Article
pubs.acs.org/JPCC
An Electrochemical in Situ Infrared Spectroscopic Study of
Graphene/Electrolyte Interface under Attenuated Total Reflection
Configuration
Yao Yao,† Wei Chen,† Yuanxin Du,‡ Zhuchen Tao,‡ Yanwu Zhu,‡ and Yan-Xia Chen*,†
†
Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, University of Science and
Technology of China, Hefei, 230026, China
‡
Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and
Engineering, iChEM, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China
ABSTRACT: The interface of electrodes composed of 2, 3, or 5
monolayers (MLs) of graphene stacked on a Si prism in 0.1 M
HClO4 is examined by electrochemical in situ infrared spectroscopy under attenuated total reflection configuration (EC-ATRFTIRS) combined with cyclic voltammetry in a wide potential
window from 0 to 3. 0 V. At 5 MLs graphene, we observe
significant oxidation current at E > 2.0 V in the first positivegoing scan. This is accompanied by the appearance of three
negative-pointing bands at 1230 cm−1 (C-O-C stretching), 1630
cm−1 (from both bending of water and CC stretching), and
3300 cm−1 (O-H stretching of C-OH and water), suggesting the
consumption of C-O-C, CC, C-OH, and water. The CVs for
the second cycle are quite similar to what was observed for the
first cycle, only that the current is ca. 10 times smaller. The general trends of the i−E curves and IR spectral behavior at 2 MLs
and 3 MLs graphene are also the same as those at 5 MLs graphene; only the current and band intensities at the corresponding
potentials are much smaller than those at the latter. Our results suggest that the edge sites and the defects of graphene are
probably the active sites for the oxidation of water at the graphene surface at E > 2.0 V, which can be easily destroyed through
oxidation at such high potentials. Within the potential region of 0.05 V < E < 1.5 V, the high stability of the graphene layer makes
it a promising support for nanocatalysts using EC-ATR-FTIRS.
copy (SEM),17 scanning tunneling microscopy (STM),18
transmission electron microscopy (TEM),19 X-ray diffraction
(XRD),20 X-ray photoelectron spectroscopy (XPS),21 infrared22,23 and Raman spectroscopy,24 and so on. In situ
information about whether or how the structure and
composition of graphene changes during the electrochemical
process is sparse. Here, we report the first results on the
electrochemical properties of the pristine monolayer, double
layer, and multilayer graphene supported on a Si prism using
electrochemical in situ infrared spectroscopy under attenuated
total reflection configuration (ATR-FTIRS). Possible reactions
and the related mechanisms are discussed based on the EC-IR
results.
1. INTRODUCTION
Because of its high surface area (2630 m2 g−1), excellent
electronic conductivity (106 S/m), broad potential window, and
unique electronic properties, graphene has been considered to
be a promising material in fuel cells,1 rechargeable Li+ ion
batteries,2 and ultracapacitors.3 It has been successfully used as
a support for electrocatalysts or an electrode for ultracapacitors
in electrochemical energy conversion systems.1,3−6 On the
other hand, graphene nanosheets or modified graphene may
also be used as electrocatalysts for some specific reactions;7,8
e.g., N-doped graphene showed a good catalytic activity for
ORR.9 Information on the morphology, structure, and chemical
compositions of graphene, especially those on their edge sites
and defects as well as their changes under operation conditions
is of great importance to understand their electrochemical
properties in such energy conversion devices.
