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Template Synthesis of Tubular Ruthenium Oxides for Supercapacitor Applications Jintao Zhang,† Jizhen Ma,† Li Li Zhang,† Peizhi Guo,‡,§ Jianwen Jiang,† and X. S. Zhao*,†,‡,§ Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576, Institute of Multifunctional Materials (IMM), Laboratory of New Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao UniVersity, and College of Chemistry, Chemical Engineering and EnVironment, Qingdao UniVersity, Qingdao 266071, P.R. China ReceiVed: June 4, 2010; ReVised Manuscript ReceiVed: July 15, 2010
Nanotubular ruthenium oxides were prepared by using manganite nanorods as a morphology sacrificial template. Experimental results showed that the template dissolved away completely during the formation of the ruthenium oxide nanotubes. A mechanism was proposed to interpret the formation of the ruthenium oxide nanotubes. The electrochemical capacitive properties of the ruthenium oxide nanotubes were investigated using cyclic voltammetry and charge/discharge techniques with H2SO4 and Na2SO4 solutions as the electrolytes, respectively. The specific capacitance of the nanotubular ruthenium oxide electrode was measured to be as high as 860 F/g at a current density of 500 mA/g in the H2SO4 electrolyte, which was higher than that obtained in the Na2SO4 electrolyte. In addition, the ruthenium oxide nanotubes were observed to exhibit a good capacitive retention at high current loads. Introduction Supercapacitors are energy storage devices that have attracted rapidly increasing attention because of a number of important features, such as high power density and long life cycle in comparison with those of batteries and conventional capacitors. Energy storage in a supercapacitor is due to either ion adsorption at the interface of the electrode and electrolyte (namely, electrical double-layer capacitors, EDLCs) or fast and reversible Faradaic reactions (namely, pseudocapacitors).1,2 The pseudocapacitance affords a higher specific capacitance than the electrical double-layer capacitance. Transition metal oxides and conducting polymers are the common electrodes for pseudocapacitors. Among transition metal oxides, ruthenium oxides with multiple redox states and good electrical conductivity offer a number of advantages as a pseudocapacitor electrode.3,4 Various structures and morphologies of ruthenium oxides have been reported in the literature.4-9 The specific capacitance of ruthenium oxides have been found to be highly sensitive to the amount of water in the structure.4,6 For example, mesoporous anhydrous ruthenium dioxide (RuO2) displayed a specific capacitance of about 185 F/g at a scan rate of 25 mV/s,10 which is considered low. In contrast, mesoporous hydrous RuO2 thin films exhibited a specific capacitance of more than 800 F/g at a scan rate of 10 mV/s.11 Nanotubular structures offer a number of advantages when used as supercapacitor electrodes, such as high surface area and fast transport of electrolyte ions.2,12-14 Hydrous RuO2 nanotube arrays have been fabricated with the assistance of anodic aluminum oxide (AAO) template by means of an anodic deposition method. The arrayed RuO2 nanotubes were observed * Corresponding author. E-mail:
[email protected]. Telephone: +65 65164727. Fax: +65 67791936. † Department of Chemical and Biomolecular Engineering, National University of Singapore. ‡ Institute of Multifunctional Materials (IMM), Laboratory of New Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University. § College of Chemistry, Chemical Engineering and Environment, Qingdao University.
