Facile Fabrication of Ultrathin Pt Overlayers onto Nanoporous Metal

Dec 8, 2008 - Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong. UniVersity, Jinan 250061, China, and Key ...
0 downloads 0 Views 2MB Size
Download PDF
Langmuir 2009, 25, 561-567

561

Facile Fabrication of Ultrathin Pt Overlayers onto Nanoporous Metal Membranes via Repeated Cu UPD and in Situ Redox Replacement Reaction Pengpeng Liu,† Xingbo Ge,† Rongyue Wang,† Houyi Ma,*,† and Yi Ding*,†,‡ Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong UniVersity, Jinan 250061, China, and Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, China ReceiVed August 19, 2008. ReVised Manuscript ReceiVed October 14, 2008 Ultrathin Pt films from one to several atomic layers are successfully decorated onto nanoporous gold (NPG) membranes by utilizing under potential deposition (UPD) of Cu onto Au or Pt surfaces, followed by in situ redox replacement reaction (RRR) of UPD Cu by Pt. The thickness of Pt layers can be controlled precisely by repeating the Cu-UPD-RRR cycles. TEM observations coupled with electrochemical testing suggest that the morphology of Pt overlayers changes from an ultrathin epitaxial film in the case of one or two atomic layers to well-dispersed nanoislands in the case of four and more atomic layers. Electron diffraction (ED) patterns confirm that the as-prepared NPG-Pt membranes maintain a single-crystalline structure, even though the thickness of Pt films reaches six atomic layers, indicating the decorated Pt films hold the same crystallographic relationship to the NPG substrate during the entire fabrication process. Due to the regular modulation of Pt utilization, the electrocatalytic activity of NPG-Pt exhibits interesting surface structure dependence in methanol, ethanol, and CO electrooxidation reactions. These novel bimetallic nanocatalysts show excellent electrocatalytic activity and much enhanced poison tolerance as compared to the commercial Pt/C catalysts. The success in the fabrication of NPG-Pt-type materials provides a new path to prepare electrocatalysts with ultralow Pt loading and high Pt utilization, which is of great significance in energy-related applications, such as direct alcohol fuel cells (DAFCs).

* Corresponding authors. Y.D.: phone, +86-531-88366513; fax, +86531-88366280; e-mail, [email protected]. H.M.: phone, +86-531-88364959; fax, +86-531-88564464; e-mail, [email protected]. † Key Laboratory of Colloid and Interface Chemistry. ‡ Key Laboratory of Liquid Structure and Heredity of Materials.

catalyst for CO oxidation at low temperatures11-13 or as the anode electrocatalyst for methanol oxidation reaction (MOR) in alkaline solutions.14,15 Considering that NPG has an interesting porous network structure with excellent electrical conductivity, it holds great promise as a nearly ideal electrode substrate material to fabricate multifunctional nanoarchitectures for electrocatalysis. One example is to construct novel bimetallic Pt-Au electrocatalysts for potential use in direct alcohol fuel cells, which is the purpose of the present work. It is well-known that Pt and Pt-based alloys are the most important anode catalysts in fuel cells. However, an obvious disadvantage of Pt catalysts is that they can be easily poisoned by strongly adsorbed CO-like species during electrooxidation of small organic molecules. The most common approach to solve this problem is to alloy Pt with oxophilic elements,16,17 such as Ru, but unfortunately, the improvement of poison resistance coming from a second composition is often balanced by a decrease in the combined catalytic activity of thus-formed bimetallic catalysts.18 In this sense, it is necessary to fabricate Pt-based bimetallic catalysts possessing both high activity and poison resistance. Since Haruta’s pioneering observation of nanogold’s catalytic activity toward CO oxidation in the late 1980s,19 much effort has been devoted to the application of Au in catalysis.