A number of studies have been carried out in order to
address the effects of oxygen-containing groups,10,11 the
structure and composition of edges of the graphene sheets,12,13
the graphitic islands,14 and the number of graphene layers15 on
its electrochemical properties. However, most of the characterizations were carried out under ex situ conditions, such as
atomic force microscopy (AFM),16 scanning electron micros© 2015 American Chemical Society
2. EXPERIMENTAL SECTION
A graphene monolayer is synthesized according to the
procedure described in ref 25. It is grown on copper foils by
chemical vapor deposition (CVD) at 1000 °C under
Received: July 1, 2015
Revised: August 18, 2015
Published: September 8, 2015
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Figure 1. Representative optical (a, c, e) and AFM (b, d, f) images of 2 MLs (a, b), 3 MLs (c, d), and 5 MLs (e, f) graphene stacked layer by layer on
a Si substrate. The scale bar in AFM images is 1 μm.
atmospheric pressure with methane as the carbon source.26
Subsequently, the graphene film is transferred onto the
reflecting plane of the Si prism by using the thermal release
tape method.25 Then, the prism supported graphene sheet is
annealed under an Ar atmosphere for 2 h at 350 °C to remove
possible impurities collected during the transfer process. We
transferred graphene layer by layer, so 2 MLs, 3 MLs, and 5
MLs graphene means twice, three, and five times transfer. Thin
films composed of 2 MLs, 3 MLs, and 5 MLs of graphene with
a size of 20 mm × 25 mm stacked layer by layer on top of the Si
prism are used as the working electrode (WE; the geometric
area exposed to electrolyte is ca. 1.76 cm2). Optical and atomic
force microscopic (AFM) images of 2 MLs, 3 MLs, and 5 MLs
graphene stacked on a Si substrate are measured by a Nikon
ECLIPSE LV100ND and MultiMode V.
The cell used for EC-ATR-FTIRS measurements in the
present study is the same as what has been reported earlier.27 A
Pt foil and a reversible hydrogen electrode (RHE) are used as
counter and reference electrodes, respectively. Millipore MilliQ water (18.2 MΩ cm) and ultrapure perchloric acid (70%,
Suprapure, Sigma-Aldrich) are used to prepare the solution.
The supporting electrolyte used in all measurements is 0.1 M
HClO4, which is constantly purged with N2 (5 N, Nanjing
Special Gas Corp) during the experiment. All potentials in this
study are quoted against the RHE. Before the experiments, the
WE is cleaned by continuously scanning the electrode potential
in the region from 0.05 to 1.0 V for ca. 30 min. Then, the
electrode potential is held at 0.05 V in 0.1 M HClO4 and a
background spectrum (reflectance of R0) is recorded. After that,
the electrode potential is scanned to 3.0 V at a scan rate of 20
mV/s; in the meantime, IR spectra are recorded with a time
resolution of 1 s/spectrum at a spectral resolution of 4 cm−1.
All spectra are presented in absorbance, A = −log(R/R0), where
R is the reflectance of the sample spectrum. A Varian FTS
7000e IR spectrometer with a mercury cadmium telluride
detector cooled by liquid nitrogen is used.
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3. RESULT AND DISCUSSION
3.1. Structure and IR Spectroscopic Characterization
of 2−5 MLs Graphene Electrodes. Representative optical
and AFM images of 2 MLs, 3 MLs, and 5 MLs graphene
stacked on a Si substrate are displayed in Figure 1. From Figure
1a,c,e, it is seen that the graphene surface is relatively uniform
and flat on a large scale. The few black dots or bumps with sizes
in the micrometer range on the surface may be dust, which
sticks to the graphene surface since the observations are carried
out under room atmosphere. The area with light color may be
the Si substrate without graphene. According to the AFM
images (Figure 1b,d,f), the height difference of the graphene
layers can be as high as 20−30 nm. The roughness of graphene
in the AFM images was mainly caused by the air, which inserted
the layers when graphene is transformed to the Si slice. In a
word, the stacked graphene layers are not flat in the nanoscale,
although it looks flat in micrometer range. Figure 2 shows the
Figure 3. IR spectra of (a) air/5 MLs graphene/Si prism, (b) water/5
MLs graphene/Si prism interface, and (c) the difference spectrum
between cases (a) and (b).
