CARBON
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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Synthesis of graphene paper from pyrolyzed asphalt
I. Francis Cheng a,*, Yuqun Xie a, R. Allen Gonzales a, Przemysław R. Brejna a,
Jency Pricilla Sundararajan b, B.A. Fouetio Kengne b, D. Eric Aston c,
David N. McIlroy b, Jeremy D. Foutch a, Peter R. Griffiths a
a
b
c
Department of Chemistry, University of Idaho, Moscow, ID 83844-2343, USA
Department of Physics, University of Idaho, Moscow, ID 83844-0903, USA
Department of Chemical & Materials Engineering, University of Idaho, Moscow, ID 83844-1021, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
A new technique for the synthesis of large sheets (>10 cm2) of multi-layered graphene is
Received 14 October 2010
presented. The condensation onto a heated surface (650 C) of fumes from the thermal
Accepted 2 March 2011
decomposition of asphalt in a ceramic crucible produces carbon films with a metallic
Available online 12 March 2011
sheen. Heating was done by a Fisher burner (natural gas/air) flame and the crucible was
covered but exposed to laboratory atmosphere. These films were determined to be multilayered graphene by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), Raman and infrared spectroscopy and X-ray diffraction. XPS
indicates that the films are primarily sp2 hybridized carbon with small amounts of sp3
C–H and C–O or C–N functionalities. Based on the D band shift (1593 cm1) and the ratio
of D band to G band (1354 cm1) of 0.93, the Raman spectrum also indicates that the material is sp2 C with some nanocrystalline features. The infrared spectrum exhibits A1U
(868 cm1) and E1U (1599 cm1) stretching of the intralayer bonds of graphene. This form
of chemical vapor deposition may be a scalable to give much larger surface areas. Furthermore, the process does not require metal substrates. Deposition onto silica nanosprings
and diatomites is demonstrated.
2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene has attracted much interest based on its observed
and calculated properties [1,2]. This two-dimensional material is an extended monolayer of graphite. In a multilayered
form it is referred to as graphene paper. This material, along
with graphene oxide paper, has also garnered much interest
in terms of its mechanical and electrical properties and as a
precursor to monolayer graphene [3–12]. Application for such
material ranges from use in Li ion batteries, supercapacitors,
optically transparent electrodes and as catalyst supports
[5,6,8,11,12]. Furthermore, graphene paper has been demonstrated to have thermal stability, and strong mechanical properties [3]. However, to exploit such opportunities a spectrum
of synthetic routes will be required. Present methods include
chemical oxidation of graphite to exfoliated aqueous graphite
oxide (GO) followed by reduction with hydrazine in the presence of surfactants [10,13–17]. This is followed by the assembly of graphene paper (GP) by filtration. Another GO
dispersion and reduction technique enhances the stabilization of aqueous colloids without surfactants [11,18–20]. Such
techniques are successful at creating large sheets of graphene
paper; however, they may prove problematic at depositing GP
onto templates, and producing nano- and micro-structures.
We demonstrate an alternative to the filtration techniques.
In the procedure that we report in this paper, tars are pyrolyzed, producing fumes that condense and rearrange into layers of graphene on heated surfaces. In this thermolyzed
* Corresponding author: Fax: +1 208 885 6173.
E-mail address: [email protected] (I.F. Cheng).
0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2011.03.020
CARBON
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asphalt reaction (TAR) no precautions were taken to exclude
oxygen from the reaction and the temperatures required for
deposition are estimated to be about 650 C. This method offers the possibilities of depositing very large sheets of GP onto
nano- and micro- scale structures and through fabrication
masks.
2.
Experimental
2.1.
Chemicals
The graphene precursor was plastic roof cement (Ace
Hardware), which consists primarily of asphalt but included
mineral spirits, clay, cellulose and water. Diatomites, which
are commonly used for the filtration of swimming pools, were
also obtained from Ace Hardware. The silicon wafer was
obtained from University Wafer (Boston, MA). It was of 111
orientations with a 300-nm layer of thermal oxide, and
resistivity of 0.001–0.002 ohm cm. Silica nanosprings were
synthesized as described by previous procedures [21].
2.2.
