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Langmuir 2008, 24, 5932-5936
Platinum-Coated Gold Nanoporous Film Surface: Electrodeposition and Enhanced Electrocatalytic Activity for Methanol Oxidation Jianbo Jia,*,† Linyuan Cao,‡ and Zhenhui Wang‡ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and College of Chemistry and EnVironmental Science, Henan Normal UniVersity, Xinxiang 453007, China ReceiVed January 17, 2008. ReVised Manuscript ReceiVed March 7, 2008 This report describes the preparation of Pt-nanoparticle-coated gold-nanoporous film (PGNF) on a gold substrate via a simple “green” approach. The gold electrode that has been anodized under a high potential of 5 V is reduced by freshly prepared ascorbic acid (AA) solution to obtain gold nanoporous film electrode. Then the Pt nanoparticle is grown on the electrode by cyclic voltammetry (CV). The resulting PGNF electrode has highly ordered arrangement and large surface area, as verified by scanning electron microscopy (SEM) and CV, suggesting that the nanoporous gold film electrode provides a good matrix for obtaining PGNF with high surface area. Furthermore, the as-prepared PGNF electrode exhibited high electrocatalytic activity toward methanol oxidation in a 0.5 M H2SO4 solution containing 1.5 M methanol. The present novel strategy is expected to reduce the cost of the Pt catalyst remarkably.
Introduction The study of direct methanol fuel cells (DMFCs) has received considerable attention over the past several decades because of their ease of handling, high-energy densities, and low operation temperatures as promising sources.1,2 Generally, the basic operations of DMFCs include methanol oxidation and oxygen reduction on precious metal catalysts, which is usually dispersed on a carbon support.3 The key problem for DMFCs with high performance is the preparation of catalysts with high electrocatalytic activity.4,5 However, the common carbon-supported Pt or Pd nanoparticle catalyst suffers from some disadvantages. For instance, poor loading, weak dispersion, and easy exfoliation of metal nanocatalyst on supporting materials usually block the commercial application of DMFCs.6,7 Therefore, many efforts have been devoted to improve the catalyst dispersion and carbonsupport design.1,8 In recent years, carbon nanotubes (CNTs) as catalyst-support material have drawn great interest because of their large accessible surface areas, high stability, and high electron conductivity. Several reports have demonstrated that metal nanoparticles or bimetallic nanoparticles are facilely adsorbed on the surface of CNTs to construct CNT-based hybrid nanomaterial as electrocatalysts.9–11 Nevertheless, the requirement of high purity and functionalized surface for the employed CNTs12 and, especially, rigorous experimental conditions make the preparation procedure complicated and time-consuming. In * Corresponding author. Tel:+86-431-85262378; fax: +86-431-85685653; e-mail address:
[email protected]. † Chinese Academy of Sciences. ‡ Henan Normal University. (1) Zhou, Y.; Fan, L. Z.; Zhong, H. Z.; Li, Y. F.; Yang, S. H. AdV. Funct. Mater. 2007, 17, 1537–1541. (2) Zheng, S. F.; Hu, J. S.; Zhong, L. S.; Wan, L. J.; Song, W. G. J. Phys. Chem. C 2007, 111, 11174–11179. (3) Gutierrez, M. C.; Hortiguela, M. J.; Amarilla, J. M.; Jimenez, R.; Ferrer, M. L.; del Monte, F. J. Phys. Chem. C 2007, 111, 5557–5560. (4) Li, L.; Xing, Y. C. J. Phys. Chem. C 2007, 111, 2803–2808. (5) Tsai, M. C.; Yeh, T. K.; Tsai, C. H. Electrochem. Commun. 2006, 8, 1445–1452. (6) Wei, Z. D.; Li, L. L.; Luo, Y. H.; Yan, C.; Xun, C. X.; Yin, G. Z.; Shen, P. K. J. Phys. Chem. B 2006, 110, 26055–26061. (7) Steigerwalt, E. S.; Deluga, G. A.; Cliffel, D. E.; Lukehart, C. M. J. Phys. Chem. B 2001, 105, 8097–8101. (8) Lin, Y. H.; Cui, X. L.; Yen, C.; Wai, C. M. J. Phys. Chem. B 2005, 109, 14410–14416.
