Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials: Application on Health, Energy and Environment (Advances in Material Research and Technology) 3031394801, 9783031394805

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Advances in Material Research and Technology

Jai Prakash Junghyun Cho Bruno Campos Janegitz Shuhui Sun   Editors

Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials Application on Health, Energy and Environment

Advances in Material Research and Technology Series Editor Shadia Jamil Ikhmayies, Physics Department, Isra University, Amman, Jordan

This Series covers the advances and developments in a wide range of materials such as energy materials, optoelectronic materials, minerals, composites, alloys and compounds, polymers, green materials, semiconductors, polymers, glasses, nanomaterials, magnetic materials, superconducting materials, high temperature materials, environmental materials, Piezoelectric Materials, ceramics, and fibers.

Jai Prakash · Junghyun Cho · Bruno Campos Janegitz · Shuhui Sun Editors

Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials Application on Health, Energy and Environment

Editors Jai Prakash Department of Chemistry National Institute of Technology Hamirpur, Himachal Pradesh, India Bruno Campos Janegitz Federal University of São Carlos Araras, São Paulo, Brazil

Junghyun Cho Department of Mechanical Engineering Binghamton University (State University of New York) Binghamton, NY, USA Shuhui Sun Center Energy, Materials and Telecommunications Institut National de la Recherche Scientifique (INRS) Montreal, QC, Canada

ISSN 2662-4761 ISSN 2662-477X (electronic) Advances in Material Research and Technology ISBN 978-3-031-39480-5 ISBN 978-3-031-39481-2 (eBook) https://doi.org/10.1007/978-3-031-39481-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 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 Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

In recent years, a great deal of research is focused on the synthesis of multifunctional nanomaterials and their potential multidisciplinary applications in a variety of fields. This book aims to provide an interdisciplinary platform for researchers covering an important class of novel materials composed of semiconductor photocatalysts and other functional nanomaterials. Owing to the synergistic effect and tunable functionality, these hybrid materials are emerging multifunctional and futuristic nanomaterials having the potential to be explored not only in engineering, energy and environment but also in biomedical fields. Since semiconductor photocatalyst nanomaterials are being used in all fields of science and have a great tendency to be modified through simple and cost-effective techniques, therefore, hybridizing with various functional nanomaterials provides advanced smart nanocomposite materials with multifunctionality. It consists of important research accomplishments of current interest based on the multifunctional application of novel nanohybrid semiconductor photocatalysts. It covers a wide range of research fields from energy and environment to the biomedical field and, therefore, we strongly believe that the present book would be appealing to the wider range of the scientific community. The present book provides a glimpse of recent advancements being carried out in the fundamental understanding of the coupling of semiconductor photocatalysts and their potential applications in sensing, energy, environmental and biomedical fields. This book consists of a total of 16 chapters covering synthesis, characterization, modification of various photocatalyst semiconductors and tailoring of their properties for producing multifunctional hybrid nanomaterials finding potential applications in health, energy and environment. Chapter 1 provides details of various semiconductor photocatalysts, their structures, the need for modifications and the formation of hybrid semiconductors. Chapter 2 discusses various synthesis techniques to produce multifunctional hybrid semiconductor nanomaterials along with their advantages and disadvantages/limitations. Chapters 3-5 are about the application of common metal oxide/hybrid photocatalysts and their health and environmental applications in the removal of bacteria/viruses and gaseous organic compounds. Chapter 6 discusses an important class of SiO2 -based hybrid multifunctional semiconductors and their applications in health, energy and the environment. Emerging hybrid semiconductor v

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materials based on graphene-based nanostructures, i.e. graphene oxide, have been discussed in Chap. 7. It includes wastewater treatment through the photodegradation of organic dye molecules and sensing toxic ions in water. Similarly, Chap. 8 discusses the sensing ability of several semiconductor hybrid nanomaterials in the field of health and environment. Chapters 9 and 10 provide potential applications of TiO2 and other metal oxides in wastewater and solar cell applications, respectively. Chapter 11 provides characteristics and potential applications of a special class of nanomaterials based on hybrid transition metal chalcogenides. Chapter 12 reports on the application of biomolecular sensing using the 3D printing technique. Chapter 13 introduces plasmonic-based hybrid semiconductor nanomaterials, their synthesis and biomedical applications in the field of bio-sensing, antibacterial activities, etc. Chapter 14 reports on a very important aspect of energy applications of photoelectrochemical water splitting particularly based on hybrid ZnO nanomaterials. In continuation to Chap. 13, plasmonic hybrid semiconductor nanomaterials have been explored in multifunctional applications in Chap. 15 that includes water splitting, solar cells, wastewater treatment as well as biomedical applications. Finally, Chap. 16 provides details on polymer-based hybrid semiconductor nanocomposite materials and their multifunctional applications. The chapters have been designed to provide the fundamental as well as technological applications of various semiconductor hybrid photocatalyst nanomaterials. It explores the capability of these special classes of nanomaterials of functioning in sunlight as well as ambient conditions due to their tailored electrical, optical and photocatalytic properties. This book covers the design and engineering of such nanohybrid semiconductor photocatalysts for improved performance, multifunctionality and durability which are of high interest to the academic, industry, energy, environmental and biomedical applications. We would like to acknowledge our students and colleagues for their support in this book project. We also acknowledge the publishing team from Springer for the kind support and cooperation throughout the completion/finalization of the book. Hamirpur, India Binghamton, USA Araras, Brazil Montreal, Canada

Dr. Jai Prakash Prof. Junghyun Cho Prof. Bruno Campos Janegitz Prof. Shuhui Sun

Contents

Introduction to Semiconductor Photocatalyst Nanomaterials: Properties, Modifications, and Multifunctional Applications . . . . . . . . . . . Sahil Thakur, Samriti, Abhijeet Ojha, and Jai Prakash Synthesis of Multifunctional Hybrid Semiconductor Nanomaterials . . . . Samriti, Rajeev Gupta, Olim Ruzimuradov, and Jai Prakash

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Application of Nanostructured Metal Oxides and Its Hybrids for Inactivation of Bacteria and Viruses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Junghyun Cho

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Advances in Semiconductor Photocatalyst Toward the Removal of Aromatic Volatile Organic Compounds in Air . . . . . . . . . . . . . . . . . . . . . . Swati Verma and Navneet Kumar

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Hybrid Semiconductor Photocatalyst Nanomaterials in CO2 Reduction and Storage Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zhangsen Chen, Shuhui Sun, and Gaixia Zhang

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SiO2 Based Multifunctional Hybrid Semiconductor Nanomaterials and Their Applications in Energy, Environment and Health . . . . . . . . . . . 127 Pratibha Sharma, Raj Kaushal, and Jai Prakash The Future of Graphene Oxide-Based Nanomaterials and Their Potential Environmental Applications: A Contemporary View . . . . . . . . . 153 Subhendu Chakroborty, Pravati Panda, and Suresh Babu Naidu Krishna Hybrid Semiconductor Photocatalyst Nanomaterials for Electrochemical Sensing Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 K. S. Shalini Devi and Seiya Tsujimura Role of TiO2 -Based Photocatalysts in Waste Water Treatment . . . . . . . . . 201 Rajpal Tyagi, Anuj Maurya, Samriti, Jai Prakash, and Seema Kohli

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Hybrid Photocatalyst Nanomaterials in Solar Cell Applications . . . . . . . . 221 Habtamu Fekadu Etefa and Vinod Kumar Transition Metal Chalcogenides-Based Nanocomposite for the Photocatalytic Degradation of Hazardous Chemicals . . . . . . . . . . . 239 Rama Gaur 3D-Printed Electrochemical (bio)sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 Luiz Ricardo Guterres Silva, Jéssica Santos Stefano, and Bruno Campos Janegitz Plasmon–Based Metal-Oxides Nanostructures for Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 Gajendra Kumar Inwati, Promod Kumar, F. Goutaland, Pratibha Sharma, and Hendrik C. Swart Recent Advances in ZnO-Based Hybrid Nanomaterials as Photoelectrodes for Photoelectrochemical Water Splitting . . . . . . . . . . . 315 Pulkit Garg, Pamisetty Tharun Sai, and Ankit Tyagi Semiconductor-Based Plasmonic Nanohybrids: Synthesis, Characterization, Mechanistic Understanding of Structure–activity, and Their Multifunctional Applications . . . . . . . . . 333 Atanu Ghosh and Tripti Ahuja Polymer-Based Hybrid Composites for Wastewater Treatment . . . . . . . . . 349 Veena Sodha, Jinal Patel, Stuti Jha, Megha Parmar, Rama Gaur, and Syed Shahabuddin Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391

Introduction to Semiconductor Photocatalyst Nanomaterials: Properties, Modifications, and Multifunctional Applications Sahil Thakur, Samriti, Abhijeet Ojha, and Jai Prakash

Abstract Semiconductor photocatalysts have attracted significant attention owing to their unique properties such as high efficiency, stability, and low cost. In recent years, advances in nanotechnology have allowed for the synthesis of semiconductor photocatalyst nanomaterials with improved properties. Among these, metal oxide semiconductors find widespread applications as photocatalysts due to their tunable properties, which enable them to absorb light and initiate a photocatalytic reaction. This chapter provides an insight into the properties, structures, and modifications of various semiconductor photocatalyst nanomaterials such as TiO2 , ZnO, WO3 , Ta2 O5 , perovskites, and some metal sulphides. It also discusses the most recent developments and modifications of semiconductor photocatalyst nanomaterials, highlighting their unique properties and enhanced performance. This chapter further examines the multifunctional applications of semiconductor photocatalyst nanomaterials in diverse fields, such as energy conversion, environmental remediation, and biomedical sciences. Overall, this chapter provides valuable insights into the potential of semiconductor photocatalyst nanomaterials in addressing critical challenges in various fields and the opportunities for further research in this area. Keywords Semiconductor photocatalysts · Metal oxides · Optical properties · Multifunctional applications

S. Thakur · Samriti · J. Prakash (B) Department of Chemistry, National Institute of Technology Hamirpur, 177005, Hamirpur (H.P.), India e-mail: [email protected] A. Ojha Department of Materials Science and Engineering, National Institute of Technology Hamirpur, 177005, Hamirpur (H.P.), India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_1

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1 Introduction Photocatalysis appears to be an emerging approach for various applications in many areas including energy, environment, and medical fields because it offers the use of solar radiations as a sustainable and renewable source of energy [1–4]. The study of chemical processes under the presence of light in the presence of a photocatalyst is known as photocatalysis. The photocatalytic reaction produces charge carriers (e+ / h− pairs) with the absorption of suitable radiation having energy larger than the band gap of the used photocatalyst as shown in Fig. 1. In photocatalysis, the choice of a suitable semiconductor is crucial for the generation of efficient photocatalysts. There are some critical conditions that must be fulfilled to design an effective and stable photocatalyst. The semiconductor must have a band gap of approximately 2–4 eV, which is sufficiently large to produce energetic electrons, but small enough to allow for efficient absorption that overlap with the solar spectrum. Semiconductor photocatalysts which are generally used for multifunctional applications include metal oxides, perovskites, metal sulphides like TiO2 , ZnO, CaTiO3, MoS2 , etc. [1, 3, 5, 6]. Out of these, in recent decades, metal oxide semiconductors have attracted a lot of attention for their potential photocatalytic activity due to their unique size and

Fig. 1 Some common metal oxide semiconductor photocatalysts along with their band gaps and multifunctional applications in different fields

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properties. Metal oxide having a band gap of 2–4 eV is a useful class of semiconductors for photocatalysis. In particular, metal oxide nanocomposites are intriguing due to their unique thermal, mechanical, optical, magnetic, electrical, and catalytic capabilities [1, 4, 7, 8]. They have the capacity to produce charge carriers when triggered with specific energy. Metal oxide-based semiconducting materials are of notable technical significance in the field of energy, health sector, and environmental remediation. Nanomaterials like TiO2 , ZnO, WO3 , Fe2 O3 , Ta2 O5 , and other nano-semiconductors have been employed in recent years in various environmental purification processes, including organic waste degradation, microbial inactivation, sensors, hydrogen production, and solar cells [9–13]. Figure 1 shows various metal oxide semiconductors along with their band gaps and their potential use in different fields of energy, environmental and biomedical such as photocatalytic water splitting, CO2 reduction, wastewater treatment, and antibacterial activity. High efficiency, stability, safety, and the potential for low-cost/scalable production are the current challenges of solar energy conversion that are being addressed by the use of metal oxide semiconductor nanomaterial in the field of energy. Perovskite solar cells, organic photovoltaics, and dye-sensitized solar cells (DSSCs) are frequently employed to address these issues. The efficient use of colloidal quantum material and nanostructured electrodes will enhance the efficiency for converting solar energy into fuel. Out of size-dependent physical properties, optical and magnetic properties of semiconductor nanomaterials are the most important for biological applications [12, 14]. In the field of medicine and biology, semiconductor nanomaterials are used in various applications like cancer treatment, cell and biomolecule manipulation, protein detection, tissue engineering, etc. [4, 7, 15]. In the civilian sector, the removal of poisonous and hazardous chemical substances from waste effluents and previously polluted areas have grown to be a serious problem. The polluted groundwater is probably the leading source of human exposure to toxic substances. The use of a metal oxide semiconductor photocatalyst for water pollution remediation demonstrates a redox reaction mechanism that has the ability to convert a wide variety of organic pollutants into biodegradable intermediates or less hazardous tiny molecular compounds. Their hybrid semiconductor nanomaterials with other functional nanomaterials show improved optical and other physical/chemical properties as a result of a combination of two different functionalities or show special qualities not found in either component [9, 16]. This book chapter covers the general properties, modifications, and applications of various metal oxide photocatalysts semiconductors as mentioned in Fig. 1. Additionally, other photocatalyst nanomaterials such as sulphides, perovskites, and carbon-based photocatalysts have also been briefly discussed with emphasis on their hybrid nanocomposite formation resulting in improved photocatalytic performance. Finally, the potential multifunctional usage of these photocatalysts in energy, environmental, and medical fields is explored.

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1.1 Titanium Dioxide: TiO2 TiO2 is a naturally occurring transition metal oxide, also referred to as titania. It is widely utilized in a variety of industrial applications, such as paints, varnishes, paper, plastics, printing inks, rubber, cosmetics, and more. TiO2 has attained considerable attention because it has unique photocatalytic characteristics that make it suitable for many energy and environment-related applications such as solar cells, fuel cells, Li-ion batteries, various sensors, and wastewater treatment, such as a self-cleaning, coating, and disinfecting materials [1, 7, 16]. In 1972, Fujishima and his co-workers found the catalytic decomposition of water on the surface of the TiO2 semiconductor with ultraviolet (UV) irradiation [17, 18]. Since then, enormous efforts have been done by researchers to know about the photocatalytic activity of TiO2 and its applications in various fields. TiO2 has three important polymorphs—anatase, rutile, and brookite (shown in Fig. 2 a–c). Among these polymorphs, rutile has the greatest thermodynamic stability. The geometry of both rutile and anatase polymorphs is tetrahedral, although rutile has a denser arrangement of atoms in its structure. The brookite polymorph has orthorhombic geometry [19, 20]. Anatase TiO2 (band gap = 3.2 eV) possesses good photocatalytic characteristics than rutile TiO2 (band gap = 3.0 eV), because it seems to be an indirect band gap semiconductor with a longer lifespan of photoexcited electrons and holes, whereas rutile and brookite belong to the direct band gap semiconductor group. In addition, when compared to rutile and brookite, anatase has the smallest average effective mass of photogenerated electrons and holes. The reduced effective mass of electrons and holes enables them to move more swiftly from the interior to the exterior of an anatase TiO2 particle, which results in a decreased rate of recombination of photogenerated charge carriers within the particle [21]. Due to the wide band gap energy (rutile: 3.0 eV; anatase: 3.2 eV), TiO2 is an active nanomaterial when exposed to UV light, which accounts for only about 5% of the solar radiation. Moreover, the quick recombination rate of the photogenerated e+ /h− pairs limits the efficiency of TiO2 . Hence, the wider band gap and rapid recombination rate of e+ /h− pairs limit its use as an efficient photocatalyst. However, there are several methods to enhance the photocatalytic activity of TiO2 , including doping with appropriate metals or non-metals, coupling with different functional nanomaterials, like carbon-based nanomaterials, creating defects, coating the surface, sensitizing with quantum dots, etc. [8]. Tauster et al. [23] in 1978 reported the first publication on doping TiO2 using noble metals. Since then, a wide number of noble metal nanoparticles, including Au, Ag, Pt, and many others, are being used for tuning the optical properties of TiO2 . The photocatalytic activity of TiO2 is enhanced by the incorporation of noble metals, which create a Schottky barrier at the interface and trap electrons. It also promotes the transfer of interfacial charge by decreasing the e+ / h− pairs recombination rate. Figure 3 illustrates the mechanism of photodegradation of organic contaminants by metal-doped TiO2 . Recently, Samriti et al. [24] reported that Ta doping can decrease the band gap of TiO2 nanorods up to 2.56 eV. They have studied the SERS and photocatalytic

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Fig. 2 Conventional cell diagram for a anatase, b rutile, and c brookite TiO2. Reprinted with permission from [22]. Copyright © (2014), Royal Society of Chemistry

Fig. 3 Photocatalytic activity of metal-doped TiO2 . Reprinted with permission from [16]. Copyright © (2018), Elsevier

decomposition of methylene blue dye molecules. Research into how to enhance the photocatalytic activity of TiO2 by immobilizing it on a material such as metallic oxides, activated carbon, and other carbon derivatives is increasing. It is well known that loading the TiO2 on these materials can overcome the problems associated with pure TiO2 photocatalysts. Recently, Wahed et al. [25] studied the removal of chemical and microbial water pollutants using TiO2 -based hybrid nanomaterials with Ag and

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rGO. The incorporation of rGO has a synergistic effect with Ag and TiO2 , which enhances the photocatalytic performance of the Ag/TiO2 composite by the formation of a hybrid system. Furthermore, microbes such as bacteria and enteric viruses were effectively destroyed in photocatalytic microbial inactivation experiments. Chen et al. [26] showed that 3D-TiO2 /graphene hybrid architecture demonstrated outstanding photoelectrochemical and photocatalytic performance, as well as the optimized material exhibited remarkable lithium storage and rate performance during cyclic operations. TiO2 nanoparticles are finding increasing application in the fields of pharmacy, particularly pharmaceutical chemistry and technology, medicine, dentistry, and surgery. The use of TiO2 nanoparticles in photodynamic therapy is widely explored. These nanoparticles are frequently utilized as drug carriers, enabling medications to reach damaged body parts while sparing healthy tissues from damage. Bone tissue engineering also uses TiO2 scaffolds to prepare implants for surgery [27]. One of the applications of TiO2 semiconductor photocatalysts is the photokilling behaviour of the photoexcited TiO2 , which has potential uses in the biological arena, particularly in the treatment of cancer. In this type of therapy, oxygen present in the human body combines with conduction band electrons of TiO2 to produce reactive oxygen species (ROS), which damages the structure of cancerous cells [28]. Thus, TiO2 nanoparticle photocatalyst holds great potential for various applications, and future research could focus on developing new synthesis methods, enhancing their photocatalytic performance, hybridization with other materials, and exploring their potential applications in water treatment and biomedical sciences.

1.2 Zinc Oxide: ZnO Zinc oxide is another inorganic compound with the molecular formula ZnO. As a wide band gap semiconductor, ZnO has drawn significant attention in many research areas because of its intriguing and distinctive qualities, such as its high photosensitivity, biocompatibility, non-toxicity, wide chemical stability, and piezoelectric and pyroelectric properties [3]. These unique properties make ZnO one of the major contenders for many applications, such as solar cells, laser diodes, thin-film transistors, UV detectors, gas sensors, and field effect transistors [29]. ZnO and TiO2 have many comparable characteristics, including a similar band gap. But there have been some instances where ZnO has outperformed TiO2 in terms of photocatalytic activity and exhibiting a better quantum efficiency, since it has the capability to absorb a greater portion of the solar spectrum than TiO2 [30, 31]. ZnO is often called II-VI semiconductor because zinc belongs to II and oxygen belongs to VI group of the periodic table. Typically, ZnO is found as a white colour powder which is insoluble in water. Pure ZnO is a naturally occurring n-type semiconductor due to the existence of zinc interstitial, oxygen vacancies, and zinc on oxygen antisite. Its direct wide band gap (3.37 eV) is in the UV region of the spectrum. ZnO has an impressive binding energy of 60 meV [3, 32, 33]. It possesses

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Fig. 4 ZnO crystal structures: a Wurtzite, b Rocksalt, and c Zinc blende. Reprinted with permission from [10]. Copyright © (2022), American Chemical Society

well-defined crystal structures, such as cubic rocksalt structure, hexagonal wurtzite, and zinc blende (Fig. 4). Highest thermodynamic stability is seen in the ZnO wurtzite structure and is thus most common at ambient conditions [34]. The typical structure of ZnO is hexagonal where zinc atoms are tetrahedrally coordinated to four oxygen atoms. A rocksalt structure can be produced under tremendous pressure conditions only, making them relatively uncommon (Fig. 4) [35]. ZnO nanoparticles have been known to have their particle form, size, and physical properties affected by the preparation methods used and the deposition parameters. Nanostructured ZnO has a larger surface area, and its higher surface/volume ratio might provide enhanced physicochemical qualities [3, 29, 36–38]. Most common photocatalytic studies of ZnO are commonly carried out under UV irradiation, but investigations have also been done in the presence of visible light. The photocatalytic efficiency of ZnO nanoparticles can be increased by doping with suitable cationic and anionic species. The introduction of non-metals with small radii into ZnO is highly advantageous. These dopants can take the place of lattice oxygen in ZnO or inhabit interstitial sites. For example, Sun et al. [39] prepared N-doped ZnO nanoparticles by the solvothermal method using melamine as a nitrogen source. The resulting N-doped nanoparticles of ZnO exhibited improved photodegradation of methyl orange under daylight irradiation as compared to undoped ZnO. Similarly, Chen et al. [40] produced N,C,Sdoped ZnO particles from thiourea and zinc sulphate through a precipitation process. N,C,S-ZnO calcinated at 500 °C exhibited a substantial photo-absorption effect in the visible light region. In addition, the dopants diminished the recombination rate of e+ /h− pairs and augmented the photocatalytic activity under UV illumination. Recently, a variety of elements such as Li, Al, Mn, Fe, Co, Ni, Ag, Au, and Zr have also been added to ZnO in order to improve the speed of photodegradation of organic pollutants. The results of this doping have been promising [41–43]. For

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example, Pd-doped ZnO was prepared by Zhong et al. [44], which showed that Pd doping reduces the BET surface area, increases the visible region absorbance of ZnO, and speeds up the rate at which charge is separated. Rajendran et al. [4] synthesized silver-doped zinc oxide nanoparticles (Ag-ZnO NPs) using the sol–gel precipitation method. With a maximal 89% degradation rate against Ponceau, these nanoparticles demonstrated good photocatalytic activity. In addition, the bactericidal activity of Ag-ZnO NPs against the selected skin opportunistic pathogen was quite impressive. Thus, Ag-ZnO NPs can be used as an antibacterial agent in various topical preparations such as drugs, cosmetics, ointments, and lotions. Further, it kills cancer cells that were caused by UVB radiation. Therefore, their research verified the excellent photocatalytic, antibacterial, and anticancer properties of the Ag-ZnO NPs. Composites of GO with ZnO also exhibit remarkable photocatalytic activity as compared to pure ZnO. Recently, Garg et al. [45] synthesized a hybrid nanocomposite of ZnO with GO, which possesses reduced band gap, improved charge carrier separation, and a large surface area. The organophosphate pesticide quinalphos was degraded up to 98% by this nanocomposite in aqueous medium. Thus, the potential of ZnO nanoparticle photocatalysts for various applications makes them an exciting area of research. As research in this area continues, it is expected that new synthesis methods and hybridization strategies will be developed to enhance their photocatalytic performance and to tailor their properties for specific applications.

1.3 Tungsten Oxide: WO3 WO3 is a highly studied metal oxide semiconductor photocatalyst because of its superior response to the solar spectrum, high efficiency, mechanical strength, fine metal interactions, and cost-effectiveness [46]. These nanomaterials are used in a variety of applications, including photocatalysis, phototherapy, electrochemistry, etc. Given its significant solar spectrum absorption (12%) with a band gap of 2.5–2.8 eV and its superior electron mobility (12 cm2 V−1 s−1 ) compared to TiO2 (0.3 cm2 V−1 s− 1 ), tungsten oxide (WOx ) has been demonstrated to have stable crystal phases and absorb light up to the near-infrared range, especially in oxygen-deficient forms such as WO2.72 , WO2.83 , and WO2.9 (where x > 3) [47]. The polymorphs of WO3 , from the least complex to the most complex, include γ-WO3 (monoclinic I), ε-WO3 (monoclinic II), δ-WO3 (triclinic), β-WO3 (orthorhombic), α-WO3 (tetragonal), h-WO3 (hexagonal), and c-WO3 (cubic) (illustrated in Fig. 5). These crystal structures, monoclinic, triclinic, orthorhombic, and tetragonal, all possess a chessboard-like arrangement of WO6 octahedra, and can thus be reversibly transformed between each other during heating, cooling, and morphology alteration [49]. These crystal structures differ only in the extent to which the tungsten atoms are displaced from the centre of WO6 octahedra. Hence, there are only three distinct crystal structures for WO3 : the monoclinic/triclinic/orthorhombic/

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tetragonal, hexagonal, and cubic. As the temperature rises, the transformation of phases happens in the following sequence: monoclinic II (ε-WO3 , colder than 43 °C) → triclinic (δ-WO3 , between -43 °C and 17 °C) → monoclinic I (γ-WO3 , between 17 °C and 330 °C) → orthorhombic (β-WO3 , between 330 °C and 740 °C) → tetragonal (α-WO3 , hotter than 740 °C) [5]. γ-WO3 is the most stable form of WOx at room temperature, while cubic WO3 is not observed usually experimentally. In the bulk form, γ-WO3 has a band gap around 2.62 eV corresponding to the energy difference between the valance band (which consists of filled 2p orbitals of oxygen) and conduction band (which consists of empty 5d orbitals of tungsten). On decreasing the size of the grains up to nano size, there is an increment in band gap due to the quantum size effect [13]. As a photocatalytic material, WO3 can be irradiated by the blue region of the visible solar spectrum, as its band gap lies in that range. It can photo-oxidize a large number of volatile organic compounds, dyes, and bacterial pollutants. It also possesses notable stability in acidic conditions due to which it is used significantly

Fig. 5 Unit cells of different phases of WO3 (Tungsten atoms are represented by the bigger grey spheres, and oxygen atoms by the smaller dark spheres). Reprinted with permission from [48]. Copyright © (2010), AIP Publishing

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for treating the water contaminated by different organic acids. Till now, only the triclinic/monoclinic/orthorhombic/tetragonal phases of WO3 have been known to act as photocatalysts [5, 50–53]. According to literature data, phase structure and calcination temperature affect the photocatalytic activity of WO3 . For example, Uresti et al. [50] prepared WO3 nanoparticles of monoclinic and hexagonal phases via a microwave-assisted hydrothermal process. As compared to monoclinic, the samples with hexagonal structures showed larger surface area and greater activity during photodegradation of rhodamine B, indigo carmine, and tetracycline hydrochloride under UV–vis irradiation. Lu et al. [54] prepared samples of WO3 with different crystalline phases, namely monoclinic (m-WO3 ), hexagonal (h-WO3 ), and mixed phase of monoclinic/hexagonal junction. The results showed that the sample with the m-WO3 /h-WO3 junction possesses the highest photocatalytic activity. The increased photocatalytic activity of mixed phase sample was attributed to the ability of phase junction to reduce the e+ /h− pair recombination rate. Pure WO3 nanomaterials often do not function as effective photocatalysts due to their high rates of e+ /h− pair recombination and low charge mobility. In fact, this is one of the major obstacles to face in using WO3 as a practical photocatalyst in many applications. Many strategies have been devised to increase its efficiency for solar energy conversion, such as substitutional doping of WO3 with cations and anions. Doping of WO3 with various elements like Fe, Co, C, N, and S have been found to enhance its photo-response to visible region [55, 56]. Doping of Fe to WO3 has drawn more and more interest due to its ability to reduce the band gap by introducing the impurity bands [57, 58]. Song et al. [57] showed that appropriate doping of Fe3+ could increase the separation and transformation of photogenerated e+ /h− pairs. Doping with optimal Fe content (5.25%) displayed exceptional activity as compared to pure WO3 . Similarly, Hameed et al. [11] observed enhanced photocatalytic activity with the doping of different transition metals like Co, Fe, Ni, and Zn in WO3 . The development of a heterojunction by mixing WO3 with another semiconductor is another effective technique for enhancing photocatalytic activity. Tahir et al. [59] studied the effect of TiO2 on WO3 nanoparticles by reducing the band gap of pure WO3 . It has been concluded that the coupling of TiO2 with WO3 enhances the photocatalytic activity. As a result of improved photocatalytic performance with 2.0% TiO2 -coupled WO3 , the degradation rate of methylene blue increased up to seven times under visible light. Similarly, Cao et al. [60] prepared a powder sample of TiO2 -WO3 photocatalyst from soluble tungstic acid and TiO2 powder by wetchemical technique. The prepared sample had good energy storage ability, and it was depending upon the crystal structure of WO3 and the molar ratio of WO3 /TiO2 . The best energy storage ability of the sample was obtained when TiO2 -WO3 were present at a 1:1 molar ratio and heated at 250 °C. Ketpang et al. [61] synthesized SiO2 / WO3 nanoparticles by in situ calcination of electrospun polyacrylonitrile nanofibers under air atmosphere. The material showed 6 times higher hydrogen evolution reaction (HER) activity, as compared to bulk WO3 . They also conclude that SiO2 /WO3 nanocomposite exhibits excellent HER stability and can be used in green energy application.

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Thus, the development of hybrid photocatalysts by combining WO3 with other materials has shown great potential for enhancing photocatalytic activity and expanding the area of applications of WO3 -based photocatalysts.

1.4 Ferric Oxide: Fe2 O3 Ferric oxide (Fe2 O3 ) is currently considered one of the most advantageous metal oxide semiconductors for photocatalytic reactions because of its low band gap (2.1– 2.3 eV) and great visible light harvesting, outstanding stability, earth-abundance, and recyclability [62]. Due to these properties, Fe2 O3 has evolved as a superb photoelectrode material for electrochemical water splitting [63, 64], an effective anodic material in lithium ion batteries [65, 66], and a photocatalyst for the destruction of various organic pollutants. Fe2 O3 exhibits various stoichiometric and crystalline polymorphs commonly known as α-Fe2 O3 , β-Fe2 O3 , γ-Fe2 O3 , and ε-Fe2 O3 . Out of these polymorphs, α- and γ-Fe2 O3 are highly crystalline and occur naturally, while β- and ε-Fe2 O3 are generally synthesized in a laboratory [67]. α-Fe2 O3 , also known as hematite has a corundumlike rhombohedral-centred hexagonal structure with a close-packed oxygen lattice in which Fe (III) ions occupy two-thirds of the octahedral sites (shown in Fig. 6). Several investigators have used α-Fe2 O3 for water purification, chemical sensing, and biomedical applications [12, 68]. It has been frequently used in wide band gap semiconductors as a sensitizer in order to enhance their ability to capture visible light [69]. β-Fe2 O3 only occurs in nanoscale form, and its abundance in nature has yet not been reported. It is the only polymorph that shows paramagnetic behaviour at room temperature. Due to its thermodynamic instability, it can be transformed into either α- or γ-Fe2 O3 on heating [70, 71]. γ-Fe2 O3 is the second most frequent polymorph of Fe2 O3 in nature and commonly known as maghemite. It is found both in bulk and nanosized forms like α-Fe2 O3 and can be obtained by a variety of reactions. γ-Fe2 O3 polymorph is thermodynamically unstable and can be converted either directly or indirectly to α-Fe2 O3 (via ε-Fe2 O3 as intermediate) when heated over a threshold value. ε-Fe2 O3 is also a rare polymorph whose abundance is low and exists only in the form of nanostructures. ε-Fe2 O3 has an orthorhombic crystal structure derived from a close packing of four oxygen layers. The crystal structure of α-Fe2 O3 consists of six non-equivalent anions and four cations (Fe1, Fe2, Fe3, and Fe4). All the cation positions are occupied by Fe3+ ions, meaning that there are no vacancies in the structure. Fe4 is coordinated in a tetrahedral manner, while Fe1, Fe2, and Fe3 are octahedrally coordinated. These polymorphs of iron oxide can be transformed into one another by various methods. Two main methods inducing Fe2 O3 transformation are thermal and mechanical treatment. Thermal treatment comprises isothermal or dynamic heating, while mechanical activation can be accomplished through milling or high-pressure treatment. The transformation mechanism of these polymorphs not only depends on the applied physiochemical circumstances but is also affected by the form of sample, i.e., thin film, crystals, nano powder, nanocomposite, and coated particles [67].

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Fig. 6 Illustrations of the basic crystalline forms of Fe2 O3 : a α- Fe2 O3 , b β- Fe2 O3 , c γ-Fe2 O3 , and d ε-Fe2 O3 [67]. (https://pubs.acs.org/doi/full/10.1021/cm200397g)

Due to the lower band gap of value 2.2 eV, α-Fe2 O3 has an advantage as compared to other commonly used materials like TiO2 , ZnO, etc., when it comes to harnessing solar energy for photocatalytic applications. Up to 600 nm, or about 40% of the solar spectrum can be collected by its absorption. Additionally, it is one of the least expensive semiconductors and is stable in the majority of aqueous solutions (pH > 3). Due to its magnetic properties, α-Fe2 O3 powder is magnetically separable from the solution. As a result, ferric oxide nanostructures with shape-dependent catalytic characteristics are emerging as key materials for the decomposition of a large number of pollutants, including rhodamine B, methylene blue, methyl orange, indigo carmine, methyl violet, rose bengal, phenols, congo red, salicylic acid, etc. [14, 72–79]. Doping of α-Fe2 O3 with other metal ions, such as Al, Ti, Cr, Co, Ni, Zn, Zr, and Rh influences its physical and photocatalytic properties. Shen et al. [80] found that doping of Zr in an array of Fe2 O3 nanorods restricts the recombination rate of e+ /h− pairs and serves as a more effective catalyst degradation of dye. Similar to this, Wang et al. [81] showed that Ti-doped Fe2 O3 has improved photocatalytic activity due to enhanced donor density and decreased recombination rate of e+ /h− pairs. Zhang and co-workers used a simple one-step combustion process to create the magnetic Fe2 O3 /TiO2 /graphene (GTF) hybrid [82]. This catalyst was employed

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to photodegrade methylene blue dye, when it was irradiated under visible light. The cycle experiments showed that this GTF hybrid exhibits a wider visible-light absorption spectrum, decreased charge recombination, and a promising stability. An external magnetic field might potentially be used to retrieve GTF photocatalyst from the reaction mixture. Thus, Fe2 O3 and its hybrids appear to be a promising photocatalytic material for various applications.

1.5 Tantalum Pentoxide: Ta2 O5 Ta2 O5 has been found as an appealing substitute for TiO2 during photocatalytic and photoelectrochemical processing. It is a notable transition metal oxide due to its amazing physiochemical characteristics, including high dielectric and refractive factors and superb photoelectric properties. Furthermore, Ta2 O5 is superior to other materials in a number of ways, including affordability, availability, nontoxicity, structural stability, and high efficiency. It finds utility in a wide variety of contexts, including photovoltaic devices, electronics, anti-reflective layering material, etc. [13]. It is generally accepted that Ta2 O5 exists mainly in two primary polymorphs, known as high-temperature (HT) and low-temperature (LT) types. The transition between HT and LT usually occurred at ∼1360 °C, and also it is a reversible process, which means that the HT phase cannot be stabilized under normal conditions. However, the addition of foreign compounds, like Sc2 O3 , can significantly help to increase its availability by reducing the temperature of transition; nonetheless, it is still too high to be adopted effectively. Orthorhombic β-Ta2 O5 and hexagonal δ-Ta2 O5 (shown in Fig. 7) are the most prevalent polymorphs in LT [83]. Ta2 O5 has a wide band gap of 3.8–5.3 eV between the O 2p orbital and the empty Ta 5d orbitals, which makes it transparent to visible light. The precise value of the gap may vary depending on the synthesis technique, the type of crystal structure, and the morphology. Ta2 O5 can be thought of as a much desirable contender to replace TiO2 . Beyond this conclusion, several important reasons are brought out. First, its

Fig. 7 Schematic diagram for a orthorhombic β-Ta2 O5 and b hexagonal δ-Ta2 O5 . Reprinted with permission from [84]. Copyright © (2010), American Physical society

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extraordinary physical, chemical, and other characteristics, such as its high dielectric and refractive coefficients, impressive photoelectric performance, and extreme chemical stability [85–87]. Second, the band gap is positioned correctly, which enables to carry out the majority of photocatalytic reactions without the usage of additional components and compounds [88]. Third and most notably, it offers more favourable conditions for the smooth initiation or continuation of reduction reactions since its conduction band is higher than TiO2 with regard to vacuum level (shown in Fig. 8). But one should also notice that like other wide band gap semiconductors Ta2 O5 also has a number of drawbacks, such as the lack of capacity to absorb visible light. It also exhibits inadequate charge carrier transportation as a result of its low conductivity and high recombination rate [89, 90]. These limitations can affect the photocatalytic action of Ta2 O5 under light-induced conditions, but these problems could be resolved by using various methods. Additionally, this positioning of the band gap (Fig. 8) allows for a great deal of flexibility in terms of adjusting and controlling the down- and up-shifts of the conduction and valence bands. It can be used to shorten the gap between them and thus elongate the absorption edge while maintaining the ability of material to carry out the necessary redox reactions. Ta2 O5 is thought to be an appealing option for effective use in a variety of photocatalytic and photoelectrochemical activities, including the breakdown of harmful pollutants in water, hydrogen production, nitridation fixation, CO2 reduction, etc. The purpose of adopting Ta2 O5 in this role becomes even more clear, because the conduction band of Ta2 O5 is around 0.3 eV more negative in relation to vacuum level than that of TiO2 . It defines the existence of enhanced kinetics for the accomplishment of reduction reaction and also has a significant impact on the capacity to generate a large number of superoxide radicals. As a result, Ta2 O5 can offer superior performance in comparison to other photocatalysts. It has been used successfully in the photocatalytic decomposition of a number of dyes [91–93]. Fig. 8 The energy differences between Ta2 O5 and TiO2 versus standard redox couples in an aqueous environment with a pH of 0. Reprinted with permission from [13]. Copyright © (2022), Elsevier

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However, Ta2 O5 is an active photocatalyst only under the effect of UV radiation. But this drawback of Ta2 O5 is not a permanent issue and, in fact, this could be fixed properly by using various strategies like defect engineering (intrinsic or extrinsic defects), metal/non-metal doping, formation of heterostructures, etc. [13]. For example, Liu et al. [94] prepared oxygen-deficient Ta2 O5 by sol–gel method and annealed it in vacuum, which showed a 48% higher H2 generation rate achieved via water splitting compared with that of original Ta2 O5 . Lou et al. [95] used a multiple metal ions absorption and templating technique to synthesize crystalline Cu2 O/amorphous Ta2 O5 heterostructures with thicknesses as thin as 8 nm. The prepared heterostructures exhibited a 3% level quantum efficiency for overall water splitting under visible light irradiation, i.e., 400–480. Similarly, N-doped Ta2 O5 also shows a decomposition of gaseous 2-propanol under irradiation of UV–vis light [96]. Ta2 O5 and its hybrid materials are highly promising for use in various applications due to high photocatalytic efficiency, abundance, non-toxicity, stability, and biocompatibility.

1.6 Cupric Oxide: CuO CuO is a significant p-type metal oxide with a constrained band gap that ranges 1.2–2.0 eV. It is a benchmark photocatalyst since it is less expensive, non-toxic, and more effective at absorbing the majority of the solar spectrum. These nanoparticles have excellent physical and chemical properties, which include high thermal and electrical conductivity, high mechanical strength, high-temperature toughness, large surface area, and proper redox potential. Due to its exceptional photoconductive and photochemical capabilities, it is widely employed in a variety of applications, including energetic materials (EMs), biosensors, magnetic storage devices, gas sensors, electronic and solar cells, etc. [97–100]. Besides this, these nanoparticles have shown promising applications in pharmacological activities, particularly in antitumor therapy [101]. The first original crystal structure of CuO was reported by Tunnel and co-workers in 1933, and it was modified by using the X-ray single-crystal method [102]. The crystal structure of CuO is shown in Fig. 9a, where the Cu atom is coordinated with 4 O atoms in a (1 1 0) plane of square planar configuration, and each O atom is surrounded by four Cu in the shape of a slightly warped tetrahedron [98]. Bourne et al. [97] revealed that CuO does not undergo phase conversion at high pressures and temperatures, as compared to other metal oxides, which undergo crystal phase transitions during annealing and cooling. CuO nanostructures with customized architecture provided viable photocatalysis due to efficient conduction band and valance band potentials of 0.46 eV and 2.16 eV, respectively, which are higher than the conventional redox potential needed for photodegradation [104]. According to studies so far, bare CuO is less likely to attain adequate stability and efficiency in photocatalysis. Building heterogeneous systems with additional components (such as CuO/ZnO, CuO/TiO2 , CuO/Fe2 O3 , CuO/Cu2 O,

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Fig. 9 Crystal structure of a CuO and b Cu2 O [103]. (https://www.nature.com/articles/srep16061)

ZnO/CuO/rGO, and CuO/g-C3 N4 ) can combine the benefits of several components to optimize the shortcomings of single component CuO photocatalyst [99, 104–107]. These heterostructures include binary, ternary, and Z-scheme-based heterojunction. They possess enhanced photocatalytic activity by enhancing charge separation and transportation, broad light absorption, and extending functional lifetime. Formation of nanocomposite of bare CuO with other metal oxides (like ZnO) provides unique morphologies to gain increased BET surface area and maintain chemical stability during the degradation process. Additionally, adding rare earth and transition metal ions as dopants to pure CuO results in structural and interstitial defects that change the crystal’s size and electrical and optical characteristics, improving the carrier stimulation that promotes the photodegradation of contaminants. A hybrid composite of g-C3 N4 with metal oxides is an example of p-n type heterojunction, which leads to increased charge carrier separation with greater stability and enhanced photodegradation efficiency [107, 108]. Thus, the development of hybrid photocatalysts by combining CuO with other materials has shown great potential for enhancing photocatalytic activity and expanding the area of applications of CuO-based photocatalysts.

1.7 Other Metal Oxides: (Cu2 O, SnO2 , CdO, and Bi2 O3 ) In addition to the metal oxide semiconductor photocatalyst mentioned above, there are also some other metal oxides such as Cu2 O, SnO2 , CdO, and Bi2 O3 that function well as photocatalysts and have received a lot of interest. These are used in a variety of sectors, including fuel cell electrodes, battery cathodes, dye-sensitized solar cells, electrochromic devices, gas sensors, catalysis, etc. Cu2 O is a p-type semiconductor with a direct band gap of 1.90–2.17 eV [109]. The unit cell of Cu2 O (shown in Fig. 9b) contains 4 copper atoms and 2 oxygen

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atoms, where copper atoms are arranged in a fcc sublattice and oxygen atoms are occupied at tetrahedral sites [103]. Cu2 O has been thought of as a potential visible light photocatalyst which initiates photocatalytic reactions when exposed to sunlight. It produces free electrons spontaneously under the influence of light and can also generate hydrogen peroxide (H2 O2 ) and strong oxidizing groups (like O2 − and OH. ) on reacting with water and oxygen. It is studied that Cu2 O particles can be synthesized in octahedral, spherical, and rhombic dodecahedra form with tuneable size by copper oxidation, electrolysis, and hydrothermal process, which shows different colours and properties [110, 111]. According to reports, Cu2 O has a high hole mobility of 50–100 cm2 V–1 s–1 at room temperature, a large carrier diffusion length of 2– 12 μm, and a high carrier concentration of 1016 –1019 cm–3 at room temperature. They combined with other semiconductors with aligned band gaps, which results in the formation of type-II band alignment [112–114]. These features are crucial for efficiently enhancing photogenerated e+ /h− pairs separation, which raises the effectiveness of the degradation process There are a lot of papers published on the PEC solar energy conversion and photocathodes of Cu2 O [115, 116]. Its efficiency for photodegradation of different contaminants can be enhanced by modifying its shape [117]. Similarly, SnO2 -based photocatalysts with high photocatalytic activity and appealing potential have a number of applications in the various domain. Due to their long-term stability, high oxidation potential, chemical inertness, corrosion resistance, non-toxicity, cost-effectiveness, and environmental protection, SnO2 has been able to pique the interest of many researchers towards itself. However, its large band gap of around 3.6 eV prevents it from absorbing visible light, and its high recombination rate restricts its photocatalytic activity [118]. So, the band of SnO2 needs to be modified to produce more electrons and holes by sunlight, carrier separation types need to be developed to improve carrier transmission, and surface modifications need to be made to improve redox reaction. Many strategies have been adopted to enhance the photocatalytic activities of SnO2 . For example, Soltan et al. [119] have found that doping of 10% Zn to SnO2 nanocrystals reduced its band gap up to 3.17 eV, which improved its efficiency to breakdown methylene blue and methyl orange under UV light. Similarly, Fe-doped SnO2 also possesses a small band gap of 2.96 eV and demonstrates more photocatalytic activity than pure SnO2 in the degradation of rhodamine B under simulated sunlight [120] (Table 1). Cadmium oxide (CdO) is also a transparent conducting oxide with a high transmission coefficient in the visible light spectrum [121]. With a high exciton binding energy of 75 meV, it possesses an optical band gap that ranges from 2.2 to 2.7 eV [122]. CdO refers to an n-type degenerated semiconductor whose conductivity is attributed to the non-stoichiometric mixture of cadmium and oxygen because of the negative oxygen vacancies (OV ) and cadmium interstitial (Cdi ) atoms [121]. CdO nanomaterials exhibit low toxicity in comparison with other toxic semiconductor nanoparticles like HgO and PbO [123]. Nowadays, cadmium is present in the majority of multivitamin tablets and dietary supplements. Additionally, a lot of cosmetics and anti-tanners include CdO. Furthermore, due to its superior

18 Table 1 List of some perovskite semiconductors with their band gap

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Perovskite oxide

Band gap

Light Source

LaFeO3 [134]

2.00

Visible light

LaCoO3 [135]

2.10

Visible light

BiFeO3 [136]

2.40

Visible light

LaNiO3 [137]

2.26

Visible light

Bi2 WO6 [131]

2.70

Visible light

SrTiO3 [138]

3.20

UV light

La2 Ti2 O7 [139]

3.28

UV light

NaNbO3 [140]

3.48

UV light

CaTiO3 [130]

3.62

UV light

optical properties including fluorescence and high resolution, it is a good option for biological applications [122, 123]. Bismuth (Bi) has a long history of use in medicine as a result of its potent antibacterial properties, and it is also regarded as one of the least poisonous heavy metals for the human body. Recently, Bi2 O3 nanoparticles have gained a lot of attention due to their excellent physicochemical features, such as high stability, large surface area, desirable catalytic activity, low toxicity, and cost-effectiveness, which include their wide range of potential uses in a variety of fields, including chemical, electrical, optical, engineering, and biomedical. These nanoparticles exhibit a blue shift in the absorption spectrum as well as potent photoluminescence and photoconductivity. Bi2 O3 NPs have a high refractive index (2.6 at 500 nm) and a large energy gap around 3.34 eV [124, 125]. These NPs are found in five different polymorphic forms such as α, β, γ, δ, and ω-Bi2 O3 , each of them having different physical properties and crystalline structure [126]. The stability of these polymorphs varies with temperature and doping. Out of these phases, the α-phase is stable at low temperatures while δphase is stable at high temperatures [126, 127]. Many of the organic-based bismuth compounds have antitumor effects. Although organic-based bismuth complexes may have anticancer potential, they may also be harmful to human health.

1.8 Perovskites: ABX3 Any substance with a crystal structure that matches the formula ABX3 is known as perovskites. In 1893, German scientist Gustave Rose made the initial discovery of perovskite material in the Ural Mountains. Rose spent a lot of time figuring out the characteristics of perovskite, and in light of that, he named this mineral after Lev von Perovski, who was a Russian mineralogist [128]. CaTiO3 was the first discovered perovskite material. This perovskite material exhibited photocatalytic activity and degraded pollutants due to its wide band gap of range 3.0–3.5, which responds only to UV light [128–130]. The fascinating physicochemical properties of perovskite and

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perovskite-related materials, such as thermal stability, electron mobility, and redox behaviour, as well as their numerous applications in various domains like catalysis, water splitting, solar cells, optical devices, and superconductors, have made them a significant new class of materials in the present day [6, 130, 131]. Binary metal oxides or perovskite oxides are common inorganic compounds with overall ABO3 cubic structure as shown in Fig. 10, where A is typically a lanthanide, alkaline, or alkaline earth cation and B is a metal element with + 3, + 4, or + 5 oxidation state [132]. An ideal perovskite has a cubic structure but perovskite-related structures may have a loss of symmetry operations in the cubic structure due to some lattice distortion at varied degrees, which results in the non-ideal crystal phases including orthogonal, rhombohedral, tetragonal, monoclinic, and triclinic phases. The difference in the radii of both cations A and B carries out distortion in the geometry or tilting of the BO6 unit. This tilting is also known as octahedral tilting [133]. The synthesis of perovskite oxides is generally carried out at high temperatures because these consist of two or more metal oxides which have high melting points [132]. However, the method used for the preparation of perovskite oxide is chosen in accordance with the unique requirements for a specific application, as activity and selectivity of the material greatly depend on how the atoms are arranged on its surface. Perovskite material provides a good framework for adjusting the band gap values to enable visible light absorption and the band edge potentials to meet the requirements of particular photocatalytic reactions. Due to their remarkable qualities and possible uses in nanotechnology, these materials have gained attention recently. There are several types of perovskite materials which are used as photocatalysts, as given in the table listed below. Perovskites have been widely employed in photocatalysis. Also, these are extensively used in energy-related fields like water splitting and solar cells in the field of PEC [141, 142]. Due to their intriguing features, perovskite and perovskite-related Fig. 10 A graphical representation of the perfect cubic structure of the ABO3 perovskite, with the cyan BO6 units and the yellow A atoms. Reprinted with permission from [6]. Copyright © (2012), American Chemical Society

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substances like BiFeO3 , LaMnO3 , and LaFeO3 are becoming more popular as ideal anodic substances for PEC in the destruction of organic pollutants [143]. Although there has undoubtedly been significant progress over the years, more work has to be done in order to fully realize the inherent properties of perovskites. Perovskites with remarkable optical and electrochemical characteristics and longterm stability are crucial for sustainability.

1.9 Metal Sulphides: MxSy Apart from metal oxides, metal sulphides are a group of semiconductor photocatalysts having narrow band gap and negative conduction band potential making them exhibit exceptional photocatalytic performance [144]. Due to their narrow band gap, they possess remarkable light harvesting abilities and are used in different fields as emerging photocatalysts. Nowadays, a number of metal sulphides like ZnS, MoS2 , Bi2 S3 , In2 S3, and SnS2 (shown in Fig. 11) are being used in different fields as successive photocatalysts. Many studies demonstrate that morphological management is an important tactic for enhancing photocatalytic activity. Moreover, the crystalline phase, nanostructure, and exposed facets affect the optical band gap and specific area, which are important factors in determining how well photocatalysts operate. Recently, Di et al. [146] reported that metal sulphide-based Z-scheme photocatalyst can successfully enhance charge separation and transfer while preserving the reduction ability of metal sulphides. As a result, sulphide-based Z-scheme photocatalysts demonstrate exceptional photocatalytic performance and have a lot of potential for use in a variety of photocatalytic applications, ranging from environmental remediation to solar fuel production. Supercapacitors made of metal sulphide are greatly favoured for usage as power storage devices because of their superior cycle life, secure operation, and higher power density than fuel cells [147].

Fig. 11 List of some selected metal sulphides along with their band gap and band positions [145] (https://www.mdpi.com/2073-4344/12/11/1316)

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Selenium, which belongs to the oxygen and sulphur family, forms metal selenides on combining with metals. The chemistry of selenides and sulphides is quite similar, which encourages researchers to investigate these materials for various applications. Various types of metal selenide nanomaterials like FeSe, CuSe, ZnSe, CdSe, and PbSe are readily used for the degradation of organic pollutants and purification of water. These materials undergo various functional modifications, and increased catalytic activity can be seen in their heterostructures [148]. Transition metal selenides typically have exceptional chemical and physical properties, including distinctive electronic structures as well as the presence of Se atom leads to good conductivity. With their well-developed porosity, nanoscale particle size, and excellent conductivity, transition metal selenide-based electrodes are widely used as promising materials for energy storage devices [149]. Unfortunately, very few investigations have been reported on metal selenides specifically for energy storage applications, despite the fact that they exhibit greater electrochemical performance than the corresponding oxides.

1.9.1

Graphene and Its Derivatives

Graphene and its derivatives, including graphene oxide (GO) and reduced graphene oxide (rGO), have shown promising potential as photocatalysts due to their unique physical and chemical properties. Graphene has an extremely high surface area per unit mass, due to its thin, two-dimensional structure. The high surface area makes it an ideal material for catalysis, energy storage, and other surface-dependent applications. It has a high electrical conductivity, making it ideal for use in electronic devices [150]. Although the high conductivity of graphene (due to p-bonds with free electrons) and its hydrophobic sp2 domain make it unsuitable for photocatalytic activity [151], doping of graphene with suitable heteroatoms can make it photocatalytically active [152, 153]. Interestingly, the oxide form of graphene is regarded as one of the emerging photocatalysts. GO and rGO are the oxidized forms of graphene, which show better photocatalytic efficiency than sole graphene. The inclusion of oxygen-based functional groups, such as carboxylic, –OH, and epoxy, in the sp3 domain contributes to its dispersible nature. GO has a band gap ranging 2.2–4 eV, which makes it suitable to use as a photocatalyst from UV to visible region [151]. For example, Singh et al. [154] demonstrated the photocatalytic behaviour of GO for the complete degradation of congo red dye in 100 min. The high photocatalytic activity of rGO was attributed to its lower band gap and higher photocurrent density. Doping with certain metals also increases the photocatalytic efficiency of graphene oxide. For example, Ag-doped rGO was prepared by Dat et al. [155] using the co-precipitation method. Ag/rGO was found as an effective photocatalyst for the complete degradation of both methyl orange and methylene blue dyes after 30 min. Additionally, the bactericidal activity of Ag/rGO NPs against S. aureus, E. coli, and P. aeruginosa was quite impressive. Hence, Ag/rGO has great potential to be used in medical fields as an antibacterial agent.

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Thus, studies have demonstrated that graphene-based materials serve as effective photocatalysts in various fields. Research into graphene and its derivatives has the potential to create more efficient and specific photocatalytic systems, which could be utilized in areas such as the environment, energy, and health.

2 Conclusion Metal oxide semiconductor photocatalysts have a range of potential applications, such as water purification, air quality improvement, and the conversion of solar energy. This chapter has examined the key principles and processes of the photocatalytic properties/activities, including the band structures of semiconductors, the production of charge carriers, and their transfer. Furthermore, varied uses of major metal oxides such as TiO2 , ZnO, WO3 , CuO, and Cu2 O and other semiconductor photocatalysts have been discussed in brief. Interestingly, some emerging photocatalysts have also been discussed which have potential future applications in a variety of fields. Strategies for enhancing their photocatalytic performance by altering their surface properties through doping, forming nanocomposites with other materials, and optimizing their compositions and structure have been discussed. Overall, the metal oxide semiconductor photocatalyst is a fascinating field of research that has the potential to address some of the most pressing environmental, biomedical, and energy challenges. With continued research and development, this technology is expected to play a significant role in the transition towards a sustainable future.

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Synthesis of Multifunctional Hybrid Semiconductor Nanomaterials Samriti, Rajeev Gupta, Olim Ruzimuradov, and Jai Prakash

Abstract A strong method for developing innovative materials with numerous capabilities results from the formation of hybrid nanomaterials, in which two or more dissimilar materials are combined to form a hybrid nanomaterial. Hybrid semiconductor nanomaterials are gaining popularity because of their wide range of applications in environmental cleanup, sensing, optoelectronic devices, energy storage, drug delivery, and photocatalysis. The synthesis methods play an important role in tuning the properties of hybrid nanomaterials. Physical, chemical, and mechanical processes are well-known methods for the preparation of nanomaterials. To fabricate hybrid semiconductor nanomaterials, different materials such as metal oxides, organic molecules, and inorganic nanostructures are mixed together. Different methods, such as hydrothermal, solvothermal, sol–gel, photoreduction, microplasma–liquid interaction, and ball milling, can be used to fabricate hybrid semiconductor nanomaterials. This chapter discusses the pros and cons of each method of making nanomaterials and the importance of controlling their size and shape. It also reviews the potential applications of hybrid semiconductor nanomaterials and the challenges associated with their synthesis and characterization. Keywords Hybrid nanomaterials · Multifunctional hybrids · Semiconductors · Photocatalysis

Samriti (B) · J. Prakash Department of Chemistry, National Institute of Technology Hamirpur, 177005, Hamirpur, India e-mail: [email protected] R. Gupta Department of Physics, School of Engineering Studies, University of Petroleum & Energy Studies, Dehradun, Uttarakhand 248007, India O. Ruzimuradov Department of Natural and Mathematic Sciences, Turin Polytechnic University in Tashkent, Malaya Kolsevaya 17, Tashkent 100095, Uzbekistan © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_2

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1 Introduction Hybrid nanomaterials have gained significance in materials science due to the growing demand for novel materials with customized physicochemical properties as a result of their outstanding new properties and multi-functionality. By joining two or more components at the nanoscale scale, hybrid nanomaterials combine the inherent qualities of their separate constituents with extra attributes as a result of the components’ synergistic interactions [1]. Thus, by modifying the composition and morphology of hybrid nanomaterials, it is possible to tune their properties, producing materials with improved performance traits like high thermal stability, strong mechanical properties, luminescence, gas permeability, electron conductivity, and ability to be wetted in a controlled manner [1, 2]. Multifunctional hybrid semiconductor nanomaterials are materials composed of two or more different semiconductors which have been combined to form a single material. These nanomaterials are capable of exhibiting multiple functionalities, such as electrical, optical, thermal, magnetic, and even chemical properties, which can be tailored to the desired application [3–5]. Synthesis of these multifunctional hybrid semiconductor nanomaterials is challenging, as it requires careful control of the different components’ particle size, shape, and composition [6, 7]. The synthesis of multifunctional hybrid semiconductor nanomaterials typically begins with the selection of the appropriate semiconductor components and their subsequent preparation [5, 8]. This includes choosing the right materials, synthesizing the materials, and purifying them. Next, the components are combined in a specific proportion to create the desired hybrid nanomaterial. The research area of creating multifunctional hybrid semiconductor nanomaterials is expanding rapidly due to its potential applications in many areas, including optoelectronics, energy harvesting, and drug delivery [5, 9, 10]. The unique properties of these materials, such as their tunable optical, electrical and mechanical properties, make them ideal candidates for a variety of applications [5]. There are numerous advantages of synthesis of multifunctional hybrid nanomaterials such as: (1) They have improved electrical, optical, and magnetic properties. (2) Increased Efficiency: The incorporation of multiple functionalities into a single material can lead to increased efficiency. This is especially true when the functionalities are mutually beneficial or complementary [11]. (3) Reduced Cost: By combining multiple functionalities into a single material, the cost of creating a functional device can be reduced. This is because the cost of creating a single functional material is much lower than the cost of creating multiple individual functional materials. (4) Increased Versatility: Multifunctional hybrid semiconductor nanomaterials offer increased versatility and can be used in a wide variety of applications. This offers manufacturers and researchers a wide range of options for creating functional devices [8, 12]. (5) Improved Durability: The incorporation of multiple functionalities into a single material can lead to increased durability and reliability. This is because the multiple functionalities are less likely to be affected by external factors such as temperature, moisture, and other environmental factors [8, 11–13].

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This chapter provides an overview of the synthesis of multifunctional hybrid semiconductor nanomaterials, including the various techniques and strategies used for their preparation. Additionally, this chapter reviews the recent advancements in the field and presents potential opportunities for future research. The synthesis of multifunctional hybrid semiconductor nanomaterials is a complex process that requires careful consideration of each step. However, the potential applications of these materials are vast, making this an exciting field of research.

2 Methods for Synthesizing Nanomaterials The control of synthesis parameters is fundamental for engineering nanomaterials of the desired size, morphology and surface area etc. Scientists can exploit this to create nanomaterials with unique characteristics for a range of applications, in energy, environment and biomedical. [8] By changing the parameters, such as temperature, pressure, reactants, and reaction time, novel nanomaterials with novel properties can be synthesized. This enables the fabrication of safe and effective materials for their intended use [5, 8]. Nanomaterials are synthesized by using two primary methods such Top-down and Bottom-up methods as shown in Fig. 1 [14]. Top-down Synthesis: Top-down synthesis involves breaking down larger structures into smaller nanostructures. Common techniques used in top-down synthesis include mechanical crushing, grinding, milling and lithography [15]. Bottom-up Synthesis: Bottom-up synthesis is the opposite of top-down synthesis, where larger nanostructures are built from smaller molecules. Common techniques used in bottom-up synthesis include solgel, hydrothermal, solvothermal, chemical vapor deposition, electrochemical deposition, and molecular self-assembly [15].