to water, two strong bands at 1637 and 3370 cm−1 from the
bending and the O-H stretching of water appear, and the latter
is superimposed on the small contribution from the C-OH or
COOH groups at the edge sites or defects in graphene. It is
interesting to note that no peaks for the D, G, and 2D bands
are observed by IR spectroscopy, which agrees well with the
selection rules for IR spectroscopy. The IR bands at the 2 MLs
and 3 MLs graphene/air interface are quite similar to that at the
5 MLs graphene/air interface, except that the intensity of the IR
bands from C-O, C-OH, and C-H is weaker. Hence, they are
not given here. The weaker IR band intensities can also be
explained by the smaller amount of edge and defects sites at the
thinner graphene electrodes, since IR spectroscopy also samples
the active vibration modes within the thin films.
3.2. Electrochemical in Situ IR Spectroscopic Studies
on 2−5 MLs Graphene/0.1 M HClO4 Interface. Figure 4
Figure 2. Raman spectra of graphene film composed of 1−5 MLs
graphene monolayers supported on a Si wafer.
Raman spectra of graphene film composed of 1−5 graphene
monolayers supported on a Si wafer. The bands at 1343, 1590,
and 2682 cm−1 are the D, G, and 2D bands of graphene film,
respectively.28 The increase in the Raman band intensities with
layer thickness is due to the fact that, for such thin graphene
films, Raman spectra reflect the vibration modes for all
graphene layers. As seen from the very weak D band in the
Raman spectra, the as-prepared graphene monolayer is rather
perfect, which only has a very small amount of defects.29 After
normalizing the intensity of Raman spectra with layer thickness,
we found that the ratio of the D band intensity to that of the G
band is roughly the same for all the graphene films with a layer
thickness from 1 to 5 MLs. This confirms that the graphene
monolayers prepared and the transfer method used are very
reproducible.
Figure 3a displays the IR spectrum recorded at the interface
between the working electrode composed of 5 MLs graphene
supported on the reflecting plane of the Si prism and air
(denoted as 5 MLs graphene/air), and the background
spectrum is taken at the Si prism/air interface without
graphene under otherwise identical conditions. The bands at
1113, 1429, 2750−2953, and 3373 cm−1 are attributed to the
C-O stretching,22 bending of C-OH,22 C-H stretching,30 and
stretching of O-H30 from the edge sites or defects of the
graphene layers, respectively. The appearance of such IR bands
suggests the existence of defects and O-containing functional
groups on the graphene layers. When exposing 5 MLs graphene
Figure 4. Cyclic voltammogram of the working electrode composed of
5 MLs, 3 MLs, and 2 MLs graphene supported on a Si prism in 0.1 M
HClO4; potential scan rate: 50 mV/s.
displays the base cyclic voltammograms (CVs) of 2 MLs, 3
MLs, and 5 MLs graphene electrodes in 0.1 M HClO4 in the
potential region from 0.05 to 1 V. The figure shows that the
current is rather low (<1 μA) and it increases slightly with
electrode potential and the thickness of the graphene
electrodes. The current is attributed to the double layer
charging current with small pseudo-capacitive contribution that
is due to Faradaic reaction at the edge and defect sites, e.g., the
redox of −C, C−, C-OH, and COOH. The increase in the
capacitive current with the number of graphene monolayers
stacked onto the Si surface suggests that the amount of oxygencontaining defects directly contacting the electrolyte increases
with graphene layer thickness.31,32 Although the amount of
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The corresponding IR spectra recorded at some selected
potentials during the cyclic voltammetric potential scans at the
5 MLs, 3 MLs, and 2 MLs graphene/0.1 M HClO4 interface are
displayed in Figures 6−8, respectively. IR spectra of 5 MLs
graphene recorded at some selected potentials during the first
positive-going potential sweeping are given in Figure 6a. It is
seen that a small and sharp peak at ca. 1580 cm−1 appears at E
> 0.8 V, and this peak is assigned to the CC stretching in the
graphene ring,22,35 or the so-called G band in Raman spectra.36
With a further positive shift in electrode potential up to 2.3 V,
the intensity and frequency of this peak do not show an obvious
change, but it disappears at E > 2.3 V. A similar phenomenon
