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Electrochemical reduction of few-layer graphene oxide (FLGO) is a simple method for a partial restoration of sp2 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.
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In situ Raman spectroelectrochemistry
of graphene oxide
Milan Bouša*
, Otakar Frank**
, Ivan Jirka
, and Ladislav Kavan
Department of Electrochemical Materials, J. Heyrovský Institute of Physical Chemistry of the AS CR, v.v.i., Dolejškova 3,
18223 Prague 8, Czech Republic
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:, Phone: þ420 266053955, Fax: þ420 286582307
e-mail:, Phone: þ420 266053446, Fax: þ420 286582307
Electrochemical reduction of few-layer graphene oxide
(FLGO) is a simple method for a partial restoration of sp
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 rst 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 rst phase is absent,
leaving only the reversible variations traceable in the Raman
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 [36]. 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
nature of
the precursor is highly distorted with sheet conductivities
dropping to 10
[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 [810], chemical reduction using hydra-
zine or other reductants (see e.g., [2, 8, 10]), or
electrochemical reduction [1119]. 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
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, 26622667 (2013) / DOI 10.1002/pssb.201300105
p s s
basic solid state physics
ß2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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
2 Experimental section
2.1 Preparation of electrodes Few-Layered Gra-
phene Oxide (FLGO, Cheap Tubes, Inc., USA, 24 layers,
>99 wt% purity, thickness <3 nm, average dimensions of
individual akes 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
) 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
uoride (PVDF) dissolved in N-methyl-2-pyrrolidone
(NMP), and doctor bladed onto F-doped SnO
glass support (FTO). The electrodes were dried at 100 8Cin
vacuum. The projected area of the electrode lm was 0.8
1.2 cm
. Electrodes for spectroelectrochemistry containing
only FLGO or GNP were prepared by repeated dipping
of platinum mesh (99.9% purity), into the respective
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
. 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
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
line of Si at 520.5 cm
. The D
and G(þD0) peaks were tted by Lorentzian and Breit
WignerFano lineshapes, respectively, according to [21].
The XPS experiments were carried out using an ESCA 3
Mk 2 spectrometer (VG) with a hemispherical analyzer in
xed transmission mode, using a band pass energy of 20 eV.
The photoelectrons were excited by the Al Ka
(1486.6 eV). The pressure during an experiment was of the
order of 10
mbar. Binding energies E
were calibrated
using the E
of the C1s photoelectron line of HOPG
(284.4 eV) [22]. A damped nonlinear least-square tting
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
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
. A clear reduction peak can be observed
in the rst cycle of FLGO at 2.3 V versus Li/Li
. 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
in 1 M LiPF
in EC/
DMC. The increase in scan number is indicated by a progressive
lightening of the line. First and tenth (fth) 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 ß2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
densities on the reduction side. These changes are reected
also on the oxidation side, indicating a lower overall
electrochemical activity in the tested potential window (1.5
4 V vs. Li/Li
) 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
CC 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
CC bonds or CH bonds [22,
23]. A weak band at 288 eV corresponds to various oxygen
bonded carbon atoms. It should be noted here, that XPS
tting 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
hybridization instead of only one asymmetrical is in line with
ndings of others [24]. For this reason we chose to include
two peaks for carbonoxygen functionalities in C1s spectra
of FLGO one for CO (in hydroxyl and epoxy groups) at
286.7 eV, and one for CO and OCO
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 OCO
at 290.6 eV) [2,
8]. The basic tting 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 rst 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 COC) and/or (ii) to
accumulation of CO
evolved during the reactions [8, 25].
The evolution of CO
is probably reected also in the O1s
spectra (insets in Fig. 2), which causes the relative increase
of the peak attributed to the CO (and OCO
)at531.1 eV
compared to the peak attributed to COat532.6 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
-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, 3034].
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
, and overlapping (but discernible in this case)
G and D0bands at 1580 and 1615 cm
, respectively.
The inset in Fig. 3 shows the 2D (or G0), D þD0and 2G
bands at 2680, 2950, and 3250 cm
, respectively. The G
band originates from a conventional rst order Raman
scattering process and corresponds to the in-plane, zone
center, doubly degenerate phonon mode (transverse (TO)
and longitudinal (LO) optical) with E
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
/C) and fraction of oxygen in the oxidized phase
) calculated from XPS peak areas in the series of investigated
sample C/O C
/C O/C
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
2664 M. Bouša et al.: In situ Raman spectroelectrochemistry of graphene oxide
ß2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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,
. The slope @v
is 50 cm
, which is
approximately one half of that of the 2D peak.
