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An environment-friendly route to synthesize reduced graphene oxide as a supercapacitor electrode material
Electrochimica Acta 69 (2012) 364–370
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
An environment-friendly route to synthesize reduced graphene oxide as a supercapacitor electrode material Dacheng Zhang, Xiong Zhang, Yao Chen, Changhui Wang, Yanwei Ma ∗ Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 October 2011 Received in revised form 5 February 2012 Accepted 3 March 2012 Available online 15 March 2012 Keywords: Graphene Glutathione Supercapacitors Specific capacitance
a b s t r a c t A large-scale, environment-friendly method to produce water-soluble reduced graphene oxide by using glutathione as a reducing and stabilization agent has been developed. The results of UV–visible absorption spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron microscopy indicate that graphene oxide is reduced to graphene nanosheets which are single-layers and exhibit good dispersion in water. A reaction mechanism is proposed. The electrochemical properties of the graphene nanosheets as electrode materials for supercapacitors are studied by cyclic voltammetry and galvanostatic charge/discharge tests. A maximum specific capacitance of 238 F g−1 in a 1 M H2 SO4 electrolyte has been obtained. Meanwhile, the material shows excellent long-term cycle stability along with the retention of 97% for specific capacitance after 1000 cycle tests. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Supercapacitors, also called electrochemical capacitors (ECs) which include pseudocapacitors and double-layer capacitors, having higher specific power density and longer cycle-life than secondary batteries and higher specific energy than conventional capacitors are attracting much attention [1]. The performance of supercapacitors highly depends on the properties of electrode materials. Additionally, the environment-friendly materials for supercapacitors are very important in response to the needs of modern society and emerging ecological concerns [2]. Graphene, a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, has attracted intense scientific interest due to its superior electrical conductivity, high specific surface area, and chemical stability [3]. With these unique properties, graphene is expected to be applied in supercapacitors [4–6]. Nowadays, many researchers obtain graphene by several approaches, including micromechanical exfoliation [7], thermal expansion [8], chemical vapor deposition [9], carbonization [10], liquid-phase reduction from graphene oxide (GO) [11], and so on. Among these methods, micromechanical exfoliation and chemical vapor deposition can produce high quality monolayer graphene but they are of low yield and uneasy operation. In contrast to them, liquid-phase reduction is the most promising techniques because the graphene derived from GO can not only be prepared
∗ Corresponding author. Fax: +86 10 82547137. E-mail address: [email protected] (Y. Ma). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.03.024
by mass production, but also shows good connectivity for supercapacitors between electrolyte and electrode materials because of the hydrophilic nature of this reduced GO. Representative work on liquid-phase reduction, including Stankovich et al. as the pioneer offers a simple and scalable method for efficient production of graphene by liquid-phase reduction with hydrazine. Our group has also developed a new approach to creating colloids of graphene monolayers with p-phenylene diamine as reducing reagent [12]. Unfortunately, the use of highly toxic reducing agent and aggregation of graphene will limit its practical applications such as supercapacitors. In response, many attempts have been made to develop environmentally friendly approaches for the reduction of GO by potassium hydroxide, l-ascorbic acid, sugar, protein, dextran, bacterial respiration, tea and photocatalytic methods [13–27]. Through their efforts, it is demonstrated that there is a possibility that graphene can be readily reduced for large-scale production compared with other methods such as micromechanical exfoliation under the environment-friendly condition. Additionally, Hu et al. have used the graphene which was produced by environment-friendly methods as electrode materials for supercapacitors [28–30]. Thus, in this work, we have developed another reduction process with glutathione to prepare the graphene materials, and fabricated supercapacitor devices using this graphene as electrode materials and investigated their electrochemical properties. Glutathione (C10 H17 N3 O6 S) is the major endogenous antioxidant produced by the cells, participating directly in the neutralization of free radicals and reactive oxygen compounds, as well as maintaining exogenous antioxidants such as vitamins C and E. Glutathione always exists in reduced (GSH) and oxidized (GSSG) states in the cells. In the
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Fig. 1. The illustration of reaction pathway for the reduction of GO with GSH and photographs of the GO and GG dispersion in water.
