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Facile Synthesis of graphene-CeO2 Nanocomposites with enhanced electrochemical properties for Supercapacitors
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a
Centre for Nano Science and Technology, Anna University, Chennai-600025, India. Centre for Materials Research, Thiruvalluvar College of Engineering and Technology, Vandavasi-604 505, India. c Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai-980 858, Japan. d Research Institute of Electronics, Shizuoka University, Johoku, Naka-ku, Hamamatsu-432 8011, Japan. *To whom correspondence should be made Email: [email protected] b
The electrochemical properties of CeO2G nanocomposites have been increased compared to pure ceria and the results suggest that it is a promising material for supercapacitor applications.
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T. Saravanana,b, M. Shanmugama, P. Anandanb, M. Azhagurajanc, K. Pazhanivelb, M. Arivanandhand, Y.Hayakawad, R. Jayavel a*
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Facile Synthesis of graphene-CeO2 Nanocomposites with enhanced electrochemical properties for Supercapacitors T. Saravanan a,b, M. Shanmugama, P. Anandanb, M. Azhagurajan c, K. Pazhanivel b, M. Arivanandhan d, Y.Hayakawa d, R. Jayavel a* Graphene - Ceria (CeO2G ) nanocomposites have been prepared by the low temperature solution process with different weight percentage of graphene and its electrochemical properties have been investigated. Structural properties of the nanocomposites were studied by X-ray diffraction, Raman and FTIR spectral analyses. FE-SEM and HRTEM images shows the wrinkled paper-like morphology of the prepared composite. Elemental mapping images were recorded by FE-EPMA technique. XPS analyses reveal the binding states of different elements present in the composites. Cyclic voltammetric studies confirm that the composite with 5 % graphene exhibits the specific capacitance of 110 F g-1, which is higher than that of pure CeO2 (75 F g-1). The significant increase in the specific capacitance suggests that the CeO2G is a promising material for supercapacitor applications.
Introduction Increased population of industries has created huge demand on energy sector. But the natural resources of energy like coal, radioactive ore and oils for the conventional production of energy are descending slowly and one day will be completely exhausted. Intensive research for the materials capable of solving the problem of energy demand is being carried, which resulted in nonconventional methods for the production of energy like Silicon solar cells, thermoelectric, photovoltaic cells and dye sensitized solar cells. However, the storage of energy is unavoidable in future for effective utilization of produced energy. Extensive investigations are being carried out to identify the electrode materials capable of storage capacity and improved efficiency. The nano sized metal oxides and its composites were reported to have electronic properties with excellent recyclability due to extended surface area and good photo catalytic activity as well.1 It is reported in a review that the transition metal oxides have the potential to overcome the low energy density limitations of electrochemical capacitors.2 After identifying a flat monolayer of carbon atoms with perfect sp2hybridized two dimensional carbon structures called graphene,3,4 researchers are interested to investigate the graphene based composites with other nanomaterials, such as noble metals, some complex oxides and metal oxides.5-9 Among them graphene-metal oxide composites have gained attention as a viable alternative to boost the efficiency of various catalytic and storage reaction in energy conversion applications.10 Ceria (CeO2) is a promising rare earth oxide material widely studied for many technological applications.11-14 In particular, the defects such as oxygen vacancies in CeO2 play a crucial role in catalytic properties as the oxygen vacancies can be easily formed and removed rapidly. The oxygen vacancies in CeO2 determine the Ce3+ and Ce4+ ratio, which leads to an enhanced visible photocatalytic
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activity by narrowing the bandgap. The oxygen vacancy in CeO2 is highly reactive with graphene and thus the grapehene-CeO2 (CeO2 G) nanocomposite can be formed and such composite could be an interesting material for enhancing the photocatalytic, electrochemical properties. However, the rare earth oxide based graphene composites were rarely investigated for their photocatalytic and electrochemical properties.15-17 Jiang et al.,15 reported the optical and catalytic properties of CeO2 G. Wang et al.,16 reported the electrochemical properties of CeO2 G nanocomposites. However, in both the studies the composites were prepared at a fixed ratio and the electrochemical studies were performed at a single scan rate and the results have not been discussed in detail. To understand the effect of graphene on the electrochemical properties of CeO2, it is essential to vary the graphene content. Therefore, further investigation is needed to understand the enhanced electrochemical properties of CeO2 G composites. In the present work, CeO2 G nanocomposites were synthesized with different weight percentage of graphene by the modified Hummer's method. The nanocomposites were characterized by various analytical techniques for structural, compositional and morphological analysis. The electrochemical performance of the composites was analyzed by cyclic voltammetry, galvanostatic charge/discharge studies and the results were discussed.