(1) Peng, X. S.; Koczkur, K.; Nigro, S.; Chen, A. C. Chem. Commun. 2004, 2872. (2) Weissmu¨ller, J.; Viswanath, R. N.; Kramer, D.; Zimmer, P.; Wu¨rschum, R.; Gleiter, H. Science 2003, 300, 312. (3) Ro¨sler, J.; Mukherji, D. AdV. Eng. Mater. 2003, 5, 916. (4) Cortie, M. B.; Maaroof, M. B.; Smith, G. B. Gold Bull. 2005, 38, 14. (5) Ding, Y.; Kim, Y. J.; Erlebacher, J. AdV. Mater. 2004, 16, 1897. (6) Cattarin, S.; Kramer, D.; Lui, A.; Musiani, M. M. J. Phys. Chem. C 2007, 111, 12643. (7) Dixon, M. C.; Daniel, T. A.; Hieda, M.; Smilgies, D. M.; Chan, M. H. W.; Allara, D. L. Langmuir 2007, 23, 2414. (8) Biener, J.; Hodge, A. M.; Hamza, A. V. Appl. Phys. Lett. 2005, 87, 121908. (9) Volkert, C. A.; Lilleodden, E. T.; Kramer, D.; Weissmu¨ller, J. Appl. Phys. Lett. 2006, 89, 061920. (10) Mathur, A.; Erlebacher, J. Appl. Phys. Lett. 2007, 90, 061910.

(11) Zielasek, V.; Jurgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Ba¨umer, M. Angew. Chem., Int. Ed. 2006, 45, 8241. (12) Xu, C.; Su, J.; Xu, X.; Liu, P.; Zhao, H.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 42. (13) Xu, C.; Xu, X.; Su, J.; Ding, Y. J. Catal. 2007, 252, 243. (14) Zhang, J.; Liu, P.; Ma, H.; Ding, Y. J. Phys. Chem. C 2007, 111, 10382. (15) Yu, C.; Jia, F.; Ai, Z.; Zhang, L. Chem. Mater. 2007, 19, 6065. (16) Iwasita, T.; Hoster, H.; John-Anacker, A.; Lin, W. F.; Vielstich, W. Langmuir 2000, 16, 522. (17) Liu, Z.; Reed, D.; Kwon, G.; Shamsuzzoha, M.; Nikles, D. E. J. Phys. Chem. C. 2007, 111, 14223. (18) Yajima, T.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2004, 108, 2654. (19) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405.

1. Introduction Nanoporous metal materials with well-defined pore sizes and ultrahigh surface area-to-volume ratio have received increasing interest in recent years because these materials are finding wide applications in a variety of areas, including selective filtration, chemical sensors, microactuators, and catalysis.1-4 Among them, nanoporous gold (NPG) made by selective dissolution of Au-Ag alloys in corrosive media5 represents an interesting class of nanostructured materials that enable us to understand the relationship between structure and function on the nanometer scale. By adjusting the applied potential or current, temperature, and other parameters, the morphology and characteristic length scale of NPG can be finely tuned during the dealloying process in a wide range from a few nanometers to many microns.5-7 At the present time, much effort in this area has been placed on studying their size-dependent mechanical properties.8-10 Very recently, it was reported that NPG can act as an unsupported

10.1021/la8027034 CCC: $40.75  2009 American Chemical Society Published on Web 12/08/2008

562 Langmuir, Vol. 25, No. 1, 2009

Many researchers have realized that Pt-Au nanostructures may work as a new type bimetallic catalysts with unusual properties.20 For example, methanol oxidation on supported and unsupported Pt-Au nanoparticle electrocatalysts was found to initiate at lower potentials than on pure Pt particles under the otherwise identical conditions,21,22 suggesting a synergistic effect between Au and Pt on alloyed nanocrystal surfaces.23 Core-shell structured Au-Pt nanoparticles have also been fabricated, typically by wet chemistry or metal underpotential deposition (UPD) mediated plating methods,24-30 and a promotional effect from the Au nanoparticle substrate was often observed for a series of electrocatalytic reactions, including methanol and formic acid oxidation and oxygen reduction reaction. While single crystal Au electrodes are widely used as substrates to observe heterogeneous crystal or thin film growth and form novel bimetallic model catalysts, especially under electrochemical conditions,31,32 it is noted that thus far NPG has not received enough attention in preparing practical bimetallic catalysts with high efficiency. In this paper, we focus on the fabrication of ultrathin Pt films in a layer-by-layer mode on the NPG substrate by means of simple Cu UPD in combination with redox replacement of UPD Cu by Pt and study the evolution of catalytic properties of the Pt-Au catalysts as a function of the Pt layer thickness, in order to have a better understanding of the structure effect of the bimetallic surface alloys on the catalytic behavior. By taking the respective advantages of Pt’s excellent catalytic activity and Au’s great tolerance to CO, our method provides a precise control to the construction of a functional, high surface area membrane catalyst with much higher catalytic activity and stronger poison tolerance than those of Pt-based commercial catalysts in electrooxidation of methanol and ethanol, which provides a sound basis for their potential applications in direct alcohol fuel cells (DAFCs).