2.1 Hydrothermal Method Aqueous solvents and mineralizers are frequently present in heterogeneous reactions that occur at high temperatures and pressures and are referred to as “hydrothermal” processes [1, 8]. Steel pressure vessels called autoclaves are employed in this synthesis procedure. These Teflon-lined autoclaves function at high temperatures and pressures, exceeding 1 atm and 100 °C, respectively, which is higher than the boiling point of water. The size and shape of the created nanostructure can be easily controlled using this method. The method uses only a small amount of energy overall. By adjusting the reaction temperature, time, pH, reactant concentration, and a large

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Fig. 1 The creation of nanomaterials using both top-down and bottom-up methods. Reprinted with permission from [14]. Copyright (2020), Springer Nature

number of other parameters, a product with regulated particle size and the necessary morphological structure can be produced. The synthesis of different dimensions nanostructures, including zero-dimensional (0D), one-dimensional (1D), twodimensional (2D), and three-dimensional, is done using this technique (3D). based on the solvent solutions that are frequently used throughout the synthesis process. It includes processes known as acid-hydrothermal and alkali-hydrothermal, which combine precursors of TiO2 with solutions of acids and alkalis [8, 16]. The hydrothermal synthesis of TiO2 nanomaterials involves the reaction of titanium dioxide precursor with aqueous solution in a high-pressure, high-temperature environment. The titanium dioxide precursor can be an organic compound, such as titanium (IV) isopropoxide, or an inorganic compound, such as titanium tetrachloride. This reaction is catalyzed by the hydroxide ions from the aqueous solution, which act as the reducing agents, and the oxygen from the aqueous solution, which acts as the oxidizing agent. The reaction products are typically TiO2 nanomaterials, such as nanotubes, nanorods, and nanoparticles. The size and shape of the nanomaterials can be controlled by adjusting the reaction temperature, pressure, and reaction duration [3, 8]. The hydrothermal method is employed in the synthesis of various hybrid semiconductor photocatalysts such as ZnO/CuO [17], CNTs@CuBi2 O4 /AgBiO3 [18],

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MoS2 /TiO2 [19], CdS–CuS decorated TiO2 NTs, [20] TiO2 /GO [21–23], ZnO/GO [24, 25], Ag or Au/metal oxide nanocomposite [26] etc. According to Fig. 2, the formation mechanism for Si-TiO2 Nanotubes (NTs) is made from SiO2 and TiO2 precursors using a hydrothermal technique. TiO2 and SiO2 first combined with NaOH to produce octahedral TiO6 , which can be found as the neutralizing complex H4 TiO6 or H4 SiO4 , the salt NanH4-nTiO6 or NanH4 -nO4 ,( n is the number of Na atoms that have taken the place of the H atoms). Two Si–O–Ti group oxygen bridges can be used to condense octahedral TiO6 and tetrahedral SiO4 together. The HCl treatment resulted in the protonation of Ti/Si–O–(Na + ) groups, thus creating Ti/Si–OH and Ti/Si–OH and leading to an uneven charge distribution on the 2D sheet. This charge imbalance caused the 2D sheet to roll into a tubular structure. Water molecules from the Si/Ti–OH structure in the tubular structures were removed through additional drying and annealing to create the Si-TNTs structure. The “build up” mechanism of Si-TNTs, which involves the even distribution of Si+4 across the TNT lattice, is explained by the formation of octahedral TiO6 and tetrahedral SiO4 units [27]. Ta doped TiO2 nanorods were created by Samriti et al. [28] using a hydrothermal technique. Prior to Ta ion doping, they optimize the post hydrothermal parameter (soaking time in acid) and adjust the band gap for pure TiO2 NRs from 3.18 to 2.95 eV. By adjusting reaction parameters including pH and temperature, Kumari et al. [17] were able to produce ZnO/CuO hybrid photocatalysts. When the temperature rises (from 160 °C to 180 °C to 200 °C), the structure of ZnO changes (Fig. 3a–e) from nanorods to nano bitter gourds, and particle size increases as a result of agglomeration. Similar to this, when the pH rises from 7 to 9 to 11, nanorods from pellet drums transform into nano bitter gourd-shaped ZnO. It is thought that the lower optical band

Fig. 2 Schematic showing hydrothermal synthesis mechanism. Reprinted with permission from [27]. Copyright (2023), Elsevier

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gap of ZnO when combined with CuO is due to interactions between the two materials or an electronic transition between the valence and conduction bands of a p-n heterojunction made of n-type ZnO and p-type CuO. Moreover, the red shift in the optical band gap, which increases the material’s capability for light absorption and scattering, may be caused by the development of surface defects in ZnO after the integration of CuO. It was shown that triclopyr’s ability to photodegrade when exposed to UV light was greatly impacted by its morphological and optical properties [17]. This method is also being employed to obtain the nanomaterial for other energy, environmental as well as biomedical related applications ranging from dye-sensitized solar cells, sensing, drug delivery and catalysis etc.

Fig. 3 FESEM picture of an 8:1 molar ZnO/CuO composite produced under varied synthesis conditions. a-c The following temperatures have pH values of 9: T = 160 °C, Zn8Cu or T = 180 °C, T = 200 °C, pH = 9, d T = 180 °C, pH = 7, and e T = 180 °C, pH = 11. Reprinted with permission from [17]. Copyright (2023), Elsevier

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Advantages [8, 16, 17, 20, 28] 1. Hydrothermal method is a fast and efficient method of synthesizing materials with a high degree of purity. 2. It can produce materials with complex structures and shapes. 3. It is a relatively low-cost method compared to other synthesis techniques. 4. It can be used to synthesize materials with nanoscale dimensions. 5. The products produced by this technique are homogeneous and well characterized. 6. It can be used to synthesize a variety of materials, including metals, alloys, ceramics, and semiconductors. 7. It is less energy intensive compared to other methods such as arc melting. 8. It is relatively easy to modify the reaction conditions to produce desired products. 9. It is a clean process with little by-product formation. 10. It is a versatile technique that can be used to synthesize a variety of materials including nanomaterials. Limitations [16, 17, 20, 28] 1. It does not apply to all types of materials, as some materials may not be able to withstand the high temperatures and pressures used in the hydrothermal method. 2. It is not suitable for large-scale production, as the process involves high equipment costs and long processing times. 3. The hydrothermal method is limited to aqueous solutions, so it is not suitable for some organic compounds. 4. It can be difficult to control the size and shape of the final product, as the process is largely dependent on the reaction conditions. 5. It is limited to a small range of temperatures and pressures, so some materials may not be able to be processed using this method.

2.2 Solvothermal Method The solvothermal method, which relies on the use of a non-aqueous solvent as the reaction medium, is built on the foundation of the hydrothermal technique. It enables the operation of reactions at higher temperatures due to the range of organic solvents with higher boiling points [7, 29]. This method can improve and control the size, shape, distribution, and crystallinity of nanoparticles, and is used to synthesize binary and ternary oxide- and sulfide-based hybrids in 2D or 3D, both layered and unlayered. The solubility, diffusion behavior, and reactivity of the reacting species can be affected by solvents of different chemical and physical properties, with the polarity of the solvent having a particular impact on the morphology [8, 29, 30]. Using a solvothermal process, heterostructures of. Ag2 WO4 and Sb2 WO6 have been reported by Rafiq et al. [31] which exhibited improved charge transfer characteristics. Alomar et al. [32] employed a two-step solvothermal approach for the creation

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of heterostructure MoS2 nanosheets coated on CdS nanoparticles, with the thickness and interlayers of the (002) MoS2 ultrathin nanosheets adjusted depending on the reaction temperature. This led to a narrowing of the MoS2 /CdS nanocomposite’s band gap, with a darker color of the nanocomposite being observed at 220 °C, which is thought to be due to the decrease in the gap and increase in the solvothermal temperature. Moreover, Cu2 O-rGO composites have been synthesized using the solvothermal approach for the photocatalytic degradation of Rhodamine B, with SEM, FTIR, and BET analysis as shown in Fig. 4a-d confirming the successful production of the composites, as well as the reduction of GO and possessing larger surface areas than pure Cu2 O [33]. Advantages [29–32, 31, 32] 1. Solvothermal method is a highly efficient, cost-effective, and environmentally friendly technique for the synthesis of a wide variety of materials. 2. It is a simple technique as compared to other synthesis methods that require complicated instrumentation. 3. Solvothermal method is capable of producing homogeneous and well-controlled nanostructured materials with improved optical, electrical, and magnetic properties. 4. The reaction conditions such as temperature and pressure can be easily controlled, allowing for precise control over the synthesis process. 5. The solvothermal method is a green synthesis technique and does not require the use of hazardous chemicals or solvents.

Fig. 4 a FT-IR spectra of as-prepared Cu2 O/rGOx% samples. b and c FESEM images of Cu2 O/ rGO 0.05, and 0.5% (wt%), respectively, and d N2-adsorption isotherms of Cu2 O/rGO0% and Cu2 O/rGO0.5% (wt). Reprinted with permission from [33]. Copyright (2023), Elsevier

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6. The reaction time is shorter than other methods, and the products can be easily separated and collected. Limitations [29–31, 31, 34] 1. The solvothermal method is time-consuming and requires strict temperature and pressure control. 2. The solvothermal method is limited to materials that can be dissolved in a solvent. 3. The solvothermal method can be expensive due to the cost of solvents and other materials. 4. The solvothermal method produces a large amount of waste that needs to be disposed of safely. 5. The solvothermal method can lead to the formation of byproducts that can interfere with the desired product. 6. The solvothermal method can be difficult to scale up to larger production levels.

2.3 Sol–Gel Method The sol–Gel process is often employed to produce hybrid metal oxide nanocomposites. This liquid-based polycondensation method is advantageous as it enables precise control of variables involved in the synthesis of a variety of metal oxides and their nanocomposites. Through this technique, the materials can be formed into powder, thin films for coatings, or nanofibers (Fig. 5a) [5, 15, 36, 37]. The sol–Gel process involves dissolving precursors in either a water or organic solvent, treating it with an acid or base reagent to form a sol, and then shaping the resultant solution by means of thermal heating or sintering. This method is beneficial as it facilitates the formation of polycrystalline particles with special properties by homogenously mixing metal ions at the molecular level. Moreover, active dopants/molecules can be added to the sol during the gelation stage to increase the photo efficiency of the hybrid photocatalysts [15, 36, 38]. Sol–Gel methods have been used to synthesize a variety of thin films, including graphene oxide-modified titanium dioxide (GO/TiO2 ), Gd–TiO2 –GO nanocomposite, etc. Hernández et al. observed a decrease in the band gap of thin films with an increase in GO content (Fig. 5b) [39], while Oppong et al. [40] observed a decrease in the band gap energy with an increase in Gd concentration. Further, Verma et al. [41] found that higher surface area TiO2 /GO nanocomposite had a higher photodegradation rate under visible light irradiation. In the sol gel process, the calcination temperature is crucial for fine-tuning the characteristics of hybrid nanomaterials. Anatase or rutile TiO2 can become porous after calcination [8, 38]. Smaller grains, lower surface area, and the anatase–rutile phase transition are frequently observed at temperatures higher than 600 °C. Mesoporous channel walls and long-aligned microporous channels characterize the anatase phase [39]. The impact of calcination temperature (TSC 300–1300 °C) on TiO2 /SiO2 hybrid photocatalytic activity was investigated by Cheng et al. [42]. As the calcination temperature was increased, the absorption edge gradually red shifted. The band

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Fig. 5 a Schematic of sol–Gel process [37]. b Tauc plot of the optical band gap for thin films of GO/TiO2 . Reprinted with permission from [39]. c The changes in the UV–vis spectrum of RhB dye when TSC-600 is used as a composite photocatalyst. Reprinted with permission from [42]. Copyright (2023), Elsevier

gap energy is usually narrowed, resulting in a decreased amount of energy needed to excite electrons from the valence band to the conduction band. The sample showed the greatest photocatalytic activity when the calcination temperature was up to 600 °C, as indicated by the results (Fig. 5c). 90% of RhB was removed by these photocatalysts after being exposed to visible light for 4 h [42]. Advantages [42–48] 1. Sol–gel technique is a simple and versatile method which offers the possibility of producing coatings with a wide range of properties. 2. Sol–gel technique can be used to produce nanosized particles with a high degree of purity and uniformity. 3. The process is cost-effective and efficient.

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4. Sol–gel technique can be used to produce coatings of various thicknesses and properties. 5. The process is relatively simple, and can be conducted at relatively low temperatures. 6. Sol–gel techniques can be used to produce coatings with improved mechanical, electrical, and optical properties. 7. Sol–gel techniques enable the production of coatings that are resistant to high temperatures, oxidation and corrosion. 8. The technique is capable of producing coatings with high adhesion and good coverage. Limitations [42–48] 1. The sol–gel process is limited to materials with low melting points and those with high melting points require high processing temperatures, which can lead to decomposition of the material. 2. The sol–gel process is time consuming and requires precise control over the chemical and physical parameters of the gelation process. 3. The sol–gel process is limited to forming materials with relatively low porosity and limited surface area. 4. The sol–gel process produces materials with a limited range of structures and shapes.

2.4 Photoreduction Method Photoreduction is a chemical reduction process that uses light energy to reduce molecules and produce hybrid semiconductor nanomaterials. This process involves a photosensitizer which absorbs light energy and transfers it to an electron donor [49, 50]. The electron donor is then reduced and the product is formed. Photoreduction is used to synthesize semiconductor materials or nanocomposites under photoirradiation and in the presence of precursors of nanomaterials. It is based on the photocatalysis phenomena in which a metal oxide semiconductor is activated by UV light to produce electrons and holes in the conduction band and valence band respectively. These ions reduce the metal ions on the metal oxide surface into a metallic form, resulting in the formation of metal nanoparticles shown in Fig. 6a [38, 49–51]. Prakash et al. [34] synthesized Ag–TiO2 nanocomposites and studied their SERS and antibacterial traits. It was found that Ag nanoparticles (7–20 nm) were tightly linked to and uniformly dispersed around the TiO2 NPs. UV absorption spectra showed a decrease in band gap energy with an increase in Ag content. This decrease was attributed to the formation of Ag metallic clusters which introduced localized energy levels into the band gap of TiO2 , and as such, electrons could be driven with less energy from the valence band to these levels than to the conduction band of TiO2 .The nanocomposites exhibited increased SERS and antibacterial activity [34].

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Fig. 6 a Synthesizing M@TiO2 nanocomposite NPs using photoreduction. Reproduced with permission from [51]. b Diagram illustrating the synthesis and photocatalytic breakdown of dye by TiO2 –Au hybrid nanoparticles. c The UV–Vis diffuse reflectance spectra of a TiO2 –Au hybrid nanostructure created by microplasma are examined. Reprinted with permission from [53]. Copyright (2023), Elsevier

Mangadlao et al. [52] prepared hybrid graphene-metal nanoparticles composed of Ag, Au, and Pd via a one-pot photochemical process. Prior to exposure to ultraviolet radiation, the solution was observed to have two distinct peaks at 231 nm and a faint peak at 300 nm, which corresponded to the −C = C− and −C = O transitions, respectively. After 30 min of UV exposure, the GO’s characteristic peaks disappeared and the spectra was red-shifted with a maximum at 264 nm. The transition from dark to black and the increased absorbance in the visible region showed the recovery of the π-conjugated network and clumping of reduced GO. The hybrid nanoparticles showed great potential for catalyzing the breakdown of environmental pollutants such as 4-nitrophenol, rose bengal, and methyl orange [52].

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Advantages [5, 50, 52] 1. Photoreduction is a simple and cost-effective method for synthesizing nanoparticles. 2. No additional reducing agents are required for the process, thus simplifying the synthesis process. 3. The process is quite fast and can be completed in minutes, depending on the type of nanoparticle desired. 4. The size and shape of the nanoparticles can be tuned by controlling the reaction parameters. 5. The photoreduction method is a green process, with no toxic byproducts. 6. The process is very flexible, allowing for a variety of precursors to be used for the synthesis of nanoparticles. 7. The method can be used to synthesize a variety of nanoparticles, such as metals, oxides, and semiconductors. 8. The nanoparticles produced by this method have a high degree of purity. Limitations [5, 50, 52] 1. Photoreduction method is limited to materials that can absorb light and have an appropriate band gap. 2. It is difficult to achieve uniformity in the thin film when using the photoreduction method. 3. The photoreduction method is time-consuming and requires a lot of energy. 4. The photoreduction method is not suitable for large-scale production of thin films. 5. The photoreduction method is limited to materials that can be dissolved in solution. 6. The photoreduction method is not suitable for controlling the thickness of thin films. 7. The photoreduction method is not suitable for producing thin films with complex structures.

2.5 Microplasma–Liquid Interaction Method The microplasma-liquid interaction process is advantageous due to its simplicity, speed, and environmental friendliness. This process allows electrons to directly enter the solution, which can reduce metal ions from the precursor solution without the use of chemical reducing agents [54–56]. As a result, bare metal particles can adhere directly to semiconductor surfaces, leading to improved electron transfer and enhanced photocatalytic activity in the hybrid nanocomposites created by this method [53, 54]. Lee et al. [53] demonstrated this by creating TiO2 -Au hybrid nanoparticles and studying their ability to degrade the organic dye methylene blue (MB) under UV and visible light irradiation as shown in Fig. 6b-c. These nanoparticles had increased

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visible light absorption at 546 nm, likely due to the localized surface plasmon resonance (LSPR) of the AuNPs, and UV light absorption at 400 nm, which was attributed to the bandgap of TiO2 [53]. Advantages [53, 55, 56] 1. Microplasma–liquid interaction is a fast, easy and affordable method for synthesis of nanoparticles with low-cost equipment. 2. This method can be used for the synthesis of nanoparticles of different metals such as gold, silver, palladium, and platinum, as well as other materials. 3. The method offers a high degree of control over the size and shape of the nanoparticles. 4. The reaction rate can be easily changed by adjusting the power supply and the duration of the plasma-liquid interaction. 5. The method is capable of generating nanoparticles with a narrow size distribution. 6. The process is relatively free of environmental pollution and produces no hazardous waste. 7. The method is suitable for the synthesis of nanoparticles in large quantities. Limitations [53, 55, 56] 1. The microplasma method is limited by the small size of the device and the limited power that can be used for plasma generation. 2. The microplasma method is limited in effectively sterilizing and purifying larger volumes of liquid. 3. The microplasma method is limited by the fact that it can only be used to treat liquid samples, not solids or gas. 4. The microplasma method is limited by the fact that it is a low temperature process and thus cannot be used to sterilize samples at higher temperatures. 5. The microplasma method is limited in effectively treating materials with high conductivity. 6. The microplasma method is limited in effectively treating samples with complex chemical compositions. 7. The microplasma method is limited by the fact that it is a relatively slow process.

2.6 Microemulsion Method The Microemulsion is a novel technique that utilizes a dispersion of two immiscible liquids stabilized by an amphiphilic surfactant to create hybrid semiconductor materials such as metal oxides, nitrides and phosphates. This microemulsion is used as a reaction medium where metal salts are combined with an inorganic precursor in an aqueous phase. The amphiphilic surfactant stabilizes the reaction medium to avoid the formation of large particles (Fig. 7) [36, 57–59]. The microemulsion method has been demonstrated to be advantageous for the synthesis of hybrid semiconductors due to its ability to offer better control of particle size, crystal structure and surface

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morphology when compared to traditional methods. Additionally, it is cost-effective and environmentally friendly [36, 57, 60]. As a result, a variety of hybrid semiconductors have been fabricated using this technique, which exhibit superior properties such as high electron mobility, low leakage current and good stability in various environmental conditions. This makes the method promising for the production of optoelectronic devices, sensors and catalysts [57, 60]. Aubert et al. [61] utilized a reverse microemulsion approach to construct stable [Re6 clusters@silica] NPs for biological applications, which were composed of

Fig. 7 Colloidal synthesis by a microemulsion technique. Reprinted with permission from [59]. Copyright (2023), Elsevier

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Re6 cluster units contained in nanodroplets of a reverse microemulsion made of nheptane/polyoxyethylene-4-lauryl ether. The A4 [Re6 Q8 L6 ]@SiO2 NPs were formed by adding silica precursor to this solution, resulting in spherical particles of 30 nm. [61] Similarly, Liu et al. [60] synthesized a bismuth oxyiodine/titanium dioxide (BiOI/TiO2 ) hybrid with enhanced photocatalytic performance under visible light irradiation.63 Additionally, Yu et al. [58] synthesized NaYF4 :Er/Yb-SiO2 particles via reverse microemulsion, observing an increase of 12.5 fold in the luminescent intensity [58]. Advantages [60, 61] 1. Microemulsion method offers a simple and economical way of synthesizing hybrid semiconductors. 2. It is a scalable process and can be used for large scale production. 3. The method is more efficient and produces homogeneous products. 4. The method is also suitable for producing nanostructured materials with high surface area. Limitations [60, 61] 1. The method is limited to the synthesis of hybrid semiconductors with specific compositions and properties. 2. The process requires careful temperature control and is prone to crystallization. 3. The reaction conditions can be difficult to control, resulting in inconsistent product quality. 4. The method is unsuitable for synthesizing materials with low melting points.

2.7 Ball Milling Process The ball milling process subjects a powder combination to high-energy collisions from the balls, resulting in the creation of new materials. Furthermore, it can change the conditions of chemical reactions through mechanical activation, which increases the speed of reactions and reduces their temperatures, or by causing reactions to occur while the solids are being ground Fig. 8a-b [62, 63]. The figure below shows the powder and ball movements. Due to the oppositedirection rotation of the bowl and turn disc, the centrifugal forces alternately synchronize. The hardened milling balls alternately rolled on the inside wall of the bowl and struck the exterior wall, creating friction and grinding the powder mixture [62].The impact energy of the milling balls can be up to 40 times more than the acceleration brought on by gravity. As a result, a planetary ball mill can be used for high-speed milling [62–64]. Advantages [62–64] 1. Ball milling is a versatile, cost-effective, and time-efficient method for producing fine particles with a high purity.

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Fig. 8 a-b Schematic view of motion of the ball and powder mixture. Reprinted with permission from [63]. Copyright (2019), Royal Society of Chemistry. (https://pubs.rsc.org/en/content/articl ehtml/2019/na/c8na00238j)

2. It produces particles in a narrow size distribution range, with an average particle size of about 10–100 nm. 3. It is a simple and inexpensive method that can be scaled up to industrial production. 4. It is a non-destructive, low-energy process with low environmental impact. 5. Ball milling can be used to synthesize a variety of materials, including nanocrystalline alloys, ceramics, and semiconductor materials. 6. Ball milling is also used to reduce the particle size of materials and to increase the reactivity of reactants. Limitations [62, 64] 1. Ball milling is time consuming and requires large amounts of energy. 2. Ball milling can produce very fine particles, but is not suitable for large-scale production. 3. The materials used in the milling process may react with the container, leading to contamination of the product. 4. The ball milling process is noisy and can produce a large amount of heat, which can be difficult to control. 5. Ball milling is a slow process, and can be difficult to scale up. 6. It is difficult to achieve uniform particle size and narrow size distribution with ball milling.

3 Conclusion This book chapter provides a complete overview of recent developments of synthesis of hybrid semiconductor and effect of synthesis parameters on their morphological and optical properties. Synthesis of multifunctional hybrid semiconductor nanomaterials has been a major focus of research in recent years. This research has enabled

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the development of novel nanomaterials with enhanced properties and functionalities. These materials have the potential to revolutionize the way we use and interact with technology. The synthesis of these materials requires careful consideration of the various components and their interactions. By combining different materials, it is possible to create nanomaterials with unique properties and functionalities. The synthesis of multifunctional hybrid semiconductor nanomaterials has opened up a wide range of possibilities for the development of new materials with improved properties and applications. Acknowledgements Author (Samriti) expresses gratitude to the MHRD and Department of Chemistry, National Institute of Technology, Hamirpur for their financial assistance during the course of her PhD research.

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Application of Nanostructured Metal Oxides and Its Hybrids for Inactivation of Bacteria and Viruses Junghyun Cho

Abstract Low-temperature solution growth of ceramic coatings made of metal oxides and its hybrids provides unique opportunities to tailor surface functionalities of the coatings. It also makes the coating surface to be toxic to microorganisms such as bacteria and viruses. In particular, TiO2 (anatase) and ZnO nanostructures have shown the ability to decompose organic dyes and contaminated species, and also to inactivate harmful bacteria and viruses. While this behavior often results from photocatalytic reactions in air or water when exposed to UV or visible light, the antimicrobial properties are also observed even with no light illumination. It indicates that the inactivation is due to a contact made between the nanostructures and the bacterial cells. This antimicrobial behavior was also shown for some viruses from limited examples. To further explore a more effective way of creating such a functional surface, a ‘core–shell’ structure consisting of a rutile nanorod core and a functionalized shell layer is proposed. Such a hybrid (nanocomposite) structure exhibited a strong antimicrobial surface that can be insensitive to the types of bacteria and viruses, thereby providing more universal protection from virus transmission through surface contact. This article reviews current progress made in lowtemperature solution processing of TiO2 , ZnO, and their doping and hybrid coatings and associated nanostructure developments that are key in the photocatalytic properties and antimicrobial surface. Keywords Ceramic coatings · Nanocomposites · Low-temperature solution processing · Photocatalytic · Antimicrobial

J. Cho (B) Department of Mechanical Engineering, Materials Science and Engineering Program, Binghamton University (State University of New York), Binghamton, NY 13902-6000, USA e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_3

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1 Introduction Metal oxides such as TiO2 , ZrO2 , SnO2 , SiO2 , and ZnO have received great attention for use as a functional ceramic film [1–15], as well as a protective and barrier coating [16–28]. More recently, these oxides have shown great potential in removing organic pollutants and microorganisms found in many wastewater treatment facilities from manufacturing plants, hospitals, farms, dye industry, and airports [29–33]. These facilities face the dual challenge of managing effective pollutant removal while also establishing recycling programs that can conserve important resources such as water. Many of these oxides are wide bandgap semiconductor that can produce chemical reactions under UV light in the presence of water to generate active hydroxyl radicals. Such radicals are strong oxidizer that attacks organic pollutants and microorganisms [34–38]. Among them, nanostructured TiO2 and ZnO have received a great deal of attention for inactivating microorganisms such as bacteria and viruses [34, 36, 37, 39, 40]. During the COVID-19 pandemic, surfaces and equipment that are frequently touched during the course of normal operations or everyday life needed to be cleaned and disinfected. Once contaminated, for example, a N95 mask can be a medium to further transmit viruses until it is disinfected. Given that, research studies have been focused on developing antimicrobial nanomaterials that can keep contact surfaces from being infected, which will provide the first level of protection by actively neutralizing viruses settled on surfaces of interest. Non-toxic/non-allergic materials not only provide long-term sanitized surfaces but also disinfect the coronavirus, thereby preventing its spread through touching surface. Furthermore, it provides more universal strategies for preventing the spread of current COVID-19 spread, and can also work for general protection against numerous microorganisms regardless of its types and strains. Advanced nanotechnology enables to engineer surface properties of the nanomaterials to make its disinfection power greatly enhanced. This could be achieved through nano-antiviral surface coatings [34–39] on the common surfaces of general use or personal protection equipment. In particular, a low-temperature processing of nanostructured oxide coatings enables their surface to be very effective in removing harmful bacteria and viruses [37, 39, 41]. For example, vertically aligned nanorod arrays have shown the maximum surface reactivity for toxic agents or bacteria [42–45]. Such nanostructured films are grown from in-situ precipitated nanoparticles when precursor solution is exposed above a critical saturation point during the hydrothermal processing. Depending on its pH, temperature, and solution chemistry, the primary nanoparticles are evolved into diverse shapes of secondary nanostructures (e.g., nanorods, nanoblades, nanoclusters) via hierarchical organization [46, 47]. TiO2 has been intensively studied for its various properties originated from its unique band gap structure and widely applied to solar energy conversions, sensors, and environmental protection applications [48–50]. As electron–hole pairs are generated upon absorption of incoming photons, the oxidizing and hydroxyl agents are generated in an aqueous environment and react with dye molecules or cell surfaces

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for their decomposition. Therefore, TiO2 provides a very promising solution for organic waste issues and antibacterial requirements. Hydrothermal method is done at a relatively low processing temperature, so can be adopted for making a film on low heat tolerance materials. ZnO has been well studied as well for its photocatalytic properties and exploited for water and chemical effluent purification, organic contamination removal, and medical applications. However, only a few studies have been made on the effects of the material under less-than-ideal illumination (i.e., ambient or low light, darkness). There is an evidence that ZnO is toxic to some bacteria under dark conditions. As shown in a recent paper [51], ZnO nanoparticles with a size-scale in the range of 10– 50 nm and specific geometries (particularly pyramidal) are very effective in inhibiting the spread of methicillin-resistant Staphylococcus aureus (MRSA). It is very likely that these nanostructured oxides with optimal features on a coating’s surface can be very effective against biofilm-forming bacteria such as E. coli, P. aeruginosa, S. pneumonia, Legionella, and other strains of bacteria and viruses. Hybrid or composite structure of such oxides offers a more efficient design by retarding the photogenerated electron–hole recombination [52–54]. It has been shown to boost the power conversion efficiency (PCE) by generating high current. There have been continuous attempts to realize the benefit for this structure and as a result, many heterogeneous systems have been developed and have achieved some enhancements in PCE, including anatase/rutile [55], ZnO/rutile [52], and SnO2 /ZnO [56]. Kawahara et al. [53] tried to generate an anatase/rutile phase-junction structure by creating a patterned bilayer and improving its dye degradation rate to 0.43/h. Later, this structure was successfully applied for a photovoltaic device and reached to ~7% PCE with a particle-based dye-sensitized solar cell [54]. Kwon et al. [57] also reported a hydrothermal method to synthesize the rutile nanorods decorated with anatase particles, but the mechanistic understanding of the rutile formation and the anatase particle quality was lacking. From the literature review, the advantage of having an anatase/rutile phase junction is generally two folds: lowering the electron/ hole recombination rate and increasing the dye load on anatase particle surface. In this review, the nanostructure features of metal oxides and its hybrids are presented, where its process schemes and structural novelties are highlighted by providing mechanisms to inactivate the microorganisms. While doing so, the nanostructure—antimicrobial property relations of the hybrid (or nanocomposite) materials are established. In order to further improve the performance of a wide bandgap semiconducting metal oxide, the use of low-energy visible light photons is essential. This can be achieved through a multiphoton excitation of the hybrid materials or doping that creates the formation of the delocalized states in the middle of the bandgap [58].

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2 Solution Processing Schemes Unlike biominerals in nature, synthetic oxide films in aqueous solution are formed under an accelerated hydrolysis environment for a relatively short period. For this, a more quantitative understanding of the hydrolysis process of precursor species is needed in order to demonstrate the effects of solution parameters such as pH, concentration and temperature on nucleation, growth and aggregation of the nanoparticles. Using equilibrium solubilities (C l ) for stable phases, the degree of supersaturation (S) is defined as: [ νi ]1/ν Πai C S= = , Cl Kp

(1)

where C is the concentration, ai is the activity of specie i, K p is the equilibrium constant and v is the number of soluble species. As the solubility of the oxides and their hydroxides are often unknown, one can use the thermodynamics data to calculate S. It is then determined through the ratio of the solution ion activity product to the equilibrium solubility product of the precipitated phase [47, 59]. For example, TiCl4 , a common precursor for TiO2 , undergoes hydrolysis in aqueous solution to form several hydroxo-, chloro-, and chloro-hydroxo-titanium species: TiCl4 + xH2 O ↔ TiCl4−x (OH) y (4−x−y)+ + xH+ + xCl− ,

(2)

The extent of association of chlorine ions is insignificant at low pH and low concentration of Cl− , in which cases the predominance of hydroxotitanium species (Ti(OH)y (4−y)+ ) has been reported [60]. Depending on the availability of OH− (i.e., with pH of solution) the extent of hydrolysis may vary. In a low pH range (250), it was mostly amorphous phase. It indicates that supersaturation not only controls the film morphologies but also the film phases. With such conditions identified, a TiO2 film consisting of a completely rutile phase was processed, as shown in Fig. 7a and b. Vertically oriented nanoblades were obtained at this low S (63.9). To further lower S ( 400 nm [49].

3 Porous Substrate (Carbonaceous)-Based for the Photocatalytic CO2 RR Carbonaceous materials can act as electron reservoirs to receive the photo-induced electrons from semiconductor photocatalysts for rapid charge transfer, accelerating the separation of photo-induced electrons and holes. Furthermore, they can improve the adsorptive properties of the hybrid photocatalysts, augmenting the photocatalytic performance. Besides, many carbon-based materials are cost-effective and

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environmentally friendly, which makes them ideal to compose highly efficient hybrid photocatalysts with other semiconductors for catalysis [50].

3.1 Carbon Nitride-Based Polymeric carbon nitride (CN) has excellent chemical stability and visible light response with the ability to donate protons and electrons for CO2 RR. It has been considered one of the most promising semiconductor photocatalysts to realize utilization at an industrial scale [51]. One of the challenges is that the binding interactions between the pristine graphitic carbon nitride (g-C3 N4 ) and CO2 are not strong, making the direct protonation process difficult. The fabrication of strong active sites on CN-based photocatalysts is of significance. Many works focus on modification of the CN-supported photocatalysts [52]. Ding et al. reported a metal-free photocatalyst of g-C3 N4 linked to melamine-resorcinol-formaldehyde (MRF) microsphere polymers for CO2 RR. The covalent linkage enables the efficient separation of the photo-induced carriers, resulting in the CN-MRF catalyst with a CH3 OH yield of 0.99 μmol h−1 and a quantum efficiency of 5.5% at 380 nm irradiation [53]. Hu et al. reported a Co2 P@BP/g-C3 N4 heterojunction for the photoreduction of CO2 to CO. The Co2 P@BP/g-C3 N4 exhibits a CO selectivity of around 96% with a CO yield of 16.21 μmol h−1 g−1 , which is more than 5 times higher than that of the original g-C3 N4 [54]. Nowadays, single-atom catalysts (SACs) show great promise in catalytic reactions for the maximum utilization of metal atoms and efficient active sites [55]. Sun et al. anchored the Cu single-atom (SA) on the P-doped CN, offering an exclusive CO production rate of 49.8 μmol h−1 g−1 under visible light irradiation. The doped P in CN upshifts the d-band center of Cu toward the Fermi level in the catalyst, which improves the CO2 adsorption and activation on Cu1 N3 active sites for the promoted CO2 RR performance [56].

3.2 Graphene-Based Graphene-based catalyst materials gain extensive interest in the field because of their fascinating electronic, thermal, and mechanical properties. Although pristine graphene has no intrinsic bandgap, the high conductivity and the high surface of graphene benefit the electron transfer and the mass transfer, thus enhancing the catalytic performance of heterogeneous catalysis [57]. Guo et al. anchored noble metal atoms on the orderly stacked graphene nanobubble arrays to mimic the natural photoreduction process. The nitrogen dopant on the graphene acts as the anchor to coordinate the noble atoms (Fig. 10a). The graphene nanobubble arrays that absorb and scatter the sunlight efficiently serve as great support for harvesting solar energy in CO2 RR. The authors tried three noble metal

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Fig. 10 a The schematic of the light absorption in graphene nanobubble arrays anchored with noble element atoms on N-dopants. The time-dependent photocatalytic CO2 RR activities of b Au/ graphene, c Ag/graphene, and d Pt/graphene. e The CO evolution on Au/graphene with different Au loadings. f The photocatalytic CO2 RR activities of Au/graphene. g The catalytic activity stability test of Au/graphene. Reprinted with permission from [58] Copyright (2022), American Chemical Society

elements (Au, Ag, Pt) on the graphene nanobubble arrays for the photoreduction of CO2 and found out that Au demonstrated the best catalytic activity towards CO production (Fig. 10b–d). The as-prepared Au atoms/graphene metamaterial offers a CO production rate of 111,360 μmol h−1 g−1 with a CO selectivity of 95%, which is claimed to be one of the highest values among metal-based catalysts to date (Fig. 10e, f). Besides, the hybrid catalyst of Au/graphene also exhibits excellent stability after a continuous test of 36 h (Fig. 10g) [58].

3.3 MOF and COF-Based Apart from C3 N4 and graphene-based materials, coordination polymers are also popular in the photocatalytic reduction of CO2 [59, 60] Metal-organic frameworks (MOFs) are one class of polymers with highly ordered crystalline coordination that is flexible for chemical modifications. They have high porosity and tunable chemical/physical properties to host active sites and reaction reagent molecules, which are appealing to catalytic reactions [61, 62]. Dai et al. encapsulated the ultrasmall Cu nanoparticles (diameter of around 1.6 nm) in the Zr-MOFs (Cu@MOF) to form the core-shell composite catalyst in a sustainable synthetic route (Fig. 11a). And the Cu@MOF catalysts showed a good CO2 photoreduction rate of around 120 μmol h−1 g−1 with an HCOOH product selectivity of around 86% (Fig. 11b) [63]. Ran et al. applied a versatile synthetic method to engineer different SA sites on covalent organic frameworks (COFs) for promoted photocatalytic CO2 RR. The SA sites (e.g., Fe, Co, Ni, Cu, Zn, Mn, and Ru) are anchored in the main chain of the

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Fig. 11 a The schematic of the Cu NCs size reduction to the Cu NCs@MOF-801 preparation. b The photocatalytic performance of Cu NCs@MOF-801 and Cu NCs@UiO-66-NH2 with control experiments. Reprinted with permission from [63] Copyright (2022), Wiley-VCH GmbH. c The synthetic process of Fe SA/COFs. d The photocatalytic CO yield and e calculated CO selectivity on COFs, 4.8 wt% Fe SA/COFs, 1.1wt% Fe SA/COFs, and 0.6 wt% Fe SA/COFs under 4 h visible light irradiation. Reprinted with permission from [64] Copyright (2022), American Chemical Society. f The schematic of the fabrication of HOF-25-Ni and HOF-25-Ni@GO for photocatalytic CO2 RR to CO. H: white; N: cyan; C: grey; O: red; Ni: green; the octyl chains of the guanine units: brown balls. g The comparison of photocatalytic CO2 RR performance between HOF-25-Ni and HOF-25-Ni@GO-10 for 2 h. Reprinted with permission from [65] Copyright (2022), Wiley-VCH GmbH

COF (Fig. 11c). The Fe SA/COF achieved a CO yield of 980.3 μmol g−1 h−1 with a 96.4% selectivity (Fig. 11d, e). The atomically dispersed SA sites and the COF support form a synergy, providing excellent catalytic activities [64]. In addition to the MOFs and COFs, hydrogen-bonded organic frameworks (HOFs) from the organic framework family also demonstrate good catalytic activity towards CO2 RR. Yu et al. immobilized Ni sites on the HOFs. The sonication method successfully exfoliates the HOFs into nanosheet structures to support the Ni sites (Fig. 11f). When dispersing on the graphene oxide, the HOF-Ni exhibits a high CO yield of up to 24,323 μmol g−1 h−1 with a CO selectivity of 96.3% (Fig. 11g) [65].

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4 The Structure-Engineering of the Photocatalysts in CO2 RR The structure of the photocatalysts plays an important role during CO2 RR, which helps to create a spatial separation of the photo-induced carriers and improve the adsorption of CO2 . Hybrid photocatalysts fabricated with 2D or 3D support catalysts are extremely popular. For example, VO4 -supported ultrathin 2D NiMgV-layered double hydroxide nanosheets demonstrate excellent photocatalytic CO2 RR performance, with C1 products (CO and CH4 ) selectivity of 99% [66]. 2D structures usually suffer from the agglomeration of the nanosheets, which prolongs the transfer of photoinduced carriers and decreases the active area of the catalysts, hindering photocatalytic performance. Wang et al. introduced negatively charged Ti3 C2 Tx into CuInZnS, causing the morphology modification of CuInZnS by interfering with nucleation and growth processes. It provides a defect regulation in CuInZnS, which results in thinner 2D nanosheets with higher specific surface area and larger pore size than the pristine CuInZnS. The hybrid photocatalyst of Ti3 C2 Tx -CuInZnS affords a CO evolution rate of 42.8 μmol g−1 h−1 in CO2 RR [67]. The photoreduction of CO2 into hydrocarbon products is a big challenge, as the pathways to long-chain carbon products require multiple steps of electron transfer and C-C coupling. Jia et al. fabricated a spatially separated Au/Cu2 O nanomaterial with the shape of a dumbbell for the plasmon-driven CO2 RR [68]. The authors selectively grew the p-type Cu2 O on the Au bipyramid with the assistance of CTAB to construct the dumbbell-shaped hetero-structure photocatalyst (Fig. 12a). Besides, the CTAB-assisted synthetic method can also fabricate Au nanoparticles with different shapes such as triangles, which gives extra options to the spatially separated structure other than the dumbbell-shaped structure. The prepared Au/Cu2 O shows good light harvesting ability toward CO2 RR in the irradiation range from violet to near-infrared (Fig. 12d–f). Notably, the evolution activity and selectivity of C2+ products augment around 15 fold and onefold under near-infrared irradiation than those under visible light irradiation, respectively (Fig. 12g). The spatially separated structure of the Au/ Cu2 O provides an interesting path of the plasmon-driven to boost the photocatalytic performance of CO2 RR (Fig. 12h). Wei et al. constructed a CeO2 @CeO2 /TiO2 with the hollow multi-shelled structure (HoMS) for the photoreduction of CO2 to CH4 . Figures 13c and d show the multishelled structure of the catalyst. The quadruple CeO2 shells are the inner shells of the catalysts, which convert CO2 into CO that is accumulated within the catalyst. The amorphous TiO2 is the outer shell of the catalyst, which further transforms the generated CO into CH4 (Fig. 13e). This tandem photocatalytic reaction by the HoMS CeO2 @CeO2 /TiO2 allows the yields of CO and CH4 of 781 μmol g−1 and 120 μmol g−1 in 8 h, respectively (Fig. 13a, b). The authors also tried to destroy the HoMS structure by grinding the catalysts into debris, which led to a dramatic decrease in the catalytic performance (27 μmol g−1 in 8 h for CH4 ). This control experiment emphasizes the importance of the HoMS structure for the tandem reaction of CO2 RR [69]. It demonstrates that the rational structure engineering of the hybrid catalysts

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Fig. 12 The schematics of the site-selective growth of a Cu2 O on Au nano-bipyramid particles (NBP) with different CTAB concentrations and b Cu2 O on Au nano-rods and c Cu2 O on Au nanoplates. The CO2 RR photocatalytic activities of d–f the product selectivity comparison on Au NBPs, Cu2 O nanosheets, Au NBP@Cu2 O core-shell structure, and dumbbell-shaped Au NBP/tip-Cu2 O structures in visible, NIR, and visible-NIR irradiations, respectively, and g the C2 product selectivity and activity of dumbbell-shaped Au NBP/tip-Cu2 O structures in visible and NIR irradiations. h the mechanisms of the photocatalytic CO2 RR on dumbbell-shaped Au NBP/tip-Cu2 O structures. Reprinted with permission from [68] Copyright (2022), American Chemical Society

holds the opportunity to combine different functional semiconductors to perform a complex reaction for the CO2 RR production of high-value-added products.

5 The CO2 Capture Coupled Photocatalytic CO2 RR While the catalytic efficiency of CO2 reduction awaits augmentation, CO2 capture and storage (CCS) plays an essential part in the mitigation of over-emitted CO2 . CCS provides potential benefits in not only the mitigation of climate change but also sustainability and energy security [70]. Sorbent materials, especially porous materials, are proven to be efficient toward CO2 captures, such as metal oxides, [71–73] covalent organic frameworks (COFs), [74–76] metal-organic frameworks (MOFs), [77–79] zeolites and amine-functionalized silicas, [80] and porous carbons [81]. The current CCS methods that rely on thermal cycles are energetically inefficient and expensive [82]. Many efforts are put into the development of other approaches such as electrochemical and photocatalytic methods to perform CCS more effectively and cost-efficiently [83, 84]. Some works also demonstrate the promising potential of integrating CCS and CO2 RR in the market. Schäppi et al. built a modular 5-kWthermal pilot-scale solar-driven fuel production system, from the CO2 captured directly from air and H2 O to the drop-in transportation fuels such as kerosene and methanol [85]. Fan et al. constructed a CO3 2− -CuCoAl-layer double hydroxide (LDH) catalyst for direct CO2 capture and photocatalytic coupling reactions (the coupling of CO2 RR and 5-HMF oxidation). On the one hand, the metal cation sites reduce the CO3 2− located in the interlayers of LDH to CO, where the consumed CO3 2− is directly supplied by

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Fig. 13 a The CO evolution and b the CH4 evolution of the corresponding samples under AM 1.5 G illumination in 8 h. c The TEM images and d The high-angle annular dark field scanning TEM (HAADF-STEM) images and X-ray energy dispersive spectral (EDS) mapping images of quadrupleshelled CeO2 @CeO2 /TiO2 . e The schematic of hetero-shells and the tandem photocatalytic CO2 RR to CH4 in CeO2 @CeO2 /TiO2 . Copyright © 2022 Wiley-VCH GmbH [69] f The schematic of the direct CO2 capture and photocatalytic coupling reactions. Reprinted with permission from [86] Copyright (2022), Elsevier Inc

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the CO2 in the air. On the other hand, the hydroxyl sites oxidize 5-HMF to a 2.5-furan dicarboxylic acid, 5-formyl-2-furanacarboxylic acid, and 2.5-furandiformaldehyde as oxidation products (Fig. 13f) [86]. Until now, biological CO2 fixation is still the most efficient approach to reducing CO2 into useful compounds at scale. Mimicking the natural photosynthetic pathways or building a CO2 fixation system based on it offers another perspective in the field. Luo et al. figured a self-replenishing, oxygen-insensitive CO2 fixation system with Opto-sensing, which includes a glyoxylate and pyruvate synthesis cycle and the production of pyruvate, acetyl-coenzyme A, and malate from CO2 through the malyl-acetyl-coenzyme A-glycerate pathway. The Opto-sensing modules control the regeneration of cofactors. Besides, the 6-h operation of the system allows a CO2 fixation rate greater than that of the typical photosynthetic CO2 fixation rates [87].

6 Conclusions and Outlook In summary, single semiconductor photocatalysts usually demonstrate unsatisfying CO2 RR catalytic performance (with the catalytic activity of the level of μmol g−1 h−1 ) due to the high recombination rate of photo-induced carriers and insufficient light harvesting capability. The hybrid semiconductor catalysts can integrate the advantages of the individual semiconductor and augment the catalytic performance of photocatalytic CO2 RR through the strategies such as co-catalyst modification and heterojunction construction. For example, the addition of co-catalysts such as Cu-Au alloys on TiO2 -based photocatalysts can not only allow the increase in the production of CH4 and C2 H4 but also broaden the light response region into visible light for the hybrid TiO2 catalyst. The Bi-based and CdS-based semiconductors have the intrinsic narrow bandgap for the visible light response, but the high recombination rate of the photo-induced electrons and holes hinders the catalytic activities. Structure engineering such as the 2D structure and layer construction helps the separation of the photo-induced electrons and holes, which improves the photocatalytic performance for Bi- and CdS-based photocatalysts and others. The doping and defect-engineering strategies can be easily applied to perovskite photocatalysts for the photo-induced carries separation, enhancing the CO2 RR catalytic activities. Besides, LDHs provide the alkaline nature to adsorb and active CO2 in the hybrid LDHs-based photocatalyst, which benefits the photocatalytic CO2 RR. The conductive carbonaceous materials such as graphene- and carbon nitride-based are great support catalysts to separate the photo-induced electrons when integrated into hybrid catalysts for CO2 RR. And the organic framework-based materials (e.g., MOFs and COFs) can provide efficient active sites such as SAs for the highly effective photocatalytic CO2 RR. With the rational design of the photocatalysts, CO2 capture coupled photocatalytic CO2 RR will be practical in the future market. In the meantime, the stability of the catalyst is always a big challenge in CO2 RR. Most of the photocatalysts show a catalytic activity duration of less than hundreds of hours, which is way too low for the industrial standard. The protective layer on the semiconductor catalyst to alleviate the degradation

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of the catalyst structure during CO2 RR could be a possible way to improve the catalyst stability [88]. Other than the issues of activity, selectivity, and stability, the current photocatalytic CO2 RR still needs to work on the following aspects: . The photoreduction of CO2 into hydrocarbon fuels is promising but challenging. A few works reveal that the hybrid photocatalysts show the ability to accomplish this goal such as P/Cu SAs@CN for C2 H6 production [89], In-Cu SA/PCN for CH3 CH2 OH production [90], NiCo SA-TiO2 for the CH3 COOH production [91]. Cu is the most known metal catalyst that can transform CO2 into hydrocarbon products. The combination of Cu with other semiconductor catalysts is a safe approach to accomplish the goal. In addition, it shows that single-atom catalysts demonstrate great potential in the photoreduction of CO2 into hydrocarbon products. As C1 products like CO and HCOOH are the easiest to achieve, a tandem transformation synthetic scheme that utilizes the C1 products from photoreduction as reagents to synthesize organic or hydrocarbon products is practical for future industrial applications [92]. . In addition, the integration of photocatalysis and other catalytic methods such as electrochemical catalysis and thermal catalysis (e.g., photo-electrocatalysis) is promising to promote both product selectivity and catalytic activities in CO2 RR [93, 94]. . Besides, state-of-art operando characterization techniques can profoundly reveal the evolution of the photo-induced carriers and reaction intermediate during CO2 RR. For example, the spatiotemporally resolved surface photovoltage measurement can directly map the trace of charge transfer on the femtosecond to the second timescale at the single-particle level [95]. The advance of the operando techniques can further demystify the underlying reaction mechanism of CO2 RR on the catalysts, hence facilitating the novel rational design of the hybrid photocatalysts. . The catalytic machinery of enzymes in nature offers a possible solution to selective and efficient catalysis. Biohybrid catalysts that involve microbes and enzymes have been proven efficient for improving the catalytic performance of CO2 RR [96– 99]. Besides, Light-harvesting artificial cells that contain cyanobacteria show the ability to fix CO2 into glucose [100]. Biomass-derived materials are also good for CO2 capture [101]. The investigations of biohybrid catalysts in the photoreduction of CO2 should be explored more. Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fonds de Recherche du Québec-Nature et Technologies (FRQNT), Centre Québécois sur les Materiaux Fonctionnels (CQMF), Institut National de la Recherche Scientifique (INRS), and École de Technologie Supérieure (ÉTS). Dr. G. Zhang thanks for the support from the Marcelle-Gauvreau Engineering Research Chair program.

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SiO2 Based Multifunctional Hybrid Semiconductor Nanomaterials and Their Applications in Energy, Environment and Health Pratibha Sharma, Raj Kaushal, and Jai Prakash

Abstract It is common practice to suggest semiconductor nanomaterials as photocatalysts for a variety of applications; however, silica nanomaterials are still largely underutilized due to the fact that their photocatalytic capabilities require development, particularly when exposed to visible light. This chapter serves as evidence that amorphous silica, once subjected to the appropriate surface modifications, can transform into an effective photocatalyst even when exposed to low-energy light sources. Because of its accessibility and adaptability, photocatalytic deterioration has become an increasingly popular approach to addressing the issue of water contamination in recent years. In the realm of environmental protection, silica and silicatebased heterostructure nanocomposites have demonstrated outstanding photocatalysis performance. The photocatalytic process has surfaced as a cutting-edge technology that is both efficient and extremely promising. After that, potential silica-based hybrid photocatalytic materials and their modification techniques for photocatalysis are presented in an organized manner for the purpose of implementation in the fields of health, energy, and the environment. Finally, a short summary of the conclusions and possibilities is presented, and the path that should be taken for the continued development of environmental photocatalysis is investigated. This chapter may provide reference instructions that can be used toward comprehending, investigating, and designing photocatalytic systems for a variety of applications. Keywords Photocatalysis · Silica based hybrid materials · Water treatment · Energy · Health

P. Sharma · R. Kaushal · J. Prakash (B) Department of Chemistry, National Institute of Technology Hamirpur, Hamirpur, Himachal Pradesh 177005, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_6

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1 Introduction Semiconductors are widely used photocatalysts because they absorb light over a broad spectral range. Silica (SiO2 ), is an oxide of silicon and a most abundant as well as thermodynamically stable component in the earth’s crust. The electrical characteristics of SiO2 -based materials have severely restricted their usage as photocatalysts. With a bandgap of roughly 9 eV and 4 eV [1, 2], for crystalline and amorphous SiO2 respectively, it is with relatively wide band gap and could only absorb shortwave UV light and therefore SiO2 is not a very effective photocatalyst. It is hard for SiO2 to get excited when exposed to natural light. So, if it is to be used as photocatalyst, SiO2 requires improvement in its electrical properties, particularly when exposed to visible light. A semiconductor material undergoes photocatalysis when it absorbs light with energy greater than or equal to the band gap, which causes valence electrons to be excited to the conduction band. This results in the formation of electron–hole pairs, which can then react with water molecules and oxygen molecules that have been adsorbed onto the surface of the catalyst to produce active free radicals known as reactive oxygen species (ROS), such as hydroxyl radicals and superoxide radical ions, both of which are highly reactive and have the ability to easily oxidize organic compounds [3]. Structural defects make SiO2 photoactive when exposed to ultraviolet (UV) light. Amorphous SiO2 nanoparticles (NPs) have been studied and characterized for a number of defects [4, 5], for example, non-bridging oxygen hole centres and neutral deficient oxygen centres. These defects have optical absorption in the Ultraviolet A (UVA having longer wavelength) and Ultraviolet B (UVB having shorter wavelength) ranges, which can improve the catalytic properties of SiO2 NPs better when they are exposed to UV light [6, 7]. Selecting a material with the proper surface area, particle size, porosity, and band gap will significantly increase its photocatalytic activity from the standpoint of the application. SiO2 , is a desirable substitute because it ensures reduced environmental impact and provides a surface that is simple to prepare for post-synthesis functionalization [8–10] which has not been taken into consideration because it possesses a larger band gap and can only receive shortwave ultraviolet radiation. One feasible strategy is to synthesize SiO2 -based composites or hybrid materials for use in photocatalytic degradation. Proper surface modifications to SiO2 can lead to a photocatalyst that is effective even when exposed to light with a minimal energy. It has been effectively used in a wide range of environmental remediation applications, such as water purification, air purification, and hazardous waste treatment. Furthermore, it has been found to have promising antimicrobial capabilities, which can be used to prevent the spread of infectious diseases. The SiO2 -based hybrid photocatalyst is also an ideal material for energy storage and conversion. Its high surface area and good adsorption properties make it suitable for solar energy storage and conversion. Additionally, its high electron mobility makes it suitable for applications in solar cells, fuel cells and other energy storage and conversion devices.

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Overall, SiO2 -based hybrid photocatalysts are a promising material with many potential applications in energy, environment, and health. As research into these materials continues, it is likely that new applications will be discovered, allowing for even greater potential impacts in these areas. This chapter deals with the multifunctional photocatalytic materials of SiO2 . These materials include the composites of SiO2 with several metals, metal oxides and non-metals. The formation of such hybrid materials has been discussed as effective photocatalyst. The application of photocatalytic activity of these materials in water treatment, energy and biomedical field have been discussed along with the mechanism of action.

