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
3.2 Experimental Section 96
v
3.2.1 Materials and Methods 96
3.2.2 Preparation of Processable Graphene Nanosheets 96
3.2.3 Instrumentation 96
3.3 Results and Discussion 97
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 Experimental Section 115
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
4.3 Results and Discussion 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
5.1 Introduction 134
5.2 Experimental Section 136
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 Results and Discussion 140
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
6.1 Introduction 158
6.2 Experimental Section 159
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 Results and Discussion 162
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
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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)
3-Nitroaniline 4-Chloroaniline 4-Bromoaniline 3,4-Dichloroaniline
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)
3-Nitroaniline 4-Chloroaniline
4-Bromoaniline 3,4-Dichloroaniline
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)
3-Nitroaniline 4-Chloroaniline 4-Bromoaniline 3,4-Dichloroaniline
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%)
3-Nitroaniline 4-Chloroaniline
4-Bromoaniline 3,4-Dichloroaniline
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
3-Nitroaniline 4-Chloroaniline 4-Bromoaniline 3,4-Dichloroaniline
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 Experimental Section
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).
3.3 Results and Discussion
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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Q. J. Phys. Chem. C 2008, 112, 16377.
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Publications from this chapter:
Vadukumpully, S.; Paul, J.; Valiyaveettil, S. Carbon 2011, 49, 198-205.
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 Experimental Section
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
AFM images were recorded using Agilent AFM with Pico plus molecular imaging
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 Results and Discussion
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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31. O’Connor, I.; Hayden, H.; O’Connor, S.; Coleman, J. N.; Gun’ko, Y. K. J.
Mater. Chem. 2008, 18, 5585.
32. Wang, G. J.; Qu, Z. H.; Liu, L.; Shi, Q.; Guo, H. L. Mat. Sci. Eng. A-Struct.
2008, 472, 136.
33. Vadukumpully, S.; Paul, J.; Valiyaveettil, S. Carbon 2009, 47, 3288.
34. Brunner, A. J. J. Polym. Sci. Part B: Polym. Lett. 1972, 10, 379.
35. (a) Veca, L. M.; Lu, F.; Meziani, M. J.; Cao, L.; Zhang, P.; Qi, G.; Qu, L. W.;
Shrestha, M.; Sun, Y. P. Chem. Commun. 2009, 2565. (b) Wang, D. W.; Li,
F.; Zhao, J.; Ren, W.; Chen, Z. G.; Tan, J.; Shuai, Z.; Wu, Z. S.; Gentle, I.; Lu,
G. Q.; Cheng, H. M. ACS Nano 2009, 3, 1745.
36. Kawasumi, M.; Hasegawa, N.; Kato, M.; Usuki, A.; Okada, A.
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37. Becker, G. W. Kolloid Z 1955, 140, 1.
38. Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney,
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40. Broza, G.; Piszczek, K.; Schulte, K.; Sterzynski, T. Compos. Sci. Technol.
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Publications from this chapter:
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 Experimental Section
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.
AFM images were recorded using Agilent AFM with Pico plus molecular imaging
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 Results and Discussion
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.
5.5 References
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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 Experimental Section
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 Results and Discussion
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.
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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).