PREPARATION, CHARACTERIZATION AND PROPERTY STUDIES … · Chapter 2 deals with the isolation and...

PREPARATION, CHARACTERIZATION AND PROPERTY

STUDIES OF CARBON NANOSTRUCTURES DERIVED

FROM CARBON RICH MATERIALS

SAJINI VADUKUMPULLY

NATIONAL UNIVERSITY OF SINGAPORE

2011

Thesis Title: PREPARATION, CHARACTERIZATION AND

PROPERTY STUDIES OF CARBON NANOSTRUCTURES DERIVED

FROM CARBON RICH MATERIALS

Abstract

Carbon nanomaterials have always been an area of interest for their applications in all

the fields of science starting from materials science to biology. The current research is

focused on low cost and simple methodologies to prepare functional carbon

nanostructures from carbon rich precursors. Towards this goal, carbon nanofibers were

isolated from soot and employed as an adsorbent for the removal of amines from waste

water. Besides, various solution phase methods for the production of processable

graphene nanosheets directly from graphite were explored. This has been achieved by

both non-covalent stabilization and covalent functionalization of exfoliated graphene

sheets. Covalent functionalizations enable the incorporation of various functional

moieties onto graphene. The applicability of these functionalized graphene sheets in

polymer composites and metal hybrids were systematically investigated. Incorporation

of graphene improves the mechanical and thermal stability of the composites, along

with an increase in the electrical conductance.

Keywords: carbon nanofibers, graphene, covalent and non-covalent functionalization,

graphene/polymer composite, graphene/metal nanocomposites.

i

Acknowledgments

I am thankful to a lot of people who supported me throughout my PhD life and it is

a great pleasure to acknowledge them for their kind help and assistance.

First of all, I would like to thank my thesis advisor Dr. Suresh Valiyaveettil for

giving me an opportunity to work with him, for the constant support, guidance and

encouragement.

I would like to extend my sincere gratitude to all the current and past lab members

for their friendship and affection. I thank Ani, Gayathri, Nurmawati, Santosh, Bindhu,

Asha, Manoj, Rajeev, Sivamurugan, Satya, Balaji, Ankur, Pradipta, Tanay, Narahari,

Yiwei, Chunyan, Kiruba, Rama and Ashok for all the good time spent in lab and for

the useful discussions. I am thankful to Dr. Jegadesan for teaching me the basic

principles and operations of AFM. I would like to thank Jhinuk for her patience in

answering all my questions related to organic chemistry. I also appreciate the

assistances from Jinu Paul with the mechanical characterization and I-V

measurements. I take this opportunity to thank all my undergraduate and high school

students for their help in the work and it was a pleasure to work with them.

Special thanks to Prof. Sow Chong Haur and his lab members for helping with the

Raman and conductivity measurements. Technical assistance from Dr. Liu Binghai,

Ms. Tang and Mdm. Loy during SEM and TEM measurements is highly appreciated.

I would like to thank NUS-Nanoscience and Nanotechnology Initiative (NUSNNI)

for the graduate scholarship and department of Chemistry, NUS for the technical and

financial assistance.

Words cannot express my gratitude to my family members for their kind

understanding, moral support and encouragement. Without their support, this thesis

would not have been materialized.

ii

Table of Contents

Acknowledgments i

Table of Contents ii

Summary viii

Abbreviations and Symbols x

List of Tables xv

List of Figures xvi

List of Schemes xxiii

Chapter 1

Introduction – A Brief Review on Carbon Nanomaterials

1 Introduction 2

1.1 Carbon Nanomaterials 3

1.2 Graphene 5

1.2.1 Preparation of Graphene 6

1.2.1.1 Mechanical Exfoliation 7

1.2.1.2 Supported Growth 9

1.2.1.3 Wet Chemical Methods 13

1.2.1.3a Graphenes from GO 14

1.2.1.3b Unoxidized Graphene Sheets Directly from Graphite 15

1.2.1.3c Molecular Approach for the Production of Graphene 17

1.2.1.4 Unzipping of CNTs 18

iii

1.2.2 Characterization Techniques 19

1.2.3 Reactions of Graphene 21

1.2.3.1 Covalent Modifications 21

1.2.3.1a Hydrogenation 22

1.2.3.1b Covalent Modifications on GO 23

1.2.3.1c Covalent Modifications on Unoxidized Graphene Sheets 26

1.2.3.2 Non-covalent Modifications 30

1.2.3.3 Molecular Doping 34

1.2.4 Properties of Graphene 36

1.2.5 Graphene Based Hybrid Materials 38

1.2.6 Applications of Graphene 41

1.3 Aim and Scope of the Thesis 44

1.4 References 45

Chapter 2

Carbon Nanofibers Extracted from Soot as a Sorbent for the

Determination of Aromatic Amines from Wastewater Effluent

Samples

2.1 Introduction 70

2.2 Experimental Section 71

2.2.1 Reagents and Materials 71

2.2.2 Sample Preparation 72

2.2.3 Isolation of CNFs 72

2.2.4 Preparation of Electrospun CNF/PVA Composite

Nanofiber Mats

72

iv

2.2.5 Materials Characterization 73

2.2.6 µ-SPE using Electrospun CNF/PVA Composite

Nanofiber Mats

74

2.2.7 HPLC Analysis 74

2.3 Results and Discussion 74

2.3.1 Characterization of the CNFs 74

2.3.2 Analytical Evaluation of Electrospun CNF/PVA

Composite Nanofiber Mats as Adsorbent for µ-SPE

77

2.3.3 Control Study 78

2.3.4 Extraction Time 79

2.3.5 Desorption Solvent 80

2.3.6 Sample Volume 81

2.3.7 Desorption Time 82

2.3.8 Effect of Ionic Strength 83

2.3.9 Effect of pH 83

2.3.10 Method Validation 84

2.3.11 Application of Method for Real Sample Analysis 86

2.4 Conclusions 87

2.5 References 87

Chapter 3

Cationic Surfactant Mediated Exfoliation of Graphite into

Graphene Nanosheets and its Field Emission Properties

3.1 Introduction 94


v

3.2.1 Materials and Methods 96

3.2.2 Preparation of Processable Graphene Nanosheets 96

3.2.3 Instrumentation 96


3.3.1 Control Studies 98

3.3.2 Exfoliation of Graphite in Presence of SDS 99

3.3.3 Exfoliation of Graphite in Presence of CTAB 99

3.3.4 Field Emission Properties of Graphene Nanosheets 105

3.4 Conclusions 108

3.5 References 108

Chapter 4

Flexible Conductive Graphene/Poly(vinyl chloride) Composite

Thin Films with High Mechanical Strength and Thermal

Stability

4.1 Introduction 113


4.2.1 Materials 115

4.2.2 Preparation of Processable Graphene Nanosheets 115

4.2.3 Fabrication of Graphene/PVC Composite Thin Films 115

4.2.4 Instrumentation 116

4.2.5 Mechanical Characterization 116

4.2.6 Electrical Characterization 117


vi

4.3.1 Characterization of Graphene and Graphene/PVC Thin

Films

117

4.3.2 Mechanical Characterization of the Graphene/PVC Thin

Films

120

4.3.3 Dynamic Mechanical Thermal Analysis 122

4.3.4 Electrical Properties 127

4.4 Conclusions 128

4.5 References 129

Chapter 5

Functionalization of Surfactant Wrapped Graphene

Nanosheets with Alkylazides for Enhanced Dispersibility



5.2.1 Materials 136

5.2.2 Analysis and Instruments 136

5.2.3 Preparation of CTAB Stabilized Graphene Sheets 137

5.2.4 General Synthetic Procedure for Alkylazides 137

5.2.5 Synthesis of 11-azidoundecanol (AUO) 138

5.2.6 Synthesis of 11-azidoundecanoic acid (AUA) 138

5.2.7 Functionalization of CTAB Stabilized Graphene

Nanosheets with 11-azidoundecanoic acid (AUA)

139

5.2.8 Preparation of Gold/Graphene Nanocomposites 139


5.3.1 Characterization of the Functionalized Graphene

Nanosheets

140

vii

5.3.2 Characterization of Gold/Graphene Nanocomposites 146

5.4 Conclusions 149

5.5 References 150

Chapter 6

Bromination of Graphite - A Novel Route for the Exfoliation

and Functionalization of Graphene Sheets



6.2.1 Materials 159

6.2.2 Bromination of Graphite 160

6.2.3 Spreading of Brominated Graphite 160

6.2.4 Alkylation of Brominated Graphene 160

6.2.5 Suzuki Coupling Reaction with Brominated Graphene 161

6.2.6 Characterization 161


6.3.1 Brominated Graphite and its Stretching 162

6.3.2 Alkylation of the Brominated Graphite 165

6.3.3 Arylation of the Brominated Graphite by Suzuki

Coupling Reaction

170

6.4 Conclusions 173

6.5 References 173

Chapter 7

Conclusions and Future Prospectives 177

viii

Appendix

List of publications and presentations

182

183

ix

Summary

Nanostructures made of carbon have always been an area of interest for researchers

for their direct applications in all the fields of science starting from material science to

biology. Most of the reported procedures for the preparation of functional carbon

nanomaterials rely on expensive equipments and resources, high temperature and

pressure etc. Besides, most of the methods generate desired nanomaterials along with

significant amount of carbonaceous impurities such as amorphous carbon and catalyst

particles. Hence a post treatment is required before the material can be employed for

certain applications. The main interest of our current work was to pay attention to low

cost, simple, environmentally friendly methodology to generate functional carbon

nanostructures.

A brief summary of all types of carbon nanostructures, mainly graphene, its

methods of preparation, characterization techniques, properties, functionalization

techniques and applications have been explained in Chapter 1.

Chapter 2 deals with the isolation and characterization of carbon nanofibers

(CNFs) obtained from soot collected from burning of plant seed oil. The isolation is

accomplished by solvent extraction. The structure of CNFs is thoroughly investigated

using various spectroscopic and microscopic techniques. Moreover, the application of

CNFs in water purification is also investigated. CNFs are incorporated into a polymer

matrix via electrospinning and the resultant composite material is used as a µ-solid

phase extraction device for the extraction and preconcentration of aromatic amines

from water samples.

In Chapter 3 we investigated the possibility of solution based exfoliation of

graphite into graphene nanosheets without any oxidative or thermal treatments. The

combined effect of ultrasonication and non-covalent functionalization using

x

surfactants are employed for the exfoliation and dispersion of graphene nanosheets in

common organic solvents.

Chapter 4 describes the use of surfactant stabilized graphene nanosheets as

reinforcing filler in poly(vinyl chloride) (PVC) matrix at very low loading levels. The

composite thin films are prepared by solution phase blending in which PVC is

blended with different amounts of graphene nanosheets. Mechanical, thermal and

electrical properties of the composite films are investigated in detail.

In Chapter 5, the applicability of covalent functionalization on surfactant stabilized

graphene nanosheets are explored, where the dispersibility of the graphene nanosheets

are considerably enhanced. A series of nitrenes are employed for the covalent

functionalization and the role of functional groups on the nitrenes in stabilizing the

graphene dispersion is also investigated. Besides, the reactive sites on graphene

nanosheets are marked with metal nanoparticles. This approach provides a useful

platform for the fabrication of graphene/metal nanocomposites.

Chapter 6 demonstrates a simple and scalable surface treatment for the exfoliation

and subsequent functionalization of graphene sheets. Bromine atoms are covalently

attached to graphite, which can be easily substituted with alkylamines or boronic

acids, which help in enhancing the dispersibility and processability. This method

provides an easy and direct route towards the preparation of highly functionalized

graphene sheets.

xi

Abbreviations and Symbols

1D One dimensional

2D Two dimensional

3D Three dimensional

2L Bilayer

Å Angstrom

ABA

AES

4-aminobenzoic acid

Auger electron spectroscopy

AFM

ARPES

Atomic force microscopy

Angle resolved photoemission spectroscopy

AUA 11-azidoundecanoic acid

AUO 11-azidoundecanol

CDCl3 Chloroform-d

C6F6 Hexafluorobenzene

C6F5CF3 Octafluorotoluene

C6F5CN Pentafluorobenzonitrile

C5F5N Pentafluoropyridine

CHCl3 Chloroform

CNF Carbon nanofibers

CNT Carbon nanotubes

CTAB Cetyltrimethylammonium bromide

CVD Chemical vapour deposition

DCB Dichlorobenzene

DCC N, N-dicyclohexylcarbodiimide

xii

DCM Dichloromethane

DMA Dimethylamine

DMA Dynamic mechanical analyzer

DMF N,N-dimethyl formamide

DMSO

DNA

Dimethyl sulfoxide

Deoxyribonucleic acid

DNT Dinitrotoluene

DSC Differential scanning calorimetry

EDX

EELS

Energy dispersive X-ray analysis

Electron energy loss spectroscopy

EG Ethylene glycol

EG Expanded graphite

EI Electron impact

F4-TCNQ Tetrafluorotetracyanoquinodimethane

FESEM Field emission scanning electron microscopy

FET

FGS

Field effect transistor

Functionalized graphene sheets

FTIR Fourier transform infrared spectroscopy

gm Gram

GNR Graphene nanoribbons

GO Graphene oxide

h Hours

HBC Hexabenzocoronene

HCl Hydrochloric acid

HF Hydrogen fluoride

xiii

H2SO4 Sulfuric acid

HOPG Highly ordered pyrolytic graphite

HPLC High pressure liquid chromatography

HSQ Hydrogensilsesquioxane

Hz Hertz

HTC Hydrothermal carbonization

ITO Indium tin oxide

KMnO4 Potassium permanganate

KOH

LCD

LEED

Potassium hydroxide

Liquid crystal display

Low energy electron diffraction

LiAlH4 Lithium aluminium hydride

LLE Liquid-liquid-extraction

LLLME Liquid-liquid-liquid micro-extraction

LOD Limit of detection

LOQ Limit of quantitation

m Multiplet

µg Microgram(s)

µ-SPE Micro-solid phase extraction

Me4Si Tetramethylsilane

mg Milligram(s)

ml Milliliter(s)

mmol

mV

Millimole(s)

Millivolt(s)

MWNT Multi-walled nanotubes

xiv

m/z Mass/charge ratio

NH3 Ammonia

NLO Non-linear optics

NMP N-methylpyrrolidone

NMR Nuclear magnetic resonance

ODA Octadecylamine

ODCB 1,2-dichlorobenzene

PANI Polyaniline

PDDA poly(diallyldimethyl ammonium chloride)

PDI Perylenediimide

PEG Polyethylene glycol

PES Photoelectron spectroscopy

PFPA Perfluorophenylazide

Phe(N3) Azido-phenylalanine

PMMA Poly(methyl methacrylate)

PP Polypropylene

PPA Poly(phosphoric acid)

PPy Polypyrrole

PS

PSS

Polystyrene

Poly(sodium 4-styrenesulfonate)

PVA Poly(vinyl alcohol)

PVC Poly(vinyl chloride)

PyS Pyrene-1-sulfonic acid sodium salt

s Singlet

RSD Relative standard deviation

xv

SAED

SDBS

Selected area electron diffraction

Sodium dodecylbenzene sulphonate

SDS Sodiumdodecyl sulfate

SEM Scanning electron microscopy

SiC Silicon carbide

Si3N4 Silicon nitride

SMA Styrene-maleic anhydride

SPANI

SPE

Sulfonated polyaniline

Solid-phase extraction

SPME Solid-phase microextraction

SPR Surface plasmon resonance

STM Scanning tunnelling microscopy

SWNT Single-walled nanotubes

t Triplet

TCB Trichlorobenzene

TEM Transmission electron microscopy

TGA Thermo gravimetric analysis

THF Tetrahydrofuran

TSG Turbostratic graphite

UV-vis Ultraviolet-visible spectroscopy

WPU Polyurethane

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

δ Chemical shift (in NMR spectroscopy)

xvi

Table No. List of Tables Page

No.

Chapter 1

Table 1.1 Summary of supported graphene growth on both metals and carbide substrates.

11

Table 1.2 Comparison of different wet chemical approaches to produce graphene suspensions.

13

Table 1.3 Comparison of different functionalization methods of graphene.

35

Table 1.4 Major applications of graphene and graphene based composite materials.

43

Chapter 2

Table 2.1 Quantitative data: linearity, precision (RSD), limit of detection (S/N = 3), enhancement factors and linear regression data obtained for anilines by the electrospun CNF/PVA composite membrane microextraction coupled with HPLC-UV.

85

Table 2.2 Concentrations of target aniline compounds (µg/l) in waste water samples and the average recoveries determined at 5 µg/l (*n.d-not detected. Relative recovery values of spiked wastewater sample at 5 µg/l compared to that of spiked pure water).

86

xvii

Figure No. List of Figures Page

No.

Chapter 1

Figure 1.1

The structures of (A) diamond (B) graphite (C) fullerene (C60) (D) CNTs and (E) graphene.

2

Figure 1.2

Schematic illustrations of the structures of (A) armchair (B) zigzag (C) chiral SWNT and (D) TEM image of a MWNT containing a concentrically nested array of nine SWNTs.

3

Figure 1.3

Atomic structure of graphene.

5

Figure 1.4 Present techniques for making graphene.

7

Figure 1.5 Schematic of “Scotch tape” peeling off method to produce graphene.

8

Figure 1.6 (A) Structure of HBC and (B) structure of the largest graphene like molecule reported with 222 carbon atoms.

18

Figure 1.7 Chemical structure of a small edge-carboxylated graphene nanoflake.

26

Figure 1.8 Schematic representation of the reaction between graphite and ABA as a molecular wedge via Friedel Crafts acylation in PPA/P2O5 medium.

27

Figure 1.9 Schematic representation of alkylation of fluorinated graphene sheets (blue: fluorine; yellow: -R group).

28

Figure 1.10 Nitrene addition to exfoliated graphite in refluxing ODCB.

29

Figure 1.11 Fabrication of covalently immobilized graphene on silicon wafer via the PFPA-silane coupling agent.

30

xviii

Figure 1.12 Schematic of self-assembly of Au NPs and PDDA-

functionalized graphene sheets. 33

Chapter 2

Figure 2.1 SEM images of the raw soot (A), extracted CNFs (B) and TEM of CNFs (C) obtained from the soot by THF extraction. Inset in C shows the SAED pattern obtained from a single nanofiber.

75

Figure 2.2

Figure 2.3

XRD pattern obtained for the isolated CNFs. FTIR (A) and the Raman spectra (B) of the CNFs.

75

76

Figure 2.4 HPLC chromatogram obtained for standard 1 mg/l. Peak (1) 3-nitroaniline; (2) 4-chloroaniline; (3) 4-bromoaniline and (4) 3,4-dichloroaniline.

77

Figure 2.5 Schematic of the extraction and desorption steps involved in electrospun CNF/PVA composite membrane µ-solid phase extraction.

77

Figure 2.6 Comparison of PVA and composite fiber mats (Extraction conditions are as follows: 10 ml spiked 25 µg/l water solution, no salt added and pH adjusted, extraction time of 30 min, desorption time of 15 min in 100 µl of acetonitrile).

78

Figure 2.7 Plot of HPLC peak area vs the extraction time (Extraction conditions are as follows: 10 ml spiked 25 µg/l water solution, no salt added and no pH adjustment, desorption time of 15 min in 100 µl of acetonitrile).

79

Figure 2.8 Comparison of different desorption solvents (Extraction conditions are as follows: 10 ml spiked 25 µg/l water solution, no adjustment of salt and pH, extraction time of 40 min, desorption time of 15 min in 100 µl of acetonitrile).

80

xix

Figure 2.9 Influence of sample volume on the extraction efficiency (Extraction conditions are as follows: 10 ml spiked 25 µg/l water solution no adjustment of salt content and pH, extraction time of 40 min, desorption time of 15 min in 75 µl of acetonitrile).

81

Figure 2.10 Plot of HPLC peak area vs desorption time (Extraction conditions are as follows: 30 ml spiked 25 µg/l water solution no adjustment of salt content and pH, extraction time of 40 min, desorption in 75 µl of acetonitrile).

82

Figure 2.11 Effect of salt addition on the extraction efficiency (Extraction conditions are as follows: 30 ml spiked 25 µg/l water solution, no pH adjustment, extraction time of 40 min, desorption in 75 µl of acetonitrile).

83

Figure 2.12 Effect of sample pH on the extraction efficiency (Extraction conditions as follows: 30 ml spiked water solution (25 µg/l), no salt addition, extraction time of 40 min, desorption time of 15 min in 75 µl of acetonitrile).

84

Chapter 3

Figure 3.1 SEM image of the exfoliated product in acetic acid. 98

Figure 3.2 FESEM (A) and TEM (B) images of SDS stabilized graphene sheets. Inset of figure 3.2B shows the HRTEM image of one the edges of the multilayered graphite flake.

99

Figure 3.3 (A) UV-vis spectrum of the CTAB stabilized graphene sheets dispersed in DMF. Inset shows a photograph of the DMF solution. SEM of (B) HOPG and (C) the CTAB stabilized exfoliated graphene.

100

Figure 3.4 (A) TEM image of surfactant stabilized graphene sheets. (B) HRTEM image of a bilayer graphene sheet. (C) STM image of a part of the monolayer graphene (inset highlights the hexagonal lattice of the monolayer). (D) Section analysis along the blue line which showing a width of 1.148 nm for 4 hexagons and (E) along the green

102

xx

line which showing a width of 0.424 nm for 3 C-C bonds.

Figure 3.5 (A) Topographic view (10 × 10 µm) of the graphene layers spin coated on mica. (B) AFM image of a single graphene sheet (1 × 1 µm). (C) Height profile of the image 3B. Statistical analysis of the AFM images of 60 nanosheets: (D) thickness, (E) length and (F) width of the flakes.

103

Figure 3.6 (A) Raman spectra of HOPG and the CTAB stabilized graphene nanosheets deposited on Si and (B) EDX spectrum.

104

Figure 3.7 (A) Schematic representation of the experimental set up for field emission measurement. (B) Graph showing the field emission current density vs applied electric field. (C) Fowler - Nordheim plot indicating the field emission characteristics of exfoliated graphene nanosheets.

106

Chapter 4

Figure 4.1 (A) AFM image of the exfoliated graphene nanosheets, (B) height profile of one of the sheets, (C) FESEM image of graphene/PVC composite thin film with 2 wt% concentration of graphene and (D) FESEM image of the fractured end of the composite film after mechanical testing.

118

Figure 4.2 XRD of graphene deposited on glass (trace a), 2 wt% graphene/PVC composite film (trace b) and pure PVC powder (trace c).

119

Figure 4.3 Raman spectra of pure graphene (trace a), 2 wt% graphene/PVC composite film (trace b) and pure PVC powder (trace c).

119

Figure 4.4 (A) Representative stress strain curves for various weight fractions of the graphene/PVC composite films and (B) the calculated Young’s modulus based on the slope of the elastic region.

120

xxi

Figure 4.5 Mechanical properties of PVC thin films with different graphene loadings when stretched at the rate of 0.01 N/min at 20 °C.

121

Figure 4.6 (A) Storage modulus and (B) Tan δ curves for the graphene/PVC films when deformed at constant amplitude of 0.1% at a frequency of 1 Hz at various temperatures.

122

Figure 4.7 Graphs showing the temperature dependence of the relative storage modulus at (A) low temperature regime and (B) glass transition temperature regime.

124

Figure 4.8 Graph showing the glass transition temperature (Tg) and the loss factor (Tan δ) values at various weight fractions of graphene in PVC matrix.

125

Figure 4.9 (A) TGA curves for graphene/PVC composite films and (B) DSC curves to determine the glass transition temperature.

126

Figure 4.10 Graph showing the influence of exfoliated graphene on the electrical conductivity of PVC.

127

Chapter 5

Figure. 5.1 (A) AFM image of the CTAB stabilized graphene sheets and (B) corresponding section analysis.

140

Figure 5.2 SEM images of (A) dodecylazide functionalized graphene sheets and (B) hexylazide functionalized graphene sheets.

141

Figure 5.3 TEM images of (A) AUO functionalized graphene sheets and (B) corresponding gold/graphene nanocomposite.

141

Figure 5.4 TEM images of AUA functionalized graphene sheets, with 1:1 weight ratio of graphene vs AUA used. Inset clearly shows the settled particles in the respective dispersed solutions in toluene. All the sheets were found to settle down within 1 h.

142

xxii

Figure 5.5 TEM images of (A) CTAB stabilized and (B) AUA

functionalized graphene sheets. (C) FTIR of AUA and AUA functionalized graphene sheets (trace A and B, respectively) and (D) Raman spectra of CTAB stabilized (trace 1), AUA functionalized graphene sheets with 1:1 and 1:10 w/w of graphene to AUA.

143

Figure 5.6 AFM images of (A) the AUA functionalized graphene sheets and (B) the section analysis.

145

Figure 5.7 Statistical analysis on the thickness of the AUA functionalized graphene sheets.

145

Figure 5.8 STM images of (A) CTAB stabilized and (B) AUA functionalized graphene sheets.

146

Figure 5.9 (A) UV-vis spectra of the AUA functionalized graphene sheets (trace A) and that of the gold/graphene nanocomposites (trace B) solutions. Inset of figure 5.9A shows the photograph of (A) functionalized graphene solution and that of (B) the gold/AUA graphene composite in DMF. (B) TEM image and (C) the AFM image with (D) the cross-section analysis of the gold/graphene nancomposite sheets.

147

Figure 5.10 I-V plots of films of (A) AUA functionalized graphene sheets and (B) gold/graphene nanocomposites.

148

Chapter 6

Figure 6.1 FESEM (A) and AFM (B) image of the brominated graphite drop-casted from toluene suspension.

162

Figure 6.2 FESEM (A) and TEM (B) image of the brominated graphite sample deposited from water-toluene interface; FESEM (C) and (D) TEM image of the 1:0.02 v/v graphene/MWNT composite.

164

xxiii

Figure 6.3 (A) AFM image of the brominated graphite sample deposited from water-toluene interface and (B) AFM image of the 1:0.02 v/v graphene/MWNT composite.

164

Figure 6.4 (A) Raman spectra of (a) pure graphite (b) brominated graphite (c) alkylated graphene and (d) debrominated graphene sheets after alkylation and (B) FTIR spectra of (a) brominated graphite (b) alkylated and (c) debrominated sheets after alkylation.

166

Figure 6.5 Weight loss of (a) brominated graphite and (b) dodecylated graphite determined by TGA analyses in nitrogen.

167

Figure 6.6 (A) FESEM and (B) AFM images of the dodecylated graphene sample (without sonication).

168

Figure 6.7 (A) AFM image of the dodecylated graphene sheets after dispersion, (B) corresponding section analysis; and (C) TEM image.

169

Figure 6.8 I-V plots of drop casted films of (a) brominated graphite (b) dodecylated graphene and (c) debrominated product after dodecylation.

170

Figure 6.9 Absorption (A) and emission (B) spectra for (a) Ph-BTP (b) G-BTP excited at 410 nm; Absorption (C) and emission (D) spectra for (a) Ph-Py (b) G-Py excited at 340 nm.

171

Figure 6.10 Raman spectra of (A) G-BTP (a - brominated graphite; b - G-BTP; c - debrominated G-BTP) and (B) G- Py (a - brominated graphite; b - G-Py; c - debrominated G-Py).

