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An environment-friendly preparation of reduced graphene oxide nanosheets via amino acid
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2011 Nanotechnology 22 325601
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 22 (2011) 325601 (7pp)
doi:10.1088/0957-4484/22/32/325601
An environment-friendly preparation of
reduced graphene oxide nanosheets via
amino acid
Dezhi Chen1,2, Lidong Li1 and Lin Guo1
1
School of Chemistry and Environment, Beijing University of Aeronautics and Astronautics,
Beijing 100191, People’s Republic of China
2
School of Environmental and Chemical Engineering, Nanchang Hangkong University,
Nanchang 330063, People’s Republic of China
E-mail: [email protected] and [email protected]
Received 24 April 2011, in final form 27 June 2011
Published 14 July 2011
Online at stacks.iop.org/Nano/22/325601
Abstract
Chemically modified graphene has been studied in many applications due to its excellent
electrical, mechanical, and thermal properties. Among the chemically modified graphenes,
reduced graphene oxide is the most important for its structure and properties, which are similar
to pristine graphene. Here, we introduce an environment-friendly approach for preparation of
reduced graphene oxide nanosheets through the reduction of graphene oxide that employs
L-cysteine as the reductant under mild reaction conditions. The conductivity of the reduced
graphene oxide nanosheets produced in this way increases by about 106 times in comparison to
that of graphene oxide. This is the first report about using amino acids as a reductant for the
preparation of reduced graphene oxide nanosheets, and this procedure offers an alternative route
to large-scale production of reduced graphene oxide nanosheets for applications that require
such material.
S Online supplementary data available from stacks.iop.org/Nano/22/325601/mmedia
(Some figures in this article are in colour only in the electronic version)
graphene-like materials [3, 8]. Generally, hydrazine [9–15],
dimethylhydrazine [2, 16], and NaBH4 [17] are used for the
preparation of RGO, but they are either toxic or hazardous.
Hence, it is very important to find an environment-friendly
and effective reductant to reduce GO for the preparation of
RGO [3, 18]. Recently, environment-friendly chemical agents,
such as vitamin C [19–21], aluminum powder [22], reducing
sugar [23], have been reported to produce RGO. However,
RGO prepared by the reduction of GO via amino acids has
not been reported in the literature up to now. For this purpose,
we propose a facile method to prepare RGO nanosheets that
utilizes L-cysteine as the reductant. L-cysteine is an amino
acid which contains a thiol group. The thiol is susceptible
to be oxidized to form the disulfide derivative cystine [24].
Due to the ability of thiols to undergo redox reactions,
L-cysteine has antioxidant properties. L-cysteine’s antioxidant
properties are typically expressed in the tripeptide glutathione,
which occurs in humans as well as other organisms [25]. By
1. Introduction
During the last half decade, chemically modified graphene
(CMG) has been studied in the context of many applications,
such as polymer composites, energy-related materials, sensors,
‘paper’-like materials, field-effect transistors (FETs), and
biomedical applications, due to its excellent electrical,
mechanical, and thermal properties [1–7]. Generally, the
reactions of chemically modified graphene oxide can be
classified into (i) reduction (removing oxygen groups from
graphene oxide) and (ii) chemical functionalizations (adding
other chemical functionalities to graphene oxide). Among
the reactions of graphene oxide, the reduction process is of
the utmost importance due to the similarities between reduced
graphene oxide (RGO) and pristine graphene. For scientists
and engineers endeavoring to use graphene in large-scale
applications, chemical reduction of graphene oxide is the most
obvious and desirable route to synthesize large quantities of
0957-4484/11/325601+07$33.00
1
© 2011 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 22 (2011) 325601
D Chen et al
7.0. Finally, one part of the as-prepared product was dispersed
in aqueous solution of pH = 10 (using NaOH aqueous solution
of 0.1 mol l−1 to adjust the pH) or N,N -dimethylacetamide
by ultrasonication (500 W) to prepare the suspension of RGO,
and the other part was used to produce the powder of RGO by
drying at 50 ◦ C for 24 h under vacuum.
virtue of the antioxidant nature of L-cysteine, we successfully
synthesized RGO nanosheets in aqueous solution under the
mild conditions.