2 SiO2 -Based Hybrid Materials as Photocatalyst for Water Treatment The SiO2 hybrid photocatalyst is an innovative technology that is designed to effectively remove pollutants from water sources. This highly efficient photocatalyst utilizes a combination of ultra-thin nanocrystalline SiO2 particles with other functional nanomaterials such as titanium dioxide (TiO2 ), carbon nanotubes, etc., to efficiently capture and break down the pollutants. The particles are suspended in a hydrophilic medium, allowing the pollutants to be readily absorbed and degraded. The efficiency of the photocatalyst is further enhanced by its ability to absorb ultra-violet light, which accelerates the reaction process. The SiO2 -based photocatalysts have been examined extensively and have shown to be extremely effective at removing a wide range of pollutants [11–13]. These are also able to remove water pollutants without producing any hazardous by-products, making it an environmentally friendly solution. The hybrid materials of SiO2 with metals and metal oxides have excellent photocatalytic properties. For example, SiO2 NPs, when grafted with silver NPs (AgSiO2 NPs) and amine groups (NH2 –SiO2 NPs) exhibited degradation properties for 9-anthracenecarboxylic acid (9ACA). NH2 –SiO2 NPs caused 9ACA degradation when exposed to 313 nm light, and Ag-SiO2 NPs enhanced the impact. On the other hand, when Ag-SiO2 NPs photocatalyst was exposed to light at 405 nm, the plasmon was activated and photodegradation took place faster and more effectively [14]. Similarly, Badr et al. used SiO2 NPs after doping with Ag+ and Au3+ ions and deposition of Ag and Au NPs for photocatalytic degradation of Methyl Red (MR) dye. The rate of photocatalytic degradation of MR was maximum for Au3+ doped SiO2 NPs. Because Au3+ consumes three electrons from conduction band and produces three ˙ ions which are accountable for the breakdown of dye molecules. While Au/Ag OH NPs deposited on SiO2 NPs act as electron–hole separation centers. The Fermi level of SiO2 is higher than that of Au/Ag NPs, so it is feasible for electrons to move from the conduction band of SiO2 NPs to Au/Ag NPs at the interface. Because of this, a schottky barrier was formed at the (Au/Ag NPs-SiO2 NPs) contact region. This

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Fig. 1 The photocatalytic process for a SiO2 NPs, b SiO2 NPs doped with Ag+ /Au3+ ions, and c Ag NPs or Au NPs deposited on the surface of SiO2 NPs. Reprinted with permission from [15]. Copyright © (2008), Elsevier

increases the distance between the charges and makes SiO2 more photocatalytic. Further, the difference in energy between the valance and conduction bands of Au NPs is lower than that of Ag NPs. Therefore, Ag NPs make SiO2 NPs more active than Au NPs do [15]. The electronic mechanism has been demonstrated in Fig. 1. SiO2 –TiO2 hybrids have excellent photocatalytic activity [16, 17]. Chatterji and co-workers synthesized TiO2 decorated SiO2 nanospheres using a simple wet chemical method. TEM pictures (Fig. 2a, b) demonstrated that most of the SiO2 nanospheres have very small TiO2 nanocrystals attached onto their surfaces. These nanocrystals typically range in size from 10 to 12 nm. Figure 2c displays the highresolution transmission electron microscopy of a TiO2 crystallite that has been grown on silicon nanospheres. Only the (1 0 1) and the (0 1 1) atomic planes with an atomic separation of less than 0.35 nm were found in this location. On the basis of this, the model structure of the synthesized material is shown in Fig. 2d, which shows prismatic TiO2 nano crystallites attached onto SiO2 nanospheres. TiO2 decorated SiO2 nanospheres (TiO2 –SiO2 ) hybrid material possesses high catalytic activity for degradation of methylene blue. The photocatalytic activity of the sample for methylene blue under the irradiation of visible light was outstanding, and the finding is attributed to the one-of-a-kind synergistic impact of the SiO2 –TiO2 nanocomposite structure, which resulted in enhanced charge separation and charge transfer [18]. The photocatalysis process is shown in Fig. 2e. Further carbon and Ag doping of TiO2 –SiO2 photocatalyst improved the photocatalytic activity of the material. Carbon doping narrowed the band gap and induced visible light absorption. While Ag doping inhibits the photogenerated electron–hole recombination. The photocatalyst was used for decomposition of Rhodamine B under visible irradiation and showed higher visible photoactivity than non-doped catalyst. The synergistic effects of Ag and carbon doping and the addition of silicon can be used to explain why the visible photocatalytic activity is better [19]. First row transition metal (Cu, Ni, Co, Cr, and Zn,)-doped TiO2 /SiO2 composite showed photocatalytic activity for methylene blue dye in sun light. The photocatalytic effectiveness of the combination was improved by the addition of metal. Ni-doped composites were found to be most effective for degradation of the dye [20]. Radwa et al. [21] used Gallium oxide@ SiO2 /polyvinyl pyrrolidone hybrid nanofluids for the degradation of malachite green dye. These showed high photocatalytic degradation efficiency

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Fig. 2 (a), (b) TEM, (c) are the HRTEM images and (d) model structure of the SiO2 – TiO2 nanocomposite (e) proposed photocatalytic degradation of methylene blue using SiO2 –TiO2 hybrid. Reprinted with permission from [18]. Copyright © (2016), Elsevier

for malachite green dye (MG) under UV irradiation. According to early research on the study on the relative stability of the pigment malachite green under the influence of photocatalysis, the photo-activity increased as the gallium oxide particle concentration rose from 0.025 to 0.1%. This may be explained by the improved dispersion of gallium oxide NPs in SiO2 nanofluids, which raised the photocatalyst and dye molecule interaction surface area. This demonstrates the important impact that adsorption capacity played in the breakdown of the dye and in the following photo-degradation of the dye. Mazhari et al. [22] synthesized Fe3 O4 @SiO2 @TiO2 nanocomposites engineered as an extremely effective heterogeneous photocatalyst for the elimination of methyl orange from water with a 74% degradation efficiency under visible light. Further photo deposition of Ag on surface of Fe3 O4 @SiO2 @TiO2 has enhanced the degradation efficiency. The degradation of methyl orange was 84% and 98% in visible and UV light respectively. Ag/Fe3 O4 @SiO2 @TiO2 nanocomposite have high photocatalytic activity, low cost, easy magnetic separation and high chemical stability, Ag/ Fe3 O4 @SiO2 @TiO2 have potential for the utilization of photocatalysts in the treatment of wastewater on a broad scale through photocatalytic reactions. Boron doped-TiO2 –SiO2 cobalt ferrite nanocomposite [23] with narrow band gap (1.5 eV) is a highly efficient magnetically visible driven photocatalyst. With a wide surface area provided more active sites on the photocatalyst surface, which resulted in the production of more hydroxyl radicals during the same amount of time that the light was exposed to the material. Not only did this nanocomposite make the reaction region to visible light larger, but it also made it less likely that photoinduced e+ and h+ couples would recombine. It has a potential photocatalytic activity for different recalcitrant organic compounds. The reduced e/h+ recombination was the cause of the improved photocatalytic activity. Under visible light irradiation, this magnetic photocatalyst has shown effective activity in the treatment of natural dye, which is generated when algal colonies develop in water; diazinon; paraquat

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dichloride; and biologically treated palm oil mill effluent; all of these chemicals are used in the production of palm oil. After three rounds, there was no leaching and the photocatalyst had not lost any of its catalytic activity. This photocatalyst had very excellent reusability. and was easily retrieved with an external magnet. Mesoporous SiO2 supported TiO2 and mesoporous SiO2 supported ZnO photocatalysts showed good photocatalytic activity against anionic dye Reactive Blue 19 at neutral pH. The adsorption and photodegradation of the dye were more in case of mesoporous SiO2 supported ZnO as compared to mesoporous SiO2 supported TiO2 . This is explained by the fact that pure ZnO NPs have a very positive zeta potential at neutral pH (+37 mV), which leads to more mixed oxide (spinel-like) ZnO/SiO2 composite behaviour when compared to TiO2 /SiO2 composites [24]. SiO2 and TiO2 capped Ag2 S nanocomposites showed strong spectral absorption capabilities in the visible regions. Consequently, they exhibited prominent photocatalytic behaviour for the degradation of Eosin Y and Safranin O dyes under sunlight irradiation [25]. Ni-loaded mesoporous SiO2 –Al2 O3 is useful for the selective elimination of methyl orange dye as well as for the photocatalytic decomposition of the dye. After irradiation, an elimination efficiency of up to 86% was achieved [26]. Cinthia et al. studied the photocatalytic properties of iron-doped SiO2 xerogels under solar radiation. For this purpose, two methods—impregnation and polymerization—were used to add Fe (III) to SiO2 xerogels (XGS) made using the sol–gel method. Model chemicals were ranitidine (RNT), ciprofloxacin (CIP), and chlorphenamine (CPM). The molecular stability of the synthesis pad, weight, material size, and reprocessing were all evaluated under conditions simulating the effects of solar radiation. It was discovered that XGS doped with Fe by impregnation (XGS-Fe-Im) were able to totally degrade CPM and RNT in 30 and 10 min, respectively, whereas for CIP it only removed 60% after 1 h of exposure to sun radiation, outperforming the parent materials and solar radiation alone. A significant contribution from the action of HO• radicals was shown by the examination of the degradation mechanism. This study presents a potential method for using XGS Fe-doped materials to remove pollutants of growing concern in settings that are close to real-world ones [27]. TiO2 -deposited SiO2 -based catalysts have been proposed as an effective heterogeneous catalyst for the degradation of 1,4-dichlorobenzene (DCB) in the aqueous phase. DCB is a typical organic contaminant. Using SiO2 -based catalysts that had been deposited with TiO2 , it was possible to demonstrate the photochemical elimination of DCB from the aqueous phase. Under photochemical reaction conditions, the transformed materials demonstrated rapid DCB disintegration kinetics, while under dark reaction conditions, the materials displayed DCB adsorption. The unmodified matrix can adsorb between 99.12 and 99.88% of the DCB depending on the reaction circumstances, which can be either dark or bright. These photocatalysts are reliable, can be used multiple times, and have the lowest quantity of leaching caused by titanium. They have a strong photocatalytic efficacy (10 mg of the catalyst without any oxidants), which makes them potential for use in environmental applications to clean wastewater containing comparable organic contaminants. The manufacture of the catalyst only requires two simple steps, and both of these factors contribute to the strong photocatalytic efficacy. These catalysts have increased activity and persistence

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for environmental catalytic contaminant degradation processes, and they can offer insights that go beyond those provided by single metal oxide catalysts in heterogeneous catalysis under a variety of different operating circumstances. Additionally, these catalysts have the ability to offer insights that go beyond those offered by single metal oxide catalysts [28]. Song et al. described a simple and efficient method for creating hollow, doubleshell hybrid spheres made of polystyrene (PS), SiO2 , and TiO2 . The cationic initiator was used in this method to create the cationic polystyrene spheres through emulsifierfree emulsion polymerization. The PS/SiO2 /TiO2 multilayer hybrid spheres were subsequently created using two-step sol–gel techniques. In connection to the amount of titanium tetrabutoxide present, the amount of surfactant present, and the thickness of the SiO2 layer in the middle, the shape, porosity and mean pore size, specific surface area, and thermal characteristic of the hybrid spheres that were generated were meticulously evaluated. The photocatalytic breakdown of methylene blue served as the model reaction to compare the photocatalytic activity of the generated hollow SiO2 /TiO2 hybrid spheres to that of commercial P-25 powder. Hybrid spheres with three layers of titania have somewhat better photocatalytic activity than P-25. Additionally, under visible light irradiation, the photocatalytic activity of hollow SiO2 / TiO2 hybrid spheres doped with nitrogen was also studied. In 5 h, the photocatalytic degradation rate of nitrogen-doped hollow hybrid sphere was approximately twice that of nitrogen-doped P-25 powder [29]. By employing ethanol and toluene as the cosolvent and solvent, respectively, a set of TiO2 –SiO2 mixed oxide materials were created. These materials underwent thorough characterization using a variety of characterization techniques, and phenol degradation under UV light was evaluated. First-order kinetics govern the degradation of phenol, and high-performance liquid chromatographic (HPLC) and atomic pressure chemical ionization mass spectroscopic (APCI-MS) techniques were used to qualitatively identify the fragmented products created during the phenol degradation. The total organic contents that were still present in the solution after the radiation treatment served as additional proof that the phenol had completely mineralized. For phenol degradation, the materials’ pore width was discovered to be the most important feature, whereas surface area and pore volume are important for mixed oxide materials. Additionally, an inverse link between the particle sizes and the efficiency of their degradation was found in the mixed oxide system. Titania in the mixed oxide material was shown to require a lower particle size in order to effectively degrade phenol. By using a cosolvent-induced gelation technique, highly active TiO2 –SiO2 mixed oxides have been successfully created using hydrothermal synthesis. When TiO2 –SiO2 semiconductor catalyst is exposed to UV light during the photocatalysis process, the catalyst is activated and a redox environment is created, which is required for degradation. The cosolvents aid in the anatase’s distribution throughout the SiO2 framework. The textural attributes of TiO2 –SiO2 mixed oxides appeared to be crucial. The conversion of phenol into CO2 and H2 O is indicated by total organic content and LC (liquid chromatography) findings. This research offers advice on the intermediates produced when these mixed oxide catalysts degrade PhOH [30]. Due to their potential for efficient degradation, photocatalytic treatment of organic pollutants in

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industrial wastewaters has attracted interest. However, the turbidity of the solution, the amount of dissolved salt, and the light absorption all pose challenges for photocatalytic slurry reactors. The invention and use of a novel, photocatalytic, porous SiO2 -based granular media (SGM) are introduced by Hart et al. Through the use of foaming agents and activation temperatures, SGM maintains the cross-linked structure created during synthesis. For continuous photocatalytic destruction of organic pollutants, a porous SiO2 -based photocatalytic granular media was created. To establish a standard procedure and technique for the SGM’s synthesis, an initial testing matrix was used. The three-dimensional structure of the polymer was preserved by adding a foaming agent and a solvent extraction technique that has never been investigated before. In order to develop a mixture design for a highly reactive and sustainable SGM, more testing was done on different ratios of titania to SiO2 . The resulting media has a high specific surface area of 150 m2 /g and 88% porosity. The resulting structure anchors the photocatalyst within the translucent matrix, significantly enhancing photocatalytic properties. SGM has the ability to perform photocatalysis together with pore-space diffusion of nucleophiles, electrophiles, and salts. The destruction of methylene blue through time and cycles was used in batch reactors to measure the photocatalytic efficiency of SGM at different SiO2 concentrations. 10 mg/L quantities of methylene blue were successfully (>90%) destroyed in 40 min. This efficiency lasted across numerous cycles and different methylene blue concentrations. SGM is a passive and economical granular treatment system technology that may be applied to industrial processes and other organic pollutants. Methylene blue was used as a stand-in for textile dye contamination. The porous structure allows for the entry of amendments that can be used as a pH buffer or to aid in the breakdown of pollutants. Over 90% of the methylene blue was successfully removed by the SGM in one hour, and it continued to operate at this capacity over several subsequent cycles. The extensive variable control of SGM design enables the media to be created for general pollutant treatment or specifically for a given purpose. The SGM is a potential media for treating organic pollutants in the aqueous phase due to its simplicity and cheap energy consumption during synthesis, diversity in application, and ongoing reactivity throughout time [31]. The effectiveness of SiO2 -functionalized graphene oxide (GO)/ZnO combined with fibreglass for the photocatalytic degradation of pirimiphos-methyl was studied. On the photocatalytic degradation, the effects of several factors including the starting pirimiphos-methyl concentration, temperature, contact time, and hydrogen peroxide concentration were studied. Additionally, the interaction between several purge gases (oxygen and nitrogen) and co-existing organic molecules (folic acid, citric acid, oxalate, phenol, and ethylenediaminetetraacetic acid) was investigated. The catalyst corrosion that happened at acidic and alkaline environments led to a reduction in efficiency, whilst the highest removal efficiency was attained at a neutral medium. Due to the catalyst expanding at higher temperatures and thus having more functional groups available, the effectiveness of photocatalytic removal utilising SiO2 -GO/ZnO increased. The electrical energy per order (EE0) increased from 95.81 to 923.12 kWh/ m3 with an increase in pollutant concentration from 5 to 60 mg/L. Under the ideal circumstances for other parameters, the removal efficiency of pirimiphos-methyl in

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the absence of hydrogen peroxide was achieved by 98%. However, the elimination effectiveness fell from 86.74% to 81.61% when the hydrogen peroxide concentration increased from 3 to 45 mM. Organic substances served as scavengers and decreased the effectiveness of elimination. The results of this investigation also revealed that oxygen gas purging offered better photocatalytic activity for the breakdown of the target pollutant than nitrogen and ambient air. In this study, SiO2 -GO/ZnO integrated fibreglass composite (SiO2 -GO/ZnO) was used to evaluate the photocatalytic degradation of pirimiphos-methyl. 98% of the pollutant was degraded after 100 min of contact time under the ideal circumstances of neutral pH (7.0), initial pollutant concentration (30 mg/L), temperature (25 °C), and oxygen gas flow (2.5 L/min). The first-order kinetic model was well-matched by the photocatalytic breakdown kinetics of the pirimiphos-methyl toxin utilising the UV-SiO2 -GO/ZnO process. Due to its potent scavenging properties, the injection of H2 O2 led to a decrease in hydroxyl radicals and, thus, a reduction in the pollutant’s ability to degrade. The active sites on the catalyst surface were occupied by various organic compounds (such as folic acid, citric acid, oxalate, phenol, and EDTA), which reduced the process’ overall effectiveness. UV-SiO2 -GO/significant ZnO’s photocatalytic capabilities will make it easier to use it in environmental photocatalysis, particularly for treating water or wastewater that contains pirimiphos-methyl [32]. Apart from composites with metal oxides and metal doping, Carbon doping to SiO2 enhanced the photocatalytic activity of SiO2 . The C-doped SiO2 become a great photocatalyst, as shown by the way it caused degradation of Rhodamine B in near-UV light. New energy levels are generated between the lowest point of the conduction band and the highest point of the valence band when carbon is incorporated into the SiO2 structure. This results in a reduction in the size of the material’s band gap. So, in a neutral environment, the C-doped SiO2 NPs have great photocatalytic properties [33]. SiO2 –carbon quantum dots (Si-CQDs) decorated into TiO2 have great photocatalytic ability. Under sunlight, Si-CQDs/TiO2 composite can completely degrade acetaminophen within 240 min (Fig. 3a). The addition of Si-CQDs to the TiO2 surface, which served as a photo sensitizer and an electron trap, had a synergistic impact that is responsible for the composite’s exceptional performance. Si-CQDs increase TiO2 ’s capacity to absorb light by lowering its band gap energy from 3.20 to 3.12 eV. Additionally, the Si-CQDs/TiO2 composite has good stability and ability to be reused as shown in Fig. 3b, which is advantageous for the elimination of pharmaceutical waste [34]. Under visible light, photocatalytic activity using methyl orange was demonstrated by gold (Au)/carbon co-doped TiO2 with fused SiO2 . The red shift to the photocatalysts’ absorption edge is caused by co-doping. The least band gap of 2.45 eV and the maximum photocatalytic activity for degradation of methyl orange and bisphenol A were attained at Au content of 1.0%, which was when the synergistic effects between Au and carbon were at their peak. Co-doping was also discovered to facilitate the transition from the anatase to rutile phases, with 0.5% of Au achieving the maximum transformation [35]. By introducing hydroxyl and mesopore-dual-confined thermal polymerization of the precursor melamine and concurrently inducing hydroxyl-induced O-doping by swapping out the edge N atoms of heptazine, hydroxyl-rich porous SiO2 nanosheets

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(SiNSs) induced the high dispersion of oxygen-doped carbon nitride (OCN) NPs. The increased photocatalytic activity of the resulting OCN/SiNS composites for the oxidation of Rhodamine B over bulk CN was primarily due to the high SiNS adsorption, increased specific surface area of highly distributed OCN NPs, as well as the modified band structure resulting from the combined impacts of nanoparticle structure and O-doping, both of which culminated in a shortened band gap with enhanced visible-light adsorption. Additionally, it demonstrated a more reactive oxygen species of singlet oxygen (main) and superoxide radicals (secondary) for efficient Rhodamine B breakdown, but it considerably decreased the holes’ ability to oxidize Rhodamine B as shown in Fig. 3c. The significant potential of SiO2 nanosheets as a supporter as well as inducer in carbon nitride dispersion and structure tweaking for improved photocatalytic property have been suggested [36]. The above discussion shows that SiO2 -based photocatalyst materials have become increasingly popular for water treatment. These materials have a wide range of applications including inactivation of harmful pollutants and decomposition of organic pollutants. SiO2 -based photocatalyst materials are highly stable and cost-effective, making them ideal for water treatment applications. Additionally, SiO2 -based photocatalyst materials have been shown to be highly effective in degrading a wide range of organic pollutants and inactivating harmful pollutants such as bacteria, viruses, and other pathogens. The materials are also known for their high surface area, allowing for efficient light absorption and increased reaction rates. These attributes make SiO2 -based photocatalyst materials a great choice for water treatment applications. Some of the important findings related to the water treatment using SiO2 -based nanocomposite photocatalysts have been summarized in Table 1.

3 SiO2 -Based Hybrid Materials as Photocatalyst for Energy Applications The contribution of solar panels to the generation of power has greatly increased during the last ten years. It is crucial to develop scalable solutions for solar energy storage in order to offset imbalances in energy output and demand and maintain net stability. Directly converting solar light into a fuel is one method of storing solar energy. (Liquid) hydrocarbons have the advantage of an established infrastructure over hydrogen, which makes their manufacture, distribution, and use as ecologically friendly solar fuels appealing. The creation of highly effective photocatalysts is necessary for potential solar to fuel converters. Given that it can simultaneously lessen the effects of a global lack of fossil fuels and global warming, photocatalytic CO2 reduction for the manufacture of solar fuel has garnered a lot of interest. Yadav and co-workers employed a non-ionic surfactant and the reverse microemulsion process to easily and environmentally create highly active carbon nanodotsSiO2 hybrid photocatalysts (CNDSH). The high-resolution transmission electron microscopy (HRTEM) pictures of carbon nanodots (CNDs) and CNDSH show that

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Fig. 3 (a) Removal of various acetaminophen concentration using Si-CQDs/TiO2 composite (b) life cycle study of Si-CQDs/TiO2 composite. Reprinted with permission from [34]. Copyright © (2020), Elsevier. (c) Photocatalytic degradation of Rhodamine B using Hydroxyl-Rich Porous SiO2 nanosheets decorated with oxygen-doped carbon nitride NPs. Reprinted with permission from [36]. Copyright © (2022), American Chemical Society

the size of well-dispersed CNDs varies from 2.9 to 3.9 nm (Fig. 4a), whereas the sizes of CNDSH range from 25.5 to 32 nm (Fig. 4b). The photocatalyst-biocatalyst system, which was created utilising CNDSH as the photocatalyst, works effectively to carry out high NADH regeneration, followed by its consumption in the manufacture of formic acid from CO2 . The whole process and the mechanism are shown in Fig. 4c. This study sets the bar for carbon nanodots-SiO2 hybrid photocatalyst

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Table 1 SiO2 hybrid materials for photodegradation of water pollutants Material

Pollutant

References

NH2 -SiO2 NPs and Ag-SiO2 NPs

9-anthracenecarboxylic acid

[14]

Ag+

and

Au3+

doped SiO2 NPs

Methyl Red

[15]

SiO2 –TiO2 nanocomposite

MB

[18]

Carbon and Ag doped titania–SiO2

Rhodamine B

[19]

Ni-doped TiO2 /SiO2 composite

MB

[20]

Gallium oxide@ SiO2 /polyvinyl pyrrolidone hybrid (SNF) nanofluids

Malecite green

[21]

Fe3 O4 @SiO2 @TiO2 and Ag/ Fe3 O4 @SiO2 @TiO2 nanocomposites

Methyl orange

[22]

Boron doped-TiO2 –SiO2 cobalt ferrite nanocomposite

Natural dyes, diazinon, paraquat [23] dichloride and biologically treated POME

Mesoporous SiO2 supported TiO2 and Reactive Blue 19 mesoporous SiO2 supported ZnO and ZnO- and TiO2 –SiO2 composites

[24]

SiO2 and TiO2 capped Ag2 S nanocomposites

Eosin Y, Safranin O

[25]

Ni doped mesoporous SiO2 –Al2 O3

Methyl orange

[26]

Iron-doped SiO2 xerogels

Ranitidine (RNT), ciprofloxacin [27] (CIP), and chlorphenamine (CPM)

TiO2 -deposited SiO2

1,4-dichlorobenzene (DCB)

[28]

Hollow, double-shell hybrid spheres made of polystyrene (PS), (SiO2 ), and (TiO2 )

Methylene blue

[29]

TiO2 –SiO2 mixed oxide

Phenol

[30]

Porous SiO2 -based granular media (SGM)

Methylene blue

[31]

SiO2 -functionalized graphene oxide (GO)/ZnO combined with fibreglass

Pirimiphos-methyl

[32]

C-doped SiO2

Rhodamine B

[33]

SiO2 —carbon quantum dots decorated into TiO2

Acetaminophen

[34]

Au/carbon co-doped TiO2 with fused SiO2

Methyl orange and bisphenol-A

[35]

Oxygen-doped carbon nitride (OCN) NPs and hydroxyl-rich porous SiO2 nanosheets composite

Rhodamine B

[36]

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Fig. 4 HRTEM of a CNDs, b CNDSH and c mechanism for production of formic acid from CO2 from CNDSH photocatalyst-biocatalyst (NAD + = Nicotinamide adenine dinucleotide; AsA = ascorbic acid; FDH = Formate Dehydrogenase). Reprinted with permission from [37]. Copyright © (2017), John Wiley and Sons

systems for solar fuel production and is anticipated to spark increased interest in the creation of effective and environmentally friendly CO2 reduction technologies [37]. In order to increase the surface area of the titania photocatalyst and serve as a recombination inhibitor, Sarkar et al. synthesized TiO2 -nanoSiO2 composites using the sol–gel method for photocatalytic and photovoltaic applications. According to the findings, an active composite photocatalyst, which is significant for applications involving photovoltaics as well, was created by the combination of TiO2 and rice husk-extracted crystalline nanoSiO2 (RHCNS). Scanning electron microscopy (SEM) images for the perovskite layer with different bottom layers are shown in Fig. 5a TiO2 , (b) TiO2 /RHAS and (c) TiO2 /RHCNS. For TiO2 /RHCNS bottom layer, order homogeneous connecting particle structure was observed compared to TiO2 / RHAS, TiO2 bottom-based layers. We also found pores in all three cases as shown in Fig. 5a–c. The TiO2 /RHCNS composite showed a significant improvement in photovoltaic parameters and dye degradation rate, which is a first step toward the further development of adsorption-assisted photocatalysts for use in both UV and visible light. This photocatalyst was created, stable, and reusable. In comparison to TiO2 /RHAS and pure TiO2 , it has been found that RHCNS-doped TiO2 powder exhibits greater visible light photocatalytic activity. The RHCNS doped TiO2 thin film composite’s

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Fig. 5 a SEM images of perovskite layer with various bottom layers TiO2 , b TiO2 /RHAS, and c TiO2 /RHCNS. Reprinted with permission from [38]. Copyright © (2017), Elsevier. d Hydrogen generation rate from pure water and 20% methanol aqueous solution with SWGT hybrid photocatalyst. Reprinted with permission from [39]. https://www.nature.com/articles/s41598-018-311 88-w

photovoltaic performance, in which perovskite quantum dots made of CH3 NH3 PbBr3 are used. It was noted that the CH3 NH3 PbBr3 bilayer on RHCNS doped TiO2 exhibits respectable results. The inclusion of rice husk-derived crystalline nano SiO2 improved the performance of the manufactured thin film solar cells in terms of short circuit current, open circuit voltage, efficiency, and maximum power. In photocatalyst and photovoltaic applications, crystalline nano SiO2 made from rice husks can therefore be utilized as “green filler.“ It can be concluded that this crystalline nanoSiO2 doping TiO2 hybrid composite has a high potential to be employed in industry due to its cost efficiency, stability, and increase photon absorption characteristic [38]. TiO2 has attracted a lot of scientific attention because it is a type of photocatalyst that is very effective, stable, and inexpensive. Due to its broad bandgap, poor carrier separation efficiency, and rapid recombination, it can only be launched under ultraviolet light irradiation. An active strategy to spread light absorption to the visible light area is to dope materials or create composites. The literature is replete with reports of crystalline TiO2 -based catalyst modifications that are effective in the light- and water-induced photocatalytic reduction of CO2 . A sustainable, self-assembled technique for incorporating a SiO2 precursor in a WSe2 -graphene-TiO2 composite with cetyltrimethylammonium bromide (CTAB) as surface active agents are put forward

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in order to deal with the restrictions of TiO2 and expand the relevance of the photocatalytic behaviour of TiO2 , WSe2 , and SiO2 . For the primary objective, the first thing that was looked into was the photocatalytic decomposition of organic dye solutions using a range of different starting pH levels and catalyst dosages. The assynthesized sample exhibited highly effective photocatalytic effects for the treatment of the SO dye solution under the ideal circumstances of this research. These conditions comprised a solution with a pH level of 11 and a dosage of 0.05 g of the catalyst. Additionally, the sample was used at a dosage of 0.05 g. Second, prior investigations on photocatalytic hydrogen production found that SiO2 /WSe2 -graphene-TiO2 (SWGT) composites performed noticeably better under ambient settings with and without 20% methanol sacrificial reagents than binary WSe2 -graphene and ternary WSe2 -graphene-TiO2 composites. The SWGT composite has a semiconductor function in photocatalytic hydrogen evolution, converting solar energy into chemical energy under ambient circumstances with and without sacrifice atmospheric pressure at room temperature. Methanol was utilized in this experiment as the sacrifice reagent, which improved the semiconductor’s catalytic activity by supplying electrons to eat the photogenerated holes and lengthening the semiconductor’s duration before recombination. When subjected to visible light, the results of the hydrogen evolution experiment are depicted in Fig. 5d for the SWGT composite both with and without the 20% methanol sacrificial chemicals. According to the data on hydrogen evolution among the survey composites (Fig. 5d), the photocatalytic H2 evolution rate was found to be at its highest when methanol was used as the sacrificial reactant in the presence of the SWGT composite. This was the case when the SWGT composite was present. The inclusion of 20% methanol sacrificial reagents did not significantly alter the photocatalytic H2 evolution rates for the SWGT example. In addition to having remarkable applicability and photocatalytic activity in a pure aqueous solution, it is a good candidate for becoming a semiconductor in a highly active photocatalyst. The results show that the best hydrogen evolution rate was reached by the SWGT photocatalyst, which is more than twice as high in both pure water and methanol aqueous solutions. The aforementioned analysis suggested that the ternary photocatalyst (WGT) and binary photocatalyst had a much higher rate of photocatalytic hydrogen evolution (WG). The SWGT composite has the potential to be a highly efficient heterosystem for the generation of hydrogen and a good photocatalytic performer [39]. SiO2 -based photocatalyst materials have been widely studied for energy applications such as water splitting for hydrogen production, CO2 reduction for solar fuels, and air purification. These materials have the advantage of being inexpensive and abundant, and their wide bandgap makes them suitable for visible light harvesting. In brief, SiO2 -based photocatalyst nanocomposites are promising in the field of energy applications as given in Table 2.

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Table 2 SiO2 -based hybrid materials for application in energy and biomedical field Material

Application

References

Carbon nanodots-SiO2 hybrid (CNDSH)

CO2 reduction

[37]

TiO2 and rice husk extracted crystalline nanoSiO2 composite

Photovoltaic application

[38]

SiO2 /WSe2 /graphene/TiO2 composite

H2 production

[39]

BiOCl–KIT-6

1. Antibacterial properties against Gram-positive S. aureus and E. faecalis 2. Detection of lead and cadmium in blood samples

[42]

TiO2 /SBA-15 with avobenzone and oxybenzone

Sunscreen

[43]

TiO2 /SBA with avobenzone

Sunscreen

[44]

aminoC60/SiO2

Inactivation of MS-2 bacteriophage

[45]

4 SiO2 -Based Hybrid Materials as Photocatalyst for Biomedical Applications The public health is currently seriously threatened by the complexity of environmental pollution, which has drawn a lot of attention to the hunt for multifunctional nanomaterials. The SiO2 hybrid photocatalyst for health applications is a revolutionary new product that promises to revolutionize the health care industry. This photocatalyst is an advanced nanotechnology-based system that is capable of producing safe and effective disinfection, sterilization, and decontamination of contaminated surfaces. It is designed to work in a variety of applications, including sterilization of medical equipment, air purification, and water purification. The SiO2 hybrid photocatalyst is a highly efficient system. It is capable of producing a high level of disinfection, sterilization, and decontamination of contaminated surfaces. It has been designed to work in a variety of applications, including medical equipment, air purification, and water purification. It is also highly efficient in terms of energy consumption, with a low energy consumption rate compared to other disinfection and decontamination systems. The SiO2 hybrid photocatalyst is also highly efficient in terms of its ability to reduce the risk of contamination and infection. It is capable of reducing the risk of transmission of bacteria, viruses, and other pathogens. This is because the photocatalyst is able to use the light energy to break down organic compounds, which can then be safely removed from the environment. A TiO2 /SiO2 nanocomposite was made using the sol–gel method. The rutile-toanatase phase transition of TiO2 NPs has been studied in relation to the varying SiO2 content (0, 5, 10, 15, 20, and 50%). The TiO2 /SiO2 with SiO2 content of 15% was chosen for making silicate coating in order to improve the photocatalytic effectiveness of the nanocomposite and lower the cost of the material. When sintering

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at temperature 600 °C, SiO2 addition prevented TiO2 NPs from transitioning from anatase to rutile. For creating a silicate coating, a TiO2 /SiO2 (15%) sample with an anatase concentration of 88.2% is used (WO and W silicate coating). With the addition of TiO2 /SiO2 (15%), the W and WO silicate coatings were effective for photocatalytic degradation of Methylene Blue (MB) and airborne antibacterial capacity. The antibacterial efficiency of the W silicate coating in the air first grows quickly, reaching a fairly high efficiency of 87.61% for 1 h, and then increases gradually after 2–3 h to 94.35% (Fig. 6c). Although the WO silicate coating exhibits the opposite tendency, the antibacterial effectiveness (5.74–6.02%) does not significantly alter in 3 h (Fig. 6b). The degradation of MB under UV-irradiation in a chamber and the technique of identifying and counting the number of colonies on plate count agar were used to evaluate the photocatalytic potential and antibacterial capabilities of WO and W silicate coatings in the air. The W silicate coating showed great photocatalytic methylene blue degradation efficiency. It increased from 96.0% (First 20 h) to 100% in 40 h. While the WO silicate coatings were less efficient and showed only about 25–30% photocatalytic activity as shown in (Fig. 6d). The higher photocatalytic and antibacterial characteristics of W silicate covering are the result of increased centers for absorption (greater surface area), a photocatalytic reaction, and the interaction between the sample and bacteria. The TiO2 /SiO2 (15%) in W silicate covering produces reactive oxygen species (ROS) when exposed to UV light with a wavelength of 365 nm. These ROS include hydroxyl radicals (OH•), oxide anion radicals, and hydrogen peroxide (H2 O2 ). In addition, the inclusion of SiO2 in TiO2 /SiO2 (15%) improves the absorption characteristics. This means that W silicate coating is capable of adsorbing a greater amount of bacteria from the air. Because of this, the TiO2 /SiO2 (15%) coating, which has a greater surface area, is advantageous for the interaction between sample and bacteria because it increased the photocatalytic and antimicrobial activity in air. This is in comparison to the WO silicate coating, which has a lower surface area. The development of W silicate coating with self-cleaning active UV and TiO2 / SiO2 (15%) can outperform organic paints in terms of heat resistance, longevity, and cost, and could find use in the environmentally friendly paint sector [40]. Visible light-induced photocatalyst was created using photocatalytic perfluorinated SiO2 -based fluorescent carbon NPs which were further combined with TiO2 (f-FNPs/TiO2 ). The material showed improved photocatalytic activity and demonstrated the use of material as antifouling substrate and hydrophobic-hydrophilic transition surface. Perfluorinated SiO2 -based fluorescent carbon NPs acting as photocatalysts changed a high-surface-energy hydrophobic surface into a low-surface-energy hydrophilic surface in response to visible light. SiO2 -carbon and deposited TiO2 worked together synergistically to promote the photocatalysis process. These surfaces have also shown progress in biomedical applications, as seen in the usage of super hydrophilic micro spots for cell screening, polyacrylamide brushes for managing protein and cell retention, and 2D scaffolds for immobilising site-selective cells. The photo-degradation of MB, which is indicated by a change in the contact angle from hydrophobic to hydrophilic nature. The activity was due to the production of reactive oxygen and hydroxide species from the TiO2 -mediated photochemical

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Fig. 6 Antibacterial assessment on plate count agar a without coating, b WO silicate coating, c W silicate coating from 1 to 3 h, d WO and W silicate layer decomposition proportion of methylene blue. Reprinted with permission from [40]. https://www.frontiersin.org/articles/10.3389/fchem.2021.738 969/full

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reaction in UV and visible light on the surface of f-FNPs/TiO2 . These species boost hydrophilicity, which leads to a lower contact angle as a direct consequence. The cell attachment study also supported the anti-wetting state’s transition to a hydrophilic state following UV–visible irradiation. These findings suggest that a transition from long-lasting super hydrophobicity to hydrophilicity may be possible in the near future due to the synergistic interaction of carbon, SiO2 , and deposited TiO2 [41]. The whole process and the mechanism are shown in Fig. 7. The multifunctional material BiOCl-KIT-6 composites have shown potential photocatalytic activity. The material was synthesized using hydrothermal process in which mesoporous SiO2 of KIT-6-encapsulated in bismuth oxychloride (BiOCl). The composite has shown multifunctionality to be used for photocatalysis, an antimicrobial test, and the simultaneous measurement of heavy metals like lead and cadmium. Images acquired through SEM analysis showed that the BiOCl that was obtained on a siliceous support is made up of microspheres that have a diameter of approximately 4 m. (Fig. 8a). In contrast to the layered assembly of BiOCl, the BiOCl-KIT-6 composites have a bunched structure and are therefore comparatively sizable. The size distributions of these composites are on the order of tens of micrometers (Fig. 8b). The composite had a bunched structure as analysed by SEM (Fig. 8). The BiOCl-KIT6 composite possessed larger and refined surface shape, which was the main cause for

Fig. 7 Representation of visible-light-driven photocatalytic process of perfluorinated luminous carbon-dot-integrated TiO2 NPs antifouling surface. Reprinted with permission from [41]. Copyright © (2016), American Chemical Society

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Fig. 8 SEM images of a BiOCl and b BiOCl–KIT-6. Reprinted with permission from [42]. https:// pubs.acs.org/toc/acsodf/3/1

the increased photocatalytic performance with a band gap of 3.06 eV. The BiOClKIT-6 composite showed antibacterial properties against Gram-positive S. aureus and E. faecalis along with the increased photocatalytic efficiency in degradation of rhodamine B. The dye degradation efficiency of the composite was ∼1.6 times higher than that of BiOCl. This suggests that the clustered arrangement of BiOCl on the mesopore substrate might make the catalysis of photons of Rhodamine B dye molecules easier. Moreover, the large surface area of the composite favoured the absorption of dye along with the photodegradation. In addition, the BiOCl-KIT-6 combination was utilized for anodic stripping analysis in the combined measurement of lead and cadmium with a linear range of 0.2–300 g/L and a detection limit of 0.05 g/L (Pb2+ ) and 0.06 g/L (Cd2+ ), respectively. This analysis had a linear range of 0.2–300 g/L and a detection limit of 0.05 g/L (Pb2+ ). This technique can be utilized for the concurrent analysis of lead and cadmium levels in actual blood samples. Therefore, it is envisaged that the BiOCl-KIT-6 composites will find widespread use for environmental and analytical reasons after undergoing additional structural modification [42]. TiO2 is a common active ingredient in many sunscreens because of its ability to reflect and scatter ultraviolet light. It acts as a physical barrier on the surface of the skin, reflecting and scattering UV rays away from the skin to protect it from UV damage. The size of the TiO2 particles used in sunscreens is important, as smaller particles can provide better protection by reflecting and scattering UV rays more effectively. Additionally, TiO2 is non-irritating and non-toxic, making it a safe and effective option for sun protection. Further, the sunscreen activity of TiO2 has been enhanced by addition of mesoporous SiO2 (SBA-15) for topical application. Then, for synergistic sunscreen effectiveness, organic ultraviolet (UV) A filters such avobenzone and oxybenzone were inserted into mesoporous support. The composite was prepared by varying the concentration of TiO2 (10–50%) in SBA-15 and it was found that the increase in concentration of TiO2 lead to a decrease in the pore size. But not much noticeable change was seen due to varying TiO2 concentration. TiO2 /SBA-15 hybrid was able to collect about 60% fluoranthene and 80% furfural within 3 h.

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Further, when avobenzone (organic UV filter) was trapped in TiO2 /SBA-15 hybrid, the sun protection efficiency was seen to increase. The photoprotection measured by UVA/UVB ratio showed an increase in Boots star rating from 2 to 5. Avobenzone and oxybenzone are two of the most commonly used chemical UV filters in sunscreen products. Avobenzone is an organic compound with a wide absorption spectrum, providing protection against UVA rays. Oxybenzone is an organic compound widely used in sunscreen to absorb UVB and short UVA rays. Both avobenzone and oxybenzone are highly lipophilic, meaning they easily penetrate the skin and are readily absorbed into the bloodstream. Studies have shown that these two compounds are deposited in the stratum corneum, the outermost layer of the epidermis, and have been detected in sweat, saliva, and urine samples. The long-term effects of avobenzone and oxybenzone deposition in the skin are still unknown, but studies have suggested that these compounds may have endocrine-disrupting activity and can cause skin irritation and sensitization. Additionally, oxybenzone has been shown to cause skin damage and increase the risk of skin cancer. The TiO2 /SBA-15 hybrid was found to reduce the Avobenzone and oxybenzone deposition by 30%. In comparison to SBA15 treatment alone, when added to SBA-15, avobenzone, and TiO2 substantially cut down on the number of skin cells that were killed and the number of neutrophils that were recruited in photoaged rodent skin. Based on the findings of the cytotoxicity experiment, skin histology, and cutaneous barrier function, the TiO2 /SBA-15 hybrid was deemed non-irritating. These findings showed that the adaptable mesoporous SiO2 is a powerful solution for topical sun protection [43]. Alalaiwe and co-workers synthesized a multifunctional TiO2 -embedded mesoporous SiO2 hybrid that also contained avobenzone to fend off environmental stress through UVA protection and pollutant adsorption. The effect of porosity of the material on the degradation of contaminants and the reduction of avobenzone skin penetration was discussed. The porosity of the material was altered by using triblock copolymers (Pluronic P123 and F68). Mesoporous SiO2 having pore volumes of 0.66 (TiO2 /SBA-L), 0.47 (TiO2 /SBA-M), and 0.25 (TiO2 /SBA-S) cm3 /g were generated by the Pluronic P123:F68 ratios of 3:1, 2:2, and 1:3, respectively. Under UVA light, the TiO2 /SBA-S was shown to have high adsorption efficiency of 43% and 53% for fluoranthene and methylene blue respectively. It was found that the less porous material showed better sunscreen activity as compared to highly porous samples. Due to the synergic effect, an UVA/UVB absorbance ratio of close to 1.5 was achieved when avobenzone was loaded into the mesoporous SiO2 . Boots star rating = 5 was achieved in this case. Additionally, the epidermal absorption of avobenzone was decreased from 0.76 nmol/mg to 0.50 nmol/mg after the avobenzone was encapsulated within the TiO2 /SBA-S. When compared to the SBA-S hydrogel, the avobenzone-loaded TiO2 /SBA-S hydrogel demonstrated a significantly greater increase in skin barrier regeneration as well as a reduction in proinflammatory mediators. After undergoing the TiO2 /SBA-S therapy, the levels of cytokines and chemokines in the photoaged skin were decreased by two to three times as much as they were in the control group that had not been treated. The mesoporous composition, which had low porosity and a specific surface area, showed effective pollutants adsorption while also providing

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UVA protection with reduced UVA filter absorption. The malleability of the newly created mesoporous structure indicated a bright future for the preservation of flesh when it was exposed to the elements [44]. C60 amino fullerene is a type of fullerene structure with an amine group attached to it. It has been studied as a photocatalyst for a variety of applications. The amine group can act as an electron donor, allowing for more efficient electron transfer from the photoexcited fullerene to the substrate. This increased electron transfer can increase the efficiency of the reaction, leading to higher yields and faster reaction times. Additionally, the amine group can also increase the solubility of the fullerene in organic solvents, which can be important for certain reactions. Jaesang and co-workers used the immobilization of photoactive C60 amino fullerene on SiO2 gel (3-(2-succinic anhydride)propyl functionalized SiO2 ) as shown in Fig. 9. The functionalized SiO2 support was attached to the C60 amino fullerene using an organic linker moiety that contained an amide group. To create an amide link between C60 amino fullerene derivatives and 3-(2-succinic anhydride)propyl functionalized SiO2 gel, a watersoluble condensing reagent called N-(dimethylaminopropyl)-N, -ethylcarbodiimide hydrochloride was used. This was done in the presence of catalytic quantities of 1-hydroxybenzotriazol hydrate. The formation of photochemical O2 under visible light irradiation is significantly increased as a result of the linker moiety’s ability to prevent aqueous C60 aggregation/agglomeration. The system provides a promising new visible-light-activated photocatalyst with minimal chemical alteration of the aminoC60/SiO2 photocatalyst after numerous cycling and no reduction in O2 production efficiency. The aminoC60/SiO2 photocatalyst is capable of efficiently and kinetically enhanced oxidation of Ranitidine and Cimetidine (pharmaceutical pollutants) and inactivation of MS-2 bacteriophage when exposed to fluorescence light illumination. This is in comparison to aqueous solutions of the C60 amino fullerene. In most cases, the inactivation of the MS-2 phage was completed more quickly when the amino fullerenes were immobilized. This photocatalyst, in contrast to aqueous solutions of the C60 amino fullerene by itself, has the potential to make it possible to clean water in areas that have not been as extensively developed by decreasing reliance on significant infrastructure, including the requirement for electricity. As a result, this photocatalyst could enable the treatment of water in less developed locations by reducing reliance on significant infrastructure, such as the requirement for electricity [45]. SiO2 -based hybrid photocatalyst materials have become increasingly popular for biomedical applications due to their light-induced activity and low toxicity. For example, SiO2 -based materials can be used in sunscreens to provide shielding from UV radiation and protect skin from sun damage. These materials can also be used to create antimicrobial coatings, which can be applied to medical devices to reduce the risk of infection. Additionally, SiO2 -based photocatalysts can be used to degrade environmental pollutants, making them a useful tool for water purification or air filtration. With their wide range of applications, SiO2 -based photocatalysts have become a valuable asset for biomedical research and practice. Some of these findings have been listed in Table 2.

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Fig. 9 Impairment of water-soluble C60 Aminofullerenes on Functionalized Silica Gel (n = 2, 4, and 6). Reprinted with permission from [45]. Copyright © (2010), American Chemical Society

5 Conclusion and Future Prospect Overall, SiO2 -based multifunctional hybrid semiconductor have been demonstrated to be a promising platform for developing various applications in energy, environment, and health. This review has summarized the applications of SiO2 -based multifunctional hybrid semiconductor. By utilizing these materials, researchers have explored the potential of SiO2 -based multifunctional hybrid semiconductor nanomaterials to develop novel approaches and devices for the development of renewable energy, environmental monitoring, and biomedical treatments. As the research in this field continues to progress, it is likely that SiO2 -based multifunctional hybrid semiconductor nanomaterials will become even more popular in the development of innovative solutions to the challenges of energy, environment, and health. However, the efficiency of the solar photocatalyst is still unsatisfactory as a result of several significant difficulties associated with composite materials based on silica. As a consequence of this, the photocatalyst technique is confronted with a number of significant difficulties, such as the instability of the material, the inefficiency of charge transition and separation, and a contradiction between the solar spectrum and the bandgap of semiconductors, which needs to be investigated further. In spite of

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the current challenges, the photocatalyst technique has tremendous potential to revolutionize energy production. Research is ongoing to develop new catalysts and techniques to improve the efficiency of charge transition and separation, and to explore novel materials with better stability and a wider bandgap. In addition, the integration of photocatalysts with other renewable energy sources, such as wind and solar, is an area of increasing interest. With further research, these issues can be resolved and the photocatalyst technique can be utilized to its full potential. The future prospects for SiO2 -based multifunctional hybrid semiconductor photocatalysts and their applications in energy, environment and health are very promising. These hybrid semiconductors can be used in a variety of ways to increase efficiency and reduce energy consumption in many sectors. For example, they can be used to create more efficient solar cells and batteries, reduce emissions from fuel combustion, and improve water and air quality. In addition, they can be used to create more efficient and cost-effective medical treatments. With further research and development, these hybrid semiconductors could become an integral part of many industries, helping to improve energy efficiency, reduce environmental pollution, and improve human health.

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The Future of Graphene Oxide-Based Nanomaterials and Their Potential Environmental Applications: A Contemporary View Subhendu Chakroborty, Pravati Panda, and Suresh Babu Naidu Krishna

Abstract Environmental safety is vital to life on Earth. Environment and life are interconnected like two sides of a coin. Pollution is a serious challenge in both developing and developed nations. Rapid rise in human civilization, together with metrological works and industrialization, affects the environment. Due to the excessive release of heavy metal ions, air, water, and soil-borne diseases as corona, cholera, cardiovascular issues, chronic conditions, and cancer increase. Different research organizations use several ways to combat environmental problems. Nanotechnologybased solutions are cost-effective and efficient. Nanomaterials’ multifaceted applications revolutionize science. Its particle-to-size ratio gives a wide surface area with several reactive sites. Carbon-based nanomaterials like graphene, fullerene, carbon nanotubes, graphene oxide, carbon-based quantum dots, etc., have received a lot of attention due to their application to combat environmental issues. Through this chapter, we want to draw researchers’ and academics’ attention to recent trends and applications of graphene oxide based photocatalysts in degradation of organic dye pollutants, biomedical significance, challenges, and future perspectives which will improve the development and application of more multidimensional nanomaterials to human health and for the development of biodiagnostics. Keywords Dye pollutants · Graphene oxide · Environment · Photocatalyst · Plasmonic nanostructures · Therapeutics

S. Chakroborty Department of Basic Sciences, IITM, IES University, Bhopal, Madhya Pradesh 462044, India P. Panda Department of Chemistry, RIE, Bhubaneswar, Odisha 751022, India S. B. N. Krishna (B) Department of Biomedical and Clinical Technology, Durban University of Technology, PO Box 1334, Durban 4000, South Africa e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_7

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Nomenclature Ag/AgCl As AmGO BET BMIMCIIL CGA CGO CS CM CMC CRZ CV DCC DLVO FSC FTIR EDX GGO GO GO/LDH(GL) GO/GCE GO-coumarin (GC) GO-SiO2 HGAAS hrGO hrGO-Trp IL LDH LOD VALUE M/GO MB MG MGO mGO MO PPD PPy PVC PS RB rGO Rh6G

Silver /Silver chloride Arsenic Amino functionalized graphene oxide Brunauer, Emmett and Teller 1- butyl 3- methyl imidazolium chloride Cellulose-graphene oxide composite aerogel Aerogels of cellulose and graphene oxide Chitosan Ceramic membrane Carboxymethyl cellulose Carbendazim Cyclic voltametric N, N , -dicyclohexylcarbodiimide Derjaguin–Landau–Verwey–Overbeek (DLVO) theory Furniture scraps charcoal Fourier Transform Infrared Energy-dispersive X-ray spectra Gd2 O3 -doped graphene oxide Graphene oxides Graphene oxide/layered double hydroxides composites Graphene oxide (GO)-based glassy carbon electrode Graphene oxide (GO)–coumarin (GC) Silica–graphene oxide nanocomposite Hydride generation atomic absorption spectroscopy Hydrothermal reduction graphene oxide Tryptophan cross-linked hrGO membranes Ionic liquid Layered double hydroxides Limit of detection Graphene oxide based magnetic nanocomposite Methylene blue Malachite green Magnetic graphene oxide Multilayer graphene oxide Methyl orange p-Phenylenediamine Polypyrrole Poly vinyl chloride Polystyrene Rose Bengal Reduced graphene oxide Rhodamine 6G

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Scanning electron microscopy Square wave voltammetry Transmission electron microscopy Thermogravimetric analysis Thymine-GO-Carbohydrazide Thiosemicarbazide Ultraviolet-visible X-ray diffractometer Zirconium (Zr) decorated with manganese dioxide (MnO2 ) nanoparticles-functionalized reduced graphene oxide (RGO)

1 Introduction Nowadays, the present generation is threatened by increasing environmental pollution, which results in a large variety of life-threatening ailments such as cardiac disease, cancer, diabetes, brain ham rage, and kidney failure [1, 2]. It is now becoming a serious matter of concern both for the developed and developing countries. Environmental pollution is not only affecting human civilization but also the aquatic animals are also suffering a lot [3–5]. To maintain high economic profile, man is setting various types of industries and companies which though make us enrich but simultaneously it causes high rate of air, water, and soil pollutions [6]. Release of highly poisonous gases cause air pollution and discharge of contaminated water goes to the water bodies and fields which simultaneously caused water and soil pollutions [7–9]. Polluted water containing organic dyes, heavy metal ions, nuclear wastes, agricultural wastes, etc., transit into the water bodies which makes a layer upon them as a result it limits the penetration of sun rays into it by which death of aquatic animals occurs as well as they also suffer from diseases due to lack of sunlight [10]. People when take the affected fish in food and water, they also get suffered from multiple waterborne diseases. Moreover, the growth of crops is also getting hampered due to loss of biogenic bacteria. Due to industries, the soil will get covered by a layer of toxic metal ions which also hampered human civilization and environment [11]. In view of the growing environmental pollution, people are developing multiple ways to tackle the challenge and invent new ways to fight the environmental problems and survive in it [12–15]. Though the developed strategies are helpful, but due to growing resistance developed by the microorganisms for different kinds of old protocols, we also need to be more attentive and work more o invent new, innovative, and cost-effective protocols which is the call of the day. Development of nanotechnology brings several opportunities for scientists as well as for the society [16–18]. Furthermore, nanotechnology has captured the attention of people all over the world because of its superior mechanical strength, small particle size with large surface area, chemical resistivity, high thickness, and other properties

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that allow it to be implemented in a variety of fields ranging from everyday life to space technology [19]. Among the varieties of nanomaterials available, graphene oxide is quite attractive because of its large surface area which can be easily functionalized by different kinds of groups which simultaneously enhances its reactivity, selectivity, and hydrophilicity [20]. The procedures for the creation of its many different derivatives are likewise quite straightforward. These procedures use reagents that are economical and far less hazardous than traditional solvents. They also save time, are simple to deal with, and produce high yields. Consequently, it has found applications in a wide variety of contexts [21–23]. Numerous implications for the environment may be found in literature. Graphene oxides (GO) capture diverse pollutants such as dyes, heavy metal ions, microorganisms, and so on through noncovalent interactions such as pi-pi interactions, H-bonding, cation-pi interactions, van der Waals interactions, and so on [24–27]. The synthesis of many GO derivatives and their various environmental applications, such as the detection of dyes, organic pollutants, heavy metal ions, microbes, agricultural wastes, and other types of contaminants, are summarized in this chapter.

2 Potential Environmental Applications of Graphene Oxide-Based Nanomaterials 2.1 Synthesis and Removal of Heavy Metal Ions from Wastewater Graphene oxide-based nanomaterials have a wide range of potential environmental applications due to their unique physical, chemical, and mechanical properties. By electrostatic self-assembly between negatively charged GO and a cationic nitrogenous coumarin-surfactant, the Peng research group designed and constructed the first photoreversible graphene oxide (GO)-coumarin (GC) composite. It was shown that exposing a GO/coumarin composite to UV light (at either 365 or 254 nm) photoreversible alters the morphological structure of the composite, hence improving its adsorption capacity and resolving the separation problem. Powerful Cd2+ adsorption capabilities (340.3 mg/g) are displayed by the GC composite [28]. Heavy metals poisoning in the water habitats has gotten worse because of quick industrialization. Therefore, it is critical to create improved heavy metal removal technology. For the adsorption of heavy metals from an aqueous solution, composite paper-like materials based on graphene oxide (GO) have been used extensively. Khan et al. used a resin-infiltration approach to create an advanced, highly ordered, and homogeneous polyvinyl chloride (PVC)/p-Phenylenediamine (PPD)/GO paperlike material for the first time. Layer-by-layer assembly, where the assembling components must interact significantly, is complemented by this approach (e.g., via hydrogen bonding or electrostatic attraction). The resulting PVC, PPD, and GO buck

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papers are extremely durable over a broad pH range and at high temperatures. They are efficient at removing Pb2+ from sewage water [29]. Abaszadeh’s research group designed and synthesized MGO@APhen (magnetic graphene oxide (MGO) functionalized with 5-amino-1,10-phenanthroline (APhen) in the presence of N, N,-dicyclohexylcarbodiimide (DCC) as a novel nanomaterial by functionalizing the GO surface with Fe3 O4 and 5-amino-1,10-phenanthroline. This nanocomposite possesses GO as a planar material that increases the reaction contact area, iron nanoparticles as a magnetic material that simplifies separation and Phen as a suitable ligand that binds to metal ions. The synthesized MGO@APhen was employed as an effective adsorbent for the adsorption of Pb2+ [30]. To prevent challenges in recovering GO powder after greener removal of heavy elements such as As (V), Pb (II), and Cr, Sodium Alginate (SA) and carboxymethyl cellulose (CMC) are used to produce granules of a graphene oxide nanocomposite doped with gadolinium oxide (Gd2 O3 ). gGO-Gd2 O3 is granular Gd2 O3 -doped-GO. gGO-Gd2 O3 adsorbs 158.23 mg/g Pb (II) [31]. Lee and his coworker devised a simple technique for manufacturing magnetic Gd2 O3 -doped graphene oxide (GGO) for Pb (II)-contaminated water treatment. Active surface functional groups increase its adsorptive capacity over plain graphene oxide (GO) and iron oxide-doped GO. GGO performance is affected by pH, adsorbent dose, starting metal concentration, and rate-limiting kinetics on homogenous surfaces. Gd2 O3 doping increased GO’s Pb adsorption capacity to 83.04 mg/g (II). Langmuir’s equation says GGO’s optimal Pb2+ uptake capacity is 83.04 mg/g. Also compares GGO’s Pb2+ adsorption capability to related materials [32]. Barik et al. synthesized silica–graphene oxide nanocomposite (GO-SiO2 ) nanocomposite via room-temperature using 1- butyl 3- methyl imidazolium chloride (BMIMCl IL). This two-step approach disperses GO in ionic liquid (IL), then decorates the GO surface with silica using formic acid in IL. After repeated washing, this compound was employed to adsorb Pb (II) and As (III) ions from aqueous systems. Novel mesoporous nanocomposite absorbed 527 mg/g Pb (II) [33]. Abubshait’s team utilizes modified graphene oxide-thiosemicarbazide (mGOTSC) nanocomposite to detect Cu2+ elimination from aqueous solution. The GO picture showed a thin, homogenous layer. Transmission electron microscopy (TEM) images of mGO-TSC showed clustered entanglement zones of thiosemicarbazide (TSC) molecules across GO sheet surface. Adsorption studies show Freundlich isotherms. Kinetic investigations showed that adsorption is driven by a pseudosecond-order model via inter-particle diffusion. The improved adsorbent (mGOTSC) was reused four times with 85% Cu2+ adsorption effectiveness. Metal ions adsorb more strongly on mGO-TSC than GO [34]. Li and his co-workers synthesized graphene oxide/layered double hydroxides GO/ LDH (GL) composites and magnetic Fe3 O4 @GO/LDH (Fe3 O4 @GL) to adsorb Cu2+ from wastewater. GL (Cu2+ : 89.26 mg/g) showed a greater maximum adsorption capacity than Fe3 O4 @GL. The adsorption rate rose progressively from 0.01 g to 0.15 g in 20 mL solution, reaching equilibrium [35]. Electrostatic self-assembly of positively charged nitrogenous coumarin surfactant and negatively charged GO creates a photoreversible graphene oxide-coumarin composite. This photoreversible

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composite removes Zn2+ from wastewater [28]. In summary, graphene oxide-based nanomaterials can synthesize and remove heavy metal ions from wastewater, making water treatment more effective and sustainable. Graphene oxide-based nanomaterials have great adsorption capacity, selectivity, and stability, making them a potential remediation choice. However, more research must be conducted to optimize the production method and examine the environmental consequences of graphene oxidebased nanomaterials. Graphene oxide-based nanoparticles could revolutionize water treatment and contribute to sustainability with further development and optimization.

2.2 Heavy Metal Removal A novel magnetic nanoparticles adsorption material based on GO, chitosan (CS), and polyethyleneimine was used to adsorb and remove heavy metals and anionic azo dyes from water. When combined with hydride generation atomic absorption spectroscopy (HGAAS) and Ultraviolet–visible (UV–Vis) to validate removal, the developed approach offers a platform for removing, analyzing, and determining harmful chemicals in aquatic ecosystems [36]. Inspired by the beautiful architecture of graphene, which has a high surface area and a wider range of oxygen-containing functionalities than other carbonaceous materials, the authors intend to use it as a useful candidate for metal ion adsorption. Considering this, graphene oxide-based nanoribbons (GONRs) were created using ultrasonication. The nanoribbons were successfully used for heavy metal ion adsorption in an aqueous environment, and the results showed that the nanoribbons were capable of absorbing As (V) from the waste water with 155.61 mg/g adsorption potential for As (V) in 12 min [37]. Hydrogels of A-GO (agar-graphene oxide) were produced through one-step jellification process and were applied for the selective removal of cationic dye Safranin-O and the drug chloroquine diphosphate. The morphology of the hydrogels was characterized through different spectroscopic measurements. The adsorption of the drug and the dye was confirmed by studying Freundlich and Sips isotherms and analyzed through Fick’s diffusion equation and driving force models which exhibited R2 > 0.98. Gradual increase in the pH increases the rate of adsorption. The developed hydrogels showed excellent potential adsorption when all of them were mixed in water with an adsorption value of ∼63 mg g−1 for chloroquine and 100 mg g−1 for safranin-O which was confirmed by Fixed-bed breakthrough curves. Mechanistically, the adsorbate gets adsorbed on the active site of the adsorbent through various types of interactions including electrostatic attractions, hydrogen bonding, or π-π conjugation interactions. In addition to this, the nanomaterial exhibited good reproducibility and recyclability and also successfully removed both the contaminants from water (Fig. 1) [38].

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Fig. 1 Mechanism of adsorption. Reprinted with permission from [38]. Copyright (2023), Elsevier

Zirconium (Zr) decorated with manganese dioxide (MnO2 ) nanoparticlesfunctionalized reduced graphene oxide (RGO) (Zr-MnO2 -RGO) based nanocomposite was synthesized by the doping of Zr and MnO2 NPs on the surface of the RGO following an easy and convenient chemical pathway. It was applied for the selective detection of As(V) in the aqueous medium. The prepared nanocomposite was characterized through X-ray diffractometer (XRD), thermogravimetric analysis (TGA), scanning electron microscope (SEM), TEM, Fourier Transform Infrared (FTIR), Energy-dispersive X-ray spectra (EDX), etc. Fabrication of MnO2 and Zr

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Fig. 2 Preparation of the Zr-MnO2 -RGO nanocomposite. Reprinted with permission from [39]. Copyright (2021), Elsevier

NPs simultaneously enhanced the specific surface area as well as adsorption affinity of RGO surface toward arsenic As(V). The optimum pH for the removal of As(V) was pH 4. The rate of adsorption was best analysed through Langmuir adsorption isotherm and followed pseudo-second order kinetics. The removal efficacy was found to be 98.5–99.3% in industrial and ground water. Moreover, it showed good recyclability and reusability (Fig. 2) [39].

2.3 Graphene Oxide-Based Nanomaterials for Removal of Organic Pollutants Water scarcity is being made worse by a wide variety of organic pollutants that are found in industrial effluents, agricultural runoff, and home discharges. These pollutants are also responsible for the spread of water-borne diseases and have a negative impact on marine ecosystems and biodiversity. Immediate attention must be paid to the development of materials that are productive, environmentally friendly, and

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economical in order to eliminate organic pollution. Aerogels of cellulose and GO (CGO) were prepared from the fruit waste by gelatinization of cellulose and GO and were subjected for the purpose of wastewater treatment. H-bonding interaction between the GO and cellulosic skeleton generated porosity in the synthesized aerogel which helped for mass transfer and diffusion of the organic dyes present in the wastewater. The organic dyes were adsorbed at the surface-active site of the cellulose-graphene oxide composite aerogel (CGA aerogel). The highly porous (96.4%), ultra-light (0.018 g/cm3 ), charge, size, and the surface-active sites play a significant role in the adsorption process. Mechanistically, it was proposed that electrostatic, dipole–dipole, π-π, n-π, cation-π interactions, and Yoshida hydrogen linkages made between the ample number of oxygen functionalities present on the CGA surface with the organic dyes are mainly responsible for the adsorption of the contaminants on the surface of the hydrogel. It displayed fast and highest cationic dye (methylene blue (MB), malachite green (MG), rhodamine 6G (Rh6G) adsorption ability over anion dyes (rose Bengal (RB) and methyl orange (MO)) due to the electrostatic interaction of the negatively charged O-atoms with the cationic dyes. The prepared aerogel was found to be an excellent candidate for the selective adsorption around >98% (MB dye) and rejection of organic contaminants from wastewater as well as it exhibited high recyclability and reusability. The adsorption tendency of the CGA toward the MB dye was found to be quite superior over simple activated carbon (46%) as well as GO powder (40%) (Figs. 3 and 4) [40]. Graphene oxide (GO) coated glassy carbon electrode (GCE) electrochemical sensor (GO/GCE) was designed, prepared by the fabrication of 2D GO over GCE and utilized toward the detection and estimation of concentration of Carbendazim (CRZ), a commonly used fungicide, in the water and soil samples. Cyclic voltammetric (CV) and square-wave voltammetry (SWV) techniques were applied for determining the voltametric behavior of the sensing material. The electro-oxidation of the CRZ followed a quasi-reversible reaction pathway with two protons and electrons

Fig. 3 a Pomelo fruit, b peeling of pomelo fruit, c peels, d cellulose extracted by chemical processing of fruit waste (peels). e Graphite powder, f representative chemical structure of GO, which is prepared by severe oxidation and exfoliation of graphite powder. g Gelatinization of GO with cellulose into a hydrogel. h Lyophilization of cellulose-GO hydrogel into CGA composite aerogel. i Digital photograph of CGA aerogel. j Microscopic view of aerogel demonstrating the porous structure. Reprinted with permission from [40]. Copyright (2022), Elsevier

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Fig. 4 Schematic representation of plausible interactions between active surface sites/chemical functionalities of CGA and MB dye molecules. Reprinted with permission from [40]. Copyright (2022), Elsevier

participation. An optimum response was recorded for GO/GCE was recorded at pH 4 when phosphate buffer solution was taken as the supporting electrolyte. It was able to detect very trace amount of CRZ present in the real samples and displayed a linear response was recorded for concentrations ranging from 1.0 × 10−7 M to 2.5 × 10−4 M. The limit of detection (LOD) value was found to be 1.38 × 10−8 M. The developed sensor displayed high selectivity toward the detection of CRZ in presence of other interfering ions and hence, can successfully apply for the estimation of CRZ in the water and soil samples (Fig. 5) [41]. Fruit waste, which generally accumulates as waste and causes environmental pollution, contains a huge number of natural reductants. Here the authors had taken advantage of the waste and reused it for the generation of highly porous adsorbent material for the effective adsorption of sulfamethoxazole, a type of antibiotic, in the aqueous environment. In this work, the authors have synthesized graphene by the biogenic reduction of GO by using the peel extracts of dragon fruit which served as a natural reductant. Betanin, a natural reductant present in the peel extract was extracted following an aqueous extraction process under optimal reaction condition, i.e., under suitable pH and kept it properly without disturbing its reducing potential. It was found that the biogenic natural reductant plays a promising role toward the reduction of GO following SN2 nucleophilic reaction pathway under a slight alkaline condition using phosphate buffer solution in 1 h. The prepared reduced graphene oxide (rGO) performed outstandingly and works as an electrochemical sensor toward the detection of antibiotic sulfamethoxazole in the aquatic medium. The silent merits of the developed process include cost-effectiveness, environmental friendliness, high stability, and quantitative production ability (Fig. 6) [42].

