Introduction to Advanced Nanomaterials

CHAPTER

INTRODUCTION TO ADVANCED NANOMATERIALS

1

Subramanian Arulmani1, Sambandam Anandan1, Muthupandian Ashokkumar2 National Institute of Technology, Trichy, India1; University of Melbourne, Parkville, VIC, Australia2

1.1 INTRODUCTION Atomic level interaction studies of matter give an extensive scope to all areas of science, i.e., chemistry, physics, biotechnology, engineering, and medicine. With a dream on promising tools for recent modernization, materials are nothing but matter that can be prepared by altering either their form or shape. This modification may provide various changes in not only their physical but also chemical properties [1]. This leads to diverse applications to promote and/or protect our environment. Recently, worldwide concern about energy-related climate change has been tied with the spiraling rate of fossil fuels. More than ever, interests in renewable energy are raised due to energy demands. In the meantime, it is essential to protect the environment from pollution as well. These problems have been encountered due to industrialization with urbanization from the past decades. Generally, materials can be classified into two main categories: natural materials such as organic matter, mineral matter, and living matter and artificial materials that could be manufactured by synthesis under known conditions. Both have chemical composition and structure that gives them particular properties or functions about. Especially, artificial materials formed in a particular environment and selected for specific properties are related to the precise field of application. Most progress occurred through the size effect which is in the form of bulk to nanoscale. Widespread approaches are broadly available to fabricate multidimensional materials with diverse properties to enhance interface performance [2]. According to their morphology, nanostructured materials can be generally classified into zerodimensional (0-D) (nanoparticles, nanospheres, etc.), one-dimensional (1-D) (nanorods, nanowires etc.), two-dimensional (2-D) (nanoplates, nanosheets etc.), and three-dimensional (3-D) (flowers, hierarchical solids). They are synthesized by various pathways, viz., chemical, physical, etc. [3]. Advancements in materials afford to underpin and facilitate revolutionary capabilities in many different areas such as energy (conversion and storage), pollution (control and alleviation), healthcare, etc. Novel materials, properties, and applications can facilitate development of competitive and economic benefits. Industrial scale production of advanced nanostructured materials such as carbonized allotropes (carbon nanotube [CNT], graphene), metals (nano/alloyed noble/non-noble), metal oxides, chalcogenides, conducting polymer dispersions, or micronized drugs has been established and is being commercialized. Innovation in technologies by utilizing Nanomaterials for Green Energy. https://doi.org/10.1016/B978-0-12-813731-4.00001-1 Copyright © 2018 Elsevier Inc. All rights reserved.

1

2

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

advanced materials will make a direct and better impact on economic development, the environment, and excellence of life, through enhanced processes and commodities. Fig. 1.1 depicts the strategy of advanced materials that may be used for key challenging areas. Our overview in this chapter can guide important progress in developing advanced nanostructured materials with various properties for energy-related utilization. “Advanced materials” have been defined in numerous ways, and these diverse emphases are on the basis of the availability and usability of materials. The most straight forward explanation for advanced materials is that these are associated with progressive technologies with the perspective to derive direct/indirect benefits in the form of highly specialized outcomes for multidisciplinary areas. The US National Institute for Standards and Technology describes these as: “The materials with unique functionalities have been identified and developed as large quantities enough for innovators and for manufacturers to investigate and authenticate in order to increase new products for consumers” [4]. Another definition about advanced materials comes from the Technology Strategy Board that these are designed for some targeted applications with better properties. They are not only new materials but also alloys or composites of graphene or high-temperature superconductors with new or superior structural (hardness, strength, flexibility) and/or functional properties (electric, electronic, magnetic, optical) [5,6]. In addition, a new type of materials that possess elevated mechanical strength, huge hardness, and more thermal, improved electrical, better optical, and superior chemical properties than the others are also called advanced materials. They are synthesized from derivatives of conventional materials

FIGURE 1.1 Strategy of advanced materials for key challenge areas.

1.1 INTRODUCTION

3

FIGURE 1.2 General characteristics of advanced materials.

with the following characteristics (Fig. 1.2): specially produced for precise usage, a great deal of process to get a high value-to-weight ratio, easy to combine with other materials to form composites. Precisely, they are designed, developed, and used in “High-Tec” applications with maximum performance. The importance of advanced materials is mainly derived from their basic properties. They are mainly employed as controlling factors for pioneering processes and play a significant responsibility in specific applications. So, the required properties of a material should be decided before designing and developing new inovative materials. For example, mostly alloys with high strength are used for stronger, lighter, and safer vehicles. The material that possesses improved properties may blinker in the modern technologies as a better performing tool in energy storage devices (batteries/supercapacitors), electronic inks (multiprinting), high-voltage transmission lines, and healthcare-associated applications. It should be noted that advanced materials should possess the features that are briefly mentioned in Fig. 1.3. They are mainly divided into four categories: chemical, physical, mechanical, and dimensional. Each category has its own subdivisions according to its important properties. Furthermore, the structure of advanced materials is classified into two main classes: (1) atomic level and (2) microscopic level. At the atomic level, atoms are arranged in different ways and show diverse activities compared with those at the bulk molecular level. For example, the property of

4

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

FIGURE 1.3 General features in advanced materials selection.

graphite differs from that of graphene. At the microscopic level, the granular arrangement of materials as small grains can be identified through microscopy. This may give evidence about different gradients and opacity levels (optical property; e.g., transparent vs. frosted glass).

1.2 HISTORY AND CLASSIFICATION OF ADVANCED MATERIALS Despite the great progress made as an outcome of the extreme focus on designing of materials, it is necessary to improve the strategy before utilizing them for various practical applications. A variety of materials obtained from rocks, minerals, and organic or biological sources due to natural evolution. Depending on growth or formation, these vary as solids or liquids or an intermediate of both. The physical and structural characteristics of the material, or a combination of both, always make a new type with peculiar properties. As per the literature, the materials are classified into various types by the priority of their usage. They are simply classified as metals, ceramics, polymers, and a mixture of these elementary types called composites. Besides this classification, other groups are also available such as semiconductors and biomaterials. Again, these are classified into metals, polymers, elastomers, ceramics, glasses, and hybrid composite materials. According to some engineering aspects, materials can be differentiated as metals and nonmetals. Nonmetals are further classified into inorganic and organic materials. Metals are additionally subdivided as ferrous and nonferrous metals. Furthermore, from a technological point of view, the materials can be classified as macro, micro, and nano. Apart from this, most kinds of advanced materials are in the span of nanomaterials due to their special properties. The role of dimensionality is the determining factor for the material’s property based on the movements of electrons/positive holes in their structures. The systematic study of the dimension of a material has an extensive history available in chemistry and physics. For fundamental

1.2 HISTORY AND CLASSIFICATION OF ADVANCED MATERIALS

5

scientific concerns with better implementation, it is necessary to find out the answer for the arrangement of atoms or another building block of elements into their prescribed structures. The specified size of lengths and diameters as well the intrinsic properties of those materials should be considered [7]. In the end of 19th century, Gleiter [2] gave an idea about the classification of nanostructured materials, which was further explained by Skorokhod et al. in 2001 [8]. However, their classifications failed to explain 0-D, 1-D, 2-D, and 3-D structures. To overcome this issue, a modified classification was stated by Pokropivny and Skorokhod with the inclusion of 0-D, 1-D, 2-D, and 3-D structured materials, which are fullerenes, nanotubes, nanoflowers, etc. [9]. In accordance with their shape/morphology, classifications are broadly varied as 0-D, 1-D, 2-D, and 3-D materials (Fig. 1.4). As per our previous statement, the essential properties such as the physical, chemical, and biological properties of a material at the nanoscale will be different in both fundamental and implementation process than the properties of individual atoms or molecules or the bulk of the same [10]. The movement of electrons, holes, excitons, phonons, and plasmons are restricted mostly regarding their physical shape that may alter the properties of familiar advanced nanomaterials. The observed changes in energy levels due to the electron confinement leads to changes in color and shape of the material [11]. Mostly, inorganic metal-based nanoparticles are obtained through the bottomeup synthetic route predominantly in the solution state. These always have tunable novel properties due to the possibility of significantly varying their dimensional shape and size. Some of the typical morphologies of solid and mesoporous/hollow inorganic nanoparticles with 0-D, 1-D, and 2-D shapes and other 3-D complex structures are displayed in Fig. 1.5 [12].

FIGURE 1.4 Diverse nanostructured materials and their directions with dimensions. 0-D, zero-dimensional; 1-D, onedimensional; 2-D, two-dimensional; 3-D, three-dimensional.

6

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

FIGURE 1.5 Typical morphologies of solid and mesoporous/hollow inorganic nanoparticles with 0-D, 1-D, and 2-D shapes and other 3-D complex structures [12]. 0-D, zero-dimensional; 1-D, one-dimensional; 2-D, two-dimensional; 3-D, three-dimensional.

1.2.1 ZERO-DIMENSIONAL MATERIALS These are the materials of sizes in the nanoscale, whose electrons are confined in all the three directions. Due to the overall dimensional confinement, the properties of 0-D materials are more or less identical. From the past decades, the field of 0-D materials has been developed and considerable work performed toward their applications. Pure nanoparticle to uniformly sized particles which are in arrays (quantum dots), heterogeneous particles as arrays, basic to coreeshell quantum dots, onions, hollow spheres, and nano lenses are the examples for 0-D materials. In 0-D materials, for example, quantum dots, electrons are not able to escape from their regions because of confinement in all directions and therefore exist only inside an “infinitely deep potential well”; hence, there is no possibility for delocalization of electrons. The length and width of these 0-D materials are predominantly similar. Amorphous or crystalline nature of 0-D materials has been observed. Generally, quantum dots are nothing but semiconductor nanoparticles. Upon promotion of an electron from the valence to conduction bands, in the bulk lattice pattern, an electronehole pair (exciton) should be produced, as per the quantum confinement theory. Due to this phenomenon, the exciton Bohr radius (rb), which is the typical physical separation formed between the electron and hole, always varies not only in semiconductors but also in nanomaterials due to the compositions. For 0-D materials, the diameter (L) of the nanocrystal is in a similar order of extent with the rb that can give the more confined state. The exciton quantum confinement effect is in discrete energy levels for the particularly small dimension of 0-D materials. At this stage, a significant change in the

1.2 HISTORY AND CLASSIFICATION OF ADVANCED MATERIALS

7

bandgap and dimensions of a nanocrystal can vary with the bringing into/out of the single atom. The discrete energy (En) for different nanomaterials can be given by the equation:  2 2   p h 2 2 2 þ n þ n For 0
D: En ¼ n x y z 2mL2 where h is the Planck’s constant, J.s; m is the mass of an electron, amu; L is the orbital perimeter, nm; n is the dimensional coordinates, dimensionless unit. As predicted by the above equation, it is easy to adjust the bandgap values for all general semiconductor crystals by simply altering the diameter of the quantum dot with controlling the size compared to rb with dimensions [13]. The electronic properties of the quantum dots are very important because of a decrease in size below 10 nm, and the electronic transition energy of the same material can increase up to 2 eV relative to the bulk material [14]. Fluorescent carbon quantum dots (CQDs) were first discovered accidentally during the purification of the single-walled CNTs from the arc-discharge soot [15]. Recently, CQDs have been a very important part of 0-D nanomaterial, which is one kind of carbon material with unique optical, electronic, and physicochemical properties. Due to the above important properties, they are active as advanced material in a variety of fields such as biological sensing, imaging, drug delivery, optoelectronics, photocatalysis, and voltaic. Easy tunability of photoluminescence, exceptional photo-induced electron transfer, and being highly capable of light harvesting makes a significant attention in the field of solar energy conversion [16].

1.2.2 ONE-DIMENSIONAL MATERIALS For decades, the system of 1-D structured materials (thin film or manufactured surfaces) has been used, and their two dimensions are in nanoscale and one other dimension that is outside of the nanoscale. The electron confinement is in only two dimensions, so the movement of electrons is restricted. Like 0-D materials, they can be crystalline (single or poly), metallic, ceramic, and polymers. Nanowires, nanotubes, nanoribbons, nanoscrolls, nanobelts, nanofibers, nanorods, and nanofilaments are the best examples of 1-D structured materials. Mostly these materials are in the purest form, but in some cases, they are impure (from doping in semiconductors). They are available as individual materials or implanted with other materials also. The most important factor of 1-D structured materials is that they have greater length than width [13]. The discrete energy (En) for 1-D nanostructured materials using the quantum confinement theory is given by:  2 2   p h For 1
D: En ¼ n2x þ n2y 2 2mL Everyone knows that the 1-D structured material is a versatile candidate for novel systems due to easy functionalization, size, and dimensionality. For energy-associated applications, these offer a variety of benefits such as easy electrical transport with direct current pathways, shorter ion diffusion length, and volume expansion when compared with nanoparticles [17]. Recently, porous 1-D nanostructured materials have been developed with more advantages using normal 1-D nanostructured materials together with organized porosity at the level of the nanoscale. This kind of porous materials opens pathways for multiapplications in a variety of sectors, for example,

8

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

conversion and storage of energy, gas sensing and storage, adsorption, catalysis, biosensing, and other areas. The combination effect of both 1-D architecture and porous properties of a material may flicker its performance towards better energy storage. Porous 1-D nanostructure, hollow 1-D geometry (also named a tubular nanostructure), hierarchical porous 1-D architecture, nanoparticles embedded in porous 1-D configuration, and porous 1-D nanoarray are available nowadays for effective utilization. Porous materials may provide high surface area with the shortest ion diffusion length, acting as a host for fillers than nonporous 1-D structures [18].