172

Figure 6.11 (A) TEM and (B) AFM image of bithiophene coupled graphene; (C) TEM and (D) AFM image of pyrene coupled graphene.

172

xxiv

Scheme No List of Schemes Page

No.

Chapter 1

Scheme 1.1 Proposed mechanism for the MWNT unzipping. First step (2) is the formation of manganate ester, followed by formation of dione (3), and subsequent induced strain causes the ring opening (4 and 5).

19

Scheme 1.2 Schematic showing various covalent functional reactions of GO.

24

Chapter 3

Scheme 3.1 Schematic of cetyltrimethylammonium bromide (CTAB) assisted exfoliation.

95

Chapter 5

Scheme 5.1 Schematic representation of the covalent functionalization of graphene sheets with various alkylazides.

135

Chapter 6

Scheme 6.1 Schematic representation of the bromination and subsequent alkylation/arylation using dodecylamine and arylboronic acids. Reaction conditions: (a) dodecylamine, (C2H5)3N, toluene, reflux, 24 h. (b) pyrene 1-boronic acid/5-hexyl-2,2'-bithiopheneboronic acid pinacol ester, Pd (0), K2CO3, DMF, 85 °C, 48 h.

159

1

Chapter 1

Introduction – A Brief Review on Carbon

Nanomaterials

2

1 Introduction

‘Carbon’ is a unique chemical element in the periodic table, which is known to man

since antiquity. Carbon and its compounds form the basis of all known forms of life

on earth. The abundance along with diverse properties makes this element a unique

one. Different allotropes of carbon exist in nature such as graphite, diamond,

fullerenes and amorphous carbon. Physical and chemical properties of carbon vary in

each of the allotropes. For example, diamond is one of the hardest materials existing

in nature and graphite is as the soft materials known. Moreover, diamond has sp3

hybridized carbon atoms forming an extended three dimensional network, whereas

carbon atoms are sp2 hybridized in graphite forming planar sheets. Figure 1.1 shows

the structures of some of the carbonaceous materials existing in nature.

A B

C D

E

Figure 1.1 The structures of (A) diamond (B) graphite (C) fullerene (C60) (D) CNTs and (E) graphene.

The importance and uniqueness of carbon have now been extended to nanomaterials.

Carbon based nanomaterials have attracted worldwide attention in the past 20 years

because of their typical chemical, physical, electrical, thermal and mechanical

properties.1-10 Some of the extensively studied structures are fullerenes, nanotubes,

3

nanofibers, nanodiamond and most recently, graphene.1,3,10-13 This chapter gives a

brief review on the structure, preparation, characterization, properties, reactions and

applications of carbon nanomaterials, in particular, graphite and graphene.

1.1 Carbon Nanomaterials

Fullerenes, carbon nanotubes (CNTs), carbon nanofibers (CNFs) and nanodiamond

are some of the well studied carbon nanostructures. Fullerenes are the newest carbon

allotrope discovered in 1985 by Kroto and co-workers. Structure of fullerenes is like

that of a soccer ball and each fullerenes (Cn) consists of 12 pentagonal rings and any

number of hexagonal rings m, such that m = (Cn − 20)/2 by Euler’s theory.14

On the other hand, CNTs are unique tubular structures made of sp2 hybridized

carbon atoms with high length/diameter ratio. These structures were first observed by

Iijima in 1991.1 There are mainly two types of CNT, the first type called multi-walled

nanotubes (MWNT) are closer to hollow graphite fibers and are made of concentric

cylinders placed around a common central hollow with spacing between the layers

close to that of interlayer spacing in graphite (∼ 0.34 nm).

A B C D

Figure 1.2 Schematic illustrations of the structures of (A) armchair (B) zigzag (C) chiral SWNT and (D) TEM image of a MWNT containing a concentrically nested array of nine SWNTs. (adapted from ref. 15)

4

The second type known as single-walled nanotubes (SWNT) is identical to a fullerene

fiber. It consists of a single graphite sheet seamlessly wrapped into a cylindrical tube.

CNFs are cylindrical nanostructures with graphene layers arranged as stacked

cones, cups or plates. They have lengths in the order of micrometers and the diameter

varies between tens of nanometers upto 200 nm. Their mechanical strength and

electric properties are similar to CNTs while their size and graphitic ordering can be

well controlled.16,17

Diamond structures at the nanoscale (length ∼ 1 to 100 nm) include pure-phase

diamond films, diamond particles, one dimensional (1D) diamond nanorods and two

dimensional (2D) diamond nanoplatelets. There is a special class of nanodiamond

material called as ‘ultra-nanocrystalline’ diamond with basic diamond constituents

having the size of just a few nanometers, which distinguishes it from other diamond

based nanostructures with characteristic sizes above ∼ 10 nm.4,18

Generally, all of these carbon nanostructures are prepared by laser ablation of

graphite or coal,19-21 metal-catalyzed arc evaporation,22-25 catalytic chemical vapour

deposition (CVD),25-28 pyrolysis,29 solvothermal reduction,30 hydrothermal

carbonization (HTC)31 and detonation of a mixture of explosives and hexogen

composed of C, N, O and H with a negative oxygen balance so that ‘excess’ carbon is

present in the system.32 The structures obtained vary upon the resources,

decomposition temperature and deposition kinetics. These materials are generally

characterized using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS),

X-ray crystallography and microscopic techniques such as scanning electron

microscopy (SEM), transmission electron microscopy (TEM), scanning tunnelling

microscopy (STM) and atomic force microscopy (AFM).

Carbon nanomaterials are widely used in various fields such as storage devices,

5

photovoltaics, sensors, field emission displays, field effect transistors, catalysts, for

bio imaging, drug delivery, water filtration, as AFM probes, high strength composites

and in CO2 sequestration.33-43 Even though these nanomaterials have many interesting

features, some of the problems involving aggregation, bundling and difficulty in

controlling the processability forced researchers to look out for alternatives with

superior properties. These investigations led to the discovery of planar single atom

thick sheet of carbon known as graphene. Discovery of graphene revolutionized the

material science community and eventually won the Nobel Prize in Physics in 2010.

Subsequent section gives an updated review on the synthesis, properties,

functionalizations and important applications of graphene.

1.2 Graphene

Graphene is an infinite single atom thick sheet of sp2 bonded carbon atoms derived

from graphite. The ideal interplanar distance of graphite is 0.345 nm, and hence the

ideal thickness of single layer graphene should be 0.345 nm.44 It can be considered as

the mother form of all other carbon nanomaterials, which can be wrapped into 0D

fullerenes, rolled into 1D CNTs or stacked to form 3D graphite.

A

B

Figure 1.3 Atomic structure of graphene. (redrawn from ref. 44)

6

As can be seen in figure 1.3, the lattice of graphene is made up of two interpenetrating

triangular sub-lattices shown in two different colours. The atoms of one sub-lattice

(A-blue) are at the centre of the triangle defined by the other lattice (B-red) with C−C

inter atomic length of 1.42 Å. One s orbital and two in-plane p orbitals in each carbon

atom contributes to the mechanical stability of graphene sheet.44 The remaining p

orbital, perpendicularly oriented to the molecular plane hybridizes to form the

conduction (π) and valence (π*) bands, which is responsible for the planar conduction.

While the term “graphene” has been known only as a theoretical concept, the

experimental investigations of graphene properties were inexistent till the recent years

because of the difficulty in identifying and isolating a single atom thick sheet. This

challenge was solved in 2004, when Geim and co-workers showed that monolayers of

graphene could be produced from graphite by simple “Scotch tape” peeling method.45

The remarkable properties of graphene reported so far include high values of its

surface area (∼ 2500 m2/g),46a thermal conductivity (∼ 5000 W/m K),46b intrinsic

charge mobility (∼ 200,000 cm2/V s),46c Young’s modulus (∼ 1 T Pa)46d and optical

transparency (∼ 97%).46e

1.2.1 Preparation of Graphene

For many years, graphene was a concept used by solid state theoreticians and later on

by some experimentalists to investigate the structure of graphitic monolayers on

certain metal substrates.44,47 Efforts to slim down graphite into graphene has started in

1960’s, when Fernandez-Moran extracted millimeter sized graphite sheets as thin as 5

nm by mechanical exfoliation.47a Later in 1962, colloidal suspensions of single and bi

layer graphite oxide were observed with electron microscopy by Boehm et al.47c The

interest in graphene was renewed after the discovery of fullerenes and CNTs in

7

1990’s, when several groups showed that graphite can be thinned down by various

techniques such as AFM manipulation and rubbing of pre-fabricated graphite pillars

on certain substrates.47d,48 But these approaches failed to produce isolated single layer

graphene sheets. Later in 2004, Geim et al, presented a simple method to produce

single layer graphene in which graphite crystal was repeatedly cleaved with an

adhesive tape followed by transfer of the thinned down graphite onto oxidized silicon

surface.45 This discovery became a landmark in graphene research. Eventually,

chemists and physicists came up with different methods for the production of

graphene such as epitaxial growth,49 CVD50 and wet chemical routes.51 Figure 1.4

shows a summary of different techniques used for the production of graphene, which

are discussed in detail in the following sections.

• GO route• Directly from graphite• Molecular approach

Preparation of graphene

Mechanical exfoliation Supported growth Wet chemical methods Unzipping of CNTs

• Epitaxial growth• CVD

2 - 3 layers, difficult to isolate monolayer, large scale production is not feasible.

Monolayers, high quality large films, requires high temperature and vacuum.

A few layered, processable graphene sheets with defects.

Monolayer thin graphenenanoribbons.

Figure 1.4 Present techniques for making graphene. 1.2.1.1 Mechanical Exfoliation

Graphite is stacked layers of many graphene sheets bonded together by weak van der

Waals forces. The interlayer van der Waals interaction energy in graphite is about 2

eV/nm2 and the force required to overcome this energy is of the order of 300

nN/µm2.52a This force can be easily achieved using mechanical or chemical energy.

8

The first attempt in this direction was by Viculis et al, who used potassium metal to

intercalate graphite and then exfoliate it with ethanol to form the dispersion of carbon

sheets.52b TEM analysis of the suspension showed presence of 40 ± 15 layers in each

sheet. Mechanical exfoliation of graphite using a Scotch tape allowed preparation of a

few layers of graphene.45

HOPG

Peeling

Transfer

Transfer

Si/SiO2 Si/SiO2

Scotch tape

Figure 1.5 Schematic of “Scotch tape” peeling off method to produce graphene.

This method consists of repeated stick and peel process using a common adhesive

tape, which brings down the thick graphite flake to a monolayer thin sample (Figure

1.5). Smooth and thin fragments on the tape are then transferred to cleaned substrates

by a gentle press of the tape. The most crucial aspect of this process is the suitable

choice of substrate to visualize graphene.53 Visibility of graphene under optical

microscope arises from an interference phenomenon in the substrate and it can be

easily changed by modifying the dielectric layer thickness (SiO2 thickness).53b Optical

images of graphene layers deposited on a 300 nm thick SiO2 layer indicated that

thicker graphitic samples (more than 50 layers) appear as yellow in colour whereas

the thinner samples appear in bluish or in slightly lighter colour shades. One

disadvantage of this method is that it leaves glue residues on the samples, which

9

adversely affect the electronic properties.46c,54 Hence, a post deposition treatment is

required, and at present the substrate is either baked under H2/Ar atmosphere or joule

heated under vacuum upto 500 °C to remove the glue residue.55 Later on, slight

variations of the original “Scotch tape method” were also reported. It was shown by

Hue and co-workers that large graphene sheets can be produced by manipulating the

substrate bonding of HOPG on Si substrate and controlled exfoliation.56a In a similar

approach, millimeter sized, single to a few layered graphene sheets were produced by

bonding bulk graphite to borosilicate glass followed by exfoliation.56b Both these

methods points to the need for suitable modification of bonding between the substrate

and graphite to generate large area graphene sheets. Even though micromechanical

exfoliation produces the best quality graphene sheets reported so far, large scale

production is expected to be difficult. Moreover, it produces uneven films which can

be easily removed from the substrates by solvent washing.57 Hence other strategies

have to be looked at for the scalable production of high quality graphene.

1.2.1.2 Supported Growth

Graphene could be grown on solid substrates via two different mechanisms, (i)

decomposition of hydrocarbons onto various metal substrates (CVD) and (ii) thermal

decomposition of metal carbides (epitaxial growth). Table 1.1 summarizes the type of

substrates and growth parameters reported so far for the supported growth of

graphene. The decomposition of hydrocarbons in presence of metal catalysts has been

extensively used for the mass production of CNTs. In an early method to produce

graphene, ethylene was decomposed on Pt(111) substrates at 800 K and it resulted in

the formation of nanometer-sized uniformly distributed graphene islands.50a Recently,

graphene monolayers were grown on Ru(0001) by thermal annealing, and it was

10

confirmed that the monolayer was formed by carbon atoms present in bulk Ru, which

then segregated and accumulated on the surface during the annealing process.50d Most

interestingly, graphene monolayers were formed continuously over areas larger than

several millimeter square. In another approach, 1-2 nm thick graphene sheets were

shown to be grown on Ni substrate by thermal CVD, in which a mixture of H2 and

CH4 were used as the precursor. The sheets were found to have smooth micrometer

size regions separated by ridges. The ridge formation was attributed to the difference

in thermal expansion coefficients of Ni and graphite.50c Similarly, growth of graphene

over polycrystalline Ni surface by thermal CVD has also been reported.50h In this

method, 500 nm thick Ni film was sputtered on Si/SiO2 substrate and was annealed in

H2/Ar atmosphere at 1000 °C for 20 min. Ni can be easily removed using dilute HCl,

which facilitates the transfer of graphene from metal substrate to any other

substrates.50h In the latest developments, different substrates such as polycrystalline

Cu, Fe-Co/MgO supported graphite, W and Mo have also been employed to grow

graphene by CVD.50b,i,j

The thermal decomposition of silicon carbide (SiC) at high temperature under

vacuum also results in the growth of graphene islands. Thermal treatment of carbides

under high vacuum result in the sublimation of Si atoms and the carbon-enriched

surface undergoes re-organization and graphitization at high temperatures.49b

Controlled sublimation result in the formation of very thin graphene coatings over the

entire surface. But the graphitization of carbon on SiC cause surface roughening and

form deep pits that induce wide graphene thickness distribution and limited lateral

extension of crystallites. Later on, it was found that the quality of epitaxial graphene

can be improved by using 900 mbar Ar atmosphere and higher annealing temperature

(1650 °C).49f

11

Table 1.1 Summary of supported graphene growth on both metals and carbide substrates.

Substrate Growth conditions Lateral

size /

thickness

Characterization Ref

Pt(111) Source: ethylene

Temp: 800 K 2-3 nm sized

graphite islands

LEED, STM, AES

50a

Ru(0001) Source: carbon

present in bulk Ru Temp: 1350 K

Pressure: 10-8 Pa

a few mm / monolayer

LEED, STM, XPS, AES

50d

Ir(111) Source: ethylene Temp: 1320 K

Pressure: 10-8 Pa

a few µm / monolayer

STM 50e

Ni Source: H2/methane

Temp: 1223 K Pressure: 104 Pa

∼ 1 µm / 1.5 ± 0.5

nm

SEM, Raman, STM

50c

Stainless steel Source: H2/methane Pressure: 4000 Pa

power: 1200 W Temp: 1073 K

2 - 40 µm / 0.5-3 nm

SEM, AFM, Raman

50f

Si, W, Mo, Zr,

Ti, Hf, Nb, Ta,

Cr, 304

stainless

steel, SiO2,

and Al2O3

Source: methane Pressure: ∼ 12 Pa

power: 900 W Temp: 953 K

∼ 1 µm / ∼ 1 nm

SEM, TEM, Raman

50b

Substrate free Source: ethanol atmospheric pressure

microwave (2.45

GHz) Ar plasma

reactor

a few 100 nm / 1-2 layers

EELS, TEM, Raman

58

Cu foil Source: H2/methane Temp: 1273 K

∼ 1 cm / 95%

monolayer

SEM, Raman 50g

Polycrystalline

Cu

Source: hexane Pressure: 1200 Pa

(Ar/H2) Temp: 1223 K

∼1.5 -�3.5 cm / 1-4 layers

AFM, STM 50i

Polycrystalline

Ni

Source: H2/methane Temp: 1173-1273 K low pressure UHV

∼ 20 µm / 1-2 layers

AFM, Raman, TEM, electron

diffraction

50h

12

Polycrystalline

Ni

Source: C2H2/H2

Temp: 973-1273 K rapid thermal CVD

∼ 1 cm / 1-3 layers

Raman, TEM, AFM

50k

Fe-Co/MgO

catalyst

supported on

graphite

Source: acetylene Temp: 973-1273 K

Argon as carrier gas

100-110 nm / 1-5 layers

Raman, TEM, STM, AFM

50j

SiC Thermal splitting of

SiC

Temp: 2273 K Pressure: 5 × 10-5 Pa

50-300 nm / 0.3 ±

0.04 nm

AFM, Raman, TEM, STM

49e

6H-SiC-

(0001)

Temp: 1300 °C UHV, sublimation of

Si

1-3 layers LEED, ARPES 49c

Amorphous

SiC deposited

on SiO2

Temp: 1373 K rapid

thermal annealing at

ambient pressure

∼ 2-15 µm / 6.5-10

nm

AFM, Raman, SEM, STM

49d

TiC(111) CVD, Temp: 1770 K ∼ 200 nm / 1-2 layers

XPS, LEED 59a

TaC(111) Source: ethylene Temp: 1570 K

1-2 layers AES, LEED, STM

59b

Metal carbides have also been used to produce supported graphene. For example,

decomposition of ethylene gas over Ta(111) and Ti(111) carbides produce graphene

monolayers.59a,b The morphology of metal carbide determines the structure of

graphene formed. In the case of terrace free TiC(111), monolayer graphene was

formed, whereas on 0.886 nm wide terrace of TiC(410), graphene nanoribbons

(GNRs) were obtained.

The islands of graphene films grown via both epitaxy and CVD produce even films

with high structural quality. Besides, these methods have opened the prospect of

growing large graphene films, which later can be transferred to any substrates.

However, the major drawback of this method is the strong metal-graphene interaction,

which will alter the transport properties of pristine graphene.60 Further, it requires

high temperature and high vacuum. Hence, it can be concluded that these methods

13

will be relevant only for high performance applications. For low end applications, wet

chemical approaches provide wide range of alternatives, which is discussed in the

subsequent section.

1.2.1.3 Wet Chemical Methods

Wet chemical approaches for the production of graphene proceeds by weakening the

interlayer van der Waals interaction forces in graphite by intercalation of reactants.

Decomposition of the intercalate releases high gas pressure, which results in

loosening and disruption of the π - stacking. Consequently, some of the sp2 hybridized

carbon atoms change their hybridization to sp3, which prevent the π - π stacking.51g

Wet chemical approach is scalable, allows the possibility of bulk production and more

versatile in terms of being well suited to chemical functionalizations. These

advantages could be utilized for a wide range of applications. Wet chemical methods

for the preparation of graphene can be generally classified into two processes based

on the precursors, one from graphene oxide (GO) and the second from graphite or its

derivatives. Table 1.2 summarizes the different wet chemical approaches to produce

graphene suspensions, which are discussed in detail in the following sections.

Table 1.2 Comparison of different wet chemical approaches to produce graphene suspensions

Starting

material

Reducing agent &

dispersible solvents

Lateral

size

Thickness Ref

GO Isocyanate Water, DMF

560 ± 60 nm

∼ 1 nm 51b,61a

GO Hydrazine vapour / NH3 Water

500-700 nm

∼ 1-1.4 nm

51c

GO Hydrazine hydrate Water

200-500 nm

∼ 1-2 nm 61b

GO Hydroquinone Water

500-1000 nm

∼ 1-1.4 nm

61c

14

GO 1-pyrenebutyrate Water

300-800 nm

∼ 1.7 nm 51d

GO ODA THF, toluene, ODCB

∼ 500 nm-2 µm

1.8 nm 51e

GO DMF, NMP, THF, EG 300 nm-2 µm

1-1.4 nm 62

GO KOH Water 1-2 µm ∼ 0.8 nm 61d

GO Allylamine/phenylisocyanate Water, DMF

∼ 500 nm- 1.5 µm

- 61e

GO quaternary ammonium salts CHCl3

several hundred nm

1.2 nm 61f

Graphite CHCl3 8-10 µm 3-4 nm 63a

Graphite liquid NH3/RI CHCl3, TCB

0.3-1.1 µm ∼ 3.5 nm 63b

Graphite sodium cholate Water

∼ 100 nm-3 µm

1-3 nm 63c

Graphite DMF, DMA, NMP several µm 1-5 nm 63d

Graphite

fluoride DCB, DCM, THF 1-2 µm ∼ 0.95 nm 64

Intercalated

graphite NMP several

hundred nm ∼ 0.4 nm 65

1.2.1.3a Graphenes from GO

One method to disrupt the π - stacking of layers is through oxidation of graphite. This

has been mainly achieved by Brodie,66 Staudenmaier67 and Hummers68 methods. All

the three methods involve oxidation of graphite in presence of strong acids or

oxidants. The level of oxidation can be controlled on the basis of the method chosen,

reaction conditions and the graphite source used. The resultant greyish brown material

can be easily separated into individual GO sheets using mild sonication. Oxidation of

graphite introduces hydroxyl, carbonyl and epoxide moieties in the basal plane and

this accounts for the colloidal stability of GO suspension in polar solvents.51f Besides,

the measurement of surface charge of GO sheets by zeta potential measurements

showed a negative charge when dispersed in water.51c In addition, XRD analysis of

GO showed absence of typical graphite interlayer spacing (0.34 nm) peak and the

15

appearance of a new peak around 0.6-1.2 nm, indicating a larger interlayer spacing.51c

Even though GO forms stable colloidal suspensions, they are electrically insulating

due to the disrupted π - conjugation. Hence, a post chemical treatment is essential to

restore the properties. GO sheets in solution are reduced using hydrazine or hydrogen

plasma.46e,69 But it results in aggregation of the reduced sheets, unless a stabilizing

agent is present. Various stabilizers such as polymers, amines, surfactants, aromatic

compounds and biomolecules have been used to stabilize the reduced GO sheets.51,70

Even though, the electronic properties can be partially restored upon reduction, it still

leaves significant amounts of defects in the lattice. Elemental analysis (atomic C/O

ratio, ~ 10) of the reduced GO measured by combustion revealed the existence of

significant amount of oxygen, indicating that reduced GO is not the same as pristine

graphene.61b But the reasonable conductivity and excellent transparency of reduced

GO thin films make it a promising component in transparent conductors as well as

reinforcement and anti-aging agent in polymer composites.51g

A few methods for producing colloidal suspensions of graphene sheets without the

help of surfactants or stabilizers have also been reported. Aqueous suspensions of

reduced GO under basic conditions do not aggregate.51c This could be attributed to the

conversion of neutral carboxylic acid groups to negatively charged carboxylate groups

under basic conditions. Thermal treatment is another method that has been used to

reduce GO. For example, rapid heating upto 1050 °C resulted in exfoliation and

reduction of GO.71

1.2.1.3b Unoxidized Graphene Sheets Directly from Graphite

An alternative strategy to produce defect free, unoxidized graphene sheets is based on

solvation, in which dispersed graphene sheets are stabilized by solvent adsorption.

16

Hernandez et al, demonstrated that simple sonication of pristine graphite in solvents

such as NMP, DMF or DMA result in well dispersed stable suspensions of unoxidized

graphene sheets.63d XPS investigations on the dispersed sheets support the conclusion

that the sheets are not oxidized. 1,2-dichlorobenzene (ODCB) also proved to be a

good solvent for the dissolution of graphite into graphene.72 The dispersions in these

solvents can be further improved by intercalating alkaline metals, surfactants or

polymers.64a,73 Even, aromatic compounds such as pyrene and perylene derivatives

with solubilizing functional groups have been employed for the exfoliation of

graphite.74 These aromatic compounds prevent graphene sheets from aggregation and

also act as healing agents to cover the defects in graphene sheets during annealing.74c

Dispersion of unoxidized graphene in water was obtained with the sonication of

graphite in water in presence of sodium cholate.73c,d It was found that the dispersed

concentration increased with sonication time and the average flake consisted of 4

stacked graphene sheets with length and width of 1 µm and 400 nm, respectively. In

addition to normal organic solvents, perflourinated solvents such as

hexafluorobenzene (C6F6), octafluorotoluene (C6F5CF3), pentafluorobenzonitrile

(C6F5CN) and pentafluoropyridine (C5F5N) have been employed for the liquid phase

exfoliation of graphite.75a The mechanism of exfoliation and solubilization involves

charge transfer through π - π stacking from the electron-rich carbon layers to the

electron-deficient solvent molecules containing electron withdrawing fluorine atoms.

Similarly, chlorosulphonic acid also proved to be a useful solvent for the exfoliation

of graphite and the solubility of graphene was found to be as high as ∼ 2 mg/ml.75b In

another approach, Billups et al, showed that reduction of graphite by lithium in liquid

ammonia yields graphite salts that can be reacted with alkyl iodides to form soluble

graphite platelets.63b The average thickness of the soluble platelets was found to be

17

3.5 nm corresponding to 10 layers of graphene. Another approach employing the

thermal exfoliation of graphite at 1000 °C, followed by re-intercalation with oleum

and expansion with tetrabutylammonium hydroxide was developed earlier.76 In

addition to organic solvents, ionic liquids and supercritical fluids have also been

reported to exfoliate graphite into individual graphene sheets either in solution or via

electrochemical methods.77,78 These solution processable graphene sheets are open to

different types of chemical functionalizations, which offer new pathways to generate

multifunctional graphene based devices or composites.

In oxidation and solvation assisted exfoliation, the separation depends on the

mechanical disruption of the graphitic network. When the samples are extensively

sonicated, it result in the fragmentation of graphene into small ribbons called as

GNRs.79 These ribbons are highly promising since electronic properties of graphene

are known to be governed by the edge states and were shown to be semiconducting.79b

1.2.1.3c Molecular Approach for the Production of Graphene

An additional option to produce defect free, solution processable graphene sheets is

by bottom-up approach, which starts with carbon precursors and to convert them to

large graphene like structures. One bottom-up approach involved the use of common

laboratory reagents ethanol and sodium, which upon solvolysis followed by low

temperature flash pyrolysis yielded single layer graphene sheets in gram quantities.80

Another strategy was thermal conversion of nano-diamond clusters into nanosized

graphene islands. Enoki and co-workers have shown that nano-diamond clusters can

be converted to nano-graphene islands by thermal graphitization process at 1600 °C.81

In contrast to the GNRs produced from graphite or CNTs, these nano-graphene

islands showed sharp arm chair or zigzag edges when imaged with STM. Another

18

method is the molecular level synthesis of large polycyclic aromatic molecules. The

synthetic routes towards graphene like molecules have been largely explored by

Müllen and co-workers.82 Hexabenzocoronene (HBC), and its derivatives upon

oxidative cyclohydrogenation reactions lead to planar graphene like discs (Figure

1.6).

