Metal Oxide Nanocomposites E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Metal Oxide Nanocomposites: Synthesis and Applications summarizes many of the recent research accomplishments in the area of metal oxide-based nanocomposites. This book focussing on the following topics: Nanocomposites preparation and characterization of metal oxide nanocomposites; synthesis of core/shell metal oxide nanocomposites; multilayer thin films; sequential assembly of nanocomposite materials; semiconducting polymer metal oxide nanocomposites; graphene-based metal and metal oxide nanocomposites; carbon nanotube-metal-oxide nanocomposites; silicon mixed oxide nanocomposites; gas semiconducting sensors based on metal oxide nanocomposites; metal ]organic framework nanocomposite for hydrogen production and nanocomposites application towards photovoltaic and photocatalytic.
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Veröffentlichungsjahr: 2021
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Table of Contents
Cover
Title Page
Copyright
Preface
1 Metal Oxide Nanocomposites: State-of-the-Art and New Challenges
1.1 Introduction to Nanocomposites
1.2 Graphene-Based Metal and Metal Oxide Nanocomposites
1.3 Carbon Nanotube−Metal Oxide Nanocomposites
1.4 Metal Oxide-Based Nanocomposites Application Towards Photocatalysis
1.5 Metal Oxide Nanomaterials for Sensor Applications
1.6 Metal Oxide Nanocomposites and its Thermal Property Analysis
1.7 Semiconducting Metal Oxides for Photocatalytic and Gas Sensing Applications
1.8 Applications of Metal Oxide-Based Nanocomposites
References
2 Introduction to Nanocomposites
2.1 Composites: An Introduction
2.2 Functions of Fibers and Matrix
2.3 Classification of Composites
2.4 Matrix Based Composites
2.5 Reinforcements
2.6 Polymer Composites
2.7 Composites Processing
2.8 Composites Product Fabrication
2.9 Application of Composites
2.10 Special Features of Composites
2.11 Composites vs Metals
2.12 Advantages of Composites
2.13 Disadvantage of Composites
2.14 Conclusion
Acknowledgments
References
3 Graphene-Based Metal and Metal Oxide Nanocomposites
3.1 Introduction
3.2 Graphene
3.3 Reduced Graphene Oxide
3.4 Graphene-Based Composites
3.5 Graphene-Based Hybrid Nanocomposites
3.6 The Mechanics of Graphene Nanocomposites
3.7 Functionalization
3.8 Thermal Properties
3.9 Conclusions
References
4 Carbon Nanotube−Metal Oxide Nanocomposites
4.1 Introduction
4.2 Synthesis Methods
4.3 Environmental Applications
4.4 Environmental Fate, Transport, and Transformation
4.5 Environmental Implications
4.6 Conclusions and Future Research Direction
References
5 Metal Oxide-Based Nanocomposites Application Towards Photocatalysis
5.1 Introduction
5.2 Nanocomposite Photocatalysts Based on Metal Oxide
5.3 Application of Metal Oxide Composites in Photocatalysis
5.4 Summary and Outlook
References
6 Metal Oxide Nanomaterials for Sensor Applications
6.1 Introduction
6.2 Binding of Metal Oxide with Imidazole
6.3 Characterizations
6.4 Absorption Characteristics
6.5 Emission Characteristics
6.6 Sensor Mechanism
6.7 Conclusions
References
7 Metal Oxide Nanocomposites and its Thermal Property Analysis
7.1 Introduction
7.2 Metal and Metal Oxide Nanoparticles in Thermal Management
7.3 Synthesis Procedures
7.4 Mechanism of Thermal Conductivity Enhancement
7.5 Thermal Conductivity Models for Nanofluids
References
8 Semiconducting Metal Oxides for Photocatalytic and Gas Sensing Applications
8.1 Semiconducting Metal Oxide as Photocatalysts
8.2 Semiconducting Metal Oxide as Gas Sensor
8.3 Conclusion
Acknowledgments
References
9 Applications of Metal Oxide-Based Nanocomposites
9.1 Introduction
9.2 Food and Agricultural Sector
9.3 Applications in Medicine
9.4 Water Barrier Properties
9.5 Thermal and Flame Retardants Apparitions
9.6 Water Disinfection Ability
9.7 Water Flux Application
9.8 Nanocomposites Membrane Apparitions
9.9 Wastewater Treatment
9.10 Non-Solvent Induced Phase Separation
9.11 Adsorption Performances Apparitions
9.12 Electrocatalytic Applications
9.13 Biosensors Application
9.14 Sensing Applications
9.15 Other Industrial Appreciations
9.16 Conclusions
References
10 Triboelectric Nanogenerators for Energy Harvesting and Sensing Applications
10.1 Introduction
10.2 What is Triboelectric Effect?
10.3 Mechanism of Triboelectric Nanogenerator (TENG)
10.4 How to Select the Materials for Your TENG?
10.5 Basic Operating Modes of TENG
10.6 TENG as Mechanical Energy Harvester
10.7 Conclusion and Future Perspectives
References
11 Metal Oxide Nanocomposites for Wastewater Treatment
11.1 Introduction
11.2 Adsorptive Removal of Water Pollutants
11.3 Photocatalytic Decomposition of Water Pollutants
11.4 Metal Oxide Nanocomposites
11.5 Removal and Decomposition of Inorganic Pollutants by Metal Oxide Nanocomposites
11.6 Removal and Decomposition of Organic Pollutants by Metal Oxide Nanocomposites
11.7 Conclusion and Outlook
References
Index
End User License Agreement
List of Tables
Chapter 3
Table 3.1 Characteristics and allied applications of graphene.
Chapter 4
Table 4.1 Synthesis methods and potential applications of CMNCs.
Table 4.2 Treatment of contaminants using CMNCs.
Chapter 8
Table 8.1 Hydroxyl radical generation in different AOP’s (Homogeneous).
Table 8.2 Some conventional semiconductors, their bandgap energies, and correspo...
Table 8.3 Main reactions occurring during photocatalysis of degradation of an or...
Table 8.4 Application of gas sensor in various areas.
Chapter 11
Table 11.1 Sources of inorganic pollutants in water along with their permissible...
Table 11.2 Application of several types of metal oxides nanocomposites for adsor...
Table 11.3 Metal oxide nanocomposites for adsorptive removal and photocatalytic ...
Table 11.4 Metal oxide nanocomposites for adsorptive removal and photocatalytic ...
Table 11.5 The representative examples of metal oxide nanocomposites for the ads...
List of Illustrations
Chapter 2
Figure 2.1 (a) Formation of a composite material using fibers and resin and (b) ...
Figure 2.2 Classification of matrices.
Figure 2.3 Types of thermoplastics.
Figure 2.4 Classification of thermosets.
