Os Bimetallic Surfaces: Spontaneous

Publication Date (Web): September 14, 2005 ... We also investigated the incremental dissolution of the Os from Au(111), to better understand electroch...
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In Situ STM Study of Au(111)/Os Bimetallic Surfaces: Spontaneous Deposition and Electrochemical Dissolution Christina M. Johnston,† Svetlana Strbac,‡ and Andrzej Wieckowski*,† Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, and ICTM-Institute of Electrochemistry, University of Belgrade, 11001 Belgrade, Serbia and Montenegro Received March 17, 2005. In Final Form: July 20, 2005 We provide an electrochemical and structural characterization by in situ STM of Au(111)/Os electrodes prepared by spontaneous deposition of Os on Au(111). Surfaces with Os coverage values up to the saturation coverage were examined, from 10%. Using comparisons to previous work on Au(111)/Ru, Pt(111)/Ru, and Pt(111)/Os, we find that we may now generalize that Os deposits spontaneously faster than Ru and has a greater tendency to form 3-D structures. Additionally, the Au(111) substrate shows preferential step and near-step decoration in both cases, although it is less pronounced for Os than Ru. We also investigated the incremental dissolution of the Os from Au(111), to better understand electrochemical dissolution processes in general and to better control the Os deposit structure. The application of controlled electrochemical treatments (cyclic voltammetry up to increasingly positive values) significantly increased the dispersion of the Os deposit by generating smaller, more widely spaced islands. Upon voltammetry up to 0.75 V, the Au(111)/Os surface showed evidence of alloying and the formation of 3-D structures suggestive of strong Os-Os (oxidized) species interactions. The CO stripping results show the Au(111)/Os is not particularly effective for this reaction, but such results help to complete the overall picture of NM-NM catalytic combinations. Although the Au(111)/Os system itself is not catalytically active, the electrochemical manipulation of the deposit structure demonstrated here may be applied to other noble metal/noble metal (NM/NM) catalytic substrates to find optimal deposit morphologies.

1. Introduction Noble metal single crystal surfaces covered by transition metal additives by the method of spontaneous deposition are a new class of electrode materials that deserve an advanced focus and research scrutiny. As frequently reported from this laboratory,1-5 the spontaneous deposition presents a very convenient method to obtain bimetallic surfaces, predominantly for modeling of fuel cell anode catalysts.6 The process generates admetal islands on the substrate surface with a simple means to control the deposit coveragesby changing the exposure time to the deposition bath (up to the saturation coverage) or by repeating the deposition several times. Studying the deposition provides new information on the ultrathin film growth process under electroless electrochemical conditions. Further, the deposition process is initiated by a chemisorption of oxide (hydroxide) adlayer precursor(s) and the metallic surfaces are obtained by a reduction of such oxidized deposits to the metallic forms. Since even the precursor films are of molecular oxide dimensions, the reductive transformation of the oxides to the metallic * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemistry, University of Illinois at UrbanaChampaign. ‡ ICTM-Institute of Electrochemistry, University of Belgrade. (1) Chrzanowski, W.; Kim, H.; Wieckowski, A. Catal. Lett. 1998, 50, 69-75. (2) Chrzanowski, W.; Wieckowski, A. Langmuir 1997, 13, 59745978. (3) Kim, H.; Rabelo de Moraes, I.; Tremiliosi-Filho, G.; Haasch, R.; Wieckowski, A. Surf. Sci. 2001, 474, L203-L212. (4) Crown, A.; Johnston, C. M.; Wieckowski, A. Surf. Sci. 2002, 506, L268-L274. (5) Crown, A.; Kim, H.; Lu, G. Q.; de Moraes, I. R.; Rice, C.; Wieckowski, A. J. New Mater. Electrochem. Syst. 2000, 3, 275-284. (6) Catalysis and Electrocatalysis at Nanoparticle Surfaces; Wieckowski, A., Savinova, E. R., Vayenas, C. G., Eds.; Marcel Dekker: New York, 2003; pp 1-970.

films occurs in a uniquely “thin” reaction confinement, leading to some surface structures that cannot be obtained by other methods. For instance, repeating the spontaneous deposition process generates many 3-D island structures,4 whereas prolonging the electrodeposition results only in further 2-D growth.7 The in situ scanning tunneling microscopy (STM) and electrochemistry project concerning the Os deposits on Au(111) surfaces is a continuation of the previous focus but with a different bent. Again, the spontaneous deposition surface reaction is used, and all phenomena related to the deposition process are of a very keen fundamental interest to us. However, as we have realized early on, we are not modeling an oxidation catalyst as Au(111)/Os is not active toward methanol dissolved in electrolytic media. However, the Au(111)/Os is active to chemisorbed CO, what was investigated and will be reported below. (Surface CO is an archetypical representative of small organic molecules of interest to fuel cell science.) Notably though, studies of osmium deposits on the inert gold surface give us a unique opportunity to look not only at the spontaneous deposition reaction, but also at the thin film noble metal electrodissolution processes, with the osmium as a case example. Therefore, in this paper, we first control the nanoisland deposits under stable conditions (and, coincidently, eliminate the Au(111) reconstruction as a factor in our observations) and then react/modify these deposits under the electrode potential cycling conditions. Here, we believe that our data are of some relevance to metal dissolution processes in aqueous corrosion environments, but certainly more results of the type we present here are needed to fully document the relevance of this research to more applied electrodissolution fields. This current project should also be viewed in the context of our much more extended work on Ru deposition on both (7) Friedrich, K. A.; Geyzers, K. P.; Stimming, U.; Vogel, R. Z. Phys. Chem. (Munich) 1999, 208, 137-150.