A B

Figure 1.6 (A) Structure of HBC and (B) structure of the largest graphene like molecule reported with 222 carbon atoms. (redrawn from ref.82a)

Till now, the largest graphene like molecule reported is the one with 222 carbon

atoms in their core.82b Although such large molecules can be synthesized; one

disadvantage associated with it is the limited solubility. Solubility can be enhanced

with the incorporation of peripheral side chains, which will allow these materials to

show liquid crystalline properties and to form thin films.82a

1.2.1.4 Unzipping of CNTs

CNTs are made up of rolled cylinders of graphene sheets. Hence, longitudinal

unzipping of CNTs will result in the formation of nanoribbons of graphene. Recently,

this strategy has been successfully demonstrated by various research groups. Tour et

al, showed that oxidized GNRs can be produced by treatment of MWNTs with conc.

19

H2SO4 and KMnO4 mixture at room temperature.83 Mechanism of the tube opening is

given in scheme 1.1. In a similar approach, Dai et al, unzipped MWNT by mild gas

phase oxidation.83b In the first method, GNRs were found to have many functional

groups at the edges, whereas in the latter, the side walls remained intact.

Scheme 1.1 Proposed mechanism for the MWNT unzipping. First step (2) is the formation of manganate ester, followed by formation of dione (3), and subsequent induced strain causes the ring opening (4 and 5). (redrawn from ref. 83a)

A few other groups also have successfully unzipped MMNT; for example,

intercalation of ammonia and lithium metal into MWNT followed by HCl and thermal

treatment led to the formation of GNRs.84 Another route is by embedding MWNT in

PMMA matrix followed by etching of the exposed surface by Ar plasma.85 Unzipping

of CNTs provide an easy way to control the structure of GNRs. Arm chair nanotubes

upon unzipping can result in zigzag GNRs.83,85 Therefore, it is possible to produce

large scale, well aligned semiconducting GNRs for practical applications such as

room temperature transistors.

1.2.2 Characterization Techniques

Even though, structure of graphene is known for years, experimental proof for the

20

existence of graphene has been difficult, because of the limitations in identifying the

single atom thick sheet. But now, there are appropriate tools to visualize and

characterize graphene, which include optical and electron microscopy, scanning probe

techniques (AFM and STM), Raman scattering and complex spectroscopic techniques

(XPS, PES etc).

Optical microscopy is one of the simplest tools to identify the presence of

monolayer graphene sheets. Normally, Si wafers with 300 nm thick SiO2 layer on the

top is widely used as the substrate, where the maximal contrast occurs at a wavelength

of 550 nm. As alternatives to silica, graphene is visible on Si3N4 using blue light,53a

72 nm Al2O3 on Si wafer53b and on 90 nm PMMA using white light.86

AFM is used to establish the thickness of graphene layers. Although AFM is

limited to lateral scans, it is the best tool to monitor the topological quality of

substrate supported graphene samples. TEM has also been widely used as a

characterization tool for colloidal graphene suspensions. Moreover, single atom thick

graphene is an excellent support film for high resolution and spherical aberration

corrected TEM studies.87 Electron diffraction studies can also be used to distinguish a

single layer from a bilayer, since the ratio of the intensities for [2110] and [1100]

spots is inverted for the two samples as predicted by Horuichi et al.88

Raman scattering provide direct insight on the crystalline and electronic structure

of graphene. Raman spectra of graphene shows three major peaks, one at ∼ 1570 cm-1

(G band) corresponding to the E2g phonon at the centre of the Brillouin zone, and the

second one at ∼ 1350 cm-1 (D band) corresponding to the out of plane breathing mode

of sp2 carbon atoms.89a Intensity of D band is an effective probe to assess the

disorders and impurities in graphene.89b The most important peak, which can be

considered as the finger print of graphene is the D’ (2D) band at ∼ 2700 cm-1. The

21

shape, position and intensity of this band is highly dependent on the number of

layers.89c Advanced spectroscopic techniques such as XPS and ARPES also provide

direct evidence for the electronic structure of graphene.49c,90

1.2.3 Reactions of Graphene

From the point of view of its electronic properties, graphene is a zero band gap

semiconductor. Before it can be employed in various applications, the problem of

absence of band gap has to be resolved. For epitaxially grown graphene sheets, the

band gap can be engineered by electronic coupling between the sheets and the

substrate. An alternative option for tuning the band gap of solution processable

graphene sheets will be the chemical functionalization or molecular doping on

graphene sheets. The main approaches for chemical modification of graphene can be

grouped into following categories.

• Covalent functionalization of the π-conjugated skeleton with different

reactions such as hydrogenation, cycloaddition reactions, nitrene and carbene

insertions.

• Non-covalent functionalization by wrapping or adsorption of different

functional molecules such as polymers and small molecules.

• Molecular doping.

1.2.3.1 Covalent Modifications

One of the convenient ways to introduce solution processability is by covalently

grafting polymers and small molecules onto graphene by hydrogenation, fluorination,

simple coupling, cycloaddition, amidation and nitrene insertion. Covalent

functionalizations on graphene can be generally classified into two. The first category

belongs to covalent attachment of different molecules on GO and the second involves

22

the use of unoxidized graphene sheets.

1.2.3.1a Hydrogenation

Hydrogenation of carbon nanomaterials are of considerable interest because the

physical properties of hydrogenated materials are entirely different from that of the

starting materials. Hydrogenated graphene has also drawn great interest because of its

predicted magnetism.91 Hydrogenation of graphene is not a simple task. The first step

requires breaking apart of the diatomic molecules of hydrogen, which has very high

binding energy (∼ 2.4 eV/atom).92 First experimental evidence for successful

hydrogenation of graphene was demonstrated by Geim et al, by exposing

mechanically exfoliated graphene to atomic hydrogen generated by low pressure (0.1

mbar) H2/Ar plasma.93 The authors also showed that the hydrogenation is reversible

and most of the properties can be restored by annealing the samples at higher

temperatures. In completely hydrogenated graphene, alternating carbon atoms are

pulled out of the plane in opposite directions with the attached hydrogen atoms.

Several alternative approaches were made to hydrogenate graphene. In one

approach, reversible hydrogenation was achieved by the dissociation of hydrogen

silsesquioxane (HSQ) coated on graphene.94a Hydrogen atoms were generated by

breaking the Si-H bonds of HSQ by e-beam lithography. It was found that monolayer

graphene can be easily hydrogenated than two layered (2L) graphene sheets. This

enhancement in reactivity is attributed to the lack of π - stacking in single layer

graphene sheets, which is required to stabilize the transition state of the hydrogenation

reaction. Another method to hydrogenate graphene was proposed by Ao et al, who

suggested the possibility of hydrogenation by exposing graphene to molecular

hydrogen in presence of a strong perpendicular electric field.94b It was found that a

23

negative electric field can act as a catalyst to suppress the reaction energy barrier

while a positive electric field has the opposite effect.

As an example of chemical species which can provide, similar to hydrogen, a

complete coverage of graphene is fluorine. Fluorine is one of the promising

candidates for functionalization since it produces a very homogeneous structure and

can be substituted with other functional groups. Fully fluorinated analogue of

graphene known as perfluorographene with the formula (CF)n has been

synthesized.95a Besides fluorine, addition of HF, HCl and methyl radicals onto

graphene have also been theoretically investigated.95b-e

1.2.3.1b Covalent Modifications on GO

Acid oxidation of graphite results in the generation of highly oxygenated species

known as GO. Epoxy, hydroxyl and carboxylic groups on the material make it

negatively charged and assist in dispersion and solvation as individual sheets. GO can

be reduced back to graphene sheets in presence of stabilizers. Reduction of GO

partially restores the π - conjugation. Different varieties of chemical and physical

methods have been developed for this purpose. Scheme 1.2 shows the different routes

followed for the covalent functionalization of GO.

Acylation reactions are one of the common approaches used for coupling

functional moieties onto GO.96a,b The acylation reaction between the carboxyl groups

in the edges of GO sheets (after activation of the COOH group by SOCl2) and amine

groups were used to modify GO with long alkyl chains, porphyrins, fullerenes and

oligothiophenes (route I in scheme 1.2). Attachment of these functional moieties

improves the solubility and dispersion stability of graphene based materials in

common organic solvents.

24

1. S

OC

l 2/D

CC

2. R

NH

2

I

N2H

4.H

2O

Na

BH

4II

Ar-

N2X

RN

H2

N2H

4.H

2O

Na

BH

4II

I1.

SO

Cl 2

or

DC

C/D

MA

P2.

RO

HIV

V

1. N

aN

3

2. L

iAlH

4

Sch

em

e1.2

Sche

me

show

ing

vari

ous

cova

lent

func

tion

aliz

atio

nre

acti

ons

ofG

O.

25

GO covalently functionalized with porphyrin and fullerenes via acylation showed

superior non-linear optical (NLO) performance in the nanosecond regime.96a,b,h

Superior NLO properties are induced by photo induced electron or energy transfer

between the attached moieties and graphene.96a,b,h

Extending this functionalization chemistry, GO can be functionalized with PEG

and poly-l-lysine to obtain biocompatible conjugates, which can be used to load

hydrophobic aromatic drugs.97,98 High density loading can be achieved, because of the

efficient π stacking. It is well known that some of the anticancerous drugs are water

insoluble. Covalent linking of these drugs to GO via PEG linkage make it highly

water soluble with excellent cancer killing potency similar to the free drugs in organic

solvents.97

Similarly, hydrazine reduced GO are functionalized by treatment with aryl

diazoinum salts (route II in scheme 1.2).96c Like acylation reactions, diazonium

chemistry also gives the capability of tailoring the functional materials by changing

the addents. Another strategy relies on epoxide nucleophilic ring opening reactions.51e

Epoxide rings in GO undergo ring opening when treated with amine terminated

organic molecules (route III in scheme 1.2). GO has been shown to functionalize with

octadecylamine (ODA) with high dispersibility in common organic solvents.51e

Another extension of this concept is the use of amine terminated ionic liquids.99 These

hybrids also showed enhanced dispersibility in organic solvents because of better

electrostatic inter-sheet repulsion provided by the ionic liquid units.

Besides small molecules, GO sheets can be functionalized with polymers such as

PVA via carbodiimide activated esterification reactions.100 Carboxylic acid groups on

the GO sheets were reacted with hydroxyl groups on PVA in presence of DCC, 4-

(dimethylamino)-pyridine and N-hydroxybenzotriazole in DMSO (route IV in scheme

26

1.2).100

Recently, another method to functionalize GO sheets have been reported in which

GO is treated with sodium azide, followed by reduction with LiAlH4 (route V in

scheme 1.2).96e It introduced free amino groups on both the sides of reduced GO

sheets. These free functional groups can form covalent bonds with isothiocyanates,

which is promising for immobilizing graphene monolayers on any isothiocyanate

treated substrates.96e Another approach involves the synthesis of

polypropylene/graphene oxide (PP/GO) nanocomposites via in situ Ziegler-Natta

polymerization.101

1.2.3.1c Covalent Modifications on Unoxidized Graphene Sheets

Majority of the covalent functionalizations use GO as the starting material. GO is

hydrophilic and hence requires more processing time. One major drawback associated

with GO is that it requires a second reduction step to remove the excess oxides

present.69 Hence attempts were made using unoxidized graphene sheets. Unoxidized

graphene sheets can be modified either through selective edge functionalization and

surface functionalization.

Figure 1.7 Chemical structure of a small edge-carboxylated graphene nanoflake. (redrawn from ref. 103a)

27

Even though, theoretical predictions of selective edge functionalization of

graphene exist,102 very few experimental evidences are reported.103 This could be due

to the difficulties in controlling the reactivity only over the edges. Salzmann and co-

workers demonstrated the synthesis of edge carboxylated graphene nanoflakes by a

single step oxidation of CNTs using nitric acid (Figure 1.7).103a The presence and

location of functional groups were determined by various spectroscopic techniques.

Tour and co-workers demonstrated that thermally expanded graphite when

functionalized with 4-bromophenyl addends using diazonium chemistry result in the

formation of exfoliated graphene sheets which are selectively functionalized over the

edges.103c Elemental mapping and energy filtered TEM measurements revealed that

majority of the Br signals came from the edges of the functionalized sheets, indicating

that the basal planes were not highly functionalized.

+

C CC

C C C

O O O

OOO

NH2 NH2 NH2

NH2 NH2 NH2

Expanded Graphite

PPA, P2O5

Edge functionalized graphene

Figure 1.8 Schematic representation of the reaction between graphite and ABA as a molecular wedge via Friedel Crafts acylation in PPA/P2O5 medium. (redrawn from ref.103d - dotted lines indicate the extended graphene network) In a slightly different approach, organic molecules were covalently grafted to the

edges of graphite via an electrophilic substitution reaction (Figure 1.8).103d 4-

aminobenzoic acid (ABA) was treated with graphite in poly(phosphoric acid)

(PPA)/phosphorus pentoxide (P2O5) medium. This resulted in grafting of ABA onto

28

the defect sites (mostly sp2 C–H) located mainly on the edges of graphite followed by

exfoliation following Friedel Crafts acylation.103d

Surface/basal plane functionalization is comparatively easier for graphene, since

all the atoms are exposed to the same chemical environment. Besides, unoxidized

sheets have extended conjugated double bonds, which can be subjected to all the

addition reactions of unsaturated aromatic compounds. In one approach, graphene

sheets were directly fluorinated with plasma enhanced CVD followed by alkylation

(Figure 1.9).104a

Fluorinated graphite

R-NH2

Alkylated graphite

Figure 1.9 Schematic representation of alkylation of fluorinated graphene sheets (blue: fluorine; yellow: -R group).104a

Alkylation of fluorinated graphite has been done in a similar manner by Billups et

al.95c ODCB suspension of graphene was functionalized with perfluorinated alkenes

via free radical additions.104b The reaction was initiated by the thermal decomposition

of benzoyl peroxide in the reaction mixture. Advantage of this kind of one-pot

functionalization procedure is that pristine graphite is used as the starting material,

which avoids the oxidation/reduction steps.104b

It has already been reported that CNTs and fullerenes can be functionalized by

nitrene additions, which are produced in situ by thermal decomposition of azido

starting materials.105 It involves [2+1] cycloaddition of nitrenes to C−C bonds,

resulting in the formation of aziridino-ring linkage. Azido starting materials can be

29

synthesized in gram scale and offer a wide range of functional group tolerance.

Exfoliated graphene

,ODCB

4 days+

N

R

N

R

Figure 1.10 Nitrene addition to exfoliated graphite in refluxing ODCB. (redrawn from ref.106a)

Barron et al, reported a high yielding method for the functionalization of graphene

sheets through addition of azido-phenylalanine [Phe(N3)] to exfoliated micro-

crystalline graphite (Figure 1.10).106a In this work, graphite was exfoliated in ODCB

with ultrasonication followed by reaction with Boc-Phe(4-N3)-OH at refluxing

temperatures. In a similar approach, epitaxial graphene was covalently modified with

azidotrimethylsilane and found that the band gap can be tuned by controlling the

amount of incorporated nitrene.106b A similar cycloaddition reaction involve aryne

cycloaddition, in which graphene was covalently functionalized with 2-triflatophenyl

silane benzyne precursors.107 The functionalized sheets were found to be well

dispersible in solvents such as ODCB, ethanol, chloroform and water, depending on

the functional groups present.

In a similar approach, a few-layer graphene suspension in NMP was functionalized

using the 1,3-dipolar cycloaddition of azomethine ylides.108 The same approach was

used for immobilizing graphene on silicon wafers using perfluorophenylazide (PFPA)

as the coupling agent (Figure 1.11).57 Graphene sheets were covalently attached to

PFPA-functionalized wafer surface by a simple heat treatment under ambient

30

conditions. Solution phase PFPA coupling to solvent exfoliated graphene is also

reported, where PFPA with different functionalities were reacted with graphene by

photochemical or thermal activation.109

G

Si Substrate

HOPG

Si Substrate

HOPG

Si Substrate

HOPG

HeatingSi substrate

Si substrate

Graphene

Figure 1.11 Fabrication of covalently immobilized graphene on silicon wafer via the PFPA-silane coupling agent. (redrawn from ref.57)

All the reported covalent approaches towards functionalization of graphene offer

many opportunities such as tailoring the optical and electronic properties,

investigating the interfacial chemistry of graphene and incorporation of various

functional groups on the surface. Besides, this gives an opportunity for the integration

of electrically conductive graphene with biological systems which can have potential

applications in medical diagnostics, bioelectronics and bio-sensors.

1.2.3.2 Non-covalent Modifications

One of the main challenges in the production of graphene is to overcome the strong

interlayer van der Waals forces. Even though micromechanical exfoliation gives the

best quality graphene samples, mass production is very tedious and time consuming.

Besides, the sheets obtained in this manner contain different number of layers and is

easily removed by solvent treatment or sonication.57 An alternative way for the

production of graphene is by thermal expansion or metal intercalation. But

31

dispersibility of the exfoliated sheets will be a challenge in such cases. In solution

phase methods, exfoliated graphene can be dispersed in solution by non-covalent

wrapping of individual sheets by various species of polynuclear aromatic compounds,

surfactants, biomolecules and polymers.51,110-112

The non-covalent interactions are very promising because it offers the possibility

to homogeneously disperse the graphene sheets without disturbing its electronic

network, unlike the covalent functionalization. It is based on van der Waal’s forces

and π - π stacking between graphene and the added moieties.110,111 Both graphene and

GO can undergo π stacking. Aromatic compounds such as coronene, pyrene,

porphyrin and perylene derivatives have been used to stabilize graphene in various

organic solvents.110 All these aromatic compounds are large structures, which can

anchor onto the hydrophobic surface of graphene sheets via π - π stacking without

disrupting the electronic conjugation. Theoretically, both donor and acceptor type

aromatic molecules can be stacked onto the graphene surface, which tunes the π -

electron density on graphene. For example, pyrene-1- sulfonic acid sodium salt (PyS)

as an electronic donor and the disodium salt of 3,4,9,10-perylenetetracarboxylic

diimide bisbenzenesulfonic acid (PDI) as an electronic acceptor were used for non-

covalently functionalizing graphene.110a Besides, the negative charges in both the

molecules act as stabilizing species that maintain a strong static repulsion force

between the negatively charged reduced graphene sheets in solution. The composites

of PDI with graphene sheets resulted in an increase in conductivity for graphene-PDI

(13.9 S cm-1) compared to pristine reduced graphene (3 S cm-1), whereas a 30%

increase in conductivity was observed for graphene-PyS composite (1.9 S cm-1).

Charge transfer interactions between graphite and aromatic compounds have also

32

been employed for non-covalent modifications.110c,d These approaches yield novel

class of dispersible materials with tunable optoelectronic properties.

Besides aromatic compounds, polymers were also used for the non-covalent

stabilization of graphene sheets in solution. Use of graphene as a viable and

inexpensive filler substitute for CNTs in nanocomposites has triggered the search for

suitable agents to disperse graphene in the polymer matrix of choice. The first work

related to this was reported by Ruoff et al, who showed that the in situ reduction of

GO in the presence of an anionic polymer, poly(sodium 4-styrenesulfonate) (PSS)

form a stable aqueous dispersion of graphene sheets.51a Following a similar approach,

Shi et al, reported that in situ reduction of GO sheets in presence of sulfonated

polyaniline (SPANI) lead to the formation of water soluble and electrochemically

active composite.111b The functionalized sheets could be dispersed in water at a

concentration higher than 1 mg/ml. Polyurethane (WPU) was also used for the non-

covalent functionalization of thermally reduced graphene sheets.111c Graphene/WPU

composite films showed an increase in the storage modulus compared with that of the

films obtained by physical blending. This reinforcing effect of graphene can be

attributed to the presence of residual oxygen and epoxides present on the sheets’

surface which might interact with the monomers. Other polymers, which have been

used for the non-covalent stabilization of graphene and GO include amine terminated

PS,111a poly(vinyl alcohol),112a polyacrylonitrile, PMMA112b and nafion.112c

Like polymers, some polyelectrolytes and ionic liquids have also been proved as

successful candidates for the non-covalent functionalization of graphene sheets.113

Niu et al, reported the supramolecular functionalization of graphene sheets with an

anionic conjugated polyelectrolyte named poly(2,5-bis(3-sulfonatopropoxy)-1,4-

ethynylphenylenealt-1,4-ethynylphenylene) sodium salt (PPE-SO3-).113b This

33

polyelectrolyte has a poly(phenylene ethylenene) based back bone, which undergoes

π - π stacking with the graphene sheets and the anionic counterpart imparts excellent

water dispersibility and the possibility for self-assembly through electrostatic

interactions. Cationic polyelectrolytes are also being employed in the non-covalent

functionalization.113c Graphene was modified with poly(diallyldimethyl ammonium

chloride) (PDDA) and used as a building block for the self assembly of Au NPs on the

surface.113c

PDDAAu NP

Figure 1.12 Schematic of self-assembly of Au NPs and PDDA-functionalized graphene sheets. (redrawn from ref.113c)

In contrast to the reported in situ synthesis of hybrid materials based on graphene,

self-assembly method provides an alternative to obtaining the graphene NP hybrids

with high-loading and uniform dispersion due to the wide adaptability of electrostatic

interactions (Figure 1.12).113c Surfactants and biomolecules were also employed for

the non-covalent functionalization. Sodium dodecylbenzene sulphonate (SDBS) was

used for the stabilization of reduced GO sheets.96c Stabilization by surfactants is

attributed to the presence of surface charge, which attracts a diffuse layer of counter

ions from the liquid to form an electric double layer. Coulombic repulsion between

the nearby charged graphene sheets prevents them from aggregation.114 Likewise,

adsorption of biomolecules such as DNA onto the surface of the graphene is

34

facilitated by non-covalent π - π stacking interactions involving both purine and

pyrimidine bases of the DNA molecules.115a On the other hand, the sugar-phosphate

backbone stays away from the surface to produce a hydrophilic outer layer, which

facilitates the dispersion in aqueous medium.115a Other biomolecules such as vitamin

C, lignin and cellulose derivatives were also reported to be used as stabilizers for

graphene.115b,c

1.2.3.3 Molecular Doping

Graphene being a zero band gap material, it is highly desirable to have a tunable band

gap because it would allow great flexibility in design and optimization of electronic

devices. One possible way to achieve this is by doping, where the concentration and

type of the charge carriers in the graphene sheets can be controlled.116 Chemical

doping has larger implications in graphene research in terms of room temperature

superconductivity and ferromagnetism.117 If graphene is covalently functionalized

with electron withdrawing functionalities, p-doping can be induced.116 Similarly,

graphene functionalized with electron donating groups can be n-doped.118 If the

interface between p-doped and n-doped graphene regions is precisely controlled, it is

possible to engineer p-n junctions.118a

In addition, graphene can also be p-doped by absorbing metals with high electron

affinity such as gold, bismuth and antimony.116 In contrast, doping with alkaline

metals results in n-doping. This is attributed to the release of their valence electrons

into the conduction band of graphene.118 In addition to such metals, other elements

such as Ca, Al, K, Mn, Cu, Pt, Ag, Si, P and S can also be used as dopants to

graphene.118 It has been theoretically shown that Ca doping induces superconductivity

in graphene, whereas transition metal doping result in ferromagnetism due to the

35

hybridization between the transition metal and the carbon orbital.117a,c,d

Alternatively, molecular doping of graphene can be accomplished via charge

transfer between electron-donor and electron-acceptor molecules such as

tetrafluorotetracyanoquinodimethane (F4-TCNQ)119a and viologen.119b This gives rise

to significant changes in the electronic structure of graphene. This is confirmed with

the changes in Raman and photoelectron spectra.49c,119c Theoretical calculations have

demonstrated that small molecules like H2O, NH3, CO, NO2 and NO can also be

adsorbed onto graphene and influences the electronic and magnetic properties.119d

Recently, Ajayan and co-workers provided experimental evidence for change in the

band gap of graphene by adsorption of H2O molecules.119e Table 1.3 summarizes the

different functionalization methods reported for graphene.

Table 1.3 Comparison of different functionalization methods of graphene.

Precursors Methods Advantages /

Disadvantages

Ref

Covalent Modifications

Graphite hydrogenation control of electronic properties / causes defects

93,94

Graphite fluorination subsequent functionalization is easier / causes defects

95a

GO acylation enhanced dispersibility 96a,b,h

GO epoxide ring opening

enhanced dispersibility 51e

GO esterification - 100

GO diazonium addition

enhanced dispersibility; properties can be tailored by

changing the addents

96c

Expanded

graphite

diazonium addition

selective on edges 103a

Graphite Friedel Crafts acylation

selective on edges 103b

Graphene fluorination followed by

single step functionalization; avoids oxidation and

104

36

alkylation reduction steps

Graphene nitrene insertion single step reaction; band gap can be easily tuned

105,106

Graphene aryne

cycloaddition enhanced dispersibility 107

Graphene ylide

cycloaddition -

108

Non-covalent Modifications

GO/graphene large polycyclic

aromatic compounds

electronic network is not disturbed; enhanced

dispersibility

110

GO/graphene polymers dispersibility in a variety of solvents; reinforcing

composites

51a,

111,112

GO/Graphene polyelectrolytes - 113

GO/graphene surfactants dispersibility in a variety of solvents depending on the tail

groups

96c

GO biomolecules dispersibility in polar solvents 114,115

1.2.4 Properties of Graphene

Monolayer, bilayer, a few-layer graphene and GO exhibits diverse set of unusual

properties, which overcome the limitations of other materials such as CNT, graphite

or ITO in many applications. The most remarkable property of graphene is the high

charge mobility at room temperature.46c This has led to the conclusion that graphene

shows anomalous quantum Hall effect.120 Improved sample preparation, post

fabrication techniques and annealing of single layer graphene sheets allowed the

mobility to reach values up to ∼ 200,000 cm2/V s, which is the highest mobility

reported for any semiconductor or semimetal.46c In addition, the mobility remains

unaffected even at the highest electron-field-carrier concentrations and also by

chemical doping.121 This suggests the possibility of achieving ballistic transport

regime on a sub-micrometer scale at 300 K.122 It shows, room temperature ambipolar

37

characteristics, i.e. the charge carriers can be altered between holes and electrons

depending on the nature of gate voltage.45

The larger size, high mobility and sensitivity to field effect make graphene a

contender for CNTs in field effect transistors.45 But the main problem associated with

graphene is the zero band gap which has to be increased for FET device fabrication.

This can be achieved either by the inclusion of sp3 defects in the sp2 lattice or by the

distortion of graphene lattice under uniaxial strain.60 The most simple approach may

be the introduction of basal interaction of graphene with various molecules or

substrates.60

In addition to these interesting electronic properties, thin films of graphene and its

derivatives possess nearly 90% optical transparency. Optical transparency along with

high conductance enables graphene to be a tough competitor to the industrial

standard, ITO.46e,123 Interestingly, the optical transmittance follows a linear relation

with the thickness of graphene films, which is related to the 2D gapless electronic

structure of graphene. The white light absorbance observed for suspended single and

double layer graphene sheets was found to be 2.3 and 4.6%, respectively, with a

minimal reflectance (< 0.1%). This linearity has been demonstrated upto five

monolayers.124

Another important property of graphene is its mechanical robustness.60 Suspended

defect free graphene sheets are reported to have the highest Young’s modulus of ∼ 0.5

- 1 TPa, which is very close to that of bulk graphite.46c Surprisingly, suspended GO

sheets retained almost the same mechanical stabilities with a Young’s modulus of ∼

0.25 TPa to that of graphene.125 High mechanical strength, thermal stability and

conductance combined with low cost and ease of blending into matrices make this

material an ideal candidate for mechanical reinforcement.126 Besides, graphene bears

38

enhanced potential as the ultimately thin material such as pressure sensors and

resonators.60

1.2.5 Graphene Based Hybrid Materials

High conductivity, large surface area, high mechanical stability and the unique

graphitized planar structure make graphene a viable substitute for other conventional

carbon nanomaterials in hybrids. Graphene based hybrid materials can be prepared

either by physical blending or by chemical methods.