reduced state, the thiol group of cysteine is able to donate a reducing equivalent (e− ) to other unstable molecules, such as reactive oxygen species [31,32]. Herein, we demonstrated that GSH could effectively remove the oxygen on the GO and its oxidized products play an important role in stabilizing graphene simultaneously. The reduced GO was stably dispersed in water more than half of a year without extra stabilizing agent. Furthermore, the supercapacitors based on as-made graphene nanosheets showed good electrochemical performances, higher capacitance, better rate capability and longer cycle life. 2. Experimental 2.1. Sample preparation
Gaussian–Lorentzian components after a Shirley background subtraction by the software of XPSPEAK41. Atomic force microscopy (AFM) images were taken out using a Nanoscope MultiMode SPM (Digital Instruments) operated in tapping mode in conjunction with a V-shaped tapping tip (Applied Nanostructures SPM model: ACTA). Raman spectra were obtained on a RM 2000 microscopic confocal Raman spectrometer (Renishaw in Via Plus, England) employing a 514 nm laser beam. Transmission electron microscopy (TEM) was carried out using a JEOL JEM-2100F TEM operating at an accelerating voltage of 300 kV. Conductivity was measured using Physical Property Measurement System (PPMS). Thermo-gravimetric analysis (TGA) was accomplished by means of an NETZSCH STA-449C under nitrogen gas flow from room temperature to 900 ◦ C at a heating rate of 5 ◦ C min−1 .
The exfoliated GO was prepared using a modified Hummers method [33]. GO (0.1 g) in water was ultrasonicated to achieve GO dispersion. The pH of the GO dispersion in water (1 mg mL−1 ) was adjusted to 10 using a 28% ammonia aqueous solution. 50 mL of GSH (0.1 g, procured from Aldrich) solution was then added to the mixture. The mixture was vigorously stirred at 90 ◦ C for 24 h. Finally, the resulting stable black dispersion was filtrated with water for three times. Then, the obtained graphene nanosheets (signed as GG) can be dried at 60 ◦ C for 12 h or redispersed in water for different use. In addition, the thin film was also prepared by vacuum filtration using a membrane filter and then dried at 60 ◦ C for 12 h. We also prepared graphene by hydrazine (HG) as a comparison for electrochemical performance according the literature [34]. 2.2. Characterization UV–visible (UV–vis) absorption spectra were detected using Ultraviolet spectrophotometer (Hitachi UV2800). X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI Quantear SXM (ULVAC-PH INC) which used Al as anode probe in 6.7 × 10−8 Pa. In the XPS data analysis, peak deconvolution was performed using
Fig. 2. UV–vis absorption spectra of GO and GG prepared from different reaction times and reactant ratios. GG1 and GG2 stand for reactant mass ratio (GSH/GO) = 1 and 2, respectively.
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2.3. Electrochemical measurements The working electrode was prepared by pressing a mixture of polyvinylidene fluoride (PVDF), acetylene black and active materials at a weight ratio of 1:2:7 onto a stainless steel with a spatula, and dried at 100 ◦ C for 12 h. The counter and the reference electrodes were Pt plate and saturated calomel electrode (SCE), respectively. All electrochemical tests were carried out using an electrochemical system in 1 M H2 SO4 at room temperature. Cyclic voltammetry (CV) was carried out at scan rates ranging from 10 to 500 mV s−1 within the potential range of 0–0.9 V (vs. SCE). Galvanostatic charge/discharge was carried out at current densities ranging from 0.1 to 5 A g−1 within the potential range of 0–0.9 V (vs. SCE). Electrochemical impedance spectrometry (EIS) was recorded under the conditions: AC voltage amplitude of 5 mV, frequency range from 105 to 0.1 Hz. Cycle-life test was carried out through galvanostatic charge/discharge at the current density of 2 A g−1 for 1000 cycles.