Experimental Techniques Synthesis of graphene and graphene-CeO2 nanocomposites Graphite powder was reduced as graphene oxide (GO) by the modified Hummer’s method as described in our earlier report.18 Then the GO was reduced using thiourea as a reducing agent to synthesize the graphene. Typically, 100 mg of GO was dispersed in 100 ml of water through ultrasonication for 1h and 100 mg of
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thiourea was added to the dispersion. The solution was maintained at 100oC with constant stirring for 24 h. The brown color of GO was turned to black color graphene due to the reduction process. Finally the black color graphene was washed with ethanol and dried in vacuum oven at 60 oC for 24 h. To prepare the CeO2G1 composite, 99 mg of as purchased cerium oxide (CeO2) (99.9 %) was dispersed in 100 ml of water by ultrasonication and 1 mg of reduced graphene was added to the dispersion. Then the solution was maintained at room temperature with constant stirring for 2 h. The final product CeO2G1 was collected and dried in vacuum at 60 C for 24 h. By the same procedure, CeO2G composites with 3 and 5 mg of graphene with 97 and 95 mg of CeO2 were also prepared and named as CeO2G3 and CeO2G5 respectively. The influence of graphene on the structural and electrochemical properties of composites has been studied by various analytical techniques. Characterization techniques CeO2G nanocomposites were characterized by various analytical techniques. Rigaku MiniFlexII-C X-ray diffractometer with CuKα ( = 1.540 nm) radiation at a scanning rate of 1deg/min was used to collect the powder X-ray diffraction data to identify the material. Raman and FTIR spectra were recorded to confirm the formation of composites JASCO Raman Spectrophotometer (NRS – 7100) and NICOLET Infrared spectrophotometer, respectively. The morphology of the composites was analyzed using FESEM and HRTEM images obtained by JEOL 7001F and JEOL-JEM 2100F electron microscopes, respectively. The elemental composition was analyzed by elemental mapping and EDS spectra obtained by JEOL JXA-8530F model field emission electron probe micro analyzer (FEEPMA) instrument. The electron binding energies of various elements in the composites were analyzed by ESCA 3400 SHIMADZU X-ray photoelectron spectrometer. The electrochemical performance of the composites was investigated by three electrode system using Bio Logic electrochemical work station. Glassy carbon, Ag/AgCl and platinum wire have been used as working, reference and counter electrodes, respectively. 1 M H2SO4 was used as electrolyte and the cyclic voltametric curves were recorded in the
2 | Dalton Transactions, 2012, 00, 1-3
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Results and Discussion The synthesized graphene, pure and CeO2G nanocomposites were subjected to powder X-ray diffraction studies and the recorded patterns are shown in Fig.1a. The diffraction pattern of graphene contains only two peaks corresponding to 002 and 100 diffraction planes at two theta values of 23.44 and 42.92 degrees, respectively. The diffraction patterns of CeO2 and CeO2G composites possess a large number of high intensity peaks confirming the good crystalline nature of the composites as shown in Fig.1a. The diffraction patterns are compared with the standard JCPDS #65-5923 for CeO2 and the peaks were indexed accordingly. From the patterns it is observed that the influence of graphene is not significant as the quantity of graphene is small. On the careful examination from the enlarged peaks at various two theta values, it was observed that the diffraction peaks were shifted reasonably in the composite compared to pure CeO2 and reduced the intensity of peaks as shown in Fig. 1b, which confirms the composite formation. Laser Raman spectra were recorded in the range of 800 to 2000 cm-1 for the graphene and CeO2 G5 composites (Fig.1 c) and CeO2G1 (Fig. 1d). The spectrum of pure graphene has two characteristic peaks called D and G bands at 1343 and 1577 cm-1, respectively as shown in Fig. 1c-2. The intensity ratio of D and G band, ID/IG for graphene is measured to be 1.11, which is higher than GO (1.03) (Fig.1e). Hence, the graphene reduced from GO ensured the sp2 bonded carbon. On the other hand, the CeO2G5 composite is having an additional strong peak at 1161 cm-1 due to CeO2 in addition to the D and G bands of graphene (Fig.1c-1). The Raman spectrum of CeO2G1 was also recorded and found that it has the intense peak at 1163 cm-1 due to CeO2 and very weak signals were observed for D and G bands of graphene. This spectrum ensures the less quantity of graphene in the composite. The band shift, increased intensity and additional peaks are the evidence for the formation of CeO2G5 composite.
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Fig.1. (a) Comparison of powder x-ray diffraction patterns of graphene, CeO2, and CeO2G composites (b) diffraction peak shifts,(c) Laser Raman spectra of CeO2G5 & graphene (d) Laser Raman spectrum of CeO2G1 and (e) Raman spectrum of Graphene Oxide and Graphene.
Fig. 2. FE-SEM images of (a & b) CeO2G1 and (c& d) CeO2G5 The morphology of the prepared composites was studied by Field emission scanning electron microscope (FE-SEM) and the images are shown in Fig. 2 (a-d). The morphology of CeO2G1 contains graphene sheets and it is observed that CeO2 is intercalated between the graphene sheets as shown in Fig.2a. Higher magnification image (Fig.2b) confirms the wrinkled paper like structure of graphene with CeO2 attached on the surface. The morphology of the CeO2G5 composite is shown in Fig. 2c. The increase in the graphene weight percent causes the aggregation of CeO2 with graphene and hence graphene structure becomes irregular flake-like morphology as shown in the magnified image (Fig. 2d).
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HRTEM images of CeO2G5 at different magnification are shown in Fig 3 a-d. It is observed that the CeO2 crystals in different sizes ranged from 100 – 250 nm were distributed on the surface of the graphene sheets as marked in Fig. 3a. Higher magnification image infers that the size of the particles is around 10 – 50 nm at different depth of graphene layers as shown in Fig 3b. Fig. 3c shows the folded graphene sheets in the 50 nm scale and the marked region is shown in 5 nm scale at Fig. 3d. Higher magnification images taken in 5-10 mm scale of marked regions of Fig. 3b are shown in Fig. 3eg. The rectangular mark shows the thick bunch of graphene sheet, which do not have any lattice fringe pattern as shown in Fig. 3e. The region marked by the continuous circle has bright lattice fringes as shown in Fig. 3f, which ensures the presence of CeO2 immediately below a layer. The region marked by the dotted circle has clearly visible bright fringes (Fig. 3g) confirming that CeO2 is on top of the graphene sheet. These images supported effectively the formation of composite. It is observed that a bunch of graphene with 5 nm thickness contains 15 sheets as shown in Fig. 3h, which confirmed that graphene has been effectively synthesized by this method. The composites were also subjected to Field emission electron probe micro analysis (EPMA) and the elemental mapping was recorded. The EPMA mapping and corresponding energy dispersive spectrum for CeO2G5 composite is shown in Fig. 4. The EPMA and EDX spectrum of CeO2G1 composite has also been recorded and given as supporting information (Fig.S1). The systematic increase of the carbon content was observed in the composites from CeO2G1 to CeO2G5. Uniform distribution of carbon, oxygen and cerium elements was observed in the composites. The energy dispersive spectra also confirm the presence of all elements in the composites.
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Fig.3. HRTEM images of CeO2G5 (a) nanocrystals of CeO2 on the top of graphene sheets, (b) CeO2 nanocrystals at different depth between the graphene sheets, (c) folded graphene sheets with CeO2 nanocrystals (d) Higher magnification image of folded graphene sheets and (e-h) magnified images of marked regions in (b and d).
