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In situ Raman spectroelectrochemistry
of graphene oxide
Milan Bouša*
,1,2
, Otakar Frank**
,1
, Ivan Jirka
1
, and Ladislav Kavan
1,2
1
Department of Electrochemical Materials, J. Heyrovský Institute of Physical Chemistry of the AS CR, v.v.i., Dolejškova 3,
18223 Prague 8, Czech Republic
2
Faculty of Science, Department of Inorganic Chemistry, Charles University, 12840 Prague 2, Czech Republic
Received 29 April 2013, revised 22 August 2013, accepted 7 October 2013
Published online 18 November 2013
Keywords electrochemical reduction, graphene oxide, Raman spectroscopy, X-ray photoelectron spectroscopy
*
Corresponding author: e-mail: milan.bousa@jh-inst.cas.cz, Phone: þ420 266053955, Fax: þ420 286582307
**
e-mail: otakar.frank@jh-inst.cas.cz, Phone: þ420 266053446, Fax: þ420 286582307
Electrochemical reduction of few-layer graphene oxide
(FLGO) is a simple method for a partial restoration of sp
2
network of the graphitic planes damaged by the previous
oxidation/exfoliation process, and it is especially interesting for
the in situ activation of FLGO in applications for energy
conversion and storage. We present a detailed study of the
structural evolution of FLGO and also non-oxidized graphene
nanoplatelets (GNP) during electrochemical treatment. Two
phases of the process can be traced tentatively in the case of
FLGO by ex situ X-ray photoelectron spectroscopy and both ex
situ and in situ Raman spectroscopy. The first phase is
irreversible and dominated by a fast removal of oxygen-bearing
functional groups accompanied by a structural ordering, while
the second phase shows only a slow irreversible progressive
reduction and the major changes in the Raman spectra caused
by lattice expansion/contraction upon doping or a mild
oxidation/reduction are reversible this time. In GNP, no
irreversible reduction is observed, i.e. the first phase is absent,
leaving only the reversible variations traceable in the Raman
spectra.
ß2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Graphene oxide (GO) [1] and its
reduced counterpart (RGO) [2] belong nowadays to
promising materials with prospective or even existing
applications in construction, energy storage and conversion,
catalysis, and many others [3–6]. GO is usually produced
from powdered graphite through a strong oxidative process,
following e.g., the protocol introduced by Hummers and
Offeman [7]. The oxidation results in graphite chunks
heavily decorated by oxygen-containing groups both in- and
out-of-plane of the graphitic layer, which not only expand
the interlayer distance but also make the product hydrophil-
ic [1, 5]. After a moderate ultrasonic agitation, a stable
aqueous dispersion of few-layered graphene oxide (FLGO)
is obtained. The exact oxidation degree of GO depends on
the particular conditions used, but in general the sp
2
nature of
the precursor is highly distorted with sheet conductivities
dropping to 10
5
Scm
1
[8]. A lot of effort has been put
recently into searching the ways how to restore the original
structure of graphene layers and thus their exceptional
properties [4, 5, 8]. Out of the most explored, we may name
thermal treatment [8–10], chemical reduction using hydra-
zine or other reductants (see e.g., [2, 8, 10]), or
electrochemical reduction [11–19]. However, none of the
so far presented methods is able to fully heal the defects
caused by oxidation. The best results are achieved by heating
in vacuum, inert or reducing atmospheres at temperatures at
least 1000 8C giving sheet conductivity up to 10
3
Scm
1
,
which is still several times smaller than the conductivity of
pristine graphene [20]. The mechanisms of GO reduction by
the two mentioned methods –i.e., thermal and chemical, are
investigated with focus on their improvement and selectivi-
ty [4, 5]. However, the electrochemical reduction of GO is
less understood at the moment. The spontaneous self-
activation of GO through its in situ electrochemical reduction
might be applicable in solar cells [6], batteries [13], and
capacitors [17].
