Reduction of graphene oxide by an in-situ photoelectrochemical method in a dye-sensitized solar cell assembly
© Chen et al; licensee Springer. 2012
Received: 17 November 2011
Accepted: 2 February 2012
Published: 2 February 2012
Reduction of graphene oxide [GO] has been achieved by an in-situ photoelectrochemical method in a dye-sensitized solar cell [DSSC] assembly, in which the semiconductor behavior of the reduced graphene oxide [RGO] is controllable. GO and RGO were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, high-resolution transmission electron microscope, and Fourier-transform infrared spectroscopy. It was found that the GO film, which assembled in the DSSC assembly as the counter electrode, was partly reduced. An optimized photoelectrochemical assembly is promising for modulating the reduction degree of RGO and controlling the band structure of the resulting RGO. Moreover, this method appeared to be a green progress for the production of RGO electrodes.
Keywordsgraphene oxide reduction photoelectrochemical J-V curve dye-sensitized solar cells
Nowadays chemical conversion of solar energy has attracted considerable attention [1–3]. Graphene is a new carbon material with diverse properties being suitable for energy conversion and storage . Graphene oxide [GO], produced by exfoliation of graphite oxide, has been traditionally considered to be a precursor for graphene [5–7]. GO has recently attracted research interest due to its good solubility in water and other solvents, which allows it to be easily deposited onto a wide range of substrates [8, 9]. Besides, GO has variable optical, mechanical, and electronic properties that can be tuned by controlling the degree of oxidation [10, 11]. Reduced graphene oxide [RGO], characterized as an incompletely reduced product of GO, is the intermediate state between graphene and GO. Because the oxygen bonding forms the sp3 hybridization on RGO  and oxygen atoms have a larger electronegativity than carbon atoms, RGO becomes a doped semiconductor where the charge flow creates negative oxygen atoms and a positively charged carbon grid [13, 14]. However, understanding the controllable semiconductor behavior of RGO is still a big challenge. Normally, the bandgap of RGO increases with the oxidation level. Controlling the ratio of sp2 carbon atoms to sp3 carbon atoms by reduction chemistry is a powerful way to tune its bandgap. Therefore, RGO can be controllably transformed from an insulator (GO) to a conductor (graphene) [10, 15]. Owing to these characteristics, RGO has great potential to be applied in biosensors , optical devices , plastic electronics , and solar cells . The reduction of GO is typically achieved by thermal annealing and exposure to hydrazine gas, as described in former cases [20–22]. These methods involved either high temperatures or a poisonous and explosive gas.
In this work, we present a green and controllable approach for the in-situ reduction of GO in a dye-sensitized solar cell [DSSC] assembly. The GO film was fabricated in the DSSC as a counter electrode. In typical DSSCs, upon illumination, photoinduced electrons from the excited dye transfer toward the conduction band of TiO2 photoanodes, accompanying the oxidation of redox species in the electrolyte (e.g., I-/I3-), and simultaneously, the reduction reaction occurs at the counter electrodes by accepting the electrons. By substituting the Pt counter electrode with the GO film, the photoinduced electrons could be captured by GO and result in the reduction of GO. Inspired by this, in this contribution, we provide an easy approach to an in-situ photoelectrochemical reduction of GO with a GO drop-cased fluorine-doped tin oxide [FTO] glass as the counter electrode in a DSSC assembly. Moreover, according to the transition mechanism, an optimized photoelectrochemical assembly can be fabricated for the controllable modulation of the band positions of RGO materials.
Preparation of GO
GO was synthesized from natural graphite powder (100 μm; Qingdao Graphite Company, Qingdao, Shandong, China) by a modified Hummers' method . In a typical experiment, the graphite powder (1 g) and NaNO3 (0.5 g) were introduced to concentrated H2SO4 (23 mL) in an ice bath. KMnO4 (3 g) was added gradually under stirring to prevent rapid temperature rise, and the temperature of the mixture was kept below 20°C. The mixture was then stirred at 35°C for 4 h. Then, deionized water (46 mL) was slowly added to the solution, followed by stirring the mixture at 98°C for 15 min. The reaction was terminated by adding deionized water (140 mL) and H2O2 (1 mL, 30 wt.%) under stirring at room temperature. The resulting graphite oxide was washed with deionized water by filtration. Graphene oxide was obtained from the graphite oxide solution by ultrasonication at room temperature for 30 min. Unexfoliated graphite oxide in suspension after ultrasonication was removed by centrifugation at 3,000 rpm for 5 min.
