Chemical Methods for Thin Film Deposition, https://doi.org/10.1007/978-981-99-0961-2_12

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vblade υ η γ F S t dry t wet x. DB DSSC ETL HTL OSC OTFT PVSC

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Blade velocity Coating speed Viscosity Surface tension Force applied Surface energy Layer thickness Wet paste film thickness Separation distance between top plate and the fixed bottom plate Doctor blade Dye sensitized solar cell Electron transport layer Hole transport layer Organic solar cell Organic thin film transistor Perovskite solar cell

1 Introduction 1.1 Definition Doctor blade is mainly defined as the popular technique for creating thin layer films over small to large area surfaces. This coating process is widely used in thin film depositions and has been initially established in the 1940s as an easy way to form thin films related to piezoelectric and capacitors materials [1]. One patent, released in 1952, has been targeted at making the usage of the aqueous and non-slurries that certainly can be used for doctor blading [2]. Slurry is well-mixed with the ceramic particles, and extra constituents like binders have been used to bind the particles, and dispersants or plasticizers to enhance the properties. The slurry is usually positioned on the substrate in front of the doctor blade which is used in the blading procedure. Each and every time a motion is established between the blades while the substrate, the slurry eventually covers the surface of the substrate yielding a thin layer coating once the gel gets dried. The process can run at an accelerated genuine level of meters every moment from single to multilayer coating which can range from some microns to several. The doctor blade method is simple, versatile, less expensive, and possible to integrate with roll-to-roll technology. This method is widely explored to coat thin layers for a variety of thin films for widespread applications including energy conversation, energy storage, sensors, microelectronics, photocatalysis, and many more. Figure 1 depicts a schematic representation of the doctor blade technique. A sharp edge blade is placed at one end of the substrate where the coating is necessary to start. Desired

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Fig. 1 Schematic illustration of the doctor bladingc

ink with proper viscosity is placed in front of the blade, and the blade can move on the substrate at a constant speed. Eventually, the ink gets spread uniformly over the substrate surface. Ink can be fallen automatically on the substrate while watching the blade. Distance between the blade and substrate, surface properties of the substrate, viscosity, and surface tension of the precursor used leading to ink plays an important role while deciding the surface wetting of the deposited layer. The width of the sample, blade-to-substrate distance, and the viscosity are internally correlated while defining the film thickness. This process utilized the ink close to a hundred percent and hence, the efficiency of this process is high as compared to the spin coating technique. As it is a moving blade on the substrate which can be a microprocessor or computer controlled and hence, can be adapted easily in the roll-to-roll process and is suitable for mass production.

1.2 Working Principle The doctor blade layer can also be referred to as the knife blade or coating finish. The principle is simple: a blade lies at a distance, ho , above the area. While watching the blade, the precursor is added then either the substrate or the blade moves at a rate that is constantly leaving a precise layer thickness behind. The mathematical estimation for the last layer that is coated t dry is expressed as [3] tdr y =

) ( ρt 1 h0 2 ρg

(1)

where ρ t is the solute concentration of the solids in the precursor in g/cm3 and ρ g is the density of the material in the final film in g/cm3 . It can be seen from Eq. (1) that the ultimate thickness, t dry , is not influenced by drawn velocity, but closely connected with the leakage associated with the precursor that is coated. The leakage of the precursor and overflow are the major disadvantages

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of doctor blade which are necessary to overcome. This problem can be overcome by changing the blade form and size and setting up a remedy bath for high-viscosity precursors [4]. In the doctor blade coating method, the ink is used over one side of the substrate and then the whole area is dragged by the doctor blade. Due to this kind of coating, the uniform film of the paste has been distributed on the substrate. After drying the paste, a uniform thin film is deposited on the substrate. In this coating process, the liquid paste is uniformly coated between the blade and the substrate surface [5, 6]. In this coating process, the velocity of the paste which is exiting from the knife is depend upon the blade velocity (vblade ) which results in the wet paste film having a thickness (t wet ) and the blade height (hblade ). It has been reported that the thickness of the film (t dry ) is dependent on the concentration of the ink paste (Φconc ) and the ink paste density (ρ solute ). The equation of t dry is given as [3] tdr y =

twet Φconc ρsolute

(2)

1.3 Strengths and Limitations 1.3.1

Strengths

The important strength of the doctor blade is that it deposits the film with a specific thickness. In this method, the distance between the sharp blade and the substrate surface is fixed. The coating solution is then positioned on one of the sides, and the blade is moved across the surface area which creates the thick film on the substrate. The strength of this coating method is that it has less solution losses ideally of about 5%. The pastes used in this process normally require high-viscosity binders. The viscosity can be increased by using the polymeric ingredients. The doctor blade is mainly introduced for depositing the liquid silicon material through the roll-to-roll processing compatible surface. Due to various advantages, this process is also suitable for slot-die coating and could be used for high-speed mass production systems.

1.3.2

Limitations

The important advantage of the doctor blade is the open ink system. However, for higher production of the higher quality devices, this advantage becomes the bottleneck. As we know, the unwanted ink in the doctor blade is removed by using the knife blade. Doctor blade should be carried out in a way that during the deposition there should not be variations in the knife and also the blade durability. The main reasons for the ink disturbances are (1) rheological properties of the ink used in the

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Fig. 2 Coating on plastic foil using Doctor Blade

process, (2) improper use of the blade, and (3) contact between the blade and the substrate. To fix doctor blade problems suitable designs and measures are required. Among the factors which are deciding the color variations could be reduced totally due to doctor blade stress. A few means are ready to accept and accomplish this aim which utilizes a pressure that is elastic in contact with the doctor blade.

2 Equipment and Design 2.1 Doctor Blade (Frame) The frame-based doctor blade (DB) can be utilized along with the liquid reservoir [7]. The doctor blade is stationary when it is used for the moving substrate. Moreover, if the doctor blade is moving then the substrate is stationary. The film thickness of the deposited layer using the doctor blade method can be adjusted by monitoring the gap between the doctor blade and the substrate. The dual doctor blades can also be used in doctor blade coating which leads to precision in the thickness of the coated layers [8]. This kind of coating has also been utilized for plastic foils [9]. Figure 2 shows the position of the blade for this coating method.

2.2 Spiral Film Applicator In this method, the spiral film applicators have also been used. These applicators are mostly used for the flexible polymerized materials having the uneven surfaces. The spiral film applicators are pressed on the substrate. The thickness of the printed paste layer is defined by using the gap between the blade and the surface. It has been found

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that the frame and the spiral coating systems depend upon the viscosity of the ink solution paste.

3 Process Adjustments: Layer Thickness 3.1 Coating Device: Geometry Solution processable material fabrication reduces the price by reducing the various steps utilized in the microfabrication of the devices and the sensors. The materials which are used are insulators, metals, and semiconductors. The typical process used in the solution-processing is spin coating. The inks may be coated for various substrates, including synthetic textiles, making use of roll-to-roll that is scalable printing solution for the large-area sensors. The improved techniques for printing the uniform layered thin films using lower viscosity inks are inkjet printing and the doctor blade [10, 11]. Although this technique is electronic and efficient for quick prototyping, for large surface area fill factors it is limited with some features [12–14]. Instead, the ink is smoothly coated over the substrate by using the doctor blade coating. The appropriate features depend upon the deposited amount of ink, the surface area of the substrate, and the number of capillaries used. This optimization relation is shown in Eq. (3), where υ represents coating speed, η represents viscosity, and γ represents surface tension. In comparison to inkjet printing the doctor blade method is more scalable for larger areas. However, the printing of the 2D patterns is difficult to achieve without fixing the ink flow to the surface [15]. In order to formulate the viscosity, the utilization of dissolvable solutes is necessary. Nevertheless, the suitable ink formula results in promising printing [15, 16]. Ca =

υη γ

(3)

The materials which are having a high paste-like viscosity are mostly used in screen printing [17, 18]. In this method, the ink is first flooded on the patterns on the substrate. The crucial parameters for optimization are viscosity and surface tension [18]. The solution processing is also incorporated for flexible substrates [19, 20].

3.2 Coating Sol: Rheological Properties Before discussing the physics of the doctor blade, it is vital to review properties which are rheological and related to the process. The important properties are viscosity, surface tension, and the wettability.

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3.2.1

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Viscosity

The viscosity is mainly recognized by analyzing the type of fluid levels. Figure 3 shows the fluid levels to study the concept of viscosity. Here, the top most effective plate can move and the bottom plate is static. The separation distance between the top plate and the fixed bottom plate is x. When the force F is applied, the top plate moves with the velocity v in accordance with the underside dish that is fixed. The dishes are intermediate with a certain velocity. Consequently, the gradient of velocity has been observed from the top plate to the bottom plate. The shear stress is described by F/A. In other words, the force applied per product area functioning on the liquid is shear stress in N/m2 or Pa. Viscosity is in connection with the shear stress, shear rate, and viscosity gradient [21]. Equation 4 shows the relationship between all the parameters. Shear Stress = Viscosity × Shear Rate

(4)

Equation 4 is certainly similar to Ohm’s law where the voltage (Shear Stress) is related to current (Shear Rate), and the opposition is described by the constant of proportionality that is among shear rate and shear stress. The kind of liquid paste which works in accordance with Eq. (4) is called a Newtonian fluid. The Newtonian liquid includes organic materials, glycerol, water, etc. However, due to the availability of the viscometers, it has been found that most of the liquids do not follow Eq. 4 and the estimation of viscosity is complex and forms the new field known as rheology. This field is associated with the fluid flow changes and the fluids are non-Newtonian. In high-speed coating, most of the inks used seems to be non-Newtonian. In these inks, the viscosity is shear-dependent. In the literature, this concept is named shear viscosity or simply viscosity. Many such kinds of liquids are classified as nonNewtonian. The shear effect is thinning where in actuality the needed shear stress reduces since the shear rate improves. This phenomenon is seen in the dispersive or emulative samples. The second could be the shear impact that is thickening where the viscosity raises because of the shear rates [23].

Fig. 3 Simple viscosity model

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The viscosity may be time-dependent also besides the reliance on the shear prices. For example, the viscosity that obviously decreases with the shear stress. Since the ink flow relies on viscosity, it is necessary to understand the rheological properties. Further, as the viscosity greatly differs with shear rates, in coating processes the approximate rate values should be known. For the high rate coatings, typical shear rates are greater. Nonetheless, the distribution after coating low shear rates is being involved. The viscosity additionally is dependent on the temperature and stress besides the shear prices and time. The majority of the fluids which are Newtonian are Arrhenius’s relationship with heat. The viscosity exponentially decreases as temperature increases. The viscosity exponentially increases with stress under high pressure, like 1 GPa. When we apply the paste for the coating, the paste should move with a certain methodology. Due to viscosity, the appropriate distribution of the paste is decided. Therefore, the lower viscosity fluid penetrated more in the material and the greater viscosity fluid is difficult to flow. Hence viscosity is the resistance provided to flow the fluid [24]. Whenever the paste has been used in the fabric coating, the shear force has been considered. This shear force varies depending upon the layer methods. In the knife layer, the paste is kept underneath the blade while in case of the rotary paste it is provided through the holes. In both the viscosity plays an important role. Figure 4 shows the basic types of rheological behaviors.

Fig. 4 Basic types of rheological viscosity behaviors

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The shear stress associated with the substance differs from the shear rates in direct percentage therefore the plot between shear stress and the shear rate moves through the origin in Newtonian behavior. The viscosity of the substance may be calculated at any given shear rate. Therefore, to understand the rheology profile, viscosity dimensions are essential at various shear rates. Hence, the dilatancy is also known as shear thickening [22]. In doctor blade, to analyze the movement of the paste, it is necessary to know the nature of the paste. The paste can be placed right in front of the blade which will be forced to flow in blade coating. If the paste is applied, the layer at the top will be coated with the help of the exerted force by the blade. Whenever particles of paste wish to flow layer by layer, the resistance is offered due to the friction at the interior layers [25].