At rst, 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
network during reduction and thus lowering
the lateral domain size L
simply following the Tuinstra
Koenig relation I
[30], where C(l) is the
excitation dependent proportionality constant. In contrast,
this I
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 TuinstraKoenig 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
, is shown to be
<3 nm in graphene and decreases further during amorph-
ization [31]. Additionally, I
is proportional to L2
this phase [21]. Increasing I
in the reduced GO is thus
reecting 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 conrms 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 conrmation 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 tting 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
and O
, 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
for Pos(D) and Pos(G),
respectively, 10 and 5cm
for FWHM(D) and
FWHM(G), respectively, and 0.05 for I
, the uctua-
tions in Raman spectra of FLGO are far more abrupt with a
considerable hysteresis. The point of a sudden change,
reected in the band narrowing by 50 and 25 cm
for D
and G bands, respectively, and I
increase from 0.9 to 1.3,
takes place at the most negative potential of the cycle.
Interestingly, Pos(D) and I
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.
Phys. Status Solidi B 250, No. 12 (2013) 2665 ß2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
acquired values with only small changes in the progressing
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 inicts the same
changes to FLGO as discussed above, i.e., substantial G and
D band narrowing, D band softening, G band stiffening and
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
, Pos(G) at 1594 cm
, FWHM(D) at
80 cm
, FWHM(G) at 67 cm
, and I
at 1.25. The next
1-day relaxation at OCP causes visible stiffening of the peak
frequencies and a lower I
ratio, together with minor
changes in the peak widths. Subsequent negative polariza-
tion, again at 1.1 V, results in peak shifts and I
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
) 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 rst phase,
Figure 5 Evolution of selected tting 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 tting 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.
2666 M. Bouša et al.: In situ Raman spectroelectrochemistry of graphene oxide
ß2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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 rst 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 reected 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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Phys. Status Solidi B 250, No. 12 (2013) 2667 ß2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
... To improve the conductivity of the GO, it needs to be partially reduced to so called reduced graphene oxide (rGO, reduced graphene) using a high temperature treatment [13][14][15] and/or a reducing agent like hydrogen, hydrazine or sodium borohydride [11,[14][15][16] and also electrochemicaly [17][18][19][20][21][22][23][24][25][26][27]. However, it is not possible to fully reduce GO back to graphene, because some oxidic-groups always remain and the sp 3 defects in the crystal lattice are not completely healed. ...
... It is fast, non-destructive and very sensitive to changes in the structure of these materials. It can provide information about the electronic structure of graphene, number of layers [65], interlayer coupling [66], structural defects [26,67], or chemical functionalization. Moreover, phonons (and thus Raman peaks) react to magnetic field, temperature, stress/strain and doping [49,68,69] and many other physical or chemical stimuli. ...
... Ref. [26]). Sometimes, Tuinstra-Koenig relation is being used for calculating the lateral domain size (L a ) in graphene using the ratio of D and G intensities {I(D)/I(G)} [71]. ...
Graphene research is nowadays one of the worldwide most prominent fields of interest in material science due to many extraordinary properties of graphene and related materials. Herein, a detailed study of the structural evolution of the graphene oxide during electrochemical treatment has been performed using X-ray photoelectron, Raman and infrared spectroscopies and the results were compared with non-oxidized graphene nano-platelets. Additionally, graphene oxide in composite with LiFePO4 olivine material, which is electrochemically almost inactive in a freshly made state, has been tested by repeated electrochemical cycling. Using various electrochemical methods, the progressive electrochemical activity enhancement has been observed and spontaneous graphene reduction was identified as responsible for this phenomenon. The second part of this work deals with mono- and bilayer graphene under uniaxial in plane loading. The behavior of various strained graphene samples transferred onto the target polymer substrates were examined by Raman spectroscopy and discussed with respect to presence of cracks, wrinkles, grain boundaries and loss of bilayer lattice periodicity. Further, the level of stress and doping transferred to the crystal from the substrate was calculated by the vector analysis method with a specific adjustment for the uniaxial strain. Finally, a new method for spectroelectrochemical characterization of isolated strained 2D crystals has been established.