10.1021/la050720p CCC: $30.25 © 2005 American Chemical Society Published on Web 09/14/2005

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Pt and Au substrates. The data show that the Ru island growth pattern is different for deposition on Au(111) versus Pt(111). For Au(111), Ru is preferentially deposited on steps,8,9 whereas on Pt(111) the Ru deposits form homogeneously across the surface.9 Os is also deposited homogeneously on Pt(111), but at a much higher rate than Ru.9,10 The results for Au(111)/Os are discussed below in light of these prior findings to better understand the morphologies obtained by spontaneous deposition and changes in the structure of the Os deposit on Au(111) induced by electrochemical processing, resulting in an increased dispersion of the Os islands. The control of the adatom dispersion by electrochemical treatment is, we believe, of general interest to catalysis, and the observations made here can be expanded readily to more relevant electrocatalytic systems. 2. Experimental Section An Au(111) single crystal, 6 mm in diameter (Metal Crystals and Oxides, Cambridge, England), cut and oriented to better than 0.5°, was used as substrate for the electrochemical measurements reported below. A 10 mm diameter Au(111) single crystal (MaTeck, Julich, Germany), cut and oriented to better than 0.1° was used for the in situ STM imaging. After mechanical and electrochemical polishing, the Au crystal was annealed in a butane flame for several minutes, cooled in air, and either mounted into the electrochemical cell of the STM or in the external electrochemical cell for cyclic voltammetry (CV) characterization. Electrochemical measurements were performed using an EcoChemie Autolab PSTAT100. All potentials in this paper are reported vs Ag/AgCl/3M NaCl. Solutions were prepared using OsCl3.×H2O (Alfa Aesar) salt, double distilled H2SO4 (GFS chemicals), and Milli-Q water. The spontaneous deposition of osmium on Au(111) electrode was performed at an open circuit potential (OCP) for the stated deposition time from an aged (over two weeks), aerated 0.1 mM OsCl3 + 0.1 M H2SO4 electrolyte. The solution is aged because the Os deposition rate increases until it reaches a steady-state after about two weeks. A single deposition was performed for a chosen deposition time from the Os- containing solution (0.01, 0.1, or 1 mM OsCl3 + 0.1 M H2SO4) at the open circuit potential, after which the solution was exchanged for 0.1 M H2SO4. Nine potential cycles from -0.2 to 0.5 V at 50 mV/s were then used to stabilize and reduce the Os deposit. Scanning tunneling microscopy measurements were performed in situ using a Molecular Imaging (MI) PicoSTM. The surface potential during imaging was -0.15 V in order to ensure that the Os deposit was reduced to the metallic phase.11 Apiezon wax coated Pt-Ir tips were used. STM images were obtained in a constant current mode. For all the images, the tip bias was held within the narrow range of 0.4-0.5 V, and the tunneling current was held between 2 and 3 nA, to avoid imaging artifacts. Data analysis was achieved using the Visual SPM software from Molecular Imaging with some additional scripts. Statistical values were calculated from several images from several places on the surface and from several (at least 3) different experiments. The images were flattened using global plane-fitting with a 1-degree polynomial and a linewise LMS correction of 1-degree. To determine coverage values, all parts of the image below a certain threshold value (0.23 nm ) Os height) were subtracted after plane-fitting and background subtraction. This procedure was compared to contouring, and the agreement was good. The island widths and island heights were determined by profile analysis. The island widths and heights were in good agreement with those determined by grain analysis using local minima to

The Au(111) surface was characterized by cyclic voltammetry in 0.1 M H2SO4 and by STM at OCP (Figure 1). As well-known,12-14 the peak at 0.3 V (Figure 1A) is due to lifting Au(111) reconstruction via sulfate adsorption, and the peak at ca. 0.80 V is associated with the formation of an ordered sulfate adlattice.15 Large terraces, 100-200 nm wide containing no additional features, indicate the proper orientation as well as the cleanliness of the initial surface. The clean Au(111) electrode was exposed to the Oscontaining solution, (0.01, 0.1, or 1 mM) OsCl3 + 0.1 M

(8) Strbac, S.; Behm, R. J.; Crown, A.; Wieckowski, A. Surf. Sci. 2002, 517, 207-218. (9) Strbac, S.; Johnston, C. M.; Lu, G.-Q.; Crown, A.; Wieckowski, A. Surf. Sci. 2004, 573, 80. (10) Johnston, C. M.; Strbac, S.; You, H.; Sibert, E.; Lewera, A.; Zhou, W.; Wieckowski, A. Langmuir, to be submitted for publication 2005. (11) Rhee, C. K.; Wakisaka, M.; Tolmachev, Y.; Johnston, C. M.; Haasch, R.; Attenkofer, K.; Lu, G.-Q.; You, H.; Wieckowski, A. J. Electroanal. Chem. 2003, 554, 367.

