Emerging Trends in Nanotechnology 9811599033, 9789811599033

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Zishan Husain Khan   Editor

Emerging Trends in Nanotechnology

Emerging Trends in Nanotechnology

Zishan Husain Khan Editor

Emerging Trends in Nanotechnology

Editor Zishan Husain Khan Department of Applied Sciences and Humanities Jamia Millia Islamia New Delhi, Delhi, India

ISBN 978-981-15-9903-3 ISBN 978-981-15-9904-0 (eBook) https://doi.org/10.1007/978-981-15-9904-0 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Nanotechnology is the emerging technology of the twenty-first century. It deals with the synthesis and investigation of ultrafine materials and their use in technology for numerous applications. It is an interdisciplinary field that combines the principles of physics, chemistry, and engineering, such as structural analysis, electrical engineering, mechanical design, and computer science and systems engineering. The ultimate goal of nanotechnology is to make the systems and devices smaller and more powerful, which requires that all the constituent materials involved have to be operated more efficiently. The discovery of the buckyball and carbon nanotubes motivated many scientists to work on the synthesis of a variety of nanostructures of different shapes and sizes. As of today, the researchers have established methods to control the size, composition, and homogeneity to explore the applications of nanomaterials in a variety of devices such as solar cells, field emission sources, single electron transistors, chemical and biological sensors, quantum dot lasers, and other electronic and optoelectronic devices. The essence of nanotechnology is the fabrication and utilization of materials and devices at the level of atoms, molecules, and supramolecular structures and the exploitation of the unique properties and phenomena of matter at the nanoscale (1–100 nm) with the fact that the materials at this scale behave differently as compared to their bulk counterpart. Though the research work on the synthesis of nanomaterials has been continuing for many years, the mass-scale production of nanomaterials and reproducibility is still an issue to be addressed. Scientists have explored both the chemical and physical approaches for the synthesis of nanomaterials. The chemical routes are cost-effective and open up the possibility of mass production, but the reproducibility of results is difficult, whereas the physical methods are reliable with higher chances of reproducibility, but the mass-scale production is an expensive business. Therefore, both the approaches have their own limitations. The factors which support the applications of nanomaterials are particularly their physical and chemical properties, high surface-to-volume ratio, and small size which provide best possibilities for manipulation and room for accommodating multiple functionalities. This book is focused on emerging trends in nanotechnology. It presents state of the art of different areas of research which includes applications of nanomaterials in solar cells and sensors, nanocomposites, biomedical applications of nanomaterials, v

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and nonlinear optics in nanocomposites. This book includes 13 chapters presented by the experts in the field of nanomaterials and their applications. Chapter 1 presents a review of the research work on synthesis and photocatalytic properties of 2D metal dichalcogenides. It presents the basic introduction of two-dimensional (2D) materials including their properties. Out of different 2D materials, this chapter is mainly focused on two-dimensional transition metal dichalcogenide (2D-TMDC) materials, which have the common formula MX2 , where X = sulfur (S), selenium (Se), or tellurium (Te), and M belongs to the elements of groups 4, 5, and 6 of the periodic table. It also includes the review of different synthesis methods, like CVD, hydrothermal, and CVT methods, which have been used to synthesize the 2D-TMDC materials. Finally, the photocatalytic properties of as-prepared two-dimensional transition metal dichalcogenides and their composites have been discussed. Chapter 2 includes the studies on dye-sensitized solar cells incorporated with perovskite as sensitizer dye. This chapter focuses on the synthesis of light sensitizers, fabrication of dye/natural dye/perovskite-based solar cells, and its characterization in an ambient air atmosphere. This chapter includes the work on the fabrication and characterization of dye, natural dye, and perovskite light sensitizer-based dye-sensitized solar cells. Use of dyes and perovskite sensitizers for developments of solar cells not only shall be cost-effective with a simpler technology of fabrication but will also be eco-friendly. Chapter 3 summarizes the research work on self-assembled growth of GaN nanostructures on flexible metal foils by laser molecular beam epitaxy. It is mainly focused on the growth of various GaN nanostructures such as islands, thin films, and nanorods on variety of flexible metal foils using laser-assisted molecular beam epitaxy (LMBE) technique, and their structural/optical properties have also been discussed in this chapter. Chapter 4 includes the review on nanostructured abrasive materials for ultraprecision finishing of high-performance materials. This chapter presents current theories associated with ultraprecision machining and the methods employed to understand microscopic interactions due to cutting, plowing, and sliding effects and how they can be used to develop nanostructured products that enhance the removal of material in ultraprecision finishing processes. Chapter 5 includes the applications of nanomaterials in dentistry. It presents the role of nanotechnology in oral health for improving the quality of dental care by manifolds. The nanodentistry thus opens new horizons for immense possibilities in dental research, but one should keep eyes on safety, efficacy, and implementation of nanotechnology. Chapter 6 includes the research work on antibacterial and anticancer activity of biologically synthesized gold nanoparticles. This chapter describes the effect of biologically synthesized Au-NPs on bacteria Pseudomonas aeruginosa and Staphylococcus aureus and on the cancer cell lines. Chapter 7 focuses on the preparation, properties, and applications of ZnCdS thin films. It includes different characteristics of ZnCdS thin films along with their synthesis process, crystal structure, energy bandgap, applications, etc. Chapter 8 presents an overview of the pharmaceutical applications of nanomaterials. This chapter is devoted to the study of nanoparticulates (synthetic, semisynthetic, and natural) with existing and potential applications in the pharmaceutical field. Chapter 9 discusses the applications of PDMS on ZnO thin film for masking of ZnO thin film-based MEMS fabrication.

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This chapter presents the research work on silicon wet etching experiments in tetra methyl ammonium hydroxide (TMAH) solution using silicon-based organic polymer as a protective mask for the zinc oxide sputtered side of wafer since it is difficult to use and remove SiO2 or Si3 N4 as an etching barrier, in multilayer structures. A comprehensive characterization of ZnO thin film is performed to demonstrate that structural, mechanical, and electrical properties of thin film remain unaltered. Chapter 10 describes the comparative study of photoluminescence and chemoresistive properties of V2 O5 nanostructures. It also includes the synthesis of V2 O5 nanostructures and their NH3 sensing properties. Chapter 11 presents the review on carbon nanotubes and their processing techniques, purification, and industrial applications. This chapter reports the advancement in processing techniques, purification, and industrial applications of carbon nanotubes and their composites. Chapter 12 reports recent advancement in nanostructured-based electrochemical genosensors for pathogen detection. This chapter also includes recent progress in the fabrication of genosensors for pathogen detection based on different nanostructures. Chapter 13 discusses the nonlinear optical properties of organic dyes and organic dye–polymer nanocomposites. It summarizes the studies on nonlinear optical responses in organic fluorescent dyes and dye-embedded polymer nanocomposite films. New Delhi, India

Zishan Husain Khan

Acknowledgements

First and foremost, I am thankful to God, who has given me wisdom to complete this book. I realized how beautiful is this gift of writing for me during the process of putting this book together. You have given me the power to believe in my passion and pursue my dreams. I could never have done this without the faith I have in you, the Almighty. My sincere thanks are also due to all the authors for their significant contributions to this book. It would not have been possible to accomplish this challenging work without your support and timely response. It is a matter of satisfaction that all the authors have justified their contribution in this book and presented the work on latest areas of nanotechnology. I am grateful to Prof. Najma Akhtar, Vice-Chancellor, Jamia Millia Islamia, New Delhi, India, for her support and encouragement to the academic environment in the university for the pursuit of higher education. Her pearls of wisdom coupled with motivation has contributed largely to the completion of this book. I would like to express my gratitude to my teacher Prof. Mushahid Husain, Former Vice-Chancellor, M. J. P. Rohilkhand University, Bareilly, Uttar Pradesh, India, for his constant support and motivation. I am also thankful to my Ph.D. students, especially Prof. Nafis Ahmad, Sultan Ahmad, Mohd. Bilal Khan, Mohd. Parvaz, Asim Khan, and Hasan Abbas, who supported a lot during the compilation of this book. My sincere acknowledgment is also due to the editorial team of Springer Nature Inc. for their support and guidance without whom this endeavor would not have been a reality. Last but not least, I am indebted to my family for their unflagging love and support throughout the process of compilation of this book. I am also indebted to my mother for her words of encouragement and support. She has been a constant source of inspiration, and I dedicate this book to her. I am grateful to my wife Rubina Mirza for her constant support and for standing beside me throughout my career. She was always very supportive and has motivated me to enrich the knowledge for the betterment of the students and society in large. At last, I also thank my wonderful

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kids Ayanab, Alina, and Ali for always making me smile and inspiring me to do my best as a teacher, a researcher, and obviously an author. As an editor, I would love to receive suggestions and feedback about this book at [email protected].

Contents

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Synthesis and Photocatalytic Properties of 2D Transition Metal Dichalcogenides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mohd. Parvaz, Hasan Abbas, and Zishan H. Khan

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Studies on Dye-Sensitized Solar Cells Incorporated with Perovskite as Sensitizer Dye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rahul, Sultan Ahmad, Pramod K. Singh, and Zishan H. Khan

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Nanomaterials: A Windfall to Dentistry . . . . . . . . . . . . . . . . . . . . . . . . . Nafis Ahmad, Zeba Jafri, Asim Khan, and Zishan H. Khan

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Nanostructured Abrasive Materials for Ultraprecision Finishing of High-Performance Materials . . . . . . . . . . . . . . . . . . . . . . . . 103 M. J. Jackson

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Self-assembled Growth of GaN Nanostructures on Flexible Metal Foils by Laser Molecular Beam Epitaxy . . . . . . . . . . . . . . . . . . . 135 S. S. Kushvaha and M. Senthil Kumar

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Antibacterial and Anticancer Activity of Biologically Synthesized Gold Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Azra Parveen, Hadeel Salih Mahdi, and Ameer Azam

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ZnCdS Thin Film: Preparation, Properties and Applications . . . . . . 185 Suresh Kumar and K. P. Tiwary

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Nanomaterials for Pharmaceutical Applications . . . . . . . . . . . . . . . . . . 221 Sundar Singh, S. B. Tiwari, and Sanjeev Tyagi

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PDMS on ZnO Thin Film: A Mask for ZnO Thin Film in MEMS Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 Priyanka Joshi and Jamil Akhtar

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10 Photoluminescence and Chemoresistive Gas Sensing: A Comparative Study Using V2 O5 Nanostructures for NH3 . . . . . . . 279 Nitu Singh, Jyoti Bamne, K. M. Mishra, Neha Singh, and Fozia Z. Haque 11 Advancement in Carbon Nanotubes: Processing Techniques, Purification and Industrial Applications . . . . . . . . . . . . . . . . . . . . . . . . . 309 Anbesh Jamwal, Muhammed Zahid Hasan, Rajeev Agrawal, Monica Sharma, Sunil Thakur, and Pallav Gupta 12 Recent Advancement in Nanostructured-Based Electrochemical Genosensors for Pathogen Detection . . . . . . . . . . . . . 339 Summaiyya Khan, Akrema, Rizwan Arif, Shama Yasmeen, and Rahisuddin 13 Nonlinear Optical Properties of Organic Dyes and Organic Dye-Polymer Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 Sana Zafar and Mohd. Shahid Khan

About the Editor

Zishan Husain Khan is Professor of Applied Physics at the Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi. He obtained his Ph.D. from Jamia Millia Islamia, New Delhi. He has almost 25 years of research experience in the fields of semiconductor physics and nanotechnology. He has published more than 130 research papers in various reputed international journals and guided a number of Ph.D. students. He has worked at several positions in the universities abroad. He is also actively involved in designing various courses in nanotechnology and energy sciences for graduate and research students. He is also the reviewer for many international journals of high repute. In addition, he has edited several special issues for reputed international journals. Dr. Khan has edited many books for reputed publishers including Springer Nature. His present research interests include hybrid solar cells, OLEDs, transition metals di-chalcogenides, carbonaceous nanomaterials and nano-sensors.

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Chapter 1

Synthesis and Photocatalytic Properties of 2D Transition Metal Dichalcogenides Mohd. Parvaz, Hasan Abbas, and Zishan H. Khan

Abstract Nanotechnology is the emerging technology of the twenty-first century. It deals with the synthesis and investigation of ultrafine materials and their use in technology for numerous applications. It is an interdisciplinary field that combines the principles of physics, chemistry, and engineering, such as structural analysis, electrical engineering, mechanical design, computer science and systems engineering. Two-dimensional (2D) materials are crystalline materials consisting of layered arranged atoms or molecules. In the last few years, 2D materials have been extensively explored for their unique 2D geometry, high surface-to-volume ratio, and nanoscale thickness. Two-dimensional transition metal dichalcogenide (2D-TMDCs) materials have the common formula MX2 , where X = sulfur (S), selenium (Se) or tellurium (Te), and M belongs to the elements of group of 4, 5, and 6 of the periodic table. MX2 layers are covalently bound by the van der Waals force between the layers. The weak van der Waals bonds between the layers facilitate separation of the layers to form 2D materials. Many synthesis methods, like as CVD, hydrothermal, and CVT method, have been used to synthesize the 2D-TMDCs materials. Titanium disulfide (TiS2 ) is an important layered material among the TMDCs family. It crystallizes in the hexagonal structure similar to CdI2 . It is a multi-layered compound with repeating subunits formed from a layer of Ti atoms and a layer of S. TiS2 has a band gap varying between 0.05 and 2.5 eV; the Bohr’s radius of approximately 6.43 nm and the lattice parameter constants a (a = b) and c of TiS2 are 3.40 A°, 5.96 A° respectively. The present chapter deals with the review of research work reported on 2D metal dichalcogenides with a special emphasis of TiS2 .

1.1 Introduction Nanoscience is the study of phenomena and manipulations of materials at the atomic, molecular, and macromolecular levels that differ in their properties at larger scale. Thus, nanoscience deals with several hundred to several thousand atoms or atomic Mohd. Parvaz · H. Abbas · Z. H. Khan (B) Department of Applied Sciences & Humanities, Jamia Millia Islamia, New Delhi 110025, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_1

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clusters. At present, there is much interest in exploring materials at the atomic level because the materials at this level show interesting properties and find applications in many areas such as medical imaging and medicine, electronics, aerospace industry, computing, optics, construction materials, biotechnology, drug delivery, and energy. Various nanoscale functional structures and devices often use light or electrical signals to interact with the macroscopic world. Ultimately, the vision of nanoscience is to combine with science, technology and technology of man-made biological objects that are controlled at the nanometer scale and then assembled into complex structures. At the atomic level, materials have significant improvements in strength, quality, toughness, and efficiency as compared to their bulk counterparts. It is a preferred technology to make things smaller, simpler and cheaper. It could possibly revolutionize the ways in which nanoscale materials are produced and may have significant economic implications that will definitely develop in the future [1]. The materials at nanoscale called nanomaterials are the cornerstones of nanoscience and technology. Nanomaterials are generally defined as a group of materials in which at least one dimension or average grain size is less than 100 nm. Each substance, when miniaturized to a length scale of less than 100 nm, has new properties, not to mention the composition. Such miniatured materials, called nanomaterials, are of great interest because of their unique properties such as magnetic, optical, and electrical properties emerge at nanoscale. Noble metal and semiconductor nanoparticles, nanocarbons such as fullerenes and carbon nanotubes are excellent examples of nanomaterials. The properties such as magnetic, electrical, optical, mechanical, and chemical can be systematically manipulated by adjusting the size, composition and shape of materials in the nanometer range. Nanomaterials are extremely small materials with at least one of the dimensions of 100 nm or less. The substances such as ceramic, polymers, glass, metals, semiconductors can be produced at nanoscale. Currently, the emerging field of nanomaterials includes (i) nanoparticles, (ii) nanocrystalline materials, and (iii) nanodevices. Nanomaterials can be classified in the zero dimension (0D) (e.g., quantum dot), in one dimension (1D) (e.g., nanowires), in two dimensions (2D) (e.g., nanosheets, or nanodiscs) or in three dimensions (3D) (e.g., particles) in individual, fused, agglomerated forms may be present with tubular, spherical, and irregular shapes. Out of these materials, 2D materials have exciting properties and are being used in different applications such as electronics, sensors, photocatalysis, and solar cell. Two-dimensional (2D) materials are crystalline materials consisting of layered arranged atoms or molecules. In the last few years, these materials have been extensively explored for their 2D geometry, nanoscale thickness, and high surface-tovolume ratio. These materials can be used for various applications, such as gas sensors [2–7], electrocatalysts [8, 9], energy storage devices [10–13], electronic devices [14, 15], and biomedical applications [16]. In particular, 2D nanostructures offer some benefits, with more active sites, larger surface area, functionality of surfaces, good compatibility with device integration, the ability to combine into three-dimensional architectures, and so on. Many 2D materials like as 2D-carbon-based materials, TMDCs, and transition metal oxides (TMO) have been synthesized and employed in

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electronic devices. In particular, materials such as graphene, silicon carbide (SiC), molybdenum disulfide (MoS2 ), molybdenum dioxide (MoO3 ), and tungsten disulfide (WS2 ) provide improved chemical and physical functionality [17–21]. 2D materials are very sensitive to the number of layers, and the material shows a transition from indirect band gap (bulk) semiconductor to direct band gap (monolayer) semiconductor as the number of layers decrease. These materials may be employed for producing transparent and flexible electronic systems for use in integrated circuits, energy storage, and solar cells [18, 20]. Due to interesting properties, these materials have opened up new possibilities for engineers and scientists to explore more in this area [20]. Some of the popular 2D materials studied so far are presented as follows.

1.1.1 Graphene Graphene isolated from graphite was the first 2D material discovered by the researchers Andre Geim and Konstantin Novoselov in 2004 [22]. It is a single sheet of closely packed sp2 carbon atoms in a hexagonal pattern where all the carbon atoms are well saturated with no dangling bonds on the surface. These properties of graphene with no interface traps make it the most suitable material for high-frequency electronics [22–24] and RF applications [25–27]. It can be easily exfoliated from a single crystal of highly ordered pyrolytic graphite (Fig. 1.1a) using scotch tape [28]. Figure 1.1b, c shows the honeycomb lattice of graphene and the linear E-k diagram of graphene with dirac cone where the electrons and holes act like mass-less particles [29]. It is an excellent conductor of electricity, with sheet resistance of less than 30 /sq, and nearly transparent, with an optical transmission of >90% [30]. This high degree of optical transmission makes it a viable competitor of ITO as a transparent conductor. Other advantages of graphene which make it a better replacement for ITO include its low cost of fabrication when using a chemical vapor deposition (CVD) process [31], high electron mobility (>3000 cm2 /Vs for CVD graphene [32, 33]), high thermal and mechanical flexibility [34]. Researchers have also used graphene electrodes in touch panels [15], displays [35], solar cells [36], and biosensor applications [37] showing the wide range of applications for this material.

Fig. 1.1 Graphene crystal, structure and E-k diagram. a Picture of highly ordered pyrolitic graphene from SPI supplies inc. b A sheet of graphene showing the honeycomb lattice of carbon atoms. c E-k diagram of graphene showing zero band gap and E-k linearity at the dirac point

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Fig. 1.2 Crystal structure of h-BN

1.1.2 Hexagonal Boron Nitride (h-BN) Hexagonal boron nitride (h-BN) is an important 2D material. It has a band gap of about 6 eV. It is an excellent dielectric material that can be used in various heterostructures for electrostatic gating [19]. It forms a 2D crystal structure, composed of boron and nitrogen atoms with lattice spacing similar to the graphene, and the crystal structure of h-BN is presented as Fig. 1.2 [38]. It is generally used as the ideal substrate and gate dielectric in field effect transistors (FETs). The doping level can also be changed by applying an external electric field and decreases with increasing thickness of the h-BN layer. It can be partially oxidized (PO-h-BN) to reduce its optical transmission (>60%) and band gap (from 3.97 to 5.46 eV) [39].

1.1.3 Transition Metal Dichalcogenides (TMDCs) Transition metal dichalcogenides (TMDCs) are MX2 -type-layered materials (M = transition metal atom and X = chalcogen atom) [40]. A layer of M atoms is sandwiched between two layers of X atoms. TMDCs materials have layered structures in the form of X-M-X, and the X atoms are located in two hexagonal planes separated by the plane of the metal atoms. The known layered TMDCs material is presented as Fig. 1.3 [41]. In general, TMDCs symmetry is either hexagonal or rhombohedral, and the metal atoms are octahedral or trigonal-prismatic coordination [42].

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Fig. 1.3 Known layered TMDs in the periodic table highlighted with shadow

TMDCs consist of hexagonal layers of metal atoms sandwiched between two layers of chalcogen atoms, with an MX2 stoichiometry that depends on the combination of X and M as presented in Fig. 1.4 [43]. Depending on the atomic stacking configuration, TMDCs can have two types of crystal structures: trigonal-prismatic (2H) and octahedral (1T) phases [44]. In the trigonal-prismatic structure, each M atom is coordinated with six surrounding X atoms and forms a stable phase, while in the octahedral phase six X atoms form a distorted octahedron around the metal atom, creating a metastable state. Both crystal structures have different electronic

Fig. 1.4 2D transition metal dichalcogenides

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Fig. 1.5 Left: top-down view of 2D TMDs with trigonal-prismatic (2H-polymorph) and octahedral coordination (1T-polymorph). Atom color code: blue, transition metal; yellow, chalcogen. Right: d-orbital splitting diagrams of 2H-polymorph and 1T-polymorph, respectively

properties. For TMDCs with metals in a trigonal-prismatic geometry, the d orbitals  are splitted into three sub-bands dz2 , dx 2 −y 2 , dx y , and dx z,yz with a sizeable band gap between the former two. In case of TMDCs with a octahedral  coordination of metal, the d orbitals are splitted into two degenerated sub-bands dz2 , dx 2 −y 2 and d yz,x z,x y (Fig. 1.5). By varying the orbital occupation, diverse electronic properties of TMDCs from metal to semiconductor and to topological insulators can be obtained. For example, 2H–TaSe2 (d 1 ) is metallic due to the partially filled d2 z, whereas 2H–WSe2 (d 2 ) is a semiconductor due to the fully occupied d 2 z.

1.1.3.1

Two-Dimensional Transition Metal Dichalcogenide Materials (2D-TMDCs)

Two-dimensional transition metal dichalcogenide (2D-TMDCs) materials are represented by the common formula MX2 (M = transition metal atom and X = chalcogen atom) with covalently bonded MX2 layers. The 2D materials are formed due to weak van der Waals bonds between the layers, which help in the separation of the layers. These materials have been synthesized using different synthesis methods such as CVD, hydrothermal, and CVT method [19]. In total, there are almost 60 TMDCs compounds. Out of 60 TMDCs compounds, two-thirds of compounds are layered and basically can be in 2D form. Some 2D-TMDCs materials like as tungsten disulfide (WS2 ), molybdenum disulfide (MoS2 ), titanium disulfide (TiS2 ), tungsten diselenide (WSe2 ), niobium disulfide (NbS2 ), niobium diselenide (NbSe2 ), zirconium disulfide (ZrS2 ), titanium diselenide (TiSe2 ), tantalum disulfide (TaS2 ), tantalum diselenide (TaSe2 ), etc. are important materials due to their different electronic properties [26– 29]. Many researchers have shown that transition metal dichalcogenides (TMDCs) have excellent electronic [45–47], magnetic [48–50], and electrochemical [51–53] properties that attracted considerable attention when used in energy-related devices,

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such as supercapacitors [54–56], solar cell [57–59], lithium-ion battery [60, 61] and water splitting to generate H2 [62, 63].

Molybdenum Disulfide (MoS2 ) Molybdenum disulfide (MoS2 ) is one of the commonly used materials in the TMDCs family [21]. It has many interesting properties, such as chemical, optical, electrical and mechanical properties, which make it an attractive material for use in a hydrodesulfurization catalyst [64–66], as an active material or as a transport material in solar cells [67, 68], as photocatalysts [69], as electrodes in lithium batteries [70] and as solid lubricants [71]. Like other TMDCs materials, MoS2 is semiconductor material with an indirect forbidden band in the bulk form with a value of 1.2 eV [72–75]. When bulk form of MoS2 exfoliated to a monolayer, the band gap changes from indirect to direct band gap semiconductor [76, 77] with a value of 1.9 eV. This effect has led to a strong revival of the investigations of the MoS2 monolayer as 2D material. Although MoS2 in its bulk form has already been extensively investigated in the past [77, 78]. The existence of this intrinsic band gap in monolayer MoS2 may be useful for the FET with a high ON/OFF ratio [79–83]. It may also be used in sensors [84, 85], integrated circuits [86] and logic operations [87]. MoS2 is a typical three-layered transition metal sulfide: A layer of Mo atoms of a metal is located between two layers of S atoms and forms a “sandwich-like” structure. The triple layer (S–Mo–S) can be arranged in three ways: hexagonal (2H–MoS2 ), rhombohedral (3R-MoS2 ) and trigonal (1T-MoS2 ). Typically, 2H-MoS2 is a stable state, while 3RMoS2 and 1T-MoS2 are metastable. For the MoS2 monolayer, only two polymorphs are observed due to different coordination methods between the Mo atom and its neighboring six S atoms: a trigonal-prismatic coordination (D3h point group) and an octahedral coordination (D3d), which are commonly referred to as 1H and 1T, respectively (Fig. 1.6a, b) [53]. With multilayer MoS2 , more polymorphs occur due to the variety of stacking sequences between the individual layers. Three intensively studied polytypisms are 1T, 2H and 3R, the letters represent different symmetry systems of trigonal, hexagonal and rhombohedral crystals, and the numbers indicate the number of layers in each unit cell (Fig. 1.6c, d) [88].

Tungsten Disulfide (WS2 ) Tungsten disulfide (WS2 ) is an inorganic semiconductor material. WS2 has a similar structure to that of MoS2 . It has a band gap (indirect) with a value of 1.3 eV in the bulk form and band gap (direct) with a value of 2.1 eV (monolayer) [89, 90]. The mobility of WS2 monolayer at room temperature was found to be 19 cm2 V−1 s−1 . The unusual and beneficial properties of WS2 are due to the high anisotropy associated with its crystalline structure. The weak bond between the layers in the basal planes leads to useful lubricating properties of WS2 . The utility of WS2 layered materials strongly depends on their crystallographic orientation, namely the morphology of the

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Fig. 1.6 Structural models and characterizations of various polymorph phases of MoS2 . a, b Atomic models showing 1H and 1T phases of monolayer MoS2 , respectively. c 3D schematic illustration of multilayer MoS2 . d Atomic models displaying three different phases, 1T, 2H and 3R, in multilayer MoS2 , respectively

crystals and the size of the crystals. Due to strong surface effects, the properties of these materials change dramatically depending on the number of layers in the sheet. Therefore, they can be customized as needed, making them a potential candidate for tunable nanoelectronics. [91, 92]

Titanium Disulfide (TiS2 ) Titanium disulfide (TiS2 ) is an important layered material among the TMDCs family. It crystallizes in the hexagonal structure similar to CdI2 . It is a multi-layered material with repeating subunits formed from one layer each of Ti and S. Figure 1.7 shows the sequential layered crystal structure of TiS2 . There is a strong bond between atoms in the Ti layer and the S layer, while weak van der Waals forces dominate the bond between the S and S layers [93]. These weak forces of attraction cause great stability of the [001] layer surface, allowing the material to place additional atoms in the van der Waals region. Sulfur atoms are octahedrally coordinated with titanium atoms, with each TiS6 octahedron is bound tightly to each other by strong covalent bonds, and each layer stacks weakly through the van der Waals force (see Fig. 1.8) [94].

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Fig. 1.7 A pictorial representation of the hexagonal layered structure of TiS2

Fig. 1.8 The crystal structure of TiS2 and an individual octahedral unit

Titanium disulfide (TiS2 ) is known to be a semiconductor material with a band gap ranging from 0.5 to 2.5 eV [95–98]. The Bohr radius of TiS2 is approximately 6.43 nm. The lattice parameter values a (a = b) and c of TiS2 are 3.40 A°, 5.96 A° respectively [94]. TiS2 is not well studied and is of particular interest due to its non-toxicity, non-hazardous property and natural abundance. The literature reports that nanostructured TiS2 can be tuned to a band gap that is optically active in the visible range, for the use in solar cells [99]. Also, the high thermopower of bulk TiS2 makes it a strong candidate for a nanostructured high ZT thermoelectric material. Thermoelectric materials are materials that can generate electricity when subject to a temperature differential. This means they can be used to recover waste heat (e.g., industrial plants, car engines) and can transform this into useable electricity, enhancing the overall efficiency of the system. Various TiS2 morphologies, such as

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nanotubes, thin films, nanoclusters, nanodiscs, fullerenes, and whiskers [98–104], have been successfully synthesized. Keeping in the view of the above, the present chapter focuses on the synthesis and photocatalytic properties of 2D metal dichalcogenides with a special focus on TiS2 .

1.1.4 Photocatalysis: Fundamentals, Processes and Mechanism Environmental pollution has become a worldwide problem, resulting in a number of diseases. Among different kinds of pollution, water pollution is one of the major problems for the entire world. With the development of new industries, most of the wastewater is discharged into rivers, lakes and fields [105, 106]. Owing to the huge amount of organic and inorganic compounds in the wastewater, this has become a serious threat to the entire world. Chemical dyes such as methylene orange (MO) and methylene blue (MB) are widely used in the textile industry, and chemical wastes from these industries are generated in the form of wastewater polluting the lakes and natural water resources [107, 108]. This wastewater, containing chemical dyes, has become one of the threats to farmers and the urban population. With this in mind, it is important to control this water pollution and remove chemical dyes from the sewage water released by the textile industry. Ion exchange, chlorination, adsorption, and reverse osmosis methods have been employed for the decoloration of dyes [109– 112]. These types of methods are very expensive. These methods do not provide a permanent solution to this problem because they transfer the pollutant from one medium to another. Due to the rapid and complete oxidation of impurities to a stable product, photocatalysis proved to be a easy and inexpensive method for the decomposition of organic dyes in waste water [113–116]. Many studies have been carried out to improve the photocatalytic properties of materials for wastewater treatment [117–119]. Catalysts are used to accelerate a chemical reaction, and similarly, the photocatalyst uses a catalyst to accelerate chemical reactions in the presence of ultraviolet radiation. As a result of the light absorption, electron and hole pairs are formed which provide chemical changes in the reactants and restore their chemical composition after each cycle of such interactions. The main characteristics of photocatalytic materials are: the band gap, material morphology, reusability and stability. Photocatalysis is often used to break down and mineralize dangerous compounds to H2 O and CO2 and thus leads to the decrease of poisonous metal ions into non-poisonous states, deactivates and destroys all aquatic microorganisms, decomposes waste plastics and green synthesis of chemicals important for industry. Photocatalysis relates to oxidation and reduction reactions on the surfaces of a photocatalyst material, which are mediated between the valence band (VB) and the conduction band (CB) which are generated by light irradiation.

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These photogenerated pairs of holes and electrons cause the formation of particles such as OH− (hydroxyl) or superoxide radicals from moisture and atmospheric oxygen. These species are strong enough to oxidize and decompose organic materials or the smell of gas and kill bacteria. Photocatalysis has been recognized as an effective method for mineralizing toxic organic compounds, dangerous inorganic materials and for microbial disinfection due to the formation of OH ions, which act as a strong oxidizing agent [120, 121]. Figure 1.9 presents the photocatalytic mechanism. Briefly, two-dimensional transition metal dichalcogenides materials (2D-TMDCs), when irradiated with an appropriate wavelength, excite an electron from VB to CB, resulting in an e− —h+ pair. When the reaction is carried out in the presence of water and oxygen, the electron is trapped in the conduction band by oxygen, thereby forming the anion of the superoxide radical, and water is oxidized at the oxidation site to form the OH radical. These are two reactive species that react with organic pollutants, resulting in complete mineralization. The reactions of this e− —h+ pair with various electron acceptors and donors as well as the e− —h+ recombination processes have been well investigated [121–123]. The formation of radical cations of organic substrates after electron transfer to excited semiconductors has been investigated in several cases using product analysis [124] and spectroscopic studies [125]. TiS2 is an important semiconductor material among the 2D-TMDCs materials. In TiS2 , sulfur (S) atoms are separated in two hexagonal planes by a plane of titanium (Ti) atoms. Titanium and sulfur are covalently bonded, and the neighboring S–Ti–S layers are connected by van der Waals forces. During the past few years, TiS2 has not been studied more, but in recent times, it has drawn a lot of attention due to its properties such as non-toxicity, non-hazardous property and natural abundance. Being an important material of TMDC’s family, it has been studied a lot and some of important and recent reports are being reviewed in this chapter.

Fig. 1.9 Photocatalytic mechanism

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Tian et al. [126] reported n-type TiS2 /organic hybrid film with exfoliation-andreassembly approach. In this article, authors outlined high electrical conductivity and low Seebeck coefficient of n-type TiS2 /organic hybrid film, synthesized at low temperature. In addition, high value of power factor (210 μW/ m1 K2 ) was observed after annealing the film under the high vacuum condition. This was later used in thermoelectric device. At a temperature gradient of 70 K, the power density of 2.5 W/m2 was observed for thermoelectric device employing n-type TiS2 /organic hybrid film. Okamoto et al. [127] synthesized a nanotitanium disulfide (nanoTiS2 )/CNT/PEDOT−PSS hybrid films. Authors reported the values of power factor (PF), electrical conductivity (σ), and Seebeck coefficient (S), (PF = 45.0 μW/(mK2 , σ = 406 S/cm, and S = 33.3 μV/K) for a nano-TiS2 /CNT/PEDOT−PSS hybrid film having a mass ratio of 1:9:23 (CNT content 70 wt%) at 50 °C. The thermal conductivity (k//) for the hybrid film was found as 0.91 Wm−1 K−1 at room temperature with a mass ratio of 1:9:23 (CNT content = 70 wt%). An enhanced ZT of nanoTiS2 /CNT/PEDOT−PSS hybrid film was observed as 0.0163. Zhang et al. [128] prepared the n-type TiS2 ceramics with p-type PbSnS3 nanoparticles and studied their thermoelectric properties in the temperature between 350 and 650 K. They reported that there is a decrease in the electrical conductivity and increase in the Seebeck coefficient after the addition of PbSnS3 nanoparticles in n-type TiS2 ceramics. An enhanced ZT of ~0.44 at 650 K was observed for TiS2 -0.5 mol% PbSnS3 , which is higher than that of pure TiS2 . Huckaba et al. [129] synthesized TiS2 and used it as the hole-transporting layer (HTL) material (HTM) in PSCs. They reported that the power conversion efficiency of the device was 13.5%. N-type 2D-TiS2 nanosheets were synthesized with the help of novel chemical-welding method by Zhou et al. [130] and studied the thermoelectric behavior of the as-synthesized material. They reported the improvement in power factor by enhancing the Seebeck coefficient and electrical conductivity. The reported power factor value was ~216.7 μW m−1 of TiS2 nanosheet-based flexible film at room temperature, which was the highest among the results reported on chemically exfoliated 2D TMDCs nanosheet-based films. The value was also similar to that of the flexible n-type thermoelectric films. Due to these interesting properties, this material was proposed as a potential candidate for application in wearable electronics. Solid-state reaction method was used by Ramakrishnan et al [131] to synthesize a series of Sn-doped TiS2 (TiS2 :Snx ; x = 0, 0.05, 0.075 and 0.1). They studied the thermoelectric properties of Sn-added TiS2 and observed that there is a remarkable decrease in electrical resistivity and slight decrease in the Seebeck coefficient, after the addition of Sn in TiS2 . They found that the value of power factor was maximum at 373 K in TiS2 :Sn0.05 and minimum at 623 K for TiS2 :Sn0.075 . This was due to the defects (excess Ti) scattering and impurity phase (misfit phase). An enhanced ZT of 0.46 was achieved at 623 K for 0.05 in Sn-added TiS2 , which is higher in comparison to a maximum value of 0.08 observed for pure TiS2 . Wang et al. [132] synthesized C60 /TiS2 hybrid films and studied their thermoelectric properties. They observed that there is an increase in the power factor and decrease in thermal conductivity after the addition of C60 nanoparticles in TiS2 . An enhanced ZT of ≈0.3 was achieved at 400 K in C60 /TiS2 material, which was higher than that of pure TiS2 . xMoS2 –TiS2 nanocomposites was prepared by Ye

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Yang et al. [133] and studied its thermoelectric properties. The authors observed that xMoS2 –TiS2 nanocomposites show the better power factor as compared to that of pure TiS2 . An enhanced ZT of 0.29 was obtained at 573 K for 3 mol% MoS2 in TiS2 , which is 60% higher than that of pure TiS2 . Sun et al. [134] synthesized the layered TiS2 materials and used it as a promising positive electrode intercalation material in Mg fuel cell. They observed a capacity of 115 mAh g−1 in Mg full cell. The voltage profiles revealed distinct Mg2+ cation ordering, unlike the solid solution behavior exhibited by Li+ . Li Rui et al. [135] synthesized Te/TiS2 heterostructure nanocables with varying sizes using two-step route. They studied the thermoelectric properties of as-synthesized materials. They reported the thermal conductivity (κ), electrical conductivity (σ ), and Seebeck coefficient (S), carried out the experiment on the same nanowires in the temperature between of 2 and 350 K. The power factor (S2 σ) was 0.58 Wm−1 K−2 for the heterostructure nanocables, which was due to strongly enhanced Seebeck coefficient and high electrical conductivity. The value of ZT was found to be 1.91. These results were interesting and opened the new possibilities for applications in TE devices. Using mechanochemical lithiation and exfoliation method, TiS2 nanosheets (TiS2 –NSs) were prepared by Yang et al. [136]. They studied electrochemical performances of sulfide solid electrolytes (SEs)-based bulk type all solid-state lithium-ion batteries (ASLBs) using TiS2 nanosheets (TiS2 –NSs). They found the superior rate capability of the TiS2 -NS as compared to that of bulk TiS2 . This can be attributed to the ultrathin 2D structure and high electronic conductivity. Hazarika et al. [137] synthesized TiS2 nanoparticles and studied the swift heavy ion irradiation effect on TiS2 nanoparticles dispersed PVA films. Various characteristics of the irradiated samples were characterized by XRD, spectroscopic and microscopic techniques. The authors found that there was red shift after swift heavy ion irradiation on TiS2 nanoparticle-dispersed PVA films. These results are very important and useful for various optoelectronic devices. PbS-nanoparticle-embedded TiS2 were synthesized by Wang et al. [138] and studied its thermoelectric properties. The authors reported large enhanced power factor values of 1 mW/(mK2 ) at 300 K and 1.23 mW/(mK2 ) in temperature between 573 and 673 K for 1%PbS-nanoparticle doped in TiS2 . On comparing with pure TiS2 , there was an increase in power factor by ~110% at 300 K, and by (35~50)% in the temperature range of 573∼673 K. T. Hexagonal titanium disulfide (TiS2 ) sheets were synthesized by Tan et al. [139]. They reported that TiS2 sheets with thicknesses of 50–200 μm and sizes of 10–20 μm were obtained at 700–750 °C. Much thicker (1–5 μm in thickness) and bigger sizes (50–100 μm in sizes) of TiS2 sheets were observed at 850 °C. The authors suggested that this methodology should be extended for preparation of large and shrill sheets of TMDs. Wan et al. [140] synthesized TiS2 /[(hexylammonium)x (H2 O)y (DMSO)z ] by facile electrochemical intercalation method and studied its thermoelectric behavior. The authors reported that electrical conductivity and power factor of TiS2 /[(hexylammonium)x (H2 O)y (DMSO)z ] were observed as 790 S cm−1 and 0.45 m Wm−1 K−2 respectively. The lattice thermal conductivity was approximately 50% in comparison to that of the thermal conductivities of the bulk and singlelayer TiS2 . Low thermal conductivity and power factor accounted for ZT, of 0.28 at 373 K. They suggested that TiS2 /[(hexylammonium)x (H2 O)y (DMSO)z ] material may

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be useful in wearable electronics. 2D nanostructures of titanium disulfide (2D-TiS2 ) were synthesized by Baraw et al. [141] using solid–gas reaction method at temperatures between 500 and 600 °C. The authors found that TiS2 hardly absorb/adsorb hydrogen for hydrogen pressures up to 80 bar and reaction temperatures up to 300 °C. Ramzy et al. [142] synthesized TiS2 material. The authors outlined studies on single crystals that are intercalated with excess Ti or Co, or substituted with Ta. They illustrated the intrinsic impact of the dopants on the thermal transport in the absence of grain boundary scattering. Bourgesa et al. [143] prepared a series of polycrystalline Ti1−x Nbx S2 (0 ≤ x ≤ 0.075) by mechanical alloying and spark plasma sintering method. Trigonal TiS2 synthesized by high energy ball-milling was characterized by XRD and TEM. It was observed that the prepared particles comprised of pseudoordered TiS2 domains between 20 and 50 nm. An enhanced ZT of 0.3 at 700 K with x = 0.05 was observed, which was higher than that of pure TiS2 . Beaumale et al. [144] prepared the compound of Ti1−x Nbx S2 (0 ≤ x ≤ 0.05) by solid–liquid– vapor reaction and spark plasma sintering. They studied the thermoelectric properties of as-synthesized materials in the temperature range between 300 and 700 K. The authors observed that the electrical resistivity and the Seebeck coefficient decreased on increasing the Nb content. This is due to an increase in the carrier concentration. An enhanced figure of merit was achieved in Ti1−x Nbx S2 compound, which was higher than that of pure TiS2 . Gupta et al. [145] prepared nanocomposites of CdS and TiS2 and used it for the visible light-induced H2 evolution reaction (HER). The authors observed that nanocomposite of cards with 0.7 eV band gap of TiS2 showed an activity of 1000 μmol h−1 g−1 . The amount of hydrogen liberated after 20 h for the CdS/TiS2 was 14,833 μmol. 2D-TiS2 nanosheets were prepared using simple solution exfoliation method by Yin et al. [146], and they were employed as an ETL in planer PSCs. The authors found higher power conversion efficiency (PCE) of 17.37%. in PSCs. They suggested that this TiS2 fabrication method can be used for scalable production of PSCs, which is capable of providing excellent UV stability and high efficiency. Titanium disulfide (TiS2 ) and MWCNTs with varying mass ratio were synthesized by Kartick et al. [147] using simple dry grinding method and used it for Li-ion batteries. The presence of interaction between the TiS2 and MWCNTs were confirmed by XRD and Raman spectroscopy techniques. The authors found the specific capacity of ≈450 mAh g−1 with 80% capacity after 50 discharge–charge cycles. These values were higher than that of TiS2 , MWCNT or other TiS2 –MWCNT hybrids. Hydric titanium disulfide (HTS) ultrathin nanosheets were synthesized and studied by Lin et al. [148]. The authors reported a high electrical conductivity of 6.76 × 104 S/m at room temperature, which was more in comparison to that of ITO (1.9 × 104 S/m), inorganic graphene analogs (5.0 × 102 S/m) and graphene (5.5 × 104 S/m). They suggested that the HTS thin films showed promising capability for the next generation conducting electrode material for nanodevices. Ryu et al. [149] synthesized Na/TiS2 composites and studied the electrochemical properties of Na/TiS2 cells. The discharge curve of cell showed two plateau regions of 1.6 and 2.1 V. The discharge capacity in the low plateau (LP) region (0.8–2.0 V) did not show decrement and was constant for 40 cycles; however, the main loss of discharge

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capacity was observed in the upper plateau (UP) region (2.0–2.6 V). Highly crystalline TiS2 nanosheets, employing low-cost molecular precursors were prepared and investigated by Plashnitsa et al. [150]. The authors reported average dimension and thickness of TiS2 NSs as ~500 nm × 500 nm and ~5 nm, respectively. TiS2 NSs exhibited broad absorption in the visible region tailing out into the near-infrared region. They suggested that the observed outcome provides the new avenues for preparing low-dimensional 2D nanomaterials for numerous photochemical energy applications. Guilmeau et al. [151] prepared Cux TiS2 bulk compounds and studied its thermoelectric properties. The authors reported that the lattice constant c expanded linearly on increasing content of Cu. The authors also reported that there was a decrease in both lattice thermal conductivity and electrical resistivity in comparison to that of pure TiS2 . An enhanced ZT of 0.45 at 800 K with x = 0.02 was observed which is higher than that of pure TiS2 . Prabakar et al. [152] prepared IF nanoparticles and IF hollow spheres of titanium dissolved by varying the injection temperature of the titanium precursor into the sulfur solution. The authors suggested that the synthesized product is very useful for hydrogen storage and lubrication. He et al. [153] synthesized TiS2 powder by a solvothermal process and studied its photocatalytic properties. Authors observed an absorption edge at 595 nm (2.08 eV) and broad absorption above 595 nm. The photocatalysis observations are in the relation with the light adsorption edge. The photocatalytic activity of pure TiS2 powder was more in comparison to that of partially S-doped TiO2 powder. Qin et al. [154] synthesized cobalt-doped TiS2 (Cox Ti1−x S2 : 0 ≤ x ≤ 0.3) using solid-state reaction method and studied its thermoelectric properties for the temperature between 5 and 310 K. The authors reported that the absolute thermopower |S| and electric resistivity (ρ) for all the doped compounds show significant decrease with increase in Co content. An enhanced ZT of 0.03 at 310 K with x = 0.3 was observed, which was 66% higher than that of pure TiS2 . Single-layered TiS2 nanodiscs of thickness 0.6 nm were synthesized by Park et al. [103] employing the wet chemical process. The authors reported that the size could be controlled by varying the experimental conditions. It was found that at room temperature, the single-layered TiS2 nanodiscs were unstable. This was confirmed by powder XRD and time-dependent EDS. They reported structural changes from TiS2 to form TiO2 , which was confirmed by the reflection of an emission at 475 nm. Ma et al. [155] prepared high-purity TiS2 dendritic crystals by simple chemical vapor transport (CVT) method employing inorganic reagents. The authors observed that the duration and reaction temperature were important parameters in monitoring the morphologies of TiS2 dendrites. The phase structure TiS2 dendritic crystals was characterized by XRD technique. The morphology of TiS2 was studied by SEM and TEM. All microscopic interpretations not only showed a dendritic morphology, but also provided direct confirmation for the growth process of TiS2 dendrites. High-quality nanosized clusters TiS2 were synthesized by Alexandru et al. [156] and studied its optical properties. The authors observed blue shift in the optical absorption with a decrease in the cluster size because of quantum confinement. Titanium disulfide (TiS2 ) whiskers on Ni-coated Si wafer were successfully synthesized at 630 °C using simple vapor transport deposition method by Zhang et al. [98]. TiS2 whiskers are single crystalline with precise stoichiometric composition. The

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authors suggested that the fruitful growth of TiS2 whiskers will help in providing important experimental and theoretical prospects for investigators. Li et al. [157] studied the thermal conductivity and structure of the bismuth-doped TiS2 (Bix TiS2 : 0 ≤ x ≤ 0.25). The structure and thermal conductivity of Bix TiS2 (0 ≤ x ≤ 0.25) were studied by various characterization techniques like Raman spectroscopy, XPS, XRD and thermal conductivity measurements. A2 and D4 modes were observed by the authors in Raman spectra. The intensity of these bands increased, by increasing the Bi content in TiS2 . The red shift of mode Eg as well as D4 and A2 reflected weakening of intra-layer bonds. The blue shift of A1g after intercalation proposed the increase in chemical binding in the van der Waals gaps because of transfer of charge. The thermal conductivity of TiS2 with Bi intercalation was recognized. This can be ascribed to the phonon scattering by “rattling” of the intercalated bismuth atoms in the van der Waals gaps of TiS2 . Li et al. [158] synthesized Nd-intercalated TiS2 (Ndx TiS2 : x = 0.025, 0.05 and 0.15) and studied its thermoelectric properties for the temperature, ranging from 5 to 310 K. The authors observed that lattice thermal conductivity and DC electrical resistivity showed a decrease in Ndx TiS2 in comparison to that of pure TiS2 . An enhanced ZT of 0.027 at 300 K with x = 0.025 was found which was 59% greater than that of TiS2 . Bi-doped TiS2 (Bix TiS2 : x = 0, 0.05, 0.15, 0.25) was synthesized by Liu et al. [159], and its temperature-dependent thermopower in the temperature between 7 and 300 K was studied. The authors reported that all samples of Bi-doped TiS2 showed a metallic behavior at high temperatures. The thermopower may be ascribed to the special lens-shaped Fermi pockets. With increase in Bi intercalation, there is expansion in Fermi pockets, and each Bi atom transferred five electrons to the host layers. Li et al. [160] synthesized Bix TiS2 (x = 0.05, 0.15, 0.25) and studied its thermoelectric properties in the temperature between 5 and 310 K. The authors observed that lattice thermal conductivity (22, 115 and 158% low at 300 K for X = 0.05, 0.15 and 0.25, respectively) and electrical resistivity (one order low for x = 0.05 and two orders low for x = 0.15, 0.25 at 300 K) of Bix TiS2 decreased as compared that of pure TiS2 . An enhanced ZT of 0.03 at 300 K (x = 0.05) was observed, which was two times higher than that of pure TiS2 . Li et al. [161] studied the thermoelectric properties of Gd intercalated TiS2 (Gdx TiS2 : x = 0.025, 0.05). The authors reported that the lattice thermal conductivity k L (k L is lowered by 20 and 46% at 300 K for x = 0.025 and 0.05, respectively) and electrical resistivity decreased for Gd intercalated TiS2 . Also the observed ZT value of Gd0.05 TiS2 at 300 K was three times more in comparison to that of TiS2 . Li et al [162] synthesized and studied the thermopower of Bix TiS2 (x = 0–0.25). The authors investigated the temperature dependence of thermopower and DC electrical resistivity in the temperature between 7 K and 300 K. The authors found that resistivity and thermopower showed decrement on intercalating Bi in TiS2 . Abbott et al. [163] synthesized TiS2 and studied its thermopower properties. The authors reported that the thermopower (α) of TiS2 showed an ntype characteristic with a value of ≈−200 μV/K at room temperature. At room temperature, the electrical resistivity (ρ) was of the order of 1 m-cm and showed a metallic behavior with dR/dT > 0 between 300 and 10 K. The authors also found a large value of power factor (PF = α 2 /ρ) with a PF ~30 μW/K2 cm at T = 300 K. They suggested that these results may be important for thermoelectric devices. Tao

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et al. [102] synthesized TiS2 nanotubes and employed them as a cathode material for rechargeable magnesium-ion batteries. They suggested that TiS2 nanotubes showed advantages for environmental considerations and safety. TiS2 films were synthesized using atmospheric pressure chemical vapor deposition (APCVD) method by Camelot et al. [164]. TiS2 films on glass substrates were prepared by reaction of HSC(CH3 )3 , HS(CH2 )2 SH and S(Si(CH3 )3 )2 with TiCl4 at 275–600 °C. Different characterization techniques such as XRD, SEM with EDX, Raman spectroscopy were used to illustrate the structural and morphological properties of TiS2 films. The authors observed that in the temperature range from 275 to 400 °C, the TiS2 film was amorphous and at temperature higher than 500 °C, the films were crystalline with lattice parameter of a = 3.405 A°, c = 5.609 A°. SEM showed a dense particulate morphology at low substrate temperatures (200–400 °C) and a needle-like mosaic at higher deposition temperatures. EDX technique provided a Ti: S ratio with 1:2. The film of TiS2 showed a band at 335 and 380 cm−1 in Raman pattern. Imai et al. [94] synthesized and studied the thermoelectric properties of TiS2 crystal. The authors found a large power factor (S 2 /ρ) at 300 K is 37.1 μW/K2 cm with thermopower (S) of −251 μV/K and resistivity (ρ) of 1.7 m cm. The ZT value of TiS2 is 0.16 at 300 K which was due to large power factor value of 68 μW/K cm. Recently, many researchers have focused on 2D-TMDCs, which have proven to be the remarkable photocatalytic materials due to their large surface area, high surface activity and adjustable band gap [165]. The size of 2D-TMDCs can be controlled by optimizing the synthesis conditions. Nanoscale 2D-TMDCs with large surface area are found to be the attractive materials for catalysis [166, 167]. A large number of 2D-TMDCs materials such as TiS2 , TiSe2 , WSe2, MoSe2 , MoS2 , WS2 , and its nanocomposites have been used as photocatalyst materials. Wu et al. [168] synthesized hyacinth flower-like WS2 nanorods with the help of hydrothermal method. They studied the photocatalytic properties of as-synthesized material and reported that the degradation of RhB in aqueous solution was 91.01% of RhB after 270 min. 2D-MoS2 was prepared with the help of hydrothermal method by Man et al. [169]. The authors investigated the photocatalytic properties under visible light irradiation of as-synthesized materials. They observed that there is a degradation of Rhodamine B under ultraviolet light. Yang et al. [170] reported synthesis of MoS2 nanosheets with diameter of 200 nm using a facile hot-injection method. The authors observed that there was a degradation of RhB and found that the maximum degradation of RhB was 97% under visible light irradiation. Ceriumdoped MoS2 composites were synthesized with the help of hydrothermal method by Wang et al. [171] and studied the photocatalytics properties. The authors observed excellent photocatalytic activity of aqueous Cr(VI). N-doped MoS2 nanoflowers were synthesized by Liu et al. [172]. The authors studied the photocatalytic performance and reusability of as-synthesized materials. The authors found that there was a completely degradation of RhB in 70 min. The outstanding photocatalytic performance of as-synthesized material found its potential application in water pollution treatment. Vattikuti et al. [173] synthesized mesoporous WS2 nanosheets with the help of hydrothermal and thermal evaporation method. They observed that the mesoporous WS2 showed superior catalytic activity as compared to WS2 nanosheets. Tahir

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et al. [174] synthesized ZnO@MoS2 composites with the help of hydrothermal and solvothermal method. The authors investigated structural and photocatalytic properties of as-synthesized materials. They observed that the 3 wt% of MoS2 –ZnO composite showed the outstanding photocatalytic properties as compared to that of pure photocatalyst. Solvothermal method was used by Xu et al. [175] to prepare a BiVO4 @MoS2 core–shell nanostructures and studied the photocatalytic properties. They reported that the degradation of RhB after 20 min. Jiang et al. [176] synthesized TiO2 /Ag/MoS2 /Ag nanocomposites with the help of hydrothermal method. The authors investigated the photocatalytic properties of as-prepared materials. They observed that TiO2 /Ag/MoS2 /Ag nanocomposites showed the excellent photocatalytic performance of Cr and RhB. Solvothermal method was used by Yin et al. [177] to synthesize the MoS2 nanosheet hybridized Bi5 O7 I(MoS2 /Bi5 O7 I) nanorods. The authors investigated the photocatalytic properties under visible light. They observed that there was a degradation of bisphenol A (BPA), tetracycline hydrochloride (TC) and ciprofloxacin (CIP). Ashraf et al. [178] synthesized BiOCl/WS2 (BWX) hybrid nanosheet with the help of sonochemical method. The authors investigated the photocatalytic properties of as-prepared materials under visible light irradiation and observed that there is a degradation of Cr(VI) ion and Malachite Green (MG). They found that the maximum degradation of MG was 98.4% in 45 min for BiOCl/WS2 (2%). RP-MoS2 /rGO nanocomposites were prepared with the help of hydrothermal method by Bai et al. [179] and studied the photocatalytic properties. The authors investigated the photocatalytic activity and observed that there is a degradation of Cr (VI). Lejbini et al. [180] prepared α-Fe2 O3 /MoS2 composites with the help of hydrothermal method and studied the crystal structure, morphology, and photocatalytic properties. The authors observed that there was a degradation of Rhudamine Blue (RhB). They found that the maximum degradation of RhB was 98% in 75 min under visible light irradiation. MoS2 /CdS composites were synthesized with the help of combination hydrothermal and solvothermal treatments by Chai et al. [181] and studied the photocatalytic properties of as-synthesized materials under visible light irradiation. They found that 5wt% MoS2 /CdS composites showed the highest photocatalytic activity. Molybdenum disulfide (MoS2 ) microspheres were synthesized by Huang et al. [182] and studied its photocatalytic activity. They observed that the degradation of thiobencarb (TBC) under visible light irradiation. The authors found that the degradation efficiency of TBC at a pH range of 6–9 is 95% in 12 h. Khan et al. [183] synthesized Ni-doped MoS2 with the help of hydrothermal method and studied the photocatalytic properties of as-synthesized materials under UV light irradiation. The authors observed that the degradation of methylene blue (MB) was 71% for pure MoS2 and 85–96% for 1–5% Ni-doped MoS2 and also observed that the degradation of RhB was 62% for pure MoS2 and 77–91% for 1–5% Ni doping in MoS2 . MoS2 @ZnO composite were synthesized by Rahimi et al. [184] and studied the photocatalytic properties of as-synthesized materials. They observed that the MoS2 @ZnO composite showed the better degradation efficiency as compared to that of MoS2 under sunlight irradiation. Saha et al. [185] synthesized the MoS2 -polyaniline (PANI) nanocomposites with the help of in situ polymerization. The authors studied the photocatalytic performance of as-synthesized materials

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and observed that there is a degradation of MB and 4-chlorophenol (4-CP). Broomshaped MoSe2 nanostructures were synthesized by Siddiqui et al. [186] with the help of hydrothermal method. As-synthesized nanostructures were characterized by different characterization techniques. They observed that the degradation of methylene Blue (MB) was ~90% in 20 min. Elangovan et al. [187] synthesized N-doped titania nanosheets and N-doped titania/tungsten dioxide nano rods with the help of two-step hydrothermal method. As-synthesized materials were characterized by XRD, FESEM with EDAX, TEM, UV-DRS, PL, FT-IR and Raman spectroscopy. They observed the photocatalytic properties under visible light irradiation and found the degradation of congo red under visible light irradiation. Hexagonal WS2 platelets were prepared with the help of liquid-phase exfoliation (LPE) process by Koyyada et al. [188] and studied the photocatalytic property. They found the degradation of Congo red (CR), Phenol red (PR), MO, and RhB pollutants on solar light irradiation. Sharma et al. [189] synthesized a Ag3 PO4 /MoS2 nanocomposites with the help of wet chemical route. The structural and optical properties of as-synthesized materials were characterized by XRD, TEM, Raman and absorption spectroscopy. The authors observed the photocatalytic properties under a low wattage lamp power (60 W). They found that the degradation efficiency was ~97.6% in 15 min of illumination. ZnO/WS2 was synthesized with the help of facile two-step method by Zhang et al. [190] and studied the optical and photocatalytic properties. The authors found the degradation efficiency of Rhodamine B (RhB) dye was 95.71% within 120 min under visible light irradiation. As this chapter is focused on two-dimensional transition metal dichalcogenides with special attention to the synthesis and photocatalytic properties of TiS2 and its nanocomposites, we shall discuss the interaction of titanium disulfide (TiS2 ) with various composites and the photocatalytic properties of TiS2 and its nanocomposites in succeeding section.

1.1.5 Synthesis and Characterization of TiS2 and Its Nanocomposites Many synthesis methods, namely CVT, CVD, and hydrothermal method, have been used for the synthesis of titanium disulfide (TiS2 ) [191, 192]. Out of the above methods, CVT has been widely used for the synthesis of TiS2 nanostructures. This method has several advantages, such as simple synthesis, mass production and controlled morphology of materials. For the synthesis of TiS2 using the CVT method, the required amounts of a metal powder of titanium (Ti) and sulfur (S) were mixed and stored in quartz ampoules and were sealed under vacuum condition. The sealed ampoules were kept in a vacuum oven at 500 °C for about 12 h followed by 24 h at 800 °C. [193]. Finally, we obtained the TiS2 nanodisc from ampoules. To synthesize of CuO–TiS2 nanocomposite and Sb@TiS2 nanocomposite, CVT method has been used. For the synthesis of CuO–TiS2 nanocomposite (wt% CuO = 30, 50%), CuO

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Fig. 1.10 Schematic representation of as-prepared pure TiS2 and Sb@TiS2 nanocomposite

nanorods and TiS2 nanodisc were dispersed in IPA and dried in a vacuum oven for 24 h at 70 °C and for the synthesis of Sb@TiS2 nanocomposite (wt% Sb = 30, 50%), the powder of TiS2 nanodisc and Sb was properly mixed and filled in a quartz ampoule. The sealed ampules were then heated for 12 h at 650 °C. Figure 1.10 presents the synthesis diagram of TiS2 nanodisc and Sb@TiS2 nanocomposite by CVT method. Figures 1.11 and 1.12 show the XRD pattern of (a) CuO–TiS2 nanocomposite and (b) Sb@TiS2 nanocomposite synthesized by CVT method. The XRD patterns for CVT synthesized TiS2 nanodisc showed the polycrystalline nature, and the diffraction peaks of TiS2 are observed at 15.404°, 31.252°, 34.054°, 44.015°, 47.742°, 53.640°, 56.166°, 57.544°, 65.310°, 72.117° corresponding to the planes of (001), (002), (011), (102), (003), (110), (111), (103), (004), (022) (Figs. 1.11 and 1.12) [194], and in CuO–TiS2 nanocomposite (Fig. 1.11), the peaks of CuO are indicated at 35.56°, 38.85° corresponding to the planes of (–111), (111) [JCPDS-ICDD no. 48-1548] [193]. Whereas in Sb@TiS2 nanocomposite (Fig. 1.12), the diffraction peaks of Sb are indexed at 28.65°(012), 40.11°(104), 42.00°(110), 48.41°(006), 51.59°(202), 59.43°(024), and 68.53°(122) [194]. In the XRD patterns of CuO–TiS2 nanocomposite and Sb@TiS2 nanocomposite, the intensity of the peaks decreases as compared to that of pure TiS2 , which indicates a decrease in the TiS2 crystallinity on adding CuO and Sb.

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Fig. 1.11 XRD spectra of as-prepared materials, a pure TiS2 , b 30% CuO–TiS2 and c 50%CuO– TiS2

Fig. 1.12 XRD spectra of as-prepared materials, a pure TiS2 , b 30% Sb@TiS2, nanocomposite and c 50% Sb@TiS2 nanocomposite

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Pure TiS2 nanodisc, CuO–TiS2 nanocomposite, and Sb@TiS2 nanocomposite were prepared by the help of CVT method, and the surface morphology was investigated with the help of FESEM. FESEM images of as-synthesized TiS2 nanodisc are shown as Figs. 1.13a–d and 1.14a–d which shows the nanodisc morphology. Figure 1.13c, d shows the FESEM images of CuO–TiS2 nanocomposite, which shows the formation of nanodiscs. The size of nanodisc is in the range of 500 nm to 5 μm. In case of Sb@TiS2 nanocomposites, the size of the nanodisc is almost same to that of TiS2 nanodisc (Fig. 1.14c, d). For more investigation, HRTEM images of TiS2 nanodisc, CuO–TiS2 nanocomposites and Sb@TiS2 nanocomposites were analyzed (Figs. 1.15a–d, 1.16a–d and 1.17a–d). HRTEM images TiS2 nanodisc (Fig. 1.15a–d) clearly show the nanodisc morphology in the range of 30–50 nm with interlayer spacing of 0.571 nm (Fig. 1.15d). From the HRTEM images of CuO–TiS2 nanocomposite (Fig. 1.16d), it can be determined that the CuO–TiS2 nanocomposite corresponds to the plane of (001) has an interlayer spacing of 0.58 nm, which agrees with the result of XRD and previously reported data [195]. Whereas in case of Sb@TiS2 nanocomposite, the diameter is in the range of 30 nm to 60 nm with interlayer spacing of 0.572 nm (Fig. 1.17d). These interlayer distances are consistent with the previously reported results [152]. The circular spot of the rings in SAED of pure TiS2 (insert in Fig. 1.15c),

Fig. 1.13 SEM micrograph of as-prepared materials, a, b pure TiS2 , c, d 50%CuO–TiS2 nanocomposite

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Fig. 1.14 SEM micrograph of as-prepared materials. a, b pure TiS2 , c, d Sb@TiS2 (Sb = 50%) nanocomposite

CuO–TiS2 nanocomposite (insert in Fig. 1.16b), and Sb@TiS2 nanocomposite (inset in Fig. 1.17c) indicates polycrystalline nature. The optical absorption spectra of as-synthesized materials have been recorded at normal temperature as a function of the wavelength in the ranges of 200–700 nm. The absorption spectra of CuO–TiS2 nanocomposites and Sb@TiS2 nanocomposites are presented as Figs. 1.18 and 1.19. Compared to the absorption spectrum of TiS2 nanodisc, the absorption peak of the CuO–TiS2 nanocomposite and the Sb@TiS2 nanocomposite shifts to a longer wavelength (red shift) with an increase in the proportion by weight of CuO nanorods and Sb in the TiS2 nanodisc [193]. The band gap of as-prepared materials has been calculated using Eq. (1.1)   Z hν − Eg = (αhν)2

(1.1)

The calculated band gap values for TiS2 nanodisc are 2.20 eV and for CuO–TiS2 nanocomposite (CuO = 30, 50%) are 1.96, and 1.85 eV. Whereas in case of Sb@TiS2 nanocomposite, the evaluated band gap values for Sb@TiS2 nanocomposite (Sb = 30, 50%) are 2.12, and 2.03 eV [95, 97, 98, 196]. The band gap decreases with increasing the weight% of CuO nanorods and Sb in TiS2 nanodiscs (Figs. 1.20 and 1.21). In case of CuO–TiS2 nanocomposite and Sb@TiS2 nanocomposites, this decrease in

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Fig. 1.15 a–d HRTEM images of as-prepared pure TiS2 nanodisc

the value of band gap may be due to the creation of intermediate states between the valance band gap and conduction band gap in TiS2 nanodiscs [95, 98]. Photoluminescence spectroscopy is another important technique for studying the optical properties of the materials such as surface optoelectronic properties, and to understand the behavior of electron hole pairs in semiconductor materials [197]. The PL emission spectra with an excitation wavelength of 312 nm are recorded at room temperature. The emission spectra of TiS2 nanodiscs, CuO–TiS2 nanocomposites and Sb@TiS2 nanocomposites are presented as Figs. 1.22 and 1.23. The PL emission peak of pure TiS2 is observed at 428 nm [192]. In the case of CuO–TiS2 nanocomposites and Sb@TiS2 nanocomposites, the emission peak shifts to a higher wavelength with lower intensity, with the increasing the weight% of CuO nanorods and Sb in the TiS2 nanodiscs. These emission bands are associated with near band emission due to the recombination of free excitons in TiS2 [198], and a lower PL peak intensity indicates slower radiation recombination [199], which plays an important role in improving photocatalytic activity.

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Fig. 1.16 a–d HRTEM images of as-prepared 50%CuO–TiS2 nanocomposite

Fig. 1.17 a–d HRTEM images of as-prepared 50% Sb@TiS2 nanocomposite

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Fig. 1.18 UV-visible spectra of as-prepared materials, a pure TiS2 , b 30%CuO–TiS2 , c 50%CuO– TiS2 nanocomposite

Fig. 1.19 UV-visible spectra of as-prepared materials, a pure TiS2 , b 30% Sb@TiS2 , c 50% Sb@TiS2

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Fig. 1.20 (αhν)2 versus hν plot of as-prepared materials, a pure TiS2 , b 30%CuO–TiS2 nanocomposite, c 50%CuO–TiS2 nanocomposite

Fig. 1.21 (αhν)2 versus hν plot of as-prepared materials, a pure TiS2 , b 30% Sb@TiS2 , c 50% Sb@TiS2

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Fig. 1.22 PL spectra of as-prepared materials, a pure TiS2 , b 30%CuO–TiS2 nanocomposite, c 50%CuO–TiS2 nanocomposite

Fig. 1.23 PL spectra of as-prepared materials, a pure TiS2 , b 30% Sb@TiS2 , c 50% Sb@TiS2

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1.1.6 Photocatalytic Properties of TiS2 and Its Nanocomposites The photocatalytic activity has been carried out using a pure synthesized TiS2 nanodisc, CuO–TiS2 nanocomposite and Sb@TiS2 nanocomposite as a photocatalyst to investigate the MB dye degradation. For the photocatalysis process, 0.2 g of as-synthesized materials with 0.5 ml MB dye are dispersed in 100 ml deionized water and the solution is stirred in visible light. After the samples have been irradiated with visible light, the complete degradation of the pure TiS2 nanodisc is observed in 210 min. Absorption spectra of MB dye using as-synthesized materials were recorded and shown in Figs. 1.24 and 1.25. The degradation of MB dyes for CuO– TiS2 nanocomposite and Sb@TiS2 nanocomposite is faster as compared to pure TiS2 nanodisc. This suggests that pure TiS2 nanodiscs show weak photocatalytic effect for MB dye. Figures 1.26 and 1.27 show the plot between C/C0 and degradation time (t), where C0 is absorption of MB before exposer and C is absorption of MB after exposer of light with degradation time.

Fig. 1.24 Time-dependent UV-visible spectra of MB dye in presence of as-synthesized materials. a Pure TiS2 , b 30%CuO–TiS2 nanocomposite, and c 50%CuO–TiS2 nanocomposite

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Fig. 1.25 Time-dependent UV-visible spectra of MB dye in presence of as-synthesized materials: a pure TiS2 , b after 20 days of pure TiS2 , c 30% Sb@TiS2 , d 50% Sb@TiS2

Fig. 1.26 Relative intensity (C/C0 ) of absorption vs time in presence of as-synthesized materials: a pure TiS2 , b 30%CuO–TiS2 nanocomposite, c 50%CuO–TiS2 nanocomposite, where C0 is the initial intensity and C is the intensity at time (t)

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Fig. 1.27 Relative intensity (C/C0 ) of absorption versus time in presence of as-synthesized materials: a pure TiS2 , b 30% Sb@TiS2 , c 50% Sb@TiS2 , where C0 is the initial intensity and C is the intensity at time (t)

The degradation efficiency of as-prepared material has been evaluated using the equation given as Eq. (1.2) [200–203]. η% = (C0 − C)/C0 × 100

(1.2)

The degradation efficiency for pure TiS2 nanodisc, CuO–TiS2 nanocomposite, and Sb@TiS2 nanocomposite is found to be 80.25% after 210 min, 83.18% after 180 min, and 78.60% after 85 min respectively. Figures 1.28 and 1.29 show the degradation efficiency of the as-synthesized materials depending on the exposure time. The percentage of degradation of the MB dye on irradiation with visible light for a pure TiS2 nanodisc is 80.25% in 210 min. Compared to pure TiS2 nanodisc, the CuO–TiS2 nanocomposite has a higher photocatalytic activity, degrading the MB dye up to 83.18% after 180 min. Whereas in case of Sb@TiS2 nanocomposite, degradation efficiency of MB dye is up to 78.60% after 85 min. This means, Sb@TiS2 nanocomposite shows the superior photocatalytic activity as compared to pure TiS2 and CuO–TiS2 nanocomposite. The photocatalytic mechanism of as-prepared materials is shown in Fig. 1.30, which have been explained using the following mechanism. On exposing visible light to the as-synthesized materials, electrons and hole are generated due to narrowing of band gap. The photogenerated holes may oxidize the water to the hydroxyl radicals, while photogenerated electrons react with the adsorbed oxygen as electron acceptor to form superoxide radicals anions, hydrogen peroxide and hydroxyl radicals.