to display a specific capacitance of 1300 F/g at a scan rate of 10 mV/s.13 Other methods, such as chemical vapor deposition and reactive sputtering have been employed to fabricate RuO2 hollow nanostructures.15,16 However, these methods involve high-temperature processing, usually leading to the formation of anhydrous RuO2, which, as has been observed before,10,11 performs more poorly than hydrous RuO2. Here, we demonstrate a simple approach to preparing nanotubular hydrous ruthenium oxides by using MnOOH nanorods as a morphology template. The novelty of the present method lies in the fact that the manganese oxide nanorods gradually dissolve away during the formation of nanotubular ruthenium oxides. As a result, the template removal step is not needed. On the basis of the experimental results, a possible mechanism for the formation of nanotubular ruthenium oxides was suggested. Formic acid was observed to play an important role in the process of depositing ruthenium oxide on the surface of MnOOH nanorods as well as forming the final tubular structure. The specific capacitance of the ruthenium oxide nanotubes was measured to be as high as 860 F/g at a current density of 500 mA/g in a H2SO4 electrolyte. Experimental Section Chemicals. Ruthenium(III) chloride hydrate, sodium sulfate, manganese nitrate, formic acid, and sulfuric acid were purchased from Merck. Ethylene diamine tetraacetic acid (EDTA) was obtained from Sigma-Aldrich. All chemicals were used as received without further purification. Preparation of Manganese Oxide Nanorods. Mn(NO3)2 (126 mg) and ethylene diamine tetraacetic acid (EDTA) (186 mg) were each dissolved in 10 mL of distilled water. The pH value of the EDTA solution was adjusted to 9.26 by adding a 10 M NaOH solution. The two solutions were mixed at room temperature under stirring for 10 min. Then, 126 mg of KMnO4 dissolved in 20 mL of distilled water was added dropwise under stirring for 20 min. The mixture was transferred to a 60-mL Teflon-lined stainless steel autoclave and heated in an oven at 100 °C. After 12 h, brown-colored solids were collected by
10.1021/jp105146c 2010 American Chemical Society Published on Web 07/28/2010
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Figure 1. XRD patterns of MnOOH nanorods (a) and RuOx · nH2O nanotubes (b).
centrifugation, washed with distilled water and finally with absolute ethanol, and then dried at 60 °C. Preparation of Ruthenium Oxide Nanotubes. The manganese oxide nanorods prepared above were employed as morphology template to direct the formation of nanotubular ruthenium oxides. First, 5 mg of the manganese oxide was dispersed in a formic acid solution (0.5 mL formic acid in 10 mL H2O). Then, 1.2 mL of a 40 mM RuCl3 solution was added. The mixture was aged at room temperature for 12 h. The resulting precipitate was separated using a centrifuge, washed with distilled water and finally with absolute ethanol, and then dried in air at 60 °C. Characterization. Samples were characterized using powder X-ray diffraction (XRD) on a Shimadzu diffractometer (XRD6000) with Cu KR radiation at a step size of 0.02 deg s-1. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI-5300 ESCA spectrometer (PerkinElmer) with an energy analyzer working in the pass energy mode at 35.75 eV. An Al KR line was used as the excitation source. Fieldemission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan) and high-resolution transmission electron microscopy (HRTEM, JEOL-2100F, 200 kV) were used to characterize sample morphologies. Energy-dispersive X-ray (EDX) analyses were recorded on an INCAX-sight EDX spectrometer equipped on the FESEM. Electrochemical Measurement. An Autolab PGSTAT302N was used to measure the electrochemical properties of the samples at ambient temperature (about 22 °C). The electrochemical cell had a three-electrode configuration with a bright Pt plate as the counter electrode and an Ag/AgCl electrode as the reference electrode. A polished glassy carbon working electrode (GCE) 5 mm in diameter was used as a current collector. A sample and acetylene black with a mass ratio of 90:10 were mixed with ethanol to achieve a homogeneous mixture. A small amount of Nafion solution (0.5 wt %) was added to paste the material on the GCE to give a mass of 0.2 mg (1 mg/cm2). Either 1 M H2SO4 or 1 M Na2SO4 solution was employed as the working electrolyte. Results and Discussion Characterization of Samples. Figure 1 shows the XRD patterns of the manganese oxide nanorods and ruthenium oxide samples. All diffraction peaks in Figure 1a can be readily indexed to a MnOOH phase (JCPDS 41-1379), showing that the sample is a pure MnOOH.17 For the ruthenium oxide sample, the XRD pattern (Figure 1b) shows a very broad diffraction peak in the range of 2θ ) 20-40°, indicating an amorphous nature of the sample.7
Figure 2. Overview XPS spectra of RuOx · nH2O nanotubes (a), highresolution XPS spectra of Ru 3d + C 1s (b), and O 1s (c) for RuOx · nH2O nanotubes.
