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High-Efficiency and Low-Cost Hybrid Nanomaterial as Enhancing Electrocatalyst: Spongelike Au/Pt Core/Shell Nanomaterial with Hollow Cavity Shaojun Guo, Youxing Fang, Shaojun Dong,* and Erkang Wang State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China ReceiVed: July 18, 2007; In Final Form: September 5, 2007
A high-efficiency and low-cost spongelike Au/Pt core/shell electrocatalyst with hollow cavity has been facilely obtained via a simple two-step wet chemical process. Hollow gold nanospheres were first synthesized via a modified galvanic replacement reaction between Co nanoparticles in situ produced and HAuCl4. The asprepared gold hollow spheres were employed as seeds to further grow spongelike Pt shell. It is found that the surface of this hybrid nanomaterial owns many Pt nanospikes, which form a spongelike nanostructure. All experimental data including scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-vis-near-infrared spectroscopy have been employed to characterize the obtained Au/Pt hybrid nanomaterial. The rapid development of fuel cell has inspired us to investigate the electrocatalytic properties for dioxygen and methanol of this novel hybrid nanomaterial. Spongelike hybrid nanomaterial mentioned here exhibits much higher catalytic activity for dioxygen reduction and methanol oxidation than the common Pt electrode.
Introduction Metal nanostructured materials possessing functional properties have been extensively studied because they have potential uses in diverse applications such as catalysis, nanodevice, optoelectronics, and biosensor. Especially nanomaterials with hollow interiors have emerged as intriguing materials for diverse applications that include drug delivery,1 bioencapsulation,2 catalysis,3 plasmonics,4 cell imaging,5 and composite electronic and structural materials6 due to their specific structures, interesting properties that differ from their solid counterparts. For example, hollow gold nanocages have tunable plasmon resonances and well-established surface chemistry, making them appropriate for use in photothermal cancer treatment.7 Hollow nanostructured materials are commonly prepared by coating the surfaces of colloidal particles (silica8 and polystyrene spheres9) with thin layers of desired materials, followed by the selective removal of the colloidal templates through calcination or wet chemical etching. However, the removal of the core will always bring a negative effect to the obtained nanoshell structure. The galvanic displacement reactions involving sacrificial metal nanoparticles and suitable metal ions provided a novel process for the synthesis of hollow nanostructured materials. For example, hollow Au,10 Ag,11 Pt,3b and Au/Pt12 alloy nanostructures have been prepared via employing Co nanoparticles in situ produced as sacrificial template. The advantage of this method is rapid, simple, and no need of complex posttreatment process. As is known, high-potential dioxygen electrocatalytic reduction and low-potential methanol electrocatalytic oxidation with high-current density are greatly pursued by the scientists in view of their important application in fuel cells. Thus, the electrochemical reduction of dioxygen or oxidation of methanol has * To whom correspondence should be addressed. E-mail: dongsj@ ciac.jl.cn.
been the essential subject for extensive studies. The development of nanotechnology provides new opportunity for searching or designing an effective nanomaterial that can accomplish the direct four-electron reduction of dioxygen to water or electrocatalytic oxidation of methanol with high efficiency. For numerous nanocatalysts, Pt and Pt-based nanomaterials are still indispensable and are the most effective catalyst for them.13 A great number of literatures have been reported to design unsupported or supported Pt catalyst.13 However, a critical problem with Pt-based catalysts is their prohibitive cost. Therefore, economical and effective alternative catalysts are required and cost-effective routes are being sought to make more-efficient Pt catalysts. Many efforts have focused on the development of a novel approach to produce hollow Pt catalysts with a high-surface area to achieve high-catalytic performance and utilization efficiency or replace it with less