Graphene materials  fundamentals and engineering applications

Preface xv Part 1: Fundamentals of Graphene and Graphene-Based 1 Graphene and Related Two-Dimensional Materials 3 Manas Mandal, Anirban Maitra, Tanya Das and Chapal Kumar Das 1.2 Prep

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Preface xv

Part 1: Fundamentals of Graphene and Graphene-Based

1 Graphene and Related Two-Dimensional Materials 3

Manas Mandal, Anirban Maitra, Tanya Das and

Chapal  Kumar Das

1.2 Preparation of Graphene Oxide by Modifi ed

1.3 Dispersion of Graphene Oxide in Organic Solvents 6

1.5 Th in Films of Graphene Oxide and Graphene 71.6 Nanocomposites of Graphene Oxide 8

2 Surface Functionalization of Graphene 25

Mojtaba Bagherzadeh and Anahita Farahbakhsh

2.2 Noncovalent Functionalization of Graphene 272.3 Covalent Functionalization of Graphene 342.3.1 Nucleophilic Substitution Reaction 342.3.2 Electrophilic Substitution Reaction 41

2.4.1 Metals NPs: Au, Pd, Pt, Ag 542.4.2 Metal oxide NPs: ZnO, SnO2, TiO2, SiO2,

RuO2, Mn3O4, Co3O4, and Fe3O4 542.4.3 Semiconducting NPs: CdSe, CdS, ZnS, CdTe and

References 58

3 Architecture and Applications of Functional

Th ree-dimensional Graphene Networks 67

Ramendra Sundar Dey and Qijin Chi

3.1.1 Synthesis of 3D Porous Graphene-Based Materials 693.1.1.1 Self-assembly Approach 693.1.1.2 Template-assisted Synthesis 703.1.1.3 Direct Deposition 71

4 Covalent Graphene-Polymer Nanocomposites 101

4.8.1 Living Radical Polymerizations 115

4.9.1 Graphene Oxide-based Chemistry 1274.9.2 Crosslinking Reactions 130

4.9.4 Other Graft ing-to Approaches 137

References 141

Part 2: Emerging Applications of Graphene in

Energy, Health,  Environment and Sensors 151

5 Magnesium Matrix Composites Reinforced with Graphene

Properties of Pure Magnesium 156

5.2.3 Microstructural Characterization 1575.2.4 Crystallographic Texture Measurements 1585.2.5 Mechanical Characterization 160

5.3 Synergetic Eff ect of Graphene Nanoplatelets (GNPs)

and Multi-walled Carbon Nanotube (MW-CNTs) on

Mechanical Properties of Pure Magnesium 164

5.4 Eff ect of Graphene Nanoplatelets (GNPs) Addition

on Strength and Ductility of Magnesium-Titanium Alloys 175

5.4.2.1 Primary Processing 1765.4.2.2 Secondary Processing 1765.4.3 Microstructure Characterization 1765.4.4 Mechanical Characterization 178

5.5 Eff ect of Graphene Nanoplatelets on Tensile

Properties of Mg–1%Al–1%Sn Alloy 180

5.5.3 Microstructure Characterization 1805.5.4 Mechanical Characterization 181

Acknowledgments 184References 185

6 Graphene and Its Derivatives for Energy Storage 191

Malgorzata Aleksandrzak and Ewa Mijowska

6.2 Graphene in Lithium Batteries 192

6.2.2 Lithium-Oxygen Batteries 2016.2.3 Lithium-Sulfur Batteries 2066.3 Graphene in Supercapacitors 212

References 218

7 Graphene-Polypyrrole Nanocomposite: An Ideal Electroactive Material for High Performance Supercapacitors 225

Alagiri Mani, Khosro Zangene Kamali, Alagarsamy

Pandikumar, Lim Yee Seng, Lim Hong Ngee

and Huang Nay Ming

7.3 Importance of Energy Storage 227

7.5 Principle and Operation of Supercapacitiors 2287.6 Electrode Materials for Supercapacitors 2307.7 Graphene-based Supercapacitors and Th eir Limitations 2317.8 Graphene-Polymer-Composite-based Supercapacitors 2327.9 Graphene-Polypyrrole Nanocomposite-based