Figure 2a shows the wide-scan XPS spectrum of the ruthenium oxide sample. Only O 1s, Ru 3p, Ru 3d, and C 1s peaks are seen. No peak due to manganese species can be seen (the absence of manganese in the ruthenium oxide sample was also confirmed by the Mn 2p XPS spectrum shown in Figure S1a, Supporting Information), indicating the sample did not contain any manganese species. Because the XPS signal of Ru 3d photoelectrons overlaps with that of C 1s, the XPS signals in the overlapping region were deconvoluted by curve fitting using the mixed Gaussian and Lorentzian line shape, and the results are shown in Figure 2b. It is shown that the C1s peak is centered at 284.5 eV. The Ru 3d5/2 and Ru 3d3/2 signals can be deconvoluted to four separate peaks. The two peaks at 281.2 and 285.6 eV are attributed to Ru (IV) of RuO2 while the other two with binding energies of 282.2 and 288.1 eV are attributed to Ru (III) from hydrous Ru (III)-OH.11,18 From the O 1s XPS spectrum shown in Figure 2c it can be seen that, except for the peak at 529.8 eV that corresponds to lattice oxygen O2-, two peaks at 531.2 and 533 eV belonging to hydroxyl groups and surface adsorbed water,19,20 respectively, are also seen. The XPS data indicate that the ruthenium oxide sample contains both hydrated ruthenium dioxide and hydrated ruthenium species (Ru (III)-OH).19,20 As a result, the sample is hereafter named as RuOx · nH2O. The FESEM image of MnOOH sample shown in Figure 3a clearly reveals a rodlike shape with smooth surface. The
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Figure 3. FESEM images of (a) MnOOH nanorods and (b) RuOx · nH2O nanotubes. (c, d) TEM images of RuOx · nH2O nanotubes. (e) EDX analysis of RuOx · nH2O nanotubes.
Zhang et al. diameters of the rods vary in the range of 20 and 40 nm, while the lengths change from several hundred nanometers to a few micrometers. Low-magnification and high-magnification FESEM images of the RuOx · nH2O sample are shown in Figure S2 (Supporting Information) and Figure 3b, respectively. It can be seen that the rod morphology of the MnOOH sample was well inherited by the RuOx · nH2O sample. The diameter of the RuOx · nH2O, however, became large, while the surface became apparently coarse in comparison with those of the MnOOH rods. A tubular structure of the RuOx · nH2O sample is clearly seen from TEM images shown in c and d of Figure 3. From the highmagnification image shown in Figure 3d, the diameter and the wall thickness of RuOx · nH2O nanotubes were estimated to be about 40 and 8 nm, respectively. For comparison purpose, the TEM images of MnOOH rods are shown in Figure S3 (Supporting Information). It is obvious that the tubular structure of RuOx · nH2O sample is different from the rodlike structure. The EDX results shown in Figure 3e confirms that the RuOx · nH2O sample contains only Ru and O elements, in good agreement with the XPS data. The Formation Mechanism of Tubular Ruthenium Oxides. To understand the formation process of the RuOx · nH2O nanotubes, evolution of the solid phase at different reaction times was investigated. Figure 4 shows the TEM images and the EDX spectra of the solid phase after 15 min of reaction. It can be seen from Figure 4a that the surface of the MnOOH rods has become very rough after 15 min. In addition, the HRTEM images shown in b and c of Figure 4 reveal that the surface of the MnOOH rods is covered with numerous nanodots (indicated by circles), which would be RuOx · nH2O nanoparticles. The EDX spectrum shown in Figure 4d indeed shows the presence
Figure 4. TEM and HRTEM images (a, b, c) and EDX spectrum (d) of the solid phase collected after 15 min reaction.
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Figure 5. TEM images of the solid phase collected after (a) 1 h, (b) 3 h, (c) 6 h, and (d) 12 h. The insets in (a) and (d) are enlarged images with the scale bars of 50 nm.