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Fig. 5 CRZ electrode mechanism. Reprinted with permission from [41]. Copyright (2022), Elsevier

Fig. 6 Proposed reaction pathway for GO reduction by dragon fruit peel aqueous extract. Reprinted with permission from [42]. Copyright (2022), Elsevier

A multifunctional nanocomposite composed of GO-Fe2 O3 was prepared and used as a sensing agent toward the dye detection in the water medium. The nanocomposite was prepared by mixing of clay and GO with Fe2 O3 NPs and annealed under 550 °C to generate the fine structure of the nanocomposite. The nanocomposite was found to have a face-centered cubic like structure with an average particle diameter of 13.31 nm. It calorimetrically detects ascorbic acid when treated with ascorbic

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acid and uric acid using paper sensor. It worked as an excellent adsorbing material whereas displayed found to have low photocatalytic ability. The prepared Clay/GO/ Fe2 O3 nanocomposite performed well toward the adsorption of the toxic dyes like methylene blue even in low concentration in the aquatic medium and followed 2nd order kinetics as confirmed through BET study. The surface adsorbing sites, electrostatic interactions, and porous nature govern the active adsorption and removal of MB dye from water samples (Figs. 7 and 8) [43]. GO fabricated polystyrene films as nanocomposite were designed and synthesized as electrospun. The GO was prepared through modified Hummers’ process and was doped on the polystyrene(PS) fibers surface. The morphology and structure prepared nanocomposite were examined through XRD, SEM, TEM, FTIR, TGA, etc., which confirmed the successful incorporation of GO over the PS surface. The smooth surface area of the PS fiber facilitates the successful incorporation of GO on it which was confirmed by SEM analysis. The nanocomposite was found to be composed of ∼87 wt.% PS and ∼13 wt.% GO and hence both displayed the same thermogravimetric behavior as examined through TGA. After successful characterization, the dye was then subjected to study for their dye-adsorbing ability. It works as an excellent adsorbent toward the detection of MB dye in wastewater. It displayed outstanding adsorptive potential around 2.3 times more as compared to other reported adsorbing material and the simple PS membranes. It can detect the dye within 30 min

Fig. 7 Schematic illustration of synthesis of clay/GO/Fe2 O3 nanocomposite. Reprinted with permission from [43]. Copyright (2022), Elsevier

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Fig. 8 Proposed mechanism for the removal of MB dye by Adsorption. Reprinted with permission from [43]. Copyright (2022), Elsevier

in the aqueous medium and has an adsorptive capacity 114 mg/g which is reached after 120 min. The adsorption process followed pseudo-second-order kinetics model which suggested 116 mg/g as the adsorptive capacity of the nanocomposite [44]. Furniture scraps charcoal (FSC) supported amino functionalized GO multilayer nanocomposite (AmGO) composite (AmGO@FSC) was prepared, well characterized, and subjected for the detection of organic dye in the wastewater. AmGO-FSC-based nanocomposite provided a two-way solution to tackle the environmental solution as a biogenic eco-friendly waste material was used to synthesize valuable nanocomposite to handle the raising water contamination problem. Also, due to the hydrophilic nature of the nanocomposite, it can be easily separated from the water medium. It possessed 54.35 mg/g maximum adsorptive tendency at monolayer with an equilibrium constant value of 0.76 L mg−1 as confirmed by Langmuir–Freundlich isotherm. The possible mechanism of adsorption was assumed to be governed by resistance to liquid–solid film and mass transfer in the bulk. Amino functionalization enhanced the adsorption tendency of the multilayer GO through pi–pi interaction and also by other possible non-covalent interactions. Cost-effectiveness, high adsorption potential (2 times more than FSC), great recyclability, and reproducibility even after six times of use were found to be the major

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advantageous features of the developed nanocomposite which can be applied as a profitable adsorbent of textile dyes (Fig. 9) [45]. To make a comparative study of the adsorbing tendency the authors synthesized of multilayer GO and magnetic GO-based nanocomposite through microwave-mediated Hummers’ process and further examined their metal ion adsorbing tendency in the wastewater. The presence of increased inter-layer spacing along the c-axis of the prepared nanocomposite was confirmed through XRD analysis. Raman, SEM, and TEM confirmed the structure, quality, and morphology, i.e., presence of wrinkles on the surface of the nanocomposite. UV–Vis analysis suggested the presence of conjugated double bonds (C = C bond) and carbonyl (C = O) groups in the nanomaterial. The manufactured nanocomposite containing a large surface area around

Fig. 9 Schematic representation of the supporting of AmGO over FSC structure. Reprinted with permission from [45]. Copyright (2022), Elsevier

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Fig. 10 SEM images of graphene oxide (GO) samples. Reprinted with permission from [46]. Copyright (2022), Elsevier

126 m2 /g with a huge quantity of surface active sites for the adsorption of the heavy metal ion was confirmed through BET analysis. Heavy metal ions like toxic Pb2+ and Cd2+ were found to be excellently adsorbed over the surface of the graphene oxide-based magnetic nanocomposite (M/GO). Experimental results suggested that prepared M/GO nanocomposite possessed superior adsorbance ability toward Cd2+ ion over simple GO nanocomposite (Fig. 10) [46].

2.4 Graphene Oxide-Based Membranes in Wastewater Treatment Graphene-based membranes can afford numerous novel mass-transport properties that are not possible in state-of-the-art commercial membranes, making them promising in areas such as membrane separation, water desalination, proton conductors, energy storage and conversion, and many more. Significant progress has been made in the design of next-generation filtration and separation membranes using graphene materials. Hydrothermal reduction GO (hrGO)-amino acid membranes

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Fig. 11 Schematic illustration of separation mechanism through GO nanochannel, hrGO nanochannel, and hrGO-Trp nanochannel. Reprinted with permission from [47]. Copyright (2022), Elsevier

toward wastewater treatment were prepared by the fabrication of amino acid over the GO produced through hydrothermal reduction process. The water permeability ability of the prepared membrane was found to be excellent as well as it displayed good potential toward heavy metal ion rejection present in the wastewater. Tryptophan cross-linked hrGO membranes (hrGO-Trp) when treated with FeCl3 solution showed 191.0 L m−2 h−1 bar−1 water permeability with 98.2% as rejection potential toward FeCl3 which was around 4.4 times more than that of pristine GO membrane and other NF membranes. The developed membrane was found to have superior water permeability, highly stable, have long half-life, and also exhibited good heavy metal ion (Fe3+ ion) rejection ability (Fig. 11) [47]. PPy (polypyrrole) coated GO/rGO based highly conductive ceramic membranes, worked like an electrode was prepared for wastewater treatment. There are a very small number of reports available in the literature regarding the wastewater remediation property of conductive ceramic membranes since they have very poor electrical conductivity. Under the applied electric field, it displayed excellent anti-fouling ability including improved contaminants removal efficiency as because of doping of highly reactive GO/rGO over it as it contains large surface area. Pyrrole gets excellently adsorbed on the surface of the GO/rGO as it makes a large number of noncovalent interactions like electrostatic interactions, pi-pi interactions, H-bonding, etc., rather than polymerization over the surface of the membrane which ultimately improved the membrane properties like hydrophilicity, flux, porosity, roughness, and zeta potential, etc. Due to the highly conductive network-like structure made between PPy and GO/rGO, it resulted decrease in the electrical resistivity of the membrane to 3.56 and 0.87 kΩ/cm from 8.46 kΩ/cm. under applied electric field, the average specific flux of rGO/PPy (GO/PPy) membrane was found to be 47.5% (33.6%) which was quite higher than GO/rGO membrane supported by CM at the time of yeast filtration which made it a more profitable candidate for water treatment. Derjaguin– Landau–Verwey–Overbeek (DLVO) theory proved that after incorporation of GO/ rGO over PPy membrane, it improved the zeta potential, hydrophilicity, weakens the roughness which in turn boosted the formation of more positive non-covalent network formation which ultimately helped in the enhancement of the anti-fouling property of the membrane (Figs. 12, 13 and 14) [48].

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Fig. 12 The separation process of a Ceramic membrane (CM) support, b conductive membrane under electric field; and c the schematic diagram of the GO/rGO reinforced PPy conductive membrane. Reprinted with permission from [48]. Copyright (2022), Elsevier

Multiplex electrochemical sensors made up of covalently functionalized GO with thymine and carbohydrazide (Thymine-GO-Carbohydrazide, T-GO-C) were prepared through an epoxide ring cleavage, followed by simultaneous-reduction approach and applied for the detection of heavy metal ion in the wastewater samples. The prepared T-GO-C-based multiplex electrochemical sensor possessed admirable electrode stability and showed high selectivity toward Hg (II) and Cr (VI) at 0.27 V and 0.9 V when silver chloride (Ag/AgCl) was taken as reference electrode. The large surface area facilitated more conductivity and high functionalization displayed superior selectivity toward the detection of Cr (VI) and Hg (II) with minimum detection limit of 20 ppb and 1 ppb respectively in real water samples. Additionally, it exhibited a linear response for Cr (VI) and Hg (II) above 5 ppb and showed high accuracy, portable, good recyclability, and reusability (Fig. 15) [49].

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Fig. 13 Schematic diagram of the preparation process for GO, rGO, and conductive membranes. Reprinted with permission from [48]. Copyright (2022), Elsevier

Fig. 14 Synthetic routes of a PPy CM, b GO/PPy CM, rGO/PPy CM; possible interactions of hydrogen bond and π bond existed between the PPy chain and GO; π bond existed between the PPy chain and rGO. Reprinted with permission from [48]. Copyright (2022), Elsevier

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Fig. 15 Schematic representation of the T-GO-C nanomaterials fabricated electrochemical sensor electrode and its SWV sensing of Hg (II) and Cr (VI). Reprinted with permission from [49]. Copyright (2022), Elsevier

3 Conclusion Environmental pollution is rising day by day due to the establishment of various environmental sectors which, though provide high economic strength but also create various lives threatening air, water, and soilborne diseases. Survival with the growing environmental pollution is the biggest challenge for human civilization. Nanotechnology and nanomaterials always mesmerized the scientific communities all over the world because of its multidimensional applications. Invention of graphene oxidebased nanomaterials brings bumper offers and opportunities with it to fight the increasing environmental problems. Due to the nano-range particle size with high

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surface area, GO and GO-based nanomaterials can be used in different forms to detect the different types of environmental contaminants. Organic pollutants, dyes, heavy metal ions, micro-organisms can make a few different kinds of interactions such as pi-pi interactions, H-bonding, cation-pi interaction, van der Waals interactions etc. with different kinds of functionalities present on the surface of the GO which in turn helps in easy detection. GO is also hydrophilic in nature which also can be recycled after multiple uses. Easy method of synthesis, avoid of toxic reagents solvents and cost-effectiveness with high yield are the major advantageous features found to be associated with the nanomaterials. Additionally, it exhibits ultra-high sensitivity and selectivity. This review will help future researchers and academicians to gain more ideas about the different methods of synthesis and applications of GO-based nanomaterials which will help them to think new and innovative protocols with different multiple novel applications. Acknowledgements PP is thankful to the Department of Chemistry, RIE, Bhubaneswar, Odisha, India. SC is grateful to the Department of Basic Sciences, IES University, Bhopal, Madhya Pradesh, India. KSBN would like to acknowledge Research fellowship from Durban University of Technology, South Africa.

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Hybrid Semiconductor Photocatalyst Nanomaterials for Electrochemical Sensing Applications K. S. Shalini Devi and Seiya Tsujimura

Abstract Hybrid semiconductor photocatalyst nanomaterials (HSPNs), an emerging class of two dimensional (2D) layered materials finds diverse application in medical and environmental, industrial applications. Some of the inorganic semiconductors such as ZnO, TiO2 , WO3 , NiS, CoS, MoS2 and CdS are the key factors among semiconductors with its respective bandgap responsible for detection mechanism. Electrode modification is a crucial part of the sensor as it is the sole deciding factor for high selectivity, sensitivity, reproducibility, dynamic ranges and ultrasensitive detection limits of sensing devices. In present chapter, the synthetic routes, and fabrication strategies of HSPNs are discussed with a focus on the synergistic effect of HSPNs in combination with redox mediators and metal nanoparticles, metal oxides, fluorophores or biomolecules. HSPNs also serve as an efficient modifier with carbon systems and their wide exploration of electrochemical detection of reactive oxygen species (ROS), hydrogen peroxide, superoxide, amino acids, hydrazine, glucose, neurotransmitters, pharmaceutical drugs, pesticide etc., have been discussed in detail in this chapter. Additionally, the HSPNs based miniaturized electrochemical sensors for various crucial analytes have been elaborated. Finally, we conclude the summary, challenges encountered in this filed with possible remedies and future scope of HSPNs in wearable sensors and electronics. Keywords Electrochemical sensing · Hybrid semiconductor photocatalyst · Analytes · Multiple analyte detection · Real sample analysis

K. S. Shalini Devi (B) Faculty of Pure and Applied Sciences, JSPS Fellow, University of Tsukuba, Ibaraki 305-5358, Japan e-mail: [email protected] S. Tsujimura Division of Material Science, Faculty of Pure and Applied Science, University of Tsukuba, 1-1-1, Tsukuba 305-5358, TennodaiIbaraki, Japan © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_8

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1 Introduction Hybrids of metal oxide nanoparticles provide a wide range of beneficial attributes and uses towards variety of fields that includes sensors [1]:, drug delivery [2], dyes and pigments [3], energy conversion [4], biomedicine [5] and electronics [6]. Utilizing numerous components of different metal oxides increases the versatility available for modifying and managing the characteristics and functions of nanomaterials. The active surface area, surface modification, functionality, and conductivity of electrode materials may all be increased to produce electrodes with improved electrocatalytic performance [7]. Metal complexes have strong electrochemical (and/ or photocatalytic) activity, hybrid systems including a semiconductor are attractive. There are variety of inorganic semiconductors involved in the detection of biomolecular analytes namely ZnO, titanium dioxide (TiO2 ), tungsten oxide (WO3 ), nickel sulfide (NiS), cobalt sulphide (CoS), Molybdenum sulphide (MoS2 ) and cadmium sulphide (CdS). Each semiconductor has its own energy bandgap which plays a major role in sensing applications. In brief, ZnO ~ 3.37 eV, TiO2 ~ 3.1 eV, WO3 ~ 2.8 eV, NiS ~ 0.9 eV, CoS ~ 2.6 eV, MoS2 ~ 1.8 eV and CdS ~ 2.42 eV [8]. It is beneficial to combine a low-band-gap semiconductor with a high-band-gap semiconductor because the continual pumping of electrons into the conduction band and the transfer of holes into the valence band lengthens the lifespan of the excited state. Low quantum efficiency, excessive electron hole recombination, and poor light capture severely impair the performance of a single photocatalyst. Hence doping of semiconductors with several carbon-based materials like graphene [9], nanotubes, polymers and other high band gap semiconductor are beneficial for efficient sensing of various analytes in the sensor field [10]. The choice of semiconducting materials is solely depending on its ease of disposal, affordable cost, need for moderate pressure and temperature conditions and complete mineralization [11]. There are two types of semiconductors mainly organic and inorganic semiconductor which have their equal advantages. Organic semiconductors depend on the different organic polymers like polypyrrole (ppy), polyaniline (PANI), Poly(3,4ethylenedioxythiophene) (PEDOT), Poly(3-hexylthiophene) (P3HT) etc. that are highly conductive offering the best interface between the biological medium, where the ion exchange produce useful electrical signals, and the devices, which rely on electron transmission [12]. The ion fluxes in the organic semiconductor support electron transfer and increase the active surface area on the sensing electrode which has widely been implemented in the potentiometric pathway for detection of different analytes. For instance, polycarbazole doped with potassium ferricyanide was studied in detail for sensing of enzymatic and non-enzymatic glucose sensor [13]. Moving on to inorganic semiconductors, p-type and n type are involved. New and timely prospects for the production of “green” chemical synthesis are provided by photocatalysis [7] (Scheme 1). The most used semiconductor photocatalyst is TiO2 [14]. ZnO, WO3 , pervoksites (SrTiO3 , BaTiO3 , CaTiO3 , CoTiO3 being the most promising) [15–17], metal sulfides including CdS, Bi2 S3 , ZnS, and MoS2 , and WS2 are materials that

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might be beneficial. Graphitic carbon nitride, g–C3 N4 , g–C3 N5 and ternary oxides as In, InNbO4 , InVO4 and TaO3 , are examples of such materials explored for sensor and biosensing applications as shown in Fig. 1. The doping mechanism of carbon materials with hybrid photocatalyst metals and metals oxides involves mixing of elements with varying chemical compositions that results in excellent electrostatic interactions, Van der Waals force, hydrogen bonds, or covalent bonds modification [18]. Notably, carbon structures such as graphite,

Fig. 1 Illustration of hybrid metal complex semiconductor photocatalyst nanomaterials (HSPNs) and its role in electrochemical sensor application Scheme 1 Potential of HSPNs for electrochemical sensing with doped materials are shown in terms of volts

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diamond, glassy carbon (GC), graphene/graphene oxide, mesoporous materials, carbon nanofiber (CNF), and carbon nanotube (CNT) are particularly intriguing materials for research, fabrication, and large-scale manufacturing [19]. Graphene with area of 2630 m2 g−1 , and CNT with the 1315 m2 g−1 in their physical form create a filamentous structure with a high specific surface area, and is only one of the numerous benefits of these materials [20]. When the nanocomposite is between 1 and 100 nm in size, this combination gives the materials new characteristics. Hence, this book covers the hybrid semiconductor photocatalyst nanomaterials (HSPNs), doping with carbon-based material preparation methods, fabrication techniques and its importance in the sensor/biosensor field for detection of various bioanalytes namely glucose, hydrazine, dopamine, H2 O2 , superoxide, amino acids, neurotransmitters, pharmaceutical drugs, pesticide, reactive oxygen species (ROS) etc.

2 Synthesis Protocols of HSPNs There are different techniques evolved for the preparation of the semiconductor photocatalyst which includes mechanical exfoliation technique, template approach (hard and soft) [21, 22], template-free approach [23], in-situ growth method [24], Ball milling [25, 26], solvothermal/hydrothermal [27], sol–gel method, thermal condensation process [28, 29], electrospinning approach [30], microwave irradiation [31], hydrolytic precipitation etc. Each method has several advantages for various applications which are illustrated in Fig. 2. To know in brief about the preparation technology, discussion of a few examples is listed. Using gold (Au) and ZnS nanocomposite, photo assisted reduction methodology was adopted. In this work, wet-chemical process has been implemented for synthesis of ZnS under Xenon arc lamp in a quartz vessel. Since, Au is expensive, precursor of Au i.e., HAuCl4. 3H2 O precursor solution was added with 30 mg/mL of ZnS/water solution. Different weight % of samples were collected. After light radiation, the Au decorated ZnS was confirmed by the change of colour from white to light purple. Conversion of metallic Au to Au3+ was observed [32]. Another example for the sol–gel procedure for the preparation of the semiconductor photocatalyst is described. Two combinations of iron nitrate nanohydrate Fe(NO3 )2 .9H2 O and Zinc nitrate Zn(NO3 )2 were used as precursors to prepare ZnO and Fe2 O3 respectively. These two metal oxides were mixed with citric acid and de-ionized water separately and stirred. For complete suspension and formation of heterojunctions between the metal oxides, it was maintained at 80 °C at pH 7. The obtained yellowish-brown sample was grinded, sintered at 500 °C [33]. Next, three different components were modified layer by layer to improve the electrical conductivity of the material preparation by etching of metal carbides and ultrasonication of metal oxide. Ti3 AlC2 MXenes was dissolved in hydrofluoric acid (HF) and completely stirred for 36 h. After continuous washing with ethanol/water, Ti3 C2 was obtained as a resultant. In order to incorporate functionalized MWCNT along with Ti3 C2 , MWCNT was treated with 80 mL of nitrating mixture of H2 SO4 /HNO3 for about 3 h. The solution was then filtered with polytetrafluoroethylene (PTFE)

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Fig. 2 Synthesis techniques involved in preparation of HSPNs with its merits and demerits representation

membrane and washed for several times, followed by drying at 60 °C. With these two components, ZnO nanospheres were developed by ultrasonication of ZnAc.H2 O dissolved in 90 mL of diethylene glycol which was refluxed for 24 h at 160 °C [34].

3 Electrochemical Approach of Different HSPNs and Its Sensing Performance Due to their high reproducibility, sensitivity and selectivity, portable field-based size, quick reaction time, and low cost, electrochemical sensors are the perfect choice for these novel applications. HSPNs have boomed up for enhancing the performance of electrochemical sensors mainly nitrogen (N), sulfur (S), carbon (C) and oxygen (O) doped semiconductors. In addition, photocatalyst metal oxide semiconductors, metal sulphides semiconductors, metal nitride semiconductors and metal carbide semiconductors play a vital role of detecting various analytes which are discussed in detail in the following section of this chapter.

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3.1 Metal Oxide Hybrid Semiconductors-Based Sensors 3.1.1

Zinc Oxide Based Electrochemical Sensors

The wide band gap (3.3 eV) and high electron–hole (e–h + ) recombination rate of ZnO are two significant limitations. To get around these restrictions, other techniques have been devised, including alloying with other semiconductors, anion and cation doping or combination of carbon matrices [35]. Two-dimensional (2D) metal carbides along with metal oxide and carbon material i.e., Ti3 C2 /grapheneMWCNTs/ZnO nanocomposites were developed using ultrasonication method for sensing of dopamine (DA), a neurological disorder biomarker. The formation of nanocomposite was confirmed by x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Cyclic voltammogram (CV) showed lower oxidation currents and poor electrocatalytic activity for DA by Ti3 C2 /GCE as shown in Fig. 3a, b, therefore combination of functionalized MWCNT/ZNO which exhibited enhanced electrocatalytic activity was used to improve the performance of Ti3 C2 . This was also evidenced by differential pulse voltammogram (DPV) and electrochemical impedance spectroscopy (EIS). Density functional theory (DFT) calculations of the Ti3 C2 /G-MWCNTs/ZnO confirms the hydroxyl groups on the benzene ring are likely to be converted to ketone in the oxidation process of DA. The prepared sensor showed concentration range of DA detection at 0–30 μM, the limit of detection (LOD) of 3.3 nM and excellent sensitivity of 16 A/M. Human serum samples were tested with the sensor with satisfactory results [34]. Similarly, the bio-friendly and robust ZnO/Fe2 O3 heterojunctions were used for the electrochemical detection of the DA and photo-degradation of the antibiotic sulfamethoxazole (SMX). The built-in metal redox Zn2+ /Zn+ and Fe3+ /Fe2+ enhance charge transfer efficiency, ease Z-scheme transfer, and lessen recombination. XPS and SEM are supported to show the formation of ZnO/Fe2 O3 heterojunctions. Magnetic, mass spectroscopy and optical characterizations allowed to know the degradation mechanism of SMX pollutant removal while, CV and DPV experiments confirm the redox peaks at—0.1 and 0.35 V. The pH experiments performed reveal the highest current value at pH 6 i.e., slightly acidic favours the detection of DA. Interference with some inorganic salts were 5% whereas biomolecules did not find to have effect with DA. Hence this sensor is highly suitable for detection in biological fluids. S2 junction has a LOD of 0.18 μM with a detection range of 1–50 μM. The stability of the sensor was found to be 15 days at room temperature as in Fig. 3c. Human urine samples were tested as real sample analysis of DA levels [33]. Glucose has tremendous interest these days due to increase of diabetic patients and its complications. The need of the glucose biosensor is increasing which is also attempted with ZnO/graphene/GOx hybrid. The confirmation of mechanistic sensor performance of H2 O2 reduction current was superior towards addition of glucose in the case ZnO coated graphene nanoparticles with glucose oxidase (GOx) than the typical graphene/GOx modified electrode. This ZNO/graphene hybrid also utilized photocatalytic degradation of methylene dye and anti-bacterial activity towards E-coli in this same work [36]. A non-enzymatic

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Fig. 3 a Schematic representation of Ti3 C2 /G-MWCNTs/ZnO/GCE modified electrode for electrochemical sensing of dopamine, Reprinted with permission from [34] Copyright (2022), Elsevier, b Illustration of ZNO nanorods/molybdenum disulfide nanosheets hybrid modified screen printed electrode and reusable electrochemical sensor for determination of anti-retroviral agent indinavir Reprinted with permission from [37] Copyright (2021), Elsevier. c Plots of a CVs and b Impedance spectra of modified electrodes in 0.1 M KCl containing 1 mM [Fe(CN)6]3/4− ; c CVs of S2 at different pH values solution of 0.01 mM DA in 0.1 M PBS solution (50 mV/s of scan rate); d Plot of Anodic peak maxima versus 0.1 M PBS solution pH. e CVs in 0.1 M PBS. f Linear plot of Ipc/Ipa versus scan rate1/2 for sol–gel synthesized electrode material; S2 modified GCEs; g DPV curves in 0.1 M PbS (pH = 6.0) at varied concentration of 1–50 μM of dopamine; h Stability up to 15 days; i Interference study in the presence of the co-analytes Reprinted with permission from [33] Copyright (2021), Elsevier

glucose sensor was developed with ceria nanoparticles and zinc oxide which was designated as ZnO–CeO2 nanocomposite whiskers. Monolayer MoS2 is an interesting semiconductor. 2D insulating and semiconducting materials are more likely to arise because most metallic materials have intrinsic chemical activity. Combination of ZnO and MoS2 was clearly utilized for effective detection of an anti-retroviral drug on a screen-printed electrode as the futuristic view of the reusable sensor with the low detection limit of 0.007 μM. The fabricated sensor showed good recovery values in the biological samples [37].

3.1.2

TiO2 Based Electrochemical Sensors

TiO2 is the most widely studied semiconductor due to its suitable band edge positions, nontoxicity, earth abundance and stability at various pH [38]. It is well known that the TiO2 has poor solubility and stability issues on the modified electrodes. But, nano-TiO2 can be prepared via dissolving in organic solvents which makes the modifications very stable. For instance, pentachlorophenol was detected using nano-TiO2 dissolved in dihexadecylphosphate(DCP) modified electrode and applied for real time water analysis [39]. In order to increase the sensitivity, in situgraphene doped

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TiO2 was prepared by the hydrothermal method. This nanocomposite preparation characterized using SEM and XRD confirms the high absorptivity and conductivity of TiO2 /graphene. The application towards effective and sensitive detection of adenine and guanine was achieved and stability of the modified electrode was monitored by storing in the refrigerator at 4 °C for two weeks, which showed the electrode is 92% stable. The adenine and guanine detection limits for the TiO2 -graphene-based electrochemical sensor are 0.10 and 0.15 μM, respectively shown in Fig. 3B, with a linear range of 0.5–200 μM. Thermally denatured cells were tested as real sample mimics the purine bases and found 80% close to the standard values [40]. Similarly, in another work, MXenes are found to enhance the conductivity of TiO2 on the modified electrodes which was prepared by the sol–gel method. To exhibit a 3D network, Poly vinyl alcohol (PVA)/GO hydrogel was made using the freeze–thaw process. The combination of PVA/GO and MXene/TiO2 provides a thick sample adsorption which acts as an efficient electrochemical modifier on a paper-based substrate for sensing of norepinephrine, a neurological biomarker. Linear range and RSD of the modified electrode was 0.01–60 μM and 4.3% respectively. As a real time application, this paper based sensor was targeted for urinary tract infections(UTIs) and compared with healthy people urine samples which showed high accuracy and satisfactory recovery results as in Fig. 4a [41]. To develop a mediator-free semiconductor modified electrochemical sensor, another attempt made with TiO2 , Ti3 C2 TX (MXene) was combined as the nanocomposite along with surfactants Cetyl Trimethyl Ammonium Bromide (CTAB) and chitosan (CS) for efficient sensing of nitrite. With a detection limit of 0.85 μM, the produced TiO2 -Ti3 C2 TX/CTAB/CS/GCE electrode revealed two linear ranges from 0.003 to 0.25 mM and 0.25 to 1.25 mM. Furthermore, the accurate nitrite detection in water and milk samples served as proof of the electrochemical sensor’s practical application [42].

3.1.3

WO3 Based Electrochemical Sensors

WO3 is naturally an n-type bulk semiconductor similar to TiO2 which is non-toxic and cheap, the extra metal present in oxygen vacancies is what causes the stoichiometric behavior. The charge carrier density at the interface is lowered in the case of n-type metal oxides as a result of the electrons coming from ionized donors via the conduction band, and a potential barrier to charge transport is developed in order to lower the band gap energy for electrochemical applications [45]. 1D WO3 nanorods were prepared hydrothermally for detection of fungicides namely carbendazim. The accumulation of this material on the WO3 nanorods/GCE varied with time and found 40 s needed for stable measurements. pH effect also plays a major role with this modified electrode. Varying different pH showed acidity of 4.2 was maximum and suitable for efficient detection of carbendazim. Electron transfer behaviour was monitored by altering the temperature from 288 to 303 K. Analytical measurements with water and soil samples by spiking fungicide proven to be satisfactory with recovery values [46]. Similar kind of pesticide detection was successfully fabricated by an acetylcholinterase (AChE) biosensor. In this work, detection of phosmet using the pencil graphite electrode (PGE) on WO3 /g-C3 N4 modification was reported by Bilal et al., The designed antibody/enzymatic sensor was useful for food samples mainly whole

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Fig. 4 a Ranolazine (RZ), an anti-anginal drug detection using WO3 decorated graphene nanocomposite based electrochemical sensor Reprinted with permission from [43]. Copyright (2019), Elsevier, b CVs curves of a 20 μM guanine and b 20 μM adenine on the TiO2 -graphene/GCE in 0.1 M acetate buffer (pH 4.5) at different scan rates from 50 to 600 mV s−1 . Insert, the plot of the oxidation peak current versus scan rate, Reprinted with permission from [40] Copyright (2011), Elsevier c illustration for the fabrication of AChE/WO3 /GCN/PGE biosensor for phosmet detection Reprinted with permission from [44] Copyright (2021), Elsevier

wheat flour (red beetle) shown in Fig. 4c. The synthesis of the nanocomposite involves thermal polymerization and ultrasonication. The electrode preparation was highly complex, WO3 /GCN was taken in 10:4 ratio and treated with acid/base medium likely NaOH/Chloroacetic acid to evolve carboxyl groups on the nanocomposite. In order to activate the PGE, 0.5 M H2 SO4 was used as a cleaning solution after each experiment. 5 μL of the enzyme, AChE, was introduced on the nanocomposite coated electrode with EDS/NHS coupled and the resultant electrode designated as AChE/ WO3 /GCN/PGE. Samples that were positively identified as being free of the pesticide were treated as blanks, while samples of poisoned wheat were utilized to calculate recovery. They were good in agreement with HPLC as well [44]. DA sensor was developed using WO3 nanoparticles/orthorhombic structures which was synthesised by the simple microwave irradiated method. A linear response with a lowest detection limit of 24 nmol L−1 for DA over a large concentration range of 0.1–600 μmol L−1 . The real samples were checked with the dopamine hydrochloride injections with good recovery values [47]. WO3 has extended applications towards drug ranolazine (RZ) detection application reported by Ansari et al., Doping of graphene and WO3 provides enhanced sensor surface area and enhanced electron transfer capacity. The sensor showed profound detection strategies in electrochemical aspects as well as the pharmaceutical samples as in Fig. 4B [43] (Table 1).

103–107 5–250

Microwave assisted radiation

Hydrothermal method

Hydrothermal method

CuO–TiO2

Ag@TiO2 @Zif-67

TiO2 NTs/RGO Aptasensor Electrochemical anodizing process

Ultrasonication

SPE/Ag–TiO2 –reduced graphene oxide

MoS2 /WO3 /GCE

Pd–WO3 /g-C3 N4 hybrid composite

WO3 /rGO

PSS-GN/WO3 /GCE

Na-doped WO3 nanorods

N-doped Graphene. Au/ ZnO hybrid

Ag–ZnO nanoflowers

3

4

5

6

7

8

9

10

11

12

13

Co-precipitation method

Homogeneous precipitation method

Hydrothermal method

Sol–gel method

Liquid-control-precipitation method

–

–

Ag doped TiO2 /CNTs

2

1–20

103, 2.00 1.03

0.0810–22.5

0.06–6.0

0.01–1.6

0.01–60

0–4800

0–2000

0.05–25

0.3–3.0

0.06–700

Seed-mediated growth/ hydrothermal

TiO2 nanotubes/Au@Pd core–shell nanoparticles

Linear range (μM)

1

Synthesis method

HSPNs Modified electrode

S.no

250

5000, 400, 800

28

40

0.24

0.03

197

40 cells/mL

990

1210

10

87.6

12

LOD (nM)

Hydrogen peroxide

Ascorbic acid, acetaminophen dopamine

Bisphenol

Puerarin

Triclosan

Paraoxon-ethyl

Hydrazine

MCF-7

Glucose

Methyl parathion

8-OHdG

Cetirizine

Hydrazine

Analyte

–

Human serum and urine sample

Tap water, Milk

Puerarin injection and human plasma

Water, soil, fruits and vegetable

Agricultural water and soil

Tap water

Breast cancer cells

–

Ground water

Urine

Tablets, human serum and urine

–

Real sample

(continued)

[60]

[59]

[58]

[57]

[56]

[55]

[54]

[53]

[52]

[51]

[50]

[49]

[48]

Refs.

Table 1 Hybrid metal oxide semiconductor photocatalyst nanomaterials for electrochemical applications and its analytical parameters are as follows

186 K. S. Shalini Devi and S. Tsujimura

Vapour to solid mechanism

ZnO/CeO2

15

1000–8000

3–30

Linear range (μM)

–

270

LOD (nM)

Uric acid Ascorbic acid

Dopamine

Analyte

–

–

Real sample

[62]

[61]

Refs.

8-OHdG—8-hydroxy-2, —deoxyguanosine, Ag-Silver, Pd-Palladium, SPE Screen printed electrodes, CuO-Copper oxide, rGO-reduced graphene oxide, TiO2 Titanium oxide, WO3 -tungsten oxide, g-C3 N4 —graphitized carbon nitride, MoS2 -Molybdenum sulphide.

Combustion method

ZnO–Fe2 O3 nano heterojunctions

14

Synthesis method

HSPNs Modified electrode

S.no

Table 1 (continued)

Hybrid Semiconductor Photocatalyst Nanomaterials … 187

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3.2 Metal Sulphide Semiconductors-Based Sensors 3.2.1

NiS Based Electrochemical Sensors

Due to Ni’s natural abundance and extraordinary capacity to catalyze glucose oxidation in alkaline conditions through redox pair, Ni-based devices have demonstrated great potential [63]. In this context, there are several articles boomed with nickel sulphide-based detection recently in electrochemical sensors not only for glucose but also for sensing variety of bioanalytes. NiS thin film was electrodeposited for the quick detection of glucose as a single step modification on indium tin oxide (ITO). The sensor had a sensitivity and detection limit of 7.43 μA μM−1 cm−2 and 0.32 μM, respectively, with a short response time of 98.4%) within 72 min. Lead (II) pollution, which is mainly caused by human activities such as municipal sewage, mines, and chemical production, has severe toxicological effects on human health and can even result in lead poisoning, which can be fatal [40]. It was uncovered in a separate study that the magnetic Fe3 O4 @C@TiO2 heterostructure could likely capture and extract up to 92% of Pb(II) within a period of three hours [41]. Industrial discharges such as chlor-alkali, plastics, batteries, electronics, and used medical devices often result in the presence of Mercury (II) in industrial wastewater, which can be hazardous to human health if inhaled as vapor or ingested through aquatic organisms, causing Minamata disease [42]. Mesoporous α-Fe2 O3 /g-C3 N4 nanocomposites, which demonstrated a 4.6-fold and 6.8-fold greater photocatalytic efficiency than individual α-Fe2 O3 nanoparticles and g-C3 N4 nanosheets, represent an intriguing utilization of Hg(II) photocatalysis [43]. Au-decorated TiO2 nanotubes exhibited the photocatalytic abatement of Hg(II) in aqueous solutions [44]. It is important to remove toxic heavy metal ions such as Arsenic (As), Uranium (U), and Cadmium (Cd) as they are hard to decompose, build up in living things and the natural world at a rapid rate, and are very toxic even at low levels. Thus, it is vital for the safety of people and the planet to eliminate these hazardous heavy metal ions [45]. It was observed that under the most suitable conditions, more than 99.97% of arsenic is removed within 120 min by a BiVO4 /TiO2 /LED system created with hydrothermal techniques at a pH of 4.5 [46]. The effectiveness of the Eosin Y-sensitized TiO2 photocatalyst was demonstrated with 100% Cd (II) removal in 3 h at a pH of 7.0. However, further research is needed to evaluate the impact of environmental factors on its performance. The use of TiO2 for the removal of heavy metal ions from wastewater has been demonstrated to be an effective way to reduce the environmental impact of these pollutants. The process can be used as a standalone or in conjunction with other treatments to reduce the concentration of heavy metals in wastewater. While the cost of the process may be a concern.

4.3 Elimination of Pharmaceutical Compounds The ecosystem has been adversely impacted by emerging contaminants, such as pharmaceutical compounds (PCs), and in recent years there has been an increase in concern about them [48]. Heterogenous photocatalysis has been found to be one of the most effective methods for degrading difficult pollutants like antibiotics, compared to traditional treatment processes including flocculation, air stripping and reverse osmosis [49].

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Fig. 4 a Efficiency of eliminating RhB, Cu(II), Cd(II), and Cr(VI) after 240 min. b The amount of four pollutants removed over time in the photodegradation process. c Efficiency of photodegradation of three heavy metal ions over a period of 240 min. d The amount of Cr over time in the photodegradation process as measured by ICP-OES. e The proposed mechanism of TNM-PFACS for photocatalysis. Reprinted with permission from [47]. Copyright (2023), Elsevier

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The toxicity, stability, and potential to interfere with the environment and ecology of antibiotics and their by-products make it essential to research effective removal methods of these compounds in wastewater. For example, Odaba¸si et al. [50] investigated the removal of Diclofenac(DFC), Ibuprofen(IBU), and Paracetamol(PRC) by the immobilized TiO2 on activated carbon(AC). As shown in Fig. 5a, Pharmaceutics are removed to a certain extent using AC/TiO2 pH = 7, initial concentration of all pharmaceuticals = 5 mg/L, dosage of AC/TiO2 = 0.025 g/L. After 180 min, 95.32% of the IBU, 83.14% of the DFC, and 80.40% of the PRC had degraded. The varied affinities of the drugs for the catalyst’s binding sites was the main reason of effective degradation. Under optimized conditions, the N-Cu co-doped TiO2 @CNTs photocatalytic system, combined with visible light and ultrasonic radiography, was found to be an efficient heterogeneous catalyst for the treatment of sewage, with removal efficiencies of 100%, 93%, and 89% for sulfamethoxazole, COD, and TOC, respectively [51]. Tetracycline (TC) is one of the most crucial antibiotics among pharmaceutical compounds, and the hydrothermal/impregnation method was used to synthesize bare TiO2 and several CuO(x)-TiO2 /MCM-41 nanocomposites with different CuO contents that were used as catalysts to degrade TC under ultraviolet light [52]. It was highlighted that h+ is capable of directly oxidizing TC, while .O2− and .OH oxidize it effectively. Furthermore, anti-inflammatory drugs, which have high polarities and strong hydrophilicity, are likely to persist in underground, surface, and drinking water sources, due to their low absorption coefficient in soil, thus posing a considerable threat to water resources. Sulfamethoxazole (SMX), metronidazole (MNZ), and ciprofloxacin (CIP) are some examples of emerging pharmaceutical organic pollutants that are harmful to the environment and the general people. Akter et al. [6] studied the individual and combined degradation of SMX, MNZ, and CIP, by UV/TiO2 photocatalyst. In 360 min, a 5 mg/L SMX solution deteriorated around 97% of the SMX, whereas an 80 mg/L SMX solution degraded 80% of the SMX at the same TiO2 dosage and photodegradation period. During 600 min of photodegradation reaction time, the highest removals of MNZ and CIP as individual components were 100% and 89%, respectively [6]. Similarly, Tran et al. [53] investigated the enhancement in UV/TiO2 photocatalysis degraded the drugs metronidazole (MNZ) and amoxicillin (AMX) when they were combined. The use of such anti-inflammatory medications as ibuprofen, naproxen, diclofenac, and ketoprofen in drinking water treatment plants is a common practice in raw water sources. Studies have shown that semiconductor photocatalysis based on titanium dioxide is an effective way to eliminate diclofenac in aqueous solutions and eliminate ibuprofen from the aquatic environment [54, 55]. It was confirmed that the TiO2 active thin layer attached to the glass substrate could be a potential solution for protecting the environment from emerging pollutants such as ibuprofen and its derivatives. The use of TiO2 in the elimination of pharmaceutical compounds from wastewater is a promising method due to its wide availability, low cost, and ability to efficiently remove molecules from solution. Its effectiveness has been demonstrated in a variety

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Fig. 5 a The removal of pharmaceuticals by AC/TiO2 , with an initial concentration of 5 mg/L, AC/TiO2 dosage of 0.025 g/L and a pH of 7, was measured. Reprinted with permission from [50]. Copyright (2023), Springer Nature. b, c Photocatalytic degradation of ortho-phenylphenol and 1,4dichlorobenzene under UV light irradiation. d Schematic showing photodegradation of pesticides by using TiO2 . Reprinted with permission from [56]. Copyright (2023), Elsevier

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of studies and is backed up by the fact that it is a safe, non-toxic technology. This method has the potential to be applied on a large scale and could be used to help reduce the number of pharmaceuticals in wastewater and improve the overall quality of our water supply.

4.4 Removal of Pesticides Pesticides are used in various ways, such as growth regulation, defoliation, desiccation, fruit thinning, ripening regulation, and preservation of goods during storage or transportation. Unfortunately, these same pesticides are a major cause of water pollution, and all of them are known to be carcinogenic and pose a risk to health [57]. The utilization of semiconductor photocatalysis technology in the field of pesticide degradation has been carried out, which exhibits chemical stability and antibiodegradation, despite the fact that even trace levels of pesticides can cause severe harm to the environment and human health due to their toxicity and biological resistance [58]. Various metal oxides, like TiO2 and ZnO, are primarily used as semiconductor materials for the photocatalytic breakdown of pesticides [59]. Pest management for a variety of crops frequently involves the use of carbamate insecticides. Carbaryl degradation was examined in the presence of TiO2 photocatalysts, ozone, and TiO2 aqueous suspension. Under ideal circumstances, carbaryl is degraded by 99% when exposed to UV radiation and suspended TiO2 particles.[60] Fiorenza et al. [56] fabricated TiO2 photocatalyst via sol–gel that were imprinted with molecules. Two typical agricultural pesticides—the insecticide imidacloprid and the herbicide 2,4-D—were utilized as templates during the synthesis process and then eliminated under UV irradiation shown in Fig. 5b, c. The TiO2 imprinted with the appropriate pesticide target was used to demonstrate a striking improvement in the photocatalytic activity. The comparison with the breakdown of pesticides not employed as a template allowed researchers to confirm the selectivity of the photodegradation process (Fig. 5d) [19]. Results indicated that the mesoporous Ag/Ag2 O-TiO2 p-n heterojunction was able to completely break down the dimethenamid-P herbicide after 180 min of exposure to visible light through photocatalytic degradation [61]. The ability of bare TiO2 and Au-modified TiO2 to degrade phenoxy acetic acid when exposed to UV and visible light was also examined in terms of their photocatalytic activity [62]. Sanguino et al. [63] studied that pesticides were completely removed in a short period of time (25 min) at realistically low concentrations of 200 μg each pesticide·L−1 . They demonstrated the effectiveness of the synthesised TiO2 -rGO nanocomposites in this pilot-plat scale solar process to mitigate refractory and bio recalcitrant contaminants on effluents as a sustainable and efficient process. The onestep hydrothermal approach was used to successfully create the Fe3 O4 -TiO2 /reduced graphene oxide (Fe3 O4 -TiO2 /rGO) nanocomposite that exhibits intrinsic peroxidase mimic activity and photocatalytic efficiency. The Fe3 O4 -TiO2 /rGO nanocomposite was effectively used for the 100% photocatalytic destruction of the atrazine molecule when exposed to sunshine [64].

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The removal of pesticides by TiO2 from wastewater has been shown to be an effective and viable solution to reduce the impact that these toxic compounds have on the environment. The use of TiO2 is an effective and cost-efficient method to remove a variety of pesticides from wastewater, and the results of this study provide further evidence that this method of removal can be successfully implemented. As more research is conducted, TiO2 will become even more efficient and effective in the removal of pesticides from wastewater, providing a safer and healthier environment for all.

5 Challenges As was previously established, only UV light can activate pure nano-TiO2 . So, it is essential to create a catalyst that can absorb visible wavelengths for indoor applications. For various uses in both private and public facilities, the most effective system configuration and substrate materials must also be identified in order to ensure the photo catalyst’s maximum longevity, efficiency, and functioning. The fact that nanoTiO2 can completely destroy all organic materials, including any organic matrix in which the nanoparticles are embedded, makes using it in inorganic environments the only feasible solution. The use of nano-TiO2 is thought to pose little risk to the environment or human health, as it tends to be embedded in or on a substrate material. However, the potential accumulation of nano-TiO2 due to its wide use, as well as its effects on workers exposed to its dust, are yet to be fully explored and understood. Health and Safety implications of nano-TiO2 are still being investigated, similarly to other nanomaterials. No regulations have been put in place yet to limit the use of nano-TiO2 for water treatment, however, standards on test methods for photocatalytic water purification are under development. The implementation of more stringent water treatments standards could necessitate new treatment methods, which may lead to a wider use of photocatalytic systems using nanomaterials. Some other challenges are listed below: 1. Low efficiency in the visible light range: TiO2 -based photocatalysts are mainly activated by UV light, which severely limits their potential applications as they are unable to utilize visible light for the treatment of wastewater. 2. Short lifetime: The photocatalytic activity of TiO2 -based photocatalysts decreases with time due to the accumulation of organic compounds on the surface, which blocks the light from reaching the catalyst and reduces the activity. 3. Low selectivity: TiO2 -based photocatalysts tend to degrade all organic compounds, including toxic and non-toxic compounds, leading to the generation of hazardous by-products. 4. High cost of production: TiO2 -based photocatalysts are expensive to produce due to the high cost of the raw materials and the complex manufacturing process.

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5. Poor stability: TiO2 -based photocatalysts are susceptible to environmental conditions such as temperature, humidity, and pH, which can significantly reduce their photocatalytic activity.

6 Summary Nano-TiO2 applied photo catalytically has promise as a low-cost, practical substitute or additional technique for the treatment of water and wastewater. The fundamental force behind the broad adoption of new technologies, such photocatalysis with nanoTiO2 in the environmental sector, is stricter laws and regulations. The use of solar photocatalysis to disinfect drinking water, particularly in developing nations, is very promising and has experimental programmes underway. Commercially available small-scale photocatalytic systems utilising synthetic UV light for wastewater treatment exist, although research on the ecotoxicity of nano-TiO2 is still inconclusive. Although immobilised nano-TiO2 is frequently utilised, there are still significant technical obstacles to overcome, such as determining the ideal system configuration, which makes it challenging to forecast the long-term effectiveness of photo-catalytic systems. In summary, this chapter has discussed the photocatalytic oxidation process, as well as the recent advances in TiO2 -photocatalytic systems for the removal of various water pollutants. It has been demonstrated that the degradation efficiency and reuse utilization of these systems are still low, and that various factors such as charge excitation, separation, transport, adsorption, and surface reaction of the semiconductor, as well as the degradation concentration, pH, temperature, the charged nature of the pollutant, the reactor, and the light source lamp all have a large impact on the photocatalytic efficiency. Consequently, these factors should all be taken into consideration when designing and producing multifunctional semiconductor photocatalysts for the treatment of organic pollutants.

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Hybrid Photocatalyst Nanomaterials in Solar Cell Applications Habtamu Fekadu Etefa and Vinod Kumar

Abstract Presently, Energy crisis and environmental pollution are major problems in the world. The fossile fuels used today are polluting the environment as well as they have limited resources. Now a days, Nanomaterials are being used to provide an alternative source of energy. The development of new hybrid photocatalytic nanomaterials for soalsolarr cell application have lot of issues and challanges. In this book chapter, different kind of nanomaterials such as zinc oxide (ZnO), titanium oxide (TiO2 ), nickel oxide (NiO) were used in dye sensitized solar cells (DSSCs), organic solar cells (OSCs), and peroskite solar cells (PSCs). Different chemical methods were used for the synthesis of these nanomaterials. The properties of these hybrid photocatalytic nanomaterials are modified by using a different approach then overall helpful to enchace the power conversion efficiency (PCE) of solar cells. These hybrid nanomaterials were used as photoanode/photcathode in DSSCs, while being used as hole transport layers (HTLs)/electron transport layers (ETL) in PSCs or OSCs. This chapter presents the hybrid nanomaterials and its technology for the development of renewable energy that will have important benefits for the society. Keywords DSSC · Pervoskite solar cell · Organic solar cell · ZnO · TiO2 · NiO · ETL · HTL

H. F. Etefa Department of Physics, College of Natural and Computational Science, Dambi Dollo University, P.O. Box-260, Dambi Dollo, Ethiopia Department of Physics, Walter Sisulu University, Private Bag X-1, Mthatha 5117, South Africa V. Kumar (B) Department of Physics, The University of the West Indies, St. Augustine 330912, Trinidad and Tobago e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_10

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1 Introduction The discrepancy between rising energy needs and Earth’s limited fossil fuel resources has become a global worry, along with the fastest-growing population and economy in history [1]. Provided these energy challenges, the development of novel, clean, and sustainable carbon–neutral energy sources that can reduce our dependence on conventional fossil fuels while also reducing emissions of greenhouse gases will undoubtedly become the most difficult scientific challenging issue of the twentyfirst century [2]. Inclined the problem facing human being association, the growth of novel, pure, and tenable carbon-impartial beginnings, which can relieve our confidence on unoriginal, non-renewable, fuel sources and cut the emission of hothouse vapour, will definitely be the 21th century’s most demanding scientific challenge [2]. Nanostructured semiconductor materials and their fabrication have prompted a lot of interest because of their good electrical, optical and spectroscopic characteristics [3, 4]. Numerous industries have made use of nanostructure photocatalysts, along with semiconductors [5]. A recently discovered photochemical technology, photocatalysis, was already studied since the 1970s. It’s also centered on the photoexcitation of electrons and holes from semiconductor materials to promote oxidation– reduction reactions. Charge separation occurs through a process known as photocatalysis when luminous energy is absorbed in a rate greater than or equal to the bandgap of the semiconductor material [5]. Photocatalysis is an environmentally friendly technology that converts plentiful photonic energy into useful chemical energy. The sudden increase of flow photoreactors over the past decade has accelerated the design and development of novel semiconductor photocatalysts. Usually, reduction and oxidation on a semiconductor nanoparticles (NPs) started with the excitation of an electron from the valence band (VB) to the conduction band (CB), followed by the generation of a hole in the VB. Nevertheless, because the energy gap of TiO2 is 3.2 eV, which is similar to a wavelength of 375 nm, the photoreaction requires ultraviolet (UV) light (wavelength 400 nm). In the solar spectrum, only 3% of UV light is available. To completely harvest energy from the sun, a visible light-driven photocatalyst is highly desired. The other metal oxide were also accordingly studded such as: zinc oxide (ZnO) [6, 7], NiO/InTaO4 [8, 9], nickel oxide (NiO) [10], copper oxide (Cu2 O) [11], titanium oxide (TiO2 ) [12, 13]. However, because of their wide bandgaps or the process of photo-corrosion, they have a low absorption of visible light. To effectively use solar energy, highly stable visible light photocatalysts must be developed. As a result, novel photocatalysts have been used in photocatalytic reactions. TiO2, ZnO and NiO, for example, are wide direct bandgap semiconductor materials. They emerge to be highly capable materials for photocatalysis because of quick formation of electron–hole pairs by photoexcitation as well as highly negative reduction potentials of excited electrons. The visible light absorption and photocatalytic performance of photocatalysts can be improved through means of different preparation methods. Metal-coated semiconducting NPs have attracted a lot of interest for a variety of applications, particularly in heterogeneous photocatalysis related to solar energy

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conversion and environmental remediation. Due to their distinct optical and electrical characteristics, carbon-based quantum dots (C-QDs) have recently created novel opportunities for the development of hybrid nanomaterials with improved photocatalytic activity [14]. The graphenne quantum dots (GQDs)/ZnO NWs demonstrated a notable enhancement in the photocatalytic degradation of methylene blue under solar irradiation due to effective light absorption. The optimized GQDs (0.4 wt%)/ ZnO NWs had the highest photoactivity when with compared to pure ZnO NWs [15]. The production of Ag NPs in the GQD solution and the development of tight linkages between them were both verified by transmission electron microscopy. The produced ternary photocatalyst also demonstrated remarkable visible-light photocatalytic activity, which was significantly better than that of pure ZnO and binary photocatalysts like Ag-ZnO and GQDs-ZnO. This was demonstrated in photocatalytic studies including the destruction of Rhodamine B. We predict that this technique will enable the “green” production of hybrid photocatalysts made of metal, carbon, and semiconductors that have higher photocatalytic activity [16]. TiO2 is a potential semiconductor for photocathodic defence, but its use is limited due to the ineffective absorption of visible light and its low quantum efficiency. The graphene quantum dots-doped TiO2 (TiO2 -GQD) with mesopores were created under the influence of ultrasonic radiation. The created TiO2 -GQD nanocomposites performed well in terms of photocathodic protection and sunlight absorption. They effectively protect the active metal and reduce corrosion rate. It is anticipated that marine engineering will use TiO2 -GQD nanocomposites to achieve the effective and long-term development of marine corrosion prevention [17]. The light-harvesting efficiency, charge recombination rate, and charge transport have a significant impact on the power conversion efficiency (PCE) of solar cells. The morphology and structure of photoanode materials are directly impacted by these variables [18]. For applications involving dye-sensitized solar cells (DSSCs), TiO2 nanoparticles are frequently employed for photoanodes in DSSCs. However, the poor electron mobility of TiO2 limits the performance of the majority of DSSCs based on this material. ZnO is used an alternate wide band gap semiconductor materials for photoanode in DSSCs due to high chemical stability, higher conductivity and resilience against photocorrosion [19–21]. Although grains present in NPs, they prevent the wasteful movement of electrons, which eventually enhances the recombination of electrons with an electrolyte. Thus, one-dimensional (1D) nanostructure morphology is recommended for improved light scattering as well as charge transport in DSSCs [22, 23]. The schematic diagram for DSSCs is shown in Fig. 1a including transparent conducting oxides (TCOs), while the schematic diagram of perovskite solar cell (PSCs) is shown in Fig. 1b including TCO, hole transport layer (HTL) and electron transport layer (ETL). The In this chapter, a details discussion is done for the application of hybrid photocatalyst nanomaterials such as TiO2 , ZnO, NiO in DSSCs, PSCs) and OSCs.

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Fig. 1 Shematic diagram of different solar cells a Dye senstizied solar cell b Pervoskite solar cell

2 Synthesis Method of Nanomaterials Applications and synthesis of nanomaterials combine current and developing advances in nanomaterial production techniques with the use of diverse technologies [24]. Depending on the sorts and nature of the nanomaterials, multiple processes are used to synthesize them [25]. Many new photocatalysts nanomaterials have been used in photocatalytic/solar cells reactions recently, and it is imperative to create new cost-efficient, long-lasting, and environmentally friendly methods of synthesising nanomaterials to meet the technology’s exponentially increasing demand for them [26]. The numerous method used to synthesize nanomaterials including the combustion method, solvothermal method, chemical vapor deposition technique, microwave method, thermal decomposition method, pulsed laser ablation, hydrothermal, templating method, and sol–gel method [25]. However, the preparation method has a large impact on the photocatalytic activity of the photocatalyst, and then several synthesis methods have been developed [27]. The catalyst preparation methods are also used for synthesis [28]. For instance, the synthesis of NiOx nanocrystals has typically been accomplished using the solvothermal technique. This method has easy processibility, and the distribution of particle size and shape including its crystallinity can be precisely controlled. Since NiOx NPs produced using this approach are already crystallized. The high-temperature annealing process is frequently omitted following spin casting. Nonetheless, to eliminate the remaining organic solvent that is present on the surface, low-temperature post-annealing is typically necessary [29, 30]. Due to its ease of usage and high level of reliability, the solgel technique [31] is most frequently employed to deposit NiOx films. Because of high-temperature annealing, manufacture on an industrial scale is not possible. The metal precursors, which are typically used as stabilizing agents, are dissolved in an organic solvent in this procedure [32]. Because, of the enormous surface-to-volume ratio of thin films, the combustion method [33] is a self-propagating high-temperature synthesis process that is restricted by the tiny quantity of precursors. Flexible plastic substrates can be employed because the substrate is not greatly changed by heat as a result [29] etc. Different synthesis method for preparation of hybrid nanomaterials for solar cell application are shown in Table 1. These techniques mentioned are used for the synthesis of hybrid photcatalysis nanomaterials, which are used in third generation for solar cell application.

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Table 1 Different synthesis methods for preparation of hybrid nanomaterials for solar cells application Materials ZnO

TiO2

NiO

Synthesis method

Types of solar cells

Refs.

Hydrothermally

DSSC

[7, 18]

Sol–gel method

DSSC

[34]

Co-Precipitation

DSSC

[35]

Spray coating method

Perovskite

[36]

Solid-state method

Perovskite

[37]

Solvothermal method

DSSC

[38, 39]

Hydrothermal sol–gel

DSSC

[40, 41]

Sol–gel/hydrolysis

DSSC

[42]

Spray pyrolysis

DSSC

[41]

Hydrothermal

Perovskite

[43, 44]

Co-Precipitation

DSSC

[7, 27]

Green synthesis

DSSC

[45]

Calcination method

DSSC

[46]

Chemical precipitation

Perosvkite

[47]

Solvothermal method

Perosvkite

[29]

Sol–Gel method

Perosvkite

[29]

Pulsed laser deposition

Perosvkite

[29]

3 Titanium Oxide (TiO2 ) for Solar Cell Application TiO2 was used in photoanodes for a long time [48, 49]. TiO2 is one of the most considered and attractive wide direct band gap semiconductors materials [50, 51]. It has good chemical and physical properties and used as a promising material for solar cell applications [51, 52]. Gratzel [50, 51] have been reported a considerable improvmente in PCE by using the TiO2 material that followed the first DSSCs in 1991 [51]. TiO2 NPs are almost accepted materials for fabrication of photo-anodes with different shapes of TiO2 nanostructures like nanorods (NRs), nanofibers (NFs), nanotubes (NTs) etc. [53–57]. The modification in morphology, crystal phase, specific surface area, and crystalline structure of TiO2 has played a considerable role in the performance of solar cell devices [58]. To develop a highly efficient and stable DSSC, many researchers are working on the fabrication of different photoanode materials with high surface area as well as high light scattering capability. Meanwhile, the packing density of the dye is also overlooked because it has also impacts on the light harvesting. The highest PCE of DSSC are now over 14% [59]. Attafi et al. synthesised anatase TiO2 NPs using solvothermally method and used these solvothermally synthesized anatase TiO2 nanoparticles (SANP) in DSSC [13]. The DSSC (an active area of 0.16 cm2 ) based on the three photoanode configurations were characterised using the D149 and N719 dye to investigate the effects of charge injection,

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Fig. 2 a J-V curve for different DSSCs b IPCE curve for all devices c The absorbance curve of desorbed N719 dye from SANP, NR18-T and WER2-O films for the thickness of the film 5.0 ± 0.3 µm [Reproduced from Ref. [48] with permission]

dye packing density, and charge collection on the device performance. The current density versus voltage (J-V) results and incident to photocurrent conversion efficiency (IPCE) graph are presented in Fig. 2a, b, respectively. The device based on SANP has shown slightly higher short-circuit current density (Jsc ) with respect to NR18-T photoanodes based devices and comparable with NR18 -T/WER2 -O based device. The DSSCs based on SANP photoanodes have recorded the PCE of 7.7%. while NR18 -T photoanodes based device observed PCE 7.2% and devices have the NR18-T/WER2-O scattering layer recorded PCE of 7.9%. The absorbance spectra of SANP, NR18-T and WER2 -O films desorbed with N719 dye are shown in Fig. 2c. Effect of doping in TiO2 synthesized by the solvothermal method to be used as a photoanode in the performance of DSSCs was reported by Dubey et al. [38]. The J-V characteristics of the doped TiO2 photoanode based DSSCs are reported in Fig. 3a. The PCE were recorded to be enchanced in the order 1.31 < 2.21 < 2.27 < 2.73 < 2.87 < 3.20 < 3.29 < 3.33 < 3.62 < 3.70 < 4.85 < 5.75% corresponding to pure TiO2 (PT) < CuN-TiO2 (CuNT) < Ba-TiO2 (BT) < Sn-TiO2 (SnT) < Ag-TiO2 (AgT) < Cr-TiO2 (CrT) < Co-TiO2 (CoT) < V-TiO2 (VT) < S-TiO2 (ST) < Zn + Mg-TiO2 (ZMT) < Fe-TiO2 (FeT) < Zn-TiO2 (ZT). The maxmimum PCE was found to be ∼6% for the devices using the ZT photoanode. The comparasion of devices parameters is depicted in Fig. 3b. The best-performing devices are ZT and FeT, which are observing the decreased series resistance (Rs ) and improved the Jsc . It was noticed that the devices based on the photanode of FeT and ZT have recoreded 27 and 33.8% enhancement in PCE with respect to the device based on PT. Nguyen et al. reported solvothermally synthesised cobalt (Co) doped TiO2 as effective electron transport layers (ETLs) in PSCs [60]. The role of Doping concentration on the device parameters is shown in Fig. 4. The solar cells based Co doped TiO2 have higher PCE when compared with commercial dyesol TiO2 . The maximum PCE was observed 15.75% with the Jsc of 24.078 mA/cm2 , Vo of 1.027 V, and fill factor (FF) of 64.95%. Carbon dots (CDs) are newest branches of carbon based nanomaterials. The size of CDs is less than 10 nm. Due to the wide absorption spectrum, high photostability, multiple exciton generations, high solubility in water sufficient fluorescent performance, and easy modification they are known as one of the excellent dopants to enchance the properties of TiO2 [61]. Rezaei et al. reported CDs doped TiO2

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Fig. 3 a J-V curve of DSSCs based on different photoanodes b comparison of different devices parameters for different devices [Reproduced from Ref. [38] with permission of CC-BY] Fig. 4 J-V characteristic of PSCs for undoped TiO2 , co-doped TiO2 and dyesol TiO2 [Reproduced from Ref. [60] with permission]

based photoanode to improve the performance of DSSCs [61]. The J-V results of DSSCs with different weight ratio of the CDs decorated on the TiO2 film used as a photoanode is shown in Fig. 5. The maximum PCE was recorded as 7.32% with the Jsc of 16.94 mA/cm2 and Voc of 0.788 mV for 5% CDs doped TiO2 photoanode based DSSC, which indicated a 2.2 times increase in the PCE with compared to TiO2 DSSC with Jsc of 7.92 mA/cm2 and Voc of 0.765 mV. It is due to the efficient charge carrier separation as well as increased lifetime of the electrons owing to the well-transferred electrons to the CB of TiO2 and parallel the accumulated holes in the VB of CDs. All the reported worked confirmed that TiO2 (wide band gap semiconductor) is the most claimed and considered by the scientific community for building blocks of future photovoltaics (PVs) technology.