(12) Scherson, D. A.; Kolb, D. M. J. Electroanal. Chem. Interfacial Electrochem. 1984, 176, 353-357. (13) Dakkouri, A. S.; Kolb, D. M. In Interfacial Electrochemistry: Experimental, Theory and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; pp 843-883. (14) Strbac, S.; Adzic, R. R.; Hamelin, A. J. Electroanal. Chem. Interfacial Electrochem. 1988, 249, 291-310. (15) Magnussen, O. M.; Hageboeck, J.; Hotlos, J.; Behm, R. J. Faraday Discussions 1992, 94, 329-338.

Figure 1. (A) Cyclic voltammogram of the clean Au(111) surface recorded in 0.1 M H2SO4 (sweep rate: 50 mV/s). (B) STM image (300 × 300) nm2 of the initial Au(111) surface recorded in 0.1 M H2SO4 at the open circuit potential. Imaging conditions: it ) 2.0 nA, Ebias ) 0.40 V. separate the islands (see Figure 4). The Os islands are complex and composed of different heights. The tallest part (excluding noise) is considered the island height in the counting schemes used in Figure 4. Given this complexity, we also report the height distribution of the Os in terms of the total deposit area in the text.

3. Results and Discussion

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Figure 2. (A) Cyclic voltammograms recorded in 0.1 M H2SO4 of the Au(111) surface after Os was spontaneously deposited from 1 mM OsCl3 + 0.1 M H2SO4 (1 mM Os solution) for 5 min (called ‘Au(111)/Os-5min/1mM’); nine cycles are shown from -0.2 V to 0.5 V; (B) steady-state CVs obtained after nine cycles from -0.2 V to 0.5 V for Au(111)/Os-1 min/0.01 mM (black line), Au(111)/Os-1 min/0.1 mM (red line), Au(111)/Os-1 min/1 mM (green line), Au(111)/Os-5 min/1 mM (blue line), and Au(111)/ Os-30 min/1 mM (purple line), sweep rate ) 50 mV/s.

H2SO4, for the chosen deposition time. Afterward, the electrode was rinsed very well, and the electrolyte was exchanged for Os-free 0.1 M H2SO4. The electrode potential was then cycled nine times from -0.20 V to 0.50 V to reduce the adsorbed Os species of high valency to stable, metallic Os islands.11 Representative CVs are presented in Figure 2A for the particular case of 5 min deposition from 1 mM OsCl3 + 0.1 M H2SO4 (referred to as “Au(111)/Os-5min/1mM”). In Figure 2B, steady-state CVs for a range of deposition times and Os-containing solution concentrations are presented: Au(111)/Os-1min/0.01mM (black line), Au(111)/Os-1min/0.1mM (red line), Au(111)/ Os-5min/1mM (blue line), and Au(111)/Os-30min/1mM (purple line). The same surfaces are explored below by STM. The CVs show a clear increase in the area of the peaks at -40 mV and the double layer thickness as the amount of Os deposit increases. The peaks at ca. -40 mV resemble the peaks at -50 mV/-115 mV obtained for the reduction/oxidation of Os precursors on Pt(111).11 Comparing the CVs in Figure 2B with the CV of the initial Au(111) electrode (Figure 1a), the peak at ca. 0.3 V is suppressed with the added Os. This shows that the electrochemically induced surface reconstruction is inhibited by the presence of the deposited Os.8,13 STM images of the Au(111)/Os bimetallic surfaces were recorded after preparing the Au(111)/Os electrodes in the STM electrochemical cell (Figure 3). After the CVs from -0.2 V to 0.5 V (Figure 2), the electrode potential was held at -0.15 V in order to keep the surface Os metallic.11 The surface reconstruction of Au(111) was not observed

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Figure 3. STM images, (80 × 80) nm2, of Os modified Au(111) recorded at -0.15 V in 0.1 M H2SO4: (A) after Os was spontaneously deposited from 0.01 mM OsCl3 + 0.1 M H2SO4 for 1 min (represented as ‘Au(111)/Os-1 min/0.01 mM’), coverage ) (9 ( 2)%; it ) 3.0 nA, Eb ) 0.4 V; B) Au(111)/Os-1 min/0.1 mM, coverage ) (17 ( 3)%; it ) 2.0 nA, Eb ) 0.4 V; (C) Au(111)/Os-5 min/1 mM, coverage ) (26 ( 5)%; it ) 2.0 nA, Eb ) 0.5 V; D) Au(111)/Os-30 min/1 mM, coverage ) (60 ( 7)%; it ) 2.0 nA, Eb ) 0.5 V. For all images: Esurf ) -0.15 V.