CeO2G5 composite was analyzed by XPS spectra of various elements as shown in Fig. 5 a-d. Figure 5a shows the 3d electron core level spectrum of cerium in the region 875-920 eV. The XPS spectrum consists of three distinct signals correspond to different ionic states of Ce3+ and Ce4+. The characteristic XPS signals of Ce3+, 3d5/2 and 3d3/2 electron states have been observed at 886 and 905 eV, respectively with a separation of 9 eV. The additional satellite signal is due to Ce4+, 3d3/2 electron state at 919 eV. The core level spectrum of 4p electron of cerium in the region 200-240 eV is shown in Fig. 5b. It is observed that the spectrum contains two peaks correspond to 4p3/2 and 4p1/2 states at 210.3 and 227.6 eV, respectively. The recorded spectra show the presence of mixed valence cerium of Ce 3+ and Ce4+. The main peaks observed at 886 and 919 eV represent the relative amount of Ce3+ and Ce4+ present in the sample. From the area of the respective peaks, it is obvious that the concentration of Ce3+ is relatively higher in the samples, which confirms the presence of oxygen vacancies in CeO2. The binding energy difference between these two states is measured to be 17.3 eV and it has good agreement with the standard values19. The core level XPS spectrum of C 1s electron is shown in Fig. 5c, which contains a prominent signal at 284.3 eV and a satellite peak at 289.4 eV. The former is due to sp2 bonded carbon (C-C) and the latter is assigned to carboxyls (C-C=O), which indicates carbon has a reasonable amount of oxygen containing functional groups.19 Two different oxygen species could be distinguished on deconvolution of core level spectrum of O1s electron in the range between 525 and 540 eV as shown in the Fig. 5d. The binding energies of 530.5 and 531.8 eV have been assigned to lattice oxygen and carboxyl groups, respectively.14 Fig. 4. FE-EPMA images of CeO2G5 and corresponding EDS spectrum.
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Fig.5. Core level X-ray photoelectron spectra of (a) Cerium 3d, (b) Cerium 4p, (c) Carbon 1s and (d) Oxygen 1s electrons
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Fig. 6. Cyclic voltammetric curves of (a) Pure CeO2, (b) CeO2G1, (c) CeO2G3 (d) CeO2G5 for different scan rates and (e) Comparison of CV of pure CeO2, CeO2G1, CeO2G3, CeO2G5 at a scan rate of 10mV/s.
Fig.7 (a) Specific capacitance of CeO2 and CeO2G composites as a function of scan rate (mV) and (b) The relation between the oxidation peak current at different scan rates and the square root of scan rate for pure CeO2 and CeO2G5.
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Fig. 8. (a) Comparison of charge –discharge curves for pure CeO2 and CeO2G5 under 0.65A/g, (b) Nyquist plots of pure CeO2 and CeO2G5 composite. Cyclic Voltammetry has been used to study the capacitance of pure and CeO2 G with various amounts of graphene at different scan rates. Figure 6 shows the cyclic voltammogram results of (a) pure CeO2, (b) CeO2G1, (c) CeO2G3 and (d) CeO2G5 in the electrolyte of 1M H2SO4. As shown in 6(a), the pure CeO2 electrode shows the capacitive-like responses in the potential window of investigation. As compared to other electrodes (Fig. 6 b & c), the CeO2G5 electrode demonstrates the highest current response with an obvious double layer current in the potential range of (-0.3 to +1.0 V) as shown in Fig. 6d. The cyclic voltammogram of the nanocompsites at the fixed scan rate is plotted in Fig.6e, which shows that the current response increases with double layer current for the nanocomposite with increasing graphene ratio. The specific capacitance values for CeO2 and CeO2G composites have been calculated under the area of the cyclic voltammogram curve for different scan rates by using equation (1) Cs = IdV/vmV. ------ (1) Where, I is the response current, V is the potential, v is the potential scan rate and m is the mass of the active material of the electrode. The calculated specific capacitance was plotted as a function of scanning rates (Fig.7a). The specific capacitance decreases with the increase of scan rates from 10 to 100 mV s-1. The specific capacitance of CeO2G5 sample is much higher than that of pure CeO2 and other composites with low graphene content at the same scan rates. The specific capacitance of 109 F g-1 is observed at 10 mV s-1 of scan rate for CeO2G5 composite when compared to 87 F g-1 for CeO2G1 composite and 75 F g-1 for pure CeO2, respectively. The specific capacitance of the CeO2 G5 samples decreases from 109 to 35 F g-1 as the scan rate increases from 10 to 100 mV/s, representing only 68 % decrease, whereas 80 % of reduction was observed in the pure CeO2. With increasing scan rate, the specific capacitance decreases gradually (Fig. 7a) and it can be attributed to the fact that the electrolytic ions easily diffuse and migrate into the active materials at lower scan rates. At higher scan rates, the diffusion is controlled, limiting the migration of the electrolytic ions, which causes some active surface areas to become inaccessible for charge storage.
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Figure 7b shows the relationship between oxidation peak current at different scan rates and the square root of scan rate, which gives the information about the electrochemical kinetics of the reaction. In general, the electrochemical double layer capacitor (EDLC) reaction is a diffusion controlled process and the corresponding CV curve is rectangular in shape, therefore I Vs V1/2 graph derived from this capacitive region is linear. For the CeO2-G nanocomposite, the enhanced specific capacitance is attributed to the combination of both EDLC and faradaic reactions. The faradaic reaction (electron transfer across the electrode electrolyte interface) caused by the presence of CeO2 has the two possibilities: (1) kinetic and (2) diffusion controlled process. From the Figure 7b, I Vs V 1/2 graph drawn for the corresponding oxidative peak current of CeO2-G composite shows the linear relationship. The graphene present in the composite might have enhanced the fast electron transport, which facilitates the charge transport kinetics of the faradaic reaction. Therefore, it is concluded that the corresponding faradaic reaction is entirely diffusion limited process rather than kinetic controlled process. Further, our experimental results also demonstrate that the composite with high graphene content (CeO2G5) is a better electrode material for supercapacitor applications. Galvanostatic charge-discharge measurements were also carried out on the pure CeO2, CeO2G1, CeO2G3 and CeO2G5 composite materials and the results are shown in Fig. 8a. It can be seen that the charging curves are symmetrical to their corresponding discharge counterparts. This indicates the excellent electrochemical reversible charge-discharge properties. Moreover, the duration of chargedischarge increases drastically when the graphene content increases, indicating the enhanced specific capacitance of the composites. The specific energy, specific power of the electrode material was calculated using the following equations: E= 1 / 2 × Cs × (ΔV)2 (2) P = E/Δt
(3)
where Cs is the specific capacitance calculated from the CV curves, ΔV is the operating voltage of the cell and Δt is the discharge time
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. The calculated values are tabulated (Table 1) for pure and composite materials. The specific energy increases and specific power decrease when the graphene content increases in the composite material. The specific energy of CeO2G5 (92.1 Whkg-1) was nearly three times higher than pure CeO2 (33.13 Whkg-1). Furthermore, the capacitive performance of the pure and CeO2G5 composites were analyzed by electrochemical impedance spectroscopy (EIS). The measurements were performed at a frequency range from 100 kHz to 100 Hz. As shown in Fig.8b, the Nyquist plots show linear region with the absence of semicircle region, which shows the low charge transfer resistance at the electrode/electrolyte interface 23. The combined resistance, including the ionic resistance of the electrolyte, the intrinsic resistance of the substrate and the contact resistance at the interface of active material/current collector can be obtained from the intercept of the linear part of the Nyquist plot. From the Fig.8b, the combined resistance of the pure and composite materials is almost the same probably due to low graphene content in the composite material. Table 1: Calculated specific capacitance, specific energy and specific power of the pure and composite electrode material. S. No
Composite material
Specific capacitanceCs ( Fg-1)
Specific Energy -E (Whkg-1)
Specific Power -P (Wkg-1)
1
Pure CeO2
75
33.13
12097.98
2
CeO2G1
86
72.67
18875.34
3
CeO2G3
106
89.57
15472.72
4
CeO2G5
109
92.10
7368.40
Conclusion Graphene-CeO2 nanocomposites were prepared with different weight percent of graphene by the low temperature solution process. Structural properties of the nanocomposites were studied by X-ray diffraction analysis. Laser Raman spectral analyses were carried out to confirm the formation of nanocomposites. The morphology of the synthesized compounds has been studied by FESEM and HRTEM analysis. Systematic variation of carbon level in the composites was observed by Elemental mapping by the FE-EPMAtechnique. The binding states of various elements of the composites were studied by XPS analysis. The electrochemical studies show that the CeO2G5 nanocomposite exhibits good capacitance behavior and the specific capacitance of CeO2G5 sample (109 F g-1) is higher than that of pure CeO2 (75 F g-1).
