Graphene Materials: Fundamentals and Emerging Applications

By Ashutosh Tiwari and Mikael Syväjärvi

Graphene Materials: Fundamentals and Emerging Applications brings together innovative methodologies with research and development strategies to provide a detailed state-of-the-art overview of the processing, properties, and technology developments of graphene materials and their wide-ranging applications. The applications areas covered are biosensing, energy storage, environmental monitoring, and health.

 

The book discusses the various methods that have been developed for the preparation and functionalization of single-layered graphene nanosheets. These form the essential building blocks for the bottom-up architecture of various graphene materials because they possess unique physico-chemical properties such as large surface areas, good conductivity and mechanical strength, high thermal stability and desirable flexibility. The electronic behavior in graphene, such as dirac fermions obtained due to the interaction with the ions of the lattice, has led to the discovery of novel miracles like Klein tunneling in carbon-based solid state systems and the so-called half-integer quantum Hall effect. The combination of these properties makes graphene a highly desirable material for applications.

 

In particular, Graphene Materials: Fundamentals and Emerging Applications has chapters covering:

•             Graphene and related two-dimensional nanomaterials

•             Surface functionalization of graphene

•             Functional three-dimensional graphene networks

•             Covalent graphene-polymer nanocomposites

•             Magnesium matrix composites reinforced with graphene nanoplatelets

•             Graphene derivatives for energy storage

•             Graphene nanocomposite for high performance supercapacitors

•             Graphene nanocomposite-based bulk hetro-junction solar cells

•             Graphene bimetallic nanocatalysts foam for energy storage and biosensing

•             Graphene  nanocomposites-based for electrochemical sensors

•             Graphene electrodes for health and environmental monitoring

• Language: English
• Publisher: Wiley
• Release date: Apr 1, 2015
• ISBN: 9781119131830

Ashutosh Tiwari

Professor Ashutosh Tiwari is Director at Institute of Advanced Materials, Sweden; Secretary General, International Association of Advanced Materials; Chairman and Managing Director of VBRI Sverige AB and AAA Innotech Pvt. Ltd; Editor-in-Chief, Advanced Materials Letters and Docent in the Applied Physics with the specialization of Biosensors and Bioelectronics from Linköping University, Sweden. Prof. Tiwari has several national and international affiliations including in the United States of America, Europe, Japan, China and India. His research focus is on the design and advanced applications of cutting-edge advanced materials for new age devices. He has more than 200 peer-reviewed primary research publications in the field of materials science and nanotechnology and has edited or authored over 50 books.

Book preview

Part 1

FUNDAMENTALS OF GRAPHENE AND GRAPHENE-BASED NANOCOMPOSITES

Chapter 1

Graphene and Related Two-Dimensional Materials

Manas Mandal¹, Anirban Maitra¹, Tanya Das² and Chapal Kumar Das*¹

¹Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur India.

²Nanyang Technological University, Singapore.

*Corresponding author: [e-mail]

Abstract

In today’s nanomaterial moderated world, besides intercalated compounds like graphite, fullerenes and carbon nanotubes; search for specialized materials (2-Dimensional) such as graphene, hexagonal boron nitride (h-BN), monolayer molybdenum disulfide, molybdenum selenide (MoSe2), molybdenum telluride, tungsten sulfide, etc., for sophisticated applications in batteries, electrochromics, integrated circuits, photovoltaic, cosmetics, catalysts, solid lubricants and supercapacitors have been a demanding field of scientific inquiry. Graphene, the most significant 2D nanomaterial having sp² hybridized carbon atoms in a honeycomb arrangement is derived from pristine graphite. It is basically a semiconductor type material having a zero band gap. Simultaneously, it has got a very high charge mobility of some higher order magnitude than silicon semiconductor. To increase the conductivity of graphene, we can dope it by using nitrogen. Moreover, it has got a very high surface area as well as excellent thermal conductivity. In the case of graphene-based polymer nanocomposites, it gives a high modulus with an excellent mechanical and thermal stability. The chapter describes preparation and properties of graphene and alike two- dimensional materials.