addition, the poisoning of Pt electrocatalysts by carbon monoxide (CO) on carbon support materials is serious.4,13 The poison species (CO, HCOOH, etc.) produced at the electrochemical scanning process could be strongly adsorbed on the Pt surface to block the active sites, which usually leads to a decrease in the activity of Pt catalysts.4 Therefore, it is necessary to develop a simple, green, and rapid method for preparing unsupported Pt nanocatalyst with reduced cost and poisoning of Pt as well as increased Pt activity. The preparation of unsupported bimetallic nanoparticles has received considerable interests in recent years.14–18 It is not surprise that most of the reports regarding bimetallic electrocatalysts are related to the combination of Pt with other metals, such as Ru and Pd. However, there are very few reports about adding Pt into Au catalysts,19–21 especially for stable Au/Pt nanoporous film. In this paper, we report a three-step strategy for obtaining a novel Pt-nanoparticle-coated gold-nanoporous film (PGNF). The Au nanoporous film possesses several obvious (9) Yen, C. H.; Shimizu, K.; Lin, Y. Y.; Bailey, F.; Cheng, I. F.; Wai, C. M. Energy Fuel 2007, 21, 2268–2271. (10) Lin, Y. H.; Cui, X. L. Langmuir 2005, 21, 11474–11479. (11) Ebbesen, T. W.; Hiura, H.; Bisher, M. E.; Treacy, M. M. J.; ShreeveKeyer, J. L.; Haushalter, R. C. AdV. Mater. 1996, 8, 155–160. (12) Hu, S. G.; Chen, Z. L.; Shen, A. G.; Shen, X. C.; Li, J.; Hu, S. S. Carbon 2006, 44, 428–434. (13) Waszczuk, P.; Wieckowski, A.; Zelenay, P.; Gottesfeld, S.; Coutanceau, C.; Leger, J. M.; Lamy, C. J. Electroanal. Chem. 2001, 511, 55–64. (14) Lou, Y. B.; Maye, M. M.; Han, L.; Luo, J.; Zhong, C. J. Chem. Commun. 2001, 473, 474. (15) Du, B. C.; Tong, Y. Y. J. Phys. Chem. B 2005, 109, 17775–17780. (16) Yajima, T.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 2654– 2659. (17) Xia, D. G.; Chen, G.; Wang, Z. Y.; Zhang, J. J.; Hui, S. Q.; Ghosh, D.; Wang, H. J. Chem. Mater. 2006, 18, 5746–5749. (18) (a) Habrioux, A.; Sibert, E.; Servat, K.; Vogel, W.; Kokoh, K. B.; AlonsoVante, N. J. Phys. Chem. B 2007, 111, 10329–10333. (b) Chan, K. Y.; Ding, J.; Ren, J. W.; Cheng, S. A.; Tsang, K. Y. J. Mater. Chem. 2004, 14, 505–511. (c) Chen, W. X.; Lee, J. Y.; Liu, Z. L. Chem. Commun. 2002, 2588–2592. (d) Higuchi, E.; Uchida, H.; Watanabe, M. J. Electroanal. Chem. 2005, 583, 69–76. (19) Liang, H. F.; Maldonado, S.; Stevenson, K. J.; Chandler, B. D. J. Am. Chem. Soc. 2004, 128, 12949–12956. (20) (a) Njoki, P. N.; Luo, J.; Wang, L. Y.; Maye, M. M.; Quaizar, H.; Zhong, C. J. Langmuir 2005, 21, 1623–1628. (b) Luo, J.; Maye, M. M.; Petkov, V.; Kariuki, N. N.; Wang, L. Y.; Njoki, P.; Mott, D.; Lin, Y.; Zhong, C. J. Chem. Mater. 2005, 17, 3086–3091. (21) (a) Guo, S. J.; Dong, S. J.; Wang, E. K. J. Phys. Chem. C 2008, 112, 2389–2393. (b) Guo, S. J.; Fang, Y. X.; Dong, S. J.; Wang, E. K. J. Phys. Chem. C 2007, 111, 17104–17109.