One class of hybrid materials, in which graphene is being used is polymer

nanocomposites, where graphene act as reinforcing filler. Pure GO is not suitable for

conventional nanocomposites, because it can be easily dispersed in water that are

incompatible with any other organic solvents. In addition, GO is electrically

insulating, which limits its application in conventional nanocomposites. GO has to be

chemically modified before it can be employed in nanocomposites. A great deal of

work has been done on graphene/polymer composites. Generally, graphene/polymer

composites have been developed using three strategies; (1) solvent processing, (2)

melt processing and (3) in situ polymerization.127 The first strategy consists of three

steps such as dispersion of graphene and the polymer in a suitable solvent, physical

blending and finally the removal of solvent. Several composites have been prepared in

using this strategy both in aqueous and organic media.126a,128 Even though, solvent

processing is a simple technique, it is associated certain disadvantages such as solvent

adsorption on graphene sheets. Melt processing involves the direct addition of

graphene into a melted polymer using a twin screw extruder. Thermally exfoliated

graphene and polymers such as polycarbonate, polyamide and polyurethane have been

mixed in melt processing.128d,129 Even though, this method is environmentally

39

friendly, one of the major disadvantages is the low degree of dispersion resulting in

poor mechanical and transport properties. In in situ polymerization, chemically

modified graphene sheets are mixed with monomers or polymer precursors, followed

by polymerization. Polymers are grafted to modified graphene sheets either by

covalent functionalization or via atom transfer free radical polymerization.130a,b The

main advantage of this strategy is the strong chemical interaction between graphene

and the polymer matrix and also the homogeneous dispersion.130 However, this

method is associated with viscosity changes that hinders higher loading.127 The

studies focussing on the mechanical and thermal properties of graphene based

polymer composites revealed an increase in the storage modulus and thermal stability

as a function of loading.126,128 Besides, the small wrinkles present on the graphene

surface is expected to enhance the mechanical interlocking and adhesion.128a

Similarly, incorporation of conducting graphene sheets in the polymer matrix

enhances the electrical conductivity with very low percolation thresholds.128,129 The

percolation thresholds vary over a wide range of loading fractions, from the lowest at

0.07 vol.% for high molecular weight polyethylene126a and 0.1 vol.% for PS129a to 3.8

vol.% for polyamide.129c Compared to mechanical reinforcement and electrical

conductivity, thermal conductivity of graphene filled polymer composites has

received minimal attention. The reason is that thermal energy is transmitted by the

interaction of adjacent particles through vibrations and free electrons, which requires

strong filler/polymer interface.127 In the case of graphene based polymer composites,

it can be only achieved with in situ polymerization. Fang et al reported an increase in

the thermal conductivity of PS films filled with 2 wt.% polystyrene-grafted graphene

from 0.158 W m-1 K-1 to 0.413 W m-1 K-1.130a Similarly, graphene/silicone foams

produced by in situ polymerization showed a 6% increase in the thermal conductivity

40

with a loading fraction of 0.25 wt.%.130f

Another class of graphene based hybrid materials is graphene/metal or metal oxide

nanosystems. In these systems, metals, semiconductors or metal oxides are distributed

onto the surface of graphene or between the layers. Generally, these are fabricated via

solution phase methods, in which the metal ions or the precursors are reduced in

presence of chemically modified graphene sheets.131,132 Compared to conventional

carbon nanomaterials, graphene provide higher surface area for better interactions.132a

The integration of graphene and functional nanoparticles such as metal nanoparticles

(Au, Ag, Pt, Pd), quantum dots (CdS, CdSe, ZnS) and metal oxides (ZnO, TiO2, CuO,

SnO2, Co3O4, Fe3O4, MnO2) present special features in the new hybrids, especially in

catalysis, optics, electronics, sensors and energy storage devices. For example,

Pd/graphene composite is used as a highly active catalyst in Suzuki-Miyaura

Coupling Reaction.131e Compared with the conventional Pd/C catalyst, graphene

based catalyst showed much higher activities with very low Pd leaching (< ppm).

Similarly, Ag/graphene hybrid systems have been used as an active SERS

substrate.131f CdSe/graphene composite films have shown improved photosensitivity

compared with pure CdSe films. Moreover, the photoconductivity and flexibility of

the films were also found to be enhanced compared with pure QD arrays.131g Hybrids

of graphene with TiO2, ZnO and other metal oxides have been used for photocatalytic

water splitting and for energy storage applications.132

Other than metal or metal oxide/graphene hybrids, graphene forms stable

composites with CNTs and fullerenes.133 CNT/graphene composites were reported to

show comparable optical transparency and conductivity to that of ITO. Besides, the

fabrication of these composites is very facile, inexpensive and is compatible with any

flexible substrates.133b

41

1.2.6 Applications of Graphene

Graphene can be used as a sensor material because all the atoms in a graphene sheet

are exposed to the environment, chemically stable and functionalization towards a

specific interaction is possible121 Moreover, mechanical robustness and transport

properties remain unchanged even after the local destruction of the sp2 lattice.121,125

The operation principle of graphene based sensors relies on the changes in electrical

conductivity due to the adsorption of molecules on graphene. Suspended graphene

sheets being under tension and impermeable provides an optimized atomically thin

supporting membrane for gas sensing.134a In the case of gas sensing, adsorption of a

gas molecule onto the graphene surface cause changes in the carrier concentration,

which can be monitored electrically in a transistor like configuration.134b In contrast to

other materials, the high mobility, large area ohmic contact and metallic conductivity

observed in graphene limit the background noise in transport measurements and this

confers high sensitivity to detect parts per billion levels or even a single molecule at a

rapid rate.121 Further, it can be chemically functionalized for specific interactions and

the variations in electronic and mechanical properties can be exploited to form the

transduction of the sensing signal.60,135 Several research groups have demonstrated the

good sensing ability of graphene towards NO2, NH3, H2O, CO and DNT.121,135 The

sensing mechanism of NO2 and DNT is based on the hole induced conduction as these

species withdraw an electron from graphene and in the case of NH3, it is electron

induced conduction as it donates an electron to graphene.121 Similarly,

electrochemical sensing ability of graphene based composite materials has also been

evaluated. Pt nanoparticle embedded graphene hybrid nanosheets were used as a new

electrode material and it showed enhanced electrocatalytic and sensing abilities

compared to pure glassy carbon electrodes.136 Graphene and its composites have been

42

used as sensors for heavy metal ions based on the fluorescence quenching ability.137

In addition to these, biosensing properties have also been demonstrated.138 For

example, Shan et al constructed a graphene based biosensor using glucose oxidase as

the enzyme model.138a A linear response upto 14 mM of glucose was observed in the

constructed device. Similarly, graphene has been used for the detection of

neurotransmitters such as dopamine and serotonine and it was found that graphene

electrodes were more sensitive than CNTs.138b Likewise, graphene hybrids were used

as high performance biosensors for DNA and organophosphates.138c-e

Another important application of graphene is in the fabrication of FET. Since

graphene being a zero band gap semiconductor, it cannot be directly employed in

FETs.45 A band gap suitable for room temperature FET applications should be

imposed on graphene. One way to achieve this is by manipulating the graphene

structure by cutting into small stripes known as GNRs.80,139 Besides being used in

FETs, graphene can be employed in LCD displays and as transparent electrodes for

solar cells.123a As described previously, extraordinary thermal, mechanical and

chemical stability of graphene along with high optical transparency make it an ideal

candidate for transparent electrodes.123

Another area in which graphene can be employed is in Li-ion batteries, where

graphite is usually employed as the anode material.140 Graphene can replace graphitic

carbon, owing to superior electrical conductivity, high surface area and chemical

tolerance. Different metal oxide/graphene composites have been synthesized for

battery applications which include SnO2/graphene,140a TiO2/graphene,140b

CuO/graphene140c and Co3O4/graphene140d hybrid systems. Though a few reports on

graphene based composite materials as electrodes in Li-ion batteries are available,

further research is needed to understand the mechanism of the electrochemical

43

processes involved. Some other notable applications of graphene are as

supercapacitors,46a,141 polymer composites,126-130,142 energy and hydrogen storage

devices.138a,143 Table 1.4 summarizes the major applications of graphene and the

graphene based composite materials.

Table 1.4 Major applications of graphene and graphene based composite materials

Source Applications Properties Ref

micromechanically cleaved

graphene; RGO; Pt embedded

graphene; graphene/metal

oxide composites

sensors highly sensitive/single molecule

detection/electrochemical response/fluorescence

quenching ability

121,125,134

-136

GNRs; graphene transistors tuned band gap/ballistic electron transport

139

graphene films; RGO films;

graphene composite films

LCD displays and transparent

electrodes

optical transparency/conductivity/ult

ra thin films/mechanical stability and flexibility

123,133

P3HT/graphene composite;

graphene films

photovoltaic/solar cells

a novel electron acceptor/ high electrocatalytic activity

and stability

13d,144

RGO films; graphene/PANI

composites;

capacitors good cycling stability/high specific capacitance/high conductivity over a wide

range of voltage scans

46a,141,145

graphene films; TiO2/graphene; SnO2/graphene; CuO/graphene; Co3O4/graphene

lithium ion batteries

high specific capacitance/enhanced Li ion insertion or extraction good

cyclic performance

140

chemically modified

graphene films; RGO films

graphene bio devices

large surface area/ biocompatible/sensitive to electrochemical changes

138d,146

44

1.3 Aim and Scope of the Thesis

Nanostructures from carbon have always been an area of interest to researchers for

their direct applications in the fields of science starting from material science to

biology. Most of the reported procedures for the preparation of functional carbon

nanomaterials rely on expensive equipments, resources, high temperature and

pressure. Besides, most of the methods generate desired nanomaterials along with

significant amount of carbonaceous impurities such as amorphous carbon and catalyst

particles. Simple and cost-effective production of carbon nanomaterials is very crucial

for the development of functional materials. The overall focus of the thesis is twofold:

� Develop simple, environmentally friendly methods to generate functional carbon

nanomaterials. Towards this goal, a novel solvent based extraction method was

employed to selectively separate carbon nanofibers from soot. The extracted fibers

were employed in µ-solid phase extraction for the separation and pre-concentration

of aromatic amines from aqueous samples. The proposed method offers advantages

such as easy operation, minimal use of organic solvent and elimination of tedious

solvent evaporation and reconstitution steps

� Build up simple solution phase approaches for the production of processable

graphene sheets. Even though micromechanical cleavage is the basic platform for

the production of high quality graphene sheets, the yield is very poor that it cannot

be used for bulk production and chemical reactions. Other reported methods also

have certain disadvantages such as lack of control over size, shape, dispersibility

and the number of layers. In order to scale up the production of graphene for

technological applications, new production methods and surface functionalization

methods have to be developed. Based on this, two different approaches have been

45

employed for the solution phase exfoliation of graphene.

(a) Surfactant assisted exfoliation of graphite into graphene without any

oxidation/thermal treatment. The effect of ultrasonication and non-covalent

stabilization were combined for the exfoliation and dispersion of graphene

nanosheets. The stabilized sheets were incorporated in poly(vinyl chloride)

matrices to generate flexible conductive composite thin films with high

mechanical strength and thermal stability. This approach might prove useful

for the use of graphene as a substitute for other carbon based fillers because of

its superior properties.

(b) Majority of the covalent functionalizations uses GO as the starting

material. GO is extremely hydrophilic and hence it requires more processing

time. In addition, one major drawback associated with GO is that it requires a

second reduction step to remove the excess oxides present. Hence, covalent

functionalizations were carried out using unoxidized graphene samples. Azide

insertion and bromination followed by alkylation/arylation were successfully

carried out on graphene samples. Chemical functionalization of the carbon

network in graphene by grafting atoms or molecules is very important because

this provides a method for the introduction of interesting photoactive

molecules for the development of optoelectronic materials.

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Papers published from this chapter:

Vadukumpully, S.; Basheer, C.; Jeng, C. S.; Valiyaveettil, S. J. Chromatogr. A 2011,

1218, 23, 3581-3587.

Chapter 2

Carbon Nanofibers Extracted from Soot as a Sorbent

for the Determination of Aromatic Amines from

Wastewater Effluent Samples

70

2.1 Introduction

Since the discovery of fullerenes1 and CNT

2, various kinds of nano-size carbon

materials are studied for different applications. A few of the best known structures are

CNTs and CNFs.3 CNTs and CNFs have been attracting a great deal of academic and

industrial interest due to their novel structural features and interesting properties.

CNFs are solid carbon fibers with lengths in the order of a few microns and diameters

below 100 nm. Owing to their unique physical and chemical properties, these have

been used in areas as diverse as electrochemical devices,4 field emission devices,5

sensors,6 gene delivery agents,7 high strength composites,8 hydrogen and charge

storage devices.9

Carbon soot collected from burning of renewable sources such as plant seed oil or

natural materials has been extensively studied during the last decade.10-12

The

nanostructures obtained vary upon the renewable resources, combustion temperature

and deposition kinetics. A variety of natural materials were explored to get good yield

of desired carbon nanostructures and the flames was called ‘sooting’ flames as they

spontaneously generated condensed carbon in the form of soot agglomerates

suspended in flame gases. Instead of using hydrocarbon as fuel, recently Sarkar et al

reported the synthesis of CNTs by pyrolysing mustard oil.13

All synthetic methods generate desired carbon nanostructures along with

significant amounts of carbonaceous impurities such as amorphous carbon, fullerene,

turbostratic graphite (TSG), polyhedral carbon nanoparticles and catalyst particles.14-

17 In order to obtain the optimum performance for carbon nanomaterials, it is

important to purify the soot and remove all unwanted materials. Several techniques

for isolation of pure materials have been applied such as chemical oxidation,17

use of

71

polymers,18 surfactant assisted extraction,19 centrifugation,20 sonication21 and

filtration.22

Carbon nanomaterials such as CNTs have been successfully used as a new sorbent

material in solid phase extraction for preconcentration of analytes.23

Aromatic amines,

widely used in the manufacturing process of pesticides, polymers, pharmaceuticals

and dyes are suspected carcinogens and are highly toxic to aquatic life.

24 Hence, it is

necessary to determine these compounds in waste water by rapid and sensitive

analytical techniques. Miniaturized extraction techniques such as solid-phase micro-

extraction (SPME) and liquid-liquid-liquid micro-extraction (LLLME) have been

employed in the extraction of aromatic amines from aqueous samples.25-27 However,

the application of CNFs in this area has not been well explored. CNFs offer large

surface area and high chemical stability which are ideal for a potential sorbent

material for solid-phase extraction. In this chapter we discuss the successful

extraction, purification, characterization of new CNFs and its performance as a

sorbent material for the extraction and preconcentration of aromatic amines.

2.2 Experimental Section

2.2.1 Reagents and Materials

Good quality edible oil was purchased from local market. Poly(vinyl alcohol) (PVA)

(average molecular weight = 124,000-186,000; 87-89% hydrolyzed) and phosphoric

acid were purchased from Sigma-Aldrich. Glutaraldehyde for cross-linking of fibers

was obtained from Alfa Aesar. HPLC-grade methanol, acetonitrile, THF, sodium

acetate and glacial acetic acid were bought from Merck. Ultrapure water was obtained

from Mili-Q (Milford, MA, USA) water purification system. Aromatic amine

standards 3-nitroaniline, 4-chloroaniline, 4-bromoaniline and 3,4-chloroaniline were

72

obtained from Fluka chemicals and were used without any further purification.

2.2.2 Sample Preparation

The standard stock solution of 1 mg/ml of each analyte was prepared by weighing

appropriate quantities of individual compounds and dissolving in methanol. A

standard solution containing all the four aromatic amines (at 5 µg/ml) was prepared

by mixing an appropriate volume of 1 mg/ml stock solutions in ultrapure water. All

solutions were filtered and stored at 4 °C. All aqueous samples with total amine

concentrations of 25 µg/l were freshly prepared before each extraction by spiking the

standard mixture into ultrapure water. The aqueous samples for calibration were

obtained by spiking ultrapure water with the standard mixture at desired

concentrations.

2.2.3 Isolation of CNFs

The oil was placed in an open container and burned using a small cotton thread at a

controlled rate in ambient conditions. The emitted carbon smoke was then condensed

onto a cold surface placed on top, but at a short distance from the flame. This process

was continued for several days to collect the carbon soot, which was used for

subsequent experiments. Briefly, about 25 mg of soot was mixed with 50 ml THF

with the aid of ultrasonication for 20 min. Only CNFs were extracted using exposure

of THF to the soot. After 6 h, the supernatant solution was collected and the solvent

was evaporated. The solid product was dried thoroughly in oven at 85 °C for

overnight and was re-suspended in THF for subsequent characterization and analysis.

2.2.4 Preparation of Electrospun CNF/PVA Composite Nanofiber Mats

An aqueous solution of PVA (10 wt%) was prepared by dissolving it in ultrapure

73

water at 50 °C with constant stirring for about 12 h. A fixed amount of CNFs were

then added to the PVA solution (3 ml). The solution was stirred for 12 h and was

further ultrasonicated for 30 min to obtain homogenous dispersions of CNFs in PVA

solution with a variation in the weight of the CNFs from 0.3-7.0 wt%. The CNF/PVA

solution was electrospun into fiber mats. In order to reduce the swelling of fibers in

water, the CNF/PVA composite fiber mats were placed in 1 vol.% glutaraldehyde-

acetone solution for 15 h to achieve cross-linking. The resultant composite fiber mat

was rinsed with ultrapure water followed by acetone and dried at ambient conditions.

For the electrospinning system, assembled parts were bought from OptroBio

Technologies Pte Ltd, Singapore. The electrospinning set up consists of a plastic

syringe with a metal syringe needle, a syringe pump, a high voltage power supply and

a grounded flat collector. The inner diameter of the syringe needle used was 0.2 mm

and the distance between the syringe tip and the grounded flat collector was

maintained at 10 cm. In the electrospinning, the CNF/PVA solution was placed into

the plastic syringe and charged with voltage (15 kV) via connecting the syringe

needle to the power supply. The flow rate of the CNF/PVA solution was optimized at

5-10 µl/min.

2.2.5 Materials Characterization

FTIR spectra were recorded at 1 cm-1 resolution with 32 scans in the wave number

range of 4000-400 cm-1 using a Bio-Rad FTS 165 FTIR spectrophotometer using

samples dispersed in KBr pellets. SEM and EDX spectra of the CNFs and electrospun

fibers were obtained using a JEOL JSM-6710F scanning electron microscope. TEM

and SAED images were taken using JEOL JEM-2010 transmission electron

microscope operating at 200 KeV. Raman spectroscopy was performed on Renishaw

74

system-2000, with an excitation wavelength of 532 nm.

2.2.6 µµµµ-SPE using Electrospun CNF/PVA Composite Nanofiber Mats

10 mg of each of the nanofibers mat was placed in the samples and stirred at 900 rpm

for 40 min. After extraction, the mat was removed, rinsed in ultrapure water, dried

with lint free tissue and placed in a 250 µl auto-sampler vial. The analytes were

desorbed by ultrasonication for 15 min in 75 µl acetonitrile, out of which 20 µl was

used for HPLC analysis. The composite membrane can be re-used after

ultrasonication in acetonitrile for 20 min to remove traces of impurities.

2.2.7 HPLC Analysis

All analyses were performed on a Shimadzu Prominence system (Shimadzu, Kyoto,

Japan) consisting of a CBM-20A system controller, a LC-20AD pump, a SIL-20A

auto sampler, a CTO-20A column oven, a DGU-20A5 degasser and a SPD-20A UV-

vis detector. Separation of the four aromatic amines at room temperature was

accomplished using a 50 mm × 3.0 mm I.D. MetaSil 5 µm ODS column (Varian, Palo

Alto, CA, USA). The detection wavelength was set at 254 nm. A mobile phase ratio

of acetate buffer (pH 3.5)-acetonitrile (85:15, v/v) at a flow rate of 0.3 ml/min was

used.

2.3 Results and Discussion

2.3.1 Characterization of the CNFs

The carbon soot is obtained via burning common edible oil in air and collecting soot

on a cold surface. The raw soot contained aggregated structures of carbon

nanoparticles (Figure 2.1A) and THF extract from the soot showed high aspect ratio

nanofibers (Figure 2.1B). From figure 2.1B, it can be seen that the THF extract

75

contained nanofibers in high yield, along with small amounts of carbon particles.

1 µµµµm

A B

100 nm

C

1 µµµµm

Figure 2.1 SEM images of the raw soot (A), extracted CNFs (B) and TEM of CNFs

(C) obtained from the soot by THF extraction. Inset in C shows the SAED pattern

obtained from a single nanofiber.

The CNFs were of several micrometers in length and expected to have very high

surface area. The CNFs isolated were of 20-50 nm in diameter. Elemental analysis

and EDX confirmed the presence of carbon (86 atom%) and oxygen (12 atom%) in

the THF extracted sample. The TEM image and SAED pattern indicated amorphous

character for the nanofibers (Figure 2.1C). There is no interlayer correlation and

therefore no significant long range of order to produce lattice diffraction (inset in

figure 2.1C).

10 20 30 40 50 60 70 8050

100

150

200

250

300

(100)

Inte

ns

ity

(2θθθθ)

(002)

Figure 2.2 XRD pattern obtained for the isolated CNFs.

XRD analysis also indicated low range of graphitization in the CNFs (Figure. 2.2).

The two predominant peaks at ~ 27 and 43.5° could be assigned for (002) and (100)

diffraction planes of hexagonal graphite, respectively. The fact that (002) diffraction

76

peak was relatively low in intensity and broad in shape, suggesting that the CNFs

have low graphitization and crystallization which in turn supports the SAED

observations. The broadening of peak also indicated the presence of disordered

structures in the products.28

FTIR spectrum of the CNFs (Figure 2.3A) showed the peaks at 2915 cm-1

and

2865 cm-1

which correspond to the aromatic –CH stretching. Bands centered at 1623,

1455 and 1386 cm-1

could be due to C=C stretching vibrations.29

Besides, it is seen

that the sample have a peak around ∼ 3400 cm-1, indicating the presence of –OH

groups. We believe that the π - π electrostatic and hydrophobic interactions between

the analytes and the large surface area CNFs facilitated the adsorption of amines,

rather than the presence of surface functional groups. Presence of surface functional

groups certainly enhances the adsorption efficiency. In the Raman spectrum (Figure

2.3B) of the isolated CNFs, there were two predominant peaks at 1347 and 1589 cm-1

respectively, corresponding to the disorder induced (D band) and E2g (G-band) mode

of graphite. It is known that the G band corresponds to stretching vibrations of sp2

hybridized carbon atoms and D band is associated with disorder-induced symmetry

lowering effects.30

Band at 1347 cm-1

arises from the amorphous graphite particles

and defect sites on the carbon nanofibers.

500 1000 1500 2000 2500

1589 cm-1

1347 cm-1

Inte

ns

ity (

a.u

)

Raman Shift (cm-1

)500 1000 1500 2000 2500 3000 3500 4000

340

0 c

m-1

10

48

cm

-1

13

86

cm

-1

145

5 c

m-1

16

23

cm

-1

23

49

cm

-1

28

65

cm

-1

% T

ran

sm

itta

nc

e (

a.u

)

Wavenumber (cm-1)

29

15 c

m-1

A B

Figure 2.3 FTIR (A) and the Raman spectra (B) of the CNFs.

77

2.3.2 Analytical Evaluation of Electrospun CNF/PVA Composite Nanofiber

Mats as Adsorbent for µµµµ-SPE

The extraction efficiency of the CNF/PVA composite nanofibers mats was evaluated

based on the peak areas obtained from HPLC analysis (Figure 2.4). Extractions were

repeated three times to obtain the statistical mean.

(1)

(2) (3)(4)

Figure 2.4 HPLC chromatogram obtained for standard 1 mg/l. Peak (1) 3-

nitroaniline; (2) 4-chloroaniline; (3) 4-bromoaniline and (4) 3,4-dichloroaniline.

Figure 2.5 gives a pictorial representation of the extraction and desorption steps

involved in the electrospun CNF/PVA composite nanofibers mat based µ-SPE.

1 µµµµm

A BElectrospun CNF/PVA composite fiber mat

Magnetic Stirrer

Stirring bar

Sample Vial

Desorption Solvent

Ultrasonicator

Water

Figure 2.5 Schematic of the extraction and desorption steps involved in electrospun

CNF/PVA composite membrane µ-solid phase extraction.

78

Cross linked CNF/PVA composite fiber mat was used for the extraction of aniline

compounds from water and the performance of CNF/PVA composite nanofiber mats

as a µ-SPE device for the determination of anilines was optimized by investigating

several factors which affect the extraction and desorption steps. The factors include

extraction time, desorption solvent, sample volume, desorption time, salting-out effect

and pH effect.

2.3.3 Control Study

The CNF/PVA nanofiber mat µ-SPE was compared with pure PVA fiber mat µ-SPE

device to prove that the extraction of anilines from water is mainly due to interaction

of anilines with the novel CNFs surface instead of the PVA support. As shown in

figure 2.6, the extraction efficiency of the four aniline compounds is greatly enhanced

with the incorporation of CNFs into PVA fibers.

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

10wt% PVA Soot +10wt% PVA

5wt% CNF+ 10wt% PVA

Pea

k A

rea

3-Nitroaniline 4-Chloroaniline 4-Bromoaniline 3,4-Dichloroaniline

Figure 2.6 Comparison of PVA and composite fiber mats (Extraction conditions are

as follows: 10 ml spiked 25 µg/l water solution, no salt added and pH adjusted,

extraction time of 30 min, desorption time of 15 min in 100 µl of acetonitrile).

A comparison of the extraction efficiency between the soot obtained after burning oil

and the CNFs extracted using THF was also carried out. The results show a better

79

extraction performance of CNFs compared to the soot particles. The extraction

efficiency depends on the interaction between anilines and the eletrospun CNF/PVA

composite fibers. The adsorption of analytes on the electrospun CNF/PVA composite

fibers is facilitated by the presence of electrostatic and hydrophobic interactions

between anilines and graphitic sp2 hybridized carbon atoms in the CNFs. In addition,

extraction efficiency is also attributed to the large surface area of electrospun

nanofibers.

2.3.4 Extraction Time

Membrane based µ-SPE is an equilibrium-dependent extraction procedure operating

with the principle of partitioning analyte on the sorbent material. The amount of

analyte extracted depends on the mass transfer from the sample solution to the

sorbent. Since mass transfer is a time-dependent process, the effect of time on the

extraction efficiency was investigated. The sample was continuously stirred at room

temperature with a magnetic stirrer to facilitate the mass transfer process and to

decrease the time required for equilibrium to be established.

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

10 20 30 40 50 60

Pea

k A

rea

Extraction Time (min)


Figure 2.7 Plot of HPLC peak area vs the extraction time (Extraction conditions are

as follows: 10 ml spiked 25 µg/l water solution, no salt added and no pH adjustment,

desorption time of 15 min in 100 µl of acetonitrile).

