Dispersion of graphene in ethanol using a simple solvent exchange method

Article abstract of DOI:10.1039/c0cc02688c

A dispersion of graphene in ethanol was achieved using solvent exchange from N-methyl-2-pyrrolidone (NMP) that enables broader application of dispersed graphene.

Full text of DOI:10.1039/c0cc02688c

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Dispersion of graphene in ethanol using a simple solvent exchange
methodw
Xiaoyan Zhang,ab Anthony C. Coleman,b Nathalie Katsonis,a Wesley R. Browne,*a
Bart J. van Wees*b and Ben L. Feringa*a
Received 20th July 2010, Accepted 23rd August 2010
DOI: 10.1039/c0cc02688c
A dispersion of graphene in ethanol was achieved using solvent
exchange from N-methyl-2-pyrrolidone (NMP) that enables
broader application of dispersed graphene.
surface deposition, low boiling point solvents are preferred.
However, most low boiling solvents, for example, water and
ethanol, have surface tension (72.8 mJ mÀ2 and 22.1 mJ mÀ2
,
respectively) that are unsuitable for the direct exfoliation of
graphene. Therefore, a more suitable method is required.
Coleman and coworkers recently reported that graphene
could be prepared by exfoliation of graphite in a surfactant/
water solution, to yield large amounts of few-layer graphene
sheets and small quantities of single-layer graphene sheet with
the aid of the surfactant.16
Since its discovery by Geim and coworkers in 2004,1 graphene
has attracted widespread attention due to its unique properties,
such as room temperature quantum effects, ambipolar electric
field effects, and high carrier mobility.2 Graphene of less than
10 layers shows properties distinct from those of bulk
graphite3 and hence single- and few-layer graphenes are highly
attractive materials for electronics, composites and devices. As
with carbon nanotubes, a major problem encountered is the
preparation of graphene on a large scale. To date, several
methods have been reported, including mechanical,1 epitaxial,4,5
reduction of graphene oxide,6–10 and solvent dispersion of
graphite.11–14 Though the mechanical method can yield high
quality single-layer graphene sheets, it is time consuming and
features low throughput, which makes it unsuitable for large
scale production. There are also reports on epitaxial growth of
graphene on metal substrates,4,5 but this process requires very
high temperature and subsequent transfer of the sample to
other substrates. Compared with these methods, the latter two
methods have obvious advantages, such as scalable production
and ease of solution processing. For example, chemical or
thermal treatment of graphene oxide provides a material that
is conductive and can be used in device applications.9,10
However, the reduced graphene oxide still contains defects
and the original properties of graphene are not recovered.15
Recently, several groups have shown that graphene can be
prepared by exfoliation of graphite in certain solvents, such as
Here, we report the discovery that graphene can form a
relatively stable high concentration dispersion in ethanol using
a solvent exchange process from NMP. The preparation of the
graphene dispersion in ethanol is confirmed by spectroscopic
and microscopic techniques, and the generated film shows
good conductivity.
The preparation of the graphene dispersion in ethanol is
shown in Scheme 1. Approximately, 200 mg of graphite in
200 ml NMP was sonicated for 2 h, and centrifuged at
4000 rpm for 30 min to remove large particles. The supernatant
of graphene in NMP was decanted, and then 150 ml of the
above supernatant was filtered through a 0.45 mm poly-
tetrafluoroethylene (PTFE) membrane. The obtained filter
cake was dispersed in 50 ml ethanol with mild sonication
and filtered again. This process was repeated five times (each
time with 50 ml ethanol and ca. 10 min of sonication) and
finally the filter cake was redispersed in 30 ml ethanol. This
suspension was centrifuged at 1000 rpm for 30 min, the
supernatant was decanted and further sonicated for several
minutes to give the required homogeneous graphene dispersion
in ethanol (the sample vial was sealed to prevent evaporation
of ethanol). The repeated washing steps yield a stable dispersion
of graphene in ethanol containing o0.3 vol% NMP (see ESIw,
Fig. S1).
N-methyl-2-pyrrolidone
(NMP),11
dimethylformamide
(DMF),12 and o-dichlorobenzene (ODCB)13 to produce single-
and few-layer graphene sheets. These above solvents have
surface tensions close to 40 mJ mÀ2, and are suitable for direct
exfoliation of graphene.11 However, these solvents hold their
own disadvantages. Their high boiling points limit their
viability for real manipulation. Thus, dispersion of graphene
in low boiling solvents without surfactants or other stabilizers
is useful in many applications. For example, in composites and
Using the solvent exchange process, graphene can form a
relatively stable dispersion in ethanol, at a concentration
of up to 0.04 mg mlÀ1 (Fig. 1a, right). Fig. 1b shows the
absorption spectrum of graphene in ethanol after dilution
with ethanol, which is flat and featureless. Over one week,
a Stratingh Institute for Chemistry, University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands.
E-mail: w.r.browne@rug.nl, b.l.feringa@rug.nl;
Fax: +31-050-363-4296
b Physics of Nanodevices, Zernike Institute for Advanced Materials,
University of Groningen, The Netherlands.
E-mail: b.j.van.wees@rug.nl
w Electronic supplementary information (ESI) available: Details of
experimental methods and additional TEM, IR data and determination
of residual NMP content. See DOI: 10.1039/c0cc02688c
Scheme 1 Procedure for the preparation of graphene in ethanol:
(a) sonicate for 2 h, (b) centrifuge at 4000 rpm/30 min, (c) filter,
redisperse the filter cake in ethanol, and repeat this process 5 times,
(d) redisperse the filter cake in ethanol, centrifuge at 1000 rpm/30 min,
then pipette the supernatant and sonicate for several minutes.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7539–7541 7539

