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Graphene
The Superior Dispersion of Easily Soluble Graphite**
Jong Hak Lee, Dong Wook Shin, Victor G. Makotchenko, Albert S. Nazarov, Vladimir
E. Fedorov, Jin Hyoung Yoo, Seong Man Yu, Jae-Young Choi, Jong Min Kim, and
Ji-Beom Yoo*
Graphene is a 2D one-atom-thick layer that has attracted
enormous scientific attention on account of its extraordinary
electronic and mechanical properties resulting from the
hexagonally arrayed sp2-hybridized carbon atom structure.[1,2]
Several efforts have been made to use graphene in devices and
composites.[3,4] The strong covalent bonds of carbon atoms
provide high mechanical and thermal properties and chemical
stability. The theoretical Young’s modulus of graphene is
approximately 1060 GPa.[5] In addition, the high electrical
conductivity (mobility: 20 000 cm2 V1 s1, velocity: c/300)
through the p-electron cloud makes graphene a promising
material in conducting composites and quantum electronics.[6–8]
As in other new materials (such as carbon nanotubes,
various quantum dots, etc.), the development of mass
production methods that can support the demand for graphene
in a variety of large-scale applications is of high priority. Thus
far, several methods for graphene production have been
developed and can be summarized into three areas: mechanical
exfoliation,[9,10] graphene in solution,[11,12] and epitaxial
growth.[13,14] Mechanical exfoliation produces the highest
quality graphene, which is suitable for fundamental studies.
Epitaxial growth provides the shortest path to graphene-based
electronic circuits. Graphene in solution can offer lower costs
and higher throughputs and may be used in a wide range of
applications because of the significant practical utility of this
material. However, the method is quite complicated and
[] Prof. J.-B. Yoo, J. H. Yoo
School of Advanced Materials Science & Engineering (BK21)
Sungkyunkwan University
Suwon 440-746 (Republic of Korea)
E-mail: jbyoo@skku.edu
J. H. Lee, D. W. Shin, S. M. Yu
SKKU Advanced Institute of Nanotechnology (SAINT)
Sungkyunkwan University
Suwon 440-746 (Republic of Korea)
V. G. Makotchenko, A. S. Nazarov, Prof. V. E. Fedorov
Nikolaev Institute of Inorganic Chemistry
Siberian Branch of Russian Academy of Sciences 3
Acad. Lavrentiev prospect
Novosibirsk, 630090 (Russia)
Dr. J.-Y. Choi, Dr. J. M. Kim
Samsung Advanced Institute of Technology
Yongin, Gyeonggi, 446-712 (Republic of Korea)
[] This research was funded by the BK21 Project of the School of
Advanced Materials Science & Engineering and Seoul Fellowship
to J.H.Lee.
DOI: 10.1002/smll.200901556
58
involves personnel-dependant procedures due to the requirement of functionalization and reduction processes. Thus far, the
most commonly used process for graphene solutions is
Hummer’s method,[15] which can produce well-dispersed
graphene oxide (GO) in water due to its hydrophilic properties.
This method involves the oxidation of graphite in the presence
of strong acids and oxidants. The level of oxidation can be
varied according to the reaction conditions and chemicals.
Recently, some researchers have reported that as-prepared
graphite oxide can be dispersed in an organic solvent such as
N,N-dimethylformamide (DMF) and N-methylpyrrolidone
(NMP) without additional chemical functionalization.[16]
However, the as-prepared GO is an electrical insulator and
therefore additional chemical reduction processes are necessary to recover its conductivity. In addition, reduced GO still
exhibits lower conductivity than pristine graphene.[17] In order
to circumvent the oxidation of graphene, some researchers
have introduced different re-intercalation or liquid-phaseexfoliation methods.[18,19] However, these methods are quite
complicated with long processing times or low yields of normal
expanded graphite in organic solvents. The production of stable
suspensions of graphene in various solvents is important in
the fabrication of graphene-based devices and composites.
Previously the preparation of easily soluble expanded graphite
(ESEG) has been reported,[20] which can be dispersed in water
with sodium dodecylbenzene sulfonate (SDBS) and NMP by
ultrasonication. Most sheets were composed of a few layers and
were well-dispersed with long-term stability in a manner
comparable to that of GO dispersions in water. In addition, due
to the absence of an oxidation state, the graphene sheet
obtained from the stable suspension showed high conductivity
compared to that produced by other solution-based methods
without any reduction process. This study examines the
dispersion behavior of easily soluble expanded graphite in a
variety of organic solvents and surfactants in water. The results
are expected to expand the applications of graphene–polymer
composites or the development of graphene-based electrical
devices.