Keywords: Nanomaterials, 2D materials, polymer nanocomposites, supercapacitors, piezoelectric, field effect transistors

1.1 Introduction

Graphene is a two-dimensional new allotrope of carbon, having monoatomic thick hexagonal (honeycomb) lattice structure with carbon-carbon distance of 1.42 Å. In other words, it is a single layer of graphite having sp² hybridized carbon atoms. Graphene is the basic building block of all other graphitic materials such as, three dimensional (3D) graphite, one dimensional (1D) carbon nanotubes and zero dimensional (0D) fullerenes [1]. Due to its attractive physical and chemical properties such as very high surface area, excellent electronic and thermal conductivities, superior mechanical and electrochemical stability, good transparency, graphene has grabbed a great scientific and technological interest in recent years [2]. Moreover, graphene can be easily produced in large scale by the reduction of graphene oxide. Because of these remarkable properties as well as ease synthesis of graphene, it has been widely used in many fields such as polymer nanocomposites, energy storage and conversion (e.g. supercapacitors, batteries, fuel cells and solar cells), chemical sensors, flexible electronic and optical devices [3–8]. Graphene shows double layer capacitance, which is resulted by the charge or ion accumulation on the surface of electrode/electrolyte interface.

Intrinsic (undoped) graphene is a semi-metal or zero gap semiconductor. It exhibits amazing electronic and mechanical properties such as, extremely high charge carriers (electrons and holes) mobility = 230,000 cm² V−1 s−1 at room temperature, thermal conductivity = 3,000 W m−1 K−1, mechanical stiffness =1 TPa with large surface area 2,600 m² g−1 [9]. Graphene is also a transparent material which can absorb 2.3% light of the optical region. In the year 2010, Andre K. Geim and Konstantin S. Novoselov were awarded a Nobel Prize for groundbreaking experiments regarding the two dimensional material graphene. They successfully synthesized free-standing graphene film for the first time by using an effective mechanical exfoliation method with a scotch tape and silicon substrate [10]. Graphene is the first two-dimensional atomic crystal [11] and it is the representative of other two-dimensional materials such as metal chalcogenides, transition metal oxides and single layer of boron nitride.

In graphite, adjacent graphene layers are bonded with weak interaction of pz orbitals. This interaction between pz orbitals restricts the complete separation of bulk graphite layers into individual graphene sheets under mechanical actions. Mechanical exfoliation of graphite results in either stack of sheets, or a small amount of detached sheets. This depends on the condition of mechanical exfoliation. Chemical oxidation and then simultaneously reduction of graphite oxide results graphene like materials termed as highly reduced graphene oxide (HRG) which contains defects and residual oxygen-containing functionalities on the periphery of the sheets.

In general, methods for preparing graphene and HRG can be classified into five categories [11, 12]:

1) Mechanical exfoliation of a single sheet of graphene from flaky like pristine graphite. 2) Epitaxial growth of graphene over SiO2 substrate. 3) Chemical vapor deposition (CVD) of graphene single layers. 4) Longitudinal unzipping of Carbon nanotubes. 5) Reduction of graphene oxide and graphene fluoride.

Epitaxial growth usually forms good quality graphene with fewer defects but it requires high-vacuum surroundings and expensive fabrication systems to generate a small size films. A CVD technique produces graphene monolayers with large surface areas. Longitudinal unzipping of CNTs can produce mainly graphene nanoribbons having width which is dependent on CNTs diameter. Nowadays, the reduction of graphene derivatives is the novel strategy for the preparation of graphene like sheets. Graphene oxide, HRG and graphene can be modified easily using a proper chemical reaction and subsequently introduced as nanofillers in composites with polymeric and/or inorganic materials. The most common route for producing large quantities of reduced graphene starts with the oxidation of graphite to graphene oxide (GO).

The graphene oxide was first invented several decades ago by Brodie, Staudenmeier and Hummer [13–15]. Scientists are still following the same synthesis procedure with minor changes. The atomic ratio of C : O indicates the extent of graphite oxidation. This solely depends on the synthesis procedure and the duration of the oxidation period [16]. The Hummers’ method is more efficient method for the preparation of graphene oxide. The two main reasons behind the huge acceptance of this method by the researchers are following: (i) it takes short time for the completion of the reaction and (ii) it does not need hazardous chlorine dioxide. One deficiency of this method is contamination by excess permanganate ions, but the problem can be eliminated by treating with H2O2, [17] followed by washing with water. The oxidation of graphite to GO breaks up the sp² hybridized structure of the stacked graphene layers [18] and increase the gap between adjacent layers from 3.35 A° in pristine graphite powder to 6.8 A° for GO powder [19]. The increment in d spacing value varies significantly depending on the amount of water introduced within the stacked-sheet structure [20] and decreases interaction between sheets and thereby facilitating the delamination of GO into separate graphene oxide sheets upon sonication. At slightly basic pH, hydrophilic oxygen-containing functional groups on the graphene oxide surface can maintain the dispersions of these sheets in aqueous media [21].