Reaction vessel and conditions
The reaction vessel was a 60 mL (70 mm) Coors casserole
crucible with an inner 5 mL crucible holding the starting
material. The inner crucible was filled with 5 g of asphalt
precursor and placed in the larger casserole crucible as
illustrated in Fig. 1. A watch glass covered the top of the
apparatus. For the TAR process, the system was heated for
12–15 min with a Fisher burner followed by cooling for 5–
10 min. The yield of TAR graphene based on the initial mass
of starting material was 0.11% (n = 3, r = 0.08%) on the flat crucible target. This yield is surface area dependant as it was
found to increase to 0.93% (n = 3, r = 0.24%) when 5.0 g of diatomites were added to the apparatus. Various target substrates
were placed on the bottom of the outer crucible, these include
a silicon wafer fragments and diatomites. A silicon wafer
acted as a flat substrate for XPS, AFM, Raman, SEM and optical microscopy studies.
2.3.
2853
Material characterization
All images (see Fig. 2C and D) were produced from a Zeiss
Supra 35 scanning electron microscope (SEM) (Carl Zeiss,
Germany). The samples were produced by depositing graphene
onto a Si wafer followed by cleaving with a diamond glass cutter. Transmission electron microscopy (TEM) observations
were conducted with a JEOL 1200 EX II TEM at 120 kV. For
the micrograph (Fig. 2E) multilayered graphene samples were
prepared by first washing the flakes in ethanol then embedding them into epoxy. The embedded sample was trimmed
with a wire saw, smoothed with a glass blade, then microtomed with a diamond blade perpendicular to the plane of
the graphene sample giving 100 nm sections.
The X-ray photoelectron spectroscopy apparatus was built
in-house at the University of Idaho. X-ray photoelectron spectroscopy (XPS) was performed in a vacuum chamber with a
base pressure of 1 · 1010 torr. Measurements were made with
the Mg Ka emission line (1253 eV) and a hemispherical energy
analyzer with a resolution of 0.025 eV. The samples were
cleaved using cellophane tape prior to insertion into the vacuum chamber. During spectral acquisition the samples were
grounded and exposed to a 500 eV electron beam to eliminate
spurious charging. All spectra were acquired at room
temperature.
The scanning confocal Raman microscope system was a
WITec Alpha300 (WITec Instruments Corp., Ulm, Germany).
The laser excitation wavelength was 532 nm and the optical
magnification at the objective was 20·, producing a spot size
of roughly 2.5 lm in diameter. Spectral scans were taken at 1s integration times with 60 averaged accumulations with a
pixel resolution of approximately 2.4 cm1 for the wide scans.
Post-acquisition data processing provides better than 1 cm1
discrimination, or effective resolution. Various incident
power settings up to roughly 25 mW were used with no instability or transient effects observed in the spectra of the sample. Multiple locations across multiple samples were
analyzed. For IR analysis, a graphene film was deposited onto
a 1.2 cm Ge disk (99.999%, 4 mm thick) (Lattice Materials LLC,
Bozeman, MT) as above. Infrared (IR) spectra were taken in
Fig. 1 – The graphene deposition apparatus.
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Fig. 2 – (A) A photograph of a 25 mm diameter graphene paper flake. (B) An optical micrograph (400·) of exfoliated graphene
paper in water. The bar is 200 lm. (C) 9.45 K · SEM of the cross-sections of folded graphene paper layers on Si. The distance
bar is 1 lm. (D) 23.08 K · SEM of the sample showing the layered characteristics. The bar is 200 nm. (E) A TEM showing the
layered characteristics of the material on the nanometer scale.
transmission with 4 cm1 resolution and 128 scans on a
Nicolet Magna-IR 760 (Nicolet Instrument Corp., Madison,
WI, USA) spectrometer equipped with a deuterated triglycine
sulfate detector.
The atomic scale structure of graphene was obtained using
a Veeco di CP-II atomic force microscope (AFM) operating in
contact mode in air at room temperature. The AFM was operated in low-voltage mode to minimize electronic noise with a
contact force (between cantilever and sample) of approximately 109 N; a 5-lm scanner was used to obtain the images.
The probes were made of non-conductive silicon nitride with
a cantilever spring constant of 0.01 N/m, nominally. Before
the AFM observation, the graphene samples were cleaved in
air to obtain a fresh surface free of secondary contamination.
The topography images were obtained in constant-height
mode where the tip-to-sample spacing is not varied, as typical where molecular or atomic accuracy is desired and at a
scan rate of 15 Hz. Use of this fast scan rate reduces the effects of thermal drift resulting in improved resolution. Micron
scale AFM of graphene was obtained using a different cantilever of spring constant 0.03 N/m. A large area scanner (100-lm)
was used to capture the layers of graphene. Before observation under AFM, the graphene sample was cleaned with IPA
and was air dried to remove any possible contaminations. A
scan rate of 0.1 Hz was used to get an optimal resolution of
step edges.