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Fig. 1 (a) Direct sonication of graphite in ethanol results in complete
precipitation after 4 h (left) while a stable graphene dispersion
in ethanol is obtained using the solvent exchange process (right).
(b) UV/Vis absorption spectrum of graphene in ethanol after dilution
with ethanol.
the sample shows at most 20% sedimentation (as determined
by UV absorption). A graphene dispersion in NMP diluted
with water (NMP : water = 1 : 99) shows 25% sedimentation
over 160 h,17 which is slightly higher than graphene in ethanol.
In addition, the sedimentation in ethanol can be redispersed by
mild sonication. By contrast, as a control, graphite which
was directly sonicated in ethanol for several hours, showed
complete precipitation on standing for 4 h (Fig. 1a, left).
Transmission electron microscopy (TEM) was performed
to investigate the quality and exfoliation behaviour of the
graphene sheets obtained in ethanol. Fig. 2a shows a single-
layer graphene sheet, which is confirmed by the corresponding
electron diffraction pattern (Fig. 2b). The inner intensity is
stronger than the outer intensity, indicating it is a single-layer
graphene sheet.11 By checking the edge of the sheets, some
bi-layer graphene sheets were also found (Fig. 2c). In addition,
a large number of few-layer graphene sheets were observed, as
shown in Fig. 2d. There are also some graphene nanoribbons
in the graphene dispersion of ethanol (see ESIw, Fig. S2). This
indicates that graphene in ethanol predominantly consists of
single- and few-layer graphene sheets.
Fig. 3 AFM image of graphene from ethanol by drop casting onto
freshly cleaved mica. The scan area is 2.0 Â 2.0 mm.
The exfoliation of graphene in ethanol is confirmed by
tapping mode atomic force microscopy (AFM). The sample
was prepared by drop casting the graphene dispersion in
ethanol onto a hot mica surface to enable rapid evaporation
of ethanol. Fig. 3 shows a large graphene flake composed of a
single layer or bilayer graphene sheet. This assignment is based
on the sheet height of ca. 1.3 nm, which is in agreement with
the height of 1–2 nm observed elsewhere for single-layer or
bi-layer graphene sheets.11,16 Analysis of the height of the
flakes indicates that most of the graphene sheets are below 10 nm.
Raman spectroscopy was employed to analyze the quality of
the graphene sheets obtained using this solvent exchange
method. The most prominent Raman features for graphitic
materials are: a defect-induced D band at 1350 cmÀ1, an
in-plane vibration of sp2 carbon at 1580 cmÀ1 (G band), and
a two phonon double resonance process at about 2700 cmÀ1
(2D band).18,19 As shown in Fig. 4, the D band of graphite is
weak, while graphene shows a small D band. The relatively
low intensity of the D band indicates that the graphene flakes
contain few defects. According to Coleman and coworkers,16
these defects are predominantly located at the edges of the
graphene flakes, and the basal plane of the flakes are relatively
defect free. In the present case, the small size of the flakes
precludes spatial mapping of the defects and hence identification
of their location on the edge or in the basal plane. Additional
evidence of the low defect content is also confirmed by FTIR
spectroscopy (Fig. S3, ESIw). The spectrum of graphene flakes
from ethanol is almost featureless, confirming the low content