1.2.3 TWO-DIMENSIONAL MATERIALS The electron confinement is only in one dimension for 2-D nanostructured materials. The size of the material lies in nanoscale at one dimension, which is an indication for surplus movement of electrons surrounded by their dimension. They always exhibit platelike shapes, including nanosheets, nanolayers, nanofilms, and nanocoatings. Like 1-D materials, 2-D materials are in crystalline or amorphous forms and also comprise metallic, ceramic, or polymeric materials. A variety of chemical compositions have been used especially for making single or multilayer sheetlike structures. Furthermore, they are also deposited on substrates like metals and ceramics and can be incorporated into other surrounding materials like carbon. Like 1-D materials, the length is more than the width, and the electrons are also in confinement with the delocalization state [13]. As per the quantum confinement theory, the discrete energy (En) for 2-D nanostructured materials can be illustrated by the equation:  2 2
p h For 2
D: En ¼ n2x 2 2mL In earlier research, 2-D nanostructured materials did not receive a lot of consideration until the discovery of graphene. The separation of graphene from graphite in 2004 is an important research toward the focus on 2-D nanostructured materials. In 2-D materials, graphene and graphene-based materials are making more evidence for fundamental research towards its unique properties and testing the practical utility for electronic and energy storage devices [19]. Unlike graphite, the ultrathin material of graphene was the focus of many research fields due to its tremendous favorable properties. In the same way, thin sheets of 2-D materials offer an extensive assortment of essential building blocks for next-generation electronic devices. Boron nitride (h-BN), black phosphors, and transition-metal dichalcogenides (TMDCs) are the newly emerging fields in 2-D nanostructured materials beyond that of graphene. The presence of vertical confinement of electrons and holes may provide an exotic physics phenomena only in the limit of monolayer [20]. The existence of an atomically thin-layered structure, it is easy to get better geometric dimension and formation of monolayered modern electronic materials with low power consumptions, lightweight, and flexible advantages [21]. For example, graphene-based materials are performing well as an outstanding conductor [22], because of the wide bandgap property, h-BN is used in gate dielectric or deep ultraviolet emitters [23] and TMDC-based semiconductors are potential candidates for numerous superior quantum efficiency optical/optoelectronic applications with the elevated oneoff ratio [24]. Scientists have developed new groups of 2-D materials from elements across the periodic table that have high conductivity, flexibility, improved strength, and the easiest chemical tunability for

1.2 HISTORY AND CLASSIFICATION OF ADVANCED MATERIALS

9

energy-associated applications, electromagnetic shielding, and environmental remediation [25]. They are classified as follows: •

•

•

•

•

MXenes (e.g., Ti3C2, Ta4C3) (Approximately 30 MXenes were developed which are based on Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, and Mo) [26]. Xenes (e.g., borophene) The arrangement of boron atoms in sheets with honeycomb pattern is called borophene which is a metallike conductor. Boron easily forms polymorphs due to its electron-deficient nature [27]. TMDCs (e.g., MoS2, WS2, ReS2) The transition metal atom is sandwiched between two chalcogenide atoms. In-between the TMDC layer, a weak van der Waals force acts that holds each TMDC layer mutually. In a single layer of TMDC, due to the covalent linkages, several stacks of polytypes and polymorphs occur. 1T, 2H, and 3R are the familiar structural polytypes of TMDC that denote to one tetragonal (1T), two hexagonal (2H), and three rhombohedral (3R) symmetries [28]. Nitrides (e.g., GaN, BN, Ca2N) The arrangement is of alternative boron and nitrogen atoms in a honeycomb pattern with a layered structure that retains its large bandgap and dielectric properties. Mostly it acts as an insulator [29]. Organic materials (e.g., covalent organic frameworks, 2-D polymers) The arrangement is that of some crystalline organic compounds as stacked molecular sheets [30,31].

1.2.4 THREE-DIMENSIONAL MATERIALS Another name of 3-D nanostructured materials is bulk nanomaterials. For these kind of materials, the nature of the particles is free. Therefore, there is no quantization of the particles during its motion. Porosity plays a vital role in 3-D nanostructured materials. In this case, nanomaterials are formed as building blocks, which are easily brought together to form hierarchical 3-D nanoarchitectures. It is described by using the term of nanocrystalline structure, and 3-D nanostructured materials are composed of numerous nanosized crystals which are in multiple arrangements with different orientations. Furthermore, 3-D nanomaterials can contain dispersions of nanoparticles, bundles of nanowires, and nanotubes as well as nanosized multilayers. At present, there are plentiful research in progress toward porous 3-D nanomaterials and 3-D porous interconnected graphene-based materials for future energy applications due to nonagglomeration with high specific surface area, physically powerful mechanical strengths, and quick mass and electron-transfer kinetics as a result of the 3-D porous arrangement and outstanding fundamental properties of graphene [32,33]. Like graphene, metaleorganic frameworks (MOFs) are porous coordination polymers consisting of metal ions or clusters that are connected by organic linkers having a 3-D porous interconnected structure with uniform cavities with long-range order. Mostly, compounds such as carboxylates, phosphonates, sulfonates, and heterocyclic rings act as the organic linker that bridges the inorganic secondary building units. By varying these two materials, it is possible to tune the size and shape and also the functional properties of the newly forming MOFs. Even though porous materials such as zeolites and carbon are present, the MOF has several distinct properties. Furthermore, the encapsulation of molecules into the MOF is different from these materials because it

10

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

FIGURE 1.6 (AeL) Different heterogeneous nanostructured materials based on structural complexity [36]. 0-D, zerodimensional; 1-D, one-dimensional; 2-D, two-dimensional; 3-D, three-dimensional.

is in a well-defined hosteguest manner. Therefore, when the MOFs are in use, they may act as a very good host to welcome the guest molecules in a well-organized arrangement all over a lattice of MOFs [34,35]. Likewise, to design and develop heterogeneous nanostructures, it is important to choose a perfect source material for getting appropriate nanostructure of particular importance. A comparative statement based on the structural complexity of various heterogeneous nanomaterials is classified into four major categories (Fig. 1.6). The combinations of multinanocomponents are available in the heterogeneous nanostructured materials, each of them tailored to address different needs (e.g., high energy density, high conductivity, excellent mechanical stability, etc.). In consequence, the ensuing composite materials will exhibit synergic properties, provide lots of benefits, and act as a promising material [36].

1.3 SYNTHETIC METHODOLOGIES Numerous synthetic approaches are available for the synthesis of advanced nanostructured materials. A number of factors such as size-controlled synthesis and shape-controlled synthesis are more

1.3 SYNTHETIC METHODOLOGIES

11

FIGURE 1.7 Techniques to convert particles to nanoscale.

significant toward the synthesis of nanomaterials. From this fact, the synthetic technologies are mainly classified into two types as follows (Fig. 1.7): • •

Topedown approach Bottomeup approach

•

Topedown approach It is the process of slicing or consecutive cutting of a bulk matter with the intention to get smallsized nanodimensional particles. Crushing, milling, or grinding are some of the methods in this approach. Bottomeup approach It is the process of stacking of a material from its bottom stage, atom by atom, molecule by molecule, or cluster by cluster.

•

Apart from these approaches, up to now, various synthetical methods are available for nanostructured advanced materials. There are especially three types that are more important because of the importance of the processed materials. They are classified as physical, chemical, and biological

12

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

FIGURE 1.8 Schematic illustration of various synthetic approaches.

methods. Controlled size, shape, perfect dimensionality, and prescribed structure are the main criteria for these kinds of new synthetical routes (Fig. 1.8).

1.3.1 CHEMICAL METHODS Chemical methods play the most important responsibility in designing and development of the new advanced materials at their nanoscale range with scientifically and technically essential properties. Adaptability of this method provides feasible functionalities in its final products and also offers a better homogeneity of chemical components through mixing. Even though it has a particular advantage, there are some limitations such as the continuous usage of toxic reagents and solvents during synthesis and the unavoidable introduction of a byproduct or its derivatives that enable consequent processes such as purification etc. that may be time consuming. On the other hand, the chemical method has blossomed recently due to features such as being simple, cheap, and easy fabrications of nanomaterials. Materials of diverse sizes and shapes are possible when using this technique by altering chemicals and conditions. For example, synthesis is feasible at any temperature (low to high) and so is the doping of foreign material (ion) during synthesis. Likewise, a lot of materials can be obtained by following this method. In view of instrumentation, the chemical method is far better than the physical methods, being relatively very simple. Self-assembly or the patterning of materials is also viable in this method. Mostly, nanomaterials which are synthesized through chemical methods is in the form of “colloids,” which consists of two or more phases such as solid, liquid, or gas. They may form particles, plates, or fibers. Various chemical synthesis approaches are available, and some of them described here in detail.

1.3 SYNTHETIC METHODOLOGIES

13

1.3.1.1 SoleGel Method It is a well-known colloidal chemistry technique. Generally, sols are solid particles in a liquid, and gels are polymers which contain liquid (nothing but a continuous arrangement of particles through pores filled with liquid). Construction of solegel generally proceeds at a low temperature that shows less energy consumption and less pollution. This process always starts from metal alkoxide (MeOeM) or metallic inorganic compounds (MeHeM), which are as a solution that easily reacts to produce particles in a colloidal state. The availability of precursor in a solvent further brings into hydrolysis and polycondensation reactions forms xerogel, which consists of the invariable inorganic lattice, for example, MeOeM or MeHeM. Concisely, the solegel process is used to construct “an oxide matrix during polycondensation reactions of a subatomic pattern in a liquid” [37,38]. Solegel synthesis generally engages with hydrolysis of precursors, condensation followed by polycondensation to form particles, and gelation with drying practices by diverse routes. The intention of controlling better size distribution and organizing stability of quantumconfined semiconductors and metal and metal oxide nanoparticles can be easily achieved by the solegel process. It offers foundation for a variety of material synthesis, including paints, ceramics, cosmetics, detergents, and cells with quite a lot of advantages such as providing ultrasmall particle size, very high specific surface area, extended triple phase boundary, management of composition at the molecular scale, and homogeneity and in being inexpensive, and involving effortless preparation. The solegel method provides a versatile approach toward the synthesis of highly crystalline birnessite (d-MnO2, layered manganese oxide minerals). In the manganese oxide family, mostly layered birnessite are getting more attention due to their usage as cathode materials for lithium-ion batteries (LIBs), as supercapacitors, as water oxidation catalysts, and act as an outstanding pioneer material to synthesize manganese oxide-based materials such as LiMn2O4 (spinel and hollandite) at low temperatures. Generally, highly crystalline birnessites are prepared either under hydrothermal conditions (for an extended period of time which ranges from days to months) or by using high posttreatment temperatures (400e500 C). In this method, highly crystalline birnessites were formed within 1 h without the need for any postprocessing to improve crystallinity. Still, there is no synthetic procedures reported without further hydrothermal posttreatment for well crystalline monoclinic birnessite as platelets. The perfect crystallinity is attained in the presence of Liþ, Naþ, and Kþ and the crystal size also be adjusted by varying the time of synthesis [39]. Likewise, thornlike ZnO nanoparticles (ZnO-NPs) were synthesized to assess their antimicrobial activities through solegel synthesis, and the effect of different stirring conditions (viz. 500, 1000, 1500, and 2000 rpm) on the size, morphology, and thermal stability was evaluated. The results revealed that the rotation speed produces a reliable impact on the aspect ratio of ZnO-NPs, and the anisotropic growth is encouraged by means of stirring which is possible through the induction created by the internal shear force. The average aspect ratios (L/D; length by diameter) were w8.6, w9, w13, and w18 nm, at 500, 1000, 1500, and 2000 rpm, respectively [40].

1.3.1.2 Microemulsion Techniques Formation of “emulsion” is nothing but the stirring or mechanical agitation of two immiscible liquids, which are in the range of up to even few millimeters from 100 nm, and is generally turbid in

14

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

appearance. Alternatively, there is an additional category of immiscible liquids called microemulsions of transparent nature, which are in the range of approximately 1e100 nm. Generally, these are optically clear fluids that are thermodynamically stable. The size/shapes of the microemulsions are stabilized by surface stabilizing active agents called surfactants. Due to their unique physical properties, microemulsions are used in various applications. This microemulsion technique has been utilized due to the formation of cavities which leads to better synthesis of nanomaterials with superior advantages such as biocompatibility (novel applications such as drug delivery) and biodegradability (to avoid environmental pollution). Morphologically different MgO nanomaterials were prepared by microemulsion-based oil/water interface precipitation by simply varying of calcination temperature. In brief, the synthesis was carried out in a paraffin-in-water microemulsion system. At first, Mg5(CO3)4(OH)2$4H2O/paraffin composite was prepared, which is the precursor for MgO, through an interface-controlled homogenous precipitation. Then the as-prepared composite was converted into MgO nanomaterials by calcination at different temperatures. At a lower calcination temperature (823 K), a flowerlike 3-D hierarchical structure of MgO was obtained which retains the morphology of the precursor (Mg5(CO3)4(OH)2$4H2O/paraffin composite). When the temperature is increased, well-defined MgO nanoparticles with diameters of 25e70 nm were obtained. The proposed formation mechanism for Mg5(CO3)4(OH)2$4H2O/paraffin composite precursor and MgO nanomaterials is shown in Fig. 1.9 [41].

1.3.1.3 Hydro-/Solvothermal Method Hydro- and solvothermal synthesis have been the most popular and gathering more interest from both scientists and technologists of diverse areas. Hydro- and solvothermal techniques are both different with only a small diversity in the usage of the solvent. The word hydrothermal is selfexplanatory, “hydro” means water and “thermal” means heat. Likewise in the word solvothermal, “solvo” means solvents. Conceptually, hydrothermal synthesis involves water as a catalyst in a closed stainless steel container at an elevated temperature of above 100 C and pressure that is greater than few atmospheres. The vessel which is generally used for this type is called an “autoclave” that provides a high temperature and pressure. Larger crystals with better quality of nanostructured materials may be attained by the hydrothermal technique. It is well known that materials with high vapor pressure close to their melting point or crystalline phases are not stable at their melting point. For these kinds of materials, the hydrothermal process provides a better synthetical approach. A variety of oxides-, sulfides-, carbonates-, and tungstates-based nanomaterials have been synthesized with uniform shape and size by altering the experimental parameters including reaction time, temperature, type of solvent, surfactant type, and precursors in hydrothermal synthesis. When comparing with other advanced methods, the hydrothermal method needs a lower cost of instrumentation, energy, and precursors. Furthermore, it offers environmental benign synthesis when compared with other chemical methods and does not need any seed or catalyst that is more harmful or any expensive surfactant/templates. It gives a promise for large-scale synthesis and high-quality crystals at low cost. Likewise, solvothermal processes also have more consideration in synthesis by utilizing a range of solvents or mixed solvents. This also engages “in situ” type of reactions such as (1) oxidationereduction, (2) hydrolysis, (3) thermolysis, (4) complex formation, and (5) metathesis reactions (double decompositions). All developments in the abovesaid reactions through the

1.3 SYNTHETIC METHODOLOGIES

15

FIGURE 1.9 The proposed formation mechanism for Mg5(CO3)4(OH)2$4H2O/paraffin composite precursor and MgO nanomaterials [41].

solvothermal process are carried out by considering the nonaqueous solvents, for getting more information about the physicochemical properties of the materials [42]. A simple one-pot hydrothermal co-reduction route was applied for synthesizing two kinds of AuePd bimetallic nanostructures (alloy and coreeshell). Here, cetyltrimethyl ammonium bromide (CTAB) plays a key role in the formation of both alloy and coreeshell. The reducing ability of poly(vinyl pyrrolidone) (PVP) molecular is rather poor, which makes the reduction of Pd (II) more difficult when compared with that of Au (IV). Therefore at first, HAuCl4 is reduced by PVP, and subsequently, H2PdCl4 is reduced. As a result, obviously, the coreeshell bimetallic nanostructures are formed. On the other hand, when CTAB is introduced into the reaction system, the reaction process gets changed. Compared with PVP, the reducing capacity of CTAB is significantly stronger in hydrothermal conditions, so that Pd(II) and Au(IV) can be reduced simultaneously. Therefore, the Au0 and Pd0 are combined together and alloy nanostructures are formed. The possible reducing mechanisms of PVP and CTAB are displayed in Fig. 1.10 [43].