80

The stirring speed was fixed at 900 rpm. The extraction profile of aniline compounds

on the CNF/PVA composite fiber mat at a range of 10 to 60 min is shown in figure

2.7. The extraction efficiency increase gradually with time and a maximum was

achieved in 40 min. Therefore, 40 min was adopted as optimum extraction time for

the subsequent experiments.

2.3.5 Desorption Solvent

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

Acetonitrile Methanol Acetate Buffer

pH 3.6

Pea

k A

rea

Desorption Solvent

3-Nitroaniline 4-Chloroaniline

4-Bromoaniline 3,4-Dichloroaniline

Figure 2.8 Comparison of different desorption solvents (Extraction conditions are as

follows: 10 ml spiked 25 µg/l water solution, no adjustment of salt and pH, extraction

time of 40 min, desorption time of 15 min in 100 µl of acetonitrile).

In order to effectively desorb the hydrophobic analytes from CNF/PVA composite

fibers, selection of suitable organic solvent is critical. Factors such as solubility of

analytes, solvent polarity, compatibility with HPLC, solubility and swelling of

composite fibers were taken into consideration during the extraction. Since the cross-

linked electrospun CNF/PVA composite fibers were insoluble in common organic

solvents, solvents such as methanol, acetonitrile and acetate buffer were investigated.

81

Figure 2.8 shows the influence of desorption solvent on µ-SPE performance. The

polarity order of the solvents investigated and the analytes are as follows; methanol >

acetonitrile > anilines. The more polar solvent (methanol) gave poor desorption than

acetonitrile. Thus, subsequent analyses were carried using acetonitrile as desorption

solvent.

2.3.6 Sample Volume

The effect of sample volume (5-30 ml) on the extraction efficiency was investigated.

Figure 2.9 shows the influence of sample volume on µ-SPE.

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

3.5E+05

5 10 20 30

Pea

k A

rea

Sample Volume (ml)



Figure 2.9 Influence of sample volume on the extraction efficiency (Extraction

conditions are as follows: 10 ml spiked 25 µg/l water solution no adjustment of salt

content and pH, extraction time of 40 min, desorption time of 15 min in 75 µl of

acetonitrile).

As shown in figure 2.9, the analyte enrichment and extraction efficiency increases

with increase in sample volume. Lower sample volumes gave poor analyte

enrichment. The latter was at its maximum for all the analytes when 30 ml of sample

was used. As explained earlier, the amount of analyte that can be extracted depends

82

on the partition coefficient of the analyte between the sample and the CNF. This type

of behavior is common in microextractions.32 With the high surface area for

adsorption of analytes, the electrospun membrane can take up significant amount of

analytes as the sample volume goes up. However, considering the increase in

extraction efficiency with the sample volume is only gradual after 10 ml and it is less

feasible to have large sample volume for the extraction set-up, 30 ml was chosen as

the optimum sample volume.

2.3.7 Desorption Time

Desorption of analyte was carried out via sonication of the fiber mat with adsorbed

analytes in acetonitrile. Figure 2.10 shows the plot of peak area vs desorption time.

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

3.5E+05

4.0E+05

4.5E+05

5 10 15 20 25 30

Pea

k A

rea

Desorption Time (min)


Figure 2.10 Plot of HPLC peak area vs desorption time (Extraction conditions are as

follows: 30 ml spiked 25 µg/l water solution no adjustment of salt content and pH,

extraction time of 40 min, desorption in 75 µl of acetonitrile).

There was a small increase in desorption peak area within 15 min for 3-nitroaniline

and 3,4-dichloroaniline, other compounds did not show any significant change in peak

area with changes in desorption time. Hence desorption time was kept as short as

possible without compromising the extraction efficiency and 15 min was chosen as

83

optimum desorption time.

2.3.8 Effect of Ionic Strength

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

3.5E+05

4.0E+05

4.5E+05

0 5 10 20 30

Pea

k A

rea

NaCl Concentration (wt%)



Figure 2.11 Effect of salt addition on the extraction efficiency (Extraction conditions

are as follows: 30 ml spiked 25 µg/l water solution, no pH adjustment, extraction time

of 40 min, desorption in 75 µl of acetonitrile).

The salting out effect has been used in SPME32

and LLE.33

Generally, addition of

NaCl enhances the extraction efficiency of some organic compounds, since the

solubility of analytes in aqueous solutions decreases with increasing ionic strength.

However, no significant variation in the extraction efficiency with increase in salt

content in the sample was observed (Figure 2.11). Another effect of salt addition is

the increase in viscosity of sample solution, which in turn, impedes the mass transfer

rate and reduces extraction efficiency. Therefore, no salt addition was used in further

experiments.

2.3.9 Effect of pH

The influence of pH in the extraction was evaluated since pH plays a significant role

in ionizable compounds such as anilines. The pH values of water sample were

84

adjusted using 1M NaOH for alkaline sample and 1M HCl for acidic sample before

spiking the analytes.

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

3.5E+05

4.0E+05

4.5E+05

2 4 7 10 12

Peak

Are

a

pH


Figure 2.12 Effect of sample pH on the extraction efficiency (Extraction conditions

as follows: 30 ml spiked water solution (25 µg/l), no salt addition, extraction time of

40 min, desorption time of 15 min in 75 µl of acetonitrile).

As shown in figure 2.12, the extraction efficiency for the four compounds

increases with the increase in pH initially and level off after pH 7. pKa values for the

deprotonation of protonated salts of 3-nitroaniline, 4-chloroaniline, 4-bromoaniline

and 3,4-dichloroaniline are 2.47, 4.15, 3.86 and 2.97, respectively. At acidic

conditions, all the four aniline compounds are expected to be protonated and the

resulting cations have poorer affinity over their neutral form for the surface of

CNF/PVA composite fibers. Thus pH 7 (ultra-pure water without salt adjustment) was

selected as optimum condition. .

2.3.10 Method Validation

To assess the feasibility of CNF/PVA electrospun nanofiber mat as extraction device,

the validation was established for the determination of several performance

parameters such as linearity, precision, limit of detection (LOD) and enrichment

factors under the optimized extraction conditions. Table 2.1 summarizes the analytical

85

data obtained using the CNF/PVA membrane extraction.

Table 2.1 Quantitative data: linearity, precision (RSD), limit of detection (S/N = 3), enhancement factors and linear regression data obtained for anilines by the

electrospun CNF/PVA composite membrane microextraction coupled with HPLC-

UV.

Analytes Enrich

ment

factor

R.S.D

(%)

(n=3)

Correlation

coefficient

(r)

Linearity

range

(µµµµg/l)

LOD

(µµµµg/l)

LOQ

(µµµµg/l)

3-nitroaniline 66 4.8 0.998 0.5-50 0.009 0.030

4-chloroaniline 57 5.8 0.989 0.5-50 0.024 0.080

4-bromoaniline 82 4.5 0.996 0.5-50 0.081 0.269

3,4-dichloroaniline 109 5.5 0.998 0.5-50 0.019 0.062

Calibration curves were obtained by plotting the peak area vs the corresponding

spiked concentrations in ultrapure water. The linearity of the calibration plot was

evaluated over a range of 0.5-50 µg/l by least square linear regression analysis. All the

four amine compounds exhibited good linearity with correlation coefficient (r) above

0.9897. This shows a directly proportional relationship between the extracted amount

of analytes and the initial spiked concentration. The precision, which is expressed as

relative standard deviation (RSD), were determined on triplicate analyses at different

concentrations. The RSD obtained were in the range of 4.5-5.8%. The good

repeatability is mainly due to simple extraction methodology and use of auto-sampler

in HPLC-UV for the analysis. The LOD determined for the aniline compounds are in

range of 0.009-0.081 µg/l and the limit of quantitation (LOQ) is within a range of

0.030-0.269 µg/l. Our data is consistent with the LOD reported for extraction of

anilines in aqueous samples27

using LLLME.

86

2.3.11 Application of Method for Real Sample Analysis

Table 2.2 Concentrations of target aniline compounds (µg/l) in waste water samples

and the average recoveries determined at 5 µg/l (* n.d-not detected. Relative recovery

values of spiked wastewater sample at 5 µg/l compared to that of spiked pure water).

Analytes Environmental water sample Relative

recovery

(%) A B C D E

3-nitroaniline 2.3 n.d n.d n.d 0.29 85

4-chloroaniline n.d n.d n.d n.d 0.81 71

4-bromoaniline 6.9 0.46 n.d n.d n.d 108

3,4-dichloroaniline n.d n.d n.d n.d n.d 70

The current method was developed as pre-treatment for analysis of aniline compounds

in environmental water samples. Water samples were analyzed to assess the

contamination of the four aniline compounds. 3 out of 5 samples were found to

contain some of the aniline compounds and results are shown in table 2.2. However,

the complex matrix of environmental sample affects the performance of the composite

fiber mat in the extraction process. A standard addition method was used to study the

matrix effect and relative recovery of the aniline compounds. From the results shown

in table 2.2, it could be seen that relative recoveries for 4-chloroaniline and 3,4-

dichloroanilines are comparatively lower, which is about 70%. The low recovery

could be due to the clogging of nanofibers surface by the particulate matters present in

the sample. It could be concluded that the applicability and performance of

electrospun composite fiber mat as µ-extraction device depend upon the types of

water sample. CNF/PVA composite nanofibers are suitable for relatively clean water

samples such as filtered water samples or treated water sample to assess the

87

performance of treatment plant in removing amine compounds. The proposed

extraction method using the CNF/PVA composite nanofibers is more cost-effective

since it is robust, durable and reusable. Furthermore, it is fast and simple involving

only a few steps.

2.4 Conclusions

CNFs were isolated from carbon soot by a simple and effective methodology in good

yield. The isolated CNFs were characterized in detail by spectroscopic and

microscopic techniques. The CNFs were of several micrometers in length and 20-50

nm in diameters. The high surface area electrospun CNF/PVA composite fiber mat

has been successfully developed as a novel sorbent material for micro-extraction of

aniline compounds. The presence of CNFs in the composite membrane ensures and

enhances the extraction efficiency of method under optimum conditions. The

composite membrane sorbent yielded satisfactory parameters for micro-extraction.

Finally, the applicability of the method was further validated to determine aniline

compounds in environmental samples. The proposed micro-extraction method offers

advantages such as easy operation, high recovery, fast extraction, minimal use of

organic solvent and elimination of tedious solvent evaporation and reconstitution

steps.

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Publications from this chapter:

Vadukumpully, S.; Paul, J.; Valiyaveettil, S. Carbon 2009, 47, 3288-3294.

Chapter 3

Cationic Surfactant Mediated Exfoliation of Graphite

into Graphene Nanosheets and its Field Emission

Properties

94

3.1 Introduction

The recent discovery of free standing graphene1 has attracted worldwide attention

because of its interesting electronic, mechanical and thermal properties.2-10 Graphene

is considered as the mother form of all the 3D carbon nanostructures, and is made up

of a monolayer of carbon atoms closely packed in a honeycomb lattice.4

One of the main challenges associated with the production of graphene sheets is to

prevent the re-stacking. Lack of suitable methods to produce quality graphene sheets

in significant quantities limits some of its applications. The interlayer attractive forces

in graphite should be prevailed over to exfoliate it into graphene. Micromechanical

cleavage of graphite using “Scotch tape” is the standard procedure used for the

preparation of graphene.5 Even though, this method gives the best quality samples

with the highest charge mobility reported so far,11,12

mass production is very tedious

and time consuming. Moreover, the sheets obtained in this manner can be easily

removed from the substrates by simple solvent treatment. Alternatively, methods such

as exfoliation of graphite by metal intercalation followed by microwave heating,13

growing graphene by CVD of hydrocarbons on metal substrates14 and thermal

reduction of SiC have also reported.15,16

All these methods require very high

temperatures and obtaining a uniform graphene layer still remains a challenge.

Recently, many solution based approaches for the exfoliation of graphite into

graphene has been reported, which mainly involves the chemical oxidation of graphite

into GO followed by reduction in presence of various stabilizers such as polymers,17

hydrazine,18

surfactants,19

pyrene and perylene derivatives,20,21

ODA22

and various

biomolecules.23 But chemical oxidation disrupts the electronic structure of graphene

and introduces many carbonyl (−C=O), hydroxyl (−OH) and epoxide groups in the

95

basal plane of the sheets. Even though, reduction can remove most of the

functionalizations, it still leaves significant number of defects which alters the

electronic properties. Very recently, sonication assisted dispersion of graphene in

NMP and DMF24,25

and ionic-liquid assisted electrochemical methods26

for the

production of graphene from graphite has reported. Graphene sheets produced via

these methods show agglomeration over a period of time which hinders the potential

applications. Hence a simple, solution phase approach for the production of

unoxidized but stabilized graphene sheets is needed to explore its potential

applications.

In this chapter, we outline a simple solution based method for preparing a few

layered graphene nanosheets directly from graphite without any oxidative or thermal

treatment. The effect of ultrasonication and non-covalent stabilization are combined

for the exfoliation and dispersion of graphene.

Graphite

Acetic acid

(CTAB)

Scheme 3.1 Schematic of cetyltrimethylammonium bromide (CTAB) assisted

exfoliation.

Mild sonication of HOPG nanosheets in presence of various surfactants (both cationic

and anionic) and acetic acid led to the exfoliation of graphite into graphene

nanosheets. The cationic surfactant CTAB stabilized graphene samples were found to

96

be more stable in solution than the anionic surfactant sodiumdodecyl sulfate (SDS)

stabilized samples. The mechanism of stabilization involves the adsorption of

amphiphilic CTAB molecules onto the electron rich graphene surface through

electrostatic interactions. The hydrophobic interaction of the alkyl chains of CTAB

molecules on the graphene surface could prevent the re-stacking and agglomeration of

the exfoliated sheets. The nanosheets thus obtained were characterized using AFM,

HRTEM, FESEM, EDX, STM, Raman spectroscopy and field emission

measurements.


3.2.1 Materials and Methods

HOPG was purchased from Good Fellow Ltd, USA. CTAB, SDS, glacial acetic acid

and DMF were purchased from Sigma-Aldrich and were used without further

purification. Milli-Q water was used for washings.

3.2.2 Preparation of Processable Graphene Nanosheets

In brief, 100 mg of HOPG nanosheets were sonicated (Elma E-30H ultrasonicator,

operating at 37 kHz ultrasonic frequency and 115 W) with 0.5 M CTAB solution in

glacial acetic acid for 4 h. The resultant solution was then refluxed for 48 h under N2

atmosphere. Reaction mixture was left to stand overnight to allow aggregated sheets

to settle down. The supernatant was decanted and then centrifuged at 20,000 rpm for

45 min. The resultant black residue obtained could be re-suspended in DMF.

3.2.3 Instrumentation

UV-vis spectrum was recorded using Shimadzu 1601 PC spectrophotometer. SEM

images and EDX spectra were obtained from JEOL JSM-6710F scanning electron

97

microscope. For SEM imaging, a dilute solution of the exfoliated sample was drop-

casted on ITO coated glass substrate and dried in the oven. EDX spectrum was

recorded from drop casted sample on conductive Si substrate. HRTEM analyses were

carried out using JEOL JEM-2010 F electron microscope, operating at 200 keV. STM

measurements were performed on Au(111) substrate (Goodfellow, UK) using a

NanoScope IIIa MultiMode SPM (Digital Instrument, Veeco Metrology Group) with

electrochemically etched Pt(.8) Ir(.2) tips (0.25 mm diameter). A drop of graphene

solution deposited on Au(111) substrate was used for STM measurements and the

images were obtained at a tunneling current of 2 pA and a bias voltage of 50 mV.

AFM images were recorded using Agilent AFM with Pico plus molecular imaging

system in the non-contact mode. Samples for AFM were prepared by spin-coating a

dilute suspension of graphene on freshly cleaved mica at a speed of 5000 rpm and

then dried at room temperature. Raman spectra were recorded with a Renishaw

system-2000 operating at an excitation wavelength of 532 nm. Sample for field

emission studies was prepared on conducting Si substrate. The Si wafers were washed

with piranha solution and Milli-Q water, respectively and air dried before drop casting

the sample. The graphene deposited on the Si wafers served as the cathode while an

ITO coated glass kept at a distance of 100 µm acted as the anode. A vacuum of ~ 10-7

torr was applied in the chamber. Voltage was supplied using a programmable high

voltage source and the resulting emitting current was measured (see figure 3.7A for

the schematic representation of experimental set-up).


Ultrasonication is a powerful tool for extracting nanomaterials from bulk.25,26

Sonication of HOPG in CTAB-acetic acid mixture facilitated the exfoliation of

98

graphite without any oxidation. It can be explained by the effect of acoustic cavitation

of high frequency ultrasound in the formation, growth and collapse of microbubbles

in solution,27 which induces shock waves on the surface of the bulk material, causing

exfoliation. Even though, surface energy of acetic acid is not very close to that of

graphite,28

dispersion quality of acetic acid is comparable with that of the best

solvents reported for a good dispersion of graphene.22

Hence acetic acid was chosen

as a solvent for the exfoliation of graphite. Both cationic and anionic surfactants

(CTAB and SDS, respectively) were used for the exfoliation and stabilization. But,

only CTAB stabilized sheets were found to be stable in solution. It could be attributed

to the strong electrostatic interactions between the quaternary cationic head groups in

CTAB and the electron rich graphene surface. Besides, the hydrophobic interactions

of long alkyl chains of CTAB molecules prevent the re-stacking and agglomeration of

the exfoliated sheets. The yield of a few layered graphene nanosheets from this

approach was found to be ∼ 10%.

3.3.1 Control Studies

Figure 3.1 SEM image of the exfoliated product in acetic acid.

Exfoliation of graphite was attempted in acetic acid without the use of any surfactants.

The resultant product was found to settle down in solution over a period of time (> 1

99

h). Figure 3.1 shows the FESEM image of the settled product obtained. From the

image, it can be concluded that sonication in presence of acetic acid exfoliates the

sheets to certain extent, but the centrifuged product seem to appear as agglomerated.

It implies that acetic acid alone is not sufficient to stabilize or prevent the re-stacking

of the exfoliated sheets.

3.3.2 Exfoliation of Graphite in Presence of SDS

2 µµµµm5 nm1 µµµµm

A B

Figure 3.2 FESEM (A) and TEM (B) images of SDS stabilized graphene sheets. Inset of figure 3.2B shows the HRTEM image of one the edges of the multilayered graphite

flake.

When SDS-acetic acid mixture was used, the exfoliated sheets were agglomerated and

precipitated when the excess SDS was removed by centrifugation and washing (SEM

and TEM image in figure 3.2A and B). This could be due to the repulsion between

negatively charged SDS molecules and electron rich graphite sheets. However, when

graphite was sonicated in presence of acetic acid and CTAB, graphite nanosheets

exfoliated to form planar a few layered graphene nanosheets and it was found to form

stable suspensions in DMF, NMP and DMSO.

3.3.3 Exfoliation of Graphite in Presence of CTAB

The exfoliated nanosheets stabilized by CTAB were characterized by various

100

spectroscopic and microscopic techniques. Figure 3.3A shows the UV-vis spectrum

and the inset shows a photograph of the surfactant stabilized graphene suspension in

DMF.

300 400 500 600 700 800

0.0

0.5

1.0

Ab

so

rba

nc

e (

a.u

)

Wavelength (nm)

A

B

1 µµµµm

1 µµµµm

C

Figure 3.3 (A) UV-vis spectrum of the CTAB stabilized graphene nanosheets

dispersed in DMF. Inset shows a photograph of the DMF solution. SEM of (B) HOPG

and (C) the CTAB stabilized exfoliated graphene.

101

The graphene suspension in DMF was stable without any visible aggregation for

several weeks. It is known from the literature that hydrazine reduction of the GO

solution cause a red shift in the absorption maximum from 231 to 275 nm, indicating

a partial restoration of the electronic structure. In the case of CTAB stabilized

graphene suspension, it showed a strong absorption band at around 280 nm, showing

the formation of stable graphene nanosheets without any disruption in the electronic

structure.

Figures 3.3B and C show a comparison of the SEM image of HOPG with that of

the exfoliated product. Several graphene layers stack each other in a highly ordered

fashion in HOPG because of the strong van der Waal’s forces of attraction (Figure

3.2B). Upon exfoliation in presence of CTAB, large number of different types of

sheets was formed. From figure 3.3C, it could be seen that most of the exfoliated

sheets were very thin and transparent to the SEM beam. Most of the thin nanosheets

were of the order of 1-2 µm in size with folds over the edges. In addition to this, some

multilayered graphitic nanosheets were also observed.

Low resolution and high resolution TEM images of the nanosheets are depicted in

figure 3.4A and B. Several nanosheets were observed on the TEM grid and were

found to be stable under the electron beam. The sheets were crumpled and it could be

due the extra thermodynamic stability of the 2D membranes arising from microscopic

crumpling involving either bending or buckling.29 Figure 3.4B shows the HRTEM

image of one end of a scrolled graphene nanosheet. The ordered graphite lattices were

clearly seen on the edges. The distance between the lattice planes (Figure 3.4B) were

found to be 0.34 nm, which corresponds to the bilayer distance between the graphitic

planes. STM image of a monolayer graphene is shown in figure 3.4C, where the

hexagonal packing of carbon atoms is clearly visible. Inset of figure 3.4C clearly

102

show the hexagonal symmetry obtained for graphene nanosheets. Presence of small

non-uniformities in the pattern could be attributed to the presence of residual

surfactants on the surface.

5 nm

B

0.34 nm

50 nm

A

C

1 nm

1.148 nm

0 2 4 6

-0.1

00

.1

0.424 nm

0 2 4 6

-0.1

00

.1

nm

nm

D

E

Figure 3.4 (A) TEM image of surfactant stabilized graphene nanosheets. (B) HRTEM

image of a bilayer graphene sheet. (C) STM image of a part of the monolayer

graphene (inset highlights the hexagonal lattice of the monolayer). (D) Section analysis along the blue line which showing a width of 1.148 nm for 4 hexagons and

(E) along the green line which showing a width of 0.424 nm for 3 C-C bonds.

Figure 3.4D is the section analysis along the blue line (Figure 3.4C) which shows a

width of 1.148 nm for 4 hexagons (hexagon width of 0.287 nm). Similarly, figure

3.4E is the section analysis along the green line which shows a width of 0.424 nm for

3 C−C bonds (C−C bond length of 0.14 nm).

Thickness and morphology of the sheets was further probed using AFM. A large

area image of the graphene nanosheets spin coated on the mica sheet from DMF

103

suspension is shown in figure 3.5A. Figure 3.5B and C represent the AFM image of

an individual graphene nanosheet and its height profile, respectively.

A

1 µµµµm

1.3 nm

C

B

200 nm 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0

5

10

15

20

Length (µµµµm)

E

0.2 0.4 0.6 0.8 1.00

4

8

12

16

Width (µµµµm)

F

0.4 0.8 1.2 1.6 2.00

3

6

9

12

15

18

Thickness (nm)

D

Figure 3.5 (A) Topographic view (10 × 10 µm) of the graphene layers spin coated on

mica. (B) AFM image of a single graphene sheet (1 × 1 µm). (C) Height profile of the

image 3B. Statistical analysis of the AFM images of 60 nanosheets: (D) thickness, (E)

length and (F) width of the nanosheets.

Contrast difference between the edge and the centre of nanosheet clearly shows the

sharp and rolled edge (Figure 3.5B). The height of the sheet was found to be ~ 1.3

104

nm, corresponding to ~ 1-3 graphene layers. AFM images clearly indicated that

nanosheets are very small with dimensions of the order of < 1 µm. Dimensions

(height, length and width) of a large number of nanosheets (60) were measured and

the statistical analyses are plotted in figure 3.5D, E and F. Average thickness of the

nanosheets was found to be 1.18 nm. Most interestingly, more than 85% of the

nanosheets were less than 4 layers thick (Figure 3.5D). The average length and width

were found to be 0.7 and 0.5 µm, respectively (Figure 3.5E and F). Decrease in the

sheet dimensions could be attributed to the breakage due to sonication.

1000 1500 2000 2500 3000

Inte

ns

ity

(a

.u)

Raman shift (cm-1)

CTAB stabilized graphene

HOPG flakes

A

D

G

2D

KeV

Co

un

ts

3.60

C Si

0.00 0.40 1.20 2.00 2.80 0

2000

4000

6000 B

Figure 3.6 (A) Raman spectra of HOPG and the CTAB stabilized graphene

nanosheets deposited on Si and (B) EDX spectrum.

Significant structural changes occurred during the exfoliation of HOPG to graphene

were clearly reflected in the Raman spectra (Figure 3.6A). Raman spectrum of HOPG

displayed mainly a prominent G band at 1582 cm-1

, corresponding to the first-order

105

scattering of the E2g mode and a 2D band at ~ 2715 cm-1.30 However, the spectrum of

the exfoliated graphene showed the presence of D band at 1352 cm-1 which indicates

the presence of defects. The D band is due to the breathing modes of sp2 carbon atoms

and requires a defect for its activation and the intensity of D band is directly related to

the amount of disorder present in the graphene sheets.30

Defects in the exfoliated samples might be due to the disorders induced by

sonication and decrease in the average size of the sp2 domains.

31 Raman spectrum of

CTAB stabilized graphene sheets also showed the presence of G band at ~ 1582 cm-1

and the 2D band at ~ 2715 cm-1

. Since no defects are required for the activation of

two phonons with the same momentum, the 2D band is always seen, even when no D

band is present.22 EDX spectrum of the exfoliated product showed absence of any

oxidized structure (Figure 3.6B). Spectrum showed peaks due to ‘C’ and ‘Si’, but not

corresponding to ‘O’. Absence of the peak corresponding to ‘O’ indicate that the

exfoliated products are not oxidized. The peak corresponding to ‘Si’ is from the

substrate.

3.3.4 Field Emission Properties of Graphene Nanosheets

The field emission characteristics of the graphene nanosheets were studied using

parallel plate diode geometry with an anode to cathode spacing of 100 µm (Figure

3.7A). A plot of current density vs the applied electric field is shown in figure 3.7B.

The turn on voltage for the onset of electron emission (10 µA/cm2) was approximately

7.5 V/µm. Emission current densities as high as 0.15 mA/cm2 was obtained at an

applied potential of 10 V/µm. The sheets were stable under high applied electric

fields.32

Plot of corresponding ln (J/E2) vs 1/E follows a linear relationship over the

field emission range as predicted by the Fowler-Nordheim relationship (Figure 3.7C).

106

Back-Gate Electrode

Si Wafer (insulator)

Graphene

High Voltage

Dielectric spacer

Drain ElectrodeSourceElectrode

0 2 4 6 8 10

0.00

0.04

0.08

0.12

0.16

C

urr

en

t d

en

sit

y,

J (m

A/c

m2

)

Applied electric field, E (V/µµµµm )

0.10 0.12 0.14 0.16 0.18 0.20

-16

-14

-12

-10

-8

-6

ln

(J/E

2)

1/E

A

B

C

Figure 3.7 (A) Schematic representation of the experimental set up for field emission measurement. (B) Graph showing the field emission current density vs applied electric

field. (C) Fowler-Nordheim plot indicating the field emission characteristics of

exfoliated graphene nanosheets.