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Fig. 5 J-V curves of DSSC based on different photoanode a TiO2 b CDs (1%)/TiO2 c CDs (2%)/TiO2 d CDs (3%)/TiO2 e CDs (4%)/ TiO2 f CDs (5%)/TiO2 g CDs (6%)/TiO2 [Reproduced from Ref. [62] with permission]

4 Zinc Oxide (ZnO) for Solar Cell Application In solar cells, ZnO plays a crucial function since it can increase light absorption and promote charge transportation. Taka et al. created a nanocrystalline ZnO coating on a glass substrate using a drop-casting method and recorded the energy band gap of 3.10 eV as well as an optical transmittance of about 80% in the wavelength range of 400 nm to 800 nm [63]. Due to its comparatively high conductivity, stability against photo-corrosion, electron mobility, and affordability, ZnO has been the attention as one of the prospective wide band gap materials for solar cell application. ZnO materials with various nanoscale-engineered nanostructures have been used as photoanodes on conducting substrates. Recently, many scholars examine current developments in ZnO nanostructured materials used in novel solar cell applications [64]. Incidentally, the study of PV places great value on metal oxides as outstanding semiconducting components, which have an adequate band gap energy and solar light absorption [65]. The benefits of ZnO as an active material for solar cell applications are shown in Fig. 6. Wide bandgap semiconductors called ZnO materials, which only absorb light in the UV spectrum, have a band gap of 3.1–3.3 eV. To increase ZnO’s light absorption in the visible spectrum, it is also possible to be linked with substances with lower energy gaps, such as dye sensitizers, organic polymers, and semiconductors with reduced band gaps [64]. A pioneering metal oxide utilized in DSSCs was ZnO. It demonstrates a remarkable combination of potentially intriguing characteristics including strong bulk electron mobility and likely the greatest variety of nanostructures based on an extremely broad range of production methods [66]. However, despite the massive amount of literature that has been created in recent years, the PCE of ZnO photoanode based DSSC are still much below that of their TiO2 counterparts. The cause of this substantial discrepancy in performance is examined and reviewed in light of potential ZnO uses in DSSCs and associated devices. In this sense, the current study on ZnO based

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Fig. 6 The merits of ZnO for solar cell application [Reproduced from Ref. [64] with permission]

DSSCs has shifted its emphasis from morphology to surface control [67]. Due to its abundance, nontoxicity, and high electron mobility, ZnO has become more important for electron transportation in solar cells based on nanostructures [68]. Figure 7a demonstrates that ZnO particles are hexagonal in form and nanoscale in dimension. SEM pictures of ZnO nanorods (NRs) produced hydrothermally at 100 °C and 120 °C are displayed in Fig. 7b, c, respectively. The SEM images demonstrate that temperature of the hydrothermal reaction has a significant impact on the growth of ZnO NRs [18]. The longer lenth of NRs was observed at the hydrothermal temperature 120 °C with respect to the synthesis temperature of 100 °C. The growth and size of ZnO NRs was directly correlated to the hydrothermal temperature, which increases with increasing temperatures [18]. However, an increase in thermal temperature above 120 °C inhibits NRs growth and ultimately hinders the generation of the photocurrent [see Fig. 7f]. As revealed from Fig. 7e, it has found that the hexagonal wurtzite phase of ZnO is consistent with the morphology of NPs and NRs as indicated by XRD examination. The randomly oriented morphology of ZnO powders was shown by SEM micrographs, while the same morphology was verified by the existence of the main XRD peak at 36.2, which corresponds to the ZnO (101) plane. The ZnO (002) plane corresponds to the high diffraction peak detected at 34.4 for NRs arrays, showing the creation of 1D NRs along the c-axis [18, 69]. The J-V characteristics of DSSC is presented in Fig. 7f. The summarized findings demonstrate how system device efficiency is impacted by the morphology of photoanodes. The ZnO NRs-based solar cell (2.08%) performs better than the device based ZnO NPs (1.19%). Because NPs include grain boundaries, which prevent the profligate passage of electrons and consequently increase the recombination of electrons with an electrolyte, ZnO NPs-based DSSCs have low efficiency. Additionally, compared to NPs, NRs have better conductivity, whose results were confirmed by Jsc data. The tabulated findings further demonstrate that the PV performance of DSSCs is influenced by the hydrothermal temperature for the synthesis of NRs. Voc and Jsc

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Fig. 7 a Morphology of ZnO NPs using SEM b Growth of ZnO NRs at 100 °C c ZnO NRs at 120 °C d ZnO NRs at hydrothermal temperature of 140 °C e XRD curve of ZnO NPs and ZnO NRs synthesis at 120 °C f J-V curve of DSSCs using by different ZnO NPs and NRs g Nyquist plots for DSSCs devices based on ZnO NPs and NRs photoanodes [Reproduced from Ref. [18] with permission]

are seen to rise together with hydrothermal temperature. Electrochemical impedance spectroscopy (EIS) analysis was used to look at the interface resistances that exist within the DSSC structure. It is also offered details on charge recombination and charge transit. Figure 7g is shown in the Nyquist charts used in the EIS study. A semicircle at an intermediate frequency displays the interface resistance between PE and electrolyte, which is inversely proportional to the semicircle’s diameter. When compared to other devices, the DSSC with ZnO NRs @ 120 °C exhibits a larger semicircle diameter, which would eventually result in greater charge recombination resistance.Therefore, greater Voc is displayed by DSSCs based on ZnO NRs @ 120 °C [18].

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5 Nickel Oxide (NiO) for Solar Cell Application Thin film solar cells, DSSCs, quantum dots synthesized solar cells, OSCs and PSCs are examples of organic–inorganic materials used in solar cells as active light-absorber surfaces. The technique allows for a variety of device topologies, which promotes the advancement of PVs and thin-film solar cells. The usage of the nanocrystalline NiO film as a hole transport substance in PSCs was reported in literature. There has been discussion of the literature on both doped and pure NiO [29, 70]. However, NiOx ’s poor electrical conductivity and misaligned energy levels have limited the effectiveness of NiOx -based devices. Doping and surface modification are seen as important countermeasures to these shortcomings. This analysis provides a vision for the future while methodically classifying the prior years’ most successful approaches to enhancing NiOx [70]. The decay curve of NiO based solar cells shows that the NiO thin film dominates the degradation of the ITO [70]. In the reference cell, the acidic PEDOT:PSS was primarily responsible for the cell’s degeneration. However, the diverging decay curves suggested that there may be additional factors operating that are not present in this cell. The NiO thin film dominates the degradation of the NiO-based devices [71], which is shown in Fig. 8. The J-V curve for OSCs with HTL of NiO baked for 3 h, 5 h, and 7 h are shown in Fig. 8a–c, respectively. The J-V curve for OSCs with HTL of PEDOT:PSS with aging time is shown in Fig. 8d. The effect of aging time on the JSC of solar cells is presented in Fig. 8e. The effect of aging time on the PCE of OSCs is shown in Fig. 8f. However, NiO NPs revealed lower PCE in different types of solar cells, nevertheless by doping or adding other metal oxides it can boost its PCE of solar cells.

Fig. 8 J-V curve for OSCs with HTL of NiO baked for a 3 h, b 5 h, c 7 h and d J-V curve for OSCs with HTL of PEDOT:PSS with aging test e Effect of aging on the JSC variation of devices f Role of aging on the PCE of different OSCs [Reproduced from Ref. [71] with permission]

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As summarized in Table 2, the PCE of the NiO@500-5 h solar cell was observed greater than that of the reference cell. The PCE of the 7-h NiO device, however, was only 1.77%. Therefore, it can be inferred that 500 °C for 5 h is the ideal baking temperature for device efficiency. NiO NPs [27] are well-known p-type transition-metal semiconductors, which have been preferred as alternative electrodes on DSSCs due to their easy and consistent manufacturing as well as distinctive electrical and optical features [72].Catching photons at the electrode in DSSCs, makes it possible to produce excitons. While a p-type DSSC utilizing NiO NPs as the working electrode only managed to achieve an efficiency of 1.3%, some research scholars have developed NiO NP-based DSSCs that have been enhanced by hybridization with other metal oxides like TiO2 and ZnO. These devices have achieved PCE values of 3.80 and 3.01%, respectively [73]. As Etefa et al. [27] have reported, the calcination procedure was used to create NiO NPs, and the TEM was used to evaluate their sizes and morphologies. The rectangular NiO NPs crystals (see in Fig. 9) were proven to have the same structure as one that had previously been published. The calcination procedure was used to create NiO NPs, and the TEM was used to evaluate their sizes and morphologies. The rectangular NiO NP crystals were proven to have the same structure as one that had previously been published. The short and long axis’ average particle sizes were 11.37 nm and 16.48 nm, respectively. Furthermore includes a TEM image of the CDs and NiO NPs composite is shown in Fig. 9b. Though NiO NPs have a distinguishable shape and size (short and long axes, 11.27 and 16.56 nm respectively), C-dots are difficult to discern on the NiO NPs and even in the background. As reported, the fact that CDs have a lower X-ray density than NiO NPs suggesting the unconfirmed presence of CDs in the TEM image, according to these findings. As revealed in Fig. 9c, the FT-IR bands displas of CDs, NiO NPs, and NiO@C-dots. The stretching mode of vibration of NiO can be attributed to the main band of NiO NPs that first appeared at 436 cm−1 . The OH stretching and bending modes, which should have originated from the hydration or hydroxylation on the NiO NP surface or the water adsorption, are which contribute to the bands at 3420 and 1644 cm−1 . Mainly five significant bands of CDs are observed at 3390, 2931, 1662, 1557, and 1390 cm−1 . These are attributed to the corresponding O–H, C-H, C-O, N–H/C–C, alkyl, amine, and graphitic vibrational modes of the above-mentioned functional groups [74]. On the other hand, NiO@Cdots exhibit distinctive bands of both NiO NPs and CDs, demonstrating the presence of both elements. Therefore, it appears from the FT-IR spectra that CDs and NiO NPs coexist. The XRD peak positions and peak heights dissipated in Fig. 8d are in Table 2 Devices performance of OSC with HTL of NiO and PEDOT:PSS Device details

JSC (mA/cm2 )

VOC (V)

FF (%)

PCE (%)

NiO@500 °C-3 h

0.4848

5.991

0.3904

1.13

NiO@500 °C-5 h

0.5662

6.666

0.5221

1.97

NiO@500 °C-7 h

0.5375

6.185

0.5329

1.77

PEDOT:PSS

0.5726

5.445

0.4882

1.52

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comparatively good agreement with the previously reported cubic phase structure of NiO NPs, which corresponds to the wurtzite structure [75]. All of the indexed diffraction peaks consequently showed up as (111), (200), (220), (311), and (222), which are in agreement with a reported constant of a = 4.175, have lattice constants of a = 4.171, and c = 2.912 (JPCDS no. 01–1239) [45, 76, 77]. Additionally, because the addition of CDs had no effect on the crystallography of NiO NPs, the XRD peaks of NiO@C-dots and those of NiO NPs were remarkably consistent with one another [7, 45, 78]. The J-V curve in Fig. 9e dissipated the calculated electrochemical parameters of NiO NPs and NiO@C-dots of DSSCs using photosensitizers (N719). A 430 nm LED source (50 mW/cm2 ) was used for PCE experiments [27]. The ethaline diamine (EDA)/citric acid (CA) molar ratios of 0.5:1, 1.0:1, 1.5:1, 2.0:1, and 2.5:1 for the N719 sensitizers, and the J-V curves for NiO@C-dots DSSCs at a 12.5 wt% C-dot content were obtained. As reported by Etefa et al. [27], only NiO NPs achieved 2.4% of PCE but after the addition of CDs its PCE is boosted to 8.62% PCE. This can be assigned the P-type of metal oxide (NiO) that can be efficient after mingling with other metal oxides or sensitizers like CDs. It is suggested that the amine (NH2 ) component in the CDs is responsible for the action, when the EDA/CA molar ratio rises, and the amine content is increased compared to the carboxylic acid content.

Fig. 9 a TEM images of NiO b TEM image of NiO@c-dots c FT-IR graph for NiO NPs, C-dots, and NiO@C-dots d XRD curve of NiO NPs and NiO@C-dots e J − V charterstics of NiO NPs and NiO@C-dots based DSSCs f PCE of N719 and Rh6G dye based DSSS at different molar ratio of EDA/CA [Reprinted (adapted) with permission from Ref. [27] (2020) American Chemical Society]

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6 Conclusions Wide band gap semiconudtor nanomaterials have huge prospective to influence the existing photovoltaic industry. Different synthesis route can be used to controlled the shape and size of these materials. The shape and size of hybrid photocatalyst nanomaterials (metal oxides) such as, TiO2 , ZnO, and NiO are confirmed by the SEM, TEM and XRD results. It was discovered that device efficiency is influenced by the morphology of photoanodematerials. These hybrid photocatalyst nanomaterials can be used as a photoanode as well as photocathode. The properties of these nanomaterial is improved by doping of different materials. The power conversion efficiency (PCE) of solar cells is recorded as enchancement after doping of C-dots (CDs) in these oxides. However, the PCE of NiO NPs based DSSCs was significantly enhanced if a sufficient amount of CDs were added. It should be mentioned that CDs frequently affect both p-type and n-type DSSCs and that they provide a promising improvement in the device performance of DSSCs, which is closely related to an increase in the PCE of these solar cells. It does, however, have the advantages of affordability, compactness, and ease of production. As a result, the special requirements for CDs-DSSCs imply that more advancement is anticipated. The hybrid photocatalyst nanomaterials will be provided a new path of research in the field of solar cell. Acknowledgements One of the authors (VK) is thankful to The University of the West Indies, St. Augustine, Trinidad and Tobago for providing CR&P fund (CRP.3.JUN23.03).

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Transition Metal Chalcogenides-Based Nanocomposite for the Photocatalytic Degradation of Hazardous Chemicals Rama Gaur

Abstract Like two sides of a coin rapid industrialization have contributed to both economic growth and environmental pollution. Chemicals are omnipresent and are being widely used in each and every segment of industries. Dyes, paints, pharmaceuticals cosmetics, and agrochemicals are the major industries in the world that are using a wide range of chemicals. At the same time, these industries are releasing tonnes of chemical waste into the water bodies, causing water pollution. Photocatalytic degradation of the hazardous chemical in wastewater is considered the most effective strategy due to its simple setup and no secondary pollution. Among different semiconductor materials reported to date transition metal chalcogenides (TMC) and their composites have served as excellent photocatalysts for the degradation of hazardous chemicals such as industrial dyes, organic molecules, and pharmaceutical and agrochemical wash-off. TMCs have received considerable attention due to their interesting and unique optical properties, tunable band gap, layered structure, variable oxidation states, and high mechanical and thermal stabilities. The present chapter discusses the specific properties and characteristics that make this semiconductor material suitable for applications in wastewater treatment. The different transition metal chalcogenides-based nanocomposite, their mechanism for photocatalytic degradation, and challenges and future prospects will be discussed in detail. Keywords Wastewater treatment · Photocatalysis · Nanocomposites · Transition metal chalcogenides · Agrochemicals · Pharmaceuticals waste · Toxic dye

R. Gaur (B) Department of Chemistry, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, 382426 Gujarat, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_11

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1 Introduction Rapid industrialization and urbanization have led to an imbalance in the ecosystem by causing environmental pollution. The industries such as textile paper, chemical, and pharmaceuticals due to the discharge of effluents into the water bodies have adversely affected the flora and fauna. The build-up of chemicals in water bodies and accumulation in the food cycle has caused serious diseases in human beings and aquatic animals. Several measures have been taken to deal with the water quality crisis. Conventional strategies involve adsorption, coagulation, chemical precipitation, reverse osmosis, photocatalytic degradation, etc. Among all the existing strategies the use of photocatalysis for the degradation of noxious chemicals in water is most sought due to obvious advantages. Photocatalysis involves abundant sunlight to irradiate the semiconductor materials and the charge carriers generated attack the pollutant molecule converting it into small non-toxic molecules. Hence, it is a facile process, low-cost, highly effective, and does not result in the formation of secondary pollutants. Many semiconductor materials such as metal oxide nanomaterials (TiO2 , ZnO, SnO2 , etc.), Carbon-based nanomaterials (graphene, graphene oxide, reduced graphene oxide, etc), doped metal oxides (Cd1−x Znx S, Cd1−x Mnx S, etc.), metal sulfides have been widely used for the application in photocatalytic degradation of noxious pollutants. Despite the wide use, the existing semiconductor materials suffer from wide or zero band gap, narrow utilization of solar spectrum, and stability issues. Recently transition metal chalcogenides (TMCs) have emerged as potential candidates for photocatalytic degradation due to their special optical and electrical properties. The present chapter discusses in detail the specific properties and characteristics that make the TMCs suitable semiconductor materials for applications in wastewater treatment. The chapter also discusses the different transition metal chalcogenides and their nanocomposites, the mechanism for photocatalytic degradation, challenges, and future prospects

2 Transition Metal Chalcogenides (TMCs) TMCs have recently emerged as an important class of semiconductor materials. Metal chalcogenides are inorganic compounds made up of one or more electropositive metal cations and at least one chalcogen anion. The group VIA are termed as chalcogens owing to their origin form. Although all the elements in VIA are known as chalcogens but the term of metal chalcogenide is limited to sulphides, selenides, tellurides and not oxides and polonides. As we move down the group the metallic properties increase. Hence due to the non-metallic property of oxygen and the metallic properties of polonium, both oxygen and polonium are not considered as metal chalcogenides.

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Fig. 1 Special characteristics of transition metal chalcogenides (TMCs) and strategies to modify their performance

The remaining members of the group (S, Se, Te) are considered as semimetals and form a variety of compounds with diverse structures and compositions. Metal chalcogenides are a large family of 2D materials with tunable and thicknessdependent band gap which circumvent the zero-band gap issue of graphene and take an important position on photo-related applications. TMDCs possess special crystal structures and based on the atomic arrangement they can be classified as monochalogenides (MX), dichalcogenides (MX2 ), and trichalcogenides (MX3 ). Figure 1 shows the different unique characteristics of TMCs and different strategies to enhance their performance.

2.1 Classification of Metal Chalcogenides In general the chalcogenides form bond with metal to form metal chalcogenides such as Na2 S, CaTe, Ti2 S, and Cu2 Se. Due to variable valency and empty d orbital transition metals make interesting, non-stoichiometric TMCs like Cu1.97 S, Cu2 S, InSe, In3 Se2 , In4 Se3 , Ta2 S, and Ta3 S. TMCs can be classified into various groups based on different criteria. They are sulfides, selenites, tellurites, and binary, ternary, and multiple chalcogenides by the number of elements. Metal chalcogenides can be classified as main group metal chalcogenides and transition metal chalcogenides based on the type of metal ion used. The general formula of TMCs is MXn where M is transition metals like Ti, Cu, Fe, Mn, and X is the chalcogen elements such as S, Se, and Te, the value of n may vary from 1-3. TMCs are a special category of material

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Fig. 2 Schematic representation of metal chalcogenides in the periodic table and classification of TMCs. Reproduced with permission from Ref. [11] Copyright (2022) Elsevier

that have received a considerable attention recently due to their promising characteristics like tunable optical band edge, excellent reversible capacity, and plenty of electroactive sites [1]. TMCs can be further classified based on their composition as mono, di, and trichalcogenides with their general formula as MX, MX2 , and MX3 (M2 X3 ). The properties of TMCs with similar composition usually possess similar crystal structures. Figure 2 shows the different metal chalcogenides present in the periodic table and their classification. Different classes of TMCs are discussed in detail as follows.

2.1.1

Monochalcogenides

Transition metal monochalcogenides (TMMCs) have a general formula of MX where M is generally group IIIA and IVA transition metals. The IIIA group TMMCs (e.g. GaS, GaTe, GaSe, InS, InSe, etc.) hexagonal ordered arrangement with alternating layers of X-M-M-X [2]. GaTe possess a distorted X-M-M-X structure, InSe has two stable β-phase and γ-phase [3, 4]. Multilayer GaSe also possesses three types of crystal phases, β-GaSe, ε-GaSe, and γ-GaSe. The IVA group TMMCs (GeS, GeSe, SnS, SnSe, etc.) possess a black phosphorous like structure with puckered M-X atom permutation [5–8].

2.1.2

Dichalcogenides

Transition metal dichalcohenides (TMDCs) are an important class of twodimensional (2D) inorganic semiconductor materials. TMDCs have attracted great

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attention owing to their special optical, electrical, thermal, and catalytic properties [9]. TMDCs are inorganic compounds with a chemical formula of MX2 , where transitional metal (M) and chalcogen atom (X) are combined in fixed stoichiometric to form 2D layered materials. The TMDCs have alternate layers of MX2 with sandwichlike arrangement of X-M-X unit assembled together to give graphitic resemblance in structure. TMDCs are reported to exhibit polymorphism in their crystal structure due to the variation of stacking of layers along z-axis. The commonly reported phases are trigonal (1T), hexagonal (2H), rhombohedral (3R), and distorted octahedral (Td) [10]. Depending on the metal atom, the MX2 family can crystallize in 1T (tetragonal), 2H (hexagonal), or 3R (rhombohedral) symmetry as shown in Fig. 3. It also shows the hexagonal Brillouin zone with the high symmetry k points. According to literature reports, there are 40 and more different types of TMDC that have been chemically fabricated in the lab. The TMDC exhibit interesting electrical properties with tunable band gap and range from superconductors, metals, semimetals

Fig. 3 Structure of transition metal chalcogenides (TMCs) of composition MX2 Reproduced with permission from Ref. [12] Copyright (2023) American Chemical Society under Creative Commons License, https://s100.copyright.com/AppDispatchServlet#formTop

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to semiconductors. The variable and tunable band gap of TMDCs ranges from 0 to 3 eV and plays a crucial role in deciding its properties and applications [10].

2.1.3

Trichalogenides

Transition metal trichalcogenides (TMTCs) are of two types (a) MX3 (e.g. NbSe3 , ZrSe3 , TiS3 , HfSe3 , etc.) and (b) M2 X3 (In2 Se3 , Bi2 Se3 , Sb2 Te3 , etc.). MX3 -type TMTCs are known to possess quasi 1D crystal structure, which involve stacking of linear chain with identical orientations. Whereas M2 X3 type TMTCs possess a quintuple layer along the c-axis with sequential atomic planes in the order X-M-XM-X [13]. The TMTCs possess variable band gap dependent on the crystal structure arrangement of the compound. For instance, TiS3 possesses a direct band gap of 1.13 eV, In2 S3 possess a band gap of 1.35 and 1.45 eV for the stable α phase and β phase while Bi2 Se3 , Bi2 Te3 , and Sb2 Te3 have insulating properties [2, 14, 15]. Table 1 lists the characteristics electrical and optical characteristics of 2D metal monochalcogenides, dichalcogenides, and trichalcogenides.

2.2 TMCs and Their Nanocomposites 2.2.1

Cadmium Based

Cadmium-based chalcogenides are CdE (where E is the chalcogen atom as sulfide, selenium, and tellurium) that possess excellent optical properties and tunable band gap. Cadmium sulfide is one of the most important TMCs that has been explored a lot due to its interesting non-linear optical and electrical properties. Owing to its excellent thermal, chemical stability, electrical, and optical properties and it is used in solar cells, photodiodes, light emitting diodes, and lasers. CdS possesses a moderate band gap of 2.4 eV, with its absorption around 515 nm which makes it an active visible light catalyst. CdS is known to possess strong reducing power owing to its low conduction band edge position compared to conventional photocatalyst semiconductors such as ZnO, TiO2, and SrTiO3 . However CdS nanomaterials suffer from the photocorrosion and fast recombination of photogenerated charge carrier. The limitation is overcome by forming composites or by the introduction of dopants. CdS acts as an excellent photosensitizer when combined with wideband gap semiconductor materials and improves the photocatalytic efficiency of the composites. [Add examples] Similarly band edge tuning of Cd-based solid solution has proved beneficial and leads to maximum utilization of the solar spectrum. Gaur et al. have reported a complete series of Cd1-x Znx S with a tunable band gap of 2.37 eV to 3.69 eV with an increase in the concentration of zinc [38].

Dichalcogenides

Monochalcogenides

Semiconductor Semiconductor n-type

Hexagonal

Monoclinic

Orthorhombic

Orthorhombic

β rhombohedral γ hexagonal

Orthorhombic

Orthorhombic

GaS

GaTe

GeS

GeSe

InSe

SnS

SnSe

Semiconductor ambipolar (1 T, ) Metal (Td) Metal

2H hexagonal 1 T

2H hexagonal 1 T

(α) 2H hexagonal (β) 1 T, monoclinic Td orthorhombic (2H)

MoS2

MoSe2

MoTe2

2H) Semiconductor ambipolar (1 T) Metal (Bulk)

1 T rhombohedral

HfS2

(2H) Semiconductor (1 T) Metal

Semiconductor

Monoclinic

GeSe2

Semiconductor p-type

Semiconductor p-type

Semiconductor p-type

Semiconductor p-type

Semiconductor p-type

Semiconductor

–

Tetragonal

FeSe

Electric conductivity

Crystal structure

Chemical formula

0.96 (Bulk) 1.07(Monolayer)

1.4 (Monolayer) 1.58

1.29 (Bulk) 1.8(Monolayer)

2.0

2.74

0.89(Bulk) 1.63(Monolayer)

1.1

1.25(Bulk) 2.6(Monolayer)

1.10 (Bulk) 1.87(Monolayer)

(Monolayer) 2.34

1.7

2.52

–

Band gap [eV]

Table 1 The electrical and optical characteristics of 2D metal monochalcogenides, dichalcogenides and trichalcogenides

[22]

[21]

[20]

[19]

[6]

[18]

[8]

[4]

[5]

[3]

[17]

[16]

Ref.

(continued)

Indirect Direct

Indirect Direct

Indirect Direct

Indirect

–

Indirect Indirect

Indirect

Direct Direct

Indirect Direct

Indirect

Direct

Indirect

–

Nature of transition

Transition Metal Chalcogenides-Based Nanocomposite … 245

Trichalcogenides

Table 1 (continued)

n-type Semiconductor Semiconductor β:

Distorted 1 T triclinic

Distorted 1 T triclinic

4H hexagonal

1 T rhombohedral

1 T rhombohedral

2H hexagonal 1 T

2H hexagonal 1 T

1 T rhombohedral

Rhombohedral

Rhombohedral

Monoclinic

Tetragonal (defective spinel)

α rhombohedral, β rhombohedral

Monoclinic

ReS2

ReSe2

SnS2

SnSe2

VSe2

WS2

WSe2

ZrS2

Bi2 Se3

Bi2 Te3

HfS3

In2 S3

In2 Se3

NbSe3

Metal

Semiconductor

Semiconductor

n-type Semiconductor

2H) ambipolar Semiconductor (1 T) (Metal

(2H) n-type Semiconductor (1 T) Metal

Metal

n-type Semiconductor

n-type Semiconductor

ambipolar Semiconductor

n-type Semiconductor

(Monolayer, bilayer) Semiconductor (≥Trilayer) Semimetal

1 T rhombohedral

PtSe2

Electric conductivity

Crystal structure

Chemical formula

–

1.3 (Bulk) 1.5(Monolayer); α: 1.3

2.02

0.15–0.3

1.7 (Bulk) 1.7–1.93(Monolayer)

1.2 (Bulk) 1.65(Monolayer)

1.3 Bulk) 2.1(Monolayer)

–

1.0 Bulk) 1.73(Few-layer)

2.308 (Bulk) 2.033(Monolayer)

1.27 (Bulk) 1.24(Monolayer)

1.50(Bulk) 1.58(Monolayer)

1.20 (Monolayer) 0.21(Bilayer)

Band gap [eV]

–

Direct direct

Direct

[32]

[31]

[13]

[30]

[14]

[14]

[29]

[28]

[27]

[26]

[7]

[9]

[25]

[24]

[23]

Ref.

(continued)

Indirect Indirect

Indirect Direct

Indirect Direct

–

Indirect Indirect

Indirect Indirect

Direct Indirect

Direct Direct

–

Nature of transition

246 R. Gaur

Table 1 (continued)

Crystal structure

Rhombohedral

Orthorhombic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Chemical formula

Sb2 Te3

TaS3

TiS3

TiSe3

TiTe3

ZrS3

ZrSe3

ZrTe3

Metal

Semiconductor

p-type Semiconductor

Metal

Semiconductor

n-type Semiconductor

Metal

–

Electric conductivity

–

0.75 (Bulk) 1.17(Monolayer)

2.56

0.21 Bulk) 0.57(Monolayer)

1.13

–

–

Band gap [eV]

–

Indirect Indirect

Direct

Indirect Indirect

Direct

–

–

Nature of transition

[37]

[35]

[36]

[35]

[35]

[34]

[33]

[14]

Ref.

Transition Metal Chalcogenides-Based Nanocomposite … 247

248

R. Gaur

Fig. 4 Schematic for CdS/Bi2 WO6 –S Nanocomposites for Photocatalytic CO2 Reduction, Reproduced with permission from Ref. [40] Copyright (2022) American Chemical Society

In this regard, researchers have reported that the performance of CdS nanostructure can be tuned by using various synthetic approaches and synthesizing nanomaterials with controlled morphology [39]. Figure 4 shows the band edge alignment of Bi2Wo6-S/CdS nanocomposite with minimum recombination of charge carriers and enhanced photocatalytic activity due to interface engineering [40]. The usage of CdSe and CdTe is limited in the area of photocatalysis due to high toxicity associated with them.

2.2.2

Cu-Based

Cu-based TMCs are considered important semiconductors with p-type and direct narrow band gap of 1.2–2.0 eV. CuS due to its variable stoichiometry holds lots of importance in photocatalytic applications. For instant Cu2-x S possesses metallic to semimetallic and semiconducting behaviour depending on the value of x. CuS has an upper hand as compared to other TMCs due its low cost, easy synthesis, and non-toxic behaviour. CuS has been explored as a promising photocatalyst due to its narrow band gap, broad visible light adsorption, low cytotoxicity, high photostability, and optical properties. The photocatalytic performance of pristine CuS is limited due to poor quantum yield, fast recombination of charge carriers, and aggregation due to high surface energy. Mohammed et al. have demonstrated the use of ZnO-CuS nanocomposites as an effective photocatalyst for the degradation of pharmaceuticals and pesticides as shown in Fig. 5 [41]. For the above limitations, Cu-based ternary metal sulfide nanocrystals are explored as an alternative solution with better stability and enhanced response. Cu-based

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Fig. 5 a Schematic representation of CuS/ZnO hybrid nanocomposite as a photocatalyst for wastewater decontamination of pharmaceuticals and pesticides. Reproduced with permission from Ref. [41] Copyright (2022) Springer Nature under Creative Commons License, https:// www.nature.com/articles/s41 598-022-22795-9

TMCs have attained a special position among all the chalcogenides due to the following reasons: (a) high abundance, (b) low toxicity as compared to Pb- and Cd-based chalcogenides, (c) composition-dependent properties, due to various stoichiometric and non-stoichiometric phases and (d) excellent electrical, optical, and thermal properties [42–45].

2.2.3

Mo Based

The two dimensional TMCs have distinct and special properties compared to their bulk counterparts. MoS2 being a layered 2D compound with tuneable and thicknessdependent optical properties has received considerable attention recently. The optical, electrical, and chemical properties of MoS2 change with the decrease in number of layers. The graphene-like layered structure of MoS2 provides high surface area and enables surface functionalization and desired improvement in the properties. The combination of Mo-based chalcogenides also opens avenues for the development of novel and efficient photocatalyst. MoS2 is known to form nanocomposites with wideband gap metal oxide semiconductors, metals, carbon-based nanostructures, with enhanced photocatalytic response.

2.2.4

Tungsten Based

Tungsten sulphide is a 2D layered TMC with S-W-S planes held together with weak van der waals forces. WS2 has been widely explored for a variety of applications such as photocatalysts, photodectors, sensors, solar cells, and electrochemical applications. The optical properties can be modulated simply by exfoliating the bulk indirect

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band gap WS2 to nanosheets with direct band gap. WS2 is known to have thicknessdependent band gap ranging from 1.3 to 2.1 eV and is a potential photocatalyst due to its layered structure, direct band gap, high surface area, strong quantum confinement, and large spin-orbit coupling. Different phases of WS2 possess different properties for instance 2H trigonal is semiconducting and 1T octahedral phase possesses metallic characteristics. Interestingly, the phase transformation enables the tunability of properties, making WS2 an excellent candidate for different applications. WS2 has upper hand over other 2D materials due to its high abundance, easy availability, low cost, and low toxicity. To add to its favour it has high surface area, tunable band gap, high carrier mobility, photocatalytic activity, and biocompatibility. The disadvantages associated with WS2 are (a) mismatch of band edge positions (b) narrow band gap, and (c) low photostability. The above said limitations can be overcome by strategic material engineering [46].

2.2.5

Doped TMCs

Metal chalcogenides due to their narrow band gaps and suitable band edge alignment have received great attention in the area of photocatalysis. As discussed in the previous section the TMCs are prone to photo corrosion and this limits their application. The limitation can be overcome by the strategy of band gap engineering by introducing suitable dopants into their lattice. The dopant not only improves the stability of the lattice at the same time but also imparts a tunable band gap. This enables the maximum utilization of the solar spectrum. For instance, CdS and ZnS have a band gap of 2.4 and 3.7 eV, respectively and have been widely studied for photocatalytic applications. CdS being a narrow band gap semiconductor is prone to photo corrosion and hence dissociates under solar irradiation. On the other hand ZnS, a wide band gap semiconductor is stable under irradiation but absorbs in UV region hence the efficiency is limited. Researchers have reported the introduction of Cd ions to ZnS lattice to form Cd-doped ZnS nanoparticles and the doping of Zn ions to CdS lattice to form Zn-doped CdS nanoparticles. The introduction of Cd/Zn to the lattice leads to modification in the band gap and hence improves the stability of the semiconductor material. Gaur et al. have reported the formation of a complete series of Cd1–x Znx S solid solution (x = 1 − 0) by thermal decomposition of bisthiourea Cadmium-zinc acetate complex [38]. The band gap of Cd1–x Znx S varied from 2.37 eV to 3.69 eV with the increased concentration of Zn in the CdS lattice[38]. Similarly Nandingana et al. have demonstrated enhanced photocatalytic activity of Sn-MoS2 towards Rhodamine B dye degradation. The introduction of Ni in MoS2 leads to improvement in photocatalytic activity from 71 % to 96 % and 62 % to 91 % for MB and RhB degradation under visible light [47]. Iqbal et al. reported enhancement in the RhB degradation using Mn-doped Bi2 S3 [48]. Figure 6a, b shows the engineered band gap structure of Mn doped Bi2 S3 and Cu2 XSnS4 doped with Zn, Ni, Mn & Co, respectively [48]. Doping on carbon in WS2 nanostructure to form C-WS2 with better efficiency compared to the pristine WS2 [49]. Selenium doping in ZnS by Datta and co-workers has resulted in seven-fold increase in the rate of

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Fig. 6 a Engineered Band gap structure of Mn-doped Bi2 S3 for the degradation of RhB. Reproduced with permission from Ref. [48] Copyright (2022) Elsevier and b Energy band position of Cu2 XSnS4 [X = Zn, Ni, Mn & Co] samples prepared by hydrothermal method Reproduced with permission from ref. [43] Copyright (2020) Elsevier

degradation of MB [50]. Band gap engineering by elemental doping has proven to be a great strategy to improve photocatalytic efficiency and resolve stability issues.

3 Remediation of Environmental Contaminants With increased comfort and status of living there has been a continuous decline in the quality of environment. Environmental pollution is a major concern for the present generation and threat to future generation. Every segment of the environment air, land, and water is severely contaminated. Several approaches such as, adsorption, electrocoagulation, osmosis, chemical precipitation, photocatalysis, etc., have been employed for the treatment of wastewater. The use of semiconductor nanomaterials as photocatalyst has received considerable attention due to its eco-friendly nature. A myriad of materials are available for the photocatalytic degradation and remediation. Material scientists are working towards modification, and surface engineering of the existing materials to improve their performance or towards development of novel materials. The following section discusses about photocatalysis, its mechanism, the criteria for a material to stand out as a photocatalyst, and various strategies to improve the photocatalytic efficiency of the semiconductor material.

3.1 Photocatalysis Shortage of energy and increased water pollution has been a major concern recently. The researchers are continuously working towards developing greener options to solve the problem of energy crisis and environmental pollution. Photocatalysis, as the name suggests, is the catalysis process in the presence of light [51]. Photocatalysis is also known as artificial photosynthesis, leading to the formation of charge

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carriers on illumination. The materials used during photocatalysis are termed as photocatalysts. The photocatalyst alters the rate of reaction in the presence of light. When irradiated with sunlight, a suitably chosen semiconductor material generates reactive species as electron-hole pairs. These electron-hole pair acts as oxidants and reductants, attacking the toxic organic pollutant and degrading it to non-toxic molecules like water, CO2 , alcohols, etc. [52]. The application of photocatalysis is not only limited to environmental degradation. The phenomenon of catalysis in the presence of light also serves as the basis for the photoelectrochemical splitting of water, photovoltaic cell, solar cells, air purification, etc. [53, 54]. The photocatalysis can be classified as homogeneous and heterogeneous catalysis depending on the state of the reactants. In homogeneous photocatalysis, both the photocatalyst and the reactant are in the same physical state. While during heterogeneous catalysis, the photocatalyst and the reactant exist in different physical states. The major advantages of the photocatalysis process are (i) it involves the use of natural sunlight as the light source, which is abundant and has an endless supply, free of cost and (ii) it is a clean process as the mineralized products obtained towards are non-toxic. Hence, photocatalysis is an answer to the existing energy crisis and environmental pollution as an economic, simple, and effective strategy for both problems.

3.2 Criteria for a Photocatalyst Semiconductor materials such as photocatalysts have received great attention. Photocatalyst is considered as the heart of the photocatalytic system. Hence, choice of material is an important aspect of photocatalytic applications. For any material to act as a photocatalyst must possess the following characteristics:

3.2.1

Suitable Band Gap

A suitable band gap facilitates sufficient light adsorption, which is the most important step during photocatalysis. The higher the light adsorption, the higher the concentration and the higher the catalyst’s efficiency. Wide band gap semiconductors like TiO2 , ZnO, ZnS, MoO3 , etc., have been widely explored for the photocatalytic degradation of organic/inorganic pollutants. But these materials suffer as they utilize only a small fraction of the solar spectrum. TMCs serve as an excellent solution to this issue. Reports are available where researchers have used narrow band gap semiconductors like CdS, SnS2 , MoS2 , PbS, etc., have been used for adsorption in the visible range to maximize the range of adsorption from the solar spectrum. The combination of wide band gap and narrow band gap semiconductors to form heterojunctions and nanocomposites has been explored to utilize the UV and visible solar spectrum

Transition Metal Chalcogenides-Based Nanocomposite …

253

Fig. 7 a Different possibilities of reactions based on the relative arrangement of the valence and conduction bands. A Reduction. B Oxidation. C Redox reaction. D No reaction and b the schematic for different types of heterojunctions

light. Figure 7 shows the possible combinations of the heterojunction of the semiconductor material depending on the relative potential of valence band (also known as the highest occupied molecular orbital (HOMO)) and conduction band (also known as lowest unoccupied molecular orbital (LUMO)). As shown in the figure, the different arrangement of the VB and CB leads to various possible reactions. Scheme A shows the CB placement towards the lower redox level, indicating the possible reduction of the substrate by the electrons. While in scheme B, when the VB of the substrate is at a higher redox level than the semiconductor, the holes result in the substrate’s oxidation. In Scheme C, both reduction and oxidation are possible due to the placement of the CB at lower redox and VB at higher redox as compared to the semiconductor. Scheme D depicts no charge transfer, and hence no reaction is possible. Based on the above possibilities, the semiconductor heterojunctions are classified as Type I, Type II, and Type III. Figure 7b shows the schematic for different types of heterojunctions.

3.2.2

High Surface Area

As photocatalysis takes place on the surface of the photocatalyst, the surface area and surface characteristics play an important role in deciding the performance of

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the photocatalyst. The higher surface area can be ensured by (a) having a smaller particle size, (b) use of porous materials, and (c) using controlled synthetic conditions (e.g., xerogels). Photocatalyst with a high surface area will have higher adsorption of the pollutant and hence higher catalytic activity. In this context TMCs with layered structure serve as an excellent material to be explored as photocatalyst.

3.2.3

High Crystallinity

Using crystalline semiconductor materials ensures the homogeneity of the catalyst surface. The presence of defects on the catalyst surface acts as a quenching site and promotes recombination of the e- -h+ pair impeding the catalytic activity of the material. Also, high crystallinity ensures high mobility of the charge carriers and hence has efficient charge separation. The higher the life of charge carriers higher the photocatalytic activity. Special efforts are paid to prevent recombination of charge carriers. The presence of various crystal structure forms for TMCs make them a versatile and potential material for photocatalytic applications. The unique crystal structure and structure-dependent optical and electrical properties of TMCs make them a suitable candidate as a photocatalyst.

3.3 Mechanism of Photocatalysis The phenomenon of photocatalysis has always been very fascinating. Be it the photocatalysis in the plant to prepare food and release oxygen or in semiconductor materials. Understanding the mechanism envisages better exploration and design of photocatalyst. When the semiconductor nanomaterials are exposed to sunlight, they absorb radiation of a suitable frequency corresponding to their band gap. The absorption of light energy leads to the promotion of electrons from the valence band to the conduction band, leading to the generation of hole/vacancies in the valence band. The electrons and holes generated are known charge carriers and are responsible for photocatalytic activity. Hence it is important to work on increasing the charge carrier concentration and improving their lifetime by preventing their recombination. The e+ -h- pair acts as oxidant and reductant, and lead to the formation of reactive oxidation species such as hydroxyl radical (HO˙), superoxide anion radical (O2 − ˙), hydroperoxyl radical (HO2 ˙), singlet oxygen (1 O2 )[55]. The ROS attack on the analyte/pollutant and metabolize it into smaller non-toxic molecules. Figure 8 depicts the fundamental steps involved in photocatalysis [56].

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Fig. 8 Schematic representation of fundamental steps involved in photocatalysis (I) generation electron and holes as charge carriers, (II) reduction reaction by electrons, (III) oxidation reaction by holes; trapping of (IV) electrons and (V) holes; (VI) recombination of electron and holes to release heat. Reproduced with permission from Ref. [56] Copyright (2017) Royal Society of Chemistry

3.4 Strategies for Improving the Efficiency of the Catalyst The properties of a material can be engineered to design a photocatalyst with desired characteristics. As mentioned in previous sections, for a material to act as an effective catalyst, it must possess a suitable band gap, high specific surface area, surface functionalization, etc. It is not possible to get all the characteristics in one single material. There are several strategies for material engineering to tune the properties, such as size tailoring, the addition of dopants, formation of hetero-nanostructures, preparation of nanocomposites, incorporation of metals in the matrix, surface functionalization, etc. (a) Tailoring the Size: Materials in the nano regime have size-dependent properties due to the quantum confinement effect. With the reduction in the dimensions, the optical and surface characteristics change drastically. Due to the quantum confinement effect, as the size of particles decreases, the effective surface area increases and the band gap of the nanoparticles increases. Both these parameters are crucial for the photocatalyst; hence, dimensionality tuning is an effective strategy to prepare a catalyst with an enormous surface area and high catalytic activity. (b) Addition of dopants: Another effective strategy for modification of the absorption characteristics of a photocatalyst is the addition of a dopant to the crystal lattice. The doping results in the modification of the band structure of the catalyst by introducing the acceptor and donor levels. This promotes the transition of electrons from VB to CB hence improving the charge transfer characteristics of the materials. The addition of dopants increases the charge carrier concentration by inducting defects and vacancies. (c) Formation of hetero-nanostructures: The combination of dissimilar materials to form heterostructures is an important strategy for the maximum utilization of the solar spectrum for absorption. As discussed above, the amount of absorption

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is a critical parameter for photocatalytic applications. The higher the photoabsorption, the higher the charge carrier concentration, hence higher photocatalytic performance. Heterostructures can be further prepared by combining two or more materials or by loading plasmonic metals on the catalyst. (d) Preparation of nanocomposites: Nanocomposites is known as composites of two or more semiconductor material with different optical characteristics and at least one having dimensions in the nano range. Generally, a wide band gap semiconductor is combined with a narrow band gap semiconductor to form a heterojunction. The narrowband gap material acts as a sensitizer and effectively absorbs visible light. The choice of two materials is a crucial factor in forming nanocomposites. The relative alignment of the band edge will help increase the concentration of charge carriers and prevent the recombination of charge carriers, increasing their lifetime. The longevity of e–h + pairs directly affects the photocatalytic properties (e) Loading of plasmonic metals: The metal nanoparticles have a special phenomenon of surface plasmon resonance due to the presence of the charged surface. The metal nanoparticles act as semiconductor materials with sizedependent band gap tunability. The loading of plasmonic metals on the catalyst improves the charge transfer characteristics by sensitizing the catalyst by injecting an electron in CB on the absorption of suitable radiation. (f) Surface functionalization: The first step in photocatalysis is the adsorption of dye/pollutant molecules on the catalyst’s surface. Adsorption is a surfacedependent phenomenon. The interaction of the analyte with the catalyst surface can be improved by surface functionalization using suitable molecules.

4 Applications of TMCs Towards Degradation of Noxious Environmental Contaminants Continuous contamination of water resource by the industrial effluents has been a major concern to be addressed. Researchers have explored various methods for the decontamination and treatment of wastewater. Among reported methods photocatalysis is considered best due to ease of operation and no generation of secondary pollutants. The advent of material science and engineering has enabled us to develop materials with desired properties to be used as a photocatalyst. From the discussion about the TMCs it is clear that they act as excellent candidate for photocatalysis. The present section deals with the treatment of different contaminants such as toxic organic dyes, agrochemical & organic molecules, pharmaceuticals, and heavy metals using TMCs as photocatalyst.

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4.1 Degradation of Toxic Industrial Dyes The textile and paper industry uses various organic dyes to make fabric and paper. Dye molecules such as alizarin, congo red, crystal violet, methyl orange, methylene blue, rhodamine B, malachite green, etc., are widely used in textile industries. These dyes are a major component of the industrial waste discharged into the water bodies [57, 58]. As per the reports, industrial dyes used in different processes are known to be carcinogenic and, once added to water bodies, flow through different domains of an ecosystem. Hence causing contamination of flora and fauna and causing scarcity of potable drinking water. The contamination of water bodies by industrial effluents has raised several concerns regarding environmental integrity and safety. The researchers are working towards (1) the development of non-toxic, non-carcinogenic, and biodegradable dyes and (2) to develop facile and cost-effective methods for treating industrial wastewater. Table 2 lists the various TMCs used as photocatalyst for degradation of toxic industrial dyes, their concentration, dosage, removal time, and removal efficiency. Figure 9 demonstrates the degradation of methylene blue dye over the multivalent charge- and solid solution-type n-MoS2 /p-WO3 -based diode catalyst under dark condition with a self-supporting charge carrier transfer mechanism [59].

4.2 Degradation of Agrochemical and Organic Molecules The presence of organic contaminants in water bodies is a serious environmental concern. These organic molecules can be naturally occurring molecules or synthetic organic molecules. The man-made organic molecules generally comprise pharmaceuticals, agrochemicals, and antibiotics used daily. The list of hazards associated with these molecules is endless. Researchers have widely explored the photocatalytic degradation of toxic organic molecules (Table 3). List of TMCs used as photocatalyst for degradation of agrochemicals, their concentration, dosage, and removal time and removal efficiency. Figure 10 illustrates the use of WS2 /Bi2 MoO6 composites under visible light irradiation for the degradation of various organic pollutants [78]. The formation of composite leads to effective charge separation and long-lived charge carriers with high life hence improved performance compared to pristine WS2 [78].

4.3 Degradation of Toxic Pharmaceuticals The use of pharmaceutical drugs for the treatment of several diseases is inevitable. A long list of prescription and non-prescription drugs, such as antipyretics, antibiotics, anticonvulsants, etc., are administered to patients in a routine course of treatment [93]. These pharmaceutical drugs and their derivatives are released into the environment as

0.2 g –

– 100 ppm 3 × 10–5 M

RhB MO

Cu2 SnS3 /rGO

–

CV RhB

MoS2

MoS2 QDs-MoS2 nanosheets@Ag3 PO4

1.2 g

–

30 mg

30 mg

20 mg

40 mg

0.2 mg/mL

0.2 g

10 mg

–

50 mg

20 mg

50 mg

–

CV:Crystal violet; MB: methylene blue; MG: malachite green; MO:methyl orange; RhB: rhodamine B

100 ppm

10 ppm

MB

MoS2 nanobox embedded g-C3 N4 @TiO2

20 ppm 20 ppm

MO MB

BiOI/MoS2

80 ppm

Lanasol Red 5B

MoS2 /Ag2 CO3

MoS2 /SrFe12 O19

10 ppm 10 ppm

MO & RhB RhB

ZnIn2 S4 /MoO3

20 ppm

TiO2 /rGO/CuS

MO

MoS2 /ZnIn2 S4

–

10 ppm

RhB MG

CdS@MoS2

MO

Ag2 S/AgInS2

CuCo2 S4 /RGO NC(@3% rGO loading)

10 ppm

tartrazine dye MB

CuFeS2

Cu2 FeSnS4

–

RhB

BaAu2 S2

– 0.1 g

– 5 ppm

RhB MO

16

50

60

60

75

25

50

80

–

360

–

–

–

40

–

240

–

20

180

2 gL − 1

Cu2 WS4 /NiTiO3

60

90

Time (mins)

3 mg

0.5 mg/mL

Sample dosage

Sb2 S3

10 ppm

RhB dye

CdS/EU-12

100 ppm –

MB E. coli

Cu2 ZSnS4

Concentration

Dyes

1 T-rich MoS2 /g-C3 N4

Photocatalyst

100%

92%

97.5%

97.0%

95.6%

95.0%

100%

98%

84%

92%

80%

80%

81%

99.1%

96% 94%

45%

47%

98%

98.62%

100%

60%

Efficiency

Table 2 List of TMCs used as photocatalyst for degradation of toxic industrial dyes, their concentration, dosage, and time and removal efficiency

[77]

[76]

[75]

[74]

[73]

[72]

[71]

[70]

[69]

[68]

[67]

[66]

[44]

[65]

[64]

[63]

[62]

[45]

[61]

[60]

[43]

Refs.

258 R. Gaur

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259

Fig. 9 Schematic representing the preparation of n-MoS2 /p-WO3 by hydrothermal method used for MB dye degradation using, p–n junction diode. Reproduced with permission from Ref. [59] Copyright (2020) Elsevier

either industrial discharge or post-treatment through bodily excreta and contaminate the water bodies [94]. The presence of such drugs in water raises health concerns for humans and animals. Long exposure and overdose of any drugs cause liver failure and several organ malfunctions due to drug toxicity [95]. In continuation to the photocatalytic degradation of organic molecules, researchers have also attempted the degradation of pharmaceutical drug molecules. Figure 11 shows the photocatalytic removal of norfloxacin from the aqueous solution using immobilized Z-scheme CdS/ Au/TiO2 nanobelt photocatalyst [96]. Table 4 lists TMCs used as photocatalysts for degradation of pharmaceutical and organic molecules, their concentration, dosage, time, and removal efficiency (Fig. 12).

4.4 Photoreduction of Heavy Metals Presence of heavy metals in water is a serious hazard to human life. Heavy metals like Pb, Cd, Zn, Hg, As, etc., may enter our food cycle and end up accumulating in the human body. The presence of such metal even in traces has toxic effect on human health and leads to the occurrence of diseases. High efficiency, low cost, avoiding secondary pollutants, and direct use of natural solar energy are preferential benefits for photocatalytic technique. Chalcogenides-based nanomaterials are widely used photocatalysts due to their narrower band gaps that correspond to the visible light

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Table 3 List of TMCs used as photocatalyst for degradation of agrochemicals, their concentration, dosage, time, and removal efficiency Photocatalyst

Agrochemicals & organic molecules

Concentration

Sample Time dosage (min)

Efficiency Ref.

MoS2 /CuO

2-mercapto benzothiazole

–

–

120

96.0%

[79]

100 ppm

1.2 g

120

100%

[80]

educed GO/ZnO/ Aniline MoS2 and CNTs/ ZnO/MoS2 MoS2 /CoTiO3

bisphenol A

10 ppm

50 mg

–

6.1%

[81]

g-C3 N4 /MoS2 polyaniline

bisphenol A

20–60 ppm

–

60

92.7%

[82]

MoS2 / BiVO4 -activated PMS

bisphenol A

–

–

20

93.3% o

[83]

bacterial cellulose/MoS2

pyrocatechol violet Not mentioned –

180

84.5%

[84]

MoS2 /ZnS-N/ S-doped graphite carbon

dicofol (pesticide)

–

–

100

84.5%

[85]

TiO2 /MoS2

Phenol

–

–

150

78.0%

[86]

MoS2 @rGO

Thiophene

14.5 nm

–

75

100%

[87]

ZnS

g naphthalene anthracene chlorophenol nitrophenol

50 ppm

10 mg

120

> 90%

[88]

CdS–TiO2

Benzene

–

–

–

95%

[89]

CuS@rGO

Atrazine

50 ppm

0.1 g

20 min

100% AZ

[90]

Ag/Ag2 S/CuS

2,4-dichlorophenol –

–

240 min 82%

[91]

MoS2 @rGO

Thiophene

600 pm

–

75

100%

[87]

100 ppm

0.2 g/L

60 min

97%

[65]

CuFeS2 modified Bisphenol A with citrate CdSe/rGO

Thiophene

600 ppm

–

90 min

100%

[92]

CuS@rGO

Atrazine

50 ppm

–

50 min

100%

[90]

absorption. These toxic materials that originated from various anthropogenic and natural sources persist in water and are discharged into streams and rivers, which compromises the overall water treatment system. Shazad et al., have reported 40% higher removal efficiency due to the presence of CuCo2 S4 in the composite. Figure 5 illustrates the mechanism for photo-induced reduction and oxidation of pollutants, and schematic representation of the preparation of SnS2 -based composites for the photoreduction of Cr(VI) [119]. Table 5 lists TMCs used for remediation (photoreduction and adsorption) of toxic heavy metals, their concentration, dosage, removal time, and removal efficiency.

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Fig. 10 a Schematic diagram of photo-generated electron-hole pairs separation and the possible reaction mechanism for photocatalytic degradation of organic pollutions over WS2 /Bi2 MoO6 composites under visible light irradiation b Degradation curves of different model pollutants, and inset demonstrating the TOC removal efficiency c TEM and HRTEM and d Cycles of RhB degradation over the hierarchical WS2 /Bi2 MoO6 composite (5 wt% of WS2 ) under visible light. Reproduced with permission from Ref. [78] Copyright (2018) Royal Society of Chemistry Fig. 11 Photocatalytic removal of norfloxacin from the aqueous solution using immobilized Z-scheme CdS/ Au/TiO2 nanobelt photocatalyst. Reproduced with permission from ref. [96] Copyright (2021) Elsevier

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Fig. 12 a Mechanism for photocatalytic reduction and oxidation of pollutant by charge carriers, b Schematic representation of visible light-induced photo reduction of Cr (VI) to Cr(III), and c a comparison of traditional charge transfer process and charge transfer process in Z scheme. Reproduced with permission from Ref. [119] Copyright (2021) Springer

5 Challenges and Future Prospects It can be summarized that TMCs have been widely used for various photocatalytic applications. TMCs have attained a bigger position as compared to the other semiconductor nanomaterials used in photocatalysis. Majority of TMCs are 2D in nature and hence possess high surface area and greater quantum confinement with high diffusion length compared to 3D structures. Other advantages associated with TMCs are their layer and thickness-dependent electronic and optical properties, high surface area, matching band edge, and exposed functionalities which make them a potential and suitable candidate for photocatalytic applications. Despite the advantages, TMCs also suffer from the limitation of low stability, susceptibility to corrosion under irradiation, and low recyclability which limits their performance and efficiency. Towards overcoming the shortcomings of TMCs, different strategies like the formation of nanocomposites, introduction of dopants, band edge engineering, and surface functionalization come in very handy in this regard. This open avenue for further exploration in developing novel materials or improving the existing materials. This can be achieved by the following ways (1) combination of suitable semiconductor materials for enhanced visible light absorption, (2) strategic band edge alignment to prevent the recombination and improved life of charge carriers, (3)

5 mg/L 0.1 g/L

10 mg/L 20 ppm 10 mg/L 10 mg/L 35 ppm 70 mg/L

10 mg/L 20 mg/L

acetaminophen

amoxicillin

amoxicillin

ampicillin

ceftriaxone sodium

ciprofloxacin

ciprofloxacin

ciprofloxacin

ciprofloxacin

esomeprazole

levofloxacin

naproxen

ofloxacin

ranitidine

tetracycline

of MoS2 /TiO2

CdS-MoS2 -coated ZnO

MoS2 @Znx Cd1–x S

C3 N4 –MoS2

CdSe QDs@MoS2

MoS2 /BiOBr

MoS2 /CoTiO3

CoS2 /MoS2 /rGO

pomelo-peel-biochar-decorated MoS2

In2 O3 /MoS2 /Fe3 O4

MoS2 /C

CeO2 –ZrO2 @MoS2

CdS/MoS2 /ZnO

Ti3 C2 /MoS2

MoS2 /BiOBr/carbon fibers

10 ppm

10 mg/L

20 mg/mL

40 ppm

not mentioned

20 ppm

2,4,5-trichlorophenoxyacetic acid (2,4-D) pesticide

CuS@ZnO

1 g/L

20 ppm

doxycycline

reactive red 141 (RR141) azo

ofloxacin

ibuprofen,

AgInS2 –TiO2

ZnO-CdS

Concentration 20 ppm

Pharmaceuticals

ceftriaxone

Photocatalyst

CuS @ ZnO

0.15 mg

20 mg

0.1 g

0.5 g/L

10 mg

0.7 g/L

20 mg

30 mg

50 mg

20 mg

12 mg

–

40 mg

–

120

60

90

40

180

50

90

75

90

300

180

120

300

60

25

20

0.2 g.L−1 25 ppm

180

240

0.10 g

10 mg L− 1

Time (min) 90

Sample dosage 0.2 gL−1

92.4%

88.4%

89.0%

96.0%

86.9%

92.9%

92.0%

94.0%

91.8%

87.0%

85.5%

74.6%

38.3%

94.0%

40%

100

95

96

90

100

Refs.

[112]

[111]

[110]

[109]

[108]

[107]

[106]

[105]

[81]

[104]

[103]

[102]

[101]

[100]

[99]

[41]

[98]

[97]

[41]

(continued)

Efficiency

Table 4 List of TMCs used as photocatalyst for degradation of pharmaceutical and organic molecules, their concentration, dosage, time, and removal efficiency

Transition Metal Chalcogenides-Based Nanocomposite … 263

0.2 g/L 20 mg/L

tetracycline

tetracycline

4-chlorophenol

MoS2 @zeolite

MnFe2 O4 /MoS2

Cu2+

–

doxycycline

norfloxacin

2- nitrophenol tetracycline

tetracycline

tetracycline

AgInS2–TiO2

CdS/Au/TiO2

AgInS2 /SnIn4 S8

u2 WS4 /g-C3 N4

Cu2 WS4 /NiTiO3 –

–

–

–

30 ppm

paracetamol

ZnIn2 S4 /MoO3

–

Lomefloxacin (LOM)

Cu2 WS4

50 ppm

20 mg/L

tetracycline

doped ZnS

Concentration

Pharmaceuticals

Photocatalyst

CoS2 /MoS2 /rGO

Table 4 (continued)

–

–

–

–

–

0.2 g

20 mg

0.1 g

20 mg

10 mg

–

Sample dosage

60 min

100 min

–

60 min

180 min

80 min

120 min

30

60 min

180

10

Time (min)

89%

798%

66%

65%

95%

98%

60%

91.5%

80.9%

87.2%

100

Efficiency

[45]

[118]

[117]

[96]

[98]

[70]

[116]

[115]

[114]

[113]

[105]

Refs.

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Table 5 List of TMCs used for remediation (photoreduction and adsorption) of toxic heavy metals, their concentration, dosage, time, and removal efficiency Photocatalyst

Heavy metals

Concentration

Sample dosage

Time

Efficiency

Refs.

Fe3 S4 hollow spheres

Pb2+

600 mg/L

92.1 mg/g

–

100%

[120]

Fe3 S4 Microcrystals

U(IV) Cr(IV)

20.0 mg/L

0.2 g/L

–

423.0 mg/ g 231.3 mg/ g)

[121]

Petal-like MoS2 nanosheets

Hg2+

100 mg/L,

289 mg/g

300 min

93%

[122]

2D MoS2 Nanosheets

Pb2+

5 mg/L

740 mg/g

2 min

99.92%

[123]

MoS2 /ZnS/ZnO

Cr(VI)

–

–

90

98.7%

[124]

MoS2 -In2 S3

hexavalent chromium [Cr(VI)]

–

Not mentioned

30 min

100%

[125]

CuCo2S4 in Z-scheme MoSe2/ BiVO4

Cu, Cd, Pb Cr Zn

2.159 ppm 0.227 ppm 0.257 ppm 0.723 ppm 0.143 ppm

0.5 mg

180 min

100%

[42]

ZnS/SnIn4 S8

Cr(vi)

–

10 mg

120 min

92.3%

[126]

SnS2 /SnO2

Cr(VI)

–

–

40 min

99%

[127]

u2 WS4 /g-C3 N4

Cr(VI)

–

–

120

75%

[118]

Cu2 WS4

Cr(VI)

–

–

150

99%

[128]

Cu2 WS4 /NiTiO3

Cr(VI)

–

–

60

86%

[45]

reduction of the particle size to improve hydrophobicity to prevent photocorrosion, and increased surface area, (4) tuning the layer dimensions to attain suitable band gap with optimum diffusion length for improved charge carrier concentration. The present chapter summarizes the specific properties and characteristics that make this semiconductor material suitable for applications in wastewater treatment. The different transition metal chalcogenides-based nanocomposite, their mechanism for photocatalytic degradation, the usefulness of chalcogens and their nanomaterials for photocatalysis (water splitting, reduction of carbon dioxide, etc.); as future guidance for researchers working in the field of chalcogen-based photocatalysis with promising future directions.