1.2 Preparation of Graphene Oxide by Modified Hummers’ Method

Graphene oxide can be easily synthesized by Modified Hummers’ method recently reported by Marcano et al [22]. Briefly, 3 g graphite fine powder was added into a mixture of concentrated H2SO4/H3PO4 (540 mL: 60 mL) and stirred for some time using a teflon coated mechanical stirrer. Then 18 g KMnO4 was added pinch by pinch to the mixture solutions because enormous heat is produced due to exothermic reaction. The solution becomes greenish. The mixture was continuously stirred in an oil bath for 12 h at a stirring speed of 340 rpm. After that the reaction mixture was cooled to room temperature. Then the mixture was poured into ice water (400 mL) containing 30% H2O2 (3 mL) and a nicely color change was observed from greenish to grey to yellowish. The graphene oxide suspension was stirred for another 4 h and centrifuged at 4000 rpm. The solid material was then washed in succession with 20% HCl, acetone, and excess water until the pH was reached about 7. Finally, the grey colored solid graphene oxide was dried at 60°C under vacuum for 48 h.

1.3 Dispersion of Graphene Oxide in Organic Solvents

Graphene oxide is hydrophilic due to oxygen containing functionalities in its surface. Its dispersion in water can be done by using ultrasonication process. However, suspending graphene oxide in organic solvent is not an easy task. This requires modification of graphene oxide with organic isocyanates type compounds [23], where the surface and edge hydroxyl and carboxyl groups of graphene oxide were transformed into amide and carbamate groups respectively. The modified with isocyanato sheets are easily dispersible in N, N-dimethylformamide (DMF), Dimethyle Sulfoxide (DMSO), and N-methylpyrrolidone (NMP) as these are polar organic solvents but not in water.

In the presence of TiO2 nanoparticle, suspensions of graphene oxides sheets are not agglomerated because TiO2 nanoparticle covers and stabilizes the surface area of graphene oxide sheets [24]. Surface modification of graphene oxide is useful for preparing organic dispersions. But the problem is that the presence of TiO2 which is coated over graphene oxide sheets during dispersion in organic solvents can change the electronic properties of graphene oxide to a great extent. Cai et al. prepared fully exfoliated grapheme oxide nanoplatelets in DMF [20], while the Paredes group increased the stability up to two to three weeks of the graphene oxide dispersions by using some polar solvents such as NMP, ethylene glycol and tetrahydrofuran (THF) [21]. The Ruoff group achieved the stable dispersions of unmodified graphene oxide by using 9:1 [v/v] organic solvent: water medium [25]. They have shown that graphene oxide could be dispersed with appropriate organic solvent via dilution process. DMF, DMSO, ethanol, NMP produces a stable dispersions of graphene oxide because of high polarity of these solvents. Similarly, the less polar organic solvent such as acetone, THF and toluene produces flocculation or aggregation of graphene oxide.

1.4 Paper-like Graphene Oxide

Recently, aqueous dispersions of lamellar clay (vermiculite and mica) into free-standing paper by the flow-directed filtration are a well-known commercialized procedure. Dikin et al. was imitated this technique for graphene oxide dispersions to give paper like shape (Figure 1.1a) [26]. Figure 1.1a shows a brownish black paper like material having a layered structure with an intersheet gap of 8.3A°, which is very close that of un-exfoliated GO (6.8A°) [13]. This is only due to the effect of intercalation of water. Figure 1.1b shows the SEM image of the edge of graphene oxide paper which consists of very closely packed sheets that cautiously form a wavy nature along the paper surface.

1.5 Thin Films of Graphene Oxide and Graphene

Another important discovery is thin films of graphene oxide. This nanometer-thick thin film consist few graphene oxide sheets which can be mono-, bi-, and tri- layers of graphene oxide. Such type of film is used as segment in field-effect transistor [27]. Ionically conductive composite film could be prepared by using an alternating uniform, single graphene oxide monolayer and polyelectrolyte layers [28, 29]. Graphene thin films are very promising materials due to their high conductivity and transparency (Figure 1.1c) [30]. But production of bulk quantities of graphene thin films is still not easy. However, reduction of as prepared graphene oxide thin film is only advantageous method to achieve large scale of graphene thin film. Mattevi et al. prepared reduced graphene oxide thin films ranging from single to few layers by solution based method and thermal annealing [31].