107

The field amplification factor β, which describes the geometrical shape and

surroundings of the emitter is calculated using the Fowler-Nordheim law.33

F

BFI

2/32

expφ

φα

where I is the emitted current, F is the local field at the emitter surface, which is

calculated as d

VF

β=

, where V is the applied potential and d is the distance between

cathode and anode, φ is the work function (5 eV for graphite) and B = 6.83 × 109 V

eV-3/2 m-1. The field amplification factor β was calculated to be ~ 1250. The field

amplification factor β of a free standing graphene sheet can also be theoretically

calculated using the empirical formula proposed by Watcharotone et al.34

Accordingly, field amplification factor across the surface of the corner βcorner can be

calculated as

( )θβ cos5.05.009.0107.2

75.0

+

+

=

w

l

t

lcorner

where l, w and t respectively represents the length, width and thickness of the

graphene nanosheets. θ is the subtended angle at the centre of curvature between the

point of consideration at the corner and the midpoint of the corner. The field

amplification factor along the edges of the sheet βedge can be calculated as

( )19.075.0

cos25.075.009.0177.1

−

+

+

=

t

d

w

l

t

ledge ϕβ

where, d is the distance from the junction of the edge and corner. ϕ is the subtended

angle at the centre of curvature between the point of consideration and the nearest

point at the middle of the rounded edge.34

The average length, width and thickness of

the graphene nanosheets were taken as 0.7 µm, 0.5 µm and 1.18 nm, respectively for

108

the calculations. The maximum field amplification factor (θ = 0) at the corner can be

calculated as 313 and at edges as 85. This is low compared to the experimentally

obtained effective value of 1250. This could be because the sheets are practically

having very sharp edges and multiple corners, a situation highly different from the

assumed rectangular geometry. Moreover, the sheets appeared to be curved at the

edges which enhance the field emission effects and are not considered while deducing

the value from the empirical formula. Field emission studies of carbon nanosheets

grown by radio-frequency plasma-enhanced chemical vapor deposition technique35

showed emission current levels of 1.3 mA. Graphene nanosheets obtained by our

method have lesser emission current, which could be attributed to the low graphene

sheet density on the substrate and also to the curved morphology.

3.4 Conclusions

A simple, solution phase method for producing graphene nanosheets directly from

graphite using sonication and CTAB as a stabilizer has been demonstrated. The sheets

could be dispersed in organic solvents such as DMF. Characterization of the sheets by

UV-vis, SEM, TEM, AFM and Raman spectroscopy showed the successful

exfoliation into graphene nanosheets of ~ 1.18 nm average thicknesses. Field

emission measurements showed a turn on voltage of 7.5 V/µm and emission current

densities of 0.15 mA/cm2. This approach for the production of graphene is expected to

be a step forward in the challenging global efforts to solubilize graphene and in the

formation of conducting composite materials. Next chapter describes the use of these

CTAB stabilized graphene nanosheets for the preparation of flexible, conducting

graphene/PVC composite films.

3.5 References

109



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Chapter 4

Flexible Conductive Graphene/Poly(vinyl chloride)

Composite Thin Films with High Mechanical Strength

and Thermal Stability

113

4.1 Introduction

In recent years, interest in polymeric nanocomposites has been increased

tremendously, since they represent an alternative to the conventional filled polymers

or polymer blends.1-3 In contrast to the conventional systems, nanocomposites use at

least one filler in the nanoscale range at very low loading levels.4 This characteristic

feature allow these nanocomposites to match with or to exceed the material

performance of conventional composites. Different types of fillers such as inorganic

and carbon based materials have been employed which provide superior mechanical,

thermal and conducting properties.2-6 At present, carbon-based filler materials used in

polymer nanocomposites are dominated by CNTs, but high cost of production,

presence of metal catalyst impurities and intrinsic aggregation impede its

applications.7 The present day challenge is to find an alternative for CNTs and

graphene can be a suitable candidate because of its outstanding mechanical properties

and ultra large interfacial surface area.8,9 Incorporation of graphene nanosheets into

elastomeric polymer matrix generates high performance composites with improved

mechanical and functional properties.4,10-12 Other interesting properties such as high

dielectric permittivity and low percolation threshold have also been observed in

graphene incorporated composites of poly(vinylidene fluoride) and PS,

respectively.13,14 Recently, GO has also been used as a filler in various polymer

matrices, due to their hydrophilicity and ease of formation of stable colloidal

suspensions.11,12 Besides, functionalized graphene sheets (FGS) are also employed as

they provide better interactions with the host polymers compared to unmodified CNTs

or traditional expanded graphite (EG).10 Functional groups on the surface of graphene

sheets can be modified to incorporate strong graphene-polymer interactions.

114

In our current investigation, poly(vinyl chloride) (PVC) is chosen as the host

polymer matrix because of its wide range of applications, low cost, chemical stability,

biocompatibility and sterilizability.15 However, PVC has low thermal stability, which

hinders some of its applications.16 The present day challenge is to introduce thermal

stability along with high mechanical strength and electrical conductivity for PVC with

the use of minimum amount of fillers. Substantial amount of work has been carried

out in the past few decades towards this goal.17-19 Fillers such as clay,20,21 wood

flour,18 wood fibers,22 agricultural residues,23 cellulose whiskers24 and calcium

carbonate25 were used to improve the thermal and mechanical stability of PVC. In

order to improve the electrical conductivity, carbon black26 and other conjugated

polymers such as PANI27,28 and polypyrrole (PPy)29 were incorporated in the PVC

matrix. Recently, CNTs have also been identified as a suitable filler material for

PVC.30 Kevlar coated CNTs used as additives to PVC resulted in composites with

improved mechanical properties demonstrating upto 50 and 70% increase in tensile

strength and Young’s modulus, respectively at very low CNT loading.31 Similarly,

CNTs grafted with styrene-maleic anhydride copolymers (SMA) was found to

enhance the interaction with PVC matrix and both thermal and mechanical stabilities

improved considerably.32 But dispersion of CNT in organic solvents is a challenge,

which is very critical for the preparation of polymer composites.11 Moreover, no

studies are available on the use of graphene as a filler material in PVC films.

This chapter elaborates on the use of CTAB stabilized dispersible graphene

nanosheets as reinforcing filler for PVC at very low loading levels. In order to have

efficient reinforcement in polymer nanocomposites, it is important to have

homogeneous dispersion in the polymer matrix. In our case, both PVC and graphene

nanosheets are readily dispersible in DMF for solution blending, which enable

115

homogeneous dispersion. The films are prepared by a solution blending route in

which PVC is blended with different amounts of CTAB stabilized graphene sheets

and the mechanical, thermal and electrical properties of the composite thin films are

investigated.


4.2.1 Materials

Graphite powder, CTAB and PVC in powder form (Mw: 120,000) were purchased

from Sigma-Aldrich. Glacial acetic acid and DMF purchased from local suppliers

were used as received. Milli-Q water was used for the washings.

4.2.2 Preparation of Processable Graphene Nanosheets

A few grams of graphite flakes were sonicated with 0.5 M CTAB solution in glacial

acetic acid for several hours and the resultant solution was then heated at 100 °C for

48 h in nitrogen atmosphere. Black powder obtained after centrifugation was washed

with distilled water to remove the excess acid and CTAB. The resultant product

formed stable dispersions in DMF.33

4.2.3 Fabrication of Graphene/PVC Composite Thin Films

Liquid phase blending method was used to make all the composite thin films. PVC

was dissolved in DMF. The graphene sheets dispersed in DMF were mixed with

appropriate weight fractions of PVC solutions and sonicated for 2 h. The mixtures

were then drop casted in glass cells and kept in an oven at 120 °C to evaporate the

solvent to get the ultrathin graphene/PVC composite films. The films were peeled off

from the cells and further annealed at 100 °C for 3 h to remove remaining traces of

solvent.

116

4.2.4 Instrumentation


system in the non-contact mode. FESEM images were recorded with JEOL JSM-6710

F field emission electron microscope. Thermal properties of the composites were

studied by TGA and DSC. TGA was done using TA instrument 2960 with heating

rate of 10 °C/min under N2 and DSC with TA instrument 2920 at a heating rate of 10

°C/min under N2 at a flow rate of 90 ml/min. Powder XRD was obtained with D5005

Siemens X-ray diffractometer with Cu Kα(1.5 Å) radiation (40 kV, 40 mA). Raman

spectra were recorded with Renishaw system-2000 with an excitation wavelength of

532 nm.

4.2.5 Mechanical Characterization

A dynamic mechanical analyzer (DMA Q800, TA Instrument) was used for

mechanical characterization of the composite thin films. Rectangular thin film strips

of width 3 mm were used for the tensile testing. The film strips were clamped on the

tension clamp such that the specimen length was 10 mm. DMA controlled force mode

was used with a preload force of 0.001 N. The temperature of the chamber was raised

to 20 °C and was kept constant during force ramping. The force was ramped at a rate

of 0.01 N/min till the fracture occurred and the obtained mechanical properties were

evaluated. For DMA, multifrequency-strain mode was utilized. A static load of 0.1 N

was used and the amplitude of dynamic strain was kept constant at 0.1%. To find the

influence of frequency, the specimen was held isothermally at 20 °C and deformed at

the constant amplitude (strain 0.1%) over a range of frequencies and the mechanical

properties were measured. To evaluate the thermal properties, the specimens were

exposed to a series of increasing isothermal temperatures. At each temperature, the

117

material was deformed at constant amplitude (strain 0.1%) over a frequency of 1 Hz

and the subsequent mechanical properties were evaluated.

4.2.6 Electrical Characterization

Film strips were accurately cut from the samples for conductivity measurements.

Silver electrodes were fabricated at the end of these strips using a conductive silver

pen (RS components). Tungsten probes with tip diameter 2.4 µm were used for two

probe conductivity studies. A source meter (Keithley 6430) was used for I-V

measurements, at room temperature. The volume fraction of graphene Vf was

calculated according to the equation,

where, Wg is the weight fraction of graphene and Wp the weight fraction of PVC.11

Density of PVC (ρp) was taken as 1.37 g/cc and the density of graphene (ρg) was

taken as 2.2 g/cc for calculations.11,13


4.3.1 Characterization of Graphene and Graphene/PVC Thin Films

Graphene nanosheets used in this method were prepared by CTAB assisted

exfoliation of graphite described in the previous chapter. Figure 4.1A displays the

tapping mode AFM image of graphene nanosheets, deposited onto the mica sheets

from DMF suspension. AFM image indicated that the sheets were very small with

dimensions of the order of < 1 µm. From the height profile (Figure 4.1B), the

thickness was found to be of the order of 1.2 nm. Average thickness of the sheets was

found to be 1.18 nm, corresponding to ~ 1-3 graphene layers. The thickness of the

118

films prepared was in the range of 5-7 µm. The dispersion state of graphene in PVC

matrix was investigated using FESEM. Since the graphene sheets were small, it was

hard to locate them.

0 500 1000 15000

1

2

3

4

nm

nm

500 nm

1.2 nm

A B

D

1 µµµµm

C

1 µµµµm

Figure 4.1 (A) AFM image of the exfoliated graphene nanosheets, (B) height profile of one of the sheets, (C) FESEM image of graphene/PVC composite thin film with 2 wt% concentration of graphene and (D) FESEM image of the fractured end of the composite film after mechanical testing.

Figure 4.1C shows the SEM image of graphene/PVC film with 2 wt% loading of

graphene. The image shows that most of the graphene nanosheets are well dispersed

in the PVC matrix, with a few restacks. It also demonstrates the random dispersion of

graphene in the 3D polymer network. It should be noted that graphene sheets tend to

organize inside the polymer matrix and are well dispersed in PVC. Figure 4.1D shows

one of the fractured ends of the film after mechanical testing, which reveals the

stacked graphene sheets. The thicknesses of the graphene sheets are increased due to

complete covering of the sheets by the polymer medium.

119

XRD and Raman spectroscopy have also been employed to evaluate the structure

of the composites. But analyses of our composite samples were difficult because of

the low X-ray diffraction intensity of PVC34 and low amounts of added graphene.

XRD of pure graphene showed a broad peak in the region of 20-30 degrees (Figure

4.2). However, after dispersing graphene nanosheets into the PVC (trace b in figure

4.2) matrix, no significant changes were observed.

10 20 30 40

c

b

Inte

ns

ity

(a

.u)

2θθθθ

a

Figure 4.2 XRD of graphene flakes deposited on glass (trace a), 2 wt% graphene/PVC composite film (trace b) and pure PVC powder (trace c).

1000 1500 2000 2500

c

b

Wavenumber (cm-1)

a

Inte

nsit

y (

a.u

)

Figure 4.3 Raman spectra of pure graphene (trace a), 2 wt% graphene/PVC composite film (trace b) and pure PVC powder (trace c).

120

The Raman spectra of the 2 wt% graphene/PVC composite films were generally

similar to that of the surfactant stabilized graphene sheets, indicating the distribution

of graphene in the polymer matrix.35 Signature features of graphene (D, G and 2D

bands) were clearly seen in the 2 wt% graphene/PVC composite films (Figure 4.3).

The Raman spectrum of PVC and the graphene/PVC composite showed a few extra

peaks around 1100, 1300, 1430 and 2250 cm-1. The peaks around 1100 and 1300 cm-1

arises from the C=C stretching of the PVC back bone and -CH bending of the –

CHCl group,respectively. The peak at 1430 cm-1 is assigned to the CH2 scissors

vibration. The origin of peak around 2250 cm-1 is not understood.

4.3.2 Mechanical Characterization of the Graphene/PVC Thin Films

It was expected that incorporation of graphene nanosheets in PVC matrix would

enhance the mechanical properties of PVC, because of the strong adhesion at the

interface.

0.0 0.5 1.0 1.5 2.0

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Yo

un

g's

mo

du

lus

(G

Pa

)

Graphene (wt%)

(B)

0.0 0.5 1.0 1.50

5

10

15

20

25

Str

es

s (

MP

a)

Strain (%)

Pure PVC

0.5 wt%

1 wt%

2 wt%

(A)

Figure 4.4 (A) Representative stress strain curves for various weight fractions of the graphene/PVC composite films and (B) the calculated Young’s modulus based on the slope of the elastic region.

The mechanical performance of graphene/PVC composite films was significantly

increased compared with pure PVC film. Representative stress-strain curves for

various weight fractions of graphene/PVC composite films are plotted in figure 4.4A

121

(plotted upto the elastic region). The slope of the curves increases with increasing the

concentration of graphene content. The presence of graphene in the PVC matrix offers

resistance to the segmental movement of polymer chains upon application of the

tensile stress which led to enhancement in modulus. The toughness of the composite

is estimated from the stress strain curve. It is seen that toughness is reduced

significantly for lower weight fractions. However for higher weight fractions (% wt

>5), the toughness is not reduced significantly. The corresponding Young’s modulus

values are shown in figure 4.4B. The addition of graphene significantly increased the

Young’s modulus. For the composite film with 2 wt% of graphene loading, Young’s

modulus increased to 2 GPa, corresponding to an increase of 58% (relative to the pure

PVC film).

0.0 0.5 1.0 1.5 2.0

20

40

60

80

100

120

140

160

Graphene (wt%)

Elo

ng

ati

on

bre

ak

(%

)

20

25

30

35

40

45

50

55

60

65

Ten

sile

str

en

gth

(M

Pa)

Figure 4.5 Mechanical properties of PVC thin films with different graphene loadings when stretched at the rate of 0.01 N/min at 20 °C.

The percentage elongation at breakage and the tensile strength of the graphene/PVC

films are summarized in figure 4.5. It is obvious that the addition of graphene has a

significant effect on the mechanical behavior of pure PVC. The average value of

percentage elongation decreased to 85% for 0.5 wt% graphene loading from 124% for

the pure PVC film. A further increase in graphene loading significantly reduced the

percentage elongation before fracture (Figure 4.5). This can be attributed to the

122

interaction between graphene and the polymer matrix which restricts the movement of

polymer chains.

On the other hand, tensile strength increases with increase in the graphene content.

The average tensile strength for pure PVC thin film is 24 MPa. The strength gradually

increased to 30 MPa for 1 wt% and further to 55 MPa for 2 wt% of graphene loading,

which corresponds to an improvement of 130%. Further increase in the graphene

content did not produce significant changes. When the graphene content was

increased beyond 2 wt%, the tensile strength increased slightly from 55 to 56 MPa

(for 5 wt% loading). Besides, the change in value of elongation at break is also not

very prominent (40% for 2 and 38% for 5 wt% graphene loading). This kind of

behavior could be due to the restacking of graphene flakes after certain level of

loading. This offers no significant improvement in mechanical properties. Beyond this

critical loading limit, tensile strength will not be significantly affected.11

4.3.3 Dynamic Mechanical Thermal Analysis

Dynamic analysis was done at various temperatures to evaluate the thermal stability.

Significant increase in storage modulus was observed (Figure 4.6A).

20 40 60 80 100 120 140

0

1

2

3

Sto

rag

e m

od

ulu

s (

GP

a)

Temperature (0C)

Pure PVC

0.5 wt%

1 wt%

2 wt%

(A)

25 50 75 100 125 1500.0

0.3

0.6

0.9

Ta

n (

δδ δδ)

Temperature (0C)

Pure PVC

0.5 wt%

1 wt%

2 wt%

(B)

Figure 4.6 (A) Storage modulus and (B) Tan δ curves for the graphene/PVC films when deformed at constant amplitude of 0.1% at a frequency of 1 Hz at various temperatures.

123

In both glassy and rubbery regions, the storage moduli of the graphene/PVC

composite thin films are higher than that of pure PVC film. For instance at 20 °C, the

storage modulus is only 0.5 GPa for pure PVC film whereas it was increased to

almost 3 GPa (approximately fivefold increase) for 2 wt% graphene/PVC films. At

lower temperatures, the material is in the glassy state and undergoes a transition from

glassy to rubbery state with increase in temperature. The storage moduli fall by a few

orders of magnitude. Energy dissipation is maximum during this transition as the loss

factor (Tan δ) peaks to maximum (Figure 4.6B). The thermal stability of the

graphene/PVC composite films have also been greatly improved as seen from the

glass transition peaks in the Tan δ curves. The glass transition occurs at 85-88 °C for

pure PVC films, whereas it shifts to temperatures as high as 105 °C for 2 wt%

graphene loaded PVC films. The maxima of the Tan δ occurs at a temperature where

the energy loss due to additional degrees of freedom takes place, which is a measure

of the Tg associated with α-relaxation of the polymeric material. The peaks also have

been significantly broadened. Both the graphs in figure 4.6 point to a rise in Tg

accompanied by a broadening of this relaxation indicative of restriction in segmental

relaxation. It could be inferred that molecular dynamics in the composites is highly

influenced by the fillers, thereby increasing the glass transition temperature and the

thermal stability of the composites as compared to pure PVC films. The well

dispersed graphene nanosheets restrict the segmental motion of polymer chains inside

the matrix.

To further analyze the influence of the graphene reinforcement, the storage

modulus of the composites relative to that of pure PVC (E’composite/E’PVC) was

measured at 1 Hz and plotted as a function of temperature.36 Moderate increase in

124

storage modulus (upto 5 times) can be seen in the glassy regime (Figure 4.7A) and

improved further in the glass transition regime (Figure 4.7B). For 1 and 2 wt% of

graphene loading, the relative modulus E’composite/E’PVC drastically increased upto a

maximum value at about 100 °C. With further rise in temperature after glass

transition, the rubber-like regime persisted and the relative moduli decreased slightly.

20 30 40 50 60 701

2

3

4

5

6

Rela

tive s

tora

ge

mo

du

lus

E' c

om

po

sit

e/E

' PV

C

Temperature (0C)

0.5 wt%

1 wt%

2 wt%

(A)

70 80 90 100 110 120

0

20

40

60

80

100

120

Re

lati

ve s

tora

ge

mo

du

lus

(E

' co

mp

osit

e/E

' PV

C)

Temperature (0C)

0.5 wt%

1 wt%

2 wt%

(B)

Figure 4.7 Graphs showing the temperature dependence of the relative storage modulus at (A) low temperature regime and (B) glass transition temperature regime.

The relative moduli above Tg are greater than the respective values below Tg. The

storage modulus values are highly influenced by the interfacial interactions between

the graphene sheets and PVC matrix. The observed results suggest a strong interfacial

bonding between graphene and the PVC segments. The polymer chain mobility was

confined near Tg by graphene so that the relative storage modulus was increased.

The Tg values at various weight fractions of graphene filling and the values of loss

factor (Tan δ) are summarized in figure 4.8. It could be seen that the loss factor

reduces significantly with increase in graphene content. Tan δ reflects the mobility

and movement capacity of molecule chain segments during glass transition. It is clear

that the movements of PVC chain segments during glass transition were significantly

limited and obstructed by the presence of graphene layers, resulting in much smaller

125

Tan δ peaks in composite samples. The graphene layers act as “physical crosslink”

which limits the movements of macromolecular chains of PVC during glass transition.

The low value of loss factor also depicts an elastic polymeric behavior, whereas the

high value shows a viscous behavior. The presence of graphene in the PVC matrix

makes the material more elastic in nature, especially in the glass transition regime,

which indicates enhanced thermal stability of the material.

0.0 0.5 1.0 1.5 2.080

90

100

110 T

g

Tan (δδδδ)

Graphene (wt%)

Tg b

y T

an

(δδ δδ)

pe

ak

0.2

0.3

0.4

0.5

0.6

0.7

Ta

n (

δδ δδ)

pe

ak

Figure 4.8 Graph showing the glass transition temperature (Tg) and the loss factor (Tan δ) values at various weight fractions of graphene in PVC matrix.

Thermal properties of the composite films were further evaluated with TGA and DSC.

The TGA curves of the pure PVC and graphene/PVC composite films are shown in

figure 4.9A. Both pure PVC and the nanocomposites follow two stage degradation

processes. The first thermal decomposition onset temperature is shown as T1 and the

second thermal decomposition onset temperature as T2. Two major weight losses were

observed in all cases (Figure 4.9A). The first weight loss was observed in the range

250-360 °C for pure PVC, which corresponds to the loss of HCl.37 At this

temperature, the Cl radicals formed from the cleavage of –C−Cl bonds abstract a

126

hydrogen from the neighbouring C−H bond resulting in the evolution of HCl

molecules from the polymer chain. Once the reaction is initiated, the “allyl” activation

continues and all the Cl atoms get dislocated along the macromolecule chain leaving

behind the polyene backbone.37

200 400 600 8000

20

40

60

80

100

T2

We

igh

t (%

)

Temperature (oC)

Pure PVC

2 wt%(A)T

1

40 60 80 100 120 140

Hea

t F

low

Temperature (oC)

Pure PVC

2 wt%(B)

Figure 4.9 (A) TGA curves for graphene/PVC composite films and (B) DSC curves to determine the glass transition temperature.

From the TGA traces, it was observed that T1 reduced to 230 °C by increasing the

graphene content from 0.5 to 2 wt%. This is because the graphene nanosheets act as

reinforcing particulate filler which attracts Cl, thus the C−Cl bonds in PVC are

destabilized at this temperature. From 360 to 420 °C, no weight loss was observed,

indicating the stability of the composite films at this temperature. The second major

weight loss (T2) was observed in between 420 and 500 °C, and it was much shorter

than T1. Thermal degradation of the polyene backbone takes place at this stage,

resulting in the formation of volatile aromatic compounds and a stable carbonaceous

residue.36 The amount of carbonaceous residue increases with increase in the amount

of graphene loading (Figure 4.9A). Tg of the composite films obtained from DSC

(Figure 4.9B) were 80.8 (0 wt% graphene) and 84.3 (2 wt%). Reinforcing effect of the

127

graphene content reduces the chain segmental mobility and hence the enhancement in

Tg was observed with increase in the graphene content. Difference in Tg values

obtained from DMA analysis and DSC measurements can be accounted with the fact

that Tg from DMA is highly dependent on the test frequency37 and variations in the Tg

values are possible. The surface area of individual graphene flakes is large,9,38 so the

interaction between PVC chains and graphene layers might limit the conformations of

polymer molecules in the composites. Moreover, if the radius of gyration of polymer

chain (a few nm) is larger than the interlayer distance of graphene in the PVC matrix,

the polymer chains intercalates between the layers and confined environment will

restrict the mobility of PVC segmental chains.

4.3.4 Electrical Properties

0 1 2 3 4 5 6

10-15

10-12

10-9

10-6

10-3

100

Ele

ctr

ical

Co

nd

ucti

vit

y σσ σσ

(S

/cm

)

Filler content/vol%

Figure 4.10 Graph showing the influence of exfoliated graphene on the electrical conductivity of PVC.

The electrical conductivity, σ of graphene/PVC composite thin films plotted as a

function of volume percentage of graphene is shown in figure 4.10. The conductivity

of pristine PVC thin film is less than 10-16 S/cm.39 At a graphene content of about

128

0.025, 0.05 and 0.075 vol%, the composite films did not show any measurable values

of conductivity. However at about 0.1 vol%, a sharp increase in the conductivity was

observed. A further increase in the volume fraction of graphene gave rise to a sharp

increase in the conductivity (Figure 4.10). At 0.12 vol%, the conductivity was 3.19 ×

10-9 S/cm, whereas at 0.3 vol% the conductivity increased to 2.14 × 10-7 S/cm.

Subsequent increase in concentration to 0.6 and 1.3 vol% led to an increase of

conductivity to 4.92 × 10-7 S/cm and 7.81 × 10-6 S/cm, respectively. A further increase

in concentration levels upto 6 vol% give only moderate rise in conductivity. From the

graph, it can be clearly seen that there is a rapid increase in the conductivity at 0.6

vol% of the filler content, which is the percolation threshold for the composite films.

Thus, it is concluded that the conductivity increases drastically upto 0.6 vol% of

graphene, above which the rate of increase was minimum. The maximum

conductivity observed was 0.058 S/cm, at 6.47 vol% of the filler content which is 10

times higher than that of the SWNT/PVC composites.40

4.4 Conclusions

Ultrathin composite films of CTAB stabilized graphene nanosheets and PVC was

fabricated and its mechanical, thermal and electrical properties were investigated. The

composite thin films were prepared by a solution blending route in which the PVC

was appropriately blended with graphene nanosheets prepared by CTAB assisted

exfoliation. A significant enhancement in the mechanical properties of pure PVC

films was obtained with a 2 wt% loading of graphene, such as a 58% increase in

Young's modulus and an almost 130% improvement of tensile strength. Thermal

stability was found to be improved to a greater extent with an increase in the glass

transition temperature. The films were found to possess high electrical conductivity

129

with a low percolation threshold of 0.6 vol%. Composites using graphene is

promising, but the availability of processable graphene sheets in large quantities is

crucial and challenging. Next chapter deals with the covalent functionalization on

CTAB stabilized graphene sheets, which enhance the processability in common

organic solvents. This is very essential for its applications in polymer and inorganic

composites.

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Macromolecules 1997, 30, 6333.

37. Becker, G. W. Kolloid Z 1955, 140, 1.

38. Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney,

E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature

2006, 442, 282.

39. Menges, G. Werkstoffkunde Kunststoffe. Carl Hanser Verlag: München 1990,

217-218.

40. Broza, G.; Piszczek, K.; Schulte, K.; Sterzynski, T. Compos. Sci. Technol.

2007, 67, 890.