Notes a
Centre for Nano Science and Technology, Anna University, Chennai600025, India. Email (T. Saravanan ): [email protected] a Centre for Nano Science and Technology, Anna University, Chennai600025, India. Email (M.Shanmugam): [email protected]. b Centre for Materials Research, Thiruvalluvar College of Engineering and Technology, Vandavasi-604 505, India. Email (P. Anandan): [email protected], (K. Pazhanivel): [email protected]. c Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai-980 858, Japan. Email: [email protected].
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d
Research Institute of Electronics, Shizuoka University, Johoku, Naka-ku, Hamamatsu-432 8011, Japan. Email (M. Arivanandhan): [email protected], (Y. Hayakawa): [email protected]. *To whom correspondence should be made (R. Jayavel) Email: [email protected]. Electronic Supplementary Information (ESI) available: [supplementary information available here].
References 1. Y. Wang, K. Takahashi, K. Lee and G. Z. Cao, Adv. Funct.Mater., 2006, 16, 1133. 2. V. Augustyn, P. Simon and B. Dunn, Energy Environ. Sci., 2014, 7, 1597. 3. A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183 4. D. A. Dikin, S. Stankovich, E. J. Zimney, R.D. Piner, G. H. B. Dommett, G. Evmenenko, S. T. Nguyen and R.S. Ruoff, Nature., 2007, 448, 457. 5. J.X. Zhu, Y. K. Sharma, Z. Y. Zeng, X.J. Zhang, M. Srinivasan, S. Mhaisalkar, H. Zhang, H. H. Hng and Q.Y. Yan, J. Phys Chem C., 2011, 115, 8400. 6. D.H. Wang, R. Kou, D.W. Choi, Z. G. Yang, Z. M. Nie, J. Li, L.V. Saraf, D. H. Hu, J.G. Zhang, G. L Graff, J. Liu, M. A. Pope and I. A. Aksay, ACS Nano., 2010, 4 1587. 7. S. X. Wu, Z. Y. Yin, Q. Y. He, X. Huang, X. Z. Zhou and H. Zhang, J. Phys. Chem.C., 2010, 114, 11816. 8. F. Ji, Y. L. Li, J. M. Feng, D. Su, Y. Y. Wen, Y. Feng and F. Hou, J. Mater. Chem., 2009, 19, 9063. 9. Y. H. Ng, A. Iwase, N. J. Bell, A. Kudo and R. Amal, Catal. Today., 2011, 164, 353. 10. C. Hu, T. Lu, F. Chen, R. Zhang and J. Chin, Adv. Mater, Soc., 2013, 1 21. 11. Z. L. Wang, G. R. Li, Y. L. Ou, P. Z. Feng, D. L. Qu and Y. X. Tong, J. Phys. Chem. C., 2011, 115, 351. 12. G. A. Deluga, J. R. Salge, L. D. Schmidt and X. E. Verykios., Science 2004, 303, 993. 13. A, Corma, P. Atienzar, H. García, and J. Y. C. Ching, Nat. Mater., 2004, 3, 394. 14. X. Wang, X. Li, D. Liu, S. Song and H. Zhang, Chem. Commun., 2012, 48, 2885. 15. L. Jiang, M. Yao, B. Liu, Q. Li, R. Liu, H. Lv, S. Lu, C. Gong, B. Z. T. Cui, and B. Liu, J. Phys. Chem. C 2012, 116, 11741. 16. Y. Wang, C. X. Guo, J. Liu, T. Chen, H. Yang and C. M. Li, Dalton Trans., 2011, 40, 6388. 17. G. Wang, J. Bai, Y. Wang, Z. Ren and J. Bai, Scripta Materialia., 2011, 65, 339. 18. K. Satheesh and R. Jayavel, Mater. Let., 2013, 113, 5. 19. Y. Chen, B. H. Song, H. S. Tang, L. Lu and J. M. Xue, J. Mater. Chem., 2012, 22, 17656. 20. T.W.Lin, C.S.Dai and K.C.Hung, Scientific Reports, 2014, 4,7274. 21. H.Kim, M.Y.Co, M.H.Kim, K.Y.Park, H. Gwon, Y.Lee, K.C.Roh and K.Kang, Adv. Energy Mater., 2013, 3, 1500. 22. R.Raja, S.Lok Kumar, K.Minami, M.Subramanian, R.Jayavel, K.Ariga, J.Mater. Chem. A, 2014, 2, 18480. 23. H.C.Gao, F.Xiao, C.B.Ching and H.W.Duan, ACS Appl. Mater. Interfaces, 2012, 4, 2801.