Figure 1.1 (a) Graphene oxide paper ribbon. (b) SEM image of the edge of graphene oxide paper [Dikin et al, Nature 2007 (ref. 25)]. (c) Graphene paper produced by filtration of an aqueous graphene solution [Li et al, Science 2008 (ref. 30)].

1.6 Nanocomposites of Graphene Oxide

Nowadays, nanocomposites of graphene oxide have attracted great importance from the researchers due to employment of the graphene oxide sheets as filler material dispersed within a continuous polymer or an inorganic polymer matrix. Graphene oxide sheets containing polymer nanocomposites have been studied for a wide range of applications in different fields [32]. Structurally carbon analogs graphene oxide sheets are very much equivalent to two dimensional montmorillonite clay. Exception is that the oxygen containing functional groups are oriented over the layers. Polymer-clay nanocomposites are mainly processed by extrusion, melt mixing, solution casting etc. Here polymers are forcefully intercalated into the layered type clay structure [33]. Unlike clay, graphene oxide has many advantages to form nanocomposites, such as high surface area to volume ratio, high dispersibility in water as well as in other organic solvent, high mechanical strength, better chemical stability etc. A large number of oxygen containing functional groups on the surface of graphene oxide facilitates dispersibility in solvent as well as reduces aggregation and enhances the interaction between fillers and polymers in nanocomposite. A large number of thin films based on graphene oxide nanocomposites have been studied for transparent and flexible electronic device. In case of conductivity studies, graphene oxide is usually reduced to graphene. Thin films are mainly prepared by spin coating or spin casting by using a proper substrate. Watcharotone et al. was fabricated a transparent and electrically conductive graphene-silica composite film on glass and SiOx/silicon substrate by using graphene oxide sheets [34].

1.7 Graphene-Based Materials

A two dimensional graphene sheet, having extraordinary electronic and mechanical properties are more preferable than carbon nanotubes. The ongoing research on graphene has established a new era in the field of materials science. Instead of direct synthesis of graphene from commercially available graphite, bulk quantities of exfoliated graphene sheets are prepared by the reduction of graphene oxide. Although it is very difficult to achieve pristine graphene, a large number of reduction strategies (thermal and chemical reduction) have been developed for the reduction of graphene oxide [35]. The obtained exfoliated graphene sheets are called reduced graphene oxide sheets which contain some residual oxygen-containing functionalities, such as periphery carboxylic groups (Figure 1.2). Due to the presence of this functional groups, the ratio of C : O of reduced graphene oxides are ranging from 10 : 1 [36] to 5 : 1 [37].

Figure 1.2 Schematic model of reduced graphene oxide sheet.

Mechanically exfoliated pristine graphene sheets possess higher mechanical strength and conductive properties than reduced graphene oxide due to highly extended conjugative structure [10, 38]. These physical properties are enhanced by the synthesis of many novel graphene-based materials. Recently reduced graphene oxide has drawn a great attention as a filler material in polymer nanocomposites as it can be easily functionalized, very high dispersibility in many polymers even it can show a synergistic properties with other nanoparticle in polymer matrices. In reduced graphene oxide–polymer nanocomposites, a very little amount of loading (0.1–5 vol%) of reduced graphene oxide leads an enormous changes in the electronic and mechanical properties [32]. For the first time, Stankovich et al. was prepared electrically conductive polystyrene-graphene nanocomposite using exfoliated phenyl isocyanate modified graphene oxide and polystyrene by solution phase mixing, followed by the chemical reduction. They achieved high dispersion of individual graphene sheets through the polymer matrices [39]. Nanoplatelets morphology without any multilayer stacking was obtained from SEM images (Figure 1.3). Electrical-conductivity measurements of nanocomposites show a gradual increase in electrical conductivity (0.1 to 1 S m−1) with increased loading of graphene sheets (1 to 2.5 vol%).

Figure 1.3 SEM images of graphene-polystyrene nanocomposites at low (a) and high magnification (b) [Stankovich et al. Nature, 2006 (ref. 39)]

An excellent improvement in thermal and mechanical properties can be done with very small amount of loading of graphene sheets to the polymer matrices. Ramanathan et al. have shown good dispersion and intimate interaction between graphene sheets and the matrix polymer can significantly enhance their performance [40]. They prepared functionalized graphene sheets of poly(methyl methacrylate) (PMMA) composite with a small loading (0.01wt%) of graphene sheets and this improves the glass transition temperature (Tg) (~30°C) as well as Young’s modulus (33%). Yuan et al. achieved a 67% increase in tensile strength of 0.5wt % graphene-PMMA nanocomposites [41]. Similar improvements in both Young’s modulus (57%) and ultimate tensile strength (70%) have been observed for polystyrene-grafted graphene nanocomposites [42].