3.2.2

Surface Tension

Besides the viscosity, surface tension is another fluid parameter that is very important, especially for imprinted electronic devices. In the flexible electronic systems, the surface tension is the key parameter in the mask creation on the silicon wafers. As can be seen from Fig. 5, the foundation of the surface tension is certainly studied by taking into consideration the difference from the molecules in bulk and at the surfaces. In fluids, the particles are attracted to each other and the force of attraction is higher in comparison to the thermal agitation. The top stress is really a measure that is direct of power and can be defined as [26], γ =

U 2a 2

(5)

where a is area per molecule and U is cohesion power per molecule. The unit of surface tension is mJ/m2 or energy/area. The definition of area power is usually used to spell out areas which can be solid. Even though surface tension originates during the level that is molecular additionally it is calculated through the capillary forces. The test starts with the metal ring of platinum the outer lining area increases since Fig. 5 Surface tension at molecular level

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the program deforms. The surface tension pulling the band reduces the area. The sum of the length is ~4πR, considering the band diameter r is smaller than outer radius R, the capillary force is 4πRγ. Therefore, one will discover the outer lining tension by [26] F = 4π Rγ ⇒ γ =

F 4π R

(6)

The temperature usually decreases the outer lining tension, because as fluid is heated, the relationship that will attract particles is paid down. There are numerous area movement phenomena being essential in imprinting [26]. The surface tension additionally forms the uniform film that is a thin paste [27]. Besides the surfacetension, the surface curvatures can increase the flows. Whenever the screen forms the curves, the force falls throughout the liquid surface. This phenomenon is known as the Laplace force.

3.2.3

Wettability

Wettability means a report of what sort of liquid is deposited for a substrate that is solid away. The doctor blade coating used for the flexible electronics requires the control on the wettability. For instance, coating a liquid on the substrates should have the control of volume per product size to reduce the discontinuity in the layers [28]. The wettability is approximately making the use of the surface energy [29]. The outer lining power associated with a state that is dry is the top power of solid–gas interface γ SG . The area powers suggested that damp is an amount of solid–liquid γ SL and liquid–gas γ LG area energy. Consequently, S is given as [29] S = γ SG − (γ SL + γ LG )

(7)

γ LG may be the area stress. Using the three-phases gas, fluid, and solid, it is categorized as partial wetting and the complete wetting. If S > 0, the liquid is distributed totally; hence it is called complete wetting. The liquid kinds a spherical cap on a substrate having a contact angle θ E having said that, if S < 0. The equation is given as [29], γ LG cos θ E = γ SG − γ SL

(8)

Young’s equation is derived with a force balance and can be seen in Fig. 6. Young’s equation is essential for imprinted electronics, specifically for the development of patterns. Nevertheless, the θ E may cause inconsistent readings called angle hysteresis [30].

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Fig. 6 Force balance equation derivation

4 Applications The doctor blade coating has a lot of applications. This process is used for solar cell developments, photonics, transistor developments, and sensor applications. The solar cell applications include organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), and perovskite solar cells (PVSCs). The photonic developments mainly include the memory applications whereas the transistor application includes organic thin film transistors (OTFTs) and the inorganic thin film transistors. Further, the doctor blade is also used for sensor applications.

4.1 Solar Cell Developments 4.1.1

Organic Solar Cells

This part will introduce the doctor blade used for fabricating the organic solar cells. Doctor blade is the straight forward process and is being used for various organic material-based devices. In the past doctor blade has been used for organic lightemitting diodes for coating the organic materials. It is reported that, for large area films up to 30 cm2 similar to the spin coating technique, the doctor blade method reduces material wastage [31–33]. Similar to organic light-emitting diodes, the doctor blade has also been used for organic solar cells (OSCs). When it comes to OSCs the blade coating is utilized for depositing both conducting and the photoactive polymers [34–36]. The OSCs having a doctor blade coated P3HT:PCBM film attracted the attention of various researchers due to larger efficiencies ~5% [37–40]. Inside this, Tsai et al. [39] successfully employed manual and auto methods to deposit active layers blade coating using chlorine-free solvents such as toluene and

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Fig. 7 a The blade coating method b The auto-blade machine [39]

xylene which were employed for PBDTTT-C-T blended with [6]-phenyl-C71-butyric acid methyl ester-based polymer solar cells. Figure 7a exhibits doctor blade and (b) shows automated blade machine. Figure 8 shows AFM topographical images of polymers in three different solvents and correspondingly achieved 3.85, 3.6, and 2.78% efficiencies in dichlorobenzene (spin), chlorobenzene (blade), and toluene (blade), respectively.

4.1.2

Dye Sensitized Solar Cells

The dye-sensitized solar cell (DSSC) was developed by using the doctor blade. Since the cost of DSSC is lower, it seems to be a promising device for photovoltaic applications [41, 42]. The DSSCs have achieved the efficiencies around 15% [43] and obtained the advantages of low-cost production procedures. The key element of DSSC is TiO2 thin film that is conventionally made by various practices. Huynth et al. [44] have modified the method. Usually in doctor-blade method uses surfactants to make TiO2 paste to enhance the porosity which in turn reduces the charge transfer across TiO2 and the used conducting electrode. To improve solar cell performance, they used the compression method and compared it with the conventional method. The surface morphologies of conventional and modified doctor blade method have been studied, where thickness found to be larger (22 micron) as compared to conventional (18 micron). Interestingly, drastic improvement in solar cell was achieved from 3.7 to 7.0% using conventional

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Fig. 8 AFM topography images and Rrms values of the PBDTTT-C-T/[70]PCBM active layers dissolved in a toluene, b chlorobenzeneand c xylene. The upper diagrams are 2-D images and the under diagrams are 3-D images (1 μm × 1 μm) [39]

and modified method, respectively. The TiO2 thin film is important from the charge storage point of view and used to improve the efficiency of the DSSCs. The DSSC performance can also be improved by depositing the crack-free TiO2 film using doctor blade [44–47]. Figure 9 shows the FESEM image of the TiO2 thin films prepared by corresponding TiO2 nano-particles.

4.1.3

Perovskite Solar Cells

Recent developments on perovskite solar cells using simple doctor blade strategy used for large surface solar cells. In these solar cells, the doctor blade was used to deposit electron transport layer (ETL), hole transport layer (HTL), cathode electrode, and the anode electrode materials. The blading process was used to fabricate CH3 NH3 PbI3−x Clx perovskite films which are thin PEDOT:PSS substrate in ambient atmosphere [48]. It has been reported that the layers deposited by doctor blading exhibits grater crystalline domains than that of the spin coating. This is mainly because of different procedures adopted in the deposition techniques. Additionally, the doctor blade is suitable for the anti-solvents and reported the PCE is ~12.21% as shown in Fig. 10. It has also been reported that, the perovskite solar cells fabricated using doctor blade achieves highly stable performance in perovskite solar cells.

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Fig. 9 FESEM image of surface (left) and cross-section (right) of TiO2 thin films prepared by corresponding TiO2 nano-particles [47]

Fig. 10 a Schematic of the doctor-blade setup including a hot plate to control substrate temperature during coating. b Typical optical microscope image of the resulting perovskite thin-films with apparent large grains. c, d and e Cross-section SEM images of the perovskite film at magnifications of 6500 × , 65,000 × and 1,00,000 × respectively [51]

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Deng et al. [49] have used doctor blade using MAPbI3 films at 100 and 125 °C to optimize the layer and studies the surface morphologies. Also they have used different PbI2 : MAI molar ratio to see the effect on the device performance. It has been shown that the grain boundary formation in this method is finished in single-step. The doctor-blading has also been used to develop the colourful 2D photonic structures the films had been formed using the Rayleigh–Bénard phenomenon. Because of the colourful perovskite films, their solar module obtained PCE of 12%. Interestingly, CH3 NH3 PbI3 the doctor blade-coated thin films have been used in planar PVSCs by Wu et al. [50]. The morphology of CH3 NH3 PbI3 layers diverse from the random nanowires and the PCE of ~11% have been obtained. Mallajosyula et al. [51] investigated the impact of environmental conditions in the development of large surface area films coated using the temperature-managed doctor blade. The developed products yield effectiveness in the I–V characteristics. The developed modules obtained the PCE of 13%. Figure 10 shows the schematic of the doctor blade setup. Further it has been shown that the doctor blading is also suited for the double step coating process. In this, thin films were deposited using double-step deposition and the PbI2 layer has been coated by utilising the doctor blade [52]. By managing the crystallization for the PbI2 with ventilation, a highly compact layer has been obtained. Figure 11 shows the comparison of figures of merit of solar cells fabricated using doctor blade and spin coating methods. Recently, PVSCs uses a combination of formamidinium (FA) and methyl ammonium (MA) while the monovalent cations for their a more suitable band gap that is optical better thermal security compared to FA/MA. The doctor blading method was used to develop the mixed FA/MA layers [53]. It was reported that doctor bladed blended cation perovskite with Cu as the cathode showed the PCE of ~18.0% and security is also good. Moreover, a doctor blading method was used to develop printable PVSCs. The PVSCs with ITO/poly(3,4-ethylenedioxythiophene):poly(4-tyrenesulfonate)(PEDOT:PSS)/CH3 NH3 PbI3−x Clx acid-phenylC61-butyric ester (PC61BM)/Bis-C60/Ag with the exception of Ag electrode has been fabricated [54]. The end result of humidity demonstrated that humidity is a key factor. The PCE of ~10% has been obtained and the fabricated flexible PVSC accomplished a PCE of ~7.14%.

4.2 Photonics Developments Hsieh et al. [55] developed a scalable technology related to macroporous shape memory photonic crystals by substituting self-assembling silica colloidal crystals in a polyurethane acrylate/polyethoxylatedtrimethylolpropanetriacrylate /poly(ethyleneglycol)diacrylate matrix. Interestingly, as-synthesized photonic crystals displayed a brilliant structural colour and observed that it was reversibly tuneable with mechanical deformation at ambient conditions. In this, roll-to-roll doctor blade coating was used to self-assemble macroporous shape memory photonic crystals

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Fig. 11 Comparison of the principal parameters of 48 solar cells made by blade (blue) and spin coating (red) the PbI2, and after 10 and 30 min of dipping in CH3 NH3 I solution [52]

[55]. In this process, a double-edged razor blade was used on the glass substrate. The substrate was under the blade with velocity 5 mm/min using a KD Scientific syringe pump. The speed was controlled by regulating the motor speed of the syringe pump. The silica particles were wet etched to create macroporous shape memory photonic crystals. Figure 12 shows the experimental setup used for the photonic crystals using doctor blade and surface morphology with cross-sectional view of the deposited material.

4.3 Transistor and Sensor Developments Doctor blade was also used to fabricate the high performance organic thin film transistors (OTFT) using molecular materials [56]. In this work, new soluble phthalocyanines (Pc), tetra-n-butyl peripheral-substituted copper (II) phthalocyanine (CuBuPc) was introduced to fabricate the OTFTs. Figure 13 shows the detailed steps and the structure of the OTFT fabricated using the doctor blade. In this method the thixotropic property of the CuBuP corganogel has been used to create the thin films of the OFETs.

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Fig. 12 Experimental set up photonic crystals using doctor blade [55]. Colloidal crystal/shape memory polymer composites fabricated by the doctor blade coating technology. Bottom a Topview b cross-sectional SEM images of a composite consisting of 370 nm silica colloid crystals coated on a glass substrate

It was reported that the OFETs produced from the gel have a charge-carrier mobility 50 times of magnitude higher than that of OFETs produced by drop-casting. It was reported that OTFTs are the promising devices for the gas sensing applications [57]. Most of the OTFTs for gas sensing applications are fabricated using the doctor blade thin-film formation. Since the humans are exposed to gases originated from the pollution, it is important to develop the gas sensors using low-cost fabrication process.