ESEG was prepared from a fluorinated graphite intercalation compound (FGIC), C2F nClF3, containing inorganic
volatile intercalating agent ClF3 as reported elsewhere.[20]
FGIC was synthesized using the single intercalation process
with ClF3. Pure natural graphite (5 g) was added to cooled
ClF3 in a Teflon reactor for intercalation and fluorine
functionalization for 5 h. After intercalation excess ClF3 was
removed and the intercalation product with a composition of
C2F 0.13ClF3 was obtained.
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small 2010, 6, No. 1, 58–62
dimethylacetamide (DMAc), and n-hexane
without a surfactant using a tip (cup-horn
type) sonication instrument for 30 min. The
as-prepared graphene suspensions, except
that with hexane, had well-dispersed states
(Figure 3, top). A few droplets of SDS in
water, DEG, and DMAc were dropped
onto the transmission electron microscopy
(TEM) grid after the sonication process to
measure the number of graphene layers per
sheet. In a previous paper, it was demonstrated that the yield of a few layers of
graphene in NMP and SDBS in water
[20]
Figure 1. Photographs of (a) volume states of ESEG (right) and commercial EG (left). b) Schematic suspensions was approximately 90%.
Figure 4 shows TEM images of the dispersed
diagram of the structure of a FGIC and thermal decomposition molecules.
ESEG in DMAc and histograms of the
number of graphene units in a sheet of SDS
Figure 1a shows the expanded volumes of commercial in water, DEG, and DMAc. The yields of a few layers of
expanded graphite (EG) and ESEG. Commercial EG (50 mg) graphene (<5 layers) were approximately 93%, 87%, and 93%,
and FGIC was placed into the bottom of a quarts tube, as respectively. However, the yields of one layer of graphene
shown in inset of Figure 1. The samples were then placed into a were approximately 17%, 7%, and 23%, which are lower than
furnace at 700 8C for a few minutes in N2 under ambient the 32% obtained for NMP. NMP has been reported to be
conditions. During the ‘‘thermal shock’’ process volume efficient in dispersing graphene because the energy required to
expansion occurred, which is explosive and accompanied exfoliate graphene is balanced by the NMP–graphene interby a flash and sound. As shown in Figure 1a, ESEG showed action, which means the surface energies match that of
higher volume expansion than commercial EG (approximately graphene.[18] In this study, the NMP suspension had the most
double that of EG). Figure 1b shows the intercalated and well-dispersed state compared to any other solution, as
functionalized graphite structure of FGIC. The main difference reported elsewhere.
in the expanded state of EG and ESEG is the thermal
decomposition molecules. In the case of EG, expansion occurs
only as a result of the rapid vaporization of volatile intercalated
substances, R (the regime of ‘‘thermal shock’’). In the case of
the ESEG, expansion of the graphite layers occurs as a result of
both the rapid increase in the vapor pressure of the volatile
intercalated substances and the formation of gaseous fluorocarbons and other gaseous products due to interaction
between the carbon matrix and fluorine atoms. Finally,
ESEG has a bulk density and specific surface of 1.3–1.8 kg m3
and 250–280 m2 g1, respectively.[20] The decomposition
process is as follows:
C2 F nClF3 ðsolidÞ ! C ðsolidÞ þ Cl2 þ Cx Fy þ Cx Fy Clz
(1)
Figure 2 shows a scanning electron microscopy (SEM)
image of the worm-like ESEG structure and the Raman
spectroscope result of ESEG, which indicates high volume
expansion and a partially destroyed graphene structure in
exfoliated graphite. In particular, in the 2D peak of ESEG, a
red-shift trend was dominant due to the larger space between
the two graphenes as compared to that of ordinary expanded
graphite.[20] This severe expansion process provides the
material properties essential for easy dispersion. This method
can be used in the low-cost mass production of graphene for a
variety of applications because of its simplicity and short
processing time.