2. Experimental details
2.1. Materials
2.4. Characterization
Graphite powder, natural briquetting grade, 100 mesh,
99.9995% (metals basis), was purchased from Alfa Aesar.
L-cysteine (purity:
97%) was purchased from Sigma.
Analytical-grade NaOH, K2 S2 O8 , P2 O5 , KMnO4 , N,N dimethylacetamide, anhydrous ethanol, 98% H2 SO4 , 36% HCl
and 30% H2 O2 aqueous solution were purchased from the
Beijing chemical reagents company and used directly without
further purification. All aqueous solutions were prepared with
deionized water.
Ultraviolet–visible (UV–vis) spectra were obtained using a
Cintra 10e spectrophotometer (GBC Scientific Equipment Pty
Ltd, Australia). The aqueous suspension of GO and RGO was
used as the UV–vis samples, and the deionized water was used
as the reference. Raman spectra were recorded from 1000 to
1900 cm−1 on a LabRAM HR800 laser Raman spectroscope
(HORIBA Jobin Yvon CO. Ltd, France) using a 514.5 nm
argon ion laser. All samples were deposited on silicon wafers
in powder form without using any solvent. Fourier transform
infrared (FT-IR) spectra of the samples were recorded on
an Avatar 360 spectrophotometer (Thermo Nicolet, USA).
The test specimens were prepared by the KBr disc method.
XRD analyses were carried out on an x-ray diffractometer
(D/MAX-1200, Rigaku Denki Co. Ltd, Japan). The XRD
patterns with Cu Kα radiation (λ = 1.5406 Å) at 40 kV
and 40 mA were recorded in the range of 2θ = 5◦ –80◦ .
Thermogravimetric analysis (TGA) was performed under a
nitrogen flow (100 ml min−1 ) using a Pyris Diamond TG/DTA
(Perkin Elmer, Inc., USA). The samples were heated from 50
to 800 ◦ C at 5 ◦ C min−1 . The x-ray photoelectron spectroscopy
(XPS) measurements were performed on a PHI Quantera xray photoelectron spectroscope (ULVAC-PHI, Inc., Japan), and
the binding energy was calibrated with C 1s = 284.8 eV.
Atomic force microscopy (AFM) images were acquired in
a tapping mode with a commercial multimode Nanoscope
IIIa (Veeco Co. Ltd). Transmission electron microscopy
(TEM) images and selected area electron diffraction (SAED)
patterns were obtained using a JEM-2100F transmission
electron microscope (JEOL Ltd, Japan) operated at 200 kV.
The electrical conductivity was measured using a SDY-6 digital
four-point probe system (Guangzhou, PR China).
2.2. Preparation of the GO
GO was synthesized using graphite powder by a modified
Hummers’ method [26–28]. In brief, 6 g of graphite powder
were added to an 80 ◦ C solution of 25 ml concentrated H2 SO4 ,
5 g of K2 S2 O8 , and 5 g of P2 O5 . The mixture was reacted for
6 h, after which it was diluted with 1 liter of water, and washed
using a 0.22 μm Nylon Millipore filter to remove the residual
acid. Afterward, this pre-oxidized graphite was put into ice
cold (0 ◦ C) concentrated H2 SO4 (230 ml). 30 g of KMnO4
were then added gradually under stirring and the temperature
of the mixture was controlled below 10 ◦ C. Successively, this
mixture was stirred at 35 ◦ C for 2 h, after which 500 ml of
distilled water were added slowly to keep the temperature
below 50 ◦ C. After further reaction for 2 h, 1.4 liter of water
and 20 ml of 30% H2 O2 were added, and the color of the
mixture changed into brilliant yellow along with bubbling. The
mixture was then centrifuged and washed with a total of 3 liter
of 10% HCl solution followed by 3 liters of water to remove
the acid. The resulting solid was subjected to dialysis for a
week to remove the remaining metal ions and acids. Finally,
the product was dried at 50 ◦ C for 24 h under vacuum. 50 mg
graphite oxide was exfoliated into deionized water (100 ml)
by ultrasonication (500 W) to form GO aqueous suspension at
room temperature. The as-obtained yellow-brown 0.5 mg ml−1
aqueous suspension of GO (figure S1-a available at stacks.
iop.org/Nano/22/325601/mmedia) was stored in a volumetric
flask, and used for the further characterizations and chemical
reduction.