8 Hydrophobic ZnO Anchored Graphene Nanocomposite

Based Bulk Hetro-junction Solar Cells to Improve Short

8.5.1 Zinc Oxide Nanoparticles 2588.5.2 ZnO Nanoparticle Decorated Graphene (Z@G)

8.6 Characterization of Synthesized ZnO Nanoparticles and ZnO Decorated Graphene (Z@G) Nanocomposite 259

8.6.6 Hydrophobicity Measurement 2668.7 Hybrid Solar Cell Fabrication and Characterization 267

9 Th ree-dimensional Graphene Bimetallic Nanocatalysts

Foam for Energy Storage and Biosensing 277

Chih-Chien Kung, Liming Dai, Xiong Yu and Chung-Chiun Liu

9.1 Background and Introduction 278

9.1.3 Bimetallic Nanocatalysts 2829.1.4 Carbon Supported Materials 2829.1.5 Rotating Disk Electrode 2849.1.6 Cyclic Voltammetry and Chronoamperometric Techniques 2869.1.7 Methods of Estimating Limit of Detection (LOD) 2889.1.8 CO Stripping for the Estimation of the Catalyst

9.1.9 Brunauer, Emmett and Teller (BET) Measurement 2889.1.10 Motivations of the Study 2899.2 Preparation and Characterization of Th ree Dimensional Graphene Foam Supported Platinum-Ruthenium

Bimetallic Nanocatalysts for Hydrogen Peroxide Based

9.2.2.3 Synthesis and Modifi cation of PtRu

Nanoparticle Catalyst 2929.2.2.4 Characterization of PtRu Nanocatalysts

with Diff erent Carbon Supported Materials 2939.2.2.5 Electrochemical measurements 2939.2.3 Results and Discussion 2949.2.3.1 Physicochemical Characterization

of PtRu Nanocatalysts with Diff erent Carbon Supported Materials 2949.2.3.2 Electrochemical Characterization and

Performance 2989.2.3.3 Electrochemical Active Surface Area

Measurement 3009.2.3.4 Amperometric Measurement of H2O2 3019.2.3.5 Interference Tests 3039.2.3.6 Stability and Durability of the PtRu/3D

GF Nanocatalyst 3049.2.4 Conclusion for H2O2 Detection in Biosensing 3079.3 Th ree dimensional graphene Foam Supported Platinum–Ruthenium Bimetallic Nanocatalysts for Direct Methanol and Direct Ethanol Fuel Cell Applications 307

of PtRu Nanocatalysts with Diff erent Carbon Supported Materials 3119.3.3.2 Surface Area Measurements 3119.3.3.3 Methanol and Ethanol Oxidation

Measurements 3129.3.4 Conclusion for Methanol and Ethanol Oxidation Reactions in Energy Storage 319

Acknowledgments 320References 320

10 Electrochemical Sensing and Biosensing Platforms Using

Graphene and Graphene-based Nanocomposites 325

Sandeep Kumar Vashist and John H.T Luong

11 Applications of Graphene Electrodes in Health and

Environmental Monitoring 361

Georgia-Paraskevi Nikoleli, Susana Campuzano,

José M Pingarrón and Dimitrios P Nikolelis

11.1 Biosensors Based on Nanostructured Materials 36211.2 Graphene Nanomaterials Used in Electrochemical (bio)

11.4.3 Detection of Organic Pollutants 38111.5 Conclusions and Future Prospects 384Acknowledgements 386

Index 393

Graphene materials constitute probably the most focused arena of als research in the present decade because of their involvement with funda-mental phenomena from the fi elds of physics, chemistry, biology, applied sciences and engineering As the fi rst atomic-thick two-dimensional crys-talline material, graphene has continuously created a wonderland in nano-materials and nanotechnology A number of methods have been developed for the preparation and rendering functional of single-layered graphene nanosheets, the essential building blocks for the bottom-up architecture

materi-of various graphene materials Th ey possess unique physico-chemical properties including large surface area, good conductivity and mechanical strength, high thermal stability and desirable fl exibility Altogether they create a new type of super-thin phenomenon, highly attractive for a wide range of applications Th e electronic behaviour 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 eff ect due

to a special type of Berry phase Th is book entitled, Graphene Materials:

Fundamentals and Emerging Applications proposes a detailed

up-to-date chapters on the processing, properties and technology developments

of graphene materials including multifunctional graphene sheets, surface functionalization, covalent nanocomposites, reinforced nanoplatelets composites etc for a wide range of applications

Graphene has created a profound interest in two-dimensional materials properties Graphene oxide has shown to be possible to reproduce in large quantities, but still the properties for its fabrication needs to be understood

in order to have reproducible material quality Still it is not clear what type

of two dimensional materials will be best for various applications Other two dimensional materials may be better suited regarding certain appli-cations, and therefore should be understood more in detail In addition, hybrids and two dimensional materials can results in extended properties