10.1021/la800163f CCC: $40.75 2008 American Chemical Society Published on Web 04/25/2008
Platinum-Coated Gold Nanoporous Films Scheme 1. The Fabrication Process of Pt-Nanoparticle-Coated Au Nanoporous Film Electrode
advantages, such as functioning as a template for the formation of porous structure, enabling the deposition of metal catalyst overlayer, and fabricating bimetallic catalysts to improve its catalytic activities.22 Most importantly, the PGNF catalyst could reduce the adsorption of CO effectively, especially at a relatively negative potential.14,15 Furthermore, the electrodeposition of Pt nanoparticles at the gold nanoporous film simplifies the experimental procedures by avoiding the tedious purification of CNTs and/or synthesis of bimetallic nanoparticles. This work replaces the common chemical synthesis approach with an electrochemical one for constructing novel PGNF, which is benefited from the eminent advantages of electrodeposition and electrooxidation, including simplicity, rapidness, and good reproducibility.23,24 In our approach, the three-step strategy to prepare PGNF was employed as follows: (1) electrochemical oxidation of Au electrode, (2) reduction of Au oxide to Au nanoporous film electrode, and (3) electrochemical transformation of the Pt(IV) complex to PGNF. We choose ascorbic acid (AA) as the reductant in the reduction of Au oxide for two reasons. First, AA is a “green” chemical reagent compared with toxic NaBH4,17 thioglycolic acid,25 and so on. The utilization of AA is environmentally benign. Second, the reaction rate for AA is faster than that of glucose, which greatly saves the reaction time. As a result, the PGNF has been prepared with a 3D morphology on the surface of Au electrode, which exhibits distinctive electrocatalytic activity for methanol oxidation.
Experimental Section Materials. Potassium chloroplatinite (K2PtCl4) and AA were purchased from Beijing Chemical, Inc. (Beijing, China) and used as received. All other chemicals were analytical grade or better. All solutions were prepared with pure water. The Preparation of Electrodes. Scheme 1 depicts the fabrication procedure of the PGNF electrode. A bulk Au electrode (diameter, 1.2 mm) is polished carefully to a mirror with emery paper and R-Al2O3 slurry (1.0 µm, 0.3 µm), and ultrasonically cleaned in water and absolute ethanol for a few minutes, respectively. The Au nanoporous film is prepared according to the procedure previously described with a little modification.26,27 The cleaned gold electrode is first anodized under a high potential of 5 V for 180 s in 0.15 M phosphate buffer solution (PBS) (pH 7.4). The color of the electrode surface turned salmon pink. The anodized gold electrode is then transferred into a freshly prepared AA aqueous solution (1.0 M) at room temperature. During the process, the color of electrode surface turns black immediately, indicating the gold nanofilm is formed on (22) Yoo, S. H.; Park, S. AdV. Mater. 2007, 19, 1612–1615. (23) Murray, B. J.; Li, Q.; Newberg, T.; Menke, E. J.; Hemminger, J. C. Nano Lett. 2005, 5, 2319–2325. (24) Dadisic, A.; Vereecken, P. M.; Hannon, J. B.; Searson, P. C.; Ross, F. M. Nano Lett. 2006, 6, 238–242. (25) Jiang, H.; Ju, H. X. Chem. Commun. 2007, 404–406. (26) Zhao, W.; Xu, J. J.; Shi, C. G.; Chen, H. Y. Electrochem. Commun. 2006, 8, 773–778. (27) Zhao, W.; Ge, P. Y.; Xu, J. J.; Chen, H. Y. Langmuir 2007, 23, 8597– 8601.