... The D and G Raman peaks refer to disorder and sp 2 hybridization of hexagonal carbon network in either graphene or graphene oxide, respectively [77]. The minimum value of intensity ratio of D to G peaks (I D /I G ) determines a larger two-dimensional layer structure with less amount of edge defects, whereas the minimum value full width at maximum height (W G ) pronounces smaller inhomogeneity between the layers [77,78]. As regards the electro-reduction of GO, most studies dealing with the reduction of GO to rGO imply an increased I D /I G ratio [78]. ...
... The minimum value of intensity ratio of D to G peaks (I D /I G ) determines a larger two-dimensional layer structure with less amount of edge defects, whereas the minimum value full width at maximum height (W G ) pronounces smaller inhomogeneity between the layers [77,78]. As regards the electro-reduction of GO, most studies dealing with the reduction of GO to rGO imply an increased I D /I G ratio [78]. Besides, Raman shifts and the distance between D and G peaks may be associated with the reduction/oxidation of graphene oxide known as another aspect of Raman spectra assessment [78]. ...
... As regards the electro-reduction of GO, most studies dealing with the reduction of GO to rGO imply an increased I D /I G ratio [78]. Besides, Raman shifts and the distance between D and G peaks may be associated with the reduction/oxidation of graphene oxide known as another aspect of Raman spectra assessment [78]. ...
Relying on the reduction of oxygenated functional groups of graphene oxide, the engineering of the morphology of Ni-based reduced graphene oxide (Ni-rGO) nanocomposite was carried out via galvanostatic electrochemical co-deposition by changing the current density in a range of 0.001–0.01⁻² and loading of 2 g.L⁻¹ of graphene oxide. The morphology has been converted to a porous, rough, and three-dimensional (3D) form by significant incorporation and simultaneous reduction of GO into the structure of Ni-rGO nanocomposite film. Study on 3D morphology by SEM, FT-IR, XRD, and Raman confocal spectroscopy approved simultaneously reduction of oxygenated functional groups. Moreover, we have discussed the impact of rGO incorporated in the structure of Ni-rGO nanocomposite onto the creation of porous 3D-morphology and the enhancement of the electroactive specific surface. This new fascinating mechanism and structure can lead to the enhancement of electroactive components in electrochemical sensors and energy conversion-storage systems.
... Mūsų atveju G smailė pasirodė maždaug 1583 cm -1 , ir tai yra daug mažiau, lyginant su labai oksiduotu grafenu ir tai labiau būdinga į neoksiduotą grafitą, kuris gali būti susijęs su žemu GO oksidacijos lygiu (C:O yra ~ 4). D ir 2D smailės kyla iš antrosios eilės dvigubo ir trigubo rezonanso proceso (Bouša et al., 2013). D smailė yra ~ 1343 cm -1 . ...
... It is more characteristic of non-oxidized graphite, which might produce a low oxidation level of GO (C/O is ~ 4). D and 2D peaks appear due to the double and triple resonance process in the second band (Bouša et al., 2013) D peak is about ~ 1343 cm -1 and it is a result of disordered or damaged structures due to attached functional oxygen groups. 2D peak is at 2683 cm -1 . ...
... In our case, the G band appeared at around 1583 cm −1 and that is much lower compared to highly-oxidized graphene and is very close to pristine graphite; this can be related to the low oxidation level of GO (C:O is ~4). The D and 2D bands originate from the second-order double and triple resonant processes, respectively [53]. The D band appeared at around 1343 cm −1 . ...
... In our case, the G band appeared at around 1583 cm −1 and that is much lower compared to highly-oxidized graphene and is very close to pristine graphite; this can be related to the low oxidation level of GO (C:O is~4). The D and 2D bands originate from the second-order double and triple resonant processes, respectively [53]. The D band appeared at around 1343 cm −1 . ...