3. Results and discussion The typical GG can be easily prepared by stirring GO and GSH aqueous dispersion at 90 ◦ C. As shown in Fig. 1, the color of reagent begins to turn black from yellow after 2 h reduction. The reduction progress of GO was first monitored by UV–vis spectroscopy (Fig. 2) with different reaction time and reactant ratios. As shown in Fig. 2, with the reduction progressing, the UV–vis absorption peak at 225 nm, which is caused by the GO, gradually red-shifted to 251 nm over the reaction time (24 h) for the reactant mass ratio (GSH/GO) = 1. This suggests that GO is reduced and the electronic conjugation within the graphene sheets is restored. While increasing the mass ratio of GSH/GO to 2, the effect of reduction is better than the former reaction because the UV–vis absorption peak shifted to 261 nm, which is comparable to the reductants sugar (261 nm) [14] and l-ascorbic acid (264 nm) [15]. We employed XPS to further confirm the reduction of GO by GSH. Fig. 3 shows the C1s XPS spectra of GO and GG. From the C1s XPS spectrum of GO (Fig. 3a), four different peaks centered at 284.5, 286.4, 287.8, and 289.0 eV, are corresponding to C C/C C in aromatic rings, C O (epoxy and alkoxy), C O, and carboxyl (COOH) groups, respectively [14]. After reduction by the GSH, the intensities of all C1s peaks of the carbon binding to oxygen, especially the peak of C O (epoxy and alkoxy), decrease remarkably (Fig. 3b), suggesting that most oxygen functional groups are successfully removed. This result shows that the as-synthesized GG has been partially repaired from sp3 hybridized carbon atoms dominated in GO to graphite structure of sp2 hybridized carbon atoms [35]. Fig. 4a and b shows the AFM image and section analysis of GO and GG deposition on a freshly cleaved mica. The average of thickness of GG is less than that of GO, indicating the loss of oxygen functional groups. The graphene nanosheet is nearly flat on the mica with a length of several hundred nanometers and a height of 0.76 nm, which confirms that GG is a single layer. On the other hand, we can see some small points on the plane of graphene, which are maybe extra GSH or its oxidation product adsorbed on the graphene and the residual hydrophilic groups. Hence, the as-made graphene can form stable dispersion more than half of a year in water without surfactant. Fig. 4c and d shows the TEM images of the large GG obtained by GSH reduction at different magnifications. It can be seen that the graphene is very thin and contains some corrugations and scrolling on the surface, which is consistent with previous work [16]. Raman spectroscopy is a non-destructive technique that is widely used to obtain structural information about carbon-based materials and recently for graphene [36–40]. Raman spectra for GO
Fig. 3. C1s XPS spectra of GO (a) and GG (b).
and obtained GG are shown in Fig. 5. The fluorescence background for the GO spectra was subtracted using the Wire 2.0 Renishaw software. It is well known that there are two typical peaks, named G and D bands, in the Raman spectra of graphitic carbon-based materials. The G band corresponds to the optical E2g in-plane vibration phonon at the Brillouin zone center, while the D band is ascribed to the breathing mode of aromatic which requires a defect for its activation [41]. The intensity of D band is therefore often used as an indicator for the degree of disorder in carbon-based materials. It can be seen that in the Raman spectrum of GG (Fig. 5), the G band is shifted upward to 1604 cm−1 , and the intensity of the D band at 1351.2 cm−1 increase substantially. The intensity of D/G ratio increases from 0.84 to 0.99, indicating that the in-plane sp2 domains of graphitic are smaller in size than those of GO, but more numerous in number [11], which is due to the presence of unrepaired defects that remained after the removal of oxygen moieties. It is also confirmed by the conductivity tests for the GG film. The conductivity of the film is 15.6 S m−1 , which is several orders of magnitude larger than that of GO. The reduced GO by using GSH as the reductant shows as good electrical properties as those produced by other environment-friendly reductants [15,19]. However, some GSH or its oxidation product adsorbed on the graphene surface and the residual hydrophilic groups on the graphene will degrade the conductivity of the GG. Moreover, it has been recently reported that the shape and position of 2D band which can estimate how many layers of graphene [42]. The 2D peaks of GG are located at about 2698 cm−1 which suggests formation of double-layer graphene. Nevertheless, AFM results show single-layer graphene has been
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Fig. 4. (a) and (b) A tapping mode AFM image and section analysis of GO and GG absorbed on freshly cleaved mica; (c) and (d) TEM images of GG at different magnifications.
obtained. The difference is mainly attributed to restack when GG powder was dried. To evaluate the thermal stability of graphene, TGA was performed to characterize the mass loss with the temperature. As seen
in Fig. 6, there is about 10% mass loss for both GO and GG below 100 ◦ C which accounts for the volatilization of water molecule. The residual mass of GG had a slow decrease with temperature elevation but still remained 65% of initial mass at 900 ◦ C while that of
Fig. 5. Raman spectra of GO and GG.
Fig. 6. TGA curves of GO and GG.
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Fig. 7. Proposed reaction mechanism for the chemical reduction of GO with GSH.
Fig. 8. (a) and (b) CV curves of GG at various scan rates with potential from 0 V to 0.9 V (vs. SCE) in 1 M H2 SO4 ; (c) galvanostatic charge/discharge curves of GG at different current densities of 0.1, 0.5 and 1 A g−1 ; (d) the specific capacitances of GG and HG at different current densities; (e) capacitance retention ratio of GG from the 1st to 1000th cycle at the current density of 2 A g−1 ; (f) the Nyquist impedance plots of GG in the frequency range of 100 kHz to 0.1 Hz (main chart) and high frequency area (inset image) after 1st cycle and 1000th cycle m2 .