after the Os deposition under our conditions, as can be seen in the images in Figure 3. The STM images show that Os coverage increases as the concentration of the Os deposition solution increases, and also as the deposition time increases. The coverage of Au(111)/Os-1 min/0.01 mM (Figure 3A) is (9 ( 2)%, compared to (17 ( 3)% for Au(111)/Os-1 min/0.1 mM (Figure 3B). A slight step preference, indicative of heterogeneous nucleation, is observed at these lower Os coverages (Figure 3A,B). For Au(111)/Os-5 min/1 mM (Figure 3C), the coverage increases to (26 ( 4)%, and no particular step decoration is observed. The deposition is faster and homogeneous from the more concentrated (1 mM) Os-containing solution. The coverage is much higher, (60 ( 7)%, for a prolonged deposition of 30 min (Au(111)/ Os-30 min/1 mM), Figure 4D. The same coverage was obtained for a 1 h deposition (not presented), demonstrating that (60 ( 7)% is the saturation coverage of the electrode. The deposited Os islands were not uniform in size, nor were they well-defined. Before analyzing the island size, islands that were close together were separated (Figure 4A,B) by using local minima to divide them. The outer green lines at the left of Figure 4A show the result of the grain analysis without splitting the islands at local minima. In contrast, the blue lines in Figure 4B show the result if local minima are used. As the Os solution concentration and the deposition time are increased, the widths and heights of the Os islands increase. The island width distribution corresponding to the STM images presented in Figure 3 is presented in Figure 4C. The range of island widths increases from 1 to 3 nm (average: 1.5 nm) for Au(111)/Os-1 min/0.01 mM, up to 2-6 nm (average 3.5 nm) for Au(111)/Os-30 min/1 mM. The island height distributions corresponding to the same STM images from Figure 3 are presented in Figure

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Figure 5. CO + Os and Os-only stripping curves for Au(111)/ Os obtained by 5 min spontaneous deposition from 1 mM Os solution. The simultaneous removal of CO and Os by oxidation is shown as a red line. The oxidative removal of only Os from a CO-free surface is shown as a black line. The difference between the curves between 0.0 and 0.7 V is approximately the CO stripping charge. CO dosing was performed for 10 min with the electrode potential held at -0.15 V vs Ag/AgCl/3 M NaCl, and the solution was purged for 5 min with Ar (at -0.15 V) before the CO stripping was performed.

Figure 4. (A and B) STM images (100 × 100) nm2 of an Osmodified Au(111) surface (Au(111)/Os-5 min/1 mM) showing how groups of Os islands are separated into individual islands before determining the island widths and the island heights. The green lines in (A) show the result of straightforward grain analysis, and the blue lines in (B) show the result if local minima are used to split the islands apart. The blue lines represent the definition of islands in this report. (C) The island width distributions of the Au(111)/Os surfaces for the conditions corresponding to Figure 3(A-D); (D) the corresponding island height distributions.

4C, using the highest part (excluding noise) of the island to define the height. The expected height of an Os atom is 0.23 nm in a 3-fold hollow Au site, close to the Au step height of 0.25 nm. The Os islands exhibited a complex structure that reasonably matched the expected height intervals, as shown in the cross sections in Figures 6 and 8. The surface of lowest Os-coverage (Au(111)/Os-1 min/ 0.01 mM) shows predominantly monolayer high islands (about 80%), but about 50% of the islands on the other three surfaces have multiple layers. The three latter surfaces show similar distributions with respect to the height of the total Os deposit area: 70-75% of the total Os area has one Os layer, 20-25% has two Os layers, and >5% has 3-5 layers. Once a certain coverage of high valency Os precursors from the Os solution is reached,11 the electroreductive formation of Os islands results in a relatively constant distribution of island surface area among the different height levels. To better understand spontaneous deposition in general, it is worthwhile to compare these results to previous data. Compared to Ru islands on Au(111) deposited from 1 mM RuCl3/0.5 M H2SO4,8 the coverage is higher and more of the Os islands have multiple layers, indicating a faster Os deposition. Namely, only ∼15% of the Ru deposit on