In the presented work, we probed GO and graphene
nanoplatelets (GNP) by ex situ Raman spectroscopy and in
situ Raman spectroelectrochemistry during voltammetric
cycling and also potentiostatic charging both under reductive
and mildly oxidative potentials. The obtained results are
discussed together with surface analysis by ex situ X-ray
Phys. Status Solidi B 250, No. 12, 2662–2667 (2013) / DOI 10.1002/pssb.201300105
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photoelectron spectroscopy (XPS). General trends were
revealed in narrowing of linewidths of the Raman bands. On
the other hand, the interpretation of other observed
parameters of the Raman spectra needs to be done carefully
to avoid misleading conclusions about the structure of the
product.
2 Experimental section
2.1 Preparation of electrodes Few-Layered Gra-
phene Oxide (FLGO, Cheap Tubes, Inc., USA, 2–4 layers,
>99 wt% purity, thickness <3 nm, average dimensions of
individual flakes range from 300 to 800 nm) and Graphene
NanoPlatelets (GNP, Cheap Tubes, Inc., average diameter
5mm, >97% purity, thickness <3 nm) were both suspended
in deionized water (1 mg mL
1
) or isopropanol, respectively.
The suspension was evaporated to dryness at room
temperature. For experiments in aprotic electrolyte solutions,
the material was mixed with 5 wt% of polyvinylidene
fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone
(NMP), and doctor bladed onto F-doped SnO
2
conducting
glass support (FTO). The electrodes were dried at 100 8Cin
vacuum. The projected area of the electrode film was 0.8–
1.2 cm
2
. Electrodes for spectroelectrochemistry containing
only FLGO or GNP were prepared by repeated dipping
of platinum mesh (99.9% purity), into the respective
suspension.
GNP or FLGO samples for XPS measurements were
deposited onto a gold foil (0.1 mm thickness, 99.95% purity)
from the dispersions in isopropanol or water, respectively.
The covered area was 0.8 cm
2
. Samples were evaporated at
70 8C in vacuum.
2.2 Methods Electrochemical measurements were
carried out in a one-compartment cell using an Autolab
Pgstat-30 (Ecochemie) controlled by GPES-4 or Nova
software. For experiments in aprotic electrolyte solutions,
1 mol L
1
LiPF
6
in ethylene carbonate þdimethylcarbonate
(EC/DMC; 1/1; w/w) was used. The reference and counter
electrodes were from Li-metal. Measurements were carried
out in a glove box under Ar atmosphere. During the
spectroelectrochemistry in aqueous electrolytes a three-
electrode cell with a silver wire as pseudoreference and
platinum wire as counter electrodes was used with 1 M KOH
electrolyte. The sample of FLGO or GNP was deposited on a
Pt mesh as a working electrode.
Raman spectra were measured by Labram HR spec-
trometer (Horiba Jobin-Yvon) interfaced to an Olympus BX-
41 microscope. Spectroelectrochemical studies used 514 nm
(2.41 eV) excitation, while 514 or 633 nm (1.96 eV) lasers
were used for ex situ measurements. The Raman spectrome-
ter was calibrated by the F
1g
line of Si at 520.5 cm
1
. The D
and G(þD0) peaks were fitted by Lorentzian and Breit–
Wigner–Fano lineshapes, respectively, according to [21].
The XPS experiments were carried out using an ESCA 3
Mk 2 spectrometer (VG) with a hemispherical analyzer in
fixed transmission mode, using a band pass energy of 20 eV.
The photoelectrons were excited by the Al Ka
1,2
radiation
(1486.6 eV). The pressure during an experiment was of the
order of 10
9
mbar. Binding energies E
b
were calibrated
using the E
b
of the C1s photoelectron line of HOPG
(284.4 eV) [22]. A damped nonlinear least-square fitting
procedure was used to distinguish partially resolved lines in
O 1s, Ti 2p, Nb 3d, and C 1s in the photoelectron spectra
using XPSPEAK 4.1 software. The spectra were approxi-
mated by a weighted sum of Gaussian and Lorentzian
functions.