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Fig. 1.28 MB dye degradation efficiency graph as function of time in as-synthesized materials: a pure TiS2 , b 30%CuO–TiS2 nanocomposite, c 50%CuO–TiS2 nanocomposite

Fig. 1.29 MB dye degradation efficiency graph as function of time in as-synthesized materials: a pure TiS2 , b 30% Sb@TiS2 , c 50% Sb@TiS2

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Fig. 1.30 Photocatalysis mechanism in CuO/Sb–TiS2 nanocomposite under visible light

These reactions have been represented by Eqs. (1.3–1.10). As a result of this reaction, the recombination of electron and holes pairs decreases, which leads to the degradation of the MB dye into a stable product.   + Ti S2 /Sb/CuO + hν → Ti S2 /Sb/CuO e− CB + hVB

(1.3)

+ h+ VB + H2 O → −OH + H

(1.4)

− h+ VB + OH → OH

(1.5)

− e− CB + O2 → O2

(1.6)

+ − O− 2 + hVB → HO2

(1.7)

2HO− 2 → O2 + H2 O2

(1.8)

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H2 O2 + O− 2 → OH + OH + O2 .

(1.9)

Dye + OH + O2 → CO2 + H2 O + other product

(1.10)

1.2 Conclusion A comprehensive review of the synthesis and photocatalytic properties was presented. TiS2 nanodiscs and its nanocomposites with CuO and Sb were synthesized with the help of low-cost CVT synthesis method, and the optical and photocatalytic properties of the as-synthesized materials have been studied. The synthesized materials were characterized using XRD, FESEM, HRTEM, UV-visible and PL spectroscopic techniques.. XRD patterns confirmed the crystallinity, formation of pure TiS2 nanodiscs, Sb@TiS2 nanocomposites and CuO–TiS2 nanocomposites, which was also confirmed by HRTEM results. The optical band gap of these nanocomposite decreased with increasing wt% of CuO nanorods as well as with the increase in the weight% of Sb. The PL of these nanocomposite showed a quenching behavior on addition of CuO nanorods. In case of Sb@TiS2 nanocomposites, the PL emission peak of pure TiS2 was observed at 428 nm, which was found to increase to higher wavelength with low intensities on increasing the weight% of Sb in TiS2 nanodisc. The degradation efficiency for MB dye of pure TiS2 nanodisc was 45.59% which was found to increase with the increase the weight% of Sb, whereas the use of CuO–TiS2 nanocomposites was found to degrade the MB dye by 83.18% in 180 min as compared to pure TiS2 which degraded MB dye 80.25% in 210 min. These nanocomposites showed faster degradation.

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190. Zhang X, Qiu F, Rong X, Jicheng X, Rong J, Zhang T (2018) Zinc oxide/graphene-like tungsten disulphide nanosheet photocatalysts: Synthesis and enhanced photocatalytic activity under visible-light irradiation. Can J Chem Eng 96(5):1053–1061 191. Vincent T, Guibal E (2003) Chitosan-supported palladium catalyst 3. Influence of experimental parameters on nitrophenol degradation. Langmuir 19:8475–8483 192. Parvaz M, Salah NA, Khan ZH (2018) Effect of ZnO nanoparticles doping on the optical properties of TiS2 discs. Optik 171:183–189 193. Ethiraj AS, Kang DJ (2012) Synthesis and characterization of CuO nanowires by a simple wet chemical method. Nanoscale Res Lett 7:70 194. Luo W, Lorger S, Wang B, Bommier C, Ji X (2014) Facile synthesis of one-dimensional peapod-like Sb@C submicron-structures. Chem Commun 50:5435–5437 195. Jeong S, Yoo D, Jang J-t, Kim M, Cheon J (2012) Well-defined colloidal 2-D layered transition-metal chalcogenide nanocrystals via generalized synthetic protocols. J Am Chem Soc 134:18233–18236 196. Let AL, Mainwaring DE, Rix C, Murugaraj P (2008) Thio sol–gel synthesis of titanium disulfide thin films and powders using titanium alkoxide precursors. J Non-Cryst Solids 354:1801–1807 197. Yu J-G, Yu H-, Cheng B, Zhao X-J, Yu JC, Ho W-K (2003) The effect of calcination temperature on the surface microstructure and photocatalytic activity of TiO2 thin films prepared by liquid phase deposition. J Phys Chem B 107:13871–13879 198. Wei C, Chen Xi, Li D, Huimin S, He H, Dai J-F (2016) Bound exciton and free exciton states in GaSe thin slab. Scient Rep 6:33890 199. Xie J, Lü X, Chen M, Zhao G, Song Y, Lu S (2008) The synthesis, characterization and photocatalytic activity of V (V), Pb (II), Ag (I) and Co (II)-doped Bi2 O3 . Dyes Pigm 77:43–47 200. Mittal H, Kumar A, Khanuja M (2019) In-situ oxidative polymerization of aniline on hydrothermally synthesized MoSe2 for enhanced photocatalytic degradation of organic dyes. J Saudi Chem Soc 201. Zhang J, Kang W, Jiang M, You Y, Cao Y, Ng T-W, Denis YW, Lee C-S, Xu J (2017) Conversion of 1T-MoSe2 to 2H-MoS2x Se2−2x mesoporous nanospheres for superior sodium storage performance. Nanoscale 9:1484–1490 202. Ye Z, Kong L, Chen F, Chen Z, Lin Y, Liu C (2018) A comparative study of photocatalytic activity of ZnS photocatalyst for degradation of various dyes. Optik 164:345–354 203. Sadaiyandi K, Kennedy A, Sagadevan S, Chowdhury ZZ, Johan MR, Aziz FA, Rafique RF, Selvi RT (2018) Influence of Mg doping on ZnO nanoparticles for enhanced photocatalytic evaluation and antibacterial analysis. Nanoscale Res Lett 13:229

Chapter 2

Studies on Dye-Sensitized Solar Cells Incorporated with Perovskite as Sensitizer Dye Rahul, Sultan Ahmad, Pramod K. Singh, and Zishan H. Khan

Abstract Over the last two decades, researchers have focused research work on photovoltaics-based solar technologies. Among different photovoltaics technologies, hybrid (organic/inorganic) photovoltaic has witnessed an enormous research effort in past years, and the present, leading to the first commercial products. In this era of solar energy, dye-sensitized solar cells (DSSC) are also playing a pivotal role. However, the maximum efficiency of DSSC is stuck around 12-14%. Perovskite was first used as a sensitizer in dye-sensitized solid-state devices in which molecular dye was replaced by perovskite in 2009. Perovskite has been found too high lightharvesting property with a high absorption coefficient and charges transporting and accumulation properties as well. Thus, perovskite can be used either as a sensitizer or light harvester. An all-solid-state perovskite solar cell based on organolead iodide demonstrated 9.7% in 2012, and a year later, its efficiency increased to more than 15%, and now it becomes which is 28% in 2020, which implies that organolead halide perovskite is a promising solar cell material. The bandgap of 1.5 eV for CH3 NH3 PbI3 can be tuned by replacing A or B or X ions in the ABX3 perovskite structure within the allowed tolerance factors, which can further improve photovoltaic performance more than 20%. Since the perovskite layer is as thin as sub-micrometer levels, a perovskite solar cell can be classified as a new type of thin-film solar cell. Because of the demand and fascinating technology of DSSC solar cells, the present chapter focuses on the synthesis of light sensitizers, fabrication of dye/natural dye/perovskitebased solar cells, and its characterization in an ambient air atmosphere. The research work presented in this chapter may be useful for the scientific society working in the field of photovoltaic devices. This work includes the fabrication and characterization of dye, natural dye, and perovskite light sensitizer-based dye-sensitized solar cells. The use of dyes and perovskite sensitizers for developments of solar cells shall not only be cost-effective with a simpler technology of fabrication but also it will be eco-friendly. Rahul (B) · S. Ahmad · Z. H. Khan Department of Applied Sciences and Humanities, Jamia Millia Islamia, New Delhi 110025, India e-mail: [email protected] Rahul · P. K. Singh Material Research Laboratory, Department of Physics, School of Basic Sciences and Research, Sharda University, Greater Noida, UP 201310, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_2

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2.1 Introduction A new type of device known as “dye-sensitized solar cells” (DSSCs) based on nanocrystalline TiO2 in 1991 was developed by Gratzel and his group [1]. These solar cells are called a third-generation photovoltaic cell as they differ significantly from other conducting semiconductor devices. A DSSC is fabricated using the nanocrystalline semiconductor oxide film as a working electrode, sensitizer dye, electrolyte with redox couple, and a platinum-coated counterelectrode. Figure 2.1 shows the component and working of DSSC. In this solar cell, the light is absorbed by the dye, which gets excited and injects the electron to the conduction band of the semiconductor (TiO2 ), which travels toward the working electrode and gets accumulated on the working electrode. Dye cation produced in this process is reduced by the iodide (I− ); I− gets converted into I3 − after a series of reactions. These I3 − ions get diffused through the electrolyte and reach to the counterelectrode. At the counterelectrode, I3 − oxidizes by the electron and reaches to the counterelectrodes from the working electrode through the external circuit and is reconverted into I− . This following process is responsible for converting solar energy into electrical energy efficiently: 1. During this operation, no exciton diffusion is needed. The electron–hole pair dissociate in dye as soon as the light gets absorbed. The electron gets transferred immediately to the TiO2 layer existed just below the dye layer. 2. The TiO2 layer features a large thickness and extent because of this plenty of electrons gets transferred into the conduction band of TiO2 , even for a thin layer of dye. Owing to these factors, the efficiencies of DSSCs are high that reach up to 12%. The voltage generated in this process is the difference between the Fermi level of the electron in the semiconductor electrode to the electrolyte redox potential. No

Fig. 2.1 Schematic diagram showing band positions of sensitizer and working electrode

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permanent chemical transformation has occurred in the entire process. The main component in DSSCs is the dye, which acts as the light sensitizer. The sensitizers should have the given blow properties given below [2–5]. It should have a chemically adsorbable group to load onto the semiconducting material. It should have appropriate HOMO and LUMO levels for the efficient energetics of the device, high extinction coefficient in visible and infrared regions for efficient light absorption. It should have to be photostable. It should be soluble in the solvent orthogonal to the semiconducting layer and should create spacing between photoanode and electrolyte. Apart from different dyes, a new class of material, i.e., the perovskites, has almost all the properties. Nowadays, these materials are being used as the sensitizers in sensitizers based on solar cells in place of dyes. But traditionally, the solar cells based on Perovskite Sensitizers are considered a variety of dye-sensitized solar cells (DSSCs). The perovskite sensitized solar cells (PSSCs) are currently one of the rigorously researched domains in the contemporary scientific world. Perovskite, named after the great Russian mineralogist L.A. Perovski, is a crystal structure with the formula ABX3 (X = halogen or oxygen), where the cation A is stabilized in a cubo-octahedral cage formed by the 12 nearest X anions, and the cation B coordinates with the six nearest X anions to form octahedral geometry. Oxide perovskite started to receive attention because of its rare physical properties such as ferroelectricity and superconductivity. Compared to oxide perovskite, halide perovskite has received relatively less attention due to the lack of unique physical properties. Replacement of an inorganic element in the A site with organic molecules was found to lead to a perovskite structure. Organotin halide compounds with layered perovskite structure gained some attention because of semiconductor-tometal transition behavior and superconductivity. The three-dimensional organolead halide CH3 NH3 PbX3 (X = Cl, Br, and/or I) was found to crystallize with a perovskite structure and showed a distinctive feature of molecular motion of methylammonium ion. However, methylammonium lead halide perovskite attracted little attention because of unusual electronic properties were not discovered. In 2009, Miyasaka et al. were the first researchers to find that CH3 NH3 PbX3 possessed light-harvesting properties when they used it as the sensitizer in a dye-sensitized electrochemical junctiontype solar cell [6]. CH3 NH3 PbBr3 -deposited nanocrystalline TiO2 showed a power conversion efficiency (PCE) of 3.1%, and CH3 NH3 PbI3 showed a little higher PCE of 3.8%. The better efficiency of iodide perovskite was due to a narrower bandgap. Two years later, a higher PCE (6.5%) was reported based on a CH3 NH3 PbI3 light harvester, in which the TiO2 surface was covered sparsely with CH3 NH3 PbI3 nanocrystals. This structure showed one order of magnitude higher absorption coefficient than that of the thick TiO2 film with a ruthenium bipyridyl-based organometallic dye molecule. Such superior light absorption property was enlarged for developing thin-film solar cells. Nevertheless, CH3 NH3 PbX3 was not used as a promising sensitizer because of its chemical instability in the polar electrolyte solution. In 2012, a long-term stable and highly efficient all-solid-state mesoscopic solar cell based on CH3 NH3 PbI3 was first developed, where a PCE of 9.7% was achieved from the submicrometric-thick TiO2 film (0.6 μm) CH3 NH3 PbI3 nanodots. The non-sensitization concept using the mixed halide perovskite CH3 NH3PbI3-x Clx coated on an Al2 O3 surface was

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proposed, and it demonstrated a PCE of 10.9%. Precipitous progress was made soon after the discovery of the so-called perovskite solar cell [7]. Organolead halide perovskite has been discovered to have a high light-gathering property with high assimilation coefficient and charges shipping and development properties too. In this way, perovskite can be utilized either as a sensitizer or a light collector. An all-solid-state perovskite sunlight-based cell dependent on organolead iodide showed 9.7% in 2012, and after a year, its proficiency expanded to over 15%, which suggests that organolead halide perovskite is a promising sunlight-based cell material. As of March 2017, researchers from KRICT and UNIST hold a record of the highest accredited efficiency of 22.1% for a single-junction perovskite solar cell. In 2020, Sriabisha et al. reported that the perovskite solar cell accomplished much higher PCEs 28% by modification of the deposition method or junction structure [8]. The bandgap of 1.5 eV for CH3 NH3 PbI3 can be tuned by replacing A or B or X ions in the ABX3 perovskite structure within the allowed tolerance factors, which can further improve photovoltaic performance more than 20%. Since the perovskite layer is as thin as sub-micrometer levels, a perovskite solar cell can be classified as a new type of thin-film solar cell and would be a big game-changer of the present era of photovoltaics.

2.2 Dye-Sensitized Solar Cells (DSSCs) In this section, a comprehensive literature survey has been presented to accumulate data on the research work undertaken in the field of DSSCs. It also incorporates the fabrication, working principle, and performance of dye-sensitized solar cells (DSSC) as well as perovskite-based solar cells. It presents a detailed explanation of the various components of DSSC, such as semiconductor, dye, electrolyte, and electrodes. Numerous designs of metal compound dyes and non-metal-based natural dyes have been deliberated in the chapter to find out the possibility of eco-friendly light-harvesting materials. Different morphologies of light-harvesting materials and their structural impact on the total presentation of solar cells have also been explained along with their synthesis methods.

2.2.1 Components of DSSC The dye-sensitized solar cells have the following components.

2.2.1.1

Redox Couple

Redox couple is described as the combination of oxidized and reduced forms of a substance that takes part in oxidation or reduction half-reaction. In DSSC, the iodine

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I3 − /I− is most used as a redox couple. It reduces the electron from the counterelectrode and oxidizes it to the working electrode by providing an electron to oxidized sensitizer. Also, scientists have tried different combinations of redox couples such as iodine-based and cobalt-based redox couple. Though the recent report by Gratzel et al. showed the highest efficiency up to 13% using Co (II)/Co (III) redox couple, it can be inferred from the literature that to date, iodine is the best material for redox reaction in DSSC [9, 10]. Therefore, we have used iodine-based redox couples in the present study.

2.2.1.2

Counterelectrode

On the one end of the cell, there is the counterelectrode, typically a film of graphite or platinum. The working electrode sends electrons through the outer circuits to the counterelectrode, which acts as a collector source of the electron to reduce the redox couple. The platinum is the most used counterelectrode, which is coated either by sputtering or by sintering a thin layer of chloroplatinic acid in the air. Also, there are a large number of reports in the literature available on replacing platinum counterelectrodes with different materials, but still, platinum gives the best match with other components of DSSC and shows a fast electron transfer process as compared to other material [11–13].

2.2.1.3

Electrolyte

An electrolyte is a solution containing free ions, which makes the solution electrically conductive. An electrolyte (in DSSC generally iodide-based) fills the space between the TiO2 electrode and the counterelectrode. It helps in transferring electrons to the dye molecules. As the conductivity of electrolyte increases, the exchange of electron from counterelectrode to dye becomes faster, and hence, it leads to an increase in the photocurrent conversion efficiency of DSSC. The electron transfer mechanism taking place in electrolyte present is given as, • The I− ion of redox mediator donates an electron to photooxidized sensitizer (S+ ), thereby allowing the regeneration of the ground state (S) and oxidizing the iodine ion from I− to I3 − .   3S+ + 3e− I− → 3S + I− 3 • The oxidized redox mediator, I3 − , is consequently diffused toward the counterelectrode leading to its reduction to I− state. − − I− 3 + 2e → 3I

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Based on the literature, there are three types of electrolytes that have been utilized in DSSC. (i) Liquid Electrolyte Liquid electrolytes are generally prepared by dissolving salt into a solvent, for example, water (organic solvent in case of DSSC) or any other polar solvent. Due to thermodynamically interactions, the individual element of salt is dissociated between the substance and the solvent molecules. In most of the conventional solar cells, the liquid electrolyte is used. The most common is KI and I2 dissolved in acetonitrile. With liquid electrolytes, the DSSC shows outstanding photocurrent conversion efficiency. In liquid electrolytes, the transmission of the electron is faster, which leads to liquid cell parameters are improved, and the solar cell shows outstanding photocurrent conversion efficiency. Recently, Gratzel and co-workers [98] reported the highest photocurrent conversion efficiency up to 13% using Co (II)/Co (III) redox couple and liquid electrolyte, which is the combination of series of ionic liquids. But liquid electrolytes have many drawbacks such as leakage of liquid, sealing problem of cell, evaporation, or self-degradation problems. These problems decrease cell performance as well as the life of DSSCs. Therefore, the high efficiency is shown by the cell generally degrades with time. These limitations decrease the efficiency as well as stability of solar cells, which are the major challenges in the commercialization of DSSC. The solid polymer-based electrolyte has resolved the issues of liquid electrolytes. (ii) Gel-Based Electrolyte Gel electrolytes are electrolytes prepared by dissolving ionic salt in the gel matrix. Many reports on the preparation and characterization of gel electrolytes are available in the literature, but their use in DSSCs is limited. Generally, gels that are used to prepare gel electrolytes are bio-, inorganic- and polymer-based gels. Like liquid electrolytes, gel electrolytes also have many limitations [14, 15]. (iii) Solid Polymer Electrolyte (SPE) Solid polymer electrolytes can overcome the limitations, which present in the liquid- and gel-based electrolytes. SPE is commonly synthesized by dissolving ionic salt in the polymer matrix. The polymers which are used to prepare electrolytes should have low Tg . Due to the solid form of the polymer matrix, polymer electrolytes have low ionic conductivity, which is in the range of 10−7 to 10−4 S/cm as the mobility of ions is low in the solid matrix. Many approaches, such as doping different salts, using plasticizer, blending, or using fillers, have been adopted to increase the ionic conductivity of SPE [16–18]. The DSSCs fabricated using solid polymer electrolytes have demonstrated high performance [19–22]. Table 2.1 is shown some results based on solid polymer electrolytes. Though the usage of solid polymer electrolytes in other electrochemical devices has been known for the last few decades, their use in DSSC is still not every day.

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Table 2.1 Different solid polymer electrolyte system and corresponding photocurrent conversion efficiencies Polymer electrolyte system

Efficiency

Author

Ref No.

Poly (epichlorohydrin-co-ethylene oxide) + NaI/I2

0.1

De Paoli et al.

[24]

PEO: KI: I2

0.1

Singh et al.

[23]

PEO + PDMS

0.48

Bhattacharya et al. [20]

PEO + IL

0.63

Bhattacharya et al. [25]

PEO: NaI: I2

1.6

Nogueira et al.

[26]

PEO + PPG:KI: I2

3.8

Kang et al.

[27]

α-meth acryloyl-ω-methoxyocta(oxyethylene)

2.6

Shozo Yanagida et al.

[28]

PEO + PEG + IL

3.02

Bhattacharya et al. [29]

PEG: LiI: I2 + PC, EC

3.6

Ren et al.

[18]

PEG: LiI: I2 + TNT, TBP

4.4

Yang et al.

[30]

PVDF-based gel polymer electrolyte

3.92

Arof et al.

[31]

Poly(caprolactone) ammonium thiocyanate and EC

Conductivity up to 3.8 × 10−5

Arof et al.

[32]

PMMA + EC +PC + Pr4 N+ I−

Conductivity up to 10−3 Scm−1

Bandara et al.

[33]

F77 Polymer gel electrolyte (IL + I2 + TBP)

2.1

Soni et al.

[34]

2.2.1.4

Working Electrode

The working electrode of laboratory fabricated DSSC is coated with wide bandgap semiconductors like Titanium dioxide (TiO2 ) or Zinc Oxide (ZnO) nanoparticles that act like roadways for the photo-generated electrons. The band position of semiconductors of the working electrode must match with the band position of sensitizer, to enable the fast transfer of electron and electron–hole separation. (i) TiO2 as a Working Electrode TiO2 is known to be chemically stable and non-toxic metal oxide. It has various crystal forms like rutile, anatase, and brookite. Among the three, the rutile phase is highly stable and formed at high temperatures, around 600 °C. Generally, the anatase phase of TiO2 is used as an electrode in DSSCs as it has a large bandgap and a more significant conduction band edge. The higher Fermi level leads to an increase in Voc [34]. TiO2 shows different spin of electrons in the conduction and valence band. It shows oxygen 2p state hybridization with 3p state of titanium in the valence band and barely 3D states in the conduction band. This electronic configuration decreases the recombination of electron–hole pair, which leads to efficient charge collection. The anatase phase, in combination with the rutile phase, is used in DSSCs. The rutile phase helps to decrease recombination. The anatase phase is more suitable for

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DSSCs application than the rutile phase because the anatase phase possesses slightly increased surface area and charge transfer characteristics [35, 36]. So far, many methods have been used for the synthesis of anatase type of TiO2 with different morphologies like nanoparticles [37, 38], nanorods [39, 40], nanowires [41], nanosheets [42–44], nanotubes [45], nanobelts [46], and nanotips [35–37, 47]. Few reports are showing that the anatase TiO2 synthesized in base-catalyzed reactions lowers the recombination, which gives higher Voc , but it lowers the dye adsorption capacity as compared to that acid-catalyzed TiO2 which results in the decrease in photocurrent. There are some reports in the literature on the modification of the TiO2 photoelectrode for harvesting maximum light with increased conversion efficiency. (ii) ZnO as Working Electrode The best alternative and competitor to TiO2 photoelectrode is the ZnO photoelectrode. ZnO is an extensively researched wide bandgap (~3.3 eV) semiconducting metal oxide. It exists in the hexagonal wurtzite phase. The edge of the conduction band and electronic structure of ZnO are very near to that of TiO2 , but ZnO possesses relatively higher mobility of electrons in the bulk state (200–300 cm2 V−1 s−1 ) equated to TiO2 (0.1 cm2 V−1 s−1 ). Though ZnO has less chemical stability as compared to that of TiO2 , its use in DSSCs has increased drastically in the last few years [48]. The first report was published in 1971 on the sensitization of the ZnO photoelectrode using merocyanine dye [49]. Several studies on the electronic properties, as well as charge transport properties of ZnO electrodes in DSSCs, have been reported [50]. The nanocrystalline ZnO is found to be longer as compared to TiO2 . In reverse bias conditions, the recombination rate of TiO2 is found to be slightly higher in ZnO electrodes, resulting in a notable decrease in Voc (open-circuit voltage) [51]. Despite improved transport properties, ZnO photoelectrode shows the difficulty in sensitizing a dye because of its inherent unsteadiness in acidic dye solutions. Though the DSSCs incorporated with nanocrystalline films of ZnO show comparatively lower efficiencies compared to TiO2 , it opens the new possibility of using the ZnO photoelectrode in DSSCs. For the first time, a significant efficiency of 5% using a nanocrystalline ZnO layer was reported on a glass substrate by employing N719 ruthenium dye as sensitizer [52]. Recently, a low-temperature compression method has been proposed for the synthesis of porous ZnO photoelectrode, which shows a high extinction coefficient with organic sensitizer and provides flexible DSSCs with photocurrent power conversion efficiency around 4% [53]. The first report on ZnO photoelectrodes based on solid-state polymer electrolyte and a ruthenium-based dye was published in 2000 by Regan et al. [54]. Recently, nanocrystalline ZnO electrodes simply grownup at low temperatures from the precursor solution have been reported using the spiro-OMeTAD as hole transport material. The spongy electrodes sensitized using N719 dye give a photocurrent transition efficiency of 0.50%. Compound sensitization process and small open-circuit voltages are the main limitations of ZnO-based DSSCs. In addition to this, the low-temperature synthesis method for the synthesis of nanocrystalline ZnO films has been used to increase solar photons energy to excite electrons utilizing various controlled nanostructures.

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Sensitizer

The sensitizer is the part of a DSSC that absorbs the maximum light to excite more numbers of electrons from the ground state. It converts photons (light) into electrons (electricity). An excited electron from the sensitizer moves to the conduction band of the working electrode. The selection of sensitizer with the working electrode is crucial for the band alignment of a semiconductor so that electron transportation from the balance band to the conduction band of semiconductor becomes appropriate. Still, much work on using and modifying different sensitizers for harvesting the maximum solar spectrum is in progress. Most common are the ruthenium-based dyes, which are known as N3 or N719 (Fig. 2.2). Also, few more dyes are commercially available and have been tested by researchers. There are many efforts on the synthesis and modification of the sensitizer, but it is found that ruthenium-based dye (Ru3+ ) demonstrates the maximum photovoltaic properties. It has a wide absorption spectrum and possesses excellent electron chemical stability also. In 1993, Gratzel et al. [55] published the work on a succession of mononuclear Ru-based complexes, that is “cis-(X)2 bis(2,2´ -bipyridyl-4,4´-dicarboxylate) ruthenium(II),” (X can be Cl, Br, I, CN, and SCN). The thiocyanate derivative, “cis(SCN)2 bis(2,2´-biprridyl-4,4´-dicarboxylate) ruthenium(II),” implied as N3 were found to demonstrate exceptional characteristics, for example, broad absorption of visible spectrum expanding up to 800 nm, appropriately extended excited-state lifetime (~20 ns). Consequently, a solar-to-electric energy conversion efficiency of 10% was achieved through N3 dye (Fig. 2.3). Fig. 2.2 Structure of N3 dye

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Fig. 2.3 Absorption spectrum of N3 DYE

Fig. 2.4 a Plot of IPCE (%) concerning wavelength and, b absorbance different dyes concerning wavelength

Dye as a Sensitizer There are reports in the literature on the use of different dyes as a sensitizer. Many researchers have shown that among all synthetic dyes, N-719 best suits with iodinebased redox couple and show excellent conversion efficiency [56–60] (Fig. 2.4).

Quantum Dots as a Sensitizer Quantum dots are a unique class of semiconductors, which are nanocrystals made from periodic groups of II-VI, III-V, or IV-VI materials and can keep electrons (quantum confinement). At the point when the size of a QD approaches, the size of the materials exciton Bohr range, quantum confinement impact gets noticeable, and electron vitality levels can never again be treated as a continuous band, they should be treated as discrete vitality levels. Subsequently, QD can be considered as an artificial atom with energy gap and energy levels spacing dispersing subject to

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Fig. 2.5 Schematic diagram showing electron transfer in QDSSC

its size. Some papers on using QD as sensitizers were also published. QD has been shown to increase the performance of photocurrent transfer [61]. The QD-sensitized photovoltaic cell’s configuration and the principle of working are almost similar to that of dye-sensitized cells. Quantum dots can be synthesized easily using many approaches [62–65]. Broad range absorption and multiphoton process make this class of materials useful for application as a sensitizer (Fig. 2.5).

Natural Pigment-Based Dye as a Sensitizer Initially, ruthenium-based dyes were favorable as photosensitizers due to their good electrochemical properties and high oxidation resistance [66]. Ruthenium (II)-based dyes along with iodide-based electrolytes were effectively used to achieve an 11.9% extreme PCE [67]. However, the availability of noble metals in addition to the high cost of Ru dyes (>$ 1000/g) limits their commercial application [68]. As a valid option, carbon-based dyes having a D-π-A or push–pull structure aimed to improve the Jsc [69]. New use of porphyrinic dyes by Simon Mathew et al. [70] yielded a notable efficiency of 13%, while a metal-less organic DSSC was recently reported with an efficiency of 12.8%. However, the structure of these dyes is complicated. In addition to that, dyes are highly hazardous for the atmosphere because of their non-biodegradable nature [71]. The representation of natural colors with standard ecological, biodegradable, and smart dyes opens another possibility for the commercialization of this innovation. Vegetable dyes that are separated from root vegetables such as turnip and pomegranate were used to achieve efficiencies of 1.7 and 1.5% [72]. Although the total efficiency performance of DSSCs incorporated in natural dyes is still low, current structural change efforts have kept productivity

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Fig. 2.6 Natural pigment-based NDSSC

expectations alive in the same class as their orchestrated counterparts. Photovoltaic use of coumarin dyes by Wang et al. [73] brought high yields of 7.6 and 6%. Anthocyanin is among the abundant and growing classes of natural pigments. They are natural dyes that are responsible for the pigmentation of many leaves, fruits, and plants [74]. Since the carbonyl and hydroxyl groups are shown in the anthocyanin particles, they can easily bind to the surface of the TiO2 nanoparticles [75]. The interest in DSSC and more precisely natural pigment-based DSSC is mainly due to its low cost of manufacture and approachable ecological tendency (Fig. 2.6).

Perovskite as a Sensitizer Since 2011, the new category of sensitizer called perovskite materials has emerged and has drawn much attention. It shows promising conversion efficiency and is considered a promising competitor for the commercialization of solar cells. Recent reports show the photocurrent conversion efficiency using perovskite as sensitizer has reached up to 22.1% [76–79]. In the present chapter, we present studies on natural dye and perovskite material as a sensitizer in DSSC.

2.3 Perovskite Sensitized Solar Cell (PSSC) The sensitizer is a primary component of DSSC as it absorbs sunlight and generates photo-excited electrons in the semiconductor interface. For optimized performance, the sensitizing component has many requirements, such as the chemically adsorbed

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Fig. 2.7 Perovskite-based dye-sensitized solar cell (PSSC)

group to charge the semiconductor material, adequate LUMO, and HOMO levels for the sufficient charge introduction within the semiconductor. The regeneration of the sensitizer electrolyte, the extinction coefficients of the high molar in the visible and near IR range for the collection of light, sound stability and solubility of salts and in few cases (e.g., where cobalt-based electrolytes are used) create the space amid the electrolyte and the anode to prevent charge recombination. Presently, perovskitebased sensitizer is commonly used as light-harvesting materials in DSSC (Fig. 2.7).

2.4 Experimental Results and Discussions In this section, we have discussed the results based on the experimental work carried on five systems, namely natural dye, CH3 CH2 NH3 PbI3, CH3 NH3 PbI3 (Powder), CH3 NH3 PbI3 (Crystal), CH3 NH3 SnCl3 (Powder), and CH3 NH3 SnCl3 (Crystal). The work presented in this section is focused on the synthesis of perovskite materials, the investigation of perovskite materials (CH3 NH3 I and CH3 NH3 Cl) after recrystallization, and the fabrication of solar cells based on these materials. A comparative study of powder and crystal perovskites is also presented based on a series of experiments performed in the laboratory.

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2.4.1 Ethyl Ammonium Iodide: Used as Light Sensitizer in Perovskite Sensitized Solar Cell Here, UV-visible measurement is used to analyze the bandgap of the perovskite (CH3 CH2 NH3 PbI3 ). The bandgap (Eg ) of bare TiO2 , perovskite, and perovskitecoated TiO2 is calculated using absorption spectra, as presented in Fig. 2.8. The photovoltaic bandgap energy (E g ) can be calculated by applying the following equation (Tauc’s plot). n  αhυ = A hυ − Eg where A is the constant conditional on transition probability, and n is an index that was associated with the optical absorption procedure. Hypothetically, n equals 2 or ½ for an indirect or direct allowed transition, respectively. The E g of the bare TiO2 determined based on indirect transition is 3.1 eV (Fig. 2.8a). In the case of CH3 CH2 NH3 PbI3 , the estimated E g was 2.6 eV (Fig. 2.8b), while E g for perovskitecoated TiO2 was 2.28 eV. These values are well consistent with the data reported by Park et al. [80–82]. Scanning electron microscopy (SEM) tests the surface morphology of perovskite (CH3 CH2 NH3 PbI3 ). Figure 2.9 shows the SEM image at 60 °C of a typical annealed sample. It is shown that large islands of precursor material (PbI2 and CH3 CH2 NH3 I)

Fig. 2.8 Band gap energy diagram of a bare TiO2 deposited on FTO, b TiO2 + perovskite on FTO, c CH3 CH2 NH3 PbI3 on FTO

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Fig. 2.9 SEM image of the perovskite

form an inhomogeneous film. It is known as the transparent, crystalline structure of materials prepared as perovskite. To investigate the crystalline nature of the material, XRD spectra were analyzed with a scan speed of 2° per minute shown in Fig. 2.10. XRD analysis indicates the

Fig. 2.10 XRD analysis of perovskite CH3 CH2 NH3 PbI3

60 Table 2.2 Miller indices (hkl), the spacing of lattice plane (d), and height of peaks of CH3 CH2 NH3 PbI3

Rahul et al. Hkl

d hkl

Height (cps)

003

10.19

212

004

11.63

359

014

15.04

45

106

20.47

41

024

25.26

1196

018

25.85

267

026

28.47

75

1 0 10

31.52

98

224

32.72

70

218

33.31

137

312

34.92

46

306

35.57

103

228

38.22

239

0 2 12

42.37

30

142

47.33

35

3 2 10

48.14

68

016

48.39

41

crystalline nature of perovskite having the orthorhombic structure (a = b = c, α = β = γ = 90°). Crystallographic analysis of perovskite accumulated at various concentrations of precursors was analyzed, and parameters such as indices of Miller, distance, and peaks are presented in Table 2.2. We carried out Energy Dispersive X-ray Spectroscopy (EDS) for the elementary study of perovskite. Figure 2.11 shows cross-sectional EDS mapping, where Pb and I in the mesoporous TiO2 film are well-distributed in three dimensions. Elemental review for Pb and I, respectively, was found to be 0.85 and 0.82%. The percentiles of each compound present in perovskite are shown in Table 2.3. The PSSC and DSSC photovoltaic parameters were measured using source meter Keithley 2400. Figure 2.12 displays the current density-voltage characteristics (J–V curve); it also shows the voltage value corresponding to the PSSC current density. Determining PSSC’s Voc, FF, and others would also be useful. Figure 2.12a displays PSSC J-V characteristics, while Fig. 2.12b shows DSSC J-V properties using solid polymer electrolytes. The detailed parameters calculated using J–V curves like Voc , Jsc , FF, and ï are presented in Table 2.4. It was clear that PSSC shows better performance than DSSC. Using a monochromator, we observed a maximum IPCE of 9.4% at 400 nm wavelength (Fig. 2.13).

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cps / eV

35 30 25 N

20

C

I

Pb

I

Pb

15 10 5 0 0

2

4

6

8

10

12

keV

Fig. 2.11 Elemental composition of CH3 CH2 NH3 PbI3

Table 2.3 Elemental composition of the perovskite CH3 CH2 NH3 PbI3 El AN

Series

unn. (1 Sigma) [wt%]

C norm. [wt%]

C Atom. [at.%]

C Error [wt%]

Pb 82

L-Series

31.67

48.84

18.83

0.85 0.82

I 53

L-Series

27.62

42.58

26.80

C6

K-Series

3.72

5.74

38.18

0.84

N7

K-Series

1.84

2.84

16.19

0.60

H1

K-Series

0.00

0.00

0.00

0.00

2.4.2 Methyl Ammonium Iodide Powder and Crystal: Used as Light Sensitizer in Perovskite Sensitized Solar Cell The Raman spectra of CH3 NH3 I precursors are shown in Fig. 2.14. It is performed under the ambient atmosphere using a 532 nm wavelength laser source of 200-mW, which is a significantly high value for photosensitive materials. The peaks emerged at 992, and 2964 cm−1 are assigned to the methylammonium iodide bonds [83]. The peak location validates the shape of the CH3 NH3 I crystal, and photo-degradation is required due to the high photosensitivity of this sensitizer, when it is exposed to the laser source. The UV measurements are used here to assess the energy bandgap of the powder and recrystallized perovskite material. The bandgap (E g ) is calculated using the spectra of the absorption curve, as shown in Fig. 2.15. Figure 2.15a shows the calculated bandgap of powder perovskite material is 2.79 eV, while the calculated bandgap of crystal perovskite material is 1.58 eV (Fig. 2.15b).

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Fig. 2.12 J-V characteristics of a PSSC and b DSSC Table 2.4 Comparison of photovoltaic parameters of CH3 CH2 NH3 PbI3 and N3 dye Light sensitizer

Electrolyte

Voc (V)

FF

Jsc (mA/cm2 )

Incident light intensity (mW/cm2 )

Efficiency (ï) (%)

CH3 CH2 NH3 PbI3

PEO

0.70

0.67

1.16

100

0.75

N3 dye

PEO

0.60

0.64

0.32

100

0.12

Fig. 2.13 Plot of wavelength versus efficiency

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Fig. 2.14 Raman spectra of CH3 NH3 I crystal

Fig. 2.15 Band gap analysis using absorption spectra of a powder methyl ammonium tri-lead iodide and b recrystallized methyl ammonium tri-lead iodide

Using SEM, the surface composition of crystal perovskite (CH3 NH3 PbI3 ) and powder is analyzed. Figure 2.16a depicts an SEM image of one unique powder sample, while Fig. 2.16b shows the SEM image of crystallized perovskite. Both perovskite materials present the significant precursor material islands (powder

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Fig. 2.16 SEM image of the perovskite material, a powder CH3 NH3 PbI3 , b crystal CH3 NH3 PbI3

perovskite and crystallized perovskite). It revealed an evident structural characteristic of perovskite materials [84, 85]. TEM image of the CH3 NH3 PbI3 drops spread on a TiO2 -coated glass substrate is shown in Fig. 2.17. Both powder and crystal CH3 NH3 PbI3 dots are periodically distributed on the TiO2 surface, as seen in TEM images (a and b). It suggests that spin coating of the perovskite solution holding recrystallized CH3 NH3 I and PbI2 leads to the formation of CH3 NH3 PbI3 dots on the TiO2 surface. XRD spectra were recorded for powder as well as crystal perovskite, as shown in Fig. 2.18. XRD spectra of crystal perovskite show almost all the peaks related to perovskite, while the XRD spectra of powder perovskite have a less intense peak with the disappearance of other prominent peaks related to perovskite (Fig. 2.18a, b). The average crystal size measured from XRD is between 30 and 50 nm. These data match well with the published work [86]. Light parameters of PSSC were calculated at room temperature using Keithley 2400 source meter. Figure 2.19 reveals the J–V curve of PSSC (black) powder and red

Fig. 2.17 TEM images of the perovskite material spread on TiO2 -coated surface, a crystal CH3 NH3 PbI3 , b powder CH3 NH3 PbI3

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Fig. 2.18 XRD patterns of a powder perovskite (CH3 NH3 PbI3 ), b crystal perovskite (CH3 NH3 PbI3 )

Fig. 2.19 Current-voltage curve of PSSC using solid polymer electrolyte

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Table 2.5 hotovoltaic parameters of CH3 NH3 PbI3 sensitized solar cell Sensitizer

Redox electrolyte

Voc (Volt)

Fill Factor (FF)

Jsc (mA/cm2 )

(ï) (%)

CH3 NH3 PbI3(Powder)

PEO:KI/I2

0.43

0.88

0.7

0.26

CH3 NH3 PbI3(Crystal)

PEO:KI/I2

0.50

0.55

3.3

0.90

crystallized perovskite using solid polymer electrolyte. Table 2.5 shows the values of photovoltaic parameters calculated using the J–V curve.

2.4.3 Methyl Ammonium Tin Chloride Powder and Crystal: Used as Light Sensitizer in Perovskite Sensitized Solar Cell UV-visible measurement was used here to determine the bandgap energy of the tin-based perovskite (CH3 NH3 SnCl3 ) powder and crystal materials. The bandgap energy (E g ) of powder perovskite and crystal perovskite is analyzed by using the absorption spectra curve and is shown in Fig. 2.20a, b. The highest absorption peak value for the powder perovskite of CH3 NH3 SnCl3 is 490 nm, and for the crystal perovskite of CH3 NH3 SnCl3 , it is 560 nm [87]. The expected band gap energy values of the sensitizer CH3 NH3 SnCl3 powder perovskite are 2.50 eV, and for the crystal perovskite (CH3 NH3 SnCl3 ), it is 2.1 eV. The surface topographies and morphological view of the CH3 NH3 SnCl3 powder perovskite and crystal perovskite are examined using SEM imaging. Figure 2.21a shows an SEM image of one distinct sample of powder while Fig. 2.21b shows the SEM image of crystallized perovskite at the same magnification. It is seen that the perovskite films are inhomogeneous. The large landforms of precursor perovskite

Fig. 2.20 Band gap analysis using absorption spectra of a CH3 NH3 SnCl3 powder perovskite and b CH3 NH3 SnCl3 crystal perovskite

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Fig. 2.21 SEM image of a CH3 NH3 SnCl3 powder perovskite, b CH3 NH3 SnCl3 crystal perovskite

material (SnCl2 and CH3 NH3 Cl) were analyzed. These landforms show clearly the crystalline structural characteristics typically observed in perovskite materials [88, 89]. For investigating the elemental analysis of CH3 NH3 SnCl3 powder perovskite and CH3 NH3 SnCl3 crystal perovskite, we have carried out Energy Dispersive Xray Spectroscopy (EDS). Figure 2.22a shows the EDS mapping of CH3 NH3 SnCl3 powder perovskite, and Fig. 2.22b shows EDS mapping of CH3 NH3 SnCl3 crystal perovskite, where Sn and Cl are well-distributed three-dimensionally in the mesoporous film. Elemental analysis for both the samples confirms the presence of Sn and Cl, respectively. The exact percentiles of each compound present in powder perovskite and crystal perovskite are given in Tables 2.6 and 2.7, respectively. TEM characterization is used to estimate the accurate particle size of the Sn-based powder perovskite of CH3 NH3 SnCl3 and the crystal CH3 NH3 SnCl3 perovskite materials. Figure 2.23a, b shows the TEM image of powder CH3 NH3 SnCl3 perovskite and crystal CH3 NH3 SnCl3 perovskite, respectively. The grain size of powder CH3 NH3 SnCl3 and crystal CH3 NH3 SnCl3 perovskite stands in between 40 and 70 nm, which are favorable for synthesized perovskite sensitizer. X-ray diffraction spectra of synthesized powder and crystal perovskite are shown in Fig. 2.24a, b, respectively. XRD data inform the crystallographic tendency and the

Fig. 2.22 EDX of a CH3 NH3 SnCl3 powder perovskite, b CH3 NH3 SnCl3 crystal perovskite

68 Table 2.6 Elemental composition of CH3 NH3 SnCl3 powder perovskite

Table 2.7 Elemental composition of CH3 NH3 SnCl3 crystal perovskite

Rahul et al. Element

Series

Weight%

N

K-Series

3.85

12.56

O

K-Series

27.05

77.21

Si

K-Series

1.32

2.14

Cl

K-Series

4.61

5.94

Sn

L-Series

70.87

27.26

H

K-Series

0.00

0.00

Element

Series

N

K-Series

5.22

18.00

O

K-Series

26.71

80.60

Si

K-Series

1.35

2.32

Cl

K-Series

3.87

5.27

Sn

L-Series

73.29

29.81

H

K-Series

0.00

0.00

Weight%

Atomic%

Atomic%

Fig. 2.23 TEM image of a CH3 NH3 SnCl3 powder perovskite and b CH3 NH3 SnCl3 crystal perovskite

monoclinic system (a = b = c, α = γ = 90°, β > 90°) of the perovskite. The lattice parameters for powder perovskite CH3 NH3 SnCl3 are calculated as a = 5.69, b = 8.23, c = 7.94, whereas for powder perovskite of CH3 NH3 SnCl3, these parameters are evaluated as a = 5.87, b = 8.38, c = 8.01. The average crystallite size of sensitizer perovskite, as evaluated using Scherrer’s equation, is 40–70 nm. Fourier transform infrared spectroscopy (FTIR) is utilized to differentiate the vibrational modes of the perovskite material and to recognize the functional groups present in the as-synthesized material. Figure 2.25a, b shows FTIR spectra of CH3 NH3 Cl powder and CH3 NH3 Cl crystal. For both CH3 NH3 Cl samples, peaks are sufficiently concerned. A full description of parameters such as frequency ranges,

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Fig. 2.24 XRD patterns of a CH3 NH3 SnCl3 powder perovskite and b CH3 NH3 SnCl3 crystal perovskite

Fig. 2.25 FTIR spectrum of a CH3 NH3 Cl powder and b CH3 NH3 Cl crystal

Table 2.8 Infrared spectroscopy data of methylammonium chloride powder and methylammonium chloride crystal CH3 NH3 Cl powder wavenumber (cm−1 )

CH3 NH3 Cl Freq. ranges Group Functional Intensity crystal (bonds) groups wavenumber (cm−1 )

Types of vibrations

498

3423

3300–3500

N-H

Amine

Medium

Stretch

1275

1300

1080–1360

C-N

Amine

Medium-weak Stretch

1574

1540

1600

N-H

Amine

Medium

Bending

658

627

600–800

C-Cl

Alkyl Halide

Strong

Stretch

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Fig. 2.26 Box chart plot of perovskite solar cell using powder and crystal Sn-based perovskite

Table 2.9 Photovoltaic parameters of CH3 NH3 SnCl3 sensitized solar cell Light sensitizer

Electrolyte Voc (V) FF

Jsc (mA/cm2 ) Incident Efficiency light (ï) (%) intensity (mW/cm2 )

CH3 NH3 SnCl3 (Powder) PEO:KI/I2

0.48

0.65 0.2

100

0.17

CH3 NH3 SnCl3 (Crystal)

0.60

0.59 1.5

100

0.55

PEO:KI/I2

functional groups, bonds, and intensity are given in Table 2.8. This data are entirely in agreement with the standard spectra. It suggests that CH3 NH3 Cl decomposes to CH3 NH2 and HCl, as reported earlier [90–95]. Under the ambient condition, photovoltaic parameters of Sn-based PSSC were measured using Keithley 2400 source meter. Figure 2.26 shows the J–V curve of Sn-based PSSC of powder perovskite and crystal perovskite using solid polymer electrolyte [100–102]. The evaluated photovoltaic parameters using the J–V curve are given in Table 2.9.

2.4.4 Comparative Photovoltaic Study of Lead and Tin-Based Perovskite Sensitized Solar Cell Using Polymer Electrolyte UV-visible measurement is used to evaluate the bandgap of the synthesized perovskite material (CH3 NH3 PbI3 ) (Fig. 2.27). The calculated band gap of methylammonium tri-lead iodide is 2.26 eV, which is in as well agreement with the reported by Gyu et al. [29, 96]. The surface topography and morphology of the perovskite (CH3 NH3 PbI3 ) are analyzed using SEM characterization. Figure 2.28 shows an SEM image of one typical sample. It was revealed that perovskite film is very homogeneous on the FTO

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Fig. 2.27 Band gap analysis of methylammonium tri-lead iodide

Fig. 2.28 SEM image of the perovskite material

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Fig. 2.29 XRD analysis of perovskite CH3 NH3 PbI3

surface. The large landmasses of precursor material (PbI2 and CH3 NH3 I) are clearly seen in the image. These landmasses show the clear crystalline structural property typically observed in perovskite sensitizers [97, 98]. Typical XRD data of a synthesized perovskite are shown in Fig. 2.29. It is observed that XRD data shows the crystallographic nature and orthorhombic structure (a = b = c, α = β = γ = 90°) of the perovskite. The average crystallite size of perovskite as evaluated using an equation is 45 nm. These data are well supported by the literature [99, 100]. EDS method is used for methylammonium tri-lead iodide elemental composition analysis, and overall EDS mapping is shown in Fig. 2.30. Based on the results, it is suggested that lead and iodine are well-allocated three-dimensionally in the mesoporous TiO2 film. Elemental analysis is found to be 0.85% for Pb and 0.82% for I, respectively, which stipulates the ratio of Pb: I. The percentage of each element present in perovskite is arranged in Table 2.10. EL = Element; AN = Atomic Number; Series = characteristic X-ray lines. unn. C [wt%] is the un-normalized concentration in weight percent of the element, norm. C norm [wt%] is the normalized concentration in weight percent of the element. C Atom. [at. %] is the atomic weight percent. C Error (1 Sigma) [wt%) is the error in the weight percent concentration at the 1 sigma level.

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Fig. 2.30 Elemental composition of CH3 NH3 PbI3

Table 2.10 Elemental composition of the perovskite CH3 NH3 PbI3 El AN

Series

unn. (1 Sigma) [wt%]

C norm. [wt%]

C atom. [at.%]

C error [wt%]

Pb 82

L-Series

34.59

45.66

17.50

0.84

I 53

L-Series

25.79

40.33

25.40

0.70

C6

K-Series

4.64

6.47

36.00

0.79

N7

K-Series

2.50

3.73

15.20

0.50

H1

K-Series

0.00

0.00

0.00

0.00

IPCE analysis delivers data about how efficiently the cell that converts photons of a given wavelength into the electrons. The maximum IPCE for the present cell is estimated at 14.9% at the wavelength of 430 nm (Fig. 2.31). Under ambient conditions, photovoltaic parameters of PSSC were analyzed by using Keithley 2400 source meter. Figure 2.32 shows the J–V curve of PSSC using solid polymer electrolyte [92, 101–109]. The estimated photovoltaic parameters using the J–V curve are shown in Table 2.11.

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Fig. 2.31 Plot of wavelength vs. efficiency to evaluate PCE values

Fig. 2.32 J–V curve of PSSC using a solid polymer electrolyte Table 2.11 Photovoltaic parameters of CH3 NH3 PbI3 and CH3 NH3 SnCl3 sensitized solar cell Light sensitizer

Electrolyte

VOC (V)

FF

Jsc (mA/cm2 )

Incident light intensity (mW/cm2 )

Efficiency (ï) (%)

CH3 NH3 PbI3

PEO:KI/I2

0.64

0.60

1.47

100

0.59

CH3 NH3 SnCl3

PEO:KI/I2

0.49

0.56

0.35

100

0.19

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2.5 Conclusion We have successfully synthesized a new nanocrystalline sensitizer, which was used efficiently as a dye in PSSC with the solid polymer electrolyte. The optical band gap was determined to be 2.28 eV. The conduction band edge position of CH3 CH2 NH3 PbI3 was 0.7 eV higher than that of TiO2 , which permits the injection of photo-excited electrons from CH3 CH2 NH3 PbI3 to TiO2 . An IPCE maximum of 9.4% was observed at the 400 nm wavelength. The fabricated PSSC showed an efficiency of 0.75% compared to DSSC containing the same solid polymer electrolyte, which showed an efficiency of 0.12% at 1 sun condition. In the case of methylammonium iodide, we have effectively synthesized a low-cost light sensitizer material in powder and crystal form and characterized by various experimental methods. The energy bandgap of crystallized perovskite was revealed to be 1.58 eV, and for powder, it was calculated to be 2.79 eV using UV-Vis absorption spectroscopy. Distribution of the CH3 NH3 PbI3 dots spread on the TiO2 -coated glass substrate was analyzed by TEM. The structural characterization using XRD showed the formation of methylammonium tri-lead iodide perovskite material, which agreed well with available literature. The perovskite sensitized solar cell using recrystallized light sensitizer and PEO-based solid polymer electrolyte achieved estimated efficiency of 0.90% and 0.26%, at 1 sun condition processed in the air for crystal perovskite and powder perovskite, respectively. An innovative CH3 NH3 SnCl3 sensitizer material has been successfully synthesized in two forms and characterized them using various experimental tools. The bandgap of perovskite was determined to be 2.5 eV for powder perovskite and 1.5 eV for crystal perovskite. The structural characterization showed the distribution of perovskite sensitizer on the titanium dye oxide surface, which agreed well with available literature. SEM image showed the perfect morphology of perovskite sensitizer film deposited on the glass substrate. FTIR analysis showed that the needed functional groups were present in as-synthesized perovskite sensitizer. The fabricated perovskite sensitized solar cell using PEO-based solid polymer electrolyte showed an efficiency of 0.17% in the case of powder perovskite, and finally, we got 0.55% for the crystal perovskite of CH3 NH3 SnCl3 at 100 mW/cm2 (1sun condition) processed in the ambient air. A comparative study of CH3 NH3 PbI3 and CH3 NH3 SnCl3 materials is presented. The bandgap of perovskite was determined to be 2.26 eV. The structural characterization showed the formation of a pure perovskite material, which matches well with the available literature. An IPCE maximum of 14.9% was observed at the 400 nm wavelength. The PSSC using lead-based light sensitizer and PEO-based solid polymer electrolyte achieved an estimated efficiency of 0.59 and 0.19% for tin-based perovskite sensitizer, at 1 sun illumination processed in air. It is a new report and opens up the possibilities of further in this area.

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Chapter 3

Nanomaterials: A Windfall to Dentistry Nafis Ahmad, Zeba Jafri, Asim Khan, and Zishan H. Khan

Abstract Nanotechnology has tremendous potential in the field of dentistry to provide a comprehensive oral health care with advanced nanomaterials, clinical devices, and advanced diagnostic tools. The impact of nanotechnology in oral health has enhanced the quality of dental care by manifolds. Nanomaterials in dental products can improve quality and have superior properties due to numerous active functional groups in nanomaterials. The nanodentistry is thus opening up new horizons for immense possibilities in dental research, but one should keep eyes on safety, efficacy, and implementation of nanotechnology. This chapter is a sincere effort to extend the horizons of nanomaterials and their windfall for dentistry.

3.1 Introduction Nanotechnology is a branch of applied science which deals in research and development of nanomaterials at atomic or molecular level employing the principles of molecular engineering [1]. Recently, there are new methods that have been developed to produce nanomaterials of different shape and size to perform assigned function. The nanomaterials exist abundantly in nature and have great utility in our everyday life. As a consumer, one utilizes many nanomaterials like titanium oxide, iron oxide, and zinc oxide in toothpastes and in cosmetic products like sunscreens, paintings in household products, and UV filters in water purifiers. Nanomaterials are available in many forms like nanorods, nanoplates, nanotubes, nanoshells, nanobelts, nanofibers, nanorings, nanoprizms, nanowires, nanocapsules, nanosheets, nanoparticles, and fullerenes [1, 4]. The main purpose of nanomaterials is to occupy every individual atom and molecule to perform its assign function as per the requirement. Nanomaterials in dental products can improve quality of dental materials and have N. Ahmad (B) · Z. Jafri Faculty of Dentistry, Jamia Millia Islamia, New Delhi 110025, India e-mail: [email protected] A. Khan · Z. H. Khan Department of Applied Sciences & Humanities, Jamia Millia Islamia, New Delhi 110025, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_3

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superior properties due to numerous active functional groups present in nanomaterials [2]. In healthcare services, the controlled use of nanomedicine can open new horizons for precise diagnosis and effective treatment planning of various dental diseases [3]. Nanotechnology has tremendous potential in the field of dentistry to provide a comprehensive oral health care with advanced nanomaterials, clinical devices, and advanced diagnostic tools. The impact of nanotechnology in oral health has enhanced the quality of dental care by manifolds. The manufacturing of advanced nanomaterials and newer dental machines like nanorobots have greatly enhanced the quality of oral health and also helped in the prevention and intervention of dental disorders [4]. Around 80% population of world has not reached to quality dental care in today’s time. Molecular nanotechnology has great potential to fulfill the hope of this marginalized section of the society, but there are some problems of human safety, drug and materials regulations, and social stigmas [5]. The manufacturing of nanomaterials is a complex and difficult process in which objects result from the merging of mainly two approaches; namely top-down approach and bottom-up approach. Top-down approach involves the production of smaller devices on nanoscale form for the bigger structures and assembly in a very precise and controlled manner. It has been used in the production of dental materials and to create nano-electromechanical system devices which are useful in cancer treatment. However, the bottom-up approach has produced more complex structures from coalescence of smaller atoms, molecules, and mono units. Bottom-up approach is basically used in repair work at cellular and nucleus level in human body.

3.2 Classification of Nanomaterials Based on Composition of Materials Nanomaterials are characterized into many groups on the basis of their composition, origin, dimensions, and particle size and shape. The capacity to calculate the distinctive properties of nanomaterials increases the importance of each classification. Further, to avoid the toxicity of nanomaterials at large-scale production, they need to classify accordingly. On the basis of origin and composition of material, nanomaterials are grouped in four categories given in Table 3.1 [6]. Based on Their Dimensions The action of nanomaterials is based on their size in three different dimensions. The nanomaterials act as a bridge between atomic or molecular structure and their bulk material. They have a great systematic value in development of dental nanomaterials and its implication on oral health. Pokropivny and Skorokhod gave an illustrative classification for nanomaterials and divided them into four categories on the basis of dimension namely: Zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) nanomaterials [7, 8].

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Table 3.1 Classification of nanomaterials based on origin and composition Carbon-based NMs

Inorganic-based NMs

Organic-based NMs

Composite-based NMs

Hollow tubes, ellipsoids or spheres, fullerenes (C60), carbon nanotubes (Cnts), carbon nanofibers, carbon black, graphene (Gr), carbon onions

Semiconductor nanoparticles, metal nanoparticles, metal oxide nanoparticles, silica nanoparticles, polyoxometalates, gold nanocrystals

Dendrimers, micelles, liposomes, polymer NPs

Any combinations of: carbon-based, metal-based, organic-based NMs with any form of metal, ceramic, or polymer bulk materials

Based on Their Origin; Natural Nanomaterials All the spheres of earth are completely packed of natural nanomaterials regardless of endless human activities. These nanomaterials are present from soil surface to deep earth crest and from hydrosphere to biosphere in huge amount like everywhere in air, soil, and water [9]. Synthetic Nanomaterials Synthetic nanomaterials are produced by physical, chemical, or biological processes carried out in factories, mills, and industries through mechanical grinding, chemical reactions, fuel combustions in machines, smoke in chimneys, and engine exhaust. Nanomaterials in the Human Body Nanostructures are enormously present in all human tissues and cells such as muscles tissue, bones, fat, and proteins. Biologically active substances or biochemical molecules like hormones, enzymes, antibodies, amino acids, nucleic acids, and DNA present in human body are nanostructures. The list of nanomaterials presents in human body along with their analogous size is given in Table 3.2 [10]. Table 3.2 Nanomaterials present in human body

Nanostructure

Size (nm)

Glucose

1

DNA

2.2–2.6

Av.size protein

3–6

Heamoglobin

6.5

Micelle

13

Ribosomes

25

Enzymes and antibodies

2–200

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3.3 Applications of Nanodentistry Researchers have developed an attention for the application of nanotechnology in dentistry. A new field called nanodentistry has emerged. The development of nanodentistry will set an ideal platform for oral health through nanomaterials and biotechnologies including tissue engineering and nanorobots. Table 3.3 is showing the application of nanodentistry, manufactured through top-down and bottom-up approaches [11]. Dental Tissues Dynamics at Nanoscale The nanoperspective of dental tissues has been explored with the advancement in dental research especially in last decade of twenty-first century. Normally, dental tissues are considered as static and stagnant, but when viewed in scanning electron microscope (SEM) at the nanoscale resolution, they have shown interesting structural dynamism. Enamel tissues are composed of hydroxyapatite crystals 92–94% by Volume. These crystals are approximately 20 nm wide and little more in length [12]. These three-dimensional structures of hydroxyapatite crystals form a superstructure which has an organized arrangement at different size scales [13]. The dentine tissues have a matrix mainly composed of type I collagen fibrils. These collagen fibrils are interacted with non-collagenous proteins and form a threedimensional organic scaffold that which is reinforced by hydroxyapatite nanocrystals. Table 3.3 Manufacturing process of nanomaterials and its applications in dentistry

Top-down approach

Bottom-up approach

Nanodiagnostics for saliva analysis

Inducing anesthesia (local anesthesia)

Nanocomposites

Hypersensitivity cure

Nanoglass ionomer composites

Tooth repair

Nanoceramic technology

Nanorobotic dentifrice (dentifrobots)

Nanosolutions

Orthodontic nanorobots

Coating agents

Dental durability and cosmetics

Impression nanomaterials

Halitosis

Nanocomposite denture teeth

Nanotech floss

Nanosurface in dental implants

Photosensitizers and carriers

Laser application for periodontology

Diagnosis of oral cancer

Nano-bone fibers

Treatment of oral cancer

Antimicrobial nanoparticles Nanorobots in orthodontics Nanomaterials in root-end sealant

Smart orthodontic brackets

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These nanocrystals separate collagen fibrils from extra fibrilar minerals present in the spaces of collagen fibrils [14]. Prevention of Micro-leakage With the invention of nanomaterials, the problem of micro-leakage during bonding in dental materials is stopped or near to stop at interface between dentin collagen and the nanomaterials. It became a complete seal to stop percolation of saliva and microorganisms. Though, some studies supported the concept of nanoleakages in hybrid layer of 20–100 nm even in the presence of nanogaps. Although these nanogaps never allow percolation of microorganisms, they can pass the enzymes and water molecules into the restoration and eventually disrupt the bond between dentine and resin in due course of time [15]. Small is Useful in Dental Materials Dental resins used as filling materials are nanofilled hybrid composites and particles ranging from 30 to 500 nm. The nanobiomaterials have strong host response at cell and tissue level which has great value of nanotechnology in osseointegration process of dental implants [16]. The osseointegration in dental implants have seen multifold improvements with nanotechnology-based approaches like improvement in bioactive coatings on implant surfaces. The basic purpose of incorporating these bio-ceramic and bio-composite surfaces on dental implant is to increase surface area and its reactivity, subsequently improving prognosis. Biofilm Formation and Treatment Nanotechnology is now promoting new concepts in oral microbiology, which has eventually changed the knowledge of biofilm formation and its prevention. Ribosomal RNA-based technologies have disclosed the range of microbial species within oral biofilm and its role in oral health and diseases [17]. Proteomic analyses by mass spectrometry which is useful in identification of proteins even in very low concentrations have great opportunities in the progress of diagnosis and treatment of dental diseases [18]. Saliva has shown its importance as an excellent investigative tool for the detection of various cancers and tumors in oral cavity and other parts of the body [19]. Saliva contains biomarkers for various diseases, but their manifestation and identity are under research at various levels. In nanodentistry, saliva has displayed an assurance for early diagnosis of the diseases and monitoring their treatment outcome through a non-invasive approach [20]. Antimicrobial Nanoparticles (NPs) Nanoparticles like silver, zinc, titanium, zirconium, copper, iron, etc., have the potential to remove all harmful microorganisms from oral cavity. The antimicrobial action of these nanoparticles has shown wide range of antimicrobial action on oral cavity’s bacteria, viruses, and fungi. These microorganisms are commonly present on denture surfaces, implant surfaces, orthodontic appliances, and in various dental infections. The mechanism of actions of most nanoparticles are mainly due to disruption of

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cell membrane and cell wall by releasing active free radicles and active metal ions. Therefore, NPs has an important role in the prevention, interruption, and treatment of most of the oral diseases [21].

3.3.1 Biocompatibility of Nanomaterials Biocompatibility is an ability of a dental material to perform its assigned function without displaying any allergic reactions to oral tissues and systemic side effects in the body of dental patients. In other words, biocompatibility is a dynamic process as material has changed its properties and host response with time because of many factors like self-life, corrosion, solubility, and aging [22]. The mechanism of toxicity of nanomaterials cannot be understood without the proper knowledge of their absorption, distribution, metabolism, and excretion in human body. Nanomaterials have the ability to cross biological barriers and easily penetrate into cells, tissues, and organs through inhalation, ingestion or even penetrate the skin. However, the surface chemistry of nanomaterials can be modified for more efficacies, but further more studies are needed for their safe use in biomedical practices [23]. Toxicology and biodynamic studies of nanomaterials have shown that silica, silicon, chitosan, etc., nanomaterials are relatively innocuous in dentistry [24]. A thorough understanding of nanotechnology and chemistry in design and kinetics of nanomaterials is needed for their dental application and their interaction with biological systems.

3.3.2 Nanorobots in Dentistry: The Futuristic Approach The robotic devices which successfully works at nanoscale is called nanorobot. Nanorobots are also known as nanobots, nanite, nanogene, or nanoant. Dental nanorobots have a spider like body for their quick moment and swift job. These nanorobots are manufactured out of diamondoid structures, which is diamond like structure made up of circular saturated hydrocarbons. Diamondoid are tough, chemically and thermally stable, and easy to assemble, but lighter than steel. Dental nanorobots have a specific mechanism to enter human tissues and reach to the target with ultraprecision and perform the assigned work in real time [25]. Application of Nanorobots in Dentistry; Dental Hypersensitivity Cure Dental hypersensitivity is very common problem, and its remedy is challenging for the dentists. Once active anesthetic dental nanorobots are placed on the patient’s gingiva or surface of the tooth crown, these automobile nanorobots will reach the dentin through gingival sulcus and cross lamina propria painlessly at cementodentinal junction and then through dentinal tubules (1–4 μm). Nanorobots proceed toward dental pulp for the anesthesia guided by positional navigation under nanocomputational simulation. Dental hypersensitivity is due to change in hydrodynamic

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pressure gradient in the pulp through dentinal tubules. The nanorobots break the pathway by occluding dentinal tubules precisely in few minutes, by using dentine material, thus patients feel a quick and long-term relief from hypersensitivity [26]. It was clinically proven that a desensitizing toothpaste having 15% hydroxyapatite nanoparticles has an effective treatment for dentine hypersensitivity even in single use monthly [23]. Digital Dental Imaging Nanodentistry is developing rapidly in every department of dentistry so in digital dental imaging. The images obtained by nanorobots with nanophosphor scintillators are superior in quality and also reduces the radiation dose significantly [27]. Diagnosis and Treatment of Oral Cancer Nano-electromechanical system (NEMS) is an ultrasensitive mass detection technology which converts biochemical energy to electrical signal. These electrical signals can be used for the detection of useful biomarkers present in saliva and other oral tissues to analyze DNA sequencing and biochemical functions which are extremely beneficial in early detection of oral cancer. Human cells contain exosomes which are secretory vesicle having proteomic and genomic markers. The level of these biomarkers is usually found elevated in tumors and malignancies. The study of theses biomarkers have been possible with the invention of atomic force microscopy (AFM), nano-electromechanical system (NEMS), oral fluid nanosensor test, and optical nanobiosensor. Other tools used in cancer therapeutics are nanoshells which have nanobeads and a metallic layer on outer surface that selectively interacts with cancer cells and eradicates them without harming normal healthy cells. There are many experiments being conducted on nanoparticle-coated, radioactive sources to kill tumor cells in its near vicinity [28]. Nanorobotic Dentifrice Nanorobotic dentifrices containing toothpastes and oral rinses are transported onto the occlusal surfaces of the teeth, and they move through all the surfaces including sub-gingival one. They can trap and remove pathogenic microorganisms from dental plaque and food debris and metabolize these organic matters into harmless and odorless byproducts. The speed of these nanorobots is quite fast (~1–10 μ/s pace). These nanodentifrobots are basically mechanical instruments that can safely auto-deactivated if accidentally swallowed [29]. Nanoneedles This is an incredible development in the field of nanodentistry in Sweden. The suture needles contain nanosized stainless steel crystals, it will be very useful to perform cell surgery in near future, and many genetic/metabolic disorders could be treated effectively. Nanotweezers are under development stage by the researchers, which will further enhance the efficiency of cell surgery [30]. Researchers have developed a “nanoneedle” with 50 nm diameter which is extremely useful in cellular and molecular biological research. The design of nanoneedle is so precise that it can penetrate

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the cell membrane for the targeted drug delivery into the cytoplasm or the nucleus. These nanoneedles are attached to atomic force microscope (AFM) to perform the process of nanosurgery [31, 32]. Nanoneedles can be used to deliver molecules, such as nucleic acids, proteins, or other chemicals to the cell and nucleus through cell surgery [33]. Nanopores A nanopore is a small hole of 1 nm diameter. Nanopore sequencing is a cheap and fast substitute to the conventional methods. Nanopores are made up of either proteins or synthetic materials. Nanopores are basically nanofilters which can detect DNA or RNA molecules. Scientists can observe the sequence and electrical properties of each base pair, and it may be effective in the detection of genetic disorders including oral cancer [34].

3.3.3 Bionanosurface Technology and Dental Implants Dental implant system is an excellent example of integrated and multidisciplinary approach of dentistry. The osseointegration is the process responsible for the success and prognosis of dental implants which is greatly dependent on topography and surface chemistry of dental implants [35]. The quality and quantity of alveolar bone around implant body has key role in the success or failure of dental implants. Further proper treatment and alteration in nanoscale topography of implant surface can enhance the process of osseointegration. Bone development and prognosis of dental implants may be increased with nanotechnology by optimizing cell colonization, surface chemistry, and wettability. The addition of nanohydroxyapatite crystals and calcium phosphate creates a favorable implant surface for osteoblast formation. These implants have long life and more stable in oral cavity as they create a better environment for chemical interaction of nanocoatings with biological materials in bone tissues [36, 37]. The aim of nanosurfaces in dental implants is to build more bioactive and smart implants structures that can easily interact with bone tissues, adjust itself to the necessary changes, and also transport suitable biomolecules or drugs and dynamically participate in cellular events [38].