Vadukumpully, S.; Gupta, J.; Zhang, Y.; Xu, G. Q.; Valiyaveettil, S. Nanoscale 2011,

3, 303-308

Chapter 5

Functionalization of Surfactant Wrapped Graphene

Nanosheets with Alkylazides for Enhanced

Dispersibility

134

5.1 Introduction

Graphene is an infinite 2D sheet of sp2 bonded carbon atoms with extended electronic

conjugation. Since its discovery in 2004, the material has triggered enormous interest

in both fundamental and applied science communities because of the unique

electrical, thermal and mechanical properties.1-12 However, use of this new material in

sensors, electronic and photovoltaic devices has been hindered by the unavailability of

stable and processable graphene structures.12c,13-16

Pure graphene sheets are highly

hydrophobic and easily agglomerates because of the strong π - π interactions.

Recently, considerable efforts have been made to improve the dispersibility of

graphene sheets via covalent and non-covalent interactions. One of the strategy relies

on the use of GO. Covalent and non-covalent modifications of GO using various

stabilizers such as polymers,17

hydrazine,18

amine derivatives,19

diazonium salts,20

phenyl isocyanates,21

aromatic compounds22

and biomolecules have been reported.23

GO is easy to stabilize in aqueous and polar conditions because it contains chemically

reactive oxygen functionalities such as hydroxyl and epoxide groups.20b Even though,

reduction can remove most of the oxygen functionalizations, significant amount of

oxygen groups still remains in the basal plane, which disrupts the π - conjugation.

Sonication assisted dispersion of graphene in DMF24

or NMP25

and electrochemical

methods26

have also been reported, however only small graphene fragments can be

obtained through this method.

Given the interest in graphene chemistry, it is desired to have convenient

dispersion routes to increase the availability and processability of graphene. To

improve the solution processability of graphene, a few methods that have been

135

employed for the functionalization and stabilization of CNTs can be adopted.27,28

Moreover, chemical functionalization of the carbon network in graphene by grafting

atoms or molecules is very important because this provides a method for the

introduction of interesting photoactive molecules for the development of

optoelectronic materials. An interesting functionalization to try on graphene is the

azide addition because many organic azides have been used for the covalent

modification of single-walled and multi-walled CNT.29 Azides are considered as one

of the useful reactive intermediates during nucleophilic aromatic substitution in

organic reactions and are frequently used in a variety of reactions.30

+

−−−−C6H13, −−−−C12H25, −−−−C11H22, −−−−C11H20 R −−−− H, −−−−OH, −−−−COOH

N

N

N

N

N

N

N

R

R

R

R

R

R

R

Scheme 5.1 Schematic representation of the covalent functionalization of graphene

sheets with various alkylazides.

Based on this concept, a mild covalent functionalization of CTAB wrapped graphene

sheets using a series of azides, which include alkylazides (hexyl and dodecyl), 11-

azidoundecanol (AUO) and 11-azidoundecanoic acid (AUA) is described in this

chapter. Besides azido group, other functional groups such as hydroxyl and carboxylic

groups are incorporated to enhance the dispersibility in common organic solvents. The

functionalized sheets are characterized by FTIR, Raman spectroscopy and various

imaging techniques. Besides, the reactive sites on the graphene sheets were marked

136

using gold nanoparticles. Free carboxylic acid groups present in the AUA

functionalized sheets bind with gold nanoparticles, indicating that the reaction has

taken place not just at the edges but also at the internal C−C bonds of graphene. In

short, our results show that the covalent functionalization using azides provides a

useful platform for the synthesis of functional graphene nanocomposites using metal

nanoparticles.


5.2.1 Materials

Sodium azide (NaN3), 11-bromoundecanoic acid, 11-bromo-1-undecanol, dodecyl

bromide, hexyl bromide, anhydrous Na2SO4, CTAB, graphite powder (> 45 µm),

chloroauric acid (HAuCl4.3H2O) and NaBH4 were from Sigma-Aldrich and used as

received. The solvents were of analytical grade and were obtained from local

suppliers.

5.2.2 Analysis and Instruments

NMR of the synthesized organic compounds was recorded using Bruker ACF

spectrometer operating at 300 MHz. Chloroform-d (CDCl3) was used as the solvent

and tetramethylsilane (Me4Si) as internal standard. Electron impact (EI) mass

spectrum was recorded using Finnigan MAT95XL-T mass spectrometer. UV-visible

spectrum was recorded using Shimadzu 1601 PC spectrophotometer. FTIR spectra

were recorded at 1 cm-1

resolution in the wave number range of 4000-400 cm-1

using

a Bio-Rad FTS 165 FTIR spectrophotometer. HRTEM images were taken using JEOL

JEM-2010 F electron microscope, operating at 200 keV. STM measurements were

performed on Au (111) substrate (Goodfellow, UK) using a NanoScope IIIa

137

MultiMode SPM (Digital Instrument, Veeco Metrology Group) with

electrochemically etched Pt(.8) Ir(.2) tips (0.25 mm diameter). A drop of graphene

solution deposited on Au(111) substrate was used for STM measurement. STM

images were obtained at a tunneling current of 2 pA and a bias voltage of 50 mV.


system in the non-contact mode. Samples for AFM were prepared by drop casting a

dilute suspension of graphene on freshly cleaved mica and then dried at room

temperature. Raman spectra were recorded with a Renishaw system-2000 operating at

an excitation wavelength of 532 nm. A source meter (Keithley 6430) was used for the

I-V measurements at room temperature. The samples for electrical characterization

were prepared by drop casting the DMF suspensions of the samples on Si substrate

with 300 nm SiO2 coating. The sheets were found to be intercalated and formed a

continuous network over the SiO2 surface. Tungsten probes with a tip diameter of 2.4

µm were precisely positioned over the thin films at a distance of 10 µm.

5.2.3 Preparation of CTAB Stabilized Graphene Sheets31

In brief, 100 mg of graphite powder were sonicated with CTAB solution in glacial

acetic acid for 4 h. The resultant solution was then refluxed for 48 h under N2

atmosphere. The reaction mixture was left to stand overnight to allow the aggregated

sheets to settle down. The supernatant was decanted and centrifuged at 20,000 rpm for

45 min. The resultant black residue obtained could be resuspended in DMF.

5.2.4 General Synthetic Procedure for Alkylazides

To a solution of alkyl bromide in DMSO, solid NaN3 was added at a time. The

solution was stirred at room temperature for 20 h before it was quenched with excess

amount of water. It was extracted with DCM. The combined organic layer was dried

138

over anhydrous Na2SO4 and evaporated under reduced pressure to get the colourless

liquid.

(1) Dodecylazide: Dodecyl bromide (1 g, 4.02 mmol), NaN3 (0.314 g, 4.82

mmol), DMSO (20 ml), Yield = 0.7 g, 83%. 1H-NMR (CDCl3, 300 MHz): δ 3.22 (t,

CH2N3, 2H) 1.64 - 1.26 (m, 10 CH2, 20H).

(2) Hexylazide: Hexyl bromide (1 g, 6.06 mmol), NaN3 (0.475 g, 7.31 mmol),

DMSO (20 ml), Yield = 0.63 g, 82%. 1H-NMR (CDCl3, 300 MHz): δ 0.88 (t, CH3,

3H), 1.41 - 1.28 (m, 5 CH2, 10H).

5.2.5 Synthesis of 11-azidoundecanol (AUO)32

A mixture of the 11-bromo-1-undecanol (1 g, 3.98 mmol), NaN3 (0.777 g, 11.95

mmol) in dry DMF (20 ml) was stirred at 80 °C overnight under N2 atmosphere. After

removing the DMF by evaporation, the residue was dissolved in DCM, washed with

water, dried over anhydrous Na2SO4 and evaporated to get a yellowish liquid (0.78 g,

92%). 1H-NMR (CDCl3, 300 MHz): δ 3.54 (t, CH2OH, 2H), 3.19 (t, CH2N3, 2H),

1.56 - 1.22 (m, 9 CH2, 18H).

5.2.6 Synthesis of 11-azidoundecanoic acid (AUA)33

NaN3 (294 mg, 4.52 mmol) was dissolved in a solution of 11-bromoundecanoic acid

(1 g, 3.77 mmol) in DMSO (20 ml). It was stirred at room temperature for overnight.

The reaction mixture was diluted with water and 1M HCl was added. The aqueous

phase was extracted (3 × 15 ml) with ethyl acetate and the combined organic layers

were washed with water, dried over anhydrous Na2SO4 and evaporated to get the 11-

azidoundecanoic acid (0.67 g, 78%) as a pale brown oil (Elemental analysis, found: C,

58.2; H, 9.2; N, 18.5; O, 14.1. C11H21N3O2 requires C, 58.12; H, 9.32; N, 18.49; O,

14.08%); 1H-NMR (CDCl3, 300 MHz): δ 10.25 (s, COOH, 1H), 3.22 (t, CH2N3, 2H),

139

2.31 (t, CH2COOH, 2H), 1.59 - 1.54 (m, CH2CH2N3 and CH2CH2COOH, 4H) and

1.27 (m, 6 × CH2, 12H); m/z (EI) 140.1 (M+- C4H7O2 C7H14N3 requires 140.12), 126

(5%), 112 (8), 98 (9), 84 (26), and 70 (100).

5.2.7 Functionalization of CTAB Stabilized Graphene Nanoheets with 11-

azidoundecanoic acid (AUA)

For AUA functionalized graphene sheets, two different weight ratios of graphene to

AUA (1:1 and 1:10) were used to check the changes in dispersibility with the extent

of functionalization. Weight of graphene was kept constant for convenience. Briefly,

20 mg of the CTAB assisted exfoliated graphene31

was dispersed in 50 ml of ODCB.

The solution was purged with nitrogen and preheated to 160 °C. Different amounts of

AUA (20 and 200 mg, respectively) in 5 ml of ODCB were then added drop wise to

the reaction mixture over a period of 20-30 min. The temperature was maintained at

160 °C for 45 min, after which the product was cooled to room temperature. The

suspension was diluted with acetone to flocculate the functionalized graphene sheets.

Subsequent washing and centrifugation resulted in the removal of all by-products. The

nanosheets were decanted and dried.

5.2.8 Preparation of Gold/Graphene Nanocomposites

Gold nanoparticles were synthesized in presence of AUA functionalized graphene

sheets. To 10 ml of the functionalized graphene suspension (∼ 0.05 mg/ml) in DMF,

HAuCl4 solution was added so that the final concentration of Au3+

was 10 mM. It was

then reduced with the addition of 500 µl freshly prepared NaBH4 (2 × 10-4

M) solution

with continuous stirring. The color of the solution gradually changed from grayish

black to dark pink. Formation of nanoparticles was confirmed using spectroscopic and

microscopic techniques.

140


5.3.1 Characterization of the Functionalized Graphene Nanosheets

Scheme 5.1 illustrates the process of covalent functionalization of CTAB stabilized

graphene nanosheets31 with various alkylazides. A large area AFM image of the

CTAB stabilized graphene nanosheets are shown in figure 5.1A.

1 µµµµm

1.1 nm 0.98 nm

0 1 2 3 4 nm0

5

10

15n

mA B

Figure. 5.1 (A) AFM image of the CTAB stabilized graphene sheets and (B)

corresponding section analysis.

When the CTAB stabilized sheets are treated with azides, nitrenes are inserted into the

double bonds on graphene and the alkyl chains with different functional groups at the

terminals. It is conceivable that the presence of long alkyl chains and polar functional

groups improves the dispersibility. The functionalization with dodecylazide, AUO

and AUA improved the dispersibility of graphene sheets in common organic solvents

such as toluene and acetone and the dispersions were stable for a few days (Figure

5.2A). In the case of hexylazide, the dispersions were not stable and the sheets were

agglomerated (Figure 5.2B). It can be attributed to the shorter alkyl chain of

hexylazide. In the case of AUO and AUA, the functionalization led to an enhanced

dispersibility in common solvents. Presence of polar functional groups on the

141

graphene surface is desirable because it assist in enhancing the dispersibility.

1 µµµµm

A

1 µµµµm

B

Figure 5.2 SEM images of (A) dodecylazide functionalized graphene sheets and (B)

hexylazide functionalized graphene sheets.

A B

200 nm 200 nm

Figure 5.3 TEM images of (A) AUO functionalized graphene sheets and (B)

corresponding gold/graphene nanocomposite.

Besides, functional groups such as −COOH and −OH can anchor metal nanoparticles

onto the surface, which may help in visualizing the reactive sites. Hence, gold

nanoparticle synthesis was attempted in presence of both AUO and AUA

functionalized graphene sheets. In presence of AUO functionalized sheets, the gold

nanoparticles were aggregated on their surface. TEM images of the AUO

functionalized sheets and the gold nanocomposites are given in figure 5.3A and B,

respectively. A possible reason for this may be the poor stabilizing/co-ordinating

142

effect of hydroxyl groups. However, gold nanocomposites with AUA functionalized

graphene sheets were found to be stable without any visible aggregation. Hence we

focused our studies with AUA functionalized graphene owing to the fact that the

presence of –COOH groups on the surface might enhance the metal

nanoparticle/graphene composite stabilities and dispersibility. Dispersibility of the

resultant product was poor when 1:1 weight ratio of graphene to AUA was used for

functionalization. It could be due to minimal functionalization on the graphene sheets.

TEM image of the dispersed sample is given in figure 5.4.

500 nm

1:1

Figure 5.4 TEM images of AUA functionalized graphene sheets, with 1:1 weight

ratio of graphene vs AUA used. Inset clearly shows the settled particles in the

respective dispersed solutions in toluene. All the sheets were found to settle down

within 1 h.

Treatment of graphene with AUA at 1:10 ratio led to the formation of considerably

dispersible solid. These sheets were found to be easily dispersible in polar solvents

with a mild sonication. These samples were used for subsequent experiments. The

dispersibility was found to be high in DMF. In the case of CTAB stabilized graphene

sheets, the maximum dispersibility achieved was ∼ 0.005-0.01 mg/ml, but for AUA

functionalized sheets, it could reach upto a maximum of ∼ 0.05-0.1 mg/ml.

143

Figures 5.5A and B show the TEM micrographs of the CTAB stabilized and

AUA functionalized graphene sheets, respectively. Due to the distortions caused

by the covalent functionalization, sheets attain a crumpled topology (Figure 5.5B).

As reported previously, crumpling and scrolling are intrinsic properties of thin

graphene sheets, which is due to the extra thermodynamic stability of the 2D

membranes arising from microscopic crumpling either by bending or buckling.34

100 nm

A B

200 nm

1000 1500 2000 2500 3000

2

3

1

2D

G

Inte

ns

ity

(a

.u)

Raman Shift (cm-1)

D D

1000 1500 2000 2500 3000

B

1128 cm-1

1630 cm-1

1729 cm-1

% T

ran

sm

itta

nc

e (

a.u

)

Wavenumber (cm-1)

2100 cm-1

A C

Figure 5.5 TEM images of (A) CTAB stabilized and (B) AUA functionalized

graphene sheets. (C) FTIR of AUA and AUA functionalized graphene sheets (trace A and B, respectively) and (D) Raman spectra of CTAB stabilized (trace 1), AUA

functionalized graphene sheets with 1:1 and 1:10 w/w of graphene to AUA.

To verify the covalent functionalization of graphene sheets with AUA, FTIR was

performed (Figure 5.5C). Traces A and B correspond to the IR spectrum of AUA

144

and AUA functionalized graphene sheets (1:10 w/w of graphene to AUA),

respectively. The peak at ~ 2100 cm-1 corresponds to the azido stretching35 and

that at 1730 cm-1 to the carbonyl stretching from the −COOH group. Absence of

azido stretching peak in AUA functionalized graphene sheets implies the covalent

functionalization.29b Azides are added onto C=C bonds on graphene with the

elimination of N2. IR spectrum of the AUA functionalized graphene sample

showed the presence of C−N (~ 1128 cm-1) and C=C stretching (~ 1630 cm-1).

Figure 5.5D shows the Raman spectra of CTAB stabilized graphene sheets

(trace 1) and the AUA functionalized sheets with 1:1 and 1:10 w/w of graphene to

AUA (traces 2 and 3, respectively). The peak centered at ~ 1350 cm-1 corresponds

to the disorder induced D band and that at ~ 1580 cm-1 to the E2g phonon of sp2

atoms.36 Intensity of the D band is directly proportional to the amount of disorder

present in the sheets. The D band intensity increased considerably with azide

functionalization, owing to the disorders induced by the addition of nitrenes to the

double bonds. Another prominent feature in the Raman spectra of graphene

samples is the 2D (or G’) peak. No defects are required for the existence of this

band and is seen even when no D band is present.25

The number of layers present

in the graphene sheets can be evaluated from the shape of 2D peak.37

Graphene

sheets with less than five layers have very broad 2D peak, which can be clearly

seen in the case of both CTAB stabilized and AUA functionalized graphene

samples.

The thickness and morphology of the functionalized graphene sheets was

evaluated using AFM (Figure 5.6A). From the cross section analysis of one of the

flakes, thickness was found to be 1.12 nm (Figure 5.6B). Analysis of a large

number of AFM images revealed graphene sheets with lateral dimensions of 500-

145

2000 nm and thickness in the range of 1-1.3 nm (Figure 5.7). Thickness of a

monolayer of graphene sheets is 0.34 nm as predicted by theory,4 while our

samples showed an average thickness of 1.3 nm, indicating the presence of 2-3

layered graphene sheets.

0.5 µµµµm0 1 2 3

0

-20

+2

0

µµµµm

nm

A B

Figure 5.6 AFM images of (A) the AUA functionalized graphene sheets and (B)

the section analysis.

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00

5

10

15

20

Nu

mb

er

Thickness (nm) Figure 5.7 Statistical analysis on the thickness of the AUA functionalized graphene sheets.

Slight increase in the thickness of the sheets as compared with the CTAB

stabilized sheets31

can be attributed to the presence of long alkyl chains present on

both sides of the sheets. Finally, STM analysis was carried out on the samples to

detect the disorders caused by functionalization. Highly ordered hexagonal lattice

146

of graphene was seen in the case of CTAB stabilized graphene flakes (Figure

5.8A). Small non-uniformities arose from the presence of loosely bound residual

surfactants.

1 nm 1 nm

BA

Figure 5.8 STM images of (A) CTAB stabilized and (B) AUA functionalized graphene sheets.

However, the STM image of AUA functionalized sheets clearly showed many

bright regions, lacking the hexagonally ordered arrangement. These disorders

could be due to the functionalization on certain areas (Figure 5.8B). The

functionalized regions are highlighted with white contours.

5.3.2 Characterization of Gold/Graphene Nanocomposites

It is conceivable that the presence of –COOH groups present in the anchored AUA

moities on graphene helps in binding the nanoparticles on the graphene surface.

Hence gold nanoparticles were synthsezied in presence of AUA functionalized

graphene sheets. Figure 5.9A shows the absorption spectra recorded for the AUA

functionalized graphene sheets (trace A) and the gold-graphene nanocomposite

solutions (trace B), respectively. Inset of figure 5.9A shows the photograph of

corresponding solutions (solutions are diluted 10 times for better contrast). Gold/

147

AUA functionalized graphene nanocomposites formed were stable for several

days, with no visible aggregation. It is well known that surface plasmon resonance

(SPR) of the metal nanoparticles depends on their size.38 In our sample, the SPR

band was around 520 nm, which corresponds to a size > 5 nm. The size and shape

of gold nanoparticles on graphene sheets were further confirmed using TEM and

AFM measurements.

0.5 µµµµm20 nm

B

0.5 µµµµm µµµµm

D

0

1

-20

+20

0n

m

1.250.750.50.250

-20

.00

20

.0n

m

C D

A BA

400 500 600 700 800

B

Ab

so

rba

nc

e (

a.u

)

Wavelength (nm)

A

Figure 5.9 (A) UV-vis spectra of the AUA functionalized graphene sheets (trace A) and that of the gold/graphene nanocomposites (trace B) solutions. Inset of

figure 5.9A shows the photograph of (A) functionalized graphene solution and that of (B) the gold/AUA graphene composite in DMF. (B) TEM image and (C) the

AFM image with (D) the cross-section analysis of the gold/graphene nancomposite

sheets.

Homogeneous grafting of gold nanoparticles on the graphene film is visible in the

TEM image (Figure 5.9B). From the TEM images, average size of the

148

nanoparticles was found to be 9.5 ± 2.2 nm. The AFM image also depicts fine

distribution of nanoparticles on the graphene nanosheets (Figure 5.9C). From the

cross sectional analysis (Figure 5.9D), the height from background was found to

be ∼ 1.26 nm, corresponding to the AUA functionalized graphene sheet thickness.

The smaller peaks seen in the cross section were from the gold nanoparticles

anchored on the graphene surface. Even though the reaction mechanism is not

clear at this stage, it is believed that the nanoparticles are adhered onto the

functionalized graphene sheets via electrostatic interactions and −COOH groups.

Uniform distribution of the nanoparticles on the AUA functionalized graphene

indicates that the reaction has taken place not just at the edges of the graphene

sheets but also at the central C−C bonds. This is very interesting because this

method of functionalization can open up new reaction routes for the synthesis of

graphene based functional composite materials.

0 2 4 6 8 100

30

60

90

B

Cu

rre

nt

(nA

)

Voltage (V)

A

Figure 5.10 I-V plots of films of (A) AUA functionalized graphene sheets and (B)

gold/graphene nanocomposites.

Primary investigations on the applicability of gold/graphene composite as a

conducting composite material were carried out using I-V measurements. Figure

5.10 shows the I-V plots of drop casted films of AUA functionalized graphene

149

sheets and gold/graphene nanocomposite. It is evident from the graph that AUA

functionalized graphene sheets show low conductivity due to the lack of extended

π - conjugation. The film showed conductance of 22 nA at an applied voltage of

10 V (trace A). In the case of gold/graphene nanocomposite, the conductivity

increased approximately three fold as compared to pure functionalized sheets.

Gold/graphene nanocomposite films showed conductance of 76 nA at an applied

voltage of 10 V (trace B). The increase in conductance for the gold/graphene

composites can be attributed to the presence of attached gold nanoparticles. Both

the drop casted samples contained the same concentration of graphene sheets.

Hence, it can be concluded that the increased conductivity is due to the embedded

gold nanoparticles. Some of the earlier works on GO based composites showed 6

fold39

and 270000 fold40

increases in conductance on reduction of GO by chemical

means. This increase can be attributed to the restoration of π - π electronic

conjugation or presence of metallic impurities on the graphene surface. In our

case, there is no restoration of the extended conjugation or use of metallic

reducing agents, therefore the increase in conductivity is mainly due to the

embedded gold nanoparticles which create the conducting pathways. The

conductivity obtained is lower than that of bulk graphite since the measurement

was done on drop-casted films with multiple discontinuities.

5.4 Conclusions

In conclusion, we have demonstrated a mild and simple functionalization

technique for dispersing graphene sheets. CTAB stabilized graphene sheets were

functionalized with various alkylazides. Sheets functionalized with azides having

longer alkyl chains and polar functional groups showed enhanced dispersibility in

150

common organic solvents such as acetone and toluene. AUA functionalized sheets

showed better dispersibility and stability in the solution state. The

functionalizations were confirmed using various imaging and spectroscopic

techniques. Thicknesses of the functionalized sheets were found to be in the range

of 1-1.3 nm. This approach can be used for the introduction of various functional

moieties into graphene sheets, this may prove to be very useful for applications in

electronic devices and graphene based composite materials. Anchoring of gold

nanoparticles onto AUA functionalized graphene sheets were also demonstrated.

This gold/graphene nanocomposite was stable in solution for several days and

showed threefold increase in conductivity as compared to the pure AUA

functionalized sheets. Such metal nanocomposites could be promising candidates

for catalytic and electronic applications.

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157

Chapter 6

Bromination of Graphite - A Novel Route for the

Exfoliation and Functionalization of Graphene Sheets

158

6.1 Introduction

In the recent years, there has been an increased interest in producing a few-layered or

individual graphene sheets.1-3

The unique electronic, thermal and mechanical

properties of graphene make it a suitable substituent for other carbon

nanostructures.2a,4-6

Graphene and its composites are used as building blocks for

different devices such as transistors, organic photovoltaic devices and transparent

anodes.7-9

In addition, graphene also provides a high surface area scaffold for the

covalent attachment of organic molecules, which is highly beneficial for the

incorporation of graphene into polymer composites with enhanced mechanical and

electronic properties.3a,10

Majority of the reports on covalent functionalizations of graphene relies on the use

of GO. GO is produced by the chemical oxidation of graphite, which introduces many

epoxide, carboxyl and hydroxyl functional groups in the basal plane.11

While covalent

functionalization using GO produce functionalized material, the processing is limited

to polar or aqueous conditions.12

Besides, an additional reduction step is required for

the removal of excess oxides present, which might leave significant amount of defects

leading to reduced electronic properties.13

Therefore, it is important to develop direct

functionalization methods for graphene to obtain processable graphene layers and to

modify the electronic properties according to the applications.

In this regard, bromination of graphene sheets is explored using the facile

intercalation of graphite by liquid bromine/bromine vapor.10b Bromination of

graphitic materials takes place via electrophilic addition of bromine molecules to the

aromatic double bonds. In this chapter, we show that the brominated graphene sheets

can be transferred to various substrates either by deposition from organic-water

159

interface or by spreading of carbon nanotubes on the surface. Besides, bromine atoms

covalently linked to graphene sheets are substituted with alkylamines and coupled

with arylboronic acids which offer an opportunity to integrate graphene into various

devices. Bromination and subsequent reactions might lead to a change in the carrier

density of graphene and may be useful to alter the electronic properties.

Scheme 6.1 Schematic representation of the bromination and subsequent

alkylation/arylation using dodecylamine and arylboronic acids. Reaction conditions: (a) dodecylamine, (C2H5)3N, toluene, reflux, 24 h. (b) pyrene 1-boronic acid/5-hexyl-

2,2'-bithiopheneboronic acid pinacol ester, Pd (0), K2CO3, DMF, 85 °C, 48 h.


6.2.1 Materials

Graphite powder (> 45 µm), MWNT, liquid bromine, DMF, dodecylamine,

triethylamine, pyrene 1-boronic acid, 5-hexyl-2,2'-bithiopheneboronic acid pinacol

ester and Pd(PPh3)4 were purchased from Sigma-Aldrich. Solvents were bought from

the local suppliers and were used without further purification.

160

6.2.2 Bromination of Graphite

Graphite powder (500 mg) was dispersed in DMF and liquid bromine (4 ml) was

added drop wise under N2 atmosphere. The reaction mixture was heated under inert

atmosphere for several days. The brominated graphite sample was washed with excess

water to remove the physisorbed bromine and DMF. The brominated graphite could

be dispersed in common organic solvents with a mild sonication.

6.2.3 Spreading of Brominated Graphite

Different methods were attempted to spread the brominated graphene sheets.

(i) Deposition from Air - Water Interface

Brominated graphite was dispersed in toluene with the aid of sonication for less than 5

min. It was then filtered to remove the insoluble larger aggregates. The clear brown

suspension was diluted with chloroform until the colour became pale brown. It was

then drop-casted onto water surface and transferred to various substrates and

characterized using various microscopic techniques.

(ii) Brominated Graphene-MWNTs Composites

MWNTs were used as such without any purification. MWNT (1 mg/ml) dispersion in

DMF was prepared and mixed with brominated graphene suspension in DMF at

various volume proportions. It was found that high MWNT content in the mixture led

to precipitation and the supernatant became colourless. A low MWNT content tend to

stabilize and solubilize graphene. It was then diluted with DMF for subsequent

characterizations.