was observed in the in situ Raman spectroelectrochemistry of
graphene in ref 37. This is probably due to the slight change of
orientation of the graphene ring, which leads the CC bond to
be closer to the direction of the surface normal. At E > 2.0 V,
small negative-pointing peaks at ca. 1320, 1630, and 3300 cm−1
appear, whose intensities increase with the positive shift in
electrode potential. The band at 1320 cm−1 is attributed to the
bending mode of C-O-C at the surface defects of graphene.23
The broad band peak at 1630 cm−1 (down to 1590 cm−1) is
attributed to superposition the bending mode of water and C
C stretching of graphene,35 and that is the reason why the
width of this peak is much broader than the bending mode of
water, as usually observed (Figure 3b). The broad band at 3300
cm−1 is attributed to the O-H stretching in water,30 which may
also include small contribution from C-OH species at the edge
sites and defects of graphene monolayers. The phenomenon
that the negative-pointing water bands at 1630 and 3300 cm−1
only appear at E > 2.2 V indicates that the water structure near
the graphene surface is changed greatly due to the oxidation of
both water and graphene. The oxidation of water to O2 is
further confirmed by the cathodic current from the oxygen
reduction current, which is observed in the negative-going scan
at E < 0.6 V (Figure 5).38−40 It should be noted that, although
the solution is constantly purged with N2, the rate for O2
evolved at E > 2.2 V is much higher than that of its removal by
purging. Some O2 produced is still left in the solution, and that
is why we still observe the reduction current of O2 in the
solution at E < 0.6 V.
In the first negative-going potential scan from 3.0 V to lower
potentials, the intensities of all three bands decrease as shown
in Figure 6b. The bands at 1630 and 3300 cm−1 disappear at E
< 2.2 V, which indicates that the water structure resumes its
original structure at the reference potential (0.05 V). A new
peak at 1590 cm−1 appears, whose band intensity only slightly
decreases with the negative-shift in electrode potential as
similar to that at 1320 cm−1. The negative-direction of these
peaks suggests that there are more such species near/on the
surface of graphene at the reference potential (0.05 V). This
means that the amount of such species decreases when the
potential increases to E > 2.0 V. This indicates that the
oxidation of graphene probably starts from the defects within
the surface of graphene, which has more C-O-C groups. We
have also checked the IR spectra recorded in the potential
region from 0.05 to 0.6 V very carefully in order to verify
whether there are any reaction intermediates related to ORR at
graphene after water oxidation at higher potentials. However,
there is no difference in the IR spectra recorded at potentials
between 0.05 and 0.6 V in the positive-going scan to that of the
reference spectrum. This result supports that ORR at graphene
probably goes through an outer-sphere mechanism.41
defects for the graphene monolayers prepared are comparable,
after stacking layer by layer, some defects in graphene beneath
the top monolayer are also exposed to the electrolyte. This is
probably due to the existence of wrinkles and possible defects
close to the wrinkles; once the graphene film is exposed to the
electrolyte, solutions may go into the interlayer through such
defects, which leads to an increase of the active surface area and
the capacitance. Anyway, the number of such defects sites is
very small, as indicated from the very small difference in the
capacitive current between electrodes composed of 5 MLs or 3
MLs and 2 MLs graphenes.33 No obvious change in the IR
spectra is observed in the potential range from 0 to 1.0 V
(Figures 6−8), this further supports that the amount of −C, 
C−, C-OH, and COOH formed/consumed at the interface is
small. The increase in cathodic current at E < 0.2 V is due to
the reduction of oxygen-containing defects on the graphene
surface.34
Figure 5 gives the first and second cyclic voltammogram of
electrodes composed of 2 MLs, 3 MLs, and 5 MLs graphene
Figure 5. Cyclic voltammograms of the working electrode composed
of 5 MLs, 3 MLs, and 2 MLs graphene supported on a Si prism in the
(a) first and (b) second potential cycles in 0.1 M HClO4; potential
scan rate: 20 mV/s.