Au(111) had two layers height, compared to 25-30% 3-D growth for Os on Au(111) in Figure 3(B,D). The Os deposition on Au(111) also shows a much higher saturation coverage than Au(111)/Ru: 60% vs 20%. The Os islands are very irregular compared to the flat, hexagonal shape of the Ru islands. (More resolved STM images showing the Os island structure in more detail are given below.) Additionally, the Au(111)/Ru substrate showed a much greater degree of preferential step decoration. The Ru species may have more mobility than the Os species to move toward and nucleate near favorable step sites, or the step sites are much more energetically favored overall for the Ru species. Taking previous data together with the Au(111)/Os data, one concludes that the Os deposition is generally faster than Ru deposition, and that the Au(111) substrate shows preferential step decoration as opposed to the Pt(111) substrate, which does not.4,8,9,16 CO stripping measurements on Au(111)/Os-5 min/1 mM did not show a distinct CO stripping peak, but rather a broad stripping region from 0.05 to 0.70 V as shown in Figure 5. The CO stripping peak overlaps the Os stripping peak to some extent, so the difference between the CO + Os stripping curve (red line) and the Os-only stripping curve (black line) was taken as the CO charge. The CO stripping charge of ca. 70 µC cm-2 (versus whole electrode area) corresponds to about 50-60% CO coverage of the Os islands, which cover (26 ( 5)% of the Au(111) substrate. (This is a somewhat lower CO coverage than that of CO/ Pt(111), which is 68%.17) The CO stripping on Au(111)/Ru initiates at a similar potential and is also very broad, continuing up to 0.9 V, making a meaningful comparison difficult.9 Although the onset potential of CO stripping is similar to Pt(111)/Os,9,10 complete CO removal requires sweeping up to 0.7 V compared to only up to 0.4 V for Pt(111)/Os. Thus, the Os deposit oxidizes CO much more effectively on a Pt substrate. According to the model of Hammer et al.,18 the expanded Os lattice should have a (16) Crown, A.; Moraes, I. R.; Wieckowski, A. J. Electroanal. Chem. 2001, 500, 333-343. (17) Gomez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J. Surf. Sci. 1998, 410, 48-61. (18) Hammer, B.; Morikawa, Y.; Noerskov, J. K. Phys. Rev. Lett. 1996, 76, 2141-2144.

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Figure 6. Os stripping from Au(111)/Os surface obtained by Os spontaneously deposited for 5 min from 1 mM Os solution. A gradual Os dissolution takes place by the opening of the positive potential limit to (red) 0.5 V; (blue) 0.6 V; (green) 0.7 V; (yellow) 0.75 V; (aqua) 0.9 V; and (purple) 1.0 V. The CV of the clean Au(111) is shown by a black line.

shifted d-band center that gives stronger Os-CO bond, in addition to the potential interlayer bonding effects. Other electronic modifications to the CO bond strength19 at either Pt or Os sites, a weakened Os-OH bond on Pt(111)/Os,19,20 or a more open CO adlattice structure21 on Pt(111)/Os versus Au(111)/Os could also explain the differences. As for methanol reactivity, the Os islands only enhanced the methanol electrooxidation rate very slightly (at 30 min g 0.2 µA cm-2) versus the bare Au(111) substrate at 0.50 V versus RHE (not shown). This result can be applied to the data for Pt(111)/Os10 that show that highly covered surfaces (65-75%) give the highest methanol oxidation rates. On the basis of the results for Au(111)/Os, it is shown that the bifunctional mechanism22,23 requiring Pt sites still applies for Pt(111)/Os, even though the number of Pt sites is reduced versus most optimized Pt/Ru catalytic surfaces.1,22,24,25 Cyclic voltammograms of Au(111)/Os-5 min/1 mM are presented in Figure 6, showing Os oxidation/reduction and subsequent gradual dissolution by the opening of the positive potential limit. The potential window was extended positively by intervals of 100 mV, and three cycles up to each new potential were performed. The third cycle of each set is shown. The oxidation and dissolution of Os are evidenced by the rise in current as the potential is swept positive of 0.5 V, and the decrease in the size of the oxidation/reduction peak couple at -40 mV. A small peak at 0.5 V on the negative sweep indicates that the remaining, highly oxidized Os is reduced again. Os is being removed with each set of cycles, and it is almost completely removed by sweeping to 0.9 V. The original Au(111) CV, shown by a black line, is nearly restored. STM images showing the structure of Au(111)/Os-5 min/1 mM after the opening of the potential limit to various (19) Babu, P. K.; Kim, H. S.; Chung, J. H.; Oldfield, E.; Wieckowski, A. J. Phys. Chem. B 2004, 108, 20228-20232. (20) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617-625. (21) Friedrich, K. A.; Geyzers, K. P.; Linke, U.; Stimming, U.; Stumper, J. J. Electroanal. Chem. 1996, 402, 123-128. (22) Hamnett, A. In Interfacial Electrochemistry: Experimental, Theory and Applications; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; pp 843-883. (23) Spendelow, J. S.; Wieckowski, A. Phys. Chem. Chem. Phys. 2004, 6, 5094. (24) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020-12029. (25) Chrzanowski, W.; Wieckowski, A. Langmuir 1998, 14, 19671970.