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3D-Printed Electrochemical (bio)sensors Luiz Ricardo Guterres Silva, Jéssica Santos Stefano, and Bruno Campos Janegitz

Abstract 3D printing technology brought a breakthrough in the way of manufacturing electrochemical sensors, giving rise to a new generation of sensors, the socalled 3D printed electrodes. 3D printing allows fast and simple manufacturing and prototyping, being able to produce in large scale and automatically, with a relatively low cost from sustainable materials. The use of 3D printing technology has gained more and more space and prominence in the manufacture of electrochemical sensors and biosensors, since it combines the advantages of 3D printing together with the quality of the sensors. In addition, 3D printing allows the production of complete electrochemical platforms, such as the most varied designs, being limited only by the operator’s creativity. 3D printed electrochemical sensors are applied for the detection of the most varied analytes, such as metals, drugs, biomarkers, neurotransmitters, biological molecules, and viruses. Therefore, this chapter will tour various aspects of 3D printed electrochemical (bio)sensors, focusing on sensors produced by the 3D printing technique called fused deposition modeling, the most used sensor production technique worldwide. Thus, the chapter will cover two important aspects, sensors produced with commercial conductive filaments, necessary surface treatments and the various applications such as sensors and biosensors. It will also address the sensors produced by new lab-made conductive filaments and their applications. In addition, future perspectives and new directions on the universe of 3D printed sensors are also present at the end of the chapter. Keywords 3D printing technology · Additive manufacturing · Fused deposition modeling · 3D printed electrodes · (bio)sensors

L. R. G. Silva · J. S. Stefano · B. C. Janegitz (B) Laboratory of Sensors, Nanomedicine, and Nanostructured Materials (LSNano), Federal University of São Carlos, Araras, São Paulo 13600-970, Brazil e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_12

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1 Introduction Three-dimensional (3D) printing technology has revolutionized several areas of society by implementing a new concept of how to produce objects, devices, and tools with the most varied designs, sizes and complexity, from the simplest to the most complex [1–3]. 3D printing technology has revolutionized the world of prototyping parts and objects due to its unique advantages of multifaceted structures with applications in the simplest and most complex areas such as aerospace, electronics, food, medicine and analytical chemistry, mainly in the production of electrochemical (bio)sensors [3, 4]. Electrochemical (bio)sensors are a powerful analytical tool that has unique advantages, such as high sensitivity, selectivity, stability, the possibility of miniaturization and portability, non-specialized operator, and low consumption of reagents and samples [5, 6]. Thus, due to their characteristics, electrochemical (bio)sensors are widely studied and researched and can be applied in the most different fields of analysis, such as environmental, food, industrial, and clinical, among others [6–8]. However, the traditional manufacturing methods of these sensors have several drawbacks, such as the need for specific and specialized equipment, highly trained operators, and excessive consumption of reagents and materials [9, 10]. However, with the advent of additive manufacturing technology (3D printing), this scenario has changed dramatically [1, 11]. The manufacturing process of electrochemical sensors has been revolutionized, allowing the integration of the unique qualities of 3D printing in the production of electrochemical sensors [3, 4]. Among these unique characteristics, one can mention rapid prototyping, large-scale production, autonomous manufacturing, and the ability to produce sensors with the most varied designs, allowing the operator to adapt the sensor format for the desired applicability [12, 13]. In 3D printing, there are several methods for printing the desired material, but there are three methods that stand out and are mostly used for the manufacture of devices and analytical apparatus. These methods are stereolithography (SLA), selective laser melting (SLM), and fused deposition modeling (FDM) [3, 14]. The SLA method is based on a photopolymerizable resin formulation controlled by optical alignment for layer-by-layer deposition until the desired object is formed. SLA is highly widespread in the manufacture of analytical apparatus with the most varied designs. However, this technology has not yet been explored for the manufacture of electrochemical sensors. SLM uses metallic powder to manufacture the desired object. However, this method requires special careful handling and has a high cost compared to SLA and FDM [3]. The manufacture of electrochemical sensors by the 3D printing method by SLM is reported in the literature as opposed to SLA, mainly in the production of metallic electrodes. Among the different works reported in the literature involving the production of electrochemical sensors by 3D printing based on the SLM method, a work by Professor Pumera’s group was a pioneer in demonstrating the application of this technology. In the article by Ambrosi et al. [15], the authors developed a 3D-printed stainless-steel electrode and demonstrated its potential as an electrode through a series of electrochemical tests. However, according to Stefano et al. [4],

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the method most used in the manufacture of electrochemical sensors is the FDM printing method due to its simplicity and highly reduced costs. This chapter will take a tour through the application of 3D printing technology based on the FDM technique for the production of electrochemical sensors and biosensors. Thus, this chapter will highlight relevant topics and works that can bring the reader an insight into the universe of 3D printed electrochemical (bio)sensors and their future perspectives.

2 Fused Deposition Modeling (FDM) The FDM 3D printing method is capable of producing complex three-dimensional structures and has been widely used by researchers for the development of new sensors or biosensors [3, 14]. In FDM printing, a thermoplastic polymer, usually acrylonitrile butadiene styrene (ABS) or polylactic acid (PLA), is heated until it reaches a semi-molten state and is then expelled through a fine metal nozzle and deposited layer-by-layer on a substrate. The nozzle moves in different directions of the plane (x, y, and z), in some cases the table moves in the z-axis, thus forming the thermoplastic polymer print layers while a constant pressure is applied [14, 16]. For a better conception, Fig. 1 shows an illustrative scheme of how 3D printing works using the FDM technique. Once the printing material (filament) is deposited, the solidification of the material occurs creating solid printing layers, and at the end of the process, the structure with the desired shape is obtained. This process is completely automated, which minimizes possible errors in the manufacturing process, and for printing, only the design of the structure to be printed is needed, which is developed by specialized software [14, 16].

Fig. 1 Schematic representation of a typical FDM setup. Reprinted from [16] with permission

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Regarding electroanalytical systems, these can still be considered a challenge for 3D printing since obtaining conductive materials is essential. However, the use of FDM for the production of 3D objects allows the extrusion of composite filaments, for example, attributing electrical conductivity to the printed material through the use of conductive materials for printing [14, 17]. In this context, there has been an expansion in the applicability of 3D printing in analytical chemistry, enabling the printing of conductive substrates, such as electrochemical (bio)sensors. The rapid rise of the FDM technique for the production of 3D-printed electrochemical sensors was mainly due to two commercial conductive filaments widely available on the market, one based on graphene and the other based on carbon black [3, 14, 18]. However, despite these commercial conductive filaments having contributed enormously to the expansion of 3D-printed electrochemical sensors, they still have some operational problems. As commercially available conductive filaments have a low % by weight of conductive material, one of the challenges in using 3D printing for the manufacture of sensors is to ensure that the electroactive material is exposed on their surface, so that they can have excellent performances on front of the analysis of the analytes of interest [18–20]. In this sense, for 3D printed electrochemical sensors to be used as analytical platforms of excellence, it is commonly necessary to apply surface pre-treatments to remove excess non-conductive material (commonly PLA) and thus expose as much of the electroactive material as possible [14, 19, 21].

2.1 Surface Pre-treatments of 3D Printed Electrochemical Sensors Different strategies are found in the literature, approaching techniques that vary from a simple mechanical polishing together with electrochemical activation in immersion in a basic medium, immersion in a solvent, or simply treatment with laser, to more elaborate techniques such as digestion of the polymeric matrix using enzymes [19, 22–24]. Therefore, this chapter will highlight two different works that present simple and efficient ways of carrying out this pre-treatment step. Ritcher et al. [21] developed a fully 3D-printed electrochemical platform for dopamine detection. The electrochemical sensors employed were manufactured using conductive filaments based on carbon black and PLA (CB/PLA). For the use of CB/PLA electrodes, it was polished in the presence of ultrapure water until the surface had a homogeneous characteristic. Thus, an electrochemical treatment was subsequently carried out, in which in the presence of a basic solution (NaOH 0.5 mol L−1 ) a constant potential of + 1.4 V was applied and followed by the application of − 1.0 V, both for 200 s. Figure 2 presents some results obtained about the electrochemical treatment carried out, as well as the design of the electrochemical platform developed. According to the authors, after the electrochemical treatment, the sensor showed significant improvements in the analytical response. This can be mainly confirmed in the cyclic voltammograms performed before and after the

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Fig. 2 Schematic diagrams of the AM electrochemical platform and the 3D-printing working electrode preparation procedure: a-b electrochemical platform; b; c polishing procedure; d rectangular printed. SEM images of the polishing 3D-printed working electrode e before and f after electrochemical activation. g Cyclic voltammograms obtained (black line) before and (red line) after surface treatment, a ascorbic acid, b uric acid, c dopamine, and d ferri/ferrocyanide redox couple. Reprinted from [21] with permission. Copyright (2019), American Chemical Society

electrochemical treatment in the presence of different analytes. Thus, the authors successfully demonstrated the manufacture of a fully 3D printed electrochemical platform and a simple and effective treatment. Following the line of surface treatment of 3D printed electrochemical sensors. Kalinke et al. [19] described the application and comparison of a series of surface treatments on a sensor based on graphene and PLA (PLA-G) from commercial conductive filaments. The performance of the sensors was evaluated after mechanical polishing, electrochemical and chemical treatments, and a combination thereof. Subsequently, the chosen treatment was used for dopamine detection. According to the authors, the surface treatment that combines immersion of the sensor in a NaOH solution for 30 min followed by an electrochemical treatment in the presence of phosphate buffer showed the best performance. Thus, it is well known that adapting and applying electrochemical treatments that aim to improve the electroanalytical properties of sensors for detecting analytes of interest is of paramount importance for the use of 3D-printed electrochemical sensors, mainly those produced from commercial conductive filaments.

3 3D Printed (bio)sensors Manufactured with Commercial Conductive Filaments The application of 3D printed electrochemical sensors produced with commercial conductive filaments is widespread in various areas of society, such as in the analysis of metals, drugs, neurotransmitters, biomarkers, viruses, peroxide hydrogen and important biological compounds. In this context, Rocha et al. [25] manufactured

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a 3D-printed electrochemical platform with sensors based on commercial conductive filament (CB/PLA) for the simultaneous detection of Cd2+ and Pb2+ in urine and saliva. The sensors were treated using the previously described treatment of applying a constant potential (+1.4 and −1.0 V) by 200 for both in the presence of a NaoH solution. Figure 3 presents an overview of the electrochemical platform and analysis method. The use of 3D printed electrochemical sensors for drug detection is vast. Among the most diverse works, João et al. [26] reported the detection of naproxen using batch injection analysis fully 3D printed with sensors based on CB/PLA. However, two 3D printed sensors were compared, one produced with a conventional 3D printer and the other manufactured with a 3D pen. Both 3D printed sensors showed similar results, demonstrating that in addition to the conventional printer, 3D pens are also usable for the manufacture of electrochemical sensors. In 2020, an interesting work was reported regarding the use of 3D printing technology with a conductive filament based on graphene and PLA (PLA-G) for the production of a sensor and a biosensor. In view of this, Silva et al. [27] produced a complete 3D electrochemical platform for the sensing and biosensing of serotonin and catechol, respectively. To this end, the authors reported that the sensors were subjected to different chemical treatments, initially by immersion in DMF to remove excess PLA on the surface. And subsequently, a treatment with nitric acid and sodium borohydride was carried out to generate reduced graphene oxide on the surface of 3D printed electrochemical sensors. Figure 4 presents a conception of the fabricated electrochemical platform and the treatment steps employed. Non-enzymatic determinations are also part of the detection hall of 3D-printed electrochemical sensors. In this context, Katic et al. [28] manufactured a 3D-printed

Fig. 3 Schematic diagram of the 3D-printed cell. Reprinted from [25] with permission

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Fig. 4 a Scheme of the procedure of the G-PLA 3D-printed electrodes. (A) the sequence of chemical treatments, (B) DMF treated; (C) HNO3 treated and sodium borohydride treated and (E) assembled 3D-printed electrochemical cell. b Scheme of the steps involved in the production of the biosensor, (A) mixture of DHP and tyrosinase, (B) drop-casting of the modifiers on the working electrode surface, (C) forming the film and (D) analytical signal. Reprinted from [27] with permission

sensor based on PLA-G conductive filaments and later modified it with Prussian blue particles for non-enzymatic detection of hydrogen peroxide in mouthwash and milk. Continuing in the line of non-enzymatic sensors, Katseli et al. [29] developed an interesting wearable electrochemical sensor (ring) fully 3D printed for nonenzymatic detection of glucose in sweat. In addition, every electrochemical device was designed to perform the analysis together with a smartphone, linking the quality of wearable sensors, 3D printing and portability of the analysis equipment. According to the authors, the set of 3 sensors was completely printed from commercial conductive filaments and the working electrode was modified with gold nanoparticles. To better illustrate to the reader, Fig. 5 presents a real conception of the 3D printed wearable electrochemical device. 3D -printed electrochemical biosensors are gaining highlights in recent years, although they are still little reported in the literature, they have great potential. One of the first works reporting a 3D printed immunosensor is presented by Martins et al. [30] In this work, the authors produced a 3D printed electrode using Carbon Black’s commercial conductive filaments to produce a Hantavirus Disease immunosensor to detect. According to the authors, the 3D immunosensor presented good selectivity and performance against human serum analysis. However, due to the pandemic of the SARS-CoV-2 virus, there was a large increase in the reported 3D-printed electrochemical biosensors. In this context, Ambrosi and Pumera [31], reported the manufacture of a 3D immunosensor modified with gold particles to detect the SARS-CoV-2 virus. Figure 6 presents an illustration of the entire process of manufacturing and production of immunosensor. Still in the context of 3D-printed biosensors for detecting the virus that causes the disease COVID-19, Silva et al. [32] developed a fully 3D-printed electrochemical genosensor for detecting the cDNA of the SARS-CoV-2 virus. For this, a set of three electrodes was obtained with the help of conductive filaments based on graphene and PLA. Later, a surface treatment was carried out by immersion in DMF and then

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Fig. 5 a Schematic illustration of the fabrication steps of the 3D-printed e-ring. b images of the 3D-printed e-ring, flexibility, and connection to the miniature potentiostat and the smartphone. Reprinted from [29] with permission. Copyright (2021), American Chemical Society

Fig. 6 a Illustration of the 3D-printed electrochemical immunosensor and b Indirect competitive assay. Reprinted from [31] with permission

modified with gold particles and capture cDNA for detection of the target cDNA of the virus. Figure 7 presents an illustrative scheme of the production process of the 3D printed genosensor. According to the authors, the 3D printed genosensor showed good selectivity and performance in the analysis of human saliva and serum. 3D printing technology together with commercial conductive filaments have revolutionized the field of electrochemical sensor production, development of entire platforms and complete sets of (bio)sensors for the most diverse purposes. In addition, the

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Fig. 7 Schematic illustration of the production of the biosensor and hybridization step. The production of the biosensor consists of the printing step, chemical treatment of the surface, modification with Au and, modification with the capture sequence. The hybridization step is carried out for 30 min. Reprinted from [32] with permission

use of this technology combined with human creativity has opened up a range of new options to be explored, from robust platforms to wearable sensors, often requiring tiny amounts of reagents and samples to carry out the analyses. However, although commercial conductive filaments play a very important role in this branch of analytical chemistry, they still have some disadvantages, such as the need to import and the low variety of available conductive material. In view of this, in recent years a new line of research and production has emerged in order to overcome these disadvantages, the production of lab-made conductive filaments.

4 3D Printed (bio)sensors Obtained with Lab-Made Conductive Filaments In 2019, Foster and collaborators observed that 20% (w/w) of graphene in a PLA filament presents sufficient conductivity for the manufacture of anodes for Li batteries in a study varying the amount of graphene from 1 to 40% (w/w) [33]. A year later, Foster and collaborators proposed the fabrication of a conductive filament containing nano graphite. The authors reported the use of this new filament on two different platforms, one for printing macro electrodes and the other for honeycomb structures for the simultaneous determination of cadmium and lead [34]. Following this same concept, Stefano et al. [18] produced a conductive filament based on graphite and

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PLA, containing 40% by mass of conductive material. In addition, the electrochemical sensors produced were ready-to-use, did not require surface treatment with any type of reagent or electrochemical technique, just simple mechanical polishing was enough to use the sensors. In addition, as a proof of concept, the authors applied the developed sensor to detect two distinct biomarkers (uric acid and dopamine) and developed an immunosensor from the 3D sensors to detect the Spike S1 protein of the SARS-CoV-2 virus. Figure 8 presents an illustrative scheme of the production steps of conductive filaments and 3D printed sensors. In this context of sensor based on conductive filament containing graphite, Silva et al. [35] demonstrated for the first time the electrochemical detection of pesticides in a series of food samples. For this, a sequential analysis of paraquat and carbendazim was performed with 3D printed sensors from the lab-made filament. Thus, the authors report that the sensor showed good selectivity and excellent performance in the analysis of the proposed samples. Also designed, the electrochemical sensor printed in 3D from the lab-made conductive filament was also given to carry out the control of coffee safety. Mutz et al. [36] applied 3D sensors together with chemometric techniques to classify highly consumed Brazilian coffees according to their origins to detect fraud. Furthermore, the sensor also can monitor if the red coffee has

Fig. 8 Representative scheme for the production of the improved conductive filaments. a incorporation of graphite powder on PLA; b recrystallization of the composite; c filtration; d drying; e cut; f composite extrusion step and g 3D printing of the electrochemical sensor. Reprinted with permission from [18]

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been adulterated with other coffees of lower quality and price. Likewise, the authors successfully demonstrated an alternative way of using two sensors, in which from a simple cyclic voltammetry it was possible to identify the origin and whether the coffee was adulterated or not. Continuing the production of both conductive filaments and 3D printed sensors based on them. Stefano et al. [20] using the same procedure described in the previous work, produced a new conductive filament. However, this new filament contained carbon black (28.5% mass). To demonstrate the application of the filaments, a set of 3 electrodes were printed and used in the analysis of hydroquinone and catechol. In addition, the authors demonstrated that the sensor is subject to surface modification, after modification with Prussian blue particles, it was used for non-enzymatic detection of hydrogen peroxide in milk. Different types of filament production have gained prominence, mainly using carbon black as a conductive material, as it is a highly accessible material with an extremely low cost when compared to other carbon materials. In this context, Singley et al. [37] presented an interesting way of producing conductive filaments by recycling coffee capsules as a polymer material, since they are manufactured from polylactic acid (PLA), together with poly(ethylene succinate) and carbon black. The conductive filaments produced were used to manufacture electrochemical sensors and later used in the detection of caffeine. The production of electrochemical (bio)sensors, whether with commercial or labmade conductive filaments, represents a new generation of sensors, with free and creative designs. In addition, which are possible to be built on a large scale and in an automated way, eliminating tedious preparation steps. In addition, the range of application of 3D printed sensors is wide and varied, being applied from the simplest analytes such as metals, to more complex matrices such as viruses. Thus, looking for new devices with new designs and varied forms of applications, such as wearables, makes this new generation of sensors and electrochemical platforms completely 3D printed even more fascinating.

5 Conclusion and Future Perspectives 3D-printed electrochemical sensors and biosensors have had a rapid rise within analytical chemistry, revolutionizing the way of producing analytical devices. This rapid ascension is due to the efforts and creativity of researchers who contribute more and more every day to the development of 3D-printed electrochemical devices for the most diverse purposes, whether in the sensing of analytes of interest or the biosensing of more complex biological species. The production of 3D-printed (bio)sensors still has much to be explored, whether new treatments that facilitate the use of commercial conductive filaments, new designs, and applications of sensors, or the production of conductive filaments aimed at developing sensors with specific and selective characteristics. In addition, there is still great potential to be explored regarding 3D printed biosensors, since it is possible to develop complete devices from 3D printing technology and apply them as point-of-care for clinical analysis. Given this, seeking new

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materials, new surface treatments, and designs for the application of sensors is of paramount importance and allows extrapolation from the laboratory to the real and commercial application. Acknowledgements The authors are grateful to the Brazilian agencies FAPESP (2017/21097-3; and 2022/06145-0), CNPq (301796/2022-0; 380632/2023-3) for the financial support.

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Plasmon–Based Metal-Oxides Nanostructures for Biomedical Applications Gajendra Kumar Inwati, Promod Kumar, F. Goutaland, Pratibha Sharma, and Hendrik C. Swart

Abstract In recent years, increasing resistance of microbes to antibiotics has caused major health problems. Researchers from various fields of science and technology have shown that by combining modern nanotechnologies and smart materials associated with the noble metals can be explored for developing the potential antibiotics. According to research findings, metal and semiconductor nanostructures are one type of hybrid material that is being researched for antibacterial capabilities. Metals with antimicrobial properties, such as gold (Au), silver (Ag), zinc (Zn), copper (Cu), and titanium (Ti), each with unique properties, have been identified and exploited for decades. In this regard, the enhanced catalytic and physico-chemical properties of noble metals (especially Ag and Au) based semiconducting nanostructures have attracted scientific attention in recent years. The surface functionalization of Au and Ag nanosystems, together with their plasmonic band and quantum confinement effects, has numerous biomedical, optoelectronic, and environmental applications. Considering these considerations, this chapter presents plasmonic-based semiconducting nanosystems in terms of their fabrication methods, plasmonic properties, and biological applications. The plasmonic properties of Ag and Au NPs, including their customizable shape, size, and surface modification, are significant assets for multiple uses. Concurrently, the focus will be on the optical, structural, and surface properties of plasmonic-based semiconducting nanosystems, and their biological implications.

G. K. Inwati · P. Kumar (B) · H. C. Swart Department of Physics, University of the Free State, Bloemfontein 9300, Republic of South Africa e-mail: [email protected] H. C. Swart e-mail: [email protected] F. Goutaland Institut d’Optique Graduate School, Laboratoire Hubert Curien, UMR 5516, Université Jean Monnet Saint-Etienne, CNRS, 42023 Saint-Etienne, France P. Sharma School of Chemical Sciences, Devi Ahilya University, Indore 452001, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_13

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Keywords Ag/Au nanostructures · Plasmonics · Defects · Fabrications · Biomedical aspects

1 Introduction Inorganic metal nanostructures of Au and Ag with size and geometry-tuned physicochemical attributes have gained much scientific and industrial interest because of their inter and cross-disciplinary applications in biology, semiconductors, photonics, and biochemistry [1–8]. Owing to plasmonic and quantum confinement-driven shape as well as size-dependent material properties, the robust and varying ratios in the bimetallic semiconducting nanostructures have permitted the accomplishing of the most critical and challenging applications. Due to their relatively low chemical activities and ease of fabrication on the laboratory scale, the plasmonic nanomaterials (NMs), primarily Au and Ag nanoparticles (NPs) have been the basis of almost every catalytic, biological, and energy conversion materials. While Ag NPs have significant antibacterial action and cytotoxicity with well-illustrated mechanism of zerovalent state prevalence, the relatively more stable Au NPs are more suited as biosensing elements and diagnostic probes or biomarkers [9, 10]. These morphological and surface-tuned characteristics of Au NPs are the basis of their surface plasmon resonance (SPR) attributes, manifesting usefulness for designing the medical impacts, visualization and analysis of biological structures, functions, and processes using various imaging techniques. For Ag/Au combined NPs, the physical and chemical reactivity (particularly their SPR spectra) can be logically varied on the basis of designed sizes, fabricated shapes, and functionalized assemblies. The researchers suggest that physical, chemical, and biological approaches can be used to produce Ag and Au-based nanostructures. Regardless of reciprocal limits and benefits, the chemical method is easily achievable on a laboratory scale. These reducing chemicals increase the stability of the manufactured NPs and protect them from agglomeration. Chemical methods of preparing Au and Ag NPs also consume less external energy and generate much lower waste, being the bottom-up approach. Besides, one always has the advantage of controlled synthesis procedure by stopping it as per the size suitability. One special significance of these nanostructures is their engineered surface which can be tailored for desired interactions based on the knowledge of the working environment. While using chemical reducing agents, one must be careful to avoid the chances of random and haphazard chemical combination which may result in aggregation-driven toxicity [10]. Phytochemicals, herein present a safer and reliable substitute as they have fewer synthetic components and largely interact with non-covalent philicphobic force gradients [11]. Additionally, Phytochemicals contain potent pharmacological compounds that can function as agents for capping with enhanced medicinal potencies, simultaneously acting as reducing agents. A number of literature attempts using Au NMs in biomedicine used the Au nanostructures photoluminescence exclusively for targeted bio-imaging. For example,

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thermal treatment dramatically improved the near-infrared luminescence of mercaptosuccinic acid and tiopronin-protected Au NPs, due to the transient non-luminescent molecules being changed into persistent luminous assemblies with restricted sizes [12, 13]. Likewise, PEGylation of Au NPs facilitates their enhanced cytoplasmic permeability, allowing them to penetrate inside the cells and nuclei in singlecell imaging [14]. Furthermore, when the diameter of the metallic core shrinks, the proportion of surface-to-core metal atoms increases, resulting in enhanced quantum yield. All these features, like the nanoclusters’ small sizes and strong emission in the near-infrared range, voiced their utility as diagnostic reagents in fluorescent bioimaging. Besides, a complex chemical composition on the surface of metallic nanoclusters offers uncomplicated techniques for attaching particular probe molecules onto them. For more than two decades, researchers have been investigating the use of nanostructures for pharmaceutical delivery systems, leading to the creation of new pharmaceutical formulations having enhanced health potential and physico-chemical properties [2, 3, 5, 15]. Varieties of semiconducting nanocomposites and associated compounds have become quite popular because of their possible antibacterial properties. Antimicrobial impacts were found in metal-based semiconducting nanosystem like Ag2 O, CuO, ZnO, MgO, CdO, TiO2, etc. These modified semiconducting plasmon-based nanoparticles suppressed various bacterial species in vitro system, observed in scientific research and findings. Because of the substantially higher ratio of surface area to volume, it is quite possible that by shrinking the size of Ag NPs to the sub-nanometer range, their antibacterial therapeutic essence can be substantially enhanced. Furthermore, these Ag NPs also provide added benefits such as easy post-functionalization, high stability, and versatile photoluminescence. It is pertinent to note here that the conclusive main constituents serve as the implicit sources for establishing an interconnection between the anti-microbial actions of Ag nanostructures and their frameworks. In a significant attempt, Liu et al. [16] reported the Ag-NP-supported hydrogel for significantly better antibacterial activity against both gram-negative and gram-positive bacteria. Thus, the hydrogel-driven Ag NPs exhibited stringent bactericidal activities rather than pure Ag nanostructures, owing to their Ag species-specific controllable release capabilities. In another attempt at eminence, Yuan et al. [17] fabricated variety of water-soluble thiolate Ag NPs with strong luminescence and tunable emissions, having better antibacterial activities against the multidrug-resistant Pseudomonas aeruginosa via high-level secretion of intracellular ROS. The investigators revealed that differing charge configurations can have a major impact on antimicrobial capabilities. For the gram-negative (P. aeruginosa and E. coli) and gram-positive (B. subtilis and S. aureus) bacteria, antimicrobial testing revealed Ag+ enriched NPs with stronger antibacterial performance than pure Ag NPs [18]. This chapter, thereby intends to shed light on the fundamental overview of the Ag- and Au-based nanostructures including their preparation approaches, structural characteristics, and biomedical uses. The information compiled herein could advance the cautionary controls for using Au and Ag nanostructures toward making biocompatible implants with a resilient utility and safety.

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2 Synthesis and Designing of Ag- and Au-Based Metal Oxide Nanosystems Biomaterials-inspired nanostructures have become relatively common over the past few years, across the multidisciplinary arenas of healthcare, nutrition, or devices. Various approaches and mechanisms are optimized to design and develop noble metal NMs having controlled morphologies and induced quantum confinement effect [19, 20]. Apart from the robust synthesis mechanisms, sophisticated characterization techniques of Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and Fluorescence Enhanced Scanning Electron Microscopy (FE-SEM) have been valuable assets to screen the morphologies and distribution via irradiation with electron beam(s) in distinct chemical environments [21]. Robustly reproducible, natural grade and sustainable NMs could be fabricated using the surface engineering and microfabrication routes. These methods evolve an optimized utility of chemical resources and biomaterials for the feasibility of the “Top Down” synthesis principle. The physical methods are more energy intensive, require sophisticated infrastructure and are complex in implementation and execution, generating larger waste materials. Following paragraphs describe the few synthesis mechanisms for optimized antibacterial and biomedical applications. Several approaches were applied to synthesize the metallic and metal-doped semiconducting nanostructures such as chemical, physical, and biological routes. The structural surfaces and band structures were altered by running these methods under the influence of various capping and reducing agents. Number of literatures are available for the synthesis of such nanomaterials, but in this section, brief accounts of chemical techniques are highlighted instead of covering all possible approaches. The chemical routes were found to be easier, and more cost-effective in terms of controlled morphologies with the higher yield. An overview of some chemical routes are discussed for the synthesis of nanomaterials with their special merits and significances.

2.1 Sol–gel Method Sol–gel technology is a moderate dispersion varied technique for producing costeffective and resilient materials with ordered molecular structures. This approach could be optimized for diverse domains including electronics, optical, space research, and energy. The sol–gel method comprises the repairing of a gel, reducing the need for atomic diffusion for a higher uniformity extent [22]. Subsequent to hydrolysis and condensation, the processed gel is subjected to solid-state calculations, generating the outcome as a mixture of suitable precursors. To make multi-component oxides, the alkoxides are frequently combined under the influence of alcohol. Additives like acetates are many times, better suited rather than alkoxides. The necessary

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requirements for the conduct of the sol–gel approach include water for disintegration, ethanol, threshold extent of alkoxides, and a regulated availability of heat.

2.2 Hydrothermal Method Owing to a close association of plasmonic attributes with the implicit size and morphology of NMs, several economical and facile approaches are in use for the preparation of Ag/Au-based nanostructures. Conventional approaches such as microemulsion and thermal degradation involve a complex procedure and conditioning of a high temperature. By combining the various wet chemical treatments, the formed particulates can be crystallized using a hydrothermal route in a closed container, arising from the melt mixture processing at a high vapor pressure. The hydrothermal approach produces granules with a graded crystal size compared to other methods. Analogous mechanism is the solvothermal regime, wherein dislocation-free specific crystalline grains, making it a dynamic approach for obtaining super solid nanostructures [23, 24].

2.3 Co-precipitation Process This method is the simplest and most efficient approach for generating Ag and Au nanostructures. The crystal formation, on the other hand, can be controlled by reaction kinetics, limiting the particle size. During the manufacturing of NMs, the shape and surface attributes can be optimized to meet the desired requirements. The implicit benefit of this approach is its suitability to generate NMs of homogeneous sizes [25].

2.4 Green Synthesis The chemically tuned production methods for plasmonic nanostructures exhibit several limitations. Of late though, the scientific attention has largely shifted toward using sustainable resources, such as biological organisms (microbes, plant extracts, fruit extracts etc.) for attuning the nanostructural conditioning. The advantage of these materials is their biocompatibility and renewable essence, wherein constituent phytochemicals are the remarkable assets of contributing bioactivities. The plant extracts for NMs formation are accompanied by the ease of synthesis, as it avoids using a separate reducing agent (of the salt precursor) and aggregation protecting agent. The typical infusion from a plant resource decreases metallic ions for a brief duration while simultaneously generating nanosized particles. The time required to form nanostructures is determined by the plant’s species and implicit phytoconstituents [26]. One would wonder about using garlic and guava extract, onion leaves

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paste and processed herbal products (aloevera, neem, and others) as benign reducing agents. The only constraint is relatively less duration of aggregation stability by these materials compared to chemically reduced NMs. For the design, a conceptual diagram demonstrates the production of Au/Ag nanomaterials in a microgel substrate (Fig. 1a). Au nanoparticles were initially produced on the shell layers of the substrates following the grafted copolymerization-based manufacture of the microgel substrates [27]. Polymerized PEI in the shell area, which includes amine groups recognized to have reduced potential to create metallic nanoparticles, makes this production conceivable. Figure 1b also provides a visual representation of this process. It has been demonstrated that Ag ions are further reduced to Ag nanoparticles using Au nanoparticles as a seed. A bimetallic hybrid nanomaterial is produced when the Ag ions are drawn to the larger Au NP. To increase the crystalline nature of the Ag/Au nanostructured materials, more heating was required. As a consequence, the heating of such nanocomposites substantially eliminates the template, exposing the nanomaterials to their natural state. Although, the hybrid Au/Ag nanomaterials could be designed by applying various chemical and physical routes. The experimental parameters and precursors could be well-tuned to fabricate the targeted nanostructures having different morphologies.

3 Ag and Au-Based Metal-Oxides Nanosystems Metal and metal oxide nanoparticles (NPs) are becoming increasingly important for various applications in industries, medicine, agriculture, and biotechnology, due to their ability to meet the growing demands and requirements for electrochemical, photonic, and kinetic stability. While oxides of Au are relatively less common (due to a higher Au stability), those of Ag are in wide use for antimicrobial applications [5, 28]. The traditional process of creating pure metal and metal oxide nanoparticles involves certain chemicals that can be harmful to both humans and other living organisms at various levels of the biological hierarchy. The toxicity threats to living beings in these circumstances, are well resolved via “green synthesis” based on minimized use of toxic chemicals and the optimization of stoichiometries through functionalization in distinct chemical environments. With the continuous success of Au and Ag NMs in cross- and inter-disciplinary biomedical applications, efforts are now intensified to engineer their functional efficacy via selective dopants. Recently, Ag and Au-doped CuO, Al2 O3 , SiO2 , TiO2, and other metal oxides have been reported as significant for diagnostic and sensing applications [1, 24]. By doping metallic ions in the host metal-oxides at various stoichiometries, the functional performance of metal-doped semiconductors could be modified. The multiple dopants induce distinctive crystal defects and shift the absorption bands of host entities, catalyzing the free electron combination in semiconductors. Analogous imperfection extents are the basis of improved physicochemical, refractive, and antibacterial capabilities, via changes in optical, electronic, and chemical properties of the bulk material. Numerous studies discovered Ag, Au, Pt, and anion-doped semiconductor NMs with modified

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Fig. 1 a Schematic diagram on the synthesis of bimetallic nanoparticles from Au/PNIPAm/PEI composite particles, b mechanism on the formation of Au@Ag nanoparticles from Au/PNIPAm/ PEI nanocomposites [27], License (http://creativecommons.org/licenses/by/3.0)

crystalline pattern to improve the intrinsic properties of pure semiconductor crystals [29]. Metal and metal-doped metal oxides, particularly plasmon-doped semiconductor nanostructures, have been widely investigated as bio-implants for correcting bone dislocations, as dental implants, in the design of cardiovascular stents, and multiple others.

3.1 Importance of Surface Plasmon Resonance for Biomedical Uses The attribute of SPR manifests a high absorption in the visible light range when nanocrystals are activated with a lighter noble metal (Au, Ag, or Cu). Noble metal NPs

296 Table 1 Percentage reduction in the number of colony forming units (CFU) for the microbial stains, on being exposed to Ag NPs functionalized dressings [39]

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Microbial strains

% CFU

Staphylococcus aureus

95

Staphylococcus haemolyticus

34

Pseudomonas aeruginosa

58

Klebsiella pneumoniae

68

Escherichia coli

64

Moraxella spp.

55

Staphylococcus aureus

95

absorb photons intensely and can squeeze light into their nanodimensional boundaries due to their plasmonic characteristics [2, 5, 30–36]. The capacity to modify and transmit light at the nanometer resolution in the noble metal NPs has opened the gates for a newly emerging field known as plasmonics. Noble metal nanocrystals have diverse applications in various fields, including surface-enhanced Raman spectroscopy (SERS), high-resolution optical microscopy based on near-field imaging, nanophotonics, biodiagnostics, photonic biosensors, and microelectromechanical systems (MEMS) design. When conductive nanoparticles are exposed to light at wavelengths shorter than the incident wavelength, they undergo a phenomenon called localized surface plasmon resonance. This is different from conventional SPR, whereby greater number of ionic nano states are generated. A range of studies has reported the antibacterial capabilities of Ag, Au, or Pt NPs, which are typical implicit functions of SPR effects [37, 38]. In one significant attempt, Alexander Yu et al. [39] have proposed an SPR-based network, wherein metal-vapor synthesis (MVS) has been used to prepare silver-based dressings and modulate their antibacterial functions. Ag NPs-based dressings have antibacterial properties against both gram-positive and gram-negative bacteria such as Streptococcus aureus, Streptococcus haemolyticus, Pseudomonas aeruginosa, Klebsiella pneumonia, E. coli, and Moraxella spp. has been discovered. Table 1 comprises the microbial strains used in this research. Since the data from regulated groups of various strains differed, a percentage reduction factor was computed to screen the antibacterial potency of AgNPs laced bandages against different pathogens. The changes in colony forming units (CFU) of screened microbes exemplify the significance of nanoscale Ag controlled and sustained toxicity. Chen et al. [40] created Ag/AgCl@plate-WO3 by performing a heterogeneous precipitation of AgCl nanocrystals on WO3 nanoplates. They then used a photoreducing mechanism to produce a lower quantity of Ag in situ species on the AgCl nano objects. Their research finding looked further into the SPR effect to understand the formation of plasmon on Ag NP surfaces attached to meta oxides like WO3 . The combined effect with size and SPR characteristics shows the potential of Ag NPs for catalytic applications.

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3.2 Optimized Optical, Structural, and Surface Studies of Au and Ag Nanostructures for Biomedical Applications The use of Ag and Au-sensitive surface texture in the health sciences dates back to more than five decades, when an old vessel of Ag lost its texture and on being cleaned, its SPR- mediated optical gradation surfaced. Similarly, the wine red appearance of Au at its nanoscale dimensions (contrary to the yellow in bilk state) caught the eyes of billions. The widespread use of Au and Ag formulations for biological applications emerged in the nineteenth century, 1970s. Since then, multiple confirmations of Ag compounds’ antiviral, antibacterial, and anti-inflammatory activities have been reported [10]. The development of antibiotics, typically more potent than antibacterial formulations, significantly reduced attention to Ag medicinal traits. Nevertheless, toward the end of the twentieth century, extensive use of antibiotics had generated manifold flaws and Ag/Au formulations gradually gained reliability for healthcare purpose. The proximity of surface engineering and the 5s1 (for Ag, atomic number 47), and 6s1 (for Au, atomic number 79) confers them versatile functionalization abilities with Au being more stable. The univalent nature of Au and Ag in their native states makes them specially suited for improved physicochemical activities and selectivity among the metallic NMs. Several research findings have been explored especially for these noble metals vis-à-vis biomedical, electronic, and photonic attributes due to their size and shape-tuned SPR. The controlled plasmon-induced light intensity enables a moderation of the dimensional fluctuations of the excited electrons on the surfaces whereby the catalytic activities are selectively controlled. This spectral concept significantly works for the plasmon-based nanosystem including metal-oxides, organic frameworks, and polymeric systems. In an important study, Mohapatra et al. [41] demonstrated the Ag NPs synthesis at room temperature using aqueous Zingiber officinale extract as a green reducing agent. The SPR spectra of Ag structures were examined via optical absorption measurements, giving the red-shift intensities on being irradiated with sunlight (refer to Fig. 2). The biosynthesized Ag NPs displayed an improved antibacterial efficacy for the waterborne bacterium Escherichia coli via involvement of SPR intensity. It is well demonstrated that distinct oxidation states of metallic NPs are responsible for unique work functions (for electronic excitation on being irradiated with external energy (photoelectric effect)), which tune the electronic confirmation of the metal-oxide NPs formation. Thereby, the complete information of formed NPs and their complexity could be gathered with several techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Energy dispersive X-ray spectroscopy (EDX). The XPS is a standardized technique to determine the elemental oxidation phase along with the implicit compositions of the designed nanocomposites. The discrete binding energy patterns help to recognize the pure metallic and the metal oxide species subsequent to their formations. Variations in these binding energies correlate for a screening of characteristic elemental compositions in nanocomposites. In general, the binding energies of Ag (3d5/2 and 3d3/2 ) are 367.73 and 373.71 eV for the Ag2 O metal-oxide states, significantly different from the pure Ag metallic nanostructures (Fig. 3) [39].

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Fig. 2 SPR spectrum of Ag NPs a without irradiations and b-d sunlight-irradiated Ag sample, variation of e SPR intensities, and f FWHM of SPR on sunlight irradiation time . [41]. Reproduced with permission from Ref. [41], copyright Elsevier, 2019

Similarly, the Au and its oxide phases could be identified using XPS techniques, vis-à-vis discrete binding energies for different structures. Other, techniques like Raman spectroscopy are also helpful in characterizing the functionalized states of nano-based materials associated with Au and Ag nanostructures. The oxidation states and the surface changes at nanoscale of the plasmon-based nanoparticles put an significant impact to enhance the antibacterial potentials, for example, Ag-doped

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Fig. 3 Binding energies for the Ag2 O a and Ag NPs b by XPS [39], license (http://creativecomm ons.org/licenses/by/4.0/)

ZnO nanostructures were produced by Swati et al. [42], by using the Moringa oleifera extract as reducing agents. The antimicrobial effect of Ag-ZnO NPs was evaluated against human pathogens such as Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Salmonella typhii, Klebsiella pneumonia, yeasts like Candida albicans, and plant pathogenic fungus, i.e., Fusarium spp., Sclerotinia sclerotiorum, and Rosellinia necatrix. These nano-objects displayed an 17 mm inhibition diameter toward Staphylococcus aureus over other bacteria, while the 18 mm inhibition zone over C. albicans. The growth inhibition of 56.8, 34.78, and 48.9% was observed corresponding to Rosellinia necatrix, Fusarium spp., and Sclerotinia sclerotiorum. The bandgap modifications by the introduction of dopants in the Ag and Au nanostructures are screened using Photoluminescence (PL) spectroscopy and UV– Vis spectroscopy. The Ag-doped ZnO and pure ZnO could be surface engineered to achieve the optimum band gap values of ZnO. The fundamental characterization for observing the bandgap energy changes in designed materials is via optical absorption spectroscopy (Fig. 4). The variations in bandgap energies help to regulate the distribution intensity of charge carriers in semiconductors [43]. The calculation of photonic energy of the plasmonic NMs is made using PL spectroscopy, identifying the populations of created imperfections in semiconducting NMs associated with the plasmon metals. Moreover, a range of plasmon-based hybrid materials was studied for biomedical applications by moderating their band gap energies and electronic bands by the concepts of ionic doping [3, 8, 11, 15, 16, 22, 29, 41, 42]. Typically, diseases like cancer, diabetes, and neurodegenerative diseases are treated with nanomedicine which takes advantage of such biochemical processes. In contrast to monometallic nanostructures, the catalytic operation is improved by the manufacturing Au/Ag core–shell nanosystem, which has potential uses in optical screening detectors, single-molecule sensors, photothermal treatments, and antiviral drugs. The accuracy in application processing is a critical factor for nanomaterials which depends on structural and morphological features. Therefore, the morphological studies using SEM and TEM techniques were applied to the nanoscopic objects

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Fig. 4 a R(%) graphs, b Band gap energies for pure and Ag-ZnO [43], license (https://creativec ommons.org/licenses/by/4.0/)

in order to determine their shape, sizes, and particle distribution patterns which play a significant role in catalytic actions. Villalobos-Noriega and colleagues [44] employed a successive production of core–shell gold-silver bimetallic nanostructures (Au@Ag NPs), having a mean size of 36 ± 11 nm for Au@Ag NPs, 24 ± 4 nm for Au NPs, and 13 ± 3 nm for Ag NPs (Fig. 5a–f). The observed nanoparticles were having a quasi-spherical shape structure for the tuned particles, synthesized by the green approach. About 6 nm thick layer of Ag NPs was coated upon the Au NPs surfaces and such functional biomolecules were used for biomedical targets. The Gompertz model was carried out to analyze the growth kinetics of microorganisms subjected to these plasmonic (Ag/Au) nanostructures. The findings show that the lag phases and rate of growth of Escherichia coli and Candida albicans are affected by Ag NPs and Au@AgNPs which depends on the doses (dose-dependent manner) of the used nanoparticles. Better antibacterial results were observed for Au@AgNPs as compared to Ag NPs due to the combined plasmonic character of the Ag and Au NPs. In order to get an HRTEM image of the shell component (Fig. 6a), the red square area is magnified. Furthermore, to confirm the crystallized shell portion, the analysis of the nanoparticles periphery zone (shown as a discontinued square, (Fig. 6b). The chosen area’s Fast Fourier Transform (FFT) picture was acquired (Fig. 6c). In Fig. 6d, it was achievable to determine the inter-planar lengths of 2.3, 2.0, and 1.4 Å by using Inverse Fast Fourier Transform. The crystallographic planes (111), (200), and (220) of face-centered cubic (fcc) silver could be assigned these distances, correspondingly [44].

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Fig. 5 TEM and histogram plot for Au NPs in a and b, AgNPs in c and d, and Au@Ag NPs in e and f [44], licence, http://creativecommons.org/licenses/by/4.0/

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Fig. 6 HRTEM images of Au@Ag NPs a. High-resolution view from a of core–shell b. FFT view having Miller index c SAED pattern d HR images with interplanar distance [44], licence, http://cre ativecommons.org/licenses/by/4.0/

4 Antimicrobial Actions of Ag and Au-Based Nano-Catalysts The textures wherein different NMs could meet the biomedical requirements include metal-based oxides, sulfides, and organic substances. Metal-based nanoparticles (NMs) primarily consist of metallic nanocrystals like Ag, Au, Cu, Pt, and Pd and their derivatives. Another class of nanocomposites, like TiO2 , ZnO, AgO/Ag2 O, and CuO, are also considered as desired nanocrystals [38, 45, 46] and are a diverse group of NMs. Among these materials, doped metal/metal oxide/metal NMs belong to a different class. The electronic properties, mechanical strength, thermochemical stability, and surface area of these altered materials have been enhanced, and they exhibit efficient optical activity. Various research studies have shown that the presence of doped ions or metals in these materials can reduce the occurrence of electron–hole pair recombination during catalytic processes by regulating the number of defects and imperfections present [35]. Therefore, modified nanostructures or nanosystems are swiftly emerging as effective tools in the biomedical and pharmaceutical sectors. The following paragraphs describe the mechanism for antibacterial and cell-damaging mechanisms of Ag NPs in distinct biochemical environments.

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The combustion approach was used to manufacture metal oxide semiconductors particularly ZnO and TiO2 doped by metallic nanoparticles (Ag, Au, and Pd) by Trilok K. Pathak and co-authors [47]. The effects of Ag, Au, and Pd additions upon morphological, surface, and antimicrobial activities were studied. In the absorbance spectra of various doped materials, the surface plasmonic effect was also detected because of noble metals. The developed compounds demonstrated remarkable antibacterial properties. All ZnO compounds demonstrated strong antibacterial action against the planktonic development of all investigated gram-positive and gram-negative strains of bacteria at certain optimized concentration ratios. All doping semiconductors had excellent anti-biofilm efficacy, with the lower required biofilm eradication concentration level recorded for ZnO doped with Au and Pd versus E. coli and ZnO loaded Ag toward Candida albicans. Therefore, the Au– and Ag-doped semiconducting nanomaterials were implemented to improve the biocatalytic actions in the area of medical sciences and pharmaceutical sectors. Kumar et al. [2] synthesized Au-doped sodium-zinc borate glasses using a combination of melt quenching technique and thermally treated ion exchange method, and studied the effect of thermal treatment on their morphological, surface, and antimicrobial properties. The researchers also investigated the potential antimicrobial applications of the prepared samples by studying their antibacterial and antifungal activities (refer to Fig. 7). The results showed that the antibacterial and antifungal activities of the samples increased as the annealing temperature increased, which was attributed to the significant generation of reactive oxygen species (ROS). The formation of ROS was observed when annealing the Au-doped sodium-zinc borate glass samples at different temperatures, which led to an increase in the size of Au nanoparticles and the concentration of Au3+ and Zn2+ ions (refer to Fig. 8). As a result, these glass samples can potentially be used as medical devices in the pharmaceutical industry. According to Lu et al. [48] a chemical route of Au/AgNPs@van was developed using NaBH4 as a reducing agent and hybrid with the Vancomycin, and the MIC was 60 nmol/mL for Au/Ag nanocrystals that were evaluated on both gram-positive and gram-negative microorganisms. The Ag/AuNPs@Van was reported to be more effective than the only vancomycin, and shows weaker drug resistance activity. However, Amina et al. [49] assessed the microwave synthesis and characterization of Au/Ag nanostructures utilizing plants (root extract). The sizes of Au/Ag nanostructures were between 10–50 nm having spherical geometry which were tested for antibacterial and immunomodulatory uses. According to the findings, A. racemosus root extracts, AgNPs, and AuNPs could all have relatively low levels of proinflammatory cytokines in macrophage cells, whereas Ag-Au alloy NPs may have the same effect on NK92 cells (refer to Fig. 9). The sulfhydryl or thiol units (−SH) on the cell membrane as well as in protein molecules of the cell surface combine with the Ag+ and Au+ ions released by the Ag-Au bimetallic composite particles to generate a stable S-metal bond. It leads to the removal of hydrogen ions by protein chains and lowers tissue porosity, which kills the cell [50]. Yet, by targeting the cellular membrane, such nanostructures increased the generation of ROS radicals. DNA, polypeptide, and lipid damage are results of

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Fig. 7 Growth inhibition at different temperatures: a 350 °C; b Au pristine sample; c 450 °C; d 500 °C; and e 550 °C 2 . Reproduced with permission from Ref. [2], copyright Elsevier, 2022

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Fig. 8 The schematic mechanism of the anti-microbial activity of the Au-doped sodium-zinc borate glass samples2 . Reproduced with permission from Ref. [2], copyright Elsevier, 2022

ROS production. The most important method by which Ag-Au bimetallic composite nanomaterials damaged the electron acceptor food route to induce cell death was discovered to be the suppression of enzymes [51].

5 Mechanism and Enhanced Antimicrobial Activities Nanocarriers are being increasingly used for drug delivery due to their unique nanoscale size, and they are also being incorporated into diagnostic instruments to improve diagnosis and develop new treatment options. The use of gold and silver nanoparticles is particularly common, as they possess properties such as photocatalysis, optics, and electricity. Plasmonic metal nanoparticles like silver, copper, gold,

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Fig. 9 (A) Minimum bactericidal doses (g mL−1 ) of Ag/Au nanoparticle against a P. aeruoginosa and b S. aureus. Results, as the mean SD of triplicate studies, (A) SEM images of without and with Ag-Au alloys [49], license (http://creativecommons.org/licenses/by/4.0/)

and platinum have been developed due to their ability to exhibit localized surface plasmon resonance, which is a resonant electron fluctuation characteristic of nanomaterials [11, 42, 46, 52]. The capacity to safely associate noble metal ions into biological systems has been the basis of accurate medical and biological screening. Major reasons for the remarkable suitability and delivery potential of Au nanocrystals in the development of diagnostic kits and as a drug carrier, has been due to their ease of synthesis, a sensitive perception of physicochemical changes (inspired by shape and size variations) and adhesion to cellular components. Various multifunctional nanoparticles have the ability to act as antibacterial agents by disrupting the permeability of microbial cell walls or plasma membranes. When nanoparticles are introduced, they can breakdown cellular components and interfere with the metabolic pathways of microbial cells. This can lead to the release of plasma and apoptosis, ultimately resulting in the abnormal inhibition of microbial growth. When there are more metal ions present in metal-oxide nanostructures, there tends to be an increase in the presence of interstitial impurities such as oxygen vacancies, as evidenced by analysis techniques like XRD, XPS, and EDX. When these metal-oxide nanoparticles with ionic imperfections are excited from the valence band to the conduction band, it results in the creation of electron–hole pairs [53]. The presence of holes likely causes the separation of H2 O molecules into OH and H+ ions, which releases electrons that combine with hydrated molecular oxygen. The resulting O2 − ions then convert into HO2 − and H2 O2 − anions, including hydrogen

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Scheme 1 Schematic representation for the production of ROS by Ag NPs [54], License (http:// creativecommons.org/licenses/by/4.0/)

peroxide. This H2 O2 can pass through cell membranes, increasing intracellular toxicity by promoting the generation of reactive oxygen species (ROS) [3, 5, 8, 11, 15, 22, 42, 46]. It is worth noting here that the rate of H2 O2 generation varied directly with the pace and intensity of generated electron–hole pairs. It has been noticed that bacteria with ubiquinone mutations, were exposed to cationic metal NPs, which were screened specifically toward an E. coli single-gene deletion library. Ubiquinone, often known as coenzyme Q10, is an electron transport system necessary for aerobic cellular respiration. It is revealed that bacteria exposed to these NPs generate ROS, resulting from intracellular oxidative stress [54] (see scheme 1).

5.1 Biomedical Impacts The unexpected variations in the physicochemical properties of NPs due to quantum confinement mandate a thorough understanding of vulnerable size-driven toxic responses. Despite multiple literature sources voicing such concerns and modellingbased conditional investigations, studies have largely been moderately successful in lessening the toxicity at the physiological scale [3, 5, 8, 11, 15, 22, 25, 29, 42, 46, 52, 54]. Efforts are indeed needed to understand the NMs behavior in different biological environments with respect to dosage, constitution, time-lapse, and geography of the region. These aspects could be indeed crucial for the future application of NPs as disease relieving agents. The most unpredictable element of NPs toxicity is

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emanated by their small sizes due to which these entities hardly exhibit any gravitational effect and are randomly engaged in non-specific interactions. This issue could aggravate oxidative stress via free radical generation. Such health risks of NPs could be resolved by their thorough capping and wholesome evaluation in distinct invitro environments. The exact requirement is to ensure that the capping layer is not affected by pH of the physiological environment other than that of the targeted site, whereby the random and continued involvement of NPs in physiological activities could be minimized [55, 56]. In a crucial attempt, the antibacterial action of Ag NPs was examined by Jo et al. [57], wherein the efficacy of Ag NPs against a variety of diseases, including soil-borne fungus that barely generates spores, was screened. Analysis revealed (20–30) nm nanocrystals as effective toward colonizing the plant tissues, whereby reduced pathogenicity for spore-forming fungal disorders was observed. The findings illustrate the usefulness of Ag NPs over conventional fungicides. In another significant attempt, Mie et al. [58] used the disc diffusion approach to assess the antibacterial activity of 19 nm Ag NPs against eight pathogens. Analysis yet again revealed promising antibacterial efficacy against gram-negative bacteria, conveying the usefulness of Ag nanostructures in medicinal and biological domains. The medicinal and biological activities of Ag NPs have been the outcomes of their surface and dimensional properties [59, 60], wherein ionic state comparable antimicrobial responses were expressed. The Ag NPs are also known for their promising antibacterial effects against drug-resistant microbes. As per the reported studies, the antibacterial actions are the outcomes of bacterial surface disruption. More exclusively, the Ag NPs can interfere with membrane permeability and cause the cell to disintegrate. The Ag+ has also been shown to react with enzymatic disulfide or sulfhydryl groups, disrupting the metabolic pathways and inducing apoptosis. Furthermore, Au NRs with a silver (Ag) structure by Jesús Mauro Adolfo Villalobos-Noriega et al. [44] form Au/Ag hybrid nanostructures with a lower photoacoustic signal. The formed structures were found stable in ambient circumstances. The discharged Ag+ ions exhibited a bactericidal activity analogous to equivalent free Ag+ of AgNO3 , causing > 99.99% damage to the gram-positive and gram-negative methicillin-resistant Staphylococcus aureus and Escherichia coli. The antibacterial activity of Ag2 O against E. coli was demonstrated by Sondi and Salopek-Sondi [61], wherein the mechanism was corroborated by interference of DNA replication which subsequently blocked the G2/M phase of the cell cycle by distorting the DNA structures. The cells were then subjected to oxidative stress, resulting in apoptotic induction. The major factors affecting the activities were homogeneity and stability against precipitation. Surface charge, hydrophilicity, durability, nature of capping moiety, extent of opsonization, and the mode of preparation of NPs were the other crucial riders. Wang et al. [62], described a simple method to create modified nanostructures with rough surfaces by coating magnetic nanoparticles with gold (Au) (see Fig. 10). To do this, they first coated the magnetic nanoparticles with a thin layer (2 nm) of polyethyleneimine using sound waves (ultrasonication). Then, they added negatively charged gold particles, which were attracted to the positively charged magnetic nanoparticles, to create a shell of Au around them. The

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Fig. 10 a Synthetic route for gold shell-coated magnetic nanoparticles and b Schematic illustration of the operating procedures for bacteria detection via a SERS method [62]. Reproduced with permission from Ref., copyright ACS, 2016

Au MNPs that were produced had a consistent and uniform size and shape, and they showed good surface-enhanced Raman scattering (SERS) capabilities and strong magnetic responsiveness. To assess the SERS effectiveness of the created Au MNPs, the researchers employed p-aminothiophenol (PATP) as a test molecule, achieving a recognition edge of 10–9 M. The Au MNPs were created and linked with antibodies that are specific to Staphylococcus aureus (S. aureus) to capture and isolate the bacteria. The Au nanorods (Au NRs) (see Fig. 11) were also created and their lightscattering properties were adjusted to match a specific laser excitation wavelength (785 nm).

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Fig. 11 TEM images of the synthesized AuNRs at different conditions: a 0.2 mL of AgNO3 , b 0.28 mL of AgNO3 , and c 0.3 mL of AgNO3 .The insets are the corresponding optical images. d UV− visible spectra of the synthesized nanoparticles. e Raman intensity of DTNB adsorbed on the three different kinds of AuNRs under the same conditions [62]. Reproduced with permission from Ref., copyright ACS, 2016

These nanorods were then linked with S. aureus antibodies to create a unique marker for the detection and identification of the target bacteria using surfaceenhanced Raman spectroscopy (SERS). A sandwich-structured immunoassay was used to indirectly detect S. aureus using SERS. The detection limit was found to be 10 cells/mL. Additionally, there was a linear relationship between the logarithm of bacteria concentrations ranging from 101 cells/mL to 105 cells/mL and the SERS intensity at the Raman peak of 1331 cm–1 . Violeta Dediu et al. [63] examined the antibacterial potential of ZnO-based nanostructures with different geometries generated using the solvothermal approach and afterward altered using Au nanoparticles via wet chemical treatment. Using the disk diffusion method and tetrazolium/formazan (TTC) tests, the antimicrobial properties of pure ZnO and ZnO/Au nanoparticles versus Escherichia coli and Staphylococcus aureus were examined. According to the findings, the recommended nanostructures had strong antimicrobial action on gram-positive and gram-negative bacteria. Additionally, ZnO nanorods having sizes less than 50 nm outperformed ZnO nanorods with larger dimensions in terms of antimicrobial activities. With the introduction of 0.2% (w/w) Au to ZnO nanorods, the antimicrobial efficacy of E. coli and Staphylococcus aureus was significantly increased.

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6 Future Perspectives and Conclusions This chapter provides a brief overview of Au and Ag-based nanostructures for biomedical applications, focusing on bioimaging and biosensing, antimicrobial applications, tumor targeting, and cancer treatment. The ultrasmall size, splendid photoluminescence, post-functionalized surface characteristics, and superior cytocompatibility of Au and Ag NPs and their hybrid semiconducting nanomaterials illustrate their significant potential for therapeutic applications. Owing to the plasmonics and unconventional physicochemical traits of Au and Ag nanostructures, they face significant hurdles before they can be used for therapeutic purpose. For instance, Ag nanocrystals exhibit good antibacterial effects but their in vivo random interactions pose a problem because they can’t be stored for long periods, such as a few months, without decomposition. In contrast, Au NPs are tough and may even prevail for months but their antimicrobial qualities are not as strong as those of Ag nanocrystals. In the regime of using metallic nanostructures for anti-microbial purpose, enhancing the stability and working states of Ag nanostructures is the next step. For Au NPs, reliable functionalized configurations that enhance the localized toxicity induction should be undertaken. Consequently, nanoscale Au/Ag loaded with metal-oxides offer manifold appealing characteristics not noticed in their bigger NP counterparts. We believe that as medical procedures and nanotechnology advance, more prospects for Au/Ag NMs in the biomedical domain will emerge. Acknowledgements This research is supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation of South Africa (Grant 84415). Authors are thankful to the Department of Physics, University of the Free State, South Africa. Conflicts of Interest/Competing Interests The authors declare that there is no conflict of interest.