Fabrication of DSSCs
To realize the in-situ photoelectrochemical reduction, GO counter electrode was used for DSSCs. The GO electrode was prepared by drop casting the GO solution of 1 mg/ml on a clean FTO glass substrate and dried in room temperature. N719-sensitized TiO2 film anode was prepared according to the literature method . In brief, 1.6 g of nanocrystalline TiO2 and 0.7 g of ethyl cellulose were suspended with 6 mL of terpilenol. Five layers of 20-nm-sized TiO2 particles and two layers of 400-nm-sized TiO2 particles were screen printed on a TiCl4-treated FTO glass. These films were heated to 500°C in air and sensitized with a 0.36 mg/ml N719 dye solution for 24 h. The cell had an active area of 0.36 cm2 and was sealed with an electrolyte solution containing 0.1 M lithium iodide, 0.05 M iodine, 0.5 M 4-tert-butylpyridine, and 0.6 M ionic liquid (1, 2-Dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide).
Solar conversion efficiency and current density-voltage [J-V] curves were measured under air mass [AM] 1.5 G light with a solar simulator (HMT Co., Bangalore, India), and a potentiostat (Keithley 2400, Keithley Instruments Inc., Cleveland, OH, USA) was used to apply various loads. The incident light intensity was calibrated using a standard solar cell composed of a crystalline silicon solar cell and an infrared cutoff filter (KG-5, Schott AG, Mainz, Germany).
Raman spectra were obtained on a Senterra R200-L dispersive Raman microscope (Bruker Optik Gmbh, Ettlingen, Germany) with a 633-nm laser source. The morphology and structure were observed by a JEM-2100F high-resolution transmission electron microscope [HRTEM] (JEOL Ltd., Akishima, Tokyo, Japan) operated at 200 kV. Fourier transform infrared [FT-IR] spectroscopy was conducted using a Fourier transform infrared spectrometer (EQUINOX 55, Bruker Optik Gmbh, Ettlingen, Germany). X-ray photoelectron spectroscopy [XPS] experiments were carried out on a RBD-upgraded PHI-5000C ESCA system (PerkinElmer, Waltham, MA, USA) with AlKα radiation (hv = 1486.6 eV).
Results and discussion
To confirm the reduction of GO, Raman spectroscopy, a powerful nondestructive tool to characterize crystal structures of carbon, was employed. The typical features for carbon in Raman spectra are the G line around 1,582 cm-1 (E2gphonon of sp2 carbon atoms) and D line around 1,350 cm-1 (κ-point phonons of A1gsymmetry). Figure 2b shows the Raman spectra of GO and RGO. The intensity ratio (ID/IG) is about 0.96 for GO, while the ratio of RGO is much higher (1.27). Comparing with the results by chemical reduction methods, such as NaBH4 (> 1) , hydrazine hydrate (1.63) , and hydrothermal reduction (0.90) , the high ratio of 1.27 implies that GO on the FTO glass was reduced significantly by the photoinduced electrons in the DSSC.
Percentages of oxygen-containing groups of GO and RGO from XPS data
RGO obtained by in-situ reduction was analyzed by HRTEM. Figure 3b shows a low magnification image of a typical RGO nanosheet. The sheets resemble crumpled silk veil waves on the carbon-coated copper grid. As reported previously, corrugation and scrolling are intrinsic to graphene nanosheets . The ordered graphene lattices are clearly visible in the HRTEM image of RGO (Figure 3c). A 0.39-nm intersheet spacing obtained from this image indicates a moderate oxidation level of RGO because the layer distance of typical oxidized graphite is between 0.6 to 0.7 nm .
RGO is obtained by the in-situ photoelectrochemical reduction of GO in a DSSC assembly. The reduction results in a partial removal of oxygen-containing groups of GO. This method avoids the high-temperature processing and the usage of harmful chemical reagents. The reduction of GO is changeable by controlling the irradiation time or substituting the reduction couples of I-/I3- in the electrolyte; a further improved performance can be achieved by the optimized photoelectrochemical assembly.
This work was financially supported by the National Natural Science Foundation of China (No. 20907031).
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