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Facile Synthesis of graphene-CeO2 Nanocomposites with enhanced electrochemical properties for Supercapacitors
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a
Centre for Nano Science and Technology, Anna University, Chennai-600025, India. Centre for Materials Research, Thiruvalluvar College of Engineering and Technology, Vandavasi-604 505, India. c Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai-980 858, Japan. d Research Institute of Electronics, Shizuoka University, Johoku, Naka-ku, Hamamatsu-432 8011, Japan. *To whom correspondence should be made Email: [email protected] b
The electrochemical properties of CeO2G nanocomposites have been increased compared to pure ceria and the results suggest that it is a promising material for supercapacitor applications.
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T. Saravanana,b, M. Shanmugama, P. Anandanb, M. Azhagurajanc, K. Pazhanivelb, M. Arivanandhand, Y.Hayakawad, R. Jayavel a*
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Facile Synthesis of graphene-CeO2 Nanocomposites with enhanced electrochemical properties for Supercapacitors T. Saravanan a,b, M. Shanmugama, P. Anandanb, M. Azhagurajan c, K. Pazhanivel b, M. Arivanandhan d, Y.Hayakawa d, R. Jayavel a* Graphene - Ceria (CeO2G ) nanocomposites have been prepared by the low temperature solution process with different weight percentage of graphene and its electrochemical properties have been investigated. Structural properties of the nanocomposites were studied by X-ray diffraction, Raman and FTIR spectral analyses. FE-SEM and HRTEM images shows the wrinkled paper-like morphology of the prepared composite. Elemental mapping images were recorded by FE-EPMA technique. XPS analyses reveal the binding states of different elements present in the composites. Cyclic voltammetric studies confirm that the composite with 5 % graphene exhibits the specific capacitance of 110 F g-1, which is higher than that of pure CeO2 (75 F g-1). The significant increase in the specific capacitance suggests that the CeO2G is a promising material for supercapacitor applications.
Introduction Increased population of industries has created huge demand on energy sector. But the natural resources of energy like coal, radioactive ore and oils for the conventional production of energy are descending slowly and one day will be completely exhausted. Intensive research for the materials capable of solving the problem of energy demand is being carried, which resulted in nonconventional methods for the production of energy like Silicon solar cells, thermoelectric, photovoltaic cells and dye sensitized solar cells. However, the storage of energy is unavoidable in future for effective utilization of produced energy. Extensive investigations are being carried out to identify the electrode materials capable of storage capacity and improved efficiency. The nano sized metal oxides and its composites were reported to have electronic properties with excellent recyclability due to extended surface area and good photo catalytic activity as well.1 It is reported in a review that the transition metal oxides have the potential to overcome the low energy density limitations of electrochemical capacitors.2 After identifying a flat monolayer of carbon atoms with perfect sp2hybridized two dimensional carbon structures called graphene,3,4 researchers are interested to investigate the graphene based composites with other nanomaterials, such as noble metals, some complex oxides and metal oxides.5-9 Among them graphene-metal oxide composites have gained attention as a viable alternative to boost the efficiency of various catalytic and storage reaction in energy conversion applications.10 Ceria (CeO2) is a promising rare earth oxide material widely studied for many technological applications.11-14 In particular, the defects such as oxygen vacancies in CeO2 play a crucial role in catalytic properties as the oxygen vacancies can be easily formed and removed rapidly. The oxygen vacancies in CeO2 determine the Ce3+ and Ce4+ ratio, which leads to an enhanced visible photocatalytic
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activity by narrowing the bandgap. The oxygen vacancy in CeO2 is highly reactive with graphene and thus the grapehene-CeO2 (CeO2 G) nanocomposite can be formed and such composite could be an interesting material for enhancing the photocatalytic, electrochemical properties. However, the rare earth oxide based graphene composites were rarely investigated for their photocatalytic and electrochemical properties.15-17 Jiang et al.,15 reported the optical and catalytic properties of CeO2 G. Wang et al.,16 reported the electrochemical properties of CeO2 G nanocomposites. However, in both the studies the composites were prepared at a fixed ratio and the electrochemical studies were performed at a single scan rate and the results have not been discussed in detail. To understand the effect of graphene on the electrochemical properties of CeO2, it is essential to vary the graphene content. Therefore, further investigation is needed to understand the enhanced electrochemical properties of CeO2 G composites. In the present work, CeO2 G nanocomposites were synthesized with different weight percentage of graphene by the modified Hummer's method. The nanocomposites were characterized by various analytical techniques for structural, compositional and morphological analysis. The electrochemical performance of the composites was analyzed by cyclic voltammetry, galvanostatic charge/discharge studies and the results were discussed.