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The current study presents research into the effect of graphene oxide (GO) with a carbon to oxygen ratio of 4:1 on the fluidity, hydration, microstructure, mechanical and physical properties of Portland cement pastes and mortars. The amounts of GO investigated were 0.02%, 0.04%, and 0.06% by weight of cement, while for mortars, an extra composition with 0.1% was also prepared. According to the results, the fluidity of cement paste and mortar increased and the hydration process was slightly retarded with the addition of GO. Despite this, improvements in compressive and flexural strength were established in the mortars containing GO. The maximum effects (~22% and ~6%, respectively) were obtained with the addition of 0.06% GO. The calculation of estimated strength proportional to samples of equal density showed that for mortars cured for 7 days the gain in strength was directly related to the gain in density. For mortar samples cured for 28 days, the estimated strength was found to be significantly higher than that of the reference sample, indicating that besides density there are other factors determining the improvement in strength of mortars modified with GO. The possible structure strengthening mechanisms are discussed.
... In a typical Raman spectrum, of the GO, the D band observed at about 1352 cm −1 that is due to sp 3 defects (A 1g symmetry), and the G band appears around 1595 cm −1 , which is related to E 2g phonons [32]. As shown, the D and G band downshift by 14 cm −1 and 16 cm −1 for the IGT-350, respectively, showing partial reduction of GO [37]. However, for the IGT-500 and IGT-650, the G band shifts to 1589 cm −1 showing the "self-healing" that recovers the hexagonal structure and reduces the GO. ...
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We present a facile method to prepare a TiO2-graphene oxide hybrid material with improved photocatalysis and photovoltaic performance. We prepared oxygen-deficient TiO2 nanoparticles (NPs) in an aqueous solution of graphene oxide (GO) by the arc-discharge method and then annealed at temperatures of 350-650 ˚C. This method unzips, reduces, and intercalates the GO sheets in titanium oxide NPs. Results show that GO sheets are decorated with oxygen-deficient TiO2 NPs, while the GO nanosheets are intercalated in NPs due to arc discharge. Raman spectroscopy confirms the GO reduction and its linkage with TiO2 NPs through Ti-O-C bonds. The hybrid materials exhibit improved photocatalytic degradation of Rhodamine B up to 52% in 2h. Our results in photovoltaic tests show improved power conversion efficiency of 8% with the ISC and VOC of 19.4 mAcm⁻² and 570 mV, respectively.
Few-layers graphene is prepared by one-step liquid-phase exfoliation with carbon quantum dots (CQDs) as exfoliating agents, which is prepared by a novel electrochemical interface technology. The morphology, structure and composition of CQDs and graphene, as well as the mechanism for the realization of large-scale preparation of CQDs and graphene was proposed. The results showed that CQDs with D50 size of 55.12 nm has excellent dispersion and exfoliation properties for graphite, and the graphene yield is as high as 97.25% (D50 size of 3.651 μm and 1∼5 layers). Further, we successfully prepared graphene film, which has high electrical conductivity (3100.45 S/cm) and thermal conductivity (950.31 W/(m·K)) after graphitization.
In this study, a novel in-fiber optofluidic trace ethanol sensor is proposed firstly. The microstructured hollow fiber (MHF) with a suspended core is a key part of the overall device which is integrated with graphene oxide (GO). The GO can be uniformly trapped on the whole surface of the suspended core in the MHF by using evanescent field inducing method. When trace microfluidic ethanol passes through the in-fiber device, the light intensity of the suspended core can be significantly modulated through the interaction between the GO on the core and ethanol. The device presents an excellent linearity on-line response with an average sensitivity of 0.16 dB/% with linear regression equation of y = 0.16x + 25.989. In general, this compact optofluidic in-fiber trace ethanol sensor can be utilized as for on-line detection of trace amounts of ethanol in special environments.
This paper reports on the electrochemical performance of activated carbon electrodes in aqueous solutionsof bromide‐based species and discusses the results in the broader context of redox‐active electrolytes in electrochemical capacitor applications. Operando techniques such as Raman spectroscopy and electrochemical quartz crystal microbalance have been implemented to describe the interactions at the electrode/electrolyte interface. Ex situ experiments, including XPS profiles at different electrode thicknesses, support the discussion by providing insights into surficial and bulk‐related processes. Finally, theelectrochemical activity of bromides and their advantages and drawbacks are discussed.