6.2.4 Alkylation of Brominated Graphene

Brominated graphite (100 mg) in toluene (10 ml) was added to a three neck flask.

161

Dodecylamine (2.3 ml, 10 mmol) and triethylamine (1.39 ml, 10 mmol) were added to

it. The resulting mixture was refluxed for 24 h, excess solvent was evaporated under

reduced pressure and the obtained solid was used for debromination.

For debromination, 50 mg of the alkylated sample was dispersed in acetic acid (5

ml) and refluxed in presence of Zn dust (50 mg) for 24 h. The reaction mixture was

cooled to room temperature, filtered, washed repeatedly with water and dried.

6.2.5 Suzuki Coupling Reaction with Brominated Graphene

Brominated graphite (20 mg) was dispersed in DMF (10 ml) for 10 min. K2CO3

(0.3455 g, 2.5 mmol) and 0.5 M solution of the respective boronic acids (0.25 mmol)

in toluene (0.5 ml) was added to the reaction mixture. The mixture was degassed and

filled with N2. Pd (0) was added to the reaction mixture under dark and stirred at 85

°C for 48 h. The reaction mixture was cooled to room temperature, diluted with THF

and the unreacted solids were removed by filtration. Excess solvent was removed

under reduced pressure and the resultant waxy solid was washed with methanol and

water. The final product was dispersible in THF, toluene, chloroform and DMF and

used for subsequent characterizations.

For debromination, 50 mg of the alkylated sample was dispersed in acetic acid (5

ml) and refluxed in presence of Zn dust (50 mg) for 24 h. The reaction mixture was

cooled to room temperature, filtered, washed repeatedly with water and dried.

6.2.6 Characterization

The functionalized samples were characterized by Raman spectroscopy, FTIR,

FESEM, AFM, TEM and EDX. FESEM images and EDX spectra were obtained from

JEOL JSM-6710F scanning electron microscope. TEM analyses were carried out

using JEOL JEM-2010 F electron microscope, operating at 200 keV. A dilute

162

suspension of the samples in THF was drop-casted on the copper grid for TEM

measurements. AFM images were recorded using commercial AFM (Dimension

3100, Digital Instruments) in the non-contact mode. Samples for AFM were prepared

by drop casting dilute suspensions of the samples on freshly cleaved mica and then

dried at room temperature. Raman spectra were recorded with a Renishaw system-

2000 operating at an excitation wavelength of 532 nm. TGA was done using TA

instrument 2960 with heating rate of 10 °C/min in N2 atmosphere.


6.3.1 Brominated Graphite and its Stretching

Bromination is one of the important halogenation reactions reported for various

carbon nanostructures such as fullerenes, graphite and carbon nanotubes.14,15,16

Graphite can be readily intercalated with bromine at room temperature when

subjected to liquid bromine or bromine vapour.15

Bromination of graphite is expected

to change the hybridization of graphitic carbon atoms from sp2 to sp

3 which changes

the planar morphologies to a rough textured sheet.15

This was confirmed from the

FESEM and AFM analysis (Figure 6.1).

10 µµµµm

A

5 µµµµm

B 2.4 nm

Figure 6.1 FESEM (A) and AFM (B) image of the brominated graphite drop-casted

from toluene suspension.

163

The samples for FESEM were prepared by drop-casting a toluene suspension. The

sheets were of 10-15 µm in size with many wrinkles which is clearly visible from the

AFM image. Section analyses of several sheets revealed that the thickness ranged

between 1.5-3 nm indicating that the sheets were crumpled and contain multiple folds.

Elemental analysis of the brominated graphite sample gave 68.1 wt% of carbon and

31.8 wt% of bromine. It indicated that the brominated sample has one bromine atom

per 14 graphitic carbon atoms.

Since the brominated graphite samples were crumpled and had a rough surface,

attempts were made to spread it into flat sheets. One approach used was to spread the

sheets at the air - water interface and collect the sheets on various substrates. Figure

6.2A shows the FESEM image of the brominated graphite collected on nitrocellulose

membrane from the interface. As compared to brominated graphite directly drop

casted from toluene suspension, the sheets were stretched to almost a flat sheet

(Figure 6.2B). The sheets were of 10-15 µm in size. An alternative approach

employed to spread the sheets was to make use of the mechanical stretching induced

by the addition of CNTs. Figures 6.2C and D shows the FESEM and TEM images of

1:0.02 v/v graphene/MWNT composite, respectively. Inset of figure 6.2D shows one

of the MWNT present on the surface of the brominated graphite. The diameter of the

MWNTs used was of the order of 10-20 nm. But when the amount of MWNT added

reached a critical concentration; a solid precipitated out of the solution and hence

1:0.02 v/v mixing of brominated graphite and MWNT was taken as the optimum

condition. The stretched sheets could be a better candidate for fabrication of devices

or composites owing to large surface area and better interaction with the host matrices

as compared with the curled and folded samples.

164

A

1 µµµµm 2 µµµµm

B

0.2 µµµµm

C

0.1 µµµµm 10 nm

D

Figure 6.2 FESEM (A) and TEM (B) image of the brominated graphite sample

deposited from water-toluene interface; FESEM (C) and (D) TEM image of the 1:0.02

v/v graphene/MWNT composite.

1 µµµµm

A

1.1 nm1.2 nm

B

1 µµµµm

Figure 6.3 (A) AFM image of the brominated graphite sample deposited from water-

toluene interface and (B) AFM image of the 1:0.02 v/v graphene/MWNT composite.

165

The thickness of the stretched sheets was further confirmed with AFM (Figure

6.3A and B). The average thickness of the sheets were found to be ∼ 1-1.2 nm in both

the cases, which supports the fact that sheets were less folded and crumpled compared

with the drop-casted samples. The small discrepancies in the height profile of figure

6.3B correspond to the MWNTs present on the graphene surface.

6.3.2 Alkylation of the Brominated Graphite

Brominated graphite was subjected to alkylation by treatment with dodecylamine. The

alkylated sample showed enhanced dispersibility in chloroform, toluene, THF and

DMF as compared to the brominated graphite. Evidence of functionalization was

determined by obtaining the Raman spectrum as shown in figure 6.4A. Pure graphite

powder showed only the G band at ∼ 1580 cm-1

, but in the case of brominated

graphite, the disorder induced D band appear at ∼ 1350 cm-1, which becomes more

prominent in the case of alkylated sample. The Raman spectra of the dodecylated

graphene were almost similar to that of brominated graphite sample, except that the D

band intensity is higher. This could be attributed to the increase in disorders caused by

the mild sonication used for dispersion. During the alkylation reaction, all bromine

atoms were not substituted with alkyl chains. Remaining bromine atoms were

removed by Zn dust treatment and confirmed by elemental analysis, which showed

only negligible amount of bromine. The ratio of the intensities of D and G (ID/IG)

bands in Raman spectra can be used as an indication of the level of chemical

modification in graphene.17

The Raman spectrum of pure graphite showed a minimal

disorder mode with ID/IG ratio of 0.12 (trace a), whereas the brominated graphite

(trace b), alkylated graphene (trace c) and the debrominated graphene after alkylation

(trace d) showed an increase in the disorder mode with increased ID/IG ratio of 0.75,

166

1.01, and 0.97, respectively (legend of figure 6.4A). This can be attributed to an

increase in the number of sp3 hybridized carbon atoms that were formed during the

functionalization.

800 1200 1600 2000 2400 2800

(c)

(b)

% T

ran

sm

itta

nc

e (

a.u

)

Wavenumber (cm-1)

(a)

1000 1250 1500 1750 2000

ID/IG = 0.97

ID/IG = 1.01

ID/IG = 0.75

(c)

(d)

(b)

Inte

ns

ity (

a.u

)

Raman Shift (cm-1)

(a)ID/IG = 0.12

BA

Figure 6.4 (A) Raman spectra of (a) pure graphite (b) brominated graphite (c)

alkylated graphene and (d) debrominated graphene sheets after alkylation and (B) FTIR spectra of (a) brominated graphite (b) alkylated and (c) debrominated sheets

after alkylation.

Furthermore, dodecylation on the brominated graphene has been supported by FTIR

investigations (Figure 6.4B). Brominated graphite exhibited IR stretching absorptions

between 600-700 cm-1 (trace a in figure 6.4B), all samples showed the C=C aromatic

stretching at ~ 1470 cm-1

indicating the existence of unreacted double bonds. In the

case of the dodecylated sample, in addition to the absence of C−Br stretching (600-

700 cm-1

), the material showed C−H stretching bands associated with the dodecyl

groups at 2800-3000 cm-1

. This is in accordance with the previous reports on the

reductive alkylation of fluorinated graphite.18

Bromination and alkylation were further investigated using TGA analyses. Traces

a and b in figure 6.5 shows the TGA curves obtained for brominated and alkylated

samples, respectively. TGA analysis of the brominated graphite showed a weight loss

of ∼ 10% in between 100-150 °C, which could be attributed to the elimination of

167

physisorbed bromine and solvent molecules (trace a). The major weight loss upto

48% take place between 200 and 400 °C, with a temperature of maximum

decomposition at 260 °C. These could be associated with the loss of chemically

bound bromine molecules.

200 400 600 80040

50

60

70

80

90

100

b

We

igh

t (%

)

Temperature (°C)

a

Figure 6.5 Weight loss of (a) brominated graphite and (b) dodecylated graphite

determined by TGA analyses in nitrogen.

Furthermore, if the high uncertainty of the TGA measurements and the removal of

physisorbed bromine and solvent molecules were taken into account, the overall

weight loss is about 40% which is in agreement with the values obtained from the

elemental analysis results (Br content 31.8 wt%). The TGA weight loss of

dodecylated graphene sheets (Figure 6.5) was near to 30%, and all the weight loss was

in between 250 and 560 °C, which is predominantly from the decomposition of

dodecyl functionalities. A similar TGA profile was observed in the alkylated graphite

via fluorination route.18

A small portion of the alkylated samples were found to be dispersible in solvents

without sonication and the sheets were of several micrometers in size. Figure 6.6

168

shows the FESEM and AFM images of the dodecylated sample. The sheet dimensions

remained the same as that of the brominated graphite suspensions. Average

thicknesses of the alkylated samples were of the order of 1-1.4 nm, which is slightly

higher than the brominated graphene suspensions. It could be attributed to the long

alkyl chains present on both sides of the sheets.

1 µµµµm

A

2 µµµµm

B

1.4 nm

Figure 6.6 (A) FESEM and (B) AFM images of the dodecylated graphene sample

(without sonication).

Elemental analysis of the dodecylated graphene sample indicated that the sample

still contained some bromine (∼ 17.3 wt%). Hence, debromination of the alkylated

sample was carried out. After debromination, the content of bromine was significantly

reduced and it was found to be only 0.5 wt%. It indicated that the π - conjugation is

partially restored with debromination, which was evident from the Raman

measurements also (trace d in figure 6.4A). When the debrominated samples were

subjected to mild sonication (less than 5 min), it resulted in better exfoliation and

dispersion, the average thickness varied between ∼ 1 to 1.2 nm. But sonication

reduced the sheet dimensions significantly and the sheets dimensions were of the

order of 1-3 µm. Additional evidence for the distribution of functionalized graphene

samples were supported with TEM and AFM measurements (Figure 6.7).

169

µµµµm40 2 86

nm

-20

20

0

1 µµµµm

1.36 nm

A

B

C

100 nm

Figure 6.7 (A) AFM image of the dodecylated graphene sheets after dispersion, (B)

the corresponding section analysis; and (C) TEM image.

Electrical properties of the dodecylated and the debrominated samples were measured

using a two - probe conductivity measurement system. Figure 6.8 shows the I-V plots

of brominated graphite (trace a), dodecylated graphite (trace b) and the debrominated

product (trace c). It is evident from the I-V plot that brominated and alkylated samples

have low electrical conductivity due to the lack of effective π - π conjugation. The

drop casted films of brominated graphite and dodecylated graphene suspensions

showed conductance of 19 and 27 nA, respectively at an applied voltage of 10 V.

There were no significant changes in conductance before and after alkylation. After

debromination of alkylated graphene, conductance was increased to 75 nA at an

applied voltage of 10 V. This increase in conductance must be due to the partially

restored extended π - π conjugation. Such condensation and coupling reactions might

be useful for the synthesis of electrically conductive polymer composites for

photovoltaic or sensing applications.

170

a

b

c

0 2 4 6 8 100

20

40

60

80

Cu

rre

nt

(nA

)

Voltage (V)

Figure 6.8 I-V plots of drop casted films of (a) brominated graphite (b) dodecylated graphene and (c) debrominated product after dodecylation.

6.3.3 Arylation of the Brominated Graphite Using Suzuki Coupling Reactions

Considering that most of the Suzuki coupling reactions requires halogenated

substrates, the coupling reactions on brominated graphene using pyrene 1-boronic

acid and 5-hexyl-2,2'-bithiopheneboronic acid pinacole ester were conducted. It was

found that the coupled products; pyrene coupled sample (G-Py) and 5-hexyl-2,2'-

bithiopheneboronic acid pinacole ester coupled sample (G-BTP), exhibited enhanced

dispersibility in THF, chloroform and toluene and the solutions remained stable for

several days without any visible aggregation. UV-vis and emission characteristics of

the products were collected in THF. In order to compare the photophysical properties,

model compounds were prepared with coupling of the respective boronic acid esters

with bromobenzene. Figure 6.9 shows the UV-vis and emission characteristics of the

coupled products. Concentrations of the model compounds and coupled product were

kept constant for all measurements. G-BTP shows an absorption maximum at 410 nm

and the model compound exhibits an absorption maximum at 350 nm. For the

emission spectra, model compound and G-BTP were excited at 410 nm, and it was

171

found that the bithiophene grafted graphene hybrid exhibits strong fluorescence

quenching, which indicates the occurrence of photoinduced charge or energy transfer

from polymer to graphene (Figure 6.9B).

350 400 450 5000.0

0.1

0.2

(a)

410 nm

Ab

so

rba

nce

Wavelength (nm)

352 nm

(b)

450 500 550 600

(a)

Inte

nsit

y

Wavelength (nm)

(b)

250 300 350 400 4500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

(a)

Ab

so

rba

nc

e

Wavelength (nm)

(b)

350 400 450 500 550

(b)

Inte

nsit

y

Wavelength (nm)

(a)

A B

C D

Figure 6.9 Absorption (A) and emission (B) spectra for (a) Ph-BTP (b) G-BTP

excited at 410 nm; Absorption (C) and emission (D) spectra for (a) Ph-Py (b) G-Py

excited at 340 nm.

Similar trend was observed in the case of G-Py (Figure 6.9 D). Further evidence for

the coupling has been provided by elemental analysis, and it was found that G-BTP

hybrid contained only the elements C (92.3%), S (5 %) and O (1.7%). Whereas G-Py

contained only C (98.7%) and O (1.3%). Presence of oxygen could be from trace

amounts of THF present in the material.

Raman spectra were recorded for the coupled products and the respective

debrominated products (Figure 6.10). Debromination of G-BTP and G-Py did not

172

have any significant influence on the ID/IG ratio (legends of figure 6.10A and B),

indicating that all the bromine atoms present on graphene were reacted with boronic

acids.

1000 1250 1500 1750 2000

(c)

(b)

Inte

nsit

y (

a.u

)

Raman Shift (cm-1)

(a)

1000 1250 1500 1750 2000

(c)

(b)

Inte

ns

ity (

a.u

)

Raman Shift (cm-1)

(a)ID/IG = 0.75

ID/IG = 1

ID/IG = 0.75

ID/IG = 1

ID/IG = 0.98

ID/IG = 0.99

A B

Figure 6.10 Raman spectra of (A) G-BTP (a - brominated graphite; b - G-BTP; c -

debrominated G-BTP) and (B) G- Py (a - brominated graphite; b - G-Py; c -

debrominated G-Py)

These results support the elemental analysis data which showed the absence of

bromine in the coupled products.

0.5 µµµµm

0.5 µµµµm

A B

C D

3.09 nm

2.98 nm

Figure 6.11 (A) TEM and (B) AFM image of bithiophene coupled graphene; (C)

TEM and (D) AFM image of pyrene coupled graphene.

173

Figure 6.11 shows the TEM and AFM images of G-BTP and G-Py drop-casted from

THF. From the AFM measurements, it was found that the thickness of the sheets

increased and was of the order of 2.5-3 nm. It could be due to accumulation of organic

moieties on the graphene surface. The thickness can be controlled with the initial

boronic acid concentration and the reaction time.

6.4 Conclusions

As a summary, we demonstrate a simple covalent functionalization method for

graphene sheets. Bromine atoms are covalently attached to the graphene sheets

through C-Br bonds which lead to stable dispersions in organic solvents. Subsequent

alkylation and arylation of these brominated sheets suggest the possibility of a broad

range of condensation and coupling reactions on the surface of graphene sheets,

leading to the production of new graphene based hybrid systems.

6.5 References

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177

Chapter 7

Conclusions and Future Prospects

178

Carbon nanomaterials have always been a fascinating area for researchers owing to

their unique properties and wide range of applications. Most of the reported

procedures for the preparation of functional carbon nanostructures rely on the use of

extreme reaction conditions such as high pressure and temperature. Simple and cost-

effective methodologies for the production of functional carbon nanomaterials are

very crucial for practical applications. In the present work, we developed simple

solution phase methods for the production of carbon nanomaterials from carbon rich

precursors.

Initial investigations were focussed on solution based extraction methods for the

isolation of novel carbon nanofibers from natural soot. The isolated fibers were

characterized using various spectroscopic and microscopic techniques. These fibers

were utilized for preparing electrospun CNF/PVA composite fiber mats as a sorbent

for microextraction of aniline derivatives from waste water. The performance of

CNF/PVA composite nanofiber mats as a µ-SPE device for the determination of

anilines was optimized by investigating several factors such as extraction time,

desorption solvent, sample volume, desorption time, salting-out effect and pH effect.

The composite membrane sorbent yielded satisfactory parameters for microextraction.

The LOD determined for the aniline compounds were in range of 0.009 - 0.081 µg/l

and LOQ was within the range of 0.030 - 0.269 µg/l, which was comparable with that

of other existing techniques. This method offer certain advantages such as easy

extraction, minimal use of solvents and elimination of tedious solvent evaporation and

reconstitution steps.

Later on, simple methods for the preparation of solution processable graphene

sheets were explored, which include non-covalent stabilization of exfoliated graphene

sheets with surfactants such as CTAB. Graphene nanosheets were exfoliated directly

179

from graphite making use of the combined effect of sonication and non-covalent

stabilization. This method eliminated the use of any oxidation/reduction steps. The

sheets could be dispersed in solvents such as DMF and were found to have an average

thickness of 1.2 nm. The sheets were characterized in detail using AFM, STM, TEM

and Raman spectroscopy. Field emission measurements showed a turn on voltage of

7.5 V/µm and emission current densities of 0.15 mA/cm2. This approach is expected

to be a step forward in the challenging global efforts to solubilize graphene and in the

formation of conducting composite materials.

CTAB stabilized graphene nanosheets were used as reinforcing filler for PVC. In

order to have efficient reinforcing effect, it is essential to have homogeneous

dispersions. In our investigations, both the polymer and graphene could be dispersed

in the same solvent, which enabled homogeneous mixing and dispersion. A

significant enhancement in the mechanical properties of pure PVC films was obtained

with very low loadings of graphene (2 wt%), such as a 58% increase in Young's

modulus and an almost 130% improvement of tensile strength. Thermal stability was

also found to be improved to a greater extent with an increase in the glass transition

temperature. Moreover, The composite films had very low percolation threshold of

0.6 vol.% and showed a maximum electrical conductivity of 0.058 S/cm at 6.47 vol.%

of graphene loading.

Use of graphene in composites is highly beneficial but it is equally challenging due

to the unavailability of solution processable graphene in larger quantities. Hence, a

different approach to stabilize graphene was adopted, which involved covalent

modifications on graphene sheets. Nitrene insertion was employed to functionalize the

exfoliated graphene sheets. The approach involved functionalization of dispersible

unoxidized CTAB stabilized graphene sheets with various alkylazides and 11-

180

azidoundecanoic acid proved to be the best azide for enhanced dispersibility. The

functionalization was confirmed by FTIR and STM. The functionalized samples

showed enhanced dispersibility and solution stability compared with CTAB stabilized

sheets. Besides, the reactive sites on azide functionalized graphene sheets were

marked with gold nanoparticles. The interaction between gold nanoparticles and the

graphene sheets was followed by UV-vis spectroscopy. TEM and AFM investigations

revealed uniform distribution of gold nanoparticles all over the surface and this

composite material was found be electrically conducting and the conductivity was

found to be three times as compared with azide functionalized graphene films. This

finding could be promising for device or sensor applications.

Another covalent functionalization, which was attempted, was the direct

bromination of graphite. Bromination of graphite takes place via electrophilic addition

of bromine molecules to the aromatic double bonds, which can be replaced with alkyl

or aryl substituents. It was seen that bromination followed by alkylation or Suzuki

coupling reactions lead to stable dispersions of graphene in common organic solvents

such as THF, toluene and chloroform. The average thickness of the alkylated

graphene sheets were found to be of the order of 1 - 1.2 nm. Graphene sheets coupled

with pyrene or bithiophene moieties showed an increased thickness of 2.5 - 3 nm,

which can be attributed to the accumulated organic molecules on the surface. Besides,

graphene coupled with pyrene and biothiophene showed fluorescence quenching. This

could be due the efficient charge or energy transfer from bithiophene or pyrene

moieties to graphene. This novel method offers an opportunity to integrate graphene

into various optoelectronic devices in an easy and direct manner.

181

Future prospects

Non-covalent and covalent interactions to stabilize graphene in common organic

solvents are a step forward in graphene based composite materials. Incorporation of

graphene sheets will enhance the mechanical, electrical and thermal properties of the

composite materials. These materials will be ideal candidates for fabricating

mechanically reinforced electro conducting plastics. Besides, improved thermal

conductivity of the graphene-polymer composite will make it suitable for aerospace

applications, where aerospace structures produce considerable amount of heat energy.

Lithium-ion batteries containing graphite anodes are now the most widely used

power sources for portable electronic devices. Recently, various anode materials have

been proposed to overcome the limited capacity of graphite (372 mAh g−1). Use of

graphene composites as anode material for lithium ion batteries can be thought of,

which is expected to show better electrochemical properties.

Attachment of Pt or Ni nanoparticles to graphene via covalent interactions is

expected to be highly impactful in catalysis such as oxygen reduction reactions.

182

Appendix

Fillers

Used

Modulus Strength Ductility Toughness Electrical

conductivity

Percolation

threshold

ref

Clay 3.6 MPa 30-50 MPa

189% 1

Wood flour - 3.1 MPa - - - - 2

Polyesters - 30-34.5

MPa

150-225% - - - 3

Layered

silica

20 MPa - - - - -

Cellulose

fibers

0.118 GPa 8.4 MPa 30-36% - - -

Lignocellul

ose fibers

43-56

MPa

87% - - - 6

Carbon

black

- - - - 0.1 S cm-1

10 wt% 7

Polyaniline 0.8 S cm-1

> 1wt% 8

polypyrrole 2140 ± 148

MPa

35 ± 2

MPa

4 ± 0.2 % - 0.1 S cm-1 - 9

MWNT - - - - 10-4 S cm-1 0.05 vol% 10

Kevlar coated

CNTs

63 MPa 13 GPa - 39 KJ/m3 - - 11

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183

LIST OF PUBLICATIONS

Refereed Journals

1. Cationic surfactant mediated exfoliation of graphite into graphene flakes.

Sajini Vadukumpully, Jinu Paul and Suresh Valiyaveettil. Carbon, 2009, 47, 3288 -

3294.

2. Flexible conductive graphene/poly(vinyl chloride) composite thin films with high

mechanical strength and thermal stability. Sajini Vadukumpully, Jinu Paul and

Suresh Valiyaveettil. Carbon 2011, 49, 198 - 205.

3. Functionalization of surfactant wrapped graphene nanosheets with alkylazides for

enhanced dispersibility. Sajini Vadukumpully, Jhinuk Gupta, Yongping Zhang, Guo

Qin Xu and Suresh Valiyaveettil. Nanoscale, 2011, 3, 303 - 308.

4. Carbon nanofibers extracted from soot as a sorbent for the determination of aromatic amines from wastewater effluent samples. Sajini Vadukumpully, Chanbasha

Basheer, Cheng Suh Jeng and Suresh Valiyaveettil. J. Chromatogr. A 2011 1218, 23, 3581-3587.

5. Charge transport studies in fluorene – dithieno[3,2-b:2’,3’ d]pyrrole oligomer using

time-of-flight photoconductivity method. Manoj Parameswaran, Ganapathy Balaji,

Tan Mein Jin, Chellappan Vijila, Sajini Vadukumpully, Zhu Furong and Suresh

Valiyaveettil. Org. Electron 2009, 10, 1534 -1540.

6. Investigations on the structural damage in human erythrocytes exposed to silver, gold,

and platinum nanoparticles. P. V. Asharani, Swaminathan Sethu, Sajini

Vadukumpully, Shaoping Zhong, Chwee Teck Lim, M. Prakash Hande and Suresh

Valiyaveettil. Adv. Funct. Mater. 2010, 20, 1233 - 1242.

7. Synthesis and property studies of linear and kinked poly(pyreneethynylene)s. Jhinuk

Gupta, Sajini Vadukumpully and Suresh Valiyaveettil, Polymer 2010, 51, 5078 -

5086.

Conference Presentations

1. Isolation and characterization of CNFs from soot – mechanical properties and

analytical applications of CNF/PVA composite membrane. Sajini Vadukumpully, Jinu Paul and Suresh Valiyaveettil. AsiaNano 2008, Singapore (poster).

2. Chemical methods for the exfoliation of graphite into graphene. Sajini Vadukumpully

and Suresh Valiyaveettil. 1st Singapore‐Hong Kong Bilateral Graduate Student

Congress in Chemical Sciences 2009, Singapore (oral).

184

3. Synthesis of processable graphene nanosheets from exfoliation of CTAB treated

graphite. Sajini Vadukumpully, Jinu Paul and Suresh Valiyaveettil. ICMAT 2009

Singapore (poster).

4. Flexible conductive graphene/PVC nanocomposites membranes with high mechanical

strength and thermal stability. Sajini Vadukumpully, Jinu Paul and Suresh

Valiyaveettil. 6th

Singapore International Chemical Conference (SICC 6), 2009, Singapore (poster).

5. Azide functionalization on graphene nanosheets. Sajini Vadukumpully, Jhinuk

Gupta and Suresh Valiyaveettil. MRS conference, 2010, San Fransico, USA (oral).

6. Processable graphene nanosheets from surfactant assisted exfoliation of graphite and its field emission properties. Sajini Vadukumpully, Jinu Paul and Suresh

Valiyaveettil. MRS-S, IMRE Singapore. 2010 (poster).

7. Carbon nanofibers for water purification. Sajini Vadukumpully and Suresh

Valiyaveettil. Singapore International Water Week (SIWW), Singapore 2010. (poster).

8. Bromination of graphite: A new route to produce graphene sheets. Sajini

Vadukumpully, Jhinuk Gupta and Suresh Valiyaveettil. Pacifichem 2010, Hawaii,

USA (poster).

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