Experimental Techniques Synthesis of graphene and graphene-CeO2 nanocomposites Graphite powder was reduced as graphene oxide (GO) by the modified Hummer’s method as described in our earlier report.18 Then the GO was reduced using thiourea as a reducing agent to synthesize the graphene. Typically, 100 mg of GO was dispersed in 100 ml of water through ultrasonication for 1h and 100 mg of
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thiourea was added to the dispersion. The solution was maintained at 100oC with constant stirring for 24 h. The brown color of GO was turned to black color graphene due to the reduction process. Finally the black color graphene was washed with ethanol and dried in vacuum oven at 60 oC for 24 h. To prepare the CeO2G1 composite, 99 mg of as purchased cerium oxide (CeO2) (99.9 %) was dispersed in 100 ml of water by ultrasonication and 1 mg of reduced graphene was added to the dispersion. Then the solution was maintained at room temperature with constant stirring for 2 h. The final product CeO2G1 was collected and dried in vacuum at 60 C for 24 h. By the same procedure, CeO2G composites with 3 and 5 mg of graphene with 97 and 95 mg of CeO2 were also prepared and named as CeO2G3 and CeO2G5 respectively. The influence of graphene on the structural and electrochemical properties of composites has been studied by various analytical techniques. Characterization techniques CeO2G nanocomposites were characterized by various analytical techniques. Rigaku MiniFlexII-C X-ray diffractometer with CuKα ( = 1.540 nm) radiation at a scanning rate of 1deg/min was used to collect the powder X-ray diffraction data to identify the material. Raman and FTIR spectra were recorded to confirm the formation of composites JASCO Raman Spectrophotometer (NRS – 7100) and NICOLET Infrared spectrophotometer, respectively. The morphology of the composites was analyzed using FESEM and HRTEM images obtained by JEOL 7001F and JEOL-JEM 2100F electron microscopes, respectively. The elemental composition was analyzed by elemental mapping and EDS spectra obtained by JEOL JXA-8530F model field emission electron probe micro analyzer (FEEPMA) instrument. The electron binding energies of various elements in the composites were analyzed by ESCA 3400 SHIMADZU X-ray photoelectron spectrometer. The electrochemical performance of the composites was investigated by three electrode system using Bio Logic electrochemical work station. Glassy carbon, Ag/AgCl and platinum wire have been used as working, reference and counter electrodes, respectively. 1 M H2SO4 was used as electrolyte and the cyclic voltametric curves were recorded in the
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Results and Discussion The synthesized graphene, pure and CeO2G nanocomposites were subjected to powder X-ray diffraction studies and the recorded patterns are shown in Fig.1a. The diffraction pattern of graphene contains only two peaks corresponding to 002 and 100 diffraction planes at two theta values of 23.44 and 42.92 degrees, respectively. The diffraction patterns of CeO2 and CeO2G composites possess a large number of high intensity peaks confirming the good crystalline nature of the composites as shown in Fig.1a. The diffraction patterns are compared with the standard JCPDS #65-5923 for CeO2 and the peaks were indexed accordingly. From the patterns it is observed that the influence of graphene is not significant as the quantity of graphene is small. On the careful examination from the enlarged peaks at various two theta values, it was observed that the diffraction peaks were shifted reasonably in the composite compared to pure CeO2 and reduced the intensity of peaks as shown in Fig. 1b, which confirms the composite formation. Laser Raman spectra were recorded in the range of 800 to 2000 cm-1 for the graphene and CeO2 G5 composites (Fig.1 c) and CeO2G1 (Fig. 1d). The spectrum of pure graphene has two characteristic peaks called D and G bands at 1343 and 1577 cm-1, respectively as shown in Fig. 1c-2. The intensity ratio of D and G band, ID/IG for graphene is measured to be 1.11, which is higher than GO (1.03) (Fig.1e). Hence, the graphene reduced from GO ensured the sp2 bonded carbon. On the other hand, the CeO2G5 composite is having an additional strong peak at 1161 cm-1 due to CeO2 in addition to the D and G bands of graphene (Fig.1c-1). The Raman spectrum of CeO2G1 was also recorded and found that it has the intense peak at 1163 cm-1 due to CeO2 and very weak signals were observed for D and G bands of graphene. This spectrum ensures the less quantity of graphene in the composite. The band shift, increased intensity and additional peaks are the evidence for the formation of CeO2G5 composite.
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Fig.1. (a) Comparison of powder x-ray diffraction patterns of graphene, CeO2, and CeO2G composites (b) diffraction peak shifts,(c) Laser Raman spectra of CeO2G5 & graphene (d) Laser Raman spectrum of CeO2G1 and (e) Raman spectrum of Graphene Oxide and Graphene.
Fig. 2. FE-SEM images of (a & b) CeO2G1 and (c& d) CeO2G5 The morphology of the prepared composites was studied by Field emission scanning electron microscope (FE-SEM) and the images are shown in Fig. 2 (a-d). The morphology of CeO2G1 contains graphene sheets and it is observed that CeO2 is intercalated between the graphene sheets as shown in Fig.2a. Higher magnification image (Fig.2b) confirms the wrinkled paper like structure of graphene with CeO2 attached on the surface. The morphology of the CeO2G5 composite is shown in Fig. 2c. The increase in the graphene weight percent causes the aggregation of CeO2 with graphene and hence graphene structure becomes irregular flake-like morphology as shown in the magnified image (Fig. 2d).
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HRTEM images of CeO2G5 at different magnification are shown in Fig 3 a-d. It is observed that the CeO2 crystals in different sizes ranged from 100 – 250 nm were distributed on the surface of the graphene sheets as marked in Fig. 3a. Higher magnification image infers that the size of the particles is around 10 – 50 nm at different depth of graphene layers as shown in Fig 3b. Fig. 3c shows the folded graphene sheets in the 50 nm scale and the marked region is shown in 5 nm scale at Fig. 3d. Higher magnification images taken in 5-10 mm scale of marked regions of Fig. 3b are shown in Fig. 3eg. The rectangular mark shows the thick bunch of graphene sheet, which do not have any lattice fringe pattern as shown in Fig. 3e. The region marked by the continuous circle has bright lattice fringes as shown in Fig. 3f, which ensures the presence of CeO2 immediately below a layer. The region marked by the dotted circle has clearly visible bright fringes (Fig. 3g) confirming that CeO2 is on top of the graphene sheet. These images supported effectively the formation of composite. It is observed that a bunch of graphene with 5 nm thickness contains 15 sheets as shown in Fig. 3h, which confirmed that graphene has been effectively synthesized by this method. The composites were also subjected to Field emission electron probe micro analysis (EPMA) and the elemental mapping was recorded. The EPMA mapping and corresponding energy dispersive spectrum for CeO2G5 composite is shown in Fig. 4. The EPMA and EDX spectrum of CeO2G1 composite has also been recorded and given as supporting information (Fig.S1). The systematic increase of the carbon content was observed in the composites from CeO2G1 to CeO2G5. Uniform distribution of carbon, oxygen and cerium elements was observed in the composites. The energy dispersive spectra also confirm the presence of all elements in the composites.
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Fig.3. HRTEM images of CeO2G5 (a) nanocrystals of CeO2 on the top of graphene sheets, (b) CeO2 nanocrystals at different depth between the graphene sheets, (c) folded graphene sheets with CeO2 nanocrystals (d) Higher magnification image of folded graphene sheets and (e-h) magnified images of marked regions in (b and d).