16

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

FIGURE 1.10 Formation process of AuePd coreeshell and alloy bimetallic nanostructures and the reducing Mechanisms of (A) PVP and (B) CTAB in Hydrothermal Conditions [43]. CTAB, cetyltrimethyl ammonium bromide; PVP, poly(vinyl pyrrolidone).

1.3.1.4 Polyol Synthesis Polyol is a kind of liquid phase synthesis that makes use of multivalent alcohols at high temperatures up to its boiling level. Ethylene glycol (EG) is the simplest representative in polyols. From the basis of EG, it is established as diethylene glycol (DEG), triethylene glycol, tetraethylene glycol, and it continues up to polyethylene glycol (PEG). Furthermore, it contains more than 2000 ethylene groups with a molecular weight of roughly up to 100,000 g/mol. These are mostly used for the synthesis of nanostructured materials. It should be noted that the boiling point of polyols plays the best role in these reactions. When the number of eOH functionalities increases, the boiling point of the polyol also increases with increase in its molecular weight. Likewise, the polarity, and viscosity, also increases with increasing molecular weight. The solubility of polyol compounds is comparable to water that provides an easy way of usage, in addition to low-cost metal salts such as nitrates, sulfates, and halides as precursors. They may be considered water equivalent but as high-boiling solvents. Additionally, polyol has the property of chelating effect which is highly advantageous in controlling the nucleation process of a particle, particle growth, and agglomeration that may arise during the synthesis. High viscosity is observed in polyols as comparable to water, which also provides better benefits. Besides, the property of excellent colloidal stabilization is

1.3 SYNTHETIC METHODOLOGIES

17

available in polyols with very high boiling points, and they are allowed for synthesis at a temperature range of 200e320 C without the help of high pressure or autoclaves. During material synthesis, an instant reduction reaction is possible in metal cations toward nanoparticles with simultaneous adequate surface functionalization in addition to stabilization due to excess amount of polyol solvent. From all the above hypotheses, polyol synthesis is a combination of numerous characteristics and can be believed to be a one-pot reaction [44]. The ultrathin diameter of 2-nm-sized palladium wavy nanowires is synthesized by using the polyol method without any template. In a typical synthesis, PVP and palladium(II) trifluoroacetate (CF3COO)2Pd were dissolved separately in DEG (solvent and reductant). The solution containing (CF3COO)2Pd was quickly injected into the PVP solution at 140 C under magnetic stirring. The reaction was terminated after 3 h, and Pd nanowires of wavy morphology were obtained. The reaction kinetics (as mediated by a precursor), the nucleation, growth, attachment, and the final morphology of Pd nanostructures in the polyol synthesis are illustrated in Fig. 1.11 [45].

1.3.1.5 Chemical Vapor Deposition Chemical vapor deposition (CVD) is a chemical method which is used to fabricate highly pure and advanced performance materials. It is a hybrid process for the coating of numerous inorganic and organic materials by using vapor phase of chemicals. Thermal decomposition or other chemical reactions of gas phase species takes place at an elevated temperature that ranges from 500 to 1000 C in the CVD process. At a fixed high temperature, generally, the reactants crack themselves to form dissimilar products which disperse on the surface of the substrate that undergoes various chemical

FIGURE 1.11 A schematic illustrating how reaction kinetics (as mediated by a precursor) affects nucleation, growth, and attachment, and thus the final morphology of Pd nanostructures in polyol synthesis [45].

18

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

reactions at a suitable site, further nucleates and grows to form the required material as a thin film in the substrate [46]. There are five main steps involved in the CVD process and are as follows: (1) the mixture of reactant gases and diluent inert gases are positioned into the chamber and the flow rate adjusted; fixed by using mass flow controller. (2) The gas species are in motion to the surface site. (3) The adsorption of gas species takes place on the surface site. (4) Nanostructured materials are produced due to chemical interactions between the reactants and substrate. (5) The final products that are in gaseous form are separated by desorption and evacuated from the chamber. The materials that are deposited by the CVD process differ as of monocrystalline, polycrystalline, amorphous, and epitaxial. It is possible to produce materials with different elemental compositions using CVD, such as silicon, carbon fiber, filament, silica (SiO2), siliconegermanium (SieGe), tungsten (W), silicon carbide (SiC), titanium nitride (Ti3N4), various high-K dielectrics, and synthetic diamonds [47]. Mostly, the CVD method is used for the synthesis of aligned CNTs, with a solution of xyleneeferrocene used as the precursor. Here, xylene acts as the carbon source, at the same time as ferrocene acted the seed catalyst as the easiest supplier of the iron metal NPs [48]. Apart from the conventional CVD process, some of the other advanced CVD techniques are now available. Moderate temperature CVD and metal organic CVD (metal organic precursors) are processes that decompose materials at a moderately low temperature of approximately 500 C. Furthermore, it is also possible to deposit nanostructures at just above the room temperature by the use of plasma or a laser beam for the activation of vapor phase, and the procedures are identified as plasma-assisted (or plasma-enhanced) CVD and laser CVD, respectively [49]. Continuous, highly flexible, and transparent graphene films that are achieved through CVD are used as transparent conductive electrodes (TCEs) in organic photovoltaic (PV) cells. Graphene films are synthesized by CVD, and the same is transferred to transparent substrates further evaluated in organic solar cell heterojunctions (TCE/poly-3,4-ethylene dioxythiophene [PEDOT]:polystyrene sulfonate/copper phthalocyanine/fullerene/bathocuproine/aluminum). The CVD process provides a minimal surface roughness of around 0.9 nm and also offers sheet resistance down to 230 U/sq (at 72% transparency), which is greatly lower than that of stacked graphene flakes, even at a similar transparency. Fig. 1.12 demonstrates the schematic representation of the energy level alignment (Fig. 1.12A, top) and construction of the heterojunction organic solar cell fabricated with graphene as anodic electrode: CVD graphene/PEDOT/CuPc/C60/block copolymers/Al, CVD graphene transfer process onto transparent substrates. Photographs showing highly transparent graphene films transferred onto glass and polyethylene terephthalate (PET) with transmission spectra for CVD graphene, indium tin oxide (ITO), and single-walled carbonnanotube (SWNT) films on glass. Furthermore, the atomic force microscopy (AFM) images of the surface of CVD graphene, ITO, and SWNT films on glass and transmission spectra of CVD graphene with a different sheet resistance (Rsheet) [50].

1.3.1.6 Electrochemical Deposition Electrochemical deposition or electrodeposition is an age-old technique to deposit metal as layers on selected conducting substrates. For the deposition process, electrical current is used. In the electrochemical deposition, the available ions in the electrolyte or from the anode due to replenishment process are going to deposit at the negatively charged cathode which carrying some amount of charge is measured in terms of current in the external circuit. To get perfect grain size during the electrodeposition process within the range of nanometers, alterations in the variables such as bath composition, pH, temperature, and current density are needed. Diverse nanostructures such as nanorods, nanowires, nanotubes, nanosheets, dendritic nanostructures, and composite nanostructures are

(A)

(B)

(E) (C)

(D)

(F)

(G)

(H)

FIGURE 1.12 (A) Schematic representation of the energy level alignment (top) and construction of the heterojunction organic solar cell fabricated with graphene as the anodic electrode: chemical vapor deposition (CVD) graphene/poly-3,4-ethylene dioxythiophene (PEDOT)/CuPc/C60/block copolymers (BCP)/Al. (B) Schematic of the CVD graphene transfer process onto transparent substrates. Photographs showing highly transparent graphene films transferred onto glass and PET are shown in panels (C) and (D), respectively. (E) Transmission spectra for CVD graphene, indium tin oxide (ITO), and SWNT films on glass. (F) Atomic force microscopy (AFM) images of the surface of CVD graphene, ITO, and SWNT films on glass. The scale bar in the z-direction is 50 nm for all images. (G) Transmission spectra of CVD graphene with a different sheet resistance (Rsheet). (H) Comparison of Rsheet versus light transmittance at 550 nm for CVD graphene and reduced graphene oxide films reported in the literature [50]. PMMA, poly (methyl methacrylate).

20

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

fabricated easily by electrochemical synthesis. It is a highly efficient method that has advantages being an economical process, a less synthetic approach, with high purity and simplicity, and an eco-friendly approach [51]. Mostly, potentiostatic and galvanostatic (GV) techniques are engaged to perform electrodeposition under different potential ranges, time durations, and current densities. The electrochemical behaviors of the deposited materials on various conductive substrates were examined through cyclic voltammetric and chronoamperometric techniques. Similarly, electroless deposition is also known as a nongalvanic type of deposition process that is also a well-recognized technique, with low cost, and is nonhazardous. Electroless plating is possible without external power, and the reaction proceeds through released hydrogen that only acts as a reducing agent and is oxidized to produce a negative charge on the surface of the substrate. Novel MnO2-CNT sponge type supercapacitors have been fabricated using a simple method. During synthesis, the commercially available sponge ribbons are subsequently coated with CNTs using a simple “dipping and drying” process with CNT ink suspension. Furthermore, the electrodeposition of MnO2 nanoparticles on the CNT-coated sponge was done by means of GV electrochemical deposition with a current density of 500 mA/cm2, providing a high specific capacitance of 1230 F/g. The macroporous nature of the sponge along with the porous nature of the electrodeposited MnO2 nanoparticles make available a double porous electrode structure, which gives excellent conductivity and full accessibility of the electrolyte. Fig. 1.13 shows the schematic synthetic procedure and fabrication of MnO2-CNT sponge-type supercapacitor and its characterizations [52].

1.3.1.7 Sonochemical Method In some typical synthesis of nanostructured materials, ultrasound irradiation is used for the nucleation process, and the so-called approach is the sonochemical method of synthesis. The irradiation frequencies are in the range of around 15 kHz to 1 MHz. The creation, growth, and collapse of a bubble in the liquid are the important features of sonochemistry. In general, ultrasound is not audible when it is transmitted through air. But in the case of a liquid medium (e.g., water), the pressure formed due to ultrasonic oscillations may possibly act as the foundation for in-phase expansion and contraction of the dissolved gas bubbles. The expansion and collapse of gas or vapor bubbles in an acoustic field is called acoustic cavitation and is due to the interaction between sound waves and bubbles in aqueous solution. When there is the possibility of collapse of cavitation bubbles, this instantly generates extreme temperatures within the cavitation bubbles. This process in due course results in the development of extremely reactive radicals. For example, upon homolysis of water molecules, the high temperature created within the bubbles generates H and HO radicals (generally referred to as primary radicals). In the meantime, the available surface-active solutes are performs the reactions by the formation of secondary reducing radicals and such are being utilized to reduce metal ions. As a matter-of-fact, the different shapes and size distributions of gold nanoparticles (GNPs) were synthesized through the sonochemical process, with the aid of high-intensity focused ultrasound (HIFU) technique. The main reason to choose the HIFU technique is that in the sonicated medium, it provides strong mechanical/ shear forces and a number of radicals to facilitate size- and shape-controlled GNPs. An operating frequency of 463 kHz and 30, 50, or 70 W of applied power is used for the synthesis of GNPs. During the sonochemical reduction reaction, different colored solutions of GNPs are obtained, which is governed by experimental conditions such as the variously applied power (30, 50, or 70 W) (Fig. 1.14A and B). The formation and growth of GNPs to diverse sizes and shapes exist. The

1.3 SYNTHETIC METHODOLOGIES

21

FIGURE 1.13 Fabrication process of MnO2-CNT-sponge supercapacitors and characterizations: (A) an overall view of three-dimensional macroporous hierarchical MnO2-CNT-sponge electrode; (B) MnO2 uniformly deposited on the skeleton of CNT-sponge; (C) high magnification of porous MnO2 nanoparticles on CNT-sponge, inset shows morphology of an individual MnO2 flowerlike particle; (D) a transmission electron microscopy (TEM) image of MnO2 shows highly porous structure; (E) high-resolution TEM image, and the inset shows the selected-area electron diffraction pattern (SAED) pattern of porous MnO2, showing the polycrystalline nature of MnO2; (F) X-ray diffraction of the as-synthesized structure, showing the ε-MnO2 phase [52]. CNT, carbon nanotube.

22

(A) Color changes observed on high-intensity focused ultrasound sonication of solutions of gold chloride at f ¼ 463 kHz and P ¼ 30, 50, and 70 W and (B) the corresponding transmission electron microscopy images with various shape and sizes [53].

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

FIGURE 1.14

1.3 SYNTHETIC METHODOLOGIES

23

reduction of Au3þ ions is aided by the sonochemically generated secondary radicals through the following reactions [53]: H2O / H þ HO H/HO þ isopropanol / secondary reducing radicals Isopropanol / R and other secondary reducing radicals Secondary reducing radicals þ Au3þ / GNPs It is also proven that monometallic, bimetallic, and coreeshell of bimetallic synthesis using water as the synthetic medium gives hydrogen radical for reduction during irradiation. The reduction of both þ AuCl
4 ions to metallic gold and Ag ions to metallic silver was accomplished at room temperature by ultrasonic irradiation in the presence of alcohols. The presence of hydrogen atoms and alcohol radicals during irradiation can reduce gold and silver ions to generate goldesilver bimetallic nanoparticles [54,55].

1.3.2 BIOLOGICAL METHODS In living cells, numerous chemical processes occur naturally, which are not fully understood until now. The progress in atomic (ionic)/molecular interactions and diffusions are thermodynamically uncontrolled but kinetically determined. By this way, nature offers a finer pathway to synthesize novel and advanced materials. From the literature, it is to be accepted that most of our biological systems perform as a “bio-lab” for the efficient eco-friendly or so-called green synthesis of various pure metals, metal oxide particles, and composites with special nanostructures by means of the biomimetic approach. The biological approach has some special advantages more than that of other chemical methods, being a greener process, economic, and energy saving. So far numerous nanostructured metals, alloys, semiconductor materials, and insulator materials or their composites have been synthesized using various techniques through biosynthetic approaches. Studies on green synthesis toward newer materials have been conducted with a variety of biological materials together with bacteria, fungi, and plant extracts. Mostly, the phytochemicals that are available in biological systems are responsible for the formation of metal/metal-oxide nanostructured materials. Even though they take part in the synthesis, it is not easy to derive the mechanism of synthesis. Mostly, the literature suggests that nanometals are formed by phytochemicals, and metal oxide formation may also be done by the same procedure with the inclusion of oxygen either from the atmosphere or from the degrading phytochemical [56]. There are three major types of biological synthesis available so far. This classification is based on the type of biological materials or the organism being used: • • •

microorganism-assisted biogenesis, biotemplates-assisted biogenesis, plant extractseassisted biogenesis.