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GO has sharply decreased to only 30% in the same condition. As a result, GSH can partially remove oxygen functional groups, especially C O (epoxy and alkoxy) and the thermal stability of GG is much better than that of GO. From the above results, it is clearly demonstrated that GO was successfully reduced by GSH. We propose a possible mechanism for deoxygenation of GO with GSH as shown in Fig. 7. Like hydrazine or l-ascorbic acid reducing GO [11,15], firstly, HS- groups of GSH attack the oxygen atom of epoxides in GO to form C S by SN 2 nucleophilic reactions, and finally, the two neighboring C S are cleaved by a thermal elimination. The reduced product was washed by water through filter. The GSSG in the resulting filtrate was detected by a biology method as shown in Fig. S1. To further estimate the performance of GG as electrode material for supercapacitors, various electrochemical tests, including CV, galvanostatic charge/discharge and EIS, were carried out. Fig. 8a and b shows that the CV curves remain rectangle shape very well from a scan rate of 10–500 mV s−1 . This pure capacitive characteristic is further proved by galvanostatic charge/discharge tests because of near triangle shapes as shown in Fig. 8c. However, an irreversible oxidation at potentials above 0.8 V is also observed at small current load because the active material has been fully reaction including some extra irreversible oxidations under this condition. As a result, the GG as electrode material for supercapacitor application should be fine at potentials equal to or below 0.8 V. Moreover, we can also find a weak redox peak at about 0.4 V. The appearance of redox peak shows that some part of capacitance is contributed by pseudocapacitance because of oxygen groups on the GG [5,43,44]. In addition, A BET surface area of 317 m2 g−1 has been obtained, which is comparable to that of other reduced graphene oxide. As shown in Fig. S2, N2 adsorption/desorption plot and pore size distribution shows that the GG is micro-porous. Micro-porous (less than 1 nm) may increase capacitance for carbon materials [45]. Thus, the capacitance of GG is the co-contribution of double layer capacitance and pseudocapacitance from oxygen groups. The specific capacitance of GG electrode using galvanostatic charge/discharge curves can be calculated according to the following formula: C=
I −(U/t)m
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transfer easier. It could be obviously seen that the impedance spectra almost have no changes, also indicating that the graphene have good cyclic stability. A semi-circle arc and a vertical line have been observed. The semi-circle arc becomes smaller with cycle numbers increase, indicating better penetration of electrolytes [50,51]. The low frequency straight line is nearly parallel to the imaginary axis, also suggesting that the capacitive behavior of GG is very good [52,53]. 4. Conclusions We have provided an environmental-friendly, cost-effective, and scalable approach for the preparation of water-soluble graphene nanosheets through the reduction of GO by using glutathione as reducing and stabilization agent. A possible mechanism for the reduction of GO by the GSH was suggested. Furthermore, the specific capacitance of GG as electrode materials for supercapacitors was as high as 238 F g−1 at the current density of 0.1 A g−1 in 1 M H2 SO4 electrolyte, and remained 97% of initial value at the current density of 2 A g−1 after 1000 cycles. Further optimize reductant and control precursor ratios to exploit better graphene-based supercapacitors are under investigation in our group. The biocompatible reduced GO could be also used in the fields of biosensors and biomaterials in the future. Acknowledgements This work was partially supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (No. KJCX2-YWW26), Beijing Municipal Science and Technology Commission (No. Z111100056011007), the National Natural Science Foundation of China (Nos. 21001103 and 51025726) Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2012.03.024. References
(1)
where I is the discharge current, U/t is the average slope of the discharge curve after the IR drop, m is the mass of active material in a single electrode. As shown in Fig. 8d, the specific capacitance of GG reaches as high as 238 F g−1 at the current density of 0.1 A g−1 , which is comparable to that of most graphene prepared by liquid-phase reduction [4,46–48], and remains 140 F g−1 at 5 A g−1 , suggesting that its rate capability is good. Additionally, we also prepared graphene by hydrazine (HG) as a comparison. As can be seen from Fig. 8d, this method obtained comparable capacitance with HG but ensures more environmental friendliness. In addition, long cycle-life and excellent stability of GG, which are required by many applications, are also demonstrated. Electrochemical stability of GG at the current density of 2 A g−1 for 1000 cycles is shown in Fig. 8e. The capacity retention of GG is as high as 97% after 1000 cycles, indicative of the excellent cyclic stability of this graphene for supercapacitor application. To further study the cyclic stability and fundamental behavior of electrode materials, we have compared the EIS of the first cycle with the last one. From the impedance plots displayed in Fig. 8f, the equivalent series resistance (ESR) of GG is only 0.58 , which is the intercept of the plot with the real impedance (Z ) and associated with the porous structure of the electrodes [49]. So small ESR reveals that graphene layer can be fully contacted with electrolyte to make the charge