3.3.4 Bone Replacement Materials Bone tissues are natural nanocomposite structures, which are mainly composed of organic collagen reinforced with inorganic filler crystals. Nanotechnology targets to imitate this natural structure for dental applications and particularly to form nanostructured bone. Nanocrystals are made up of loose microstructures having numerous nanopores dispersed in between the crystals. The modified surfaces of these

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nanopores are capable to adsorb bone protein, with the addition of silica molecules, it forms nanohydroxyapatite crystals to treat the defects in bone [39].

3.3.5 Dental Caries Nanosized calcium carbonate present in some toothpastes assist in remineralization of early carious lesions. The Streptococcus mutans are one of the main causative organism in dental caries. Scientific studies have shown that some nanoparticles like silver, zinc oxide, and gold have bacteriostatic action on S. mutans. However, silver nanoparticles had shown bactericidal effect even in low concentration and displayed least toxicity [28].

3.3.6 Tissue Engineering and Tooth Replacement Nanodentistry has tremendous hope for teeth repair and even whole tooth replacement through nanorobots and other technological development in nanotechnology like genetic engineering and tissue engineering. Tissue engineering can grow tooth bud into new teeth in vitro and implant them at new edentulous space. An in-vivo study on animals has shown that dental tissue engineering is capable to form a biomimetic root canal and develop the dentine while placing growth factor-beta (TGF-β), bone morphogenic proteins (BMP) onto the dental pulp. A flexible approach is used in vitro for dental pulp regeneration in nanodentistry with the cultured human pulp cells grown in a polyglycolic acid matrix [40]. Nanodentistry has tremendous opportunities in tissue engineering and stem cell research for the treatment of dental diseases like pulp regeneration, dentine repair, and growth of periodontal tissues, bone augmentation, and osseointegration in implants. Tissue engineering work in the treatment of orofacial tissues has shown superior results in biological and physiological aspects. Dental implants placed with nanocrystalline hydroxyapatite surface treatment are always more stable and capable for immediate loading [40, 41].

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3.3.7 Nanotechnology in Prosthodontics Prosthodontics has the widest range in dentistry starting from the use of dental materials as filling material to complete rehabilitation of lost orofacial structures. Nanomaterials are used in treatment of removable prosthesis, fixed prosthesis, implant prosthesis, and maxillofacial prosthesis [42]. In Removable Prosthodontics Polymerization shrinkage of heat cure resin denture base was reduced while carbon nanotubes incorporated into heat cure monomer and also improve their mechanical properties. The incorporation of metal oxide nanoparticles in polymethyl methacrylate-based denture material has improved strength by reducing porosities and enhanced antimicrobial property. The mechanical failure occurs in maxillofacial prostheses due to presence of tensile and tearing load, but it can be overcome with a reinforcing agent called polyhedral oligomeric silse squiox. Nanocomposite denture teeth have shown stain and impact resistance, more durable, and more esthetic in appearance [43]. In Fixed Prosthodontics The nano-optimized moldable ceramics contains nanofillers which enhance the polishing property and reduce occlusal wear in ceramic restorations. Addition of nanopigments in ceramic veneers and crown increases esthetics, and nanomodifiers improve the handling properties of dental ceramics [44]. The incorporation of nanomodifiers in newer resin luting agents has improved the mechanical properties including bonding [30]. Nanofillers are incorporated in the vinylpolysiloxanes and form a better addition silicone impression material with good hydrophilic properties, improved flow, and enhanced surface detail. Nanocare gold as a base in composite/ceramic restorations inlays or onlays enhanced adhesive and antibacterial properties [45].

3.3.8 Dental Cements The incorporation of 5% w/w TiO2 nanoparticles have improved the properties of glass ionomer cement like bond strength, fracture toughness, compressive strength, and antimicrobial property [46]. Incorporation of Ag nanoparticles in resin luting cements has shown antibacterial activity against S. mutans and also improved mechanical properties [47]. The nanoparticles loaded luting agents have better bonding to both enamel and dentine. There have been significant increment in both

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compressive and tensile strength of zinc polycarboxylate cements with ZnO and MgO nanoparticles [48]. Nanoceramics It was observed that nanoceramics are tough, strong, and more ductile than conventional ceramics [49]. The nanoglass ceramics produced from zirconia-silica system are tough, hard, corrosion resistant more translucent but low in modulus of elasticity [50]. It was observed in a study that addition of 20% nanozirconia significantly improved toughness and hardness of dental ceramics [51]. In another in-vitro study, adding 4% carbon nanotubes to dental ceramic has significantly improved their wear resistance and some mechanical properties [52]. Dental Adhesives Dental adhesives are materials that increase the adhesion and cohesion forces between two different substances or between a material and natural tooth structure. Silane coupling agent (a polymer) added in dental adhesives to increase the cohesive forces. The coupling agent has impregnated with nanoparticles of silica and zirconia, and adhesive property has increased many folds due to settling of the filler particles. These nanoadhesives have displayed stronger bond strength to both dentine and enamel, more shelf life, sturdy marginal seal, and no need of separate etching [53]. Tissue Conditioners and Soft Liners Incorporation of Ag nanoparticles in tissue conditioners has shown significant antibacterial activity against S. mutans and Staphylococcus aureus at 0.1% and antifungal activity against Candida albicans at 0.5% concentration [54]. Chlorhexidine mixed with sodium triphosphate (TP), trimetaphosphate (TMP), and hexametaphosphate (HMP) have displayed antifungal property on soft liners used in maxillofacial prosthesis. Combination of chlorhexidine and HMP coatings has shown most effective antifungal action, and it increases shelf life of the prosthesis [55]. Maxillo-Facial Prosthesis Silicones are the best suitable materials for maxillofacial prostheses. Silicones completely rehabilitate the lost orofacial tissues in trauma or diseases and also maintain the heath and esthetics of facial structures. But silicones are susceptible to infections and contamination, so nanomaterials have been added to overcome this problem and maintaining the quality and stability of the maxillofacial prosthesis [56, 57]. It has been seen in the studies that the addition of silver nanoparticles to silicone polymers inhibits adherence of fungal infection including C. albicans at the surface of maxillofacial prostheses but no toxicity to oral tissues [58]. Nanoparticles like ZnO and TiO2 are opacifiers in silicone elastomers and titanium dioxide and cerium dioxide nanoparticles have shown good color stability [59]. The nanoparticles of 3% silicone dioxide are added in silicone material to improve the tear strength of silicone elastomers and other mechanical properties [60].

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3.3.9 Nanotechnology in Orthodontics Nanomaterials in Orthodontic Brackets and Wires Undue orthodontic forces lead to the loss of anchorage and start root resorption. Researchers have reported in a study that there is a significant reduction in frictional forces during orthodontic treatment if orthodontic wires are coated with inorganic fullerene like tungsten disulfide nanoparticles [61]. It was clinically observed that sanitization of the orthodontic steel wires significantly increased the surface hardness, increased elastic modulus, and reduced the surface roughness [62]. Scientists have observed that the orthodontic brackets coated with the nitrogen-doped titanium oxide thin film have shown significant antimicrobial properties against oral pathogens. This is an effective tool in the prevention of gingivitis and enamel demineralization during orthodontic treatment [63]. In a clinical study it was observed that incorporation of tungsten disulfide (WS2) nanoparticles to Ni–W–P alloy coating results significant drop in friction and improvement in the corrosion resistance of wire coating [64]. Fabrication of Hollow Wires Hollow wires are formed from a polymer coated with nickel titanium nanoparticles through an electro-spinning process, the polymer is removed during this process and produce a hollow wire for orthodontic use. The wire potentially has the shapememory and super elasticity properties but it is difficult to produce pure NiTi nanoparticles wires for desired function with the available precursors, reaction gas and collection medium. So further research is needed to be conducted for new precursors and collection media [65]. Nanorobots in Orthodontics Conventional orthodontic treatment needs months to years for its completion, but with the invention of nanorobots, the same treatment could be possible in hours and days. Nanorobots are special machines that can directly manipulate all periodontal tissues, like gingiva, periodontal ligament, cementum, and alveolar bone and enable fast and painless orthodontic tooth movements like intrusion, extrusion, rotation, and straightening, as well as rapid tissue repair in much shorter time [30, 61]. Smart Orthodontic Brackets The concept of a smart bracket is based on the feedback of an integrated sensor system. These nanomechanical sensors provide feedback to orthodontist about applied three-dimensional forces and tooth movement. Orthodontists have placed nanomechanical sensors into the base of orthodontic brackets and received actual feedback about the magnitude of applied orthodontic forces. These signals allow the doctors to regulate the applied force within biological range for desirable and effective results with minimum error [66, 67].

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3.3.10 Nanotechnology in Periodontology Nanomaterials for Periodontal Drug Delivery Some nanomaterials like nanotubes, hollow tubes, and nanospheres are biodegradable polymers and very useful in local drug delivery. These nanomaterials can carry dental medicines to dead periodontal tissues and nanosphere degraded there, and the medicine is released at target. The triclosan-loaded nanospheres have proved an effective anti-inflammatory agent in gingival swellings. Microspheres loaded with tetracycline are already available as arestin for local drug delivery into periodontal pockets. An experiment was carried out on rats to treat periodontal diseases with 8.5% nanostructured doxycycline gel and results were very exciting [68]. Laser Application in Periodontics When a gel like emulsion of titanium dioxide (TiO2 ) nanoparticles irradiated with laser pulses on human skin or gingiva, these nanoparticles are optically broken down with shock wave of laser and exhibited some interesting properties like microabrasion of hard tissue and stimulation of collagen production. This process has some exciting clinical applications in periodontal treatment, depigmentation, and anesthesia-free soft tissue incision and cavity preparation [30]. Periodontal bone grafts with nanoporosity allowed ideal bone regeneration as it provide more surface area [68]. Nano-bone Fibers Nano-bone fibers have a significant role in local drug delivery system due to their suitable properties. Nano-bone fibers are very strong and possess a superior tensile strength, e.g., polyphosphazene nanofibers [69].

3.3.10.1

Nanotechnology in Endodontics

Antimicrobial Agents in Root Canals Nanomaterials have well-known antimicrobial effect on oral biofilm, and similarly they have shown antimicrobial properties in root canal, displaying cleansing effects in root canals along with other medicaments. Nanomaterials disinfected the root canal by removing food debris and pathogens from the canal and enhanced antimicrobial action of other canal-irrigating agents [70]. It was concluded in an in-vitro study that nano-silver particles have shown an effective antimicrobial property against Enterococcus faecalis but for short term, and their efficiency is questionable after one week [71]. Nanomaterials in Root-End Sealant Nanomaterial-enhanced retrofill polymers (NERPs) are nicely adapted to the root canal contour and also provide superior properties including strength and antipercolation seal. In-vitro study, nanoparticle ceramic cement (a bio-aggregate of calcium silicate, calcium hydroxide, and hydroxyapatite) has been proved a new-end filling material [72].

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Nanomaterials in Restorative Dentistry

Dental Nanocomposites The development of resin-based composite technology has a breakthrough impact in restorative dentistry. Nanocomposite dental filling materials are widely accepted restorative materials with superior mechanical properties in heavy occlusal load areas. Nanocomposites contains nanofiller particles like nanomeric and nanoclusters. These nanofiller particles are non-aggregated and non-agglomerated silica nanoparticles with particle size ranges from 2 to 75 nm [73]. Nanocomposites have excellent mechanical properties like strength, hardness, and esthetics. They also possess high polishability, lower polymerization shrinkage, and superior manipulating properties that allow enough working time for material placement and buildup [74]. Nanoglass Ionomer Composites Nanotechnology was used to develop a newer type of glass ionomer cement with fluoroaluminosilicate (FAS) glass that provided some value-added features like fluoridereleasing capacity, and some bonded nanofillers and nanocluster filler particles were added for strong mechanical properties. This product has a wide range of clinical implication as restorative material in Class I, II, V, and core buildup. Nano-GIC has been proven a perfect restorative material for general dentistry because of its high wear strength, excellent esthetics, very good polishability, and easier to mix and dispense [41]. Nano-bonding Agents Advanced bonding agents like silica nanofillers have been prepared from nanosolutions. They are quite stable and never form clusters in the solution so provide the superior bond strength. Bonding area present in nano-interaction zone (NIZ) of less than 300 nm produces an insoluble calcium compound which is resistant to salivary enzymes, and there are minimal chances of decalcification and degradation of collagen fibers of the tooth material [75]. Coating Agents Coating agents are basically thin solution of nanosized filler particles. They can be used as polishing agents, varnishes, paints and bases in different restorations like composite restorations, glass ionomer cements, jacket crowns, and veneers. Addition of nanofillers in these coating agents provides a protective polish layer on different restorations to prevent staining and increases longevity [76].

3.4 Challenges and Acceptance of Nanodentistry Biocompatibility Nanomaterials should be compatible with living tissues and ensure no adverse effects on human body. In general, miniature of matter is always more bioactive therefore

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more chances of toxicity. The reason for their super activity is that being in smaller size, nanoparticles have ability to penetrate the skin, lung, and other tissues even blood brain barriers and interact them. Inside the human body, nanoparticles may further release or create many biochemical and biomodulators like free radicals, which can damage cells. The body has its own defense mechanism against natural materials, which come across during development, but nanoparticles are new for the body, so it may recognize them as foreign materials. This is main concern in the use of nanotechnology [27]. Mass Production Nanomaterials have basic problem in engineering work at large scale due to ultraprecision in production techniques. Exact positioning and collection at molecular level is huge challenge in mass production. The simultaneous coordination and manipulation of large number of independent molecules at nanoscale is always a tough task. Although macrorobots have several resemblances in the design and control technique with nanorobots, there is huge difference in the scale and material used [77]. Cost Effectiveness Nanotechnology is boon for mankind in terms of cost effectiveness and updated technology in every field. It increases the capabilities and quality of the work done. The use of nanotechnology is very common in medicine including dentistry and also lessens the manufacturing budget and the time. The emerging applications are expected even more efficient, low cost, and long shelf life. In near future, nanomedicine will revolutionize the methods of diagnosis and treatment planning of the diseases at very low cost [78]. Social Issues and Public Compliance The public acceptance of nanotechnology will depend on its more positive aspects and minimum drawbacks. The successful integration of nanotechnology into human life will come from advanced research and continuous monitoring of consumer products and useful applications. The governments and local bodies need a concrete answer on safety concerns of the nanotechnology on environment, health, and sustainable development before implementing in medicine [79]. Human Safety Many research papers have shown their concern about nanotoxicity. There are possibilities that some nanomaterials may produce health risk because of their high reactivity. The small size of nanomaterials allowed them to penetrate the body in such a noble ways that they provide maximum benefit to the body. The development of targeted nanomedicine in cancer therapy, gene therapy, and microvascular surgery of heart. All nanomaterials are not toxic in nature, but toxicity depends on nature of material [80].

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3.5 Conclusion The future of nanotechnology is fascinating and is all set to revolutionize medical science including dentistry. In dentistry, it will depend on development of nanomaterials with improved quality, more success, and less toxicity. The recent development of nanoparticles in orthodontics, prosthodontics, operative dentistry, endodontics and periodontal management, will play a pivotal role in the growth of overall dentistry. The manufacturing of advanced nanomaterials and dental machines like nanorobots further increased the expectation of improved oral health care delivery and prevention of oral diseases. The nanodentistry is thus opening up new horizons for immense possibilities in dental research, but one should keep eyes on safety, efficacy, and implementation of nanotechnology. This chapter is a sincere effort to extend the horizons of nanomaterials and their windfall for dentistry.

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46. Elsaka SE, Hamouda IM, Swain MV (2011) TiO2 nanoparticles addition to conventional glass ionomer restoratives: influence on physical and antibacterial properties. J Dent 34:589–598 47. Yoshida K, Tanagawa M, Matsumoto S, Yamada T, Atsuta M (1999) Antibacterial activity of resin composites with silver containing materials. Euro J Oral Sci 107(4):290–296 48. Karimi MA et al (2011) Synthesis and Characterization of nanoparticles and nanocomposites of ZnO and MgO by sonochemical method and their application for Zinc polycarboxylate dental cement preparation. Int Nano Lett 1(1):43–51 49. Wang W, Liao S, Liu M, Qian H, Fu Y (2015) Recent applications of nanomaterials in prosthodontics. J Nanomater. Article ID 408643. https://doi.org/10.1155/2015/408643 50. Persson C et al (2012) Nanograin sized zirconia silica glass ceramics for dental applications. J Eur Ceram Soc 32(16):4105–4110 51. Li CH, Hou YL, Liu ZR, Ding YC (2011) Investigation in temperature field of nanozirconia ceramics precision grinding. Int J Abrasive Technology 4(1):77–89 52. An JW, You DH, Lim DS (2003) Tribological properties of hot pressed alumina-CNT composites. Wear 255(1–6):677–681 53. Silikas N, Masouras K, Satterthwaite J, Watts DC (2007) Effects of Nano fills in adhesives and aesthetic properties of dental resin composites. Int J Nano Biomater 1(2):116–127 54. Nam K-Y (2011) In vitro antimicrobial effect of the tissue conditioner containing silver nanoparticles. J Adv Prosthodont 3:20–24 55. Garner SJ, Nobbs AH, McNally LM, Barbour ME (2015) An antifungal coating for dental silicones composed of chlorhexidine nanoparticles. J Dent 43(3):362–372 56. Lewis DH, Castleberry DJ (1980) An assessment of recent advances in external maxillofacial materials. J Prosthet Dent 43(4):426–432 57. Montgomery PC, Kiat- AS (2010) Survey of currently used materials for fabrication of extra oral maxilla-facial prostheses in North America, Europe, Asia and Australia. J Prosthodont 19(6):482–490 58. Zhala M, Alexandros B, Tracy De P,Richard D H. Antifungal properties and biocompatibilty of silver nanoparticles coatings on silicone maxillofacial prostheses. J Biomed Mater Res Part B Appl Biomater 106B:1038–1051 59. Han Y, Zhao Y, Xie C, Powers JM, Kiatamnuay S (2010) Colour stability of pigmented maxillofacial silicone elastomer; Effects of nano-oxides as opacifiers. J Dent 38(2):100–105 60. Zayed SM, Alshimy A, Fahmy AE (2014) Effect of surface treated silicon dioxide nanoparticles on some mechanical properties of maxillofacial silicone elastomer. Int J Biomater. Article ID 750398 61. Redlich M, Katz A, Rapoport L, Wagner HD, Feldman Y, Tenne R (2008) Improved orthodontic stainless steel wires coated with inorganic fullerene-like nanoparticles of WS(2) impregnated in electroless nickel-phosphorous film. Dent Mater 24:1640–1646 62. Alcock JP, Barbour ME, Sandy JR, Ireland AJ (2009) Nano indentation of orthodontic arch wires: the effect of decontamination and clinical use on hardness, elastic modulus and surface roughness. Dent Mater 25:1039–1043 63. Cao B, Wang Y, Li N, Liu B, Zhang Y (2013) Preparation of an orthodontic bracket coated with an nitrogen-doped TiO (2–x) N (y) thin film and examination of its antimicrobial performance. Dent Mater J 32:311–316 64. Ranganatha S, Venkatesha TV, Vathsala K (2012) Electroless Ni–W–P coating and its nanoWS2 composite: preparation and properties. Ind Eng Chem Res 51(23):7932–7940 65. Panchali B, Anam M, Jahirul m, Meryam SR, Ragini M. Nanoparticles and their Applications in Orthodontics. Adv Dent Oral Health 2(2) 66. Lapatki BG, Paul O (2007) Smart brackets for 3D-force-moment measurement in orthodontic research and therapy developmental status and prospects. J Orofac Orthop 68(5):377–396 67. Lapatki HG, Bartholomeyczik J, Ruther P, Jonas IE, Paul O (2007) Smart bracket for multidimensional force and moment measurement. J Dent Res 86(1):73–78 68. Kim JS, Cho BH, Lee IB, Um CM, Lim BS, Oh MH et al (2005) Effect of the hydrophilic nanofiller loading on the mechanical properties and the microtensile bond strength of an ethanolbased one-bottle dentin adhesive. J Biomed Mater Res B Appl Biomater 72:284–291

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69. Slavkin HC (1999) Entering the era of molecular dentistry. J Am Dent Assoc 130:413–417 70. Shrestha A, Fong SW, Khoo BC, Kishen A (2009) Delivery of antibacterial nanoparticles into dentinal tubules using high intensity focused ultrasound. J Endod 35:1028–1033 71. Mozayeni MA, Haeri A, Dianat O, Jafari AR (2014) Antimicrobial effects of four intracanal medicaments on Enterococcus faecalis: an in vitro study. Iranian Endod J 9(3):195–198 72. Yuan Z, Peng B, Jiang H, Bian Z, Yan P (2010) Effect of bioaggregate on mineral-associated gene expression in osteoblast cells. J Endod 36:1145–1148 73. Mitra SB, Wu D, Holmes BN (2003) An application of nanotechnology in advanced dental materials. J Am Dent Assoc 134:1382–1390 74. Rybachuk AV, Cekman IS (2009) Nanotechnology and nanoparticles in dentistry. Pharmocol Pharm 1:18–21 75. Khosla R (2009) Nanotechnology in dentistry. Famdent Pract Dent Handb 9:69–84 76. Atabek D, Sillelioglu H, Olmez A (2010) Th e efficiency of a new polishing material: nanotechnology liquid polish. Oper Dent 35:362–369 77. Mehra P, Nabhi k (2016) A nanorobotics—the changing face of dentistry. Int J Sci Res 5:192– 197 78. Roco MC, Mirkin CA, Hersam MC (2011) Nanotechnology research directions for societal needs in 2020: retrospective and outlook summary 79. www.nano.wustl.edu/doc/Forms/NRF%20Nano%20Ethics.pdf. 80. Brianna S (2009) Perspectives on FDA’S regulation of nanotechnology: Emerging challenges and potential solutions. Compr Rev Food Sci Food Saf 8:375–393

Chapter 4

Nanostructured Abrasive Materials for Ultraprecision Finishing of High-Performance Materials M. J. Jackson

Abstract This chapter focuses on the development of nanostructured abrasive materials that are required to produce engineered surfaces of the highest integrity possible when processing difficult-to-machine materials such as those used in the aerospace sector. The chapter describes the challenges manufacturers have when dealing with abrasive cutting tools that fail to maintain roundness and suffer from “wheel collapse,” and localized pitting that affects the roundness of the wheel and the integrity of the ground surface that eventually has to be rectified by dressing methods that are nonproductive in terms of cycle time and generate waste products. Subsequent discussions focus on the manufacture and use of new abrasive products by fusion and sintering that are nanostructured such that wheel pitting is virtually eliminated, wheel collapse is rare, and cycle time is reduced when grinding difficult materials such nickel and titanium alloys. Case studies are referred to throughout the chapter as a way to explain how nanostructured materials can change the way abrasive materials operate especially in the aerospace sector and beyond. Keywords Nanotechnology · Abrasives · Materials · Finishing · Grinding · Machining

4.1 Introduction Nanostructured abrasive particles engineered to cut precision surfaces without the need to slide and plow the surface (focusing on cutting surfaces, or interaction 1.1, shown in the microscopic interaction chart (Fig. 4.1)) are rapidly in development and focus on optimizing plowing and sliding interactions 1.2 (t = 0) and 1.3 (t = 0) (Fig. 4.1) to minimize the resultant threshold power, Pth (t = 0), according to Jackson [1], Subramanian [2] and other researchers [3–8]. Owing to the reduction in cycle time to eliminate non-cutting operations such as dressing and spark-in and spark-out cycles, abrasive grits are engineered to fracture at the micro- and M. J. Jackson (B) School of Integrated Studies, College of Technology and Aviation, Kansas State University, Salina, KS 67401, USA e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_4

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Fig. 4.1 Microscopic interactions associated with abrasive finishing processes: (1) abrasive/workpiece interactions of cutting (1.1), plowing (1.2), and sliding (1.3); (2) chip/bond interactions; (3) chip/workpiece interactions; and (4) bond/workpiece interactions [1, 2]. Inspired and developed by Dr. Subramanian, STIMS Institute [2]

nanoscales [9–12]. Engineered grit shapes are available to grind many newly developed difficult-to-grind materials [13–17], and production strategies are being adapted to apply these new types of abrasive shapes [18–25]. Controlled strength, sharp, and permeable agglomerated grits have been developed to minimize burn and frictional effects in finish grinding [26–28], but the issue of pitting of wheel surfaces persists even in highly controlled grinding environments. The erosion, or localized pitting, of wheel surfaces is a phenomenon that plagues the precision machining of engineered surfaces. This chapter focuses on explaining the principles of ultraprecision finishing with grinding wheels and explains how advances in science of materials and processing are producing the next generation of micro- and nanostructured abrasive products that are capable of finish difficult-to-machine materials.

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4.2 Ultraprecision Finishing Using Grinding Wheels Surface pitting of the grinding wheel is a term which denotes a type of wear where groups of grits fracture at once and are removed from the surface of the wheel. Owing to the removal of clusters of grits at the wheel’s circumference, the wheel loses its roundness (wheel collapse). The most possible reason for wheel pitting is inappropriate self-sharpening. In this case, wear by attrition is more dominant than grit and bond fracture, so grits become dull over time. There are several negative effects of this effect. Firstly, dull grits create higher cutting forces. Secondly, dull grits produce more heat which can cause material softening when plowing, and rubbing becomes dominant. When the force increases over a certain threshold, especially with softened materials, bond bridges break not just between grits on the outer surface but deeper inside the wheel, leading to a cluster of grits that break off. Also, if the self-sharpening does not occur in a timely manner, freshly removed material (chips) collects between grits, or clusters of grits, and then parasitic wear takes place. This can increase forces acting on the wheel, especially normal forces, and leads to an increase in wedge pressure. This can lead to chipping or removal of whole grit(s) and clusters of grits. The increased normal force is considered the main factor in wheel pitting. It is well known (Hertz equation) that normal contact forces produce maximum stresses at some depth below the surface. Cyclic loading leads to fatigue, the final result being pitting. Increased tangential forces contributes to faster removal of fatigued material, in other words faster pitting. Intuitively, one expects more grits to be in contact with the surface if they are not self-sharpened, so not only normal force per grit increases but the total normal force is further increased due to number of grits in contact with the surface. According to its description, there is a strong resemblance between the final outcome in wheel collapse (espoused by Badger [29]) and pitting. The difference is due to the random structure of a grinding wheel. It is expected that the depth of the maximum contact stresses may vary widely, depending on changes in grit orientation and the shape of a particular grit and the surrounding grits, the position of the bonding material between grits, the relative volume (thickness) of the bond, and many other factors at that particular scale. One cannot expect all grits to be dulled to an equal level as a function of time. Also, the cutting geometry varies between different grits. Depending on how broad the variation is, clusters that chip off the grinding wheel will vary in size and as a function of depth into the wheel surface. The approach to wheel collapse as a form of pitting is unique and gives a lot of options to contribute to solving this problem. It also provides the possibility of applying mathematical models associated with pitting of surfaces. In this particular case, existing mathematical treatment of pitting and contact mechanics can be used and modified for a porous composite material such as a grinding wheel. However, we must understand the sequence of events leading up to pitting. Figure 4.2 shows the proposed sequences of wheel pitting. If grits are sharp (Fig. 4.2a), they cut material as a function of materials removal rate. Normal forces that develop are relatively low. If grits are dull (Fig. 4.2b), then

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Fig. 4.2 Proposed mechanism of wheel collapse (localized pitting): a grinding wheel in operation with new or self- sharpened grits; b operating with dull grits where normal forces, F n , are higher in the case of dull grits (tangential forces not shown in the figures for the purpose of brevity) and It should be noted that maximum tangential stresses develop beneath the contact point shown; c bonding material cracks in the region around the application of maximum tangential stresses, while the applied moment, M Ft , is initiated by tangential forces acting about a distance between abrasive grit and the workpiece surface; d broken clusters are ejected from the wheel under the action of centrifugal forces and the wheel loses roundness and wheel collapse is said to have occurred, and e if clusters are not completely detached, or cannot be pulled out by the centrifugal force, they can be crushed by another loading cycle and pushed out as shown

the material of the work piece is plastically deformed far too much. If grits are dull enough, the grinding wheel cannot remove material as dictated by the feed rate, and clusters of abrasive grits will be removed by high forces. If grits are still able to keep pace with the feed, they will be heavily loaded. It is clear that the tangential component of the resultant force will be much higher. The increase of the normal component of the force is also expected, but it is believed that it is responsible for the wheel collapse. Tangential force alone should break bonds next to the surface, or at least crack grits that are in the contact. On the other hand, it is known that maximum shear stresses, τ max, develop at some distance from the surface, d τmax, (right-hand side of Fig. 4.2b). Although this is valid for continuous materials, this can be also assumed for porous composite materials. The depth, d τmax, depends on the magnitude of the normal force. Since the normal force increases as the grit goes toward the end of the contact, so does d τmax (Fig. 4.2b). A combination of the tangential and normal components results in shear stresses which develop within the wheel. Orientation of grits and bond is also important. Grits, or more probably bonding material, will crack at depths around d τmax (Fig. 4.2c). This makes groups of grits to detach from the wheel. Detached clusters of grits can be ejected by the centrifugal force (Fig. 4.2d), or by subsequently entering the contact zone (Fig. 4.2e). It should be noted that the

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Fig. 4.3 Shearing forces appear in two directions: tangential and radial. Shear in the tangential direction is due to F n and F t , and in the radial direction, it is due to the difference of the normal force between two adjacent grains separated by an angle θ

bond is susceptible to breaking not only in the tangential direction, but also in the radial direction. There is also shear created in the radial direction (Fig. 4.3), which is due to the difference in the normal force between two adjacent grains. Figure 4.2c shows only cracks caused by the action of tangential shear. The accumulation of the removed material decreases the accommodation required for new chips, which can further increase cutting forces. The possible mechanisms of wheel collapse are described by Badger [29]: • Thermal—An increased temperature may induce changes in the material of the wheel and/or the method that the grinding zone is cooled. Phase changes in grits and bonding material can lead to loss of mechanical properties. Nucleate boiling is the most effective way for cooling, since the heat transfer is highest due to the heat of evaporation. However, if vapor nuclei fail to initiate or the rate of their production is too high, film boiling occurs when the vapor phase represents an insulator. This situation creates a further increase of the wheel temperature; • Loading—Increased accumulation of freshly removed material between grits leads to higher forces, pulling out whole grits and clusters of grits before they fracture; • Mechanical—This mechanism is connected to the dull geometry of grits, which leads to increased cutting forces. The first mechanism is related to a negative change in material properties, while the other two have the same outcome—increased forces imposed on grits. Since wheel collapse does not introduce some substantial changes on the ground surface, the only noticeable negative effect is somewhat stronger chatter of the spindle. It seems that the loss of roundness has been known for decades, but no substantial attention has been paid to it. It is also interesting to note that increased grinding power has not drawn much attention either. From the power profile described in reference [29], one might make a hypothesis that the main reasons for the loss of roundness (wheel collapse) are the main reasons for increased grinding power. Solving this issue could make savings in the power consumption by 50% or more. A higher G-ratio wheel could solve the issue of premature wheel collapse, and it has two effects: (1) the grinding power increases at slower rate, and (2) the collapse of the wheel’s roundness occurs at the higher power recorded during grinding.

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Good cooling reduces the grinding power drawn and delays the onset of wheel collapse [29], and reduces the magnitude power at collapse. This means that one or both of these affect the collapse of the wheel: thermal effects (due to overheating and film boiling), and chip collection and crowding between abrasive grits. Intensive cooling prevents vapor sticking to the surface and provides a higher rate of heat removal. Faster flows of coolant wash away removed material (chips) from the cutting zone avoiding chip crowding and subsequent “aggressive cutting.” Grinding aggressiveness [29] is defined as: vtable Aggressiveness = 10 vwheel 6



ae ds

(4.1)

Here, vtable is the table speed, vwheel is the peripheral speed of the wheel, ae the depth of cut, and d s the wheel diameter. Badger shows that the increased value of aggressiveness delays the onset of the wheel collapse [29]. The ratio (vtable /vwheel ) is shown to be directly proportional to the feed rate, which means that increased feed rate should delay the onset of collapse. Increased values of aggressiveness were achieved only by varying, ae , so one might argue whether, or not, ae and aggressiveness equally affect wheel collapse. However, it seems that harder working conditions delay the onset of wheel collapse [29], which has many implications associated with the design of the abrasive wheel to grind such materials. The answers are provided by better control of the microstructure during manufacture of the grits and the correct selection of raw materials for bonding the abrasive grits together. Nanotechnology is providing a greater role in developing the next generation of abrasive products specifically to offset the phenomenon of wheel collapse.

4.3 Ultraprecision Finishing: Development of Nanostructured Abrasive Materials 4.3.1 Fused Abrasive Grits The variability in chemical and mechanical properties of abrasive grits prior to controlled fusion processes produced differences in wheel formulations especially in vitrified grinding wheels. Mined bauxite is used in fused aluminum oxide grits. Mined bauxite is composed of ~60% alumina as gibbsite Al(OH)3 , boehmite γ-AlO(OH), and diaspore α-AlO(OH), goethite, hematite, kaolinite, anatase, and titania (TiO2 ). Bauxite plus iron and coke is fused to create brown-fused alumina (BFA) abrasives that contain titania. Bauxite is purified before fusion in a process known as the Bayer process. Bauxite is mixed and heated with sodium hydroxide solution at 150–200 °C. After separation of the red-mud residue, gibbsite is precipitated and seeded with aluminum hydroxide. Gibbsite is converted to aluminum oxide by calcination. The Bayer process removes most impurities in bauxite, but leaves ~0.1% to

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0.4% Na2 O in calcined alumina (white-fused alumina (WFA)). The Bayer process increases the cost of WFA feed by a factor of 5 compared with bauxite feed for BFA. The Higgins electric arc furnace consists of a thin steel structure on a metal hearth. Feed material is poured into the bottom of the furnace, and a carbon rod is placed in situ. Two or three vertical carbon rods are lowered to touch the surface of the bauxite, and a large current is applied. The starter rod is rapidly consumed, and the bauxite is melted, which becomes an electrolyte. Feed material is added continually to increase the volume of melt to ~20 tons. A typical fusion weight of 30 tons takes ~20 h to completely fill and melt the contents of the furnace pot. Cooling takes ~100 h to complete and is directional; the insulating outer layer of the melt is microcrystalline. Large crystalline, dendritic, growth regions occur inside the pot in a radial direction with solidification toward the center as heat flows. Measurements conducted in cooling pots show that diffusion coefficients are in the region of 8 × 10–10 m3 /s [30], using methods developed by Universal Abrasives and mathematical analyses developed by Hitchiner and Hunt at the University of Oxford, UK, and shown in Appendices 4.1 and 4.2 [30]. Current research directions aim to increase the cooling rate toward the center of the melt to produce the same microcrystalline structure that is produced in the insulating outer layer of the cast melt. The pot has an aspect ratio of 1:1. Impurities concentrate in the liquid phase. The ingot is broken up after cooling then sorted to remove primary concentration of impurities. The conversion of residual Na2 O from the Bayer process into sodium βalumina which crystallizes as soft hexagonal plates in the alumina is critical. Sodium β-alumina has a lower melting point than alumina, so it will concentrate in specific areas of the ingot. The design of pour pots for use with tilting furnaces has a major impact on grit structure and chemistry. For a WFA fusion pour into a high profile pot, the cooling process and output produces a large dendritic alumina crystal and low sodium βalumina content. However, when pouring from a pot with aspect ratio 1 to a cold hearth, cooling rates are faster which results in a fine crystalline structure with evenly dispersed sodium β-alumina content. A high thermal gradient when cooling with a WFA ingot in a deep profile pot causes crystallization of α-alumina in the form of a dendritic habit composed of rhombohedra extending along a thermal gradient. This type of rhombohedron grows much faster than its faces, and abrasive grits made from this tend to fracture in large fragments along defined planes, but can be self-sharpening. Block melting results in uneven refining of the alumina melt and produces alumina blocks with heterogeneous composition and structure. The heterogeneity is exposed when the block is broken up with a hammer. A regular alumina block consists of the following parts, each with a different appearance, structure, and mineralogical composition: the so-called cap, sub-cap, center, side, and bottom (Fig. 4.4). The appearance of lump alumina, in which these block parts typically appear, is described in Table 4.1. The mineralogical composition of alumina shown below was established by detailed chemical and microscopic analysis of the block material and by synthesizing and examining the individual minerals.

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Fig. 4.4 Cross-sectional diagram of a regular alumina block: 1—Cap; 2—Sub-cap; 3—Center; 4—Side; 5—Bottom; 6—Main ferroalloy area; 7—Hearth residue; 8—Shrink hole. (Courtesy of Motim Electro α-alumina and Prof. Bockuchava)

Microscopic investigation and preparations of samples showed that the alumina contained both α-alumina and a number of transparent minerals: calcium hexaluminate (CaO·6Al2 O3 ·SiO2 ), anorthite (CaO·Al2 O3 ·2SiO2 ), mullite (Al2 O3 ·2SiO2 ), and spinel ((Mg, Fe, Mn)O·Al2 O3 ). The cast blocks contained opaque, round, or irregular inclusions, dendrites, rods, and needles, which appeared black in transmitted light. The first three types used are to be classified as ferroalloy, and the last two as titanium minerals of unknown composition. Their exact analysis became possible thanks to the development of a method of preparation of alumina slides, and their analysis in reflected light. The opaque phases contained in an alumina block are ferroalloys and titanium minerals: titanium sesquioxide Ti2 O3 , a variety of anosovite, Ti2 O3 ·2TiO2 (Ti4 O7 ), Ti3 O5 , TiO, rutile TiO2 , titanium nitride (TiN), and titanium carbide (TiC), and the solid solutions TiO-TiN and TiN-TiC. The mineralogical description of the various components of an alumina block shown in Fig. 4.4 are: 1. The cap consists of microcrystalline skeletal, laminar or isometric α-alumina, with crystal dimensions 150–350 μm. The crystals contain impregnations of titanium carbide and solid solutions of TiO-TiN and TiN-TiC, less often single grits of carbon materials in the titanium carbide margin. The spaces between αalumina crystals contain glass interspersed with anorthite, titanium sesquioxide, TiO, spinel, and titanium nitride. Less often, abutting crystal faces contain tabular aggregates of calcium hexaluminate.

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Table 4.1 Structure and color of typical cast alumina blocks Block section Sample No Lump structure and color Cap

1

Dense material. Porous in the lump. Color varies from light to dark gray with a range of muddy tones. Some of the surface reddish-brown

2

Dense in the lump (no porosity). Laminar structure. Dark gray in color

3

Spongy, equi-granular lump. Color pink, pale blue, rarely brown

4

Porous lump. A dense granular microcrystalline material, wine-brown in color

Center

5

Dense solid lump, chiefly and meso- and macrocrystalline. Waxy luster. Color is brown to pinky brown. The body of the lump contains ferroalloy inclusions, showing silvery with a yellow tinge in the fracture

Side

6

Dense lump, predominantly microcrystalline, in places meso- and macrocrystalline. Fracture plane rough, matt, less often with a waxy luster. Color dark brown to bluish-black

Bottom

7

Dense solid lump, microcrystalline to macrocrystalline structure, the latter more common. Significant number of ferroalloy inclusions of various dimensions. Waxy luster in the fracture. Color dark gray-brown to pale pink-brown

Bottom

8

A dense lump with granular or laminar structure, pinky-gray or pale gray in color. Typically contains carbon inclusions and irregular ferroalloy slabs

Sub-cap

2. The sub-cap consists of 500–600 μm isometric α-alumina crystals, frequently with rounded fused edges. Abutting crystal faces contain small lamellae of glass interspersed with oxygen compounds of titanium, and less often, the α-alumina crystals are cemented with foliated aggregates of titanium nitride or lamellae of titanium ferroalloys. α-alumina crystals contain impregnations of titanium carbide and inclusions of fine ferroalloy beads. 3. The center consists of medium- and coarse-grained α-alumina in the form of rhombohedral or thick tabular crystals and isometric grits. α-alumina crystal diameters vary within 10–150 μm. Fine glass pieces are interspersed between the crystals or enclosed within the body of the crystal. The glass contains anorthite, calcium hexaluminate, anosovite, and Ti2 O3 , while the α-alumina crystals contain infrequent ferroalloy beads. 4. The sides of the cast block consist of medium- and coarse-grained isometric and elongated tabular α-alumina crystals. The crystals are interlayered with glass containing anorthite, anosovite, Ti2 O3 , and less frequently calcium hexaluminate. Occasionally, the α-alumina crystals contain ferroalloy beads. Crystal dimensions are typically 500–750 μm. 5. The bottom of the cast block consists of coarse- and medium-grained isometric α-alumina crystals, 700–2000 μm in size, occasionally with ferroalloy bead inclusions. The α-alumina is accompanied by calcium hexaluminate, anorthite,

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and titanium minerals, predominantly titanium sesquioxide and anosovite. The mineral and chemical compositions of samples of regular alumina taken from several blocks are given in Tables 4.2 and 4.3, respectively. The central section of the alumina blocks contains the most α-alumina and has a coarse-grained structure. The α-alumina content of a block varies from 72% on the margins to 98% in the central sections. The marginal sections contain various impurities: the α-alumina crystals of which they are composed frequently have a defective, elongated, or skeletal shape and are separated by smaller, or larger, bands of glass minerals. As a rule, impurities occur in the largest quantities in the cap. After fusion and cooling, the block casting is broken up and sized according to the type of grit required and the size of grit. These grits are used with combinations of other grits such as engineered ceramic grits to achieve a particular level of performance during the grinding of difficult-to-machine materials.

4.3.2 Sintered Abrasive Grits Engineered abrasive grits have much higher grinding ratios (G-ratio) than fused grits owing to their ability to fracture in an economic way by self-sharpening at the microscale. Higher threshold forces are needed to produce the effect. By reducing mesh size ( 0) = Cm1

(4.4a)

C(x = +∞, t > 0) = Cm2

(4.4b)

C(x = 0 − δx, t > 0) = C1

(4.4c)

C(x = 0 + δx, t > 0) = C2

(4.4d)

  ∂C  ∂C  − D1 (C1 − C2 )V = D2 ∂ x 2 ∂ x 1

(4.5)

The interface flux:

where V is the velocity of the moving interface. Fick’s law of diffusion is solved by using the Boltzmann substitution: ε=

x − xM x √ = √ for xM = 0 2 t 2 t

Because the diffusion coefficient is not the same for the two phases in contact owing to differences in composition, the substitution is: x ε1 = √ 2 D1 t

(4.6a)

x ε2 = √ 2 D2 t

(4.6b)

At the interface between two phases, the parameters ε1 and ε2 become:

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xI η1 = √ 2 D1 t

(4.6c)

xI η2 = √ 2 D2 t

(4.6d)

For the derivation of the diffusion equation 4.8f, a substitution without coefficients is used: x ε= √ 2 Dt

(4.6e)

From previous equations, the following relationship will be used in (4.7a): 4D1 ε12 4D2 ε22 4Dε2 1 = = = 2 2 t x x x2

(4.6f)

Equations 4.3a and 4.3b are solved separately for both phases, and then, Eq. 4.5 is used to combine them. Therefore, Eq. 4.6e will be used to derive parts common for both phases, and then, subscripts are added together with the appropriate integration limits. Using Eqs. 4.6c and 4.6d, the left-hand side of Eqs. 4.3a and 4.3b is: ∂C ∂ε ∂C ∂ ∂C = = ∂t ∂ε ∂t ∂ε ∂t



x √ 2 Dt

 =−

 ε 2 ∂C 1 x 1 ∂C = −D (4.7a) 2ε √ 2 2 Dt t ∂ε x ∂ε

The right-hand side can be transformed in a similar way: D

      ∂ 2C ∂ ∂C ∂ε ∂ 1 ∂C ∂ ∂C = D = D = D √ ∂x2 ∂x ∂x ∂ x ∂ε ∂ x ∂ x 2 Dt ∂ε    2 2 1 ∂C ε ∂ C ∂ε ∂ =D =D √ ∂ x ∂ε 2 Dt ∂ε x ∂ε2

(4.7b)

Setting Eq. 4.7a equal to Eq. 4.7b, Eqs. 4.3a, 4.3b in the transformed case: −2ε

∂ 2C ∂C = ∂ε ∂ε2

(4.8a)

This equation can be solved by decreasing its order: ∂ 2C dp ∂C ∂p =p = = ∂ε ∂ε2 ∂ε dε

(4.8b)

The second-order partial differential equation is reduced to the first-order ordinary differential equation:

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dp = −2εp dε

(4.8c)

Applying integrals:

dp = −2 p



p = Ae−ε

εdε

(4.8d)

2

(4.8e)

Using the definition of p from Eq. 4.8b: C1

ε1

C0

ε0

∫ dC = A ∫ e−ε dε 2

(4.8f)

Here, A is a constant which has to be determined for particular integration limits. Integration boundaries are different for primary and secondary phases. The following two definitions are used to solve integrals: 0

∫ e

−aε2

−∞

∞

dε = ∫ e

−aε2

0

1 dε = 2



π a

(4.9a)

2 x 2 √ ∫ e−ε dε = erfx π 0

(4.9b)

For the primary phase, the lower limits are at x = - ∞: C0 = Cm1 ; ε0 = ε1 |−∞ = −∞C0

(4.10a)

The upper limits are set at x = xI : ε1 = ε1 |x I = η1

(4.10b)

Therefore, Eq. 4.8f for the primary phase is: C1

η1

cm1

−∞

∫ dC = A1 ∫ e

−ε2

 dε = A1

0

∫ e

−∞

−ε2

η1

dε + ∫ e

−ε2

 dε

(4.10c)

0

Hence, (C1 − Cm1 ) = A

√ √  √ A1 π π π + erfη1 = (1 + erfη1 ) 2 2 2

The integration constant for the primary phase is:

(4.10d)

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M. J. Jackson

2(C1 − Cm1 ) A1 = √ π (1 + erfη1 )

(4.10e)

The concentration throughout the primary phase is obtained substituting the general coordinates and concentration for the upper limit of the integral shown in Eq. 4.10b: C1−phase

∫

Cm1

2(C1 − Cm1 ) εα −ε2 ∫ e dε A dC = √ π (1 + erfη1 ) −∞   ε1 0 2(C1 − Cm1 ) 2 2 ∫ e−ε dε + ∫ e−ε dε =√ π (1 + erfη1 ) −∞ 0 √ 2(C1 − Cm1 ) π =√ (1 + erfε1 ) π (1 + erfη1 ) 2

(4.10f)

The solution for the primary phase: C1−phase = Cm1 + (C1 − Cm1 )

(1 + erf ε1 ) (1 + erf η1 )

(4.10g)

For the secondary phase, the upper limits are at x = + ∞: C1 = Cm2 ; ε1 = ε2 |+∞ = ∞

(4.11a)

The lower limits are set at x = xI : C0 = C2 ε0 = ε2 |x I = η2

(4.11b)

Therefore:   η2 ∞ ∞ 2 2 2 ∫ dC = A2 ∫ e−ε dε = A2 ∫ e−ε dε − ∫ e−ε dε

Cm2 C2

η2

0

(4.11c)

0

Hence: (Cm2 − C2 ) =

√ A2 π (1 − erf η2 ) 2

(4.11d)

The integration constant for secondary phase: 2(Cm2 − C2 ) A2 = √ π (1 − erf η2 ) The general upper limits applied to Eq. 4.8f:

(4.11e)

4 Nanostructured Abrasive Materials for Ultraprecision Finishing …

2(Cm2 − C2 ) ∞ −ε2 ∫ e dε A dC = √ π (1 − erf η2 ) ε2 C2−phase   ε2 2(Cm2 − C2 ) ∞ −ε2 −ε2 ∫ e dε − ∫ e dε =√ π (1 − erf η2 ) 0 0 √ π 2(Cm2 − C2 ) =√ (1 − erf ε2 ) π (1 − erf η2 ) 2

123

Cm2

∫

(4.11f)

The solution for the secondary phase: C2−phase = Cm2 − (Cm2 − C2 )

(1 − erf ε2 ) (1 − erf η2 )

(4.11g)

The interface velocity is the first derivative of the interface coordinate xI shown in Eqs. 4.6c and 4.6d: dx I = V = dt



D1 η1 = t



D2 η2 t

(4.12a)

Hence: 

D 1 η1 =



D 2 η2

(4.12b)

Equation 4.5 contains first derivatives of Eqs. 4.10g and 4.11g. For the secondary phase in general coordinates:  ∂C2−phase ∂ε2 ∂C2−phase ∂C  = = ∂ x 2 ∂x ∂ε2 ∂x     ∂ x ∂ erf ε2 (Cm2 − C2 ) Cm2 − = + (Cm2 − C2 ) √ ∂ε2 (1 − erfη2 ) (1 − erf η2 ) ∂ x 2 D2 t

2 ε √2 ∫ 2 e−ε dε 1 ∂ π 0 = √ (Cm2 − C2 ) ∂x (1 − erf η2 ) 2 D2 t e−ε2 1 =√ √ (Cm2 − C2 ) (1 − erf η2 ) π D2 t 2

(4.13a)

Since Eq. 4.5 is expressed for the interface, a derivative for the interface is expected:  2 ∂C  1 e−η2 = − C √ (C ) √ m2 2 ∂ x 2,x I (1 − erfη2 ) π D2 t For the primary phase in general coordinates:

(4.13b)

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 ∂C1−phase ∂ε1 ∂C1−phase ∂C  = = ∂ x 1 ∂x ∂ε1 ∂x     x ∂ erf ε1 ∂ (C1 − Cm1 ) Cm1 + = + √ ∂ε1 (1 + erfη1 ) (1 + erf η1 ) ∂ x 2 D1 t

ε1 −ε2 2 √ ∫ e dε 1 ∂ π 0 = √ (C1 − Cm1 ) ∂x (1 + erf η1 ) 2 D1 t e−ε1 1 =√ √ (C1 − Cm1 ) (1 + erf η1 ) π D1 t 2

(4.13c)

At the interface:  2 ∂C  1 e−η1 = − C √ (C1 √ m) ∂ x 1,x I (1 + erfη1 ) π D1 t

(4.13d)

From Eq. 4.5 1 (C1 − C2 )V



1 e−η2 D2 √ √ (Cm2 − C2 ) (1 − erfη2 ) π D2 t 2

1 e−η1 −D1 √ √ (C1 − Cm1 ) (1 + erfη1 ) π D1 t 2

=1

(4.14a)

√ e−η1 1 (Cm1 − C1 ) = π D1 t (C1 − C2 ) (1 + erfη1 )

(4.14b)

Substituting Eq. 4.12a for the interface velocity, V: 1 (Cm2 − C2 ) e−η2 D2  √ D2 D2 t (C1 − C2 ) (1 − erfη2 ) η t 2 2

1

+ D1 

2

1 D1 η t 1

√

√ e−η2 1 (Cm1 − C1 ) e−η1 1 (Cm2 − C2 ) + = π η2 (C1 − C2 ) (1 − erfη2 ) η1 (C1 − C2 ) (1 + erfη1 ) 2

2

(4.14c)

Hence: √ 1 (Cm2 − C2 ) e−η2 1 (Cm1 − C1 ) e−η1 + = π η2 (C1 − C2 ) (1 − erfη2 ) η1 (C1 − C2 ) (1 + erfη1 ) 2

2

(4.14d)

Assuming the primary phase to be liquid (L = 1), and the secondary phase to be solid (S = 2), and if conditions at the solid–liquid interface exhibit non-equilibrium conditions:

4 Nanostructured Abrasive Materials for Ultraprecision Finishing …

C2 CS = = k C1 CL

125

(4.15)

where k is the equilibrium partition coefficient. Instead, a non-equilibrium partition coefficient would apply: CS = kv CL

(4.16)

The coefficient, k v, depends on the velocity of the expanding solid phase. However, this type of coefficient is used for rapid solidification processes where the trapping of solutes has a significant effect on the solidification process. It is used when interface melting and solidification is collision controlled rather than diffusion controlled, which is normally the case for slowly cooled abrasive materials that may take up to 100 h to completely solidify once cast into the appropriately sized pour pots. Faster cooling rates may be collision controlled depending on the type of solute(s) present in the melt.

Appendix 4.2 Assuming that the advancing solid planer front is moving in the direction of the shrinking liquid phase, a number of pertinent questions arise especially when applied to the manufacture of abrasive grits from a slowly cooled block of alumina: (1) How to use these equations to measure the diffusion coefficient of a solute element in the liquid phase? (2) What are the problems associated with using these equations for determining the diffusion coefficient of a solute element in the solid phase itself? If Eq. 4.14d is simplified assuming that DL = ∞, then there is no solution for the diffusion in the solid. The solution is contradictory for the growth of the solid phase (ηS < 0). For the melting process (ηS > 0), allowed values for ηS are in the range where there is no solution. Therefore, Eq. 4.14d has to be used with no underlying assumptions. In this case, a set of parameters can be found when DS is obtained. To answer question 1, the primary phase = L (liquid), and the secondary phase = S (solid). Equations 4.12b and 4.14d become: 

D L ηL =



D S ηS

(4.17a)

√ e−ηL e−ηS (CmS − CS ) (CmL − CL ) + = π ηL (CL − CS ) (1 + erf ηL ) ηS (CL − CS ) (1 − erf ηS ) 2

2

(4.17b)

There are three main steps in determining the diffusion coefficient in the liquid phase, based on Eqs. 4.17a and 4.17b: (1)Measurement of the coordinate of the solid/liquid interface, x I , and time, t. The parameter, x I (t), is measured;

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(2)Combination of Eqs. 4.17a and 4.17b provide a system of two equations with two unknowns, ηL and ηS . Therefore, unique values of ηL and ηS are obtained. Since Eq. 4.17b contains transcendental numbers, a numerical solution is inevitable. (3)In Eqs. 4.6c and 4.6d, x I and t are measured in step 1, and ηL and ηS are calculated in step 2. The only unknowns are DL and DS , which can be calculated from these two equations. It is usual to neglect the diffusion coefficient in the solid, DS . In this case: √ e−ηL (CmL − CL ) = π ηL (CL − CS ) (1 + erf ηL ) 2

(4.18)

According to Hunt, if DL /DS ≥ 1000 (which is a usual range for the ratio between the liquid and solid), the error will be less than 1%. Therefore, Eq. 4.18 can be used for step 2. To determine the diffusion coefficient in the solid phase, then one assumes no concentration gradient in the liquid (diffusion infinite relative to the solid phase), then C L = C mL . Equation 4.17b is reduced then to: √ e−ηS (CmS − CS ) = π ηS (CL − CS ) (1 − erf ηS )

(4.19a)

√ (C L − C S ) e−ηS = π η S (1 − erfη S ) (Cm S − C S )

(4.19b)

2

2

The right-hand side of the equation is constant, so: e−ηS = const ηS (1 − erf ηS ) 2

(4.19c)

For easier reference, the left-hand side of Eqs. 4.19b and 4.19c is: e−ηS ηS (1 − erf ηS ) 2

y=

(4.20a)

The abscissa is the value of the Boltzmann substitution for the interface: x = ηS

(4.20b)

Values of the constant on the right-hand side of Eqs. 4.20b and 4.20c will be denoted as z: z=

√

π

(C L − C S ) (Cm S − C S )

(4.20c)

Figure 4.10 shows the function on the left-hand side of 4.20c as a function of ηS for five values of the constant at the right-hand side of Eqs. 4.20b or 4.20c. Figure 4.11

4 Nanostructured Abrasive Materials for Ultraprecision Finishing …

127

is a plot for negative values of z (Eq. 4.20c). The function (y) exists only for ηS < 6, and is very unstable for ηS ~ 0. It should be noted that the solutions are points on the curves for different values of z when y = z. This is shown in Fig. 4.10. For example, there is no solution for z = 0.1, since the horizontal line at y = 0.1 does not intersect the curve at z = 0.1. There is a solution for ~z ≥ 1. However, when z = 4, the solution becomes unstable since small variations in x(ηS ) leads to a large change in y. On the other hand, there is no solution for z < 0 (Fig. 4.11). For ηS > 0 all values y > 0, then y = z (Eqs. 4.20b or 4.20c) and is in contradiction. If C mS = 0, or C mS = C S , then z < 0. Figures 4.13 and 4.14 are the same as Figs. 4.11 and 4.12, with the difference that negative parts of x and y axes are present. Positive x axis (ηS > 0), according to Fig. 4.1, is for melting of the solid phase, while negative x axis (ηS < 0) is for solidification of the melt on the solid phase. It can be seen from Fig. 4.12 that there is no solution for ηS < 0, since values of y are negative, while z > 0. This renders Eq. (4.20c) contradictory. For z < 0, values of y are positive for all values of ηS , and hence, (4.20c) is always in contradiction. The conclusion is that there is a theoretical possibility of solution to the diffusion equation in the solid phase, and it is only for melting of the solid. One might note that this would not be feasible, since the melting rate is most likely greater than the diffusion rate. It will be proven by comparing with the values of ηL (ηS > 6), when there is no solution for the diffusion in the solid phase. Equation 17 for the liquid phase can be rearranged: √ (CL − CS ) e−ηL = π ηL (1 + erf ηL ) (CmL − CL )

(4.21a)

x  = ηL

(4.21b)

2

e−ηL ηL (1 + erf ηL ) 2

y = z =

√

π

(CL − CS ) (CmL − CL )

(4.21c) (4.21d)

It can be seen in Fig. 4.14 that the diffusion in the liquid phase for z > 0 has solutions for all y values only when x  > 0 (melting). It is evident from Fig. 4.14 that diffusion in the liquid can be described only when η\L < 1. In that case any z -curve has a corresponding y -ordinate. Now from Eq. 4.17a, we can relate ηS with ηL (Fig. 4.15):  ηS = Using usual ratio DL /DS = 103 to 104 :

DL ηL DS

(4.22a)

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ηS = (30..100)ηL

(4.22b)

From Fig. 4.10, the maximum value for ηS ~ 5 to 5.5: ηL ≤ 0.05..0.17

(4.22c)

In order, diffusion in the solid phase can have a solution. Equation 4.22c applies to very low growth rates (Eq. 4.12a), and it is unstable in regions prone to perturbations. Since determining DS from Eq. 4.19b has proven to be not feasible, the most appropriate course is to estimate Eq. 4.14b and check its feasibility in determining DS . Typically, the liquid diffusion coefficient is between 1000 and 10,000 times the magnitude of the diffusion coefficient, and the maximum value of the square root of their ratio is:   DS 1 ≈ 0.01 (4.23a) ≈ DL 10, 000 The minimal ratio being:  DS ≈ DL



1 ≈ 0.03 1000

(4.23b)

Using Eq. 4.12b and assuming Eqs. 4.23a and 4.23b applied, we can estimate Boltzmann substitution at the interface, and its minimum value is: (ηL )max ≈ 0.01 · ηS

(4.23c)

Fig. 4.10 Plot of (y) (Eq. 4.19a) as a function of (x) (Eq. 4.20b) for positive values of z (Eq. 4.20c)

4 Nanostructured Abrasive Materials for Ultraprecision Finishing …

129

Fig. 4.11 Plot of (y) (Eq. 4.20a) as a function of (x) (Eq. 4.20b) for negative values of z (Eq. 4.20c)

Fig. 4.12 Plot of y (Eq. 4.20a) as a function of x (Eq. 4.20b) for positive values of z (Eq. 4.20c). Figure 4.10 is only a portion of this plot for x > 0

The maximum value being: (ηL )min ≈ 0.03 · ηS

(4.23d)

Using Eq. 4.23c, Eq. 4.23d can be rearranged to look similar to: √ (CL − CS ) e−(0.03·ηS ) e−ηS (CmL − CL ) + = π (CmS − CS ) (0.03 · ηS )(1 + erf(0.03 · ηS )) ηβ (1 − erfηS ) (CmS − CS ) (4.24a) 2

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Fig. 4.13 Plot of y (Eq. 4.20a) as a function of x (Eq. 4.20b) for negative values of z (Eq. 4.20c). Figure 4.11 is only a portion of this plot for x > 0

y' z' > 0

x'

Fig. 4.14 Plot of y (Eq. 4.21c) as a function of x  (Eq. 4.21b) for positive values of z (Eq. 4.21d). The red box represents the range of allowed solutions. Since there is no solution for z < 0 (solid), the case z < 0 (liquid) is analyzed, except by providing a plot of that case (Fig. 4.15)

The left-hand side is a transcendental function that depends solely on the value of ηS : e−(0.03·ηS ) e−ηS (CmL − CL ) + = y(ηS ) (CmS − CS ) (0.03 · ηS )(1 + erf(0.03 · ηS )) ηβ (1 − erf ηS ) 2

(4.24b)

4 Nanostructured Abrasive Materials for Ultraprecision Finishing …

131

y' z' < 0

x'

Fig. 4.15 Plot of y (Eq. 4.21c) as a function of x  (Eq. 4.21b) for negative values of z (Eq. 4.21d). The red box represents the range of allowed solutions (ηL > −0.5)

The right-hand side of Eq. 4.24a is constant, depending on the experimental conditions (molten alloy used, constant temperature, and the composition of the immersed solid: √ (CL − CS ) = z  π (CmS − CS )

(4.24c)

  In the function, y ηβ , there is a constant that depends on the experimental conditions: (CmL − CL ) =A (CmS − CS )

(4.24d)

y(ηS ) = z

(4.24e)

Equation 4.24a has the form:

As before, this function can be plotted versus ηS , but now it depends on more free parameters: z (Eq. 4.24c), A (Eq. 4.24d), and the Boltzmann substitution for liquid which usually ranges from (Eq. 4.23c) to (Eq. 4.23d). Figures 4.16 and 4.17 are shown to illustrate that it is possible to find sets of experimental parameters (parameters A and z) when it is possible to determine DS . However, there are also sets of parameters when it is hard to find a solution to DS . Diffusion in the liquid and the solid are analyzed separately in this text using Eq. 4.14d with the following assumptions:

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z = - 10 z = - 10

z = - 0.1

z = - 0.1

Fig. 4.16 Plot of y(ηS ) versus ηS for A = +0.5 and negative values of z (−0.1, −1, −2, −4, − 10). The solution is possible to find for all negative ηS (growth from the melt)

z = 10

z = 0.1

z = 10

z = 0.1

Fig. 4.17 Plot of y(ηS ) versus ηS for A = −0.2 and positive values of z (+0.1, +1, +2, +4, +10). The solution is possible to find for all values of ηS (both growth from the melt and melting of the solid into the melt)

• Assuming DS = 0 implies that C mS = C S, which renders the second term on the left-hand side equal to zero, leading to Eq. 4.19a. The diffusion in the liquid is analyzed to estimate ηL ; • Assuming C mL = C L (complete mixing in the liquid, DL = ∞) renders the first term of 4.14d to be zero, reducing Eq. 4.14d to Eq. 4.18. In this context, ηL is estimated.

4 Nanostructured Abrasive Materials for Ultraprecision Finishing …

133

It is possible to obtain solutions for certain experimental combinations of temperatures and compositions of molten alloy. However, the method is used extensively in the development of abrasive materials in order to control the microstructure, so that abrasive grits can be controlled in terms of their cutting and frictional sliding abilities based on the workpiece being ground and environmental conditions.