24

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

1.3.2.1 Microorganism-Assisted Biogenesis Organisms such as bacteria, fungi, or yeasts, which are observed only through a microscope, are called microorganisms. These are generally classified into two types on the basis of their usefulness. For example, for the process and development of bread, curd, cheese, vaccines, and alcohols, some of the bacteria are used, whereas some are more dangerous, in addition, that they are mainly responsible for spoiling foodstuffs or laying the foundation for so many diseases. During the synthesis, the microorganisms are interacting with metals through their cell walls and this kind of interactions is unable to understand the reason behind that complexity of cells. When using microorganisms, especially bacteria, the ambient temperature of the environment, pH of the reaction, and pressure are altered that leads to materials produced by this process becoming shape-/size-controlled and possessing higher catalytic activity with specific better surface area [57]. In the progress of microorganism-assisted biosynthesis, initially, the microorganisms are taken hold of the target ions from their surroundings and subsequently execute the metal ions into their corresponding element metal through enzymes which are produced by the activities of cells. This formation is classified by the location of the newly formed nanomaterial as intracellular synthesis (transfer ions into the microbial cell) and extracellular synthesis (catch the metal ions on the surface of the cells and reduce) in the presence of the corresponding enzymes [58]. In most cases, bacteria are used for the synthesis of nanomaterials such as gold, platinum, palladium, silver, cadmium sulfide, cadmium telluride, etc. To a certain extent, mycosynthesis provides an easy corridor for the synthesis of stable nanomaterials. Due to the superior bioaccumulation capability of fungi and downstream processing, these are the important strategies toward efficient culture design, in addition to economic fabrication of nanoparticles. Fungi have more tolerance capacity as well as binding potential with metal salts in the direction of the superior yield of nanomaterials compared to microorganisms [59]. Likewise, actinomycetes are also used for the synthesis of nanomaterials, but this method is not established even if it provides excellent monodispersity, superior stability, and considerable biocidal performances against a variety of pathogens [60]. Yeasts have also been extensively examined for the large-scale production of nanoparticles through extracellular processes, with basic downstream processing. Besides, it is possible to utilize viruses for the synthesis of nanowires with functional components that are brought together for diverse applications, for example, PV devices, electrodes for battery, and supercapacitors. Even though microorganisms have been used for effective synthesis, it is a time-consuming process provides less productivity, and is the main requirement of a downstream process for the recovery of nanoparticles. Moreover, complex steps involved in the synthesis, such as microbial sampling, isolation, culturing, and maintenance, are the major problems in microbial synthesis [59].

1.3.2.2 Biotemplates-Assisted Biogenesis Some of the biological systems that have the tendency of self-assembled hierarchical structures that are especially obtained in nature are called biotemplates that permit specific dimensions and arrangement of nanostructured materials for enormous fabrication. These biotemplates are extensively used not only to produce nanoparticles but also to obtain arrays, superlattices, or hierarchical structures of various inorganic materials. So far, a lot of biotemplates have been available, such as bacteria, textiles/paper, hair cells, insect wings, spider silk, wool, wood, onion, eggshell membranes, DNA, viruses, and diatoms. Even though, a large list of templates is available,

1.3 SYNTHETIC METHODOLOGIES

25

only a few of them show an economic feasibility that is widely used for technology. Likewise, DNA, S-layers (i.e., surface layers of cell walls of some bacteria), and some membranes posses orderly arrangement in nature due to some of their constituent groups. Due to this factor, a number of periodic active positions created for the anchoring of nanomaterials on the basis of the interactions occur between the templates and materials. When proteins are used as biotemplates, they offer numerous binding sites that easily attach to a metal ion and further reduce. By comparing other biotemplates, proteins provide outstanding scaffolds for the template-driven arrangement of nanostructured materials with special shape and size [61]. Up to now, numerous types of proteins have been in use as templates for the synthesis of various nanostructured materials. These materials are classified into two types regarding their structure/shape (i.e., spherical proteins [used for the synthesis of spherical particles, clusters, nanoplates, and microspheres] and filamentous proteins [used for the synthesis of nanowires or nanotubes]). Ferritin and bovine serum protein are the most-used spherical proteins, whereas collagen, silks, wool, elastins, actins, keratins, myosins, and flagellins are the widely used filamentous proteins for the synthesis of diverse structured nanomaterials [62]. Besides, it is easy to modify the biologically templated nanomaterial to alter their usual functions and properties to produce novel kinds of extremely competent electronic devices such as dye-sensitized solar cells (DSSCs) or LIBs, with lightweight and improved performance.

1.3.2.3 Plant ExtractseAssisted Biogenesis Recently, significant attention has been given to the plant-mediated approach for extracellular synthesis of nanostructured materials because of the nature of sample and effective alternative scaffolds. By this way, the utilization of plants varies as live plants, plant biomass, and biomolecules extracted from plants or its biomass. A novel research is now going on, which is named phytonanotechnology that provides a new possibility toward eco-friendly, simple, fast, stable, and economical synthesis of nanostructured materials with advantages such as using the universal solvent (water) as a medium for reduction, biocompatibility, and large scalability. For plant-mediated synthesis, the mechanism to be elucidated illustrates reduction with the formation of specific shapes/structures and the components that are involved. Moreover, it is only proposed because proteins, amino acids, organic acid, vitamins, and secondary metabolites such as flavonoids, alkaloids, polyphenols, terpenoids, heterocyclic compounds, and some of the other polysaccharides are responsible for metal reductions, and they perform the duty of capping and a stabilizing agent [63].

1.3.3 PHYSICAL METHODS The physical method gives an ecological and environmental pathway to construct surface clean nanostructured materials. It is a time and energy consuming method. To avoid this, nanomaterials may be synthesized at high temperatures. These processes are under the topedown approach, and the advantages of this method are it is solvent free and produces uniform monodispersed particles.

1.3.3.1 Mechanical Methods The mechanical method is one of the synthetic approaches under the physical method of synthesis and is used to synthesize nanostructured materials as powders. It is again subclassified into two types.

26

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

1.3.3.1.1 High-Energy Ball Milling For making nanoparticles in the form of powder, high-energy ball milling is one of the finest techniques. For this process, numerous types of mills are in use such as planetary, vibratory, rod, tumbler, etc. Depending on the need of nanomaterials, one or more containers are used to make large quantities of fine particles. The advisable mass ratio of balls to the material is typically 2:1 for this synthetic approach. Half-filled containers are advisable because if the quantity exceeds, the efficiency of the milling process will be reduced during collision. To get lesser grain size, it is better to use larger balls for milling. Generally, a rise in temperature from 100 to 1100 C during collision takes place and amorphous particle formation is also possible at lower temperatures. In some cases, liquids are also used at some stage in the process of milling. Mostly, the containers are rotated on their own axis with high speed and sometimes around their central axis also. This method of rotation is called “planetary ball mill.” To get fine and uniform-sized material, parameters such as the speed of rotary motion of the central axis and the container with the time span of milling should be controlled. It is possible to get material quantity of few milligrams to several kilograms by this method. Materials such as Co, Cr, W, NieTi, AleFe, and AgeFe are prepared as nanocrystalline materials by using the ball milling process. For example, large-scale synthesis of nitrogen-doped carbon nanoparticles was done through the high-energy ball milling technique, as a powerful green method. For synthesis, graphite is used as the source, and the milling process proceeds for 24 h at a rotation speed of 150 rpm. The milled samples are then heated in a tubular furnace up to 700 C at a rate of 25 C/min and then annealed for 3 h in the presence of N2e15% H2 gas mixture. The obtained nitrogen-doped carbon nanostructures show signs of high catalytic activity and excellent tolerance to methanol in comparison with other nonprecious metal catalysts in oxygen reduction reactions after a structural refinement through controlled thermal annealing. As the synthesized calcined material after ball milling shows high activity compared with the others and also with commercial Pt/C catalyst [64].

1.3.3.1.2 Melt Mixing The most commonly used mechanical process is melt mixing, which is one of the oldest methods used especially for the preparation of polymer composites with nanoparticles in the form of fillers. It involves the mechanical mixing of a polymer with modified nanofillers through extrusion or kneading. It is possible to attain the required material characteristics by this process and provides an eco-friendly approach for commercial and current industrial applications [65].

1.3.3.2 Inert Gas Technique Most commonly, vapor condensation is used for the synthesis of fine or amorphous alloys depending on the temperature of the substrate and the reaction conditions. But this technique is mainly used for the synthesis of high-quality pure nanostructured materials, especially for metals. Recently, ambient stable complex coreeshell and three-layer MneBi nanoparticles were synthesized through a singlestep inert-gas condensation method. A mixture of Ar and He gas was used for sputtering, in addition to a carrier gas. The coreeshell and three-layer structure were achieved by controlling the thermal environment of the nanoparticles. In this, there are two forms of particles: (1) a crystalline Bi core with an amorphous Mn-rich shell and (2) a crystalline Bi annular shell between two amorphous layers with high Mn concentration. These particles show significant magnetic hysteresis, possibly

1.3 SYNTHETIC METHODOLOGIES

27

arising from a change in the bond length between the Mn atoms introduced by the Bi atoms in the bonding environment of the Mn atoms [66].

1.3.3.2.1 Inert Gas Condensation In this process, an inert gas (usually argon or helium) is periodically admitted into a vacuum chamber, which consists of vapors of an inorganic material. For the vaporization process, an evaporation boat, a sputtering target, or a laser-ablation target is typically used. On boil-off, the atoms rapidly collide with the inert gas by losing their energy. Furthermore, the vapor cools quickly and supersaturates to form the desired nanoparticles collected on a finger with the help of liquid nitrogen which is mostly obtained in the range of 2e100 nm. Finally, the particles are scrapped and collected for further processing. The whole process proceeds under inert gas only. It is also possible to get the alloys using the same procedure by utilizing dual metal sources.

1.3.3.2.2 Inert Gas Expansion This process is generally called inert-gas or free-jet expansion in which the evaporated atoms are transferred through a high-pressure helium gas stream that pushes the atoms to a low-pressure chamber with supersonic velocities. Due to sudden cooling, adiabatic expansion occurs in the evaporated atoms, leading to the formation of clusters which are few nanometers in diameter. Even though this is a good technique, it has some limitations such as agglomeration, etc.

1.3.3.3 Pulse Vapor Deposition It is the process which is used to deposit thin layers of material onto the substrate surface usually in the range of few nanometers to several micrometers. A collection of procedures and technologies are collectively called physical vapor deposition (PVD). Mostly, it covers technologies such as • • • •

pulsed laser deposition (PLD), cathodic arc deposition from the cathode, electron beam PVD, sputtering depositiondthe deposition consists of bombing a material with ions.

A vacuum chamber is needed for all the PVD methods, and very high vacuum should be created in the chamber. For the propagation of the reaction, vacuum is necessary to prepare the free space in the chamber when the atoms are emitted from the target.

1.3.3.3.1 Pulsed Laser Deposition This method is otherwise called the laser ablation method, which uses high-energy laser pulses to evaporate the material from the target, which is a solid source. If the usage of laser is in the form of pulses, then it is called PLD, and sometimes it may be continuous. The laser ablation method offers a flexible move toward the construction of micro- and nanostructures of polymeric materials. In the case of PLD, laser pulses that have high power strike the surfaces of the target, which leads to the melting process and evaporation, and at last, ionization of the material occurs. Finally, the ablated materials are deposited onto the specified substrate. Generally, either excimer laser or Nd: YAG (neodymium-doped yttrium aluminum garnet; Nd:Y3Al5O12) laser is used for the ablation process, and this PLD process is

28

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

mostly used for the synthesis of various nanostructures including oxides, metallic systems, fullerenes, polymers, carbides, nitrides, etc. Recently, a successful demonstration has been provided about the hybrid system, which is the combination of PLD and magnetron sputtering (MS) for the deposition of high-quality thin films. Both the PLD and MS were used together in the same target, which lead to an enhancement in the deposition rate. A deposition of titanium dioxide and bismuth-based perovskite oxide Bi2FeCrO6(BFCO) thin films on Si(100) and LaAlO3 (LAO) (100) was achieved by a technique that demonstrates the performance of the PLD and MS combination. The results showed the improved quality of the deposited films, and increasing film uniformity and deposition rate when using the hybrid technique [67].

1.3.3.3.2 Cathodic Arc Deposition In this method, the material is evaporated from the target by using electric arc that generally acts as the cathode. The effect of electric arc proceeds toward the formation of materials, which is acknowledged as the “cathode spot.” This is the small spot where electric charge has been imposed and evaporation of particles takes place from the target. During the process, the arc is moved across the whole region of the target to keep away from burning or making a hole in it. During the progress of deposition, enormous power with highly moving particles is observed. Mostly this method is used for the deposition of metallic, ceramic, and composite materials with enormously hard layers (tools for cutting, drilling, etc.), which enhances their effective usage and for extending their service life.

1.3.3.3.3 Electron Beam Physical Vapor Deposition It is a vacuum-based PVD process for the effective synthesis of nanostructured materials. The common electron beam system consists of a vacuum unit and an electron beam source and target materials. A charged tungsten filament is usually used as the electron source and is heated through the passing current for the generation of the electron beam, as the generated electron beam is purposefully directed onto the target material by using magnets on both sides. Concisely, high kinetic energized electrons strike the target material, heating it at a spot and making it melt and sublimate, which is then used for further deposition. The advantage of this method is the high deposition rate, in addition to the possibility of depositing materials which range from conducting to insulating states.

1.3.3.3.4 Sputtering Deposition Sputtering is a vacuum-based PVD method, which involves the ejection of electrons from a target or by striking the target with high-speed ions. The principle of sputtering is the transfer of momentum by which the atoms of the target are ejected by ion bombardment. The atomic weight of the gas used for sputtering may be close to the atomic weight of the target material, which is the most significant criterion. For example, the synthesis of oxide materials through sputtering, where only oxygen should be used as the reactive gas. The sputtering deposition of materials takes place in the following steps: 1. Mostly Ar-like neutral gases are used to generate plasma that is in-between the two electrodes due to the collision of electrons with gaseous molecules. 2. While applying the potential among the two electrodes, the ions are accelerated toward the target material.

1.3 SYNTHETIC METHODOLOGIES

29

3. The ions that have suitable energy may strike the target species that leads to ejection of the material easily. 4. Finally, the ejected material is easily transported and deposited onto the desired substrate. Materials that have a low melting point can also be sputtered by this process, which is the main advantage of this method. This method is mostly used in semiconductor industries and for antireflective covering of optical lenses.