5 Advantages and Disadvantages 5.1 Advantages (1) Doctor blade coated surface is flat, and it is not affected by the environmental conditions.

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Fig. 13 Detailed steps and the CuBuPc OTFT structure fabricated using doctor blade [56]

(2) (3) (4) (5) (6) (7) (8)

High speed deposition is possible Easy, low cost, small area to large area is possible Multilayer deposition is possible Can be coated any material where gel of that material is available as raw material Easily possible to integrate for roll-to-roll technology Any substrates are possible to coat where surface is flat Use of large area is possible in which either single blade or the dual blade can be used.

5.2 Disadvantages (1) Impurity is easily incorporated in film from precursors source of by external means. (2) If the external particles mix up into the coating layer, then these particles stick to the blade knife edge. Therefore, the blade knife needs to be frequently replaced before the start of the actual deposition of the layer. (3) Further, due to thickness of the layer the uniformity in the layer may also be affected. (4) The ink-delivering technology as technique is open, the substrates are exposed to the surrounding environment. Therefore, the ink may contain some contaminants such as dust particles, dirty particulates, scraps which leads to either defects on the coated layer or block the ink pumps. (5) Due to open environment it is also difficult to maintain the ink viscosity.

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6 Summary The doctor blade deposition made a remarkable breakthrough to fabricate thin film transistors, solar cells, and the sensors. The deposited thin films offer the most promising options for reducing the cost of the solution processing-based systems. The doctor blade progresses lot in the field of organic solar cells, dye sensitized solar cells, and the perovskite solar cells. Further, there are a lot of opportunities to develop high-performance tandem solar cells which uses both perovskite and the existing technologies. To get the uniformity in the scalable large area surfaces the doctor blade seems to be more suitable over the spin coating method. However, because of low viscosity solutions used in doctor blade to dewetting is the major hurdle. It has also been noticed that, for highly scalable large surface area devices with lower manufacturing cost the doctor blade coating is the promising process and is more precise to investigate the performance of the large area devices having the uniformity in the fabricated layers. Further, by controlling the active layer thickness the good reliability in the devices can also be achieved. Surface energy-patterning is one of the methods used in the doctor blade to create the uniform thin films required for the high-performance thin film transistors. Further, uniform layers of the channel leads to lesser variability on the organic thin film transistors. Due to less variability the thin film transistors fabricated using the doctor blade are mostly used for the image-sensing applications.

7 Future Prospects The fabrication of the devices and sensors requires the solution processing of the material layers on the substrates by using the various material deposition techniques. The fabrication processes used must not have the harmful toxic solvents and the chemicals. Further, the developed devices and the sensors must have the lower environmental impact and a higher reproducibility. Doctor blade seems to be a promising method to develop the efficient devices and the sensors at the laboratory scale. The future commercial devices and the sensors will lead to a dramatic change in the current industries for developing the thin-film devices at lower cost by using high throughput industrial processes. The doctor blade kind of roll-to-roll printing processes on the flexible substrates will be used to develop the future low-cost devices and the sensors. To enhance the upcoming research and developments, it is essential for the researchers to develop the suitable materials to address the issues faced by the device and the sensor community.

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Dr. Ganesh C. Patil is working as an Associate Professor at the Center for VLSI and Nanotechnology at Visvesvaraya National Institute of Technology, Nagpur. Dr. Patil received a Bachelor’s degree in Electronics and Telecommunication Engineering from the University of Pune and an M.Tech degree from the College of Engineering Pune (COEP) in 2002 and 2007, respectively. After M.Tech he completed his Ph.D. in the area of Microelectronics and VLSI from the Indian Institute of Technology Kanpur, India in Jan 2014. Dr. Patil Bagged Best Student Award at 4th International Student Workshop on Electrical Engineering, at Kyushu University, Fukuoka, Japan, and also received Cash Award (Rs 20,000/-) twice by IIT Kanpur for publishing the papers in reputed journals. He is the reviewer of various SCI listed journals and also worked as Technical Program Chair for various IEEE international conferences. The area of his research work is Device Physics and Modeling, Novel Nanoscale MOSFETs, Analog/Digital CMOS circuits, VLSI System Design, and organic electronics. Dr. Patil has published several research papers in various reputed journals and more than 20 research papers in peer-reviewed international conferences held at USA, Japan, Egypt, Singapore, India, and China.

Chapter 13

Sol–Gel Derived Thin Films Nikila Nair

Abstract Sol–gel process is a flexible methodology for obtaining materials with higher surface area and great stability via the ‘chemie douce’ approach. A wide range of material morphologies with dimensions in nano and micro range is possible to synthesize through this approach. It realizes the synthesis of various composite materials with complex structures at low temperatures. The present chapter comprehensively explores the sol–gel method, its chemistry, and its various mechanisms. This chapter gives an in-depth idea of different parameters that can be controlled to generate advanced materials with customized morphologies which are otherwise difficult to synthesize using other prevalent methods. A considerable amount of research articles employing sol–gel method for the synthesis of nanostructured materials are reviewed for this chapter. Potential applications of the sol–gel method in current research interests like solar cells, gas sensors, and supercapacitors are also elaborated.

Abbreviations Aº AAO DI DTA EEL FESEM FF FTIR H HRTEM LR-TEM

Angstrom Anodic aluminium oxide Deionised water Differential thermal analysis Electron extraction layer Field emission scanning electron microscopy Fill factor Fourier transform infrared Hour High-resolution transmission electron microscopy Low-resolution transmission electron microscopy

N. Nair (B) Department of Mechanical Engineering, Indian Institute of Science, Bengaluru 560012, India e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 B. R. Sankapal et al. (eds.), Simple Chemical Methods for Thin Film Deposition, https://doi.org/10.1007/978-981-99-0961-2_13

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mA/cm2 mV OR PCE RE S/cm SEM SPME TEM TGA XRD

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Milli ampere per centimeter square Millivolt Alkoxide group Power conversion efficiency Rare earth Siemens per centimeter Scanning electron microscopy Solid-phase micro extraction Transmission electron microscopy Thermogravimetric analysis X-ray diffraction

1 Introduction Preparatory methods used for the synthesis of materials play a pivotal role in defining their morphology or architecture [1]. These preparatory methods can be classified into physical and chemical types, wherein most of the physical processes require high temperatures and complex processing. Besides this, it also incurs a high cost which acts as a challenge for industrial scaling up [2]. In the 1970s, low-temperature chemical methods were explored for the synthesis of glasses and ceramics. Especially, Dislich’s [3] report on the fabrication of Pyrex-type transparent glass lenses by hot pressing method at 630 °C caught the attention of most researchers worldwide. This process usually requires a very high temperature of about 1500 °C for meltquenching. By 1990s, this technology of low-temperature quenching was practised for the fabrication of functional materials applicable in optical, electrical, chemical, mechanical, and biomedical applications [4]. Later on, this method was termed as ‘Sol–Gel’. The sol–gel method, often considered as “soft chemistry”, is a bottom-up approach to the solution-based synthesis of materials at low temperature. This process requires simple instrumentation and a short reaction time [5, 6]. In general, it includes the transformation of solution into a gel and then depositing it over any substrate [7]. It is a more flexible method when compared to most of the other physical deposition processes. Materials like glasses, ceramics, and organic–inorganic hybrids can be prepared by using the colloidal solutions (sol) of this method [8]. Single or multicomponent powder or fibre form materials can also be synthesized using this process [9]. Figure 1 shows the metal-based materials that can be synthesized using the sol–gel process and the different applications in which these materials can be used. Conventionally, in this method molecular precursors are prepared either by dissolving metal alkoxides in water or in alcohol. These are further heated or undergoes hydrolysis/alcoholysis process such that a gel form is obtained which is later dried through appropriate methods. The benefit of this method is that it is a lowtemperature process and hence, can give more control over the material design or

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Fig. 1 Materials synthesized using sol–gel process and their applications

architecture. The dried gels are widely used for surface encapsulation and building protective coatings over cloths [10]. In 2006, Schmidt [11] proposed that this method should be explored more as there exists tiny or discrete particles in precursors or the sol matrix, which through careful reaction scrutiny can produce nanomaterials of various morphologies. Nowadays, this method is highly prevalent in industries for low-temperature synthesis of high-quality and uniformly sized nanoparticles [12– 14]. Sol–gel method is more commonly used in cement science to improve the physical and chemical properties of the cement mixture by delivering more complex silica-based materials [15]. It has found wide-scale application in medical field where targeted therapies are of absolute importance. The pores generated in gels while drying process has an added advantage in targeted therapies as it can entrap biologically active materials and release them near the targeted cells in a controlled manner [10]. The sol–gel method is particularly well-known for the deposition of thin films on substrates like ceramics, glass, metals, and polymers of various shapes and sizes. The thickness of the film can be controlled from micrometre to nanometre range without using expensive instrumentation. Thin films with desired mechanical and chemical properties can be developed just by varying the precursor composition, its amount, and the type of functional groups used. This method is highly recommended in photonics, solar cells, magnetic films, electrochromic filters, and microelectronic

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industries. Display industries adopted this method for the fabrication of anti-glare and anti-reflection screens. Nanomaterials synthesized using sol–gel method are more favoured compared to the inorganic binders as the former requires only a UV curing system while the latter requires thermal curing which ultimately reduces the production speed. Due to the ease of fabrication using sol–gel method, the global market for sol–gel coatings has already reached US$ 2.5 Billion in 2020 which is estimated to reach about US$ 4.8 Billion in 2027 [16]. However, there exists certain obstructions for the complete adoption of sol–gel method in industries, like batch reproducibility, agglomeration of nanoparticles, and availability of precursors by virtue of which large-scale production is always restrained. Issues pertaining to large-scale production is resolved to some extent through controlled scale-up methods whereas agglomeration of nanoparticles and batch reproducibility can be fixed by the use of appropriate surfactants and coupling agents [17]. The precursors used for sol–gel method are mainly metal alkoxides which are very difficult to obtain and manage, because of their high sensitivity to the atmospheric moisture. For multi-component ceramics, the rate at which alkoxide hydrolysis occurs should be controlled, which is not a very easy task to accomplish. Metal salts are the best alternative for alkoxides, as these are less expensive and can be easily managed. These are readily solvable in many organic solvents and even in water within which they form metal complexes [18].

1.1 Basic Approaches Figure 2 shows the schematic of various processes involved in the sol–gel method and also provides an insight into different stages where the morphology of the final product can be controlled [19, 20]. The steps involved are as follows [21]: (1) Preparation of initial solution i.e. sols. Sols are prepared by mixing metal alkoxides or metal halides in a solvent which is highly volatile and of low surface tension. The rate at which the solvent evaporates is depended on its porosity and available OH− traps. Occasionally, acids or any other chelating agents can be used to manage the condensation process to stabilize the sols. (2) Next, the prepared sol is deposited over any substrate by some simple coating methods like dip-coating or spin coating. These methods form a very thin layer of sol over the substrate surface whose thickness can be altered depending on the precursor concentration, solution density or viscosity, temperature, spinning rate, extraction speed, etc. (3) Third step involves heat treatment under a controlled atmosphere. A hightemperature annealing can provide a more stable end product [22]. (4) Last step is the removal of the template, if required.