expensive materials.14 Recently, Jang et al.14c demonstrated a novel process to the preparation of hollow Pt nanosphere via excavating metal silver core of a Ag/Pt core/shell nanoparticle with picosecond laser pulses of 1064 nm. However, it must be noted that some facts, such as reducing the cost of preparation, how to utilize Pt with high efficiency, while in the meantime making the nanomaterial obtained own mutlifunctional properties, are greatly required for the social development in the future. Thus, it is necessary to design a cheaper and more effective electrocatalytic nanomaterial, which can not only complete the direct four-electron reduction of dioxygen but also be useful for the methanol electrocatalytic oxidation with high efficiency. In this paper, we first have explored a facile, efficient, and economical route to obtain a novel low-cost nanostructured spongelike Au/Pt core/shell hybrid electrocatalyst with hollow cavity. A two-step strategy has been employed to design this novel electrocatalyst. A detailed preparation process is shown in Scheme 1. First, hollow gold nanospheres were produced via the galvanic displacement reactions involving Co nanopar-
10.1021/jp075625z CCC: $37.00 © 2007 American Chemical Society Published on Web 10/13/2007
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SCHEME 1: Procedure to Design Hybrid Au/Pt Nanostructure with Hollow Cavity
ticles and HAuCl4. Then, the hollow gold nanospheres obtained were used as seeds to grow spongelike Pt shells with high surface-to-volume ratio and catalytic activity. These novel spongelike hollow nanospheres were initially exported as catalysts for electroreduction of dioxygen and electrooxidation of methanol. It is found that this hybrid nanomaterial exhibits much higher electrocatalytic activities for enhancing dioxygen reduction and methanol oxidation than the common Pt electrode. This research may open a new approach for designing other hybrid nanomaterials with hollow cavity. Furthermore, due to its excellent electrocatalytic properties, this novel nanomaterial will find a potential application in fuel cells. Experimental Section Materials. Sodium citrate, CoCl2‚6H2O, ascorbic acid (Vitamin C), methanol, HAuCl4‚4H2O, and H2PtCl6‚6H2O were purchased from Shanghai Chemical Factory (Shanghai, China) and used as received without further purification. NaBH4 was obtained from ACROS (New Jersey). Water used throughout all experiments was purified with Millipore system. Fabrication of Gold Hollow Nanosphere. For the synthesis of gold hollow nanospheres, the Co nanoparticles were first fabricated. The preparation process of Co nanoparticles was employed by the method reported by Wan et al.10b with a slight modification. Briefly, 0.2 mL of 0.4 M CoCl2 solution was added into 200 mL of deaerated aqueous solution containing 8 mM NaBH4 and 1 mM sodium citrate to fabricate the Co nanoparticles. To avoid the oxidation of the Co nanoparticles in the presence of oxygen, nitrogen was bubbled through the solution during the whole preparation. A black-brown solution was obtained and marked as solution I. In the meantime, 0.55 mL of a HAuCl4 solution (24.3 mM) was diluted to 30 mL (denoted as solution II). Solution II was then mixed with solution I (50 mL) with rapid agitation. The resulting solution was then stirred for 30 min and kept at the ambient condition for use. Fabrication of Hybrid Electrocatalyst with Hollow Cavity. The above gold hollow sphere solution (80 mL) was heating to boil. Then 2.5 mL of 0.1 M ascorbic acid was added to the above solution, followed by adding 1 wt % H2PtCl6 (2 mL). Herein, ascorbic acid acted as a reducting agent for reduction of H2PtCl6. After heating for 30 min, the novel Au/Pt hybrid electrocatalyst with hollow cavity was obtained. Electrocatalytic Experiment. The above product was collected by centrifugation at 10 000 rpm and dissolved in 20 mL of water by sonicating. The gold electrode (1.1 mm) was loaded with hybrid nanomaterial (2 µL). Electrocatalytic dioxygen reduction measurements were carried out in a solution of H2SO4 (0.5 M) at the scan rate of 50 mV/s; Methanol electrocatalytic oxidation measurements were carried out in a solution of H2SO4 (0.5 M) containing 1 M methanol at the scan rate of 50 mV/s.
Figure 1. Typical TEM image of gold hollow spheres.