2. Experimental Section All chemicals were of analytical grade and were used as received from the suppliers. The aqueous solutions were prepared with AR reagents and ultrapure water (>18.2 MΩ). NPG membranes were prepared by dealloying 100 nm thick 12 carat white gold leaves (Au50Ag50 wt %, Sepp Leaf Products Inc.) by floating them on concentrated HNO3 (65%) at 30 °C for 1 h.5 The as-prepared NPG membranes were transferred and carefully rinsed with ultrapure water at least 5 times and then preserved on ultrapure water before use. For surface modification, NPG membranes were first loaded onto the surface of a glass carbon electrode (GCE) and then fixed by dropping 2.0 µL of diluted Nafion solution (0.1 mL 5 wt% Nafion solution, DuPont, mixed with 10 mL ethanol) onto the electrode surface. Cu UPD was carried out in the potentiostatic mode in a mixed solution of 0.5 M H2SO4 + 0.5 mM CuSO4. Afterward, the NPG electrodes modified with Cu UPD were rapidly transferred (20) Pedersen, M. Ø.; Helveg, S.; Ruban, A.; Stensgaard, I.; Lægsgaard, E.; Nørskov, J. K.; Besenbacher, F. Surf. Sci. 1999, 426, 395. (21) Choi, J. H.; Park, I. S.; Kim, K. J. Electrochem. Soc. 2006, 153, A1812. (22) Park, I. S.; Lee, K. S.; Jung, D. S.; Park, H. Y.; Sung, Y. E. Electrochim. Acta 2007, 52, 5599. (23) Mott, D.; Luo, J.; Njoki, P. N.; Lin, Y.; Wang, L.; Zhong, C. J. Catal. Today 2007, 122, 378. (24) Zeng, J.; Yang, J.; Lee, J. Y.; Zhou, W. J. Phys. Chem. B 2006, 110, 24606. (25) Zhao, D.; Xu, B. Q. Angew. Chem., Int. Ed. 2006, 45, 4955. (26) Park, I. S.; Lee, K. S.; Choi, J. H.; Park, H. Y.; Sung, Y. E. J. Phys. Chem. C 2007, 111, 19126. (27) Kristian, N.; Yan, Y.; Wang, X. Chem. Commun. 2008, 353. (28) Zhai, J.; Huang, M.; Dong, S. Electroanalysis 2007, 19, 506. (29) Kumar, S.; Zou, S. Langmuir 2007, 23, 7365. (30) Sasaki, K.; Mo, Y.; Wang, J. X.; Balasubramanian, M.; Uribe, F.; McBreen, J.; Adzic, R. R. Electrochim. Acta 2003, 48, 3841. (31) Kim, J.; Jung, C.; Rhee, C. K.; Lim, T. H. Langmuir 2007, 23, 10831. (32) Shao, M. H.; Huang, T.; Liu, P.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Adzic, R. R. Langmuir 2006, 22, 10409.