Figure 2.5 Types of reinforcements in composites.
Figure 2.6 Classification of composites fabrication techniques.
Chapter 3
Figure 3.1 Pictorial representation of allotropes of carbon.
Figure 3.2 Synthesis approaches for graphene.
Figure 3.3 Types of Graphene-based composites and their synthesis approaches.
Figure 3.4 Schematic illustration of layer-by-layer deposition of GO sheets onto...
Figure 3.5 Types of Graphene-based hybrid nanocomposites and their synthesis app...
Chapter 4
Figure 4.1 Synthesis methods of carbon nanotube−metal oxide nanocomposites (CMNC...
Figure 4.2 Covalent linkage of titanium tetraisopropoxide- and tetraethyl orthos...
Figure 4.3 Schematic illustration of self-assembly of CMNCs includes: 1) CNTs; 2...
Figure 4.4 Schematic illustration of synthesis of CNT/pHBP hybrids via noncovale...
Figure 4.5 Proposed schematic of the methylene blue (MB) degradation processes o...
Figure 4.6 Schematic illustration of potential colloidal stability and aggregati...
Figure 4.7 Schematic illustration of potential transport of CMNCs in aqueous por...
Figure 4.8 Schematic illustration of potential chemical and biological transform...
Chapter 5
Figure 5.1 Comparison of the photocatalytic mechanism: (a) for UV irradiation of...
Figure 5.2 Primary steps of the postulated mechanism for titania modified with P...
Figure 5.3 Balance of the semiconductor–metal nanocomposite with the redox coupl...
Figure 5.4 VB and CB energy levels of many semiconductors. Copy or adopt with pe...
Chapter 6
Figure 6.1 Electron–hole pair generation in a photo illuminated n-type semicondu...
Figure 6.2 Powder XRD spectrum of (a) Unmodified Fe
2
O
3
nanoparticles; (b) APTS g...
Figure 6.3 SEM images of (a) Unmodified Fe
2
O
3
nanoparticles and (b) APTS grafted...
Figure 6.4 AFM (a) 3D image of unmodified Fe
2
O
3
Nanoparticles; (b) topographic i...
Figure 6.5 TEM images of (a) Unmodified Fe
2
O
3
nanoparticles; (b) APTS grafted Fe...
Figure 6.6 (a) SEM image of virgin NiO; (b) AMB-functionalized NiO.
Figure 6.7 (a) EDS spectrum of virgin NiO; (b) AMB-functionalized NiO.
Figure 6.8 Powder XRD spectrum of n-ZnO.
Figure 6.9 (a) AFM image of n-ZnO; (b) AFM image of f-ZnO.
Figure 6.10 XRD pattern of Ag
3
O
4
nanoparticles.
Figure 6.11 (a) SEM image of bare Ag
3
O
4
nanoparticles; (b) SEM image of AMB func...
Figure 6.12 (a) EDS spectrum of bare Ag
3
O
4
nanoparticles; (b) EDS spectrum of AM...
Figure 6.13 (a) UV–Visible spectrum of AMP; (b) AMP functionalized NiO nanoparti...
Figure 6.14 Absorption spectra of APTS in presence and absence of various concen...
Figure 6.15 Absorption spectra of APTS in presence and absence of various concen...
Figure 6.16 (a) Absorption spectrum of AMB; (b) AMB functionalized Ag
3
O
4
nanopar...
Figure 6.17 Fluorescence spectra of APTS in the presence and absence of various ...
Figure 6.18 (a) Fluorescence spectrum of AMP; (b) AMP functionalized NiO nanopar...
Figure 6.19 HOMO and LUMO orbital diagrams of AMB.
Figure 6.20 Fluorescence spectra of APTS in the presence and absence of various ...
Figure 6.21 Mechanism of electron transfer.
Figure 6.22 Molecular electrostatic potential map of AMB.
Figure 6.23 (a) Fluorescence spectrum of AMB; (b) AMB functionalized Ag
3
O
4
nanop...
Chapter 7
Figure 7.1 Two-step method for nanofluids production.
Figure 7.2 One-step nanofluid production system [154].
Figure 7.3 Schematic illustration representing the clustering phenomenon [152, 1...
Figure 7.4 Schematic illustration representing the liquid layering around nanopa...
Figure 7.5 Liquid layering around an Al
2
O
3
nanoparticle coated by surfactants in...
Figure 7.6 Interaction energy for the stable dispersion of nanoparticles in a li...
Chapter 8
Figure 8.1 The various sources of generation of a large amount of wastewater.
Figure 8.2 Photo-activation of a semiconductor and primary reactions occurring o...
Figure 8.3 Change in the electronic structures of a semiconductor compound as th...
Figure 8.4 The fundamental mechanism of heterogeneous photocatalysis. Different ...
Figure 8.5 Schematic illustration of the band structures of different type of se...
Figure 8.6 The various indoor sources of various organic gases.
Figure 8.7 Schematic of the measurement chain for MOx gas-sensing systems.
Figure 8.8 Schematic representation of band bending after chemisorption of charg...
Figure 8.9 Influence of sensor material and the nature of detecting gas on senso...
Figure 8.10 Factors effecting gas sensing performance.
Chapter 10
Figure 10.1 Triboelectric series of some materials [34]. Reprinted with permissi...
Figure 10.2 Vertical contact separation mode of TENG.
Figure 10.3 Lateral sliding mode of TENG.
Figure 10.4 Single electrode mode of TENG.
Figure 10.5 Free standing triboelectric layer mode of TENG.
Figure 10.6 Schematic of the generator’s structure [35]. Reprinted with permissi...
Figure 10.7 (a) Schematic of the arch-shaped TENG. (b) Fabrication flowchart for...
Figure 10.8 (a) Schematic of a double layered TENG. (b) Scanning electron micros...
Figure 10.9 (a) Schematic structure of the ITNG. (b) Optical image of the ITNG. ...
Figure 10.10 Schematic structure of a BN-TENG and SEM, AFM images of natural bio...
Figure 10.11 Structure of slinky TENG [81]. Reprinted with permission from Ref. ...
Chapter 11
Figure 11.1 Photocatalysis mechanism and applications of treated wastewater afte...
Figure 11.2 Microstructure of metal oxide nanocomposites and their applications ...
Figure 11.3 Plausible photocatalytic mechanism for reduction of Cr (VI) using 2-...
Figure 11.4 The fluoride removal via continuous fixed-bed column using the rGO/Z...