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potentials are presented in Figures 7-9. The images were taken at -0.15 V after three potential cycles from -0.2 V to the chosen positive potential limit and back. Cycling up to -0.5 V can be repeated without changing the island morphology, as shown by Figure 7A, which as expected appears the same as Figure 3C. After cycling up to 0.6 V (imaged at -0.15 V), Figure 7C shows the appearance of several 1-1.5 nm islands that are dispersed between the main island groupings. For example, many (5 × 5) nm2 areas of bare substrate can be seen in Figure 7A, but after cycling to 0.6 V (Figure 7C) the entire surface is peppered with small islands. The deposit has reorganized during the Os oxidation/reduction and dissolution/redeposition processes shown in the CV in Figure 6. The surface coverage remains similar at (28 ( 4)%, due to the simultaneous, compensating effects of Os dissolution and the formation of the smaller, 1 layer high islands. The large island groupings retain their multilayer height but lose some of their width, decreasing in average area by about 1 nm2. The osmium species are apparently removed from the outer edges of the island groupings, and then renucleate as small islands on the bare areas of the substrate. Perhaps the 3-D Os are more stable to oxidation due to the perhaps stronger Os-Os bond versus the AuOs bond. Os has a higher surface energy than Au (1.81 eV/atom versus 0.72 eV/atom),26 so the Os-Os bond is favored. Also, the 3-D growth on the islands may be preserved by the Ehrlich-Schwoebel barrier for the Os species to cross the Os step down to the Au surface, which should be relatively high at these rather positive potentials, based on analogy to Ag on Au(111).27-29 The pzc of Os is not well established, but the work function of Os is similar to Ag(111) (4.8 eV vs 4.74 eV), so the pzc and the surface charge dependence on potential (hence the ES barrier height) should also be similar.30 When the positive potential limit is increased to higher values, more significant changes to Os deposit structure take place. The STM images (100 × 100) nm2 in Figure 8 show the Au(111)/Os-5 min/1 mM surface after sweeping the potential to 0.7 V (Figure 8A) and 0.75 V (Figure 8B), respectively. After sweeping to 0.7 V (Figure 8A), the island coverage of the Au(111) surface continues to remain about the same at (26 ( 4)% due to the simultaneous Os dissolution and the smaller island formation from Os reorganization/redeposition. However, the fraction of smaller (1 to 1.5 nm wide, 1 layer high) Os islands increases by about 15-20% compared to the original Au(111)/Os-5 min/1 mM surface, resulting in 80-90% of the Os deposit now having 1 layer height. About 15-20% of the Os deposit has been removed from the surface as a reduction of 3-D height, but the islands are not simply dissolving away in place. The distribution across the surface of the islands is increasingly different from the original Au(111)/Os-5 min/1 mM surface. Note that in Figure 4A (image is the same size), there are many areas as large as (10 × 10) nm2 that are island-free. There are fewer and smaller islandfree areas in Figure 8A due to formation of new Os islands. The island dispersion has clearly increased due to the electrochemical processing, which is due to the increased adspecies mobility and/or dissolution/redeposition processes. At the higher potentials used, the Os species from the edges and the taller centers have been mobilized to (26) Ruban, A. V.; Skriver, H. L.; Norskov, J. K. Phys. Rev. B: Condens. Matter 1999, 59, 15990-16000. (27) Haftel, M. I.; Rosen, M. Phys. Rev. B 2003, 68. (28) Corcoran, S. G.; Chakarova, G. S.; Sieradzki, K. Phys. Rev. Lett. 1993, 71, 1585-1588. (29) He, Y.; Borguet, E. Faraday Discuss. 2002, 121, 17-25. (30) Hamm, U. W.; Kramer, D.; Zhai, R. S.; Kolb, D. M. J. Electroanal. Chem. 1996, 414, 85-89.

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Figure 7. STM images of the Au(111)/Os surface obtained by the deposition of Os from 1 mM OsCl3 + 0.1 M H2SO4 for 5 min after the potential was cycled: (A) from -0.2 V to 0.5 V (40 × 40) nm2; imaged at -0.15 V; (B) profile showing the multilayer heights of the islands; (C) from -0.2 V to 0.6 V (40 × 40) nm2; imaged at -0.15 V; (D) profile showing the appearance of many smaller (1-1.5 nm), 1 layer high islands. For both images: it ) 2.0 nA, Eb ) 0.50 V.

diffuse across the surface and renucleate as small 1-1.5 nm, 1 layer high islands. The formation of smaller, more isolated islands compared to the initial islands indicates a different balance of forces determining the final ideal size of the islands (e.g., stronger adsorbate-substrate interaction, less adsorbate aggregation, or an initially higher diffusion rate quenched by the potential sweeping). This can be true because the Os species generated by electrooxidation are likely not the same as the original Os species from the spontaneous deposition. This assumption is based on previous experience with Ru species, which are less strongly bound to the surface and can be partly displaced by CO immediately after spontaneous deposition.9 However, because of the complex conditions of island formation involving a mixture of unknown species of different oxidation states with different degrees of mobility (perhaps no mobility for some), a specific mechanism is difficult to postulate. After cycling the potential further to 0.75 V (Figure 8B) (imaging afterward at -0.15 V), the overall Os coverage remains about the same at (28 ( 4)%, but the island morphology continues to change. Although most of the islands are still 1-1.5 nm wide and 1 layer high, there are now several large 8-20 nm wide islands (compare Figure 8B to Figure 4). The 8-20 nm islands have a smooth base that is 1 layer high, on top of which there are structures that are 2-4 layers high. The tallest spots (3-4 layers) only account for about 1-3% of the surface, whereas 8090% of the total island area remains only 1 layer high. A smaller scale STM image and a corresponding cross section