References 1. Jackson MJ (2020) Recent advances in ultraprecision abrasive machining processes. SN Appl Sci 2(7):1–26. https://doi.org/10.1007/s42452-020-2982-y 2. Subramanian K et al (2017) Microscopic interactions in surface generation processes using abrasive tools. ASME J Manuf Sci Eng 139:1–17. 3. Suh NP (1986) Tribophysics. Prentice-Hall, Englewood Cliifs 4. Subramanian K (1990) Finishing using multiple cutting edges. ASM Handbook 5:107 5. Malkin S, Guo C (2008) Grinding technology: theory and application of machining with abrasives. Industrial Press, New York, pp 319–321 6. Lindsey RP (1986) Principles of grinding. In: King R, Hahn R (eds) Handbook of modern grinding technology. Chapman and Hall, New York, pp 30–71 7. Linke BS et al (2017) Grinding energy modelling based on friction, plowing and shearing. ASME J Manuf Sci Eng 139:1–11 8. Subramanian K (2016) Bringing the science to shop floor manufacturing. Eff Manuf 42–46. https://www.industr.com/en/EM-Magazine/_storage/asset/1984959/storage/ master/file/13764293/EM%20Feb%202016.pdf. Accessed 27 July 2020 9. Csillag F (2012) Engineered to cut above all. In: Saint-Gobain symposium on grinding science, Northborough R&D Laboratory, Saint-Gobain Norton Abrasives, Massachusetts, USA, 8 Nov 2012 10. Saint-Gobain Abrasives (2019) AZ-40 for coated abrasives product codes: 1565 and 1575. Saint-Gobain Abrasive Materials. Accessed 27 July 2020 11. Saint-Gobain Abrasive Materials (2019) Cerpass DGE from https://www.abrasivematerials. saint-gobain.com/sites/imdf.abrasivematerials.com/files/cerpass_dge_for_bonded_abrasive_ tools_70357.pdf. Accessed 27 July 2020 12. Diamond Innovations (2020) Borazon CBN from https://myaccount.diamondinnovations.com/ en/product/mbs/cbn/b.htm. Accessed 27 July 2020 13. Batako ADL, Morgan MN, Rowe BW (2013) High efficiency deep grinding with very high removal rates. Int J Adv Manuf Technol 66(9–12):1367–1377 14. Salmon SC (2004) Creep-feed grinding is surprisingly versatile. Manuf Eng 133(5) 15. Jackson MJ, Hitchiner MP (2014) High performance grinding and advanced cutting tools. In: Springer Briefs in applied sciences and technology. Springer Nature, New York, pp 45–94 16. Benes J (2007) All about abrasives: an array of abrasive-grit types meets any grinding or finishing requirement. American Machinist, from https://americanmachinist.com/features/allabout-abrasives. Accessed 27 July 2020 17. Marinescu ID, Rowe WB, Dimitrov B, Ohmori H (2012) Tribology of abrasive machining processes. William Andrew, pp 369–456 18. Subramanian K, Jain A, Rajagopal V, Bushan BM (2015) Tribology as an enabler for innovation in surface generation processes. In: Proceedings of the ASME 2015 international mechanical engineering congress and exposition (IMECE 2015), Houston, Texas, USA, 13–19 Nov 2015 19. Subramanian K, Ramanath S, Tricard M (1997) Mechanisms of material removal in the precision production grinding of ceramics. J Manuf Sci Eng 119(4A):509–519 20. Subramanian K (1988) Precision finishing of ceramic components with diamond abrasives. Ceramic Bull 1–9

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21. Subramanian K, Webster JW, Caputa P (2010) Method for grinding complex shapes. U.S. Patent No. 7708619 B2, Assignee: Saint-Gobain Abrasives, Worcester, MA (US) 22. Besse JR, Graham DC, Subramanian K, Ramanath S, Lamoureux MA (2014) Abrasive tool and a method for finishing complex shapes in workpieces. U.S. Patent No. 8911283 23. Besse JR, Graham D (2009) Grinding turbine rotors has advantages. Modern Machine Shop. https://www.mmsonline.com/articles/grinding-turbine-rotors-has-advantages. Accessed 27 July 2020 24. Willcutt R (2020) Grinding big gears from blanks. Article From: 3/2/2015 Modern Machine Shop. https://www.mmsonline.com/articles/grinding-big-gears-from-blanks. Accessed 27 July 2020 25. Hitchiner M, Graham D, Plainte P (2013) Advances in abrasive technology for grinding gears from solid. Gear Solutions. https://www.gearsolutions.com/article/detail/6368/advances-in-abr asive-technology-for-grinding-gears-from-solid#comments. Accessed July 2020 26. Hitchiner M (2015) Precision grinding faster than machining. In: United Grinding North America Grinding conference, Miamisburg, OH, USA, 16–17 Sept 2015, pp 1–32 27. Ahmed W, Jackson MJ (2015) Emerging nanotechnologies for manufacturing – 2nd Edition. Elsevier, New York, pp 1–551 28. Jackson MJ, Ahmed W (2018) Micro and nanomanufacturing, vol II. Springer-Nature, New York, pp 1–570 29. Badger J (2009) Factors affecting wheel collapse in grinding. CIRP Ann 58:307–310 30. Hitchiner MP, Hunt JD (1985) Solidification processing of fused alumina and associated experimental and mathematical analyses. Universal Abrasives Limited Internal Report, Stafford, pp 1–253 31. Bohr S (2017) Analytical considerations for highly porous bonding systems. Norton SaintGobain Abrasives: https://www.nortonabrasives.com/en-emea/resources/expertise/analyticalconsiderations-highly-porous-bond-systems. Accessed 28 July 2020 32. Huber C (2012) Innovative grinding wheels for a cooler grinding strategy. Seminar “Moderne Schleiftechnologie und Feinstbearbeitung”, Germany 33. Bohr S (2019) High Performance grinding—rapid development of various technologies. In: United Grinding Group symposium, pp 44–46. https://cdn.grinding.ch/fileadmin/content_live_ 2019/www.grinding.ch/01_pdf/10_motion/01_2019_motion/Motion_01_2019_EN_72_dpi. pdf. Accessed 28 July 2020

Chapter 5

Self-assembled Growth of GaN Nanostructures on Flexible Metal Foils by Laser Molecular Beam Epitaxy S. S. Kushvaha and M. Senthil Kumar

Abstract Low-dimensional semiconductor structures such as thin films, nanorods, nanowires and zero-dimensional (0-D) quantum dots or islands possess exotic electrical and optical properties compared to their bulk counterpart. Here, 1D nanorods draw a special attention due to their high aspect ratio for potential applications in the field of sensors and other nanoscale devices. Among various semiconductors, GaN having a direct wideband gap has stimulated a great deal of research interest due to the applications in the area of light-emitting diodes, solar cells, high-power electronics devices, laser diodes, ultraviolet photodetectors and water splitting, etc. Due to the advances in flexible or wearable optoelectronic devices, it is required to fabricate inorganic semiconductors hybrid devices directly on flexible substrates in near future. Here, we report the direct growth of various GaN nanostructures such as islands, thin films and nanorods on variety of flexible metal foils using laser-assisted molecular beam epitaxy (LMBE) technique and studied their structural and optical properties. Cubic and wurtzite mixed-phase GaN thin film and island structures have been obtained on the thin Cu and graphene/Cu metal foils under nitrogen-rich growth condition. Interestingly, the growth of high optical quality wurtzite GaN nanorods on bare and nitridated W foil is achieved at a low temperature of 600 °C. Vertically self-aligned GaN nanorods are successfully grown on flexible Ti metal foils at growth temperature of 650–700 °C by tuning the pre-nitridation condition of Ti foils. On the other hand, vertically well-oriented, high-density GaN nanorods have been achieved at 700 °C on bare Ta foil without any surface treatment. Raman spectroscopy, highresolution X-ray diffraction and high-resolution transmission electron microscopy studies revealed the c-axis growth of high structural quality wurtzite GaN nanorods on these flexible metal foils. The photoluminescence spectroscopy measurements exhibit a near band edge emission around ~3.4 eV with a full width at half maximum value of ~100 meV for densely grown GaN nanorods. Our studies disclosed that various GaN nanostructures were grown directly on different flexible metal foils by S. S. Kushvaha (B) · M. Senthil Kumar CSIR- National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India e-mail: [email protected] Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_5

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S. S. Kushvaha and M. Senthil Kumar

tuning the surface treatment process by LMBE technique for developing futuristic flexible optoelectronics devices. Keywords Gallium nitride · Nanostructures · Flexible foil · Laser molecular beam epitaxy · Field emission scanning electron microscopy · Raman spectroscopy · Photoluminescence spectroscopy · High-resolution transmission electron microscopy

5.1 Introduction Among various semiconducting materials, group III-nitride materials (InN, GaN, AlN) are the well-known semiconductors for various applications in the field of light-emitting diodes (LEDs), high-electron mobility transistors (HEMTs), ultraviolet (UV) photodetectors, laser diodes (LDs) and solar cells [1–14]. Especially, GaN has a wide and direct band gap of 3.4 eV which can be tuned between 0.7 and 6.2 eV by proper alloying with indium and aluminum metals with a minimum lattice mismatch. Consequently, it will cover a large electromagnetic spectrum for various optoelectronics applications [15]. Apart, GaN possesses high mechanical strength, radiation hardness, high saturation velocity, large critical electric field (3 MV/cm), stability at high-temperature operation and chemical stability. High bright GaN blue LED devices are the backbone for development of recent solid-state lighting [11, 16]. Due to miniature in size, high efficiency, lightweight and long lifetime, GaN-based LEDs have a wide range of fruitful applications in the field of traffic lights, displays, automobiles, room lighting, ultraviolet lights for water purification, etc. Currently, these III-nitrides can be used for their future applications in the field of HEMTs; especially AlGaN/GaN-based two-dimensional electron gas (2DEG) is studied toward quantum hall resistance and high-power electronic applications [13, 17–19]. Due to the robust nature, GaN-based power electronics and HEMTs can be used in space, satellite and military as well as in extreme harsh environmental conditions [20, 21]. The GaN-based devices are mostly fabricated on various other substrates due to unavailability of large GaN wafers. For example, the single crystal growth of GaN with size of 6–10 mm requires a high-temperature (1100–1200 °C) process in the presence of high-pressure nitrogen for more than 60–100 h, and it is similar to grow diamond [22]. To overcome with this issue, mostly hydride vapor phase epitaxy (HVPE), metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) techniques have been employed to grow large size GaN on various foreign high lattice mismatched substrates [23–30]. However, these epitaxial growth techniques have their limits and advantages. For example, the GaN growth rate in HVPE process is very high (10-100 μm/h). On the other hand, it requires hazardous gases/chemicals and high growth temperature process (1000–1100 °C) and very difficult to grow monolayer thin film [25, 26]. In case of MOCVD, the GaN growth rate is moderate, and it requires high growth process (800–1100 °C) with involvement of

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various chemical processes [23, 24]. In this respect, plasma-assisted MBE has advantages over these techniques as it is capable of growing monolayer thin film with sharp interface at moderate growth temperature of 750–850 °C [29, 30]. These conventional growth techniques employ a high/moderate temperature for the growth of GaN and their alloys which limit the choice of use of closely thermal- and lattice-matched substrates like single metal crystals, ZnO, etc., since they are reactive at temperatures above 600–700 °C [31]. Hence, there is a high demand to use a low-temperature GaN growth process on unconventional substrates to explore the advanced applications and further improve the device performance. In this respect, laser-assisted molecular beam epitaxy (LMBE), i.e., the combination of MBE and pulsed laser deposition (PLD), is one of the low-temperature growth techniques for GaN-based materials as film precursors produced by laser ablation have high-kinetic energized Ga and GaN1-x adatoms or species [32–34]. Low-temperature LMBE growth of epitaxial GaN has been reported on various substrates such as silicon, sapphire and small lattice-mismatched ZrB2 and SiC substrates [35–38]. The GaN-based LED structures mostly grown on sapphire and silicon-based substrates possess a high density of threading dislocation (TD) and stress in the films due to large lattice and thermal mismatch [39]. The dislocation and stress in the films can be well controlled by introducing substrate nitridation, buffer layers, etc., in the growth process or by employing lattice-matched substrates for the growth [37, 38]. In this respect, metallic substrates have drawn a special interest recently for fabrication of GaN-based LED structures [40, 41]. Some of the metallic substrates such as Mo (110), W (110) and Al (111) have lower lattice mismatch compared with Si and sapphire [40]. Due to their high thermal conducting nature, the heat generated in LED can be reduced which can lower the temperature of devices, and subsequently, the lifetime of LED will be improved. Another benefit of metallic substrates includes the strong reflection of light and excellent electrical properties which improve the light extraction efficiency and direct cathode fabrication in LED devices, respectively [40– 43]. However, the limitation of metallic surfaces includes the unstability/diffusion of surface atoms at high temperature, expensive single crystal metal substrates and reactive/interfacial compound or alloy formation at high growth temperature [44]. Alternatively, several researchers use the stable AlN, HfN, etc., buffer layers to efficiently block the metal atom diffusion and interfacial reactions at the interface of GaN and metals [40, 45]. Recently, flexible electronic and photonic devices based on III-nitrides have immensely attracted due to their roll-to-roll processing with new functionalities, and these are not possible on conventional substrates [46]. In this respect, the selection of flexible substrates for GaN-based devices is crucial as the growth of GaN requires high growth temperature which limits the use of organic- or polymer-based flexible substrates. In recent years, flexible thin metal foils have drawn special interest for the fabrication of GaN-based LED devices due to their enhanced photon-harvesting, low-cost, high thermal and electrical conductivity, flexibility and scalability. The LEDs were fabricated using III-nitride nanowires (NWs) on metal-coated substrates; however, it required transferring on stretchy substrates for development of roll-to-roll flexible devices [47]. In this respect, the growth of GaN nanostructures on flexible

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metal foils is one of the alternate ways toward fabrication of large-scale roll-to-roll flexible devices [48–51]. Recently, GaN NWs-based ultraviolet LEDs grown directly on flexible Ta metal foil at a temperature of 700 °C have been reported, and more applications remain to be demonstrated in near future [51]. GaN nanostructures such as thin films, NWs, nanowalls and nanocolumns are gaining a significant research interest due to their large surface to volume ratio. Onedimensional (1D) GaN nanostructures such as NWs, nanorods (NRs) and nanotubes (NTs) have enormous potential as a building block for various nanoscale electronic and optoelectronic devices due to their exciting electronic, exotic physical, electrical and optical properties [49, 52]. Several devices such as LEDs, transistors, lasers and photo-detectors have been successfully fabricated using 1D GaN NRs and NWs [53, 54]. The advantage of 1D NRs over other nanostructures is related with the large aspect ratios which favor the growth of low or nearly TD-free GaN NRs in the upper part as the growth proceeds [51]. In other words, TD-free GaN NRs growth is possible on other substrates in which the lattice mismatch is high. Alternatively, apart from conventional substrates, flexible metal foils can be also utilized for the development of GaN NRs-based devices due to enhanced optical reflection, reduction of ohmic loss, low cost, excellent thermal and electrical properties and flexible nature of the substrates [48–51]. Spontaneous formation of GaN NWs or NRs on flexible metal foils has been reported using conventional MBE and LMBE techniques [48–55]. Here, we report the LMBE growth and characterization of various GaN nanostructures such as thin films, island, random and vertical aligned NRs on various thin metal foils (Cu, W, Ti and Ta). In case of Cu and graphene/Cu metal foil, mostly GaN thin films and islands have been grown at 700 °C, and these films possess mixed wurtzite and cubic GaN structures. Randomly oriented wurtzite GaN NRs were grown on bare and nitridated W foil at 600 °C. Vertically self-aligned wurtzite GaN NRs have been obtained on Ta and Ti foils and show an intense band-to-band light emission at ~3.41 eV along with minimum optical defect levels.

5.2 Experimental Section Various GaN nanostructures were grown using LMBE system (base pressure: ~2 × 10–10 Torr) on variety of flexible metals foils under ultra-high vacuum (UHV) condition. LMBE system is equipped with RF nitrogen plasma cell to supply nitrogen radicals for the surface nitridation or GaN growth surface, residual gas analyzer, reflection high energy electron diffraction and various laser ablation targets [33, 35]. The polycrystalline solid GaN target (99.9999%) and semiconductor grade nitrogen gas were used as the precursors for GaN. The substrates were heated inside the main growth chamber using a resistive heater. The metal foils (Cu, W, Ta and Ti) were cleaned using acetone and isopropyl-alcohol followed by de-ionized water. The metal foils were then degassed for few hours at 230–250 °C in the load lock chamber. The graphene/Cu foil was directly loaded in the load lock chamber after chemical

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vapor deposition (CVD) of graphene on Cu foil. The Cu and graphene/Cu substrates were thermally cleaned at 750 °C, whereas metal foils (W, Ta and Ti) were thermally cleaned at 850 °C in the growth chamber for 10–30 min using resistive heater. The pre-nitridation of metal foils was performed at 800–850 °C for 10–30 min using RF nitrogen plasma with RF plasma power of 400 W and N2 gas flow of 1.1 sccm. A KrF excimer laser (wavelength = 248 nm, pulse width = 25 ns) was used to ablate the solid GaN targets with an energy density of ~3–5 J/cm2 and a laser repetition rate ranging 10–30 Hz, depending on the requirement of the growth rate and nature of the foils. An RF nitrogen plasma source was used to supply nitrogen radicals during the ablation of GaN target to avoid any nitrogen deficiency [33, 35]. The liquid nitrogen was circulated to the cryo shroud during thermal cleaning, nitridation and GaN growth process to avoid any cross-contamination from growth chamber to growing surface of metal foils. The structural properties of the GaN nanostructures were characterized using xray diffraction (XRD), field emission scanning electron microscopy (FESEM) and Raman spectroscopy. A XRD system was employed to characterize the crystalline nature of LMBE grown GaN nanostructures on flexible metal foils using CuKα1 radiation. The plan- and 45° tilt view images of the LMBE grown GaN nanostructures on various metal foils were obtained using a FE-SEM operated at 5 kV. Raman spectra were collected in backscattering geometry at room temperature using an excitation laser source with wavelength of 514.5 nm. The high-resolution transmission electron microscopy (HR-TEM) characterization was performed with an operating voltage of 300 kV on the dispersed GaN NRs on Cu grid. The optical properties of GaN nanostructures were characterized with photoluminescence (PL) spectroscopy using solid-state semiconductor or He-Cd lasers excitation source.

5.3 Result and Discussion The key factor for a controlled growth of GaN nanostructures depends on several parameters such as the nature of metal foil, buffer layer and nitridation of the metal foil, as well as the LMBE growth parameters. Here, we have studied the physical properties of GaN nanostructures grown on Cu, W, Ti and Ta thin metal foils using LMBE technique by introducing pre-nitridation/buffer layer on metal foils. The detailed study of the GaN grown on these metal foils is discussed in Sects. 5.3.1 to 5.3.4.

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5.3.1 Growth of GaN Islands and Films on Cu and Graphene/Cu Foils GaN was grown on flexible Cu foil, graphene (Gr)/Cu and low-temperature (LT) grown GaN buffer/Gr/Cu foils at 700 °C under N-rich growth condition using LMBE technique. Figure 5.1 represents Raman spectrum of CVD graphene on thin Cu foil, and it consists of three peaks: a high intensity peak at 2674.62 cm−1 , a relatively low intensity peak at 1581.08 cm−1 and a lowest intensity peak at 1339 cm−1 [56–58]. These peaks correspond to 2D-band, G-band and D-band of graphene confirming the growth of graphene on the Cu foil [56]. The 2D peak observed in this spectrum is broader and has full width at half maximum (FWHM) of ~ 39.5 cm−1 (>24 cm−1 ), reflecting the heterogeneity nature of the polycrystalline surface of Cu foil used. However, the low intensity for defect-related D-band in spectrum affirms a less number of defects within the graphene [56]. Intensity ratio for 2D to G peaks (I2D /IG ) is obtained as 2.784 (>2.0) which discloses the growth of monolayer graphene [57]. Inset of Fig. 5.1 represents the schematic of graphene/Cu metal foil which was used to grow GaN with and without LT-GaN buffer layer. Figure 5.2 (a) shows FESEM image of LMBE grown GaN on Cu foil, and it revealed the formation of uniformly distributed small-sized islands throughout the surface. Average size of GaN islands is about 100–120 nm. The Cu foil possesses polycrystalline grains/grain boundaries on its surface which may act as potential site for nucleation. Thus, when an adatom arrives on its surface, it has both chances, i.e., to undergo nucleation or get attached to an existing island at a nucleation site, resulting in the formation of uniformly distributed nano-sized islands. For GaN growth on Gr/Cu, large interconnected islands with random grain sizes of ~200–600 nm were obtained as shown in Fig. 5.2b. When GaN is grown on graphene, the low migration barrier of Ga or GaN adatoms on graphite surface promoted the formation of larger islands [56, 58]. For GaN growth on LT-GaN buffer/Gr/Cu foil, large terrace like structures were obtained (Fig. 5.2c). The LT-GaN buffer layer was grown on graphene/Cu foil at 500 °C for 10 min before the main growth under similar growth conditions. The Fig. 5.1 Room temperature Raman spectrum of CVD grown graphene on thin Cu metal foil. Inset: the schematic represents the graphene layer on Cu metal foil

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Fig. 5.2 45° tilt FESEM image of LMBE grown GaN at 700 °C on: a bare Cu: b Gr/Cu metal foil and c LT-GaN/Gr/Cu metal foil

GaN growth on LT-GaN buffer layer reduces the activation energy for atomic bonding between the grown film and buffer layer and a formation of smooth and large islands occurs [56]. Raman spectra of GaN on flexible Cu, Gr/Cu and LT-GaN/Gr/Cu foils are presented in Fig. 5.3. Several Raman scattering phonon modes for hexagonal wurtzite GaN such as A1 (LO), A1 (TO), E1 (LO), E1 (TO) and E2 (high) are available [58– 65]. The E1 (LO) and E1 (TO) modes are forbidden for c-axis orientation but these modes may appear for highly defective GaN nanostructures [60]. The E2 (high) and A1 (LO) are prominent in the backscattered geometry of GaN Raman measurement. The FWHM value, peak position and shift of E2 (high) peak position disclose

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Fig. 5.3 a Room temperature Raman spectra of LMBE grown GaN film on bare Cu, Gr/Cu and LTGaN/Gr/Cu metal foils. GaN-related Raman modes are denoted in the graph. b–d Lorentzian fitted Raman spectra of E2 (high) and c-GaN:TO for GaN film grown on Cu, Gr/Cu and LT-GaN/Gr/Cu metal foils, respectively

the crystalline nature, structural quality and stress present in GaN, whereas carrier concentration, electron mobility and doping level can be indirectly estimated with the characteristics of A1 (LO) peak [61, 62]. Here, one dominant Raman peak was observed for GaN grown on these modified Cu foils at ~570 cm−1 along with a clear shoulder peak at ~ 555 cm−1 . Prominent Raman peak positioned at ~570 cm−1 corresponds to E2 (high) mode of wurtzite GaN (h-GaN), whereas shoulder peak around 555 cm−1 is mostly observed for the cubic phase GaN (c-GaN) [58, 66–68]. For GaN grown on Cu and graphene/Cu foils, another additional Raman peak was obtained at ~732 cm−1 for A1 (LO) mode of GaN structure. These findings showed that GaN film on these surface-modified Cu foil possesses mixed phase (cubic and wurtzite), which is in good agreement with the other reports of GaN growth on graphene-based substrates [66, 69]. The peak position of GaN E2 (high) and transverse mode (TO) of c-GaN were deduced by using Lorentzian fitting of the experimental data as shown in Fig. 5.3b–d for all samples [58, 59]. Presence of wurtzite E2 (high) and c-GaN:TO confirms the

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mixed phase of GaN on these foils [58, 59]. The blue shift of E2 (high) peak represents the compressive stress, whereas red shift confirms the presence of tensile stress. The estimated Raman E2 (high) peak positions for LMBE grown GaN on bare Cu, Gr/Cu and LT-GaN/Gr/Cu foils are 570.05, 569.67 and 567.03 cm−1 , respectively. The in-plane biaxial stress is deduced by following equation [63, 64] σ =

ω − ω0 4.3

(5.1)

where ω is Raman shift of E2 (high) mode of grown GaN and ω0 is E2 (high) peak position for stress-free thick GaN film (567.6 cm−1 ) [63, 64]. The obtained stress values are 0.57 (compressive), 0.48 (compressive) and 0.13 GPa (tensile) for GaN films on bare Cu, Gr/Cu and LT-GaN/Gr/Cu foils, respectively. The nearly stressfree GaN growth on LT-GaN/Gr/Cu foil could be associated with the LT buffer GaN growth. The estimated c-GaN: TO peak positions for GaN film grown on Cu, Gr/Cu and LT-GaN/Gr/Cu foils are 555.05, 555.75 and 555.3 cm−1 , respectively, and these values are similar to the MBE grown GaN on Gr/Cu metal sheet [66]. The optical nature of LMBE GaN on Cu and Gr/Cu and LT-GaN/Gr/Cu foils was characterized by PL using 325 nm laser as an excitation source (Fig. 5.4a). The PL of hetero-epitaxial wurtzite GaN films mainly consists of two peaks: the main near band edge emission (NBE) peak at ~3.4 eV and defect-related broad peak in range of ~2.1–2.6 eV [70–72]. Here, a strong NBE emission peak at 3.37 eV arises due to the band-to-band transition for GaN film on Cu, Gr/Cu and LT-GaN/Gr/Cu foils. For GaN growth directly on Cu foil, an intense wide green luminescence (GL) band is observed at 2.67 eV due to luminescent center created at dislocation edges introduced by Ga and N vacancies [73]. In the PL spectrum of GaN grown on graphene/Cu foil, a yellow luminescence (YL) peak is found around 2.25 eV and the origin of YL peak is explained by the combination of Ga vacancy clusters and VGa -ON complexes, which arise due to the presence of defects or dislocations in the film [73]. To resolve the PL transition peak, we have fitted the PL data with Gaussian fitting, and it is presented in Fig. 5.4b–d for GaN on bare Cu, Gr/Cu and LT-GaN/Gr/Cu foils, respectively. The PL emission peaks at 3.26 and 3.40 eV were obtained for all samples which are related to c-GaN and h-GaN, respectively. For GaN growth on LT-GaN/Gr/Cu foil, the defect-related peak is obtained at 3.01 eV. Raman and PL spectroscopy studies revealed that the mixed-phase GaN was grown on bare Cu, Gr/Cu and LTGaN/Gr/Cu foils [58]. These observations reveal that the size of the GaN islands critically depends on graphene layer present on Cu foil.

5.3.2 Growth of GaN Nanorods on Flexible Tungsten Metal Foil Due to advancement in GaN-based flexible devices for the futuristic applications in the field of space, satellites, etc., and for harsh environment, there is a demand of

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Fig. 5.4 a Room temperature normalized PL spectra of LMBE grown GaN film on bare Cu, Gr/Cu and LT-GaN/Gr/Cu metal foils. b–d Gaussian fitted PL spectra for GaN film grown on Cu, Gr/Cu and LT-GaN/Gr/Cu metal foils, respectively

flexible substrates that can sustain in the harsh environment. The W metal is one of the suitable materials as it has a higher chemical and thermal stability which can resist forming any interfacial compounds in harsh environment [40]. Additionally, it possesses good optical reflectivities which can improved the light harvesting in LED and solar cells applications. The schematic of GaN growth on bare and pre-nitridated W foils at 600 °C is presented in Fig. 5.5. Inset of Fig. 5.5 shows the FESEM image of bare W foil after thermal cleaning, and it revealed a flat surface of W foil decorated with several pits and deep trenches. Figure 5.6a, b represents the 45° tilt FESEM images for GaN on bare and nitridated W foil grown by LMBE at 600 °C, respectively. It is interesting to observe the growth of GaN NRs instead of thin films or islands as we obtained for Cu foils. Several FESEM images have been analyzed, and the statistical analysis disclosed that the density, width and length of GaN NRs are ~ 3–4 × 109 cm−2 , 40–70 nm and 400–530 nm, respectively. For GaN growth on pre-nitridated W foil under the similar growth conditions (Fig. 5.6b), the pre-nitridation of W foil was performed at 850 °C for 40 min under RF nitrogen plasma. The statistical analysis revealed

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Fig. 5.5 Schematic diagram of GaN growth sequence on bare and nitridated flexible W metal foils using LMBE technique. Inset represents the FESEM image of thermally cleaned W foil at 850 °C under UHV condition

Fig. 5.6 45° tilt FESEM image after LMBE GaN growth at 600 °C on: a bare W and b pre-nitridated W metal foil

that the density, width and length of GaN NRs fall in range of ~2.4 × 109 cm−2 , 40–80 nm and 350–540 nm, respectively. In comparison with bare W foil, the degree of random orientation of GaN NRs is lower for the nitridated W foil, and it is due to polycrystalline nature of metal foil [48–51]. It has been reported that the nitridation of Ti metal foils promotes the aligned growth of GaN NWs due to reduction of surface roughness by the transformation of uppermost layers into TiN and TiOx Ny [50]. In

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Fig. 5.7 XRD 2 theta–omega scan of LMBE grown GaN nanorods on W metal foil. The inset shows the clear observation of GaN (0002) peak

case of W (110), the suppression of interfacial layer occurs when AlN film is grown on W (110) substrate at 450–600 °C [45]. The GaN NRs density obtained on bare W foil compared to the nitridated W foil is significantly higher as the nucleation occurs on grains/grain boundaries of the W foil, whereas the nitridation generally alters the surface composition and roughness of metal foil that may affect NR nucleation [49]. The crystalline property of GaN NRs was investigated using 2θ-ω scan XRD pattern, and it is presented in Fig. 5.7. It is seen that only GaN (0002) plane is observed along with some unidentified and W (110), (200) and (211) peaks. This observation reveals that the GaN NRs have grown along the c-axis on polycrystalline W foil. Figure 5.8a shows the TEM image of GaN NRs obtained on W foil and the measured width and length of GaN NR are in good agreement with the FE-SEM analysis. Further, HR-TEM image has been taken on a single GaN NR, and we clearly observed the atomic arrangement of GaN with the lattice interspacing of ~0.26 nm, similar to GaN (0001) (Fig. 5.8b). From XRD and TEM analysis, it is clear that the c-axis-oriented GaN NRs are grown on W foil by LMBE at low growth temperature. The possible mechanism for growth of GaN NRs has been also proposed [48– 51]. It is well known for plasma-assisted MBE GaN growth process that the N to Ga flux ratio primarily dictates the growth mode and the structure/roughness of GaN films. When Ga to N ratio flux is larger than 1, a smooth 2D thin-film growth occurs, whereas N-rich growth (N/Ga > 1) condition promotes a rough 3D or island growth mode [74, 75]. It is believed that the MBE and LMBE growth processes are very similar except the use of Knudsen cell for Ga flux in MBE instead of GaN target for laser ablation that produces Ga and GaN1-x species in LMBE [32]. In the LMBE growth process, N to Ga ratio will be more than one since a solid HVPE grown stoichiometric GaN target was laser ablated in the presence of additional supply of RF nitrogen plasma [76]. Recently, GaN NWs have been grown on polycrystalline Ti foil at 800 °C using plasma-assisted MBE under N-rich growth condition [51]. These observations disclosed that N-rich condition is essential for formation of GaN NRs.

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Fig. 5.8 a Typical bright field TEM image of LMBE grown GaN NR on W metal foil. b HR-TEM image of the single GaN NR revealing the c-axis growth of single crystalline structure

The random orientation of GaN NRs on W foil is related to the roughness and polycrystalline grains present on as-received W foil. The degree of orientation of GaN NRs on nitridated W foil improved likely due to the reduced surface roughness compared to bare W foil, similar to the GaN NWs on Ti foil after surface nitridation [50]. The orientation of GaN NRs mainly depends on the roughness as epitaxial relation between GaN and oriented metal grains are less sensitive to NR orientation and density [50]. The growth of GaN NRs is mainly driven by self-induced formation of NR nuclei with a critical radius under N-rich growth. Initial nucleation of GaN NRs on substrates plays the crucial role as it determines the density, diameter and length of the NRs [77]. The side walls of GaN NRs consist of non-polar m-plane and a-plane facets, the surface diffusion of Ga atom along these non-polar planes determines the growth direction (lateral or vertical) of NRs, and the low nucleation energy along c-axis promotes the polar c-axial growth [78, 79]. The diffusion length of GaN species on active growth substrate is restricted under excess nitrogen atoms which form 3D islands under N-rich growth condition [49, 77, 80]. The 3D islands with various shapes will transform from hemispherical to tapered shape and finally 1D elongated GaN NRs, depending on the surface activation energy and surface diffusion [77–80]. Further, the elongation of GaN NRs is mostly determined by the surface diffusion of Ga adatom as well as the direct impinging of the flux on the top of NR facet [77]. Generally, the GaN NRs are formed at a growth temperature ≥700 °C in MBE process. However, the low-temperature (600 °C) growth of GaN

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NRs on W foil is related to the generation of high energetic Ga and GaN1-x species by laser ablation of GaN target in LMBE process [32, 79]. Figure 5.9 shows the optical quality of GaN NRs on bare and pre-nitridated W foil measured using room temperature PL spectroscopy. In both cases, the sharp NBE peaks are obtained at ~3.41 eV, similar to bulk GaN [70]. The intensity of NBE peak for GaN NRs grown on nitridated W foil is lower in comparison with GaN NRs grown on bare W foil, and it is likely related to the low density of GaN NRs obtained on nitridated W foil (Fig. 5.9a). To further understand the optical quality of GaN NRs, NBE peaks have been analyzed with the help of Lorentzian fittings. The FWHM values of NBE peaks for GaN NRs on bare and nitridated W foils are nearly same about 100 meV and are comparable to GaN NWs grown on gold-coated Si (111) using MOCVD technique [81]. The defect-related broad YL peaks centered at 2.18 eV have been observed in both cases for the GaN NRs grown on bare and nitridated W foils. The origin of YL peak can be understood by radiative transition from shallow donors or conduction band to deep accepter bands on top of the valance band [79, 82, 83]. The peak intensity ratio of NBE to YL for these GaN NRs is ~10 with a narrow NBE peak FWHM of ~100 meV at room temperature which revealed Fig. 5.9 Room temperature PL spectra of LMBE grown GaN at 600 °C on bare and nitridated W foils: a As-measured PL and b normalized PL spectra

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the excellent optical quality. It showed the capability of LMBE to produce highquality GaN NRs at relatively low temperatures on flexible metal foils for futuristic flexible optoelectronics devices [51]. We have also studied the polarity of GaN NRs using wet chemical etching at room temperature with 5 M KOH aqueous solution. The FESEM images of etched GaN NRs for etching time of 20 and 30 min are presented in Fig. 5.10a, b, respectively. The statistical analysis on different FESEM images disclosed that the density, width and length of etched GaN NRs for 20 min are ~2 × 109 cm−2 , 40–65 and 250– 400 nm, respectively [79]. For wet KOH etching for 30 min, the density, width and length of the etched GaN NRs fall in the range of ~1.2 × 109 cm−2 , 35–55 nm and 150–330 nm, respectively. The shape and size of the etched GaN NRs changed after wet chemical etching and particularly, the shape changed to pencil-like structure and length of NRs decreases with etch time. With further increase of 5 M KOH wet etching to 2 h, we only obtained bare W foils. It is well established for GaN that the KOH wet etching changes the surface features/roughness for N-polar GaN, whereas almost no change in surface morphology is observed for Ga-polar GaN structures [48, 84]. These observations revealed the growth of N-polar GaN NRs on W foil by LMBE technique [48, 79, 84]. Fig. 5.10 45° tilt view FESEM images after wet chemical etching of GaN NRs on W foil at room temperature using 5 M aqueous KOH solution for duration of: a 20 min; b 30 min

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5.3.3 Growth of GaN Nanorods on Flexible Titanium Metal Foil In order to extend the study of GaN on flexible foils, we have employed 0.127mm-thick Ti metal foil, similar to the previous reports [50, 51]. As per the available literature, Ti foil has been mostly used for the growth of GaN NRs for futuristic flexible LED device fabrications due to its ohmic contact nature with GaN [51]. It has been found that the substrate nitridation plays a crucial role in determining the orientation and density of the GaN NWs on Ti metal foils [50, 55]. While sparse GaN NRs were grown on bare Ti, highly dense GaN NRs had been obtained on pre-nitridated Ti metal foil [50, 55]. However, the density of the GaN NRs critically depends on the pre-nitridation temperature of Ti metal foil [55]. The formation of discrete and probe-shaped GaN NRs is found at 700 °C on pre-nitridated Ti foil [49]. Compared with the plasma-assisted MBE growth of GaN NWs on Ti metal foils at 730–800 °C [48, 50, 51], the LMBE technique offers low-temperature growth. The effect of pre-nitridation time on the quality of GaN on Ti foil has been studied here. Keeping the substrate temperature (700 °C) and plasma parameters (1.1 sccm nitrogen flow, 400 W) constant, the Ti nitridation has been carried out for different durations such as 20 and 40 min. Figure 5.11a shows the 45° tilt view FESEM image after LMBE growth of GaN at 650 °C on Ti metal foil which was pre-nitridated for Fig. 5.11 45° tilt view FESEM images of LMBE grown GaN NRs at 650 °C on pre-nitridated Ti foil and the pre-nitridation time durations are a 20 min; b 40 min

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20 min. Mostly, individual along with few coalesced NRs has been obtained on Ti metal foil, and the statistical analysis revealed that the density, width and length of GaN NRs obtained are ~4.5 × 109 cm−2 , 50–100 nm and 100–200 nm, respectively. For 40 min pre-nitridation under the similar growth condition, GaN NR growth with improved surface coverage is obtained on Ti metal foil as shown in Fig. 5.11b. It clearly shows two different features: boundary wall-type coalesced GaN NRs and individual GaN NRs with a length of ~220 nm [49]. Statistically, density of the individual GaN NRs is ~6.2 × 109 cm−2 which is higher than the density obtained on 20 min nitridated Ti metal foil. The possible growth mechanism is related to the nitridation of topmost Ti layer for a long time as well as the composition/orientation of TiNx grains for the nucleation of GaN nanostructures [49, 50]. The structural and stress analysis of GaN on pre-nitridated Ti metal foils for 20 and 40 min durations have been analyzed from the Raman studies, and the related spectra are presented in Fig. 5.12a. Presence of E2 (high) peak resembles the wurtzite phase of GaN, and it is highly sensitive to the biaxial stress [61, 62]. The peak positions (FWHM) of E2 (high) for the samples grown on 20 and 40 min nitridations are found to be 566.41 (6.32) and 566.66 (5.98) cm−1 (cm−1 ), respectively. The E2 (high) mode was shifted toward lower side with respect to stress-free for both the samples, and the estimated tensile biaxial stress using Eq. 5.1 disclosed that sparse GaN NRs on Ti Fig. 5.12 a Raman spectra and b PL spectra of LMBE grown GaN NRs at 650 °C on pre-nitridated Ti foil at 700 °C for duration of 20 and 40 min

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foil for 20 min nitridation has stress of 0.28 GPa which is slightly high as compared with the GaN on 40 min pre-nitridated Ti metal foil (0.22 GPa). The optical nature of GaN NRs on 20 and 40 min nitridated Ti metal foils is studied using PL spectroscopy, and the graphs are presented in Fig. 5.12b. Both samples exhibit a high intensity NBE emission peak around 3.41 eV, similar to bulk GaN [70]. The NBE FWHM values are 240 and 205 meV corresponding to the GaN NRs grown on 20 and 40 min nitridated Ti metal foil, respectively. These observations disclose that the optical properties of GaN NRs are enhanced with nitridation time of Ti metal foil under the similar growth conditions. With the pre-nitridation time of 20 min, we increased the Ti nitridation temperature to 800 °C to see the effect of nitridation temperature on quality of GaN growth at 700 °C. In addition, the influence of low-temperature (LT) GaN buffer grown at 600 ºC has also been examined in details. Figure 5.13a, b shows the surface morphology of LMBE grown GaN on the pre-nitridated Ti foil without and with LT-GaN buffer layer, respectively. Vertically oriented GaN NRs were obtained on the Ti foil without LTbuffer layer. The density, width and length of GaN NRs are obtained from statistical analysis of several FESEM images as ~3.47 × 109 cm−2 , 90–150 nm and 220– 320 nm, respectively [55]. It is noted that the dimension of NRs increased with increase of nitridation temperature from 700 to 800 °C but with a slightly reduced aerial density. In case of LT-GaN buffer on nitridated Ti foil, surprisingly, GaN film Fig. 5.13 45° tilt view FESEM images of LMBE grown GaN NRs at 700 °C on pre-nitridated Ti foil at 800 °C: a without any LT-buffer; b with LT-GaN buffer

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consisting of nano-islands was obtained instead of GaN NR growth (Fig. 5.13b). The lateral size of GaN islands was measured using FESEM images in range of 50–170 nm. The growth of island-like GaN thin film can be understood in terms of a large amount of buffer GaN nucleation at low temperature and subsequent lateral growth at 700 °C [49]. In case of GaN NRs grown without any buffer layer on pre-nitridated Ti foil, the NBE peak position and FWHM value are calculated as 3.42 eV and 230 meV, respectively, from PL spectrum (Fig. 5.14a). PL spectrum of GaN island-like film grown on pre-nitridated Ti metal foil with LT-GaN buffer is given in Fig. 5.14b. The intense NBE peak position is obtained at 3.36 eV which is 40 meV red shifted in comparison with the bulk GaN (3.4 eV) [70]. The FWHM value of NBE peak is estimated to be 340 meV using Lorentzian fitting of NBE PL peak. The broadening of NBE peak toward lower energy is related to point defects in GaN film during coalescence process [55, 85]. These observations suggest that morphology of GaN nanostructures can be tuned by appropriately modifying the surface condition of Ti metal. For example, vertically oriented GaN NRs can be achieved on nitridated Ti foil, whereas a compact GaN island film is achievable on GaN buffer layer grown Ti foil. Fig. 5.14 Room temperature PL spectra of LMBE grown GaN NRs at 700 °C on pre-nitridated Ti foil at 800 °C: a without any LT-buffer; b with LT-GaN buffer

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5.3.4 Vertical Self-aligned GaN Nanorods on Tantalum Metal Foils Self-induced vertically oriented GaN NRs have been grown on 0.127-mm-thick Ta metal foils using LMBE technique with laser repetition rate of 20 Hz at an optimized temperature of 700 ºC without any buffer layer. The effect of pre-nitridation of Ta metal foil on the formation of GaN NRs is also discussed. Ta nitridation was performed using an RF plasma supply with a nitrogen partial pressure of ~6.5 × 10–5 Torr at a plasma power of 400 W for 30 min. The GaN growth was carried out for duration of 2 h by laser ablation of GaN target on Ta foil without and with nitridation under RF plasma ambient. GaN surface morphology on bare and nitridated Ta foil was examined using FESEM as shown in Fig. 5.15a, b, respectively. As seen, a high density of vertical self-aligned GaN NRs is observed on Ta without nitridation. The NRs have a well-defined hexagonal facet with six sidewalls implying the caxis-oriented growth. From the statistical analyses of several FESEM images, NRs diameter, length and density were calculated. Diameter of NRs slightly varied in the range of 70–90 nm with uniform coverage over the entire surface of the Ta foil. The length and density of NRs are obtained to be about 300 nm and 1–2 × 1010 cm−2 . The GaN growth on nitridated Ta foil also yielded NRs but their density decreased about one order as compared to that grown on bare Ta foil. However, the diameter and length of NRs significantly increased in the range of 100–150 nm and 500– 600 nm, respectively. The GaN NRs are found to be oriented along different angles Fig. 5.15 FESEM images (45° tilt view) of GaN NRs grown on flexible Ta foil using LMBE technique a without and b with pre-nitridation of Ta foil

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Fig. 5.16 HR-TEM image of GaN NR grown Ta metal foil without nitridation. Inset shows the selected area electron diffraction (SAED) image

with respect to the substrate normal including a considerable amount of vertical alignment. The growth of three NRs out of a single nucleation site at a mutual angle of approximately 120° is also noticed at several places. For the fabrication of devices, vertically well-aligned GaN NR ensemble is most preferable [86]. The crystalline structure and growth direction of GaN NR grown Ta metal foil without nitridation were analyzed with HRTEM measurements with selected area electron diffraction (SAED) pattern (inset) as presented in Fig. 5.16. It is observed the growth of single crystalline GaN NRs grows along (0001) direction. The interplanar lattice spacing is measured to be 0.519 nm as indicated in Fig. 5.16 which implies the growth of single crystalline GaN (0001) structure [86]. Raman spectra of GaN NRs on Ta without and with nitridation are shown in Fig. 5.17a. Two Raman modes corresponding to E2 (high) and A1 (TO) modes have been observed. The intense and narrow peak of E2 (high) mode are positioned around 566 cm−1 with a FWHM value of 4–5 cm−1 , which implies the fine structural quality of the grown NRs that have crystallized in wurtzite phase. The appearance of low intensity A1 (TO) modes could be associated with any misalignment of vertically grown GaN NRs as evident from the result that the peak intensity of A1 (TO) mode significantly increased for the randomly oriented GaN NRs on the nitridated Ta foil. Optical emission properties of the GaN NRs were characterized by PL, and the data are given in Fig. 5.17b. The GaN NRs exhibit a sharp and strong UV emission peak around 366 nm (~3.4 eV) without any deep bands related to defect levels. The UV emission belongs to the NBE of GaN caused by the band-to-band transition of excited carriers. Using Lorentzian fitting, the FWHM of the UV peak is estimated to be about 100 meV, which is comparable to the GaN NRs grown on gold-coated Si substrate using MOCVD [81]. It disclosed that the GaN NRs on Ta foil using LMBE process have a superior optical quality suitable for optoelectronic applications. The high intensity PL signal of GaN NRs grown on bare Ta foil can be assigned to the high density of NR growth. The mechanism underlying the formation and alignment of GaN NRs on flexible metal substrates is not well understood since the NR growth shows a high sensitivity toward substrate nature and experimental parameters. In case of Ta foil, a high density of vertically aligned growth of GaN NRs is achieved on bare Ta foil without any

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Fig. 5.17 Room temperature a Raman and b PL spectra of GaN NRs grown on flexible Ta foil using LMBE technique without and with pre-nitridation of Ta foil

surface treatment in contrast to the GaN NR growth on Ti metal foil [51]. The GaN NR formation along c-axis is most likely ruled by the self-induced, diffusioncontrolled mechanism [77]. In case of GaN growth on Ta foil, the substrate nitridation disturbs the density and alignment of GaN NR growth due to possible modification of chemical composition of Ta surface and is not favorable, and it is quite opposite to GaN NRs grown on Ti foil [87].

5.4 Conclusion and Future Remarks In summary, we have explored various thin Cu, W, Ti and Ta metal foils for the growth of GaN nanostructures using LMBE growth technique. Various kinds of GaN nanostructures such as islands, thin films and random/aligned nanorods have been obtained on the flexible metal foils. The wurtzite and cubic mixed-phase GaN thin film and island-like structures are grown on flexible Cu, graphene/Cu and LT-GaN/graphene/Cu metal foils at 700 °C under nitrogen-rich growth condition. Growth of wurtzite GaN NRs on bare and nitridated W foil has been achieved at a low growth temperature of 600 °C compared to the conventional plasma-assisted MBE technique. The PL spectroscopy exhibited that the dense GaN nanorods possess

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the NBE at 3.4 eV with a FWHM value of ~100 meV. Vertically, GaN NRs are grown on pre-nitridated Ti foils at 650 °C and NR density increased with the increase of nitridation time. On the other hand, Ta metal foil offers the growth of vertically well-aligned GaN NRs with a high density on bare substrate, whereas the density of nanorods decreases with the nitridation of Ta foil. Raman investigations revealed wurtzite nature of GaN NRs on these flexible metal foils. The room temperature PL study showed the NBE peak at ~3.41 eV for GaN NRs on Ti and Ta foils along with weak defect-related bands. From these observations, we conclude that the shape and size of nanostructures primarily depend on nature of metal foils and the surface treatment given during the growth process. It should be noted that there is no universal recipe to grow a particular nanostructure on all kinds of metal foils rather the growth sequence needs to be figured out for individual metal substrate. The LMBE growth of GaN nanostructures on flexible metal foils with high structural and optical quality paves the way for futuristic integrated flexible LEDs, solar cells, sensors, water-splitting systems, etc. In future, there are few challenges in developing a commercial GaN-based flexible devices as they require processes different from that used in the current GaN-based technology. For example, the control of GaN nanostructures shape and sizes on these flexible substrates is one of the major challenges compared to the epitaxial GaN on rigid substrates as it demands a better understanding on the growth mechanism [51]. Another challenge is related with the fabrication of electrical contacts since there are openings in between GaN NRs on metal surface which can short the devices. To overcome this, some dielectric or insulating buffer layers are required to be grown before the contact fabrication. In addition, the process for device fabrication on flexible foil will be quite different than the conventional processes and packaging. By suitably addressing these issues, one can succeed in the fabrication of roll-to-roll III-nitride-based optoelectronic and photocatalytic solar water-splitting devices [86]. Acknowledgements The authors thank the director, NPL, for the constant encouragement and support. The authors are also grateful to Dr. N.D. Sharma, Dr. G. Gupta, Dr. K. Subhedar and Dr. M. Kaur of CSIR-NPL, Dr. B. S. Yadav of SSPL, New Delhi, and Dr. S. Ojha of IUAC, New Delhi, for their help in different sample characterizations. Authors would like to acknowledge the group members Mr. C. Ramesh and Mr. P. Tyagi for their contribution in carrying out this work.

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68. Sun XL, Yang H, Zheng LX, Xu DP, Li JB, Wang YT, Li GH, Wang ZG (1999) Stability investigation of cubic GaN films grown by metalorganic chemical vapor deposition on GaAs (001). Appl Phys Lett 74:2827 69. Araki T, Uchimura S, Sakaguchi J, Nanishi Y, Fujishima T, Hsu A, Kim KK, Palacios T, Pesquera A, Centeno A, Zurutuza A (2014) Radio-frequency plasma-excited molecular beam epitaxy growth of GaN on graphene/Si (100) substrates. Appl Phys Exp 7:071001 70. Strite S, Morkoc H (1992) GaN, AlN, and InN: a review. J Vac Sci Technol B 10:1237 71. Grieshaber W, Schubert EF, Goepfert ID, Karlicek RF Jr, Schurman MJ, Tran C (1996) Competition between band gap and yellow luminescence in GaN and its relevance for optoelectronic devices. J Appl Phys 80:4615 72. Saarinen K, Laine T, Kuisma S, Nissila J, Hautojarvi P, Dobrzynski L, Baranowski JM, Pakula K, Stepniewski R, Wojdak M, Wysmolek A, Suski T, Leszczynski M, Grzegory I, Porowski S (1997) Observation of native Ga vacancies in GaN by positron annihilation. Phys Rev Lett 79:3030 73. Santana G, de Melo O, Aguilar-Hernández J, Mendoza-Pérez R, Monroy BM, EscamillaEsquivel A, López-López M, de Moure F, Hernández LA, Contreras-Puente G (2013) Photoluminescence study of gallium nitride thin films obtained by infrared close space vapor transport. Mater 6:1050 74. Heying B, Averbeck R, Chen LF, Haus E, Riechert H, Speck JS (2000) Control of GaN surface morphologies using plasma-assisted molecular beam epitaxy. J. Appl. Phys. 88:1855 75. Tarsa EJ, Heying B, Wu XH, Fini P, DenBaars SP, Speck JS (1997) Homoepitaxial growth of GaN under Ga-stable and N-stable conditions by plasma-assisted molecular beam epitaxy. J. Appl. Phys. 82:5472 76. Kushvaha SS, Kumar MS, Shukla AK, Yadav BS, Singh DK, Jewariya M, Ragam SR, Maurya KK (2015) Structural, optical and electronic properties of homoepitaxial GaN nanowalls grown on GaN template by laser molecular beam epitaxy. RSC Adv. 5:87818 77. Consonni V (2013) Self-induced growth of GaN nanowires by molecular beam epitaxy: A critical review of the formation mechanisms. Phys. Status Solidi RRL 7:699 78. Li H, Geelhaar L, Riechert H, Draxl C (2015) Computing Equilibrium Shapes of Wurtzite Crystals: The Example of GaN. Phys. Rev. Lett. 115:085503 79. Ramesh C, Tyagi P, Mauraya A, Kumar MS, Kushvaha SS (2019) Structural and optical properties of low temperature grown single crystalline GaN nanorods on flexible tungsten foils using laser molecular beam epitaxy. Mater. Res. Exp. 6:085919 80. Consonni V, Dubrovskii VG, Trampert A, Geelhaar L, Riechert H (2012) Quantitative description for the growth rate of self-induced GaN nanowires. Phys. Rev. B 85:155313 81. Ra YH, Navamathavan R, Lee YM, Kim DW, Kim JS, Lee IH, Lee CR (2010) The influence of the working pressure on the synthesis of GaN nanowires by using MOCVD. J. Cryst. Growth 312:770 82. Reshchikov MA, Korotkov RY (2001) Analysis of the temperature and excitation intensity dependencies of photoluminescence in undoped GaN films. Phys. Rev. B 64:115205 83. Liao H, Li J, Wei T, Wen P, Li M, Hu X (2019) First-principles study of CN point defects on sidewall surface of [0 0 0 1]-oriented GaN nanowires. Appl. Surf. Sci. 467:293 84. Fernandez-Garrido S, Kong X, Gotschke T, Calarco R, Geelhaar L, Trampert A, Brandt O (2012) Spontaneous Nucleation and Growth of GaN Nanowires: The Fundamental Role of Crystal Polarity. Nano Lett. 12:6119 85. Gorczyca I, Christensen NE, Svane A (2002) Influence of hydrostatic pressure on cation vacancies in GaN, AlN, and GaAs. Phys. Rev. B 66:075210 86. Tyagi P, Ramesh C, Kaswan J, Dhua S, John S, Shukla AK, Roy SC, Kushvaha SS, Muthusamy SK (2019) Direct growth of self-aligned single-crystalline GaN nanorod array on flexible Ta foil for photocatalytic solar water splitting. J. Alloys Compd. 805:97 87. Ramesh C, Tyagi P, Kaswan J, Yadav BS, Shukal AK, Kumar MS, Kushvaha SS (2020) Effect of surface modification and laser repetition rate on growth, structural, electronic and optical properties of GaN nanorods on flexible metal foil. RSC Adv. 10:2113

Chapter 6

Antibacterial and Anticancer Activity of Biologically Synthesized Gold Nanoparticles Azra Parveen, Hadeel Salih Mahdi, and Ameer Azam

Abstract The rapid developments in the field of nanotechnology have spurred interest in many metal nanoparticles. Among many other uses, these nanoparticles are increasingly providing helpful ways to tackle-resistant bacteria. This research was aimed to identify the effect of biologically synthesized Au-NPs on bacteria Pseudomonas aeruginosa and Staphylococcus aureus and on the cancer cell lines. The antibacterial effect of Au-NPs against P. aeruginosa and S. aureus was studied by scanning electron microscopy (SEM) analysis. The bacterial culture (P. aeruginosa and S. aureus) was exposed to sublethal concentrations of Au-NPs (25 µg/mL) and incubated at 37 °C for 12 h. Antibacterial activity was observed to increase as AuNPs increased in concentration from 25 to 100 µg/mL. The antimicrobial efficacy of Au-NPs was fairly different in Gram-positive and Gram-negative bacteria. The rigid structure of Gram-positive bacteria is likely responsible for this as it contains high peptidoglycan content that provides superior protective ability and enhances pathogenicity. To determine the effect of Au-NPs on the cell lines used in this study, the cell lines were grown in RPMI 1640 medium containing 10% fetal bovine serum and 2 mM L-glutamine. The study shows that the treatment with Au-NPs was notably effective against the proliferation of MDA-MB-231 cancer cells in a dosedependent method. The cytotoxicity assays revealed the concentration-dependent cytotoxic effects of Au-NPs in range of 0–80 µg/ml. The results obtained confirm the strong activity of biologically synthesized Au-NPs against bacteria and cancer cell lines. Keywords Pseudomonas aeruginosa and Staphylococcus aureus bacteria · SEM and MDA-MB-231 cancer cells

A. Parveen (B) · H. S. Mahdi · A. Azam Department of Applied Physics, Z. H. College of Engineering & Technology, Aligarh Muslim University, Aligarh 202002, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_6

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6.1 Introduction The application of nanoparticles transcends the field of research. Size reduction ability of these nanoparticles leads to improved physical and chemical properties and has ensured the sustained relevance of nanoparticles in biomedical and pharmaceutical fields. Nanoparticles have a wide-ranging applications such as biosensing to catalysis, medical diagnosis, chemical sensing and optoelectronics [1]. While many studies have been conducted on numerous nanostructured materials, Au-NPs, in particular, gain interest due to their inertness, stability and wide range of applications in the field of biotechnology [2]. Au-NPs are also known to efficiently bind with amine and thiol groups and this enhances surface modification, making them valuable for biomedical purposes. Au-NPs have recently been used in drug encapsulation, cancer cell detection, in luminescent biomarkers, sensor analysis, nerve cell signaling stimulators, nanobarcodes, as antimicrobial agents and in drug delivery [3]. Antimicrobial agents have traditionally been employed for fighting infectious diseases so far but the growing rise of resistance to these drugs has galvanized global attention over the years. Antimicrobial drug resistance is linked to the sustained and broad abuse of these drugs. Hence, otherwise, treatable diseases have become increasingly difficult to cure, spurring global scale health challenge for many infectious diseases. Mutation is the key mechanism through which resistance is developed. Once resistance is developed, infectious bacteria can exchange, share or transfer infectious properties to their progeny vertically, or do so horizontally via transformation, conjugation or transduction [4]. Multiple drug resistance is not the only challenge in developing traditional antimicrobial agents, but the main focus nowadays is to reduce side effects. Owing to drug resistance, high-dose management of antibiotics may occur, and this often results in excessive toxicity that is harmful to cells, hence, the need to develop alternative and viable approaches for treating bacterial diseases. This need for alternative strategies has brought the potentials of nanoscale materials as antimicrobial agents to the fore. A wide range of unique properties has made metal nanoparticles a resourceful tool in biosensing, optoelectronics [5], therapeutics, catalysis and as antimicrobial agents [6, 7] cell labeling and imaging, drug delivery, etc. [8]. Among other metals, silver has especially been pertinent as an antimicrobial agent for combating diseases, as well as in water and food purification [9]. The advent of nanotechnology and the high surface to volume ratio of Au-NPs have coincided with the re-emergence of gold on the antimicrobial activities [10]. The effectiveness of Au-NPs has been established following consistent studies on its antimicrobial activities. In a study conducted by a team of Indian researches, Au-NPs in the range of ~(1–22) nm were synthesized using citric acid and CTAB as reducing agents [11]. The potency of the nanoparticle was put to the test against Escherichia coli, and the result was a zone of inhibition of ~22 mm, thus echoing the high antibiotic potential of the metal nanoparticle. Other workers [12] have considered Au-NPs as possible drug delivery vehicles with potential use in treating a wide range of medical conditions. A confluence of features such as the exceptional surface functionalization chemistry, non-toxicity and the optical and electronic properties of Au-NPs has since

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been identified as reasons for their many applications in the biomedical industry [13, 14]. Metal nanoparticles generated microbially have been subject to extensive studies over the years. The small size and large surface area of nanoparticles are important in its antimicrobial activity [15]. Recently, it has been discovered that the resistance of abundant enteric human pathogenic bacteria against several synthetic drugs has been developed day by day [8]. Studies on the efficiency of Au-NPs as antimicrobial agents against human bacterial pathogens show the zone of inhibition for E. coli and Staphylococcus aureus as 7 mm and 16 mm, respectively [16, 17]. The results obtained herein will, therefore, show how this pure metal nanoparticle can help in the global challenge of antibacterial resistance. Although gold lost ground after an early rise as a therapeutic agent, it is relevance in nanoparticles has significantly revived its medical relevance in the diagnosis and treatment of diseases. Gold nanoparticles may be used intravenously or intramuscularly to treat deleterious circumstances like cancers [18]. In spite the fact that the use of metal nanoparticles in medicinal field are generally obscure, recent developments may ultimately comprehend the probability of these nanoparticles as therapeutic compounds in the treatments of diseases like cancer [19]. The advantage of gold nanoparticles over any other metal nanoparticles is remarkably due to its non-cytotoxicity and biocompatibility. The chemical inertness of gold is also responsible for its use over the last 50 years for treating internal conditions in humans [20]. Apart from enteric conditions, Au-NPs have shown relevance in cancer treatment. Cancer remains a global challenge, responsible for many terminal conditions and deaths of millions of sufferers worldwide. Cancers, being an extremely challenging disease, are difficult to cure. They remain the second most common cause of death in developed countries, while deleteriously plaguing many other countries worldwide. When used to treat tumor cells, Au-NPs accumulate and show optical scattering, thereby enabling the microscopic study of cancer cells using the nanoparticles. Au-NPs are also good agents in chemotherapy and cancer cell diagnosis [21]. Although the dynamic growth of nanoscale technologies has not coincided with a reduction in cancer burden, metal nanoparticles, such as Au-NPs, have huge potentials in checking cancer-related mortalities [22]. Au-NPs are intrinsically able to deliver large biomolecules, as opposed to exclusively restricting themselves as carriers of small molecular drugs. In previous researches, the anticancer activity of silver and Au-NPs against Dalton’s lymphoma ascite (DLA) cell lines, human leukemic monocyte lymphoma and human laryngeal (Hep-2) cell lines have been studied [23–25]. Nam et al. designed functionalized Au-NPs using dendrimers to combat cancer cells [26]. The cytotoxic abilities of silver nanoparticles have been reported against human breast cancer (MCF 7) cell line, human laryngeal cancer (Hep-2) cell line and human colon cancer (HT 29) cell line [24, 27]. This chapter presents the effect of biologically synthesized Au-NPs on bacteria Pseudomonas aeruginosa and S. aureus and on the cancer cell lines using different plant extracts.

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6.2 Sample Preparation and Its Analysis 6.2.1 Antibacterial Study of Au-NPs Antibacterial properties of synthesized Au-NPs were studied by disk diffusion method using two test bacteria, Gram-negative—P. aeruginosa and Gram-positive— S. aureus. These bacteria were grown in 10 ml of Luria Bertini broth media in an incubator shaker at room temperature for 24 h. A specific weight of 2.8 g of nutrient agar medium was taken and dissolved in a flask having 100 ml of sterile distilled water. The conical flask was placed inside an autoclave and sterilized for 30 min at a temperature of 120 °C. Following sterilization, the agar medium was left to cool for 30 min at room temperature, and then 100 ml nutrient agar medium was dispensed into Luria Bertini (LB) agar plates and left for 4 h to allow broth solidification. Lawn of bacterial culture was then arranged by spreading 1 mL broth culture of each test organism on solid agar plates. The cultured plates were kept for 45 min to allow absorption. Then after, 5 mm size disks were pressed into the agar with the head of sterile micropipette tips. Further, the synthesized nanoparticle of desired concentration (100 µg/ml) was put on the filter paper, and the LB agar plates were subsequently incubated for 24 h at 35 °C. After incubation, the extent of zone of inhibition was then determined in millimeters [28].

6.2.2 Bacteria and Au-NPs Association by Scanning Electron Microscopy (SEM) Scanning electron microscopy (SEM) analysis was performed to study the antibacterial effect of Au-NPs against P. aeruginosa and S. aureus. The bacterial culture (P. aeruginosa and S. aureus) was exposed to sublethal concentrations of AuNPs (25 µg/mL) and incubated for 12 h at 37 °C. Afterward, both samples were centrifuged at a speed of 3000 rpm for 5 min, placed in a 2.5% concentration of glutaraldehyde solution for 4 h and subsequently dehydrated in progressive concentrations of aqueous ethanol solutions and then they were placed in absolute ethanol. After this process was completed, the specimens were dried on glass cover, and finally, the examination was performed by a JSM 6510LV scanning electron microscope (JEOL, Japan) at an accelerating voltage of 15 kV [29]. Additional analysis of the antibacterial efficacy of Au-NPs against the select organisms was also done using the Kirby-Bauer disk diffusion method [30].

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6.2.3 Cell Culture and Exposure to Au-NPs To determine the consequence of Au-NPs on the cell lines used here, the cell lines were grown in RPMI 1640 medium comprising 10% fetal bovine serum and 2 mM l-glutamine. Then after, the cells were inoculated into 96 well microtiter plates and incubated for 24 h at 37 °C. Following this, different concentrations of Au-NPs were added to the plates and incubation was done at typical conditions for 48 h. The assay was ended by adding cold trichloroacetic acid (TCA). Cells were static in situ after mild addition of 50 µl of cold 30% (w/v) TCA (final concentration, 10% TCA) and incubating the plates at 4 °C for 60 min. After washing, sulforhodamine B (SRB) solution (50 µl) at 0.4% (w/v) in 1% acetic acid was added to each of the wells. Then, the plates were incubated at room temperature for 20 min. Five times washing was done to remove residual dye, with 1% acetic acid. The plates were then air dried. The bound stain was later eluted, and the absorbance was measured using a plate reader at a wavelength of 540 nm [31, 32].

6.3 Results and Discussion 6.3.1 Antibacterial Activity of (Au-NPs) The antimicrobial activity of Au-NPs against multidrug-resistant P. aeruginosa and S. aureus suggests that the size of inhibition zones significantly increases when treated with Au-NPs as compared with the zones of inhibition in the case of control as shown in Figs. 6.1, 6.3, 6.5, 6.7 and 6.9 for Albizia lebbeck, NaBH4 , lemongrass, carrot and stachys, respectively. Antibacterial activity was observed to increase as

Fig. 6.1 Zone of inhibition of biologically synthesized Au-NPs using A. lebbeck (i) P. aeruginosa and (ii) S. aureus

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Fig. 6.2 Zone of inhibition at different concentration of biologically synthesized Au-NPs A. lebbeck

Au-NPs increased in concentration from 0 to 100 µg/mL. This could be linked to the increase in particle size of nanoparticles against Gram-positive S. aureus and Gramnegative P. aeruginosas, as ROS caused oxidative stress and cell destruction. From Tables 6.1, 6.2, 6.3, 6.4 and 6.5 for A. lebbeck, NaBH4 , lemongrass, carrot and stachys, respectively, it can be inferred that the Au-NPs appreciably inhibited the growth of both Gram-negative and positive bacteria. As it can be seen in Figs. 6.2, 6.4, 6.6, 6.8 and 6.10 for A. lebbeck, NaBH4 , lemongrass, carrot and stachys, respectively, that the increase in inhibition zone paralleled the increase in the concentration of Au-NPs. The antibacterial effect of Au-NPs against P. aeruginosa and S. aureus was studied by SEM analysis and the results are shown in Figs. 6.11, 6.12, 6.13, 6.14 and 6.15 for A. lebbeck, NaBH4 , lemongrass, carrot and stachys, respectively. SEM image revealed that the Au-NPs were effective, causing structural disfigurements and cellular damages compare against the untreated controls of P. aeruginosa and S. aureus cells. These results are consistent with previous works which reported the effective antibacterial properties of Au-NPs [8, 33]. The results, therefore, tend to give valuable information that can be helpful in the future development of potential biomaterials against multidrug-resistant microorganisms.

6.3.1.1

Differences in Au-NPs Efficacy

The antimicrobial effect of Au-NPs was significantly different in Gram-positive and Gram-negative bacteria. This difference is chiefly linked to the variance in the construction of the membrane; Gram-positive bacteria contain a thick peptidoglycan layer that confers extra protection to the cell membrane, a feature that enhances

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Fig. 6.3 Zone of inhibition of Au-NPs synthesized using NaBH4 (i) P. aeruginosa and (ii) S. aureus

Fig. 6.4 Zone of inhibition at different concentration of Au-NPs synthesized using NaBH4

pathogenicity. Gram-positive bacteria contain a 50% higher thickness of peptidoglycan compared with Gram-negative bacteria [34]. This ensures that only higher doses of nanoparticles would be effective against Gram-positive bacteria. The size of the nanoparticle is another deciding (factor) in this regard. Hence, the antibacterial mechanism of Au-NPs on Gram-positive and Gram-negative bacteria is a function of the concentration of the gold NPs [35]. Results show that the antibacterial activity of the Au-NPs increases with a increase in its average size [8].

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Fig. 6.5 Zone of inhibition of biologically synthesized Au-NPs using lemongrass (i) P. aeruginosa and (ii) S. aureus

Fig. 6.6 Zone of inhibition at different concentration of Au-NPs synthesized using lemongrass

6.3.1.2

Mechanism of the ROS Generation and Antibacterial Activity

From the antibacterial analysis of the five samples of different sized Au-NPs, it can be concluded that the identical dose of both samples of NPs provided maximum inhibition against each pathogen. Interaction of Au ions attached to the cell membrane with thiol in protein groups and inactivate respiratory enzymes leads to the production of reactive oxygen species (ROS) [36].The nanoparticles also change the membrane

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Fig. 6.7 Zone of inhibition of biologically synthesized Au-NPs using carrot (i) P. aeruginosa and (ii) S. aureus

Fig. 6.8 Zone of inhibition at different concentration of Au-NPs synthesized using carrot

potential. As they decrease cellular ATP levels and make it difficult for tRNA to bind with ribosomal subunit, they affect the process of translation [37]. Additionally, AuNPs are capable of generating holes in the cell wall, causing the cell contents to leak. By binding with the DNA, the nanoparticles inhibit transcription [38]. Aggregating within biofilms, Au-NPs cause distortions on the bacterial cell wall; this feature can be exploited to minimize duration of treatment and prevent side effects [39]. Oxidative stress resulting from the formation of ROS is initiated by non-toxicity, which often

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Fig. 6.9 Zone of inhibition of biologically synthesized Au-NPs using stachys (i) P. aeruginosa and (ii) S. aureus

Table 6.1 Diameter of the inhibition zone with the different concentration of biologically synthesized Au-NPs using A. lebbeck S. No.

Concentration of Au-NPs (µg/mL)

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Control

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Diameter of inhibition (mm) P. aeruginosa (gram-negative)

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Table 6.2 Diameter of the inhibition zone with the different concentration of Au-NPs synthesized using NaBH4 S. No.

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leads to bacterial cell death [40]. Bacteria death and an increase in production of ROS species is a likely hood of the interaction between exceedingly small Au-NPs and bacteria, an association which triggers imbalance in the metabolic processes of the bacterial cell [41].

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Table 6.3 Diameter of the inhibition zone with the different concentration of biologically synthesized Au-NPs using lemongrass S. No.

Concentration of Au-NPs (µg/mL)

Diameter of inhibition (mm) P. aeruginosa (gram-negative)

S. aureus (gram-positive)

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Table 6.4 Diameter of the inhibition zone with the different concentration of biologically synthesized Au-NPs using carrot S. No

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S. aureus (gram-positive)

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Table 6.5 Diameter of the inhibition zone with the different concentration of biologically synthesized Au-NPs using stachys S. No.

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Diameter of inhibition (mm) P. aeruginosa (gram-negative)

S. aureus (gram-positive)

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6.3.2 Anticancer Activity of Au-NPs Cultured MDA-MB-231 cells exposed to Au-NPs (0–80 µg/mL) for 24 h. Exhibited cytotoxicity as shown in Figs. 6.16, 6.18, 6.20, 6.22 and 6.24 for A. lebbeck, NaBH4 , lemongrass, carrot and Stachys, respectively. The Au-NPs dose-dependent decrease in cell viability is shown in Figs. 6.17, 6.19, 6.21, 6.23 and 6.25 A. lebbeck, NaBH4 , lemongrass, carrot and stachys, respectively. The anticancer activity of AuNPs against MDA-MB-231 is to increase with an increase in the concentration of Au-NPs, thus, signifying the effectiveness of Au-NPs as anticancer agents against MDA-MB-231 cells lines. The effect of concentration on anticancer activity can be

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Fig. 6.10 Zone of inhibition at different concentration of Au-NPs synthesized using stachys

Fig. 6.11 Morphological (i) P. aeruginosa, (ii) P. aeruginosa+Au-NPs, (iii) S. aureus, (iv) S. aureus +Au-NPs following Au-NPs treatment (Au-NPs synthesized using A. lebbeck)

seen as Au-NPs was the most potent against MDA-MB-231 at a concentration of 80 µg/mL. Although significantly reduced, strong activity was followed by 40 and 20 µg/mL concentrations of Au-NPs. In the cytotoxicity assay, cell viability showed a signed decrease of 58, 44 and 30% at 20, 40 and 80 µg/mL, respectively, as shown in Fig. 6.17. Reduction of 83, 64 and 55% at concentrations of 20, 40, 80 µg/mL was obtained in Fig. 6.19. There was 88, 82 and 74% cell viability reduction at 20, 40

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Fig. 6.12 Morphological (i) P. aeruginosa, (ii) P. aeruginosa+Au-NPs, (iii) S. aureus, (iv) S. aureus +Au-NPs following Au-NPs treatment (Au-NPs synthesized using NaBH4 )

Fig. 6.13 Morphological (i) P. aeruginosa, (ii) P. aeruginosa+Au-NPs, (iii) S. aureus, (iv) S. aureus +Au-NPs following Au-NPs treatment (Au-NPs synthesized using lemongrass)

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Fig. 6.14 Morphological (i) P. aeruginosa, (ii) P. aeruginosa+Au-NPs, (iii) S. aureus, (iv) S. aureus +Au-NPs following Au-NPs treatment (Au-NPs synthesized using carrot)

Fig. 6.15 Morphological (i) P. aeruginosa, (ii) P. aeruginosa+Au-NPs, (iii) S. aureus, (iv) S. aureus +Au-NPs following Au-NPs treatment (Au-NPs synthesized using stachys)

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Fig. 6.16 Microscopic images of MDA-MB-231 cells after treatment with Au-NPs synthesized using A. lebbeck for 24 h

and 80 µg/mL for Fig. 6.21. But there was 90, 86 and 74% at 20, 40 and 80 µg/mL, respectively, were recorded in Fig. 6.23. The last Fig. 6.25 shows 81, 72 and 65% decrease in cell viability at concentrations of 20, 40 and 80 µg/mL, respectively. Fig. 6.17 Cytotoxicity assays in MDA-MB-231 cells exposed to various concentrations of Au-NPs synthesized using A. lebbeck for 24 h

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Fig. 6.18 Microscopic images of MDA-MB-231 cells after treatment with Au-NPs synthesized using NaBH4 for 24 h

Fig. 6.19 Cytotoxicity assays in MDA-MB-231 cells exposed to various concentrations of Au-NPs synthesis using NaBH4 for 24 h

Cell viability (%)

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6.4 Conclusions Results from this study are consistent with previous findings on the potency of Au-NPs in treating medical conditions, especially those of bacterial origin such as P. aeruginosa and S. aureus which are of human relevance. The strong effect of Au-NPs can be a springboard for future research on other bacterial infections. The results achieved in this study also specify that the activity of Au-NPs increases

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Fig. 6.20 Microscopic images of MDA-MB-231 cells after treatment with Au-NPs synthesized using lemongrass for 24 h

Fig. 6.21 Cytotoxicity assays in MDA-MB-231 cells exposed to various concentrations of Au-NPs synthesis using lemongrass for 24 h

100

Cell viability (%)

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Concentration (µg/ml)

as their concentration increases, thus, potent concentrations should be used when treating medical conditions. Also, the highest level of inhibition was obtained from the nanoparticles synthesized using A. lebbeck as shown in Table 6.1. At the highest concentration of Au-NPs, the diameter of inhibition was 23 mm for P. aeruginosa and 31 mm for S. aureus. In all controls, there was no inhibition of bacterial growth since no inhibitory substance was available to disrupt cell processes. Au-NPs synthesized

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Fig. 6.22 Microscopic images of MDA-MB-231 cells after treatment with Au-NPs synthesized using carrot for 24 h

Fig. 6.23 Cytotoxicity assays in MDA-MB-231 cells exposed to various concentrations of Au-NPs synthesis using carrot for 24 h

Cell viability (%)

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from NaBH4 were the most potent after A. lebbeck, as is reflected in the maximum inhibition obtained at the highest concentration of 100 µg/mL. The diameter of inhibition was 16 mm for P. aeruginosa and 25 mm for S. aureus. The least inhibitory Au-NPs were obtained using carrot, where, at the maximum concentration of AuNPs, only a 12 mm of inhibition was recorded for P. aeruginosa and 18 mm of S. aureus. The Au-NPs were used at different concentrations and found to show

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Fig. 6.24 Microscopic images of MDA-MB-231 cells after treatment with Au-NPs synthesized using stachys for 24 h

Fig. 6.25 Cytotoxicity assays in MDA-MB-231 cells exposed to various concentrations of Au-NPs synthesis using stachys for 24 h

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activity against MDA-MB-231 cancer cell in a dose-dependent manner. Thus, the anticancer activity of Au-NPs against MDA-MB-231 increases with the increase in the concentration of Au-NPs. The results of the antiproliferative investigations clearly demonstrate that treatments with Au-NPs sensitize the cancer cells.