Apparatus. A XL30 ESEM scanning electron microscope (SEM) equipped with an energy-dispersive X-ray analyzer was used to determine the morphology and composition of hybrid nanomaterial. Transmission electron microscopy (TEM) measurements were made on a JEOL 2000 transmission electron microscope operated at an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of prepared solution on carbon-coated copper grid and dried at room temperature. X-ray photoelectron spectroscopy (XPS) measurement was performed on an ESCALAB-MKII spectrometer (United Kingdom) with Al KR X-ray radiation as the X-ray source for excitation. UV-vis spectra were collected on a CARY 500 Scan UV-vis-near-infrared (UV-vis-NIR) spectrophotometer. Electrochemical experiments were performed with a CHI 832 electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China). A conventional three-electrode cell was used, including a Ag/AgCl (saturated KCl) electrode as reference electrode, a platinum wire as counter electrode, and a bare or modified gold electrode as working electrode. The working electrode was a gold disk with a diameter of 1.1 mm, polished with Al2O3 paste, and washed ultrasonically in Millipore water. Results and Discussion In the present work, the Co nanoparticles were synthesized by the reduction of Co2+ by NaBH4. Several literatures10b,11 have dealt with the synthesis of inorganic hollow nanospheres employing the galvanic displacement reactions. The galvanic replacement reaction driving force comes from the large reduction potential gap between the AuCl4-/Au and Co2+/Co redox couples. Because the reduction potential of the AuCl4-/ Au (0.935 V versus SHE) redox couple is much higher than that of Co2+/Co (-0.377 V versus SHE), the following replacement reaction can occur spontaneously as soon as a AuCl4- ion contacts with metallic Co:
3Co +2 AuCl4- ) 2Au + 3Co2+ +8 ClThus the gold hollow nanospheres were successfully fabricated by subsequent addition of Co colloidal solution into aqueous HAuCl4 solution. Figure 1 shows a typical TEM image of gold hollow spheres, where there is a strong contrast difference in all of the spheres with a bright center surrounded by a much darker edge, confirming their hollow structure. The inset of Figure 1
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Figure 2. Typical SEM images of the as-prepared spongelike Au/Pt hybrid nanospheres at different magnifications.
obviously shows hollow structure of gold spheres. The average of outer and inner diameters of these gold hollow spheres were measured to be about 30 and 15 nm, respectively. To construct Au/Pt hybrid electrocatalyst with hollow cavity, the above gold hollow sphere was employed as a seed to further grow spongelike Pt shell. Figure 2 shows typical SEM images of the as-prepared spongelike Au/Pt hybrid nanosphere at different magnifications. A great deal of nanospheres with the diameter of about 70 nm are observed, indicating Pt nanoshell grows on the surface of gold nanosphere. To show more detail structure of Au/Pt hybrid electrocatalyst, TEM was further employed to characterize the obtained hybrid nanosphere. Figure 3A shows the typical TEM images of hybrid electrocatalyst with the diameter of 60-70 nm. A strong contrast difference in all of the spheres with a dark center surrounded by a much lighter edge is observed, confirming core/shell nanostructure. Because of the different contrast of Au, Pt, and cavity, it is hard for us to distinguish the hollow structure. To prove hollow structure of hybrid electrocatalyst, Figure 3B shows the typical TEM image of hybrid electrocatalyst obtained from another contrast, which obviously reveals hollow structure. Observing the hollow hybrid nanostructure carefully here, it is found that Pt shell on the surface of hollow gold sphere owns spongelike nanostructure, which is expected to have good electrocatalytic activity. The chemical composition of hybrid nanostructure was determined by energy-dispersive X-ray spectroscopy (EDX) (Figure 4). The EDX spectrum with two main peaks (Au and Pt) was observed (other peaks originated from ITO glass substrate), indicating that the hollow hybrid nanostructure was made up of metallic gold and platinum. It is known that X-ray photoelectron spectroscopy (XPS) is valuable for detecting the surface composition of samples, and depending on the experimental setup and elements presented this technique has a detectable depth of 2-10 nm. Thus, XPS was further employed
Figure 3. Typical TEM images of the as-prepared spongelike Au/Pt hybrid nanospheres at different magnifications.