during the potential scan from 0.05 to 3.0 V. From the first CV
with 5 MLs graphene, it is seen that a small oxidation current
appears at E > 1.6 V, which increases significantly at potentials
higher than 2.2 V and reaches a maximum at E > 2.6 V. After
that, the current drops in the positive scan until the potential
approaches the upper limit. In the negative-going scan from 3.0
V to lower potentials, the anodic current decreases further with
a negative shift in the electrode potential and it drops to zero at
E < 1.8 V. With further negative potential sweeping, a cathodic
current appears at E < 0.55 V, whose amplitude increases with
the negative shift in electrode potential. The general trend of
the i−E curve at electrodes composed of 2 MLs and 3 MLs
graphene is the same as that at 5 MLs graphene, while the
current at the corresponding potential is much smaller than that
at the latter and there is no obvious ORR current when
scanning negatively. Since the geometric areas of those three
working electrode are the same and the oxidation currents are
not proportional to the amount of layers, the oxidation of
graphene contributes only a small part of the oxidation current
under high potential (E > 2.0 V). The CVs for the second cycle
is quite similar to what is observed for the first cycle, only that
the current is ca. 10 times smaller.
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Figure 6. IR spectra of the 5 MLs graphene/0.1 M HClO4 interface recorded at some selected potentials during the (a) first positive-going, (b) first
negative-going, and (c) second positive-going potential scans in the region from 0.05 to 3.0 V; other conditions are the same as those in Figure 5.
In the first negative-going potential scan, the lower intensity
of the negative-pointing bands at 1320 and 1590 cm−1 with the
negative shift in electrode potential (Figure 6b) suggests that a
small fraction of C-O-C and CC structure resumes during
the cathodic scan through reduction of the defects on graphene.
The survival of the negative-pointing bands at 1320 and 1590
cm−1 with potentials down to 0.05 V (Figure 6b) indicates that
a certain amount of C-O-C and CC at the defects are
irreversibly oxidized at higher potentials.
In the subsequent second positive-going scan from 0.4 to 1.9
V (Figure 6c), a significant decrease of the intensity of the
negative-pointing bands at 1320 and 1590 cm−1 is observed,
indicating further destruction of C-O-C and CC at the
defects. With a further increase in the potential from 2.0 to 3.0
V, the intensity of the negative-pointing bands at 1320 and
1590 cm−1 decreases. This suggests that subsequent production
of C-O-C and CC defects come from the oxidation of the
carbon ring in the graphene. The anodic current at E > 2 V is
much smaller than that for the first positive-going potential
scan, and no obvious spectral change is observed. This is
probably due to that water oxidation to O2 mainly occurs at the
defects, such as defects with C-O-C and C-OH groups that
initially existed on the imperfect graphene layers, and they are
oxidized and detached from the graphene layer at high
potential, leaving near perfect graphene rings.
IR spectra of 3 MLs graphene recorded at some selected
potentials during the positive scan are given in Figure 7a.