Chapter 1 presents fabrication of graphene oxide and two dimensional

materials, like tin selenides, SnS2, MnO2, NO BN, MoS2 and WS2, the latter

xv

which can tune electrical properties from metallic and semiconducting by changing the crystal structure and the amount of layers, but it may also act as a lubricant material for use in high temperature and high pressure applications In comparison, MoS2 is one of the transition metal dichalco-genides and applicable as battery, electrochemical capacitor, memory cell, catalysts, and composite Th e chapter also introduces the concept of WS2nanosheets hybridized with reduced graphene oxide nanosheets to achieve

a good catalytic activity

Novel features may be obtained combining graphene nanosheets and graphene oxide with other new nanomaterials such as magnetic nanopar-ticles, carbon dots, carbon nanotubes, nanosemiconductors, quantum dots Th e requirement is that the graphene surfaces must be rendered functional Th e noncovalent and covalent functionalization of graphene

nanosheets and graphene oxide are presented in Chapter 2 Noncovalent

functionalization involves hydrophobic, π-π, Van der Waals, and static interactions In this, there is a physical adsorption of suitable mole-cules on the graphene surface Covalent functionalization can take place at the end of the sheets and/or on the surface Th e combination of inorganic nanoparticles with graphene oxide may be either as a pre-graphenization (graphene oxide is mixed with the nano particles) or post-graphenization (where nanosheets and graphene oxide are prepared separately) process

electro-Th e functionalized graphene nanosheets may be applied into sional porous graphene networks that have large surface areas, good con-ductivity and mechanical strength, high thermal stability and fl exibility

dimen-In Chapter 3, the most widely-used methods for assembling

three-dimensional porous graphene networks and their structural tics are presented Examples are given of their applications in sensors and energy devices Graphene-based composites have a large specifi c surface area, porous structure, and fast electron transport kinetics, providing unique physicochemical properties that are mechanically robust, with high conductivity and thermal stability combined with fast mass and electron transport properties Th e challenges lie in controlling pore size and func-tionality so as to enjoy fl exibility in the development of frameworks for mechanically robust materials while maintaining structural integrity, sta-bility and conductivity

characteris-Graphene-based nanocomposites may act as both graphene fi ller and polymer host Th ese are known for their enhanced performance in many applications such as fl exible packaging, structural components for trans-portation or energy storage, memory devices, hydrogen storage and printed electronics Polymers covalently reinforced with graphene may be best when homogeneously dispersed in the matrix with a strong fi ller/polymer

interface without phase segregation, especially in direct covalent binding between polymers and graphene Th e graft ing-from (graphene as a mac-

romolecular initiator for growing polymer brushes from its surface) and

graft ing-to (combining graphene and polymers through a chemical

reac-tion) approaches to bind polymers to graphene are presented in Chapter 4

In Chapter 5, metal matrix composites, oft en used in aerospace and

automobile industries, are investigated using graphene and magnesium matrix composites reinforced with graphene nanoplatelets Th e mechani-cal properties of Mg-graphene composites show that there is a poor response of graphene nanoplatelet additions on tensile strength of pure

Mg matrix, while addition of graphene nanoplatelets into Mg alloys matrix leads to signifi cant improvement in mechanical strength In addition, there

is higher tensile failure strain in the synergetic eff ect of graphene and bon nanotubes in the Mg-1Al alloy matrix relative to those reinforced with individual graphene nanoplatelets and multi-wall carbon nanotubes

car-Th e increase in energy saving need pushes the graphene to be explored

in batteries and supercapacitors Graphene with its electron transfer behavior and unique two-dimensional surface is acknowledged as a poten-tial electrode material Th is becomes attractive since graphene improves conductivity, charge rate, energy capacity Th e excellent chemical stability, high electrical conductivity, and large surface area of graphene makes it attractive in reduction of volume expansion of electrode materials in lith-ium batteries and graphene-based supercapacitors which may exhibit high storage capacity, fast energy release, quick recharge time, and a long life-

time Chapter 6 furnishes insights in intrinsic challenges of poor kinetics,

large volume expansion, and dissolution of polysulfi des in the electrolyte

in graphene based batteries, and V2O5/reduced graphene oxide posites, Co3O4 nanoplates/reduced graphene oxide composites and gra-phene/NiO as well as graphene–MnO2 hybrids together with some other material approaches as electrode materials for supercapacitors Th e poor stability of conducting polymers during charging/discharging is a major challenge in high power supercapacitors In addition, the low conductiv-ity of conducting polymer also results in high ohmic polarization and a declining reversibility and stability