to investigate the composition of hybrid nanosphere. XPS pattern of these hybrid nanomaterials shows significant Pt 4f signals corresponding to the binding energy of metallic Pt (Figure 5B). The peaks located at 71.2 and 74.5 eV are ascribed to Pt 4f5/2 and Pt 4f7/2, respectively. Also, note that the introduction of a strong reducing reagent such as NaBH4 into as-formed hybrid nanomaterials cannot result in further change of the solution color, suggesting that the Pt salts have been totally reduced by ascorbic acid during the thermal process. In addition, it is hard to observe Au4f signal characteristic of metallic Au (Figure 5A), which indicates that gold hollow spheres inside the hybrid nanomaterials are hard to be detected in this case because the thickness of the spongelike Pt shell exceeds the detectable depth of X-ray. Thus, we can further affirm that spongelike Au/Pt hybrid nanomaterials can be obtained via the above method. It is known that the surface plasmon resonance (SPR) property of gold nanostructure is discernible in the UV-vis region. Thus, the growth of spongelike Pt shell on the gold hollow spheres could be readily monitored by UV-vis spectroscopy. Figure 6 shows the UV-vis spectra of the as-prepared gold hollow spheres and hybrid nanospheres. Before Pt growth, the solution displays characteristic SPR peak at 555 nm (Figure 6a). Upon the Pt overgrowth, the SPR of hybrid nanostructure did not show any adsorption peak in the region employed. This may be attributed to the following facts. First, spongelike Pt shell does not have any characteristic absorption in the vis-NIR spectroscopy, which is of importance to change the optical absorption of Au/Pt hybrid nanostructure. Second, the change in the dielectric that surrounds the gold may be an important factor as well as the large scattering by the spongelike Pt shell.15
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Figure 4. EDX image of as-prepared Au/Pt hybrid hollow nanospheres.
Figure 5. XPS spectra of as-prepared Au/Pt hybrid hollow nanospheres.
Figure 6. UV-vis spectra of as-prepared hollow gold nanospheres (a) and spongelike Au/Pt hybrid nanospheres (b).
The large overpotential associated with the oxygen reduction reaction (ORR) is one of the major challenges that call for the development of a high-performance cathode catalyst. The ORR is a reaction of indispensable importance in metal air batteries, fuel cells as well as in oxygen sensor.16,17 In the above part, we have obtained and characterized the hollow Au/Pt hybrid nanospheres. The spongelike hybrid nanostructure is observed, which is expected to have good and high-efficiency electrocatalytic activity. Therefore, the spongelike hybrid nanostructure mentioned here for application in fuel cell is an extremely promising prospect. Of central interest here are the electrocatalytic applications of these hybrid nanostructures. The electrocatalytic activity of the hybrid nanostructure has first been investigated for dioxygen reduction. Figure 7a,b shows the typical cyclic voltammograms (CVs) of dioxygen reduction at
the Au/Pt hybrid nanostructure modified gold electrode in a 0.5 M H2SO4 solution in the presence of air and saturated dioxygen. In the presence of air, a remarkable catalytic reduction current occurs at 0.56 V (Figure 7a) at a scan rate of 50 mV/s. Higher catalytic current for dioxygen reduction is observed at 0.48 V in the presence of saturated dioxygen (Figure 7b), while no catalytic reduction current can be observed at the bare ITO(Figure 7d) and hollow gold nanospheres- (Figure 7c) modified electrode in the potential range employed. It is noted that the hybrid hollow nanostructure-modified gold electrode exhibits more positive potential for dioxygen reduction than that of the common Pt electrode (see inset). The above results indicate that these hybrid nanostructures have excellent electrocatalytic activity for dioxygen reduction. It is expected that this hybrid nanomaterial can be used for new functional building block to assemble new three-dimensional complex multicomponent nanostructures, which are believed to be useful for the electrochemical nanodevices and fuel cell, etc. In recent years, direct methanol fuel cells (DMFCs) have been intensely studied because of their numerous advantages, which include high-energy density, the ease of handling a liquid, lowoperating temperatures, and their possible applications to microfuel cells.18 The performance of fuel cells such as DMFCs are known to be strongly dependent on electrocatalytic materials used. Accordingly, for the best DMFC performance, it is essential to develop a good electrocatalytic material for exploring the performance of methanol electrocatalytic oxidation, which is at the heart of DMFC application in the anodic half-cell reaction. The electrocatalytic activity of the spongelike hybrid nanospheres for the oxidation of methanol was initially demonstrated and compared with that of common Pt electrode in a
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Guo et al. oxidation peaks of methanol were observed, indicating that electrocatalytic current observed in Figure 8d can be ascribed to the high-electrocatalytic activity of spongelike hybrid nanosphere with high surface-to-volume ratio. Considering the particular structure of spongelike hybrid nanomaterial, two important factors should be responsible for the above high-electrocatalytic activities. First, the Pt shell on the surface of gold nanoparticles is in the form of irregular one-dimensional Pt nanospikes (or nanorods). These small nanospikes (Figure 3) have much higher surface-to-volume ratios and surface activities. Second, these hybrid nanospheres own spongelike structure (porosity), which will also increase the efficient surface-to-volume ratios and utilization efficiency of Pt.