1.8 Other Two-dimensional Materials

With increasing the research interest on graphene, the other two dimensional materials such as transition metal dichalcogenides (TMD) [WS2, MoS2, SnS2, SnSe and SnSe2], transition metal oxides [MnO2, NiO], hexagonal boron nitride [h-BN] have got emerging attention from the scientific community due to their extraordinary novel properties. Here we will briefly discuss about their synthetic procedure, properties and applications.

1.8.1 Tungsten Sulfide

In recent days, dichlacogenides of higher atomic weight transition metals have created a profound impact in advance materials research fields owing to its single layer array. In 1992, Tenne et al. first achieved the stable polyhedral and cylindrical structures of tungsten disulfide by heating the tungsten film in hydrogen sulfide atmosphere [43]. Generally, transition metal chalcogenides show graphene like layered structure, with transition metal atom placed in trigonal prismatic coordination sphere. Figure 1.4 represents the structure of a hexagonal TMD monolayer. The electrical properties of these dichalcogenides depend on their composition, structure of the crystal and the number of layers [44].

Figure 1.4 Structure of a hexagonal TMD monolayer (a) perspective view and (b) along the perpendicular axis.

1.8.1.1 Different Methods for WS2 Preparation

Tungsten sulfide can be synthesized in a numerous number of methods:

1) Hydrothermal preparation. 2) Reducing ammonium tetrathiotungstate [(NH4)2WS4] at ~1200°C in presence of hydrogen gas. 3) Gas phase reaction of hydrogen sulfide with tungsten metal in presence of argon atmosphere. 4) Decomposition reaction of various tetraalkylammonium tetrathiotungstate precursors in presence of inert gas. 5) Microwave treatment of a concentrated solution of tungstic acid, elemental sulfur and mono ethanolamine. 6) Heating WS3 in absence of oxygen atmosphere (otherwise the product will be tungsten trioxide). 7) Melting a proportionate mixture of WO3, K2CO3 and sulfur. 8) Mechanical exfoliation of tungsten sulfide in a liquid phase in presence of chlorosulfonic acid.

Typically monolayers and stacked few layers of tungsten sulphide can be synthesized by mechanical exfoliation and chemical vapor deposition (CVD) procedure by using WOCl4, WO(CO)6, or WCl6 with HS-(CH2)2-SH or HSC(CH3)3 as precursors [45]. Seo et al. synthesized 2D WS2 nanosheet crystals having lateral dimensions of less than100 nm can be synthesized from one-dimensional (1D) W18O49 by applying a rolling out method using surfactant-assisted solution process [46]. Recently, Wu et al. obtained WS2 nanosheets with less than 10 nm thickness from tungsten oxide (WO3) and sulfur powder by a mechanical activation strategy. The total reaction process involves a ball-milling process followed by annealing at 600-700°C in argon atmosphere [47]. The reaction between tungstic acid and thiourea in presence of nitrogen gas at 773 K results uniform graphene-like layered form of WS2 [48].

The most advantageous procedure is the hydrothermal method for the large scale synthesis of transition metal dichlacogenides at low temperature. But the synthesis of tungsten sulfide nanosheets by the hydrothermal method is still challenging. The prime factor behind the formation of nanosheets is due to the precursor WOx needed during the formation of WS2 which does not arise in two dimensional forms. WOx usually adopt one dimensional or very rarely in zero dimensional nanostructure forms. That is why the sulfurization of the WOx produces either zero dimensional fullerenes-like or one dimensional nanotube/nanorods like structures [49]. Recently, Cao et al. successfully prepared various kinds of morphologies such as nanoparticle, nanorod, nanosheets and nanofibres of WS2 by using different surfactants and discussed their possible growth mechanisms of different nanostructures [50]. However, synthesis of 1D WS2 nanocrystal or nanotube by hydrothermal process has been first reported in 2005. The quasi 1D WS2 nanocrystal and multiwalled nanotube were prepared by using Na2WO4 or (NH4)10W12O41 as precursors which reacts with acid to form WOx nanoparticles first then obtained trioxide was sulfurized to give WS2 [51, 52]. Generally, condensed WOx nanoparticles act as templates during the formation of tungsten sulfide nanosheets [49].