nanocom-Chapter 7 presents conducting polymers including polypyrrole,

poly-aniline and polyethenedioxythiophene with superior electrical ity and large pseudo capacitance have aroused great interest as electrode materials for supercapacitors as a consequence of their high conductivity and fast redox electroactivity

conductiv-Chapter 8 deals with ZnO/graphene nanocomposite-based bulk

hetero-junction solar cells, deliberating upon carrier diff usion length,

recombination losses, device architect limitations, effi cient charge ration and transport to respective electrodes and possible restriction of organic photovoltaic effi ciency, dielectric constant value and charge carrier mobility

sepa-Bimetallic nanocatalysts may give a large surface excellent dispersion

and high degrees of sensitivity Chapter 9 describes

hierarchically-struc-tured platinum–ruthenium nanoparticles incorporated in sional graphene foam as electrode materials for fuel cells with enhanced performance by decreasing particle size, increasing number of active sites for methanol or ethanol, and increasing the resistance against CO poison-ing, as well as detection of H2O2 in biosensing by Pt active binding sites that are able to interact with H2O2 to enhance the catalytic activity of the

three-dimen-H2O2 detection Graphene and graphene-based nanocomposites may be platforms for electrochemical sensing and biosensing Th ese can lead to biosensors with superior analytical performance, high sensitivity, low detection limit, high precision, high specifi city, low working potentials and prolonged stability

Direct electrochemical detection or enzymeless sensing of glucose is feasible using nanocomposites of graphene decorated with metal nano particles and nanowires that can be operated at low applied potentials In particular, graphene with exposed edge-like planes off ers several advan-tages over other electrode materials for the catalytic oxidation of the DNA

bases, as described in Chapter 10 Th is has also been used to demonstrate how graphene can be used as a biocompatible substrate to enhance cell adhesion and growth to form a basis for the detection of cells

Chapter 11 describes graphene approaches that have been adopted for

improving the performance of graphene nanomaterials-based ized electrochemical biosensors that may be binding of various enzymes

miniatur-Th is may lead to utilizing graphene as a transducer in bio-fi eld-eff ect transistors, electrochemical, impedimetric, electrochemiluminescence, and fl uorescence biosensors, as well as biomolecular labels Further on, graphene-nanostructured biosensors have broad applicability for environ-mental monitoring purposes, particularly in toxic gases, heavy metal ions and organic pollutants detection

EditorsAshutosh Tiwari, PhD, DScMikael Syväjärvi, PhD

LinköpingFebruary 2015

Graphene is a monolayer of carbon atoms in a densely-packed sional (2D) honeycomb crystal structure It can be considered a building block of three-dimensional (3D) graphite, quasi one-dimensional (1D) carbon nanotubes and quasi zero-dimensional (0D) fullerenes Graphene

two-dimen-is a semi-metal with a tiny overlap between the valence and the tion band (zero-gap semiconductor) Graphene was not known to exist in

conduc-an isolated form until 2004 Before that, it was known to exist only in the 1D or 0D form, or even better known in its 3D structure as graphite, which consists of graphene sheets with strong in-plane bonds and weak van der Waals-like coupling between layers Moreover, it was presumed, that

a single 2D graphene sheet would be thermodynamically unstable Only

in 2004, researchers from Manchester — Kostya Novoselov and Andre Geim — demonstrated that it is indeed possible to realize stable single and few layer graphene sheets Th ey were awarded the Nobel Prize in Physics

2010 for groundbreaking experiments regarding the two-dimensional material graphene Graphene was fi rst obtained by delicately cleaving a sample of graphite with sticky tape

Th e direct observation of the isolated graphene monolayer has sparked exponentially growing interest Just a few years were enough to gather sev-eral scientifi c communities to investigate the properties of this unusual material About 3500 scientifi c articles were published in 2010 Owing to its peculiar electronic behavior under magnetic fi eld and at low tempera-ture, graphene has attracted the curiosity of mesoscopic physicists Th e investigation and tailoring of its transport properties from macroscopic

to molecular scales captures a large share of the current research eff ort Materials scientists have rapidly grabbed some of the assets of graphene and are already exploring the ways of incorporating graphene into applied devices and materials