Figure 11.5 Adsorptive removal of Methylene blue dye using the Fe
2
O
3
/ZrO
2
nanoco...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Scrivener Publishing
100 Cummings Center, Suite 541J
Beverly, MA 01915-6106
Publishers at Scrivener
Martin Scrivener ([email protected])
Phillip Carmical ([email protected])
Metal Oxide Nanocomposites
Synthesis and Applications
Edited by
B. Raneesh and Visakh P. M.
This edition first published 2021 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-36357-6
Cover image: Pixabay.Com
Cover design by: Russell Richardson
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
Printed in the USA
10 9 8 7 6 5 4 3 2 1
Preface
Recently, there have been a number of research accomplishments in the field of metal oxide nanocomposites and their synthesis and applications, many of which are summarized in this book. Besides discussing the state of the art in the field, new challenges and opportunities regarding their use are also presented. Along with the structure-property relationships of metal oxide nanocomposites, the book also includes introductory material on nanocomposites, graphene-based metal and metal oxide nanocomposites, carbon nanotube–metal oxide nanocomposites, and metal oxide-based nanocomposites. In addition to the application of metal oxide nanocomposites in photocatalysis and sensors, among the other topics presented are thermal analysis of the properties of metal oxide nanocomposites, semiconducting metal oxides for photocatalytic and gas-sensing applications, applications of metal oxide-based nanocomposites, triboelectric nanogenerators (TENG) based on nanocomposites, and metal oxide nanocomposites for water purification. Since this book is an up-to-date record of the major findings and observations in the field—with chapters contributed by prominent researchers from industry, academia and government/private research laboratories across the world—it will act as a very valuable reference source for university and college faculties, professionals, post-doctoral research fellows, senior graduate students, and researchers from R&D laboratories working in the area of metal oxide nanocomposites.
The introductory chapter discusses the scope, state of the art, preparation methods, new challenges and opportunities concerning metal oxide nanocomposites. Chapter 2 gives a more detailed review of structure-property relationships. Included in the many topics discussed in this chapter are the functions of fibers and matrix, classification of composites, matrix-based composites, reinforcements, polymer composites, composites processing, product fabrication, application, and special features of composites, along with their advantages and disadvantages compared with metals.
Graphene-based metal and metal oxide nanocomposites are introduced in Chapter 3. In this chapter, the authors discuss graphene, reduced graphene oxide, graphene-based composites and hybrid nanocomposites, mechanics of graphene nanocomposites, covalent and noncovalent functionalization and thermal properties. Chapter 4 on carbon nanotube–metal oxide nanocomposites introduces carbon nanotubes, including packaging applications, followed by different topics such as synthesis methods, environmental applications, environmental fate, transport, transformation, and environmental implications; and concludes with the direction that future research might take.
Two main topics are addressed in Chapter 5. The first is nanocomposite photocatalysts based on metal oxide, including those based on TiO2, ZnO and WOx. The second topic, the application of metal oxide composites in photocatalysis, includes a discussion of water splitting for hydrogen generation, photodegradation of pollutants and wettability patterning based on photocatalysts. Chapter 6 is a structural analysis of metal oxide nanocomposites for sensor applications. Following an introduction to the subject matter, the chapter goes on to discuss binding of metal oxide with imidazole and silane, characterizations, absorption and emission characteristics, and sensing mechanisms.
Thermal property analysis of metal oxide nanocomposites is discussed in Chapter 7. In this chapter the authors explain the use of metal and metal oxide nanoparticles in thermal management and synthesis procedures, the mechanism of thermal conductivity enhancement and thermal conductivity modeling of nanofluids. In Chapter 8 there is a two-part discussion on the topic of semiconducting metal oxides for photocatalytic and gassensing applications. The first part covers topics such as organic dyes as a major source of water pollution, conventional methods used for dye degradation, advanced oxidation processes, the role electronic structure of semiconducting metal oxide plays in photocatalysis, basic principles of photocatalysis, oxidizing species generation mechanism, semiconductor photocatalysts, kinetic studies of semiconductor photocatalysis and parameters affecting dye degradation. The second part of the chapter covers topics such as the evolution of gas sensors and their necessity, metal oxides as gas sensors, the gas-sensing mechanism of metal oxides and factors influencing sensor performance.
Metal oxide-based nanocomposites and their many applications are the subject of Chapter 9. It covers their use in the food and agricultural sector and in medicine due to their water barrier, thermal and flame-retardant properties, and their ability to disinfect water and improve water flux. Also discussed are nanocomposite membrane assemblies for water treatment prepared by non-solvent induced phase separation method, their adsorption performances, and their suitability for electrocatalytic, biosensor and sensing applications. Next, in Chapter 10, energy harvesting and sensing applications of triboelectric nanogenerators (TENG) are presented. Among the topics discussed are the working mechanism of a triboelectric nanogenerator, how to select materials for your TENG, basic operating modes of TENG, TENG as mechanical energy harvester, and TENG based on vertical contact separation mode, lateral sliding mode, single electrode mode and free-standing triboelectric layer mode. The final chapter on metal oxide nanocomposites for water purification includes several topics such as adsorptive removal of water pollutants; and the use of metal oxide nanocomposites for photocatalytic decomposition of water pollutants, removal and decomposition of inorganic pollutants, and decomposition of organic pollutants.
In conclusion, we would like to express our sincere gratitude to all the contributors to this book, whose commitment and sincerity shown towards their contributions, along with their enthusiasm and excellent support, enabled the successful completion of this venture. We would also like to thank all the reviewers who have taken their valuable time to make critical comments on each chapter. Finally, our thanks go out to Scrivener Publishing for recognizing the demand for such a book and realizing the increasing importance of the area of metal oxide nanocomposites and their synthesis and applications.
Dr. B. RaneeshDr. Visakh P. M.November 2020
1Metal Oxide Nanocomposites: State-of-the-Art and New Challenges
Visakh P.M.1* and B. Raneesh2
1Department of Physical Electronics, TUSUR University, Tomsk, Russia
2Department of Physics, Catholicate College, Pathanamthitta, Kerala, India
Abstract
This first chapter discusses about the metal oxide nanocomposites: synthesis and applications with many different topics such as introduction to nanocomposites, graphene-based metal, metal oxide nanocomposites, carbon nanotube–metal oxide nanocomposites, metal oxide-based nanocomposites application towards photocatalysis, metal oxide nanomaterials for sensor applications, metal oxide nanocomposites and its thermal property analysis, semiconducting metal oxides for photocatalytic and gas sensing applications, and other potential applications of metal oxide-based nanocomposites.
