In Situ Electrochemical STM Study of the Coarsening of Platinum

Several interesting surface island reconstruction processes ... island growth; and (3) growth of platinum islands accompanied by disappearance of near...
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Langmuir 2007, 23, 9098-9103

In Situ Electrochemical STM Study of the Coarsening of Platinum Islands at Double-Layer Potentials Qingmin Xu,†,‡ Ting He,*,† and David O. Wipf‡ Honda Research Institute USA, Inc., 1381 Kinnear Road, Columbus, Ohio 43212, and Department of Chemistry, Mississippi State UniVersity, Mississippi State, Mississippi 39762 ReceiVed February 12, 2007. In Final Form: June 6, 2007 In situ electrochemical scanning tunneling microscopy is used to study the coarsening of platinum islands at potentials of 0.4, 0.5, and 0.6 V in the double-layer region. Several interesting surface island reconstruction processes were observed, namely, (1) growth of small polycrystalline platinum islands; (2) shape- and size-limited platinum island growth; and (3) growth of platinum islands accompanied by disappearance of nearby islands. It is evident that these potential-induced coarsening processes can be explained by Gibbs-Thomson theory as a variant of Ostwald ripening. Details of the island reconstruction processes are described, and the possible influences of these phenomena on fuel cell operation are discussed.

Introduction The proton exchange membrane fuel cell (PEMFC) is expected to be a next-generation power source used for vehicles and homes, offering high-energy conversion efficiency and minimal pollutant emissions.1,2 The high efficiency of PEMFC arises from the direct conversion of chemical energy to electricity without the Carnot limitation that applies to thermal engines. However, high-surfacearea carbon-supported platinum nanoparticles, which serve as state-of-the-art cathode catalysts for oxygen electroreduction, lose their electrochemical surface area (ECSA) during operation, resulting in severe performance degradation. One most important reason for Pt ECSA loss under fuel cell operation is sintering and/or agglomeration of Pt nanoparticles in the electrode.3-10 Extensive performance degradation studies have been performed in the past for phosphoric acid fuel cell (PAFC), where highsurface-area carbon-supported Pt or Pt alloy nanoparticles have also been used as catalysts.11-14 The Pt ECSA loss there was associated with Pt crystal growth ascribed to the following possible processes: (i) Pt dissolution and redeposition (traditional Ostwald ripening); (ii) coalescence of Pt nanoparticles via Pt nanocrystallite migration on the carbon support; and (iii) agglomeration of Pt nanoparticles triggered by corrosion of the carbon support or detachment of Pt nanoparticles from the carbon support. For * Corresponding author. E-mail: [email protected]. † Honda Research Institute USA, Inc. ‡ Mississippi State University. (1) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1, 5. (2) Okada, O.; Yokoyama, K. Fuel Cells 2001, 1, 72. (3) Bett, J. A.; Kinoshita, K.; Stonehart, P. J. Catal. 1974, 35, 307. (4) Bett, J. A. S.; Kinoshita, K.; Stonehart, P. J. Catal. 1976, 41, 124. (5) Blurton, K. F.; Kunz, H. R.; Rutt, D. R. Electrochim. Acta 1978, 23, 183. (6) Gruver, G. A.; Pascoe, R. F.; Kunz, H. R. J. Electrochem. Soc. 1980, 127, 1219. (7) Aragane, J.; Murahashi, T.; Odaka, T. J. Electrochem. Soc. 1988, 135, 844. (8) Honji, A.; Mori, T.; Tamura, K.; Hishinuma, Y. J. Electrochem. Soc. 1988, 135, 355. (9) Landsman, D. A.; Luczak, F. J. In Handbook of Fuel Cells-Fundamentals, Technology and Applications; Vielstich, W., Gasteiger, H., Eds.; John Wiley & Sons: Chichester, U.K, 2003; Vol. 4, p. 811. (10) Ferreira, P. J.; Ia O, G. J.; Shao-Horn, Y.; Morgan, D.; Makharia, R.; Kocha, S.; Gasteiger, H. A. J. Electrochem. Soc. 2005, 152, A2256. (11) Borodovsky, L.; Beery, J. G.; Paffett, M. Nucl. Instrum. Methods Phys. Res. 1987, B24/25, 568. (12) Bett, J. A.; Kinoshita, K.; Stonehart, P. J. Catal. 1974, 35, 307. (13) Blurton, K. F.; Kunz, H. R.; Rutt, D. R. Electrochim. Acta 1978, 23, 183. (14) Honji, A.; Mori, T.; Tamura, K.; Hishinuma, Y. J. Electrochem. Soc. 1988, 135, 355.