Because of its linear energy–momentum dispersion relations, which cross at the Dirac point, graphene holds open great promise for future electronics technology as well as fundamental physics applications Two of the most extraordinary properties of graphene are its absolute

xix

two-dimensionality and the behavior of its charge carriers as Dirac ticles, which obey the Dirac equation rather than usual Schrödinger equa-tion As a result, many well-known eff ects in the fi eld of solid state physics are expected to be modifi ed

par-Graphene's exceptional electronic properties (e.g high carrier mobility) along with transparency make it an extremely attractive material candidate for a wide range of applications - in electronics, optoelectronics, sensing and fundamental studies of the way electrons behave when confi ned in two dimensions Concomitantly, the light weight, mechanical strength and high conductivity of graphene are perfectly suited for composite and light polymer materials

Graphene can be fabricated by many diff erent ways: from exfoliation

to chemical synthesis and thermal decomposition of SiC exploiting solid, liquid or vapor phase Th e thinnest-ever crystal graphene is a versatile material promising many applications for mankind’s benefi t Th ese will contribute to the solution of existing acute problems related to health, energy saving and ecology Depending on targeted applications diff erent types of graphene are used In this book, the reader will fi nd useful infor-mation on most of these aspects

November 27, 2014Rositsa YakimovaLinköping, Sweden

Rositsa Yakimova is Professor Emerita in material science, Linköping University She is an internationally recognized expert in the fi eld of semi-conductor crystal and nanostructure growth Since 1993 she has had a substantial contribution to the development of the sublimation growth process of SiC Her major eff orts recently have been in research of gra-phene on SiC Yakimova has pioneered a novel method for fabrication of large area uniform epitaxial graphene on SiC and since 2008 she is leading the research of graphene on SiC at Linköping University

FUNDAMENTALS OF GRAPHENE AND GRAPHENE- BASED NANOCOMPOSITES

1 Materials Science Centre, Indian Institute of Technology Kharagpur,

of graphene-based polymer nanocomposites, it gives a high modulus with an excellent mechanical and thermal stability Th e chapter describes preparation and

p roperties of graphene and alike two- dimensional materials

Keywords: Nanomaterials, 2D materials, polymer nanocomposites,

supercapacitors, piezoelectric, fi eld eff ect transistors

*Corresponding author: chapal12@yahoo.co.in

1.1 Introduction

Graphene is a two-dimensional new allotrope of carbon, having omic 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

monoat-hybridized carbon atoms Graphene is the basic building block of all other graphitic materials such as, three dimensional (3D) graphite, one dimen-sional (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 scientifi c 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 fi elds such as poly-mer nanocomposites, energy storage and conversion (e.g supercapacitors, batteries, fuel cells and solar cells), chemical sensors, fl exible 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 stiff ness =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 sional material graphene” Th ey successfully synthesized free-standing gra-phene fi lm for the fi rst time by using an eff ective mechanical exfoliation method with a scotch tape and silicon substrate [10] Graphene is the fi rst 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

dimen-In graphite, adjacent graphene layers are bonded with weak tion of pz orbitals Th is interaction between pz orbitals restricts the com-plete 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 Th is depends

interac-on the cinterac-onditiinterac-on of mechanical exfoliatiinterac-on Chemical oxidatiinterac-on and then simultaneously reduction of graphite oxide results graphene like mate-rials termed as highly reduced graphene oxide (HRG) which contains

defects and residual oxygen-containing functionalities on the periphery

Epitaxial growth usually forms good quality graphene with fewer defects but it requires high-vacuum surroundings and expensive fabrication sys-tems to generate a small size fi lms A CVD technique produces graphene monolayers with large surface areas Longitudinal “unzipping” of CNTs can produce mainly graphene nanoribbons having width which is depen-dent 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 modifi ed easily using a proper chemi-cal reaction and subsequently introduced as nanofi llers in composites with polymeric and/or inorganic materials Th e most common route for pro-ducing large quantities of reduced graphene starts with the oxidation of graphite to graphene oxide (GO)

Th e graphene oxide was fi rst invented several decades ago by Brodie, Staudenmeier and Hummer [13–15] Scientists are still following the same synthesis procedure with minor changes Th e atomic ratio of C : O indi-cates the extent of graphite oxidation Th is solely depends on the synthesis procedure and the duration of the oxidation period [16] Th e Hummers’ method is more effi cient method for the preparation of graphene oxide