1.8.1.2 Properties of WS2

Tungsten sulfide is usually obtained in dark grey color having a hexagonal crystal structure. They are very much chemically inactive and can only dissolve in a quantitative mixture of nitric and hydrofluoric acids. Tungsten sulfide converts into corresponding tungsten trioxide while burning in presence oxygen. Tungsten sulfide does not melt during heating in absence of oxygen gas. It disintegrates to elemental tungsten and sulfur near about 1250 °C [53]. Tungsten disulfide acts as a lubricant material as its coefficient of friction is 0.03. The lubricating properties of tungsten sulfide are admirable under vigorous conditions of load, vacuum and temperature. Tungsten disulfide also makes its profound impressions in high temperature and high pressure applications. It offers a wide range temperature shield from –240 °C to 650 °C in normal atmosphere and from –170 °C to 1316 °C in vacuum. Load bearing ability of tungsten sulfide incorporated film is as high as 300,000 psi. Tungsten disulfide can replace molybdenum disulfide and graphene in certain field of applications like electrical and electronic industries, sound detection, production of electronic frequency and high voltage. Tungsten disulfide is piezoelectric material because it has an ability to produce electric charge under an external mechanical stress. This is a reversible process. When a mechanical stress (such as, deformation, bending force, pressure) is applied to these materials, the charge symmetry within the crystal structure has been disrupted, which results an external electric field and vice versa [54]. NASA, military, aerospace and automotive industries are also using this material expensively.

1.8.1.3 WS2 and Reduced Graphene Oxide Nanocomposites

As the synthesis of WS2 nanosheets is problematic in hydrothermal process, many researches have tried to make a hybrid nanocomposite of WS2 nanosheets with reduced graphene oxide by in situ reduction of graphene oxide for numerous applications [49, 55–57]. Tungsten sulfide/reduced graphene oxide (WS2/rGO) hybrid nanocomposites show good catalytic activity for hydrogen evolution as well as it is used for energy storage and conversion such as supercapacitor, Na-ion battery and solar photovoltaic applications. In terms of Impedance spectroscopic measurements, it is concluded that modified catalytic activity of WS2/rGO nanocomposites appears mainly due to charge transfer phenomenon. Efficient charge transfer occurs mainly due to an intimate contact in between tungsten sulfide and the reduced graphene oxide components. As mentioned earlier that hydrothermal preparation of tungsten sulfide is sensitive towards temperature. Tungsten sulfide/reduced graphene oxide nanocomposite sheets were then dried at 300°C to boost the crystallinity of the nanosheets [49].

Figure 1.5 (a) and (b) depicts the SEM images of WS2 and WS2/rGO hybrid nanocomposites respectively [49]. The surface of tungsten disulphide is quite ruff with a large number of micro voids and pores in its surfaces. The as prepared tungsten disulfide/reduced graphene oxide hybrid nanocomposites shrinks immediately after freeze-drying which perhaps due to the removal of water adsorbed on reduced graphene oxide. The high resolution transmission electron microscopic image of tungsten disulfide/reduced graphene oxide hybrid nanocomposite is shown in Figure 1.6 which displays the overlapping nanosheets morphology with bilayer WS2 nanosheets in some areas. The tungsten disulfide/reduced graphene oxide hybrid nanocomposites imparts promising catalytic properties. The tungsten disulfide/reduced graphene oxide hybrid nanocomposites exhibits a potential window ranging from 150–200 mV versus reversible hydrogen electrode (RHE). It also shows the high sodium storage capacity of 590 mA h g−1 with excellent performance and cyclability [55].

Figure 1.5 SEM images of WS2 (a) and WS2/rGO hybrid nanosheets (b) [Yang et al. Angew. Chem. Int. Ed. 2013 (ref. 49)].

Figure 1.6 HRTEM image of WS2/rGO hybrid nanosheets [Yang et al. Angew. Chem. Int. Ed. 2013 (ref. 49)].