Figure 7. CVs of O2 reduction at spongelike Au/Pt hybrid nanostructure (a,b); hollow gold nanospheres (c) modified gold electrode; bare gold electrode (d) in air-saturated (lines a,c,d) and in O2-saturated (line b) 0.5 M H2SO4 solution. Scan rate: 50 mV/s. (The inset shows CVs of O2 reduction at the Au/Pt hybrid nanostructure (a) modified gold electrode and common Pt electrode (b) in O2-saturated 0.5 M H2SO4 solution).
Conclusions In summary, a novel approach has been successfully developed to design Au/Pt hybrid electrocatalyst, which owns spongelike and hollow structure. This particular architecture (spongelike and hollow) is quite suitable for catalytic purposes in the view of high-efficiency and low-cost. The applications as anodic and cathodic electrocatalysts of this novel hybrid nanomaterial were initially investigated. It is found that the hybrid nanomaterial exhibits higher catalytic activity for dioxygen reduction and methanol oxidation than the common Pt electrode. The study is of significance in shape-controlled synthesis of hybrid nanomaterials and is of importance in electrocatalysis, sensors, and fuel cells. Acknowledgment. This work was supported by the National Science Foundation of China. (No. 20575064, No. 20427003) References and Notes
Figure 8. CVs of methanol oxidation at spongelike Au/Pt hybrid nanostructure (d); hollow gold nanospheres (a) modified gold electrode; bare gold electrode (b); common Pt electrode (c) in 0.5 M H2SO4 solution containing 1 M methanol. Scan rate: 50 mV/s.
solution of H2SO4. A gold plate electrode with a diameter of 1.1 mm was used. From the voltammograms in Figure 8d, the spongelike Pt hybrid hollow nanospheres show catalytic behavior for the electrooxidation of methanol by the appearance of an oxidation current in the positive potential region. The onset potentials are around 0.3 V (versus Ag/AgCl). The current peak at about 0.67 V (versus Ag/AgCl) in the forward scan is attributed to methanol electrooxidation on the hybrid electrocatalyst.3b,19 This peak is significantly higher than that obtained from the common Pt electrode (Figure 8c). This significant improvement in the catalytic performance can be attributed to particular morphology of spongelike hybrid hollow nanospheres designed here. In the reverse scan, an oxidation peak is observed around 0.50 V, which is probably associated with the removal of the residual carbon species formed in the forward scan.20 As is known, the ratio of the forward oxidation current peak (If) to the reverse current peak (Ib), If/Ib, is an index of the catalyst tolerance to the poisoning species, PtdCdO.13e A higher ratio indicates more effective removal of the poisoning species on the catalyst surface. The If/Ib ratio of spongelike hybrid hollow nanosphere is 2.57, which is higher than that of the E-TEK catalyst (0.74),13e showing better catalyst tolerance of hybrid nanosphere. In comparison, methanol electrocatalytic oxidation was also investigated on gold hollow sphere-modified gold electrode (Figure 8a) and bare gold electrode (Figure 8b). No