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2855
Fig. 3 – (Top) A 3D micron-scale AFM of graphene paper demonstrating the flat and layered characteristics of material. Each
division on X–Y axes is 4.0 lm. (Bottom left) A 2D representation of the above. (Bottom right) A height profile analysis of the
above, the two steps of 2.439 and 0.784.6 lm were measured between points A and B, respectively.
X-ray diffraction (XRD) analyses were carried out by
employing a Siemens D5000 Diffractometer (Germany)
equipped with an FK 60-04 air insulated X-ray diffraction tube
with Cu anode, the XRD spectrums were taken with Cu Ka
radiation (1.5406 nm) at 40 kV and 30 mA in the range of
2b = 2–60. The graphite powder (300 mesh) samples were obtained from Johnson Matthey Inc. (Seabrook, New Hampshire).
3.
Results
The TAR procedure outlined above led to the formation of
films with a shiny metallic appearance at the bottom of
the outer crucible shown in Fig. 1. This is consistent with vi-
sual observations regarding GP [2,9,11]. In some cases, the
film was observed to be strongly adhered to the ceramic.
However, if the cooling rate was rapid it was possible to
exfoliate large flakes of GP from the crucible. Fig. 2A shows
a photograph of an approximately 25 mm diameter GP flake.
Much larger flakes are anticipated by simple scale-up of the
apparatus. The visual features of the GP sheets in this work
seem relatively flat when compared to GP produced by filtration methods. The optical micrograph (Fig. 2B) demonstrates
the flat nature of the sample. The flakes were suspended in
water to ease handling; this caused the GP to curl in the
fashion noted in that photograph. Scanning electron micrographs of this material are shown in Fig. 2C and D. The SEMs
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Fig. 4 – (Top) 2.2 · 2.2 nm atomic-scale AFM topography image of graphene in 2D and 3D. (Bottom) Line analysis of topography
showing the periodicity of atomic structure. The inter-atomic distance of 0.1485 matches that of sp2 carbon.
demonstrate the planar nature of the GP sample, with flatness down to approximately 10 nm or less. It appears much
flatter than the SEM’s of other GP materials [2,9,11]. The
material tends to fold back upon itself, hence the two layers
with a thickness of approximately 1 lm each with the bottom one on a silicon substrate (Fig. 2C). The micron-level
layered characteristics are noticeable under higher magnification in Fig. 2D. This layered characteristic is seen with
other SEM of GP and graphene oxide papers samples in the
literature [3,7,22–25]. Under the higher magnification of
TEM (Fig. 2E) layered characteristics are apparent at the
nanometer scale. It is also noteworthy to add that the GP
of this work tended to cleave in flat planes as evident in
the micrographs.
The temperature required for deposition was estimated
based on the melting point of aluminum which is 660 C. When
aluminum foil is introduced into the apparatus of Fig. 1, it was
observed to soften but not melt. We therefore estimate the temperature for graphene deposition to be about 650 C.
3.1.
Characterization by AFM
The most notable feature of the micron scale AFM of the
graphene sample is its flatness (±0.01 lm) which was evident
over several mm2 (not shown). This is in agreement with the
optical micrographs, TEM and SEM’s of Fig. 2. Visual observations indicate that this flat nature extends indefinitely. Fig. 3
demonstrates the step-like features that form when the
material is cleaved. The TEM of Fig. 2E illustrates the same
feature on the nano-scale. These steps are observed by AFM
with materials such as highly ordered pyrolytic graphite
(HOPG) which has graphene layers with low mosaic spread
[26–28].
The atomic-scale AFM image shows the sample topography in 2D and 3D (Fig. 4) obtained in contact mode under
ambient conditions. The image matches the expected hexagonal lattice and interatomic C–C distance (0.148 nm) of graphene and graphite [29–32]. That distance was from line analysis
shown in Fig. 4 (bottom) for a scanning area of
2.2 nm · 2.2 nm and a scan speed of 15 Hz.
Care was taken to ensure that anomalous effects did not
alter the atomic force micrographs [33,34]. These effects include repulsive force associated between the tip and the sample of approximately 1–5 nN. Generally, under ambient
conditions the surface is covered by one or more layers of absorbed water and other low molecular weight airborne contaminants leading to substantial capillary forces pulling the
probe towards the sample with pressures of the order of
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1593
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1354
1300
Int
1290
1280
1270
1260
3000
2000
1000
Relative Wavenumber (cm -1)
Fig. 5 – (Top) Raman spectrum for the graphene material
acquired in ambient air using 532 nm excitation; G mode
peak at 1593 cm1 and D mode peak at 1354 cm1 at
maximum intensity. (Bottom) IR spectrum of graphene on a
germanium disk. The 861 and 1576 cm1 bands correspond
to the A1u and E1u intralayer graphene stretches.