Th e 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 defi -ciency 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 Th e 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] Th e increment in “d spacing” value ies signifi cantly 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]

var-1.2 Preparation of Graphene Oxide by Modifi ed

Hummers’ Method

Graphene oxide can be easily synthesized by Modifi ed Hummers’ method

recently reported by Marcano et al [22] Briefl y, 3 g graphite fi ne powder

was added into a mixture of concentrated H2SO4/H3PO4 (540 mL: 60 mL) and stirred for some time using a tefl on coated mechanical stirrer Th en 18

g KMnO4 was added pinch by pinch to the mixture solutions because mous heat is produced due to exothermic reaction Th e solution becomes greenish Th e mixture was continuously stirred in an oil bath for 12 h at

enor-a stirring speed of 340 rpm Aft er thenor-at the reenor-action mixture wenor-as cooled to room temperature Th en 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 Th e graphene oxide suspension was stirred for another 4 h and centrifuged at 4000 rpm Th e 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 Th is requires modifi cation of graphene oxide with organic iso-cyanates type compounds [23], where the surface and edge hydroxyl and carboxyl groups of graphene oxide were transformed into amide and car-bamate groups respectively Th e modifi ed with isocyanato sheets are eas-ily dispersible in N, N-dimethylformamide (DMF), Dimethyle Sulfoxide (DMSO), and N-methylpyrrolidone (NMP) as these are polar organic sol-vents 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 modifi cation of gra-phene 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 gra-

phene oxide to a great extent Cai et al prepared fully exfoliated graphene

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 tetrahydrofu-ran (THF) [21] Th e Ruoff group achieved the stable dispersions of unmod-ifi ed graphene oxide by using 9:1 [v/v] organic solvent: water medium [25]

Th ey 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 fl occulation 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 fl ow-directed fi ltration are a well-known com-

mercialized procedure Dikin et al was imitated this technique for

gra-phene 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] Th is is only due to the eff ect 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

Another important discovery is thin fi lms of graphene oxide Th is meter-thick thin fi lm consist few graphene oxide sheets which can be mono-, bi-, and tri- layers of graphene oxide Such type of fi lm is used

nano-as segment in fi eld-eff ect transistor [27] Ionically conductive composite

fi lm could be prepared by using an alternating uniform, single graphene oxide monolayer and polyelectrolyte layers [28, 29] Graphene thin fi lms are very promising materials due to their high conductivity and transpar-ency (Figure 1.1c) [30] But production of bulk quantities of graphene thin fi lms is still not easy However, reduction of as prepared graphene oxide thin fi lm is only advantageous method to achieve large scale of gra-

phene thin fi lm Mattevi et al prepared reduced graphene oxide thin fi lms

ranging from single to few layers by solution based method and thermal annealing [31]

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 fi ller 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 diff erent fi elds [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 force-fully 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 fi llers and polymers in nanocomposite A large number of thin fi lms based on graphene oxide nanocomposites have been studied for transparent and

fl exible electronic device In case of conductivity studies, graphene oxide

is usually reduced to graphene Th in fi lms 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 fi lm on glass and SiOx/silicon substrate by using g raphene oxide sheets [34]

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 fi ltration

of an aqueous graphene solution [Li et al, Science 2008 (ref 30)].

1.7 Graphene-Based Materials

A two dimensional graphene sheet, having extraordinary electronic and mechanical properties are more preferable than carbon nanotubes Th e ongoing research on graphene has established a new era in the fi eld of materials science Instead of direct synthesis of graphene from commer-cially available graphite, bulk quantities of exfoliated graphene sheets are prepared by the reduction of graphene oxide Although it is very diffi cult to achieve pristine graphene, a large number of reduction strategies (thermal and chemical reduction) have been developed for the reduction of graphene oxide [35] Th e 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]

Mechanically exfoliated pristine graphene sheets possess higher ical strength and conductive properties than reduced graphene oxide due

mechan-to highly extended conjugative structure [10, 38] Th ese physical ties are enhanced by the synthesis of many novel graphene-based materi-als Recently reduced graphene oxide has drawn a great attention as a fi ller material in polymer nanocomposites as it can be easily functionalized, very high dispersibility in many polymers even it can show a synergistic proper-ties 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

proper-mechanical properties [32] For the fi rst time, Stankovich et al was

pre-pared electrically conductive polystyrene-graphene nanocomposite using exfoliated phenyl isocyanate modifi ed graphene oxide and polystyrene by solution phase mixing, followed by the chemical reduction Th ey achieved high dispersion of individual graphene sheets through the polymer matri-ces [39] Nanoplatelets morphology without any multilayer stacking was obtained from SEM images (Figure 1.3) Electrical-conductivity measure-ments 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 1.2 Schematic model of reduced graphene oxide sheet.