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Chapter 7

ZnCdS Thin Film: Preparation, Properties and Applications Suresh Kumar and K. P. Tiwary

Abstract Energy crisis is one of the serious problems around the world, but it can be normalized by renewable energy sources such as solar, wind, etc. Solar cell is a device that receives sunlight and converts it into electricity. Requirement of semiconductors in devices depends upon amplification characteristics, switching behaviour, etc. Metal chalcogenides (sulphide, selenides and tellurides) are studied in the form of thin film and is considered as heart of semiconductors due to its applications in photovoltaic cells, photoconductors, optical filters, solar cells, sensors, variety of optoelectronic devices, etc. Cadmium sulphide is a n-type semiconductor material, and it has energy bandgap 2.42 eV. Zinc is a transition element added to CdS, and its bandgap changes from 2.42 to 3.5 eV. Due to increase in bandgap, opencircuit voltage together with circuit current density of device also increases, which is attributed to greater conversion efficiency of CIGS solar cells. The preparation of ZnCdS thin films along with different Zn concentration (0.2, 0.3, 0.4, 0.5 wt%) by chemical bath deposition method has been elaborated. In this chapter, we have discussed different characteristics of ZnCdS thin films along with their synthesis process, crystal structure, energy bandgap, applications, etc.

7.1 Introduction In present years, the energy requirements are not fulfilled by fossil fuel. Electricity is dependent on coal, gas and oil for cooperation and development. Sun sends a mixture of heat and light in the form of energy on earth surface, and both are essential for living things. Solar, wind, etc., are the most important parameters for renewable energy which would be required to minimize energy crisis. With the energy crisis in the world, recently growing research on solar technology has been rapidly progressive. S. Kumar Department of Physics, Chaibasa Engineering College, Kelende, Jhinkpani 833215, India K. P. Tiwary (B) Department of Physics, Birla Institute of Technology Mesra, Patna Campus, Patna 800014, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_7

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A part of non-conventional energy is solar cell, and the regenerated energy is abundant as well as pollution free. In 1839, E. Becquerel invented solar cell and defined that it is a device that captured sunlight and converted it into electricity. Thin film solar cells can be developed because it decreases the cost of solar energy reaches to the grid parity level. Recently, solar cell industries are developed due to attenuation on renewable energy and difficulties related to global climate change.

7.2 Semiconducting Materials During the few decades, electronics is more central to our lives. Semiconducting materials are the building blocks of electronics and optoelectronics devices. It was discovered in the mid-nineteenth century, and over the world, semiconductor industry products are spread and helpful for our everyday life and can be used by our society. Semiconductor material is one whose electrical properties lie between an insulator and conductor. The resistivity of semiconductor is less than an insulator but greater than conductor, and it has bandgap between 1 and 4 eV. The process which involves mixing of impurities with a semiconductor is said to be doping. S. Bidwell and B. Gudden observed that the properties of semiconductors are due to impurities within them. Semiconductors are helpful in devices because of their amplification characteristics, switching, energy changing, behaviour, etc. Development and application for semiconducting materials has great considerable interest because it can be used with wide area such as diode, photodetectors, transistors, solar cells. Semiconductors can be attributed in various forms: (i) Elements—Si, Ge, etc. (ii) Compounds—CdSe, CdTe, etc. (iii) Alloys—GaAsx P1−x , HgCdx Te1−x , etc. Classifications of semiconductor are given below: (a) Intrinsic semiconductor The elemental form of pure silicon (Si), germanium (Ge) is intrinsic and is helpful. (b) Extrinsic semiconductor Impurity added semiconductors are called extrinsic semiconductors. It also belongs to the family of alloys and compounds. Impurity added semiconductors provide a large number of holes, referred to as “p-type semiconductor” and if this involves number of electrons is referred to be “n-type semiconductor” (Tables 7.1 and 7.2).

7 ZnCdS Thin Film: Preparation, Properties and Applications Table 7.1 Conductivities (σ = L/RA): Insulator, superconductors, metals, semimetals and semiconductors at room temperature

Table 7.2 At room temperature, variety of solids based on their energy bandgap (eV) and carrier density (n)

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Variety of solid

σ (Sm−1 )

Example

Insulator

1010

Pb, YBa2 Cu3 O7 , etc.

Metal

105 to 1010

Cu, Ag, etc.

Semimetal

102 to 105

Graphite(C), HgTe, etc.

Semiconductor

109 to 102

Germanium, Silicon, ZnSe, GaAs, etc.

Variety of solid

Energy bandgap (E g ) in eV

Carrier density (cm−3 )

Insulator

Energy bandgap greater than and equal to 4

=Much less than 1

Metal

Not available energy bandgap

=1022

Semimetal

Energy bandgap less than and equal to zero

=1017 to 1021

Semiconductor

0 < energy bandgap < 4

=Less than 1017

7.3 Nanotechnology Nanotechnology has an ability to create wide range of some new and innovative uses with an area such as medicines, communications, electronics [1–4]. From past decades, the fabrication of nanocrystalline material is interested due to their physical as well as chemical properties. Increasing the ratio of surface–volume has more attenuation because it could be subjected to arrangement of atoms, so it changes optical characteristics of materials prepared in nano-range. The material which have one and two dimensions in scale is referred to as “quantum wire” and “quantum well”. The size reduction of semiconductor in which all the three dimension in low range of nanometre is called “quantum dots”. Nanocrystalline materials are categorized into following forms: (a) Lameller structures (b) Filamentary structure (c) Equiaxed structures

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7.4 Thin Film According to thickness, material layer possesses fractions of nanometer to some micrometres is said to be “thin film”. If property of a film is different from bulk significantly, then it can be considered as “thin” due to: (i) As the film thickness decreases, surface-to-volume ratio increases. (ii) Deposition is one of the important parameters which indicate the microscopic structure. In the present day, thin film technology is more interested because it can be new for sciences along with old for arts. Thin film technology is one of the oldest arts and one of the newest sciences. Nanocrystalline thin film is considered as building blocks of solid-state technology due to its applications in photovoltaic cells, electronic components, photoconductors, sensors, optical filters, solar cells, etc. [5]. The fabrication of thin film in the nano-range is considerable interest because of their uses in solar energy conversion, devices of optoelectronic, etc., and can be fabricated by multi-layers and multi-compounds on different substrates with various shapes and sizes. In modern science, since vacuum system thin films have been prepared, deposition of the films for device application was synthesized over past 40 years. Past 25 years, thin film deposition processes have developed, especially for the purpose of semiconductor devices. Film growth technique used can be described in terms of three main events: (i) Nucleation, (ii) crystal growth and (iii) grain growth.

7.5 Deposition Technique for Thin Film The following steps are taken for film deposition: (a) Synthesis of film producing particles like atoms, molecules and cluster. (b) Particles are moved from source to substrate. (c) Particle adsorption onto substrate, so it depends upon the film growth. According to the synthesis of thin film semiconductor, many physical as well as chemical methods are reported. With the help of different deposition situations during the development of the film, various properties of the films should have more attention (Fig. 7.1).

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Fig. 7.1 Thin film deposition techniques

7.5.1 Physical Process 7.5.1.1

Thermal Evaporation

Deposition of metal, alloy, compounds thin films would be done with the help of “thermal evaporation method”. Its principle states that material is kept inside the vacuum chamber and is to be heated; its atoms under the surface could have much energy for emitting from surface. So it moves from the vacuum chamber and coated on substrate over evaporated material. According to this process, solid material is heated at a temperature in the inner part of the high vacuum chamber and is to be produced vapour pressure. Internal part of the vacuum creates vapour cloud into chamber due to availability of low vapour pressure. The material (evaporated) forms the stream of vapour moves up to the chamber, hit with the substrate and sticking to it as coating. Thermal evaporation system can allow number of parameters of the film like structure of grain, its thickness, optical and electrical characteristics, etc.

7.5.1.2

Sputtering

Sputtering is another technique which is used for thin film deposition. In this process, inside a vacuum chamber inert argon is filled as well as electrically energizing a cathode to create self-sustaining plasma. Cathode exposed surface known as “target”, platform of material and is to be coated over the substrates. Inside this plasma loss of electrons by gas atoms, it became positively charged ions so it can be accelerated

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towards the target and hits with more kinetic energy to dislodge molecules of target material. The sputtered materials form vapour stream, can move towards the chamber and then strike with substrate, sticking to it act as coating.

7.5.2 Chemical Process 7.5.2.1

Chemical Vapour Deposition (CVD) Method

CVD is a common technique and is required for preparation of materials. This method consists of deposition layer of thin film over the surfaces and form bulk materials which have greater purity, through infiltration method and it could also prepare composite materials. This technique involves precursor gas flow to the chamber which takes heated objects as single/more in numbers, and they are to be coated. Chemical reactions should be done in front of warm surfaces; hence, deposition of thin film also occurs at the surface. It is due to formation of chemicals through products and is exhausted out of chamber which is associated with unreacted precursor gas. Films which are to be prepared by chemical vapour deposition are normally quite conformal because its inside or underside thickness of features comparable with the thickness of the top. It can be concluded that films could be applied as an elaborately shaped pieces together with sidewalls of features; with increasing holes ratio, several features are to be fully filled. A different material prepared by chemical vapour deposition has more purity. Hence, conclusion arises the relative ease with removal of gas precursor impurities with the help of distillation method.

7.5.2.2

Chemical Bath Deposition (CBD) Method

CBD is the simplest technique used for thin film deposition and is referred to as solution growth, method. In 1884, thin films deposited by CBD were first synthesized, its area of deposition was limited to lead sulphide (PbS) and lead selenide (PbSe). Around 1980, chemical bath deposited films were involved in the area of solar energy. The principle of this method is based on the fact that the controlled precipitation of a compound with solution is deposited over a substrate, and it has ability for wide area coating and is one of the cheapest techniques. Basic principle of CBD states that for getting precipitation of a compound with solution its ionic product should be greater than the solubility product. When this situation reached in bath solution, then suitable complexing agent is introduced into the bath solution because it maintains the metal ions concentration. This is required for removal of spontaneous precipitation in such a way that thin film is maintained with ion-by-ion reaction. ‘M for a metal complexing agent “A”, the existence of metal ions contained in the bath solution could be described using the following reaction:

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M(A)2+ ↔ M2+ + A It is one of the best methods for deposition of films on substrate due to low-cost technique, versatile deposition over large area, does not need sophisticated instrument such as vacuum system or other expensive instruments, and required chemicals are commonly available, working at lower temperature and able to generate fine quality of thin films. This technique involves precursor solution of metal ions complexed through ligands, and complex solution is formed using the solution of ammonia, citric acid, etc. After complexation is to be completed, sulphur/selenium or other anions are introduced to deposit the required chalcogenides. In CBD method, substrates and solution both are important and solution can be stirred using magnetic stirrer. Aqueous/non-aqueous baths along with stirring are helpful for heating the bath solution at required temperature under continuous stirring. Substrates are inserted vertically into the bath solution for specific span of time and removed; hence, the required film thickness can be found which may depend upon the deposition parameters like stirring rate of the solution, temperature of the bath, content of solution, etc. The development of the thin film could be done through ion-by-ion condensation of materials on the substrate.

7.6 Different Processes of Thin Film Deposited with the Help of CBD Technique 7.6.1 Ion-by-Ion Process It is the process in which ions are condensed at the reacting surface and the films are formed due to condensation of ions. The mechanism which is used for the formation of binary metal chalcogenide is given below (Fig. 7.2): MA+ + XB− ↔ MA XB

Fig. 7.2 Thin film deposition using CBD method through ion-by-ion process

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Fig. 7.3 Thin film deposition using CBD method through cluster by cluster process

7.6.2 Cluster-by-Cluster Process In this process, due to homogenous reactions of bath solution, particles are formed in the solution and deposited on the face of substrate to make thin layers (Fig. 7.3).

7.6.3 Mixed Process It is the steps in which predominance of a method upon another method is subjected with the help of homogenous and heterogeneous nucleation. Homogenous nucleation can be done within bulk solution but the heterogeneous nucleation occurs on the surface of substrate (Fig. 7.4).

7.7 Solar Energy One of the most important parts of renewable energy sources is solar energy that can supply a significant portion of our electricity needs. Using solar cell, electricity could be generated from solar energy. Presently, required technology used for solar cell device depends upon inorganic semiconductors like (Ge) germanium, (Si) silicon and gallium arsenide. Among them, silicon is one of best materials for conversion of solar energy; it has an indirect bandgap which indicates inefficient optical absorption. The organic semiconducting materials properties can be developed using various simple and low-cost techniques. The organic semiconductors are proved to be beneficial because it has low price synthesis, simple, production in the form of thin film devices with the help of printing technologies, and it shows absorption coefficients above than 105 cm−1 , so it is suitable for optoelectronic applications [6, 7]. An organic and inorganic semiconductor constitutes “hybrid solar cell”. The characteristics of Fig. 7.4 Thin film deposition using CBD method through mixed process

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Fig. 7.5 Modes of solar energy utilization

inorganic semiconductor as well as film make the organic semiconductor properties like polymers. However, solar cell in the form of hybrid produces hetero-junctions along with inorganic semiconductors and polymers.

7.7.1 Solar Energy Utilization Modes of utilization of solar energy can be classified into two categories: • Direct method—the direct use of solar radiations incident on the surface of the earth. • Indirect method—the radiations incident on the earth lead to water power, wind power, biomass, etc. (Fig. 7.5).

7.7.2 Types of Solar Cells Today, energy crisis can be minimized by renewable energy, and in this direction, the research is rapidly progressing. The need of present time is pollution free and cheaper energy sources, which can be harnessed from sun, wind and oceans, etc. Among these sources, solar energy is most attractive. However, solar cells are too much expensive for creating electricity. But, mainly due to the advancement of nanotechnology, it seems to be easier for creation of low cost as well as capable for more efficient solar cell. Photovoltaic, photochemical, photoelectrochemical, photothermal

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Fig. 7.6 Classification of various solar cells

and photosynthetic are the different process to achieve energy conversion. Different solar energy conversion devices are classified into the following categories (Fig. 7.6). p-type semiconductor and n-type semiconductor constitute to form p-n junction. Electrons from n-type semiconductor tend to diffuse into p-type side, leaving positively charge donor ions towards n-region. Again, holes from p-type side flow to ntype side, left negatively charged acceptor ions towards p-region. Therefore, transfer of carriers creates an electric field () proposed into diffusion of carriers. Electric field region () is called “depletion region”. Potential (v) exist across the depletion region due to the electric field (), and their relation is  = dv/dx ‘x’ is the length of the depletion layer. At thermal equilibrium, conduction bands and valence bands act upwards in ntype semiconductor, but as the case of p-type semiconductor, it bends downwards near the interface. In this interface, Fermi energy of constant value reaches the entire p-n junction (Figs. 7.7 and 7.8).

7.7.2.1

Schottky Junction

The semiconductor metal rectifying interface gives to the Schottky junction devices. It is also used to harness the solar energy, so this device is referred to as Schottky junction solar cells. It can be fabricated by using the rectifying contact of metal

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Fig. 7.7 p-n junction in thermal equilibrium

Fig. 7.8 Schematic band diagram of p-n junction

and semiconductor. If one junction is metal, the absorption of incident radiation and photogeneration of charge carriers take place in the semiconductor regions.

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Metal–Oxide–Semiconductor (MOS)

It can be also used in the solar energy conversion process. Major absorption of light takes place in the semiconductor, generating the charge carriers. So, the efficient absorption of incident radiation semiconductor governs the photogeneration mechanism of charge carriers.

7.7.2.3

Metal–Insulator–Semiconductor (MIS)

Metal–insulator–semiconductor (MIS) devices are more or less analogous to the MOS devices. The effective photogeneration of the charge carriers takes place in semiconductor dominate the photoconversion process.

7.7.2.4

Semiconductor–Insulator–Semiconductor (SIS)

Semiconductor–insulator–semiconductor (SIS) is also helpful for generation of electricity from solar energy. Here, insulator is between two semiconductor regions and the photogeneration of charge carriers occurs in semiconductor region.

7.7.3 Photoelectrochemical (PEC) Solar Cell In recent years, photoelectrochemical systems are of considerable interest because of converting solar energy directly into electrical energy. Photochemistry has created a link between photovoltaic devices and electrochemical devices, subsequent developments known as photoelectrochemical solar cells (PEC). The photoelectrochemical cells phenomena state that the semiconductor absorbs the photon in the enhancement of charge carriers in it through the generation of electron hole pairs (Fig. 7.9). The following parts are needed for working of photoelectrochemical solar cell.

7.7.3.1

Semiconductor Electrode

The good performance of PEC solar cell satisfy the below mentioned requirements. 1. Optical absorption coefficient of semiconductor electrode material should be high. 2. It should have direct bandgap along with absorption coefficients above than 105 cm−1 . 3. Diffusion length for minority carriers and width of space charge layer must be large.

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Fig. 7.9 Schematic diagram of photoelectrochemical (PEC) solar cell

4. Thickness and area of the photoelectrode must be large due to absorption of all the incident radiation. 7.7.3.2

Electrolyte

The properties of electrolyte which satisfy PEC solar cell are given as. (a) The electrolyte should have minimum optical absorption. (b) Electrolyte should not react with the semiconductor electrode and should be non-corrosive to the electrode as well as container material. (c) Electrolyte should be in the form of ohmic losses. (d) Cost, toxic nature, environmental aspects, etc., should be low. 7.7.3.3

Counter Electrode

Need of counter electrode for performance of PEC solar cells is as follows. (a) Counter electrode can be chemically inert. (b) Counter electrode is to be electrically active. (c) Area of the counter electrode must be larger than the semiconductor electrode which improves the efficiency. (d) Counter electrode can be low over potential for the reduction reaction. Many counter electrode materials have been evaluated electrochemically by Allen and Hickling. A photoelectrochemical (PEC) solar cell has the irradiation by light of appropriate frequency of an electrode in contact together with suitable electrolyte, making a

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change in the electrode potential with respect to reference electrode (under opencircuit situation) or making a change for flow of current in the galvanic cell contained with the electrode (under short-circuit conditions).

7.7.4 Parameters Used for PEC Solar Cell The following parameters are required for characterization of PEC solar cell. (a) (b) (c) (d) (e)

Short-circuit current (I sc ) Open-circuit voltage (V oc ) Maximum power (Pm ) Fill factor (FF) Efficiency (ï).

7.7.5 Photoelectrolysis Solar Cell The solid–liquid interface used in the solid–liquid structure devices which can be used for the electrolysis of water consequently generates the hydrogen and oxygen. Electrolyte is to be transparent to the incident solar radiations.

7.8 Different Types of Semiconducting Materials Semiconductors are classified into binary, ternary and quaternary type when it is in the form of compound, whereas if it is in alloys form, these are available in different types such as homogenous and heterogeneous. A detailed classification of various types of semiconducting materials is given in Table 7.3 [8]. Table 7.3 Classification of semiconductor materials

Variety of semiconductors

Examples

(i) Compounds in the form of (II-VI as a binary)

Zns, ZnTe, CdS, CdTe, etc.

(ii) Compounds in the form of (III-V as a binary)

GaN, InP, InSb, etc.

(iii) Compounds in the form of (IV-IV as a binary)

SiC, SiGe, etc.

Compounds in the form of “ternary”

ZnCdS, GaAsP, AlGaAs, etc.

Compounds in the form of “quarternary”

InGaAsP, etc.

7 ZnCdS Thin Film: Preparation, Properties and Applications

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Fig. 7.10 Sketch over a CIGS/CZTS thin film solar cell

In recent years, semiconducting materials have great attention due to their novel properties, and it can be used in a system of photovoltaic in addition to cadmium telluride (CdTe), copper indium diselenide/sulphide and copper indium gallium diselenide/sulphide (CIGS) as solar cells [9] (Fig. 7.10). Over the past few years, metal chalcogenides (sulphide, selenides and tellurides) are studied in the form of thin film due to their applications as a photovoltaic solar cell, photoconductive cell, as well as with variety of optoelectronic devices. Semiconductors belonging to “II-VI along with III-V group” have more attraction due to their potential applications in photoconductors, detectors, optoelectronic devices, etc. [10–12]. Binary compounds of group IIB and VIA elements, commonly known as IIVI compound semiconductor, exhibit larger degrees of ionic bonding than IIIV compound semiconductor. Till today, “Cadmium-”based chalcogenide family of group II-VI semiconducting material is more attracted towards thin film by researchers because of their well characteristics behaviour and their uses in the area of electrical, energy source for the production of solar cells, optoelectronic devices [13, 14], light emitting diodes, optical filters, gas sensors, etc. Most important member of group II-VI compound is to be represented as “cadmium sulphide (CdS)”. It is n-type semiconductor material, and its energy bandgap equals to 2.42 eV [15, 16]. It is available in the form of cubic as well as hexagonal phase. These phases depend upon various features together with deposition process. The energy bandgap shows the light absorption edge and then the spectrum of visible light changes into electricity with the help of solar cell. Impurities added into binary semiconducting material cause dramatic change in their increasing properties. Semiconducting material lying in group (II-VI compound as ternary) has taken much attention than the group (II-VI compound as binary) because their physical characteristics maintain shape of particle together with the surface morphology, mole fraction, etc. A lot of focus has been lately given to the doping of CdS nanostructures like “indium (In3+ ), aluminium (Al3+ ), gallium (Ga3+ ), boron (B2+ ), manganese (Mn2+ ), iron (Fe2+ ), zinc (Zn2+ ),” etc., which change the

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optical, electrical and magnetic properties. Among these, transition metal such as zinc (Zn2+ ) whose ionic radius (=0.074 nm) can be less than ionic radius (= 0.097 nm) of cadmium (Cd2+ ). Therefore, in an easy way, Zn2+ could be incorporated with CdS crystal lattice by replacing “cadmium” ion which improves its properties. When zinc ion is added with CdS, its energy bandgap changes from 2.4 to 3.5 eV [17]. In place of CdS together with greater bandgap of ZnCdS can be attributed to decrement of window absorption losses as well as increment of short-circuit current density for solar cell [18–24]. The narrow bandgap partially blocks the transmission of high energy photon to the absorber layer below and decreases the power conversion efficiency. When CdS is replaced by Znx Cd1−x S, the wider bandgap leads to higher quantum efficiency in blue region of spectrum. Due to incorporation of zinc with cadmium sulphide buffer layer, decrement of lattice constant along with lattice is matched with CIGS absorber to produce suitable alignment of conduction band. So, this is to be confirmed that addition of Zn improves open-circuit voltage together with circuit current density of device which could be assigned to greater conversion efficiency of CIGS solar cells [25, 26]. ZnCdS is also useful for creation of p-n junction without lattice mismatch in devices on “quarternary materials such as CuInx Ga1−x Se2 or CuIn(Sx Se1−x )2 ” [27, 28].

7.8.1 Importance of ZnCdS Film Recently, various works have been done by several researchers on “synthesis, their characterization” of wide bandgap semiconductors like ZnO, SnO2 , etc., because of its uses on photovoltaic, photoelectrochemical for converting energy, photoconductors, etc. Interest in ZnCdS ternary material can be considerable due to expected increasing development which is based on working condition of thin film photovoltaic cells as: cadmium telluride along with CuInSe2 -based cells with the help of ZnCdS in comparison to CdS. This expectation arises because of the increasing bandgap of the zinc containing solid solution, as a result in increasing transparency to shorter wavelength of light. For heterojunction formation, as the zinc concentration increases, electron affinity related to semiconductor decreases. Measurement of position for conduction band with respect to vacuum energy level can be referred to as “electron affinity of semiconductor”. Hence, the alignment of the conduction band of ZnCdS together with second semiconductor can be maintained to a large extent through different film composition (Table 7.4).

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Table 7.4 Different characteristics of zinc, cadmium and sulphur Characteristics

Zinc

Cadmium

Sulphur

Symbol

Zn

Cd

S

Appearance

Silver grey

Silver bluish grey metallic

Lemon yellow sintered microcrystals

Atomic weight

65.38

112.41

32.065

Atomic number

30

48

16

Block

d-block

d-block

p-block

Electron configuration

3d10 4s2

4d10 5s2

3s2 3p4

Electrons per shell

2,8,18,2

2,8,18, 18,2

2,8,6

M.P.

692.68 K

594.22 K

388.36 K

B.P.

1180 K

1040 K

717.8 K

Density

7.14

g/cm3

8.65

g/cm3

2.07 g/cm3

Heat of Fusion

7.32 kJ/mol

6.21 kJ/mol

1.727 kJ/mol

Thermal conductivity

116 w/(m-k)

96.6 w/(m-k)

0.205 w/(m-k)

Thermal expansion

30.2 μm/(m-k) at 25 °C

30.8 μm/(m-k) at 25 °C

–

Electrical resistivity

59 n-m at 20 °C 72.7 n-m at 22 °C

2 × 1015 -m at 20 °C

Young’s modulus

108 GPa

50 GPa

–

Bulk modulus

70 GPa

42 GPa

–

Shear modulus

43 GPa

19 GPa

Poisson’s ratio

0.25

0.30

–

Structure of crystal

Hexagonal (close-packed)

Hexagonal (close-packed)

Orthorhombic

7.8.2 Chemical Reactions Involved for the Synthesis of ZnCdS Thin Films with Aqueous Medium The role of ammonia is suitable for synthesis of ZnCdS thin films. Ammonia solution will combine into the precursor of cadmium salt and zinc salt bath solution to create 2+ ions of “cadmium tetraamine [Cd(NH3 )2+ 4 ] as well as zinc tetraamine [Zn(NH3 )4 ]”. The chemical reaction during the process of deposition of ZnCdS thin film is given as the formation of ammonia ion. − NH3 + H2 O → NH+ 4 + OH

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The introduction of ammomium hydroxide into precursor of cadmium salt and zinc salt bath solution, generate ions as “cadmium tetraamine [Cd(NH3 )2+ 4 ] as well ], and the chemical reaction is given as: as zinc tetraamine [Zn(NH3 )2+ 4 CdCl2 + 2NH4 OH → Cd(OH)2 + 2NH4 Cl Cd2+ + 4NH3 → Cd(NH3 )2+ 4 2+ + 4NH3 [Cd(NH3 )]2+ 4 → Cd

ZnCl2 + 2NH4 OH → Zn(OH)2 + 2NH4 Cl ZN2+ + 4NH3 → Zn(NH3 )2+ 4 Zn(NH3 )2+ 4 → Zn + 4NH3 The sulphide ions are removed because in alkaline medium, thiourea (precursor of sulphur) decomposes and the chemical reaction may be given as: − CS(NH2 )2 + 3OH− → 2NH3 + CO2− 3 + HS

HS− + OH− → S2− + H2 O These three source ions are added into mixed reaction to form ZnCdS, and the reaction is shown below:     + x Zn(NH3 )2+ + S2− + NH3 → Cd1−x Znx S (1 − x) Cd(NH3 )2+ 4 4

7.8.3 Growth Process Through CBD Growth process can be achieved using CBD method and have three regions as given in Fig. 7.11. 1. Period A—is an induction period 2. Period B—one species at a time reacts 3. Period C—colloids formed in the solution.

7 ZnCdS Thin Film: Preparation, Properties and Applications

203

Fig. 7.11 Growth process of ZnCdS thin films by CBD method

7.8.4 Preparation of ZnCdS Films Using CBD Method in Non-aqueous Medium The ZnCdS thin films can be developed using several techniques like spray pyrolysis, sol–gel technology, ion beam deposition, vacuum evaporation, molecular beam epitaxial growth, CBD, etc. Among them, chemical baths deposition (CBD) process has great interest because it is low-cost technique, versatile, and deposition over large area can be achieved to deposit ZnCdS films. This process requires a magnetic stirrer, glass or metal plate acts as substrates, thermometer, low cost chemicals, etc. Large number of researchers have used aqueous medium for the deposition of ZnCdS film using this technique. But flexibility of higher temperature, deposition, solubility of the solutes, deposition over large area, evolution of hydrogen atom, etc., is the constraints for deposition. The limitations of the aqueous systems are overcome by using non-aqueous medium. ZnCdS films had been grown on glass substrates using CBD technique in an aqueous medium containing precursor of cadmium like CdCl2 , CdSO4 , Cadmium acetate, etc., and for zinc like ZnCl2 , ZnSO4 , zinc acetate, etc., and precursor of sulphur like thiourea, Na2 S, etc. The setup consists of borosil made glass reaction cell containing the electrolyte. A magnetic stirrer/hot plate is used for agitation of the electrolyte. The substrates used in this work are glass and molybdenum plates. The substrate has been fixed vertically with the help of stand. Initially, the glass substrates are successfully washed with hot soap solution, distilled water and finally rinsed with pure water. The substrates are then kept in an oven for drying (Fig. 7.12).

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Fig. 7.12 Experimental set-up of ZnCdS films by CBD method

7.8.5 Preparation of Bath The electrolyte has been prepared by taking AR grade of 0.2 M cadmium chloride (CdCl2 ), 0.2 M zinc chloride (ZnCl2 ) and is dissolved with 40 ml of ethylene glycol. The electrolyte temperature is controlled at 160 °C under continuous stirring for two hours. The molybdenum substrates whose dimension 1.5 cm × 1 cm × 0.1 cm is polished using 600 grit carborundum paper, continuously washed with the help of hot soap solution, boiled distilled water and then with distilled water. With the help of rigid support, molybdenum substrate is inserted vertically into the electrolyte. At time t = 0, 0.4 M thiourea (precursor of sulphur) is mixed with the electrolyte under moderate stirring and the film is deposited for 15 min. After completion of the deposition, films are cleaned with the help of distilled water in order to release of the counter ions and organic impurities. The ZnCdS film is deposited on molybdenum substrate which is physically stable and well adhesive. The deposited films are found to be free from voids, pits, pin holes, etc. Again, the concentration of Zn is varied from 0.2 to 0.5 wt% by the same procedure with the help of following formula: X = ZnCl2 /CdCl2 + ZnCl2

7.9 Properties of ZnCdS Thin Films 7.9.1 Structural Properties X-ray diffraction (XRD) is an important characterization technique which is used to know the structure, nature, quality, lattice parameters, orientation, defects stress, strain, etc., inside the materials. It is also an important tool to know the relative abundances of the phases in a mixture and their qualification. The basic principle for XRD is related to Bragg’s law.

7 ZnCdS Thin Film: Preparation, Properties and Applications

205

Fig. 7.13 X-ray diffraction from different planes following Bragg’s law

When monochromatic X-ray impinges on the atoms of crystal lattice, each atom behaves like a scattering source but crystal lattice behaves as series of parallel reflecting planes. The intensity of reflected beam at definite angles has been maximum if the path difference between two reflected waves from two different planes is an integral multiple of λ. The statement can be referred to as Bragg’s law. Regular array of spherical waves are obtained due to regular array of scatterers. In most of the directions these waves cancel out each other due to destructive interference. They add some particular directions which can be determined through Bragg’s law: 2d Sin θ = nλ where θ is angle of incidence, λ is wavelength of the X-ray, n is an integer and “d” is the spacing between diffracting planes (Fig. 7.13). The XRD pattern of ZnCdS films for structural analysis is reported. The structural analysis of ZnCdS film has been carried out by XRD along with Cu K α wavelength 1.54 Å. Sharp peaks available in XRD gives information that the deposited film are polycrystalline in nature. The as deposited ZnCdS thin films are either cubic (zincblende) or wurtzite (hexagonal) phase. XRD pattern confirms that the compound transformed into binary to ternary. Due to annealing, the deposited film changes its phase and crystallite size. Using Scherer’s formula, crystallite size of as deposited as well as annealed ZnCdS thin films can be calculated by the formula as given below [29]. The reported crystallite size of ZnCdS thin film prepared by above method is found to be 36 nm [30] (Table 7.5). D=

Kλ β cos θ

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Table 7.5 Reported standard value along with observed XRD data [30] JCPDS File no.: 49-1302 CdZnS (2θ value) (degree)

hkl values (JCPDS)

Observed (2θ) (degree)

26.54

(002)

26.539

43.77

(110)

43.7459

52.89

(201)

52.831

Reprinted from S. Kumar, S. Rajpal, S. K. Sharma, D. Roy, S. R. Kumar, Chalcogenide Letteres, vol. 14, No. 1, (2017) p. 17–23 with kind permission from the publisher

where K = Constant (=0.94), λ = X-ray wavelength (=1.54 Å), β = Full width at half maxima, θ = Diffracted angle.

7.9.2 Influence of Annealing Annealing is required because for heat treatments of the prepared samples at desired temperature focus to elaborate his characteristics. It is done because the crystalline of the deposited films improves. Annealing of material can be attributed to phase transitions, recrysytallization, polygonization, removal of after effects of cold plastic deformation, rearrangement of defects, homogenization and so on. Annealing depends on its kinetics: the rate of heating and cooling and the time of exposure at a given temperature. Due to annealing, average crystallite size may increase which confirms the improvement of the crystalline nature of the deposited film. As the grain boundary reduces, average grain size increases along with recombination centre decreases in grain boundaries. According to research, it can be attributed as increment of annealing temperature, grain size increases, but strain and dislocation density of films decreases (Table 7.6). Microstrain () of both the deposited films is calculated by the given equation: ε=

β cos θ 4

Table 7.6 Observed 2θ, d value and miller planes of as deposited and annealed ZnCdS films using CBD method in non-aqueous medium [30] Compound

Observed value

Standard value

(2θ in degree)

“d” value

Miller indices

(2θ in degree)

“d” value

Miller indices

ZnCdS As deposited

26.72

3.42

(002)

26.48

3.36

(002)

ZnCdS Annealed

28.42

3.13

(111)

28.20

3.16

(111)

7 ZnCdS Thin Film: Preparation, Properties and Applications Table 7.7 Average crystallite size, microstrain of as deposited along with annealed ZnCdS thin films using CBD method carried on non-aqueous medium [30]

207

Compound

FWHM (degree)

Average crystallite size (nm)

Microstrain (×10−3 )

ZnCdS as deposited

0.2378

36

1.008

ZnCdS Annealed

0.2080

41

0.882

Reprinted from S. Kumar, S. Rajpal, S. K. Sharma, D. Roy, S. R. Kumar, Chalcogenide Letteres, vol. 14, No. 1, (2017) p. 17–23 with kind permission from the publisher

In case of annealed ZnCdS films, the microstrain decreases that supports the recrystallization process in polycrystalline material (Table 7.7).

7.9.3 Influence of Varying Zn Concentration 0.2 to 0.5 Wt% on ZnCdS Thin Films Using the Relation (X = ZnCl2 /CdCl2 + ZnCl2 ) in Chemical Bath Films characteristics are affected because influence of solution content can be measured with the help of different Zn content of solutions carried out in the chemical bath. Dislocation density (δ) of deposited ZnCdS films has been measured with the help of following relation: δ=

1 D2

where D is the grain size. It may be also possible that the increase in grain size and defects such as dislocation density decreases along with the film thickness. Relative percentage error has been calculated [31] using the formula R=

ZH − Z X 100 Z

where Z H is the observed “d” value and Z is the standard value. The reported crystallite size of ZnCdS with varying Zn concentration (0.2– 0.5 wt%) has been found 30, 29, 28 and 23 nm [32]. From the reported data, it can be concluded that decrease of grain size depends upon the increase of Zn concentration (Tables 7.8 and 7.9).

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Table 7.8 Average crystalline size, microstrain of varying Zn concentration (0.2 to 0.5 wt%) as deposited ZnCdS films using CBD method in non-aqueous medium [32] Varying Zn 2θ (degree) concentration x = ZnCl2 /(CdCl2 + ZnCl2 )

‘d’ (Å)

Full width at Grain size plane half maxima (nm) (hkl) (in degree)

Micro-strain (× 10−3 )

0.2

26.64

3.343

0.2789

30

002

0.678

0.3

26.64

3.342

0.2941

29

002

0.715

0.4

26.70

3.335

0.3038

28

002

0.738

0.5

26.68

3.337

0.3689

23

002

0.897

Reprinted from “Digest Journal of Nanomaterials and Biostructures”, vol. 12, No. 2, 2017, p. 339– 347 with kind permission from Publisher

7.10 Surface Morphological Properties 7.10.1 Field-Emission Scanning Electron Microscopy (FESEM) It is one of the important characterization methods for knowing the parameters like surface morphology of the deposited films; i.e., it indicates that the surface is smooth or rough. It is also an important tool to know the idea about the surface which may be free from voids, pits, pinholes, etc. It has taken a beam with energetic electrons in place of light and is emitted through source of emission. Due to interaction with electron on the sample, signal has to be observed which gives suggestion about crystalline structure, composition of chemical, morphology, etc. FESEM works on the principle “field-emission cathode of SEM which consists of thick probing electron beams with small as well as greater energy, so both developed spatial resolution along with sample charging and breaking (Fig. 7.14). The working principle of FESEM has been shown by schematic diagram in Fig. 7.14 where the electron beams are emitted from a cold source. It consists of a tungsten needle (sharp, thin, tip dia = 10−7 to 10−8 m) kept near primary and secondary anode which acts as a cathode. Between cathode and anode, voltage is taken to be 0.5–30 kV. The beam of electron is focused through electro-magnetic lenses along with the apertures on column to a tiny spot. Object has to be coated through a conductive layer and covered over a holder. Object has been placed by an exchange chamber onto high vacuum section of microscope with anchored in rotating condition. Bombardment of primary probe to object results the removal of secondary electrons by object surface with definite velocity and can be measured through levels along with angles at surface of object. Both angles as well as velocity of secondary electrons are correlated with surface structure of object. Electronic signal can be generated if secondary electrons are captured by detector, so it can be amplified as well as send to a video scan image which is seen in monitor or digital image is to be seen.

3.586

3.359

3.163

2.451

2.058

1.899

(100)

(002)

(101)

(102)

(220)

(103)

1.891

2.062

2.443

3.148

3.344

0.42

0.19

0.32

0.47

0.44

1.892

2.063

2.446

3.149

3.342

3.566

Obs. ‘d’ (in Å)

d % error 0.36

Obs. ‘d’ (in Å)

3.573

Content (0.3)

Content (0.2)

Zn concentration (x = 0.2, 0.3, 0.4 and 0.5)

0.36

0.24

0.20

0.44

0.50

0.55

d% error

1.891

2.059

2.443

3.145

3.342

3.576

Obs. ‘d’ (in Å)

Content (0.4)

0.42

0.04

0.32

0.56

0.50

0.27

d% error

1.888

2.044

–

3.132

3.337

–

Obs. ‘d’ (in Å)

Content (0.5)

Reprinted from “Digest journal of Nanomaterials and Biostructures”, vol. 12, No. 2, 2017, p. 339–347 with kind permission from Publisher

Stnd. ‘d’ values (in Å)

Plane (hkl)

0.57

0.68

–

0.98

0.65

–

d% error

Table 7.9 Relative percentage error of varying Zn concentration (0.2 to 0.5) as deposited ZnCdS films using CBD method carried on non-aqueous method [32]

7 ZnCdS Thin Film: Preparation, Properties and Applications 209

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Fig. 7.14 Schematic diagram of FESEM

Figure 7.15 shows the surface morphology of the as deposited and annealed ZnCdS film. Performance of field-emission scanning electron microscope (FESEM) creates images, however gives the idea about surface of the film as well as orientation and distribution of grains. It is clearly seen that surface morphology of as deposited together with annealed ZnCdS film is taken at a magnification of 15,000×. Indication of image (a) that surface of film is well covered with the homogenous hexagonal morphology structure deposited material without any void, pinholes, cracks free, etc. In film surface, grains are compact and densely distributed over the surface. As deposited ZnCdS film also informs that the film surface is not rough, uniform- and spherical-shaped grains are directed either in single form as well as in cluster forms.

Fig. 7.15 FESEM images of a as deposited and b annealed ZnCdS films [30]. Reprinted from S. Kumar, S. Rajpal, S.K. Sharma, D. Roy, S.R. Kumar, “Chalcogenide Letters”, vol. 14, No. 1, 2017, 17–23, with kind permission from the publisher

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211

From image (b), it has been concluded that the grains are spherical, densely packed, uniform, crack free, bigger size as compared to as deposited films. Annealing helps in evaporation of excess sulphur and results of thinning of film. It is noticed that due to annealing, the films conversion of cluster grains into isolated grains took place. More compaction is attributed and indicates that annealing of films at certain temperature, grains in cluster form, changes into isolated grains.

7.10.2 Influence of Varying Zn Concentration (0.2–0.5) on Surface Morphology of ZnCdS Thin Films From the micrograph study, it reveals that the films are uniform, grains are distinct, spherical and uniformly distributed throughout the analysed area. It is clear from the images that the surfaces of the films are compact and grains are interconnected to each other throughout the film. Distributions of grains are very much dependent on Zn concentration. Figure reveals that grains density increases as concentration of Zn varies from 0.2 to 0.4. The grains are unevenly distributed over the surface because of generation of soluble particle into solution. Surface of ZnCdS film gives clear information that it is not rough, uniform along with same size of grains whose size and shape are spherical. Zn = 0.3 concentration coated on CdS film assigned that equal size grains are distributed among the whole surface area with cluster forms. Increment of Zn = 0.4 concentration introduced on CdS film can be attributed to big sphericalshaped grains in cluster form with the entire area, and unavailability of cloudy surface can be noticed. Again, increment of Zn = 0.5 concentration introduced on CdS film gives the information that the spherical grains were observed and considered as fibres structure, completely spread over the film surface (Fig. 7.16).

7.11 Optical Properties Optical characteristics of semiconductors consists refractive index (n) and extinction coefficients (k) or absorption coefficient (A) associated with dispersion relations; i.e., it depends upon the wavelength (λ), energy of photon (hν), and changes involve the dispersion relations with temperature, pressure, alloying, impurities, etc. Absorptions are classified as mentioned below: (i)

Lattice absorption—the radiation has to be absorbed due to the vibrations of crystal ions. (ii) Free-carrier absorption—the decreasing effect is observed with increasing photon energy due to availability of free electrons and holes. Such type of absorption results lattice phonons excitations, accelerates free electrons in conduction band or formation of an exciton. (iii) Impurity absorption band—It is because of the several dopants.

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(a) Zn=0.2

(c) Zn=0.4

(b) Zn=0.3

(d) Zn=0.5

Fig. 7.16 FESEM images of as deposited ZnCdS films for varying Zn concentration [32]. Reprinted from S. Kumar, S. Rajpal, S.K. Sharma, D. Roy, S.R. Kumar, “Digest Journal of Nanomaterials and Biostructures”, vol. 12, No. 2, (2017), P. 339–347, with kind permission from the publisher

(iv) Exciton absorption—The peaks are mainly attributed near the absorption edge with small temperature. (v) Fundamental absorption process—Involvement of photons absorption whose energies are equal or greater than E g (bandgap energy) and is referred as fundamental absorption process. It can be accompanied through an electronic transition over the forbidden gap, and hence, the formation of excess electron-hole pairs takes place in the semiconductor.

7.11.1 Optical Absorption and Energy Bandgap Present days, thin films are considered in several forms: “binary, ternary together with quaternary” have been working successfully, and they are reported in the field of photovoltaic solar cells because in the semiconductor technology its manufacturing price is low and range of energy bandgap (1–2 eV).

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213

Absorption coefficient (A) helps for carrying out the explanation about the optical absorption and can be derived through transmission or absorption measurements. If I o = Intensity of incident light, I = Intensity of transmitted light and R = Reflectivity, then Transmission (T ) = I/I o which can be written in the form T = (1 − R)2 exp(−αt)/1 − R 2 exp(−2αt)

(i)

where t be the thickness of the material. For high At, the expression can be reduced to T = (1 − R)2 exp(−αt)

(ii)

If the reflection is not present, it is again reduced to I = Io exp(−αt)

(iii)

α = (2.303 × A)/t

(iv)

In terms of absorbance (A),

In a direct transition, inter-relation with absorption coefficient and incident photon energy is given by: n  αhν = A hν − E g

(v)

where A = constant, E g = energy bandgap. The value of n = 1/2, 3/2 for direct allowed and direct forbidden transitions. Hence, the graph plotted between (Ahν)2 versus hν allows the determination of energy bandgap. Figures 7.17 and 7.18 indicate the absorption spectra of as deposited and annealed ZnCdS thin films. Absorption coefficient (A) and bandgap (E g ) are related in direct bandgap materials. According to the Tauc’s relation graph plotted between (Ahν)2 with hν, extrapolation of curve gives the information about the energy bandgap (E g ) of ZnCdS films. It is reported to be 2.45 eV in the case of as deposited film and 2.52 eV for the annealed film [30]. The electronic energy bandgap parameter of semiconductor like alloys together with its dependency over alloy composition is most valuable. The energy bandgap of a ternary alloy is dependent on the relative concentration of the constituent elements. The optical properties of the film produce fruitful significance which can be attributed to development of optical devices. The study of optical absorption of materials gives information for measure necessary feature related with materials band structure. One of the most essential characteristics of semiconductor is “optical

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Fig. 7.17 Bandgap of as deposited ZnCdS film. Reprinted from S. Kumar, S. Rajpal, S.K. Sharma, D. Roy, S.R. Kumar, “Effect of annealing on the surface and optical properties of ZnCdS Nanocrystalline Thin Films”, Chalcogenide Letters, vol. 14, No. 1, (2017) p. 17–23 with kind permission from the publisher (Ref. [30])

Fig. 7.18 Bandgap of annealed ZnCdS film. Reprinted from S. Kumar, S. Rajpal, S.K. Sharma, D. Roy, S.R. Kumar, “Effect of annealing on the surface and optical properties of ZnCdS Nanocrystalline Thin films”, Chalcogenide Letters, vol. 14, No. 1, (2017) p. 17–23 with kind permission from the publisher (Ref. [30])

bandgap” because it can be helpful to construct a photovoltaic, photoelectrochemical solar cell. The information related to optical bandgap and behaviour of transition involved optical properties of ZnCdS films. The values of energy bandgap depend with film crystal structure, its placing, division of atoms upon crystal lattice, and so this is to be affected through crystal regularity. Using UV-visible absorption spectroscopy, optical bandgap of semiconductor can be successfully measured. The optical bandgap of some compound semiconductors is given in Table 7.10 [33]. As per the one reported data mentioned above, energy bandgap of the as deposited and annealed ZnCdS films synthesized through CBD method in non-aqueous medium is found to be 2.45 and 2.52 eV using Tauc’s relation. The reported values when Zn content varied from x = 0.2, 0.3, 0.4 and 0.5%, the energy bandgap of deposited

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215

Table 7.10 Energy bandgap of some II-VI, III-V and I-VII semiconductors at RT [33] II-VI

Energy bandgap (E g ) eV

III-V

Energy bandgap (E g ) eV

I-VII

Energy bandgap (E g ) eV

CdS

2.50

GaSb

0.71

AgI

3.02

CdTe

1.43

InAs

0.36

CuBr

3.07

CdSe

1.75

InP

1.26

CuCl

3.39

ZnSe

2.67

GaAs

1.43

AgBr

2.68

ZnTe

2.28

GaN

3.44

–

–

ZnS

3.56

GaP

2.27

AgCl

3.25

A-HgS

2.1

InSb

0.18

CuI

3.11

β-HgS

–

AlN

6.2

–

–

ZnO

3.20

AlSb

1.63

AgF

2.8

HgSe

–

AlP

2.51

–

–

HgTe

–

AlAs

2.15

–

–

films changes and their values are 2.54, 2.65, 2.77 and 2.85 eV. It indicates that bandgap increases due to increase in Zn concentration, which is due to formation of solid solution. Higher Zn content increases the energy bandgap which suggest the availability of secondary phase with small amount. Increase in bandgap is attributed due to the replacement of Cd2+ ions by Zn2+ in the CdS lattice.

7.12 Applications of ZnCdS Thin Film Due to technological applications, requirement of thin film on the basis of “crystalline and amorphous nature” are most important because it is used in the broad field of optoelectronics, semiconductor instruments, photovoltaic solar cells, LEDs, LCDs, infra-red detectors, films act as superconductor, photoconductors, transistors, wireless communications, telecommunications, works on satellite to maintain temperature, as a recorder instruments (like magnetic thin film), microelectronic instruments, both magnetic and gas sensor, interference filters, integrated circuits, computer chips, rectifiers, audio with video system, electro-optic and decorative coatings and so on. Among these applications of thin films, ternary ZnCdS thin film is one of them, and their various roles are mention below: 1. Ternary ZnCdS thin films acts as buffer or window layers over optoelectronic devices like CIGS and CdTe solar cells. Growing interest towards the ZnCdSbased solar cell is due to their performance, and also, it is suitable material for p-n junction formation.

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2. ZnCdS thin film is a direct bandgap semiconducting material, and at room temperature, its bangap varies from 2.4 to 3.7 eV. So, it can be used in the field of interference filters, optical fibres, optical instruments, coated glazing for windows, solar energy collectors, flat panel solar cells, photoluminescent, photoconductor, photodetectors, photoresistors. [34]. 3. In hetrojunction solar cells, decrement of window absorption losses through replacing CdS (2.4 eV) with greater bandgap of ZnCdS (2.4–3.7 eV) shows the possibility for increment of short-circuit current density. 4. ZnCdS thin film photoelectrode assigned together with the energy bandgap equals to 2.69 eV can be useful to 0.11% power conversion efficiency [35]. 5. ZnCdS thin films act as a role of electro catalytic and photoelectrocatalytic.

7.13 Applications of Thin Film Technology 7.13.1 Engineering/Processing • Tribological Applications: Protective coatings to reduce wear, corrosion and erosion • Surface passivation • In refractory metals, it is used for pipes, rocket nozzles, etc. • Hard coatings for cutting tools • Decorative coatings • Protection affiants high temperature corrosion • Catalysing coatings.

7.13.2 Optics • • • • •

Very high reflected coatings Beam splitter as well as thin film polarizers Antireflex coatings Integrated optics Interference filters.

7.13.3 Optoelectronics • • • •

LCD/TFT Photodetectors Optical memories Image transmission.

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7.13.4 Electronics • • • •

Charge coupled device Active thin film elements (transistors, diodes) Passive thin film elements (resistors, condensers, interconnects) Integrated circuits (VLSI).

7.13.5 Cryotechnics • • • • • • • • •

Superconducting thin films, switches, memories Superconducting quantum interference devices New materials Superhard carbon (Diamond) Amorphous silicon Metastable phases: metallic glasses Ultrafine powders Spheroidization of high M.P. materials (Diameter-1–500 μm) High purity semiconductors (GaAs).

7.13.6 Magnetic Applications • Audio, video and computer memories • Magnetic read/write heads.

7.13.7 Sensorics • Data acquisition in aggressive environments and media • Telemetry • Biological sensorics.

7.13.8 Biomedicine • Biocompatible implant coatings • Neurological sensors • Cladding for depot pharmaca.

7.14 Conclusion The syntheses of semiconductor thin film by various methods have been extensively discussed in this chapter. The emphasis is given to study the synthesis process and

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different properties of ZnCdS thin film. In this chapter, the structural, morphological and optical behaviours of the ZnCdS thin film have been discussed in detail. When Zinc is added to CdS, its bandgap changes from 2.42 to 3.5 eV. Due to increase in bandgap, open-circuit voltage together with circuit current density of device also increases and attributed to greater conversion efficiency of CIGS solar cells. Requirement of semiconductors in devices depends upon amplification characteristics, switching behaviour, etc. Due to technological applications, thin film semiconductors are most important because it is used in the broad field of optoelectronics, semiconductor instruments, photovoltaic solar cells, LEDs, LCDs, infra-red detectors, photoconductors, magnetic and gas sensor, interference filters, integrated circuits and many more applications.

References 1. Colvin VL, Schlamp MC, Alivisatos AP (1994) Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer. Nature 370:354–357 2. Devoret Michel H, Schoelkopf Robert J (2000) Amplifying quantum signals with the singleelectron transistor. Nature 406:1039–1046 3. Gao X, Cui Y, Levenson RM, Chung LWK, Nie S (2004) In vivo cancer targeting and imaging with semiconductor quantum dots. Nat Biotechnol 22:969–976 4. Kumar R, Anjum KN, Rani S, Sharma K, Tiwary KP, Kumar KD (2019) Material properties of ZnS nanoparticles incorporated soy protein isolate biopolymeric film. Plast Rubber Compos 48(10):1–8 5. Pawar SM, Pawar BS, Kim JH, Joo O-S, Lokhande CD (2011) Recent status of chemical bath deposited metal chalcogenide and metal oxide thin films. Curr Appl Phys 11:117–161 6. Fritz KP, Guenes S, Luther J, Kumar S, Sariciftci NS, Scholes GD (2008) IV–VI nanocrystal– polymer solar cells. J Photochem Photobiol A Chem 195(1–5):39–46 7. Arici E, Hoppe H, Schäffler F, Meissner D, Malik MA, Sariciftci NS (2004) Hybrid solar cells based on inorganic nanoclusters and conjugated polymers. Thin Solid Films 451–452:612–618 8. Kumar S, Tiwary KP (2020) ZnCdS thin film chalcogenide by chemical bath deposition method. Nanotrends A J Nanotechnol Appl 22(1):19–27 9. Repins I, Glynn S, Duenow J, Coutts TJ, Metzger WK, Contreras MA (2009) Required material properties for high-efficiency CIGS modules. Thin Film Solar Technol 7409:74090 M-1–74090 M-14 10. Britt J, Ferekides C (1993) Thin-film CdS/CdTe solar cell with 15.8% efficiency. Appl Phys Lett 82(22):2851–2852 11. Tiwary KP, Choubey SK, Sharma K (2013) Structural and optical properties of ZnS nanoparticles synthesized by microwave irradiation method. Chalcogenide Lett 10(9):319–323 12. Kaur I, Pandya DK, Chopra KL (1980) Growth kinetics and polymorphism of chemically deposited CdS films. J Electrochem Soc 127(4):943–948 13. Gu Y, Kwak ES, Lensch JL, Allen JE, Odom TW, Lauhon LJ (2005) Near-field scanning photocurrent microscopy of a nanowire photodetector. Appl Phys Lett 87:043111–043113 14. Pillai PKC, Shroff N, Tripathi AK (1983) A study of the photoconducting properties of CdSSe(Cu) with a view to its use in solid-state image intensifiers. J Phys D Appl Phys 16(3):393–399 15. Lee J-H (2006) Structural and optical properties of CdS thin films on organic substrates for flexible solar cell applications. 17:1103–1108 16. Choubey SK, Tiwary KP (2016) Structural, morphological and optical investigation of CdS nanoparticles synthesized by microwave assisted method. Dig J Nanomater Biostructures 11(1):33–37

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17. Kumar SR, Kumar S, Sharma SK, Roy D (2015) Structure, composition and optical properties of nonaqueous deposited ZnCdS nanocrystalline film. Mater Today Proceed 2:4563–4568 18. Ramakrishna Reddy KT, Jayarama Reddy P (1992) Studies of Znx Cd1−x S films and Znx Cd1−x S/CuGaSe2 heterojunction solar cells. J Phys D Appl Phys 25(9):1345–1348 19. Kim HS, Im HB, Moon JT (1992) Effects of cell width on the photovoltaic properties of sintered Cd1−x Znx S/CdTe solar cells. Thin Solid Films 214(2):207–212 20. Jun YK, Im HB (1988) Effects of thickness and sintering conditions of CdS films on the photovoltaic properties of CdS/CdTe. J Electrochem Soc 135(7):1658–1661 21. Basol BM (1984) High-efficiency electroplated heterojunction solar cell. J Appl Phys 55(2):601–603 22. Mitchell KW, Fahrenbruch AL, Bube RH (1977) Evaluation of the CdS/CdTe heterojunction solar cell. J Appl Phys 48(10):4365–4371 23. Torres J, Gordillo G (1992) Photoconductors based on Znx Cd1−x S thin films. Thin Solid Films 207(1–2):231–235 24. Yamaguchi T, Yamamoto Y, Tanaka T, Demizu Y, Yoshida A (1996) (Cd, Zn)S thin films prepared by chemical bath deposition for photovoltaic devices. Thin Solid Films 281–282:375– 378 25. Selin Tosun B, Pettit C, Campbell SA, Aydil ES (2012) Structure and composition of Znx Cd1–x S films synthesized through chemical bath deposition. ACS Appl Mater Interfaces 4(7):3676– 3684 26. Song J, Li SS, Yoon S, Kim WK, Kim J, Chen J, Craciun V, Anderson TJ, Crisalle OD, Ren F (2005) Growth and characterization of CdZnS thin film buffer layers by chemical bath deposition, pp 449–452. IEEE 27. Yamaguchi T, Matsufusa J, Yoshida A (1992) Optical transitions in RF sputtered CuInX Ga1-x Se2 thin films. Jpn J Appl Phys 31(6A):L703–L705 28. Walter T, Ruckh M, VeIthnus KO, Schock HW (1992) In: Proceedings of 11th EC photovoltaic solar energy conference, p 124. Montreux 29. Raza khan Z, Zulfequar M, Shahid khan M (2010) Effect of thickness on structural and optical properties of thermally evaporated cadmium sulfide polycrystalline thin films. Chalcogenide Lett 7(6):431–438 30. Kumar S, Rajpal S, Sharma SK, Roy D, Kumar SR (2017) Effect of annealing on the surface and optical properties of ZnCdS nanocrystalline thin films. Chalcogenide Lett 14(1):17–23 31. Padiyan DP, Marikani A (2002) X-ray determination of lattice constants of Cdx Sn1-x Se mixed crystal systems. Cryst Res Technol 37(11):1241–1248 32. Kumar S, Rajpal S, Sharma SK, Roy D, Kumar SR (2017) Effect of Zn concentration on the structural, morphological and optical properties of ternary ZnCdS nanocrystalline thin films. Dig J Nanomater Biostructures 12(2):339–347 33. A Thesis (Master of Science), “Investigation the optical properties of Cd1-X ZnX S thin films deposited by the dip technique. Thesis submitted to the Department of physics, Nkrumah University of Science and Technology, in the year February 2010 34. Sain S, Patra S, Pradhan SK (2012) Quickest ever single-step mechanosynthesis of Cd0.5 Zn0.5 S quantum dots: nanostructure and optical characterizations. Mater Res Bull 47(4):1062–1072 35. Dongre JK, Chaturvedi M, Patil Y, Sharma S, Jain UK (2015) Dip coated nanocrystalline CdZnS thin films for solar cell application. In: International conference on emerging interfaces of plasma science and technology (EIPT-2015), AIP conference proceedings, vol 1670, pp 030007-1–030007-4

Chapter 8

Nanomaterials for Pharmaceutical Applications Sundar Singh, S. B. Tiwari, and Sanjeev Tyagi

Abstract The field of medicine and pharmaceuticals has experienced revolutionary changes due to the development of nanotechnology. Materials or their constructions with size less than 100 nm along at least one dimension are termed as nanoparticulates. Nanoparticulates possess a greater surface area to volume ratio in comparison with bulk materials with the same composition to provide them enhanced selective therapeutic activity and are useful in the pharmaceutical field besides many other applications. Novel formulations of nanomaterials are being developed for drug encapsulation, targeted drug delivery systems (TDDs), and diagnostic purposes. Examples include polymeric nanoparticles in the form of nanospheres and nanocapsules; colloidal drug carriers such as micelles, dendrimers; phospholipid-based drug delivery systems, e.g., liposomes, phytosomes, ethosomes, etc.; solid-lipid nanoparticles (SLNs); niosomes; biphasic systems such as nanoemulsions and nanocomposite hydrogels; quantum dots; carbon nanotubes (CNTs), etc. TDDs involve direct application of drugs to the desired individual tissues with minimal damage to non-target tissues and organs resulting in the lesser requirement of drugs with enhanced biological response and protection from physical and chemical degradation. Nonmaterials not only increase the therapeutic value of drugs but also reduce their toxicological effects compared to conventional therapies. Nanomaterials, especially lipidic systems have excellent biotransformation response besides enhancing solubility and bioavailability of drugs. Nanoparticle-based dosage forms utilizing enhanced permeation and retention (EPR) effect through the biological barriers help in improving the pharmacokinetic profile and pharmacodynamic activity of the drugs. This chapter is devoted to various nanoparticulates (synthetic, semi-synthetic, and natural) having existing and potential applications in the pharmaceutical field.

S. Singh (B) Department of Physics, Bareilly College, Bareilly 243005, India e-mail: [email protected] S. B. Tiwari Department of Pharmacy, MJP Rohilkhand University, Bareilly 243006, India S. Tyagi Faculty of Engineering & Technology, MJP Rohilkhand University, Bareilly 243006, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_8

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8.1 Introduction The scientific study of materials and structures made out of these materials having sizes of the order of nanometers (billionth part of a meter) comes under the purview of nanoscience, which is a fast-developing and most fascinating discipline of scientific and academic discipline. The application part of principles and concepts developed in nanoscience manifests itself into what we call nanotechnology, which is the combination of, or you can say integrates several technologies used in research and industry for designing and making functional objects and materials at the nanoscale [1]. In nanotechnology, at least one dimension of structure or material lies between 1 to 100 nm. Norio Taniguchi, a Japanese scientist, is credited for using the term nanotechnology for the first time around 1974; though people were using nanotechnology in its different forms much before than that. The research field of nanotechnology being multidisciplinary attracts and engages not only persons from physics, chemistry, and engineering streams but also bio-scientists and environmentalists and is affecting or has the potential to greatly influence every aspect of human life [2]. Although nanoscience and nanotechnology have existed in nature from the very beginning of life on earth, as is manifested by several natural structures (adhesive and self-cleaning property of Gecko foot, self-healing underwater property of diatoms, self-cleaning properties of leaves of lotus flower, etc.), its evolution and recent progress are mainly attributed to renowned scientist R. P. Feynman. In 1959, Feynman at a meeting of American Physical Society delivered a famous lecture entitled, “There’s Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics.” Through his talk, Feynman proposed that atom-by-atom manipulation of the matter might be possible in the future. He floated the idea that the existence of nanosized materials would one day become a reality. K. Eric Drexler, a great theoretical physicist from Germany did a lot of work for the development of nanotechnology and in 1986 wrote a book entitled, “Engines of Creation: The Coming Era of Nanotechnology.” He contributed significantly to the further development of nanotechnology. Drexler’s approach was based on a nanoscale assembler capable of assembling atoms and molecules into larger structures. The observation of phenomena on the nanoscale was not possible until the invention of advanced instruments like scanning tunneling microscope (STM) and atomic force microscope (AFM). These ultra-high resolution microscopes have made possible the visualization at nanoscale, and especially STM has led to successful atom-by-atom manipulation. The invention of such high powered instruments resulted in the tremendous rate of development in the field of nanotechnology. In the modern era, the rapid growth seen in the field of nanotechnology has its basis into two important breakthroughs that occurred in the 1980s. The invention of STM in 1981 by Gerd Binnig and Heinrich Rohrer of IBM Zurich research laboratory proved to be the first step forward in this direction. The STM with its unparalleled observational capabilities led not only to the possible visualization on the individual atomic and bonds level but also imaging of surfaces of the samples. Manipulation of individual atoms could be successfully done for the first time in 1989 by employing the

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STM. The other important breakthrough in the developmental journey of nanotechnology occurred in the year 1989 with the discovery of fullerene (C60 ) molecule, an allotrope of carbon, by a group of three scientists namely Harold W. Kroto, Richard E. Smalley, and Robert F. Curl. Fullerene was found to form graphene sheets that can be rolled into graphene tubes called carbon nanotubes (CNTs) with diameters of few nanometers. In 1991, CNTs were actually discovered by a Japanese scientist S. Iijima. These cylindrical macromolecules of carbon with several potential applications in nanoscale electronics and devices resulted in great advancements on the horizon of the material. The CNTs, an allotrope of carbon with a very large aspect ratio (i.e., length/diameter), were found to have amazing mechanical, optical, thermal, and electrical properties. Nanoscience and nanotechnology are for most of the part concerned with the crystalline solids lying in the nanometer size regime. Two basic approaches in nanotechnology are top-down and bottom-up for the synthesis of nanomaterials, which are materials at the nanoscale and possess very different electrical, mechanical, thermal, optical, magnetic, and electronic properties as compared to the same materials in the bulk form. A three-dimensional material whose all external dimensions are larger than the exciton Bohr radius for that material is said to be a bulk material. Any material with at least one external dimension lying in the size range from 1 to 100 nm may be classified as a nanomaterial. It should be noted that even a nanomaterial can have an internal structure lying in the nanoregime. Nanomaterials might also possess surface structure but that too must lie in the nanosized range. Although nanomaterials are very small in size even this small size is much larger than the size of individual atoms and molecules. In fact, even the smallest nanomaterial is made up of at least thousands of atoms and molecules. Therefore, nanoparticles consist of a certain number of atoms or molecules. This is why on the basis of the size, nanomaterials lie between isolated atoms or molecules and bulk materials (which consist of thousands of atoms). Stated another way, nanomaterials (or nanoscale materials) have a physical size that is smaller than bulk macroscopic materials but larger than that of molecular compounds. Nanomaterials can be broadly classified into two categories: • Non-intentionally made nanomaterials—These nanomaterials are of two types. The first category of such nanomaterials is directly available in nature in the form of proteins, viruses, nanoparticles produced during volcanic eruptions, etc. The other category of nanomaterials is created unintentionally by humans, the representative examples of which are nanoparticles created out of the combustion of diesel. This second category of nanomaterials is produced as a by-product of some important process. • Engineered nanomaterials—These are one of the important classes of nanomaterials produced through deliberate human activities for some specific purpose. In most cases, engineered nanomaterials are developed for their use in medical applications for the purpose of diagnostics and therapy. Nanomaterials exhibit very different properties in comparison with the corresponding bulk materials because of two major factors that are:

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(i) For the same volume, nanomaterials have a greater surface area in comparison with the corresponding bulk materials. In other words, nanomaterials have a higher surface area to volume (S/V) ratio. This is because with the decrease in size of a material an increasing number of atoms lie on the surface of the material. As a result, the surface area increases for the same mass of the material. Larger S/V ratio for nanomaterials makes them very useful for applications such as catalysis. Materials become more reactive on the nanoscale. (ii) Quantum effects begin to dominate at the nanoscale. These effects give rise to very unusual properties to nanoscale materials. Commercialization of products manufactured by using nanotechnologies includes cosmetics, sunscreen lotions, paints, medicines, etc. Titanium dioxide (TiO2 ) and zinc oxide (ZnO) nanoparticles are used in many sunscreen lotions as a means of blocking ultraviolet (UV) radiation more effectively [3]. Silver nanoplatform use silver nanoparticles (NPs) which are potential antibacterial agents [4]. Carbon nanotubes are being used for producing stain-resistant textiles. A substance consisting of nanoparticles is termed as being nanoparticulates. The properties of nanoparticulates substances differ greatly when compared with the bulk form of the same substance. Nanoparticulates are suitable for their use in medicines and also in drug delivery systems which are being used in the treatment of various diseases such as cancer. The use of techniques involved in nanotechnology for the development of drugs makes the whole process safe, cost-effective, and highly sensitive. The focus of current research and development in the pharmaceutical field is on personalized medicine which integrates therapeutics with diagnostics. For the effective treatment with lesser side effects of patients suffering from serious diseases, several drug delivery systems are being developed. A drug delivery system represents the method or process of administering a pharmaceutical compound (i.e., a dose) to produce a therapeutic effect not only in humans but also in animals. For this purpose, several drug delivery systems have been formulated. Lack of choosiness, inadequate efficacy, and lower biodistribution are some of the major limitations associated with conventional drug delivery systems. Targeted drug delivery system primarily aims at concentrating the medication in the tissues of interest but simultaneously tries to reduce the relative concentration of the medication in the non-target tissues [5]. The branch of science which is concerned with the discovery of compounds known as drugs for the treatment of patients is termed as pharmaceutics. In the pharmaceutical field research and development mainly targets several issues, e.g., (i) the discovery of novel functional nanomaterials, (ii) identifying nanoparticles with controlled and targeted drug delivery characteristics, and (iii) investigation of biofunctionalized materials and nanomaterials suitable for diagnostic purposes [6]. Some of the commonly used drug delivery systems include liposomes, proliposomes, niosomes, microspheres, hydrogels, prodrugs, cyclodextrins, and solid-lipid nanoparticles, etc. Nanoparticulates due to their small size have greater activity and enhance the bioavailability, solubility, and efficacy of the drug [7]. Polymeric nanoparticles in the form of nanocapsules and nanospheres have been researched greatly as drug

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delivery agents. Liposomes, micelles, dendrimers, and solid-lipid nanoparticles are some of the other nanoparticulates that are found to be suitable for their use in targeted drug delivery systems. Their use leads to the reduced loading of the drug.

8.2 Role of Nanoscience and Nanotechnology in Pharmaceuticals Each pharmaceutical dosage form is designed and developed by four basic criteria which are as follows: • Therapeutic constituent/drug/active moiety—Smallest chemical pharmaco element or structure responsible for the physiological or pharmacological action of the drug substance • Solvent—Any material that is capable of dissolving some other material in it is known as the solvent. The material which gets dissolved in the solvent is called the solute. Both the solvent and the solute may be in either of the three physical states, i.e., solid, liquid, or gas. Some of the common solvent materials include water, methanol, ethanol, toluene, chloroform, acetone, etc. • Carrier—High selectivity, improved effectiveness and/or adequate safety of drug administration must be the prime concern for any treatment protocol. These issues are taken care of by using certain substances, known as carriers in the process of drug delivery. Another role played by the carriers is the controlled release of drug into the systemic circulation. Carrier material improves the pharmacokinetic parameter and bioavailability of the drug. • Excipient or diluents—Excipients are the materials that ensure an enhancement in the therapeutic index of the active ingredients that are in the final dosage form. In the field of nanotechnology or nanoscience, the range of active moiety is taken from a nanometer to several micrometers. Important properties to be considered during the synthesis of new pharmaceutical compounds include permeability, solubility, amount of ionization, stability in biological fluids, gastrointestinal metabolism, systematic pharmacokinetics and pharmacodynamic and protein binding characteristics. Carriers used in the transport of drugs to the targeted sites have to overcome challenges posed by their drawbacks because of the physicochemical properties of drugs like short half-life, poor solubility, low permeability, and high molecular weight. The increased surface area on the nanoscale increases the absorption of the drug. The presence of both hydrophilic and hydrophobic environments on the nanodrugs gives rise to an enhanced solubility. A point of major concern about the drugs is that many drugs have a very narrow therapeutic window. So the therapeutic concentration (i.e., the concentration of drug needed for producing therapeutic effects) is not much lower than the toxic one. This necessitates the requirement for administering drugs in very accurate amounts to avoid their toxic effects.