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Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
An environment-friendly route to synthesize reduced graphene oxide as a supercapacitor electrode material Dacheng Zhang, Xiong Zhang, Yao Chen, Changhui Wang, Yanwei Ma ∗ Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 October 2011 Received in revised form 5 February 2012 Accepted 3 March 2012 Available online 15 March 2012 Keywords: Graphene Glutathione Supercapacitors Specific capacitance
a b s t r a c t A large-scale, environment-friendly method to produce water-soluble reduced graphene oxide by using glutathione as a reducing and stabilization agent has been developed. The results of UV–visible absorption spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron microscopy indicate that graphene oxide is reduced to graphene nanosheets which are single-layers and exhibit good dispersion in water. A reaction mechanism is proposed. The electrochemical properties of the graphene nanosheets as electrode materials for supercapacitors are studied by cyclic voltammetry and galvanostatic charge/discharge tests. A maximum specific capacitance of 238 F g−1 in a 1 M H2 SO4 electrolyte has been obtained. Meanwhile, the material shows excellent long-term cycle stability along with the retention of 97% for specific capacitance after 1000 cycle tests. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Supercapacitors, also called electrochemical capacitors (ECs) which include pseudocapacitors and double-layer capacitors, having higher specific power density and longer cycle-life than secondary batteries and higher specific energy than conventional capacitors are attracting much attention [1]. The performance of supercapacitors highly depends on the properties of electrode materials. Additionally, the environment-friendly materials for supercapacitors are very important in response to the needs of modern society and emerging ecological concerns [2]. Graphene, a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, has attracted intense scientific interest due to its superior electrical conductivity, high specific surface area, and chemical stability [3]. With these unique properties, graphene is expected to be applied in supercapacitors [4–6]. Nowadays, many researchers obtain graphene by several approaches, including micromechanical exfoliation [7], thermal expansion [8], chemical vapor deposition [9], carbonization [10], liquid-phase reduction from graphene oxide (GO) [11], and so on. Among these methods, micromechanical exfoliation and chemical vapor deposition can produce high quality monolayer graphene but they are of low yield and uneasy operation. In contrast to them, liquid-phase reduction is the most promising techniques because the graphene derived from GO can not only be prepared
∗ Corresponding author. Fax: +86 10 82547137. E-mail address: [email protected] (Y. Ma). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.03.024
by mass production, but also shows good connectivity for supercapacitors between electrolyte and electrode materials because of the hydrophilic nature of this reduced GO. Representative work on liquid-phase reduction, including Stankovich et al. as the pioneer offers a simple and scalable method for efficient production of graphene by liquid-phase reduction with hydrazine. Our group has also developed a new approach to creating colloids of graphene monolayers with p-phenylene diamine as reducing reagent [12]. Unfortunately, the use of highly toxic reducing agent and aggregation of graphene will limit its practical applications such as supercapacitors. In response, many attempts have been made to develop environmentally friendly approaches for the reduction of GO by potassium hydroxide, l-ascorbic acid, sugar, protein, dextran, bacterial respiration, tea and photocatalytic methods [13–27]. Through their efforts, it is demonstrated that there is a possibility that graphene can be readily reduced for large-scale production compared with other methods such as micromechanical exfoliation under the environment-friendly condition. Additionally, Hu et al. have used the graphene which was produced by environment-friendly methods as electrode materials for supercapacitors [28–30]. Thus, in this work, we have developed another reduction process with glutathione to prepare the graphene materials, and fabricated supercapacitor devices using this graphene as electrode materials and investigated their electrochemical properties. Glutathione (C10 H17 N3 O6 S) is the major endogenous antioxidant produced by the cells, participating directly in the neutralization of free radicals and reactive oxygen compounds, as well as maintaining exogenous antioxidants such as vitamins C and E. Glutathione always exists in reduced (GSH) and oxidized (GSSG) states in the cells. In the
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Fig. 1. The illustration of reaction pathway for the reduction of GO with GSH and photographs of the GO and GG dispersion in water.