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Recent Advances in ZnO-Based Hybrid Nanomaterials as Photoelectrodes for Photoelectrochemical Water Splitting Pulkit Garg, Pamisetty Tharun Sai, and Ankit Tyagi

Abstract Photoelectrochemical (PEC) water splitting (WS) is a viable method for generating renewable hydrogen and resolving global energy and environmental crises. Zinc oxide (ZnO) is an appropriate semiconductor material for photoanodes with a bandgap of 3.37 eV. In recent years, significant progress has been made in developing ZnO-based photoelectrodes for PEC-WS. However, ZnO has some inherent limitations that impede its application for solar water splitting. These drawbacks include limited visible light absorption, rapid charge carrier recombination, and material degradation by photo corrosion. This chapter focuses on the performance of the PEC-WS system by altering the ZnO surface through anionic or cationic material doping, heterojunction creation, and coupling other semiconductors with ZnO, tandem semiconductors or by the synthesis of nanocomposites with ZnO. We also discussed the future potential of ZnO-based photoelectrodes for PEC-WS to encourage additional study in this rapidly developing area. Keywords Photoelectrochemical · Water splitting · Hydrogen synthesis · Heterojunction · Zinc oxide

1 Introduction Emerging global energy crises have led to the invention of clean, renewable, and sustainable energy technologies. The most environmentally friendly and cleanest fuel for the future among all renewable resources is hydrogen (H2 ). It is also a powerful energy carrier, with a maximum energy density of about 140 MJ/kg. The P. Garg · A. Tyagi (B) Department of Chemical Engineering, Indian Institute of Technology Jammu, Jammu 181221, India e-mail: [email protected] P. T. Sai Department of Chemical Engineering, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra 440010, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_14

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straightforward and practical method of producing H2 is photoelectrochemical water splitting (PEC-WS). PEC-WS is believed to effectively transform renewable solar power into chemical energy [1]. The PEC-WS system consists of three electrodes, i.e., a reference electrode, a counter electrode and working electrode, an electrolyte, and a light source. Numerous studies have been done on the PEC-WS system but selecting efficient photoactive electrode materials has received the most attention. In 1972, Fujishima and Honda made the first public demonstration of the environmentally friendly PEC-WS method using titanium dioxide (TiO2 ) as a photoanode material. However, TiO2 has a high rate of charge carriers recombination and a large bandgap (~3.2 eV) which only accounts for UV light absorption, which is only 4% of the total solar spectrum. Hence, scientists began focusing more on other photoactive materials. The selection of photoactive materials as a working electrode (photoanode or photocathode) was based on several factors, such as corrosion-resistant nature, appropriate band structure, and efficient light absorption [2]. Numerous investigations on various photoactive materials, such as ZnO [3–5], Fe2 O3 [6–8], WO3 [9], BiVO4 [10], and TiO2 [11], have been conducted. Among these metal oxides, ZnO has undergone extensive research and proved to be an effective photoelectrode material due to its low cost, high electrical conductivity, and high free exciton binding energy (60 meV at ambient conditions). Moreover, it has a superior electron transport rate (transfer of an electron from photoanode material to the surface of externally incorporated material); for instance, ZnO has an electron transport rate of 200–300 cm2 /V s, while for TiO2 it is 0.1–4 cm2 /V s [12]. However, due to the large bandgap of ZnO (3.37 eV), the charge generation occurs in ultraviolet (UV) light, which typically makes up 4% of the solar band. Therefore, for proper utilization of solar energy, numerous attempts have been made over the years to properly activate ZnO and bring its bandgap under the visible light range. Apart from bandgap, other factors also play a crucial role in improving the photoelectrode efficiency, such as catalytic reaction on the surface of ZnO-based electrodes, electron–hole separation, and visible light absorption. It is widely recognized that efficient and limited doping with different materials can change the optoelectronic properties of ZnO photoelectrodes [5]. Therefore various modifications have also been done for structural regulation [13, 14], element doping [15, 16], and heterojunctions [17, 18] to enhance the performance of photoelectrode for the PEC-WS system. This chapter first briefly discusses the WS mechanism and then summarizes the various modifications done on the surface of ZnO. Doping is one of the primary methods for modifying a material’s optical and electrical properties. Different metallic and nonmetallic ions can be added to the structures, changing the behavior of the material/electrolyte interface and the density of charge carriers (electrons and holes) [19]. Another method is heterojunction formation, where the heterostructure’s energy-band structures prevent the recombination of photogenerated carriers and extend the lifespan of electron–hole pairs [20]. Other techniques, like ZnO-coupled semiconductors, tandem semiconductors, and ZnO-based nanocomposites, are also discussed. The emphasis has been given to improving optoelectronic properties by various modifications.

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2 Water Splitting Mechanism In PEC-WS, the first step begins with solar energy illumination of ZnO-based photoelectrodes. Suppose the incident energy is greater than that of the bandgap energy of ZnO. In that case, the electron jumps from the valence band (VB) to the conduction band (CB), and the photogenerated holes will be left in the valence band, creating an exciton (electron–hole pair). Then the photogenerated electrons migrate from the bulk to the photoelectrode’s surface and then move towards the counter electrode to take part in hydrogen evolution reaction (HER). HER occurs at the counter electrode, where water reduction occurs to produce H2 , as shown in Eq. (2). The holes move from the bulk to the photoanode’s surface and take part in an Oxygen Evolution Reaction (OER), which occurs as a reaction of water with the holes at the electrode’s surface, as shown in Eq. (3). To drive these reactions, the semiconductor should be able to meet two conditions, (i) The valence band of the semiconductor must be at a higher potential than the water oxidation potential (E° O2 /H2 O) (ii) the conduction band must be at a lower potential than the water reduction potential E° (H+ /H2 ) [21, 22]. Under normal conditions, the Gibbs free energy change for WS is ΔG° = 237.2 kJ/ mol. According to the Nernst equation, the ΔG° value corresponds to a standard potential of 1.23 V. Therefore, a theoretical potential of 1.23 V is required to drive the reaction. However, the potential needed for WS is higher than 1.23 V due to the excess heat released during the reaction (48.6 kJ/mol). Hence, the WS reaction thermodynamically requires 285.8 kJ/mol of energy, corresponding to the necessary potential of 1.48 V. The difference between the theoretical and required potentials is known as overpotential (η). The reactions involved in PEC-WS sequentially are as follows [23]: Catalyst (Photoelectrode): − + PE + hν → eCB + hVB

(1)

Photocathode (Hydrogen Evolution Reaction): o 2H+ + 2e− → H2 E reduction = 0.0V

(2)

Photoanode (Oxygen Evolution Reaction): 1 o = 1.23V H2 O + 2h+ → 2H+ + O2 E Oxidation 2

(3)

1 H2 O + hν → H2 + O2 2

(4)

Net Reaction:

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3 Modifications on ZnO Two main strategies can be discussed to create photoactive materials with increased incident photon conversion efficiency (IPCE). The first strategy is to modify the ZnO structure to increase its capacity to collect photons in the visible light range. It includes modification in the ZnO crystal’s shape, change in the number of charge carriers (electrons and holes) within the crystal structure (doping), or functionalization of the ZnO surface with dye molecules to increase its light-harvesting capacity. The second strategy involves enhancing efficient photogenerated charge separation by regulating crystal lattice defects or adding electron transfer agents [24]. Modifications on ZnO for improving water oxidation processes include (i) doping, (ii) heterojunctions, (iii) nanocomposites, (iv) tandem PEC cells, (v) coupled semiconductors, (vi) other modifications as shown in Fig. 1.

3.1 Doping According to the dopant type, doping is categorized into anionic and cationic doping.

Fig. 1 Schematic diagram of different types of modifications on ZnO a undoped ZnO, anionic doping and cationic doping, b Heterojunctions, c couple semiconductors

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Anionic Doping

Anionic doping is a process in which the stable ion of an electrolyte is replaced by an anion [25]. Doping of anion moves the VB a little closer to CB, thereby reducing the bandgap energy, resulting in a comparatively lower energy requirement for photoexcitation of electrons than that of pristine ZnO [26]. To overcome the problem of low absorption of visible light, Yang et al. [27] prepared N-doped ZnO nanowires in which the doping of N to ZnO crystal structure adds an energy level between VB and CB. Wang et al. [28] noted that optimized doping nitrogen to ZnO increases the photocurrent density. In contrast, at the higher concentrations of N in ZnO, the photocurrent density decreases due to the higher recombination rate of holes and electrons. Chen et al. [29] investigated the CdTesensitized N-doped ZnO nanowire arrays in PEC-WS, which has reduced the rate of recombination of charges as nanowire allows one-directional electron mobility and also increases visible light absorption. Thus, affecting the PEC-WS by improving photocurrent density. Gadisa et al. modified the ZnO@Ni-foam by doping graphitic carbon (g-C) consisting of O, S, and N heteroatom moieties via the hydrothermal treatment. These heteroatoms offer more active sites for OER, enhancing the PECWS performance. The graphical representation of the mechanism is shown in Fig. 2a, where electron migration takes place from ZnO to g-C, and then the electrons transmit to Ni-foam and go through the circuit to the Pt electrode to take part in the reduction reaction. Furthermore, the modified photoelectrode required a lower overpotential of 317 mV than unmodified ZnO/NF (398 mV) at 10 mA/cm2 current density for OER [30]. Thus, anionic doping can reduce charge carrier recombination and provide more active sites for OER.

Fig. 2 a Schematic representation of PEC-WS over the surface of ZnO-C@NF [30]. b Mechanism of different kinds of heterojunction [31]

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Cationic Doping

Cation doping is the voluntary introduction of cation impurities to the semiconductor to modify its structural, electrical, and optical properties. Like anion doping, doping of cation shifts the conduction band maximum downwards, reducing the bandgap energy required for the photoexcitation of electrons [26]. Kant et al. [32] studied the application of Al-doped ZnO (AZO) in PEC-WS at different wt.% of Al. It was found that at low and moderate amounts of doping, the effect of doping was not as significant as that of higher concentration on PEC-WS. But exceeding the optimized concentration reduces PEC activity as it disintegrates the crystal structure and increases the charge recombination rate. Commandeur et al. [15] used a microwave-assisted procedure of doping ZnO with Yttrium where an optimized concentration of Yttrium (0.1%) produces the photocurrent density of 0.84 mA/cm2 , which is 47% more than that of the pristine ZnO and N [27], Al [33], Na [34] doped ZnO produces 0.3, 0.3, 0.5 mA/cm2 respectively at similar conditions. However, by the chemical bath deposition method, Wei et al. incorporated various dopants such as Na, K, Co, and Ni in ZnO nanostructures, and the photocurrent density obtained was 0.90, 0.62, 0.58, 0.48, and 0.42 mA/cm2 , respectively. Thus, For Na-doped ZnO, photocurrent density was more than twice that of pristine ZnO (0.42 mA/cm2 ) [34]. Banerjee et al. co-doped Sn and Al on ZnO and found that the inclusion of dopants in the crystal structure of ZnO reduced the bandgap from 3.26 eV to 2.87 eV and exhibited a photocurrent density of 1.87 mA/cm2 , which is ten times more than that of pure ZnO nanorods (0.18 mA/cm2 ) [5]. Das et al. synthesized Ag-doped ZnO and suggested that adding Ag was vital in developing the Schottky junction and defect concentration. They reported that the synergistic enhancement of the electron–hole separation process and the optical response was primarily made by the Ag-ZnO catalyst’s surface plasmon resonance [35]. Li et al. prepared Au-modified ZnO/TiO2 nanorods as described in Fig. 3i, exhibiting 3.14 mA/cm2 photocurrent density. They observed that the Au-decorated nanorods have an increased rate of electron generation and long decay time, which indicates successful charge transfer and prevention of charge coalescence [36]. Similarly, Park et al. synthesized Au/ ZnO-TiO2 on Si wafer by chemical vapor deposition and photodeposition method, as shown in Fig. 3(ii) [37]. Rabell et al. incorporated Al in ZnO via the sol–gel method and observed that the bandgap of ZnO decreases on increasing the concentration of aluminum, and the PEC experiments exhibited enhancement in photocurrent density. The H2 production reached the highest (~33 μmol) for 5wt.% of Al in ZnO, which is 6.6 times more than H2 obtained from ZnO at similar experimental conditions [38]. Thus, the introduction of cation in ZnO reduces the bandgap energy, resulting in increased generation of charge carriers and enhancing the synthesized electrode’s photocurrent density.

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Fig. 3 i Schematic of the synthesis of ZnO-TiO2 /Au nanorods [36] and ii HR-TEM images of (a) ZnO (b) ZnO-TiO2 c–f) ZnO@TiO2 /Au with 1, 2, 3 and 4 h photodeposition time. Reprinted with permission from [37]. Copyright (2019) Elsevier

3.2 Heterojunctions A heterojunction is an interface between two regions or layers of different semiconductors (SC-I and SC-II). These will have unequal bandgaps, unlike homojunction [39]. No semiconductor will simultaneously utilize the maximum sunlight and have a high redox potential. Heterojunctions allowed it to absorb broad light and possess a strong redox potential. Two or more materials involved in heterojunction formation must have similar crystal assembly, coefficient of thermal expansion, and lattice spacings. Heterojunctions consist of three configurations (i) straddling gap (type I), (ii) staggered gap (type II), and (iii) broken gap (type III). The staggered gap type heterojunction proved the most efficient among other heterojunctions. The general mechanism of these types of heterojunctions is shown in Fig. 2(b). Under sunlight exposure, a cliff-like junction will be formed at the interface resulting in easy photoexcitation of electrons in both semiconductors. Due to the difference in CB and VB energy levels of semiconductor (SC-I) and SC-II, the electrons at a higher energy level in the CB of SC-II jump down to the CB of SC-I at a lower energy level, and the holes in the VB of SC-I transfers to VB of SC-II which reduces the energy bandgap for excitation along with the reduction in charger carrier’s recombination rate. Thus, the strategy increases the photocurrent density enhancing PEC-WS [31]. The p-n heterojunction of CuO/ZnO showed improved charge carrier separation and light absorption properties, thus enhancing the PEC performance four times more than the pure ZnO nanorods [20]. Hou et al. developed rGO electrodeposited ZnO/Cu2 O heterojunction, which exhibited ~3 and ~9 times higher photocurrent density of ZnO/Cu2 O/rGO than the respective ZnO and Cu2 O [40]. Similarly, CoPi/BiVO4 /ZnO [41], Ov -CoOx /ZnO [42], TiO2 /ZnO [43] heterojunctions exhibited photocurrent of densities 3.5, 2.1, ~2.5 mA/cm2 which are comparatively higher than that of pristine ZnO.

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Recently Cai et al. prepared hierarchical structures of plasmonic Au nanoparticles (NPs) on the surface of p-NiO/n-ZnO. This NiO/ZnO/Au p–n junction array photoanode was subjected to a PEC experiment and exhibited a photocurrent density of 1.78 mA/cm2 [44]. Fareza et al. reported the hydrothermal synthesis of ternary heterostructures comprised of ZnS NPs and ZnO nanorods (NRs) covered in MoS2 nanosheets (ZnO/ZnS/MoS2 , ZSM), which exhibited a photocurrent density of 0.72 mA/cm2 [45]. Kaur et al. constructed ZnO/In2 S3 heterojunction in two steps, where In2 S3 2D nanolayers are synthesized by chemical vapor deposition (CVD), and the second is physical vapor deposition for ZnO. The photocurrent density at the ZnO/In2 S3 heterojunction was 2.4 mA/cm2 , and the IPCE value was 43%. They claimed that more absorption at shorter wavelengths and improved charge segregation at the boundary at longer wavelengths were the leading causes of the heterojunction’s higher current density [46]. Xie et al. decorated CdS on ZnO nanotube arrays via the magnetron sputtering method to form a heterojunction and then deposited Ag2 S NPs by ion exchange as shown in Fig. 4. Ag2 S was deposited to reduce the photo corrosion of CdS and to construct two heterojunctions. This photoanode showed a high photocurrent density of 6.82 mA/cm2 , 3.69% applied bias photon-to-current efficiency, and 68.38% IPCE [47]. Hence, heterojunction formation improves the optoelectronic properties and enhances the charge carrier separation, thereby improving the overall PEC-WS performance.

Fig. 4 a Schematic diagram of ZnO/CdS/Ag2 S nanorod fabrication. SEM images of b ZnO, c ZnO/ CdS, and d ZnO/CdS/Ag2 S nanorods. Reprinted with permission from [47]. Copyright (2022) American Chemical Society

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3.3 Nanocomposites Nanocomposites are materials composed of heterogeneous solids in which one of the components should have a size of less than 100 nm. Quantum dots, clusters, and atomic doping mechanisms can form nanocomposites. After the preparation of nanocomposites, new electronic states will be formed in the space between the CB and VB of the host material, leading to increased photosensitivity. Greater spectral sensitivity increases the absorption of light within the broader range of spectra, increasing the rate of photoelectron generation, thus enhancing PEC-WS [48]. Experimental results show the composite structure of ternary nanocomposite of NiO/CdS@ZnO lets the composite utilize the incident light more effectively than pristine ZnO or CdS. The composite’s lower photoluminescence intensity inhibits the recombination rate of charges. All these factors effectively increased the photoelectron density by 678% [49]. Bai et al. decorated CdS NPs on 3D ZnO nanowires to make ZnO/CdS nanocomposites, as shown in Fig. 5 (i). The CdS NPs were deposited by successive ionic layer atomic reaction method. Their results indicated that the photocurrent density of 3.58 mA/cm2 and 3.1% solar to H2 conversion (STH) efficiency were achieved [50]. Wang et al. [51] synthesized ZnO@ sulfur-doped ZnO aiming to suppress the recombination of charges by incorporating sulfur atoms on the surface of ZnO, which will introduce impurity bands within the bandgap and traps the photogenerated holes for improvement of PEC activity. In Au/ZnO nanocomposite, gold NPs trap the photogenerated electrons, increasing the exciton pair’s separation efficiency. In the same way, many Ag/ZnO nanocomposites improved the capability of visible light and reduced the rate of electron–hole recombination by enhancing PEC performance [52]. Thus, ZnO-based nanocomposites showed increased photosensitivity and higher electron generation rate by utilizing a broad range of solar spectrum and exhibited higher photocurrent density.

Fig. 5 i Schematic diagram of ZnO@CDS fabrication process a photoresist template on an AZO substrate, The SEM images of b top-view of ZnO NWs, c the cross-section of ZnO NWs, d topview of 3D ZnO NWs, e the cross-section of 3D ZnO NWs, and f top-view of 3D ZnO NWs–CdS. Reprinted with permission from [50]. Copyright (2015) Wiley Online Library

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3.4 Tandem Semiconductors Several studies have demonstrated that having a semiconductor with a particular bandgap and efficient PEC-WS behavior is difficult. The strategy of tandem photoanode with different semiconductor layers is introduced to overcome this behavior. Tandem PEC cells are a series of two or more photoelectrodes (anode and cathode) with different bandgaps arranged in a stack to increase the efficiency limits of a single semiconductor [53]. When sunlight falls on a photoelectrode in which the different light absorbers/semiconductors are arranged in the decreasing order of their bandgap, at first, the photoactive material with the maximum bandgap absorbs the photons of energy greater than or equal to its bandgap. The photons left unabsorbed by the first material will strike the adjacent material. Here the photons of less energy conforming to the bandgap of the second material will be absorbed and so on by releasing photogenerated electrons, which plays a leading role in improving PECWS performance. Thus, tandem semiconductors improve the efficient utilization of sunlight [54]. Liu et al. demonstrated a tandem triple junction PEC cell of Ag/Cu2 O/ZnO (Fig. 6), which showed enhanced photocurrent generation as compared to Cu2 O and Cu2 O/ ZnO, as the density of photocurrent generated by the electrode Ag/Cu2 O/ZnO is 1100 μ A/cm2 which is 11 times and almost 7.5 times greater than that of Cu2 O and Cu2 O/ZnO respectively [55]. Similarly, Bai et al. presented a tandem PEC cell in which Au/ZnO photoanode and Ni(OH)2 /Cu2 O photocathode formed a Z-scheme system, and under solar illumination, the photovoltage was generated due to the fermi level difference in Cu2 O and ZnO [56]. Therefore, it can be concluded that tandem photoelectrodes widen the solar spectrum’s absorption and improve the STH efficiency.

Fig. 6 Ag/Cu2 O/ZnO tandem triple junction photocathode [55]

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3.5 Coupled Semiconductors As the name suggests, a coupled semiconductor indicates the joining/ coupling of two or more semiconductors that acts as a promising photocatalyst. Although doping provides a low energy gap by providing the vacant states nearer than the conduction band, which enhances the visible light absorption, these states sometimes act as recombination centers to overcome this and coupling of the semiconductor comes into the picture. Intrinsic properties of semiconductors, such as low bandgap, large surface area, crystallinity, high electrical conductivity, and optimum particle size, are the main characteristics of an excellent semiconductor for PEC-WS. However, these criteria are insufficient to categorize semiconductors as efficient photocatalysts. Along with these properties, charge generation and low charge recombination rate are also considered [57]. Coupling semiconductors with appropriate bandgap minimize the recombination of charges, thus improving the PEC-WS efficiency. For instance, Ji et al. [58] synthesized 1D core/shell structured ZnO/TiO2 with CdSe, which generated a photocurrent of density five times more than that of CdSe/TiO2 . Li et al. grew nanosheets of CdS in situ on ZnO via the sequential self-assembly method for the first time, as shown in Fig. 6. The PEC results showed the highest photocurrent density of 9.1 mA/cm2 and 3.72% photoconversion efficiency [59]. Therefore, coupled semiconductors can be a promising approach for improving PEC-WS performance.

3.6 Other Modifications Other ZnO modifications were being done to enhance PEC-WS performance, such as metal oxide, alkaline earth metal, semi-metal, graphene, nitride materials, nonmetal, and rare earth element doping. Abbas et al. synthesized ZnO-GaN/ZnO 3D branched nanowires photoanode through chemical vapor deposition [60]. Rakibuddin et al. doped Samarium (Sm) into ZnO via the wet chemical method and coupled Sm/ ZnO with g-C3 N4 by solid-state transformation method. They achieve 10,250 μmol/ g.h H2 generation by using photoanode with 10 wt.% Sm doping [61]. Wannapop et al. fabricated ZnWO4 -ZnO nanorods and studied the effect of citric acid on the prepared photoanode. Their findings demonstrated that the ZnWO4 was grown on hexagonal ZnO and converted into a hierarchical structure when the citric acid content was increased [62]. Elrahim et al. deposited the ZnO-rGO nanosheets on a titanium sheet via a kinetic spray process. The results showed that the photoanode with 50 wt.% rGO exhibited 4.82 mA/cm2 photocurrent density and 3.25% photoconversion efficiency[63]. Kim et al. fabricated a hierarchical photoanode by electrospraying MnO2 on ZnO/ITO and then deposited an ultrathin layer of TiO2 for passivation, reducing the photocorrosion [64]. Swathi et al. doped phosphorus into ZnO. It was suggested that phosphorus is very good as a doping substance and can enhance

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PEC performance owing to its covalent P-O bonds with five valence electrons, which could prevent the formation of electron-trapping states [65]. Salem et al. synthesized Germanium (Ge) doped ZnO nanorods on FTO as a reliable photoactive material for PEC-WS. It was stated that Ge might substitute Zn in the ZnO structure, enhancing the material’s electronic characteristics. Additionally, Ge is soluble in ZnO, which should aid its growth along the lowest energy state [66, 67]. The presence of Ge improved the photoanode activity and was responsible for the outstanding 12 mA/cm2 photocurrent density and 3.6% photoconversion efficiency [68]. Yarahmadi et al. synthesized Strontium (Sr) doped ZnO by mechanical milling method and observed that 3 wt.% doping of Sr showed the best performance and lowered the bandgap from 3.34 eV to 3.17 eV [69]. Wei et al. doped Yttrium into ZnO by hydrothermal method. The doped sample maintained the same nanorod morphology and wurtzite structure [70]. Cocatalyst loading is the most popular method for achieving a high PEC performance since it can speed up processes and reduce charge recombination. Zhong et al. synthesized Ti3 C2 TX (MXene) flakes, which were spin-coated onto the surface of ZnO to create a unique ZnO/Ti3 C2 TX cocatalyst photoanode [71]. The performance of different types of ZnO-based photoelectrodes is summarized in Table 1. It includes the materials combined with ZnO, the synthesis method of the photoelectrodes and photocurrent density comparison of pure ZnO and ZnO modified with different materials (M1-M2-ZnO), where M1 and M2 are material 1 and material 2.

4 Conclusion Due to their excellent physical and chemical properties, ZnO-based photoelectrodes have shown significant potential for PEC-WS. Recent advances in the synthesis, fabrication, and functionalization of ZnO-based photoelectrodes have significantly improved their PEC performance. However, several challenges, such as poor stability and low efficiency, still need to be addressed to make ZnO-based photoelectrodes practical for large-scale PEC-WS applications. Nevertheless, the performance of ZnO-based photoelectrodes can be enhanced by employing various strategies such as nanostructuring, doping, surface modification, and bandgap engineering. Doping ZnO with different elements has been shown to improve its PEC performance by altering its electronic structure and enhancing its charge separation. Heterojunctions between ZnO and other semiconductors have also been investigated to improve the photo response of ZnO-based photoelectrodes, resulting in enhanced photocurrent and photo conversion efficiency. Similarly, coupling ZnO with other semiconductors has enhanced the photocatalytic activity of ZnO-based photoelectrodes. The use of tandem ZnO photoelectrodes allows for efficient utilization of the solar spectrum by combining the absorption of different light wavelengths in each photoelectrode layer. However, developing

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Table 1 Performance of different modified ZnO photoelectrodes for PEC Strategy

Photoelectrode materials

Synthesis method

Photocurrent density (mA cm−2 ) Pure ZnO

M1-M2-ZnO

Ref

Anion Doping

CdTe/N/ZnO

Modified hydrothermal method and chemical bath deposition

0.111 (nanowires)

0.46

[29]

Cation Doping

Yt-ZnO

Spin coating and chemical bath deposition

0.394 (nanorods)

0.84

[15]

Cation Doping

Au-ZnO/TiO2

Hydrothermal, spin coating, sol–gel, magnetron sputtering

3.14

[36]

Heterojunction

ZnO/ Cu2 O-rGO

Cathodic deposition Facile hydrothermal and electrodeposition

1.06

10.11

[40]

Heterojunction

CuO/ZnO

Hydrothermal Facile method

0.24

0.97

[20]

Heterojunction

Ov -CoOx /ZnO

Hydrothermal 1 solution impregnation & solvothermal

2.1

[42]

Hydrothermal

Heterojunction

TiO2 /ZnO

–

~2.5

[43]

Heterojunction

ZnO/CdS/Ag2 S Magnetron sputtering and cation exchange

0.88

6.82

[47]

Nanocomposite

NiO/ CdS@ZnO

Hydrothermal

0.1401

0.95

[49]

Core–Shell Nanocomposite

S doped ZnO

Spin coating annealing

~0.1

1.08

[51]

Tandem Triple Junction PEC cell

Ag/Cu2 O/ZnO

Vacuum Evaporation Electrochemical Deposition

–

1.11

[55]

Couple semiconductor

CdS/ZnO

SILAR deposition

3.15

9.1

[59]

Other

ZnO-rGO

Kinetic spray process

4.82

[63]

Ge-ZnO

Rf sputtering

12

[69]

tandem ZnO photoelectrodes presents challenges, including optimal bandgap engineering and the efficient coupling of the different layers. The recent advancements in these approaches provide a foundation for future research to explore their potential for practical applications.

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Semiconductor-Based Plasmonic Nanohybrids: Synthesis, Characterization, Mechanistic Understanding of Structure–activity, and Their Multifunctional Applications Atanu Ghosh and Tripti Ahuja

Abstract Semiconductor-based plasmonic nanohybrids are gaining increasing interest as photocatalysts in healthcare, sustainable environment, and energy-related applications due to their tunable electronic and optical properties. Plasmonic nanostructures (Gold and silver nanostructures) can be integrated with semiconductors, such as 2D materials, graphene, quantum dots, etc. that aid in improving their electro-optical properties and thereby enhancing the photocatalytic efficiency of such integrated plasmonic nanohybrids. The integrated nanohybrids due to their unique structural and functional properties serve as better photocatalysts and photodetectors. Controlling the structure and morphology of these materials along with the fundamental understanding of their structure–property correlation are pivotal in improving their efficacy. These semiconducting plasmonic hybrids have also been efficiently used for water and air purification, pollutant degradation, solar cells, hydrogen production, and so forth. This chapter discusses the synthetic protocols, characterization, mechanistic understanding of structure–activity relating photocatalysis, and significant applications of semiconductor-based plasmonic nanohybrids with an emphasis on their challenges and future impact on society. Keywords Plasmonic nanohybrids · Photocatalysis · Semiconductors · Electron–hole charge separation · Hydrogen production

A. Ghosh Department of Chemistry, Michigan State University, 578 South Shaw Lane East Lansing, MI 48824, USA T. Ahuja (B) Center for Nanosciences, Indian Institute of Technology Kanpur, Kanpur, U.P. 208016, India e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_15

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1 Introduction The conversion of light energy to chemical potential is one of the fundamental processes on the earth. Photosynthesis is a prime example of that, where solar energy results in charge separation that creates the chemical potential needed for the synthesis of adenosine triphosphate (ATP) molecules [1, 2]. Getting inspired by nature, harvesting solar energy is one of the promising ways for tackling the future energy crisis. Photocatalysis (PC) and photovoltaics (PV) are two major processes for solar energy conversion, which convert light energy as a chemical bond to electricity [3, 4]. In PC, the photoexcited electron–hole pair is utilized in dark reactions, whereas in PV, the current is generated from the charge carriers [1]. In both processes, the crucial initial steps are light absorption and charge separation. However, the light energy conversion efficiencies of PC and PV are yet to be improved. Two key things which contribute as hurdles to energy conversion efficiencies are—light capture efficiency and charge recombination [5, 6]. Light capture is a material extensive property, thus materials with higher molar absorption coefficients in the visible and near-infrared region of the solar spectra are ideal for light absorption. Typical PV materials have high bandgaps which capture the higher energy photons from the solar energy spectrum, leaving the visible and near-infrared photons unused [1, 6]. Charge recombination, where the excited electrons and holes, created upon photoexcitation recombine before efficient charge separation, decreasing the light conversion efficiency [7]. In this milieu, plasmonic nanomaterials have been integrated with semiconductors and utilized as effective photocatalysts that have a strong potential to capture photons of visible and near-infrared regions and thereby decrease the charge recombination efficiency [8–12]. Plasmonic nanomaterials are metal nanoparticles that show exciting phenomena at the nanoscale with the interaction of photons in the visible and near-infrared regions of the solar spectrum. However, such plasmonic materials suffer from ultrafast recombination of charge carriers [13–15]. Due to the rapid recombination of charge carriers, the photocatalytic efficiency is reduced and this can be delayed by integrating the plasmonic materials with semiconductors of high band gaps [14]. Plasmonic metal–semiconductor composites have proven to be a more efficient solution to this problem as compared to traditional semiconductors because of their unique surface plasmon resonance (SPR) effect [16, 17]. SPR is the collective and coherent electron cloud oscillations on the surface of metal nanostructures as an effect of electromagnetic field interaction (Fig. 1A) [16, 17]. SPR effect has been utilized in various applications, including sensing, bioimaging, surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS), plasmon-enhanced spectroscopies, etc. [18–26]. SPR enhances the local electromagnetic field enormously, and the corresponding enhancement factor could rise to ~1014 , which depends on several factors including the type of metal, size, and shape of the nanostructures, sample preparation methods, etc. Typically, gold (Au) and silver (Ag) nanostructures are well-explored as efficient plasmonic materials for their relatively easy sample preparation, functionalization,

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Fig. 1 A Schematic illustration of LSPR phenomenon in plasmonic nanoparticles, and B LSPR tuning in Ag nanoparticles of various morphologies. Figure reprinted from [29]. Copyright (2006) Chemical Society Reviews, The Royal Society of Chemistry

synthetic control, and high electromagnetic enhancement via SPR. Confinement of SPR phenomena on the surface of plasmonic nanostructures is known as localized surface plasmon resonance (LSPR). Like SPR, the LSPR effect can also be tuned by controlling the size, shape, and morphology of the plasmonic nanostructures. LSPR is a boon in various applications while thinking about light absorption of plasmonic materials [27, 28]. The LSPR effect of Au nanostructures with the variations in the shapes of nanoparticles (NPs) is exhibited in Fig. 1B [29]. Integration of plasmonic nanostructures (AuNPs and AgNPs) with semiconductors such as titanium dioxide (TiO2 ), zinc oxide (ZnO), cuprous dioxide (Cu2 O), silicon dioxide (SiO2 ), aluminum oxide (Al2 O3 ), and so forth help to concentrate the low energy photons to utilize the visible and near-infrared photons of the solar spectrum [30]. In addition, the collective oscillations of electron clouds in SPR are known to have a positive effect on the electron–hole separation in semiconductors, decreasing the charge-recombination [31]. In the past, various semiconductor-based plasmonic PC and PV materials, such as Ag/Au@TiO2 [6, 32], Ag/Au@SiO2 [33, 34], Ag/Au@ZnO [35], etc. have been developed and their photocatalytic efficiency were explored due to their potential applications in contaminant degradation [36–38], water splitting [28, 39–42], and photocatalytic organic transformations [4, 43, 44]. Au/Ag nanostructures with TiO2 exhibits better response to visible light and are useful in PV cells and simple plasmon-aided sensors [32]. Singh et al. reported Au-TiO2 nanohybrids that have 3.3 times better photocatalytic efficiency than pristine TiO2 for pollutant degradation, taking methylene blue (MB) and methyl orange (MO) as examples [45]. Reduction of 4-nitroaniline has also been studied using CdS-Au nanorods (NRs) composite under vis-NIR light irradiation, and increased efficiency has been observed compared to bare CdS nanowires (NWs) and Au NRs [46]. Several reports have shown enhanced performance in photocatalytic hydrogen/ oxygen evolution reactions with semiconductor-plasmonic nanohybrids—paving a

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promising direction toward the energy crisis solution. Few such examples of manyfold enhancement on photocatalytic hydrogen evolution have been achieved by incorporating Au nanoparticles onto TiO2 nanosheets with exposed (001) facets [46, 47]. Given the applied importance of nanohybrids, various microscopy and spectroscopy tools have been exploited to characterize and understand the underlying photocatalytic mechanisms of such integrated hybrids. In the semiconductor-integrated plasmonic nanostructures, the photoexcited electrons of plasmonic nanostructures can be directly injected into the conduction band of semiconductors to generate electricity or perform chemical reactions. Plasmonic heating, dipolar energy transfer, and light scattering are other mechanisms that could potentially operate at the interface of plasmonic-semiconductor nanohybrids, making the working mechanism incredibly difficult to comprehend [6, 13, 48–50]. Several research groups tried to disentangle these mechanisms over the years by smart chemical engineering, which will be discussed in this chapter briefly. In this book chapter, we discuss the synthetic protocols and characterization methods of a few common plasmonic nanostructures integrated with semiconductors, with a particular focus on the several proposed working mechanisms on the structure–activity correlation from the literature. The schematic presentation of broad areas of semiconductor-integrated plasmonic photocatalysts discussed in this chapter is shown in Fig. 2. Moreover, specific examples of the applications of integrated plasmonic nanohybrids in the areas of water and air purification, hydrogen/oxygen evolution, environment pollutants remediation, and biomedical applications have been discussed briefly. In the end, we have also mentioned the challenges associated with and future perspectives of semiconductor-integrated plasmonic nanohybrids.

2 Synthetic Methods and Characterization The physicochemical and functional properties of nanostructures depend on their structural properties such as periodic arrangement of atoms, crystallinity, size, shape, interface, and overall surface area. Additionally, the structural properties of nanomaterials depend upon the way they are synthesized. Various approaches and synthetic protocols have been pursued in the synthesis pathway to provide precise functionalization of semiconductor-plasmonic photocatalysts which possess better efficiencies than conventional semiconductor photocatalysts. These efficiencies become better as different synthetic protocols lead to varying excited-state dynamics of hot carriers between plasmonic metals and semiconductors [14]. Some of the synthetic methods such as hydrothermal, solvothermal, thermal evaporation, electrochemical, etc. that are commonly used for semiconductor-plasmonic photocatalysts are represented schematically in Fig. 3. Generally, there are two categories of semiconductor-based plasmonic photocatalysts as mentioned below:

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Fig. 2 Schematic overview of the synthesis, characterization, photocatalysis mechanism, and application of semiconductor-based photocatalysts as discussed in the book chapter. The figure of the photocatalysis mechanism is reprinted with [30]. Copyright (2011) Journal of Materials Chemistry, The Royal Society of Chemistry

A. Plasmonic photocatalysts based on silver nanoparticles (AgNPs) B. Plasmonic photocatalysts based on gold nanoparticles (AuNPs) In both these categories, plasmonic photocatalysts with semiconducting materials such as TiO2 , SiO2 , and ZnO have been discussed in this book chapter. The plasmonic nanoparticles can be disordered or ordered on the surface of these semiconductor materials. The composites of AuNPs and AgNPs with semiconductors are majorly synthesized using wet-chemical and photo-reduction methods. In wetchemical methods, precursors of Ti, Zn, and Si are mixed with reducing agents to produce hybrid semiconducting plasmon materials of desired morphology. For

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Fig. 3 Various synthetic methodologies for the synthesis of semiconductor-plasmonic photocatalysts

instance, 1D anatase, Ag/TiO2 heterojunction was fabricated by a facile wet impregnation method and the fabricated composite was found to enhance the photodegradation of 2,4-dichlorophenol (2,4-DCP) under visible light [51]. Z. K. Zhand et al. prepared plasmonic M/TiO2 (M = Au, Pt, Ag) photocatalysts by the in-situ photoreduction of plasmonic NPs with the assistance of Ti3+ under UV-light irradiation. These composites exhibited the highest yield and extensive selectivity for the oxidation of benzene to phenol in aqueous phenol [30]. Zhu et al. fabricated Au/SiO2 NPs which had activities in the decomposition of formaldehyde (HCHO) under visible-light illumination [52]. Another important method to fabricate plasmonic composite photocatalysts is to deposit the semiconducting layer physically and chemically on the surface of plasmonic nanostructures. The first reported semiconductor-based plasmonic photocatalysts was an Ag@SiO2 –TiO2 film, which was fabricated by depositing TiO2 films on a layer of silica-encapsulated AgNPs. The presence of AgNPs in this composite led to the enhancement of photocatalytic efficiency. Besides, AgNPs, the silica shell has played a crucial role in photocatalytic reactivity, however with the increased thickness of the silica shell, the reactivity decreases drastically [34]. In the evaporation process, plasmonic hybrid materials are prepared via thermal and chemical evaporation methods. Sini. K. et al. prepared Au@ZnO rods via a twostep process using carbothermal evaporation followed by thermal annealing at 400 °C for 1 h in an oxygen atmosphere [38]. Besides, evaporation, wet-chemical, deposition, and photo-reduction methods, hydrothermal, and sol–gel approaches are more convenient in terms of experimental parameters and procedures. The hydrothermal and sol–gel synthesis methods yield nanostructures with randomly distributed structures. The sol–gel process involves three steps which are as follows. The first step

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includes the mixing of precursors of noble metals and semiconductors in appropriate ratios followed by the addition of acid/base for making sol and finally, heat treatment to obtain the desired integrated hybrids. Prakash et al. used the sol–gel method to produce Ag–TiO2 nanocomposites and studied their multifunctional applications in photocatalysis, antimicrobial activity, and SERS [53]. The hydrothermal process is similar to sol–gel where first all precursors are mixed and then a quartz autoclave is used for heat treatment. Chanchal Mondal et al. have synthesized Au–ZnO nanocomposites through a facile, inexpensive, and one-pot hydrothermal approach. The synthesized nanocomposites exhibit excellent sunlight-driven photocatalytic activity that can effectively decompose various kinds of organic dyes and maintain a high level of photoactivity even after four cycles [36]. Similarly, AgNPs/N-doped TiO2 and Au/TiO2 composite electrodes were prepared via the wet-chemical and sol–gel processes by Linic et al. and Cronin et al., respectively [5, 54]. In this milieu of synthetic approaches, sputtering is an effective method for producing thin films on a substrate. In this method, atoms are ejected from a solid target due to the bombardment of atoms by energetic particles. Because they are in the gas phase and not in an equilibrium state, sputtered atoms are easily deposited on substrates under vacuum conditions [37]. K. Awazu et al. synthesized TiO2 embedded Ag photocatalyst where Ag@SiO2 core–shell particles were prepared via sputtering followed by spin coating of thin TiO2 film on the former particles [34]. Photo deposition and ion exchange methods have also gained the interest of researchers for the synthesis of semiconductor-based plasmonic photocatalysts. Jia Liu et al. has judiciously synthesized metal@semiconductor core–shell nanocrystals with an atomically organized interface, quasi-monocrystalline shell, and diverse controllable structures and morphologies via ion-exchange process, which are hardly tractable by conventional synthetic strategies [55]. In recent times, electrochemical synthetic methods have also played a crucial role in the synthesis of core–shell plasmonic-semiconductor nanocrystals. Hui Wang et al. reported a very simple, cost-effective, and universal electrochemical synthesis approach for core–shell structured Au@semiconductor (Au@SC) which benefited from the full advantages of Schottky junction formation with enhanced electron mobility and can be monitored at the single-particle level [56]. It has been justified that different synthetic methodologies lead to plasmonic hybrids of varied morphologies, sizes, crystallinity, and defects that need various characterization tools to obtain a complete and thorough understanding of such integrated hybrids to utilize them as efficient PV and PC devices. In the past few years, great progress has also been made in the characterization of plasmonic photocatalysts that have aided in generating excellent photocatalytic reactivity. In this regard, optical and electron microscopies such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) will provide extensive details on the morphological and structural determination. The surfaces and surface areas of nanohybrids play a crucial role in conducting the photocatalytic reactions and such surface characterization can be performed efficiently via scanning probe microscopies such as atomic force microscopy (AFM), scanning tunneling electron microscopy (STEM),

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tip-enhanced Raman spectroscopy (TERS), etc. Various other techniques such as Xray photoelectron spectroscopy (XPS) and X-ray diffractometer (XRD) are required for surface and crystallinity characterization. The thorough characterization of plasmonic nanohybrids is indeed necessary to understand the nature of the material and thereby propose the mechanism of photocatalysis in such nanohybrids.

3 Mechanisms Understanding the structure–activity mechanisms of semiconductor-based plasmonic photocatalysts is pivotal for designing new photocatalytic systems with improved efficiencies. Some primary mechanisms that are known in the literature to explain the surface plasmon-enhanced photocatalytic activities of semiconductorplasmonic hybrids are; (A) hot-electron (or hole) injection, (B) local electromagnetic field enhancement, (C) dipolar resonant energy transfer, (D) light scattering, and (E) plasmonic heating. It is worth mentioning that in most examples multiple pathways contribute to the overall mechanism. All the mechanisms are discussed briefly here. A schematic explaining the different mechanisms of photocatalysis in semiconductor-plasmonic nanohybrids is shown in Fig. 4. (A) Hot-electron injection: Photoexcited plasmon decays through both radiative and non-radiative pathways. Non-radiative decay generates electron–hole pair, which is known as hot electrons and hot holes, respectively. The hot electron with sufficient momentum can overcome the Schottky barrier created at the junction of the plasmon and semiconductor materials, increasing the number of charges. On the other hand, the electron can flow from the semiconductor to the plasmonic nanostructure, however, this flow will eventually reach an equilibrium because of the additional negative potential of the fermi level created by the hot electrons on the photoexcited plasmonic nanostructure (Fig. 4A). This equilibration increases the charge separation and decreases the recombination of the electron–hole pair of the semiconductor, increasing the overall photocatalytic efficiency [57]. This has been achieved by modifying the size and shape of the plasmonic nanostructures. In addition, using the hot-electron injection, it is possible to sensitize large band-gap semiconductors, creating an excellent opportunity to increase the low energy solar light absorption cross-section of semiconductors by leveraging high molar absorptivity of plasmonic nanostructures, which is easily tunable. (B) Local electromagnetic field enhancement: This mechanism is also known as near-field electromagnetic enhancement, which is based on the interaction of the semiconductor material with the enhanced SPR of the plasmonic nanostructures. Plasmonic nanostructures give rise to an intense electromagnetic field enhancement with the interaction of photons with a specific frequency, and this increases the photon absorption rate of the semiconductor present in the vicinity of the surface of the plasmonic nanostructures. This further increases the charge generation and separation rate in semiconductors, increasing the photocatalytic efficiency (Fig. 4B).

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Fig. 4 Different photocatalytic mechanisms of semiconductor-based plasmonic photocatalysts. Figure reprinted from [57]. Copyright (2018) Chem, Elsevier Inc.

(C) Dipolar resonant energy transfer: In this working mechanism, a resonance condition between the plasmonic band of the metal nanostructure and the semiconductor absorption band is necessary. Initially, the photoexcited localized plasmonic resonance decays non-radiatively, generating an electron–hole pair in the adjacent semiconductor. Since this is a dipole-induced energy transfer process, physical contact between the metal nanostructure and semiconductor is not a prerequisite (Fig. 4C). (D) Light scattering: For large (>50 nm) nanoparticles, resonantly scattered light increases the possibility of light absorption by the semiconductor, which can make more charge carriers. A schematic of the process can be seen in (Fig. 4D). (E) Plasmonic heating: Plasmonic heating occurs during the non-radiative decay of LSPR. It was suggested that non-radiative decay increases the local temperature significantly at the metal–semiconductor junction, which increases the probability of crossing the activation energy barrier and thus enhances the photocatalytic behavior and a similar mechanism is shown in (Fig. 4E).

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4 Applications Semiconducting plasmonic hybrids owing to their unique optical, catalytic, and multifunctional properties have been rapidly evolving in various fields with potential applications such as water purification, air purification, solar cells, photocatalytic hydrogen production, photocatalytic self-cleaning surfaces, recyclable catalysts, environmental remediation, biomedical applications, and so on. These nanohybrids have opened new horizons in photocatalytic devices, photosensors, and photodetectors. The hot electrons generated by the plasmonic nanomaterials that were destroyed without semiconductor materials via thermalization can be easily harnessed in the presence of semiconductor materials for photodetection and photocatalytic reactions. Some of the common applications of plasmonic photocatalysts are discussed below: (a) Water purification: Semiconductor-based plasmonic hybrids can be used to purify water by dissociating pollutants and bacteria under sunlight or ultraviolet (UV) light. Such integrated photocatalysts enhance their photoresponse toward visible light. These hybrids also help in water splitting, photocatalytically. Many scientists have explored the use of TiO2 and ZnO semiconductors, with Ag and Au nanomaterials for enhanced water splitting efficiency. Similarly, other research groups have prepared various morphologies of AuNPs on TiO2 that have improved the efficiency of water splitting for H2 and O2 production [34, 40, 42, 51, 58–60] The major contaminants of water are categorized as; (i) Contaminants of major concern such as additives, dyes, preservatives; (ii) Endocrine disruptive compounds that are alkyl phenols, bisphenol A, pesticides, heavy metals, etc.; (iii) Pathogenic germs such as E. coli.; (iv) Cyanotoxins that are Microcystins, Cylindrospermopsin, Nodularins, etc. These water contaminants are regularly treated and degraded via photocatalysis, particularly with semiconducting plasmonic photocatalysis with enhanced efficiencies. (b) Photocatalytic hydrogen production: Plasmonic photocatalysts can be used to produce hydrogen from water using sunlight or UV light. TiO2 was the first photocatalyst used to produce H2 from water. Later, several semiconductor materials such as metal oxides, sulphides, nitrides, and their plasmonic hybrids have been explored for enhanced efficiencies of H2 production. (c) Solar cells: Plasmonic photocatalysts can be used to improve the efficiency of solar cells by increasing the absorption of light and the separation of charge carriers. Solar cells using a plasmonic-photoconductor serves as an ultimate solution to the energy problem that requires only solar energy. (d) Environmental remediation and air purification: Many plasmonic hybrids of Ag and Au nanostructures with semiconductors such as TiO2 , ZrO2 , SiO2 , etc. serve in environmental remediation processes such as the removal of pollutants from soil and water. Some of the plasmonic hybrids such as (AuNPs on ZrO2 ) have the potential to break down toxic pollutants such as volatile organic compounds (VOCs) and nitro compounds (NOx) under the presence of visible or UV light to less toxic substances [52]. They can also perform several environmental-based important chemical reactions like CO2 reduction and N2 fixation.

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Besides H2 production and water splitting, plasmonic photocatalysts have been utilized for the reduction of CO2 that have addressed the problem of harmful emissions to the environment and reduced global warming. Several plasmonic photocatalytic microstructures such as (Au/TiO2 , Al/Cu2 O, Au-Rh/ SBA-15 catalyst, etc.) have been described to provide high adsorption of CO2 followed by a rapid charge separation and migration that enhanced CO2 photoconversion. For the sustainability of the environment, various plasmonic photocatalysts have been utilized for the degradation of toxic and harmful dyes such as Rhodamine 6G, Methylene blue, Methyl orange, Rhodamine B, etc. [49]. (e) Photocatalytic self-cleaning surfaces: Plasmonic photocatalysts can also be used to create self-cleaning surfaces that break down dirt and grime under sunlight or UV light. (f) Biomedical applications: Plasmonic photocatalysts have the potential to be used in a wide range of biomedical applications, such as in the treatment of cancer, bacterial and viral infections, wound healing, and drug delivery [61]. Plasmonic photocatalysts under light irradiation generate charge carriers that can produce reactive oxygen species (ROS) in the presence of oxygen and water for the treatment of bacterial and viral infections. Hydroxyl free radical (OH•) is known to be the most active reactive oxygen species (ROS), however O2 •– , and H2 O2 are also ROS which has been proposed to play a crucial role in bacterial inactivity. It is also proposed that specific interactions between the photocatalyst and the bacteria play a crucial role in bacteria deactivation. For example, Ag/ TiO2 (ST41) composite increases the effectiveness of the process compared to bare TiO2 in the presence of visible light. SEM images shown in the following Fig. 5 unequivocally shows that 1 h of irradiation starts the destruction of the healthy bacterial cells. Whereas Au/TiO2 composite decreases the efficiency of the cell damage under visible light irradiation. It was proposed that Au particles disturb the binding between TiO2 and bacterial cells, whereas the presence of Ag facilitates the binding. Since it is possible to tune the absorption profile of silver nanoparticles by tuning the size, thus a promising future lies in this area of research utilizing very low-energy photons for medical treatments [62].

5 Conclusions and Future Perspectives Semiconductor-plasmonic photocatalysts are a type of material that combines the properties of plasmonics of metal nanostructures and the electron–hole generation phenomenon of semiconductors to design new materials with unique optical, electronic, and catalytic properties. Huge developments have been pursued to design enormous photocatalytic hybrids that have better photocatalytic efficiencies than conventional semiconductors. To obtain a morphological, structural, and mechanistic understanding of photocatalytic hybrids, various microscopy and spectroscopy tools such as TEM, SEM, XRD, XPS, Raman, etc. have been utilized as characterization tools and discussed briefly in this chapter. The fundamental concepts and

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Fig. 5 SEM images of the decomposition of bacterial cells under vis (λ > 420 nm) irradiation on Ag/TiO2 (ST41) photocatalyst. Figure reprinted from [62]. Copyright (2018) Creative commons @CC

principles of the structure–activity mechanism of photocatalysis have been comprehensively described. Although huge efforts have been progressed in the fabrication, designing, characterizing, and mechanistic understanding of semiconductor-based plasmonic photocatalysts, certain challenges persist in the development of low-cost, high-processing, and efficient photodetectors and photocatalysts devices in the visible range of the solar spectrum. The interface of semiconductor-plasmonic hybrids has not been fully explored. Hence, there is a need to understand and explore the interface structure and chemistry of such hybrid systems. Moreover, these integrated hybrids can potentially be useful in solar energy harvesting for artificial photosynthesis, which is a hot area of interest for scientists to exploit renewable resources of energy.

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55. Liu, J., Feng, J., Gui, J., Chen, T., Xu, M., Wang, H., Dong, H., Chen, H., Li, X., Wang, L., Chen, Z., Yang, Z., Liu, J., Hao, W., Yao, Y., Gu, L., Weng, Y., Huang, Y., Duan, X., Zhang, J., Li, Y.: Metal@semiconductor core-shell nanocrystals with atomically organized interfaces for efficient hot electron-mediated photocatalysis. Nano Energy 48, 44–52 (2018). https://doi. org/10.1016/j.nanoen.2018.02.040 56. Wang, H., Zhao, W., Xu, C.H., Chen, H.Y., Xu, J.J.: Electrochemical synthesis of Au@semiconductor core-shell nanocrystals guided by single particle plasmonic imaging. Chem. Sci. 10, 9308–9314 (2019). https://doi.org/10.1039/c9sc02804h 57. Zhang, N., Han, C., Fu, X., Xu, Y.J.: Function-oriented engineering of metal-based nanohybrids for photoredox catalysis: Exerting plasmonic effect and beyond. Chem. 4, 1832–1861 (2018). https://doi.org/10.1016/j.chempr.2018.05.005 58. Bian, Z., Tachikawa, T., Zhang, P., Fujitsuka, M., Majima, T.: Au/TiO2 superstructure-based plasmonic photocatalysts exhibiting efficient charge separation and unprecedented activity. J. Am. Chem. Soc. 136, 458–465 (2014). https://doi.org/10.1021/ja410994f 59. Fragua, D.M., Abargues, R., Rodriguez-Canto, P.J., Sanchez-Royo, J.F., Agouram, S., Martinez-Pastor, J.P.: Au-ZnO nanocomposite films for plasmonic photocatalysis. Adv. Mater. Interfaces. 2, 1–10 (2015). https://doi.org/10.1002/admi.201500156 60. Lu, J., Wang, H., Peng, D., Chen, T., Dong, S., Chang, Y.: Synthesis and properties of Au/ ZnO nanorods as a plasmonic photocatalyst. Phys. E Low-Dimensional Syst. Nanostructures 78, 41–48 (2016). https://doi.org/10.1016/j.physe.2015.11.035 61. Endo-kimura, M., Kowalska, E.: Plasmonic photocatalysts for microbiological applications. Catalysts 10, 824 (2020). https://doi.org/10.3390/catal10080824 62. Endo, M., Wei, Z., Wang, K., Karabiyik, B., Yoshiiri, K., Rokicka, P., Ohtani, B., MarkowskaSzczupak, A., Kowalska, E.: Noble metal-modified Titania with visible-light activity for the decomposition of microorganisms. Beilstein J. Nanotechnol. 9, 829–841 (2018). https://doi. org/10.3762/bjnano.9.77

Polymer-Based Hybrid Composites for Wastewater Treatment Veena Sodha, Jinal Patel, Stuti Jha, Megha Parmar, Rama Gaur, and Syed Shahabuddin

Abstract Over the past few decades, extensive research has been focused on polymers and polymer-based composites for environmental remediation focusing on wastewater treatment and desalination. Polymers are synthetic organic macromolecules that offer controllable properties, enhanced processability, porous surface for enhanced adsorption, high surface-to-volume ratios, selectivity in pollutant removal, and cost-effectiveness. Thus, polymer-based composite materials are the ideal materials for wastewater treatment, which can provide quality treatment in a facile manner. Thus, considerable scientific interest is diverted to the synthesis, characterizations, and water treatment efficacy of polymeric composite materials which may be in the form of membranes, photocatalysis, adsorption, etc. The following chapter highlighted recent trends in designing, fabrications, and applications of various polymeric composites which have been exploited for water treatment. The main focus has been emphasized on photocatalysis and adsorption via polymer and polymer-based composite materials for the effective removal of industrial pollutants, dyes, pesticides, and various other harmful organic pollutants. The chapter comprehensively reviews the recent research conducted by researchers for wastewater treatment via polymeric materials focusing on mechanisms of photocatalysis, adsorptions, kinetics, chemisorption, physisorption, etc. Also, the future prospects and challenges have been discussed for better research outputs from this chapter. Keywords Polymers · Photocatalysis · Nanocomposites · Adsorption · Wastewater treatment

V. Sodha · J. Patel · S. Jha · M. Parmar · R. Gaur · S. Shahabuddin (B) Department of Chemistry, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat 382426, India e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2_16

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1 Introduction Among the water available on earth, only 0.07% of freshwater is available for human consumption. This generation is facing a big issue of water scarcity and according to the present case scenario, if some legal actions and measures will not be taken, by 2080, 4.6 billion people will be suffering from the water crisis. There are varieties of reasons for water getting polluted. A major reason is the direct dumping of effluents, hazardous wastes, and chemicals into the natural-bodies without pre-treatment. Removing toxic contaminants like heavy metals, dyes, and biological waste is a task for industries as well. After use for several activities like bathing, and washing, water gets contaminated and is directly disposed into the natural water bodies that contaminate the freshwater as well. There are a variety of pollutants present in the wastewater like suspended solids, colored pollutants, organic and inorganic pollutants, synthetic waste, etc. [1] Wastewater causes many problems to not only humans but plants and the environment as well. It can cause several health risks to humans like polio, cancer, jaundice, cholera, etc. The marine plants get deprived form oxygen when river bodies get contaminated by hazardous pollutants. To overcome the problem, there are many treatment methods such as adsorption [1, 2], reverse osmosis [3], membrane filtration [4, 5], and photocatalysis [6, 7]. Among them, adsorption is a method that is sustainable, environmentally friendly, and cost-effective. Despite these advantages, the use of adsorbents suffers from some limitations like high-cost, low efficiency, recyclability, the problem of controlling pore size, etc. These limitations can be overcome by using polymer-based composite. Polymers are excellent materials having stability, flexibility, high surface area, etc. Polymer-based composites combine the adsorptive property of materials in a polymer blend. These composites are easy to prepare and have excellent stability. Polymers are combined with several adsorbent materials like graphene, zeolites, biochar, silicates, clay, and so on to prepare composites [8]. Polymer-based composites are the materials that are in demand for many applications including wastewater treatment using adsorption [1] and photocatalysis [6] techniques. Activated carbon [9, 10], zeolites [11, 12], biochar [13, 14], and chitosan [14] are some examples of adsorbent materials that are commonly used for wastewater treatment. Polymer-based composites are a blend of polymer and adsorbent materials hence combining the advantageous properties of both materials shows some exceptional usage and application in various sectors. Polymer-based materials consist of organic or inorganic polymer as a matrix and small nano-size materials in the nanoscale as fillers that act as reinforcement material. These composites can be used in automobile, aerospace, military, marine, wastewater treatment, and several other applications. Figure 1 shows the applications of polymer-based composites in the removal of water pollutants such as heavy metals, dyes, etc.

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Fig. 1 Applications of polymer-based composites for pollutants removal in water treatment

2 Classification of Polymer-Based Composites Polymer-based composites are the leading materials for the present world and are considered excellent candidates as compared to conventional adsorbent materials. Polymer-based composites can be synthetic, entirely natural, or a blend of synthetic and natural as well. Polymer-based composites are materials that contain polymer as a matrix or additives as reinforcement(filler) material. The additives could be onedimensional, two-dimensional, or three-dimensional, depending upon the requirement of the composite. The polymer matrix binds the fibers together and distributes the load applied among them uniformly. The choice of the material depends upon the role that they need to play. Together, or after merging they exhibit properties like facile synthesis, stiffness, anticorrosive, fatigue resistance, tuneable size, and shape, etc. [15]. These properties make them very demanding for several applications. As compared to other composites, polymer-based composites have low processing temperatures. Composites that are synthesized from biodegradable and natural sources are preferable choices compared to conventional ones because of their sustainability. Polymer-based composites are used in different sectors like sports, aerospace, automobile industry, wastewater treatment, military, etc. Different natural polymers like chitosan, guar gum, agar, cellulose, and synthetic polymers like polyamide, polyglycolic acid, and polylactic are used in biomedical applications. Based on the matrix and filler, polymer composites are classified into four major categories: (a) polymer–polymer composites in which the matrix and filler, both are polymeric materials; (b) polymer–carbon-based material composites in which carbon material is used as filler to provide strength, shape, and thermal stability to the composite; (c) polymer–zeolite composites where polymeric chains are developed in cavities of zeolite; and (d) polymer–metal oxide composites where metal oxide is converted to the nanoscale, they exhibit better hybrid properties. There is an

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Fig. 2 Classification of polymer-based composites

electrostatic interaction between positively charged metal oxide and lone pair of polymers. Polymer-based composites are the future materials for a variety of applications in various sectors. Figure 2 shows the generalized classification of polymer-based composites.

2.1 Polymer–polymer Composites The blending of different polymers has several advantageous properties and enhanced functional properties. Polymer is used both as a matrix and filler in polymer–polymer composites. Blending different polymers results in the improvement of functional properties, but this is only possible in miscible polymers pairings that are not stable thermodynamically. Blending immiscible polymers is not considered suitable because it leads to the formation of composites having weak mechanical strength. These limitations can be overcome by blending those polymers that have compatible morphology. Just as in ordinary composites, the matrix of polymer–polymer composites constitutes the main framework of the structure, and the reinforcement material gets dispersed into it. The quality and characteristics of this dispersion decide the final functionality and properties of the composite. If the polymer blend contains spherical particles, then the matrix impact resistance property gets enhanced and if there is a fiber-like structure, it enhances the unidirectional strength [16]. Fibrillar polymer–polymer composites are in trend because they offer several advantages compared to other composites such as easy processing, lightweight, no mineral additives required, sustainable and recyclable [17]. Polymer–polymer composites can be synthesized using in-situ polymerization. The blend of many polymers is considered sustainable because they can easily be broken down and recycled [16]. Polymer– polymer composites can be used for the design of biomaterials. Polymeric films can be prepared by combining different polymers using extrusion [18]. Polymer–polymer composites are used to remove toxic pollutants from wastewater. Co-polymer of acrylonitrile and styrene was synthesized using the polymerization technique. Using

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an electrospinning technique, this co-polymer was then fabricated on fiber. After functionalization, it was used for dye removal from water. Cyclodextrin and polydopamine were combined and used as a composite that showed good results for dye removal [19]. Polymer–polymer composites can be used to make membranes for water filtration and further can be used to remove heavy metal form wastewater. Two polymers can be combined using co-polymerization technique and the resultant material is made having high absorptivity and reduces the limitation of polymer aggregation. A porous polymeric composite was synthesized using chitosan and cyclodextrin that showed good results for heavy metal removal form wastewater [20]. The functional group of the polymeric composites are responsible for the interaction of the pollutants with the surface of the composite. Hence, these are emerging materials that can be used in various applications.

2.2 Polymer–Carbon-Based Materials Composites Carbon, which has different structures like fullerene, graphite, and carbon black, exhibits important physical and chemical properties. It can be used directly or used as a filler in composites of polymer. In the polymer matrix composite, different carbon-based materials like carbon fillers, and carbon fiber nanotubes are immobilized to make the polymeric material multifunctional. Polymer–carbon composite shows superior properties like low density, flexibility, high thermal stability, etc. Polymer–carbon composites show similar chemical properties to organic polymers. Carbon-based materials have strong and stable mechanical properties and these properties remain stable even after the introduction of polymer into it. The properties like the stability of the pores against chemical treatments, mechanical compression, and thermal stability are greatly enhanced when combined with carbon-based filler and converted to composite. Organic polymers have a limitation of variation in size, structure, and tuneable pore sizes. These limitations can be overcome in several inorganic materials exhibiting variations in shapes and pore size as well. The composite has dual properties of the polymer as well as carbon. The composite shows electrical properties like the conductivity of the carbon framework and the surface properties of the polymer. The polymer–carbon composites can be synthesized by having uniform pore sizes, different shapes, and structures. It can be synthesized using in-situ polymerization technique, melt mixing and filler dispersion [21]. Polymer–carbon composites have a variety of applications. In electrical properties, these composites can be used in solder joints, heat sinks, thermal interface material, electrically conductive adhesives etc. [22].