An excellent improvement in thermal and mechanical properties can

be done with very small amount of loading of graphene sheets to the

poly-mer matrices Ramanathan et al have shown good dispersion and

inti-mate interaction between graphene sheets and the matrix polymer can signifi cantly enhance their performance [40] Th ey prepared functional-ized graphene sheets of poly(methyl methacrylate) (PMMA) compos-ite 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-graft ed graphene nanocomposites [42]

1.8 Other Two-dimensional Materials

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

dimen-1.8.1 Tungsten Sulfi de

In recent days, dichlacogenides of higher atomic weight transition metals have created a profound impact in advance materials research fi elds owing

Figure 1.3 SEM images of graphene-polystyrene nanocomposites at low (a) and high

magnifi cation (b) [Stankovich et al Nature, 2006 (ref 39)]

to its single layer array In 1992, Tenne et al fi rst achieved the stable

poly-hedral and cylindrical structures of tungsten disulfi de by heating the sten fi lm in hydrogen sulfi de atmosphere [43] Generally, transition metal chalcogenides show graphene like layered structure, with transition metal atom placed in trigonal prismatic coordination sphere Figure 1.4 repre-sents the structure of a hexagonal TMD monolayer Th e electrical proper-ties of these dichalcogenides depend on their composition, structure of the crystal and the number of layers [44]

tung-1.8.1.1 Diff erent Methods for WS2 Preparation

Tungsten sulfi de can be synthesized in a numerous number of methods:1) Hydrothermal preparation 2) Reducing ammonium tetrathiotung-state [(NH4)2WS4] at ~1200°C in presence of hydrogen gas 3) Gas phase reaction of hydrogen sulfi de with tungsten metal in presence of argon atmo-sphere 4) Decomposition reaction of various tetraalkylammonium tetra-thiotungstate precursors in presence of inert gas 5) Microwave treatment

of a concentrated solution of tungstic acid, elemental sulfur and mono anolamine 6) Heating WS3 in absence of oxygen atmosphere (otherwise the product will be tungsten trioxide) 7) Melting a proportionate mixture

eth-of WO3, K2CO3 and sulfur 8) Mechanical exfoliation of tungsten sulfi de 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 (WO) and

Figure 1.4 Structure of a hexagonal TMD monolayer (a) perspective view and (b) along

the perpendicular axis.

sulfur powder by a mechanical activation strategy Th e total reaction cess involves a ball-milling process followed by annealing at 600-700°C in argon atmosphere [47] Th e reaction between tungstic acid and thiourea

pro-in presence of nitrogen gas at 773 K results uniform graphene-like layered form of WS2 [48]

Th e most advantageous procedure is the hydrothermal method for the large scale synthesis of transition metal dichlacogenides at low tempera-ture But the synthesis of tungsten sulfi de nanosheets by the hydrother-mal method is still challenging Th e 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

Th at 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 nanofi bres of WS2 by using diff erent surfactants and discussed their possible growth mechanisms of diff erent nanostructures [50] However, synthesis of 1D WS2 nanocrystal

or nanotube by hydrothermal process has been fi rst reported in 2005 Th e 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 fi rst then obtained trioxide was sulfurized to give

WS2 [51, 52] Generally, condensed WOx nanoparticles act as templates during the formation of tungsten sulfi de nanosheets [49]

1.8.1.2 Properties of WS2

Tungsten sulfi de is usually obtained in dark grey color having a hexagonal crystal structure Th ey are very much chemically inactive and can only dis-solve in a quantitative mixture of nitric and hydrofl uoric acids Tungsten sulfi de converts into corresponding tungsten trioxide while burning in presence oxygen Tungsten sulfi de does not melt during heating in absence

of oxygen gas It disintegrates to elemental tungsten and sulfur near about

1250 °C [53] Tungsten disulfi de acts as a lubricant material as its coeffi cient of friction is 0.03 Th e lubricating properties of tungsten sulfi de are admirable under vigorous conditions of load, vacuum and temperature Tungsten disulfi de also makes its profound impressions in high temper-ature and high pressure applications It off ers 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 sulfi de incorporated

fi lm is as high as 300,000 psi Tungsten disulfi de can replace molybdenum

disulfi de and graphene in certain fi eld of applications like electrical and electronic industries, sound detection, production of electronic frequency and high voltage Tungsten disulfi de is piezoelectric material because it has

an ability to produce electric charge under an external mechanical stress

Th is is a reversible process When a mechanical stress (such as, tion, bending force, pressure) is applied to these materials, the charge sym-metry within the crystal structure has been disrupted, which results an external electric fi eld and vice versa [54] NASA, military, aerospace and automotive industries are also using this material expensively