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Fig. 2 Schematic representation of various steps involved in sol–gel process [19–21]

1.2 Literature Review A wide range of reports on sol–gel based semiconductor oxide nanostructures is available wherein the micro-structuring of the nanostructures governs their physical and chemical properties. The colloidal species dispersed in the sol regulates the micro-structuring of the nanoparticles [23–27]. Sol–gel method is feasible for the growth of sub-micron particles with tiny particle size distribution [28, 29]. Monoliths, fibres, powders, and films of high surface area can be prepared using this method. Kafshagari et al. [30] found that the particle size can be controlled just by varying the synthesis method. Apart from it, the morphology was also observed to change as the process varied from hydrothermal to co-precipitation to sol–gel method. The particle size increased as the process changed from hydrothermal to co-precipitation to sol–gel method [30]. Zarrin et al. [31] were able to prepare Co doped LaCrO3 with a crystallite size of 10–50 nm using the sol–gel method. Shandilya et al. [32] were successful in the fabrication of lead-free perovskite material using the sol–gel method in conjunction with the hydrothermal method which otherwise requires a very high temperature for synthesis. Ali et al. [33] prepared Cu2 ZnSnS4 thin film which is a non-toxic, efficient, and cheaper counterpart of Copper, Indium, and Gallium using low-temperature sol–gel method for photovoltaic applications. Ishikawa et al. [34] reviewed the use of environmental benign and effective sol–gel process for the synthesis of nano-dimensional and nanocrystalline form of biocompatible material, calcium phosphate.

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2 Principles of the Sol–Gel Method Sol–gel methodology is a wet chemical procedure that includes hydrolysis and polycondensation, gelation, aging, drying, densification, and at last crystallization. Manipulating the precursor treatment at the initial phase of the process can generate morphologies like nanotubes, nanowires, nanofibers and spherical nanoparticles [35, 36]. The initial phase of the method implies the formation of sols in which the particles are bonded together by van der Waals force. At this stage, the interface atoms possess high Gibbs free energy as compared to the internal atoms making it thermodynamically unstable [37, 38]. As in case of gels, it is the covalent type of interaction that dominates among the atoms. Once the gel is formed the process is irreversible. Through this process, the multicomponent compounds can be mixed at the atomic level to form small particles [38]. The two main reactions observed in the sol–gel process are (1) hydrolysis of the precursor in acidic or basic medium and then (2) polycondensation of the hydrolyzed product [39]. Both hydrolysis and condensation processes are not simultaneous reactions but a series of initial and propagating reaction wherein the hydrolysis is the initial step followed by the condensation [40]. Hydrolysis: During the hydrolysis process, the water molecules replaces the alkoxide (OR) groups attached to the metal ions forming hydroxyl groups (OH). It is a slow process under neutral environment, to fasten this process either acid or base catalysts are added to the solution. Equation (1) shows the hydrolysis process observed in metal alkoxides where M indicates the metal ions and R is the alkyl group [41].

(1)

Condensation: During the condensation process, water or alcohol solvents are removed from the colloids. With the release of water molecules, the colloids aggregate to form larger particles containing molecules of the form M–O–M, through polymerization. The resultant polymer is an amorphous network consisting of a large number of small molecules called monomers [36]. Equation (2a) shows the condensation process observed in water solvent while Eq. (2b) shows the condensation process observed in alcohol solvent [41].

(2a)

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(2b) Hydrolysis and condensation kinetics are mainly depended on the pH of the solution [15]. The rate at which these processes occur determines the micro-structuring of the gel [42]. If the rate at which hydrolysis occurs is changed, an inhomogeneity will persist in the subsequent gel [9]. At lower pH, the hydrolysis process increases and the condensation process reduces while in case of higher pH the reverse will occur. Indicating that for lower pH, weakly branched gels are formed while for increased pH, densely branched gels are formed. A mild condensation process allows more cross-linked networks to be formed which are mechanically stable and easier to handle while an extensive condensation process results in shrinkage of the gel [43]. This is true when we consider the formation of xerogels, aerogels, and thin films. The sol–gel route can be categorized under aqueous and non-aqueous methods. If water is used as a solvent then it is termed as aqueous method and if solvents like ethers, alcohols, ketones, or aldehydes are used then it is termed as non-aqueous method. The aqueous method is more popular for the synthesis of bulk materials like metal oxides while in cases where the structure and dimensions of the particles are to be controlled, the non-aqueous method is more reliable. The major limitation of the aqueous method is that water is a highly complex solvent and it can act both as a ligand and a solvent. Another major limitation in utilizing the aqueous method is that it is highly reactive when metal oxide solutions are prepared. This method of sol–gel preparation causes hydrolysis and condensation or agglomeration to occur simultaneously making it difficult to control the reactions individually and affecting the particle morphology and its reproducibility [44, 45]. But still aqueous method can help to retain the stoichiometry of the material [43]. The non-aqueous process can upgrade the role of organic components in the reaction and overcome the major limitations aroused due to the aqueous media during nanostructure synthesis. The non-aqueous processes can be further classified as surfactant or solvent-based. Sol dynamics such as its viscosity, stability, and its ability to wet the substrate will impact the structural order of the sample. An increase in its viscosity will slow down the condensation process leading to smaller grain size and lesser crystallinity with thick film formation, containing organic residues in the final film [46]. However, a decrease in its viscosity will enhance its stability and improves the wetting of the substrate [43].

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3 Basic Terminology 3.1 Sol Gel Sol–gel, as the name suggests, exhibits two phases before the formation of the final product. The initial form ‘Sol’ consists of solid colloidal particles of approximately 1–100 nm dimension, homogeneously dispersed in a liquid medium. The density of colloidal particles is found to be greater than that of the liquid in which they are dispersed. These colloidal particles perform Brownian motion in the liquid medium [36]. On the contrary, the gels are three-dimensionally interconnected, rigid solid network with polymeric chains of length greater than a micrometre and contains continuous pores of submicrometric dimensions [47, 48]. These are mostly amorphous in nature which upon heat treatment promotes crystallinity [36]. Depending on the order of the polymeric chains, the gels can be of (1) lamellar form (ordered), (2) covalent polymeric form (completely disordered), (3) polymer networks formed through physical aggregation (predominantly disordered), and (4) particular structures (disordered) [49]. Gel is basically a biphasic system in which both liquid and solid medium co-exist in such a way that it does not flow instantaneously and remains in the equilibrium state. Due to the co-existence of two mediums the gel can have two forms as shown in Fig. 3: (1) when the liquid medium is greater than the solid medium, and (2) when the liquid medium is less than the solid medium. If the liquid medium present in the network is water, then it is called ‘Aquagel’ or ‘Hydrogel’ and if the liquid is alcohol, it is called ‘Alcogel’. Further, depending on its drying methodology, it can be differentiated as Aerogel, Xerogel or Cryogel. During the drying process, gel allows desorption of water that

Fig. 3 Classification of gels depending on the liquid medium present and the drying method

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is physically bounded transforming it into a dry form. This deforms the porous backbone of the gel inducing cracks and making them brittle [8, 48]. When the gel is dried in an uncontrolled manner at ambient pressure, disordered porosity is generated due to the absence of structure-directing agents. Such products are called as ‘Xerogel’. When the gels are dried under supercritical conditions ‘Aerogels’ are formed whose porosity is negligibly affected by the drying process. If the gels are dried through the freeze-drying method, then the product is termed as ‘Cryogels’ and has high porosity [4, 48].

3.2 Growth Mechanism of Sol Gel A sol can exist either in colloidal gel form or in polymeric gel form when it transforms from a viscous liquid to gel [50]. The major dissimilarity between the colloidal and polymeric gel is that in the former case the bonds are not of permanent nature while in the latter it is of permanent nature. Thus enabling them to reform and rearrange according to the mechanical deformation and thermal fluctuations [51]. Other steps involved in sol–gel process apart from precursor selection are colloidization, flocculation, gelation, superfluid processing, and drying by evaporation [52]. Figure 4 shows the various stages involved during this process and the various factors influencing these stages. Freshly synthesized sol–gel films are much sensitive to the ambient conditions and even a slight variation in the temperature or humidity can alter their properties. The gels exist in an unstable state when they are going through drying, condensation, and shrinkage process. As a result, the final thickness of the film is measured after it attains stabilization i.e. through thermal treatment [53].

3.3 Effect of Preparative Parameters As shown in Fig. 4, the films obtained using sol–gel methods are affected by its preparative parameters such as the (1) type and concentration of the precursors used, (2) solvent nature, (3) water/acid ratio, (4) solution pH, (5) type of additives (catalysts, surfactants, structure directing agents) added and their concentration, (6) heat treatment provided to the materials, and (7) aging [54]. The substrate on which the

Fig. 4 Factors affecting each stage of sol–gel route

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material is deposited also plays an important part in the crystallization and proper adhesion of the film. It should have an appropriate wetting property to prevent any peeling of the suspended sols [55]. Effect of the precursor type and its concentration Crystallization of the material prepared using sol–gel method is mostly reliant on the initial precursors and its concentration. Based on these precursors the synthesis can be classified into three types: (1) metal salts in aqueous solution, (2) metal alkoxide solution, and (3) mixed organic/inorganic solutions. The hydrolysis and condensation rate in the sol–gel method is governed by the size of the alkoxy ligands because of their steric and inductive effects. The texture of the material will be coarser if the alkyl group of the alkoxide is larger [56]. Bahadur et al. [57] analysed the effect of various precursors on the morphology of the ZnO thin film. They used zinc-based acetate and nitrate as two precursors and two different solvents for each material as they have different solubility. ZnO thin film grown using zinc nitrate (Fig. 5a) showed nanograin structures with agglomerated dendritic morphology showing polycrystalline nature while the material prepared using zinc acetate showed smooth film with crystalline nature (Fig. 5b). They concluded that the morphology of the material synthesized depends on the speed of the spinner, sol concentration, and annealing temperature etc. [57]. Likewise, Karkare et al. [58] demonstrated that the precursor type does not hamper the dimensions of the material synthesized during the sol–gel method but it does influence the morphology of the final product as they obtained a film and spherical granular-like structures when they synthesized anatase TiO2 using titanium isopropoxide and titanium butoxide, respectively. Effect of solvent Siddiqui et al. [59] analysed the influence of parameters like precursor concentration, solvent, and gelating agent on the formation of CuO particles, prepared using the sol–gel method. It was observed that during low concentration of precursors,

Fig. 5 TEM of ZnO grown using a zinc nitrate and b zinc acetate as a precursor with corresponding electron diffraction [57]

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no significant formation of particle nucleus were observed in the precursor solution instead the solution was homogeneous. As the copper concentration increased, Cu2+ starts to nucleate and form nanospheres which on further increase results into an irregularly shaped material without any preferential growth due to the uncontrolled hydrolysis/condensation process. It was observed that isopropyl alcohol had better control over the crystallinity of the particle as compared to the ethanol. In aqueous solution, the particle showed rod like morphology which changed to nonuniform, densely packed nanorods in ethanol while in isopropyl alcohol it changed into flakes consisting of individual particles forming large particles with citric acid as a chelating agent. The reaction between the solvent i.e. isopropyl alcohol and the citric acid reduced the solution pH eventually altering the morphology of CuO particles. As the solution became more acidic it suppressed the growth of CuO along (111) plane affecting its morphology, changing from irregular crystals to homogeneous nanoflowers to plate-like structure [59]. Effect of pH on sol–gel pH in the sol–gel method affects the polymerization of metal–oxygen bonds, influencing the hydrolysis and condensation process and thereafter manipulating the morphology of the material [60]. It also determines the ability of the sol–gel to form hydroxides or oxides as the final product. This is mainly because the ratio between the charge of the material and the radius of its cation is dependent on the pH of the sol [61]. As stated before, the formation of pores in gel is inevitable, whereas the size of the pores and the particle size in the final product is decided by the solution pH [8, 62]. Figure 6a shows the XRD analysis of TiO2 particles synthesized using the sol–gel method at pH 2, 3, 6, 8, and 9. Although the peaks formed become more prominent as the pH increases, the width of the peak is prominent in the case of pH 2 as compared to other pH values. The graph between the crystallite size and the pH of the sol (Fig. 6b), indicates that these are directly related to each other i.e. as the pH increases the TiO2 crystal size also increases. In acidic solutions, the ionic strength is more, and this reduces the growth and rate at which nuclei are formed affecting the particle size. An analysis of calcination on the particle size prepared in acidic medium revealed that as the temperature increased it promoted the growth of particle (Fig. 6c). It also reduced the phase transformation temperature due to the larger surface energy of the particles. TiO2 particles synthesized at pH ~ 2 and calcined at 100 °C showed spherical morphology with reduced agglomeration and aggregation whereas for pH 9, highly agglomerated particles with non-spherical morphology were observed [63]. This study shows that as pH reduces the particle size reduces, increasing its surface energy. On heating such material, crystal growth is induced and it tries to attain minimum surface energy by forming a spherical morphology. Similarly, Alias et al. [64] prepared ZnO powder from zinc acetate dihydrate (Zn(CH3 COO)2 . H2 O) in methanol and varied its pH from 6 to 11 by adding NaOH. After the gelation and hydrolysis process the XRD analysis of the sample showed that for pH ≥ 8, highly crystalline peaks were obtained while for pH 9 these peaks were intense. Figure 7a and b shows the agglomerated nanoparticles of ZnO in acidic and neutral sols. In alkaline condition (Fig. 7c–f), a uniform homogeneous spherical ZnO nanoparticles