Langmuir, Vol. 24, No. 11, 2008 5933 the surface of the gold electrode. In order to make sure the oxidized Au is reduced to Au(0) completely, the electrode is kept in the AA solution for about 30 s. PGNF is obtained by electrochemical deposition according to a one-step process reported by Wang et al.28,29 Cyclic voltammetry (CV) is carried out in electrolyte solution (0.02 M K2PtCl4 + 0.50 M H2SO4) from -0.2 to 0.5 V at a scan rate of 10 mV/s. The structure and loading of the Pt nanoparticles are adjusted by changing the circles of the potential. Apparatus. Scanning electron microscope (SEM) measurements are conducted with a JEOL 6700 SEM (JEOL Ltd., Japan) operated at an accelerating voltage of 15 kV. The samples for SEM characterization are prepared with Au plate. X-ray photoelectron spectroscopy (XPS) measurement is performed on an ESCALABMKII spectrometer (VG Co., U.K.) with an Al KR X-ray radiation as the X-ray source for excitation and a chamber pressure of 3.5 × 10-7 Pa. The crystallographic information of porous Au nanoporous film and Au/Pt composite nanoporous film is carried out with X-ray diffraction (XRD, Philips X-ray diffractometer, model PW1700 BASED, Cu KR radiation λ ) 1.5406 Å). Electrochemical measurements are performed with a CHI 830B electrochemical analyzer (CH Instruments, Inc., Shanghai). A conventional threeelectrode cell is used, including a pretreated gold electrode as the working electrode, an Ag/AgCl (saturated KCl) electrode as the reference electrode, and platinum foil as the counter electrode. The potentials are measured with an Ag/AgCl electrode as the reference electrode. All the experiments are carried out at ambient temperature.
Results and Discussion Characterization of Pt-Nanoparticles-Coated Gold Nanoporous Film Electrode. SEM was used to characterize the Au nanoporous film and PGNF electrodes. Figure 1 displays the typical SEM images of the Au nanostructure before (A) and after (B) the coating of Pt. As shown in Figure 1A, the Au substrate presents an irregular porous structure consisting of Au nanoparticles. On the other hand, as shown in Figure 1B, the Au nanoporous film is uniformly covered by Pt nanoparticles, suggesting that the present electrochemical method is effective for the deposition of Pt nanoparticles on the surface of Au nanoporous film. XPS is a valuable technique for detecting the surface composition of samples. Figure 2 shows the XPS spectra of the Au nanofilm before (Aa) and after the electrodeposition of Pt (Ab and B). Two peaks in Figure 2Aa at 83.5 and 87.1 eV can be assigned to Au 4f, which indicates that the oxidized Au has been deoxidized by AA completely.30 On the contrary, the XPS spectra of the Pt-electrodeposited material exhibit significant Pt 4f signals corresponding to the binding energy of metallic Pt (Figure 2B). The peaks located at 71.0 and 74.4 eV are ascribed to Pt 4f7/2 and Pt 4f5/2, respectively.9 In addition, the characteristic of metallic Au 4f signals are not observed (Figure 2Ab), which indicates that the thickness of the covered Pt exceeded the detectable depth of the X-ray. Thus, it can be concluded that a compact Pt nanoparticle film has been electrodeposited at the Au nanoporous film surface. XRD was also carried out to study the crystalline nature of Au nanoporous film and PGNF. The peaks between 35° and 90° are indexed to face-centered cubic Au crystals. Figure 3A depicts the XRD spectrum of Au nanoporous film. The peaks at 2θ ) 37.5°, 43.7°, 64.1°, 77.1°, and 81.3° are assigned to the (111), (200), (220), (311), and (222) planes of Au, respectively.31,32 Figure 3B exhibits the XRD pattern of PGNF. Except for the (28) Wang, Z. H.; Qiu, K. Y. Electrochem. Commun. 2006, 8, 1075–1081. (29) Wang, Z. H.; Zhang, L. L.; Qiu, K. Y. J. Power Sources 2006, 161, 133–137. (30) Liu, Y. C. J. Phys. Chem. B 2004, 108, 2948–2952.