1.3.3.4 Laser Pyrolysis In contrast to other vapor phase methods, the laser pyrolysis provides highly restricted (confined to a small area) and quicker heating (leads to speedy nucleation), with faster quenching of particle growth (in few ms). In addition, this method provides more possibilities for the narrow size distribution of nanoparticles, with a range from 5 to 60 nm. The reaction proceeds in-between the laser beam and molecular flow of gaseous/vapors phase reactants, and condensable products are generated at the interface. The main criterion is that either the precursor or the reactant should be able to absorb the energy that is supplied through the resonant vibrational mode of infrared CO2 laser radiations. Alternatively, some of the other chemicals such ammonia (NH3), sulfur hexafluoride (SF6), ethylene gas (C2H4), etc are in usage. A large variety of oxide nanomaterials (TiO2, SiO2, Al2O3, Fe2O3), nonoxide (Si, SiC, Si3N3, MoS2) and ternary composites such as Si/C/N and Si/Ti/C may be prepared via laser pyrolysis. At present, the direct laser writing (DLW) method has come into sight as a novel technique toward the fabrication of various nanostructures, with low cost, high efficiency, and flexible designability. The DLW method provides in-situ growth and patterning capability with high designability that enables growing materials at required locations. By this way, MoS2/carbon hybrid materials have been synthesized and used as electrocatalysts for hydrogen evolution reactions. In the DLW method, for the synthesis and patterning of MoS2/carbon hybrids, cheap starting materials composed of citric acid (C6H8O7), ammonium molybdate tetrahydrate ((NH4)6Mo7O24$4H2O), and sodium sulfide (Na2S) were used. Using a computer-controlled laser scribing, MoS2/carbon hybrids were readily written into delicate geometry. It shows two distinct areas: the black Mizzou tiger logo after the precursor was exposed to the laser and the brown unexposed area. Scanning electron microscope images also show the perfect patterning of MoS2/carbon hybrids [68].

1.3.3.5 Flash Spray Pyrolysis It is one of the flame aerosol technologies, which is a one-step incineration method, where the precursor is in liquid form, with notably high combustion enthalpy (more than 50% of total energy of combustion), and is usually kept in an organic solvent. In favor of nanoparticle formation, the liquid precursor is able to follow any one of these routes: the droplet-to-particle route or gas-to-particle route. However, at the end, the obtained particle has better homogeneity in size and morphology. For the formation of materials through precursor spraying series, the below mentioned sequential steps were followed: 1. 2. 3. 4.

formation of metal vapors from precursors either evaporation/decomposition, nucleation stage as a result of supersaturation, further growth due to coalescence and sintering, and either aggregation (via chemical bonds) or agglomeration (via physical interactions) of particles

30

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

The flash spray pyrolysis method is used for the construction of complex and functional nanostructured materials. This technique is used massively to design and fabricate various oxide and nonoxide ceramic nanomaterials, e.g., CeO2, SiO2, Al2O3, MgO, CeCu, BaCO3, CaO, CaCO3, CaSO4, etc. Very recently, dense, homogeneous CuBi2O4 thin films were prepared for the first time through the spray pyrolysis method. For spray pyrolysis, the precursor solution was prepared by dissolving Bi(NO3)3$5H2O in acetic acid and Cu(NO3)2$3H2O in ethanol. Both solutions were mixed together to form Cu(NO3)2$3H2O: Bi(NO3)3$5H2O in the molar ratio of 1:2 to match the stoichiometry of CuBi2O4. Here, triethyl orthoformate (TEOF) acts as a water scavenger to avoid fast hydrolysis and polycondensation of bismuth ions in the precursor solution. In addition, it reduces powder formation during the spray deposition process. PEG is used to improve the spreading behavior of sprayed droplets over the entire CuBi2O4 film surface, which also prevents powder formation and allows deposition of dense, homogeneous films with thicknesses of over 420 nm. The optimal TEOF concentration of 5% was confirmed by measuring surface roughness on a series of films made by varying the precursor solutions with 0%, 1%, 5%, and 10% TEOF. The lowest root mean squared roughness of 34.5 nm was observed for CuBi2O4 thin films synthesized with 5% TEOF (Fig. 1.15). The obtained thin films are well suited for fundamental studies on optical and photoelectrochemical properties [69].

FIGURE 1.15 Atomic force microscopy images of w50 nm CuBi2O4 thin films deposited on fluorine doped tin oxide substrates at 450 C using 20 mM precursor solution with 0%, 1%, 5%, and 10% triethyl orthoformate (TEOF) [69]. RMS, root mean square.

1.4 PROPERTIES OF NANOSTRUCTURED MATERIALS

31

1.4 PROPERTIES OF NANOSTRUCTURED MATERIALS Recently, numerous materials have been used for multiapplications, especially nanostructured materials having the most significance in a variety of fields due to their unusual properties compared with bulk material. For example, all materials when becoming very small in size exhibit their sizedependent properties. Properties of materials are generally described by how the material reacts with its surroundings. For example, mechanical, electrical, and magnetic properties are provides the interactions between the materials and the environment. Thermal property of materials is also important which is observed as transmission of heat and heat capacity. Likewise, optical property, occurring due to absorption, transmission, and scattering of light, and chemical stability, occurring when in contact with the environment such as corrosion resistance, are the main properties of the materials.

1.4.1 PHYSICAL PROPERTIES It is the important property observed without any change in identity. Regardless of their modest alchemical beginnings, the physical properties of GNPs actually differ from both small molecules and bulk materials, in addition to the particles that are available in nanoscale. Because of the beautiful colors and unique electronic properties, GNPs have attracted tremendous attention for historical applications in art and ancient medicine and in recent applications for enhancements in optoelectronics, sensors, and PVs. Their unique combination of properties is just beginning to be fully realized in a range of medical diagnostic and therapeutic applications. The morphology (size and shape) of the GNPs and their surface/colloidal properties play a vital role in various applications. The methodology for the synthesis of GNPs with controlled size and shape and exceptional colloidal stability in an aqueous medium provides better applications. The diversity in sizes and shapes of GNPs are listed in Fig. 1.16, which is widely used in biomedical applications [70]. The physical properties of nanomaterials are especially due to various effects such as the ultrahigh surface effect, ultrahigh volume effect, and quantum size effect. Ultrahigh surface effect: The remarkable increase in surface atoms is explained by this effect during size reduction. Atoms that are present on surfaces contain only fewer adjacent atoms than atoms in bulk materials. With the existence of lower coordination and unsatisfied bonds, surface atoms are not as much stabilized as atoms available in bulk. Ultrahigh volume effect: on size reduction, electrons are rendered more movable than those that are present in bulk materials. It also provides a low mass, porosity, or density when the particles are very small. Quantum size effect: when the size of the material is reduced, energy will be increased and so will the quasi-discrete energy of the electron orbital around the Fermi energy level. In addition, when the dimension of the material is about to decrease, a new variable will be available in the length scale to control the properties of the material. In continuation of the decrease in size up to nanometer scale, there are numerous possibilities in the density of electronic states [71,72]. Likewise, the optoelectronic properties of semiconductor nanocrystals are also powerfully shape dependent. In the quantum confinement regime, both the size and shape of semiconductor nanocrystals have an impact on the exciton fine structure [73].

32

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

(A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

(J)

(K)

(L)

(M)

(N)

(O)

(P)

FIGURE 1.16 Gold nanoparticles of various sizes and shapes with potential applications in biomedicine. Small (A) and large (B) nanospheres, (C) nanorods, (D) sharpened nanorods, (E) nanoshells, (F) nanocages/frames, (G) hollow nanospheres, (H) tetrahedron/octahedron/cubes/icosahedron, (I) rhombic dodecahedron, (J) octahedron, (K) concave nanocubes, (L) tetrahexahedron, (M) rhombic dodecahedron, (N) obtuse triangular bipyramids, (O) trisoctahedron, and (P) nanoprisms [70].

1.4 PROPERTIES OF NANOSTRUCTURED MATERIALS

33

1.4.2 MECHANICAL PROPERTIES Nanostructured materials exhibit different mechanical properties compared with bulk materials. They also provide alternative options for surface alterations in numerous devices to get excellent mechanical strength or to develop high quality of nanomanufacturing/nanofabrication processes. Exploration of mechanical properties at the nanoscale provides better results in strength and ductility of materials. These kinds of developments could be attained through modification, decoration, and refinement of microstructural materials [74,75]. A variety of mechanical properties, such as durability, are extremely based on the defects that are created inside the material. On decreasing the size of materials, the ability to reinforce the defects will be correspondingly improved. As a result of this phenomenon, the mechanical property of nanomaterials gets altered. Therefore the arrangements of both atomic and molecular structures of nanomaterials differ from bulk materials. This is also the reason for the observance of diversity in the mechanical properties of nanomaterials. For example, the mesoporous silica nanoparticle (MSN)emodified magnesium (Mg) matrix (Mg-xMSN) plays a crucial role in mechanical properties. The composite Mg-xMSN was synthesized by spark plasma sintering (SPS) subsequent to the high-energy wet ball milling method (HEWBM). Regarding the results of mechanical properties within MSN content, Mg-8wt% MSN exhibited the highest compressive strength of 414  7.50 MPa, the highest bending strength of 210  14.38 MPa, and the highest specific compressive strength of 217.88 N m/kg. As a consequence, excellent mechanical performances were mainly endorsed to the formation of Mg2Si during sintering and the homogeneous dispersion of doped and in-situ formed particles (Fig. 1.17) [76].

1.4.3 MAGNETIC PROPERTIES During the rotation of atoms, electric current is produced which is the main source of magnetism. Sometimes the external magnetic forces also create magnetism when applied to other materials, particularly magnetic materials. When an external magnetic force is applied to the material, the magnetic moments of the substance will be reoriented to align itself across the direction of the external field. Regarding the property of electron pairing, materials are distinguished as either paramagnetic or diamagnetic. Materials that possess one or more unpaired electrons are called paramagnetic materials and the process is called paramagnetism. These will be easily attracted by an external magnetic field. Likewise, materials that have fully paired electrons are called diamagnetic materials and the process is called diamagnetism. Most of the diamagnetic materials will repel the applied magnetic fields. Magnetic nanoparticles that are smaller in size will show superparamagnetism, and they have coercivity property (the resistance of a magnetic material changes its magnetization, corresponding to the field strength, which is essential to demagnetize the completely magnetized material), which is the most significant. Whenever the magnetic field of superparamagnetic particles is withdrawn, the particles freely reorient their spins due to thermal energy, and there is no need of external energy to demagnetize the system. Iron oxide (Fe2O3 and Fe3O4), cobalt ferrite (CoFe2O4), iron platinum (FePt), and manganese ferrite (MnFe2O4), are the extensively investigated magnetic nanoparticles. For example, magnetite (Fe3O4) and its oxidized form of

34

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

FIGURE 1.17 Ultimate compressive strength (UCS), ultimate bending strength (UBS) and specific compressive strength (SCS) of all the compacts prepared by high-energy wet ball milling method (HEWBM) and spark plasma sintering (SPS) [76]. DC, direct current; MSN, mesoporous silica nanoparticle.

maghemite (g-Fe2O3) are the important superparamagnetic iron oxide nanoparticles that are used in medical imaging, magnetic field assisted transport, and environmental separation process due to their inherent superparamagnetic properties. The magnetic properties of nanostructured materials are characterized using vibrating-sample magnetometer technique. It is easy to distinguish paramagnetic, diamagnetic, ferromagnetic, and other samples through plotting a graph of magnetization versus the applied magnetic field. For example, the magnetic hysteresis loops of attapulgite/Fe3O4/ polyaniline (APT/Fe3O4/PANI) nanocomposites are seen in Fig. 1.18A, which exhibits superparamagnetic behavior without remanence and coercivity. The saturation magnetization of APT/ Fe3O4/PANI1, APT/Fe3O4/PANI2, APT/Fe3O4/PANI3, and APT/Fe3O4/PANI4 is 3.23, 39.79, 37.05, and 13.95 emu/g, respectively. Thermogravimetric analysis (TGA) results show that the content of Fe3O4 nanoparticles of APT/Fe3O4/PANI2 is higher than that of other APT/Fe3O4/PANI nanocomposites when the contents of APT is same (Fig. 1.18B). The color of HAuCl4 solution fades after

1.4 PROPERTIES OF NANOSTRUCTURED MATERIALS

35

FIGURE 1.18 (A) The magnetic hysteresis loops, (B) thermogravimetric analysis curves of (1) attapulgite/polyaniline (APT/ PANI), (2) APT/Fe3O4/PANI1, (3) APT/Fe3O4/PANI2, (4) APT/Fe3O4/PANI3, and (5) APT/Fe3O4/PANI4. Digital photograph of the solution of (C) 100 mg/L of CR and (D) 10 ppm of HAuCl4 before and after addition of APT/Fe3O4/PANI2 nanocomposites [77].

adding APT/Fe3O4/PANI2 nanocomposites, indicating that AuCl4 has been adsorbed and reduced to elemental gold to form APT/Fe3O4/PANI2esupported Au (APT/Fe3O4/PANI2/Au) nanocomposites (Fig. 1.18C and D) [77].

1.4.4 OPTICAL PROPERTIES In general, change in absorption would have an attractive consequence on the originally colored materials. When the dimensions of a material are decreased, it creates some impacts on their electronic structure and hence energy changes occur in the highest occupied molecular orbital (HOMO; valence band) and in the lowest unoccupied molecular orbital (LUMO; conduction band). The HOMO and LUMO are the main features for optical properties such as absorption and emission transitions, as well as for line strength. The above characteristics play a crucial role toward the changes of HOMO and LUMO levels. Likewise, changes in the size of nanoparticles also effectively modify the physical and electronic properties. Mostly color and conductivity in semiconductors and metals change with varying sizes (Fig. 1.19). For example, in semiconductor quantum dots, the optical absorption and emission shift to the blue region (higher energies) when the size of the dots decreases. In contrast to

36

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

FIGURE 1.19 Energy gap increases with decreasing particle size.

metals, the size reduction is more prominent in semiconductors, that is, the quantum size confinement effects become more significant in metals at smaller sizes than in semiconductor crystals. Predominantly, nanomaterials show signs of improved luminescence which compared with their bulk counterparts. The property of luminescence is further classified as: • • • •

photoluminescence, electroluminescence, cathodoluminescence, thermoluminescence.

1.4.4.1 Photoluminescence The electromagnetic radiation or photons are the external stimulus for some kinds of luminescence, which is identified as photoluminescence. When the photon has sufficient energy, the electrons are excited to the conduction band from the valence band, and this process leaves a hole. The photon is emitted in a comparatively shorter time (w1012e1013 s) due to the loss of energy by the excited electron prior to relaxation and making a radiative transition. Compared with nonradiative transition, the lifetime of the radiative process is, to a large extent, w107 to few milliseconds (fluorescence) or even few seconds (phosphorescence).

1.4.4.2 Electroluminescence Luminescence observed in various materials once the electric field is applied is called electroluminescence. Electroluminescence is an efficient progression by reason of high quantum efficiency of luminescence.

1.4.4.3 Cathodoluminescence Luminescence produced due to the striking of extremely high-energy electrons with semiconductor material is known as cathodoluminescence. The field emission cathode is the source of the incident electrons, and they hit the luminescent material at high energy in the presence of vacuum. Cathodoluminescence phenomenon is used in oscilloscope and screens of older television displays.