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Drug delivery systems based on lipids and polymeric nanoparticles are capable of improving the therapeutic properties of the drugs and enhancing their pharmacological response. Micelles, liposomes, SLNs, polymeric nanoparticles, nanoemulsions, and dendrimers are some of the important nanoparticulates used for drug delivery systems. Nanomaterials having different shapes and properties (physical, chemical, and biological) and varied functionalities can be created by using the principles of nanoscience and nanotechnology. The nanomaterials with above-mentioned distinctive characteristics play a key role in the field of pharmaceuticals. A particular class of nanomaterials for which the chemical structure or properties provide them the capability of responding to the biocatalytic action of enzymes is called smart nanomaterials. These smart nanomaterials are sensitive to the enzymes present in the host environment and therefore can serve as a promising tool in diagnostic and therapeutic applications [8]. Polymer materials, phospholipids, and inorganic materials belong to the category of enzyme responsive smart nanomaterials. A certain class of nanomaterials is sensitive to the pH of the host environment and is therefore called pH-responsive nanomaterials. This kind of nanomaterials has been used to design sensitive drug delivery nanosystems for use in cancer therapy. These pH-responsive nanomaterials are capable of stabilizing the drug at the level of physiological pH and then releasing the drug at the pH trigger point [9]. Let us now discuss some of the nanoparticulates materials which are very important for pharmaceutical applications.

8.3 Nanomaterials in Imaging and Diagnostics Nanomaterials are the materials with unique physical and chemical properties like very small size, large surface area to mass ratio, and high reactivity. There occurs a drastic change in properties in the bulk to nanotransition. For instance, bulk gold is yellow in color, whereas gold nanoparticles may have different colors depending upon their size. Nanomaterials, because of the existence of unique physicochemical properties, are capable of overcoming some of the limitations of traditional therapeutic and diagnostic agents which makes them very suitable for their use in applications such as in medicines and pharmaceuticals. Some of the nanoparticles that are suitable for imaging purposes include dendrimers, quantum dots, mesoporous silica, liposomes, gold nanoparticles, micelles, iron oxide nanoparticles (IONPs), etc. Nanoparticles that can be used in diagnostic imaging may be of organic (e.g., liposomes, dendrimers, polymers) as well as the inorganic type (e.g., IONPs, gold NPs, carbon-based nanomaterials, silica, etc.). Nanoparticle-based therapeutic and diagnostic agents have been developed by several research groups and are being used in the treatment of diseases such as diabetes, cancer, asthma, and allergy. Some nanomaterials can be used as probes for bioimaging for early stage diagnoses of diseases [10]. The use of nanoparticles provides us better imaging techniques useful for diagnostics. Some of the important nanoparticle-based imaging techniques employ light, magnetic field

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of nuclei, ultrasonic waves, and radioactive nuclei. Accordingly, the imaging techniques are called photo (or optical)-imaging, magnetic resonance imaging, ultrasound imaging, and radionuclide imaging. The substances that are capable of emitting their own light after being excited with light of suitable wavelength are called fluorophores or fluorochromes. These substances possess adequate absorption properties as well as emission quantum yields. The absorption and subsequent emission of light radiations by certain organic and inorganic substances are called fluorescence or phosphorescence. Fluorescence occurs almost instantaneously after the excitation of the sample with light (time lag is less than 1 μs), whereas, in phosphorescence, the light emission persists long after the removal of excitation. The optical imaging (OP) technique is based on fluorescence emitted from fluorophores such as semiconductor nanocrystals called quantum dots (QDs), certain protein molecules with fluorescent properties, and complexes of transition metals with luminescent properties. The optical imaging technique possesses certain advantages such as high temporal and spatial resolutions. Some nanomaterials when doped with lanthanide (Ln) ions acquire amazing optical properties such as the capability to convert two or lower energy photons into a single high energy photon. This unique class of nanoparticles is known as upconversion nanoparticles (UCNPs). These lanthanide-doped upconversion nanoparticles are very useful for biomedical field, especially for optical imaging technique because they exhibit several advantages over conventional fluorophores, e.g., lower cardiac toxicity, absence of photobleaching property (chemical destruction of fluorophores taking place during excitation is called photobleaching which can severely reduce the time of observation of the sample), high spatial resolution, and the non-existence of random switching between ON and OFF states of the fluorophores [11]. Another category of efficient upconversion nanoparticles is that of fluoride-based upconversion nanoparticles which can also be used as fluorophores in the optical imaging technique. One of the important routes for the treatment of tumors is through the application of photodynamic therapy (PDT). A. V. Lantsova et al. studied the response of tetra-3phenylthiophthalocyanine aluminum hydroxide (lipophthalocyan), a liposome-based drug, specifically for its anti-tumor activity. The findings of their study are suggestive of the possible use of this drug for the photodynamic therapy of tumors of surface localization [12]. Semiconductor nanocrystals or quantum dots are important zerodimensional nanomaterials which possess amazing and very useful optical properties like minimal photobleaching, higher fluorescence, and longer systemic circulation times [13]. Most quantum dots have core–shell structure (in which smaller bandgap semiconductor nanoparticles, i.e., the core is encapsulated with another semiconductor material, called the shell having larger bandgap) and are useful nanomaterials for imaging applications as they exhibit broad excitation bands and narrow emission bands. Quantum dots conjugated with certain other materials such as folate, antibodies, or antigens have shown better imaging performance. Another widely used imaging technique is that of magnetic resonance imaging (MRI). This imaging technique is non-invasive (i.e., carried out without cutting or putting instruments inside the patient’s body) and makes use of magnet of high strength and radiofrequency (RF) waves for producing image films of internal parts

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of a patient’s body. The orientation of nuclei with an odd number of protons or neutrons in an external magnetic field provides a non-zero magnetic moment to the nuclei which the basis of this imaging technique. Several types of nanoparticles have been explored to be used as MRI contrast agents. Notable among them are colloidal nanoparticles of iron oxide, e.g., superparamagnetic iron oxide nanoparticles and nanoparticles of ultra superparamagnetic iron oxide. Both active targeting and passive targeting have been explored with these nanoparticles in MRI imaging. For example, active targeting of superparamagnetic iron oxides can result in an increase in the contrast enhancement of tumors as compared to the normal tissues (i.e., non-targeted tissues). In active targeting, nanoparticle surfaces are decorated with ligands binding to receptors that are overexpressed on the tumor (i.e., target) cells. Active targeting enhances drug penetration and increases the therapeutic efficiency of the drug. However, in passive targeting, small drug molecules are encapsulated within nanosized drug carriers which not only improve the pharmacokinetics of the drug but also provide tumor selectivity to some extent besides reducing its side effects. The tumor targeting by iron oxide nanoparticles can be improved by attaching molecules such as proteins, peptides, antibodies, and oligosaccharides to their surface [14]. An example of a substance used as ligand is folic acid which is a vitamin that has been widely used as a targeting agent. Some nanomaterials have been used in ultrasound imaging and radionuclide imaging. Ultrasound imaging provides the real-time imaging of internal organs of the human body and makes use of sound waves of high frequency. The radionuclide imaging technique utilizes a small quantity of a substance called tracer which is a radioactive isotope and is capable of detecting certain disorders of the human body such as trauma, cancer, and infection. In this imaging technique, the radionuclide is most commonly injected into a vein but can also be inhaled, swallowed, or injected under the skin. Different radionuclides are used depending upon which part is to be imaged. Technetium is the most commonly used tracer and is most suitable for the imaging of bones. Advancements in biotechnology and nanotechnologies could provide us for the development of multifunctional nanoparticles that are capable of performing several tasks synergistically, for example, simultaneous delivery of multiple bioactive with imaging agents, site-specific delivery by means of surface ligand decoration, and in theranogtics. Such multifunctional nanoparticles besides accomplishing multiple objectives can also perform a single advanced function by the integration of multiple functional units. Synergetic means involving the interaction of two or more agents to produce a combined effect which is more than the sum of their individual effects. Thus, multimodal imaging involves the integration of several imaging agents with diverse properties so as to provide us with much faster and precise diagnostics of diseases. Multifunctional nanoparticles of gold can perform the function of contrast agents for photoacoustic imaging, optical imaging, and computed tomography (CT). Similarly, nanoshells and nanocages of gold can be used in photothermal therapies. The multimodal imaging integrates the advantages of different imaging techniques.

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8.4 Nanomaterials in Therapeutics Nanomaterials not only have diagnostic applications but also many therapeutic applications because they can overcome certain limitations of conventional therapeutic agents. Some of the useful characteristics of nanoscale materials which are invaluable for the therapeutic applications include their ability, (i) to penetrate and spontaneously accumulate at biological sites because of EPR effect, (ii) across the physiological barriers freely, and (iii) for recognizing and binding with the desired site through specific surface attaching ligands. Because of these abilities, nanomaterials find applications in various therapies for curing different kinds of diseases. One of the important therapies used for the treatment of cancer is photodynamic therapy (PDT). In this treatment methodology, disease-affected cells and tissues are destroyed with the help of reactive oxygen species (ROS) which can be generated using a combination of light of suitable wavelength and photosensitizers [15]. The photosensitizer molecules are instrumental in the transfer of photon energy to the surrounding oxygen molecules which leads to the creation of reactive oxygen species. These ROS upon being irradiated with light of suitable energy destroy cancer cells. PDT has improved selectivity and lesser side effects over traditional chemotherapies or radiotherapies. There are numerous advantages associated with photodynamic therapy, e.g., it can be used even after performing other therapies such as radiotherapy, chemotherapy, and surgery. Further, PDT can be used time and again without producing immunosuppressive or myelosuppressive effects [16]. Nanomaterials help in increasing the water solubility of photosensitizers and in improving their delivery to cancer affected cells. Prominent among them are the upconversion nanoparticles (UCNPs). Such nanoparticles are capable of converting the light of lower energy (near-infrared light) to that of higher energy (visible light). These higher energy visible photons are capable of activating photosensitizer molecules that are absorbed on the NPs and produce reactive oxygen species. Then, the ROS molecules destroy the cancer cells. Gold nanoparticles are also very useful for photodynamic therapy. Their characteristics make them capable of maintaining the stability and activity of water-hating (i.e., hydrophobic) photosensitizer molecules in aqueous environments. The limitations of photosensitizers such as lower solubility and smaller selectivity of cancer affected cells can be overcome by the use of gold nanoparticles as their delivery vehicles [17]. Quantum dots can be employed for PDT of cancer patients in which they are absorbed in the tumor and makes the absorption of radiation by these cancer affected cells more readily than their surroundings [18]. Further, it has been observed that quantum dots can also form complexes with photosensitizer molecules. The advantages associated with the conjugations of photosensitizers and quantum dots over photosensitizers alone are: (i) higher efficiency of photoluminescence emission, (ii) tunable optical properties, and (iii) better imaging performance. Quantum dots when coated with peptides become capable of forming stable conjugates with photosensitizer molecules. This makes them very suitable to be used as targeted multifunctional probes for targeting and imaging of the living cells and also for photodynamic therapy [19].

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Electromagnetic radiation can also be applied in the treatment of various diseases like dermatological disorders and cancers with much fewer side effects. This approach which utilizes vibrational (i.e., heat) energy of photosensitizer molecules is termed as photothermal therapy (PTT) and is possible with longer wavelength (i.e., less energy) light. Because of the use of lower energy light, PTT causes less harm to those cells and tissues which are not affected by the disease. Carbon nanotubes are sensitive enough for the absorption of near-infrared light and then producing heat which can be used for destroying cancer cells. According to some study, it has been observed that multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) can be given as intra-tumor injections which when irradiated with near-infrared laser light at the power levels of 2.5 and 3.8 W/cm2 becomes capable of destroying tumors developed in mice [20]. PEGylation functionalized single-walled carbon nanotubes acquire characteristics that make them highly suitable for in vivo cancer treatment. Another nanomaterial useful from the PTT point of view is that of nanographene because it can also absorb near-infrared light. PEGylated nanographene sheets (NGS) show higher efficiency as compared to PEGylated CNTs when used in passive targeting of tumors. Moreover, PEGylated NGS shows relatively low retention in RES when compared with PEGylated CNTs. Gold nanoparticles in the form of nanorods, nanoshells, and hollow nanoparticles have also been found to be useful for PTT because of very fast absorption of light and exhibiting red-shifted properties. These materials are capable of converting nearly 100% absorbed light into heat through non-radiative transitions and therefore can function as photothermal contrast agents for PTT. Au nanoshells have also shown highly efficient near-infrared hyperthermia ability because of its strong near-infrared absorption. Au nanorods have also been shown to work as near-infrared hyperthermia agents for PTT [21]. Magnetic nanoparticles (MNPs) inside a tumor in the patient’s body in presence of alternating magnetic field (AMF) of suitable amplitude and frequency are capable of raising the temperature in the tumor so as to kill cancer cells. This kind of therapy is termed as magnetic hyperthermia (MH), in which MNPs such as those of magnetite convert electromagnetic energy into heat and make it a powerful, non-invasive therapy for curing diseases. The effect of MNPs on tumors in the presence of AMF has been studied extensively. The findings of these studies show that the MNPs can cause the temperature at the tumor sites to increase up to 42–46 °C, which leads to a reduction in the growth of cancer cells. Studies have shown that the specific absorption rate (SAR) varies according to the size of nanoparticles. Monodispersed MNPs have a larger SAR as compared to polydispersed MNPs [22]. The performance of MNPs can further be improved by the optimization of surface coating. Liu et al. [23] synthesized monodispersed magnetite nanoparticles coated with mPEG and observed that it results in a significantly high thermal conductivity and high SAR which directs their use as high-performance hyperthermia agents. Another important therapy for the treatment of cancer is sonodynamic therapy (SDT) which utilizes low-intensity ultrasound and specialized chemical compounds, called sonosensitizers. The ultrasound can be focussed onto a specific small region of the tumor and can penetrate deep into the tumor. The function of ultrasound in SDT

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is to activate the sonosensitizers which are capable of removing solid tumors noninvasively. In other words, the ultrasound activates the cytotoxic effect of sonosensitizers. TiO2 nanoparticles and nanopowder have been found to be very useful for SDT because of their ability to generate various reactive oxygen species upon irradiation with ultrasound [24].

8.5 Nanomaterials in Drug Delivery Nanomaterials can be used for the efficient delivery of certain drugs. The use of nanomaterials for the delivery of drugs involves two steps. The first one is that of drug targeting, and the second one is concerned with the controlled release of these drugs. Nanomaterials used for drug targeting must be capable of avoiding the normal immune response of the body so that they can reach their desired sites. Nanoparticles drug delivery systems can be either passive target type or active target type. Passive targeting is possible due to the EPR effect. Doxil, a PEGylated liposomal doxorubicin delivery system is used for cancer therapy and abraxane which is an albuminbound paclitaxel nanoparticles can target metastatic breast cancer. Attaching target molecules to the nanoparticles surface results in active targeting which overcomes limitations of passive targeting. In active targeting, the delivery system (nanoparticles) has a homing device with the function of recognizing or binding to the complementary molecules (i.e., receptors) found on the tumor surface. This homing device is capable of guiding the carrier to the desired site. The addition of such molecules to the drug delivery nanoparticles (e.g., Au NPs) causes them to target the tumor cells. It also enhances the efficacy of the treatment along with reduced toxic effects on the healthy tissues. Controlled release of drugs is another important issue that must be taken care of while using nanomaterials for pharmaceutical applications. Drug delivery carriers not only possess the property of high drug load but also the drug must be released in a controlled manner, otherwise, it might lead to toxic effects to healthy or normal tissues. Nanomaterials used as drug delivery vehicles avoid these limitations. Several nanomaterials, such as polymeric nanoparticles, micelles, dendrimers, liposomes, niosomes, and solid-lipid nanoparticles, are suitable as delivery vehicles for various drugs used in the treatment of diseases. We shall discuss in detail some important nanomaterials for pharmaceutical applications in this chapter. The drug delivery systems must have certain characteristics such as (i) being structurally stable, (ii) the ability to deliver large amounts of the drug, and (iii) no premature release of the drug. These characteristics of nanomaterials used in the delivery of drugs help in increasing the efficacy of the treatment. The release of the drug may be responsive to various factors such as pH or stimuli. Several chemical substances can be used as gatekeepers which control the encapsulation and release of the drug. Mesoporous silica nanospheres (MSN) with the facility of gatekeeping by certain nanomaterials such as gold NPs, silver NPs, and dendrimers have been explored by researchers [25]. An advanced concept of drug delivery is based on multifunctional nanocarriers that

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have the ability to deliver therapeutic payloads and/or agents for the enhancement of contrast in the images used for diagnostic purposes to the desired sites. Paclitaxel is an anticancer drug that is near-infrared fluorescent and when it is loaded into nanoparticles of chitosan then it provides us with the simultaneous facilities such as cancer diagnosis in the early stage, delivery of drugs, and monitoring in real time [26]. Multifunctional nanomaterials have been developed which are capable of performing several functions simultaneously such as in vivo imaging, delivery of siRNA, and silencing in tumors. Let us now discuss different nanomaterials used as drug delivery vehicles.

8.5.1 Micelles A surface active, amphiphilic chemical compound which increases the reactivity of the precursors, is called a surfactant. A surfactant can be used as soaps or detergents. Surfactants can also perform the wetting action, foaming action, or may be used for emulsification. Each surfactant molecule has two parts—hydrophilic (i.e., waterloving) head and hydrophobic (i.e., water-hating) tail. A colloid is a mixture in which one substance is spread out uniformly inside another substance. Colloidal particles have diameters between 1 and 1000 nm. Micelles are nanosized colloidal dispersions that are formed by the aggregation of a large number of molecules of the surfactant which are dispersed in a liquid colloid. They are very small in size which may vary from 2 to 80 nm. Micelles were first discovered by Ringsdorf in 1984 and are vesicles in which drugs can be trapped and administered more efficiently. They are lipid molecules with a spherical shape. In fact, micelles have amphiphilic character (i.e., having both hydrophilic and hydrophobic parts) and present an example of colloidal structures that are formed in aqueous solutions. There always exists equilibrium between the molecules or ions of the solution forming the micelles and the micelles itself. There are two types of micelles—normal phase micelle and reverse-phase micelle. In the normal phase micelle (or oil-in-water micelle), the surfactant molecules aggregate in such a manner so that their hydrophilic heads lie in contact with the surrounding solvent and the hydrophobic tails arrange themselves in the core of the micelle as shown in Fig. 8.1a. Besides water, amphiphilic molecules are also capable of forming micelles in nonpolar organic solvents such as carbon tetrachloride and benzene. But the micelles so formed are just opposite of those formed in water and are therefore called reverse micelles or inverse micelles. These are water-in-oil micelles and are formed when hydrophilic parts of surfactant molecules aggregate in the core region with the hydrophobic parts constituting the shell of the micelle as shown in Fig. 8.1b. Since in the inverse micelles, polar heads lie in the core region, therefore, they are capable of holding a relatively large amount of water in their core. This is just like a pocket present inside the inverse micelles. Reverse micelles, because of the presence of a pocket-like structure in their core become highly suitable for dissolving and transporting a polar solute through a non-polar solvent. Micelles can also be formed by

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(b) Reverse Micelle

Fig. 8.1 Micelles [27]

placing phospholipids (which are building blocks of biological membranes) in an aqueous solution (e.g., water). The process of formation of micelles is called micellization. This micellization occurs mainly as a result of balance between two important effects: (i) water-hating tendency of hydrophobic tails, and (ii) the repelling tendency among the polar (or charged) heads which destabilizes the aggregation process. When the concentration of surfactant molecules in the solution is low, no micelles are formed. If this concentration is increased then at a critical value, surfactant molecules aggregate together in the liquid to form groups, called micelles. This critical value of concentration at which micellization just begins or the first micelle appears in the solution is known as the critical micelle concentration (CMC). When the concentration of surfactant molecules in the solution is higher than CMC, micelles are formed spontaneously. This causes the number of micelles in the solution to increasing but the concentration of free surfactant molecules remains unaffected. However, when the concentration of surfactant molecules in the solution is lower than CMC, no micelles are formed. Critical micelle concentration is different for different types of surfactants depending upon their ionic or non-ionic nature. Non-ionic surfactants generally have lower CMC values (10−4 to 10−3 M) compared to ionic surfactants (10−3 to 10−2 M). The formation of micelles takes place spontaneously as a result of balance between two physical characteristics of the solution, viz., entropy and enthalpy. The net change in the entropy (which is a measure of disorder present within the system) of the aqueous solution is negative during the process of micelle formation. The change in entropy, in this case, is due to two effects. Above CMC, there is a decrease in entropy due to the assembly of surfactant molecules. As a second effect, there is an increase in the entropy because of freedom given to those water molecules which were bound in the salvation shells of the surfactant molecules. This increase in entropy is smaller than the decrease in entropy due to aggregation of surfactant molecules, thereby making the net change in entropy negative. During the micelle formation, the temperature of the system should be maintained higher than the critical micelle temperature which is known as the Krafft temperature. However, the micelle

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formation cannot take place when the temperature of the solution is higher than a certain temperature. There are certain factors that determine the shape and size of a micelle. These are (i) molecular geometry of its surfactant molecules, and (ii) characteristics of the solution such as the concentration of surfactant, pH, and ionic strength. When water is used as a solvent, the hydrophobic interaction is the main driving force behind micellization. Hydrophobic parts of the surfactant are repelled by the water molecules and hence avoid contact with the molecules of solvent (i.e., water) and point toward the core region of the aggregate. As a result, there is a lack of water in the core region of the micelle. The presence of counterions on the surface of micelle causes a decrease in the repulsive force between the polar heads and favors the micelle formation. The hydrophobic interaction between hydrophobic parts (tails) of surfactant molecules promotes their association in the core region of the micelle and leads to the formation of micelles. The surfactant molecules which are although present within the aqueous solution but do not specifically belong to any micelle are called monomers. These molecules are in the free form. When micellization is in progress, the monomers and micelle are always in equilibrium with each other. The number of surfactant or polymer molecules (i.e., number of monomers) in a micelle is termed as the aggregation number, which is an important factor for determining the size of the micelle. The aggregation number generally lies between 50 and 10,000. In most cases, micelles acquire spherical shape but other shapes can also be found for them. Some of the other possible shapes of micelles include worm-like micelle, vesicles, bilayer fragments, inverted structures, etc. The aggregation number is quite high for the rod-shaped or worm-like micelles in comparison with that for spherical micelles. The ratio between the number of counterions bound on the surface of the micelle and the total number of counterions present within the system gives the degree of counterion binding. Its value lies in the range of 0.2–0.8. The degree of counterion binding has a prominent effect on the shape and size of the micelles. Further, the structure and properties of a micelle can vary according to the counterion bound to the micelle surface. Micelles are capable of dissolving and moving non-polar substances through an aqueous medium because of which they find extensive applications in the industrial and biological fields. Micelles can be formed from detergents, emulsifiers, various wetting agents as well as certain copolymers. Colloidal micelles because of their small size have little circulation time which makes them enter effortlessly inside tumor cells as a consequence of the EPR effect. The micelles work as efficient nanocarriers when they are used in diagnostic imaging and for the delivery of drugs and genes. Colloidal micelles are particularly suitable for the delivery of drugs whose solubility in water is very small. Polymeric micelles (5–50 nm), also known as block copolymer micelle, are organized auto-assembly formed in a liquid. Such polymeric micelles have a structure of core–shell type and can be formed from amphiphilic di- or tri-block copolymers consisting of solvophilic (i.e., having affinity to a particular solvent) and solvophobic (i.e., lacking an affinity to a particular solvent) blocks. The hydrophilic blocks in polymeric micelles are longer than the hydrophobic blocks. A comparison of polymeric micelles with surfactant micelles tells us that they are more stable than surfactant

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micelles and have relatively longer circulation time. Because of these characteristics, polymeric micelles are best suited for applications such as the delivery of drugs. There use for drug delivery leads to improved biodistribution. Block copolymer micelles can entrap the drugs physically inside their core and can transport them at concentrations which can be higher than their solubility in the water [28]. Polymeric micelles have generally low CMC values as compared to colloidal micelles and serve as efficient nanocarriers for applications such as in diagnostic imaging and drug and gene delivery (i.e., a non-viral carrier system). Polymeric micelles generally suffer from the problem of drug leakage during storage. Hoda Soleymani Abyaneh et al. developed three-layered block copolymer micelle having drug compatible blocks at the inner core and those incompatible with the drug at the outer core. As per their observation, these block copolymer micelles can lower the initial burst release of the drug [29]. The use of micelles as delivery vehicles requires that they should be sufficiently stable in blood circulation. No disintegration of micelles should take place when they are brought into contact with blood components. This is the reason why a very low CMC value is desirable. The basic difference between surfactant micelle and polymeric micelle is because of the different sizes of their building blocks (i.e., surfactant molecules or block copolymers). The molecular weights of monomers in the polymeric micelles (i.e., block copolymers) are much higher than the molecular weights of surfactant molecules (nearly a few hundred grams per mole). Polymeric micelles are called dynamic micelles, whereas surfactant micelles are kinetically frozen micelle. Block copolymers can have amphiphilic nature to a greater degree as compared to surfactant molecules. Polymeric micelles can undergo spontaneous aggregation in aqueous solutions and are capable of overcoming certain limitations of drugs such as insolubility and increased circulation half-life [30]. Another class of micelles is lipid-polymer conjugate micelles that show better stability and longevity. Micellar compositions can be used for parenteral, oral, nasal, and ocular applications. As a separate media, micelles find their use in the process of electrophoresis and chromatography [31]. They act as emulsifiers if the concentration of surfactant molecules is higher than CMC. This results in the dissolution of a compound that is usually insoluble. One important application of reverse micelles is their use for the synthesis of quantum dots or quantum wires. Quantum dots are zero-dimensional nanomaterials in which charge carriers are quantum confined in all three spatial dimensions. These are actually semiconductor nanocrystals (e.g., CdSe, CdTe, etc.) which possess important properties such as high quantum yield (number of photons emitted for every photon absorbed) and high extinction coefficient. They are sometimes referred to as artificial atoms or fluorescent nanoparticles. Quantum dots are particularly well suited for optical applications because of the property of high quantum yield [32]. Quantum wires are one-dimensional nanomaterial in which charge carriers are quantum confined along two dimensions but free to move along the third dimension and possess several important applications such as in LEDs, solar cells, and efficient sensors for toxic gases. For the synthesis of quantum dots, first of all, precursors which are surfactants or capping agents like bis (2-ethylhexyl) phosphate also

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termed as AOT and are utilized in the creation of reverse micelles of small size with hydrophilic parts lying in the core region and hydrophobic parts in contact with the surrounding solvent. Metal salts in the form of aqueous solutions are then introduced into the compartments containing water. After this, the reactions are allowed to proceed in the aqueous phase. The reduction of the metal precursor with the help of sodium borohydride as well as its reaction with another chalcogen like S, Se, or Te, result into the creation of metal or semiconducting nanoparticles (i.e., quantum dots). Nanoparticles possess the natural tendency of agglomeration, which must be discouraged for keeping their size small. The coating of nanoparticles with capping agents prevents their agglomeration. After the reaction, the agglomeration of nanoparticles can be stopped by adding some chemical which passivates their surface. The surface passivating agent also helps in extracting the quantum dots from the solution. The nanomaterial formed in this process is in the powder form which is again dissolved in some solvent for preserving it. The initial ratio of water to surfactant which is known as W or  value decides the average size of the quantum dots formed in this process. The nanoscopic size of micelles makes them suitable for holding the drugs tightly in their core region and for the subsequent controlled release of the drugs. These nanoparticles serve as excellent drug carriers because the controlled release of drugs can be accomplished by causing the molecular structure of the inner cores to change. These nanocarriers are suitable for the delivery of sparingly water-soluble anticancer drugs and for drugs that are less permeable through the intestinal lining. For this purpose, micelles are preferred over liposomes (phospholipids bilayer-based small spherical vesicles) due to easy encapsulation of drugs and easy surface manipulation. The therapeutic index of any drug is an important parameter to be considered for deciding the dose of the drug to cure some disease without producing toxic effects on normal tissues. It is measured by the pharmacological response of the drug and associated safety to the patient. Micelles are capable of carrying lipophilic drugs inside their core, whereas polar molecules can be attached to their surfaces whenever they are used as carriers for therapeutics in the aqueous media. Polymeric micelles when used as carriers provide improvement in the aqueous solubility of the drugs and also result in higher intestinal permeability. Drug molecules can be protected from being degraded because of the hydrolysis or other physicochemical reactions when they are administered through the micelles used as delivery vehicles. Micelles help in enhancing the shelf life of the drugs and also results into increased stability during use. Micelles are suitable nanocarriers to be used in the targeted drug delivery systems which aim at delivering the drugs to the targeted sites for producing therapeutic effects. These nanocarriers are not recognized as foreign material by the immune system of the patient’s body. In the human body, micelles help in the removal of complex lipids and of those vitamins which are soluble in the fat. The pH-sensitive micelles possess certain advantages over traditional nanoparticles because these micelles can give rise to the existence of the drug-loaded selfassemblies containing an amphiphilic di-block copolymer as the only excipient. Stimuli-responsive polymeric micelles loaded with therapeutic agents are seen as

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promising candidates for the treatment of tumor. This is because there are differences in physicochemical properties of tumor tissues and normal tissues and the sensitivity variations exist according to a change of endogenous or exogenous environments [33]. There are certain advantages associated with micelles when used as drug carriers. Some of these advantages are—(i) Micelles cause an increase in the water solubility of the poorly soluble drug and also leads to an improvement in the bioavailability of the drug, (ii) micelles result into a reduction of toxicity and other side effects, (iii) they cause the permeability through the biological barriers to increase, and (iv) micelles can protect the drug from being inactivated due to the effect of biological surrounding and avoids undesirable side effects on non-target organs or tissues. Micelles not only have advantages, but they do also have some disadvantages. An obvious limitation associated with micelles used as drug delivery carriers is that they have very low stability against encountering changes in the environment. A hopeful treatment strategy for cancer involves small interfering RNA (siRNA) which is a relatively new therapy and makes use of nucleic acid to treat diseases like cancer. For a sufficient increase in the accumulation of polymeric micelles, in vivo half-life of siRNA should be extended. This can be accomplished by the incorporation of hydrophilic groups in the polymeric micelles. Further, they must be exchanged with cations that have an electrostatic interaction with siRNA or they must be attached to different ligands for cell-specific targeting [34]. A change in the physicochemical properties and structures of polymers constituting the polymeric micelles can bring about a change in the several characteristics of polymeric micelles such as particle size; their stability, capacity for loading of drugs, and kinetics of the drug release.

8.5.2 Dendrimers A dendrimer is a nanosized, highly branched biological macromolecule (i.e., a polymer) that has a specific chemical composition. It is also known by the other names like “arborols” or “cascade molecules.” The term dendrimer combines within itself two Greek words “dendri” and “meros.” The word dendri means “tree-like” and the meaning of meros is a “part of.” Therefore, dendrimers have a tree-like shape as seen from the top which justifies their name. A dendrimer is not a new compound rather it is just an architectural design. Dendrimer synthesis is similar to self-assembly processes (i.e., a bottom-up approach). Fritz Vogtle synthesized one of the first dendrimers in 1978 using a divergent synthesis approach. A better approach for the synthesis of dendrimers is termed as convergent synthesis. Dendrimers are in general spherical in shape and the central unit from which they emerge out is known as the core. The dendrimer’s core is filled with a solvent and its exterior surface is homogeneous and has attached with it, several functional groups. Dendrimer nanomaterial has very low cytotoxicity (i.e., toxicity associated with cells) and high biopermeability. A dendrimer consists of a large number of branching levels and the number of branching levels between the interior core of the dendrimer and its surface is called the generation number of the dendrimer. Stated another way, the generation of

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a dendrimer can be defined as the number of repeated branching cycles that have to be performed during the synthesis of the dendrimer. The generation of dendrimers is denoted as G-0, G-1, G-2,… depending upon the number of repeated branching cycles while synthesizing them. Accordingly, the dendrimers are classified as zerogeneration dendrimer, first-generation dendrimer, second-generation dendrimer, and so on. Dendrimers are three-dimensional, well-organized, strong, and covalently fixed nanoscopic macromolecules and possess a low value of polydispersity index ( MMWn > 1, but low) which makes them very useful for the field of nanomedicine. Dendrimer nanoparticles belong to the category of functional nanomaterials are endowed with structural, electronic, optical, optoelectronic, magnetic, biological, and chemical properties that are unique to them. These multifunctional nanomaterials provide for targeting, imaging, diagnostic, and therapeutic applications. As per another definition dendrimers are artificially created macromolecules which are characterized by the presence of a very large number of functional groups on their surface with the molecular structure being very compact. The end groups serve as receptors for the attachment of functional groups such as catalysts, molecular switches, or light-sensitive chromophores. The dendrimer family is very exhaustive and consists of representative members known as dendrimers, dendrons, hyperbranched polymers, dendigraft polymers, and dendritic linear polymers with examples like Poly (propyleneimine), i.e., PPI, Poly (amidoamine), i.e., PAMAM, Poly (lysine), i.e., PLL, Poly 2, 2-bis (methylol) propionic acid (PBis MPA), etc. Dendrimer nanomaterials have a number of important applications which include their use in anticancer drugs, targeted drug delivery, cosmetics, biomedical field, diagnostic imaging, transdermal drug delivery, gene delivery, for mimicking micelles, and as nanoscale building blocks of polymers used in high-level performance. One of the most commonly used dendrimers is poly (amidoamine) or (PAMAM). The other name for PAMAM dendrimers is the starburst dendrimers and these were discovered by Dr. Tomalia and others in 1985. PAMAM dendrimer is available up to tenth generation (G-10) which consists of 6141 monomer units and has a diameter of 12.4 nm The PAMAM dendrimers by their nature are non-immunogenic and are soluble in water. Moreover, they possess end functional groups that are capable of binding various guest molecules. PAMAM dendrimers with a large number of applications in the biomedical and pharmaceutical fields have been found to be very useful for the delivery of genes and diagnostic agents. Dendrimers with end groups containing catalytic sites are sometimes referred to as dendralysts. Due to their large size, dendralysts can be easily separated from that of the reaction mixture when the reaction is completed. Thus, dendralysts, though being heterogeneous catalysts, can function in solution as homogeneous catalysts. As another member of the dendrimer family, dendrigrafts belong to a class of dendritic polymers just as dendrimers do. Dendrigrafts can be constructed with a specific molecular structure and are monodisperse (i.e., all nanoparticles having the same size). These are similar in structure to dendrimers and can be constructed in several stages by grafting pieces onto an existing structure.

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Fig. 8.2 Basic structure of dendrimer [35]

Dendrimers are biofriendly and are made predominantly from the amino acid (e.g., lysine). The preparation of dendrimers can be undertaken with great control, and therefore they acquire the characteristics such as being nearly monodisperse, macromolecules with globular shape, and consisting of a large number of surface functional groups. Dendrimers provide efficient nanotechnology platforms for the delivery of drugs for curing various diseases such as cancer. The number of surface groups available per unit molecular volume of the dendrimer is very large which provides the possibility of using them as a synthetic vector for the delivery of genes. Figure 8.2 shows the basic structure of a dendrimer. The three main parts in the structure of dendrimer are as follows: (i)

Core—The core of a dendrimer is a linear polymer molecule in the heart of the dendrimer from which symmetric branches emerge out to form the dendrimer. It determines the three-dimensional shape of the dendrimer (spherical, ellipsoidal, cylindrical, etc.). A change in unique properties (e.g., cavity size) of the core can provide absorption capacity and capture-release characteristics to the dendrimer. (ii) Interior Structure—It consists of the dendritic branches emanating from the central core. The function of the interior is to affect the host-guest properties in the dendrimer. The interior structure is of paramount importance with regard to the drug encapsulation. (iii) Exterior Surface—The exterior surface of a dendrimer consists of functional groups or end groups that are responsible for providing solubility and chelation ability (i.e., ability to remove certain heavy metals) to the dendrimer. It should be noted that (i)

The morphology of the dendrimer depends on both the core and number/type of interior branching units. (ii) Steric (i.e., spatial) crowding at the dendrimer’s surface is due to the fact that the diameter of dendrimer increases linearly with its generation, whereas

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the number of functional groups increases exponentially with increasing generation number of the dendrimer. Thus, low generation dendrimers are open and flexible, whereas higher generation dendrimers acquire dense and three-dimensional shapes. Generation number also affects the rigidity of the dendrimer. (iii) By suitable control of the composition of the core, interior structure, and surface, it is possible to design dendrimers with specific structure as well as specific composition. Dendrimer molecules being synthetic and are equivalent to peptides or polynucleotides in many respects. The ability to control the structural features can give rise to altered physical, chemical, or rheological properties (e.g., fluidity) of the dendrimers. Functionalized dendrimers are suitable for the creation of several nanoplatforms to be used in multimodal diagnostics (e.g., optical imaging, magnetic resonance imaging, thermal imaging, radionuclide imaging, etc.) and combining a number of therapies (e.g., chemotherapy, photothermal therapy, radiotherapy, gene delivery, etc.) effectively, comprehensively and simultaneously [36]. Synthesis of Dendrimers—Controlled synthesis of dendrimers produces dendrimers having almost perfect structure, exact molecular weight, and size. Cascade or iterative methods such as polymerization are used for the synthesis of dendrimers. Polymerization provides them spherical structure with cavities inside. Such cavities are used for carrying drugs by dendrimer molecules due to which they are used in drug delivery systems. Dendrimer’s surface groups due to their high chemical activity attach other molecules with them. These end groups can be given desired properties and transport attached molecules at the desired sites. These characteristics provide dendrimers various functionalities useful for biomedical and pharmaceutical applications. Two main approaches are possible for the synthesis of dendrimers, viz., divergent synthesis and convergent synthesis approach. These are discussed below: Divergent method of synthesis of dendrimers was introduced by Dr. Tomalia and was used in early periods. This synthesis method starts with the core consisting of a multifunctional molecule and the dendrimer grows outward layer after layer. A series of reactions are involved in the whole synthesis. The most common reaction involved therein is the Michael reaction. In this reaction, the molecule at the core of the dendrimer makes reaction with monomer molecules which contain not only one reactive group but also two dormant groups. As a result of this reaction, we get the first-generation dendrimer, i.e., G-1 dendrimer. After the completion of first level of reactions, the molecule acquires the new surface. This new surface of the molecule becomes active now and allows for fresh reactions with more monomers. This process is repeated at every generation. In this manner, a dendrimer is formed layer after layer. In this kind of synthesis of dendrimer, the number of functional groups on the surface of the dendrimer increases exponentially as is evident from Fig. 8.3. At each of the steps, the reactions must be fully completed so as to avoid defects in the structure of the dendrimer. This means that the next level of reactions must

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Fig. 8.3 Divergent synthesis of dendrimer [37]

take place only after the full completion of previous level reactions. Incomplete reaction steps may lead to trailing generations in which all the branches are not equal instead some branches may be shorter as compared to the rest of the branches. These kinds of irregularities in the structure of dendrimer are called defects which may result in a reduction not only in the functionality but also in the symmetry of the dendrimer. Purification of dendrimers synthesized by a divergent approach is relatively difficult due to smaller size difference between perfect and imperfect ones. Polyamidoamine (PAMAM) dendrimers with symmetry in the radial direction were synthesized by the divergent method of synthesis in which ammonia was used as the trivalent core molecule. One important advantage of divergent synthesis methods is that they are suitable for producing dendrimers on a large scale. A drawback of divergent synthesis methods is that the surface groups may undergo some side reactions as well as incomplete reactions leading to the defects in the structure of the dendrimer. In the divergent methods, this problem of defects in the structure of dendrimer can be avoided by using an excess concentration of monomers and involving lengthy chromatographic separations. This is particularly suitable for producing higher generation dendrimers. The convergent method of synthesis was initiated and developed by Hawker and Frechet. This method advances from outside toward the interior. In this method, the molecular structure that we have in the beginning ultimately becomes the outermost arm of the final dendrimer as shown in Fig. 8.4. When the growing branches, called dendrons, become sufficiently large, they attach themselves with a multifunctional molecule in the core. Thus, this method involves the reactions which proceed inward, build inward, and get finally attached to a core. Advantages associated with convergent synthesis methods include more monodisperse final dendrimers, relatively easy purification of the final product and a lesser number of defects in the final dendrimer. The main drawback of convergent methods is that they lead to smaller dendrimers

Fig. 8.4 Convergent synthesis of dendrimer [37]

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as compared to dendrimers produced by divergent methods because of the presence of less steric crowding along with the core of the dendrimer. Thus, convergent synthesis methods can only be used for the construction of small dendrimers or lower generation dendrimers. Dendrimers have inherent toxicity which is charge-dependent, concentrationdependent, and generation dependent. It is observed that cytotoxicity of cationic dendrimers is greater than neutral or anionic dendrimers. Cationic dendrimers are more hemolytic too in comparison with neutral or anionic dendrimers. Further, the toxicity of dendrimers increases with an increase in the generation of the dendrimer, i.e., lower generation dendrimers are less toxic as compared to the dendrimers of the higher generation. The cytotoxicity of PAMAM dendrimers depends on their concentration as well as on their generation. PAMAM dendrimers also exhibit hemolysis which is a function of their concentration and generation. Dendrimer’s toxicity arises as a result of the interaction of cationic charge on dendrimer’s surface with the negatively charged biological membranes in vivo and leads to membrane disruption. The toxicity of dendrimers can be minimized by (i) using neutral or anionic dendrimers and (ii) by shielding surface charge through chemical modification. Due to toxicity, their use in biological systems is limited. Functionalized dendrimers have toxicity which is much lower than those of native dendrimers. There may exist an interaction between nanosized dendrimers nanometer-sized cellular components, e.g., cell membrane, cell organelles, proteins, etc. The presence of cationic surface groups in the dendrimer gives rise to an interaction between dendrimer and the lipid bilayer. It leads to an increase in permeability and a decrease in the integrity of the biological membrane. Dendrimer molecules, in general, have a size less than or equal to 5 nm. The biological properties of dendrimers include polyvalency, self-assembling, electrostatic interactions, chemical stability, a low value of cytotoxicity and solubility, and nearly perfect structures. Dendrimer possesses a compact nanoscale structure that provides its characteristics like high solubility and low solution viscosity because of which dendrimers become highly suitable for applications such as rheological modifiers. The dendrimers have a core–shell molecular architecture in which the core and the surface of the dendrimer are chemically distinct. This feature of dendrimers can be utilized to encapsulate and release molecules that are chemically incompatible with the surrounding environment of the dendrimer, for instance, molecules like catalysts, drugs, or chromophores. Several active functional groups may exist on the surface of the dendrimer simultaneously. This is called the property of the polyvalency of the dendrimer. In the convergent synthesis method of a dendrimer, if the branching reactions are performed on the core molecule twice, then we get a second-generation (G-2) dendrimer. If these reactions involve three convergent steps, then it will produce a third-generation dendrimer (G-3), and reactions with four repeating steps result in fourth-generation dendrimer (G-4) and so on. A twofold increase in the molecular weight of the dendrimer is observed in going from dendrimer of one generation to the next generation. The simultaneous presence of active groups can lead to completely new or enhanced activity in comparison with the single presence of the same active group. It is

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helpful in producing multiple interactions with biological receptor sites. Polyvalency provides versatile functionalization to the dendrimer and can be utilized for achieving concentrated payloads of drugs or imaging labels that are chemically grafted to the surface of the dendrimer. Surface groups in the dendrimers can be changed into those biocompatible compounds which have a low value of cytotoxicity and a high biopermeability. Dendrimers are suitable for a number of pharmaceutical applications such as diagnostic imaging, anticancer therapy, and drug delivery. Metal chelates in the form of dendrimer are used as contrast agents for magnetic resonance imaging (MRI). They are gaining importance as macromolecular nanoscale delivery devices. Dendrimers are promising candidates to be used in the controlled and specified drug delivery of anticancer, anti-viral, and anti-tubercular drugs. Dendrimer molecules possess high entrapment efficiency for drugs. There are mainly two modes of attaching drugs to the dendrimer. In one approach, drugs can be physically trapped inside the core of the dendrimer, whereas, in the other approach, the drug molecules can be adsorbed on the dendrimer surfaces through various means such as through electrostatic interaction, hydrogen bonding, or Van der Waals force. In a still, another approach drug molecule can be attached with the surface of dendrimer through covalent interaction which provides dendrimer-drug conjugates [15]. The conjugates of drug with the dendrimer lead to an increase in the solubility, reduction in the systemic toxicity, and also promote selective accumulation inside solid tumors. Dendrimer-PEG conjugates are capable of creating hydrogels, and the nanomaterials which can have applications such as tissue production in the cartilage and also for closing ophthalmic injuries [38]. Dendritic polymers have features similar to certain biological molecules such as those of protein, viruses, and enzymes. These macromolecules have nearly the same size, i.e., are monodisperse and can be easily functionalized. These characteristics make them extremely popular for biomedical applications such as the delivery of drugs, diagnostic imaging, and therapies like photodynamic therapy and neutron capture therapy, etc. PAMAM dendrimers have properties that allow them to be used as a substitute for the blood [39] and it has been observed that these dendrimers are capable of carrying the anticancer drug methotrexate and fluorescein for the purpose of tracking [40]. Dendrimers are used in hair, skin and nail care, and other cosmetic products. Dendrimer nanoparticles are useful for different drug delivery routes such as oral, ocular, and also suitable for transdermal drug delivery systems, etc. When used for the delivery of bioactive drugs consisting of two equal hydrophobic parts in their structure and having low solubility in water, dendrimer nanomaterials work very efficiently. Dendrimers find a number of applications in the supramolecular chemistry, especially to participate in host-guest reactions and self-assembly processes. Bigger dendrimers, i.e., higher generation dendrimers because of possessing threedimensional structures and due to the presence of multiple internal and external functional groups can act as the host for various ions and molecules. In the hostguest reactions, the substrate molecule (i.e., guest) is allowed to bound to a receptor molecule (i.e., host). Dendrimer nanomaterials can also be used for synthesizing metal nanoparticles (NPs) through encapsulation. The nanoparticles so obtained

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have diameters below 10 nm and are known as dendrimer encapsulated nanoparticles. The dendrimer-based NPs do not agglomerate. A major part of their surface becomes unpassivated because of which such nanoparticles acquire characteristics ensuring their participation in catalytic reactions. Further, dendrimer nanomaterials are suitable to be used as templates in the control of the size, the stability and solubility of the small metal nanoparticles. The presence of a large number of surface functional groups on the dendrimer leads to an enhancement in its sensitivity when it is used as a sensor. The properties of dendrimers such as large solubility in water, biocompatibility, polyvalency, and specific molecular weight make them very suitable for their use in different biological applications. Dendrimer nanoparticles serve as ideal carriers for applications like the delivery of drugs and targeting applications. When dendrimers are used as agents for the delivery of drugs, it is observed that lower generation (i.e., smaller) dendrimers come out of the body in a short time as they undergo fast renal clearance. Further, the dendrimers having charged surface or hydrophobic surfaces undergo fast clearance by the liver. The site-specific delivery of drugs can be accomplished by modifying the surfaces of dendrimers using different targeting moieties, e.g., folic acid, peptides, monoclonal antibodies, sugar groups, etc. New genetic material is given to the host for the treatment of diseases in the treatment methodology of gene delivery. There are two approaches to gene delivery to accomplish the transfer of genetic material to the target cells. These two approaches are called viral-based and non-viral-based. Vectors for non-viral gene delivery make use of natural or synthetic molecules or physical forces for transferring the genetic material to the targeted cells. The cationic nature of PAMAM dendrimers makes them the most used non-viral gene delivery agents. The advantages associated with the non-viral gene delivery method include simplicity in fabrication, low immune response, targeting ability, and potential for repeat administration.

8.5.3 Liposomes A bilayer is made up of amphiphilic phospholipids in which there is a hydrophilic part (head) of phosphate and a hydrophobic tail. These hydrophilic and hydrophobic parts are formed of two chains of fatty acids. The presence of two fatty acid chains makes phospholipids immiscible with water. Bilayers possess important characteristics of self-healing and fluidity. Liposomes are artificial vesicular structures of spherical shape and are very small in size. These nanomaterials contain phospholipids bilayer surrounding an aqueous internal cavity (i.e., a membranous structure). Liposomes were invented by Alec D. Bangham with his co-workers in the early 1960s and are traditional lipid-based (natural or synthetic) formulations. These biological macromolecules have one or more lipid bilayers which enclose aqueous parts inside them. Liposomes are selfassembled colloidal particles that occur in nature and can also be produced by synthetic means. Liposome properties are a function of several factors such as their

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Fig. 8.5 Liposomes [41]

size, charge on their surface, methods of preparation, and composition of lipids. The characteristics of liposomes such as the colloidal size, structure, and chemical composition remain under control at every stage while they are being prepared (Fig. 8.5). Carrier or the prominent lipid components in the liposomes are generally phospholipids or PEGylated phospholipids, being amphiphilic in nature and consisting of a hydrophilic head and two hydrophobic chains (tails). The phospholipids carriers constitute a closed spherical bilayer structure that covers (shields) its hydrophobic part from the surrounding aqueous medium. The size of liposome vesicles can be as small as 25 nm to as large as 2500 nm. The size of the active constituent particle in liposomes lies between 30 nm to several micrometers. In the application of liposomes as a carrier of the drugs, the size becomes an important parameter to determine their circulation half-life. The quantity of the drug to be encapsulated depends upon the size and number of the bilayer contained in the liposome. The liposome of a specific size range serves as viable targets for natural macrophage phagocytosis which is used to transform DNA into the host cell. This process is called lipofaction. There are some liposomes for which the body’s immune system, e.g., reticuloendothelial system (RES) is incapable of detecting. Such liposomes are called “stealth liposomes.” In fact, a stealth liposome is nothing but a spherical vesicle having a membranous structure made up of phospholipid bilayer and can be used for the delivery of drugs or genetic material into a cell. Such long-circulating liposomes have high stability and can be obtained by changing the composition of the lipids as well as the size and charge on the vesicle. Liposomes having long circulation half-lives are suitable to work as a reservoir for releasing a therapeutic agent for a longer duration. The characteristics of the surface of liposomes and its membranous structure include charge density, phase behavior of its bilayer, and presence of grafted polymers or

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Fig. 8.6 Classification of liposomes

Liposomes

Unilamellar vesicles (ULVs)

Small Unilamellar vesicles (SUVs) Diameter < 100 nm

Multilamellar vesicles (MLVs) Diameter > 200 nm

Large Unilamellar vesicles (LUVs) Diameter 100 to 1000 nm

those bound to its surface, permeability, and certain mechanical properties such as stiffness. Liposomes exhibit amphiphilic character because of which they serve as a strong system for making soluble a variety of compounds. These vesicles can be classified on the basis of their size and the number of lipid bilayers contained in them. This classification of liposomes is as follows (Fig. 8.6): Different methods of preparation of liposomes include mechanical agitation, solvent evaporation, solvent injection, and detergent solubilization. Other better methods of preparation of liposomes include extrusion and Mozafari. Different kinds of active constituents are encapsulated in the different parts and manner in the liposomes. For example, the active constituents which are hydrophilic in nature are carried and encapsulated within the aqueous phase, and lipidic constituents are intercalated into the bilayer, whereas lyophilic and amphiphilic constituents interact with the surface of the liposome. Therefore, these vesicular structures serve as carriers for all types of drugs—hydrophilic, hydrophobic, and amphiphilic. The phospholipids carrier is made from natural, semi-synthetic, and synthetic sources, which are biologically inert, non-immunogenic, and exhibit lower inherent toxicity. Liposomes are best suited for their use as a delivery vehicle for those drugs which are potentially toxic with a narrow therapeutic index. They enhance efficacy and reduce related toxicity of pharmaceuticals. The new generation of liposomes, called polymersomes, can also be formed of polymers, which are biocompatible and biodegradable. The advanced or next generations of delivery systems for drugs include molecular targeting; immunoliposomes and other ligand-directed constructs. These advanced systems will be constituted by integrating biological components that can recognize tumors and the technologies concerned with the delivery of drugs. There are two mechanisms through which drugs can be loaded in the liposomes. These are passive loading and active loading mechanism. In the passive loading technique, the drug is

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encapsulated itself during the formation of liposome, whereas in the active loading method the drugs are loaded after the formation of liposomes. Liposome-based delivery systems for drugs give rise to drug formulations which are stable. Such delivery systems also result into an improved pharmacokinetics of the drug. The surface of liposomes is coated with polymers for providing the stability to liposomes in normal body milieus, to prolong the release of drugs, and to deliver the loaded drug at the desired target site [42]. Chitosan is a natural polymer that possesses several important characteristics such as cationic nature, bioadhesive property, being biodegradable and biocompatible, and properties for increasing absorption. Therefore, chitosan-coated liposomes, called “chitosomes” are very promising carriers for the purpose of drug delivery and to facilitate absorption of drugs with prolongedrelease time [43]. In a study, it has been observed that the physical stability of liposomes can be improved by the presence of chitosan on the outer surface of the liposomes [44]. Encapsulation of liposomes with chitosomes can greatly improve the water solubility of curcumin (CURC) which is a hydrophobic molecule. Xue-Qin Wei et al. studied the stability characteristics of liposome-based curcumin by modulating the pH of the inner aqueous chamber in the liposomes. They prepared CURC-loaded liposomes having three different internal pH values (pH 2.5, 5.0, or 7.4) and made observations with regard to the properties such as the size of particles, zeta-potential, morphology, the efficiency of entrapment, and physical stabilities. Out of these three preparations, CURC-LP (2.5) was found to have the highest entrapment efficiency, largest physical stability, and longest release time, i.e., slowest release rate in vitro [45]. Liposomes are used for delivering the active constituents efficiently and in this role they serve as an alternative to the conventional drug delivery systems such as polymeric nanoparticulates (PNPs) which work in a controlled and sustained manner. They can release the entrapped drugs at the designated targets and are very efficient and effective drug carriers for the treatment of cancer. Low toxicity along with properties of biocompatibility and biodegradability and the ability for trapping hydrophilic as well as lipophilic drugs leads to the delivery of drugs to specific sites such as cancer affected cells or tissues. Liposomes shield their contents from different types of biological chemicals produced in different parts of our body by forming a barrier around it. For example, this barrier cannot allow the enzymes produced in the mouth and stomach to pass through it. Several other biological chemicals such as alkaline solutions, digestive juices, bile salts, and intestinal flora produced in the human body, as well as free radicals, cannot also penetrate through this barrier. Thus, the drugs or other chemicals enclosed within liposomes are shielded from undergoing the processes of oxidation and degradation. The shielding barrier is so firm that it is not damaged until the delivery of the contents of the liposomes to the desired site whether it is a tissue, a gland, or a system. This property of liposomes makes them very useful for the applications of the delivery of drugs. In principle, any delivery system of drugs, such as for delivering anticancer agents, can enhance their efficacy and/or reduce the toxicity. Macromolecular carriers like liposomes that have long circulation times can make use of the EPR effect for selective extravasations from tumor vessels causing considerable anticancer activity with a

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reduction in the cardiotoxicity (i.e., toxicity to the heart). Anticancer drugs daunorubicin as well as PEGylated doxorubicin as a liposomal preparation possess highly prolonged circulation times. In particular, PEGylated doxorubicin as a liposomal preparation has been found to be very effective in the treatment of breast cancer when used as an independent therapy or when used along with other chemotherapeutics. The advanced forms (or next-generation) of drug carriers are being developed on the basis of features such as the direct molecular targeting of cancer cells through the interactions which are mediated by antibodies or by other ligands. The treatment methodology based on immunoliposomes provides us in principle several advantages in comparison with other strategies which are based on antibodies. Liposomes possess both diagnostic as well as therapeutic applications. In diagnostic applications, liposomes can work as multitasking agents. They can serve as a tool, a model, or reagent in the fundamental studies concerned with interactions among cells, recognition processes, and for knowing the mode of action of certain substances. Liposomes are very useful when used as carriers for a number of cosmetic molecules and also for pharmaceuticals. Liposome mediated drug delivery has been successfully applied in removing obstacles to cellular and tissue uptake of drugs with improved biodistribution. The drug encapsulation characteristics of liposomes make them very promising carriers for anticancer drugs [46] and are widely used to enhance the transportation of various anticancer, antifungal, and antimicrobial drugs. These composite structures made of phospholipids are suitable for gene delivery and drug delivery (especially for anticancer drugs). They are also suitable carriers for the delivery of several other agents such as dyes to textiles, pesticides to plants, enzymes and nutritional supplements to foods, and cosmetics to skin. As a carrier of pharmaceuticals, a number of advantages are associated with liposomes, e.g., protecting the drug from enzymatic degradation, flexibility, biocompatibility, low toxicity, non-immunogenicity, and biodegradability. Liposomes help in improving the therapeutic index of the drugs through a change in the drug absorption, reduction in metabolism and toxicity, and increasing the biological half-life. Some of the disadvantages associated with liposomes are being sparingly soluble in water, having short shelf life and low encapsulation efficacy, fast removal by the reticuloendothelial system (RES), inter-membrane transfer, cell interactions or adsorption, etc. Liposomes can remove some of the problems faced by conventional aerosol delivery because they can perform as a solubilization matrix for sparingly soluble agents. Liposomes can also be used to act as a pulmonary sustained-release reservoir and to provide for intracellular delivery of drugs. According to the study made by Kibria et al. [47], the cationic liposomes which were modified by double-ligands indicated an increase in the cellular uptake into integrin αvβ3-expressing cells and better transfection of luciferase-encoding plasmid DNA. The use of cationic liposomes can produce nucleic acid pulsed-dendritic antitumor vaccines [48]. Iron liposomes were prepared through rotator-evaporated film ultra-sonicating techniques [49]. This group prepared heme liposomes and ferric citrate liposomes using the above technique and evaluated certain properties such as particle size distributions and zeta-potentials, efficiency for entrapping drugs, and

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the structure. Their studies established that both types of ironic liposomes exhibited stable physical properties. Their study helped to conclude that the encapsulation of ironic agents with liposomes enhances the absorption of iron and hence cures iron-deficiency ailments efficiently. The drug delivery systems based on liposomes leads to an increase in the efficacy of treatment and cause a reduction in the toxicity with regard to various anticancer and antimicrobial agents. The current aim of pharmaceutical science is to utilize liposomes to improve the pharmacodynamic and pharmacokinetic parameters of various drugs. The challenges for conventional therapies such as low solubility, short halflife, poor bioavailability, strong side effects, and large toxicity can be removed by employing the concept of liposome-based drug delivery systems. Liposomes offer a great opportunity in the hands of the investigator to find the unexplored potential of nanotechnology applied to the field of pharmaceuticals. Andang Miatmoko et al. studied liposomes loaded with primaquin (PQ) and chloroquin (CQ). This simultaneous loading of PQ and CQ into liposomes caused a reduction in the drug release rate and in the capacity for drug encapsulation. This might increase the efficacy of PQ in the treatment of malaria with a simultaneous reduction in its toxicity [50]. Liposomes, a bilayer of phospholipids, are different from micelles which are a monolayer of surfactants. Micelles and liposomes both are vesicles composed of amphipathic molecules and drugs can be trapped and administered in them with great efficiency. Drugs with low solubility and which are less permeable through the intestinal lining are administered using micelles. On the other hand, antifungal drugs and those used for curing cancer are administered using liposomes. Liposomes and polymeric micelles increase the time a particular drug may be present in the system. They both help in increasing the efficiency of the drug. To protect the personnel from severe disease threats in various regions of the world, army vaccine scientists have developed useful liposome-based vaccine adjuvants (medicines given after initial treatment). Army liposome formulation (ALF) is made up of liposomes that contain components such as saturated phospholipids, cholesterol, and monophosphoryl lipid A (MPLA) and work as an immunostimulant [51].

8.5.4 Polymeric Nanoparticles The nanoparticles are used as a carrier system for drugs and can be made from different materials like, e.g., polyalkycynoacrylates (PACAS), polyacetates, polysaccharides, and copolymers. A nanoparticle is a general term used for different types of polymeric nanoparticulates and has size between 10 and 1000 nm. Several types of nanoparticles can be formed such as coated nanoparticles, PEGylated nanoparticles, solid-lipid nanoparticles, and nanogels. Polymeric nanoparticles that are used for the delivery of active pharmaceutical ingredients are found in two important geometries which are nanospheres and nanocapsules. These nanoparticles possess properties like biodegradability and biocompatibility, non-toxicity, and non-antigeneticity. Nanocapsules, being vesicular, are used as a reservoir of drug, whereas nanospheres

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are solid matrix-based system made of polymer. Drugs in the nanospheres are either present within their interior or are adsorbed to their surface. Polymers used for the preparation of nanoparticulates must possess properties such as biocompatibility and biodegradability and drugs can be encapsulated, entrapped, or dissolved within the polymer matrix. Polymeric nanoparticulates because of controlled release characteristics are suitable carriers for delivering the vaccines; agents used in cancer therapy, contraceptives, and targeted delivery of antibiotics and can also be used in tissue engineering [52]. Polymeric nanoparticles which are particles made of artificial polymers can possibly be used as drug carriers to the brain. Nanoparticles and nanoformulations have shown tremendous success when used for the purpose of delivering the drugs; and drug delivery systems based on nanoparticulates are supposed to have a much higher potential in several of the applications. Nanoparticles possess great advantages when used in applications such as a targeting agent, and for delivering and releasing the drug. Moreover, they are capable of combining diagnosis and therapy because of which nanoparticles emerge out as one of the many possible tools in the field of nanomedicine. Metal-organic nanocapsules (MONCs) have potential applications in catalysis, gas adsorption and separation and sensing [53]. Due to the characteristics of possessing a larger surface area nanoparticles can work as efficient catalysts and can help in various chemical reactions. For example, ferrite nanoparticles find applications as nanocatalysts in some of the organic reactions. The main function of catalysts is to improve the rate of reaction and their yields at low temperatures and they also promote the enantioselectivity [8]. These nanoparticles have a very narrow range for their sizes and are highly stable against chemical degradation, and the characteristics that are very valuable while considering their use for drug designing. Wei et al. prepared silver nanoparticles employing several means such as physical, chemical, and biological and examined them for their use in applications such as in various cancer treatments in the role of anti-angiogenesis agent or as photosensitizer molecule or like a radio-sensitizer agent [54]. Kaur H et al. synthesized and studied tropicamide-loaded nanoparticles of carboxymethyl tamarind kernel polysaccharide using the iontropic gelation process for their use in ocular delivery. These nanoparticles, according to the observations made by Kaur et al. exhibited characteristics, such as great opthalmic tolerance, mucoadhesion, and large corneal permeability in comparison with traditional opthalmic formulations [55]. The toxicity to the cells produced due to the nanoparticles or by the products obtained because of their degradation remains a major cause of concern and this issue should be addressed sincerely. Marta Szczech and Krzysztof Szczepanowicz have prepared polymeric nanoparticles with core–shell structure by using the method of spontaneous emulsification solvent evaporation and were given functional behavior through the layer-by-layer method. The properties of their polymeric core–shell nanoparticles were optimized for bioimaging, passive, and magnetic targeting [56].

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8.5.5 Solid-Lipid Nanoparticles Nanoparticles of lipids (LNPs) are an important class of advanced colloid-based drug carrier systems that are available in two categories namely solid-lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs). The first category of lipidbased nanoparticles, i.e., the solid-lipid nanoparticles are innovative nanocarriers that have non-toxic nature and are spherical in shape lying in the size range of 50 to 1000 nm. The second category of lipid-based nanoparticles, i.e., the nanostructured lipid carriers is obtained by modifying the solid-lipid nanoparticles up to that extent so that some of the problems associated with SLNs are alleviated. Thus, nanostructured lipid carriers are actually the modified solid-lipid nanoparticles, i.e., MSLNs. Now both these categories of lipid nanoparticles were invented around 1990 as a substitute for colloid-based drug carriers like, for example, micelles, liposomes, emulsions, and micro- and nanoparticles of polymers. Solid-lipid nanoparticles and nanostructured lipid carriers are green nanovehicles. The SLNs have size-dependent properties. Solid-lipid nanoparticles have small size and relatively narrow size distribution, and easy surface modification which makes them suitable for site-specific delivery of drugs. The advantages shown by SLNs are actually the combined set of those associated with liposomes, fat emulsions, and polymeric nanoparticles. The SLNs do not face problems that are associated with polymeric nanoparticles and other colloidal carriers. The SLNs are well protected against chemical degradation and are cheaper due to the non-use of organic solvents. Solid-lipid nanoparticles form a fast-developing part of nanotechnology having numerous potential applications such as in the delivery of drugs, nanomedicine, and research. Since drug delivery systems based on lipids makes use of the physiological lipid, therefore, they possess an obvious advantage of having lower toxicity. Other advantages associated with them include increased bioavailability and productivity, greater reproducibility, and the possibility to incorporate hydrophilic as well as hydrophobic compounds as drugs. SLNs can be prepared from physiologically well tolerated and highly biocompatible lipids and consists of a solid-lipid core and a single layer of phospholipids in the form of coating (shell). The core region is hydrophobic in nature. Thus, in SLNs, the dispersed phase consists of fat which is in the solid state and the surfactant is used for the purpose of emulsification. The lipids suitable for use in SLNs may be triglycerides in their extremely pure form, mixtures of complex glyceride, steroids, fatty acids, waxes, etc. The composition of SLNs is such that they contain about 0.1–30% solid fat which is dispersed in an aqueous phase. Besides it, the SLNs also contain the surfactants in about 0.5–5% concentration which enhances their stability. Physicochemical properties associated with SLNs such as the size of a particle, drug loading capability, and remain stable for longer periods while storage, and also release characteristics, depend upon the proper choice of lipids and surfactants and are highly regarded for the formulation of targeted delivery of a drug to the brain. SLNs provide controlled as well as the prolonged release of drugs and by attaching ligands; targeted drug delivery is made possible (Fig. 8.7).

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Fig. 8.7 Basic structure of SLNs [57]

Solid-lipid nanoparticles facilitate incorporation of both hydrophilic as well as lipophilic drugs in quantities much larger than that possible with biopolymer-based nanoparticles, and therefore SLNs can be used for the delivery of proteins and peptides [58]. SLNs get preference over polymeric nanoparticles because of the possibility of high scale production and high drug loading with negligible toxicity. SLNs exhibit all the merits associated with important nanocarriers such as liposomes, emulsions, and polymeric nanoparticles [59]. The advantages associated with the use of solid-lipid nanoparticles in drug delivery systems as compared to using polymeric nanoparticles for the same purpose is because of causing lesser toxicity to cells, the ability to incorporate drugs in larger quantities, and easily possible scalable production [60]. Since the interior of SLNs contains a solid matrix core of hydrophobic nature, therefore, they are capable of entrapping higher amounts of hydrophobic drugs in their core in comparison with usual liposomes. The solid-lipid nanoparticles can be prepared easily and at a low cost. Moreover, their preparation can be scaled up to high volume easily and possess a high level of reproducibility. They show exceptionally high physical stability and since no organic solvent is used in their preparation, therefore, they do not cause any toxicity. The lipids used in their preparation are biodegradable. Because of the numerous benefits provided by SLNs, these lipid carriers are very reliable and easier to get approval. The SLNs have more drug stability and prolonged release as compared to liposomes and because of the absence of organic solvents during their preparation, their use is relatively safe as compared to polymeric carriers. However, SLNs do possess certain disadvantages such as the growth of lipid-particle, the unforeseeable tendency of solidification by freezing, and drug leakage during storage. Further, SLNs have inherently low rates of incorporation which is the consequence of the crystalline structure of the solid lipid and dynamics of polymorphic transitions. High-pressure homogenization (HPH) and ultrasonication or high-speed homogenization are important preparation methods for SLNs. Some other methods for the preparation of solid-lipid nanoparticles include solvent emulsification/evaporation, supercritical fluid extraction of emulsions (SFEE), and spray drying method, each one having their advantages and drawbacks. The HPH is a powerful method for producing SLNs in large quantities and its reliability is quite high. This method

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requires a high pressure (about 100–2000 bars) for its proper working. It produces an average particle size in the submicron region. This method is based upon the principle in which the drug is mixed in the bulk of lipid melts. The challenges involved herein are the requirement of specifically large pressure and high temperature. This method can progress in two different ways involving different conditions of temperature and the way of mixing the drug in the lipid melts. Accordingly, the two types of this method are called hot homogenization and cold homogenization. In the hot homogenization technique, the temperature must be maintained higher than the melting point of lipids and this method is suitable for lipophilic and insoluble drugs. However, this method encounters certain problems such as degradation of the drug depending upon the temperature, leakage of the drug into aqueous phase while homogenization is under progress and difficulty in the crystallization step of the nanoemulsions. This method involves low entrapment efficiency for loading hydrophilic drugs into SLNs and is therefore not suitable for such drugs. The cold homogenization method progresses by dissolving the drug in the lipid melt and then it is allowed to cool rapidly using liquid N2 or dry ice (solidified carbon dioxide). The cooled and solidified substance so formed is converted into nanoparticles in the size range 50–100 nm through the milling process. It resolves the problems of the hot homogenization method and is therefore a better approach than hot homogenization. The solvent emulsification/evaporation method involves dissolving the lipid in an organic solvent such as cyclohexane and toluene which cannot be mixed with water. The solvent is then removed from the emulsion by using the process of evaporation under reduced pressure. The size of nanoparticles created in this method depends upon the solid lipid and surfactant used in the process. If the lipid load is very small as compared to the organic solvent, then we get nanoparticles of very small size. Higher toxicity due to the use of an organic solvent is an obvious drawback of this method. The ultrasonification or high-speed homogenization technique uses a large quantity of surfactant in its aqueous phase in which the lipid phase is also dispersed. A pre-emulsion is formed which is ultrasonicated. Nanoparticles produced in this technique have a broad size distribution which is the major drawback of this preparation technique. An innovative method for preparing SLNs involves the use of a supercritical fluid (e.g., CO2 ) for solvent extraction from O/W emulsions and is therefore called supercritical fluid extraction of emulsions (SFEE). When the temperature and pressure of any fluid are maintained above their respective critical values, then this fluid is said to be supercritical. The advantage of this approach is solvent-less processing which results in less toxicity. The spray drying method works as a substitute for lyophilization (freeze-drying) method as a means of transforming aqueous SLN dispersion into a drug. It involves high temperature and shear forces which lead to particle aggregation. The advantage of the spray drying method is that it can prepare solid-lipid nanoparticles at a low cost. Several drugs are poorly soluble in water which is an obvious limitation for them. Drug delivery systems based on lipids are capable of improving the bioavailability

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of drugs. These systems involve dissolving the drugs in the lipid. SLNs not only produce improved but also prolonged therapeutic effects which cause an increased interval between two consecutive doses and better acceptance by the patients. The release profile of drugs entrapped in SLNs can be better controlled and their release can be prolonged as compared to other polymeric materials. Other advantages of SLNs in comparison with polymeric nanomaterials include rapid degradation within living organisms and superior endurability. SLNs are used either in the form of dry powder formulations or aqueous suspensions. Solid-lipid nanoparticles have the capability of directly bonding with DNA and therefore are suitable for their use in gene transfection. Cationic SLNs have great physical stability and are biocompatible because of which they possess great potential for their use in targeted delivery of genes. Cationic solid-lipid nanoparticles have the ability to form complexes with DNA because of which they are recommended for getting preference over liposomes for non-viral gene delivery [61]. Since SLNs possess UV blocking and skin hydration features so they are talented carriers for cosmetic applications such as in sunscreens. As far as the encapsulation of bioactive compounds is concerned, SLNs work as unusually important delivery systems. SLNs can also serve as potential carriers for the delivery of delicate compounds in the food industry. The research group of P. Vijayananda has observed that when solid-lipid nanoparticles are loaded with the extracts of hibiscus rosa-sinensis, they give rise to an increase in the antidepressant activity within living organisms [62]. These hibiscus rosa extracts loaded solid-lipid nanoparticles (HSLNs) were evaluated in terms of size, surface charge, and framework and tested them for activity against depression in male Swiss albino mice. As per their observation to produce similar pharmacological effects, much smaller doses were required as compared to the native crude extract dose. Arif et al. prepared curcumin-loaded SLNs through the techniques of microemulsion and ultrasonication for evaluating their action on liver-spleen scintigraphy. Further, they labeled a radioisotope Technetium-99 m on SLNs loaded with curcumin. Their results have shown that Technetium-99 m labeled SLNs containing curcumin were promising imaging agents [63]. Harshita Krishnatreyya et al. prepared piroxicam loaded solid-lipid nanoparticles through the method of high-speed homogenization followed by ultrasonication technique and tested them for the size of particles, zeta-potentials, entrapment and release profile, drug loading capability, etc. Their results proved the suitability and capability of fabricated piroxicam loaded SLN gel system in treating arthritis pain and inflammation when delivered topically [64]. Nanostructured lipid carriers (NLCs) are members of the succeeding generation of SLNs in which disadvantages of SLNs such as the issue of improving the stability, low loading capability, and drug expulsion while being stored, have been removed. The lipid phase in NLCs can be formed of solid as well as liquid lipids. Three different types of possible NLCs are formless, imperfect, and multiple types. The imperfect type of NLCs consists of fats both in the solid and liquid phase which is mixed in various kinds of lipid-based structures. Formless (or amorphous) type of NLCs does not possess a crystalline structure and does not allow the drug to leak out while it is stored. In the third category of NLCs, i.e., multiple types of NLCs the solubility

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of a drug in the liquid lipid is higher in comparison with that in the solid lipid. The contrasting features of NLCs as compared to SLNs are: (i) NLCs are capable of entrapping higher quantities of some drugs; (ii) they have a smaller amount of water in the dispersion, and (iii) smaller leakage of drugs during storage. But the level of biotoxicity caused due to the two types of lipid-based nanocarriers is almost the same. Drugs can be incorporated in SLNs in three different ways. As per the first scheme, the drug is homogeneously dispersed throughout the lipid melt. This scheme of incorporation of drugs is called the solid solution model and is shown in Fig. 8.8a. For incorporating drugs according to this model, SLN must be produced through the cold homogenization method. The second scheme of incorporation of drugs in SLNs is called the drug-enriched shell model in which the shell portion consists of a higher concentration of drugs as compared to the core region as shown in Fig. 8.8b. This can be accomplished by producing SLNs through the hot homogenization method. The third scheme of incorporation of drugs into SLNs, called the drug-enriched core model, consists of incorporating a higher concentration of drugs in the core (inner) region as compared to the shell (outer) region. Such SLNs have a greater amount of active ingredient lying within the lipid melt as shown in Fig. 8.8c. Different types of drug delivery routes possible with solid-lipid nanoparticles and nanostructured lipid carriers are parenteral, dermal, pulmonary, and topical. The use of SLNs and NLCs as carriers of drugs leads to a reduction in the toxicity caused by drugs of high potency and enhances the efficacy of the treatment. The presence of such characteristics provides them to have important applications like transfer of genes, and use in the cosmetic and food industry.