PEMFCs, Ferreira et al.10 studied the Pt ECSA loss under steadystate conditions and proposed that the ECSA loss occurred via two different processes: (i) Ostwald ripening on the carbon support at the nanometer scale, which is responsible for Pt particle coarsening; and (ii) migration of soluble (oxidized) Pt species into the ionomer phase on the micrometer scale. Recently, Komanicly et al.15,16 investigated the dissolution of Pt singlecrystal and nanofaceted surfaces with ex situ atomic force microscopy (AFM) and inductively coupled plasma mass spectrometry (ICP-MS). They found that Pt oxide formation passivates the low-index surfaces, resulting in lower overall dissolution rates at higher potentials. In contrast, the nanofaceted surface dissolves faster at higher potentials, indicating that the edges and corners are the main sources of dissolution. Although considerable effort has been made to understand the mechanisms of Pt ECSA loss and extensive experiments have been performed to verify the proposed mechanisms, direct in situ observation of the electrochemically induced processes is still lacking. Electrochemical scanning tunneling microscopy (STM) is a powerful tool for the in situ characterization of surfaces under potentiostatic control at atomic-scale resolution.17-20 It not only determines the electrode nanostructure at the electrode/electrolyte interface17-25 but also monitors electrochemical reactions on the electrode, such as the dissolution and deposition of metals and semiconductors.18,20,26-30 Examples of such studies include the (15) Komanicky, V.; Chang, K. C.; Menzel, A.; Markovic, N. M.; You, H.; Wang, X.; Myers, D. ECS Trans. 2006, 1, 167. (16) Komanicky, V.; Menzel, A.; Chang, K. C.; You, H. J. Phys. Chem. B 2005, 109, 23543. (17) Siegenthaler, H. In Scanning Tunneling Microscopy II; Springer-Verlag: New York, 1992; p 7. (18) Bard, A. J., Rubinstein I., Eds. Electroananlytical Chemistry (V21); Marcel Dekker, Inc.: New York, 1999. (19) Oppenheim, I. C.; Trevor, D. J.; Chidsey, C. E. D.; Trevor, P. L.; Sieradzki, K. Science 1991, 254, 687. (20) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (21) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (22) Wan, L. J.; Yau, S. L.; Itaya, K. J. Phys. Chem. 1995, 99, 9507. (23) Wan, L. J.; Wang, C.; Bai, C. L.; Osawa, M. J. Phys. Chem. B 2001, 105, 8399. (24) Xu, Q. M.; Wang, D.; Wan, L. J.; Bai, C. L.; Wang, Y. J. Am. Chem. Soc. 2002, 124, 14300. (25) Xu, Q. M.; Wang, D.; Wan, L. J.; Wang, C.; Bai, C. L.; Feng, G. Q.; Wang, M. X. Angew. Chem., Int. Ed. 2002, 41, 3408. (26) Gewirth, A. A.; Niece, B. K. Chem. ReV. 1997, 97, 1129.

10.1021/la700415w CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007

Coarsening of Pt Islands at Double-Layer Potentials

Figure 1. In situ STM images (constant height mode) of polycrystalline Pt film under double-layer potential control at 0.5 V: (a) immediately after electrochemical cleaning; (b) 22 min later; (c) 3 h 6 min later; (d) 5 h 3 min later; and (e) 7 h 32 min later than image a. (f) High-resolution image taken on a flat terrace showing the Pt lattice and orientation. The image sizes of (a-e) are 100 nm × 100 nm; image size of (f) is 6 nm × 6 nm.

thermally driven equilibrium step fluctuations on Ag and Cu surfaces, where the steps appeared to be “frizzy” due to rapid kink motion,29,30 and the effect of potential on the equilibrium step fluctuations on immersed Ag(111) surfaces, where step fluctuations increased sharply as the potential was moved toward the reversible potential of the Ag/Ag+ electrode.29 Therefore, the electrochemical STM is an ideal technique to monitor reconstructions and reactions of surface Pt islands under electrochemical control. This paper reports our initial results of an in situ electrochemical scanning tunneling microscopy study of the coarsening of platinum islands at potentials of 0.4, 0.5, and 0.6 V (vs reversible hydrogen electrode, RHE) in the double-layer region in 0.1 M perchloric acid. Several interesting phenomena have been observed, and possible mechanisms are discussed. To the best of our knowledge, this is the first report of in situ observation of platinum island coarsening while being held at double-layer potentials. (27) Wan, L. J.; Moriyama, T.; Ito, M.; Uchida, H.; Watanabe, M. Chem. Commun. 2002, 58. (28) Wan, L. J.; Itaya, K. Langmuir 1997, 13, 7173. (29) Dietterle, M.; Will, T.; Kolb, D. M. Surf. Sci. 1995, 327, L495. (30) Giesen, M. Surf. Sci. 1997, 370, 55.