1.5 ENERGY APPLICATIONS

37

1.4.4.4 Thermoluminescence If large bandgap semiconductors are excited at very low temperatures through photons in the UV range or in the meantime on heating to various temperatures which depend on the dopant ions, light is emitted even in the absence of any other stimulus. Such a phenomenon is known as “thermoluminescence” or “after glow” process.

1.5 ENERGY APPLICATIONS Due to unique properties, nanostructured materials are used for diverse applications. It is well known that natural energy resources such as coal, oil, and natural gas that are widely used in transportation, communication, agriculture, and industries are restricted and extremely exhausted. Hence for future needs, it is necessary to focus on alternative sustainable energy sources such as nuclear, geothermal, wind, solar energy or hydrogen-based fuel cells that easily satisfy the requirements. Considerable amount of research is in progress for the conversion and storage of energy.

1.5.1 SOLAR CELLS PV cells or solar cells are defined as the devices that spontaneously convert light energy into direct electricity. PVs are widely used due to improved features such as being greatly reliable and requiring minimum maintenance, being economically feasible to build and operate, and virtually not creating any impact on the environment. By absorbing sunlight spontaneously, PV cells generate electricity and also produce zero or near-zero air pollution and hazardous wastes. An important feature of the PV cell is without the usage of liquid or gaseous fuels. At present, three types of PVs are available: the first, second, and third generations. First generation cells are made up of crystalline silicon wafers (p-n diodes). This technique is the oldest and the most accepted technology as a result of high power efficiencies and is further characterized into two subgroups, namely single/mono-crystalline silicon solar cell and poly-/multicrystalline silicon solar cell. The silicon solar cells provide somewhat better efficiency but are of high cost. Solar cells that come under the second generation are based on thin films of crystalline or amorphous silicon, CuInSe2-based cells, and many other thin films. These are more economical than the first generation cells (silicon wafer solar cells) but not much efficient. Third generation cells are more innovative and that showing better potential technologies for harvesting the solar energy. Nanocrystal-based solar cells, polymer-based solar cells, DSSCs, and concentrated solar cells are the most popular third-generation solar cells. They are efficient and cost-effective. The developmental history of solar cells can be broadly classified into three different generations, as shown in Fig. 1.20A. For instance, Quantum dot sensitized solar cells (QDSSCs) have become very popular over the past few years. The interesting properties such as size-dependent bandgap, high molar extinction coefficient, solution processability, large dipole moments, photo-stability, high optical absorption coefficients (a ¼ w1,00,000 cm
1) and the tendency of multiple exciton generations are given further importance in quantum dots toward solar cell applications. They enhance the possibility of PV devices to achieve energy conversion efficiency (w42%) beyond traditional Si-based solar cells and even S-Q limits. Fig. 1.20B, shows a schematic representation of QDSSCs along with their different components and shuffling of charge carriers between different components. In general, QDs

38

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

(A) Mono-crystalline Si Solar Cells

First Gen Poly-crystalline Si Solar Cells CIGS Solar Cells

Solar Cells

CdTe Solar Cells

Second Gen

a-Si Solar Cells

Cu In S

Single Core Structure

Photoanode DSSCs QDSSCs

Third Gen

Organic Solar Cells

Generations

Types

CuInS2

Alloy Structure

Electrolyte Perovskite Solar Cells

Choice of New Materials

Core/Shell Structure

Sensitizer

Counter Electrode

Graphene

2D TMDs

Choice of Structures

Components

(C)

(B) e

-

-4.0

ITO

-4.3 hv

FTO Redox

Ec QDs (PbS)

FTO Energy (eV)

Metal

Eg

ZnO

e. Metal

Ev

Semiconductor Oxide Nanoparticles

Counter Electrode

h

+

e.

QD’s

P-i-n-QD Array Solar Cell

e.

e.

e. h+

h+

Ef h+

hv

e.

h+

FTO

Red/ TiO2 QD Ox QD-Sensitized Nnocrystalline TiO2 Solar Cell

Pt/ FTO

h

e.

+

h+

h+ QD

ITO

Metal

Light harvester Hole Polymer conductor layer

Hybrid QD:Polymer Bulk-Heterojunction Solar Cell

FIGURE 1.20 (A) Schematic representation of solar cell technology. The solar cell technology has been classified into first, second, and third generations and further classified into different types based on sensitizer material used for the fabrication of a solar cell. (B) Schematic representation of quantum dot sensitized solar cells (QDSSCs) along with their different components and shuffling of charge carriers between different components [78]. (C) The different quantum dot (QD) solar cell configurations (top), and the energy diagrams of each one (bottom) [79]. a-Si, amorphous silicon; CdTe, cadmium telluride; CIGS, copper indium gallium selenide; DSSC, dye-sensitized solar cell; FTO, fluorine doped tin oxide; Gen, generation; ITO, indium tin oxide; TMD, transition metal dichalcogenide.

1.5 ENERGY APPLICATIONS

39

work as light absorbers and leads to the generation of charge carriers through the transfer across elective surrounding phases. Generated electrons (due to incident photons) are transferred to a semiconducting oxide layer (TiO2, ZnO, SnO2, etc.) with QDs attached and then transferred to the conducting ITO/FTO glass. Then, QDs regain their original position through an electrolyte (solid/liquid) containing a reversible redox couple. The counter electrode (metal or semiconducting electrode) should have fast kinetics for the electrolyte redox couple. Efficient QDSSCs require high open circuit voltages, large incident photon-to-current efficiency, and a high fill factor [78]. As a foundation, there are three different types of quantum dotsebased molecular solar cell configurations: photoelectrodes composed of QD assemblies, QD-sensitized solar cells, and QDs dispersed in organic semiconductor polymer matrices (Fig. 1.20C) [79]. Recently fourth generation solar cells are developed which merges the low cost and flexible polymer thin films with a stable of novel inorganic nanostructures. The main aim is to improve the optoelectronic properties of the low-cost thin-film PVs [80].

1.5.2 FUEL CELLS These are devices that generate electric energy through electrochemical reactions involving an oxidizing agent and a fuel with a byproduct of water and heat. Fuel cell technology shows a prospective way to offer energy for rural regions, wherever there is no access to the public grid or where there is an enormous price for electric wiring and transferring electricity. Fuel cells have simple design and fabrication, and reliable operation as well. Furthermore, the usage of hydrogen as the reactant makes the energy systems most eco-friendly and noiseless. In addition, these have special features such as high energy conversion efficiency, zero emission, modularity, scalability, and quick installation [81]. Mostly, a typical fuel cell consists of an electrolyte layer which is in contact with two different electrodes on either side (anode and cathode). As in the working procedure of the common fuel cell, hydrogen fuel is always fed to the anode electrode and the oxidant (or) oxygen from air is fed continuously to the cathode electrode. During the reaction, the hydrogen fuel is decomposed at the anode terminal into positive ions along with negative ions. The positive ions are only allowed to flow from the anode to cathode side by the intermediate electrolyte membrane, and it acts as an insulator for electrons. For the stable recombination process of electrons, they tend to move through an external circuit to the cathode side. The recombination takes place at the cathode, the positive and negative ions with an oxidant to form depleted oxidant (or) pure water [82]. In accordance with their operating temperature, efficiency, applications, and costs, fuel cells are of different categories. Especially on the basis of the choice of fuel and electrolyte, they are classified into six major groups: • • • • • •

proton-exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), direct methanol fuel cell (DMFC).

As a comparison of the six major fuel cells based on their inlet fuels, electrolyte material, cost, advantages, disadvantages, and their suitability for applications are discussed in Table 1.1. As is

Table 1.1 Comparison of Different Fuel Cells [83] Fuel Cell Type

Serial No

Parameters

PEMFC

AFC

PAFC

MCFC

SOFC

DMFC

1

Electrolyte

Solid polymer membrane (Nafion)

Liquid solution of KOH

Phosphoric acid (H3PO4)

Operating temperature ( C) Anode reaction Cathode reaction Charge carrier Fuel

50e100

50e200

w200

Stabilized solid oxide electrolyte (Y2O3, ZrO2) 800e1000

Solid polymer membrane

2

Lithium and potassium carbonate (LiAlO4) w650

60e200

H2 / 2Hþ þ 2e


H2 / 2Hþ þ 2e


1/2O2 þ 2Hþ þ 2e
/ H2O Hþ

H2 þ 2OH
/ 2H2O þ 2e
1/2O2 þ H2O þ 2e
/ 2(OH
) OH


H2O þ CO2
3 / H2O þ CO2 þ 2e
1/2O2 þ CO2 þ 2e
/ CO2
3 CO
3

H2 þ O2
/ H2O þ 2e
1/2O2 þ 2e
/ O2
O


CH3OH þ H2O / CO2 þ 6Hþ þ 6H
3O2 þ 12Hþ þ 12H
/ 6H2O Hþ

Pure H2

Pure H2

Pure H2

H2, CO, CH4, ether, hydrocarbons

CH3OH

O2 in air 40%e50% e Yes

O2 in air w50% e Yes

O2 in air 40% Yes Yes

O2 in air >50% Yes e

H2, CO, CH4, ether, hydrocarbons O2 in air >50% Yes e

O2 in air 40% No e

1.1 3.8e6.5

1.0 w1

1.1 0.8e1.9

0.7e1.0 1.5e2.6

0.8e1.0 0.1e1.5

0.2e0.4 w0.6

<1500

w1800

2100

w2000e3000

3000

e

30 W, 1, 2, 5, 7, 250 kW

10e100 kW

100, 200 kW, 1.3 MW

155, 200, 250, 1, 2 kW

1, 25, 5, 100, 250 kW

1 W to 1 kW, 100 kW to 1 MW (research)

3 4 5 6

7 8 9 10 11 12 13

14

Oxidant Efficiency Cogeneration Reformer is required Cell voltage Power density (kW/m2) Installation cost (US $/kW) Capacity

1/2O2 þ 2Hþ þ 2e
/ H2O Hþ

15

Applications

16

Advantages

17

Drawbacks

Residential; Uninterruptible power supply (UPS); emergency services such as hospitals and banking industry; transportation; commercial applications High power density; quick start-up; solid noncorrosive electrolyte

Transportation; space shuttles; portable powers

Transportation; commercial cogeneration; portable power

Transportations (e.g., marine ships; naval vessels; rail); industries; utility power plants

Residential; utility power plants; commercial cogenerations; portable power

It is used to replace batteries in mobiles, computers, and other portable devices

High power density; quick startup

Produce highgrade waste heat; stable electrolyte characteristics

High efficiency; no metal catalysts needed

Reduced cost due to the absence of fuel reformer

Expensive platinum catalyst; sensitive to fuel impurities (CO, H2S)

Expensive platinum catalyst; sensitive to fuel impurities (CO, CO2, CH4, H2S)

Corrosive liquid electrolyte; sensitive to fuel impurities (CO, H2S)

High cost; corrosive liquid electrolyte; slow startup; intolerance to sulfur

Solid electrolyte; high efficiency; generates highgrade waste heat High cost; slow startup; intolerance to sulfur

Lower efficiency and power density

AFC, alkaline fuel cell; DMFC, direct methanol fuel cell; MCFC, molten carbonate fuel cell; PAFC, Phosphoric acid fuel cells PEMFC, proton exchange membrane fuel cell; SOFC, solid oxide fuel cell.

42

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

evident from Table 1.1, PEMFC is more suitable for residential and commercial applications due to low working temperature (50e100 C) and fast startup properties. Compared with other fuel cells, MCFC and SOFC are better for large power applications [83]. At present, flexible devices are attractive and get more attention due to their numerous advantages such as less weight and smaller in size. The flexible PEMFCs mostly differ from other traditional PEMFCs in many ways, including structure, size, components, etc. For traditional PEMFCs, rigid, heavy, and expensive metal or graphite bipolar plates are used as flow field and current collector for both anode and cathode, but in flexible PEMFCs, the plate on the cathode side is totally removed, and the plate on the anode side is replaced by a light and cheap plastic plate (Fig. 1.21A). Recently, a newly designed light and flexible air-breathing PEMFC has been developed by using a flexible composite electrode (working area size is 1  1 cm2, lightweight is 0.065 g, and thin is 0.22 mm). This new type of flexible PEMFC shows evidence of remarkable specific volume power density (5190 W/L) and specific mass power density (2230 W/kg), which is much higher than traditional (air-breathing) PEMFCs and outstanding in that it retains 89.1% of its original performance after 600 times, being bent as well (Fig. 1.21B). Furthermore, it retains its original performance even after being dropped from a height of about 30 m, five times. As in the form of the stack in the bending state, this device can light up 53 LED lights (volume is 5.7 cm3; weight is 2.18 g.). The flexible stack consists of eight single PEMFCs and is lightweight (3.70 g), small (9 cm length and 3.5 cm width), thin (1 mm), and occupies a small volume (3.15 cm3) that can power some electronic devices such as mobile phones (Fig. 1.21C) [84].

1.5.3 BATTERIES An electrochemical power source that suddenly converts chemical energy into electrical energy is called a battery, which consists of self-contained voltaic cells arranged in series and is composed of a cathode, electrolyte, and anode [85]. Completion of the electrical circuit, maintaining the charge neutrality, and allowing ions to move between the electrodes and terminals are the main functions of the electrolyte. The electrochemical reactions proceed at the two interfaces through the transfer of electrons between the electrode surface and ions from the solution. Usually, two types of batteries are available depending upon their usage: (1) primary battery: when the cell reaction has reached its equilibrium, these cannot be recharged repeatedly, and the battery is “dead”; and (2) secondary battery: these are rechargeable many times, and their original composition of the electrodes can be restored by reverse current. Large numbers of battery types are available in the following five subsections, and the chemical reactions proceeding in these battery types are listed in Table 1.2 [86]. Apart from other types, the development of the next-generation lithium-based rechargeable batteries (LIBs) is a great challenge with high energy density, low cost, and improved safety. Recently, LIBs have played a vital role in sustainable energy landscape, as the facilitating technology for many kinds of applications such as electric vehicles and grid-scale storage. The state-of-the-art LIBs comprise the anode part (graphite) and cathode part (Li transition-metal oxide [LTMO]/ phosphate) that reversibly intercalate Li ions with minimal structural change recognition to the low atomic ratio of Li ions to host atoms. Next-generation batteries call for a hypothesis that shifts to electrodes side with high Li-to-host ratios on the basis of a conversion or alloying mechanism. In addition, the relative volume expansion is also an important factor with the Li-to-host atomic ratio. So

1.5 ENERGY APPLICATIONS

43

FIGURE 1.21 (A) Structures of different proton exchange membrane fuel cells (PEMFCs). Traditional PEMFC (air or O2 is pumped into PEMFC), traditional air-breathing PEMFC (air naturally diffuses into PEMFC), and flexible airbreathing PEMFC (air naturally diffuses into PEMFC). (B) Photograph and performance of flexible and traditional air-breathing PEMFCs with the same working area (1  1 cm2) with polarization curves. (C) Application demonstrations of flexible PEMFC stacks. Schematic illustrating a flexible PEMFC stack, 53-LED lights powered by a stack with four single cells when bent and charging a mobile phone with a flexible PEMFC stack of eight single cells (84). PEM, proton exchange membrane.