Keywords: Metal oxide nanocomposites, carbon nanotube, graphene, photocatalysis, gas sensor
1.1 Introduction to Nanocomposites
The composite materials composed of polymer matrix are easily processible and readily available and therefore widely used in various industries. In composite materials, the fiber acts as load carrier with its strength being greatest along the axis of the fiber. The long continuous fibers aligned in the direction of the load leads to the formation of composite with much enhanced properties than the pure matrix material. Research studies reveal that the spider’s web fibers are much stronger than man-made processed fibers. In ancient civilizations across the world, husks or straws mixed with clay have been widely utilized to build houses that last longer for several hundred years [1]. The matrix material in composites serves two important purposes: (a) binding the reinforcement phases in place and (b) uniformly distributing the stresses among the constituent reinforcement materials in the event of an applied force. The matrix offer weight advantages and ease of handling. The inorganic materials, polymers and metals can be utilized as matrix materials in the designing of structural composites [2]. Thermoplastics resins are generally used as molding compounds. The fibers are randomly dispersed in thermoplastics, and so the reinforcement is isotropic but directionality can be achieved using molding processes [3]. Thermosets are retained in a partially cured condition over prolonged periods of time to induce flexibility in them [4]. Generally, the condition of fiber material (chopped, aspect ratio) in epoxy, polymer and phenolic polyamide resins decides the final application of thermosets. Most of the metals and alloys can be used as matrices, however, they often require compatible reinforcement materials which are stable over a range of temperature and also non-reactive [5].
The addition of reinforcements in ceramic overcomes the problems related with high modulus of elasticity and low tensile strain to obtain strength improvement. The addition of reinforcements in adequate amount causes the ceramics to effectively transfer quantum of load to the reinforcement thereby reducing the chances of ceramics rupture at high stress levels. The carbon–carbon composite can be synthesized using compaction of carbon or multiple impregnations of porous frames with liquid carbonizer precursors and subsequent pyrolization or through chemical vapor deposition of pyrolytic carbon [6]. In a 2-D composite, the strength remains only one-third to the strength of a unidirectional fiber-stressed in the direction of fibers. But, in a 3-dimension, less than one-fifth of the strength is obtained. The fiber composites can be either continuous or short fibers. It is generally observed that the continuous fibers exhibit better orientation in matrix. The major proportion (>95%) in reinforced plastics are glass fibers. They are inexpensive, have low density, resistant to chemicals, insulation capacity, easy to process with high strength/stiffness than the plastics with which they are reinforced [7]. However, they are more prone to breakage when subjected to high tensile stress for a long time. Metal fibers when amalgamated with refractory ceramics improve performance by improving their thermal shock and impact resistance properties. The resulting composites possess high strength, light weight and good fatigue resistance.
The properties of boron fibers depend upon their diameter due to the changing ratio of boron to tungsten and the associated surface defects that change according to size. The boron fibers are known for their remarkable stiffness and strength [8]. The uncoated boron-tungsten fibers do not react with molten aluminum and also withstand high temperatures for utilization in hot-press titanium matrices. However, silicon carbide-tungsten fibers are dense and prone to surface damage and require careful, delicate handling, during fabrication of the composite [9]. Quartz fibers can withstand high temperatures, while silica cannot [10]. Quartz fibers are highly elastic and can be stretched to 1% of their length before break point. Laminar composites comprises of layers of materials bonded together and can exists in as many combinations as the number of materials. In laminar composites, several layers of two or more metal materials can occur alternately or in a definite order, and in as many numbers as required for a specific purpose. Both clad and sandwich laminates follow the rule of mixtures from the modulus and strength point of view [11]. Flakes composites have densely packed structures. Metal flakes in polymer matrices can conduct electricity or heat, whereas, mica and glass flakes can resist both. Flakes are much cheaper than fibers. More often, the flakes fall short of expectations while controlling the size, shape and hence produce defects in the end product. The infiltrate can be independent of the matrix which binds the components like powders or fibers, or they could just be used to fill voids [12]. The matrix is not naturally formed in the honeycomb structure, but specifically designed to a predetermined shape.
Reinforcement can be of the square, triangular and round shapes, and the dimensions of all their sides are more or less equal [13]. The dispersion size in particulate composites is in microns range whereas volume concentration is greater than 28%. Their potential properties are based on the relative volumes of the metal and ceramic constituents [14]. Cermets are produced by impregnating the porous ceramic structure with a metallic matrix binder. Cermets can also be used as coating in a powder form where the powder is sprayed through a gas flame and fused to a base material. In a polymer composite, either the constituent matrix material or the fiber is a polymer. The polymer matrix composites (PMCs) compose of a polymer resin as the matrix material and fibers as the reinforcement medium [15]. The techniques to produce carbon fibers are relatively complex. Rayon, polyacrylonitrile (PAN), and pitch are used as organic precursor materials for producing carbon fibers. The processing techniques for composites are different than those for metals processing because composite materials involve two or more different materials [16]. Substantial changes in technology and its requirement in the past three to four decades have created many new needs and opportunities, which has fostered the need of advanced materials in associated manufacturing technology.
1.2 Graphene-Based Metal and Metal Oxide Nanocomposites
Carbon materials exist in all dimensionalities including zero-dimensional (0D) i.e. fullerenes, quantum dots, one-dimensional (1D) carbon nanotubes i.e. CNTs, two-dimensional (2D) i.e. graphene and three-dimensional (3D) i.e. graphite. Graphene is the appellation given to a two-dimensional sheet of sp2-hybridized carbon atoms with exceptionally high crystallinity and electronic property [17]. Lately, the nomenclature ‘‘graphene’’ was acclaimed by the commission of IUPAC as a substitute to the older name ‘‘graphite layers’’, for the reason that graphite is three-dimensionally (3D) stacked carbon structure. It has arose as a speedily growing wonder material in the field of material science due to its thinnest and the sturdiest structure [18]. In early 2004, it gained high significance after the studies presented by Geim’s group, who demonstrated the graphene sheets and stated their unparalleled electronic properties. Later, in 2010, Physics Nobel Prize for pioneering research highlighting the two-dimensional material graphene presented by the Royal Swedish Academy to pioneers namely, Andre Geim and Konstantin Novoselov [19]. The progress in research till date, on graphene is mainly focused on the chemical and physical route of synthesis of pristine graphene, its chemical modification, detailed characterization of its chemical and physical properties and functions, synthesis and characterization of graphene-based polymer composites and metal-oxide nanocomposites, aiming to exemplify the impression of graphene-based nanomaterials on the development of novel analytical developments and its applications. Despite the fact, Geim and coworkers also employed the use of mechanical exfoliation which led to many stimulating investigations on graphene and its electronic and mechanical properties [20]. Recently, a facile and green synthetic stratergy for the large scale production of graphene by means of ball-milling of graphite flakes with carbohydrates namely sucrose, Producing graphite through epitaxial growth under the effect of ultrahigh vacuum (UHV) annealing conditions of SiC surface has attracted many researchers and technologists for the semiconductor industry [21]. The practice of epitaxial growth of graphene layers on silicon carbide (SiC) substrates give an impression of being highly capable approach for the fabrication of electronic devices. Likewise, Berger and De Heer in the early days of graphene research provoked the usage of epitaxial graphene on SiC substrates [22].