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Figure 2. (a) Size (diagonal length) of island “A” as a function of time during the reconstruction process. The open squares are the sizes of island A measured from STM images, and the solid line is a simple fitting of these data points; (b) a log-log plot of data from (a). A slope of about 3 indicates that surface-diffusion transport is the dominating mechanism.

Experimental Section 1. Preparation of Platinum Thin Films. Polycrystalline Pt films were deposited onto highly polished graphite substrates (10 × 10 mm2) by plasma sputtering. Plasma sputtering is an atomistic deposition process in which metal is vaporized from a target in the form of atoms and is transported through a low-pressure gaseous environment to the substrate where it is condensed. Prior to film deposition, the graphite substrate was prepared and cleaned by mechanical polishing with diamond paste down to 0.25 µm followed by thorough ultrasonic cleaning and repeated rinsing in ultrapure water and subsequent in situ oxygen and argon plasma treatments (3 min each). The films were deposited at a deposition rate of 0.3 Å/s controlled by plasma power (40 W) and argon gas pressure (3 m Torr). The total thickness is about 100 nm. 2. Electrochemical Scanning Tunneling Microscopy. The deposited polycrystalline Pt film was used as a working electrode for both electrochemical characterization and in situ STM imaging. The geometric surface area of the working electrode (the Pt film exposing to acidic electrolyte) is about 0.28 cm2. High-purity Pt wires were used as quasi-reference (ca. -0.8 V vs RHE) and counter electrodes. All electrode potentials are reported with respect to reversible hydrogen electrode (RHE). The electrolyte used in all experiments was 0.1 M perchloric acid (optima grade) in ultrapure Millipore water (18.2 MΩ; TOC < 5 ppb). Perchloric acid was selected as the electrolyte to minimize anion adsorption on the Pt surface. In situ electrochemical STM experiments were carried out at room temperature in both constant-current (height image) and constantheight (current image) modes with an Agilent Technologies PicoPlus scanning probe microscope. The STM tips were homemade by electrochemically etching 0.25 mm tungsten wires in 0.6 M KOH and coating with clean nail polish to minimize the faradic current.

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Figure 3. In situ STM images of the growth of a large Pt island under potential control at 0.4 V: (a) immediately after electrochemical cleaning; (b) 1 h 24 min later; (c) 3 h 19 min later; and (d) 7 h 14 min later than image a. All scan sizes are 200 nm × 200 nm. The tunneling current used for imaging varied between 0.5 and 1.5 nA. All STM images are shown as collected without any data processing. 3. Electrochemical Cleaning and Potential Control. Since the Pt film surface will be contaminated by organic adsorbates after exposure to air, it is necessary to refresh/clean the film surface under in situ conditions. In situ electrochemical cleaning was performed by potential cycling between 0.05 and 1.00 V at a scan rate of 200 mV/s in 0.1 M perchloric acid. Typically, a steady-state cyclic voltammogram (CV) with standard polycrystalline Pt features could be achieved after about 20 cycles. Hydrogen adsorption and proton oxidation are observed at lower potentials (0.75). The featureless region between these potentials is the “double-layer” region, and potential control at 0.4, 0.5, and 0.6 V in this region was selected to monitor the coarsening of Pt islands on the electrode surface.

Results and Discussion The results and discussion of this work is divided into four sections. The first section describes the phenomena of potentialinduced Pt island growth in the double-layer potential region. In the second section, shape- and size-limited Pt island growth is presented. The third section describes the growth of Pt islands accompanied by the disappearance of nearby islands. The last section discusses the possible influence of these phenomena on fuel cell operation. 1. Growth of Platinum Islands. The surface of freshly sputterdeposited polycrystalline Pt film is very rough, and highmagnification STM imaging of this surface is difficult. Therefore,