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The combination of electrochemistry and spectroscopy, known as spectroelectrochemistry (SEC), is an established technique. By combining these two techniques, the relevance of the data obtained is greater than what it would be when using them independently. A number of review papers have been published on this subject, mostly written for experts in the field and focused on recent advances. In this review, written for both the novice in the field and the more experienced reader, the focus is not on the past but on the future. The scope is narrowed down to four techniques the authors claim to have most potential for the future, namely: infrared spectroelectrochemistry (IR-SEC), Raman spectroelectrochemistry (Raman-SEC), nuclear magnetic resonance spectroelectrochemistry (NMR-SEC) and perhaps slightly more controversial, but certainly promising, electrochemistry mass-spectrometry (EC-MS).
Hybrids consisting of supercapacitive functionalized graphene (graphene oxide; GO reduced graphene oxide; rGO multilayer graphene; MLG, electrochemically reduced GO; ErGO) and three-dimensional graphene scaffold (rGOHT; hydrothermally prepared) decorated with cobalt nanoparticles (CoNP), nanostructured cobalt (CoO and Co3O4) and manganese (MnO2) oxide polymorphs, assembled electrochemically facilitate chemically bridged interfaces with tunable properties. Since Raman spectroscopy can capture variations in structural and chemical bonding, Raman spectro-electrochemistry in operando i.e. under electrochemical environment with applied bias is employed to 1) probe graphene/metal bonding and dynamic processes, 2) monitor the spectral changes with successive redox interfacial reactions, and 3) quantify the associated parameters including type and fraction of charge transfer. The transverse optical (TO) and longitudinal optical (LO) phonons above 500 cm⁻¹ belonging to Co3O4, CoO, MnO2 and carbon-carbon bonding occurring at 1340 cm⁻¹, 1590 cm⁻¹ and 2670 cm⁻¹ belonging to D, G, and 2D bands, respectively, are analyzed with applied potential. Consistent variation in Raman band position and intensity ratio reveal structural modification, combined charge transfer due to localized orbital re-hybridization and mechanical strain, all resulting in finely tuned electronic properties. Moreover, the heterogeneous basal and edge plane sites of graphene nanosheets in conjunction with transition metal oxide ‘hybrids’ reinforce efficient surface/interfacial electron transfer and available electronic density of states near Fermi level for enhanced performance. We estimated the extent and nature (n− or p−) of charge transfer complemented with Density Functional Theory calculations affected by hydration and demonstrate the synergistic coupling between graphene nanosheets and nanoscale cobalt (and manganese) oxides for applied electrochemical applications.
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Electrochemically reduced graphene was used as the key element in the preparation of electric double-layer capacitors where the thickness of the electrode was only a few hundred nano-meters. The resultant electrodes showed different specific capacitances after pre-reduction with scanning potential windows of −1.0 to 1.6 V, −1.5 to 0 V and −1.0 to 1.0 V. Also, a specific capacitance of 246 F/g was obtained as the graphene oxide electrode was reduced with an applied potential of −1.0 to 1.0 V for 4000 s. The influence of the residual oxygen functional groups and sp2 domains in electrochemically reduced graphene were investigated for capacitance performance.
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Graphene oxide is electrochemically reduced which is called electrochemically reduced graphene oxide (ER-G). ER-G is characterized with scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The oxygen content is significantly decreased and the sp 2 carbon is restored after electrochemical reduction. ER-G exhibits much higher electrochemical capacitance and cycling durability than carbon nanotubes (CNTs) and chemically reduced graphene; the specific capacitance measured with cyclic voltammetry (20 mV/s) is ~165 F/g, ~86 F/g, and ~100 F/g for ER-G, CNTs, and chemically reduced graphene,1 respectively. The electrochemical reduction of oxygen and hydrogen peroxide was greatly enhanced on ER-G electrodes as compared with CNTs. ER-G has shown a good potential for applications in energy storage, biosensors, and electrocatalysis.
