Solid-phase electrochemical reduction of graphene oxide films in alkaline solution
© Basirun et al.; licensee Springer. 2013
Received: 21 July 2013
Accepted: 12 September 2013
Published: 24 September 2013
Graphene oxide (GO) film was evaporated onto graphite and used as an electrode to produce electrochemically reduced graphene oxide (ERGO) films by electrochemical reduction in 6 M KOH solution through voltammetric cycling. Fourier transformed infrared and Raman spectroscopy confirmed the presence of ERGO. Electrochemical impedance spectroscopy characterization of ERGO and GO films in ferrocyanide/ferricyanide redox couple with 0.1 M KCl supporting electrolyte gave results that are in accordance with previous reports. Based on the EIS results, ERGO shows higher capacitance and lower charge transfer resistance compared to GO.
Graphene, a one-dimensional carbon sp2-bonded compound is finding considerable attention in the development of advance nanomaterials. Chemically modified graphene is studied for their importance in biomedical sensors, composites, field-effect transistors, energy conversion, and storage applications due to its excellent electrical, thermal, and mechanical properties. Reduced graphene oxide (RGO) can be produced by the reduction of graphene oxide (GO) by various methods. High temperature annealing of GO above 1,000°C is an effective method to produce RGO . Sodium borohydride  and hydrazine [3–5] are also acceptable chemical methods for the reduction of GO to produce the RGO. Among the methods to synthesize RGO are by chemical exfoliation of GO in propylene carbonate followed by thermal reduction [4, 5]. Another method of reduction of GO is by using hydrohalic acids . Nutrients such as vitamin C [7, 8] and metallic element such as aluminum powder  are also viable reducing agents for the production of RGO from GO. Hydrothermal reduction is also an effective method for the reduction of GO to RGO . Electrochemical reduction to produce RGO or better known as electrochemically reduced graphene oxide (ERGO) is considered a green method which offers safer procedures compared to other chemical methods of reduction without the use of dangerous chemicals such as hydrazine. A suspension of GO was evaporated on glassy carbon and used as an electrode and reduced by voltammetric cycling in 0.1 M Na2SO4 solution to produce ERGO films . Electrochemical reduction of GO suspensions were also done in acidic media using phosphate buffer solution at pH 4  and basic pH at 7.2 . Direct electrochemical reduction of GO onto glassy carbon has also been reported  in sulfuric acid  and in NaCl solution . Electrophoretic deposition of GO to produce ERGO is also an effective method to produced solid films of ERGO . Several authors [11, 13, 14, 18] have performed voltammetric cycling of exfoliated GO sheets from colloidal suspensions and found that electrochemical reduction for different functional groups in GO are dependent on the reduction potential. In this work, voltammetric cycling was used to electrochemically reduce GO films to ERGO in KOH solution.
All chemicals such as KOH, KCl, K4[Fe(CN)6], and K3[Fe(CN)6] were of Analar grade and procured from Sigma Aldrich (St. Louis, MO, USA).
Synthesis of GO
GO was synthesized using a modified Hummers' method . GO was dispersed in a beaker filled with distilled water and sonicated for 5 h. GO dispersion with a concentration of 0.3 mg cm-3 was poured on a graphite sheet in the jar and evaporated overnight in an oven at 60°C.
Field emission scanning electron microscopy (FESEM) using a Quanta 200F instrument (FEI, Hillsboro, OR, USA), was used to capture the images of the evaporated GO and ERGO layers on the graphite sheet. Fourier transformed infrared (FTIR) spectroscopy was carried out using Spectrum 400 instrument while Raman spectroscopy was done with a Renishaw inVia Raman microscope (Wotton-under-Edge, UK) using (λ = 514 nm) laser excitation.
Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were done using a potentiostat / galvanostat, Autolab PGSTAT-302N from Ecochemie (Utrecht, the Netherlands). A general purpose electrochemical software installed in the computer interfaced with a USB card (USB_IF030) was used to run the CV experiments while frequency response analysis (FRA) software was used to run the EIS experiments. The CV and EIS experiments were done in a single compartment cell. A mercury oxide (Hg/HgO) reference electrode (RE) and graphite rod counter electrode (CE) was used in the voltammetric cycling for the reduction of GO films in 6 M KOH solution at a scan rate of 50 mV·s-1. The CV experiments performed in 6 M KOH solution and [FeII(CN)6]3-/4- redox couple in 0.1 M KCl supporting electrolyte were done on stationary electrodes. A two-electrode configuration was used in the EIS experiments using the working electrodes (WE), and a saturated calomel electrode (SCE) as the reference and counter electrode (RE-CE). The EIS measurements were performed over a frequency range of 100 kHz to 10 mHz, with an acquisition of 10 points per decade, and with a signal amplitude of 5 mV around the open circuit potential. Analysis of the impedance spectra was done by fitting the experimental results to equivalent circuits using the nonlinear least-square fitting procedure with the chi-squared value minimized to 10-4. All experiments were performed at room temperature 300 K.