CeO2G5 composite was analyzed by XPS spectra of various elements as shown in Fig. 5 a-d. Figure 5a shows the 3d electron core level spectrum of cerium in the region 875-920 eV. The XPS spectrum consists of three distinct signals correspond to different ionic states of Ce3+ and Ce4+. The characteristic XPS signals of Ce3+, 3d5/2 and 3d3/2 electron states have been observed at 886 and 905 eV, respectively with a separation of 9 eV. The additional satellite signal is due to Ce4+, 3d3/2 electron state at 919 eV. The core level spectrum of 4p electron of cerium in the region 200-240 eV is shown in Fig. 5b. It is observed that the spectrum contains two peaks correspond to 4p3/2 and 4p1/2 states at 210.3 and 227.6 eV, respectively. The recorded spectra show the presence of mixed valence cerium of Ce 3+ and Ce4+. The main peaks observed at 886 and 919 eV represent the relative amount of Ce3+ and Ce4+ present in the sample. From the area of the respective peaks, it is obvious that the concentration of Ce3+ is relatively higher in the samples, which confirms the presence of oxygen vacancies in CeO2. The binding energy difference between these two states is measured to be 17.3 eV and it has good agreement with the standard values19. The core level XPS spectrum of C 1s electron is shown in Fig. 5c, which contains a prominent signal at 284.3 eV and a satellite peak at 289.4 eV. The former is due to sp2 bonded carbon (C-C) and the latter is assigned to carboxyls (C-C=O), which indicates carbon has a reasonable amount of oxygen containing functional groups.19 Two different oxygen species could be distinguished on deconvolution of core level spectrum of O1s electron in the range between 525 and 540 eV as shown in the Fig. 5d. The binding energies of 530.5 and 531.8 eV have been assigned to lattice oxygen and carboxyl groups, respectively.14 Fig. 4. FE-EPMA images of CeO2G5 and corresponding EDS spectrum.
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Fig.5. Core level X-ray photoelectron spectra of (a) Cerium 3d, (b) Cerium 4p, (c) Carbon 1s and (d) Oxygen 1s electrons
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Fig. 6. Cyclic voltammetric curves of (a) Pure CeO2, (b) CeO2G1, (c) CeO2G3 (d) CeO2G5 for different scan rates and (e) Comparison of CV of pure CeO2, CeO2G1, CeO2G3, CeO2G5 at a scan rate of 10mV/s.
Fig.7 (a) Specific capacitance of CeO2 and CeO2G composites as a function of scan rate (mV) and (b) The relation between the oxidation peak current at different scan rates and the square root of scan rate for pure CeO2 and CeO2G5.
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Fig. 8. (a) Comparison of charge –discharge curves for pure CeO2 and CeO2G5 under 0.65A/g, (b) Nyquist plots of pure CeO2 and CeO2G5 composite. Cyclic Voltammetry has been used to study the capacitance of pure and CeO2 G with various amounts of graphene at different scan rates. Figure 6 shows the cyclic voltammogram results of (a) pure CeO2, (b) CeO2G1, (c) CeO2G3 and (d) CeO2G5 in the electrolyte of 1M H2SO4. As shown in 6(a), the pure CeO2 electrode shows the capacitive-like responses in the potential window of investigation. As compared to other electrodes (Fig. 6 b & c), the CeO2G5 electrode demonstrates the highest current response with an obvious double layer current in the potential range of (-0.3 to +1.0 V) as shown in Fig. 6d. The cyclic voltammogram of the nanocompsites at the fixed scan rate is plotted in Fig.6e, which shows that the current response increases with double layer current for the nanocomposite with increasing graphene ratio. The specific capacitance values for CeO2 and CeO2G composites have been calculated under the area of the cyclic voltammogram curve for different scan rates by using equation (1) Cs = IdV/vmV. ------ (1) Where, I is the response current, V is the potential, v is the potential scan rate and m is the mass of the active material of the electrode. The calculated specific capacitance was plotted as a function of scanning rates (Fig.7a). The specific capacitance decreases with the increase of scan rates from 10 to 100 mV s-1. The specific capacitance of CeO2G5 sample is much higher than that of pure CeO2 and other composites with low graphene content at the same scan rates. The specific capacitance of 109 F g-1 is observed at 10 mV s-1 of scan rate for CeO2G5 composite when compared to 87 F g-1 for CeO2G1 composite and 75 F g-1 for pure CeO2, respectively. The specific capacitance of the CeO2 G5 samples decreases from 109 to 35 F g-1 as the scan rate increases from 10 to 100 mV/s, representing only 68 % decrease, whereas 80 % of reduction was observed in the pure CeO2. With increasing scan rate, the specific capacitance decreases gradually (Fig. 7a) and it can be attributed to the fact that the electrolytic ions easily diffuse and migrate into the active materials at lower scan rates. At higher scan rates, the diffusion is controlled, limiting the migration of the electrolytic ions, which causes some active surface areas to become inaccessible for charge storage.
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Figure 7b shows the relationship between oxidation peak current at different scan rates and the square root of scan rate, which gives the information about the electrochemical kinetics of the reaction. In general, the electrochemical double layer capacitor (EDLC) reaction is a diffusion controlled process and the corresponding CV curve is rectangular in shape, therefore I Vs V1/2 graph derived from this capacitive region is linear. For the CeO2-G nanocomposite, the enhanced specific capacitance is attributed to the combination of both EDLC and faradaic reactions. The faradaic reaction (electron transfer across the electrode electrolyte interface) caused by the presence of CeO2 has the two possibilities: (1) kinetic and (2) diffusion controlled process. From the Figure 7b, I Vs V 1/2 graph drawn for the corresponding oxidative peak current of CeO2-G composite shows the linear relationship. The graphene present in the composite might have enhanced the fast electron transport, which facilitates the charge transport kinetics of the faradaic reaction. Therefore, it is concluded that the corresponding faradaic reaction is entirely diffusion limited process rather than kinetic controlled process. Further, our experimental results also demonstrate that the composite with high graphene content (CeO2G5) is a better electrode material for supercapacitor applications. Galvanostatic charge-discharge measurements were also carried out on the pure CeO2, CeO2G1, CeO2G3 and CeO2G5 composite materials and the results are shown in Fig. 8a. It can be seen that the charging curves are symmetrical to their corresponding discharge counterparts. This indicates the excellent electrochemical reversible charge-discharge properties. Moreover, the duration of chargedischarge increases drastically when the graphene content increases, indicating the enhanced specific capacitance of the composites. The specific energy, specific power of the electrode material was calculated using the following equations: E= 1 / 2 × Cs × (ΔV)2 (2) P = E/Δt
(3)
where Cs is the specific capacitance calculated from the CV curves, ΔV is the operating voltage of the cell and Δt is the discharge time
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. The calculated values are tabulated (Table 1) for pure and composite materials. The specific energy increases and specific power decrease when the graphene content increases in the composite material. The specific energy of CeO2G5 (92.1 Whkg-1) was nearly three times higher than pure CeO2 (33.13 Whkg-1). Furthermore, the capacitive performance of the pure and CeO2G5 composites were analyzed by electrochemical impedance spectroscopy (EIS). The measurements were performed at a frequency range from 100 kHz to 100 Hz. As shown in Fig.8b, the Nyquist plots show linear region with the absence of semicircle region, which shows the low charge transfer resistance at the electrode/electrolyte interface 23. The combined resistance, including the ionic resistance of the electrolyte, the intrinsic resistance of the substrate and the contact resistance at the interface of active material/current collector can be obtained from the intercept of the linear part of the Nyquist plot. From the Fig.8b, the combined resistance of the pure and composite materials is almost the same probably due to low graphene content in the composite material. Table 1: Calculated specific capacitance, specific energy and specific power of the pure and composite electrode material. S. No
Composite material
Specific capacitanceCs ( Fg-1)
Specific Energy -E (Whkg-1)
Specific Power -P (Wkg-1)
1
Pure CeO2
75
33.13
12097.98
2
CeO2G1
86
72.67
18875.34
3
CeO2G3
106
89.57
15472.72
4
CeO2G5
109
92.10
7368.40
Conclusion Graphene-CeO2 nanocomposites were prepared with different weight percent of graphene by the low temperature solution process. Structural properties of the nanocomposites were studied by X-ray diffraction analysis. Laser Raman spectral analyses were carried out to confirm the formation of nanocomposites. The morphology of the synthesized compounds has been studied by FESEM and HRTEM analysis. Systematic variation of carbon level in the composites was observed by Elemental mapping by the FE-EPMAtechnique. The binding states of various elements of the composites were studied by XPS analysis. The electrochemical studies show that the CeO2G5 nanocomposite exhibits good capacitance behavior and the specific capacitance of CeO2G5 sample (109 F g-1) is higher than that of pure CeO2 (75 F g-1).