3. Results and discussion
The UV–vis absorption spectrum of GO shown in figure 1(a)
is characterized by the π –π ∗ of the C=C plasmon peak around
230 nm and a shoulder around 300 nm which is often attributed
to n –π ∗ transitions of the carbonyl groups [29]. While reduced
by L-cysteine (figures 1(b)–(f)), the plasmon peak gradually
red-shifts to ∼270 nm with the increase of the reduction time,
reflecting increased π -electron concentration and structural
ordering, which is consistent with the restoration of sp2 carbon
and possible rearrangement of atoms [30]. It implies that the
GO might be reduced and the aromatic structure might be
restored gradually, and the degree of reduction was gradually
improved with the increase of reaction time. Similar features
and trends are observed for the reduction of GO by L-ascorbic
acid [19, 21].
The Raman spectra further support the structural change
before and after the reduction of GO. Figure 2 shows the
2.3. Reduction of the GO
Typically, 0.2 g of L-cysteine was put into 20 ml GO aqueous
suspension of 0.5 mg ml−1 . The mixture was kept in a
tightly sealed glass bottle and stirred for 12, 24, 48, and 72 h
respectively at room temperature (26 ± 2 ◦ C). Firstly, the
black product was isolated by centrifugation at 4000 rpm, and
then 20 ml NaOH aqueous solution of 0.1 mol l−1 was added
into the product to dissolve L-cystine. Then, the solution was
centrifuged at 10 000 rpm, and the obtained black slurry was
washed with adequate deionized water and ethanol up to pH =
2
Nanotechnology 22 (2011) 325601
D Chen et al
Figure 1. UV–vis spectra of aqueous dispersion of GO before (a) and
after being reduced by L-cysteine for different reduction times
12 h (b), 24 h (c), 48 h (d), 72 h (e).
Figure 3. FT-IR of the GO before (a) and after reduction by
L-cysteine for different reduction times 12 h (b), 24 h (c), 48 h (d),
72 h (e).
respectively. Aromatic C=C stretching vibration shows at
1625 cm−1 , and the peaks at 1220 cm−1 and 1053 cm−1 can
be attributed to the epoxy C–O stretching vibration and the
alkoxy C–O stretching vibration, respectively [32]. While
being reduced by L-cysteine, the peaks for oxygen functional
groups gradually decreased with reaction time, and some of
them disappeared completely. These observations confirmed
that most oxygen functionalities in the GO were removed [20].
The curve is very similar to the curve of RGO by vitamin
C [19], indicating the high efficiency of reduction by Lcysteine. However, the peak at 1625 cm−1 attributed to the
aromatic C=C group still exists. It suggests that the frame of
sp2 carbon atoms after reduction by L-cysteine is retained well,
as before.
The distance between two layers is an important parameter
to evaluate the structural information of the graphene. The
XRD patterns of graphite and GO are compared with those
of the samples reduced by L-cysteine for different reduction
times in figure 4. Owing to the presence of oxygen-containing
functional groups attached on both sides of the graphene
sheet and the atomic-scale roughness arising from structural
defects (sp3 bonding) generated on the originally atomically
flat graphene sheet [33], the d -spacing of the GO (figure 4(b))
is about 0.78 nm (2θ ≈ 11.3◦ ), which is significantly larger
than the d(002) value of graphite (figures 4(a), d ≈ 0.34 nm,
2θ ≈ 26.2◦ , thickness single-layer pristine graphene). With
the increase of reaction time, the (002) peak of GO gradually
disappears whereas the broad diffraction peak at 24.0◦ (d ≈
0.37 nm) progressively becomes prominent (figures 4(c)–(f)).
This shift in the interlayer spacing can be attributed to the
reduction of the GO, where the reduction makes the RGO pack
tighter than the GO [11]. Though there is a decrease in the
interlayer spacing compared with GO, the basal spacing of
RGO is higher than that of well-ordered graphite (single-layer
pristine graphene). The higher basal spacing may be due to
the presence of residual oxygen functional groups, indicating
incomplete reduction of GO. The fact that (002) reflection
Figure 2. Raman spectra of graphite (a), GO before (b) and after
reduction by L-cysteine for different reduction times 12 h (c),
24 h (d), 48 h (e), 72 h (f).
typical Raman spectra of graphite, GO, and after reduction
by L-cysteine (figure 2). The Raman spectrum of the pristine
graphite, as expected, displays a prominent G (the E2g mode
of sp2 carbon atoms) peak as the only feature at 1582 cm−1 .