Liu et al. into 1 mM K2PtCl4 + 0.1 M HClO4 solution and held for 10 min for redox replacement of Cu layer by Pt, to form a Pt monolayer decorated NPG sample. Because Cu UPD can occur on both Au and Pt surfaces,33 this kind of operation can be repeated to form ultrathin Pt films in a precise manner from one to several atomic layers, and the bimetallic Pt-Au nanostructures obtained in this way were written as NPG-Ptn, where n is the number of Cu-UPD-RRR (redox replacement reaction) cycles. For TEM measurements, samples were prepared by ultrasonically dispersing NPG-Ptn in ethanol and then putting a drop of the suspension onto a standard microscope grid. Electron micrographs were taken with a JEOL-2100 transmission electron microscope (TEM) operating at 200 kV. All electrochemical experiments were performed with a CHI 760C electrochemical workstation in a homemade three-electrode cell at room temperature (∼25 °C). A 1.5 × 1.5 cm2 Pt plate was used as the counter electrode, and a reversible hydrogen electrode (RHE) or a mercurous sulfate electrode (MSE, 0.5 M H2SO4, 0.68 V vs RHE) was selected as the reference electrode, depending on the experimental requirements. The potential values measured versus MSE were converted to values with reference to RHE. The reference electrode used was led to the surface of the working electrode through a Luggin capillary. The electrochemical behavior of NPG-Ptn was characterized by means of voltammetry in 0.5 M H2SO4 solutions. Electrochemical active surface areas (ECSAs) of Pt layers were determined on the basis of the charge associated with the hydrogen adsorption and desorption. The voltammetric measurements for methanol and ethanol oxidations were carried out in 0.5 M H2SO4 + 1.0 M CH3OH (or 1.0 M C2H5OH) mixed solutions. To compare the mass efficiency, the obtained current data were normalized by the mass of Pt (Pt loading) on the NPG substrates. The CO stripping experiments were also conducted in 0.5 M H2SO4. The potential was first held at 0.1 V vs RHE for 15 min in CO-saturated solutions in order to allow sufficient CO adsorption onto the surface of NPG-Ptn catalysts. Subsequently, the solution was deaerated with N2 for 20 min to remove freely dissolved CO molecules. The bubbling of N2 was maintained throughout the experiment. The presented CO stripping curves have subtracted the base lines from the second cycles, so all current density information can be attributed to CO electrooxidation.

3. Results and Discussion 3.1. Cu UPD on NPG Electrode. The deposition of a foreign metal with atomic precision onto the NPG substrate is of great significance and also technically challenging in surface science and heterogeneous catalysis. Such bimetallic systems are expected to exhibit physicochemical properties quite different from their bulk counterparts. Traditional electrochemical deposition of a metal onto a substrate will lead to the formation of a new solid phase, which usually takes place at the thermodynamically reversible potentials or more negative potentials. However, sometimes the electrochemical adsorption of a metal (M, such as Cu) on another metal (N, such as Au) substrate may occur in a potential region positive to the reversible potential for the bulk deposition due to the stronger interaction between M and N than that of M-M. This unusual phenomenon is called underpotential deposition.33 Under these conditions, M atoms may deposit onto the N substrate in a form of a monolayer or submonolayer. The UPD process is very important from the electrochemical point of view, and by taking advantage of UPD, one is able to construct a monolayer of precious metal atoms on a foreign metal surface with great ease. For example, Adzic34 and Weaver35 were among the first to prove that metal UPD could be used as a media in in situ redox replacement reactions to deposit precious metals such as Pt and Pd onto the substrate electrode surfaces with (33) Herrero, E.; Buller, L. J.; Abrun˜a, H. D. Chem. ReV. 2001, 101, 1897. (34) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Surf. Sci. 2001, 474, L173. (35) Mrozek, M. F.; Xie, Y.; Weaver, M. J. Anal. Chem. 2001, 73, 5953.

Facile Fabrication of Ultrathin Pt OVerlayers

Langmuir, Vol. 25, No. 1, 2009 563

Figure 1. Cyclic voltammograms (CVs) of NPG in a mixed solution of 0.5 M H2SO4 + 0.5 mM CuSO4. Scan rate: 2 mV/s.

Figure 3. Dependence of Cu stripping charge as a function of deposition potential. The data used here were extracted from Figure 2.

Figure 2. Cu stripping profiles at different deposition potentials with the same deposition time of 120s.