Notes a
Centre for Nano Science and Technology, Anna University, Chennai600025, India. Email (T. Saravanan ): [email protected] a Centre for Nano Science and Technology, Anna University, Chennai600025, India. Email (M.Shanmugam): [email protected]. b Centre for Materials Research, Thiruvalluvar College of Engineering and Technology, Vandavasi-604 505, India. Email (P. Anandan): [email protected], (K. Pazhanivel): [email protected]. c Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai-980 858, Japan. Email: [email protected].
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d
Research Institute of Electronics, Shizuoka University, Johoku, Naka-ku, Hamamatsu-432 8011, Japan. Email (M. Arivanandhan): [email protected], (Y. Hayakawa): [email protected]. *To whom correspondence should be made (R. Jayavel) Email: [email protected]. Electronic Supplementary Information (ESI) available: [supplementary information available here].
References 1. Y. Wang, K. Takahashi, K. Lee and G. Z. Cao, Adv. Funct.Mater., 2006, 16, 1133. 2. V. Augustyn, P. Simon and B. Dunn, Energy Environ. Sci., 2014, 7, 1597. 3. A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183 4. D. A. Dikin, S. Stankovich, E. J. Zimney, R.D. Piner, G. H. B. Dommett, G. Evmenenko, S. T. Nguyen and R.S. Ruoff, Nature., 2007, 448, 457. 5. J.X. Zhu, Y. K. Sharma, Z. Y. Zeng, X.J. Zhang, M. Srinivasan, S. Mhaisalkar, H. Zhang, H. H. Hng and Q.Y. Yan, J. Phys Chem C., 2011, 115, 8400. 6. D.H. Wang, R. Kou, D.W. Choi, Z. G. Yang, Z. M. Nie, J. Li, L.V. Saraf, D. H. Hu, J.G. Zhang, G. L Graff, J. Liu, M. A. Pope and I. A. Aksay, ACS Nano., 2010, 4 1587. 7. S. X. Wu, Z. Y. Yin, Q. Y. He, X. Huang, X. Z. Zhou and H. Zhang, J. Phys. Chem.C., 2010, 114, 11816. 8. F. Ji, Y. L. Li, J. M. Feng, D. Su, Y. Y. Wen, Y. Feng and F. Hou, J. Mater. Chem., 2009, 19, 9063. 9. Y. H. Ng, A. Iwase, N. J. Bell, A. Kudo and R. Amal, Catal. Today., 2011, 164, 353. 10. C. Hu, T. Lu, F. Chen, R. Zhang and J. Chin, Adv. Mater, Soc., 2013, 1 21. 11. Z. L. Wang, G. R. Li, Y. L. Ou, P. Z. Feng, D. L. Qu and Y. X. Tong, J. Phys. Chem. C., 2011, 115, 351. 12. G. A. Deluga, J. R. Salge, L. D. Schmidt and X. E. Verykios., Science 2004, 303, 993. 13. A, Corma, P. Atienzar, H. García, and J. Y. C. Ching, Nat. Mater., 2004, 3, 394. 14. X. Wang, X. Li, D. Liu, S. Song and H. Zhang, Chem. Commun., 2012, 48, 2885. 15. L. Jiang, M. Yao, B. Liu, Q. Li, R. Liu, H. Lv, S. Lu, C. Gong, B. Z. T. Cui, and B. Liu, J. Phys. Chem. C 2012, 116, 11741. 16. Y. Wang, C. X. Guo, J. Liu, T. Chen, H. Yang and C. M. Li, Dalton Trans., 2011, 40, 6388. 17. G. Wang, J. Bai, Y. Wang, Z. Ren and J. Bai, Scripta Materialia., 2011, 65, 339. 18. K. Satheesh and R. Jayavel, Mater. Let., 2013, 113, 5. 19. Y. Chen, B. H. Song, H. S. Tang, L. Lu and J. M. Xue, J. Mater. Chem., 2012, 22, 17656. 20. T.W.Lin, C.S.Dai and K.C.Hung, Scientific Reports, 2014, 4,7274. 21. H.Kim, M.Y.Co, M.H.Kim, K.Y.Park, H. Gwon, Y.Lee, K.C.Roh and K.Kang, Adv. Energy Mater., 2013, 3, 1500. 22. R.Raja, S.Lok Kumar, K.Minami, M.Subramanian, R.Jayavel, K.Ariga, J.Mater. Chem. A, 2014, 2, 18480. 23. H.C.Gao, F.Xiao, C.B.Ching and H.W.Duan, ACS Appl. Mater. Interfaces, 2012, 4, 2801.
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