Contrasting Electrochemical Behavior of CO, Hydrogen, and Ethanol

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Contrasting Electrochemical Behavior of CO, Hydrogen and Ethanol on Single-Layered and Multiple-Layered Pt Islands on Au Surfaces Jandee Kim, Jaesung Lee, Sechul Kim, Young-Rae Kim, and Choong Kyun Rhee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5062557 • Publication Date (Web): 30 Sep 2014 Downloaded from http://pubs.acs.org on October 5, 2014

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The Journal of Physical Chemistry

Contrasting Electrochemical Behavior of CO, Hydrogen and Ethanol on Single-Layered and Multiple-Layered Pt Islands on Au Surfaces

Jandee Kim1, Jaesung Lee1, Sechul Kim2, Young-Rae Kim2, and Choong Kyun Rhee1, 2*

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Department of Chemistry, Chungnam National University, Daejeon, 305-764, Korea

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Gruaduate School of Analytical Science and Technology, Chungnam National University,

Daejeon, 305-704, Korea

Corresponding Author *E-mail: [email protected]. Tel: 82-42-821-5483. Fax: 82-42-821-8896.

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ACS Paragon Plus Environment


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Abstract This work presents formation of single-layered Pt islands on Au electrodes using CO route and the electrochemical behavior of CO, hydrogen, and ethanol as investigated with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. Conventional route, consisting of irreversible adsorption of Pt precursor ions (10-3 M PtCl42- in 0.05 M H2SO4) and subsequent electrochemical reduction, resulted in multiple-layered Pt islands; CO route, utilizing CO adsorption to protect pre-existing Pt islands from irreversible adsorption of Pt, exclusively produced single-layered Pt islands. Furthermore, STM results implied that single-layered Pt islands on Au(111) were islands of alloyed Pt in a (√3×√3)R30° arrangement, while multiple-layered islands were stacked layers of Pt in an (1×1) array. The coverages of deposited Pt estimated from STM and XPS measurements were quantitatively consistent with each other to confirm existence of the single-layered Pt islands. Coulometric analyses of adsorbed CO and hydrogen indicated lower adsorption stoichiometry of hydrogen on Pt islands prepared by the two deposition routes, especially when the deposited amount of Pt was small. Comparison of the coulometric coverages of CO and hydrogen with electrochemically active Pt coverages estimated with STM results supported that the adsorption stoichiometries of CO and hydrogen were higher on single-layered Pt islands than on multiple-layered ones, roughly by factor of ~1.8. Also, ethanol oxidation was enhanced on single-layered Pt islands approximately ~4 times in average referring to Pt(poly), while the enhancement factor on multiple-layered ones was ~1.5. Thus, this work demonstrated that CO route exclusively produced single-layered Pt islands on Au, contrasting with multiple-layered islands in various electrochemical aspects. Keywords: Pt, Au, irreversible adsorption, CO, hydrogen, ethanol

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Introduction Recent approaches toward Pt-based electrocatalysts for fuel cells have focused on Pt films of nanometer thickness on various substrates, particularly in the form of core-shell1-4. The Pt films (or shells on nanoparticle cores) are monometallic5-10 or alloyed ones with other metals8,9,11,12, and their physical shapes are stacked layers or islands on substrate surfaces2,4. Such designs aimed to dual targets: reduction of required contents of precious Pt in catalysts and enhancement of their electrocatalytic efficiencies including long-term stabilities1,6,7. Various synthetic routes were utilized to achieve Pt films on numerous substrates or cores: segregation of Pt from Pt-containing alloys to the surfaces by heat9 or adsorbate13, dealloying of Pt-containing alloys to leave Pt at the tops of surfaces14-16, galvanic replacement of active metals (e.g., Cu and Pb) with Pt precursor ions17-32, electrochemical deposition of Pt precursor ions33-39, direct chemical reduction of Pt precursor ions with reducing agents10,40,41, and irreversible adsorption of Pt precursor ions on substrates followed by reduction42-45. The basic idea behind tailoring Pt surface layers is to manipulate proper electronic and crystallographic structures of Pt for aimed electrocatalytic reactions5-10,14. The d-band center of a Pt surface layer determines the adsorption energies of reaction intermediates via positioning anti-bonding band relatively to Fermi level, so that reaction rates are regulated by the coverages of reaction intermediates46-50. By manipulating the d-band center of a Pt surface layer with alloying with other metals or depositing on various substrates (ligand and geometrical effects51), a volcano-plot appears as a function of the d-band-center or adsorption energy of an intermediates (descriptor52) to determine the most efficient catalyst for a given reaction. For example, a density functional theory work48 demonstrated that a Pt monolayer on Pt (i.e., bulk Pt) was near the top of a volcano-plot for oxygen reduction reaction (ORR),