1 GPa. Also, there is often an appearance of atomic structure
without attaining true atomic resolution for materials of planar anisotropy such as HOPG and mica, where the molecular
layers are known to translocate in a corrugated fashion moving in registry with the AFM tip. Imaging can further be complicated by the electrostatic forces between the tip and
sample. Atomic repeat structures in the graphene layer were
obtained by minimizing the tip force with a softer probe of
high resonant frequency operating at a low setpoint. As the
analysis was performed in air, the image is dominated by
the topmost atomic layers.
3.2.
Raman and IR analyses
TAR samples gave two observable and relatively broad bands
in the Raman spectrum centered at 1593 and 1354 cm1
(Fig. 5, top). These two bands are located near to the typical
G (E2g) and D (A1g) peaks, respectively. The G and D bands
are associated with the ordered sp2 carbon, and disordered,
defects and edge carbons, respectively [35]. The wavenumber
positions and relative peak intensities of the D and G bands
(I(D)/I(G) = 0.93) indicate that the carbon in this sample is
nearly all sp2 in hybridization and is graphitic with some
nanocrystalline features (see Fig. 7 of Ref. [35]).
The IR analyses were conducted by depositing graphene
onto germanium and measuring the transmission spectrum.
Fig. 6 – (Top) A wide XPS scan of cleaved graphene. (Bottom)
XPS spectrum of the C 1s core level state of graphene.
The peaks at 284.24, 285.15 and 286.15 eV are assigned to
sp2 C–C, C@N sp2 and sp3 C–O or sp2 C–N, respectively.
Table 1 – XPS peak assignments.
XPS peak
284.24 eV
285.15 eV
286.15 eV
Possible assignment
2
C@C, sp
C@N sp2
C–OH, C–O–C, or C@N sp2
Reference
[45–49]
[45,46,49]
[45–49]
The spectrum appears in Fig. 5. Two bands appear at 868
and 1599 cm1. The position of these bands, respectively,
matches those of the A1U out of plane and E1U stretch of the
intralayer bonds of graphene [36–39]. There are no peaks that
match the expected positions for C–O or C@O stretches (1715–
1740 and 1050 cm1, respectively) [40,41] nor between 3100
and 2800 cm1, which would have indicated the presence of
C–H bonds. Thus the Raman and IR spectra both indicate that
2858
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3.3.
Fig. 7 – XRD patterns of natural graphite (top) and graphene
paper (bottom) product from the TAR process.
the GP is mostly carbon in the sp2 form, and that oxides are
not the predominant form of this specimen.
XPS analysis
Wide XPS scan (Fig. 6) reveals peaks that correspond to C 1s
(284.2 eV), O 1s (533.3 eV), Si 2p (103.7 eV), Si 2s (155.6 eV)
and N 1s (401.2 eV). Given that the samples were cleaved in
air prior to insertion into the vacuum chamber, it is difficult
to ascertain if the O 1s peak is due to adsorbed H2O, CO, or
CO2. In order to resolve the origin of O, i.e., in situ incorporation, or ex situ adsorption, the sample was annealed to
800 C in vacuum. No change in the binding energy or intensity of the O 1s core level state was observed, indicating that
O is due to in situ incorporation into the graphene films. The
presence of broad features between 5 and 20 eV in the valence
band spectrum is attributed to semi-metallic graphite, as opposed to diamond for which these features are much more
pronounced [42]. However, in the case of graphene XPS of
the valence band alone is not sufficient for evaluating the
metallic nature of the samples. The large peak at 25.5 eV is
unassigned. However, Schafer et al. have suggested that the
appearance of O on the surface of graphene the O 2p state
mixes with the graphene valence band resulting in a feature
around a binding energy of 26 eV [43]. This assignment appears to be consistent with the present study when taken in
conjunction with the observation of O incorporated into the
graphene film.
Curve-fitting of the XPS spectrum (Fig. 6, bottom) demonstrates the presence of bands at 284.24, 285.15 and
286.15 eV. The 284.24 eV band agrees well with literature
regarding with the sp2 hybridized carbon–carbon bond. These
include carbon nanotubes (CNT), graphite, graphene and
Fig. 8 – (A) SEM of untreated diatomites at 700· magnification. Au sputter coating was required to prevent charging. (B)
Graphene coated diatomites at 721· magnification. The sample was conductive enough not to require Au sputtering. Also
note that the diatomites retain their pore structure. (C) SEM of the diatomite pore structure after GP deposition at 24.4 · 103·.