44

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

Table 1.2 Chemical Reactions and Single Unit Voltages of Available Main Batteries [86] Battery Type Leadeacid Lithium-ion Sodiumesulfur Nickelecadmium Nickelemetal hydride Sodium nickel chloride

Chemical Reactions at Anodes and Cathodes
Pb þ SO2
4 5PbSO4 þ 2e þ þ 2e
5PbSO þ þ 4H PbO2 þ SO2
4 4 C þ nLiþ þ ne
5Lin C LiXXO2 5Li1
n XXO2 þ nLiþ þ ne
2Na52Naþ þ 2e
cS þ 2e
5cS2


Unit Voltage 2.0 V

2H2 O

Cd þ 2OH
5CdðOHÞ2 þ 2e
2NiOOH þ 2H2 O þ 2e
52NiðOHÞ2 þ 2OH
H2 O þ e
51=2H2 þ OH
NiðOHÞ2 þ OH
5NiOOH þ H2 O þ e
2Na52Naþ þ 2e
NiCl2 þ 2e
5Ni þ 2Cl


3.7 V w2.08 V 1.0e1.3 V 1.0e1.3 V w2.58 V

far, the existing electrode materials have had less than 10% volume changes with a small Li-to-host ratio (1:6). Increase in specific capacity is also well accompanied by a significant increase in volume change and makes it challenging toward practical applications of advanced electrode materials (Fig. 1.22A and B) [87]. Likewise, among the other anode materials, Silicon (Si) is the most outstanding anode material for next-generation batteries. High theoretical capacity (4200 mA h/g based on the mass of Si) which is greater than 10 times that of graphite (372 mA h/g) gives more attention in the direction of practical utilization. This high capacity originates from its alloying mechanism, where each Si can host up to 4.4 Li-ions. The usage of Si anode is an exemplary case to illustrate the power of nanotechnology to address the material challenges facing battery research. Until now, 11 generations of nanostructural designs and approaches are made toward improving Si for realistic applications (Fig. 1.22C).

1.5.4 SUPERCAPACITORS For near future energy demands, faster and higher-power energy storage systems are needed. Batteries suffering relatively slow power delivery or uptake therefore focus on another type of device called supercapacitors. These are devices that are getting more attention nowadays due to the provision of high power density (w410 kW/kg), quick charge/discharge process (within seconds), and extremely extended cycling life (>105 cycles). The energy, as well as power densities of the supercapacitors, can fill the gap between the batteries and conventional (solid state and electrolytic) capacitors. Generally, two types of supercapacitors are available depending upon their storage mechanism: electrical doublelayer capacitors (EDLCs) and pseudosupercapacitors. The EDLC capacitance is ascribed due to the charge separation along with accumulation at the electrode/electrolyte interface and mainly from carbonaceous materials. Unlike EDLCs, the pseudocapacitance originates from the fast and reversible Faradaic process of electroactive species, for example, conducting polymers and transition metal oxides/hydroxides [88e91].

1.5 ENERGY APPLICATIONS

100

Li Host atom 6 1:

(C)

Li to host atomic ratio amorphous carbon Si microparticles

Gen 1: Si Nanowires

2009 2008

self-healing coating

Gen 9: Porous Si Microparticles

Gen 5: Yolk-Shell Gen 7: Self-Healing Polymer Coating

Gen 3: Hollow Nanospheres 2011 2010

2013 2012

crystalline Si core

amorphous Si shell

Gen 4: Double-Walled Nanotubes

Gen 11: Conformal Graphene Cages 2015

2014 Gen 6: Conducting Hydrogel Framework

Gen 2: Core-Shell Nanowires

ss

0

Li-air

le

0 Li-ion Si-LTMO Li-LTMO Si-Li2S Li-S

LiC6

st

0

200

1

300

Li2O+M

Ho

250

LiCoO2 300

4:

600

Li2S Li2O

1

500

Li3P

400

3:

900

LiFePO4

1

750

∞

2:

1200

1

1000

a-Li4.4Si Metallic Li a-Li4.4Ge a-Li4.4Sn

1:

1500

Relative volume change (%)

(B)

1250

Energy density (Wh l-1)

Specific energy (Wh kg-1)

(A)

45

2016 Timeline Gen 10: Prelithiation

Gen 8: Pomegranate Hierarchial Structure

LixSi nanoparticles

mechanical constraining layer

FIGURE 1.22 (A) Practical specific energies and energy densities of the state-of-art LiBs (Li-ion), and advanced battery chemistries including Si
LTMO, Li metal
LTMO, SieLi2S, Li metal
S, and Li
air. Casings, separators, and electrolytes are all considered, and Li metal cells are calculated on the basis of 100% excess Li. (B) Increased specific capacity is accompanied by significantly increased volume changes, bringing challenges to the practical applications of advanced electrode materials. (C) Roadmap of nanostructural designs for Si anodes [87]. LIB, lithium ion batteries; LTMO, Li transition-metal oxide.

Furthermore, the charge storage mechanism can be classified as non-Faradaic capacitive, Faradaic capacitive (pseudocapacitive), or Faradaic noncapacitive (Nernstian), and the combinations are given different names (Fig. 1.23A). On the basis of the mechanisms, four schematic illustrations of cyclic voltammograms (CV) are available through single electrode configuration when using three electrode

46

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

FIGURE 1.23 (A) Charge storage mechanisms for electrochemical energy storage and their possible device classification. (B) Schematic representations of the voltammetric features for Nernstian (b1, b2), mixed Nernstian, and capacitive (b3) and capacitive (b4) charge storage mechanisms. (C) Schematic illustrations of the band model for chemical bonding between metal atoms that are separated and noninteractive (i), and forming clusters of 2 (ii), 5 (iii), 20 (iv), and 1020 (v) atoms. (D) The corresponding energy levels of the valence electrons as a function of the degree (or zone size) of delocalization of valence electrons in the respective clusters of metal atoms [91].

1.5 ENERGY APPLICATIONS

47

systems (Fig. 1.23B). The electrode materials which display the Faradaic capacitive or pseudocapacitive storage mechanism can be described with the assistance of the semiconductor band model (Fig. 1.23C and D). The reversible electron transfer reactions which are available in redox-active molecules or ions at a liquid electrolyte or confined on the surface of the electrode can be illustrated by using the Nernst equation. As the development of storage technology, asymmetric electrode designs are developed by combining two different technologies and those are mainly with different storage mechanisms. Mostly, a capacitive EDL electrode and a battery-type faradaic or pseudocapacitive electrode are in touch for making new devices and this type of design and fabrications are called as hybrid supercapacitors some time known as supercapbatteries. When the Li-intercalating phase is used as a battery-type electrode, these systems are also referred to as lithium-ion capacitors. In this type, typically, the battery-type electrode offers a high energy density at the same time the capacitor type electrode facilitates a high power capability. The possible hybridization approaches between supercapacitor and battery electrodes and materials are explained through Schematic representations (Fig. 1.24) [92]. Recently, an elegant thermally-responsive supercapacitor was fabricated a facile and cost-effective approach by loading a thermosensitive polymer P(NIPAM-co-SPMA) onto the surface of NiAl-layered double hydroxide (LDH) nanowalls grown on a flexible Ni foil substrate. It demonstrates the temperature-triggered oneoff ion channels which en route to controlling the electrochemical behavior. The smart supercapacitors can be switched by reversible on-off ion channels at 20 and 40 C that is

FIGURE 1.24 Schematic representations of different possible hybridization approach between supercapacitor and battery electrodes and materials [92].

48

(A) Schematic illustration of the LDH@P(NIPAM-co-SPMA) film on a flexible Ni foil substrate serving as a smart supercapacitor with thermallyresponsive ion channels. (B) Temperature-dependent (left to right) CVs; EIS; GV discharge curves; specific capacitance in the range 20e40 C of the LDH@P(NIPAM-co-SPMA) electrode; the GV charge-discharge curves with the increase of temperature from 20 to 40 C; the specific capacitance of the thermally-responsive LDH@P(NIPAM-co-SPMA) switch between 20 and 40 C for eight heating-cooling cycles. GV, galvanostatic.

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

FIGURE 1.25

REFERENCES

49

expected to achieve thermal safety and capacitance control in an electrical energy storage device. For the investigation regarding the thermally-responsive capacitive behavior, all the electrochemical tests are done as a function of temperature. In CV, a continuous decrease in both the anodic and cathodic peak currents is observed when the temperature increases from 20 to 40 C. Likewise, In electrochemical impedance spectroscopy, the diameter of the semicircle becomes generously proportioned upon rising the temperature and in constant-current GV charge-discharge study, the electrode exhibits a constant decrease in the specific capacitance from w518 to w38 F/g. Further, while switching at 20 and 40 C, in the GV charge-discharge curves a largely-depression is observed at 40 C and moreover, it evident a highly-reversible transition during heating and cooling Fig. 1.25 [93].

1.6 SUMMARY In this chapter, an attempt is made to summarize the importance of advanced nanostructured materials, various synthetic protocols, special properties and the energy-related applications. The strategies to design and develop advanced materials together with nanostructuring, nano-/ microcombination, an alternative to 2d materials, hybridization, core-shell control, porous nature, and surface modifications are discussed at a fundamental level. Based on an in-depth perceptive on the principles and fundamentals it is possible to make new technologies in both economical as well as eco-friendly approaches.

REFERENCES [1] S.C. Tjong, Nanocrystalline Materials: Their Synthesis-Structure-Property Relationships and Applications, Elsevier, Boston, 2006. [2] H. Gleiter, Nanostructured materials: basic concepts and microstructure, Acta Mater. 48 (2000) 1e29. [3] J.N. Tiwari, R.N. Tiwari, K.S. Kim, Zero-dimensional, one-dimensional, two-dimensional and threedimensional nanostructured materials for advanced electrochemical energy devices, Prog. Mater. Sci. 57 (2012) 724e803. [4] NIST, Manufacturing and Biomanufacturing: Materials Advances and Critical Processes, 2011. [5] A synthetic biology roadmap for the UK (TSB, 2012). [6] C. Featherston, E.O. Sullivan, A review of international public sector strategies and roadmaps: a case study in advanced materials, (2014). [7] J. Hu, T.W. Odom, C.M. Lieber, Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes, Acc. Chem. Res. 32 (5) (1999) 435e445. [8] V. Skorokhod, A. Ragulya, I. Uvarova, Physico-chemical Kinetics in Nanostructured Systems, Academperiodica, Kyiv, 2001, p. 180. [9] V.V. Pokropivny, V.V. Skorokhod, Classification of nanostructures by dimensionality and concept of surface forms engineering in nanomaterial science, Mater. Sci. Eng. C 27 (2007) 990e993. [10] H.D. Tran, D. Li, R.B. Kaner, One-dimensional conducting polymer nanostructures: bulk synthesis and applications, Adv. Mater. 21 (14e15) (2009) 1487e1499. [11] P.R. Sajanlal, T.S. Sreeprasad, A.K. Samal, T. Pradeep, Anisotropic nanomaterials: structure, growth, assembly, and functions, Nano Rev. 2 (2011) 5883. [12] Z. Wu, S. Yang, W. Wu, Shape control of inorganic nanoparticles from solution, Nanoscale 8 (2016) 1237e1259.

50

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

[13] S. Bashir, J. Liu, Chapter 2 e Overviews of synthesis of nanomaterials, advanced nanomaterials and their applications in renewable energy, 2015, 51e115. [14] S.B. Brichkin, V.F. Razumov, Colloidal quantum dots: synthesis, properties, and applications, Russ. Chem. Rev. 85 (12) (2016) 1297e1312. [15] X. Xu, R. Ray, Y. Gu, H.J. Ploehn, L. Gearheart, K. Raker, W.A. Scrivens, J. Am. Chem. Soc. 126 (2004) 12736. [16] R. Wang, K.Q. Lu, Z.R. Tang, Y.J. Xu, Recent progress in carbon quantum dots: synthesis, properties, and applications in photocatalysis, Mater. Chem. 5 (2017) 3717. [17] L. Mai, X. Tian, X. Xu, L. Chang, L. Xu, Nanowire electrodes for electrochemical energy storage devices, Chem. Rev. 114 (2014) 11828. [18] Q. Wei, F. Xiong, S. Tan, L. Huang, E.H. Lan, B. Dunn, L. Mai, Porous one-dimensional nanomaterials: design, fabrication and applications in electrochemical energy storage, Adv. Mater. 29 (2017) 1602300. [19] L. Ji, P. Meduri, V. Agubra, X. Xiao, M. Alcoutlabi, Graphene-based nanocomposites for energy storage, Adv. Energy Mater. 6 (2016) 1502159. [20] H. Li, Y. Li, A. Aljarb, Y. Shi, L.J. Li, Epitaxial growth of two-dimensional layered transition-metal dichalcogenides: growth mechanism, controllability, and scalability, Chem. Rev. Article ASAP (2017), https://doi.org/10.1021/acs.chemrev.7b00212. [21] H.N. Li, Y.M. Shi, M.H. Chiu, L.J. Li, Emerging energy applications of two-dimensional layered transition metal dichalcogenides, Nano Energy 18 (2015) 293e305. [22] Y.M. Shi, K.K. Kim, A. Reina, M. Hofmann, L.J. Li, J. Kong, Work function engineering of graphene electrode via chemical doping, ACS Nano 4 (5) (2010) 2689e2694. [23] C.R. Dean, A.F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K.L. Shepard, J. Hone, Boron nitride substrates for high-quality graphene electronics, Nat. Nanotechnol. 5 (10) (2010) 722e726. [24] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (3) (2011) 147e150. [25] https://cen.acs.org/articles/95/i22/2-D-materials-beyond-graphene.html. [26] B. Anasori, M.R. Lukatskaya, Y. Gogotsi, 2D metal carbides and nitrides (MXenes) for energy storage, Nat. Rev. Mater. 2 (2017), 16098. [27] G. Tai, T. Hu, Y. Zhou, X. Wang, J. Kong, T. Zeng, Y. You, Q. Wang, Synthesis of atomically thin boron films on copper foils, Angew. Chem. Int. Ed. 54 (2015) 15473e15477. [28] Y. Shi, H. Zhang, W.H. Chang, H.S. Shin, L.J. Li, Synthesis and structure of two-dimensional transitionmetal dichalcogenides, MRS Bull. 40 (2015) 566e576. [29] G.R. Bhimanapati, N.R. Glavin, J.A. Robinson, Chapter three e 2D boron nitride: synthesis and applications, Semiconduct. Semimet. 95 (2016) 101e147. [30] S.L. Cai, W.G. Zhang, R.N. Zuckermann, Z.T. Li, X. Zhao, Y. Liu, The organic flatland-recent advances in synthetic 2D organic layers, Adv. Mater. 27 (2015) 5762e5770. [31] M.J. Bojdys, 2D or not 2D-layered functional (C, N) materials “beyond silicon and graphene”, Macromol. Chem. Phys. 217 (2016) 232e241, https://doi.org/10.1002/macp.201500312. [32] X. Cao, Z. Yin, H. Zhang, Three-dimensional graphene materials: preparation, structures, and application in supercapacitors, Energy Environ. Sci. 7 (6) (2014) 1850e1865. [33] S. Mao, G. Lu, J. Chen, Three-dimensional graphene-based composites for energy applications, Nanoscale 7 (16) (2015) 6924e6943. [34] L. Chen, R. Luque, Y. Li, The controllable design of tunable nanostructures inside metal-organic frameworks, Chem. Soc. Rev. 46 (2017) 4614e4630. [35] R. Wang, D. Jin, Y. Zhang, S. Wang, J. Lang, X. Yan, L. Zhang, Engineering metal-organic framework derived 3D nanostructures for high-performance hybrid supercapacitors, Mater. Chem. A 5 (2017) 292.