deforma-1.8.1.3 WS2 and Reduced Graphene Oxide Nanocomposites

As the synthesis of WS2 nanosheets is problematic in hydrothermal cess, many researches have tried to make a hybrid nanocomposite of WS2

pro-nanosheets with reduced graphene oxide by in situ reduction of graphene

oxide for numerous applications [49, 55–57] Tungsten sulfi de/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 modifi ed catalytic activity of WS2/rGO nanocomposites appears mainly due to charge transfer phenomenon Effi cient charge trans-fer occurs mainly due to an intimate contact in between tungsten sulfi de and the reduced graphene oxide components As mentioned earlier that hydrothermal preparation of tungsten sulfi de is sensitive towards tempera-ture Tungsten sulfi de/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] Th e surface of tungsten disul-phide is quite ruff with a large number of micro voids and pores in its sur-faces Th e as prepared tungsten disulfi de/reduced graphene oxide hybrid

Angew Chem Int Ed 2013 (ref 49)].

nanocomposites shrinks immediately aft er freeze-drying which perhaps due to the removal of water adsorbed on reduced graphene oxide Th e high resolution transmission electron microscopic image of tungsten disulfi de/reduced graphene oxide hybrid nanocomposite is shown in Figure 1.6 which displays the overlapping nanosheets morphology with bilayer WS2nanosheets in some areas Th e tungsten disulfi de/reduced graphene oxide hybrid nanocomposites imparts promising catalytic properties Th e tung-sten disulfi de/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]

1.8.2 Molybdenum Sulfi de

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

Th e structure of MoS2 is composed of three atom layers (S-Mo-S) ated 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 dimen-

associ-sional high performance piezoelectric materials Wu et al reported that

oscillating piezoelectric voltage and current outputs depends on the ber of atomic layers present in thin MoS2 fl ake with applied strain [54, 59]

Ed 2013 (ref 49)]

Because of this layered structure it has been used in many application

fi elds, such as, Li-ion battery, electrochemical capacitor, memory cell, lysts and composites

cata-Due to similar layered structure of graphene, MoS2 nanocomposite with graphene has been used as extraordinarily high performance anode mate-rial for Li-ion battery [60, 61] Th e fi rst Li-ion battery using MoS2 was pub-lished in a patent in 1980 Th e 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 capac-ity (~ 840 mA h g-1) by Du et al [62] Whereas, Wang et al have used

single layer MoS2-graphene nanosheets composites for high cal 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 nanocom-posites are shown in Figure 1.7 which indicates the fl owery architecture composed by nanopetals in the form of nanosheets Both MoS2 and rGO nanosheets were obtained as intercalated state [63]

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

1.8.3 Tin Sulfi de

Recently, layered tin sulfi de has attracted a great interest because of its sive structural characteristics It is an n-type semiconductor Th e 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

IJLRST 2014 (ref 63)].

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 decom-

position [64] or hydrothermal synthesis [65] Seo et al [64] have

synthe-sized 2D layered SnS2 nanoplates by thermally decomposing the precursor, e.g., Sn(S2CNEt2)4, in presence of an organic solvent at 180 °C Th ey 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 nal, highly crystalline SnS2 nanoplates Th e lateral size of the nanoplates is about 150 nm and the thickness of SnS2 nanoplates is around 15 nm Gao et

[Wang et al J Mater Chem A 2013 (ref 61)].

2008 (ref 64)].

al have synthesized SnS2 nanosheets by a simple single step hydrothermal process [65] Th ey have used tin chloride pentahydrate (SnCl4·5H2O) and thioacetamide (TAA) as precursor agents Th ey have demonstrated the fer-romagnetic behavior of porous hexagonal disulfi de at room temperature due to disordered grain boundary, defects or edges.

fl ake is about 600–700 nm in side length and 30–45 nm thickness

Two dimensional layered semiconductor materials have been signifi cantly 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 bat-tery 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 paring electrode materials for supercapacitor applications Actually birnes-site-type manganese dioxide (MnO2) having a layered nanosheet structures

(b) [Liu et al Mater Lett 2009 (ref 66)].

actuates the acceptance of numerous number of metal cations from an trolyte to move in and out of the interlayer region Th e movements of the metal cations do not make any structural changes of MnO2 [68].

elec-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 Th e morphology of the MnO2 nanosheets depends on the processing conditions Figure 1.11 shows the SEM micrograph of MnO2

nanosheets on carbon fi ber, synthesized via anodic electrodeposition using

0.1 M MnSO4 precursor in 0.1 M H2SO4 solution [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

dif-ferent magnifi cations Zhu et al reported a cost eff ective microwave

syn-thesis 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