(D) Graphene coated silica nanosprings. The distance bar is 100 nm without the requirement for Au sputter coating.
CARBON
4 9 ( 20 1 1 ) 2 8 5 2–28 6 1
reduced graphene oxide [44–49]. The peak at 285.15 eV holds
two possibilities, C–H sp3, as has been noted with graphite,
or C@N sp2, as has been suggested for nitrogen doped graphene and CNT [45,46,49]. The latter is worthy of consideration
based on the appearance of the N 1s peak (401.2 eV) in the
wide scan XPS of Fig. 6 and the lack of C-H stretches in the
IR spectrum. The 286.15 eV peak is associated with C-O sp3,
or other forms of C@N sp2 [45–49]. These assignments are
summarized in Table 1.
SEM of the untreated diatomites, which require a Au sputtered coating to prevent charging effects. On the other hand,
graphene coated samples did not require this treatment. This
gives the intriguing possibility of coating a variety structures
and patterns with GP that cannot be done by present methods. Such applications may include composite materials, Li
ion batteries, supercapacitors, RF shielding, thermal cooling
and electronic devices.
5.
3.4.
Conclusion
X-ray diffraction
Fig. 7 shows the comparison of XRD diffraction patterns of
natural graphite (top) and graphene paper (paper). The 002
XRD peak corresponds to 3.35 Å interspacing between the
graphene layers. The sharp (0 0 2) peak at 26.5 is representative of graphite [50]. The broader diffraction (0 0 2) peak at
26.6 is typical of multilayered graphene indicating that the
TAR process does indeed produce multilayered graphene
[50–54].
4.
2859
Discussion
The various methods of physical characterization in this work
all indicate that the deposited films by the TAR procedure of
Fig. 1 are multilayered graphene (graphene paper). Visual
observations (Fig. 2A and B) show a metallic appearance consistent with previous observations [2,3,11]. Figs. 2 and 3 show
a layered and essentially an atomically flat surface. This layered characteristic is observable at both the nm (TEM of
Fig. 2E) and the lm-scales (AFM, Fig. 3). The atomic-scale
AFM image (Fig. 4) indicates an atomic structure consistent
with the sp2 hybridization of graphite/graphene [29–32]. This
hybridization is verified by XPS (Fig. 5 and Table 1). Since
the apparatus (Fig. 1) for vapor deposition is open to laboratory atmosphere, there is a possibility that the graphene is
oxidized (to form graphene oxides). However, the elemental
composition as indicated by XPS is nearly pure carbon
(Fig. 6) with minor peaks to higher binding energy that can
be visualized on curve fitting. Of these, the band at
286.15 eV is associated with C–O–C, however it is minor in
comparison with reduced graphene oxide materials in the literature [45–49]. Furthermore, the expected binding energy for
C@O in graphene oxide is 288.5 eV, which is absent in the XPS
spectrum of Fig. 5. The IR (Fig. 4) spectrum indicates only an
extremely weak C@O stretch and insignificant C–O stretch,
confirming the XPS interpretation. The XRD of Fig. 7 is consistent with graphene [50–55]. Thus all these measurements
strongly indicate a layered carbon material primarily of sp2
hybridization, i.e., graphene paper (GP).
The estimated temperature for deposition of 650 C
compares well with contemporary techniques [55]. However,
the TAR method does not require Fe patterned films as a
substrate. Furthermore, other techniques require higher temperatures, typically 800–1000 C. The TAR process also another interesting advantage over others, the ability to
deposit graphene onto porous, textured and nanostructures.
Fig. 8B–D illustrates the results of the deposition of graphene
paper onto diatomites and silica nanosprings. Fig. 8A is the
The demonstrated (TAR) process is significant in several respects: (1) this is the lowest reported temperature for a chemical vapor deposition of grapheme; (2) it is conducted under
atmospheric pressures and in open containers without need
to exclude oxygen; (3) no specialized apparatus is required
and (4) no catalysts are required. Foremost, the simplicity of
the technique will allow for the immediate implementation
by other groups. Also, the large GP flakes produced by this
pyrolysis method may provide an economical path to monolayer graphene through exfoliation. Further studies will be
aimed at strategies for control of depth of layers, including
monolayers, electrical conductivity studies and control of
dopant concentrations through precursor purity.
Acknowledgements
The authors thank Dr. J. Franklin Bailey of the University of
Idaho Center for Electron Microscopy and Microanalysis and
to the internal Y-accounts of Drs. I.F. Cheng and D. McIlroy.
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