analysis are presented in Figure 9(C,D). A possible explanation for the formation of the smooth domains is that some highly mobile Os species created by the more extreme oxidation process form larger Os islands at the expense of smaller Os islands by an Ostwald ripening mechanism.31,32 However, the coverage of the surface by islands is higher than expected given the extent of dissolution indicated by the CVs in Figure 6. It is necessary to speculate that Au adatoms have detached from the steps and participated in the formation of new islands. The mobility of Au atoms should increase with potential above the pzc based on previous work with Au(100),32,33 and if there is significant detachment of Au atoms from the steps, then capture by Os islands on the terraces is conceivable. The first layer of the large islands is morphologically smoother than the Os islands observed previously, so it is possible that the features are composed of Au atoms rather than Os atoms, or a mixture thereof. Since the Os islands are morphologically rough (see smaller-scale images below), the precise height of Os was not determined well enough to distinguish Os from Au based only on the apparent height. Indeed, alloy formation with Au(111) has been observed previously after the electrodeposition of Pb,34,35 Cd,36-42 and Sb43-46 on Au(111). In UHV, the (31) Venables, J. A. Introduction to Surfaces and Thin Film Processes; Cambridge University Press: Cambridge, UK, 2000. (32) He, Y.; Borguet, E. J. Phys. Chem. B 2001, 105, 3981-3986. (33) Ikemiya, N.; Nishide, M.; Hara, S. Surf. Sci. Lett. 1995, 340, L965-L970. (34) Green, M. P.; Hanson, K. J. Surf. Sci. 1991, 259, L743-L749.

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STM images recorded on a smaller scale of (27 × 27) nm2 in Figure 9 enable a closer look at the structure and the distribution of islands over the Au(111) surface before and after the oxidation/reduction cycles up to 0.75 V. Before the partial dissolution of Os the islands are mostly 1-4 nm wide (see Figure 3C), and many are grouped closely together. The line profile in Figure 9 (B) shows that these Os islands are composed of even smaller parts that are about 1 nm wide. Examples of islands with 1, 2, and 3 layers are shown in the profile plot. These Os islands are completely rearranged and partly dissolved by the oxidation/reduction cycles up to 0.75 V, as shown in Figure 9(C-D). The islands are less tightly clustered together in Figure 9C, and most of them are smaller, 1-1.5 nm width. Smaller parts of about 1 nm width are again visible within the islands in Figure 9(C). These sub-structures are too large to be single Os atoms of 0.27 nm diameter, and may represent a close-packed adsorbate layer of hydroxyl or other species as suggested previously for images of electrodeposited Ru on Au(111).48 Also notable is the layered structure of the large island in the lower right corner. This is shown clearly by the line profile in Figure 9D which shows that different parts of the island have 1, 2, and 4 layers of Os. The bottom layer is postulated to be at least partly composed of Au atoms, as mentioned above. The formation of the very tall structures may result from the possible tendency of highly oxidized Os species to aggregate. If such aggregates of oxidized Os species have formed by sweeping to 0.75 V, it is possible that the mobile Au atoms attached to these aggregated Os structures to form the complex islands observed here. 4. Summary and Conclusions Figure 8. STM images, (100 × 100) nm2, showing the change in Os deposit morphology due to the opening of the positive potential limit. Potential cycled three times (sweep rate ) 50 mV/s) from -0.2 V to: (A) 0.7 V; (B) 0.75 V. For both images: Esurf ) -0.15 V, it ) 2.0 nA, Eb ) 0.50 V.

noble metal Pt was deposited on Au(111) at room temperature and formed a 0.03 ML alloy followed by the formation of mixed Pt-Au islands.47 Thus, the formation of an Au-Os alloy seems possible and it is the best explanation for the unexpectedly high coverage of islands. Unfortunately, it was not possible to get STM images after the potential was cycled to higher values to demonstrate the complete dissolution of Os species to leave only the Au structures behind, most likely due to the deposition on the tip of dissolved Os, which caused noise and decreased the tip quality. (35) Green, M. P.; Hanson, K. J.; Carr, R.; Lindau, I. J. Electrochem. Soc. 1990, 137, 3493-3498. (36) Inzelt, G.; Horanyi, G. J. Electroanal. Chem. 2000, 491, 111116. (37) Lee, D.; Rayment, T. Electrochem. Commun. 2002, 4, 832-837. (38) Kawamura, H.; Takahasi, M.; Mizuki, J. i. J. Electrochem. Soc. 2002, 149, C586-C591. (39) Lay, M. D.; Stickney, J. L. J. Am. Chem. Soc. 2003, 125, 13521355. (40) Lay, M. D.; Varazo, K.; Srisook, N.; Stickney, J. L. J. Electroanal. Chem. 2003, 554-555, 221-231. (41) Maupai, S.; Zhang, Y.; Schmuki, P. Surf. Sci. 2003, 527, L165L170. (42) Maupai, S.; Zhang, Y.; Schmuki, P. Electrochem. Solid-State Lett. 2003, 6, C63-C65. (43) del Barrio, M. C.; Garcia, S. G.; Salinas, D. R. Electrochem. Commun. 2004, 6, 762-766. (44) Wu, Q.; Shang, W.-H.; Yan, J.-W.; Mao, B.-W. J. Mol. Catal. A: Chem. 2003, 199, 49-56. (45) Jung, C.; Rhee, C. K. J. Electroanal. Chem. 2004, 566, 1-5. (46) Jung, G.; Rhee, C. K. J. Electroanal. Chem. 1997, 436, 277-280. (47) Pedersen, M. O.; Helveg, S.; Ruban, A.; Stensgaard, I.; Laegsgaard, E.; Norskov, J. K.; Besenbacher, F. Surf. Sci. 1999, 426, 395409.