reduced state, the thiol group of cysteine is able to donate a reducing equivalent (e− ) to other unstable molecules, such as reactive oxygen species [31,32]. Herein, we demonstrated that GSH could effectively remove the oxygen on the GO and its oxidized products play an important role in stabilizing graphene simultaneously. The reduced GO was stably dispersed in water more than half of a year without extra stabilizing agent. Furthermore, the supercapacitors based on as-made graphene nanosheets showed good electrochemical performances, higher capacitance, better rate capability and longer cycle life. 2. Experimental 2.1. Sample preparation
Gaussian–Lorentzian components after a Shirley background subtraction by the software of XPSPEAK41. Atomic force microscopy (AFM) images were taken out using a Nanoscope MultiMode SPM (Digital Instruments) operated in tapping mode in conjunction with a V-shaped tapping tip (Applied Nanostructures SPM model: ACTA). Raman spectra were obtained on a RM 2000 microscopic confocal Raman spectrometer (Renishaw in Via Plus, England) employing a 514 nm laser beam. Transmission electron microscopy (TEM) was carried out using a JEOL JEM-2100F TEM operating at an accelerating voltage of 300 kV. Conductivity was measured using Physical Property Measurement System (PPMS). Thermo-gravimetric analysis (TGA) was accomplished by means of an NETZSCH STA-449C under nitrogen gas flow from room temperature to 900 ◦ C at a heating rate of 5 ◦ C min−1 .
The exfoliated GO was prepared using a modified Hummers method [33]. GO (0.1 g) in water was ultrasonicated to achieve GO dispersion. The pH of the GO dispersion in water (1 mg mL−1 ) was adjusted to 10 using a 28% ammonia aqueous solution. 50 mL of GSH (0.1 g, procured from Aldrich) solution was then added to the mixture. The mixture was vigorously stirred at 90 ◦ C for 24 h. Finally, the resulting stable black dispersion was filtrated with water for three times. Then, the obtained graphene nanosheets (signed as GG) can be dried at 60 ◦ C for 12 h or redispersed in water for different use. In addition, the thin film was also prepared by vacuum filtration using a membrane filter and then dried at 60 ◦ C for 12 h. We also prepared graphene by hydrazine (HG) as a comparison for electrochemical performance according the literature [34]. 2.2. Characterization UV–visible (UV–vis) absorption spectra were detected using Ultraviolet spectrophotometer (Hitachi UV2800). X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI Quantear SXM (ULVAC-PH INC) which used Al as anode probe in 6.7 × 10−8 Pa. In the XPS data analysis, peak deconvolution was performed using
Fig. 2. UV–vis absorption spectra of GO and GG prepared from different reaction times and reactant ratios. GG1 and GG2 stand for reactant mass ratio (GSH/GO) = 1 and 2, respectively.
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2.3. Electrochemical measurements The working electrode was prepared by pressing a mixture of polyvinylidene fluoride (PVDF), acetylene black and active materials at a weight ratio of 1:2:7 onto a stainless steel with a spatula, and dried at 100 ◦ C for 12 h. The counter and the reference electrodes were Pt plate and saturated calomel electrode (SCE), respectively. All electrochemical tests were carried out using an electrochemical system in 1 M H2 SO4 at room temperature. Cyclic voltammetry (CV) was carried out at scan rates ranging from 10 to 500 mV s−1 within the potential range of 0–0.9 V (vs. SCE). Galvanostatic charge/discharge was carried out at current densities ranging from 0.1 to 5 A g−1 within the potential range of 0–0.9 V (vs. SCE). Electrochemical impedance spectrometry (EIS) was recorded under the conditions: AC voltage amplitude of 5 mV, frequency range from 105 to 0.1 Hz. Cycle-life test was carried out through galvanostatic charge/discharge at the current density of 2 A g−1 for 1000 cycles.