Later, much research efforts have been dedicated to recognize the synthetic and mechanistic of graphene growth and detailed experiential studies to optimize the thickness of graphene layers. The merits of the Plasma enhanced chemical vapor deposition (PECVD) embraces the short deposition time around <5 min and a temperature of 650 °C for the growth of graphene layers (contrary to 1,000 °C for CVD) [23]. The aim of regulating pH with ammonia solution resulted in deprotonation of the carboxylic acid functionalities leading to the formation of well-dispersed graphene colloids [24]. Likewise, Athanasios and coworkers investigated the synthesis of reduced graphene oxide by engaging the use of sodium borohydride [25]. Apart from these, there are other chemical routes employing the use of hydroquinone [26] and strong alkaline stock solutions [27], thermal reduction and solvothermal methods have also been vastly studied. The thermal reduction of GO to form rGO, utilizes the thermal treatment (at high temperatures) to eradicate the oxide functional moieties from the surface of GO [28]. Sol–gel process is engaged for the synthesis of TiO2-GO/rGO composites by the chemical reaction of titanium isopropoxide and GO/rGO sheets which ensued the surface hydroxyl groups on GO/rGO that necessitate nucleation sites [29]. The ss-DNA/Graphene nanocomposite offers a novel bio-graphene hybrid electrochemical platform for the biological determination of redox enzymes [30]. The covalent functionalization in graphene is commonly headed by a chemical oxidation of the graphite with the effect of strong acids and oxidants to acquire oxygen-rich functional moieties that assist as pioneers for the chelation of organic molecules. As detailed synthesis of GO outcomes in extremely functionalized oxygen surface moieties, thereby reaching the C/O fraction of 2:1 [31].
1.3 Carbon Nanotube–Metal Oxide Nanocomposites
Carbon nanotubes (CNTs) have received intense and growing interests since the first discovery in 1991 [32]. Structurally, CNTs are rolled-up graphene nanosheets, the ends of which are capped with a hemisphere of a buckyball [33, 34]. CNTs exist either as single-walled or multi-walled CNTs (SWCNTs or MWCNTs), and MWCNTs are composed of a series of concentric SWCNTs with an inter-tube distance of approximately 0.34 nm [35]. Depending on the atomic arrangement of the hexagonal rings (graphene structure) along the tubular surfaces, CNTs can be metallic and/or semiconducting. SWCNTs are generally regarded as a mixture of metallic and semiconducting material, while MWCNTs are metallic conductors [36]. CNTs offer tremendous exciting opportunities for physicists, chemists, biologists, engineers, and material scientists to develop fundamentally novel material systems, better serving the human being in an accelerating and sustainable fashion. Technically, CNTs can be hybridized with a wide spectrum of inorganic compounds such as oxides, nitrides, carbides, and ceramics, in which metal/metal oxides are currently the most widely exploited species due to their high modulus and strength especially at high temperatures. Consequently, CNT–metal/metal oxide NCs (CMNCs) are considered as promising candidates for many potential applications. CMNCs can be tailored to equip qualities such as lightweight (low density), low thermal expansion coefficient, and high thermal conductivity suitable for use in aerospace, automobile, and many other industries. For example, it is projected that applying CNT–Al and CNT–Mg NCs in automobiles would decrease CO2 emission by 50% per year, based on the report of Japan Automobile Manufacturers Association (JAMA). Prior to introducing the synthesis methods of CMNCs, it is informative to briefly retrieve the methods for preparation of CNTs first. Various approaches such as arc discharge [37], laser ablation [38], gas phase catalytic growth [39], and chemical vapor deposition (CVD) [40–42] have been commonly used to produce CNTs. To prepare CNTs for application in the NCs, the production of large quantities of CNTs is a prerequisite.
Due to the limitations and high costs associated to large-scale production, the arc discharge and laser ablation techniques are not promising. Often, as produced CNTs contain various degrees of impurities such as fragments of wrapped-up graphene sheets, soot, amorphous C, fullerene, and metal catalyst particles [43], thus purification is needed to purify CNTs because these impurities deteriorate the desired and promising properties and performance of CNTs [44]. Given that gas phase techniques such as CVD method can produce large quantities of CNTs with fewer impurities and particularly, they are superior for in situ assembly of metal oxide nanoparticles (NPs) with CNTs, the gas phase techniques hold the greatest potential for scaling-up manufacturing of CNT-based NCs. Alternatively, derivatization of CNTs with other families of organic interlinker molecules particularly biomolecules (e.g., DNA and biotin streptavidin) offer another promising and versatile route in the assembly of CNTs and metal oxide NPs [45, 46]. The advantage of covalent approach is that various well-defined, structurally tunable interlinkers excel in hybridizing metal oxide NPs and CNTs, projecting the flexibility and versatility of this approach. However, as mentioned above, the electronic and mechanical properties of CNTs would be disrupted because sp2-hybridized C atoms are converted to sp3-hybridized analogues after functionalization. Hydrophobic interactions between long chains of aliphatic compounds and CNTs’ hydrophobic surfaces can be used to assemble metal oxide NPs and CNTs. For instances, phosphonic acid- and alkoxysilane-functionalized MWCNTs were templated to hybridize TiO2 and SiO2 NPs; and the resulting NCs show great promise in building blocks for sensors, nanoscale switches, and other nanodevices [47]. Surfactants such as sodium dodecyl sulfate (SDS) and Triton X are also powerful binding motifs for connecting metal oxide NPs (e.g., Pd and ZnO) and CNTs [48, 49]. Particularly, a combination of hydrophobic interaction and hydrogen bonding improves the assembly of CNTs and metal oxide NPs. Au NPs (2–5 nm diameter) covered by a mixed-monolayer of decanethiol and mercaptoundecanoic acid were adjoined strongly with acid-treated CNTs containing carboxylic groups [50]. Both hydrophobic interactions arising from alkyl chains and hydrogen bonding due to carboxylic groups contributed to the formation of stable NCs. The major advantage of hydrophobic assembly is that, in some cases, simple and non-specific physical mixing is suffice to obtain desirable CMNCs [51]. CNTs can be modified to capture either negative or positive surface charges through polyelectrolyte or polymer-wrapping, thereby connecting positively or negatively charged NPs via electrostatically attractive interaction. For example, chemically oxidized CNTs were capped with a thin film of cationic poly(diallyldimethylammonium) (PDDA), serving as the template for anchoring the negatively charged Au NPs (10 nm) [52].