Graphene oxide (GO) has attracted intense interest for its use as a precursor material for the mass production of graphene-based materials, which hold great potential in various applications. Insights into the structure of GO and reduced GO (RGO) are of significant interest, as their properties are dependent on the type and distribution of functional groups, defects, and holes from missing carbons in the GO carbon lattice. Modeling the structural motifs of GO can predict the structural evolution in its reduction and presents promising directions to tailor the properties of RGO. Two general reduction approaches, chemical and thermal, are proposed to achieve highly reduced GO materials. This review introduces typical chemical oxidation methods to produce GO from pure graphite, then summarizes the modeling progress on the GO structure and its oxidation and reduction dynamics, and lastly, presents the recent progress of RGO preparation through chemical and thermal reduction approaches. By summarizing recent studies on GO structural modeling and its reduction, this review leads to a deeper understanding of GO morphology and reduction path, and suggests future directions for the scalable production of graphene-based materials through atomic engineering.
A broad review of recent research work on the preparation and the remarkable properties of intercalation compounds of graphite, covering a wide range of topics from the basic chemistry, physics and materials science to engineering applications.
Electrochemically reduced graphene oxides (ERGO) are obtained under various reducing potentials in the phosphate buffer solution (PBS). Different characterization methods are used to analyse the changes of structure and surface chemical condition for graphene oxide (GO). The results show that GO could be reduced controllably to certain degree and its electrochemical activity towards VO2+/VO2+ and V3+/V2+ redox couples is also tunable using this environmentally friendly method. The catalytic mechanism of the ERGO is discussed in detail, the CO functional groups other than the C–O functional groups on the surface of ERGO more likely provide reactive sites for those redox couples, leading to a more comprehensive understanding about the catalytic process than previous relevant researches. This controllable modification method and the ERGO as electrode reaction catalyst with enhanced battery performance are supposed to have promising applications in the all vanadium redox flow battery.
Electrochemical reduction of graphene oxide (GO) films cast on tin oxide glass substrates were carried out in aqueous solutions and in propylene carbonate and acetonitrile. Cyclic voltammetric measurements indicate successful reduction of GO films. The reduction process begins in aqueous solutions at less negative potentials than in organic solutions. Additionally the pH value of the aqueous solutions influences the reduction potential. According to spectroscopic analysis the reduction process of the GO film can be controlled by the choice of reduction potential and electrolyte medium. The potential window in this work was made broader by increasing pH or by using organic electrolyte media. Infrared and energy-dispersive X-ray spectroscopy measurements show that the use of more negative potentials lead to more efficient reduction of the GO films and that also the used solvent has an effect on reduction.
Graphene oxide (GO) cast on conductive substrates was electrochemically reduced in some organic solvents. The amount of electricity required for the almost complete reduction of GO was 2.0C for 1mg GO, corresponding to attaching of a one-electron reducible species to each benzene ring in graphene. The electrochemically reduced GO film gave an electrical conductivity of about 3Scm−1 and exhibited a relatively high specific capacitance of 147.2Fg−1 in propylene carbonate. The electrochemical reduction of GO was feasible on Al foils as well.
When following by XPS the evolution of a series of graphitic materials submitted to either mechanical, chemical or thermal treatment, the addition of a “defect peak” to the classical pristine asymmetric line appears imperative in the fitting. This new peak is broader than and very slightly shifted from the graphitic peak. Its intensity depends on the consequence of the treatment: creating or destroying the crystalline/molecular order. It may be in confidence attributed to carbon atoms located in defective regions and surely out of sp2 graphitic configuration.Fitting the C1s XPS spectrum in such a way is proven to be accurate during graphitization of anthracene semi-cokes and during milling of graphite powders under different atmospheres.
A theory is formulated that explains the observed bond-length change in graphite intercalation compounds in terms of the charge transfer f. The values of f obtained via the bond-length changes are in good agreement with those derived with other methods. The present analysis also provides information on the electron-phonon coupling that defines the maximum conductivity.
The model and theoretical understanding of the Raman spectra in disordered and amorphous carbon are given. The nature of the G and D vibration modes in graphite is analyzed in terms of the resonant excitation of π states and the long-range polarizability of π bonding. Visible Raman data on disordered, amorphous, and diamondlike carbon are classified in a three-stage model to show the factors that control the position, intensity, and widths of the G and D peaks. It is shown that the visible Raman spectra depend formally on the configuration of the sp2 sites in sp2-bonded clusters. In cases where the sp2 clustering is controlled by the sp3 fraction, such as in as-deposited tetrahedral amorphous carbon (ta-C) or hydrogenated amorphous carbon (a-C:H) films, the visible Raman parameters can be used to derive the sp3 fraction.