Results and discussions
Voltammetric reduction of GO to ERGO
It should be noted that different types of graphene such as graphene nanosheets  and porous graphene  are also good electro-catalysts for ORR in lithium-air cells. Graphene-based materials are also finding importance in the ORR such as chemically converted graphene , nitrogen-doped graphene , polyelectrolyte-functionalized graphene , and graphene-based Fe-N-C materials . Therefore, the higher current and charge for each scans for the oxygenated solutions are due to the ORR which occurs concurrent with the reduction of GO to ERGO. When the solution was deoxygenated, the total charge for the negative scan was always higher than the total charge for the positive scan. This trend reveals that there was a net reduction current for each scan that could be attributed to the electrochemical reduction of GO to ERGO in the deoxygenated solution.
FTIR and Raman spectra
Figure 2b shows the Raman spectra for GO and ERGO, respectively, where two typical peaks for GO can be found at 1,361 and 1,604 cm-1, corresponding to the D and G bands, respectively. The D band is assigned to the breathing mode of A1g symmetry due to the phonon interaction near the K zone boundary, while the G band is attributed to the E2g phonon mode of the sp2-bonded carbon atoms . The D and G bands of ERGO were shifted to lower wave numbers of 1,352 and 1583 cm-1, respectively, compared to GO. The intensity ratio of the D to G peak (ID/IG) is an indication of the degree of defects in graphene-related materials where the intensity of the D band is related to the disordered structure of the sp2 lattice . For example, pristine graphite which has the lowest disorder density in the sp2 lattice gave a ratio of 0.23, while thermally reduced graphene oxide which has the highest disorder density gave a ratio of 1.35 . In this work, the ratio of the ID/IG peak for ERGO is 1.03, while the ID/IG peak for GO (measured from the nearest baseline) is 1.02. This result is in accordance with previous reports of 1.08 and 1.05 for ERGO and GO, respectively . This result indicates that GO reduction to ERGO did not increase the defect density significantly. It can be suggested that the sp2 lattice was maintained even after reduction of GO to ERGO and this is also in accordance with the FTIR of ERGO where the sp2-hybridized C=C bonds are still present in ERGO at around 1,610 cm-1.
FESEM and EIS
The equivalent circuit model can be explained as follows: the R1 is the solution resistance between the RE-CE and the WE. The R1 is in series with a parallel arrangement of the constant phase element (CPE - denoted as Q in the equivalent circuit) and the R2-W elements. The CPE was introduced instead of a pure capacitor in the simulations to obtain a good agreement between the experimental and simulation data. The CPE impedance can be defined as ZCPE = Q-1.(jω)-n where “n” is related to the slope of log Z vs. log f in the Bode plot, ω is the angular frequency and Q is the combination of properties related to both the surface and the electro-active species, and is independent of frequency. The CPE depends on both the parameter Q and the exponent “n,” but it should be stressed that Q is often approximated to capacitance. The CPE is in parallel arrangement with R2-W elements, where R2 is the charge transfer resistance which is in series with the Warburg element W. The circuit diagram is consistent with earlier results using the [FeII(CN)6]4- / [FeIII(CN)6]3- redox couple in solution .
Parameters of GO and ERGO obtained using EIS
Q(S·s n )
1.5 × 10- 6
1.99 × 10- 3
6.66 × 10- 7
8.04 × 10- 6
3.47 × 10- 3
3.30 × 10- 6
Cyclic voltammetry in [FeII(CN)6]3-/4- redox couple
Solid-phase electrochemical reduction of GO films on graphite in alkaline solution produced ERGO which was confirmed with FTIR and Raman spectra. The EIS results obtained using [FeII(CN)6]4-/[FeIII(CN)6]3- redox couple in 0.1-M KCl supporting electrolyte indicated that the charge transfer resistance for ERGO is lower than GO and is consistent with the higher electrical conductivity of ERGO. The results also reveal that the capacitance of ERGO is larger than GO, due to its higher polarity of ERGO. This result is also supported by voltammetry of both GO and ERGO in [FeII(CN)6]4-/ [FeIII(CN)6]3- redox couple in 0.1-M KCl supporting electrolyte, where ERGO surface has a larger separation of the anodic and cathodic baseline currents due to the larger capacitance compared to the GO surface.
The authors would like to thank University Malaya and Ministry of Higher Education for providing financial assistance with grant number FP033-2013A and RG181-12SUS for this work.
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