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Fig. 6 a XRD of TiO2 particles in different pH solutions. b Crystallite size (from XRD) as the pH varies (b) particle size variation as calcination temperature varies [63]

with low agglomeration is obtained which is attributed to the centrifugation of the final sols. The crystallite size showed an increase in size as pH varied from 8 to 9 (Fig. 7c and d). On further increasing the pH from 9, the crystallite size was reduced owing to the dissolution of ZnO particles (Fig. 7e and f). The ZnO particle size kept on reducing as the pH was varied from 8 to 11. The maximum particle size of 49.98 nm (crystallite size = 24.96 nm) is obtained for pH 8 while for pH 11 it is found to be 36.65 nm (crystallite size = 18.37 nm) [64]. Azis et al. [61] evaluated the variation in the morphology of SrFe12 O19 with respect to the pH of the sol. They used Sr(NO3 )2 and Fe(NO3 )3 as the precursors and citric acid as the chelating agent. Ammonium hydroxide (NH4 OH) is used to vary the pH of the sol from 0 to 8. Although, the sol–gel method provides highly crystalline and pure material at pH 8, a minute amount of secondary phase, hematite Fe2 O3 , is observed which subsidized the purity of SrFe12 O19 to 87.8%. Such impure phase formation at pH 8 has been attributed to the insufficient calcination temperature provided to the sample. High acidity favoured the formation of highly crystalline SrFe12 O19 particles. As the pH increased, the iron gel formed a negative charge which was equilibrated by the adsorption of the positively charged Sr ions. However, aggregates were formed in the immediate vicinity of the complex during the polymerization process due to the localized shifts, inhibiting the crystal growth and reducing the crystallinity of the product from pH 4. The microstructural analysis showed that

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Fig. 7 FESEM images of ZnO at pH a 6, b 7, c 8, d 9, e 10 and f 11 [64]

the grain size agglomerated as the pH increased. Figure 8 shows that as the solution pH increases from acid to base the grain size increases except for pH 4 where it reduces and then again increases with pH [61]. Effect of catalysts on sol–gel The final structure of the gel depends on the catalyst used which can be either acidic or basic. Their role is critical as they control the relative kinetics of hydrolysis, nucleation, growth, and percolation before deposition [65]. Figure 9 shows various stages of sol and gel when acid and base catalysts are added. Gel synthesized in acidic conditions consist of smaller particles as compared to the base [62]. Acid catalysts exhibit weakly branched sol which on gelation shows larger pores while in case of base, the sol is densely branched, forming a highly porous gel [66]. In acidcatalysed solutions, the hydrolysis kinetics is predominant than the condensation process while in alkali-catalysed conditions, it is the condensation process that is faster. This results in the formation of highly dense species that agglomerates into

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Fig. 8 Grain size distribution of SrFe12 O19 at different pH [61]

fine particles [67]. In acid catalysts, a microporous chain-like structure with pore-size < 2 nm is formed in the material. If the water/alkoxide ratio is low a weakly branched polymeric network is formed in the case of acid medium while in case of base, a more connected and branched network is obtained [15, 68, 69]. These catalysts should be eliminated from the final product to obtain a pure material. Usha et al. [70] prepared CuO nanoparticles using two different catalysts: NaOH and KOH. Figure 10a and b exhibits the XRD analysis of CuO nanoparticles obtained in both the catalysts. It shows that CuO nanoparticles have more crystalline nature in NaOH than in KOH. The Cu(OH)2 colloidal clusters act as a nuclei for the growth of CuO NPs. During the sol–gel process, Cu2+ and OH− reach a supersaturation value and CuO nuclei are formed in the aqueous media. The crystallite size of CuO from the NaOH catalyst is calculated to be 95.1 and 47.5 nm while in case of KOH it was found to be 19.8 and 42.9 nm, respectively. This implies that NaOH stimulates an easy growth of CuO NPs as compared to KOH. NaOH-assisted CuO NPs show minimum dislocations as compared to KOH-assisted CuO NPs. Figure 10c and d exhibits surface morphologies of CuO NPs respectively. The surface morphology shows leafy grain-like structures which tends to agglomerate on annealing, forming a flower-like structure [70].

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Fig. 9 Sol and Gel formation in acid and base catalysts [62]

Effect of aging ‘Gel’ is a solid matrix with solvent encapsulated in it, so it requires to be dried to remove the solvent from the final product. The time taken, in between its gel form to its drying process is termed as aging. During the aging process, gel is not in a stationary condition but it endures hydrolysis and condensation process. Effect of water/acid ratio Different amounts of water/acid ratio relatively change the hydrolysis and condensation process of the sol, eventually affecting the pore size of the resultant material. Effect of post heating Post heating method seems to densify the grains as it reduces the grain boundaries and increase the particle size. Typically, as the temperature increases the material exhibits preferential orientation along one plane [71]. Gels when treated from 100 to 180 °C, it evacuates most of their physically absorbed water making them dry [48]. The time and temperature for which the material has been calcined affects the grain size. At low temperature, small crystallites are obtained while at higher temperature,

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Fig. 10 a XRD of CuO NPs with NaOH and KOH catalysts. (b) Magnified XRD peaks. c SEM images of CuO NPs. d magnified CuO NPs image [70]

the molecular diffusion and crystal growth fuses the small crystallites forming a well-defined crystal [72]. Zak et al. [73] reported enhancement in the crystallinity of ZnO xerogels with annealing temperature. It is evident from Fig. 11 that there is an initial weight loss as the temperature varies from 50 to 190 °C due to the loss of absorbed water. From 190 to 750 °C combustion of organic materials induces another major weight loss. Further, above 750 °C the material undergoes complete decomposition. From Fig. 12 it can be inferred that the particles acquire a hexagonal shape with smooth surface. The particle size increased from 32 ± 4 nm, 38 ± 6 nm to 41 ± 9 nm as the temperature varied from 600, 650 to 750 °C, respectively [73]. Quintero et al. [74] studied the influence of parameters like stirring time, catalysts type, and calcination temperature on the morphology, size, and crystallinity of TiO2 particles. Ultrasonication of 3 h showed the existence of a combination of nanorods and nanoparticles. After 6 h of agitation nanoparticles of diameter 10 nm were obtained. Increasing the ultrasonic stirring increased the kinetic energy of the particles in the solution generating both elastic and inelastic collisions in the colloidal suspension. This induced implosive collapses which favoured the formation of structural arrays. Larger particles continued the collisions which broke the structure into small particles. The catalysts study showed that the use of strong acids promote smaller nanoparticles whereas the weak acid favours larger nanostructures.

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Fig. 11 Thermogravimetric and DTA curves of ZnO xerogels from 50 to 900 °C [73]

Fig. 12 TEM images of ZnO NPs at a 600 °C, b 650 °C and c 750 °C [73]

An increase in the calcination temperature is responsible for the enhancement in nanoparticle size and it also influences the crystalline phase. The use of catalyst like acetic acid favours the formation of anatase phase as compared to nitric acid since the former allows the condensation process which initiates the nucleation and growth of the crystalline lattice. The ultrasonic stirring process for two different time duration does not show any remarkable change in the phase of the material. However, the calcination process influences the crystalline phase [74].

4 Experimental and Working of Sol–Gel 4.1 Experimental Design As stated before, the preparation of sol starts with the selection of precursors like metal alkoxides or halides mixed with water or alcohols. A small amount of catalysts

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can adjust the reaction to either form particles or to promote the gel formation. The metal alkoxides denoted as M(OR) with M as the metal atom and R as the alkyl group, initially undergoes partial or total hydrolysis as shown below [75] M(OR)n + x H2 O → M(OH)x (OR)n−x + x ROH, x ≤ n

(3)

Here, the hydrolysis process is dependent on the metal precursor, water content, catalysts, and the relative rate of hydrolysis and polycondensation. After hydrolysis, the material undergoes a condensation process as shown below [75]: −M − OH + H − O − M → −M − O − M − + H2 O

(4)

Metal-alkoxide precursors form oxide particles interacting through van der Waals force or H-bonding. The hydrolysis in water forms a colloidal gel while polymeric gel is formed in presence of acid or base, respectively. Smaller nanoparticles are obtained when the hydrolysis rate is reduced [76]. The growth and aggregation of the particles take place in the sol in such a way that they grow as one-dimensional, two-dimensional, or three-dimensional materials [75].

4.2 Particulates of Sols and Gels Gels are formed from sol by aggregation and gelation process. Aggregation implies the cluster formation of smaller entities and gel is a semi-rigid dispersion of a solid in liquid or gas form which spans the entire volume. In case of aggregation the particles are relatively very far apart while in case of gelation they are closer [77]. The preparative conditions of sols, its drying and sintering processes determines the pore size, its distribution, and volume in the resulting material. As a result, the final product may have varied physical and chemical properties. The stability of sol and particle size plays a vital role in constituting the characteristics of the yield. The initial stage of the particulate sol–gel route necessitates uniformly distributed nanoparticles with size comparable to the pore size [78]. Just by changing certain parameters of the hydrolysis-peptization process, sols of different particle size can be obtained [79]. It was found that the peptization temperature had a greater influence on the sol stability followed by water/alkoxide and acid/alkoxide molar ratio. Mohammadi et al. [80] analysed the effect of several preparative parameters on the particulate sol–gel process. They found that for higher molar ratio of acid to alkoxide, a high surface charge is acquired by the particles inhibiting the clotting and flocculation of particles by electrostatic repulsion. As the acid concentration increased the interparticle distance reduced resulting in agglomeration. On varying the annealing temperature, the crystallite size increased as the temperature increased. However, the particle size was reduced on heat treatment. Enhancement in the surface area has been observed due to reduction in average crystallite and grain sizes with decreasing peptization temperature along with an increase in phase

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Fig. 13 Gel time effect on the surface area and pore volume of ZrO2 aerogels. (Reproduced from ref 81 with permission. Copyright 1993 American Chemical Society)

transition temperature as the particle size decreases [80]. The surface area and the pore volume are depended on the gel time as shown in Fig. 13. Time zero, in graph, represents the formation of precipitates instead of gel when no acid is added to the prepared solution. When acid is added protonation of the functional group begins which eventually slows down the condensation process allowing the branching to occur. This increases the surface area and pore volume of the product on calcination. After an optimum time i.e. optimum addition of acid, there is a decrease in the condensation process forming a weak branched network. This network collapses during calcination, reducing the surface area and the pore volume of the prepared material [81].