1.8.2 Molybdenum Sulfide

MoS2 is one of the family members of the transition metal dichalcogenides (TMDs) which have an analogous structure of graphene and has attracted much more importance due to its unique chemical and physical properties. The structure of MoS2 is composed of three atom layers (S-Mo-S) associated by weak van der Waals interactions, where the hexagonal Mo atom layers is sandwiched between two hexagonal S atom layers [43, 58]. Single layer of MoS2 is strongly piezoelectric in parallel with other two dimensional high performance piezoelectric materials. Wu et al. reported that oscillating piezoelectric voltage and current outputs depends on the number of atomic layers present in thin MoS2 flake with applied strain [54, 59]. Because of this layered structure it has been used in many application fields, such as, Li-ion battery, electrochemical capacitor, memory cell, catalysts and composites.

Due to similar layered structure of graphene, MoS2 nanocomposite with graphene has been used as extraordinarily high performance anode material for Li-ion battery [60, 61]. The first Li-ion battery using MoS2 was published in a patent in 1980. The main advantage is Li+ ions easily intercalate and exfoliate through the layers. An exfoliated–restacked MoS2 electrode material was reported for a tremendously high lithium ion storage capacity (~ 840 mA h g−1) by Du et al. [62]. Whereas, Wang et al. have used single layer MoS2-graphene nanosheets composites for high electrochemical reversibility for Li+ storage capacity (~ 825 mAh g−1), where graphene nanosheets improve the conductivity in the electrode and as well as the rate of electrons transfer during electrochemical reactions in the electrode [61]. FESEM images of MoS2 and MoS2/reduced graphene oxide nanocomposites are shown in Figure 1.7 which indicates the flowery architecture composed by nanopetals in the form of nanosheets. Both MoS2 and rGO nanosheets were obtained as intercalated state [63].

Figure 1.7 FESEM images of MoS2 (a) and MoS2/rGO nanocomposites (b) [Mandal et al. IJLRST 2014 (ref. 63)].

Figure 1.8 shows the HRTEM images of MoS2 and MoS2-graphene nanosheets composite which revealed the well layered structure with a lattice spacing of (002) plane is of 0.62 nm and 1.15 nm, respectively.

Figure 1.8 HRTEM images of MoS2 (a) and MoS2-graphene nanosheets composites (b) [Wang et al. J. Mater. Chem. A 2013 (ref. 61)].

1.8.3 Tin Sulfide

Recently, layered tin sulfide has attracted a great interest because of its exclusive structural characteristics. It is an n-type semiconductor. The structure of SnS2 is very similar to MoS2. SnS2 shows layered CdI2 like structure. In each and every layer, Sn atoms are stacked in between two layers of hexagonally close-packed S atoms and the nearest sulfur layers are connected through weak van der Waals interactions. Because of this 2D layered structure, it intercalates alkali metal and shows electric and photoelectric conductivity.

A large number of methods have been developed for the synthesis 2D SnS2 nanoplates or nanosheets. It can be prepare by using thermal decomposition [64] or hydrothermal synthesis [65]. Seo et al. [64] have synthesized 2D layered SnS2 nanoplates by thermally decomposing the precursor, e.g., Sn(S2CNEt2)4, in presence of an organic solvent at 180 °C. They have shown extraordinary high irreversible discharge capacity (~1311 mA h g−1) for lithium ion batteries due to extended surface area of SnS2 nanoplates and greater access of lithium ions.

Figure 1.9 demonstrates the TEM and FESEM images of 2D hexagonal, highly crystalline SnS2 nanoplates. The lateral size of the nanoplates is about 150 nm and the thickness of SnS2 nanoplates is around 15 nm. Gao et al. have synthesized SnS2 nanosheets by a simple single step hydrothermal process [65]. They have used tin chloride pentahydrate (SnCl4·5H2O) and thioacetamide (TAA) as precursor agents. They have demonstrated the ferromagnetic behavior of porous hexagonal disulfide at room temperature due to disordered grain boundary, defects or edges.

Figure 1.9 TEM (a) and FESEM (b) images of SnS2 nanoplates [Seo et al. Adv. Mater. 2008 (ref. 64)].

1.8.4 Tin Selenide

Generally, tin selenides are stoichiometrically two types: SnSe and SnSe2. Among of them SnSe2 has hexagonal layered structures. Liu et al. have synthesized hexagonal nanoflakes of SnSe2 by hydrothermal method by using SnCl2·2H2O and SeO2 as precursors at 180°C [66]. Figure 1.10 shows the FESEM images of hexagonal nanoflakes of SnSe2. Each hexagonal nanoflake is about 600–700 nm in side length and 30–45 nm thickness.

Figure 1.10 FESEM images of hexagonal SnSe2 nanoflakes: Top-view (a) and side-view (b) [Liu et al. Mater. Lett. 2009 (ref. 66)].