Figure 7. Cyclic voltammograms of nanotubular RuOx · nH2O electrode in H2SO4 (a) and in Na2SO4 (b) at scan rates of 50, 100, 250, 500, and 1000 mV/s, respectively.
Figure 6. Schematic illustration of the formation of RuOx · nH2O nanotubes templated by MnOOH nanorods in formic acid solution.
of Ru, Mn, and O elements in the sample. The atomic content of Ru is estimated to be about 7%. Thus, it can be concluded that the nanodots are ruthenium oxides. Figure 5 compares the TEM images of the solid phases collected after different reaction times. It is seen from Figure 5a and the enlarged image (inset) that a rodlike structure formed after 1 h. However, the composition is different from that of the sample obtained after 15 min reaction. The EDX data (Figure S4a, Supporting Information) reveals that the sample contains only about 2.3% Mn, indicating that the sample is mainly ruthenium oxide. After 3 h of reaction, the sample still appeared as rods (Figure 5b). In contrast, the sample exhibited a partially hollow tubular structure (Figure 5c) after 6 h. When the reaction time was increased to 12 h, a complete tubular structure was obtained (Figure 5d). The inset in Figure 5d clearly shows a tubular structure of the sample. The EDX data depicted in Figure S4b (Supporting Information) confirmed that no manganese species was present in the solid phase after 12 h of reaction. On the basis of the observations on the morphological evolution as a function of time, a mechanism is proposed to explain the formation of the RuOx · nH2O nanotubes as schematically illustrated in Figure 6. It is believed that the surface of alkali manganese oxide nanorods generated many hydroxyl groups at the initial stage.21 The hydroxyl groups would combine with the colloids of ruthenium ions to form Ru(OH)x · yH2O and further transform to RuOx · nH2O nanoparticles at the solid (MnOOH)-liquid interface.3,5,7,22 Meanwhile, formic acid corrosively etched the inner part of the MnOOH nanorods, which facilitated the diffusion of Ru3+ ions for the continuous formation of RuOx · nH2O nanoparticles. These small RuOx · nH2O nanoparticles self-assembled and merged together due to their high surface energy and low stability in the presence of formic acid,23,24 With the dissolving of the manganese oxide rods, RuOx · nH2O nanotubes eventualy formed. Here, formic
acid is favorable to the formation of RuOx · nH2O tubular structures. Our experimental results revealed that no RuOx · nH2O nanotubes formed in the absence of formic acid under otherwise the same experimental conditions. Instead, only small RuO2 nanoparticles formed after 12 h reaction (Figure S5, Supporting Information). Electrochemical Properties. Both cycle voltammetry and charge/discharge techniques were used to evaluate the electrochemical properties of the nanotubular RuOx · nH2O. As the annealing temperature and time of RuO2 nanoparticles are important factors in terms of electrochemical capacitance,25,26 the RuOx · nH2O sample was annealed at 150 °C for 2 h before electrochemical measurements. Figure 7a shows the CV curves in 1 M H2SO4 electrolyte at various scan rates. The nearly perfect rectangle shape of CV curves at scan rates of 500 and 1000 mV/s shows an ideal pseudocapacitive behavior of the electrode materials with a high rate capability. When 1 M Na2SO4 solution was used as the electrolyte, a similar ideal pseudocapacitive behavior was observed (Figure 7b), but with a slight decrease in the current density. The capacitances of electrode in different electrolytes were calculated using equationCarea ) I/V, where Carea in mF/cm2 is the capacitance normalized to the geometric area, and V in V/s is the scan rate. In 1 M H2SO4 electrolyte, the capacitances were measured to be about 118 and 31 mF/cm2 at the scan rates of 50 and 1000 mV/s, respectively. When 1 M Na2SO4 solution were used as electrolyte, The capacitances were measured to be about 39 and 27 mF/cm2 at the scan rates of 50 and 1000 mV/s, respectively. The capacitance values are higher than that of ruthenium oxide electrode materials reported in the literature.7,27 It is practically more useful to calculate the gravimetric capacitance based on the mass of electrode materials. Therefore, the relationship between mass current density and specific capacitance of the tubular RuOx · nH2O was investigated in H2SO4 and in Na2SO4 electrolytes, respectively. The charge/ discharge curves at different current densities in 1 M H2SO4 electrolyte are shown in Figure 8a. With the equation Cm ) I∆t/m∆E (Cm in F/g stands for the specific capacitance of the
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Figure 9. Specific capacitances of nanotubular RuOx · nH2O electrode at different current densities in Na2SO4 (a) and H2SO4 (b). Capacitance retention of nanotubular RuOx · nH2O at different current densities in H2SO4 (c) and Na2SO4 (d).