To examine the solubility of ESEG, 0.5 mg of ESEG was
dispersed in 50 mL aqueous solutions of both SDBS and sodium
dodecyl sulfate (SDS) and 50 mL solutions of diethylene
glycol (DEG), DMF, NMP, dichloromethane (DCM), toluene,
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Figure 2. a) SEM image of ESEG and b) Raman results of ESEG and EG.
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properties. However, the long term stabilities of SDS and SDBS are quite different.
Even the yield of a few layers of graphene
from a just-sonicated graphene suspension
in SDS was approximately 93%. The
difference in molecule composition between
SDS and SDBS is the additional benzene
ring of SDBS, which means that the dispersion mechanisms and early states of SDS and
SDBS suspensions are similar. However, the
steric hindrance effect of the additional
benzene ring is a critical factor for the longterm stability of a graphene dispersion.
Using the suspensions after 3 weeks, the
dispersibility of each solvent was examined
Figure 3. Digital images of the as-prepared ESEG dispersed in aqueous SDS and SDBS
from the relationship between the absorsolutions and seven organic solvents. Top: suspensions immediately after sonication. Bottom: bance (A) and concentration (c). According
3 weeks after sonication.
to the Lambert–Beer law,[22] A of the
suspension is proportional to the absorption
coefficient (a), the cell length (l), and c:
A ¼ alc. This means that if c can be obtained
for one suspension, c can be determined for
the other suspensions because a and l are
constant. This method is quite useful for
well-dispersed suspensions with no sedimentation. However, there is usually some
sediment in carbon nanotubes or graphene.
Therefore, it is very difficult to measure
c because of the uncertainty of measuring
the amount of sediment. Although some
researchers use a filtration method, which
compares the weight difference between the
clean and filtered membrane, the method is
quite risky because only 0.5 mg of graphene
was used and some surfactant or solvent still
remained. Fortunately, there was no sediment in the graphene suspension in NMP
even after 3 weeks. It could be assumed that
the graphene concentration of the suspension was 10 mg mL1. Before using the
Figure 4. a) TEM and atomic force microscopy (AFM) images of one, two, and a few layers of Lambert–Beer law, it is important to confirm
graphene from the DMAc suspension. The scale bar of the AFM image is 1 mm. b) The number of
the uniform dispersion of the suspensions.
graphene layers of DEG, SDS, and DMAc.
First, the baseline was made using pure
solvents for each solvent and the quartz cell
The bottom part of Figure 3 shows the long-term stability of filled by a mixture of the graphene suspension and pure solvent
ESEG in various solvents. After 3 weeks, NMP, DMAc, DMF, at different ratios such as 1:2, 2:1, and 3:0. Figure 5b shows the
SDBS in water, DEG, and toluene still showed a very good Lambert–Beer behavior and different slopes of each suspendispersion state, whereas n-hexane, SDS, and DCM had low sion, which means that each solvent has a uniform dispersion
dispersibility. Interestingly, there was a significant difference and different dispersibility. The dispersibility of each suspenbetween the SDS and SDBS solutions. SDS and SDBS are well- sion was calculated using A for the pure graphene suspension in
known and commonly used surfactants for dispersing carbon NMP and the other suspensions and the Lambert–Beer law.
nanotubes due to their amphiphilic properties.[21] The hydro- After 3 weeks, the NMP, DMAc, DEG, toluene, DMF, SDBS,
phobic tails of SDS and SDBS molecules interact with the DCM, and SDS suspensions gave A ¼ 0.629, 0.565, 0.458, 0.38,
hydrophobic surface of the nanotubes and the hydrophilic 0.315, 0.238, 0.092, and 0.012 and dispersibilities of 10, 8.98, 7.28,
moieties face outward. The dispersion mechanism of ESEG 6.04, 5.0, 3.78, 1.46, and 0.19 mg mL1, respectively.
using SDS or SDBS surfactants should be similar to the carbon
Using the graphene suspension in NMP, transparent
nanotube case because the lattice structures of graphene and conducting films (TCFs) were made in order to evaluate their
carbon nanotubes are basically the same with a hexagonally use as a transparent electrodes using a filtration and wet transfer
arranged carbon structure and both have hydrophobic method.[23] Figure 6a shows SEM images of the filtered ESEG
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Figure 5. Optical characterization of the ESEG dispersions.
a) Transmittance spectra of ESEG dispersed in various solvents. b) Optical
absorbance slopes at excitation wavelength 660 nm as a function of the
graphene concentration in the each solvent showing Lambert–Beer
behavior. c) Calculated solubility of each graphene suspension using the
Lambert–Beer law.