Fig. 8.8 Drug incorporation models of SLNs [65]

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8.5.6 Niosomes Niosomes are an important class of hopeful drug carriers that can be formed through the self-association of synthetic non-ionic surfactants with cholesterol in an aqueous phase. Niosomes, an innovative delivery system for drugs, are capable of improving certain characteristics, e.g., solubility and stability of pharmaceutical compounds of natural origin. They have a long shelf life, high stability, low toxicity and are biological non-immunogenic, biocompatible, and capable of controlled and sustained delivery of all types of drugs at the desired site. Niosomes help in increasing the stability of encapsulated medication. In this drug delivery system, carriers of drugs are non-ionic surfactant-based vesicles combined with cholesterol and some other excipient. Lipid compounds (e.g., cholesterol) and synthetic non-ionic surfactants are the two major constituents that are used in the preparation of niosomes. Several types of surfactants are suitable for the preparation of niosomes, e.g., alkyl ethers and alkyl glycerol ethers, polyoxyethylene fatty acid esters, block copolymers, etc. The surfactants like spans (span 60, 40, 20, 80, 85), tweens (tween 20, 40, 60, 80), and brijs (brij 30, 35, 52, 58, 72, 76) are suitable for preparing niosomes. These carriers have much more penetration power than emulsions. By modifying surfaces of drugs and restricting the effects of drugs only to the target cells, niosomes cause an improvement in the therapeutic performance of the drug. This in turn reduces the clearance of the medication or in other words, increases the circulation time of the drugs. Based on particle size, niosomes are of three types. The first category of niosomes, called small unilamellar vesicles, has diameters in the range 10–100 nm. The second category of niosomes, known as the large unilamellar vesicles, has diameters lying between 100 and 300 nm. The third type of niosomes is termed as multilamellar vesicles which have diameters between 100 and 1000 nm. Niosomes construction depends upon certain factors such as a method of preparation, drug entrapment, type and amount of surfactant, lipids hydration temperature, and the packing factor. An important parameter which forms the basis for the selection of non-ionic surfactant to be used in the preparation of niosomes is the hydrophilic-lipophilic balance (HLB) value and must lie in the range of 4–8. The HLB value is a measure of the balance between the hydrophilic and the lipophilic portions of the non-ionic surfactant. Another crucial parameter for the selection of surfactants in the niosomes is the critical packing factor (CPP) which could also be considered as a tool to define their geometry and also for a complete description of the self-assembled structure and its structural transformation in an amphiphilic solution. The HLB value of the surfactant used is the determining factor for the size of niosomes and an increase in HLB value means an increase in the size. However, the entrapment efficiency of niosomes increases with a decrease in the HLB value of surfactant. The entrapment efficiency of niosomes is directly proportional to the important characteristics of the surfactant such as their concentration and affinity toward lipids. Sahinwala reported niosomes of nimusulides incorporated using film hydration technique [66]. If CPP ≤ 1/3, we get spherical niosomes, and for 1/2 ≥ CPP ≥ 1/3, nonspherical niosomes

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are formed. Bilayer niosomes are obtained when 1/2 ≤ CPP ≤ 1, whereas CPP ≥ 1 gives inverted niosomes. The cholesterol constituent in this dosage form is responsible for the physical properties like the efficiency of entrapping drugs, long time stability, release of entrapped drugs and bio-stability and rigidity of vesicles to stabilize the niosomes, reduce the destabilization effect of dosage from plasma and serum, decrease the permeability of vesicles, and breakdown of niosomes. The entrapment efficiency of niosomes is directly proportional to the concentration of its cholesterol component. The concentration of cholesterol also affects the time of circulation and release of entrapped drugs. The steadiness of niosomes can be affected by the addition of some charged molecules in niosomes. Both positively and negatively charged molecules can be used for this purpose. The steadiness and capacity for encapsulation of drugs in the niosomes depend upon the size and charge of vesicles. There exist several microscopy techniques for the determination of the size of niosomes. The preparation methods for niosomes include thin-film hydration, sonication, ether injection method, microfluidazation, transmembrane pH gradients, supercritical carbon dioxide fluid, lipid layer hydration, etc. The hydrophilic drugs are entrapped within the space surrounding the vesicle, i.e., within the aqueous core, whereas hydrophobic drugs are entrapped in the bilayer of niosomes. Amphiphilic drugs are compatible with the lipophilicity of the drug that lies between hydrophobic core and lipophilic tail. Niosomes are very much similar to liposomes in many respects. However, some differences in their properties arise because niosomes are formed from non-ionic single-chain surfactants and cholesterol, whereas liposomes are prepared from double-chain phospholipids which can be neutral or charged. In comparison with liposomes, niosomes possess certain advantages, e.g., good stability, being costeffective, easy formulation, and scaling-up. Niosomes have higher stability than liposomes because their constituent surfactants are more stable than those of lipids of liposomes in terms of physical stability. Niosomes have been developed as the best alternative nanocarriers for liposomes. Drawbacks of niosomes include (i) the aqueous suspension of niosomes may have insufficient shelf life because of combination, aggregation, the permeability of captured drugs, and hydrolysis of encapsulated drugs, (ii) the preparation of multilamellar vesicles is time-consuming and requires distinct tools [67]. Niosomes, when used for the transdermal route of delivery suffer from the problem of slow penetration of drugs through the skin which is their major drawback for the transdermal route. Niosomes can be used for sustained release of transport of peptides and proteins via oral administration. Niosomes are useful for the treatment of pathological disease and the efficacy of treatment can be increased by targeting previous constituents passing through tissues via blood vessels. These can also serve as a transporter of hemoglobin because liposomal vesicles can absorb oxygen. Lipid components in niosomes are significant as they impart unbending nature, proper shape, and acclimatization to the niosomes, whereas surfactants undertake the main part in the development of niosomes [68]. Niosomes when used as drug carriers are osmotically active, show less toxicity and work as a chemically active agents and

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can be easily subjected to surface modification because of the functional groups. Mukesh Mohite and Tanvi Kumbhar formulated niosomes by sonication technique and then converted it into a gel drug delivery system to study the controlled release of ketoconazole. Their study showed that a gel formulation containing niosomes loaded with ketoconazole provides control release and prolonged its action as an antifungal drug than ketoconazole in non-niosomal form [69].

8.5.7 Hydrogels Hydrogels represent to the three-dimensional matrix of hydrophilic polymers with cross-linking. Because of the hydrophilic structure, hydrogels do not dissolve in water but can hold large quantities of water or we can say that they can swell in water. Hydrogels are capable of responding to the fluctuations of the external impetus like pressure, temperature, electric and magnetic fields, pH, specific chemical composition, etc. [70]. The hydrogels can be formed by homopolymers (consisting of only one type of monomers) or copolymers (consisting of more than one type of monomers). They maintain their structural identity in bulk water and are treated as insoluble which is a consequence of physical or chemical cross-linking. Hydrogel nanoparticles are smart 3D nanomaterials with probable applications like catalysis, delivery of drugs, and other biomedical applications (e.g., tissue engineering, wound dressing, cell encapsulation, etc.). Hydrogels can be natural, semi-synthetic as well as synthetic polymer networks which have the ability to absorb 90% of water. Synthetic hydrogels possess certain advantages in comparison with natural hydrogels. For example, they possess tunable mechanical properties and their structural architecture can be controlled easily. The high cost of production is an obvious limitation for the commercialization of hydrogels. Hydrogels were first given by Wichterle & Lim in 1960. They proposed that the hydrogel must constitute at least 10% of the water proportion of the total hydrogel polymer. Hydrogels resemble living tissues on the basis of possessing a degree of flexibility (i.e., softness) comparable with natural tissue. This resemblance between the two is a consequence of their appreciable water content and soft constancy. The hydrophilicity of the hydrogel network is provided by the hydrophilic groups like NH2 , –COOH, –OH, –CONH2 , –CONH, and –SO3 H present in their structure [71]. Hydrogels can be prepared by using the techniques such as physical cross-linking (e.g., cooling a hot polymer solution; freeze-thawing, etc.), chemical cross-linking (e.g., grafting of monomers on the backbone of polymers; using some cross-linking agent for linking two polymer chains, etc.), radiation cross-linking, i.e., irradiation of solid polymer with high energy sources like X-ray, gamma ray, etc., and grafting polymerization, etc. Some of the examples of smart hydrogels include thermosensitive (i.e., sensitive to temperature changes) hydrogels such as collagen, agarose, hyaluronic acid, poly, and chitosan-poly (acrylamide); pH-sensitive (i.e., sensitive to changes in pH of

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solution) like carrageenan; pressure-sensitive such as poly (N-isopropylacrylamide), etc. Two important types of hydrogels are—physical hydrogels and chemical hydrogels. Weak mechanical properties, e.g., lack of mechanical strength are the major drawback of hydrogels. Hydrogels are categorized on the basis of (i) structure (amorphous, semi-crystalline, and hydrogen-bonded), (ii) charge (neutral, anionic, cationic, and ampholytic), (iii) mechanism of drug release (diffusion control, swelling control, chemically controlled, and environment controlled release), (iv) mono-, co-, and multipolymer systems. Hydrogels are nowadays used to deliver all types of pharmaceuticals because they possess various properties like swelling and mechanical properties to absorb the maximum amount and broader spectrum of the drug and can also be used in tissue engineering. An advantage of hydrogel materials is that they are biocompatible in nature, so hydrogels are used to produce soft contact lenses, films for biosensors because of exhibiting highly open structure and large inner surface, liners for artificial hearts, constituents for artificial skin due to soft mechanical properties and high water content, and drug delivery. Further, the hydrogels are suitable for use as biosensors because of certain advantages such as: greater specific surface area as compared to one-dimensional and two-dimensional nanomaterials as they have nanoscale pores in their structure (core–shell blue hydrogel particles have pores between 3.2 and 3.9 nm in diameter); excellent conducting properties of polymers used in them; good biocompatibility and excellent processability. Easy injection and modification of hydrogels are possible. The hydrogels are used for controlled, sustained, prolonged, and targeted drug delivery systems with fewer side effects. Drawbacks associated with hydrogels include difficulty in loading and handling, high production cost, low mechanical strength, and surgical risks involved in device implantation and retrieval. Nanocomposite hydrogels are nanomaterials-filled, hydrated polymeric networks exhibiting higher elasticity and strength relative to traditional hydrogels [72]. Natural polymers forming hydrogels include proteins (e.g., collagen, albumin, elastin, and gelatine) and polysaccharides (e.g., starch, alginate, chitosan, and agarose). Synthetic polymers forming hydrogels are obtained through chemical polymerization techniques. Hydrogels can be used for dispersing nanomaterials in which a greater part of the surface of the nanomaterial gets exposed. This results into a superior performance as a catalyst as well as a sensor. Hydrogels have been successfully employed as electrochemical biosensors because of better sensing abilities at a low cost. Some hydrogels exhibit faster drug release and swelling characteristics which makes them suitable for the delivery of antibiotics at the desired site in the gastric area, within the confines of gastric draining period [73]. Ana Torres-Martinez et al. prepared liposome enveloped molecular nanogels through pH-triggered molecular gel formation within liposomes with a gelator of low molecular weight which was derived from L-valine [74]. These carriers were found to be suitable for environmentally sensitive drug release. M. W. Amjad and M. A. G. Raja made a study about the development and characterization of ketoprofen liposomal hydrogels enriched with liposomes to know in vitro release profile. Their study suggests that such liposomes of ketoprofen are promising candidates for sustained and effective delivery of drugs [75].

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8.5.8 Nanoemulsions Nanoemulsions are a biphasic colloidal particulate system with a size range lying in the submicron region (about 10–1000 nm). In this system, one phase is faithfully dispersed in the other phase as very small droplets (with size lying in the range 20–200 nm). The purpose behind manufacturing nanoemulsions is to improve the delivery of active pharmaceutical components. Nanoemulsions carriers are in the form of solid spheres and their surface does not possess crystalline structure and is lipophilic having a negative charge. Nanoemulsions carriers raise the therapeutic efficacy of the drug with simultaneously reducing the side effects and reactions causing toxicity. Nanoemulsions constitute an advanced drug delivery system which is thermodynamically unstable but can be stabilized by surfactant or co-surfactant working as emulgent. These nanocarriers possess certain advantages such as low irritation on the skin, large capacity for drug loading, and possibly may lead to an increase in skin moisturization and drug permeability. Because of the droplet size being very small in comparison with the wavelength of visible light, nanoemulsions are transparent. As far as systemic delivery of active pharmaceuticals is concerned, nanoemulsions are the most progressive nanoparticle system for the controlled and targeted drug delivery. Nanoemulsions can be used for topical and transdermal routes of administration of active ingredients in cosmetics, and also for the delivery of drugs and genes. These are an ideal substitute for the oral route of administration of drugs. They are capable of improving the lymphatic absorption and therefore can avoid the first-pass metabolism. Important applications of nanoemulsions are the treatment of infection of the reticuloendothelial system (RES), enzyme replacement therapy in the liver, and treatment of cancer, etc. [76]. Three types of possible nanoemulsions are (i) oil-in-water (O/W) nanoemulsions consisting of oil phase dispersed in the continuously distributed aqueous phase, (ii) water-in-oil (W/O) nanoemulsions having water droplets dispersed in the continuously distributed oil phase, and (iii) bicontinuous nanoemulsions consisting of small droplets of both the component phases (i.e., oil and water) interdispersed in the system. Nanoemulsions are prepared through the methods like high energy emulsification (e.g., HPH and microfluidazation) and low energy emulsification (e.g., phase inversion temperature, and spontaneous emulsification). The spontaneous emulsification method for the preparation of nanoemulsions has a great perspective for the development of nanoemulsion-based drug delivery systems. The main constituents of nanoemulsions are oil, emulsifying agents, and aqueous phases. Emulugents or emulsifying agents provide stability to these systems and are surfactants such as spans, and tweens, hydrophilic colloids, and finely divided solids. Nanoemulsions, an advanced alternative to liposomes, are capable of improving the bioavailability of drugs, cause no toxicity and no irritation, and exist in different types of formulations, e.g., creams, liquids, and sprays, etc. The principal application of nanoemulsions is to obscure the displeasing taste of oily liquids. Nanoemulsions have the ability to protect drugs that are vulnerable to

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hydrolysis and oxidation. These provide suitable formulations for both hydrophobic as well as hydrophilic drugs. Nanoemulsions can be employed in targeted delivery of various anticancer drugs, and also for photosensitizing molecules or therapeutic ingredients. Paclitaxel is a major anticancer drug. Lina Chen et al. developed paclitaxel (PTX)-laden lipid nanoemulsions (TPLEs) having three different particle sizes as 110 nm (designated as TPLE-1), 220 nm (designated as TPLE-2), and 380 nm (denoted as TPLE-3) and assessed them in terms of their pharmaceutics, pharmacokinetics, and biodistribution, and also for in vitro and in vivo efficiency against cancer. From various observations, they concluded that TPLE-1 displayed significantly higher drug loading rate, slower release of PTX, and a greater PTX distribution in the oil phase and results into higher in vitro and in vivo efficacy against tumors in comparison with TPLE-2 and TPLE-3 [77]. Huihui Bu et al. tried for developing TPGS integrated nanoemulsions of paclitaxel (NE-PTX) to evade drug resistance in the treatment of breast cancer. They prepared NE-PTX through a self-assembly process and undertook the characterization of the physicochemical properties. The in vitro and in vivo measurement was carried out for determining the efficacy of NEPTX in avoiding the paclitaxel resistance. Their measurements led to the conclusion that NE-PTX was in the form of nanometer-sized droplets having a mean diameter of 24.93 ± 3.45 nm and the resistance of paclitaxel could be convincingly lowered due to NE-PTX [78]. Nanoemulsions can improve the drug’s efficacy against Parkinson’s disease. Bharti Gaba et al. developed Vitamin E laden naringenin (NRG) nanoemulsions to be used in direct delivery from nose-to-brain for better control of Parkinson’s disease. The nanoemulsions prepared for their optimum benefits were assessed for their effectiveness in Parkinson’s disease using multifarious actions studies in a rat model. Their observations could provide support for the viable success of a state-of-the-art no protruding intranasal delivery system of NRG for the control of Parkinson’s disease [79]. The current focus of researchers is on developing multifunctional nanoemulsions to be used for the treatment of various types of cancer [80].

8.5.9 Nanosuspension The nanosuspensions are developed as carriers for the delivery of drugs in the hydrophobic dosage forms. Nanosuspensions, an enabling technology, is a suitable carrier for those drugs which can be strongly bound to the hydrophobic receptor sites and accordingly relented drug loads which are scarcely soluble in water. Water insoluble drugs, in general, suffer from the problem of poor bioavailability. Nanosuspensions, a formulation approach, provide a typical commercially viable technique for hydrophobic drugs to solve some of their problems such as poor solubility and poor bioavailability [81]. Nanosuspensions also change the pharmacokinetics of drugs which enhances its safety and effectiveness.

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These are in fact a submicron colloidal dispersion of drug molecules which are stabilized by the use of surfactants. Nanosuspensions have a submicron particle size (i.e., size less than 1 μm) which is highly suitable for intravenous drug delivery of drugs, and moreover, there is a possibility of high drug loading through crystal phase because of which these nanomaterials are very suitable drug carriers. The reduction in particle size causes a significant increase in the dissolution rate which results into an enhancement in the bioavailability of the drug. Another advantage of nanosuspensions is that no organic solvent is used in their preparation which leads to lesser toxicity. Nanosuspension formulations require very less amount of drugs and therefore found to be very suitable in the screening stage. Drugs used in the form of nanosuspensions need less excipient. However, a drawback of nanosuspensions is that they need proper care for handling and transport as they are bulky. Nanosuspensions can be prepared through both the bottom-up and top-down approaches used in nanotechnology. The bottom-up approach of preparation of nanosuspensions includes different methods based on precipitation technique which involve nucleation and growth of particles phenomena. The top-down approach for the preparation of nanosuspensions includes disintegration or size reduction methods such as through milling or high-pressure homogenization in water and also in aqueous media or a combination of homogenization and precipitation based method. Topdown methods of preparing nanosuspensions are preferred over bottom-up methods. Nanosuspensions can also be prepared by using microemulsions and emulsions as templates. Nanosuspension formulations find their use in different delivery systems, e.g., oral, brain, ocular, topical, nasal, transdermal, etc., for delivering drugs [82]. This approach is particularly useful for those drug compounds, which are needed in higher quantities as doses, have high melting points, and a high value of log P. Nanosuspension used with receptor-mediated endocytosis in the oral delivery route is capable of resolving the problem of less absorption due to less permeability and can also fix hepatic first-pass metabolism-related issues which badly affect the bioavailability [83]. Particularly, nanosuspensions are useful for overcoming problems faced by protein and peptide-based dosage forms. Boedeker et al. prepared a lecithin-coated tetracaine-HI nanosuspension using the sonication method and assessed it for its anesthetic effects. But their observations could not reveal any anesthetic effect in negative controls of lecithin casings, without drug and 5% dextrose [84]. Karan et al. utilized phospholipid-coated nanosuspension of dantrolene and sodium dantrolene for assessing its role in the treatment of malignant hyperthermia. They made measurements regarding dose-response curves using strain gauge transducer of forelimb adduction. Their observations were in favor of showing a faster dissolution for both the nanosuspensions [85]. Ward and Yalkowski made an observation that if the speed of the injection of drugs is equal to the blood flow rate, then it leads to the emergence of plug flow. A rapid IV bolous of nanosuspension of unfiltered dantrolene can be injected into dogs which results into no observable change in pressure of pulmonary artery but in comparison with a dog; swine can serve as a better model [86].

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Docetaxel (DOX) is a chemotherapeutic drug which is an important anticancer drug for treating cancer of several kinds. The lower clinical effectiveness of DOX is a consequence of its drawbacks such as low solubility, poor selective distribution, and rapid elimination in vivo. Mohamed A. Ibrahim et al. prepared DOX nanosuspensions by solvent precipitation technique and determined its particle size and zeta-potential (for knowing the physical stability of the nanosuspension). The cytotoxic activity of DOX was found to be remarkably refined in its nanoparticle formulation [87].

8.6 Conclusion In nanotechnology, constituent particles in any nanomaterial lay in the size range 1– 100 nm. At such a small scale (i.e., termed as nanoscale), materials acquire altogether different properties as compared to their bulk counterparts. Because of their unusual properties, nanomaterials can be used for a range of applications, e.g., in automobiles for increasing fuel efficiency, efficient energy production and storage, reducing environmental pollution, catalysis, cosmetics, drug delivery, other biomedical applications, etc. Especially, nanomaterials have shown great potential for curing various diseases such as cancer when used as carriers for site-specific (i.e., targeted) delivery of drugs. Conventional methods of delivering drugs such as oral delivery, intravenous delivery, and sub-cutaneous delivery suffer from a number of drawbacks, e.g., poor effectiveness, reduced biodistribution, and lower selectivity. But nanoparticulates used as carriers of drugs possess several benefits such as enhanced physicochemical and biological control and therefore can enter the cells more favorably as compared to particles of larger dimensions. Multifunctional nanoparticles equipped with the capability of simultaneously performing multifarious activities such as targeting, imaging, and therapies are being researched extensively to make improvements in the field of nanomedicine. Nanoscience principles have been manifested by nature in various structures ever since life began on earth. Nanotechnology is making inroads into our life in several ways. It has the potential to completely change the face of electronics and other industries in a manner beneficial to mankind. Nanomaterials have also significantly affected the pharmaceutical industry because of their simultaneous use for diagnostics and therapy with enhanced efficacy and lesser side effects. Nanomaterials provide several benefits in pharmaceutical applications such as improvement in the solubility of poorly soluble drugs, enhance their bioavailability, reduce drug loading, increase dosing intervals, and reduce toxic effects to normal tissues. Thus, nanomaterials increase the therapeutic index of a drug and enhance the efficacy of the treatment. However, feasible solutions to the toxicity problems introduced by nanotechnology and its social implications have to be taken care of to optimize nanotechnology benefits to humanity. In this chapter, we have tried to explore the developments in the nanomaterials field particularly useful for pharmaceutical applications.

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Conflicts of Interest The authors declare that there are no conflicts of interest involved in this work.

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Chapter 9

PDMS on ZnO Thin Film: A Mask for ZnO Thin Film in MEMS Fabrication Priyanka Joshi and Jamil Akhtar

Abstract Zinc oxide (ZnO) is a multifunctional material and flaunts optical, piezoelectric and semiconducting properties. ZnO thin film, as a basic layer, is used in micro-electro-mechanical systems (MEMS)-based devices for sensing and actuation purpose. ZnO-based micro-electro-mechanical structures such as cantilevers and membranes require single-side processing of Si wafer. Dry etching process (DRIE) is desired way to etch silicon. Preferably, wet chemicals like potassium hydroxide (KOH), ethylene diamine pyrochatechol (EDP) or tetramethylammonium hydroxide (TMAH) can also be used to etch silicon in a more economical way. However, ZnO film is not amicable to such chemicals and dissolves in no time. Therefore, the protection of ZnO thin film in etching solvent is a very crucial issue while releasing this kind of structures. The chapter presents silicon wet etching experiments in tetramethylammonium hydroxide (TMAH) solution using silicon-based organic polymer as a protective mask for the zinc oxide sputtered side of wafer since it is difficult to use and remove SiO2 or Si3 N4 as an etching barrier, in multilayer structures. A comprehensive characterization of ZnO thin film is performed to demonstrate that structural, mechanical and electrical properties of thin film remain unaltered. Keywords ZnO thin film · Tetramethylammonium hydroxide · MEMS fabrication · Polydimethylsiloxane · Wet etching · Silicon bulk micromachining

9.1 Introduction Zinc oxide (ZnO) is a multifunctional material and flaunts optical, piezoelectric and semiconducting properties [1]. ZnO thin film as a basic layer is used in micro-electromechanical systems (MEMS)-based devices for sensing and actuation purpose. ZnO material can be incorporated in variety of devices such as microwave filters, sensors, P. Joshi (B) Dibrugarh, India e-mail: [email protected] J. Akhtar SEECE, Manipal University, Jaipur, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_9

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acoustic resonators and optical waveguides [2, 3]. Although ZnO is easy to deposit and compatible with semiconductor and micro-electro-mechanical systems (MEMS) fabrication techniques [4–7], reliability, stability and device technology are important technical issues to be explored yet. ZnO thin film is very susceptible to almost all wet chemicals and reacts rapidly. The release of some ZnO-based micro-electro-mechanical structures, such as cantilevers and membranes, is typically the last process step in 3D microfabrication process and it requires single-side processing of Si wafer [8–11]. Dry etching process (DRIE) is desired way to etch silicon. Preferably, wet chemicals like potassium hydroxide (KOH), ethylene diamine pyrochatechol (EDP) or tetramethylammonium hydroxide (TMAH) can also be used to etch silicon in a more economical way [12, 13]. However, ZnO film is not amicable to such chemicals and dissolves in no time. Therefore, in the process, protection of the side other than that to be etched is very important. Usually, Si3 N4 and SiO2 layers are used as masking layers for silicon during the anisotropic etching. But, for multilayer structures, removal and deposition of Si3 N4 or SiO2 in the lithography sequence are difficult due to the process complexity of multilayer stack. Teflon holders can be used as an alternative approach to backside protection but sometimes it can produce accidentally the penetration of etchant through the cavities to the other side of wafers or splits in them, during etching. Consequently, it damages the active devices. SU-8, a high contrast negative epoxybased photoresist can also be used as masking layer for the bulk etching of silicon, but unlike positive photoresist, it does not dissolve in alkaline solution like TMAH rather it peels off because it has poor adhesion for any material in an aqueous environment. Though an adhesion promoter can be used to improve the adhesion, but in that case, etchant will attack it and SU-8 tends to lift off. So, SU-8 is preferably used for dry etching of silicon [14]. In this report, this approach is replaced by an elastomer polydimethylsiloxane (PDMS) [15–17] coating on the wafer. PDMS is an important elastomer which is used in the development of microelectronics, MEMS/NEMS, soft robotics and microfluidics [18–22]. Xia et al. reported some applications, where photolithography falters or fails; soft lithography works and it offers some advantages like fabrication of 3D structures, patterning on nonplanar surface, process below 100 nm scale and patterning of functional materials other than photoresist. Optical transparency, biocompatibility and chemical inertness are some properties of PDMS which makes it a suitable material for transfer printing process [18]. Liu reviewed polymer materials and described that PDMS is widely used for microfluidic devices, valves, mixers and pumps [22]. Peng et al. and Ko et al. used PDMS as transfer substrate to obtain arrays of Si micro/nanowires and ribbons using transfer printing method [23, 24]. Peng et al., Zhou et al., Lee and Choi, Kim and Meng, Liu et al., and Xue et al. demonstrated that PDMS is the most promising material for large deflection applications of pressure sensor [25–30]. For the fabrication of micromachined structures, sensors and actuators silicon bulk micromachining is required. Kovacs et al. reported various methods for bulk etching of the silicon [31]. Prasad et al. fabricated ZnO thin film-based acoustic sensor. First diaphragm was made and then Al/ZnO/Al layers are deposited [32]. Polla et al. fabricated surface micromachined ZnO-based sensor [33]. ZnO thin film-based acoustic

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sensor on SOI wafer was fabricated by Prasad et al. [34]. Kim et al. reported ZnO thin film bulk acoustic wave resonator by surface micromachining using porous silicon layer etching [35]. Most of the literatures demonstrate the fabrication of ZnO-based MEMS sensor by first making the membrane followed by deposition of ZnO thin film and other metal/oxide layers to make an active device. This chapter reports an approach to protect ZnO thin film during anisotropic etching of silicon and this method allows single-sided processing of wafer, while shielding the active devices on the other side. This inexpensive unconventional approach may be preferred over DRIE. Since MEMS structures like micro-cantilever [36–41] -based sensors having zinc oxide as a sensing layer usually are thin film multilayer profiles. The protection of ZnO thin film in etching solvent is a very crucial issue while releasing this kind of structures. Therefore, in the present work, the experimentation for the protection of ZnO thin film in TMAH (25 wt% in H2 O), a Si anisotropic etching solvent [42, 43] using the PDMS approach, is discussed. Some characterization methods like scanning electron microscopy, X-ray diffraction, energy dispersive scattering, atomic force microscopy, film stress measurement and four-point probe were utilized to validate that the properties of ZnO film are not significantly affected.

9.2 Experimental Work 9.2.1 Sample Preparation Double side polished single crystal Si (100) wafer was cleaned by standard semiconductor cleaning process. For degreasing, beaker containing organic solvent such as trichloroethelene (TCE) was heated until it boils and wafer was immersed in this. Now, the same procedure was repeated with acetone and then methanol. After that, wafer was rinsed with DI water. Heavy organic residues were removed in piranha solution (H2 SO4 :H2 O2 = 3:1). Native oxide was removed in 2% HF solution and then DI water rinse. Dried wafer was loaded in oxidation furnace for ~1 μm thick thermal silicon oxide (SiO2 ) growth. The SiO2 layer works as an isolation layer between ZnO thin film and underlying Si substrate. Reactive magnetron sputtering system was used for the deposition of ZnO thin film. Zn target was used in combination with flow of O2 admixed to Ar. The oxygen flow was adjusted for the best stoichiometry of the ZnO films [44]. The 1.4 μm thick ZnO layer was grown on SiO2 /Si substrate by radio frequency (RF) magnetron sputtering while RF power was maintained at 450 W with a frequency of 13.56 MHz. The growth temperature was 25 °C and substrate to target distance was 11 cm. The sample was grown and all deposition was done with a rate of 933 Å/min. No special surface treatment was performed except degrease in acetone and methanol prior to the ZnO deposition. After that, the characterization of RF sputtered zinc oxide thin film was carried out.

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Next step is PDMS coating on the ZnO deposited wafer. In order to prepare polymer thin film, elastomer base and curing agent (Sylgard 184) were blended in a ratio of 10:1 (w/w). After a degassing process, the mixture was transferred onto the ZnO deposited side of wafer and spin coated at 3000 rpm for 30 s. After this, wafer was placed in a mechanical convection oven at 90 °C for 2 h to obtain a thick PDMS film on ZnO. After PDMS cure process, TMAH (25 wt% in H2 O) solution was heated up to 90 °C, and then, wafer was put in the solvent for 16 h to examine if PDMS is capable to shield the ZnO thin film in the etchant. The experiment was carried out for 16 h and solution temperature was maintained at 90 °C because for releasing the MEMS cantilevers or diaphragms, generally silicon micromachining is carried out for 12 h at 85 °C or more.

9.2.2 Cured PDMS Removal The common method for removing cured PDMS includes mechanical peel off followed by media blast and water rinse. Organic solvents are also used to remove PDMS by swelling of PDMS, and then, it can be removed by scraping or other mechanical means. For micro/nanostructures, such methods are not acceptable in whatever way as they tend to harm the structures/surface and result in incomplete removal of PDMS. In the present work, a dilute solution of tetrabutylammonium fluoride (75% TBAF in water) in propylene glycol 1-monomethyl ether 2-acetate has been used to remove cured PDMS from the ZnO deposited Si wafer [45, 46]. The solution can be heated up to around 50 °C for faster etching. Immersion time of about 10 min was sufficient to remove PDMS. After this, the wafer was put in PMA solvent bath for around 5 min and finally rinsed with methanol. Since ZnO is a very reactive material but it is compatible with the above-said materials. After the step above, again characterization of ZnO thin film was carried out was and compared with the previously obtained data. Various steps of the process are illustrated in Fig. 9.1. The whole experiment was repeated for about five times and the average data of each characterization is given in Sect. 9.3.

9.2.3 Characterizations In order to determine the effect of TMAH etching the electrical, structural and mechanical properties of ZnO thin film deposited over SiO2 /Si substrate were examined. The resistivity of the deposited ZnO thin film was measured using a four-point probe system. The crystallographic structure of samples was examined by X-ray diffractometer in the angle (2θ) range of 30–40 °C using CuKα radiation of wavelength 0.15405 nm. The scanning electron microscope (SEM) was used for morphological observations of the ZnO. Energy dispersive spectroscopy (EDX) was also

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Fig. 9.1 Process flow

conducted in the same chamber for qualitative analysis of material composition of the sputtered ZnO film. The topography and surface roughness of the ZnO thin film was obtained using atomic force microscopy (AFM). In-plane stress in ZnO thin film was measured by laser scanning method using film stress measurement (FSM).

9.3 Results and Discussion Diffraction patterns for the sputtered ZnO samples were obtained through X-ray diffraction (XRD) and indicated in Fig. 9.2. For as deposited sample and TMAH etched sample, (002) diffraction peak is shown in the XRD pattern. As can be observed, the only diffraction peak seen is the peak of ZnO thin film corresponds to the peak of standard ZnO (JCPDS 36-1451). The prominent peak obtained (002) which exhibited as hexagonal wurtzite structure and is preferably oriented along the c-axis [47]. The as sputtered sample has the higher (002) peak intensity and a low FWHM, which indicates an extremely narrow peak with excellent crystalline quality. The TMAH etching experiment tends to reduce the intensity of the peaks and also shifts the (002) peak angle slightly, but the FWHM values do not seem to follow the same trend. It is found that the FWHM decreases with no apparent peak shift. However, a slight increase of crystallite size is observed for the sample after TMAH etching. The grain size D of crystallites is evaluated using well-known Scherrer’s formula [48]. The dislocation density δ which defined as the length of dislocation lines per unit volume and can be calculated from the formula δ = 1/D2 [49]. The lattice constants of ZnO thin film are determined for the peak (002) and Zn–O bond length L is given by [50]. The calculated structural parameters of ZnO thin film are given in Table 9.1. In the table, the samples referred to as (a) and (b) are ZnO as deposited and thin film after TMAH etching, respectively. The larger D and smaller FWHM values denote better crystallization of thin film. It is noticed that the grain size values upgrade with the experiment, but the variation is very small which clearly reveals

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Fig. 9.2 XRD spectra of ZnO thin film

Table 9.1 Structural parameters of ZnO thin film S. No.

ZnO samples

FWHM(in degree)

D (nm)

δ (nm)−2

a (nm)

c (nm)

L(Å)

1

(a)

0.27110

31.957

9.7919

30.819

53.3817

1.927

2

(b)

0.26662

32.954

9.2083

30.780

53.3132

1.925

that there is no deterioration in the crystallinity after TMAH etching. AFM was used for morphological analysis of the film and images are shown in Fig. 9.3. The average roughness Ra and root mean square (RMS) roughness Rq are obtained for ZnO thin film, and as per the analysis result, it is found that after TMAH etching avg. and RMS roughness of the film increased from 5.52 and 6.95 nm to 7.77 and 9.47 nm, respectively. Figure 9.4a presents the surface morphology of ZnO thin film (as deposited) captured at 50,000 magnifications and 30 kV and (b) shows the SEM image of the ZnO thin film after the experiment using scanning electron microscopy (SEM). For

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Fig. 9.3 AFM images for surface topography of ZnO thin film a before, b after TMAH etching and c RMS and average roughness

Fig. 9.4 SEM images of ZnO thin film deposited on SiO2 /Si substrate a as deposited and b after the experiment

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as deposited ZnO thin film, smaller granular elements are detected and the results are correlated with the XRD data. In order to examine for the elemental composition of ZnO thin film, the energy dispersive scattering (EDS) analysis was done. Figure 9.5 shows the EDS spectrum of ZnO film at Ar pressure of 20 mTorr. The EDS spectra have prominent peaks of zinc (Zn), oxygen (O) and silicon (Si) in the samples. The Si contents are neglected assuming that the Si signals are coming from the substrate and are hence not included

Fig. 9.5 EDS of ZnO thin film deposited on SiO2 /Si substrate a as deposited and b after the experiment

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Fig. 9.6 Plots of weight and atomic percentage of Zn and O; before and after the experiment

in the analysis. The spotted Zn and O validate the presence of sputtered ZnO. The EDS spectra measured can be further identified for the particulars on atomic and weight percentage of Zn and O. The plots of atomic% and weight% of Zn and O are shown in Fig. 9.6. The features of ZnO thin film, before and after the experiment are approximately similar. Table 9.2 shows the electrical properties of ZnO thin film sputtered at room temperature. In the table, the sample referred to as (a) is for as sputtered and (b) is for the ZnO thin film after 16 h of anisotropic wet chemical etching of silicon in 25% TMAH solution. From the measured ρ, σ and Rs values listed in Table 9.2, it can be seen that the variation in the values is very less. The experiment of TMAH etching therefore did not affect the electrical properties of film in a very significant way. The residual stress of ZnO thin film was measured by measuring the curvature of wafer using film stress measurement (FSM) tool. Initially, the curvature of SiO2 /Si wafer was measured. After ZnO deposition, the same wafer was mapped and measured stress profile is shown in Fig. 9.7a. Further, in-plane stress profile of ZnO thin film after TMAH etching experiment is shown in Fig. 9.7b. The film stress can Table 9.2 Resistivity, electrical conductivity and sheet resistance of ZnO thin film deposited over SiO2 /Si wafer S. No.

ZnO thin film samples

Average electrical resistivity ρ (Ohm-cm)

Average electrical conductivity σ (mho/cm) × 10−6

Average sheet resistance Rs (Ohm/sq.)

1

(a)

15.8242 x 106

0.06319

7.9121 x 1010

0.2373

2.1063 x 1010

2

(b)

4.2127 x

106

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Fig. 9.7 Stress mapping profile of ZnO/SiO2 /Si a before and b after the experiment

be evaluated from Stoney’s equation [51]. The stress of the film is varied from 1.226 × 109 dyne/cm2 to 3.974 × 108 dyne/cm2 with compressive in nature. The variation in the stress might be due to the high temperature of TMAH, i.e., 90 °C. Therefore, it is conceived that there is no remarkable variation in the composition, electrical, structural and mechanical properties of the ZnO thin film with the TMAH protection experiment.

9.4 Conclusion The experimental results revealed that the PDMS layer assures the protection of ZnO thin film during the etching process of silicon in TMAH solution. Also, it showed good adhesion on silicon and on zinc oxide thin film as well. Based on the technique, the ZnO MEMS structures can be fabricated on silicon substrate using PDMS as a masking layer for the multilayer structure. Acknowledgements The authors wish to acknowledge the DST-FIST support X-ray diffraction facility of the physics department, BITS Pilani. Dr Jitendra Singh is thanked for his help in the experimental work.

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Chapter 10

Photoluminescence and Chemoresistive Gas Sensing: A Comparative Study Using V2 O5 Nanostructures for NH3 Nitu Singh, Jyoti Bamne, K. M. Mishra, Neha Singh, and Fozia Z. Haque

Abstract In this study, pristine and tin-doped vanadium pentoxide nanoparticles were synthesized via hydrothermal method and characterized using X-ray diffraction (XRD), Raman, transmission electron microscopy (TEM) and photoluminescence spectroscopy confirmed the orthorhombic structure, single phase and chainlayered structure. Microstructural analysis revealed the morphology of V2 O5 as chain-layered-like structure. The crystallite size with microstrain with the peakbroadening of doped and undoped V2 O5 nanoparticles was analyzed by William– Hall (W-H) method. The effect of Sn dopant on the crystallite size, morphology and luminescence property showed that the crystallite size reduces to 53.45 nm in the presence of Sn dopant. Optical sensing study on pure and Sn-doped V2 O5 at room temperature showed the maximum 79.94% sensitivity for 45 ppm ammonia. Keywords Photoluminescence sensing · Ammonia · V2 O5 nanoparticles · Chemoreceptive sensors

10.1 Introduction At the present time, various concerns regarding environmental protection, inflammable gases in industry and increasing demands for gas sensors have attracted attention of researchers and scientists. Metal oxides stand for a miscellaneous as well as interesting group of compounds whose characteristics envelope the complete spectrum of existing materials including insulators, conductors and semiconductors. These oxides cover approximately all features of physics and material science N. Singh · J. Bamne · N. Singh · F. Z. Haque (B) Optical Nanomaterials Lab, Department of Physics, Maulana Azad National Institute of Technology, Bhopal, MP, India e-mail: [email protected] K. M. Mishra Department of Physics, D.D.U. Gorakhpur University, Gorakhpur, UP, India J. Bamne Department of Physics, BUIT, Bhopal, MP, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_10

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inclusively taking care of magnetism and superconductivity. These oxides of metals exhibit various wide-ranged properties such as physical, electronic and chemical. These properties are mostly subtle to the chemical milieu and their changes. Semiconductor behavior, structural simplicity and low cost of thin films of these metal oxides have driven many scientists to work on films as electronic resources. It is noticed in the field of chemicals as well as gas sensing that for semiconductors, the electrical conductivity differs from the configuration of the atmosphere (chemicals and gases) around them. Overviewing environment through rigorous monitoring, community safekeeping, domiciliary security, air and space craft and house conditioning, sensor networks are the areas significantly affected by the presence of gas sensors. This impact is due to advantage of small size, low cost, simple operation and good reversibility. Vanadium pentoxide (V2 O5 ) is the sensitive material for resistivetype SO2 sensor and good stability. Inorganic V2 O5 has wedged much attention in the form of cathodes used in rechargeable batteries, discerning gas sensing devices because of high surface-to-volume ratio, redox action and composite edifices of V2 O5 /conducting polymer that control inner morphology. Electrical transformations of V2 O5 nanoribbon phase depend not only on the width but also on the thickness of the nanoribbons. Vanadium pentoxide (V2 O5 ) wide band gap is 2.6 eV and n-type semiconductor material and orthorhombic structure. There exist two main categories of sensors based on semiconductor metal oxide, including n-type whose majority carrier is electron and p-type whose majority carrier is hole [1, 2, 3]. The metal oxide gas sensor works on the principle of chemoresistance, i.e., alteration in electronic properties, specifically conductivity/resistivity, of films with thickness in microns or nano, when exposed to a specific gas. Molecules of the gas interact with the oxides of the metals in the form of donor/acceptor of carrier charges [4]. The chemoresistance characteristics of the oxides of the metals may be related to the breadth of conducting bands produced at the crystallites because of the electron transitions all through the adsorption process and process of desorption of molecules of the gases. The space charge region band behaves as a potential barrier to the conduction of charge carriers between the grain boundaries. This inducts variations in Fermi level. The extermination of charge carriers form the band of conduction and give rise to band bending and modulation of Fermi level. Gas sensing oxides of metals find broad applications in portable gas sensing schemes due to costeffectiveness, ease of production, compact size and modest electronic measurements [5, 6]. Nevertheless, shape, size and structure of sensitive materials remarkably affect the performance of sensors. This constrains bulk material or dense film-based gas sensors to attain standard requisites. Briefly, the retort of a sensor for a particular gas is decided by the rate of chemical reaction at the adsorptive surface of the sensor and the rate of intermingling of the molecules of the gas with the surface. At this stage, low temperature restricts response of the sensor due to the speed of the chemical reaction and high temperature limits the speed of diffusion of gas molecules. So, a transitional temperature provides equal speed to the two processes, best response of sensor [7]. Several metal oxides demonstrate a response in their electrical conductivity such as In2 O3 , Cr2 O3 , Co3 O4 , CuO, WO3 , V2 O3 , Ta2 O5 , TiO2 , La2 O3 , Mn2 O3, CeO2 , NiO, Nd2 O3 [8]. Selection of metal oxides for gas sensing can be done on the basis of

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electronic configuration. Based on electronic configuration, oxides were categorized as follows: [9]. (1) Transition metal oxides (Fe2 O3 , NiO, Cr2 O3 , etc.) (2) Non-transition metal oxides, which include a. Pre-transition metal oxides (Al2 O3 , etc.) and b. Post-transition metal oxides (ZnO, SnO2 , In2 O3 , etc.). Oxides of metals are suitable as gas sensors. These can be used in either bulk, thick or thin film form. The principle of working of sensors to detect gas depends on the variations in the following. i.

Work function: Work function is very easy and rapid process to speculate the changes in the potential of surface all through desorption and adsorption of gases from the surrounding atmosphere. ii. Resistance: Gas sensor response needs to be measured with an automatic logging meter which can record the changing values of the resistance of the gas sensor with time. iii. Sensing material’s mass after gas adsorption: Alteration in electrical characteristics of semiconductor metal oxide (SMO) due to adsorption of gas. Definition of sensors identifying gases (chemoresistive) describes it as a device which transmutes information related to chemical configuration of a specific sample at certain concentration or its total conformation analysis into an analytically advantageous signal [10]. Classification of chemical sensor is given below. A. Physical Sensor: These sensors are frequently utilized to study freshwater resources like lakes, etc. These vary from other sensors such as chemical sensors/biological sensors which measure temperature or other such parameters that circulate the transference or flux of mass/energy inside and into a freshwater body. B. Chemical sensor: A chemical indicator in the form of a signal is created when analyte binds to the identifier element. This signal is then transformed into a computable output signal using a transducer. C. Biochemical sensor: A device capable of translating a biological (or chemical) unit into an output signal (electrical) is called a biochemical sensor. The sensor consists of basic constituents including molecule of an analyte, transducer and a layer sensitive to chemicals. The consequential changes in these properties (any) are observed and evaluated to find the existence and fraction of the gas in the sensor/ambient. Conventional type of sensors uses, most often, bulk or thick/dense films as a sensing material. Recently, nanostructured thin films have replaced conventional ones, as it provides improvised control on properties of a gas sensing material. According to the sensing material used, gas sensors, based on thin films, are of three types as given below. a. Metal oxide thin film sensors (SnO2 , ZnO, Ga2 O3 , etc.)

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b. Catalytic metal thin film sensors (Pd and Pt) c. Special class of organic material thin film sensor (phthalocyanine and polypyrrol). Due to certain drawbacks like (i) selectivity issue in catalytic thin film sensor and (ii) short life time & compatibility issue in organic materials thin films sensors, the metal oxide thin film are now being preferred for commercial usage.

10.1.1 Gas Sensing Mechanism in Metal Oxides The working principle of metal oxide requires divergences in the depletion layer at the boundaries of grain when reducing or oxidizing gases are present. This gives modulations in the energy barriers allowing free charge carriers to cross the barrier and flow. As a result, conductivity of the sensing material changes. Sintered sensors or sensors with thick film consist of an active sensing layer which is formed by plentiful interconnected grains of metal oxide. Figure 10.1 exhibits that depending on the temperature, the adsorption of oxygen molecule, O2 , eliminates electrons (charge carriers) from surface of the grain to give oxygen ion species. This removal of electron gives a depletion layer near the boundaries of grain. Partial pressure of the oxygen and surface characteristics of oxide materials determine the depth of the depletion layer. Figure 10.1 shows the schema corresponding to band, which bends after chem− 2− ical adsorptions of charged species, (oxygen has three species O− 2 , O and O ). Oxygen molecules, existing on the surface of oxides of metals due to adsorption, extract electrons from the band for conduction. It then traps the electrons on the exterior of material in ion form, thus transferring electron from conduction band

Fig. 10.1 Schema of bending of band after chemical adsorption of charged species

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to valance band. When these oxygen species or ions come in contact with gases that give reduction on reaction, or a comparable adsorption, then replacement of the adsorbed oxygen by others decreases. This, in turn, can inverse the bending of band, providing enhanced physical properties. O− ion is observed to be dominant at the functioning temperature, 300–450 °C [11]. When gas molecules interact with the surface complexes of the metal oxides, dynamic material of the sensors, they bring change in numerous physical characteristics, for example, charge carriers’ density, including variation in the electrical and optical emission [12]. Sensor exposure to flammable gases allows the adsorbed oxygen species to react and form inflammable products. This reduces the coverage of the oxygen species, giving free charge carriers to the bulk oxide material, and hence electrical conductivity is finally increased. This conductance modulated by surface reaction provides a gas sensor signal. Figure 10.1 depicts energy corresponding to conduction and valence band, and the Fermi level by EC, EV, and EF, respectively. The thickness of layer of space charge is denoted by air, and potential barrier is shown by eV. e– and + represent conducting electrons and the donor sites, respectively (adapted from).

10.2 Gas Sensing Methods Out of various gas sensing methods, we discuss only two methods for comparison.

10.2.1 Photoluminescence Optical Gas Sensing Noninvasive, remote measurements and operating in perilous environment possibilities were permissible due to optical sensing techniques. Such and many more significant benefits over various other sensing technologies fascinated scientists over the past decades. It is because of these reasons that it has gone through major development. Various transduction mechanisms, e.g., resonance, absorption, photoluminescence, can be used in optical sensors. PL-based optical sensor utilizes emission of light by a material which is due to its absorption at lower excitation wavelengths. Luminescence can be classified as fluorescence or phosphorescence depending on persistence of emission. Various parameters such as oxygen, temperature or metal ion concentration can increase its emission lifetime or quenching. Development of luminescence-based sensors uses cadence of the intensity by these external parameters. Some important parameters of optical sensors such as low power consumption, stability, sensitivity and safety have recently earned them significant interest of scientists and researchers over others. However, complex structures due to optical fibers have made optical sensors troublesome in integrating with electronic circuits. The likelihood to process easy high compact and high sensitivity sensor method is based on photoluminescence shown in Fig. 10.3.

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Optical gas sensing is divided into two types; the first is based on optical adsorption when NH3 is exposed to the light source such as lasers [13, 14]. Second type of optical gas sensing is based on a change in absorption/emission of active materials when ammonia reacts with it. The emission of light corresponds to the charge transfer from oxygen to vanadium ions [15, 16]. V5+ − O2−hν → V4+ − O−

(10.1)

An extremely sensitive thin layer of sensing material, placed on quartz substratum, was fitted to a quartz gas chamber (500 ml). A thin pipe for the supply of NH3 gas was coupled to it. After exciting the sample film by 273 nm, the respective PL emission spectra were noted for the presence of ammonia and air. Photoluminescence sensor sensitivity is directly proportional to the number of active atoms on the film surface. Higher the film surface and density of active centers, higher the sensitivity of the materials. ⎫ − ⎪ 2NH3 + 3O− (adsorbed) → N2 + 3H2 O + 3e ⎬ Or ⎪ → 2N2 + 6H2 O + 6e− ⎭ 4NH3 + 3O− (adsorbed)

(10.2)

− 2NH3 + 4O− (adsorbed) → N2 O + 3H2 O + 4e

(10.3)

− 2NH3 + 5O− (adsorbed) → 2NO + 3H2 O + 5e

(10.4)

The optical sensing response or sensitivity of the sensing element is defined as the ratio of intensity in air and gas. Sensing response (S) of the sensing material can be calculated using the sensitivity formula for reducing ammonia gas [17, 18]. S% =

Ig − Ia × 100 Ig

f or I g > I a

(10.5)

where I g is the intensity of material in the presence of gas and I a is the intensity of material in the presence of air at room temperature.

10.2.2 Chemoresistive Gas Sensing Adsorption process determines sensing response of n-type metal oxide. Conduction band of sensing materials exchanges electrons/charge carriers (mobility) with the adsorbents in air/atmosphere. Transfer of electron between gas molecules, as the electron donor to n-type metal oxide matrix, takes place at room temperature, decreasing concentration of electron and resistance [19]. n-type metal oxides like V2 O5 , TiO2 , WO3 , ZnO and SnO2 are based on the interchanging of electrons

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flowing into molecules/absorbents present in air atmosphere and valance band to band of conduction of recognizing/identifying constituent. Various oxygen adsor− 2− corresponding to molecular and atomic ions, bent species, namely O− 2 and O , O respectively, adsorb the electron [20]. This forms a broad space charge band leading to increased or decreased resistance. Oxygen species increase the concentration carriers of the sensing material and then increase the conductance giving increased radiative property [21]. The probable sensing mechanism is as follows O2 (gas) → O2 (ads)

(10.6)

O2 (ads) + e− → O− 2 (ads),

(10.7)

− − O− 2 (ads) + e → 2O (ads)

(10.8)

Throughout the interface between ammonia and oxygen molecules adsorbed on the metal oxide material exterior, the surface resistance of film of metal oxide got decreased because of the reducing property of gas. The variation in the electrical resistance of this metal oxide semiconductor (MOS) film indicating the existence and nonexistence of ammonia gas can be utilized in detecting numerous concentrations of NH3 ppm ratio [22]. The sensitivity formula of the prepared thin films for various concentrations of ammonia gas can be calculated from the equation [23]. S% = (Rair − Rgas )/Rair × 100

(10.9)

where Rair represents sensor’s electrical resistance in the presence of dry air and Rgas represents sensor’s electrical resistance in the presence of the gas at varying concentration of ammonia. The changes in the sensor’s resistance in the presence of ammonia gas may be due to surface reactions ascribed to adsorbed oxygen ions O− (ads) and molecules of ammonia due to its gas phase [24] (Table 10.1). − 2NH3 + 3O− (ads) → N2 + 3H2 O + 3e

(10.10)

Numerous oxidation reactions take place at surface, for the ammonia sensor mechanisms, of n-type or p-type metal oxides. − 2NH3 + 4 O− (ads) → N2 O + 3H2 O + 4e

(10.11)

− 2NH3 + 5 O− (ads) → 2NO + 3H2 O + 5e

(10.12)

Thin film

Nanorods

Spherical nanoparticles

Nanoparticles

Nanoparticles

WO3

TiO2

SnO2 :Mg

SnO2 :Ni

PL

PL

PL

Resistive

Resistive

Resistive

Resistive

Resistive

Resistive

Resistive

Resistive

Resistive

Resistive

Resistive

PL

PL

Sensing method

RT Room temperature, PL photoluminescence

Thin film

V2 O5

Thin film

Pt/SnO2

Pd/SnO2/RGO

Nanospheres

Co3 O4 /SnO2

Particles

Nanofiber

V2 O5

CeO2 @PANI

Nanoparticle

MoO3

Nanosheets

Nanotubes

SnO2 /SnS2

Nanofibers

Spherical nanoparticles

TiO2 /SnO2

In2 O3 /PANI

Nanoparticles

SnO2

SnO2 -SnS2

Structure

Material

RT

RT

RT

180

RT

25 °C

RT

RT

RT

230 °C

200 °C

RT

RT

RT

RT

RT

Temp. (°C)

10

80

25

100

10

100

2

1000

100

450

50

100

500

100

20

25

Concen. (ppm)

–

–

–

57

60

19.6%

7.5%

1.2

2.0

25.7

13.6

–

69% (Ra– Rg )/Ra

2.48 (Ig /Ia )

63

–

Sensitivity

15

17

07

60

300

>300

400

500

200

1

4

100

60

21

12

14

Response time(s)

Table 10.1 Comparison of the sensing performance of various nanoparticle-based NH3 optical and resistive gas sensors

200 600

500

–

–

30

49

05

>500

–

17

180

180

110

50

11

Recovery time(s)

[18]

[18]

[37]

[36]

[35]

[34]

[33]

[32]

[31]

[30]

[29]

[28]

[27]

[26]

[25]

[15]

References

286 N. Singh et al.

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Fig. 10.2 Thin film prepared by doctor-blade method. a Sn-doped V2 O5 NPs, b Sn-doped Cr2 O3 NPs and c Sn-doped WO3 NPs

10.3 Experimental Details 10.3.1 Synthesis of Materials Analytical grade of Ammonium Metavanadate (NH4 VO3 ) and Tin Chloride (SnCl4 .5H2 O), NH4 OH, Nitric acid (HNO3 ) & deionized water (CDH) were used as the precursor materials for synthesis of Sn (0.5%, 1.0%, 1.5% and 2.0%) doped V2 O5 , nanoparticles (NPs) by the Hydrothermal method at 180 °C for 15 h. Then the sample was allowed to cool at room (nearly 28–30 °C), afterward the cooled sample was rinsed repetitively by ethanol & water and dried before the calcinations.

10.3.2 Thin Film Preparation for Sensing Study For promoting formation of metal oxide films of thickness in microns or nano with good integrity, the samples were individually dissolved in acetone and the resulting solution is sonicated for 15 min at room temperature. The samples were then deposited on quarts substrate by spin coating in Fig. 10.2.

10.3.3 Gas Sensing Setup Photoluminescence Ammonia Gas Sensing Study Figure 10.3 shows the optical ammonia gas sensor setup, and the gas chamber has an inlet to which a gas source is attached, an outlet with gas collector and a thin film holder at the bottom. This gas chamber was connected to the F-7000 Hitachi system which in turn was connected with the computer, and the spectrophotometer gives comparative data between sample in air and sample in ammonia gas for different ppm.

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Fig. 10.3 Photoluminescence ammonia gas sensing setup at room temperature

Fig. 10.4 Sensor schematic setup of NH3 gas molecule on Sn4+ -doped metal oxide thin films at room temperature

Chemoresistive Ammonia Gas Sensing Study and Thin Film Preparation Figure 10.4 shows resistive sensor setup.

10.4 Results and Discussion 10.4.1 X-Ray Diffraction Analysis The crystallinity and crystal structures of the synthesized products were examined by X-ray diffractometer (XRD; Bruker D8 Advance) at room temperature using Cu-Kα radiation (λ = 1.5406 Å). The XRD pattern of the pure and Sn4+ (0.5 to

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Fig. 10.5 a XRD pattern of Sn-doped V2 O5 micro-/nanostructures with JCPDS card # 77-2418 and (a1) graph between crystallite size and doping concentration

2.0wt%)-doped V2 O5 nanoparticle recorded diffraction angle 20° –80° and is shown in Fig. 10.5a. The observed X-ray diffraction (XRD) patterns are the characteristic peak of orthorhombic structure of V2 O5 and are in conformity with the JCPDS card standard (77–2418) with the space group Pmmn (59) and lattice parameters, a = 11.512Å, b = 3.564Å and c = 4.368Å. Moreover, no Sn4+ compounds and other impurity found XRD response, showing the high purity of the as-prepared samples. The diffraction intensity of doped V2 O5 nanoparticles for (001) plane was found to be smaller than the pure V2 O5 nanoparticles. This indicates that quality and crystallinity of the material are changed after doping [38]. Scherrer’s method Eq. (10.13) was applied to evaluate the sizes of unit crystals of the samples [39]. Calculated results show that V2 O5 nanostructure doped with Sn4+ (2.0%wt) drastically led to reduction in the average crystallite size from 61.36 nm to 53.45 nm; refer Table 10.2. This reduction is attributed to distortion of the crystal lattice after the Sn4+ dopant was incorporated into the V2 O5 crystal. The average inter-crystallite separation (R) is calculated by Eq. (10.13) [40, 41]. R =

5λ 8 sin θ

(10.13)

where “R” is the average inter-crystallite separation and it is used to find the position of maximum hole. Addition of Sn4+ (0.5 and 1.0wt%) was found to increase R from 3.11 to 3.41 Å. Further increase in Sn4+ concentration (1.5 and 2.0wt%) the average inter-crystallite separation was decreased from 3.20to 3.0 5Å respectively. The dislocations in the crystal system can be represented as dislocation density. The dislocation density (δ) is calculated by Eq. (10.14) [42, 43]. δ =

1 D2

(10.14)

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Table 10.2 Crystallite size from Debye–Scherrer method and “d” spacing of pure and doped V2 O5 nanoparticles (hkl) Plane

Pure and Sn4+ (0.5 to 2.0wt%)-doped V2 O5 nanoparticles 2θ

FWHM (°)

d (Å) observed

d (Å) calculated

Primitive (h2 + k2 + l2 )

BCC

FCC

200

15.364

0.096

5.756

5.767

4

BCC

FCC

001

20.265

0.117

4.368

4.382

1

–

–

101

21.698

0.115

4.084

4.095

2

BCC

–

110

26.149

0.105

3.404

3.408

2

BCC

–

301

31.034

0.157

2.878

2.883

10

BCC

–

011

32.372

0.132

2.761

2.765

2

BCC

–

111

33.319

0.124

2.685

2.689

3

–

FCC

310

34.314

0.123

2.611

2.613

10

BCC

–

002

41.239

0.162

2.184

2.189

4

BCC

FCC

102

42.038

0.147

2.146

2.150

5

–

–

411

45.473

0.140

1.992

1.994

18

BCC

–

600

47.352

0.153

1.918

1.919

36

BCC

–

012

48.812

0.154

1.862

1.865

5

–

–

020

51.235

0.162

1.782

1.783

4

BCC

FCC

601

52.004

0.120

1.756

1.758

37

–

FCC

021

55.646

0.132

1.649

1.651

5

–

–

611

58.982

0.151

1.576

1.566

38

BCC

–

420

61.111

0.171

1.515

1.516

20

BCC

–

710

62.121

0.164

1.493

1.494

50

BCC

–

620

72.509

0.406

1.306

1.303

40

BCC

–

a (Å)

11.512

Space group = Pmmn (59)

b (Å)

3.564

Orthorhombic structure

c (Å)

4.368

JCPDS No. 77-2418

Aveg. crys. size D’(nm)

61.36

67.01

73.19

66.65

53.45

Aveg. inter-crys. separation

3.11

3.24

3.41

3.20

3.05

2.656 × 10−4

2.227 × 10−4

1.867 × 10−4

2.251 × 10−4

3.501 × 10−4

(Å) Dislocation density lines/m2

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where D is the average crystallite size. The dislocation density decreased with increasing crystallite size for Sn4+ (0.5 and 1.0wt%)-doped V2 O5 , but at higher concentration of Sn4+ (1.5 and 2.0wt%) dislocation density increases with the decrease in crystallite size. Calculated average inter-crystallite separation and dislocation values are given in Table 10.2. Figure 10.5b shows the relationship between the crystallite size vs doping concentration.

10.4.2 UV–Visible Spectroscopy Analysis The band gap properties of the synthesized Sn-doped V2 O5 nanoparticles were evaluated by UV–visible spectrophotometer (UV–Vis; Lambda 950, PerkinElmer). The effect of Sn4+ substitution on orthorhombic structure of V2 O5 was confirmed. UV– visible absorption spectroscopy exhibited a well-defined and sharp absorption band in the range 200-800 nm. The mentioned absorption in undoped and doped V2 O5 nanoparticles was due to charge transfer from O–2p valance band to empty V-3d orbitals [44]. E g was calculated by considering transition between bands (conduction and valence) as direct transition. The band gap energy for pure and doped V2 O5 nanoparticles was calculated by drawing a plot between (αhν)2 vs hν using the Tauc Eq. (10.15) [43]. αhν = B(hν − Eg)n or 1/n αhν = B(hν − Eg)

(10.15)

where α is an absorbance coefficient, h is Planck constant, B is constant, ν is the photon frequency, Eg is the optical absorbance energy gap and n is a number describing transitions of various types. The transitions that took places are: directly and indirectly allowed as well as forbidden states for n = 1/2, 2, 3/2 and 3, respectively [38]. In the present study, by increasing the doping concentration of Sn4+ (0.5 and 1.0wt%) energy band gap linearly increased from 2.91, 2.95 and 3.18 eV but at higher concentration of Sn4+ (1.5 and 2.0wt%) energy band gap suddenly decreased from 3.15 and 2.76 eV [45]. The spectrum corresponding to absorption for undoped as well as doped V2 O5 attains the UV region of photon energy. UV–visible spectroscopy analysis was in good agreement with XRD and AFM results because absorption position depends on the morphologies and sizes of doped and undoped V2 O5 . The blue region of the band gap and crystallite size confirms the lattice of V2 O5 and supports XRD results as shown in Table 10.3.

292 Table 10.3 Absorption and energy band gap of pure and doped V2 O5 nanoparticles

N. Singh et al. Doping concentration

Absorption wavelength (nm)

Energy band gap (eV)

Crystallite size (nm)

Pure V2 O5

426

2.91

61.36

0.5wt% Sn4+ doped

420

2.95

67.01

1.0wt% Sn4+ doped

390

3.18

73.19

1.5wt% Sn4+ doped

393

3.15

66.65

2.0wt% Sn4+ doped

449

2.76

53.45

10.4.3 High-Resolution Transmission Electron Microscopy (HRTEM) Analysis The cryo-HRTEM instrument was used to determine both the average size and shape of the nanoparticles prepared by hydrothermal method. The HRTEM image shown in Fig. 10.6a represents the micrograph of pure V2 O5 nanoparticles. It has been observed that the obtained particles are in the range of few nanometers (25–144 nm). It has been observed that the obtained particles show the sphere-like particles which were joined together as sintered necks giving chain-type aggregates [46]. Figure 10.6b demonstrates the discrete interplanar “d” spacing of 0.34 nm, in correspondence with the (110) plane of orthorhombic structure V2 O5 , although Fig. 10.6c depicts the selected area electron diffraction (SAED) of V2 O5 , showing the polycrystalline behavior of the sample. Particle size distribution graph is shown in Fig. 10.6d. The lattice parameter of “d” spacing is in agreement with standard JCPDS file. 77-2418 and in accordance with the XRD results.

10.4.4 Atomic Force Microscopy (AFM) Atomic force microscopy (NT-MDT Solver Next) was used to examine the particle size distribution and surface topography of pure and tin-doped V2 O5 nanocrystalline thin films on selected places with 1.0 × 1.0 μm2 area. Figure 10.7 shows the morphology, topology and quantitative roughness (2D and 3D) of all prepared pure and doped V2 O5 thin films analyzed by atomic force microscopic. In addition, Sn4+ -doped sample displayed plane and condensed surface as final particle increases and reduces the surface roughness. Figure 10.7 depicts that root mean speed (rms) values of the surface of pure and doped V2 O5 significantly reduce from 0.68 nm to 0.29 nm (2D) and 0.88 nm to 0.27 nm (3D), with decrease in the average grain size from 14.83 to 6.79 nm as shown in Table 10.4. The average mean width and average

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Fig. 10.6 HRTEM images of V2 O5 nanoparticle scale bar are a 100 nm, b lattice fringes confirm the partly crystallization of V2 O5 , c SAED pattern and d graph between number of particles vs particle size (nm)

diameter decrease from 9.56 nm to 4.86 nm and from 27.14 nm to 11.52 nm at 49.7% room humidity. Atomic force microscopy (AFM) analysis presented the fact that the distribution of the local surface is normal at all the points of the images [47]. A comparison of morphology and roughness between pure and doped V2 O5 thin films prepared at RT can be evidently observed from these images. It is also obvious that grains are much bigger; width and diameter were formed on the films deposited at RT with a high homogeneity indicating the uniform film growth. The WSX M software provides clear images and calculates different parameters of AFM data. The observed nano-faceted crystalline nanoparticles were due to the high surface energy which further increases with the Sn4+ doping [8]. By increasing the doping concentration of Sn4+ , average grain size, average roughness (2D and 3D), width and diameter decrease significantly.