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Figure 1. SEM images of Au nanoporous film (A) and PGNF (B). (A) Au nanoporous film was prepared by the electrooxidation of a gold plate under 5 V for 180 s in 0.15 M PBS buffer. (B) PGNF was prepared by electrodeposition from the electrolyte solution containing 0.02 M K2PtCl4 and 0.50 M H2SO4 with five cycles of potential scan. Scan rate, 10 mV/s.
Figure 2. XPS spectra of Au nanoporous film (curve a) and Pt nanoparticle coated Au nanoporous film (curves b and c). (A) Au4f peak, (B) Pt4f peak.
peaks assigned to Au, other peaks are observed in the XRD patterns. The characteristic of Pt nanoporous film (Figure 3B) is evident as indicated by the orientations along the Pt (111), Pt (200) and Pt (220) directions.33 According to Scherrer’s equation,34 the sizes of Pt nanoparticles are about 17.3 nm (111), 11.6 nm (200), and 20.8 nm (220), which are consistent with the results obtained by the SEM images. Therefore, it can be concluded that Pt nanoparticles have been deposited on the surface of Au nanoporous film successfully. Additionally, the electrodeposited Pt nanoparticles do not change the crystalline nature
Figure 3. XRD patterns of Au nanoporous film (A) and PGNF (B).
of Au nanoporous film, which is well consistent with the SEM data. CVs of the PGNF electrode in a 0.50 M H2SO4 solution were performed at a scan rate of 50 mV/s. As shown in Figure 4a, the PGNF electrode produces typical peaks similar to that of polycrystalline Pt electrode, indicating that the Pt nanoparticles cover the whole surface of the Au nanoporous electrode. Two (31) Wang, C. H.; Yang, C.; Song, Y. Y.; Gao, W.; Xia, X. H. AdV. Funct. Mater. 2005, 15, 1267–1275. (32) Li, Y.; Song, Y. Y.; Yang, C.; Xia, X. H. Electrochem. Commun. 2007, 9, 981–988. (33) Hayashi, Y.; Matsushita, Y.; Soga, T.; Umeno, M.; Jimbo, T. Diamond Relat. Mater. 2002, 11, 499–503.
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Figure 4. CVs of a PGNF electrode (a), a bare gold electrode (b) and a gold nanoporous film electrode (c) in 0.50 M H2SO4 at a scan rate of 50 mV/s.
typical redox peaks at about 0-0.1 V are ascribed to the hydrogen adsorption/desorption events on the PGNF electrode. The peak corresponding to the formation of platinum oxide appears at 0.85 V, and that corresponding to the reduction appears at 0.48 V. The CVs of polycrystalline gold electrode (b) and porous Au nanofilm electrode (c) under the same conditions are also displayed in Figure 4. The peak currents of the gold nanoporous film electrode are much higher than those of polycrystalline gold electrode. The electrochemically active surface area (ECSA) of the PGNF electrode was measured by using the integral of the charge under the Pt-H redox peaks. The hydrogen desorption peaks formed in the lower potential region are used to calculate the ECSA.35–37 It was found that the PGNF electrode has a 29fold higher ECSA (0.364 cm-2) compared to that of the bulk Pt electrode (0.0125 cm-2) with the same geometrical area of 0.0113 cm-2 under identical conditions. Higher ECSA could be ascribed to the formation of abundant active adatoms (Au*) during the Au nanoporous film fabrication process,26,38 which could produce more active sites on the electrode surface26 and thus lead to higher active surface for the deposition of Pt. Electrocatalytic Oxidation of Methanol. The electrocatalytic behavior of the PGNF electrode toward the methanol oxidation was studied by CV in detail. The typical CVs are shown in Figure 5. In the absence of methanol, the peaks for the formation of platinum oxide and the corresponding reduction process are observed at 0.85 and 0.57 V, respectively. Furthermore, there is one pair of redox peaks at about 0.05 V, which arises from the adsorption/desorption of hydrogen in acidic solution. The results are consistent with previous reports.28,39,40 In the presence of 1.50 M methanol, the oxidation peak at 0.75 V is attributed to the oxidative removal of adsorbed/dehydrogenated methanol fragments by