Figure 8. Charge/discharge curves of nanotubular RuOx · nH2O electrode in H2SO4 (a) and in Na2SO4 (b) at different current densities.
electrode, I in A is the charge/discharge current, ∆t in s is the charge or discharge time, m in g is the mass of the active electrode material, and ∆E in V is the voltage range of charge or discharge),28 the corresponding specific capacitances of the RuOx · nH2O nanotubes at current densities of 4, 2, 1, and 0.5 A/g were calculated to be 654, 722, 787, and 861 F/g, respectively, which are comparable or superior to the reported values in the literature.29 As a comparison, RuOx · nH2O nanoparticles were synthesized by a modified sol-gel method (in Supporting Information).4,5 The RuOx · nH2O nanoparticles exhibited a network structure composed of many small nanodots with an average diameter of about 2 nm in Figure S6 (Supporting Information). The capacitive performances were investigated by cycle voltammetry and charge/discharge methods (Figures S7 and S8, Supporting Information). Based on the charge/discharge curves the specific capacitances of RuOx · nH2O nanoparticles at current densities of 4, 2, 1, and 0.5 A/g were 596, 623, 634, and 646 F/g, respectively. The values of specific capacitances of nanotubular RuOx · nH2O are larger than these of RuOx · nH2O nanoparticles. The high specific capacitances of nanotubular RuOx · nH2O are believed to be related to unique tubular structure. The specific capacitance of ruthenium oxide electrodes involves two contributions: electrical double layer capacitance (ECDL) and pseudocapacitance.30,31 The predominant contribution of ruthenium oxide comes from pseudocapacitance due to reversible redox reactions in a proton-rich electrolyte. The hydrous nature of the RuO2 · xH2O electrode plays an important role in the charge/discharge processes versus insertion/extraction of protons with concomitant redox transitions between Ru(II)/ Ru(III), Ru(III)/Ru(IV), and Ru(IV)/Ru(VI) pairs.26,32,33 The charge storage mechanism of RuO2 · nH2O can be expressed as:1,34
RuOx(OH)y + δH+ + δe- S RuOx-δ(OH)y+δ
tubular architecture of the present RuOx · nH2O can on one hand reduce the resistance of electrolyte penetration and diffusion,2,13 and on the other hand, the hydrous nature is important to promote the mass transfer of protons and accelerate the redox transitions of RuO2.4,26,33 It is thus not surprising that the tubular RuOx · nH2O displays such excellent capacitive behaviors in H2SO4 electrolyte. The importance of using a proton-rich electrolyte was further confirmed by the capacitive data of the nanotubular RuOx · nH2O in Na2SO4 electrolyte (Figure 8b). The specific capacitances of the RuOx · nH2O nanotubes at current densities of 4, 2, 1, and 0.5 A/g were estimated to be 273, 281, 293, and 313 F/g, respectively, which are much lower than these in H2SO4 electrolyte. The pseudocapacitance of RuOx · nH2O incorporating the proton exchange with redox transitions of RuO2 results in a large capacitance in acid electrolyte.31,32 In contrast, due to the scarcity of protons the capacitance in neutral Na2SO4 mainly originates from electrochemical double layer capacitance (the electro-adsorption of ions at the surface of RuOx · nH2O electrode), resulting in a lower capacitance value.35,36 It was reported that the anhydrous ruthenium oxides also showed low capacitances resulting from the prevention of proton transfer in the inside of anhydrous ruthenium oxides due to the anhydrous nature.31,33 Therefore, the proton-rich electrolyte and hydrous nature of RuOx · nH2O