nanoscopically smooth regions were chosen to study potentialinduced Pt island coarsening. Figure 1 shows a typical set of STM images under potential control at 0.5 V after an initial in situ electrochemical cleaning as described in the Experimental Section. Immediately after surface cleaning, Pt island “A” with several oriented crystalline steps was formed near some irregular islands (circled area in Figure 1a). Island A grew in both length and width with time and is easily identified by the top lamellae (see Figure 1b-e). After 22 min (Figure 1b), island A grew larger, and the edges of islands B, C, and D are getting sharper. Islands A and B merged with time and grew over islands C and D as shown by Figure 1c-e. Intriguingly, the shape and size of island A after 7 h 32 min (Figure 1e) is almost identical to that seen after 5 h 3 min (Figure 1d), except that the top lamellae grew and the edges of the oriented crystalline steps became more prominent, which means a stable shape has been formed. The step lines of these domains cross each other at 60° or 120°, indicating well-ordered crystalline facets with a (111) orientation. In addition, an island marked by E above island A in Figure 1c-e showed similar growth phenomena (partly cropped out for simplicity). Figure 1f displays a representative STM image with atomic resolution acquired in a wide terrace of island A. A hexagonal structure is evident and is indicated by the six red circles on the STM image. The atomic rows are in threefold symmetryswith the direction of the atomic rows almost parallel to the step directions. The interatomic distance is measured to be 0.277 nm, exactly the same as expected for a Pt (111) surface. Similar (111)

Coarsening of Pt Islands at Double-Layer Potentials

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Figure 4. In situ STM images of a large Pt island under potential control at 0.6 V: (a) immediately after electrochemical cleaning; (b) 32 min later; (c) 1 h 12 min later; and (d) 3 h 30 min later than image a. All scan sizes are 200 nm × 200 nm.

surfaces were observed on other terraces of islands A and B, and no ordered (100) and (110) surfaces have ever been observed on the Pt films under double-layer potential controlsindicating the superior thermodynamic stability of the (111) hexagonal arrangement, which guides the reconstruction of the surface Pt atoms.31 Moreover, Figure 1f displays an atomic image with extraordinary stripes (indicated by white arrows) revealing misalignment of the atomic rows from both sides (outlined by the dotted line). Usually, the atomic spacing at the strain region differs from the ideal. Since the dislocation defect phenomenon has not been observed at Pt single-crystal surfaces, it may result from film reconstruction and crystallization, e.g., the nucleationand-growth process. Figure 2a shows the growth (diagonal length) with time of island A in Figure 1. The island grew rapidly at an initial rate of 0.3 nm/min; however, once the size and shape was established, the growth was much slower and eventually stopped. More experiments will be described in the next section regarding this size- and shape-limited growth. While the island grew under double-layer potential control, no coalescence of smaller islands was seen. Ostwald ripening can take two different forms in this case corresponding to two distinct mass-transport mechanisms, which can occur at the island boundary similar to the vacancy diffusion described in ref 32:32 (i) surface-diffusion transport, (31) Yau, S. L.; Moriyama, T.; Uchida, H.; Watanabe, M. Chem. Commun. 2000, 2279. (32) Wagner, K.; Brankovic, S. R.; Dimitrov, N.; Sieradzki, K. J. Electrochem. Soc. 1997, 144, 3545.

where smaller islands emit adatoms which diffuse on the film surface to larger island; (ii) dissolution-redeposition process, where atoms undergo random detachment and attachment at the solid/liquid interface. Each mechanism has a relaxation time which scales with a power law of the change of island size,33 i.e., t ∝ ∆Rn, where t represents relaxation time, ∆R is the change of island size, and n ) 3 and 2 for the case of surface-diffusion transport and the dissolution-redeposition process. Figure 2b displays a log-log plot of the island growth shown in Figure 2a. It turns out from a linear fitting that n ≈ 3, meaning that surfacediffusion transport is the dominating mechanism for the ripening. This conclusion is also based on the fact that the dissolution kinetics is orders of magnitude slower at double-layer potentials (dissolved Pt concentration is in the order of 10-9 mol or less) compared to higher potentials and the slow dissolution kinetics cannot make up the drastic island growth, 0.3 nm/min at the beginning. To further verify the origin of Ostwald ripening, a freshly sputter-deposited Pt film was immersed into the same solvent for various lengths of time without potential control (open circuit). No noticeable morphological change of surface Pt was observed, indicating that the Pt film was stable in the solvent and the coarsening was caused by the electrochemical potential. Therefore, it can be concluded that the Ostwald ripening observed in (33) Mullins, W. W. In Metal Surfaces: Structure, Energetics and Kinetics; American Society of Metals: Metal Park, OH, 1967; p 17.