Figure 4. Linear potential scan curves of NPG in 0.5 M H2SO4+ 0.5 mM CuSO4 after deposition for different periods of time at 0.24 V.

atomic precision. Considering that Cu UPD can occur on both Au and Pt surfaces,33 herein a purposeful combination of UPD and RRR is employed as a convenient way to prepare the Pt ultrathin films onto NPG with well-controlled thickness. In order to find the optimum deposition condition such as deposition potential and time, we first investigated the electrochemical behavior of Cu UPD on NPG. Figure 1 shows the cyclic voltammograms (CVs) of NPG in 0.5 M H2SO4 solution containing 0.5 mM Cu2+ ions. Three pairs of current peaks are observed in this CV profile: two reaction peaks at ∼0.275 (a) and 0.42 V (b) at more positive potentials are due to the UPD of Cu, and two oxidation peaks at 0.33 (a′) and ∼0.5 V (b′) are the corresponding UPD stripping peaks, while another pair of peaks located at 0.19 (c) and 0.24 V (c′) are ascribed to the bulk deposition and stripping of Cu, respectively. Although this CV profile is slightly different from those of the electrochemical deposition of Cu on single crystal Au surfaces, the two pairs of peaks for the UPD of Cu on the NPG electrode almost appear at the identical potentials as on single crystal Au(111) electrodes.33 This CV curve also indicates that UPD of Cu on NPG takes place in a wide potential region between 0.24 and 0.5 V. To select the optimum potential value to ensure a nearly ideal coverage of a monolayer of Cu adatoms, the relations between the deposition potentials/time and the related stripping charges for the deposited Cu layers were determined by holding at different potentials for 120 s or holding at a fixed potential for a different time period. As shown in Figure 2, holding the potential at 0.22 V results in significantly higher current intensities for both Cu stripping peaks. Holding the potential at four different values positive to 0.24 V

generates nearly similar stripping behavior at 0.5 V, but markedly different stripping characteristics at 0.33 V. In general, holding the potential at 0.24 V gives a most well-defined Cu UPD stripping profile. More detailed analyses were done by integrating the stripping charges generated at different holding potentials. As seen in Figure 3, the stripping charge increases slowly with the decrease of potential at more positive potentials (g0.24 V), whereas at more negative potentials ( 4. More specifically, CO stripping on the NPG-Pt1 electrode shows a nonsymmetric broad anodic wave peaked around ∼0.80 V. For NPG-Pt2 and NPG-Pt3 samples, this anodic wave gradually increases its intensity in lower potentials, so the entire curve seems to consist of two poorly separated peaks centered at 0.7 and 0.78 V. After four cycles of Pt deposition, this broad peak eventually evolves into one well-defined symmetric peak centered at 0.73 V. With the further increase of the thickness of Pt films, the bigger the n value, the sharper the CO stripping peak. The changes of the peak currents and peak potentials with the thickness of Pt layers are closely associated with the amount of Pt deposited and the bonding strength of adsorbed CO species on the NPG-Ptn electrodes. Du and Tong40 have also examined the CO oxidation on the Pt modified polycrystalline Au and found that CO oxidation took place at more positive potentials at low Pt loadings, which is attributed to the stronger adsorption of CO and OH on Pt-Au surface alloys than on pure Pt. 3.5. Electrooxidation of Methanol. Figure 9 shows CVs for methanol electrooxidation on NPG-Ptn electrodes in a mixed solution of 0.5 M H2SO4 and 1.0 M CH3OH. For comparison, the activity of commercial Pt/C catalyst (Johnson Matthey) was also tested and included. The currents were normalized with respect to the ECSAs and the Pt loading, respectively. As shown in Figure 9a, the ECSA-normalized activity of Pt orderly increases (38) Ding, Y.; Chen, M. W.; Erlebacher, J. J. Am. Chem. Soc. 2004, 126, 6876. (39) Ding, Y.; Mathur, A.; Erlebacher, J. Angew. Chem., Int. Ed. 2005, 44, 4002. (40) Du, B. C.; Tong, Y. Y. J. Phys. Chem. B 2005, 109, 17775.

Facile Fabrication of Ultrathin Pt OVerlayers

Langmuir, Vol. 25, No. 1, 2009 565

Figure 7. TEM and HRTEM images for NPG-Pt2 (a, d), NPG-Pt4 (b, e), and NPG-Pt6 (c, f). Insets are the respective electron diffraction patterns for these samples.