Attributed to the robustness of regulation of nucleation and growth processes, electrochemistry is demonstrated as a powerful way for depositing metal NPs onto CNTs directly, especially for noble metals such as Pd [53, 54], Pt [55], Au, Ag, and bimetallic Pt–Co [56] with potential implications in heterogeneous catalysis, electrocatalysis, biosensors, and fuel cells. Typically, metal NPs can attach onto CNTs’ surface via reduction of metal salts (e.g., AuCl−, PtCl62−, and PdCl42−), with the aid of reducing agents such as H2 [57], NaBH [58], citric acid [59], or ethylene glycol [60]. The size of the resulting metal NPs and their assembly onto CNTs’ surface can be tailored by adjusting reactant concentration, reaction time and temperature, and nucleation potential and voltage, or by introducing surfactants [61]. An earlier review has addressed potential environmental applications of C-based NMs including CNTs and their composites [62]. Deposition of transition metals (e.g., Au, Ni, Pd, Pt, and Ti) has been achieved on perfect and defective MWCNTs via thermal evaporation of different amounts of metals onto the substrates [63]. Other metal and metallic NMs have also been decorated onto CNTs’ surface including Ag, Cu, and PdO [64]. Confinement of other NPs inside CNTs has been reported including Se [65], Co [66, 67], Pd [68], and magnetite [69]. A more comprehensive review has showcased dozens of inorganic compounds used in CMNCs including synthesis methods, and tested and potential applications. Another review has provided a thorough CNT characterization summary and discussion of adsorption mechanisms of organic contaminants by CNTs as well as the statistical adsorption model development efforts. Other reviews have demonstrated the importance of surface modification of CNTs for removal of heavy metals and organics from industry wastes [69–71]. Specific sensors can be developed for detecting specific analytes. A novel nitrite sensor has been developed by electropolymerization of alizarin red on the surface of glassy C electrode modified with MWCNT–Fe3O4 composite nanofilm [72].
1.4 Metal Oxide-Based Nanocomposites Application Towards Photocatalysis
Photocatalysis has long been studied and is expected to make a great contribution to both environmental treatment (emission cleaning and water purification) and renewable energy. Over the past few decades, the number of applications based on photocatalysis increased sharply; while a wide range of materials systems have been developed [73]. Photocatalytic H2 production from water is one of the most promising ways to realize a hydrogen economy for three reasons. (1) This technology is based on photon (or solar) energy, which is a clean, permanent source of energy, and mainly water, which is a renewable resource; (2) it is an environmental safe technology without undesirable by-products and pollutants; and (3) the photochemical conversion of solar energy into a storable form of energy, i.e. hydrogen allows to deal with the intermittent character and seasonal variation of the solar influx. Nano science and nanotechnology have boosted the modification of existing photocatalysts and the discovery and development of new candidate materials [74]. The rapidly increasing number of scientific publications constitutes clear bibliographical evidence for the significance of this hot topic. The valence holes are good oxidants and the conduction electrons are reductants. In most cases, the organic compounds are degraded by oxidation reaction involving photogenerated holes. Since 1972, scientists discovered the photocatalytic splitting of water on the illuminated single crystal TiO2 electrodes [75], the photocatalysis as a research area is of significant interest and photocatalytic processes have been extensively studied.
The adoption of TiO2 photoanode as well as a Pt counter electrode soaked in an aqueous electrolytic solution made it possible for the water to be incised, which was caused by UV light. This was conducted by Honda and Fujishima in 1972. It has been issued that the charge clipping and photocatalytic performance can be improved by combination of semiconductor with a wide band gap like SnO2 with TiO2 [76, 77]. There is an effective of promoting the photocatalytic performance of visible light, namely TiO2 matrix is inserted with the combination of metal ions. The principle of this approach is through blocked charge carrier recombination [78]. The insertion of the metal ion promotes the shape of Ti3+ ions, thus improving the photocatalytic performance. Transforming the TiO2 through incorporation of two or more than two dopants is issued, which makes great combination influence. On the contrary, the undoped TiO2 or the TiO2 with one ion incorporated is less effective [79, 80]. Surface spots, oxygen vacancies and polar planes contribute the difference in photocatalytic performance of ZnO. With solvothermal technique, Xu et al. synthesized carious forms of ZnO and adopted them as photocatalyst to degrade the phenol [81].
These researchers proposed that nanoflowers and NPs indicated boosted photodegradation outcomes in comparison to nanoflowers, nanorods, nanotubes, as well as hour-glass-like ZnO spheres. To photodegrade phenol, Liu et al. used TiO2 nanostructures with various forms such as microspheres, NPs and nanorods though hydrothermal approach [82]. With nanorods to be photocatalyst, this group of researchers gained marvelous photodegradation outcomes. ZnO has come into the researchers’ focus since 1935, but its excellent features are discovered through modern methods and improved equipment [83]. Liang et al. [84] found that the generated graphene–TiO2 nanocrystal combination featured advanced photocatalytic performance in contrast to other TiO2 materials like P25, bare TiO2 and mixture of P25 and GO handled by hydrothermal procedure, in the splitting process of rhodamine B with UV irradiation, boosting a three-fold photocatalytic influence on P25. Metal oxide appearances indicate that it is good at decomposing organic molecules with great oxidizing ability for and superhydrophilicity [85, 86] and such traits could be adopted to generate wettability patterns, which have been adopted in many areas like in printed-circuit boards and offset printing, and can be used for fluid microchips in the future [87, 88].
1.5 Metal Oxide Nanomaterials for Sensor Applications
Research into imidazole chemistry has been quite comprehensive, single-site functionalizations and substitution as well as various annulation strategies has been well documented [89]. Recently, the interest in this heterocyclic system has been widened as it is a precursor to a class of compounds, called room temperature ionic liquids. Microwave-assisted synthesis of the substituted imidazoles on a solid support under solvent-free conditions in a three-component reaction [90] and efficient synthesis of imidazoles from aldehydes, 1,2-diketones and ammonium acetate in acetic acid have been reported [91]. Noble metal Au, Ag, or Pt coated ZnO is important for photoelectron transfer (PET) in the bulk and interface of ZnO semiconductors. Under illumination of UV light, the exciton absorption bands of ZnO are strongly bleached due to the accumulation of conduction band electrons. Thus, the efficiency of both the photocatalysis and photoelectric energy conversion can be greatly enhanced by depositing noble metals on the surface of ZnO [92–94]. The principle terms involved in a photoactive semiconductor are conduction band (CB), valence bands (VB), band gap, trap sites and Fermi level. The bands are the allowed energy states that an electron can occupy in a material. The highest energy band occupied by an electron is called the valence band while the next available lowest empty energy level, next to valence band is called the conduction band. The metal oxide samples were deoxygenated by bubbling with pure nitrogen gas. An ethanolic solution of the imidazole derivative of required concentration was mixed with nanoparticles dispersed in ethanol at different loading and the absorbance and emission spectra were recorded. The nanocrystals were dispersed under sonication in ethanol using ethylene glycol followed by dilution with ethanol. Surface modification of Fe2O3 has been performed as follows [95]. 2 g of Fe2O3 nanoparticles has been kept in a vacuum chamber at 110 °C for 75 min and then dispersed in acetone by stirring for 1 h at ambient temperature and finally has been sonicated for 20 min. Then, 1 g APTS (50 wt%) has been gradually added to the dispersed solution and stirred for further 24 h at ambient temperature.