film and a piece of ESEG (inset). After conformal contact
between the filtrated ESEG film and polyethylene terephthalate (PET), lateral transfer of the graphene film occurred as a
result of water flow through the filter channels in the deionized
water bath. The film thickness was controlled simply by varying
the filtration volume of the suspensions. As shown in Figure 6c,
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Figure 6. a) SEM image of the as-prepared graphene film on an anodized
aluminum oxide (AAO) membrane. Inset: a piece of graphene on an AAO
membrane.b) Digital image of the transparent conducting flexiblegraphene
films on PET. c) Sheet resistance and transparency of the NMP solution.
the sheet resistance of the ESEG film was measured and
compared with those of the as-made and acid-treated films.
After a nitric acid treatment for a few minutes, the remaining
organic material was removed. The obtained multilayered
flexible graphene films on the PET substrates had sheet
resistances of 2320, 39.3, 8.5, and 4.23 kV sq1 at room
temperature and transparencies (defined as transmittance at
a wavelength of 550 nm) of 88%, 84.4%, 78.5%, and 72.6%,
respectively.
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In summary, ESEG was synthesized by a one-step
exfoliation process using FGIC. Due to the severe expansion
state, the obtained ESEG was sufficiently dispersed in an
aqueous solution using an ordinary surfactant and various
organic solvents. NMP, DMAc, DEG, toluene, DMF, SDBS,
DCM, and SDS had dispersibilities of 10, 8.98, 7.28, 6.04, 5.0,
3.78, 1.46, and 0.19 mg mL1, respectively. A sheet resistance
of 4.23 kV sq1 and a transparency of approximately 72%
were obtained using filtration-transfer and acid-treatment
methods. This one-step exfoliation process can allow the lowcost mass production of graphene because of the very simple
procedure and short processing time. In addition, welldispersed graphene in water and organic solutions have
potential use in high-performance, scalable graphene-based
applications.
Experimental Section
FGIC production: Step 1: Synthesis of the FGIC C2F nClF3. A
Teflon reactor was filled with 30 g of liquid ClF3 and cooled with
liquid nitrogen. Pure natural graphite (5 g; ash content
<0.05 mass%, particle size ¼ 200–300 mm; Zaval’evsk coal field,
Ukraine) was added to the cooled ClF3. The reactor was then
hermetically sealed. The temperature was increased slowly to
22 8C and kept at that temperature for 5 h. The excess ClF3 was
removed in a nickel vessel cooled with liquid nitrogen until a
constant mass was measured. The intercalation product (approximately 11 g) had an approximate composition of C2F 0.13ClF3.
Elemental analysis data (mass%): C 44.22; F 44.79; Cl 12.49. Step
2: Synthesis of ESEG. The intercalation compound, C2F 0.13ClF3
(200 mg), was thermally decomposed in a quartz reactor with
volume 500 mL and heated to 600–700 8C. After 4 min, the
compound was decomposed using a thermal shock process and
ESEG filled the larger volume of the reactor. Elemental analysis
(mass%): C 94.1; F 3.0; Cl 1.9; H 1.0.
Dispersion, film formation, and transfer process: ESEG
(0.5 mg) was dispersed in 50 mL of 1% SDBS, 2% SDS dissolved
water, DEG, DMF, NMP, DCM, toluene, DMAc, or n-hexane, and
sonicated using a tip (bar-type) sonication instrument at 400 W for
30 min. A few droplets of SDS in water, DEG, and DMAc were
added to the Cu micro-TEM grid to measure the number of
graphene layers per sheet. After 3 weeks, A was measured for
each suspension. After making a baseline with each pure solvent,
the quartz cell was filled with the graphene suspension and pure
solvent with different concentrations such as 1:2, 2:1, and 3:0. A
uniform flexible graphene film was obtained by vacuum filtration
using an anodic membrane (Whatman International Ltd.) and
transferred to a PET film. The transmittance and sheet resistance
of the obtained film were examined by UV–Vis spectroscopy and a
4-point probe, respectively.
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dispersions . exfoliation . graphene . graphite . solvent
effects
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Received: August 19, 2009
Revised: September 29, 2009
Published online: November 18, 2009
small 2010, 6, No. 1, 58–62