3 Results and discussion Redox behavior of both
GNP and FLGO was studied by cyclic voltammetry in
aqueous as well as organic solvents. Figure 1 shows an
evolution of voltammograms of GNP and FLGO during
successive cycles in an aprotic environment at a constant
scan rate 1 mV s
1
. A clear reduction peak can be observed
in the first cycle of FLGO at 2.3 V versus Li/Li
þ
. The peak
quickly vanishes in a few following cycles and is completely
irreversible. However, ongoing changes in the material can
be traced even further by a gradual decrease of the current
Figure 1 Cyclic voltammograms of FLGO (10 scans) and GNP (5
scans) electrode at a scan rate of 1 mV s
1
in 1 M LiPF
6
in EC/
DMC. The increase in scan number is indicated by a progressive
lightening of the line. First and tenth (fifth) voltammograms are
labeled for FLGO (GNP). The vertical dashed arrow in the top
graph marks the reduction peak of FLGO.
Phys. Status Solidi B 250, No. 12 (2013) 2663
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densities on the reduction side. These changes are reflected
also on the oxidation side, indicating a lower overall
electrochemical activity in the tested potential window (1.5–
4 V vs. Li/Li
þ
) caused by the loss of functional groups with
lower overpotentials. The cyclic behavior is essentially
similar for both media (not shown for an aqueous electrolyte
here), which has been documented in recent work [12]. In the
case of GNP, the main difference consists in the absence of
the pronounced reduction peak, hence in the absence of
easily reduced oxidic functional groups.
A more detailed insight into the changes caused by the
above described electrochemical cycling is provided by
XPS. Both carbon and oxygen 1s core level spectra are
shown in Fig. 2.
The photoelectron spectra of GNP correspond to
microcrystalline graphite [22]. The graphitic C1s peak is
asymmetric towards higher binding energies, with the main
C–C peak at 284.6 eV being accompanied by a pronounced
shoulder peak at 285.7 eV originating from various defects
stemming from non-ideal sp
2
C–C bonds or C–H bonds [22,
23]. A weak band at 288 eV corresponds to various oxygen
bonded carbon atoms. It should be noted here, that XPS
fitting was done mainly to illustrate general trends rather than
to provide precise quantitative data, which are always
burdened by the impossibility to deconvolute heavily
overlapping peaks. Application of several peaks with
symmetrical lineshapes for the states related with sp
2
hybridization instead of only one asymmetrical is in line with
findings of others [24]. For this reason we chose to include
two peaks for carbon–oxygen functionalities in C1s spectra
of FLGO –one for C–O (in hydroxyl and epoxy groups) at
286.7 eV, and one for CO and OC–O
at 288.2 eV. In the
latter one, two types of groups are often further simulated in
the literature (CO at 287.5 eV and OC–O
at 290.6 eV) [2,
8]. The basic fitting parameters are evaluated in Table 1. We
can trace a gradual increase of the C/O ratio with prolonged
cycling of FLGO, however, the change is much more
pronounced in the first 10 cycles than in the following 60. On
the other hand, the fraction of oxidized carbon atoms
diminishes appreciably even in the later cycles. We ascribe
this discrepancy (i) to selective reduction of functional
groups containing less oxygen (like C–O–C) and/or (ii) to
accumulation of CO
2
evolved during the reactions [8, 25].
The evolution of CO
2
is probably reflected also in the O1s
spectra (insets in Fig. 2), which causes the relative increase
of the peak attributed to the CO (and OC–O
)at531.1 eV
compared to the peak attributed to C–Oat532.6 eV [8, 25].