Fluorescence enhancing arises due to formation of APTS–ZnO complex. The energy transfer competence is related not only to the distance between the acceptor ZnO and donor APTS (r0) but also to critical energy transfer distance (R0). The critical energy transfer distance (R0), , where, K2 is the spatial orientation factor of the dipole, N is the refractive index of the medium, φ is the fluorescence quantum yield of the donor and J is the overlap integral of the fluorescence emission spectrum of the donor and the absorption spectrum of the acceptor. On the relative positions basis of APTS and interfacial energy levels, the interfacial electron injection would be thermodynamically allowed from the excited singlet of APTS to CB of ZnO. On lighting at excitation wavelength both the APTS and ZnO are excited. Dual emission is anticipated due to LUMO → HOMO and CB → VB electron transfer. The probable is electron jump from the excited APTS to ZnO. Electron in LUMO of the excited APTS is of higher energy compared to that in the CB of ZnO. The inorganic nano size fillers with organic functional groups that attach to their surface by strong chemical bonds can be finally obtained. The interaction increases the surface tension of inorganic nano particles results f-ZnO. Binding interaction studies of NiO with AMB shows the unexpected results. The increased absorption observed with the dispersed NiO is due to the adsorption of AMB on the surface of NiO. Fluorescence enhancement resulted while adding the NiO to AMB. Azomethine nitrogen is involved in the binding process with Ag3O4 nanoparticles which was proved by molecular electrostatic potential. Morphological changes of Ag3O4 nanoparticles to AMB modified Ag3O4 nanoparticles confirm the binding of Ag3O4 nanoparticles to AMB. EDX spectrum of AMB modified Ag3O4 nanoparticles shows incorporation of AMB to Ag3O4 nanoparticles. The interaction between AMB and Ag3O4 occurs through static quenching mechanism.
1.6 Metal Oxide Nanocomposites and its Thermal Property Analysis
Metal nanoparticles, due to their special properties and also small dimensions, find important applications in optical, magnetic, thermal, sensoric devices, catalysis, etc. Metals are dominated by the collective oscillation of conduction electrons resulting from the interaction with electromagnetic radiation. In addition to these, many production processes include heat transfer in various forms; it might be the cooling of a machine tool, pasteurization of food, or the temperature adjustment for triggering a chemical process. In most of these applications, heat transfer is realized through some heat transfer devices; such as, heat exchangers, evaporators, condensers, and heat sinks [96–98]. The feasibility of the usage of such suspensions of solid particles with sizes on the order of 2 mm or μm was previously investigated by several researchers and significant drawbacks were observed. These drawbacks are sedimentation of particles, clogging of channels and erosion in channel walls, which prevented the practical application of suspensions of solid particles in base fluids as advanced working fluids in heat transfer applications [99, 100]. Two-step method is the most widely used method for preparing nanofluids. Nanoparticles, nanofibers, nanotubes or other nanomaterials used in this method are first produced as dry powders by chemical or physical methods. Metal, metal oxide and various carbonaceous and nanocomposite based nanofluids are prepared by this method. The surfactants used in this method for stabilization can lower the thermal conductivity [101]. So the nanofluids prepared by this method exhibit lower thermal conductivity compared to the ones prepared by one-step method.
The formation of silver nanowires was explained by the in situ precipitation of a lamellar silver thiolate, Ag(SC12H), acting as a template for the 1D coalescence of isotropic Ag nanoparticles. Heterogeneous nucleation has also proved to be an efficient method for the control of the particle growth. Using Ruthenium as a seed (Ru is more easily reducible than cobalt or nickel), allows to get nickel–cobalt or cobalt nanowires [102, 103]. The synthesis of metal nanoparticles can be commonly carried out by chemical reduction, electrochemical reduction [104] and thermal decomposition [105] also induced by microwave heating [106–108]. Through the synthetic methodology, it is possible to vary the morphology, size and size distribution of M-NPs. Strong reducing agents such as sodium hydride seldom control the size and shape of M-NPs but mild reducing agents such as sodium citrate and ascorbic acid [109] which simultaneously act as coordinating capping ligands can control the size and shape of metal Nps. The biphasic synthesis can be carried out quickly under ambient conditions, permits the use of a variety of phosphines as passivating ligands, and provides control over particle core size to produce 1.5-nm nanoparticles [110].
Phosphine-stabilized nanoparticles are precursors to other functionalized nanoparticle building blocks where nearly any functional group can be introduced into the ligand shell, and the metal core size can be tuned from 1.4 to 10 nm in diameter through ligand exchange reactions [111]. Prasher et al. [112] compared the effect of translational Brownian motion and convection induced by Brownian motion. They also considered the existence of an interparticle potential. Evans et al. [113] theoretically showed that the thermal conductivity enhancement due to Brownian motion is a very small fraction of the thermal conductivity of the base fluid. This fact was also verified by molecular dynamics simulations. The clustering effect as the main reason of thermal conductivity enhancement was made by Keblinski et al. [114]. They analyzed the experimental data for thermal conductivity of nanofluids and examined the potential mechanisms of anomalous enhancement. Enhancement mechanisms such as microconvection created by Brownian motion of nanoparticles, nanolayer formation around particles, and near field radiation were concluded not to be the major cause of the enhancement. Turanov and Tolmachev used NMR technique to measure the self-diffusion coefficient of water in silica nanoparticle suspensions, and they confirmed the reduced mobility of water molecules in the proximity of solid-liquid interface. Besides density and self-diffusivity, several works also focused on the water viscosity and solid–liquid interfaces, shedding light on the differences from the bulk region [115–120]. Prasher et al. [121] demonstrated that the aggregation of nanoparticles can significantly enhance the thermal conductivity of nanofluids. They assumed that a fractal cluster is embedded within a sphere with a radius equal to the radius of gyration Rg and is composed of several approximately linear chains, which span the entire cluster and side chains. These linear chains are called the backbone of the cluster, while the other particles are called dead ends [121, 122].