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2.3 Polymer–Zeolite Composites Zeolites are inorganic, hydrated aluminosilicate minerals having tetrahedral structures. They are porous structured material that has an intra-crystalline cage-like structure. They are used as adsorbent materials for removing various contaminates like dyes and heavy metals from water. The chains of polymer can be introduced into the cavities of the zeolite which improved their alignment and conductivity. The surface of the zeolite is acidic which makes the adhesion of the two materials easy. The polymer/zeolite system combines the mobility of the polymer with the property of the zeolite of exchanging cations. Various methods for the preparation of the polymer– zeolite composite are in-situ polymerization, solid-dispersion, mechanical mixing of polymer with the zeolite, etc. Several examples of polymer–zeolite composites are polypyrrole–zeolite composite, polythiophene–zeolite composite, etc. Polyaniline–zeolite composite is one such example that is most widely explored because of its environmental sustainability, facile synthesis and controllable properties. An efficient method to control the properties of the polyaniline is introducing the polymeric chains into the zeolite structure on a nanometre scale. One choice would be the polymerization of the aniline into the structural pores of the inorganic host like material like zeolite. Polypyrrole can be synthesized using chemical oxidative polymerization [23]. Various other polymers like polyfuran, poly (N-vinyl-carbazole), etc. can also be combined with the zeolite to make the composite as well but polyfuran would be the least opted material because of its complex synthesis and moisture sensitivity [24].

2.4 Polymer–Metal Oxide Composites Among the several nanomaterials, metal oxides are of great interest because of their technological applications. Metal oxides as pure materials do exhibit good properties but when they are converted to the nanoscale, they show increased hybrid properties. Metal oxides are considered one of the most useful materials because of their unique properties like photoelectronic, thermal, electronic, magnetic, and several other properties [25]. There are mainly three methods of synthesizing metal oxide/polymer composite. The first one is in-situ polymerization [26]. In this method, a dispersion of metal oxide is made in the monomer and polymerization will take place. The second method is mixing or blending both components via melting. The last method is the sol–gel process [27]. Metal oxides have a variety of applications in gas sensing [28], catalytic application [29], and the energy sector. The application of metal oxide as adsorbent has several limitations. When metal oxide is converted to nanoscale, its surface area does increases but the increased surface area leads to agglomeration as metal oxides have wander wall interactions, making the metal oxide less stable.

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To overcome these limitations, metal oxides are combined with polymeric materials. In composites, polymers provide some exceptional characteristics such as flexibility, easy processing, tuneable electrical properties, photoluminescence, etc. Metal oxide provides enhanced magnetic properties, tuneable band gap, and mechanical stability. The properties of the composites are greatly influenced by the interaction between the polymer and metal oxide. These composites have a variety of applications in adsorptive technologies, membrane technologies, ion-exchange, etc. Polymeric materials like polypropylene, polyimide, and polysulfide are used in membrane materials owing to their excellent mechanical stability and chemical resistance [26]. Incorporation of metal oxide into the polymeric membrane shows photodegradation of the contaminants present in wastewater. TiO2 was immobilized into the polyethylene membrane and it shows good degradation of 1,2 dichlorobenzene. TiO2 was also combined with the polyaniline and it also reduces the agglomeration and antifouling of membranes [25]. Polymers like polypyrrole, and polythiophene has a strong affinity towards cations as polymers have lone pair. There is a strong electrostatic attraction between the positively charged metal oxide and lone pair of polymers. Nowadays, several ion-exchange membranes are fabricated with a polymeric material that forms very unique and new ion-exchange membranes that offers good conductivity and stability [25]. Polyimide(PI)/ZnO, polypyrrole(PPy)/ZnO, Au/TiO2 /polyaniline(PANI) are some examples of polymer–metal oxide composites that are currently in demand for a variety of applications [25]. When the non-conducting polymers are combined with another non-conducting material, they are unable to show the photocatalytic behavior and only show adsorption. For example, polyethylene glycol–chitosan composite [30]. When the polymers are used with the semiconducting materials such as metal oxides [31], carbon-based materials [32] etc., the resultant composites show a good photocatalytic behavior and can be applied in degradation processes. For example, PANI–CdO [33], PANI–TiO2 – rGO [34] composite. When the composite is made up of one conducting polymer, for example, polyaniline–chitosan [35] composites, etc., the materials are capable of showing the photocatalytic activities. Conducting polymers with metal oxides and carbon-based materials is of great interest due to their increased degradation efficiencies in a binary composite photocatalytic system.

3 Wastewater Treatment by Polymer-Based Hybrid Composites Polymer-based hybrid composites have been widely explored for the treatment of wastewater via three processes, namely (a) adsorption, (b) photocatalysis and (c) desalination. These processes are discussed in detail as follows.

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3.1 Adsorption of Contaminants Adsorption is one of the most viable and efficient methods for wastewater treatment. It is a trending method for the past many decades since it has low operational cost, simple experimental setup, and an economically friendly method for removal of contaminants from wastewater [13, 36]. Both natural and synthetic polymers have been extensively used for the mitigation of the contaminants present in the water. Many reports are available in the literature for the removal of pollutants like dyes, organic molecules, heavy metals, oil spills, agrochemicals, pharmaceuticals, etc. Polymer-based composites can be used in powdered form, as membranes, pellets, etc. based on the application and feasibility [37, 38]. Cellulose was grafted with 2-acrylamido-2-methylpropane sulfonic acid and acrylic acid in order to prepare a polymer-based composite for adsorption of malachite green (36.80%) and crystal violet (50.17%). Thus, they concluded that the polymer composite can be a potential adsorbent for the adsorption of cationic and anionic dyes [39]. Fe3 O4 coated polymer clay composite was synthesized by Arya et al. The composite was used for the removal of atenolol, ciprofloxacin and gemfibrozil. It exhibited 15.6, 39.1 and 24.8 mg/g removal for atenolol, ciprofloxacin and gemfibrozil, respectively [40]. Starch-SnO2 nanocomposite was prepared by and evaluated for the adsorption of Hg (II). It showed an adsorption efficiency of 192 mg/g [41]. Permethrin was adsorbed by using chitosan–zinc oxide composite as demonstrated by Dehaghi et al. The composite showed an excellent adsorption performance (99%) in 45 min [42].

3.2 Photocatalytic Degradation A few percent of the world’s population are privileged to have access to safe drinking water. Thousands of individuals die each year from deadly waterborne diseases, as reported by the WHO. This data may rise in the upcoming decades due to the increased release of pollutants and micropollutants into the water supply. Membrane filtration, distillation, sedimentation, adsorption, and many other processes have higher operating costs and can release more pollutants into the environment that could be dangerous. To tackle the increasing clean water crisis, photocatalysis technology with excellent photocatalytic efficiency and low costs needs to be developed. A class of polymers includes conducting polymers which are of high interest due to their electrical conductivity, facile synthesis, optical properties, and high mechanical strength. The presence of conjugated π-bonds in the conducting polymers is responsible for their electrical and optical properties [43]. These conducting polymers are able to excite their electrons in the presence of light and generate electron–hole pairs, similar to a semiconductor. Some examples of conducting polymers are PANI, PPy, polyacetylene (PA), polythiophene (PTH), polyfuran (PF) etc. Conducting polymers acquire HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) similar to the VB (Valance Band)

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and CB (Conduction Band) of the semiconducting materials. When exposed to the light source the electrons from HOMO of conducting polymers get excited and reach their LUMO. This way the electron–hole pair is generated and which in turn is responsible for the generation of (Active Oxygen Species) AOS. Though, the electron–hole recombination greatly affects the photocatalytic activity of conducting polymers, which can be overcome via a combination of some other photoactive materials. The mechanism of photocatalysis is discussed in Sect. 4.1. Gilja et al. synthesized the PANI/ZnO composite via in-situ polymerization and evaluated it for the photocatalytic degradation of Acid Blue 25 (AB25) dye. Within 60 min of irradiation time, 90% removal of 30 ppm AB25 dye with 1 mg/ml catalyst dosage was achieved under simulated solar irradiation [44].

3.3 Desalination and Water Treatment Desalination is the process of removing dissolved salts from saltwater. For desalination, there are three main types of water purification technologies are used: membrane technique, distillation techniques, and chemical approaches [45]. In Pervaporation (PV) processes, hydrophilic membranes are used to increase the water flux. While in membrane distillation processes, mostly hydrophobic membranes are used. Hydrophilic polymers have a high-water absorption capacity, which causes them to have uncertain desalination performance and a short lifespan. Higher swelling levels can also cause the hydrophilic polymeric barrier to leak to the penetrating side. As a result, using a hydrophobic mesh can be an important strategy to increase membrane longevity [46]. Additionally, there have been applications for pure polymeric membranes, inorganic material loaded polymeric membranes, composite membranes containing multiple polymers, and inorganic membranes which will be discussed in detail in further sections [47]. Polymer-based composites have shown good potential for the removal of other water pollutants such as oxygen-demanding wastes, nitrates, agrochemicals etc. Therefore, the polymer-based composites can be employed in water treatment technologies.

4 Mechanism of Photocatalysis and Adsorption 4.1 Photocatalysis Mechanism Photocatalysis refers to a photochemical reaction that gets catalyzed in the presence of a photocatalyst. It entails the absorption of photons with energies (hv) that are equal to or higher than the bandgap energy (Eg) of material, followed by the transfer of electrons (e− ) from its valence band to the conduction band, resulting in the

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simultaneous production of a hole in the valence band. These electron–hole pairs react with available oxygen and generate AOS (Active Oxygen Species) such as hydroxyl and superoxide radicals. These AOS species immediately attack the organic moiety and break them down into smaller molecules. The fundamental principle of photocatalysis is that when organic molecules are exposed to UV light, they come into contact with the surface of the photocatalyst, which starts a chain reaction of oxidation and reduction (redox) processes. At least two simultaneous reactions oxidation from photo-generated holes and reduction from photo-generated electrons must take place during this procedure. Reactive hydroxyl radicals are created as a result of the degradation of difficult molecules. There are two types of photocatalytic reactions one is heterogeneous photocatalysis and homogenous photocatalysis. The catalyst and substrate are in different phases in heterogeneous photocatalysis. In homogeneous photocatalysis, the catalyst and the substrate are both elements that comprise the same phase. AOPs include heterogeneous photocatalysis, which entails the interaction of materials with light photons to cause photocatalytic degradation. As photosensitizers, a variety of semiconductor materials have been explored including polymers, carbon-based materials, metal oxides, metal sulfides, and polymer–carbon composites [48]. Due to their sizeddependent characteristics, the use of nanomaterials allows researchers to properly modify and tune the band gap of the photocatalyst. The efficiency of a single-component photocatalytic system is low due to the recombination of electrons and holes. The electron–hole recombination can be reduced significantly by using a multi-component system. In a multi-component system, materials work synergistically enhancing the efficiency of photocatalytic reactions as shown in Fig. 3. In a binary composite system, when the light strikes, electrons from the valance band of a material having a lower band gap excite to its conduction band. Instead of going to its own valance band, electrons jump to the conduction of another component. This phenomenon causes reduced electron–hole recombination and faster generation of AOS, which ultimately leads to faster degradation of organic contaminant.

Fig. 3 Schematic illustrations of single and multi-component photocatalysis

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The photocatalysis mechanism can be explained as below from equations (1) to (7). In which, charge carriers are produced, followed by radical formation. Photocatalyst→hv≥E g e− + h +

(1)

h + + H2 O → O H • + H +

(2)

h+ + O H − → O H •

(3)

e− + O2 → O2•−

(4)

O2•− + H + → O O H •

(5)

2O O H • → H2 O2 + O2

(6)

H2 O2 + O2•− → O H − + O H • + O2

(7)

) ( P O Ps + h + , O H • , O2•− , O O H • or H 2 O2 → Degraded pr oducts

(8)

4.2 Adsorption Mechanism In the adsorption process, there are various interactions occurring between the target molecule and the surface of the adsorbent. These interactions mainly depend on textural and chemical properties like the nature of the analyte to be adsorbed, functional groups on the surface of the adsorbent and adsorbate, pH of the solution, surface charge on the adsorbent, etc. Thus, the mechanism involved for each type of analyte is different. A detailed understanding of the adsorption mechanism enables the researchers to enhance the application as well as the efficiency of the adsorbent material. Mechanisms like hydrogen bonding, complexation, hydrophobic interactions, pore-filling, precipitation, electrostatic interactions, and ion-exchange, are responsible for the adsorption process. Figure 4 shows the mechanism of adsorption on the surface of adsorbent via different interactions [49].

4.2.1

Hydrogen Bonding

Hydrogen bonding is a dipole–dipole interaction between hydrogen atom and an electronegative atom having lone pair of electrons like O, N, Cl or F [50]. In this

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Fig. 4 Adsorption mechanism for adsorption of organic molecules on the adsorbent surface. Reproduced with permission from Springer, 2019 [49]

type of bonding, the molecule having the hydrogen atom acts as an electron acceptor while the molecule having electronegative atoms functions as a donor. Hydrogen bonding is stronger than Van der Waals force but weaker than covalent and ionic bonds [51]. Different functional groups present on the surface of adsorbent and the pollutant molecules are responsible for the formation of hydrogen bonding during the adsorption process [52]. The involvement of hydrogen bonding was discussed by Chen et al. in their study regarding sorption-induced swelling of polymers.

4.2.2

Complexation

Generally, the adsorption of heavy metals occurs via complexation. This mechanism involves the formation of co-ordination bond between the transition metal ion and ligand present on the surface of the adsorbent. Functional groups containing oxygen, nitrogen, and sulphur serve as a bonding site for the adsorption of metals. ZnO nanoparticles were used as an adsorbent for the removal of radionuclides and heavy metals by Akpomie [53]. Huang et al reported the adsorption of Hg (II) by sulfurized biochar. Both the adsorption processes were found to occur via complexation mechanism [54].

4.2.3

Pore–Filling

Pore–filling is considered as one of the prime mechanism associated with the adsorption of organic contaminants. It is based on the porous network and pore dimension

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of the adsorbent. The adsorbent material is categorized into microporous, mesoporous, and macroporous based on the pore size. This mechanism relies upon the size of the model pollutant. The adsorbent gets adsorbed on the surface of the adsorbent by getting trapped in the pores present on the surface of adsorbent. Based on the desired analyte to be treated the pore size can also be modified by altering the preparation reaction conditions, pre-treatment and post-treatment of the adsorbent. Liu et al. in their report explained in detail the mechanism of adsorption on a porous adsorbent [55]. Adsorption of hydrocarbons on wood-derived biochar was studied by Nguyen et al. In their study, they provided evidence for the involvement of pore–filling interaction in the adsorption process [56].

4.2.4

Precipitation

A solid ionic compound formed due to reactions between cations and anions in a medium is known as a precipitate and the process is termed as precipitation. The solid precipitates are either formed on the surface of the adsorbent or in the aqueous medium. The formation of precipitates depends on the pH of the solution, as its solubility changes in acidic and basic conditions [57]. Selective precipitation can also be carried out depending on the nature of the analyte and adsorbent by performing the adsorption process in a specific environment and controlled condition. The adsorption of heavy metals usually takes place by precipitation or salting out. [58]. Hashim et al. reported the precipitation mechanism to be the driving factor behind adsorption of petroleum sulfonate on limestone. Ca2+ released from the limestone leads to the precipitation of petroleum [59]. The role of precipitation in lead ions getting immobilized in soil was explained in detail by Zeng et al. [60].

4.2.5

Electrostatic Interaction

Electrostatic interaction is based on the attraction between opposite charges and repulsion between like charges [61]. Readily ionizable pollutants adhere to the surface of the adsorbent through attraction forces between unlike charges. The strength of electrostatic bond depends on the magnitude of the charge and the distance between the charged atoms. The role of electrostatic interactions in the adsorption of proteins on a charged surface was documented by Hartwig et al. [62]. Electrostatic interaction was found to be the mechanism of adsorption of different cationic and anionic dyes on the surface of MXenes as demonstrated by Lim et al. [63]. The adsorption of nanoparticles at fluid–fluid interface occurring due to electrostatic interaction was reported by Dugyala et al. [64].

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Hydrophobic Interaction

When water-repellent, non-polar groups are present in the adsorbent material it is likely to participate in hydrophobic interactions during the adsorption process. It is a property of non-polar molecules to avoid or have a low affinity for water [65]. It is also a measure of non-polar group to aggregate in an aqueous solution [66]. It is one of the major driving forces for the adsorption of oil, hydrophobic molecules, and neutral organic compounds. Thus, if the targeted analyte to be adsorbed is hydrophobic or has low solubility in water, the hydrophobic interaction predominates [67]. Tilton et al. in their study correlated the adsorption of protein with hydrophobicity of the adsorbent [68]. The role of hydrophobic forces on the oil spill cleanup using adsorption was well described by Zhou et al. They stated that the presence of hydrophobic sites on the adsorbent surface enhanced the sorption capacity of oil [69]. The adsorption of proteins is also facilitated on a hydrophobic adsorbent surface owing to the hydrophobic nature of protein [70].

4.2.7

Ion-Exchange

Ion-exchange refers to a process where interchange (exchange) of ions takes place between the solid and liquid interface. In terms of adsorption, it is the swapping of ions between the adsorbent and the aqueous medium containing the pollutant [71]. The ions with similar size and charge get exchanged in a way that the number of ions leaving the adsorbent surface and coming from the aqueous medium remains the same. Electrical neutrality of the medium is maintained during the ion-exchange process. Mostly, the adsorption of heavy metals occurs via ion-exchange mechanism. This mechanism depends on the pH of the aqueous medium and the point of zero charge (PZC) of the adsorbent [72]. At pH lower than PZC the adsorbent has positive charge which promotes the exchange of cationic (positively charged) pollutant molecules. In contrast to this, at pH greater than PZC the adsorbent has negative charge on its surface [73]. This leads to the exchange and thus adsorption of positively charged pollutant molecules. Polymeric resins were used for the adsorption of phenol in a study by Caetano. They demonstrated the role of ion-exchange mechanism by varying the pH in the adsorption process [1].

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5 Applications of Various Polymer Composites in Wastewater Treatment 5.1 Polymer–polymer Composites Polymers have a wide range of applications because they can be molded easily based on the desired application. However, in certain domains using only polymer does not serve the complete purpose. Fabrication of composite enables us to overcome the gap by providing superior and enhanced properties as compared to the parent compounds. A polymer–polymer composite is a material in which a polymer functions as a matrix and reinforcement, both. It offers added advantages like high modulus, tuneable properties, controlled molecular weight, high strength to weight ratio, etc. Polymer– polymer composites are used for the adsorption of various noxious pollutants like dyes, heavy metals, and organic molecules as mentioned below (summarized in Table 1).

5.1.1

Polymer–polymer Composites for Dye Removal

Various advancements and novel technologies have been adopted by scientists all over the world for the adsorption of organic dyes. Cationic and anionic dyes like methylene blue, methyl orange, rhodamine B, congo red, crystal violet, etc. are released from the industrial sector. Untreated dyes persist in the environment causing water, land, and air pollution. Adsorption has been one of the widely adapted techniques for dye removal. Polymer–polymer-based composites have attracted a lot of attention in recent years for their contribution as an adsorbent and photocatalysts in dye removal [74, 75]. Conducting polymers and organic dyes share a similar structure which increases their chances of interaction with each other [76]. The adsorption of dyes on such types of composites mostly occurs via hydrogen bonding and electrostatic interactions [77]. If one or both of the polymers in the composite is a conducting polymer then the material can work as a good photocatalyst and can be employed for dye degradation [35]. Liu et al. demonstrated the removal of methylene blue and methyl orange dye as individual components and also simultaneously using polymer–polymer composite as an adsorbent [78]. The schematic of the synthesis and removal of dyes is shown in Fig. 5. Firstly, they synthesized microspheres of poly (styrene–methyl methacrylate– acrylic acid) (PSMA) via emulsion method. PANI was then coated on the surface of the microsphere to prepare a composite. The composite microsphere showed selectivity towards methyl orange, eliminating about 90% of dye from the dye mixture. From the result, they also concluded that the composite was an effective adsorbent for the selective removal of anionic dye from a mixture of anionic and cationic dyes present in wastewater [78] Majhi et al. synthesized polyaniline–sodium alginate composites for removal of different organic dyes. Incorporation of sodium alginate into polyaniline reduced

Photocatalysis 20

Poly(vinyl alcohol)–chitosan

Chitosan–PANI

Eosin yellow, methylene blue

Methyl orange

Polyethylene glycol–chitosan

Adsorption

Adsorption

Polyamine-type starch/glycidyl methacrylate

Cr (III)

Adsorption

Lignin-poly(3,4-ethylenedioxythiophene)–polystyrene Adsorption sulfonate

Other Phosphate contaminants anions

200

2

10

–

1

Photocatalysis 0.1

Pb (II)

Poly-ε-caprolactone–chitosan

Adsorption

Direct blue 67 Poly(vinyl alcohol)–alginate-rice husk

2.5

Adsorption

Poly(vinyl alcohol)–poly(acrylic acid)

Methylene blue

1 –

–

Light source

60 min

12 h

13 min

3.02 mg/g

34.20 mg/g

27.27 mg/g

Efficiency

–

–

–

[2]

[91]

[35]

[75]

[90]

[89]

[88]

Ref

74.85 mg/g

[30]

0.56 mmol/ [92] g

452.8 mg/g

165.59 mg/ g

Sunlight 89.5%

Sunlight 95%

120 min –

90 min

2h

400 min –

–

60 min

Dosage Time (mg/ml)

Polyaniline–chitosan composites

Adsorption

Mechanism

Acid black-234

Heavy metals Zn (II)

Dyes

Contaminants Material

Table 1 Removal of various pollutants using polymer–polymer composites

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Fig. 5 PANI coated microspheres of poly (styrene–methyl methacrylate–acrylic acid) for adsorption of methyl orange and methylene blue. Reproduced from MDPI, licensed under Creative Common License 4.0 (Creative Commons—Attribution 4.0 International—CC BY 4.0) [78]

the cost and increased the biodegradability and selectivity of the prepared material. Methylene blue (555.5 mg/g), Rhodamine B (434.78 mg/g), Orange-II (476.19 mg/ g), and Methyl orange (416.66 mg/g) dyes were found to be effectively removed by the polymer–polymer composite within 240 mins [79].

5.1.2

Polymer–polymer Composites for Desalination

Desalination is the process of removing dissolved mineral salt from seawater. Apart from salts, it also means the exclusion of harmful bacteria, metals, and chemicals [80]. This allows the production of more fresh water to make it suitable for various purposes. Membrane desalination is one of the established methods for desalination [4]. Ashraf et al. synthesized a membrane of polyvinyl alcohol, cellulose acetate, polyethylene glycol, etc. and evaluated it for desalination of groundwater, saline water, and extremely saline water. The membrane was shown to exhibit excellent results for all the water samples analyzed [81]. Many reports are available on the use of polymer–polymer composite for desalination purpose by fabricating it into a membrane. Koriem et al. prepared a membrane of polyvinylidene fluoride (PVDF) and cellulose acetate composite. The composite membrane was further impregnated with UiO-66 nanofiller to enhance the performance of the membrane in terms of salt rejection. The membrane was found to exhibit an excellent salt rejection of 90.2%, proving it to be a potential candidate for water desalination. The entire work is represented in Fig. 6 [82].

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Fig. 6 Graphical representation of cellulose acetate–polyvinylidene fluoride membrane for water desalination. Reproduced with permission from Springer, 2023 [82]

5.1.3

Polymer–polymer Composites for Heavy Metal Removal

Application of polymer–polymer-based adsorbent for removal of heavy metal has been on forefront due to their multiple advantages. The composites have enhanced chemical, physical and mechanical properties as compared to both counterparts [83]. Different functional (organic and inorganic) groups present in the composite material promote the removal of toxic heavy metals like chromium, lead, arsenic, mercury, etc. [84]. Chromium and copper were adsorbed using a polymer–polymer composite as reported by Zeng et al. In their study, they prepared magnetic chitosan– polyethyleneimine–sodium alginate composite and used is as an adsorbent for Cr(VI) and Cu(II) heavy metal ions. The composite showed adsorption capacity of 87.53 mg/ g and 351.03 mg/g for Cr(VI) and Cu(II), respectively. The adsorption of these metal ions was then carried out in the presence of azo dyes (methyl orange and methylene blue). The adsorption of heavy metals was enhanced in the presence of dyes. Thus, an effective adsorbent was prepared to solve the problem of simultaneous removal of heavy metals and dyes [85]. A novel polyaniline–magnetic chitosan composite was explored for the treatment of wastewater containing chromium. The composite exhibited 80% adsorption efficiency within 15 min and easy magnetic separation properties. The nitrogen containing functional groups present in the composite contributed to the adsorption process. The functional groups interacted with chromium through electrostatic forces and also served as a redox pair for the reduction of Cr(VI) as reported in the study by Lei et al. [86]. Figure 7 shows the synthesis and removal mechanism of Cr(IV) ions via magnetic chitosan composite [86].

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Fig. 7 Magnetic chitosan–polyaniline composite for reduction and adsorption of chromium from wastewater. Reproduced with permission from Elsevier, 2020 [86]

5.1.4

Polymer–polymer Composites for Removal of Other Contaminants

Besides dyes and heavy metals various harmful organic molecules, inorganics, CO2 , and oil are also effectively removed by treating the wastewater using polymer– polymer composites. Phosphates, ammonia, chlorides, fluorides, oil-spills, phenols, nitrates, etc. have been reported to be successfully removed by polymer–polymer composites through adsorption, membrane filtration, and photocatalysis. Polyvinyl alcohol–chitosan–TiO2 composite was synthesized by Neghi et al. via precipitation method. The composite was then used for photocatalytic degradation of metronidazole which is an imidazole antibiotic. 100% removal of metronidazole within 120 mins using 32 W UV radiation as a source was demonstrated in the study. The schematic representation of the work is as shown in Fig. 8. Thus, the composite was reported to be a suitable candidate in removal of pharmaceuticals from the wastewater [87].

5.2 Polymer–Metal Oxide Composites Semiconductor materials have drawn increased attention in research and development in the field of photocatalysis and adsorption. The incorporation of metal oxides to polymers leads to the uniform dispersion of metal oxides nanoparticles on the surface of polymer. This phenomenon causes an increase in total surface area which leads to increased adsorption and photocatalytic efficiency. In non-conducting polymer and

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Fig. 8 Removal of metronidazole using polyvinyl alcohol–chitosan–TiO2 composite. Reproduced with permission from Elsevier, 2019 [87]

metal oxide composites, the polymer matrix acts as a support for metal oxides thereby increasing their surface area along with adsorption and photocatalytic efficiencies. In the conducting polymer–metal oxide system, both materials work synergistically in electron–hole transfer, which increases the photocatalytic efficiency (Fig. 9) [93]. The major drawback faced by photocatalysts is that they are only active in UV light of the solar spectrum [94]. Hence, the research in this area is focused on the development of such materials which are active in the solar spectrum. Herein we have reviewed the combination of polymer along with semiconductor materials for the treatment of pollutants by adsorption and photocatalysis approach.

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Fig. 9 Photocatalytic degradation mechanism in polymer–metal oxide composites system. Reproduced with permission from Elsevier, 2016 [93]

5.2.1

Polymer–Metal Oxide Composites for Dyes Removal

Dyes are organic molecules, hence their removal mechanism by polymer–metal oxide composites can be adsorption as well as photocatalytic degradation. The industrial sector produces cationic and anionic dyes such as malachite green, methylene blue, methyl orange, rhodamine B, etc. Untreated dyes are still present in the environment, and water and pollute the air. The most commonly used methods for dye removal are photocatalysis and adsorption. Eskizeybek et al. show 99% removal of dyes within 5 h in the presence of natural sunlight using PANI–ZnO [31]. Wang et al. reported 81% removal of methylene blue in 2 h using PANI–TiO2 composites in the presence of visible light [95]. Gulce et al. show the different dyes in different lights. Gulce et al. show the removal of methylene blue in UV light at 92% and in sunlight, it shows 98% removal. Gulce et al. shows the removal of other dye malachite green in UV light at 97% and in sunlight, it shows 99% removal [33].

5.2.2

Polymer–Metal Oxide Composites for Heavy Metal Removal

Via polymer–metal oxides composites, heavy metals are typically removed through adsorption process. Table 2 provides a thorough review of the photocatalytic and adsorption functions that polymers containing metal oxides as composites for environmental applications can carry out. Fallah, Z et al. reported removal of 3 different heavy metals with the help of TiO2 -grafted cellulose composites [96]. The composite showed 102.04 mg/g, 102.05 mg/g and 120.48 mg/g efficiency for Zn+2 , Cd+2 and Pb+2 in just 60 min, respectively. Naushad et al. reported Hg+2 at 94% removal within 60 min using starch-SnO2 which is shown in Fig. 10 [41].

Other contaminants

Heavy metal

Dyes

Magnetic imprinted PEDOT-CdS nanoreactor

Danofloxacin mesylate

Starch-SnO2

Hg2+

ZnO-PANI nanocomposite

TiO2 -grafted cellulose

Zn2+ , Cd2+ , Pb2+

Ampicillin

PANI-CdO

MB and MG

TiO2 -PEDOT

PANI-ZnO

MB and MG

Bisphenol-A

PANI-TiO2

Material

MB

Contaminant

Photo Catalysis

Photo Catalysis

Photo Catalysis

Adsorption

Adsorption

Photo Catalysis

Photo Catalysis

Photo Catalysis

Mechanism

Table 2 Removal of various pollutants using polymer–metal oxide composites

0.02

0.01

–

0.4

0.5

0.4

0.4

–

Dosage (mg/ml)

60 min

120 min

60 min

60 min

60 min

4h

5 h for both dyes

2h

Time (min)

Visible

Sunlight

Simulated sunlight

–

–

UV and natural sunlight

Natural sunlight

Visible

Light source

84.8%

41%

56%

(continued)

[100]

[99]

[98]

[41]

[96]

Zn2+ = 102.04 mg/g Cd2+ = 102.05 mg/g Pb2+ = 120.48 mg/g 192 mg/g 94%

[33]

[31]

[95]

Ref

MB 92% (UV), 98% (Sunlight) MG 97% (UV), 99% (Sunlight)

99%

81.74%

Efficiency

370 V. Sodha et al.

TiO2 @V2 O5 -PPy

TC, DC and OTC

Photo Catalysis

Mechanism 0.6

Dosage (mg/ml) 120 min

Time (min)

MB-methylene blue, MG-Malachite green, TC-Tetracycline, DC-Doxycycline, OTC-Oxytetracycline

Material

Contaminant

Table 2 (continued)

Visible

Light source

Ref

98%, 96% and [97] 85% for TC, DC, and OTC respectively

Efficiency

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Fig. 10 Schematic image of removal of Hg2+ using Starch/SnO2 nanocomposite. Reproduced with permission from Elsevier, 2016 [41]

5.2.3

Polymer–Metal Oxide Composites for Other Pollutant Removal

Other pollutants that are harmful to human beings and the ecosystem include agrochemicals, organic molecules, medicines, and drugs. These pollutants have also been efficiently removed by researches using polymer–metal oxide composites. As the agrochemical, drugs, and pharmaceutical wastes are the organic molecules they can be easily adsorbed and degraded by using polymer–metal oxides composites. The inorganic pollutants such as ammonium ions, chlorides, etc. can easily be removed by using adsorption technique. Liu et al. developed TiO2 @V2 O5 -Polypyrrole which exhibits highly photocatalytic performance in visible light for removal of TC, DC and OTC [97]. Katanˇci´c et al. reported 56% removal of Bisphenol-A in the presence of light in 60 min using TiO2 –PEDOT composite [98]. Nosrati et al. reported 41% removal of Ampicillin in the presence of sunlight in 120 min using ZnO–PANI nanocomposite [99]. Lu et al. demonstrated 84% removal of Danofloxacin mesylate using Magnetic imprinted PEDOT-CdS nanoreactor in just 60 min [100].

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5.2.4

373

Polymer–Metal Oxide Composites for Water Treatment and Desalination

There are many processes that use nanomaterials for a variety of purposes, such as adsorption, photocatalysis, desalination, or membrane technologies, is the purification of water. Desalination is the process that is used to purify saltwater of dissolved mineral salts. Under the pressure of a water crisis, effective and environmentally friendly desalination technologies have been developed. To address this issue, several approaches have been put up. In Comparison with conventional materials, carbonbased materials desalination process has attracted a lot of interest recently [101]. Vahid Vatanpour et al. reported the use polymer–metal oxide composite (TiO2 coated MWCNTs with PES) membranes prepared by the phase inversion process for desalination [102]. The primary objective is to improve efficiency and also get clear of membrane technology difficulties like pollution and poor durability. The properties of many metal oxide polymeric membranes, such as TiO2 , Al, ZrO2 , etc., have been explored after facing various modifications. Nanomaterials are able to do this through improving the surface area, selectivity, permeability, resistance, strength, and other possible qualities [103]. Membrane technologies are utilized for the desalination of water as well as for the treatment of microorganisms [103].

5.3 Polymer–Carbon-Based Material Composites Because of their wide range of applications and flexibility, polymers are able to serve a variety of fields. However, in some areas, using only polymers shows less efficiency along with some drawbacks. Carbon is one of the attractive materials that had gained a lot more attention because of its exceptional chemical and physical properties which can be employed in a broad range of applications. Polymer–carbon-based composites are the ones in which a polymer serves as both a matrix and a stabilizer. By offering better and increased qualities than the parent materials, the fabrication of composites helps us overcome the drawbacks. Hazardous water pollutants such as dyes, heavy metals, and other organic pollutants can be adsorbed and degraded using polymer–carbon composites. In polymer–carbon-based materials composites, the photocatalytic mechanism is widely explored due to the great photocatalytic activities of carbon-based materials. As shown in Fig. 11, in g-C3 N4 and polymer, both materials can be stimulated by visible light because of their lower band gaps [32]. Table 3 displays the photocatalysis and adsorption of various pollutants, including dyes, heavy metals, organic compounds, and other pollutants, using polymer and carbon-based composites.

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Fig. 11 Photocatalytic degradation mechanism of polymer–carbon-based composites system. Reproduced with permission from Royal Society of Chemistry, 2012 [32]

5.3.1

Polymer–Carbon-Based Material Composites for Heavy Metal Removal

A list of polymer–carbon composites using adsorption for removal of heavy metal can be found in Table 3. One of the methods for removal of heavy metal which is commonly applied is adsorption. There has also been extensive research on composite of polymer with carbon components. Li et al. developed magnetic cyclodextrin– chitosan/GO for removal of Cr (VI) which is highly toxic in the environment [104]. Li et al. demonstrated the removal of chromium ions with 61.31 mg/g efficiency within 5 h contact time using magnetic cyclodextrin–chitosan-GO [104]. Alghamadi et al. reported the removal of Pb+2 using Polypyrrole-activated carbon with 50 mg/g efficiency with 4 h contact time [9].

5.3.2

Polymer–Carbon-Based Material Composites for Dye Removal

Researchers across the world have explored a variety of technologies for the removal of organic dyes. To remove dyes from water, three main processes are available such as photocatalysis, adsorption, and membrane separation. The researchers have extensively used photocatalysis for the removal of organic dyes which are present in water environment. Ge et al. shows 92% removal of dyes using PANI–g-C3 N4 composites in the presence of visible light [32]. Ameen et al. reported 56% removal of rose Bengal dye in the presence of visible light using PANI–Graphene composites as shown in Fig. 12 [105]. Ma et al. reported a ternary system for removal of dye in

PPy@Ag-g-C3 N4

PPy Grafted, Chitosan Decorated CDs

EH

2-chlorophenol

Photocatalysis

PPy@Ag-g-C3 N4

PPy@Ag-g-C3 N4

CIP

GFLX

PPy@Ag-g-C3 N4

PPy@Ag-g-C3 N4

DM

TC

Photocatalysis

Photocatalysis

Photocatalysis

Photocatalysis Photocatalysis

g-C3 N4 - and PANI-co-modified TiO2 nanotube arrays (NTAs)

Photocatalysis

Adsorption

Adsorption

TBBPA

Magnetic cyclodextrin–chitosan/ GO

Cr(VI)

Photocatalysis

PANI TiO2 -rGO

Polypyrrole-Activated Carbon

RhB

Pb2+

Photocatalysis Photocatalysis

PANI-g-C3 N4

PANI-Graphene

MB

Rose Bengal dye

-

1

1

1

1

1

–

1

5

0.5

2

Dosage (mg/ml)

> 96.7%

60 min

60 min

60 min

60 min

60 min

>94%

5h

4h

90 min

3h

120 min

Time (min)

Sunlight

Visible

Visible

Visible

Visible

Visible

Visible

Visible

Visible light

Visible

Light source

90 min

95%

90%

93%

95%

90%

120 min

61.31 mg/g

50 mg/g

90.5%

~56%

92.8%

Efficiency

[108]

[107]

[107]

[107]

[107]

[107]

[106]

[104]

[9]

[34]

[105]

[32]

Ref

MB-Methylene blue, RhB-Rhoda mine-B, DM-Danofloxacin mesylate, TC-tetracycline, CIP-ciprofloxacin, GFLX-gatifloxacin, EH-Enrofloxacin hydrochloride, TBBPA-tetrabromobisphenol A

Other Contaminants

Heavy Metal

Dyes

Mechanism

Material

Contaminant

Table 3 Removal of various pollutants using polymer–carbon-based material composites

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Fig. 12 Schematic image of photocatalytic degradation of RB dye over the surface of PANI-Gr nanocomposite. Reproduced with permission from Elsevier, 2012 [105]

the presence of visible light. The composite shows the 90% removal pf rhodamine-B in 90 min using PANI–TiO2 –rGO [34].

5.3.3

Polymer–Carbon-Based Material Composites for Other Pollutants Removal

Groundwater contains a variety of hazardous pollutants, including metals, pesticides, and substances that drain from contaminated areas. Zhou et al. discovered that within 120 min of visible light irradiation, tetrabromobisphenol A (TBBPA) was removed photocatalytically to a level of over 90% using g-C3 N4 –polyaniline–TiO2 nanotube composite. In addition, they claimed that holes, hydroxyl radicals, and superoxide radicals all contributed significantly to photocatalysis [106]. Zhu et al. demonstrated the removal of different pollutants removal using PPy@Ag-g-C3 N4 composites in 60 min in the presence of visible light. The removal efficiencies for DM (Danofloxacin mesylate), TC (tetracycline), CIP (ciprofloxacin), GFLX (gatifloxacin)and EH (Enrofloxacin hydrochloride) were 90%, 95%, 93%, 90% and 95%, respectively [107]. Midya et al. reported the 96% removal of 2chlorophenol using PPy grafted, chitosan decorated CDs in 90 min in sunlight [108].

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5.3.4

377

Polymer–Carbon-Based Material Composites for Water Treatment and Desalination

For water treatment techniques like adsorption, photocatalysis, desalination and ionexchange membrane filtration, many forms of carbon-based materials have been employed. The use of carbon-based materials has attracted a lot of interest because of their unique qualities such as chemical, high thermal stability, mechanical and antibacterial activities, all of which serve to preserve the balance of nature [109]. The most critical matter in the world right now is the lack of clean water. Due to their superior and special benefits, such as good water quality with simple maintenance, low chemical sludge effluent, and excellent separation efficiency are gaining more and more attention from researchers. A novel kind of desalination membrane with excellent characteristics and efficiency is the thin-film nanocomposite (TFN) membrane, which includes a nanoparticles filler such as MWCNTs, graphene, GO, metal oxide and zeolite within the active layer of the TFC membrane [110]. Yaqin wang et al. reported carbon-based polymer composite (rGO-modified graphitic carbon nitride) membranes prepared by the phase inversion process for desalination [111].

5.4 Polymer–Zeolite Composites Zeolites can be defined as aluminosilicate crystalline materials having microporous pore sizes and well-defined structures. Zeolites are naturally available and are also being synthesized chemically. Zeolites are vastly investigated for the removal of pollutants due to their excellent adsorption and ion-exchange efficiencies. Zeolites have a negatively charged framework which makes them suitable for the removal of cationic pollutants very efficiently. Zeolites mostly attract pollutants via physical adsorption forces such as electrostatic attractions [6]. The adsorption in zeolites mostly takes place via electrostatic adsorption and ion-exchange behavior. The adsorption capacity of the zeolites is greatly dependent on the pore size of zeolites and pH of the solution. Some examples of zeolites are mordenite. Clinoptilolite, ZSM-5, zeolite X, zeolite Y and zeolite A. Polymers can be classified into two categories i.e., conducting and nonconducting. When the composite of zeolite is prepared with a non-conducting polymer, adsorption of contaminants takes place. On the other hand, when zeolites are used with the conducting polymers, the underlying removal mechanism can be adsorption as well as photocatalysis. Polymer–zeolite composites can be synthesized by different approaches such as in-situ polymerization [6], cross-linking [112], mechanical mixing [113], solid dispersion [114], etc. In an in-situ polymerization method, zeolite is mixed along with the monomer. The polymerization is then performed by using an initiator to prepare a zeolite–polymer composite. These methods are more attractive as the polymeric chains are developing inside the cage of the zeolites that provide enhanced mechanical, thermal, and optical properties. In a mortar grinding process, parent materials

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Fig. 13 Schematic representation of synthesis of zeolite–polymer composites

i.e., zeolite and polymer are mixed together by agate-mortar grinding process. In a cross-linking process, the zeolite–polymer composite can be obtained by using crosslinking agents such as N, N methylene bisacrylamide, glutaraldehyde, irgacure, etc. In a cross-linking process, the two materials are linked with each other via covalent or ionic bonds. In a solid dispersion method, parent materials are stirred together for the desired time interval. Figure 13 shows the synthesis of polymer–zeolite composite by three different synthetic approaches.

5.4.1

Polymer–Zeolite Composites for Dyes Removal

Two of the most used techniques for the removal of dyes using polymer–zeolite composites are photocatalysis and adsorption. The adsorption of dyes generally takes place via electrostatic attractions and hydrogen bonding. Khanday et al. synthesized beads of activated oil palm ash derived zeolite and chitosan via cross-linking process [112]. These beads were then employed for the adsorption of methylene blue and acid blue 29 dyes. Polyaniline and polypyrrole are two conducting polymers that are good photocatalysts and can degrade the dyes into smaller molecules. An experimental investigation has been done for the degradation of methylene blue dye by using ZSM-5/PANI composite [6]. The study showed a 99% degradation of methylene blue dye within 210 min. In the photocatalysis mechanism, both the materials, ZSM5 and PANI works synergistically in the electron–hole transfer as shown in Fig. 6b. Abukhadra et al. synthesized heulandite/polyaniline@nickel oxide composite by two step method constating in-situ polymerization and nickel oxide impregnation [115]. The composite was employed for the photocatalytic degradation of safranin T dye.

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Fig. 14 Schematic representation of a adsorption and b photocatalysis mechanism in zeolite-based composites. Reproduced with permission from Springer, 2023 [6]

The composite showed 100% degradation of safranine T under 1 min of solar irradiation. The mechanism of adsorption and photocatalysis in zeolite–polymer composite can be given as shown in Fig. 14.

5.4.2

Polymer–Zeolite Composites for Heavy Metal Removal

The term “heavy metals” refers to a class of metals and metalloids that have a relatively high density and are poisonous at concentrations as low as ppb. Arsenic (As), Lead (Pb), Nickle (Ni), Copper (Cu), Chromium (Cr), Iron (Fe), Zinc (Zn), Cadmium (Cd), and Mercury (Hg) are a few examples of heavy metals [116]. Zeolite– polymer composites have been successfully employed for heavy metal removal as well. Porous structure, functions groups interactions and electrostatic attractions allow zeolites–polymer composites to attract in heavy metal ions. In addition, cations present within zeolite cavities may induce the ion-exchange process with heavy metal ions. Truong et al. synthesized chitosan–zeolite composite membrane and employed this for the efficient removal of Cr, Cd, As, Hg, Pb and Cu ions [117]. Saez et al. synthesized Co+2 imprinted composites beads of chitosan and natural zeolite and employed it for the selective separation of Co+2 from its mixture with Cu+2 , Cd+2 , Ni+2 , and Fe+2 [118]. Chirino et al. utilized industrial waste coal fly ash for the synthesis of zeolite followed by the incorporation with polyether sulfone via phase inversion. The zeolite–polyamide–polyether sulfone nanofiltration membrane was then prepared by performing the interfacial polymerization of zeolite–polyether blend. The membrane was then employed for the removal of Cr(III) ions with 98% of removal efficiency [119].

5.4.3

Polymer–Zeolite Composites for Other Contaminant Removal

Other than dyes and heavy metals, there are some other contaminants which cause harm to human health and environment. These pollutants include pharmaceutical

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waste, agrochemicals such as chlorpyrifos, glyphosate, Formaldehyde, 2.4D, cations and anions such as ammonium and chloride, nitrate and sulphate ions, etc. These pollutants can be removed via adsorption, photocatalysis and ion-exchange. The pollutants in persistent organic pollutants (POPs) categories can be most effectively treated via photocatalysis. Polymer–zeolite composites are good adsorbents as well as photocatalysts and hence can be used for the remediation of a wide category of pollutants. Rakic et al. synthesized PANI/FeZSM-5 composite via in-situ polymerization of PANI with FeZSM-5. HZSM-5 was synthesized via hydrothermal treatment and then was ion-exchanged to form FeZSM-5. The composite was then evaluated for the oxidative degradation of herbicide glyphosate via hydrogen peroxide. The composite showed 80.4% degradation of glyphosate within 4 h [120].

5.4.4

Polymer–Zeolite Composites for Water Treatment and Desalination

The zeolite–polymer composites are also effective at treating other water-related issues like BOD, COD removal, desalination etc. Fixed pore sizes and hydrogen bonding capacities of zeolites make them suitable to be used with polymeric membranes in desalination [47]. Sihombing et al. prepared polystyrene–zeolite membrane via electrospinning method and successfully used it for desalination purpose [121]. Zendehdel et al. removed 85% of COD (Chemical oxygen Demand) from real wastewater sample by using polyethersulphone/Ag-clinoptilolite composite [122]. Nigiz et al. synthesized sodium alginate blended zeolite–polyvinyl alcohol (PVA) membrane via cross-linking and solution-casting. The 5% wt. zeolite loaded PVA membrane showed 100% salt retention at 293 K temperature. Also, on increasing the temperature the retention values were decreased [123]. Jamshid et al. synthesized zeolite-modified polyvinyl chloride-co-vinyl acetate/cellulose acetate (PVCA/CA) membrane via dissolution casting method. Both pristine zeolite and APTS functionalized zeolite was incorporated in a membrane and employed in the desalination experiment. The maximum salt rejection of 99.56% was found in PVCA/CA membrane with APT functionalized zeolite. The flux recovery ratio increases with increasing the zeolite ratio. This phenomena might be due to the fact that the functionalized zeolites have amine end groups, which tend to enhance the hydrophilic nature of the resulting membranes. [124]. Figure 15 shows the diagram of reverse osmosis model used for desalination purpose. Table 4 summarizes the literature on polymer–zeolite composites for wastewater treatment.

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Fig. 15 Prototype diagram for desalination via reverse osmosis. Reproduced with permission from Elsevier, 2020 [124]

6 Conclusion and Prospective Polymer-based composites have been vastly investigated as adsorbents and photocatalysts for wastewater treatment. Metal oxides, carbon-based materials, polymers, zeolites, etc. are being used with the polymers for the preparation of polymerbased composites. In most cases, these composites show better removal performances than bare polymers. This phenomenon occurs due to the increased surface area, porosity, reduced electron–hole recombination and charge modifications in composite materials. Though, polymer-based composites are potential materials for water and wastewater treatment, there are some gaps that need to be filled. The developed material needs to be low cost, efficient, easy to synthesis, recyclable, and most efficiently, can be applied on large-scale applications. To tackle the increasing issues of wastewater handling, the treatment of real wastewater samples should be done. Some other

ZSM-5/polyaniline

Heulandite/polyaniline@ nickel oxide

Chitosan/zeolite A

Chitosan/zeolite A

Chitosan/zeolite A

Acephate

Omthosate

Methyl parathione

Cr (VI)

Glyphosate

Zeolite/polyaniline

Cd+2

Safranin T

Alginate–clinoptilolite

Alginate–clinoptilolite

Cu+2

Alginate–clinoptilolite

Pb+2

Adsorption

Adsorption

Adsorption

Photocatalysis

Adsorption

Adsorption

Adsorption

Adsorption

Adsorption

Adsorption

Adsorption

Zeolite/chitosan

Zeolite/chitosan

MB

Adsorption Photocatalysis

Zeolite/Polypyrrole

Heulandite/Polyaniline

RR

LGSF

AB29

Photocatalysis

Mechanism

ZSM-5/PANI

MB

Material

LGSF-Light Green SF, AB 29-Acid Blue 29, MB-Methylene Blue, RR-Reactive Red

Other contaminants

Heavy metals

Dyes

Contaminant

Table 4 Removal of various pollutants using polymer–zeolite composites

0.7

0.7

0.7

0.35

–

4

2

2

2

1

1

0.3

1.8

0.5

Dosage (mg/ml)

480 min

480 min

480 min

1 min

2h

10 min

24 h

24 h

24 h

27 h

27 h

589 min

75 min

210 min

Contact time

560.8 mg/g, 74.3%

506.5 mg/g, 57.6%

650.7 mg/g, 78%

100%

98.5 mg/g

100 %

32%, 75

57%, 94

214.5 mg/g, 98%

212.76 mg/g

151.51 mg/g

97%

88.3%

99%

Efficiency

Solar irradiation

VIS

VIS

Light source

[129]

[129]

[129]

[115]

[128]

[127]

[126]

[126]

[126]

[112]

[112]

[125]

[11]

[6]

Ref

382 V. Sodha et al.

Polymer-Based Hybrid Composites for Wastewater Treatment

383

factors such as regeneration, evaluation of degradation pathways, and establishment of photocatalysis mechanism in photodegradation reactions can be studied in the future.

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Index

A Adsorption, 90, 91, 102, 104, 106, 108, 109, 111, 113, 116, 128, 131, 132, 136, 139, 147, 184, 202, 207, 210, 217, 232, 240, 248, 251, 252, 254, 256, 260, 343, 349, 350, 355–357, 359–370, 373–375, 377–379, 382 Air purification, 128, 141, 142, 203, 252, 333, 336, 342 Anisotropic growth, 61, 63, 74 Antimicrobial actions, 53, 129, 302, 310 Antimicrobial behavior, 53, 67, 71, 307, 308

B Biomarkers, 182, 184, 194, 275, 279, 284, 290 Biomedical applications, 11, 33, 36, 142, 143, 148, 153, 201, 289, 292, 294, 297, 299, 311, 336, 342, 343, 351 Biosensors, 15, 180, 182, 184, 185, 275, 277, 280, 281, 283, 285, 296 BTX, 81–83, 86, 87, 91, 92

C Cadium Sulphide (CdS), 38, 99, 100, 104, 109, 110, 119, 177, 178, 189–192, 211, 226–228, 232–234, 244, 248, 250, 252, 258–261, 263, 264, 322, 323, 325, 327, 335, 370, 372, 375, 376

Carbon-based material, 4, 112, 153, 180, 223, 226, 240, 351, 353, 355, 358, 373, 374, 376, 377, 381 Chalcogenides, 105, 239–244, 249, 250, 259, 265 Co-precipitation, 21, 84, 93, 186, 191, 225, 293 CO2 reduction, 3, 14, 99, 100, 103, 107, 117, 136, 139, 141, 142, 202, 248, 342 Coupled semiconductors, 111, 113, 194, 316, 318, 325 Cupric oxide, 15

D Degradation mechanism, 55, 81, 90, 92, 119, 132, 182, 206, 207, 209, 239, 240, 252, 257, 261, 265, 293, 336, 369, 374 Desalination, 167, 349, 355, 357, 365, 373, 377, 380, 381 Doping, 4, 7, 10, 12, 15, 17, 18, 21, 22, 35, 53, 55, 74, 81, 83, 86–89, 92, 105, 110, 111, 119, 129, 130, 135, 140, 157, 159, 168, 178–180, 182, 185, 205, 208, 226, 231, 234, 250, 294, 299, 303, 315, 316, 318–320, 323, 325–327 Dye-Sensitized Solar Cell (DSSCs), 3, 221, 223, 225–234 Dyes removal, 136, 156, 160, 161, 164, 203, 208, 209, 257, 342, 369, 378

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 J. Prakash et al. (eds.), Multifunctional Hybrid Semiconductor Photocatalyst Nanomaterials, Advances in Material Research and Technology, https://doi.org/10.1007/978-3-031-39481-2

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392 E Electrochemical sensing, 161, 162, 171, 177–179, 183, 190, 193, 194, 280, 285, 294 Energy, 1–4, 6, 9, 10, 12, 14, 17–19, 21, 22, 31–33, 36, 37, 39–41, 43, 46, 47, 54–57, 63, 66, 71, 72, 83, 84, 86, 87, 89, 90, 92, 127–130, 134–136, 141–143, 149, 150, 159, 167, 178, 184, 189, 193, 201–204, 207, 208, 210, 221, 222, 228, 239, 248, 251, 252, 254, 259, 290, 292, 297, 299, 315–317, 319–321, 324, 325, 333–335, 340–344, 349, 354, 357 Energy applications, 10, 31–33, 54–56, 136, 141, 193, 207, 221, 222, 228, 231, 239, 248, 251, 254, 290, 292, 297, 315, 320, 321, 333, 334, 336, 354 Environment, 2, 4, 14, 22, 33, 34, 54, 56, 62, 68, 69, 71, 81, 87, 89, 91, 127, 129, 133–135, 142, 149, 150, 153, 155, 156, 158, 162, 201, 203, 207–210, 213, 215, 216, 221, 257, 290, 292, 294, 302, 307, 308, 333, 336, 343 Environmental applications, 31, 36, 54, 132, 153, 155, 156, 171, 172, 177, 193, 201, 202, 207, 221, 223, 239, 240, 251, 252, 256, 276, 289, 315, 342, 369 Environmental remediation, 1, 3, 20, 82, 128, 153, 206, 223, 251, 342, 349 G Graphene, 6, 12, 21, 22, 39, 42, 99, 113, 114, 119, 134, 138, 140–142, 153, 155–158, 160–162, 167, 171, 178, 180, 182–188, 190, 192, 215, 223, 240, 241, 249, 278–281, 283, 325, 333, 350, 374, 375, 377 Graphene oxide, 21, 39, 115, 134, 153, 155–162, 167, 171, 180, 215, 240, 280, 325, 377 Green synthesis, 38, 128, 132, 178, 225, 293, 294 H Hazardous chemicals, 3, 38, 82, 130, 156, 203, 239, 350, 373 Health, 3, 18, 22, 81, 82, 127, 129, 142, 149, 150, 153, 202, 207, 211, 215, 216, 259, 289, 291, 297, 350, 379

Index Heavy metal ions, 145, 153, 155, 156, 158, 167–169, 172, 204, 210, 211, 342, 366, 379 Heterojunctions, 10, 16, 36, 180, 182, 187, 190, 205, 252, 253, 315, 316, 318, 319, 321, 322, 326, 327, 338 Hybrid semiconductor nanomaterials, 3, 31–33, 41, 47, 48, 99, 127, 149, 177, 180, 194, 221, 223, 289, 294, 311, 315, 354 Hybrid Semiconductor Photocatalyst Nanomaterials (HSPNs), 177, 179–181, 186, 187, 194 Hydrogen production, 3, 14, 141, 158, 179, 207, 281, 285, 305, 307, 320, 333, 342 Hydrothermal method, 33, 34, 37, 55, 84, 87, 180, 184, 186, 189, 190, 213, 215, 224, 225, 229, 251, 259, 293, 326, 327, 336, 338, 339, 380

L Layered double hydroxides, 111, 157

M 2D materials, 111, 190, 241, 250, 333 Metal nitride, 44, 110, 111, 133, 181, 193, 325, 342, 377 Metal-Organic Framework (MOF), 107–109, 114, 115, 117, 119 Metal oxide photocatalysts, 16, 67, 105, 129, 133, 135, 215, 222, 240, 325, 342, 381 Metal oxides, 1–4, 11, 13, 15, 16, 19, 20, 22, 31, 35, 39, 41, 44, 53–55, 59, 66, 69, 74, 105, 117, 129, 135, 156, 177, 178, 180, 184, 215, 228, 232, 234, 240, 294, 295, 316, 342, 354, 355, 358, 367, 369, 372 Metal sulphide, 181, 188, 191, 240, 342 MoS2 , 2, 20, 34, 38, 177, 178, 183, 186, 187, 190, 191, 245, 249, 250, 252, 257–260, 263, 265 Multifunctional applications, 1, 2, 129, 142, 149, 193, 217, 333, 339, 342

N Nanocomposites, 3, 8, 10, 16, 22, 35, 38, 39, 41, 43, 53, 55, 71, 74, 90, 127, 130–132, 138, 141, 142, 157, 159, 160, 163–167, 182, 211, 213, 215,

Index 223, 239, 240, 244, 248, 249, 252, 255, 256, 262, 291, 294, 297, 302, 315, 316, 318, 323, 339, 350, 356, 370, 372, 376, 377 Nickel oxide, 221, 222, 231, 378 O Organic dyes, 53, 132, 136, 155, 161, 202, 203, 205, 208, 210, 256, 257, 339, 363, 374 Organic pollutants, 3, 7, 11, 20, 21, 54, 82, 133, 134, 136, 156, 160, 172, 201–203, 208–210, 213, 252, 257, 342, 349, 350, 373, 380 P Perovskites, 1–3, 18–20, 110, 111, 119, 139, 140, 193, 194, 223, 225 Pesticides, 8, 177, 180, 184, 185, 203, 208, 214–216, 248, 249, 260, 263, 284, 342, 349, 376 Pharmaceutical compounds, 135, 208, 211, 213, 249, 291, 303 Photocatalytic degradation, 38, 66, 81, 90–92, 111, 119, 128–131, 133–135, 137, 143, 182, 193, 208, 209, 214, 215, 223, 239, 240, 251, 252, 257, 259, 261, 265, 333, 335, 338, 343, 356–358, 367, 369, 374, 376, 378 Photodegradation, 4, 7, 10, 15–17, 39, 66, 111, 120, 129, 132, 138, 146, 153, 182, 193, 208, 210, 212–215, 239, 240, 251, 252, 315, 338, 355, 383 Photoelectrochemical water splitting, 252, 315 Photoelectrodes, 11, 315–317, 324, 326, 327 Photoreduction, 31, 41, 43, 99–104, 106–108, 111, 113, 114, 116, 120, 180, 201, 210, 259, 260, 265, 337, 338 Photoreduction method, 41, 43, 116, 120 Plasmonic nanohybrids, 194, 333, 335, 336, 340 Plasmonic nanomaterials, 116, 153, 290, 334, 342 Plasmonic photocatalysts, 153, 194, 256, 336–339, 341–344 Pollutants, 3, 5, 7, 9, 11, 12, 14, 18, 20, 21, 42, 54, 72, 82, 90, 91, 129, 132–134, 136, 147, 148, 153, 156, 160, 172, 182, 190, 201–203, 205, 207–211,

393 213, 217, 240, 252, 254, 256, 257, 259–262, 333, 336, 342, 349–353, 356, 357, 360–364, 368, 370, 372, 373, 376, 377, 379, 380, 382 Polymer-based hybrid composites, 349, 355 Polymers, 65, 106, 113, 114, 134, 178, 228, 277, 285, 349–358, 363, 367, 369, 373, 377, 378, 381 Power Conversion Efficiency (PCE), 55, 221, 223, 225–228, 231–234 3D printing, 210, 275–278, 280, 282, 285

R Reduced graphene oxide, 21, 159, 162, 186, 187, 190, 215, 240, 280

S Self-cleaning surfaces, 4, 143, 204, 342, 343 Semiconductor photocatalyst nanomaterials, 1, 81, 99, 150, 177–180, 186, 191, 194, 217, 222, 336 Sensing applications, 11, 31, 36, 171, 177, 178, 194, 294, 334, 354 Sensors, 3, 4, 6, 15, 16, 45, 54, 161, 162, 164, 169, 171, 177–179, 181–185, 188, 189, 193, 194, 249, 275–286, 299, 335 Silica (SiO2 ), 10, 45, 54, 117, 128, 149, 157, 203, 294, 338 Solar cell applications, 36, 128, 140, 150, 221, 223, 224, 228, 229, 249, 252, 333, 342 Sol-Gel Method, 15, 39, 85, 86, 89, 132, 139, 142, 180, 184, 186, 215, 224, 225, 292, 320, 339, 354 Solvothermal method, 7, 37–39, 84, 180, 189, 194, 224–226, 293, 310, 327, 336 Storage applications, 21, 99, 100, 128, 216 Surface-Enhanced Raman Spectroscopy (SERS), 4, 41, 296, 309, 310, 334, 339 Surface Plasmon Resonance (SPR), 44, 256, 290, 295–297, 306, 320, 334, 335, 340

T Tandem semiconductors, 117, 119, 315, 316, 324

394 Titanium dioxide, 4, 34, 39, 46, 82, 86, 129, 178, 207, 213, 221, 222, 225, 316, 335 Transition Metal Chalcogenides (TMCs), 239–244, 249, 250, 252, 254, 256, 259, 262, 263 Tungsten oxide, 8, 178, 187, 249

Index Water purification, 11, 22, 128, 142, 148, 202, 204, 216, 252, 342, 357 Water splitting, 3, 11, 15, 19, 141, 202, 265, 315–317, 335, 342, 343 Water treatment, 3, 4, 6, 54, 127, 129, 136, 157, 158, 168, 202, 206, 207, 213, 216, 260, 279, 342, 349–351, 357, 363, 373, 377, 380

V Volatile organic compounds, 9, 81, 82, 342 W Waste water treatment, 4, 54, 161, 167, 168, 201–203, 207, 217, 239, 240, 265, 349–351, 355, 363, 380, 381

Z Zeolite, 117, 264, 350, 351, 354, 377–382 Zinc oxide, 6, 8, 182, 183, 221, 222, 228, 315, 335, 356 Z-scheme, 16, 20, 182, 259, 261, 265, 324