3. Results and discussion The typical GG can be easily prepared by stirring GO and GSH aqueous dispersion at 90 ◦ C. As shown in Fig. 1, the color of reagent begins to turn black from yellow after 2 h reduction. The reduction progress of GO was first monitored by UV–vis spectroscopy (Fig. 2) with different reaction time and reactant ratios. As shown in Fig. 2, with the reduction progressing, the UV–vis absorption peak at 225 nm, which is caused by the GO, gradually red-shifted to 251 nm over the reaction time (24 h) for the reactant mass ratio (GSH/GO) = 1. This suggests that GO is reduced and the electronic conjugation within the graphene sheets is restored. While increasing the mass ratio of GSH/GO to 2, the effect of reduction is better than the former reaction because the UV–vis absorption peak shifted to 261 nm, which is comparable to the reductants sugar (261 nm) [14] and l-ascorbic acid (264 nm) [15]. We employed XPS to further confirm the reduction of GO by GSH. Fig. 3 shows the C1s XPS spectra of GO and GG. From the C1s XPS spectrum of GO (Fig. 3a), four different peaks centered at 284.5, 286.4, 287.8, and 289.0 eV, are corresponding to C C/C C in aromatic rings, C O (epoxy and alkoxy), C O, and carboxyl (COOH) groups, respectively [14]. After reduction by the GSH, the intensities of all C1s peaks of the carbon binding to oxygen, especially the peak of C O (epoxy and alkoxy), decrease remarkably (Fig. 3b), suggesting that most oxygen functional groups are successfully removed. This result shows that the as-synthesized GG has been partially repaired from sp3 hybridized carbon atoms dominated in GO to graphite structure of sp2 hybridized carbon atoms [35]. Fig. 4a and b shows the AFM image and section analysis of GO and GG deposition on a freshly cleaved mica. The average of thickness of GG is less than that of GO, indicating the loss of oxygen functional groups. The graphene nanosheet is nearly flat on the mica with a length of several hundred nanometers and a height of 0.76 nm, which confirms that GG is a single layer. On the other hand, we can see some small points on the plane of graphene, which are maybe extra GSH or its oxidation product adsorbed on the graphene and the residual hydrophilic groups. Hence, the as-made graphene can form stable dispersion more than half of a year in water without surfactant. Fig. 4c and d shows the TEM images of the large GG obtained by GSH reduction at different magnifications. It can be seen that the graphene is very thin and contains some corrugations and scrolling on the surface, which is consistent with previous work [16]. Raman spectroscopy is a non-destructive technique that is widely used to obtain structural information about carbon-based materials and recently for graphene [36–40]. Raman spectra for GO
Fig. 3. C1s XPS spectra of GO (a) and GG (b).
and obtained GG are shown in Fig. 5. The fluorescence background for the GO spectra was subtracted using the Wire 2.0 Renishaw software. It is well known that there are two typical peaks, named G and D bands, in the Raman spectra of graphitic carbon-based materials. The G band corresponds to the optical E2g in-plane vibration phonon at the Brillouin zone center, while the D band is ascribed to the breathing mode of aromatic which requires a defect for its activation [41]. The intensity of D band is therefore often used as an indicator for the degree of disorder in carbon-based materials. It can be seen that in the Raman spectrum of GG (Fig. 5), the G band is shifted upward to 1604 cm−1 , and the intensity of the D band at 1351.2 cm−1 increase substantially. The intensity of D/G ratio increases from 0.84 to 0.99, indicating that the in-plane sp2 domains of graphitic are smaller in size than those of GO, but more numerous in number [11], which is due to the presence of unrepaired defects that remained after the removal of oxygen moieties. It is also confirmed by the conductivity tests for the GG film. The conductivity of the film is 15.6 S m−1 , which is several orders of magnitude larger than that of GO. The reduced GO by using GSH as the reductant shows as good electrical properties as those produced by other environment-friendly reductants [15,19]. However, some GSH or its oxidation product adsorbed on the graphene surface and the residual hydrophilic groups on the graphene will degrade the conductivity of the GG. Moreover, it has been recently reported that the shape and position of 2D band which can estimate how many layers of graphene [42]. The 2D peaks of GG are located at about 2698 cm−1 which suggests formation of double-layer graphene. Nevertheless, AFM results show single-layer graphene has been
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Fig. 4. (a) and (b) A tapping mode AFM image and section analysis of GO and GG absorbed on freshly cleaved mica; (c) and (d) TEM images of GG at different magnifications.
obtained. The difference is mainly attributed to restack when GG powder was dried. To evaluate the thermal stability of graphene, TGA was performed to characterize the mass loss with the temperature. As seen
in Fig. 6, there is about 10% mass loss for both GO and GG below 100 ◦ C which accounts for the volatilization of water molecule. The residual mass of GG had a slow decrease with temperature elevation but still remained 65% of initial mass at 900 ◦ C while that of
Fig. 5. Raman spectra of GO and GG.
Fig. 6. TGA curves of GO and GG.
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Fig. 7. Proposed reaction mechanism for the chemical reduction of GO with GSH.
Fig. 8. (a) and (b) CV curves of GG at various scan rates with potential from 0 V to 0.9 V (vs. SCE) in 1 M H2 SO4 ; (c) galvanostatic charge/discharge curves of GG at different current densities of 0.1, 0.5 and 1 A g−1 ; (d) the specific capacitances of GG and HG at different current densities; (e) capacitance retention ratio of GG from the 1st to 1000th cycle at the current density of 2 A g−1 ; (f) the Nyquist impedance plots of GG in the frequency range of 100 kHz to 0.1 Hz (main chart) and high frequency area (inset image) after 1st cycle and 1000th cycle m2 .