Raman spectroscopy represents the mostly employed
characterization method for carbonaceous materials. In
pristine graphene, Raman spectroscopy can distinguish the
number of layers [26], their doping [27, 28] or strain [29]. In
defective sp
2
-based materials, Raman spectra can provide
qualitative information on the disorder degree [21, 30], and
with some caution even quantitative data on the crystallite
sizes can be obtained [21, 30–34].
Figure 3 shows ex situ Raman spectra of FLGO
electrodes before and after chronoamperometric cycling. We
might observe all characteristic Raman features of disor-
dered carbonaceous materials: broad and intense D band at
1350 cm
1
, and overlapping (but discernible in this case)
G and D0bands at 1580 and 1615 cm
1
, respectively.
The inset in Fig. 3 shows the 2D (or G0), D þD0and 2G
bands at 2680, 2950, and 3250 cm
1
, respectively. The G
band originates from a conventional first order Raman
scattering process and corresponds to the in-plane, zone
center, doubly degenerate phonon mode (transverse (TO)
and longitudinal (LO) optical) with E
2g
symmetry [30]. The
D and 2D modes come from a second-order double and triple
resonant process, respectively, between non-equivalent
Figure 2 C1s XPS spectra of graphene nanoplatelets and graphene
oxide before and after electrochemical cycling. The corresponding
O1s spectra are plotted in insets. Presumed graphene-like structures
of carbon atoms in different chemical states label the peaks in
FLGO. Signs of K-contamination from electrolyte are visible in the
bottom spectrum.
Table 1 Overall C/O stoichiometry, fraction of total oxidized
carbon atoms (C
ox
/C) and fraction of oxygen in the oxidized phase
(O/C
ox
) calculated from XPS peak areas in the series of investigated
samples.
sample C/O C
ox
/C O/C
ox
FLGO raw 2.75 0.87 0.42
FLGO 10 cycles 3.55 0.62 0.46
FLGO 70 cycles 3.72 0.41 0.65
GNP 13.89 0.09 0.79
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K points in the Brillouin zone (BZ) of graphene, involving
two zone-boundary phonons (TO-derived) for the 2D and
one phonon and a defect for the D band [35, 36]. Both modes
are dispersive spectral features, i.e., their frequencies vary
linearly as a function of the energy of the incident laser,
E
laser
. The slope @v
D
/@E
laser
is 50 cm
1
eV
1
, which is
approximately one half of that of the 2D peak.
At first, the FLGO samples were checked by Raman
spectroscopy ex situ before and after chronoamperometric
cyclic (Fig. 3). As has been observed in the majority of
works dealing with GO reduction, most of the treatment
protocols induce an increased D/G ratio. It is, however, often
misinterpreted as a sign of a progressive disruption of the
remaining sp
2
network during reduction and thus lowering
the lateral domain size L
a
simply following the Tuinstra–
Koenig relation I
D
/I
G
¼C(l)/L
a
[30], where C(l) is the
excitation dependent proportionality constant. In contrast,
this I
D
/I
G
increase has to be percepted in the frame of the
amorphization trajectory introduced by Ferrari and Rob-
ertson [21], and recently corroborated by others [31, 32, 34].
In principle, the Tuinstra–Koenig relation fails when a
certain defect density is reached and the aromatic rings are
opening up, in the so-called stage 2 [21]. In this stage,
the average distance between defects, L
D
, is shown to be
<3 nm in graphene and decreases further during amorph-
ization [31]. Additionally, I
D
/I
G
is proportional to L2
ain
this phase [21]. Increasing I
D
/I
G
in the reduced GO is thus
reflecting the structural ordering in stage 2, i.e., the opposite
of amorphization. Alongside with the relative G and D
intensity change, both peaks are downshifted in the reduced
FLGO. The D band redshift is caused by increasing the size
of small aromatic clusters, which have higher modes [21].