the oxygen-containing species PtOH. During the process, CO, CO2, HCOOH, HCOH, and HCOOCH3 are formed, and the CO molecules might be readsorbed and poison the electrode surface.2 The peak at 0.58 V is attributed to the reoxidation of CO and other adsorbed species. The great increase of the oxidized peak at 0.75 V and the deoxidized peak at 0.58 V indicates that the PGNF electrode has good electrocatalytic (34) Huang, H. Z. The Analysis of Nanomaterial; Chemical Industry Press: Beijing, 2003, p 244. (35) Parsons, R.; Vander Noot, T. J. Electroanal. Chem. 1988, 257, 9–45. (36) Masahiro, W.; Mistuhiro, T.; Satoshi, M. J. Electroanal. Chem. 1985, 195, 81–93. (37) Masahiro, W.; Kazuyuki, K.; Hiroyuki, U.; Satoshi, M. J. Electroanal. Chem. 1986, 197, 195–208. (38) Adzic, R. R.; Hsiao, P. R.; Yeager, E. B. J. Electroanal. Chem. 1989, 260, 475–481. (39) Feltham, A. M.; Spiro, M. Chem. ReV. 1971, 71, 177–193. (40) Breiter, M. W. Electrochim. Acta 1963, 8, 447–456.
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Figure 5. CVs of methanol oxidation at the PGNF electrode in a 0.50 M H2SO4 solution in the absence (a) and presence (b) of 1.50 M CH3OH (scan rate, 50 mV/s). Inset: bare Au (c), Au nanoporous film (d), and bare Pt (e) electrode in 0.50 M H2SO4 in the presence of 1.50 M CH3OH under the same conditions.
activity toward methanol oxidation. In addition, the hydrogen adsorption/desorption peaks at -0.05 V decrease compared to that in the absence of methanol, which can be attributed to the fact that the competitive absorption of methanol on the surface of Pt nanoparticle decreases the adsorption quantity of hydrogen.28 To discern the contribution of individual components and the possible synergistic effect among them, the control experiments were carried out on the bare gold, gold nanoporous film, and Pt electrodes (diameter, 1.2 mm). The electrocatalytic activities of these electrodes were investigated by CV, and the results are shown in the inset of Figure 5. For the methanol oxidation at a polycrystalline gold electrode in 1.50 M CH3OH and 0.50 M H2SO4, no oxidized or deoxidized peaks have been observed, indicating that the Au electrode is inert for methanol oxidation under the present conditions. For the gold nanoporous film electrode, the redox peaks of methanol oxidation appear at 0.65 and 0.50 V, respectively, indicating that the electrochemical properties of Au electrode have been changed by the fabrication of Au nanoporous film. The results agree well with previous reports.26 For the bulk Pt electrode (the geometric area is equal to the area of Au electrode), two peaks related to the oxidation of methanol and intermediates appear at 0.70 and 0.52 V, respectively. However, the peak corresponding to the methanol oxidation is 120 times lower than that of PGNF electrode, indicating that PGNF posesses excellent electrocatalytic activity toward methanol oxidation. As is known, a lower anodic overpotential of the catalyst indicates higher electrocatalytic activity.2 Compared to other DMFC catalysts, the as-prepared PGNF electrode exhibits the equivalent or better catalytic performance under optimal experimental conditions. For example, Yang et al. reported that the oxidation potential for CH3OH at the Pt/CNTs electrode was at about 0.68 V,1 which was very close to our results. The oxidation potential of methanol is at about 0.8 V with PtRu/carbon as anode catalysts.41 Furthermore, previous literature has shown that the ratio of the forward anodic peak current (If) to the reverse anodic peak current (Ib) could be used to gauge the tolerance of the catalyst to CO.42,43 The If/Ib values of PGNF and bulk Pt electrode are 1.9 and 1.3, respectively, (41) Tripkovic, A. V.; Popovic, K. D.; Grgur, B. N.; Blizanac, B.; Ross, P. N.; Markovic, N. M. Electrochim. Acta 2002, 47, 3707–3714. (42) Huang, J. C.; Liu, Z. L.; He, C. B.; Gan, L. M. J. Phys. Chem. B 2005, 109, 16644–16649. (43) Liu, Z. L.; Ling, X. Y.; Sun, X. D.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 8234–8240.