In this study, Os islands were deposited spontaneously on Au(111) single-crystal surfaces, and characterized by in situ STM. Many new details of such nanosized islands were revealed. Comparing the data presented here to previous results obtained on Pt(111)/Ru, Au(111)/Ru, and Pt(111)/Os substrates, we have shown that Os spontaneously deposits faster and generates more 3-D islands than Ru in the all cases considered. The Au(111) substrate also shows a tendency for step and near-step decoration in both cases, although it is more pronounced for Au(111)/ Ru. The coverage of the Au(111) with the deposited Os islands can be increased controllably up to (60 ( 7)%, with the island size generally increasing with Os coverage. These data create a complete set of comparative data for the spontaneous deposition of two of the most important admetals in noble metal catalyst research in general.49 In a different direction, we have investigated the island structures observed after dissolving the Os incrementally. After a few potential cycles up to 0.70 V during the “electrochemical treatment” process, most of the islands become smaller in width and height and they are spaced farther apart, giving a greater dispersion of the Os deposit. Such structures must result partly from remnant islands after Os dissolution and partly from the renucleation of new islands from the mobile Os species. After cycling further up to 0.75 V, some larger, 8-20 nm islands are also observed, which may result from an Ostwald ripening process of the Os islands or the participation of Au atoms in the structures. The latter explanation seems more likely given the mobility of Au atoms and the tendency of Au to form alloys. Overall, the island morphology changes (48) Strbac, S.; Maroun, F.; Magnussen, O. M.; Behm, R. J. J. Electroanal. Chem. 2001, 500, 479-490. (49) Bond, G. C. In Chemistry of the Platinum Group Metals; Hartley, F. R., Ed.; Elsevier: New York, 1991; Vol. 11, pp 32-59.

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Figure 9. Smaller scale STM images (27 × 27) nm2 of the Au(111)/Os surface for the deposition of Os from 1mM OsCl3 + 0.1 M H2SO4 for 5 min, (A) after the electrode potential was cycled (sweep rate 50 mV/s) several times from -0.2 V to 0.5 V; (B) cross section along the line presented in the STM image; (C) after sweeping the potential incrementally up to 0.75 V (smaller-scale version of Figure 8B); (D) cross section along the line presented in the STM image. Images were taken with Esurf ) -0.15 V. For both images: it ) 2.0 nA, Eb ) 0.50 V.

dramatically due to the Os oxidation/reduction cycles in a very complex manner due to the various oxidation states of Os that are simultaneously present and the possible participation of Au adatoms. Such effects deserve further investigation to better understand electrodissolution processes in general and to lead to more effective manipulation of admetal structure. Combined with previous results for Pt(111)/Ru that also showed the ability to manipulate the island morphology with electrochemical processing,9 the possibility for using such treatments to optimize the nanoscale structure of modified electrodes is increasingly appealing. In the catalytic perspective, the CO electrooxidation performance is worse on Au(111)/Os than on Pt(111)/Os, and the Au(111)/Os surface sustains very low rates of methanol electrooxidation. The combination of Au and Os does not appear promising as a candidate for catalyzing methanol oxidation in relation to fuel cell needs for new anode materials. However, learning about the interaction of a wide variety of NM/NM combinations with small

organic molecules increases the understanding of catalysis of this entire class of materials, which overall are technologically relevant. Additionally, the manipulation of the structure of noble metal deposits on noble metal substrates by electrochemical treatment is one of general interest to catalysis, and the observations made here can be expanded readily to more relevant systems. Acknowledgment. This work was supported by the National Science Foundation grant number Grant CHE 03-4999. This material is also based upon work supported by the U.S. Department of Energy, Division of Materials Sciences under Award No. DEFG02-91ER45439, through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign. Svetlana Strbac acknowledges also support by the MSEP of Republic Serbia by the MSEP of Republic Serbia (project 2018). Christina M. Johnston acknowledges the National Science Foundation for a fellowship. LA050720P