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and suggested that tuning the electronic property by placing the Pt monolayer over a Co monolayer would lower the binding energy of O and OH (indeed, the oxygen binding energy on a Pt skin on Pt3Co(111) became lower than that on Pt(111)53) to approach to the volcano summit. In reality, (annealed) Pt skins on Pt3M (M = Ni, Co, Fe, Ti, V) surfaces exhibited a volcano-plot for ORR against d-band center, whose maximum activity was for Pt3Co (the enhancement factor was 3 versus polycrystalline Pt.)9. Furthermore, the d-band centers of (sputtered) Pt skeleton layers on the Pt3M surfaces were found to be closer to Fermi level than those of the corresponding un-sputtered Pt skin layers, so that stronger adsorption of oxygen, oxides and anions on skeleton layers limited the availability of adsorbate-free Pt sites to reduce their ORR reactivity. On the other hand, Pt-rich shells on Pt-Cu core formed by dealloying exhibited compress strains to modify Pt d-band centers, thus ORR activities14. These examples clearly demonstrated that development of various routes producing Pt surface layers on appropriate substrates is demanding in manipulating the physical and chemical features of Pt to tailor electrocatalysts. Recently, our group demonstrated a new multiple deposition method of Pt on Au(111) using irreversible adsorption of Pt and selective adsorption of CO on Pt to form exclusively single-layered Pt islands on Au(111)54. The new method (CO route) is contrasting with conventional multiple deposition method (conventional route) producing multiple-layered Pt islands. Figure 1 describes briefly the differences between the two methods. Both routes start with an identical deposition cycle consisting of irreversible adsorption of Pt precursor ions on a Au surface and subsequent electrochemical reduction of the Pt precursor ion adlayers to Pt monatomic islands as shown at the left-hand side of Figure 1(a). Conventional route is a route simply repeating the deposition cycle several times to increase the amount of deposited Pt on

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Au surface. This particular simple multiple deposition method is named “conventional method” because it is a generally adopted way to build up a significant amount of metal spontaneously depositing or adsorbing irreversibly on another metal substrate as illustrated for the first time by Wieckowski55. In conventional route, Au surface is eventually covered with multiple-layered Pt islands because pre-existing Pt islands and bare Au sites are simultaneously exposed to Pt precursor ions in solution phase (the right hand-side of Figure 1(a)). In CO route, on the other hand, a CO adsorption step is performed before exposing Au surface covered with pre-existing Pt to Pt precursor ions, so that selectively adsorbed CO molecules only on pre-existing Pt islands protect them from further Pt deposition (the lefthand side of Figure 1(b)). Consequently, CO route ultimately covers Au surface with singlelayered Pt islands (the right-hand side of Figure 1(b)). Similar tactics using CO adsorbed on Pt as a Pt-protecting layer have been reported independently on electrochemical and chemical deposition of Pt on Au39,41. In this work, we present enhancement of ethanol oxidation on single-layered Pt islands on Au exclusively prepared using CO route, including more quantitative details. A rapid increase in the amount of Pt deposits was achieved by employing a Pt precursor solution of a higher concentration. Comparison of quantitative results derived from X-ray photoelectron spectroscopy (XPS) with electrochemical scanning tunneling microscope (STM) images at atomic level enriches structural details of the Pt deposits to understand their morphological differences depending on deposition route. Distinguishing electrochemical characteristics of single-layered Pt islands resulted from CO route (adsorption of CO and hydrogen, and ethanol oxidation) are discussed in terms of morphological differences.

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Figure 1. Schematic illustration of conventional and CO routes.