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Fig. 10.7 Atomic force microscopic images of pure (a) and Sn4+ (b-0.5, c-1.0, d-1.5 and e-2.0wt%)doped nanoparticle thin films on quartz glass substrate

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Table 10.4 Atomic force microscopic analysis results of pure and doped V2 O5 NPs-based films on quartz glass substrate Doping concentration

Grain size (nm)

Roughness 2D (nm)

Roughness 3D (nm)

Mean width (nm)

Diameter (nm)

Pure V2 O5

14.83

0.68

0.88

9.56

27.14

0.5wt% Sn4+ doped

11.56

0.67

0.75

7.63

20.65

1.0wt% Sn4+ doped

10.52

0.53

0.62

6.73

19.31

1.5wt% Sn4+ doped

7.34

0.50

0.49

5.12

12.77

2.0wt% Sn4+ doped

6.79

0.29

0.27

4.86

11.52

10.4.5 Photoluminescence Analysis The room-temperature photoluminescence properties were evaluated by roomtemperature photoluminescence (PL; F-7000 Hitachi). Figure 10.8 shows photoluminescence spectra of pure and doped V2 O5 nanoparticles. In the photoluminescence spectra, the excitation wavelength of 320 nm was used for all the samples. The observed PL spectra exhibited seven well-defined luminescence peaks originated at 382, 397, 451, 469, 491, 535 and 557 nm with photon energies of ~3.246, 3.123,

Fig. 10.8 PL spectrum of undoped and doped V2 O5 nanoparticles

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2.749, 2.644, 2.525, 2.177 and 2.226 eV, respectively. All the samples validate the luminescence for UV (385 nm and 397 nm) and the visible regions (451–557 nm). The observed photoluminescence spectra for the doped V2 O5 NPs show various bands in the violet, blue and green regions. All bands appearing in various regions originated at 382 and 387 nm (violet region), 451, 469 and 491 nm (blue region) and 535 and 557 nm (green region). NUV photoluminescence band was explained as the near band edge emission (NBE), which was applied to recombine electrons and holes in the conduction and valence bands, respectively [48, 49]. The luminescence properties of vanadium pentoxide strongly depend on the recombination of electrons, and luminescence peak intensity depends on the wt% concentration of Sn4+ in the V2 O5 matrix. Photoluminescence of V2 O5 nanoparticles is closely associated with the V–O bond, because of the weakness of the V–O, its oxygen was effortlessly removed at relatively low temperature [50]. The PL intensity of as-synthesized samples increased with increasing Sn4+ (0–1.5wt%) doping concentration and decreased at higher (2.0wt%) concentration. Pure and doped V2 O5 nanoparticles illustrate a broad band of emission around 382 nm wavelength to 557 nm wavelength that may be attributed to the self-trapped excitation due to their process of recombination which arise as a result of charge transferred to excited state of the V2 O5 [51]. The self-trapping of excitations causes loss of excitation energy through lattice relaxation [52]. This makes all pure and doped sample a best nominee for optical sensor at room temperature with layered structure of V2 O5 nanoparticles.

10.4.6 Gas Sensing Study and Comparison Thin films of all the samples were utilized to investigate gas sensing features. Films were prepared on quartz substrate using Holmarc spin coater (Model No: HO-TH-05) operated at the speed of about 6000 rpm for 60 s in a single step.

10.4.6.1

NH3 Gas Sensing by PL Sensing Method at Room Temperature

Optical gas sensing mechanism can be divided into two types: First type of sensing is based on adsorption when target gas is exposed to the light source such as lasers [53], while the second type is based on a change in emission of active materials when analyzed gas reacts with it. The optical sensor used here is of second type where the emission of light corresponds to the charge transfer from oxygen to vanadium ions [50] and reduces the charge of some ions from V5+ to V4+ . Easily detectable changes, and charge carrier density was favorable for optical ammonia sensing. V5+ − O2− hv V4+ − O− − →

(10.16)

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The charge transfer explains the sensor response of ammonia interacting with a network of pure and doped V2 O5 nanoparticles [54]. Room-temperature PL sensing spectra of all as-prepared samples were measured with excitation at a wavelength of 320 nm in the presence of NH3 gas at different concentrations (5 ppm to 200 ppm) for photoluminescence optical gas sensing spectra of pure and doped V2 O5 nanoparticles. Figure 10.9 shows the overlapped PL spectra of the pure V2 O5 and several Sn4+ -doped V2 O5 at zero ppm to 200 ppm. When ammonia gas is introduced, interaction between the molecules of the gas and already adsorbed oxygen species takes place releasing charge carrier (electrons) once again to the band of conduction that leads to increase the concentration of the carrier. This describes the functionality of sensors that gives the identification of gases reduced by n-type V2 O5 . The change in the concentration of the carrier is unswervingly associated with the number of molecules of ammonia adsorbed on the surface of the sensor and increases the intensity. The intensity increases with the increase of ammonia for all the samples as shown in Fig. 10.9. When the reducing gas (donor) molecules interact with the V2 O5 surface, it reacts with oxygen, removes it by adding electron and decreases oxygen traps to increase the luminescence intensity due to reduction in non-radiation transition. When all the prepared samples interact with the ammonia gas, the solitary noteworthy consequence detected in the PL intensity is increased in the PL intensity up to 45 ppm due to reduced oxygen traps. As it cannot overcome or compensate, the electrons trap completely so the intensity of PL decreases from 50 ppm to 200 ppm. But the increase of ammonia is from 0 ppm to 45 ppm (in step of 5 ppm), the gas overcomes and goes on decreasing the traps to increase PL intensity as shown in Fig. 10.9. The emission increases till 45 ppm, and after that it decreases maybe because access electrons flooded in the materials have dissipated the large energy in the form of non-radiation mode. This increase and decrease in the luminescence intensity may be related to the response of ammonia gas molecules when it interacts with all of the synthesized samples. In case of reversible absorption process, re-exposure of active material to dry air will result in leaving the surface of the adsorbed molecules. Due to this, recovery of the photoluminescence signal takes place which can be seen in time response graph shown in Fig. 10.10.

10.4.6.2

Time Response Measurement by PL Sensing Method

Figure 10.10 shows the time response curve of PL intensity vs time during the sensing measurement. All these samples responded immediately when NH3 gas was introduced into the testing chamber. Each time, the concentration of NH3 was same ppm at which the respective sample has given its maximum sensitivity. When ammonia gas enters inside designated chamber, integrated intensity of PL decreases sharply, which confirms its n-type semiconductor behavior, reaching the rather stable values. When normal atmospheric air is glided through the chamber again, the integral intensity recovers its initial values, exhibiting the excellent characteristic of the sensors. The time essentially required by the sensitivity factor to attain the 90% variation with

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Fig. 10.9 Photoluminescence gas sensing graph of pure and doped V2 O5 nanoparticles

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Fig. 10.10 Response, recovery and sensitivity of pure and doped V2 O5 NPs (PL method)

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Table 10.5 Comparison of PL and chemoresistive gas time of response, time to recover and sensitivity of pure and doped V2 O5 nanoparticles S. No. Doping Method concentration PL sensing

Chemoresistive sensing

Response Recovery Sensitivity Response Recovery Sensitivity time (s) time (s) % time (s) time (s) % 1

Pure V2 O5

36

54

68.37

15

26

84.45

2

0.5wt% Sn4+ doped

34

51

72.52

14

26

85.12

3

1.0wt% Sn4+ doped

32

44

74.03

14

22

85.65

4

1.5wt% Sn4+ doped

26

43

75.68

14

24

85.43

5

2.0wt% Sn4+ doped

36

54

68.37

14

20

86.61

respect to equilibrium is represented by response time and rise time, whereas the recovery time relates to the time requisite by the sensitivity factor to attain a value 10% below the equipoise value in the air. Sn4+ (2wt%)-doped V2 O5 nanoparticles show lower response time (24 s) and recovery time (36 s) at 45 ppm. This may be attributed to the reason that the decreased particle size has a larger surface area and also to the high accessibility to the largely available vacant sites on the surface of gas for adsorption. Increased recovery time may be attributed to the weightier characteristics of NH3 and to decreased desorption rate due to the reaction products which do not leave the interface instantly as the reaction ends. All the samples show response and recovery time much lower than the reported ones at room temperature only [55]. The response and recovery times signify that the sensor exhibited significant sensing properties toward NH3 at room temperature. Table 10.5 shows the calculated response time along with recovery time of the uncontaminated and doped V2 O5 nanoparticles.

10.4.6.3

Sensitivity Measurement by PL Sensing Method

The evaluation of optical sensor sensitivity was to calculate the percentage variation of ratio of the PL intensity, when sensor was brought close to dry (without gas) and to the fixed concentration of gas, respectively. Sensitivity (%) of sensor can be calculated by the sensitivity formula given by Eq. (10.5) [56, 57]. The sensitivity of the sensor was found to be increasing with the increase in dopant concentration. The maximum sensitivity of 79.94% Sn4+ (2.0wt%) was observed while sensing ammonia at a room temperature. Optimum doping was observed for Sn4+ (2.0wt%) ions. Substantial amount of surface defects due to Sn4+ doping in 2.0wt% gave better sensitivity compared to pure and other doped V2 O5 nanoparticles [56].

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Table 10.5 summarizes features of optical ammonia gas sensor, pure and nobbled (doped) with V2 O5 nanoparticles, and reveals the typical graph for the observed response time, recovery time and sensitivities of optical ammonia gas sensors based on pure and doped V2 O5 thin films, Fig. 10.10.

10.4.6.4

Chemoresistive NH3 Gas Sensing at Room Temperature

The electrical response of pure and Sn4+ -doped V2 O5 nanoparticles to ammonia (NH3 ) was studied at different concentrations (5 ppm to 200 ppm) at room temperature. The increase or decrease in resistance in the presence of gas depends upon the nature of gas (reducing or oxidative). The sensor response has been measured prior and after the exposure to various concentrations of ammonia with room temperature shown in Fig. 10.11.

10.4.6.5

Time Response Measurement by Chemoresistive Method

The times of response and recover were correspondingly the time taken by the sensor response to achieve almost 90% of the saturation value and the recovery time required to reach to its initial value after vapor evacuation. The finest times to respond and recover were obtained at exposure duration of 2 min. The electrical response of the films was studied at RT in air and ammonia (NH3 ) for 5–200 ppm as displayed in Fig. 10.11. The response and recovery times were determined to be 26–17 s and 2–14 s for NH3 at room temperature shown in Fig. 10.12 and Table 10.5. The response and recovery time decreased with increasing doping concentration. The response (14 s) and recovery times (20 s) with the best sensing parameter for Sn4+ (2.0wt%)-doped V2 O5 suggest that the Sn4+ (2.0wt%)-doped V2 O5 nanoparticle sensing material was also beneficial for ammonia detection at room temperature as shown in Figs. 10.11 and 10.12.

10.4.6.6

Sensitivity Measurement by Chemoresistive Method

The sensitivity was defined as the ratio of the sensor resistance at the various concentrations of the gas to the sensor resistance in air and calculated using Eq. 10.9 [58]. The sensing response of the pure and doped V2 O5 thin films deposited at quartz substrate glass slide was investigated for 5 ppm to 200 ppm for ammonia at RT as shown in Fig. 10.12. The sensor sensitivity increases of pure and doped V2 O5 nanoparticles with an increasing gas concentration. The response and recovery times were found to be in decreasing order when the doping concentration and ammonia gas concentration were increased. The sensing behavior (sensitivity), and response and recovery times are in Table 10.5. The results for sensitivity, and response and recovery times were significantly better than that of accessible results of doped and

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Fig. 10.11 Resistance versus time (s) and sensitivity versus doping concentration of pure and doped V2 O5 nanoparticles

pure V2 O5 nanoparticles. It was also noticed that the Sn4+ (2.0wt%)-doped V2 O5 nanoparticles were also beneficial for ammonia detection at room temperature as shown in Fig. 10.12.

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Fig. 10.12 Response and recovery time vs doping concentration of pure and doped V2 O5 NPs

10.5 Conclusion Hydrothermal technique/route is found to be an effective process to synthesize pure and Sn4+ -doped V2 O5 nanoparticles. The XRD patterns show that the samples are orthorhombic in nature. No impurity phase due to dopant has been detected in XRD patterns. The crystallinity, particle size and lattice constant are increased and found

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to be (61.36–73.19 nm) with the increase in dopant (0–1.0w%) concentration. But for higher concentration of Sn4+ (1.5 and 2.0wt%), the crystallite size decreases continuously at very slow rate up to 53.45 nm. HRTEM image shows the morphology of V2 O5 nanostructure as a layered structure. The selected area electron diffraction (SAED) patterns illustrate the polycrystalline nature of vanadium pentoxide NPs. AFM results confirm significant decrease in average grain size, average roughness (2D and 3D) and diameter as the doping concentration of Sn4 increases. Energy band gap of pure and doped V2 O5 linearly increased with increasing doping concentration (0– 1.0wt%), but at higher concentration energy band gap diminishes. PL spectrum gives emission in visible region around two distinguished wavelengths, blue and green in all the nanoparticles attributed to the formation of luminescent centers formed by defects. The intensity of doped V2 O5 is found to be significantly lower than pure one. It is observed that there exists a wide-ranging emission peak in visible region of the fluorescence spectra probably due to surface defect levels. Increase in the concentration of the dopant leads to the increases in the intensity of emission in visible range. Therefore, Sn4+ dopant controls the structural/physical properties, band gap and visible luminescence of the V2 O5 nanostructure. The maximum optical sensitivity (PL) was observed for Sn4+ (2.0wt%)-doped V2 O5 nanoparticles which was 79.94%, the response time is 24 s, and recovery time is 36 s for 45 ppm ammonia concentration. The chemoresistive sensitivity for Sn4+ (2.0wt%)-doped V2 O5 is found to be highest 86.61% with a remarkable response time 14 s and recovery time 20 s. Since we do not observe much peak shift in PL graphs, we can conclude that this material is stable and can be used again with authenticity. Acknowledgements Authors are pleased to acknowledge the Director, MANIT, Bhopal, for providing necessary facilities for this work, and the Director, UGC-DAE-Consortium for Scientific Research, Indore Centre (M.P.), India, for XRD, FTIR, micro-Raman, thin film analyzer and UV–Vis measurements. Mr. Nitu Singh is thankful to MANIT for providing Institute Fellowship. Competing interests The authors declare that they have no competing interests and are alone responsible for the content and the writing of this chapter.

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Chapter 11

Advancement in Carbon Nanotubes: Processing Techniques, Purification and Industrial Applications Anbesh Jamwal, Muhammed Zahid Hasan, Rajeev Agrawal, Monica Sharma, Sunil Thakur, and Pallav Gupta Abstract Right from the starting, carbon nanotubes (CNTs) have been given special attention. Numerous researchers have investigated the processing techniques, purification as well as mechano-physical properties of this advanced form of Carbon (C). At present, CNTs and its composites have gained industrial importance due to its better mechanical, optical and thermal properties when it is compared with other materials. Development in carbon nanotubes-based composites has opened up scopes for their utilization in engineering applications. Various properties such as physical, structural, thermal and mechanical have been improved due to the utilization of CNTs as the reinforcement phase in the composites. The aim of the present chapter is to report the advancement in processing techniques, purification and industrial applications of carbon nanotubes and their composites. Among all the processing techniques, chemical vapor deposition (CVD) is widely used to synthesize carbon nanotubes and oxidation techniques is used for purification purposes. This work also examines the reported literature on the processing and purification of carbon nanotubes as well as the use of carbon nanotubes in the development of composites. Keywords Carbon nanotubes (CNTs) · Processing techniques · Purification · Industrial applications

A. Jamwal · R. Agrawal · M. Sharma Department of Mechanical Engineering, Malaviya National Institute of Technology, J.L.N. Marg, Jaipur, Rajasthan, India M. Z. Hasan · S. Thakur Department of Mechanical Engineering, Alakh Prakash Goyal Shimla University, Shimla, Himachal Pradesh, India P. Gupta (B) Department of Mechanical Engineering, A.S.E.T., Amity University Uttar Pradesh, Noida, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_11

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11.1 Introduction In the present time, carbon nanotubes (CNTs) and carbon nanofibers are promising materials for engineering applications [1]. The market development of carbon nanotubes depends on two factors, i.e., cost of material and availability of materials in local market. At present, CNTs have many applications areas in chemical industry, energy, aerospace and automobile component manufacturing industry in which they are being used as composites reinforcement, templates, gas adsorbents, probes, nano-reactors and chemical sensors [2, 3]. Good structures of carbon can be formed in sp2 hybridization, and it can also form both open and closed cages with honeycomb atomic structure (HAC) apart from graphite [4]. C60 was the first structure discovered by Kroto and also C as fullerenes [5]. “In the starting of 1991s, Iijima observed the first tubular carbon structure” [6]. Properties of CNTs, i.e., thermal conductivity (k), lattice structure, density (p) and electrical conductivity (σ ) are highly influenced by its structure in which both diameter and type of structure are important [7]. Generally, carbon nanotubes having higher structural perfection can be classified into two basic categories which are as follows: (1) Single-walled carbon nanotubes (SWCNTs), which consists of a singular sheet of graphite wrapped in a cylindrical tube. Single-walled carbon nanotubes are the allotropes of sp2 hybridization of carbon (C) which are similar to fullerenes [8]. (2) Multiwalled nanotubes, in which concentric walls vary from 6 to 25 nm or more. The diameter of the multiwalled nanotubes is large (30 nm) when compared to single-walled carbon nanotubes (0.7–2.0 nm) [9].

11.1.1 Single-Walled Carbon Nanotubes (SWCNTs) In the last few years, single-walled carbon nanotube properties have become popular in the engineering and industrial applications because of their excellent properties, i.e., thermal, physical, optical, mechano-tribological and electrical [10]. The conductivity of single-walled carbon nanotubes depends on the chiral angle which decides that the nanotubes are semiconductors or metallic in nature [11]. Singlewalled carbon nanotubes are inert which can also be used as reinforcement phase in composites to improve their properties and develop materials for industrial and automobile applications to meet the present generation expectations [12]. Physical properties of single-walled carbon nanotubes (SWCNTs) depend on the shape, size and on sp2 hybridization of atoms present in hollow structure [13]. Different types of chiral angles are formed from the graphite sheets during the synthesis of single-walled CNTs, and hence, there are different types of SWCNTs [14]. Generally, single-walled nanotubes show three types of structures which are (1) Armchair [15] (2) Zigzag [16] and (3) Chiral [17]. The structure of carbon nanotubes is shown in Fig. 11.1.

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Fig. 11.1 Carbon Nanotubes structure

Table 11.1 Electrical behavior of CNTs (Carbon nanotubes) with respect to differing values

Differing value

Electrical behavior

m=n

Metallic

(n–m) is a multiple of 3 and (n = m) and nm = 0

A small band gap with semi-electrical behavior

Electrical and other properties of single-walled carbon nanotubes (SWCNTs) can be determined by length, diameter and chiral structure of nanotube. The diameter of nanotube in the nano-range gives the quantum effect to the tube [18]. Carbon nanotubes can either be conductive or it may be semi-conductive depending upon the chirality [19]. The properties of SWCNTs change with change in differing (m, n) values [20]. The band gap in SWCNTs varies from 0 to 2 eV. Further, electrical behavior with respect to differing values is presented in Table 11.1.

11.1.1.1

Optical Properties

Generally, at present, nanotubes are used for their fluorescence, light absorption and transparent conductive properties. Nanotubes are known as light absorbing (super black) materials when they are being used as a carbon nanotube array because once the light gets trapped, it cannot reflect back from individual CNT [21].

11.1.1.2

Thermal Properties

CNT has excellent thermal properties. Thermal conductivity (k) of single-walled carbon nanotubes (SWCNTs) along its axis at normal room temperature is about 3500 W m−1 K−1 which is higher than the copper thermal conductivity at room temperature. In some studies, it is observed that thermal conductivity and other thermal properties are strongly affected by structural defects which leads to phonon scattering and hence reduce the thermal conductivity [22].

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Mechanical Properties

It is found that SWCNTs are the stiffest and strongest materials when compared with other available materials for both industrial and engineering applications. Physically strong and mechanically flexible in nature makes CNT as a promising choice for materials in many engineering applications. Covalent sp2 hybridization bonding between the chains of carbon atoms with other atoms in structure provides strength to the CNTs. It is observed in some studies that advanced materials, i.e., composite materials reinforced with CNTs result in good strength and tribological properties over base material [23].

11.1.2 Multiwalled Carbon Nanotubes (MWCNTs) Multiwalled carbon nanotubes exhibit excellent properties which make them a promising choice for industrial, engineering and transportation applications [24]. The various properties of MWNTs are Electrical properties: Generally, multiwalled nanotubes are highly conductive when used as reinforcement material in the composite. It is found that the inner wall is not conductive as compared to the outer wall [25]. Physical properties: Composite reinforced with multiwalled nanotubes exhibits excellent tensile strength. It can help to fabricate defect-free material [26]. Thermal and Chemical properties: Multiwalled nanotubes can withstand temperature up to 600 °C. Multiwalled nanotubes are the allotropes of sp2 hybridization of carbon which are almost similar to fullerenes and graphite, so the chemical stability of multiwalled nanotubes is good. Both dispersibility and strength of composites can be enhanced by functionalizing the nanotubes [27]. Challenges which resist the commercialization of multiwalled nanotubes are also investigated such as:

11.1.2.1

Purity

As many multiwalled carbon nanotubes extraction processes result in the residual metallic catalyst which affect the performance of carbon nanotubes in its application areas [28].

11.1.2.2

Dispersion

When compared to single-walled nanotubes, multiwalled nanotubes have better dispersibility into polymers or solutions. It is found that the quality of dispersion in the polymer or solution is considered as one of the critical factors that influences the performance of CNTs in the final material and its applications [29].

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11.1.2.3

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Defects

Studies reported that number of defects present are dependent on the large number of layers which are present in multiwalled nanotubes. Higher aspect ratio of multiwalled nanotube contributes more to the value of their use [30]. Excellent properties and the nature of CNTs lead toward the vast use of CNTs at present in industries [31]. Over time, the research area of the CNT-based studies is gaining popularity, and the researchers are having an impact on the place of their choice where fields have grown very diversely in the past years [32]. Lot of studies has been done in past few decades in the area of CNT which is discussed in Table 11.2.

11.2 Production of CNTs As investigated, many production processes have already been introduced in the past few years. Some processes need physical attributes of carbon conversion into nanotubes, and some are chemically synthesized. CNT production processes are scaled up into two categories which is shown in Fig. 11.2.

11.2.1 Large-Scale Production Techniques 11.2.1.1

Arc Discharge

Carbon arc discharge method previously aimed at the production of C60 fullerenes which is regarded as the simplest and general method for manufacturing carbon nanotubes. “This technique results in a mixture of components and requires nanotubes to separate from soot and other catalytic metals that come out with the crude product” [55]. In this method, the apparatus is made up of an enclosed vacuum chamber where two carbon rods are placed in the range of 1–2 mm distance between them, and nanotubes are formed. Recent experiments reported the use of liquid nitrogen as a medium to create nanotubes by arc discharge method. Direct current arc discharge (50–100 A) carried out by around 20 V is induced between the electrodes that create a significantly high temperature of approximately 4000–6000 K. The discharge vaporizes carbon anode, and subsequently, it is accumulated at cathode losing temperature forms columnar structure-shaped nanotubes of thickness 6 mm categorized as MWNTs. Higher production of nanotubes requires the plasma arc uniformity as well as temperature of the accumulation formed on C electrode [55]. The main objective of growth mechanism is dependent on the different diameter distributions of Helium (He) and Argon (Ar) mixtures. These blends have different thermal conductivities and dispersion that influence the speed which makes C and catalyst diffuse and cool which impacts nanotube width in the process. It is possible to generate SWNTs and MWNTs selectively [55]. Li et al. synthesized with N-doped-based graphene sheets

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Table 11.2 Past studies carried out in the area of CNTs S. No.

Study

Research area

1.

“Partition effects (corrugated conductive) of CNT water nano fluid under cavity on MHD free convection” [33]

“CNT water nano-fluid”

2.

“Polymer composites (reinforced carbon fiber with, g-C3 N4 by in-situ method)” [34]

“CNTs/composites”

3.

“Vibrational analysis of CNT (functionally graded) reinforced composite (truncated conical plane/boundary condition)” [35]

“CNTs/composites”

4.

“Achieving strength and toughness along with electrical conductivity though trace electro sprayed nano polystyrene Facilitated Dispersion in MWCNTs” [36]

“Multiwalled CNTs”

5.

“Enhancing interfacial properties through grafting CNTs layer by layer on the carbon-fiber surface” [37]

“Core CNTs”

6.

“For lithium-Sulphur batteries, CNTs (3d interconnected) as polysulfide reservoir” [38]

“CNTs in battery”

7.

“For lithium ion storage, nano-TiNb2 O7 /CNTs composites as anode” [39]

“CNTs in storage”

8.

“Scaling CNTs transistor (complementary) with gate length 5-nm” [40]

“CNTs in transistor”

9.

“Interwoven MXene (Nanosheet)/CNTs Composites as Li–S Cathode” [41]

“CNTs/composites as cathode”

10.

“Metal organic frameworks/CNTs (Foldable & interpenetrated) thin film for batteries of lithium-Sulphur” [42]

“CNTs in batteries”

11.

“Co@Co3 O4 Encapsulated in CNTs/Grafted Nitrogen-Doped Carbon Polyhedra as Oxygen Electrode” [43]

“CNTs as electrode”

12.

“CNTs/Graphene for Energy Storage (Electro-chemical)” [44].

“CNTs as energy storage”

13.

“Composed of Co3 O4 Hollow Nanoparticles/CNTs by Hierarchical Tubular Structures for Lithium Storage” [45]

“CNTs/composites”

14.

“High volumetric capacitance for flexible MXene/CNTs composite paper” [46]

“CNTs/composites”

15.

“Biosensors fabrication by graphene, CNTs, Au, Zinc oxide for healthcare” [47]

“CNTs in health science”

16.

“Cobalt embedded Ni-Rich CNTs efficiently catalysed by Hydrogen evolution reaction at different pH” [48]

“CNTs in chemical science”

(continued)

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Table 11.2 (continued) S. No.

Study

Research area

17.

“CNTs decorated/CoP nanocrystals as electro catalyst for Hydrogen evolution” [49]

“CNTs as an electro catalyst”

18.

“CNT computers” [50]

“Computer with CNTs”

19.

“Ultrahigh conductive, multifunctional, strong and light CNTs” [51]

“Core CNTs”

20.

“Toxicity of CNTs” [52]

“Impact of CNTs”

21.

“Surface modifications in solvents and polymers by dispersion of CNTs” [53]

“Core CNTs”

22.

“Photo catalysed degradation of dye “CNTs in chemical science” methyl-orange by composite with MWCNTs & TiO2 ” [54]

Fig. 11.2 CNT Production methods for large scale and small scale

which consist of multiple layers on a large scale for industrial purposes with the help of the DC arc discharge production method [56]. At first, hydrogen was used as buffer gas in this method, but no significant change was observed. Later, NH3 , a reactive gas, was used as buffer gas without any additional nitrogen source. Sheets of graphene were of bi to hexa layered and having dimensions of 100–200 nm. Setup for arc discharge production method and TEM images for multi-layered graphene sheets is shown in Fig. 11.3.

Helium Arc Discharge This method was introduced in 2006 by Goddard Space Flight Centre. It is safe and economical for mono-walled CNT production [57] and is used to vaporize an unstructured C rod that allegedly formed nanotubes by amassing vapor on a C cathode

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a

b

Gas out

Gas in

Electric Motor

c Anode

Cathode

Power Supply CNT

Graphite

Fig. 11.3 a Basic setup for arc discharge method. b The transmission electron microscopy images of the MLG sheet fabricated with the help of arc discharge method (DC) [23]. C HRTEM figures represent the edge of MLG sheets which consists of two (a), three (b) and four (c) layers [56]

with cooled-water mechanism. CNT produced in this process is of rope or bundle shape. In addition, no metal catalyst is used in this phenomenon. This technique is still under further research for impending use on the commercial scale [58, 59].

Hydrogen Arc Discharge The hydrogen arc discharge technique is widely used for yielding nanotubes since the year 1992 [60]. It is found that nanotubes produced via inert gas medium found capped at their outer ends. The capped ends of CNTs are opened with the help of the post oxidation process. In addition, carbon nanotubes produced synthesized by inert gas are defective containing amorphous carbon and rough graphite sheets [61]. Inert gas acts as an extinguisher for carbon vapor, and this undergoes supersaturation after being nucleated and thus yields carbon nanotubes. As Hydrogen (H2 ) is known as the lightest among all the elements having higher thermal conductivity. This property of hydrogen acts as extinguisher more efficiently for condensing carbon vapor and generates nanotube. Furthermore, hydrogen is highly reactive as they react with another carbon to structure well-built covalent C–H bonds [62]. Hutchison et al. [63] synthesized DWNTs by hydrogen arc discharge in a medium of Ar and H2 mixture with a proportion of 1:1. Occasionally, SWNTs were observed under highresolution electron microscopy (HREM), but it also yields DWNT as a by-product. The HRTEM micrographs showed most of the tubules structured with two concentric layers (Fig. 11.4). The presence of catalytic particle was along with the DWNTs bundles, discrete DWNTs were found, and these were easily distorted (Fig. 11.4) [63].

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B

B

a

b

c

Fig. 11.4 a DWNTs, catalytic particle. b General view of bundles B and separate DWNTs. c DWNT distorted regions [64]

11.2.1.2

Laser Ablation

In 1995s, Rice University Smalley’s group discovered carbon nanotube synthesized with the help of laser vaporization method. A continuous or pulsed beam of laser (Yttrium, Aluminum, Garnet or CO2 laser) when incidents on a graphite rod is used for vaporization and mixture of Argon and Helium buffer gas is used to maintain the pressure (500 Torr) in the laser ablation process [64]. Higher light intensity is required for pulsed laser in comparison with continuous laser. Further, an extreme hot vapor plume expands and cools rapidly. When the vapor is cooled, then tiny carbon particles are quickly condensed to large clusters in the form of fullerenes. The catalysts at the same time begins to condense initially, gets attached to carbon clusters and prevent it from cage structure generation. Open cage structures may be generated in catalysts when comes in contact with them. These underlying structures create a rounded structure that bit by bit shapes single divider carbon nanotubes until the impetus particles gets excessively huge. It might likewise be conceivable that particles gets covered with such a large number of carbon layers, and thus, they cannot ingest more, and nanotubes start quit developing. In this, SWCNTs produced are bundled by van der Waals forces [65]. Depending upon the wavelength of the laser, the yielded carbon soot differs in properties. Continuous wave lasers as well pulsed laser can be used as the synthesis method for CNTs. Single-walled CNTs (SWCNTs) produced by the pulsed laser vaporization (PLV) method are found to have firm structural integrity with long and absolutely negligible impurity. “The laser wavelength of 1064 nm with different conditions shows different properties of CNTs. The first pulse energy was of 25 mJ and it showed a change, whether the second pulse energy were in between 16 mJ to 32 mJ, showed no significant influence on it. Laser wavelength of 355 nm at fluence F = 1 J.cm−2 indicates the metallic properties of nanotubes; predominantly semi-conducting.” CNT properties are highly dependable

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Fig. 11.5 Diagram for laser ablation synthesis method

on laser fluence than infrared laser radiation [66]. Figure 11.5 shows the diagram for laser ablation synthesis method.

11.2.1.3

Chemical Vapor Deposition

Chemical vapor deposition (CVD) method for CNTs production is widely used nowadays due to its simpler construction and cost efficient nature in comparison with other methods. In CVD, substrate surface undergoes pre-treatment that allegedly helps in governing of different structures [60]. Chemical vapor deposition (CVD) synthesis is of thermal catalytic type. It is obtained by having a carbon source decomposing at a higher temperature which ranges between 500 and 1200 °C when diluted under a stream of flowing noble gases where the energy source is namely used as a plasma or resistively heated coil [67]. Organo-metallocene generated metal catalysts, namely cobaltocene, nickelocene, ferrocene, are used in the CVD method [79]. Till a state of supersaturation is obtained: Disintegrated carbon will consequently dissolve the metal particle where carbon precipitates in the form of fullerene. CVD method is highly beneficial in yielding CNT in various forms. At low temperature (ranges from 600 to 900 °C), CVD yields MWNTs, and on the other hand, high temperature (ranges from 900 to 1200 °C) helps in yielding SWNTs [68]. In addition, plasma enhanced CVD (PECVD) is another technique where thermal energy is replaced by the energy sources that decompose hydrocarbon. The controlled size catalyst particle determines the nanotube diameter that is produced. Moreover, carbon nanotubes are formed when an accurate parameter is followed like proper alignment as well as positional control on the nanometer scale. In PECVD, hydrocarbon remains ionized and is applied over transition metal. Different plasma energy sources are radio frequency PECVD and microwave PECVD, direct current PECVD and hot filament PECVD, [64]. Plasma-assisted CVD is efficient for generating vertically grown nanotubes by placing reactor geometry properly. Despite in absence of arc, it is possible to generate vertical aligned CNT under specific controlled conditions. Recently, researchers of

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Fig. 11.6 Arrangement for chemical vapor deposition method

UC Berkeley confirmed the generation of vertically aligned CNT from CVD [69]. Double-walled carbon nanotubes production via CVD has also been reported from the University of California, San Diego [70]. Post processing methods are required to fabricate CNTs. Li et al. spun fibers and ribbons of carbon nanotubes straight from CVD synthesis zone of a furnace. To do this, liquid source of carbon and an iron nano catalyst were used. MWNTs in fibers spun at high temperatures were of higher purity 85–95%, whereas materials collected from the furnace without spinning were of nearly 70–85% purity. In 3D, it was seen like the fibers were twisted as a rope [71]. Figure 11.6 shows the arrangement for chemical vapor deposition method.

11.2.1.4

High-Pressure Carbon Monoxide Reaction (HiPco)

This method was developed at Rice University in 1999 for producing CNT [13]. Unlike other methods where catalysts were accumulated, in this method, catalyst goes through gas state. The hydrocarbon gas along with the catalyst is fed into the furnace after catalytic reaction at gas state. This method is applicable for large-scale synthesis as reaction keeps on going when nanotubes are remaining apart from catalytic aid. CO is used as hydrocarbon gas that reacts to iron pentacarbonyl Fe(CO)5 to generate SWCNT. This phenomenon is regarded as the HiPco process. Another way of HiPco process is available where SWCNT synthesized in a mixture of benzene and ferrocene, Fe(C5 H5 )2 allowed to go through a hydrogen gas which flows to generate SWCNT [72]. In all variants of the HiPco method, catalyst nanoparticles are formed by thermal decomposition of organo metallic compounds like iron pentacarbonyl and ferrocene [73]. A highpressure CO disproportionation (HiPco) process was used to produce nanotubes with

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Fig. 11.7 a HiPco process. SEM images of HiPco tubes, b raw HiPco SWCNTs, c purified SWCNTs. TEM images of HiPco SWCNTs. Raw HiPco tubes d magnification 100 X, e magnification 500 X. Purified HiPco tubes, f magnification 100 X, g magnification 500 X [74]

purities of >90% atomic percent SWCNT. Raw HiPco tubes were subjected to under Ar/O2 . Low-temperature catalytic oxidation of carbon was enhanced by the addition of water [74]. Figure 11.7 shows (a) HiPco process. SEM images of HiPco tubes (b) raw HiPco SWCNTs, (c) purified SWCNTs. TEM images of HiPco SWCNTs. Raw HiPco tubes (d) magnification 100 X, (e) magnification 500 X. Purified HiPco tubes (f) magnification 100 X (g) magnification 500 X [74].

11.2.1.5

CoMoCAT Process

An effort at the University of Oklahoma recently paved a way to develop this process where cobalt and molybdenum catalyst and CO gases were used [75]. In this method, CO disproportioned into C and CO2 in the presence of CoMo catalyst at 700–950 °C in a stream of CO in the pressure of 1–10 atm. This process produces a significant amount of SWCNT in the shortest span of time. The synergistic effect of Co and Mo fastens the whole process. The silica substrate with a low ratio of Co-Mo makes the catalyst most effective. The diameter distribution produced by the HiPco process is broader than that of the CoMoCAT process. CoMoCAT holds a strong prospect of large-scale production of SWCNT [76]. Figure 11.8 shows the CoMoCAT process arrangement.

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Gas out

CO2 Traps

Product out Fresh catalyst in Recycled CO

Makeup CO

Recirculating Pump Fig. 11.8 CoMoCAT process arrangement

11.2.2 Small-Scale Production Techniques 11.2.2.1

Electrolysis

By this process, G Kaptay and J Sytchey produced CNT at the University of Miskolc [77]. By accumulating alkaline metal on graphite cathode in a molten salt system keeping at a high temperature, they produced carbon nanotubes. The disbursed metallic atoms interspersed into the space between the graphite sheets and dissipated toward the bulk of the graphite cathode, and this induces some mechanical stress inside graphite. Later on, this stress provokes the removal of distinct graphitic sheets that turns into carbon nanotubes because of interfacial forces. This method is not scalable to large-scale production of CNT [78, 79]. It may have a different arrangement. A basic diagram of electrolysis process for CNT production is shown in Fig. 11.9. CNTs were synthesized from CO2 using the electrolytic method. The electrolysis was done under current and potential controls. Most of the nanotubes produced like this are MWCNTs, and most of the time these are agglomerated into bundles. Production of MWCNTs is found about 40% volume in the cathode product. The metallic phase regards as a catalyst for the production of nanotubes in the electrolytic synthesis method from molten salts [80]. Figure 11.9 shows the electrolysis process for CNTs production.

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Power Supply Stainless Steel Cover O-ring Reactor with water cooling Molybdenum wire quasireference electrode Thermocouple Graphite rod cathode Graphite rod anode Lithium chloride melt Ceramic insulator Fig. 11.9 Electrolysis process for CNTs production

11.2.2.2

Flame Synthesis

This phenomenon is based upon SWCNT synthesis in a regulated flame ambience that provokes the high temperature state in carbon atoms by applying economic hydrocarbon fuels that results in forming an aerosol metal catalyst island. SWCNT generated are alike that are done in laser ablation and arc discharge. The metal catalyst can be produced by coating cobalt catalyst on meshing, in which isle of metals like droplets is formed by physical vapor deposition. Consequently, droplets turn into aerosol when exposed to a flame. When a filter paper rinsed with a metal ion (iron nitrate) is burnt, it produces aerosol too. In addition, thermal evaporating metal powder (e.g., Fe or Ni) also does it as well [81, 82]. On a stainless steel substrate, it is possible to grow iron-oxide nanoparticles (α-Fe2 O3 ) nanocrystals and CNTs directly by open atmosphere flame synthesis technique. Upon increasing the temperature, it is possible to generate larger γ -Fe2 O3 nanocrystals from (α–Fe2 O3 ) with few discrete CNTs [83].

11.2.2.3

Substrate Method

For better quality and high yield of CNTs, catalysts require opposite material. The yield and quality of CNT are affected by surface morphology and texture properties of the substrate material. This works as a supporting medium as well as interlinks catalyst to growth ambience. Physical or chemical interlink may take place between them. The catalysts are nanoparticles. The tube structure determined by substrate and catalyst hence requires special focus. For generating self-oriented nanotubes,

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porous silicon is regarded as the ideal substrate, and it is found that nanotubes yield a higher ratio when plain silicon is used as a substrate. It is found that the oxide substrate is mainly used as physical support which can assist in CNT production by Noda et al. in 2007 [84]. There are numerous substrates found, which are used in CVD for generating CNT such as silicon, quartz and graphite. [85].

11.2.2.4

Sol-Gel Techniques

The template synthesis method was first introduced by Martin and Allies, and it paved the way for generating nanotubes that are monodispersed in nature and of any size irrespective of any material [86]. The sol-gel method requires silicone gel, and it is considered an ideal carrier as it is easy to make [87]. In addition, they possess a cross-linked structure and are preferable for forming delicate recognition sites [78]. Yang et al. proposed a CNT composite where Pt nanoparticles doped sol gel acted as an electrode binder. A synergic effect is obtained by combining electro-catalytic property of Pt nanoparticle with CNT along with hydrogen peroxide [88].

11.2.2.5

Mechano Thermal Process

Mechano thermal synthesis of CNT holds two prime stages; at first, amorphous carbon is yielded, and consequently, it goes through annealing in vacuum furnace by nailing them [89]. Carbon gets formless by high energy ball milling in the inert or aerobic medium. With the passage of time, carbon crystal turns into an amorphous structure. Depending upon elongated milling, metal powders perchance aid to nucleate, and generation of CNT in the thermal stage is done. Yielded amorphous carbon is kept in the furnace at 1400 °C for several hours that actuates the connection of atoms resulting in the formation of CNT. CNT produced via mechano thermal process is of multiwalled and spring shape. Mechano thermal method is simple as it does not require any special equipment. It is of low cost and well fit for mass production. On the other hand, this process is time consuming and discontinuous [90]. In this field, the most relevant work has been experienced by Chen et al. (2004) as investigated. Significant growth of carbon nanotube has been identified by ball milling and by thermal annealing up to 1400 °C. Mostly, the absence of carbon vapor forms two types of a multiwalled nanotube (diameter containing more than 20 nm and less than 20 nm). TEM images with the growth of carbon have been shown below in Fig. 11.10 [91].

11.3 Purification of CNTs SWCNT produced is filled with impurities like granular carbon, graphite sheet, metal catalyst and fullerenes. These impurities hamper the properties of SWCNT. It is

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a

b

Fig. 11.10 a TEM image of deformed graphene layers in the graphite sample after ball milling treatment. The arrows indicate heavily bent monolayers. b TEM image of the metal catalytic formation of thick carbon nanotubes containing a Fe particle at the tip. The insert high-resolution TEM image was taken from the wall area of one nanotube [91]

always preferable to yield SWCNT as pure as possible without altering its properties. In addition, these are always required to be homogeneous in nature. Strong oxidation and refluxing are the most available technique which is used in industries for producing homogenized CNT. These methods work in two ways namely structure selective and size selective operations. Structure selective separates SWCNT from impurities, while size selective provides homogeneous diameter [92]. Below mentioned are the techniques which are adopted for purification of CNTs:

11.3.1 Oxidation Oxidative treatment is quite effective in removing carbonaceous impurities of SWCNT. On a different note, oxidation treatment oxidizes impurities as well as SWCNTs. Despite oxidizing SWCNT, it, fortunately, causes less damage. Moreover, the efficiency and output of this procedure acutely depend on several factors like oxidation time, metal properties, ambience, oxidizing agent and temperature [93]. Dementev et al. used UV-VIS-NIR spectroscopy and found that dynamically oxidized SWCNTs have lower carbon impurities. It is also found that SWCNTs purity improved by six times [94].

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11.3.2 Acid Treatment Mainly acid treatment removes metal catalysts. The metal catalyst is exposed to acid and solvated followed by oxidizing or sonicating. Treatment with HNO3 only affects metal catalyst, while SWNTs and other carbon particles remain unaffected. On the other hand, HCl treatment affects slightly SWCNTs and other carbon particles. 4 M HCl reflux is as of HNO3 [92]. Acid-treated CNTs by in situ polymerization are found better in mechanical properties as compared to CNT composites. An addition of 1.5 wt% of A-CNT raises electrical conductivity by about nine orders of magnitude [95].

11.3.3 Annealing CNTs get rearranged, and defects get vanished at a higher temperature around (873– 1873 K). High temperature pyrolysis is done for producing graphitic carbon and fullerene. Metal gets melted and removed by applying vacuum treatment at 1873 K [92]. MWCNTs produced by CVD are being purified efficiently by annealing that provides about 99.9% of purity with both metal and its oxide content. Catalysts, i.e., Fe–Mo and Al2 O3 , are sublimated at a pressure not more than 10 bar and temperature of about 1500 °C. This removes both residual metal and its oxides [96].

11.3.4 Ultrasonication By applying ultrasonic vibrations, particles are forced to move apart. By ultrasonication, nanoparticles agglomerate and are forced to pulsate and dissipate. The separation of particles is acutely reliable on several parameters like surfactant, solvent and reagent. Although attached to metal, SWNTs are more stable in a poor solvent. In acid, SWNT’s purity is effected by exposure time. Metal solvates are exposed to acid for a short time [92]. After synthesizing polyacrylonitrile (PAN) in aqueous deposition polymerization via ultrasonication, it is found by Zhang et al. that there had an interfacial relation between CNTs and PAN macromolecules [97].

11.3.5 Magnetic Purification Graphitic shells are freed from ferromagnetic particles (catalytic) mechanically. Ultrasonic effect actuates inorganic nanoparticle to remove ferromagnetic particles. Later on, particles are placed with magnetic poles. Lastly, chemical treatment yields high purity SWCNT material [98]. On purifying SWCNTs by magnetic purification, it is found that absorption background is dipped while fluorescence efficiencies

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increase. Ghosh et al. also observe optical properties also levelled up to supernatants that are formed by ultra centrifugation [99].

11.3.6 Micro Filtration Micro filtration is predicated on particle filtration. CNT along with carbon nanoparticle is placed in a filter. Other nanoparticles are allowed to flow through in filter at the same time. Drenching SWCNTs in CS2 solution causes the removal of fullerenes from it. The fullerenes solvated in CS2 flows through the filter, while insoluble CS2 is trapped [100].

11.3.7 Functionalization Functionalization mainly focuses on making SWCNTs more soluble by adding various groups to the tubes. Another variant of functionalization makes SWCNTs more soluble for chromatographic size operation without altering them at all. Purified SWCNTs are recovered by removing added functional groups applying heat treatment like annealing [101].

11.3.8 Cutting CNTs can be cut down by either chemically or mechanically or by a combination of two. Partially functionalizing with tubes like fluorine, CNTs can be cut chemically. The fluorinated carbon forms CF4 and COF2 by pyrolysis, while driving off the wall results in the chemically cut nanotube. By dint of ball milling: mechanical cutting of nanotubes takes place. Bonds break owing to high friction between the nanoparticles. Chemical and mechanical allied cutting is an ultrasonic induced cutting taking place in acid medium. Nanotubes energized by ultrasonic vibration help them to leave the catalyst surface [102].

11.3.9 Chromatography CNTs with a small fractional limit of small length and diameter distribution are being filtrated by this method. The CNTs are allowed to pass through a column with a permeable material. Gel permeation chromatography (GPC) and high performance liquid chromatography-size exclusion chromatography (HPLC-SEC) types of the column are used. The CNT’s size determines the pores they will pass through.

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From a very recent study, on MWCNTs-based sorbent, dSPE and liquid chromatography mass spectrometry methods are successfully applied to analysis β-blockers at combined [92, 103].

11.4 CNTs in Composite Materials In the last few decades’, lots of carbon fibers have been developed and at present widely utilized as reinforcement in composite materials for various engineering applications [104]. Carbon nanotubes (CNTs) are introduced in 1991 by Iijima [105]. CNT has a nano structure with special physical and mechanical characteristics. As compared with traditional fillers for composites, carbon nanotubes can exhibit excellent mechanical, electrical, optical and thermal properties as compared to conventional materials thus inspired to use CNTs as reinforcement material to develop new material with required properties as well as to increase the commercial and potential value [106–112]. In polymer matrix, composite CNTs are used as reinforcement to obtain an ultra-light structure with onward optical, thermal and electrical properties [113, 114]. Moreover, CNT reinforcements are capable of increasing load bearing capability by providing high strength, thermal and electrical conductivity. Primarily with the variation of matrix, CNT-based composites can be scaled up with CNTpolymer composites, CNT-metal composites, CNT-ceramic composites and CNTcarbon matrix composites [115]. As specifically, the development of CNT-metal matrix composite may have significant progress on improvement of strength in materials [116–119], wear properties in materials [120–123] and EDM tool development [124–126].

11.4.1 CNT-Based Metal Matrix Composites CNT-based metal matrix composites are less enhanced than ceramics-based metal matrix composites. It is found that metals and alloys are not enough to provide high strength and high stiffness which limited their use in many engineering applications and CNTs-based metal composites might be the solution for engineering applications where high strength with high thermal stability is required. CNTs-based metal matrix composites have a wide range of applications in automobile and aerospace science. Generally, CNTs reinforced with Al, Cu, Ni and Mg provide extensive properties that are far away better than base metal or alloy. [127]. Powder metallurgy, melting and solidification, thermal spray, electrochemical deposition, vapor deposition, molecular level mixing techniques are the processes that have been already adopted for CNT reinforced metal matrix composites [128]. In the processing of CNTs-based metal composites, chemical stability, structural solidity and surface bond strength between phases are the major challenges including homogeneous dispersion inside the matrix. CNT reinforcement in the metal can enhance mechanical properties by improving

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the hardness and stiffness of the composite. It is found that 1.6% volume of CNTs in Al increase 35% of yield strength in compression [129]. Tribological properties can also be enhanced by reinforcing CNTs in metal matrix composites. As investigated, the effect of 1 vol. % CNT reinforcement with SiC in Al matrix composites leads to an improvement in mechanical properties as well as also helps in achieving a low thermal coefficient of expansion. It is also found that yield strength and ultimate tensile strength are improved by 25% and 33.5%, respectively, [130]. Thermal and electrical properties with tribological behavior can be improved by reinforcing CNTs as reinforcement materials in copper. It is found that composites reinforced with CNTs up to 16% vol. show a 14% less wear rate with 91% less coefficient of friction [131].

11.4.2 CNT-Based Ceramic Matrix Composites Though, ceramic matrix composites show good thermal stability, high stiffness, good tribological properties and low density. Optimum reinforcement is possible for ceramic due to the notable resilience of CNTs. Improved thermal stability, toughness, creep resistance than ceramic matrix composites are possible by reinforcing CNTs in the matrix phase which shows excellent electrical and thermal conductivity. Reinforcing CNTs in the matrix phase of ceramic like alumina or silica is critical and more difficult than polymers. Spark plasma sintering [132], sol gel techniques [133], etc., are being noticed as suitable methods for good dispersion of CNT reinforcement in the matrix phase. The uniform distribution of CNTs throughout the matrix is a critical issue that is highly influenced by many factors. It is found that high temperatures and sensitive reactive environments can damage CNTs. Pyrolytic carbon on to the CNTs may be employed, and this makes an interface of CNT-ceramic weak to provide debonding. The weak interface is also necessary to provide the sliding nature of CNTs in the matrix. Properties of ceramic like mechanical or electrical can be enhanced in a CNT-ceramic matrix, and material damage tolerance can also be improved by adding CNTs [134]. Adding CNTs as reinforcement in the matrix phase can improve the crack behavior of composites [135]. CNTs also are reinforced to improve toughness because of having a high capacity of carrying load [136]. “Sword-in-sheath” type of failure in CNT-alumina is observed which shows the remarkable enhancement in mechanical properties [137]. But many more influences of CNT-ceramic reinforcement on various properties are not yet revealed clearly as it makes a change in the matrix such as grain size along with hardness in CNT-alumina composite [138]. Grain size, heating rate, densities, Vickers hardness can also be investigated. In recent research, B4 C-CNT composite is prepared under 1590–1700 °C where the density reached maximum and sintering grade powders (particle size—small) gives relatively higher density. On the other hand, increasing CNT in ceramic (B4 C-CNT) improves both vickers hardness and fracture toughness which is about 12% higher as compared to original values obtained in pure B4 C [139].

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11.4.3 CNT-Based Polymer Matrix Composites Having CNTs as effective filler for polymer, CNTs must be dispersed uniformly in the matrix and also needs good surface interactions. Along with homogeneous dispersion, an arrangement of CNT in matrices is another factor affecting the quality of the CNT-polymer composites. Melt-blending, solution mixing, chemical modification, in situ polymerization are some of the known methods for processing [140–142]. The legacy has started a study on CNT-epoxy composites by Ajayan et al. [143]. With the progress of research in this field, the number of CNT-polymer composites has increased. Epoxy, polystyrene, polyethylene terephthalate, polypropylene, polycarbonate, polyurethane, etc., are some of the polymer matrices used for CNT reinforcement [144–149]. Very significantly CNTs reinforced with textile composites mark wide attention in some of the fields of aerospace and military because of their enhanced structural, mechanical, thermal and electrical properties [150, 151]. It is investigated that CNT composite now deals with experimental techniques where special focus is given to elastic properties or damage propagation. For example, a study of Tarfaoui et al. in the CNT-polymer field. But still, it is challenging due to its multiple behaviors. Even small CNT concentration in polymer matrices can also influence the mechanical behavior [152]. Toughness can also be improved for CNTpolymers. Increasing 47% of toughness has been achieved in CNT/PI nanocomposite where long CNT-actuated three-dimensional, ceaseless and heterogeneous system is shaped to toughen the nanocomposites [151]. These all are the sign of enhanced mechanical properties. When it comes to stiffness, a very recent study place that 2wt.% addition of graphene in carbon fiber epoxy composite is capable to increase stiffness (axial) about 10 GPa which is about an increasing rate of 9% [153]. This technology has been reported as being used for ultra-lightweight “graphene watch” produced by Richard Mille, the Swiss luxury watchmaker, in conjunction with McLaren Formula 1 motor racing [154]. The most promising character for the CNT-polymer composites is that CNT fillers provides electrical conductive behavior where most of the polymers are insulators. So, to achieve thermal conductivity, there is some dependency on filler type, its shape, its size and also on the loading level. High filler loading is needed for high thermal conductivity. But at low filler loading, hybrid fillers (random type, shape, size) may provide higher thermal conductivity. As for nanoscale, filler properties (like the melting point) can also be changed [155].

11.5 Industrial Applications The superior properties of carbon nanotube and its composites have a wide range of applications at the present time. The applications of carbon nanotube and its composites are as follows: 1. 2.

Carbon nanotubes for sensing applications. Carbon nanotubes for displaying applications.

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Development of stimuli-responsive materials. Development of wearable CNT devices for sensing applications. CNT applications in lithium-ion batteries. Electrochemical capacitors. Biomedical applications. Energy storage materials. Electrical actuators. Water treatment.

11.6 Conclusion A systematic study is done on the processing techniques, purification techniques and industrial application of carbon nanotubes and its composites. It is found that there is a lot of advancement in the processing techniques for carbon nanotubes in the last few years. The development of carbon nanotube-based composites exhibits better mechanical and optical properties. Based on the present investigation, the following conclusions can be drawn: 1. A number of processing and purification techniques for the carbon nanotubes have been developed in the last few years. Among them, chemical vapor deposition processing technique is widely used to synthesize the carbon nanotubes, and the oxidation technique gives better purity level for SWCNTs purification. However, annealing is better for MWCNT’s purification. 2. With the advancement of CNT in recent days, the horizon of its application broadened widely. Previously, a lot of research work was done only on CNT advancement, but now, this field paved the way for CNT-metal composite to CNT-ceramic composite and further towards nano-fluid field. 3. Electrical property of CNTs are significant which means it was improvised to apply for energy resources and also for its modification. 4. CNT-based composites exhibit excellent mechanical properties as compared to conventional materials. The uniform dispersion of CNTs in the matrix material improves the mechanical properties of a material. 5. CNTs-based composite finds wide range of applications in sensing applications, electrochemical applications and also in energy storage materials.

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Chapter 12

Recent Advancement in Nanostructured-Based Electrochemical Genosensors for Pathogen Detection Summaiyya Khan, Akrema, Rizwan Arif, Shama Yasmeen, and Rahisuddin Abstract Pathogen detection is a critical issue to minimize the mortality caused due to different pathogenic diseases. Biosensors provide the most attractive and alternative method for the fast, selective and reliable detection of pathogens as compared to conventional methods such as PCR, ELISA and FISH which have some limitations. Biosensors in which DNA/RNA were used as recognition element are known as genosensors. Genosensors provide wide range of applications in diagnosis of diseases including infectious diseases, cancer, autoimmune diseases and much more. Recent advancement in the incorporation of nanostructures for the fabrication of genosensors had raised lot of attention. These nanostructured materials provide large surface area, biocompatibility, nontoxicity and surface defects which have led to the development of successful electrochemical genosensors. This chapter includes recent progress in the fabrication of genosensors for pathogen detection based on different nanostructures. Keywords Pathogen detection · Genosensors · Infectious diseases · Nanostructured materials

12.1 Introduction Food—and waterborne illnesses caused by toxins produced by pathogenic bacteria are very serious problems and threat to human health. Most common techniques used for the safety of food for the detection of Escherichia coli are conventional culture method which possesses enumeration of bacterial colonies, and molecular method is very sensitive technique used for the specific nucleic acid sequences in which enumeration is done by polymerase chain reaction. Due to disadvantages of conventional methods like bulky instruments and proper training for handling, the S. Khan · Akrema (B) · R. Arif · S. Yasmeen · Rahisuddin (B) Department of Chemistry, Jamia Millia Islamia, New Delhi 110025, India e-mail: [email protected] Rahisuddin e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 Z. H. Khan, Emerging Trends in Nanotechnology, https://doi.org/10.1007/978-981-15-9904-0_12

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result interpretation often waste culture time and enumeration. In this respect, there is always a need to develop new methods with improved specificity and feasibility for the E. coli detection [1]. Developing biosensors is very promising approach as they are very popular and possess many special features viz. small size, high sensitivity and selectivity, high detection capacity, easy analysis and relatively low-cost instrumentation. Design and development of biosensors of desired characteristics may involve combinations of different disciplines with selectivity and sensitivity for the biological components. Cancer is a serious disease which increases the mortality rate and can only be reduced by early detection and is only factor to avoiding future problems. Synthesis of electrochemical nanobiosensors as portable device is very significant approach as these sensors have been now used very frequently for the diagnosis of cancer and in clinical medicine. In the last few years, researchers are working to design and develop electrochemical biosensors with large surface, high electrical conductivity and more repeatability. Biosensor, a diagnostic tool, has been used for the determination of the concentration of target substance and has been found significant applications in drug detection, biomedical assessment and environmental monitoring [2–5]. There was an inaccuracy in the cancer detection few years back due to low amount of cancer biomarkers in blood. In this respect, researchers have been attracted and developed new approaches for the improvement of detection process. Less number of highly sensitive electrodes with unique recognition was the main problem for the electrochemical biosensors. This problem has been removed after the incorporation of NPs along with biomarkers [6]. Nazari and his team fabricated the gold nanostructures based on highly sensitive electrochemical DNA biosensor which is based on Enterococcus faecalis (E. faecalis) genome after immobilization of a thiolated E. faecalis-DNA probe (Fig. 12.1) [7]. Hybridization of the probe, like complementary DNA (c-DNA), 1 base (1-missDNA), 2 base (2-miss-DNA), 3 base mismatched complementary DNA (3-missDNA), non-complementary DNA (nc-DNA) and EF genomic DNA was evaluated using toluidine blue (TB) as marker. Results showed that biosensor showed good sensitivity and selectivity with a low limit of detection [7]. For rapid detection of pathogenic bacterial diseases, electrochemical geno-sensing is one of the most significant methods. An efficient approach for the detection of Staphylococcus aureus using PbS nanoparticles (as electrochemical genosensor) has been reported. Some researchers have also designed a genosensor of the gold electrode surface by the immobilization of complementary DNA. Sensitivity of genosensor was increased by using carbon nanotubes after the modification of chitosan for the electrochemical detection of pathogen in contaminated beef samples. In comparison with other electrochemical biosensor, this genosensor was found to be very efficient for the pathogenic bacterial detection. Hence, the developed genosensor exhibited significant selectivity and sensitivity with a low limit of detection. Synthesized genosensor could be used for EF detection without PCR amplification [7]. DNA hybridization detection is an important approach for the diagnosis of genetic diseases and pathogen agents which included electrochemical methods [8], surface plasmon resonance and absorption spectroscopy [9]. Brucellosis is an infectious disease which spreads by the contact of animals and its products on contamination

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Fig. 12.1 Fabrication of the DNA biosensor and c-DNA detection. (Reproduced with permission from [7]). Reproduction Link (https://s100.copyright.com/CustomerAdmin/PLF.jsp?ref=a2b8f4b0aaa2-41e3-8988-560f9074afaa (License Number: 4795910467091))

of bacteria [10]. Brucellosis is diagnosed in bacterial cultural medium, and many other techniques for diagnosis such as serum agglutination test (SAT), PCR, enzymelinked immuno-sorbent assay (ELISA), Raman spectroscopy and serological tests have also been reported. Development of rapid, easy and low-cost methodologies has been attracted researcher for direct detection of Brucella. Rathi et al. developed palladium nanoparticle-based electrochemical genosensor for a quantitative detection of Brucella organism [11]. Many researchers have developed gold, cobalt and palladium nanoparticle-based genosensor for the detection of Leishmania major, human parasitic infections, mycardial infections and various other dangerous diseases [12–17] (Fig. 12.2).

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Fig. 12.2 Fabrication protocol of the genosensor and detection of t-ssDNA. (Reproduced with permission from [16]). (Reproduction Link https://s100.copyright.com/CustomerAdmin/PLF.jsp? ref=a2b8f4b0-aaa2-41e3-8988-560f9074afaa (License Number: 4795910467091

Mobed and co-worker developed and fabricated Electrochemical DNA-based geno-assay for Legionella bacteria detection using gold nano as transducer, toluidine blue (redox marker), mixture of beta-cyclodextrin (stabilizer) and poly (dopamine-βcyclodextrin) as dopamine [18]. Authors found that the genosensor is able to detect the complementary sequence in 1 μM-1 ZM range with low limit of quantification (LLOQ) 1 ZM. The performance of genosensor viz. selectivity, sensitivity, low detection limit, stability and reusability has been improved significantly due to the combination of nano-Au with polymeric interface (P(DA-β-CD) [18]. Ye et al. reported a sandwich-type genosensor for the detection and quantification of cauliflower mosaic virus 35S (CaMV35S) using Au- and Ag-loaded Fe3 O4 nanocomposite (Fe3 O4 –Au@Ag) as signal DNA probe (sDNA) with stability, selectivity and reproducibility. The developed genosensor successfully detected the target in genetically modified tomato samples with ultralow detection limit of 12.6 aM [19]. Hepatitis C virus (HCV) has spread all over the world which causes various infectious disease like cirrhosis, hepatocellular carcinoma and chronic hepatitis. Chronic hepatitis C infection is very dangerous disease which results in deaths of as many people out of about 71 million people; those are suffering from this disease. Oliveira and co-worker developed a geno-biosensor for the specific detection of hepatitis C virus (HCV) through modification of gold electrodes with grapheme oxide and functionalization of ethylenediamine (GO-ETD). HCV-infected sera and viral genomic

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RNA has been evaluated by differential pulse voltammetry and electrochemical impedance spectroscopy which revealed that after the incorporation of HCV genomic RNA, charge transfer resistance values have been increased by twice. Differential pulse voltammetry shows that the genosensor discriminated the HCV positive and HCV negative serum samples [20].

12.2 Pathogens The word pathogen means “anything that causes disease.” The term pathogen came into use in the 1880s. Pathogen includes different infectious agents such as virus, bacteria, protozoans, fungi or any other microorganism [21]. Pathogens are microbes which are proficient of causing illness in the host species such as man, animals and plants. They are spread in many ways, with the blood, air, sex and other human fluids or through the fecal-oral way. A significant number of microbes are depicted as zoonotic (directly communicable to man from animals). Salmonellosis and cryptosporidiosis diseases are the examples of zoonotic. This is especially a significant factor while considering the dangers to human well-being emerging from the use of muck in cultivation [22]. The human body is an intricate and flourishing biological system. It comprises around 1013 human cells as well as furthermore around 1014 bacterial, fungal and protozoan cells, which denote numerous microbial species. These microorganisms, known as the normal flora, are normally restricted to specific regions of the body, comprising the skin, mouth, digestive organ and vagina. Pathogens are generally dissimilar from the normal flora. Our typical microbial occupants possibly aim inconvenience if our immunity is debilitated or in the event that they access an ordinarily sterile piece of the body. They have grown profoundly specific mechanisms for crossing cellular and biological walls and for producing certain responses from the host creature that afford to the existence and development of the microbes [23]. Each pathogen desires to flourish and live in a host. At the point when the microorganism sets itself up in a host’s body, it achieves to stay away from the body’s resistant reactions and utilizations the body’s assets to recreate before leaving and spreading to another host. A few signs of bacterial diseases, including inflammation and irritation at the site of diseases as well as the manufacture of discharge, are the immediate consequence of immune system cells trying to destroy the attacking microbes. Fever is a protective response, for example, rise in body temperature is able to constrain development of certain organisms. Pathogens fall into main categories, primary pathogens which cause disease in normal proportion of individuals and other category is opportunistic pathogens that invade immunocompromised individuals. Although there are various types of pathogens, we are going to emphasis on the four most common types that are bacteria, viruses, fungi and parasites.

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12.2.1 Bacterial Pathogens Bacteria can be pathogenic as well as beneficial. About 100 species of bacterial pathogens are known that are harmful and cause different diseases [24]. According to World Health Organization (WHO), about 17 million individuals are affected with different bacterial diseases per year [25]. Bacterial pathogen can be gram positive or gram negative. Gram-negative bacteria are more pathogenic compared to gram-positive bacteria, due to impenetrable cell walls and presence of unique lipopolysaccharide (LPS) found in gram-negative bacteria. The presence of LPS in gram-negative bacteria serves as virulence factor that causes pathogenicity. Individuals are more susceptible to a bacterial disease once their immune system had been recently debilitated because of an infection.

12.2.2 Viral Pathogens Virus is another pathogen that can replicate only inside the host. About 5000 species of viruses are studied in detail. They cause various diseases such as influenza, hepatitis and human immunodeficiency virus (HIV). When viruses exist as independent particles they are known as virions. Viruses can be of two main types—deoxyribonucleic acid (DNA) virus or ribonucleic acid (RNA) virus based on the genetic material they carry [26].

12.2.3 Fungal Pathogens Fungi are eukaryotes; their cell contains a nucleus along with supplementary constituents that are surrounded inside membranes. Consequently, it is difficult to destroy them, and most of the antibiotic drugs are ineffective for the treatment of viral infections and cause side effects. Ringworm, histoplasmosis and vaginal yeast are the good examples of fungal infections. Candida, Aspergillus, etc., are the common fungal species that cause diseases [27]. But some species can be source of pharmaceutical drugs. About 300 species of fungi are known that are harmful [28].

12.2.4 Parasites Pathogens A parasite is an entity that is alive on or in a host body and acquires its nutrition from or at the expenditure of its host. The three primary kinds of parasites responsible for human disease are protozoa, helminths and ectoparasites. Occurrences of parasites affecting human disease are tapeworm that are responsible for the digestive illness,

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ticks which causes Lyme disease and plasmodium that are accountable for malaria [27]. They can be spread in a few different ways, as well as through tainted soil, food, water and blood also with the sexual contact and by means of creepy crawly chomps.

12.3 Nanostructured Materials The physics and chemistry of the solids depend on the chemical composition, arrangement of atoms and size in one, two or three dimensions. A nanometer (nm) is an international system of units that represents 10−9 m in length. They are described as materials with length in the range of 1–100 nm in at least one of the dimensions. But they are however commonly defined as materials in diameter in the range of 1–100 nm [29]. According to the environmental protection agency (EPA), nanomaterials possess exclusive properties which are different from bulk properties with larger dimensions. Similarly, the International Organization for Standardization (ISO) has termed nanomaterials with any external nanoscale dimension or having internal nanoscale surface structure [30]. Based on ISO, nanofibers, nanoplates, nanowires, quantum dots and other related terms have been defined [31]. Also, the term nanomaterial is defined as fabricated or natural material that possesses unbound, aggregated or agglomerated particles in which dimensions are between 1–100 nm size ranges [32].

12.3.1 Classification of Nanostructured Materials Currently, nanoparticles/nanostructured materials can be organized into four material-based categories:

12.3.1.1

Carbon-Based Nanostructured Materials

Carbon-based nanostructured materials exist in different structures such as ellipsoids, hollow tubes or spheres. Fullerenes (C60), carbon nanotubes (CNTs), carbon nanofibers, graphene (Gr), carbon black, etc., are included in the carbon-based NMs category. Generally, carbon-based NMs are synthesized using laser ablation, arc discharge and chemical vapor deposition (CVD) [33]. Since the discovery of carbon nanotubes (CNTs), they are proved to be one of the efficient carbon-based nanomaterials [34, 35]. CNTs are explored in many industrial applications [36]. Graphene is the latest carbon-based nanomaterial. Graphene and CNTs possess similar properties, but the two-dimensional atomic sheet structure enables more diverse electronic characteristics. Graphene oxide (GO) possess advantages over pure graphene such as dispersibility in aqueous medium, presence of hydrophilic functional groups and

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structural heterogeneity [37]. Zhang et al. functionalized GO with folic acid (FA) as a cancer-targeting molecule and load them with doxorubicin and camptothecin, a wellknown cancer drug, onto the GO surface [38]. Sun et al. conjugated polyethylene glycol with GO to be used as a cellular sensor by using the intrinsic photoluminescence property of GO at the NIR region [39]. In another study, Zhang et al. integrated GO into poly (vinyl alcohol) hydrogels to improve their mechanical strength [40]. The noncovalent (weak) interactions for the self-assembly and design of molecules were performed to change the organic NMs into required structures such as dendrimers, micelles, liposomes and polymer NPs [41].

12.3.1.2

Inorganic-Based Nanostructured Materials

These nanostructured materials include metal and metal oxide nanoparticles and nanostructured materials. These nanostructured materials can be synthesized into metals such as Au or Ag nanoparticles or as metal oxides such as TiO2 and ZnO nanoparticles, and semiconductors such as silicon and ceramics. Metal oxide nanostructures, such as ZnO [42], CuO [43, 44], SnO2 [45], In2 O3 [46] and TiO2 [47], have been attracted extensive attention because of their optical, electrical and magnetic properties. They have wide applications in the field of optoelectronic devices, such as photocatalyzers [48], solar cells [49], photodiodes [50] and humidity sensors [51]. Recently, the important CuO–ZnO–based compounds and CuO/ZnO heterojunction nanostructured materials have been characterized and reported [52, 53].

12.3.1.3

Composite-Based Nanostructured Materials

Composite nanomaterials are multiphase nanostructured materials with one phase in the nanoscale dimension that can either combine nanoparticles with other nanoparticles or nanoparticles combined with larger or with bulk-type materials or more complicated structures, such as a metal–organic framework [41]. Nanocomposites are being used in many biomedical applications. They are also important in drug delivery system [54–56], stem cell therapy [57] antimicrobial properties [58, 59], biosensors [60–62] and cancer therapy [63].

12.3.2 Classification of Nanomaterials Based on Dimensions The classification of nanomaterials is based on the number of dimensions of a material, which are outside the nanoscale (