are important for the good capacitive performance. The nanotubular structure is to optimize electron and proton conducting pathways, leading to enhanced capacitive performances of nanotubular RuOx · nH2O.12,13,37 The specific capacitance and capacitance retention of the tubular RuOx · nH2O electrode in H2SO4 and in Na2SO4 solutions are presented in Figure 9. At the current density of 4 A/g, the RuOx · nH2O electrode exhibited about 76% of the initial capacitance obtained at the current density of 0.5 A/g in H2SO4 solution, showing an acceptable capacitance retention. The effect of current density on specific capacitance may be caused by the transport of effective ions into active materials. Higher concentration polarization at the large current density allows the charging process to reach completion in a short time. Protons in the vicinity of the electrode-electrolyte interface would be exhausted, thus retarding the redox transitions of electroactive species at a high current density.13 The RuOx · nH2O electrode in Na2SO4 electrolyte showed a better capacitance retention (about 87%), increasing in the current density from 0.5 A/g to 4 A/g. This insignificant dependence of specific capacitance upon current density is due to the fact that electrochemical double layer capacitance is the main contribution to the observed overall capacitance. Conclusions
Therefore, it is necessary to provide effective electron and proton transport pathways in the supercapacitor electrode. The
In summary, a simple method has been demonstrated to synthesize tubular RuOx · nH2O with superior electrochemical
Template Synthesis of Tubular Ruthenium Oxides performance as a supercapacitor electrode. According to the experimental observation, the formation process of the tubular RuOx · nH2O underwent a two-step mechanism: (1) the gradual deposition of RuOx · nH2O nanoparticles surrounding MnOOH nanorods and (2) the ripening of RuOx · nH2O nanoparticles along with the dissolution of MnOOH, leading to RuOx · nH2O nanotubes in the presence of HCOOH during the long-time aging. The unique tubular structure and hydrous nature of RuOx · nH2O contributed to the observed excellent electrochemical performance. The maximum specific capacitance of tubular ruthenium oxide electrode in the H2SO4 electrolyte was measured to be 861 F/g at a current density of 500 mA/g, much larger than that measured in the Na2SO4 electrolyte. The results confirmed the important contribution of the redox reactions of hydrous RuOx · nH2O with protons in charge/discharge processes. The synthesis method described here opens up a facile route to making nanotubular RuOx · nH2O materials for other applications, such as catalysts and chemical sensors. Acknowledgment. Financial support from the Ministry of Education Tier 2 Grant (MOE2008-T2-1-004) is appreciated. Supporting Information Available: SEM, TEM, EDS data, a detailed description of synthesis and capacitive performances of RuOx · nH2O nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhang, L. L.; Zhao, X. S. Chem. Soc. ReV. 2009, 38, 2520. (2) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. (3) Zheng, J. P.; Jow, T. R. J. Electrochem. Soc. 1995, 142, L6. (4) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699. (5) Hu, C. C.; Chen, W. C.; Chang, K. -H. J. Electrochem. Soc. 2004, 151, A281. (6) Chang, K.-H.; Hu, C.-C.; Chou, C.-Y. Electrochim. Acta 2009, 54, 978. (7) Lin, Y.; Zhao, N.; Nie, W.; Ji, X. J. Phys. Chem. C 2008, 112, 16219. (8) Mondal, S. K.; Munichandraiah, N. J. Power Sources 2008, 175, 657.