4.3 Selection and Optimization of Materials Apart from emphasis on the morphological aspect of the film, there are other factors like uniformity and crack free coating which needs to be evaluated for the successful deposition or coating of any substrate using the sol–gel method. These films are expected to be highly adhesive to the surface and do not peel-off in long run. The method used to dry the gels may initiate such problems since the heating process of gel aids shrinkage. Shrinkage induces tensile stress on the film which ultimately forms cracks on the film surface. Atkinson et al. [82] found a relationship between the mechanical stability of the film and its thickness. They prepared sols of ceria and coated it on stainless steel substrate using the spin coating method. The thickness of the coating was varied from 0.34 nm to 2.07 μm for the same gel density through controlled speed of the spin and sol viscosity. The films gelled as soon as they were coated on the substrate. It was found that for thickness 0.61 μm the cracks

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were initiated which increased further as the layer deposition increased. The spacing between the cracks and gel thickness showed linear relation [82]. Thus, optimization of the film thickness during coating purposes plays a pivotal role during the sample preparation.

4.4 Strategies to Improve the Sol–Gel method There are several modifications done to advance the final products of the sol–gel process. A few are discussed below: (1) Non-hydrolytic sol–gel process The non-hydrolytic sol–gel process is the same as that of the non-aqueous process. If solvents like alcohols, ketones, esters, and aldehydes are used for dissolving metal precursors, it is called a non-hydrolytic approach of the sol– gel process. It is a slow process and provides a better control over the particle homogeneity and size which are also reproducible [43]. (2) Pechini method Pechini method is used to synthesize thin films, nano-crystalline powders, and bulk materials [43]. Pechini [83, 84] added citric acid and ethylene glycol to metal cation solution to synthesize polymeric resin precursor that decomposes at 573 K. The polymeric resin reduces the segregation of the cations. The two main chemical reactions of the Pechini process are (1) chelation of metal ions using carboxylic groups like citric acid or EDTA and (2) polyesterification of excess hydroxycarboxylic acid with ethylene glycol. During the chelation process, the metal cations are coated by complex ring-shaped compounds which allows homogeneous distribution of the chelated metal ions in the polymeric network. The prepared viscous liquid can be converted into a gelatinous precursor by drying it in vacuum. The precursors are further calcined and pulverized to remove any organic substances and to break down any agglomerations formed, respectively. In the Pechini method, the particles are trapped in polymer gel while in the traditional sol–gel method it is a part of the gel structure. This induces limitations on the Pechini method where the particle morphology and sizes are not controlled. However, sintering, initial metal concentration, and intermediate resin formed can provide some control over the reaction [85].

5 How to Apply Sol–gel to Get Thin Film The advantages of the sol–gel method are that the sols can be deposited on different substrates before gelation. Different morphologies like thin films, nanofibers, nanowires, nanotubes, and spherical nanoparticles can be obtained from sol–gel precursors by using different approaches like dip coating, spin coating, doctor blade, electrodeposition, electrospinning, flow coating, blow spinning, spray drying and

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Fig. 14 Steps in dip coating method

spray coating methods [28, 30, 83, 84]. All these methods have been briefly elaborated in the following section.

5.1 Dip Coating Dip coating is a simple method for excellent reproducibility of thin film from sol–gel solutions. During this process the substrate is consecutively immersed into the sol and taken out at a constant speed as shown in Fig. 14. By controlling the withdrawal speed and the concentration of the solution the thickness of the film can be controlled. As withdrawal speed and concentration increases the film thickness also increases [53]. Microstructures in thin film are controlled by the morphology of the inorganic precursors, the reactivity of the precursors, reaction time, and the amount of shear field and capillary forces applied during film deposition [88].

5.2 Spin Coat Figure 15 depicts the stages involved during the spin coating process. It includes dispensing of sol on the substrate, providing high-speed rotation to remove extra solutions, and finally evaporating the film. Thickness and microstructures of the film rely mostly on the solution dispensed, spinning time, speed, atmosphere, sols surface tension, viscosity, temperature, and the duration for which the heat treatment is provided [8, 48]. During the spin coating process, the centrifugal force which

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Fig. 15 Stages of spin coating process

tries to spin off the material and the viscosity which resists such process, are in equilibrium. The main drawback associated with the dip coating and spin coating method is that very thin deposition is obtained on the surface [88].

5.3 Doctor Blade Under this technique, a uniform film of well-defined thickness can be obtained on the surface of the substrate. In this process, films are obtained by placing a sharp blade at a fixed distance from the surface of the substrate. The sol to be coated is placed in front of the blade and is moved inline to the surface creating a wet film. It is ideally believed that there is only 5% loss of chemicals in this process, but it takes some time to obtain the optimal condition. The sol used for deposition is a mixture or slurry of a large amount of binders and thickeners to produce high viscosity which is required to generate reproducible and reliable films [89].

5.4 Electrodeposition In this method, the film thickness and morphology are controlled by changing the voltage applied, deposition time, and concentration of the suspension. Both anodic and cathodic potentials allow the electrodeposition of sols on the substrate.

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5.5 Electrospinning The purity of electrospun material depends on the quality of the precursor material used and the careful handling of the conversion process. The salt or precursors and the catalysts used must be mixed very well at the molecular level. The pH of the sol should be maintained at a low level, also the salts used must be of high solubility since during the evaporation process if the solvent is highly volatile and the metal salts are sparingly soluble in it then it may crystallize thereby destroying the homogeneity of the film. Figure 16 shows the schematic of the electrospinning method. The disadvantage associated with this method is its low output. Although through high voltage this can be increased but the combination of sparks and flammable solvents restricts its use [90]. The materials prepared by the electrospinning process are highly influenced by the relative humidity and the chemical nature of the precursor solution. It can manipulate the morphology and mechanical properties of the material. As the humidity increases, the precursor solution absorbs the ambient water and does not allow the drying process to complete when the solution jet reaches the substrate. Vrieze et al. [91] in their study found that the diameter of cellulose acetate nanofibers increases with relative humidity while the diameter of poly(vinylpyrrolidone) (PVP) nanofiber decreases as the humidity increases [91]. Pelipenko et al. [92] obtained thicker nanofibers of homogeneous size at lower relative humidity while at higher relative humidity they obtained more heterogeneously distributed thinner nanofibers. As reported before this is also attributed to the polymers compatibility with relative humidity [92].

Fig. 16 Electrospinning method [90]

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5.6 Blow Spinning It is a rapid technic for the synthesis of nano and micro fibers without the use of any electric field. However, in this method more inter-fiber pores are observed as compared to electrospun materials [93]. In this process high-speed gas stream is used on polymer solution to obtain elongated nanofibers. These nano-fibers become solid as the solvent evaporates during the spinning process [94].

5.7 Spray Coating Used mainly to coat any irregular surface with a protective layer. During the spray coating process very, fine droplets are sprayed onto the hard surface. These droplets are not fine liquid drops but small dried particles, in the nanometer range.

5.8 Roll Coating In roll coating process, the sols or liquid are allowed to pass through a narrow gap between two cylinders which are rotating. The surfaces of these cylinders either roll in the same direction or in opposite directions forming two films on each cylinder.

5.9 Flow Coating In this process, the sol is poured over the substrate as shown in Fig. 17. The coating thickness is depended on the inclination of the substrate, viscosity of sol, and evaporation of the solvent. The advantages of such processes are that any non-planar surface can be coated. But the drawback associated with this process is the nonuniform thickness of the film. Since the liquid flows from top to bottom the thickness decreases as it flows from to top to bottom. So, this process can be used in conjuncture with the spin coating to obtain a uniform coating on the surface.

6 Advantages and Disadvantages • The advantages of the sol–gel method are [7] – Low temperature reactions – Large scale deposition

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Fig. 17 Flow coating method

– Good compositional control which is essential to modulate the morphology of the material – Allows the production of complex organic–inorganic composite materials – Provides a high-purity product which can be processed for large-area applications. • Disadvantages of the sol–gel process are – – – – –

Other methods are required for the deposition of thin film from gel material Sensitive to atmospheric condition Expensive raw materials Use of toxic solvents Volume shrinkage and cracking during the drying process. Such cracks or pores may absorb the moisture and other impurities which weakens the bonds in the product and eventually degrades the quality of the synthesized material [7, 9]. – During chemical reactions between the precursors, the incorporation of impurity in terms of undesired atoms, molecules, ions, and functional groups can be observed in the final product. Thus, worsening the physicochemical properties of the coatings. A high-temperature annealing may be required to discard such impurities [22]. – During post-thermal treatments, uncontrolled growth of nanocrystals or carbon doping is observed which eventually hampers the material phase, size, and shape, tailoring the exposed surface.

7 Sol–Gel Literature Review 7.1 Nanostructure Metal Oxides Metal oxide nanoparticles have unique physicochemical properties that can be influenced by their surface area, shape, size, conductivity, anti-corrosivity, crystallinity, and stability [95, 96]. Lakshmi et al. [97] used the sol–gel route to prepare TiO2 , WO3, and ZnO nanofibrils and nanotubules within the pores of an alumina template

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membrane just by dipping the porous alumina membrane into the sol. By controlling the temperature and immersion time both the structure and thickness of the material were controlled [97]. The sol–gel process can help in the crystallization of the metastable phase because of the low temperature associated with the synthesis process [98]. Li et al. [71] studied the effect of annealing temperature on the ZnO film microstructure, morphology, and its transparent characteristics. The particle size seems to increase as the post-heating temperature increased. This is due to the enhancement in the crystal orientation as the post-heating temperature increased. Increase in temperature-initiated grain densification and augmented the crystal growth reducing the grain boundary density. Packing density increased initially with temperature but later it was reduced due to the evaporation of ZnO. The grain size became larger forming a hexagonal shape with the evaporation of ZnO [71]. Hasnidawani et al. [1] prepared ZnO nano powders using zinc acetate dihydrate, NaOH, and ethanol with water as the solvent medium. Zinc acetate, due to heating undergoes hydrolysis forming zinc hydroxide acetate which is transformed into ZnO at higher temperatures and prolonged refluxing. The morphological and XRD analysis showed that ZnO nanorod with good crystallinity were formed, with particle size less than 100 nm [1]. Amri et al. [22] investigated the structural and optical features of nanostructured Cu-oxide thin films prepared using a sol–gel dip coating route. Figure 18 depicts the SEM images of CuO thin film deposited over aluminium. SEM image shows agglomerated nanoparticles forming a porous structure which are essential for solar selective absorber [22]. Kuang et al. [99] prepared 12 rare-earth-based metal oxide nanotubes using nitrate solutions of the rare earth materials via the sol–gel route with the help of anodic aluminium oxide (AAO) template. Figure 19 shows the TEM images of various synthesized rare earth oxides. It shows bamboo-like nanostructures divided into many cavities and also ordinary nanotubes with an entire hollow interior. Advantages of using this process are homogeneity of the constituent material at the molecular level which is obtained by controlled hydrolysis and condensation process. Figure 20 shows the HRTEM image of Ho2 O3 rare earth oxide nanotube. The height of the separated cavities is not uniform whereas in some it remains uniform. Only drawback associated with this method is the existence of alumina from the template on the nanostructures even after dissolving the template. Hence proper dissolving of the template becomes essential to obtain a pure nanotube. The sols are usually viscous which certainly influences the quantity of sol that fills the AAO pores. Calcination action converts sols into gels which decomposes and crystallizes to form RE oxides, along with the evaporation of the by-products. Figure 21 shows the schematic of RE oxide tube formation. The mechanism proposed by the authors is that non-availability of organic components in the sol allows strong interaction between AAO and the gel which allows the filling of the pores. The viscosity of the sol and the gas released during the reaction governs the creation of straight and bamboo-like nanotubes. Released gas acts as the template for bamboo-like gel nanotubes. The exterior of the straight nanotubes is denser than the bamboo-like nanotubes indicating the dissimilarity in the viscosity of the sol when preparing the two structures [99].