REFERENCES

51

[36] R. Liu, J. Duay, S.B. Lee, Heterogeneous nanostructured electrode materials for electrochemical energy storage, Chem. Commun. 47 (2011) 1384e1404. [37] M. Tadic, M. Panjan, D. Markovic, I. Milosevic, V. Spasojevic, Unusual magnetic properties of NiO nanoparticles embedded in a silica matrix, J. Alloy. Compd. 509 (2011) 7134e7138. [38] V. Bognor-Legare, P. Cassagnau, In situ synthesis of organic-inorganic hybrids or nanocomposites from solgel chemistry in molten polymers, Prog. Polym. Sci. 39 (2014) 1473e1497. [39] S. Ziller, J.F. von Bu¨low, S. Dahl, M. Linde´n, A fast solegel synthesis leading to highly crystalline birnessites under non-hydrothermal conditions, Dalton Trans. 46 (2017) 4582. [40] M.F. Khan, A.H. Ansari, M. Hameedullah, E. Ahmad, F.M. Husain, Q. Zia, U. Baig, M.R. Zaheer, M.M. Alam, A.M. Khan, Z.A. AlOthman, I. Ahmad, G.M. Ashraf, G. Aliev, Sol-gel synthesis of thorn-like ZnO nanoparticles endorsing mechanical stirring effect and their antimicrobial activities: potential role as nano-antibiotics, Sci. Rep. 6 (2016) 27689, https://doi.org/10.1038/srep27689. [41] S. Li, B. Zhou, B. Ren, L. Xing, L. Tan, L. Dong, J. Li, Preparation of MgO nanomaterials by microemulsion-based oil/water interface precipitation, Mater. Lett. 171 (2016) 204e207. [42] G. Demazeau, Solvothermal reactions: an original route for the synthesis of novel materials, J. Mater. Sci. 43 (7) (2008) 2104e2114. Springer Verlag. [43] L. Kuai, X. Yu, S. Wang, Y. Sang, B. Geng, Au
Pd alloy and core
shell nanostructures: one-pot coreduction preparation, formation mechanism, and electrochemical properties, Langmuir 28 (2012) 7168e7173. [44] H. Dong, Y.C. Chen, C. Feldmann, Polyol synthesis of nanoparticles: status and options regarding metals, oxides, chalcogenides, and non-metal elements, Green Chem. 17 (2015) 4107e4132. [45] Y. Wang, S. Choi, X. Zhao, S. Xie, H.-C. Peng, M. Chi, C.Z. Huang, Y. Xia, Polyol synthesis of ultrathin Pd nanowires via attachment- based growth and their enhanced activity towards formic acid oxidation, Adv. Funct. Mater. 24 (2013) 131e139. [46] J.P. Espinos, A. Fernandez, A. Caballero, V.M. Jimenez, J.C. Sanchez-Lopez, L. Contreras, D. Leinen, A.R. Gonzalez-Elipe, Ion-beam-induced CVD: an alternative method of thin film preparation, Chem. Vap. Depos. 3 (1997) 219e226. [47] D. Mohapatra, L. Jain, P. Rai, K. Hazra, I. Samajdar, D. Misra, Development of crystallographic texture and in-grain misorientation in CVD-produced single and polycrystalline diamond, Chem. Vap. Depos. 17 (2011) 107e113. [48] J. Liu, Y. Yuan, S. Bashir, Functionalization of aligned carbon nanotubes to enhance the performance of fuel cell, Energies 6 (2013) 6476e6486. [49] J. Mazumder, A. Kar, Theory and Application of Laser Chemical Vapor Deposition, Springer, 1995. [50] L.G. De Arco, Y. Zhang, C.W. Schlenker, K. Ryu, M.E. Thompson, C.W. Zhou, Continuous, highly flexible, and transparent graphene films by chemical vapor deposition for organic photovoltaics, ACS Nano 4 (2010) 2865e2873. [51] G.R. Li, H. Xu, X.F. Lu, J.X. Feng, Y.X. Tong, C.Y. Su, Electrochemical synthesis of nanostructured materials for electrochemical energy conversion and storage, Nanoscale 5 (2013) 4056e4069. [52] W. Chen, R.B. Rakhi, L. Hu, X. Xie, Y. Cui, H.N. Alshareef, High-performance nanostructured supercapacitors on a sponge, Nano Lett. 11 (2011) 5165e5172. [53] N.S.M. Yusof, M. Ashokkumar, Sonochemical synthesis of gold nanoparticles by using high intensity focused ultrasound, ChemPhysChem 16 (2015) 775e781. [54] S. Anandan, F. Grieser, M. Ashokkumar, Sonochemical synthesis of Au-Ag core-shell bimetallic nanoparticles, J. Phys. Chem. C 112 (2008) 15102e15105. [55] S. Anandan, M. Ashokkumar, Sonochemical preparation of monometallic, bimetallic and metal-loaded semiconductor nanoparticles, in: Pankaj, M. Ashokkumar (Eds.), Theoretical and Experimental Sonochemistry Involving Inorganic Systems, Springer, Dordrecht, 2010.

52

CHAPTER 1 INTRODUCTION TO ADVANCED NANOMATERIALS

[56] J. Jeevanandam, Y.S. Chan, M.K. Danquah, Biosynthesis of metal and metal oxide nanoparticles, ChemBioEng Rev. 3 (2016) 55e67. [57] R. Bhattacharya, P. Mukherjee, Biological properties of “naked” metal nanoparticles, Adv. Drug Deliv. Rev. 60 (11) (2008) 1289e1306. [58] X. Zhang, S. Yan, R.D. Tyagi, R.Y. Surampalli, Synthesis of nanoparticles by microorganisms and their application in enhancing microbiological reaction rates, Chemosphere 82 (4) (2011) 489e494. [59] P. Singh, Y.J. Kim, D. Zhang, D.C. Yang, Biological synthesis of nanoparticles from plants and microorganisms, Trends Biotechnol. 34 (7) (2016) 588e599. [60] P. Golinska, M. Wypij, A.P. Ingle, I. Gupta, H. Dahm, M. Rai, Biogenic synthesis of metal nanoparticles from actinomycetes: biomedical applications and cytotoxicity, Appl. Microbiol. Biotechnol. 98 (2014) 8083e8097. [61] M.B. Dickerson, K.H. Sandhage, R.R. Naik, Protein- and peptide-directed syntheses of inorganic materials, Chem. Rev. 108 (2008) 4935e4978. [62] J. Huang, L. Lin, D. Sun, H. Chen, D. Yang, Q. Li, Bio-inspired synthesis of metal nanomaterials and applications, Chem. Soc. Rev. 44 (2015) 6330e6374. [63] A.K. Mittal, Y. Chisti, U.C. Banerjee, Synthesis of metallic nanoparticles using plant extracts, Biotechnol. Adv. 31 (2013) 346e356. [64] T. Xing, J. Sunarso, W. Yang, Y. Yin, A.M. Glushenkov, L.H. Li, P.C. Howlett, Y. Chen, Ball milling: a green mechanochemical approach for synthesis of nitrogen doped carbon nanoparticles, Nanoscale 5 (2013) 7970e7976. [65] B. Lin, U. Sundararaj, P. Potschke, Melt mixing of polycarbonate with multi-walled carbon nanotubes in miniature mixers, Macromol. Mater. Eng. 291 (2006) 227e238. [66] P. Mukherjee, B. Balamurugan, J.E. Shield, D.J. Sellmyer, Direct gas-phase formation of complex coree shell and three-layer MneBi nanoparticles, RSC Adv. 6 (2016) 92765e92770. [67] D. Benetti, R. Nouar, R. Nechache, H. Pepin, A. Sarkissian, F. Rosei, J.M. MacLeod, Combined magnetron sputtering and pulsed laser deposition of TiO2 and BFCO thin films, Sci. Rep. 7 (2017) 2503. [68] H. Deng, C. Zhang, Y. Xie, T. Tumlin, L. Giri, S.P. Karna, J. Lin, Laser induced MoS2/carbon hybrids for hydrogen evolution reaction catalysts, J. Mater. Chem. A 4 (2016) 6824. [69] F. Wang, A. Chemseddine, F.F. Abdi, R. Van de Krol, S.P. Berglund, Spray pyrolysis of CuBi2O4 photocathodes: improved solution chemistry for highly homogeneous thin films, J. Mater. Chem. A 5 (2017) 12838. [70] E.-C. Dreaden, A.M. Alkilany, X. Huang, C.J. Murphy, M.A. El-Sayed, The golden age: gold nanoparticles for biomedicine, Chem. Soc. Rev. 41 (2012) 2740e2779. [71] https://www.electrical4u.com/physical-properties-of-materials/. [72] M.S. Dresselhaus, G. Chen, M.Y. Tang, R. Yang, H. Lee, D. Wang, Z. Ren, J.P. Fleurial, P. Gogna, New directions for low-dimensional thermoelectric materials, Adv. Mater. 19 (8) (2007) 1043e1053. [73] F.T. Rabouw, C.M. Donega, Excited-state dynamics in colloidal semiconductor nanocrystals, Top. Curr. Chem. 58 (2016) 374. [74] S. Cuenot, C. Fretigny, S.D. Champagne, B. Nysten, Surface tension effect on the mechanical properties of nanomaterials measured by atomic force microscopy, Phys. Rev. B 69 (16) (2004) 165410. [75] D. Guo, G. Xie, J. Luo, Mechanical properties of nanoparticles: basics and applications, J. Phys. D: Appl. Phys. 47 (2014), 013001 (25 pp.). [76] F. Zeng, Q. Cai, X. Liu, Y. Yao, F. Zhuo, Q. Gao, Y. Xie, Y. Wang, Fabrication and mechanical properties of mesoporous silica nanoparticles reinforced magnesium matrix composite, J. Alloy. Compd. 728 (2017) 413e423. [77] B. Mu, A. Wang, One-pot fabrication of multifunctional superparamagnetic attapulgite/Fe3O4/polyaniline nanocomposites served as an adsorbent and catalyst support, J. Mater. Chem. A 3 (2015) 281e289.

REFERENCES

53

[78] S. Kumar, M. Nehra, A. Deep, D. Kedia, N. Dilbaghi, K.-H. Kim, Quantum-sized nanomaterials for solar cell applications, Renew. Sustain. Energy Rev. 73 (2017) 821e839. [79] J. Albero, J.N. Clifford, E. Palomares, Quantum dot based molecular solar cells, Coord. Chem. Rev. 263e264 (2014) 53e64. [80] K.D.G.I. Jayawardena, L.J. Rozanski, C.A. Mills, M.J. Beliatis, N.A. Nismy, S.R.P. Silva, ‘Inorganics-inorganics’: recent developments and outlook for 4G polymer solar cells, Nanoscale 5 (2013) 8411e8427. [81] S. Arulmani, S. Krishnamoorthy, J.J. Wu, S. Anandan, High-performance electrocatalytic activity of palladium- copper nanoalloy towards methanol electro-oxidation in an alkaline medium, Electroanalysis 29 (2017) 433e440. [82] S. Mekhilef, R. Saidur, A. Safari, Comparative study of different fuel cell technologies, Renew. Sustain. Energy Rev. 16 (2012) 981e989. [83] A. Kirubakaran, S. Jain, R.K. Nema, A review on fuel cell technologies and power electronic interface, Renew. Sustain. Energy Rev. 13 (2009) 2430e2440. [84] F. Ning, X. He, Y. Shen, H. Jin, Q. Li, D. Li, S. Li, Y. Zhan, Y. Du, J. Jiang, H. Yang, X. Zhou, Flexible and lightweight fuel cell with high specific power density, ACS Nano 11 (2017) 5982e5991. [85] K.C. Divya, J. Østergaard, Battery energy storage technology for power systems: an overview, Electr. Power Syst. Res. 79 (4) (2009) 511e520. [86] X. Luo, J. Wang, M. Dooner, J. Clarke, Overview of current development in electrical energy storage technologies and the application potential in power system operation, Appl. Energy 137 (2015) 511e536. [87] Y. Liu, G. Zhou, K. Liu, Y. Cui, Design of complex nanomaterials for energy storage: past success and future opportunity, Acc. Chem. Res. 50 (2017) 2895e2905. [88] L. Liu, Z. Niu, J. Chen, Unconventional supercapacitors from nanocarbon-based electrode materials to device configurations, Chem. Soc. Rev. 45 (2016) 4340e4363. [89] B. Gnana Sundara Raj, S. Bhuvaneshwari, J.J. Wu, A.M. Asiri, S. Anandan, T. Sivasankar, Sonochemical synthesis of Co2SnO4 nanocubes for supercapacitor applications, Ultrason. Sonochem. 41 (2018) 435e440. [90] S. Arulmani, J.J. Wu, S. Anandan, Amphiphilic triblock copolymer guided polyaniline embraced CNT nanohybrid with outcropping whiskers as an energy storage electrode, Electrochim. Acta 246 (2017) 737e747. [91] B. Akinwolemiwa, C. Wei, G.Z. Chen, Mechanisms and designs of asymmetrical electrochemical capacitors, Electrochim. Acta 247 (2017) 344e357. [92] D.P. Dubal, O. Ayyad, V. Ruiz, P.G. Romero, Hybrid energy storage: the merging of battery and supercapacitor chemistries, Chem. Soc. Rev. 44 (2015) 1777e1790. [93] Y. Dou, T. Pan, A. Zhou, S. Xu, X. Liu, J. Han, M. Wei, D.G. Evans, X. Duan, Reversible thermallyresponsive electrochemical energy storage based on smart LDH@P(NIPAM-co-SPMA) films, Chem. Commun. 49 (2013) 8462e8464.