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Figure 6. CVs of methanol oxidation at different PGNF electrodes in a 0.50 M H2SO4 solution in the presence of 1.50 M CH3OH. Panels a-e denote that the electrode was deposited with Pt after the potential scan for 1, 2, 5, 7, and 10 cycles, respectively (scan rate, 50 mV/s).
suggesting that the PGNF is more efficient in reducing the adsorbed carbon monoxide than the bulk Pt. The Effect of Pt Nanoparticles on Methanol Electrocatalytic Oxidation. Generally, small particle size and high dispersion of the nanocatalyst on the supporting material may result in a large electrochemical active area for methanol oxidation. The mean size of the Pt nanoparticles increases upon increasing the amount of platinized platinum.29,44 Therefore, the electrocatalytic activities of the PGNF with different Pt loadings were compared by changing the cycles of the potential scan under the same conditions. Figure 6 depicts that the current densities of methanol oxidation are affected by the varying cycles of the potential scan. As suggested by Budevski et al.,45 the metal platinum elec(44) Takasu, Y.; Iwazaki, T.; Sugimoto, W.; Murakami, Y. Electrochem. Commun. 2000, 2, 671–674. (45) Budevski, E.; Staikov, G.; Lorenz, W. J. Electrochim. Acta 2000, 45, 2559–2574.
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trodeposition is a crystal growth process. Platinum ions are electroreduced to atoms, absorbed, and assembled on the surface of Au nanoporous film by diffusing to dislocation or kink sites under the influence of electric field. The current density first increases with the successive cyclic scans. It reaches the maximum when the potential cycle scan is 5, indicating that the loading of Pt nanoparticles on the Au nanoporous film electrode reaches a maximum value. When the cycles exceed five, the peak current decreases rapidly, implying that the catalytic activity of the modified electrode decreases. This is possibly due to the fact that the Pt particles with larger size tend to form in this case, and also the number of nanoporous structure with high surface area is reduced. Thus, the efficient surface area of PGNF electrode is reduced, leading to lower electrocatalytic activity. As a result, the potential scan for five cycles is chosen for the best electrocatalytic activity toward methanol oxidation.
Conclusion We have demonstrated a simple, “green”, and rapid approach to prepare PGNF on a bare gold electrode using a three-step process. The electrocatalytic activity for methanol oxidation on the PGNF electrode was compared with bare Au, Au nanoporous film, and bare Pt electrode under the same conditions. It is found that the PGNF electrode exhibits the best electrocatalytic activity. The loading of Pt nanoparticles on the surface of gold nanoporous electrode has a significant influence on electrocatalytic methanol oxidation. Furthermore, this approach is potentially applied to prepare other gold-based hybrid nanoporous film electrode such as Au/Pd and Au/Ag. Acknowledgment. We thank Professors E. K. Wang and S. J. Dong for helpful suggestions. This project was supported by the National Natural Science Foundation of China (No. 20645004), Changchun Institute of Applied Chemistry, and State Key Laboratory of Electroanalytical Chemistry. LA800163F