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Fig. 18 FESEM micrographs of Cu-oxide thin film coatings [22]

7.2 Nanostructure Metal Sulfides Qian et al. [100] prepared a highly crystalline and stable multimetal sulfide/TiO2 heterostructure through sol–gel method for photocatalytic analysis. Mixed morphologies of Bi2 S3 , CdS, and MoS2 nanoparticles were grown over the porous TiO2 . The length of the Bi2 S3 nanowires were 300 nm, and MoS2 and CdS nanoparticles were 30 nm on average [100]. Du et al. [101] prepared pyramid-like nanoparticles of CdS over TiO2 monolith using a sol–gel process where polystyrene spheres were used as the template. Figure 22 shows the TEM images of CdS-TiO2 material. On closer observation it shows randomly distributed CdS nanoparticles on the porous TiO2 column surface. EDS mapping shows a homogeneous distribution of elemental O, Ti, Cd, and S (Fig. 22d). TEM analysis shows CdS with pyramidal morphology exhibiting a hexagonal plane. Self-assembly of small CdS nanoparticles forms the pyramidal morphology of CdS [101]. Sonker et al. [102] used sol–gel method for the synthesis of CdS nanoparticles of spherical morphology with a grain size of ~127 nm for low-temperature NO2 sensor.

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Fig. 19 TEM images of various rare earth oxides [99]

The normal diameter of the particles was about 40–60 nm having a hexagonal shape uniformly distributed [102]. Yang et al. [103] used sol–gel method for the preparation of ZnS nanoparticles on nitrogen and sulfur-doped carbon nanosheets (ZnS@NSC). Figure 23a shows the schematic for the synthesis of ZnS@NSC using thiourea, glucose, and Zn(NO3 )2 .6H2 O of different concentrations in a mixture of DI water and ethanol followed by stirring and heating. SEM images in Fig. 23b, c, and d show crumbled nanosheet morphology. The crumpled morphology can increase the surface area of the material. TEM images (Fig. 24) show that ZnS nanoparticles having diameters of 50–100 nm homogeneously covered the nitrogen and sulphur doped graphene analogues carbon nanosheets. Also, the ZnS nanoparticles are coated with ultrafine carbo shell of thickness 3–4 nm. Such morphology provides lower resistivity for the ZnS nanoparticles. The sol–gel method attributes to such a fine structure in which the elements are able to bond in liquid form [103]. Sankar et al. [104] used

13 Sol–Gel Derived Thin Films

Fig. 20 HRTEM images of HO2 O3 nanotubes at different magnifications [99]

Fig. 21 Schematic of the possible two mechanisms for RE oxide nanotube growth [99]

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Fig. 22 TEM images of CdS nanoparticles on TiO2 surface [101]

Fig. 23 a Schematic of the ZnS nanoparticles doped on Nitrogen and Sulfur doped carbon nanosheets. SEM images for ZnS@NSC treated at 700 °C with Zn precursor mass, b 0.8 g. c 1.2 and d 1.8 g [103]

sol–gel process towards the synthesis of cadmium sulfide (CdS) nanoparticles by Ni doping and studied it for methylene blue and methylene orange degradation. The XRD analysis showed hexagonal structures of CdS nanoparticles without any impurity except for Ni ions. The particles obtained were defect free. As the doping concentration of Ni is varied, the average crystallite size decreased initially and then increased (9.86, 8.87, 8.70, and 9.93 nm). The minimum crystallite size of 8.7 nm has been achieved through the sol–gel process. Figure 25 exhibits surface morphology of 1.5% Ni doped CdS with anisotropic growth of rod and cube like structures. These morphologies were overlapped because of the high surface energy. The TEM image

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Fig. 24 TEM and HRTEM analysis of ZnS@NSC [103]

Fig. 25 SEM images of CdS nanoparticles doped with Ni (1.5%) [104]

shows porous spherical clustered nano-assemblies of Ni-doped CdS nanoparticles [104].

7.3 Nanostructure Metal Telluride Grishanov et al. [105] prepared graphene oxide supported tellurantimony (Sb2 Te3 ) anode electrodes for sodium and lithium-ion batteries using sol–gel method. The addition of hydrogen peroxide (H2 O2 ), hydrogen donor, stabilized the nanoparticles and they further attached to the graphene oxide surface due to the O-bonds. Figure 26 showed an ultrathin and homogeneous Sb2 Te3 -rGO film deposited on the graphene oxide with a crystal size of 300 °C the sample showed crystal formation. At temperatures ≥400 °C the samples showed the formation of α, β, and γ- phases of In2 Se3 thin films. However at 600 °C a peculiar behaviour were observed as peaks corresponding to γInSe3 were quenched and peaks corresponding to α-In2 Se3 became more prominent [112]. The following table summarizes the literature review of the reported materials (Table 1).

Titanium isopropoxide, zinc acetate and WCl3 with ethanol

Zinc acetate dihydrate, 2-methoxyethanol and monoethanola- mine (MEA)

Zinc acetate dihydrate, NaOH and ethanol with water

Copper nitrate, propionic acid and ethanol

Rare earth oxide and nitric acid

Cd(Ac)2 ·2H2 O (AR), Sol–gel with Bi(NO3 )3 ·5H2 O (AR), hydrothermal Na2 S·9H2 O (AR), (NH4 )6 Mo7 O24 ·4H2 O (AR), ice acetate acid (AR), thiourea (AR), K2 S2 O8 (AR), tetrabutyl titanate (TBOT, AR), styrene (AR), alpha-methyl acrylic acid (CP), and absolute ethyl alcohol (AR)

TBT in ethanol and ice acetate acid in DI water

TiO2 , WO3 and ZnO

ZnO

ZnO

CuO

Rare earth oxides

Bi2 S3 -CdS-MoS2 -TiO2

CdS@TiO2

Sol–gel with hydrothermal sulfuration process

Template assisted dip coating

Dip coating

Precipitate formation

Spin coating

Template assisted dip coating

Precursor materials and solvents Deposition process

Material

Table 1 Literature survey of various deposited materials using sol–gel method

Pyramid like CdS over TiO2 monolith

Porous nanostructure

Nanotubes

Agglomerated nanoparticles

Nanorod

Round shaped particles at temperature 400 and 500 °C. At 600 °C the grains become hexagonal shape

Nanofibrils and nanotubules

Morphology

Application

(continued)

[101]

[100]

[99]

[22]

[1]

[71]

[97]

Ref

13 Sol–Gel Derived Thin Films 565

Nanoparticles

Thiourea, glucose and Sol–gel Zn(NO3 )2 ·6H2 O were dissolved in a solution containing DI and ethanol

Cadmium acetate (Cd Sol–gel (CH3 COO)2 )Nickel chloride (NiCl2 ), Sodium hydroxide (NaOH), acetone (CH3 COCH3 ), absolute ethanol (C2 H5 OH)

SbCl5 , Te(OH)6 , Sol–gel tetramethylammonium hydroxide, ammonia, H2 O2 , GO

Lead acetate trihydrate (Pb(OAc)2 .3H2 O) tellurium powder, 4-fluorothiophenol, 1-octadecene and tetranitromethane; trioctylphosphine; triethylamine, oleic acid, and H2 O2

ZnS nanoparticles on Nitrogen and Sulfur doped carbon nanosheets (ZnS@NSC)

CdS

Sb2 Te3 on r-GO

PbTe

Sol–gel process

Sphere and cubes

Nanocomposite

Nanoparticles and nanorods

Nanoparticles encapsulated by carbon sheet on graphene like carbon nanosheets

Morphology

Cadmium acetate hydrate, diaminobenzene and thioacetamide

CdS

Sol–gel

Precursor materials and solvents Deposition process

Material

Table 1 (continued)

Thermoelectric device

Na and Li ion batteries anode

Application

(continued)

[106]

[105]

[104]

[103]

[102]

Ref

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Bismuth neodecanoate, bismuth Sol–gel method acetate, antimony acetate, tellurium powder, 1-dodecanethiol, diphenyl ether, and tetranitromethane trioctylphosphine and triethyl amine, oleic acid

Selenium acid, TEOS, Water, Ethanol, nitric acid

Bismuth nitrate pentahydrate (Bi(NO3 )3 .5H2 O), elemental selenium powder, solvent DMF (dimethylformamide),ethanol, nonidat, NaOH

Tetraethoxysilane, HCl, aerosol, Sol–gel ammonia, Cu(NO3 )2 , Se

Cu(NO3 )3 , Ga(NO3 )3 , citric acid, ethylene glycol

SeO2 , HCl, InCl3 , glacial acetic Sol–Gel and dip acid, ethanol and methanol coating

Bi2 Te3 and Bi2-x Sbx Te3

Se QDs in SiO2 network

Bi2 Se3

Cux Se

CuGaSe2

In2 Se3

Sol–gel and selenization

Comparison of particles prepared by sol–gel and hydrothermal method

Solvothermal assisted sol–gel method

Precursor materials and solvents Deposition process

Material

Table 1 (continued)

–

Nanoparticles

Nanoparticles

Sol–gel: nanoparticles Hydrothermal: nanorods

QDs

Nanoparticles

Morphology

–

Antibacterial activity

Thermoelectric device

Application

[112]

[111]

[110]

[109]

[2]

[107]

Ref

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8 Applications 8.1 Solar Cells Anti-reflection (AR) coating for photovoltaic cells plays an important role as these can transmit infrared radiations and prevent losses. High refractive index materials which are used in infrared regions face reflection loss at their surface which can be avoided through the coating of AR. Usually, the AR coatings are realized through physical methods which are expensive and stress induced. These can also be easily peeled off under high-energy irradiation. Hence, the sol–gel method can be used for the formation of these AR films. In case of silicon photovoltaic cells, the sol–gel route were employed to cover the glasses, to protect the silicon, and to form an anti-reflecting film which avoids loss of light [4]. Oh et al. [113] studied ZnO, TiOx, and aluminium doped ZnO (AZO) as electron extraction layer (EEL) in organic solar cells. ZnO and TiOx films were amorphous in nature when dried in air, wurtzite phase of ZnO and anatase phase of TiOx were obtained. AZO showed a preferred orientation along the c-axis i.e. along (002) plane. The calculated average crystallite size of ZnO, AZO, and TiOx were 4.2, 4, and 12.5 nm, respectively. The conductivity study showed that the ZnO due to the presence of oxygen molecules showed lower conductivity as compared to TiOx . The conductivity as such is weakly dependent on the doping ratio but it increases as the doping concentration increases which eventually reduces after reaching an optimum value. Devices with ZnO in acetone and TiOx in EtOH showed performances between 2.5% efficiency after optimization. This was much better than the results obtained for ZnO in EtOH or isopronol. Al-doped ZnO exhibits less scattering with a good fill factor (FF). Table 2 shows the J-V characteristic of inverted organic solar cells having different interfacial layers. The J–V characteristics as a function of doping concentration (Fig. 28a) demonstrate strong photoshunt resistance which is independent of the doping ratio. Al doping variation does not make any major impact on the device’s performance. The J-V characteristics predict the dependency of power conversion efficiency on the annealing temperature (Fig. 28b). For higher temperatures the PCE obtained Table 2 J–V characteristic of inverted organic solar cells having different interfacial layer [113] Samples

V oc (mV)

J sc (mA/cm2 )

FF (%)

PCE (%)

ZnO in EtOH (non-optimized)

0.51

6.63

48.5

1.64

ZnO in acetone (non-optimized)

0.52

8.02

46.3

1.93

ZnO in IPA (non-optimized)

0.52

7.11

43.0

1.59

ZnO in acetone (optimized)

0.56

8.33

56.5

2.62

TiOx in EtOH

0.54

7.02

50.5

1.91

TiOx in EtOH

0.57

8.38

54.4

2.58

1 at% AZO_EtOH at 260°C

0.54

8.97

55.0

2.65

13 Sol–Gel Derived Thin Films

(a)

569

(b)

Fig. 28 J–V characteristics at a varied Al concentration, b at various temperature [113]

was 2.59% [113]. Sun et al. [114] synthesized CZTS thin films using a sol– gel spin coating process and high-temperature sulfurization annealing method. CZTS is regarded as one of the most promising low-cost light harvest absorbers. The SEM analysis for different ratios of Cu/Sn subjected to sulfurization at a high temperature of 560 °C shows inhomogeneous grains with the largest grain size of 2000 nm. As the Cu/Sn ratio increases, the graph shows smaller voids with better homogeneity. This is attributed to the volatile nature of SnS during sulfurization annealing. As the ratio increased the grain size decreased to 500–1000 nm with uniform high-quality film formation. The Voc of the samples with higher Cu/Sn ratio shows 130 mV higher values than the smaller ratio. As the ratio increases, the fill factor and power conversion efficiency increases due to reduced shunting issue caused by pinholes. Cu/Zn = 1.93 showed a peak EQE of 90% at 570 nm wavelength [114].