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GO has sharply decreased to only 30% in the same condition. As a result, GSH can partially remove oxygen functional groups, especially C O (epoxy and alkoxy) and the thermal stability of GG is much better than that of GO. From the above results, it is clearly demonstrated that GO was successfully reduced by GSH. We propose a possible mechanism for deoxygenation of GO with GSH as shown in Fig. 7. Like hydrazine or l-ascorbic acid reducing GO [11,15], firstly, HS- groups of GSH attack the oxygen atom of epoxides in GO to form C S by SN 2 nucleophilic reactions, and finally, the two neighboring C S are cleaved by a thermal elimination. The reduced product was washed by water through filter. The GSSG in the resulting filtrate was detected by a biology method as shown in Fig. S1. To further estimate the performance of GG as electrode material for supercapacitors, various electrochemical tests, including CV, galvanostatic charge/discharge and EIS, were carried out. Fig. 8a and b shows that the CV curves remain rectangle shape very well from a scan rate of 10–500 mV s−1 . This pure capacitive characteristic is further proved by galvanostatic charge/discharge tests because of near triangle shapes as shown in Fig. 8c. However, an irreversible oxidation at potentials above 0.8 V is also observed at small current load because the active material has been fully reaction including some extra irreversible oxidations under this condition. As a result, the GG as electrode material for supercapacitor application should be fine at potentials equal to or below 0.8 V. Moreover, we can also find a weak redox peak at about 0.4 V. The appearance of redox peak shows that some part of capacitance is contributed by pseudocapacitance because of oxygen groups on the GG [5,43,44]. In addition, A BET surface area of 317 m2 g−1 has been obtained, which is comparable to that of other reduced graphene oxide. As shown in Fig. S2, N2 adsorption/desorption plot and pore size distribution shows that the GG is micro-porous. Micro-porous (less than 1 nm) may increase capacitance for carbon materials [45]. Thus, the capacitance of GG is the co-contribution of double layer capacitance and pseudocapacitance from oxygen groups. The specific capacitance of GG electrode using galvanostatic charge/discharge curves can be calculated according to the following formula: C=
I −(U/t)m
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transfer easier. It could be obviously seen that the impedance spectra almost have no changes, also indicating that the graphene have good cyclic stability. A semi-circle arc and a vertical line have been observed. The semi-circle arc becomes smaller with cycle numbers increase, indicating better penetration of electrolytes [50,51]. The low frequency straight line is nearly parallel to the imaginary axis, also suggesting that the capacitive behavior of GG is very good [52,53]. 4. Conclusions We have provided an environmental-friendly, cost-effective, and scalable approach for the preparation of water-soluble graphene nanosheets through the reduction of GO by using glutathione as reducing and stabilization agent. A possible mechanism for the reduction of GO by the GSH was suggested. Furthermore, the specific capacitance of GG as electrode materials for supercapacitors was as high as 238 F g−1 at the current density of 0.1 A g−1 in 1 M H2 SO4 electrolyte, and remained 97% of initial value at the current density of 2 A g−1 after 1000 cycles. Further optimize reductant and control precursor ratios to exploit better graphene-based supercapacitors are under investigation in our group. The biocompatible reduced GO could be also used in the fields of biosensors and biomaterials in the future. Acknowledgements This work was partially supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (No. KJCX2-YWW26), Beijing Municipal Science and Technology Commission (No. Z111100056011007), the National Natural Science Foundation of China (Nos. 21001103 and 51025726) Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2012.03.024. References
(1)
where I is the discharge current, U/t is the average slope of the discharge curve after the IR drop, m is the mass of active material in a single electrode. As shown in Fig. 8d, the specific capacitance of GG reaches as high as 238 F g−1 at the current density of 0.1 A g−1 , which is comparable to that of most graphene prepared by liquid-phase reduction [4,46–48], and remains 140 F g−1 at 5 A g−1 , suggesting that its rate capability is good. Additionally, we also prepared graphene by hydrazine (HG) as a comparison. As can be seen from Fig. 8d, this method obtained comparable capacitance with HG but ensures more environmental friendliness. In addition, long cycle-life and excellent stability of GG, which are required by many applications, are also demonstrated. Electrochemical stability of GG at the current density of 2 A g−1 for 1000 cycles is shown in Fig. 8e. The capacity retention of GG is as high as 97% after 1000 cycles, indicative of the excellent cyclic stability of this graphene for supercapacitor application. To further study the cyclic stability and fundamental behavior of electrode materials, we have compared the EIS of the first cycle with the last one. From the impedance plots displayed in Fig. 8f, the equivalent series resistance (ESR) of GG is only 0.58 , which is the intercept of the plot with the real impedance (Z ) and associated with the porous structure of the electrodes [49]. So small ESR reveals that graphene layer can be fully contacted with electrolyte to make the charge
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