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Figure 5. In situ STM images of the disappearance of a polycrystalline Pt island with time under potential control at 0.4 V: (a) immediately after electrochemical cleaning; (b) 1 h 5 min later; (c) 1 h 35 min later; and (d) 2 h later than image (a). All scan sizes are 200 nm × 200 nm.

our experiment is not the traditional one, but rather a potentialinduced surface-diffusion transport, a variant of Ostwald ripening. 2. Shape- and Size-Limited Island Growth. In the previous section, we described the growth of a Pt island with time. As can be seen from Figures 1 and 2, the shape and size of the island do not appear to change significantly once an initial growth phase has occurred. To verify this phenomenon, the growth of a bigger island (over 100 nm) was monitored by in situ imaging immediately following electrochemical cleaning of the Pt surface. Figure 3 shows a set of STM images of the maturation of this island at a controlled potential of 0.4 V. The initial size of the “leaf-shaped” island is about 130 nm (Figure 3a). After 1 h 24 min, the “leaf” structure became more pronounced as indicated by much clearer edges and steps (Figure 3b). Thereafter, there was almost no change in shape except that the top layer became much rougher after 7 h 14 min, as indicated in Figure 3d. Figure 4 displays another representative set of STM images of the growth of a bigger island (about 100 nm) under potential control of 0.6 V. After the standard electrochemical cleaning, the island region resembles a “leaf skeleton” (Figure 4a). The whole “leaf” shape was rapidly fleshed out, as evidenced by the STM image after 32 min (Figure 4b). Thenceforth, the “leaf” grew in the vertical direction and eventually formed a moundlike structure; see Figure 4c,d. From these and other experiments, it has been found that the polycrystalline Pt island growth is shape- and size-limited under double-layer potential control. Once the geometric size reaches about 100 nm, its growth stabilizes in the horizontal directions but continues to grow slowly in the vertical direction, leading

to “fatter” structures. It will be very interesting to determine if future theoretical or simulation work can be used to predict the upper limit of island coarsening in addition to predicting the critical island size according to the Gibbs-Thomson formula.34 Unfortunately, most theoretical work has been focused on thin film growth, where no electrochemical force was applied.30,35 3. Coarsening and Decay of Pt Islands. So far, it has been demonstrated that the polycrystalline Pt islands grow via potentialinduced surface diffusion of Pt atoms, and further growth is limited once the island reaches about 100 nm in size. A question thus arises about the stability of the Pt island under double-layer potential control. To answer this question, a group of Pt islands was chosen, and the morphological changes of these Pt islands were followed by in situ electrochemical STM while under doublelayer potential control. Figure 5 displays a typical set of in situ STM images of competitive growth of a group of “mound”-shaped Pt islands marked by A, B, C, D, and E under potential control at 0.4 V. At the beginning of the surface reconstruction, island A was as large as islands B, C, and D, while island E was relatively smaller (Figure 5a). After 1 h 5 min, islands A and D became smaller, whereas islands B and E grew larger (Figure 5b). After an additional 30 min, island D joined the group of growing islands, while island A continued to shrink (Figure 5c). After about 2 h, island A had almost entirely vanished (Figure 5d). It is clear that, (34) Krishnamachari, B.; McLean, J.; Cooper, B.; Sethna, J. Phys. ReV. B 1996, 54, 8899. (35) Kodambaka, S.; Khare, S. V.; Petrov, I.; Greene, J. E. Surf. Sci. Rep. 2006, 60, 55.

Coarsening of Pt Islands at Double-Layer Potentials

Figure 6. In situ STM images of the disappearance of polycrystalline Pt islands with time under potential control at 0.6 V: (a) immediately after electrochemical cleaning; (b) 1 h 25 min later; (c) 2 h later; (d) 2 h 38 min later; (e) 3 h 28 min later than image a; and (f) high-resolution image taken on a flat terrace showing the Pt lattice and orientation. The scan sizes of (a-e) are 200 nm × 200 nm; and image size of (f) is 4 nm × 4 nm.