(1) Mathiowitz, E.; Jacob, J. S.; Jon, Y. S.; Carino, G. P.; Chickering, D. E.; Chaturvedi, P.; Santos, C. A.; Vijayaraghavan, K.; Montgomery, S.; Bassett, M.; Morrell, C. Nature 1997, 386, 410. (2) (a) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C.; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (b) Marinakos, S. M.; Anderson, M. F.; Ryan, J. A.; Martin, L. D.; Feldheim, D. L. J. Phys. Chem. B 2001, 105, 8872. (3) (a) Kim, S.-W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (b) Liang, H. P.; Zhang, H. M.; Hu, J. S.; Guo, Y. G.; Wan, L. J.; Bai, C. L. Angew. Chem., Int. Ed. 2004, 43, 1540. (4) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (5) Chen, J.; Saeki, F.; Wiley, B.; Cang, H.; Cobb, M. J.; Li, Z.; Au, L.; Zhang, H.; Kimmey, M. J.; Li, X.; Xia, Y. Nano Lett. 2005, 5, 473. (6) Cochran, J. K. Curr. Opin. Solid State Mater. Sci. 1998, 3, 474. (7) Chen, J.; Wang, D.; Xi, J.; Au, L.; Siekkinen, A.; Warsen, A.; Li, Z.-Y.; Zhang, H.; Xia, Y.; Li, X. Nano Lett. 2007, 7, 1318. (8) Lu, L.; Zhang, H.; Sun, G.; Xi, S.; Wang, H.; Li, X.; Wang, X.; Zhao, B. Langmuir 2003, 19, 9490. (9) (a) Pol, V. G.; Grisaru, H.; Gedanken, A. Langmuir 2005, 21, 3635. (b) Zhang, J.; Liu, J.; Wang, S.; Zhan, P.; Wang, Z.; Ming, N. AdV. Funct. Mater. 2004, 14, 1089. (10) (a) Sun, Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 3892. (b) Liang, H. P.; Wan, L. J.; Bai, C. L.; Jiang, L. J. Phys. Chem. B 2005, 109, 7795. (11) Chen, M. H.; Gao, L. Inorg. Chem. 2006, 45, 5145. (12) Liang, H.; Guo, Y.; Zhang, H.; Hu, J.; Wan, L.; Bai, C. Chem. Commun. 2004, 1496. (13) (a) Brussel, M. V.; Kokkinidis, G.; Vandendael, I.; Buess-Herman, C. Electrochem. Commun. 2002, 4, 808. (b) Coutanceau, C.; Croissant, M. J.; Napporn, T.; Lamy, C. Electrochim. Acta 2000, 46, 579. (c) Gutierrez, M. C.; Hortiguela, M. J.; Amarilla, J. M.; Jimenez, R.; Ferrer, M. L.; del Monte, F. J. Phys. Chem. C 2007, 111, 5557. (d) Kongkanand, A.; Vinodgopal, K.; Kuwabata, S.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 16185. (e) Mu, Y.; Liang, H.; Hu, J.; Jiang, L.; Wan, L. J. Phys. Chem. B 2005, 109, 22212. (14) (a) Markovic, N. M.; Schmidt, T. J.; Stamenkovic, V.; Ross, P. N. Fuel Cells 2001, 1, 105. (b) Shao, M.-H.; Sasaki, K.; Adzic, R. R. J. Am. Chem. Soc. 2006, 128, 3526. (c) Kim, S. J.; Ah, C. S.; Jang, D. J. AdV. Mater. 2007, 19, 1064. (d) Chen, G.; Xia, D.; Nie, Z.; Wang, Z.; Ang, W. L.; Zhang, L.; Zhang, J. Chem. Mater. 2007, 19, 1840.
Hybrid Nanomaterial as Enhancing Electrocatalyst (15) Song, J. H.; Kim, F.; Kim, D.; Yang, P. D. Chem.sEur. J. 2005, 11, 910. (16) (a) Wakabayashi, N.; Takeichi, M.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 5836. (b) Yang, C. C. Int. J. Hydrogen Energy 2004, 29 135. (c) Fernandez, J. L.; Walsh, D. A.; Bard, A. J. J. Am. Chem. Soc. 2005, 127, 357. (d) Yang, J. S.; Xu, J. J. Electrochem. Commun. 2003, 5, 306. (17) (a) Zhang, D.; Chi, D. H.; Okajima, T.; Ohsaka, T. Electrochim. Acta 2007, 52, 5400. (b) El-Deab, M. S.; Ohsaka, T. Electrochim. Acta 2007, 52, 2166. (c) El-Deab, M. S.; Sotomura, T.; Ohsaka, T. Electrochim. Acta 2006, 52, 1792.
J. Phys. Chem. C, Vol. 111, No. 45, 2007 17109 (18) (a) Reddington, E.; Sapienza, A.; Gurau, B.; Viswanathan, R.; Sarangapani, S.; Smotkin, E. S.; Mallouk, T. E. Science 1998, 280, 1735. (b) Hyeon, T.; Han, S.; Sung, Y. E.; Park, K. W.; Kim, Y. W. Angew. Chem., Int. Ed. 2003, 42, 4352. (19) (a) Chen, W. X.; Lee, J. Y.; Liu, Z. L. Chem. Commun. 2002, 2588. (b) Yu, J. S.; Kanf, S.; Yoon, S. B.; Chai, G. J. Am. Chem. Soc. 2002, 124, 9382. (20) (a) Yajima, T.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 2654. (b) Zhu, Y. M.; Uchida, H.; Yajima, T.; Watanabe, M. Langmuir 2001, 17, 146.