The apparent redshift of the G þD0peak is contradictory
to [21] and its reason is unclear at the moment, but it may be
rationalized by variations in relative G and D0intensities
together with narrowing. Such shape alterations may explain
the inconsistencies regarding the G peak positions through-
out the literature, with some of the works reporting G band
downshift after reduction [2, 15, 37], while others showing
the opposite [8] or no appreciable shift at all [18]. On the
other hand, it is the peak narrowing, which confirms clearly
the structural ordering upon reduction. Full-width-at-half-
maxima of both G and D (denoted further as FWHM(G)
and FWHM(D), respectively) decrease considerably during
cycling. Further confirmation follows from the inset to
Fig. 3, where the individual overtone and combination bands
emerge from the hardly resolved bump.
The evolution of Raman spectra obtained in situ during
electrochemical cycling of FLGO in 1M KOH is shown in
Fig. 4 and the respective fitting parameters in Fig. 5, together
with GNP treated under the same conditions. The potential
range (1.1 to þ0.4 V versus Ag/AgCl) was chosen in order
to avoid electrolysis of water accompanied by evolution of
H
2
and O
2
, which could damage the examined material.
Figure 5 demonstrates clearly the different impact of
electrochemical cycling on the two tested materials, FLGO
and GNP, similarly to Fig. 1. While the spectra of GNP
undergo almost perfectly reversible changes in a very limited
range of 2 and 3cm
1
for Pos(D) and Pos(G),
respectively, 10 and 5cm
1
for FWHM(D) and
FWHM(G), respectively, and 0.05 for I
D
/I
G
, the fluctua-
tions in Raman spectra of FLGO are far more abrupt with a
considerable hysteresis. The point of a sudden change,
reflected in the band narrowing by 50 and 25 cm
1
for D
and G bands, respectively, and I
D
/I
G
increase from 0.9 to 1.3,
takes place at the most negative potential of the cycle.
Interestingly, Pos(D) and I
D
/I
G
values return gradually close
to the initial values, indicating a partial reversibility of the
process. On the other hand, the linewidths keep the newly
Figure 3 Ex situ Raman spectra of FLGO before (solid line) and
after (dashed line) chronoamperometric cycling (50 cycles). The
inset shows Raman 2D bands. The spectra are normalized to the D
band intensity. Excitation wavelength is 633 nm.
Figure 4 In situ Raman spectroelectrochemistry of FLGO during
one electrochemical cycle, starting at the open circuit potential
(OCP, 0.2 V vs. Ag/AgCl), with the potentials tagged on the right
side of the spectra. The spectra are offset for clarity and normalized
to the G band intensity (amplitude). Excitation wavelength is
514 nm.
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acquired values with only small changes in the progressing
cycle.
A more detailed account of the spectral changes during
prolonged electrochemical treatment is given in Fig. 6. The
initial stage of reduction at 1.1 V versus Ag/AgCl (0.9 V
negative of OCP) lasting approx. 1 h inflicts the same
changes to FLGO as discussed above, i.e., substantial G and
D band narrowing, D band softening, G band stiffening and
I
D
/I
G
ratio increasing.
However, ongoing reduction for additional 4 h at the
same potential induces only small or even opposite effects.
After 5 h of reduction, following values are reached and kept:
Pos(D) at 1345 cm
1
, Pos(G) at 1594 cm
1
, FWHM(D) at
80 cm
1
, FWHM(G) at 67 cm
1
, and I
D
/I
G
at 1.25. The next
1-day relaxation at OCP causes visible stiffening of the peak
frequencies and a lower I
D
/I
G
ratio, together with minor
changes in the peak widths. Subsequent negative polariza-
tion, again at 1.1 V, results in peak shifts and I
D
/I
G
change
close to the previous values at this potential. The small
differences between the spectra at 1.1 V may be caused by
structural reorganization during the relaxation linked to a
slow desorption of gases (like CO
2
) evolved during
reduction. The next relaxation (1 week), followed immedi-
ately by negative polarization at 1.1 V (2 h) and another
relaxation (2 h) cause almost exclusively sole “switching”
between the two respective states with little hysteresis
regardless the duration, even though a tiny progressive
reduction can still be observed. However, a subsequent
positive polarization at þ0.3 V versus Ag/AgCl (0.6
positive of OCP) has a pronounced and gradual effect on
the FLGO spectra. The changes are in part irreversible, as
evidenced by the next 1-day relaxation. The irreversibility
can be understood as physical oxidation associated with
renewed creation of defects.