Figure 8. CO stripping curves of NPG-Ptn samples in 0.5 M H2SO4. Scan rate: 20 mV/s.

from the minimum in the NPG-Pt1 to the maximum in the NPG-Pt6 on the forward scan. It is generally accepted that the activity of catalysts depends on the physical and chemical nature of active surface atoms, which is often affected by the support material and the adsorbed species. This support effect may explain why all NPG-Ptn samples can exhibit better performance than the commercial Pt/C catalyst. Actually, Au has been found to have a promotional effect on Pt atom for methanol oxidation in acidic and alkaline solution.24,41 On the other hand, during methanol oxidation, the performance of a catalyst is extremely sensitive to the actual surface structure and arrangement of Pt atoms, well-known as the “ensemble effect”.42,43 It is expected that the ECSA-normalized activity would markedly increase from (41) Luo, J.; Maye, M. M.; Kariuki, N. N.; Wang, L.; Njoki, P.; Lin, Y.; Schadt, M.; Naslund, H. R.; Zhong, C. J. Catal. Today 2005, 99, 291. (42) Chang, S. C.; Ho, Y.; Weaver, M. J. Surf. Sci. 1992, 265, 81. (43) Park, S.; Xie, Y.; Weaver, M. J. Langmuir 2002, 18, 5792.

NPG-Pt1 to NPG-Pt4, because the more Pt deposited and the larger Pt particles formed, the more chance that Pt atoms can form continuous overlayers and clusters that are essential for methanol electrooxidation.40 For NPG-Pt5 and NPG-Pt6, the similar activities are indicative of their similar structures. As for the mass specific activities (Figure 9b), the NPG-Pt1 electrode displays the highest catalytic activity among all NPG-Ptn samples because monatomic Pt overlayer spreads over the entire NPG substrate surfaces; thus, all Pt atoms are available in the oxidation of methanol. In other words, here the deposited Pt atoms reaches their full utilization. However, this situation greatly changes with the continuous deposition of Pt. When the thickness of Pt films increases from one to six layers, the catalytic activity of NPG-Ptn decreases accordingly in general. From a structure point of view, compared to NPG-Pt1, 50% of Pt atoms in NPG-Pt2 will not participate in the methanol oxidation reaction, because they are probably not exposed on the surface. Despite all this, being the least efficient, NPG-Pt6 still exhibits higher catalytic activity than the commercial Pt/C catalysts. It is interesting that with a higher Pt loading, NPG-Pt4 exhibits a little higher mass specific activity than NPG-Pt3. This unusual phenomenon can be attributed to the formation of small Pt nanoparticles with size less than 3 nm, since after four Cu-UPD-RRR cycles, the deposited Pt can no longer adopt a two-dimensional growth mode due to the accumulated misfit strain in the system (lattice mismatch between Au and Pt, 3.78%), but rather they start a three-dimensional islanding growth, as indicated by electron microscopy observations discussed above. While a simple calculation for 3 nm spherical Pt nanoparticles (or hemispherical nanoislands) shows that the surface atoms can count ∼50% of the total atoms, which is equivalent to an ideal bilayer structure, this change in morphology indicates that NPG-Pt4 may have a slightly higher utilization than NPG-Pt3.

566 Langmuir, Vol. 25, No. 1, 2009

Figure 9. ECSA (a) and mass (b) normalized CVs of NPG-Ptn for methanol electrooxidation in 0.5 M H2SO4 + 1.0 M CH3OH. CV curve (dotted black line) for a commercial Pt/C catalyst (Johnson Matthey, 20 wt %) is also included for comparison. The inset in part a shows the If/Ib values of NPG-Ptn along with that for the commercial Pt/C catalyst (red line). Scan rate: 20 mV/s.