In the Raman spectrum of GO after reduction by L-cysteine,
the G band is broadened and shifted to around 1597 cm−1 .
In addition, the D band (the symmetry A1g mode) becomes
prominent. Noticeably, the Raman spectrum of GO after
reduction by L-cysteine shows a higher D/G intensity ratio than
GO (0.94), and the D/G intensity ratio increases (0.98, 1.02,
1.08 and 1.17) with the increase of reduction time (12, 24, 48
and 72 h) step by step. The variation of the relative intensities
of G band to D band in the Raman spectra of the GO during
the reduction usually reveals the change of the electronic
conjugation state. This change suggests an increase in the
number of sp2 domains with the reduction of GO [10, 31].
Figure 3 shows the FT-IR spectra of GO after reduction by
L-cysteine. For GO, the characteristic peaks for C=O stretching
vibration appear at 1746 cm−1 , and for O–H the stretching and
deformation vibration appear at 3420 cm−1 and 1395 cm−1 ,
3
Nanotechnology 22 (2011) 325601
D Chen et al
The above data suggest that we can produce RGO
nanosheets through the reduction of GO by L-cysteine for 72 h.
TGA measurements provide further proof of the preparation
of RGO. Figure 5 shows the TGA curves for GO and RGO.
On the one hand, GO is thermally unstable. There are two
steps for mass loss with increasing temperature. The first mass
loss is about 10% around 100 ◦ C, which can be attributed to
the removal of adsorbed water. The second mass loss around
200 ◦ C is about 30% because of the decomposition of labile
oxygen functional groups [35]. On the other hand, the removal
of the thermally labile oxygen functional groups by L-cysteine
results in much increased thermal stability for the RGO, and
only 10% of the mass was lost at around 200 ◦ C [10]. This
result is also supported by XPS measurements (figure 6), which
show that the C/O ratio in the GO increases remarkably after
reduction by L-cysteine for 72 h (figure 6(a)). Furthermore,
a significant decrease of oxygenated carbon-related signals at
286–289 eV after reduction (figure 6(b)) reveals that most
of the epoxide, hydroxyl, and carboxyl functional groups are
removed after the reduction.
AFM is a powerful tool to measure the thickness of the
samples. Figure 7 shows the typical AFM height images of GO
and the RGO nanosheets prepared by L-cysteine. The average
thickness of the GO and RGO nanosheets obtained is about
1.0 nm and 0.8 nm respectively. The data match well with the
previous reports [9, 10, 19, 36]. Figure 8 presents TEM images
and a SAED pattern of GO and RGO nanosheets. Figure 8(a)
displays that GO nanosheets are corrugated, but the SAED
pattern (in the inset) indicates a crystalline structure. The
SAED pattern contains information from many GO grains. A
typical sharp, polycrystalline ring pattern is obtained. The first
ring comes from the (1100) plane, and the second ring comes
from the (1120) plane. Strong diffraction spots are observed
on the ring. These results imply that the GO nanosheets
before reduction are not randomly oriented with respect to one
another, and the inter-layered coherence is not destroyed at this
stage [37]. The TEM image of the RGO nanosheets shows a
wrinkled paper-like structure. The SAED pattern in the inset
of figure 8(b) shows a typical sharp polycrystalline ring pattern
composed of many diffraction spots, indicating the loss of long
range ordering between the RGO nanosheets.
Figure 4. XRD spectra of graphite (a), GO before (b) and after
reduction by L-cysteine for different reduction times 12 h (c),
24 h (d), 48 h (e), and 72 (f).
Figure 5. Normalized TGA plots for GO (a) and RGO (b).
in these samples is very broad suggests that the samples are
very poorly ordered along the stacking direction. It indicates
that these samples comprise largely free RGO nanosheets
(figure 4(f)) [34].