Qualitatively, it shows the same phenomenon as that with 5
MLs graphene. By carefully comparing the spectra given in
Figures 6 and 7, the following differences are discerned: (i) the
CC stretching in the graphene rings at ca. 1580 cm−1
becomes more obvious than that of 5 MLs graphene; (ii) the
intensity of the negative-pointing bands for the bending water
and stretching of O-H from both water and C-OH is much
weaker; and (iii) the C-O-C band only appears at E < 2.6 V in
the subsequent negative-going scan. The much smaller
intensities of the stretching vibration of C-O−, C-OH, and
water bands as well as the slightly higher band intensity of C
C stretching at 1580 cm−1 from the graphene ring correspond
well to the much smaller anodic current for the oxidation of
water and graphene itself. This indicates that the electrode with
3 MLs of graphene has fewer defects than that with 5 MLs
graphene. This is also well supported by previous studies by
Raman spectroscopy, which found that the higher the amount
of defects, the higher the ratio between the band intensities of
the D band and G band. A slight blue-shift in the peak
frequency of the G band was also observed.36 The C-O-C band
only appears at 2.6 V in the subsequent negative-going scan,
which further supports that it takes a longer time to break the
C−C bond in the ring and produce the C-O-C structure at the
defects. All such phenomena confirm that the edge and defects
sites are probably the active sites for such processes.
Figure 8 shows the IR spectra of 2 MLs graphene, and there
are no obvious spectral features compared with those of 3 MLs
and 5 MLs graphene. This also correlates well to the much
smaller oxidation current observed at this electrode comparing
to that of 3 MLs and 5 MLs graphene. It should be mentioned
that the small current is not due to the high resistance of the
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Figure 8. IR spectra of the 2 MLs graphene/0.1 M HClO4 interface
recorded at some selected potentials during the (a) first positive-going
and (b) first negative-going potential scans in the region from 0.05 to
3.0 V; other conditions are the same as those in Figure 4a.
Figure 7. IR spectra of the 3 MLs graphene/0.1 M HClO4 interface
recorded at some selected potentials during the (a) first positive-going
and (b) first negative-going potential scans in the region from 0.05 to
3.0 V; other conditions are the same as those in Figure 4a.
graphene at high potentials. The electrochemical performance
is dependent on the density of edge plane sites at the graphenebased electrode, which increases with the coverage of graphene
defects and the number of graphene layers from two to five
monolayers. Furthermore, we found that, after the destruction
of such defects (by oxidizing at high potentials), the CVs for
the rest of the graphene electrodes are very stable in the
potential region of 0.05 V < E < 1.5 V. The good stability and
electronic conductivity, optical transparency, and their high IR
sensitivity of interfacial species suggest that such graphene
layers can serve as a stable support for loading of nanocatalysts.
What’s more, how to load the catalyst on the infrared window is
not an easy task for ATR-FTIR studies. Even though sputtering
and chemical deposition are widely used, it is still difficult to
maintain a good stability and a well-defined structure. However,
with several layers of graphene on the prism, nanocatalysts can
be studied in ATR-FTIR. Further studies on such systems are
underway in our lab.
thinner graphene electrode, since Ohmic compensation has
been applied to all of these electrodes, and the resistances of
those three working electrodes were about tens of ohms. As
revealed in Figure 4, the number of edge sites and defects are
also smaller in the 2 MLs graphene electrode. This agrees with
previous reports that monolayer graphene exhibits slower
heterogeneous electron transfer kinetics toward OER, and
increasing the number of graphene layers improved the OER
activity.42 All of these phenomena suggest that the edge plane
sites and the defect sites are the predominant origin of fast
electron transfer kinetics at graphitic materials. The slow ET
kinetics at pristine single layer graphene electrodes are likely
due to graphene’s fundamental geometry, which comprises
small edge plane and large basal plane contributions.
■
4. CONCLUSION
Electrochemistry of large-scale graphene with 2−5 MLs has
been examined by EC-ATR-FTIRS. We observe significant
spectral changes together with strong oxidation current at E >
2.0 V, which are attributed to the oxidation of water and
graphene itself at the edge and defect sites at the interface.
Oxygen-containing functional groups such as C-OH, CO,
and −COO− at the edge and defects of graphene layers are
probably the active centers for the oxidation of water and
AUTHOR INFORMATION
Corresponding Author
*E-mail: yachen@ustc.edu.cn. Tel/Fax: +86-551-63600035.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (no. 21273215), the National Instru22457
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