Another reaction for the observed higher mass specific activity is that NPG-Pt4 has much higher ECSA-normalized activity as compared to NPG-Pt3 (Figure 9a). In a word, there are more surface atoms that are more active available for the MOR reaction in NPG-Pt4 than NPG-Pt3, which leads to the rapid rising of the current. For other samples, the utilization of Pt plays a dominant role in mass specific activity. Accordingly, the electrocatalytic activities of NPG-Pt5 and NPG-Pt6 electrode are naturally lower than those of other samples. For methanol electrooxidation, the ratio of the current densities between the primary and secondary anodic peaks (If/Ib) is an important parameter to evaluate the poison resistance of Ptbased catalysts. We found that all NPG-Ptn bimetallic catalysts show higher If/Ib ratio than the commerical Pt/C catalyst, demonstrating better tolerance to poisoning from carbonaceous species generated during the electrocatalytic reactions due to the synergistic effect of Pt and Au in NPG-Ptn. In particular, NPG-Pt2 has a highest If/Ib ratio of 1.44, indicating its greatly enhanced tolerance to poisoning. 3.6. Electrooxidation of Ethanol. Besides methanol, ethanol is another attractive liquid fuel in DAFCs due to its higher energy density, lower toxicity, and greater availability. The catalytic activity of NPG-Ptn toward ethanol electrooxidation was thus tested by cyclic voltammetry in a mixed solution of 0.5 M H2SO4 and 1.0 M C2H5OH, which is illustrated in Figure 10. Generally, ethanol oxidation exhibits a very similar voltammetric feature to that of methanol, i.e., an oxidation peak on the positive-going scan at ∼0.9 V and a reactivation peak on the negative-going scan at ∼0.7 V. As expected, NPG-Pt1 presents the highest mass specific activity (Figure 10a), which is easily understandable

Liu et al.

Figure 10. ECSA (a) and mass (b) normalized CVs of NPG-Ptn for methanol electrooxidation in 0.5 M H2SO4 + 1.0 M C2H5OH. Scan rate: 20 mV/s.

because it has the highest Pt utilization. Interestingly, NPG-Pt1 also possesses a markedly higher ECSA-normalized current density than other NPG-Ptn samples (Figure 10b), which is somewhat different from its performance in methanol oxidation. The decrease of peak current during forward scan with n, to a certain extent, reflects that Au substrate has a positive effect on surface Pt atoms in ethanol electrooxidation. Compared with methanol, the current peak of ethanol oxidation shifts to a slight positive potential, e.g. 0.93 V versus 0.87 V for the NPG-Pt1 sample, possibly because a higher activation energy is needed for breaking the C-C bond than C-H bond.44 The different voltammetric features between the electrooxidation of methanol and ethanol suggest that a distinct reaction pathways occurred on the NPG-Ptn surfaces.45 Spectroscopic investigations and theoretical studies are needed in order to clarify the underlying mechanisms, which is currently underway in our laboratory.

Conclusions We have developed a simple but very effective method to fabricate platinum-functionalized nanoporous gold (NPG) membranes by means of repeated UPD-redox replacement procedure. One special advantage of the present method over the traditional chemical and electrochemical deposition methods is that there is essentially no precious metal loss during the whole process, and there is no organic reagents such as reductants and surfactants used for structure formation and surface passivation. The facile fabrication process also allows the resulting composite nanostructures to well preserve the fine structure of the porous precursors. These novel Au-Pt nanocomposites display high (44) Nonaka, H.; Matsumura, Y. J. Electroanal. Chem. 2002, 520, 101. (45) Xia, X. H.; Liess, H.-D.; Iwasita, T. J. Electroanal. Chem. 1997, 437, 233.

Facile Fabrication of Ultrathin Pt OVerlayers

catalytic activity for methanol and ethanol oxidation reactions. Moreover, they present the stronger tolerance to carbonaceous poisoning than the commercial Pt/C catalysts due to the synergistic effect of Pt and Au in NPG-Ptn. The fabrication of NPG-Ptntype materials based on NPG provides a new path to construct electrocatalysts with ultralow precious metal loading, high catalyst utilization, and high catalytic activity in a controlled way. These materials should find immediate applications in energy-saving technologies such as direct alcohol fuel cells (DAFCs).

Langmuir, Vol. 25, No. 1, 2009 567

Acknowledgment. Financial supports from the National 863 (2006AA03Z222) and 973 (2007CB936602) Program Projects of China, the Key Project of Chinese Ministry of Education (108078), and the Natural Science Foundation of Shandong Province (2007ZRB01117, 2006BS04018) are greatly appreciated. Y.D. is a Tai-Shan Scholar supported by the SEM-NCET and SRF-ROCS Programs. LA8027034