Figure 6. XPS spectra of GO and RGO, survey scan (a); C1S (b). The Na signal in RGO is attributed to residual NaOH.
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Nanotechnology 22 (2011) 325601
D Chen et al
Figure 7. AFM images and section analysis of GO (a) and RGO (b) nanosheets absorbed on freshly cleaved mica.
Figure 8. TEM image and a typical SAED pattern (inset) of the GO (a) and RGO (b) nanosheets.
To determine the extent to which the chemical reduction
of exfoliated GO nanosheets restores the electrical properties
of the graphitic network, we measured the room-temperature
electrical conductivities of compressed-powder samples of GO
and the RGO via L-cysteine for 72 h. The conductivity of
compressed-powder samples of GO is 7.97 × 10−7 S m−1 ,
while the RGO is as high as 0.124 S m−1 . It reveals that the
conductivity of RGO by L-cysteine increases about 106 times
in comparison to that of GO. Therefore, it indicates that RGO
produced in this way has unique electrical properties that are
the same as those produced via other methods [20].
The mechanism for the chemical reduction of GO is still
an open question. A possible mechanism for the chemical
reduction of GO by L-cysteine is shown in scheme 1. We
speculate that it should be a two-step SN2 nucleophilic reaction
followed by a thermal elimination. L-cysteine is an amino
acid which contains a thiol group. The thiol is susceptible
to oxidization and ready to release a proton, functioning as
a nucleophile [24]. The proton has commonly high binding
affinity to the oxygen-containing groups, such as hydroxyl
and epoxide groups on GO to form H2 O molecules [19].
Finally, the L-cysteine is oxidized into L-cystine, leading
to the formation of RGO nanosheets. Actually, the XRD
spectra (figure S2 available at stacks.iop.org/Nano/22/325601/
mmedia) of the RGO sample without any washing also
confirms the above deduction.
It is well known that the L-cystine is an amino acid
insoluble in water and alcohol. Practically, however, it can
5
Nanotechnology 22 (2011) 325601
D Chen et al
Scheme 1. Possible mechanism of reduction of GO with L-cysteine.
be dissolved in dilute solutions of alkali hydroxides. Hence,
a proper amount of NaOH aqueous solution of 0.1 mol l−1
was added into the product to remove L-cystine. Surprisingly,
the NaOH aqueous solution can also disperse the precipitation
of the RGO nanosheets for being less hydrophilic as a result
of oxygen removal. With the aid of ultrasonication, we
can prepare well dispersed RGO nanosheets in an aqueous
solution of pH = 10 (figure S1-b available at stacks.iop.org/
Nano/22/325601/mmedia). This may be attributed to residual
neutral carboxylic groups being converted to negatively
charged carboxyl groups in alkaline solution [9, 38], and the
electrostatic repulsion makes the RGO nanosheets’ dispersions
stable. The presence of the remaining carboxyl groups on RGO
nanosheets indicates the ineffective and incomplete reduction
which greatly compromises their conductivity. Actually, the
conductivity reported here is not high. Although a few of
the RGO nanosheets were precipitated after two weeks, we
still observed other RGO nanosheets dispersed well at the
top of liquid. Furthermore, the RGO nanosheets also can
be dispersed well in N,N -dimethylacetamide (figure S1-c
available at stacks.iop.org/Nano/22/325601/mmedia). These
results indicate that we can prepare uniformly dispersed and
conductive RGO nanosheets through gentle reduction of GO
by L-cysteine without any stabilizer.
Acknowledgments
This work was supported by the National Basic Research
Program of China (2010CB934700) and the National Natural
Science Foundation of China (50725208, 20973019).
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In summary, we have introduced an environmentally friendly
approach for the reduction of GO by using L-cysteine as the
reductant. During the reduction of GO, the thiol group of
L-cysteine releases protons, and the protons commonly have
high binding affinity to the oxygen-containing groups of GO,
such as hydroxyl and epoxide groups, to form water molecules.
This is the first report about the use of amino acid as reductant
for the preparation of RGO sheets. The conductivity of RGO
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with that of GO. Furthermore, this reduction method avoids the
use of any toxic reagents, and both reductant and the coproduct
during the reduction of GO are biocompatible. This procedure
offers an alternative route to large-scale RGO production for
applications that require such material.
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