of defects in the graphene sample.
The conductivity of the film from graphene in ethanol
was also determined using the standard four-point probe
method. The film was made by vacuum filtration of graphene
dispersion in ethanol. The film shows conductivity of 1130 S mÀ1
after drying under vacuum at room temperature for 24 h
(without annealing). This value is comparable with the reported
Fig. 2 TEM images of a single-layer graphene sheet (a), the corres-
ponding electron diffraction pattern of the sheet in (b), a bi-layer
graphene sheet (c), and few-layer graphene sheets (d).
c
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The Zernike Institute for Advanced Materials (XYZ, ACC),
NWO-Veni (NK) and NWO-Vidi (WRB) are acknowledged
for financial support.
Notes and references
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Fig. 4 Raman spectra of graphene from ethanol dispersion (solid
line) and graphite (dotted line).
conductivity value (1500 S mÀ1) of an annealed graphene film
(250 1C in Ar/N2) from the surfactant/water system,16 and also
the conductivity value (1200 S mÀ1) of graphene powder using
the wet ball milling exfoliation method.20
The same solvent exchange process was also applied using
other solvents, such as methanol, dichloromethane (DCM)
and toluene. However, graphene was found to completely
precipitate after the final centrifugation process (1000 rpm/
30 min) in these three solvents (see ESIw, Fig. S4). The reason
for the difference with ethanol is rationalized as follows. NMP
is a good solvent for the exfoliation of graphene, which
produces single- and few-layer graphene sheets.11,17 After the
filtration process, the filter cake still contains single- and
few-layer graphene flakes. During the solvent exchange
process, the above materials were redispersed into the corres-
ponding solvents. However, the interactions of graphene
with different solvents are not the same, so the dispersion
behaviours in these solvents are different. Ethanol shows better
dispersion ability and stability than the other three solvents.
However, the stability mechanism in ethanol is still not clear, and
needs further study. Importantly it should be noted that direct
exfoliation in ethanol leads to much reduced stability compared
with initial exfoliation in NMP followed by solvent exchange
with ethanol. This indicates that the solvent properties that are
optimum for initial exfoliation are not necessarily the same as
that required to maintain the dispersion.
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In summary, we present a simple solvent exchange method
to prepare graphene in ethanol, which consists of single- and
few-layer graphene sheets. The dispersion is relatively stable
and the small amount of sedimentation can be redispersed by
mild sonication. The graphene sheets contain few defects and
the conductivity of the film is 1130 S mÀ1 after drying under
vacuum at room temperature for 24 h. Considering the low
boiling point of ethanol, this method will be very useful in the
preparation of composites, surface deposition and fabrication
of graphene based materials, and will help to find other low
boiling point solvents to disperse graphene.
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c
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Chem. Commun., 2010, 46, 7539–7541 7541