during this period of potential control, surface Pt atoms, even those already attached to a sizable island, can still diffuse to energetically more favorable locations. In addition, the growth following the decay of island D, for instance, was repeatedly observed for “mound”-shaped islands under double-layer potential control. For these kinds of islands, no Pt lattice was ever observed, indicating that these islands are likely assemblies of polycrystalline Pt clusters. Thermodynamically, these polycrystalline Pt clusters have much lower bonding energy between them than single crystals, resulting in unstable “mound”-shaped structures under electrochemical forces. Figure 6 shows another representative set of in situ STM images of the coarsening/decay of polycrystalline Pt islands under potential control at 0.6 V. Unlike the “mound”-shaped islands shown in Figure 5, this group of islands are more lamella-shaped. Figure 6a is the first STM image after in situ electrochemical cleaning, where a group of islands marked by A, B, C, and D are clearly visible. After 1 h 25 min, the outlines of these islands are getting much sharper (Figure 6b). Thereafter, islands C and D have started to decay (see Figure 6c,d) and have completely vanished after 3 h 28 min (Figure 6e). Unlike Figure 5, the decay of islands C and D in this case may result from the coverage of tiny Pt clusters. This is evidenced by a close examination of edges of island B. Figure 6f is a high-resolution STM image taken from a flat terrace on island A. Similar to the surface in

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Figure 1f, it shows a hexagonal (111) Pt facet with an interatomic distance of 0.277 nm. The polycrystalline Pt island growth is apparently a variant of Ostwald ripening, where surface Pt adatoms or Pt adatoms from unstable islands diffuse to energetically more favorable locations under electrochemical potentialsforming big islands. This process may be described by the Gibbs-Thomson effect,35,36 where smaller islands (with higher curvatures) have higher adatom concentrations than larger islands. These adatoms diffuse on the film surface to large islands. It is simply a potential-induced curvature-driven mass transport involving desorption of the diffusing atoms from a parent island, diffusion across the terrace, and attachment at the neighboring step edges. At present, we are not aware of what causes the “mound-shaped” Pt islands in opposition to the highly oriented lamella-shaped Pt islands, at which ordered Pt atoms can be imaged. It is also not clear what kind of interactions build these interesting 2D and 3D surface morphological patterns. Theoretical studies are necessary to explore the mechanisms involved. 4. Potential Influence of Surface Pt Coarsening on Fuel Cell Operation. As described in the Introduction, high-surfacearea carbon-supported Pt nanoparticles are state-of-the-art catalysts for oxygen electroreduction in PEMFC. Their chemical stability under thermodynamic and electrochemical forces is a serious concern. In fact, Pt particles coarsening on electrodes and precipitating inside membranes are well-known. Although Pt alloys exhibit superior catalytic activities over pure Pt,37,38 their chemical stabilities are not yet well studied. Recently, the corrosion of the carbon support has attracted more attention. The causes of carbon gasification are fuel starvation and start/stop cycling. The latter lifts the open circuit potential well above 1.2 V.39 Since fuel-cell-powered vehicles cannot avoid start/stop cycling, there is a likelihood that unsupported Pt or Pt alloy nanoparticles (Pt black or Pt alloy black) will eventually be used. In that case, this experiment may well mimic the electrochemical potential influence on catalyst coarsening. Under the assumption that the present experiments mimic fuel cell catalyst behavior under potential polarization, our results indicate that operating fuel cells in the double-layer-potential range (0.4-0.6 V) may cause severe Pt particle coarsening, though low voltages (e.g., 0.6 V) may deliver much higher power density. Our ongoing experiments show that there may be a way to passivate Pt island coarsening. Results such as this remain to be confirmed and will be reported in an upcoming paper.

Conclusion The coarsening of surface Pt islands in perchloric acid while under electrochemical double-layer potential polarization was investigated by in situ electrochemical STM. Potential-induced surface Pt island growth is shape- and size-limited and is realized by surface Pt adatom diffusion and not through coalescence of much smaller islands. It has also been observed that the growth of a Pt island could be accompanied by the disappearance of a nearby islandsindicating that surface Pt adatoms prefer energetically favorable locations, not edges or defects. These surface island reconstruction processes can be explained by the wellestablished Gibbs-Thomson theory as a potential-induced Ostwald ripening. Acknowledgment. The authors acknowledge Eric Kreidler for preparing the platinum films. LA700415W (36) Pelce, P. Dynamics of CurVed Fronts; Academic: San Diego, 1988. (37) He, T.; Kreidler, E.; Xiong, L.; Luo, J.; Zhong, C. J. J. Electrochem. Soc. 2006, 153, A1637. (38) He, T.; Kreidler, E.; Xiong, L.; Ding, E. J. Power Sources 2007, 165, 87. (39) Meyers, J. P.; Darling, R. M. J. Electrochem. Soc. 2006, 153, A1432.