Two dimensional layered semiconductor materials have been significantly useful as electrode materials for lithium ion batteries because Li+ ions can be easily inserted into the weakly interacting layers and come out during electrochemical reactions. Recently, pure SnSe2 or SnSe2 nanoplate-graphene composites have been playing an important role in Li+ ion battery due to its two dimensional layer morphology [67].

Some metal oxides such as MnO2, NiO etc. also have been playing the important role in nanotechnology as two-dimensional material.

1.8.5 Manganese Dioxide

MnO2 is another important inorganic material which is used mainly for preparing electrode materials for supercapacitor applications. Actually birnessite-type manganese dioxide (MnO2) having a layered nanosheet structures actuates the acceptance of numerous number of metal cations from an electrolyte to move in and out of the interlayer region. The movements of the metal cations do not make any structural changes of MnO2 [68].

A large capacitance value can be obtained by changing the design of the electrode as well as the morphology and crystal structure of the MnO2 nanosheets. The morphology of the MnO2 nanosheets depends on the processing conditions. Figure 1.11 shows the SEM micrograph of MnO2 nanosheets on carbon fiber, synthesized via anodic electrodeposition using 0.1 M MnSO4 precursor in 0.1 M H2SO4 solution [69].

Figure 1.11 SEM micrograph of MnO2 nanosheets on carbon fiber. HRSEM of an individual MnO2 nanosheet cluster (insect) [Hsu et al. Chem. Commun. 2011 (ref. 69)].

1.8.6 Nickel Oxide

NiO, a two-dimensional nanomaterial has achieved a huge potential for energy storage application. Ultrathin nano dimensional sheets of NiO have similar to graphene like morphology with a sheet thickness of around 2 nm [70]. Figure 1.12 shows the FESEM images of NiO nanosheets at different magnifications. Zhu et al. reported a cost effective microwave synthesis root for large scale preparation of ultrathin 2D NiO nanosheets. One can design an anode by incorporating this electrode material for lithium ion batteries. It exhibits a reversible lithium ion storage capacities with the discharge capacity of 1574.7 mA h g−1 at 200 mA g−1 current with excellent cycling stability. Lee et al. prepared disorderly interconnected thin and highly vented NiO nanosheets with 10-30 nm thickness for supercapacitor application. The capacitance comes from pseudocapacitive capacitance based on a fast redox reaction : NiO + OH− ↔ NiOOH + e− [71].

Figure 1.12 FESEM images at low (a) and high (b) magnifications of NiO nanosheets [Zhu et al. J. Mater. Chem. A 2014 (ref. 70)].

1.8.7 Boron Nitride

Generally, hexagonal shaped boron nitride (h-BN) can be found in sheet form which has attracted a great interest in materials research field because it is the structural analogue of graphene. It is also called ‘White Graphene’. By substituting all C atoms of graphene with alternating B and N atoms forms honeycomb lattice of BN [72]. However, h-BN is a wide band gap (~6.00 eV) semiconductor due to ionic character of B-N bond. For this reason, h-BN sheets have more advantages than graphene [73]. It is thermally stable up to 1000 K and more resistant to oxidation. Single layer of h-BN is also piezoelectric [54]. Lee et al. achieved three-fold higher mobility for a graphene device on high quality h-BN nanosheets, grown by chemical vapor deposition technique [74]. Hexagonal boron nitride is also used as dielectric material for electronic device like field-effect transistors (FETs). Lee et al. made heterostructure devices based on a stack of MoS2/h-BN/graphene by a mechanical stacking process [75].

Two dimensional h-BN nanosheets were first prepared by decomposition of borazine with the help of metallic substrates. The small quantity of h-BN flakes can be easily prepared by chemical exfoliations of h-BN. Figure 1.13 shows the typical SEM images of boron nitride-carbon phase-separated composite nanosheet.

Figure 1.13 SEM image of the BN-C composite (a). Compact structure of the nanosheets observed at a higher magnification (b) [Pakdel et al. J. Mater. Chem. 2012 (ref.73)].

1.9 Conclusion

In conclusion, graphene and related 2D materials such as transition metal dichalcogenides (MoS2, WS2, SnS2 and SnSe2), transition metal oxide (MnO2, NiO) and hexagonal boron nitride (h-BN) discussed in this chapter, have intense and increasing applications in various field of materials research for example energy storage application, catalyst application, piezoelectric devices, FETs etc.

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