1.7 Semiconducting Metal Oxides for Photocatalytic and Gas Sensing Applications
One of the vital classes of the water contaminations is ‘dyes’. Worldwide, more than 0.75 million tons of synthetic dyes are fabricated every year for the most part for use in the leather goods, plastics, textile, modern painting, products, electronic sectors and cosmetics [123–126]. About 1–20% of the total worldwide production of dyes is lost in the dyeing process and released into the environment as textile effluent. The effect of these dyes on the earth is a noteworthy concern as a result of the conceivably carcinogenic properties of these compounds. The wastewater which is colored due to the presence of these dyes can block both oxygen dissolution and sunlight penetration, which are necessary for aquatic life [127–129]. Apart from the aesthetic point of view, dyes are undesirable because they can affect the living creatures in water, when discharged as effluent into the environment. Industrial effluents containing synthetic dyes diminish light penetration in rivers and other water bodies and in this manner influence the photosynthetic activity of aquatic flora, thereby severely affecting the food source of aquatic organisms [130, 131]. Colored effluents from textile, dye, paper and pulp industries are somewhat hard to treat as they contain compounds with complex aromatic structures, rendering them quite difficult to treat [132, 133]. Oxidation of organic compounds proceeds through a number of free radical reactions, producing a large number of intermediates, which in turn, undergo oxidative cleavage, ultimately resulting in the formation of carbon dioxide, water and inorganic ions [134].
In the photocatalytic degradation of pollutants, when the reduction process of oxygen and the oxidation of pollutants do not advance simultaneously, there is an electron accumulation in the CB, thereby causing an increase in the rate of recombination of and . An important step in photocatalysis reactions is light absorption in the catalyst particle and to design properly a photo-reactor, the kinetics of the photo reaction should be known. Most kinetic models used in photocatalysis are based on the Langmuir–Hinshelwood (L–H) model [135]. This model explains the kinetics of reactions that occur between two adsorbed species, a free radical and an adsorbed substrate, or a surface-bound radical and a free substrate. The initial rate of substrate varies proportionally with the catalyst surface coverage and the adsorption equilibrium of the substrate follows a Langmuir isotherm. The experimentally determined dark adsorption coefficients are reported to differ from those determined during illumination [136]. Many techniques are used for the purpose of gas detection and each technique has certain advantages and disadvantages as well. Depending on the nature of material, each sensor is known to be sensitive to a group of a family of gases and similarly each gas can be detected by different materials [137]. Gas sensors have a great influence in many important areas namely environmental monitoring, domestic safety, public security, automotive applications, air-conditioning in aeroplanes, spacecrafts, etc. Semiconducting metal oxides possess a broad range of electronic, chemical, and physical properties that are often highly sensitive to changes in their chemical environment [138, 139]. Many scientists and engineers have studied metal oxide thin films as electronic materials due to their semiconducting behavior, structural simplicity and low cost. Numerous researchers have shown that the reversible interaction of gas with the surface of material is the characteristic of conductometric semiconducting metal oxide gas sensors. This reaction can be influenced by many factors, including internal and external causes, such as the natural properties of base materials, surface area, microstructure of sensing layers, surface additives, temperature, humidity, etc. [140].
1.8 Applications of Metal Oxide-Based Nanocomposites
Metal oxide nanocomposites of ZnO, CeO and CuO used as slow and controlled release of fertilizers provide nutrients to plants for prolonged period and also helps in prevent of soil degradation and improvement of sustainable agriculture [141, 142]. Nasiri et al. [143] discussed with enhanced oxygen barrier properties, and their application as packaging. LDPE films-based TiO2 nanocomposites have been reported to preserve other foods including strawberries, cheese, and shrimp [144, 145]. Cerrada et al. [146] prepared TiO2 nanoparticles based ethylene–vinyl alcohol (EVOH) nanocomposites. This nanocomposites with self-sterilizing properties and showing effectiveness against Gram-positive and Gram-negative bacteria. The starch/iron oxide nanocomposites for MRI and drug delivery applications [147, 148], chitosan/CuO nanocomposites are using as photocatalyst and antibacterial agent [149], epoxy/ZnO nanocomposites for light-emitting diodes based applications [150] and PMMA/ZnO nanocomposites using for Memory cells [151]. Skorenko et al. [152] have been prepared ZnO–chromophore nanocomposite, these nanocomposites demonstrates a thermal stability up to 300 °C. ZnO by a polar covalent metal-to-chromophore bridge which minimizes thermal degradation pathways of the organic molecules. Jo et al. [153] prepared an ultrafiltration membrane of poly(1-vinylpyrrolidone-co-acrylonitrile)-g-ZnO and poly(ether sulfone)-g-ZnO with both membranes have high antibacterial performances and both membranes show improved water flux, high antibacterial activities, and antifouling characteristic. The hybrid reverse osmosis membranes nanocomposite [154] of aromatic polyamide thin films and TiO2 particles were prepared through a self-assembly route, this membrane irradiated UV light showed enhanced photocatalytic bactericidal efficiency. Polymer nanoocomposites with TiO2 effectively degrade contaminants in water, for example chlorinated compounds in the water [155].
Teli et al. [156] have synthesized polyaniline-based TiO2 metal oxide nanocomposites. This nanocomposites were synthesized using the in situ polymerization to enhance the property of membrane antifouling. A hybrid SiO2/polyvinylchloride nanocomposites was prepared using the phase-inversion technique [157]. This nanocomposites shows better performance toward bacterial attachment and protein absorption, higher flux recovery ratio, and better antifouling performance compared to the neat polyvinylchloride. Feng et al. [158] have been synthesized many metal oxide based nanocomposites such as acetate/polypyrrole/TiO2, succinic-polypyrrole/TiO2, tartaric/polypyrrole/TiO2 and citric/polypyrrole/TiO2 and the results was excellent, the hydroxyl group significantly influenced the adsorption capacity and the better physicochemical properties of the nanocomposite were occurred. The electrical properties of polymer nanocomposites depends their nanoparticles size are in nano scale for many reasons. Anand et al. [159] have been synthesized Graphene/zinc oxide (ZnO) nanocomposite by in situ reduction of zinc acetate and graphene oxide (GO) during refluxing, structural, morphological and elemental analysis, the synthesized samples were characterized by X-ray diffraction, field emission scanning electron microscopy, X-ray analysis (EDX) and Fourier transform infrared spectroscopy. Several metal oxide nanoparticles and their composites have been widely used for the wastewater treatment by adsorptive removal and the photocatalytic degradation [160].
In the field of energy harvesting applications, nanogenerators (NG) have resulted a revolution [161]. The first NG was introduced by Wang and his group in 2006, which successfully used AFM tip to harvest mechanical energy by deflecting mechanism of ZnO nanowires in contact mode [162].
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