On the other hand, there might be several phenomena
contributing to the reversible behavior of Raman features
during this phase for both FLGO and GNP. The reversible
peak softening at negative potentials and stiffening at
positive potentials can be explained by changes in bond
lengths upon charging [38], which causes lattice expansion/
contraction in the presence of excess/lack of electrons.
Such effects are well known from the studies of graphite
intercalation compounds [39]. The more pronounced
downshift during negative charging is probably due to a
larger potential difference than during positive charging,
when compared to OCP (0.9 V vs. þ0.6 V), hence
the negative doping is larger. Another contribution to the
observed variation of D and G band frequencies as well as of
I(D)/I(G) ratio can come from a mild reversible reduction/
oxidation. It should be emphasized that we do not observe
(either in FLGO or GNP) the characteristic pattern of
pristine graphene, where the G band stiffens under both
electron and hole doping caused by the nonadiabatic removal
of the Kohn anomaly from the Gpoint [40].
4 Conclusions We have studied the behavior of few-
layer graphene oxide and graphene nanoplatelets during
electrochemical treatment by X-ray photoelectron spectro-
scopy and in situ Raman spectroscopy. The evolution of
FLGO can be divided into two phases. During the first phase,
Figure 5 Evolution of selected fitting parameters of Raman G and
D bands measured in situ during voltammetric cycling of FLGO
(full rectangles) and GNP (empty diamonds) as a function of
electrochemical potential (V vs. Ag/AgCl) in the cycle progression.
The full sequence is: 0.3, 0.5, 0.8, 1.1, 0.8, 0.5, 0.3,
0.0, and þ0.4V. Arrows inside the plots emphasize the sudden
changes observed in FLGO Raman spectra. Excitation wavelength
is 514 nm.
Figure 6 Evolution of selected fitting parameters of Raman G and
D bands measured in situ during chronoamperometric charging of
FLGO. The three individual vertical dashed lines indicate longer
time breaks, during which the electrodes were left for relaxation:
from left to right 1 day, 1 week, 1 day. Apart from the breaks, the
timescale in the plots is indicated by the bar in the left bottom
corners (5 h). Excitation wavelength is 514 nm.
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both under moderately fast cycling (voltammetric or
chronoamperometric) and prolonged negative polarization,
FLGO is reduced rapidly. The reduction, accompanied by
structural ordering, is documented by lowering peaks
belonging to oxygen-containing groups in XPS C1s and
O1s spectra and by abrupt narrowing of Raman D and G
linewidths. In this phase, the increase of the I(D)/I(G) ratio
evidences the structural ordering as well. The reduction
degree can be compared to that achieved by a mild heating
treatment in vacuum [8, 25]. This first phase in FLGO is
irreversible and it is completely missing in the evolution of
GNP. On the other hand, the second phase, both in FLGO
and GNP, is dominated by only minor changes in the
monitored parameters of Raman spectra. The G and D band
shifts are governed by mild oxidation/reduction and/or by
charging-induced lattice expansion during negative doping
and lattice contraction during positive doping. In turn,
the lattice expansion (contraction) is reflected in downshift
(upshift) of the band frequencies. These variations are
reversible. A small contribution of progressing reduction can
still be traced in the FLGO in the second phase, again with no
signs of such process in GNP.
Acknowledgements Financial support was provided by the
Grant Agency of the Czech Republic (Contract No. 13-07724S) and
FP7-Energy-2010-FET project Molesol (Contract No. 256617).
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