8.2 Gas Sensors Metal oxide-based sensors have fascinated much courtesy due to their high response, and low cost. NO2 is one of the most hazardous gases emitted during the combustion process and exhaust of cars. It also contributes to the formation of hazardous gas like O3 (ozone) [115]. Zhao et al. [116] synthesized p-type nanocrystalline NiO thin films on ITO substrate by sol–gel dip coating method for NO2 sensor. Synthesis by sol– gel method helps in the preparation of excellent nanocrystalline, highly porous, and uniform film which is essential for an enhanced sensing performance. The FESEM analysis of the synthesized NiO film showed a uniform crack free homogeneous film with a porous surface embedded with nano crystalline particles whose size ranged from 20 to 30 nm providing a huge specific surface area to absorb enough quantity of gas molecules thus enhancing the gas sensing properties. The TEM (Fig. 29a and b) analysis of the NiO nanocrystalline particles shows polycrystalline structure

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Fig. 29 NiO TEM images [116]

with an average particle size of about 23 nm. The interplanar spacing of 0.24 nm confirms (111) plane of cubic phase. Highest response was achieved at operating temperature of 150 °C with 20 ppm exposure of NO2 gas (Fig. 30a). Dynamic gas response at different NO2 gas concentration operated at 150 °C is shown in Fig. 30b [116]. Hsu et al. [117] studied the response of CuO-doped ZnO nanofibers for H2 S gas sensing. They used sol–gel and electrospinning method to form CuO-doped ZnO heterojunction nanofibers. Figure 31 shows the SEM images of various ratios of CuO doping on ZnO nanofibers after annealing at 550 °C for 4 h. The average diameter of fiber was found to increase with the concentration of CuO and their surface remained smooth and uniform. Figure 32a and b shows the LR-TEM image of CuO/ZnO NFs at different magnifications. The result shows that the nanofiber is composed of closely packed nanoparticles forming a porous structure. The porous morphology of the fiber has been attributed to the PVP decomposition at high temperatures. The gas sensing ability of 0.15 mol CuO doped ZnO was evaluated at different temperatures of 150, 175, 200, 225, 250, and 275 °C. At 200 °C CuO doped ZnO nanofibers showed the optimum response (Fig. 33a). 0.15 mol CuO doped ZnO nanofibers showed 84% response for 1 ppm H2 S gas at 200 °C. Figure 33b shows the response of the material for different ppm of H2 S gas at 200 °C. The sensor showed an increase in response as the concentration of the gas varied from 1, 2, 3, 4, to 5 ppm indicating a linear relation. Such a good response of CuO/ZnO nanofibers has been attributed to the reduction–oxidation or sulfuration-desulfuration process of the material [117].

8.3 Supercapacitors As material plays an important role in the charge storage capability of supercapacitors Jeyalakshmi et al. [118] studied V2 O5 thin film for electrochemical applications. The thickness of V2 O5 was varied on FTO glass using the sol–gel spin coating method. The spin coating process was repeated to get 6, 8, 10, and 12 layers of uniformly coated films dried and annealed in air at 300 °C for an hour. The SEM analysis showed the formation of nanorods, as the layer coating increases the aggregation of nanorods changes the morphology to nanosphere. The cyclic voltammetry analysis in

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Fig. 30 Graph a NiO thin film response on exposure to 20 ppm of NO2 gas at different operating time, b response at different concentrations of NO2 gas [116]

Fig. 31 SEM image of a 0, b 0.1, c 0.15, d 0.2 and e 0.25 molar ratio of CuO doped ZnO nanofibers [117]

1 M LiClO4 with propylene carbonate showed that the area enclosed by coating with 8 layers within a potential window of −0.8 to 1.2 V was larger than that observed in the case of 6, 10, and 12 layers. This indicates that the charge intercalation and deintercalation process is more effective in the case of 8-layered films. The peaks observed in the graph are due to the partial oxidation/reduction of vanadium ions i.e. V5+ to V4+ and V3+ ions. The 8-layered coating showed a maximum specific capacitance of 346 F g−1 where the nanorods just begin to form. This value was greater than the nanorod and the other layer of the V2 O5 on FTO [118]. Baig et al. [119] developed spherically shaped NiFe2 O4 nanoparticles dispersed in a SiO2 matrix as an electrode material for supercapacitor application. They used the Stober method with the sol–gel process to synthesize the material and obtained a maximum specific capacitance of 925 F g−1 . Figure 34a shows the schematic of the process undertaken. Figure 34b shows the SEM images of NiFe2 O4 /SiO2

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Fig. 32 LR-TEM images of 0.15 mol CuO doped ZnO nanofibers after annealing at 550° for 4 h [117]

Fig. 33 Response of CuO/ZnO nanofibers to H2 S gas at different temperatures (b) response to different ppm of H2 S gas at 200 °C [117]

at different magnifications. The image shows roughly spherical-shaped NiFe2 O4 uniformly distributed in the SiO2 matrix. The size of NiFe2 O4 /SiO2 varies between 25 ± 5 nm and 36 ± 5 nm. Figure 35a shows the CV response of Ni foam, NiFe2 O4 /Ni, and NiFe2 O4 @SiO2 /Ni materials. NiFe2 O4 @SiO2 /Ni shows two peaks indicating that SiO2 does take part in the electrochemical performance of the material. The peak current values are delivered by NiFe2 O4 @SiO2 /Ni compared to that of Ni foam and NiFe2 O4 indicating that SiO2 does contributes towards the efficiency of the device. NiFe2 O4 @SiO2 /Ni shows capacitive behaviour and this increases as the scan rate increases. The capacitance of NiFe2 O4 /Ni was calculated to be 525 F g−1 at 1 A g−1 while that of NiFe2 O4 @SiO2 /Ni electrode was found to be 833.3 F g−1 at the same current density. The superior performance of the later has been attributed to the SiO2 coating which provides a stable electrode/electrolyte interface and contains the volume change in NiFe2 O4 . As the current density increased from 1 to 2, 3, 4, and 5 A g−1 , the specific capacitance decreased to 833.3, 575, 366.6, and 333.3 F g−1 ,

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Fig. 34 a Schematic of the work undertaken during the synthesis of NiFe2 O4 /SiO2 material, b SEM images NiFe2 O4 /SiO2 material at different magnifications [119]

Fig. 35 a CV analysis of Ni foam, NiFe2 O4 /Ni, NiFe2 O4 @SiO2 /Ni at 100 mV/s [119]

respectively. The cyclic stability of NiFe2 O4 @SiO2 /Ni was about 95.5% even after 5000 cycles [119]. Table 3 includes a comparative detail about materials prepared by the sol–gel method and other methods, for supercapacitor application.

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Table 3 Literature details on the materials fabricated using different methods and there corresponding Ragone plot details Material

Method

Capacitance

Energy density (Wh kg−1 )

Power density (W kg−1 )

Ref.

Zno/PEDOT:PSS

Electrospinning method

1.22 mF cm−2

5.685

0.81 k

[120]

Ni doped β-V2 O5

Sol–gel

417 F g−1

192

3.46 k

[121]

g−1

Ni doped V2 O5

Electrodeposited

323 F

α-Fe2 O3

Sol–gel

582.80 F g−1 g−1

3.5 k

[122]

74

0.5 k

[123]

Fe2 O3

SILAR

180 F

16

800

[124]

MgFe2 O4

Auto combustion method

428.9 F g−1

18.01

21,468

[125]

MgFe2 O4

Sol–gel

61 F g−1

41

550

[126]

ZnO/AC composite

Hydrothermal method followed by chemical etching

126 F g−1

25.2

896.44

[127]

ZnO/AC composite

Sol–gel

118.122 mF cm−2

3.41

0.45

[128]

Li doped NiFe2 O4 /SiO2

Sol–gel

31.4 F g−1

2.45

[129]

9 Conclusions Sol–gel method is a more feasible process for the synthesis of thin films, powder, xerogels, aerogels, and glasses. This process provides full control over the size and shape of the particles synthesized since the nucleation and growth of the particles can be regulated at the preliminary stage itself. It is a low-temperature method. Small variations in the selection of precursors, solvent used, their ratio, etc. can play a vital role in the synthesis of material. Inorganic and organic materials can be brought together by using this method. Due to its versatility, simplicity, and cost effectiveness, this method is so far prevalent in thin film and coating industries. However, its potential is far beyond just coatings. These are being used in textile industries, pervaporation membrane formation, sensors, powder making, bioactive glasses, optoelectronics, semiconductors, biotechnology, composites, etc. The main constraint for extensive application of sol–gel method is the raw materials. Conventionally used precursors like metal alkoxides are expensive and for some metals these are not available. Moreover, these are not readily soluble in mild diluents. They mostly require alcoholic solutions which are toxic when used in plenty. Nevertheless, the by-products released during the synthesis process may incur environmental threats. As there is very little control over the porosity of the film formed, the rate at which the solvent is evaporated or removed from the gel is declined.

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Besides these long preparation time, aging and curing time act as the limitation in the use of the sol–gel method for industrial purposes. The possible solutions for some of the above-mentioned perils are the use of metal salts instead of metal alkoxides as these are less expensive. Implementing new methods for the removal of solvents or alcohols like rotavaporization or the use of any other alternatives for alcohols which can easily dissipate. Controlling the porosity by selecting appropriate precursors and thermal curing.

10 Challenges and Future Scope In sol–gel method, the number of steps involved makes it an inconvenient process. Other factors which act as a challenge for the large-scale application of this method are the expensive precursors, adhesive properties of the material, cracks generated in the film, etc. Still, as per market research the demand for the sol–gel method will increase in near future. This method is worthwhile for optical and opto-mechanical industry. This technology is used to make indium tin and oxide films for low-cost solar cell applications. This method of material synthesis is highly recommended in the medical field like tissue engineering and drug deliveries. It needs to be further probed in textile industries for UV protection, oil repellence, and anti-wrinkle and antibacterial cloths. Since the main advantage of this process is the synthesis of inorganic materials, the development of a synthesis strategy needs to be found.

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Dr. Nikila Nair is currently a post-doctoral research associate working in the Department of Mechanical Engineering, Indian Institute of Science, Bengaluru. Formerly she was working as a Lecturer at NSS College, Kerala from 2016 to 2019. She completed her Ph.D. from the Department of Physics, Visvesvaraya National Institute of Technology (VNIT), Nagpur (India) in 2017. She completed her master’s degree in physics from The Rashtrasant Tukadoji Maharaj, Nagpur University. On research front, she has worked as a lab engineer in the Center for VLSI and Nanotechnology, VNIT Nagpur. She has an hand on experience of working on VNL, Guassian, and VASP software. Her focus is on the synthesis of nanomaterials and their applications as supercapacitors and sensors.