J. Phys. Chem. C, Vol. 114, No. 32, 2010 13613 (9) Hu, C.-C.; Liu, M.-J.; Chang, K.-H. J. Power Sources 2007, 163, 1126. (10) Lin, K.-M.; Chang, K.-H.; Hu, C.-C.; Li, Y.-Y. Electrochim. Acta 2009, 54, 4574. (11) Capucine, S.; Christel, L.; Hung Le, K.; Sophie, C.; Ce´dric, B.; Markus, A.; Cle´ment, S. AdV. Funct. Mater. 2009, 19, 1922. (12) Cho, S. I.; Lee, S. B. Acc. Chem. Res. 2008, 41, 699. (13) Hu, C.-C.; Chang, K.-H.; Lin, M.-C.; Wu, Y.-T. Nano Lett. 2006, 6, 2690. (14) Kim, S.-W.; Han, T. H.; Kim, J.; Gwon, H.; Moon, H.-S.; Kang, S.-W.; Kim, S. O.; Kang, K. ACS Nano 2009, 3, 1085. (15) Xiong, Y.; Mayers, B. T.; Xia, Y. Chem. Commun. 2005, 5013. (16) Zheng, J. P.; Jow, T. R. J. Power Sources 1996, 62, 155. (17) Hu, C.-C.; Wu, Y.-T.; Chang, K.-H. Chem. Mater. 2008, 20, 2890. (18) Moulder, J. K.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Eden Prairie, 1995. (19) Hu, C.-C.; Huang, Y.-H. J. Electrochem. Soc. 1999, 146, 2465. (20) Mun, C.; Ehrhardt, J. J.; Lambert, J.; Madic, C. Appl. Surf. Sci. 2007, 253, 7613. (21) Trasatti, S. Electrochim. Acta 1991, 36, 225. (22) Barranco, V.; Pico, F.; Ibano`ez, J.; Lillo-Rodenas, M. A.; LinaresSolano, A.; Kimura, M.; Oya, A.; Rojas, R. M.; Amarilla, J. M.; Rojo, J. M. Electrochim. Acta 2009, 54, 7452. (23) Zhan, Y.-J.; Yu, S.-H. J. Phys. Chem. C 2008, 112, 4024. (24) Fan, H. J.; Go¨ele, U.; Zacharias, M. Small 2007, 3, 1660. (25) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699. (26) Fu, R.; Ma, Z.; Zheng, J. P. J. Phys. Chem. B 2002, 106, 3592. (27) Ye, J.-S.; Cui, H. F.; Liu, X.; Lim, T. M.; Zhang, W. D.; Sheu, F.-S. Small 2005, 1, 560. (28) Subramanian, V.; Zhu, H.; Vajtai, R.; Ajayan, P. M.; Wei, B. J. Phys. Chem. B 2005, 109, 20207. (29) (a) Hu, Y.-S.; Guo, Y.-G.; Sigle, W.; Hore, S.; Balaya, P. B. Nat. Mater. 2006, 5, 713. (b) Oh, S. H.; Nazar, L. F. J. Mater. Chem. 2010, 20, 3834. (30) Rishpon, J.; Gottesfeld, S. J. Electrochem. Soc. 1984, 131, 1960. (31) Sugimoto, W.; Yokoshima, K.; Murakami, Y.; Takasu, Y. Electrochim. Acta 2006, 52, 1742. (32) Dmowski, W.; Egami, T.; Swider, K. E.; Love, C. T.; Rolison, D. R. J. Phys. Chem. B 2002, 106, 12677. (33) Chang, K.-H.; Hu, C.-C.; Chou, C.-Y. Chem. Mater. 2007, 19, 2112. (34) Trasatti, S.; Buzzanca, G. J. Electroanal. Chem. 1971, 29, A1. (35) Sugimoto, W.; Iwata, H.; Yokoshima, K.; Murakami, Y.; Takasu, Y. J. Phys. Chem. B 2005, 109, 7330. (36) Susanti, D.; Tsai, D.-S.; Huang, Y.-S.; Korotcov, A.; Chung, W.H. J. Phys. Chem. C 2007, 111, 9530. (37) Reddy, A. L. M.; Shaijumon, M. M.; Gowda, S. R.; Ajayan, P. M. Nano Lett. 2009, 9, 1002.
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