Experimental Au electrodes of two types were employed in this work: Au single crystal beads for STM works and Au polycrystalline hemispheres (Au(poly)) for XPS and electrochemical measurements. The Au single crystal beads were formed by melting a gold wire of 0.5 mm in diameter (99.999%, Aldrich) in a hydrogen-oxygen flame; the polycrystalline Au disks were produced by cutting Au polycrystalline spherical beads to Au hemispheres of ~2 mm in diameter and subsequently polishing to mirror like surfaces. In STM experiments, one of the (111) facets of a single crystal bead was positioned to face an STM tip, while in 6


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electrochemical investigations, only the polished surfaces of the hemispheres were exposed to electrolytes by keeping a meniscus position. Clean Au surfaces were routinely prepared by annealing in a hydrogen-air flame and quenching in water saturated with hydrogen. The protocols for conventional and CO routes are described as below. The first deposition cycle of the two routes was identical. In the first cycle, irreversible adsorption of Pt on Au surface was performed by immersing a Au electrode in a solution of 10–3 M H2PtCl4 (99.98%, Aldrich) in 0.05 M H2SO4 (Suprapur, Merck) for 10 min without potential control. After rinsing the electrode with water to remove any remnants of the Pt-containing solution, electrochemical reduction was performed at 0 V (versus a Ag/AgCl electrode with [Cl−]=1.0 M) in 0.05 M H2SO4 solution for 10 min. Conventional route was a simple repetition of the two operations. In CO route, the deposition cycle after the first cycle changed to a sequential operation of CO adsorption onto pre-existing Pt islands, irreversible adsorption of additional Pt precursors, and electrochemical reduction of additional Pt precursor adsorbates at 0 V for 10 min. CO adsorption was carried out by immersing the Au electrode with reduced Pt deposits into a CO−saturated 0.05 M H2SO4 solution for 3 min at 0 V. The Au electrode with adsorbed CO on Pt deposits was transferred to the Pt precursor solution through the air for additional irreversible adsorption of Pt for 10 min without potential control. Again, a rinsing process with water and an electrochemical reduction of the additional Pt precursor ions adsorbed on the Au electrode followed. For STM measurements, a Au single crystal bead after appropriate deposition cycles was assembled into an electrochemical STM cell, and the solution compartment was filled immediately with 0.05 M H2SO4 solution. The employed STM tips were etched W wires (0.25 mm in diameter, Sigma-Aldrich) coated with molten polyethylene. The set current and

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tip bias of the employed STM instrument (Nanoscope V, Bruker) were 0.5−3.5 nA and −0.35 − −0.02 V, respectively. During imaging, the potential was held at 0 V. For potential control of Au electrodes in STM and electrochemical works, a conventional three-electrode system was employed. The reference electrode was a Ag/AgCl reference electrode ([Cl−] =1.0 M), and the counter electrode was a Pt wire (99.9%, Aldrich). The reported potentials in this work are the measured ones against the reference electrode. In ethanol oxidation investigations, a solution of 2.0 M ethanol (99.5%, Sigma-Aldrich) in 0.5 M H2SO4 (96%, Merck) was used. The surface areas of Au(poly) electrodes were estimated by converting the reduction charges of surface Au oxides (formed during anodic scan up to 1.35 V at 50 mV/sec) using a conversion factor of 400 µC/cm2.56 The coverage values of CO and hydrogen in this work were obtained by dividing their stripping charges with the surface oxide reduction charge of unmodified Au. In XPS measurements, Au(poly) with Pt deposits was transferred from aqueous solutions through the air to an ultrahigh vacuum chamber equipped with an XPS instrument (MultiLab 2000, Thermo Electron Co., Waltham, MA). We employed a Mg Kα excitation source incorporated with a monochromator and a hemisphere electron energy analyzer with pass energy of 20 eV. The atomic ratios of Pt/Au were calculated using the peak areas of Pt(4f7/2) and Au(4f7/2) and their sensitivity factors (Pt(4f7/2): 8.890, and Au(4f7/2): 9.790)57. The atomic ratios were converted to coverage values by employing a standard of Pt monolayer on polycrystalline Au as prepared galvanic replacement of underpotentially deposited Cu layer with PtCl4- ions30,58,59(detailed in Supporting Information). Results and discussions 8


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1. Structural investigations: STM Figure 2 shows the STM images of Pt islands on a Au(111) surface prepared using conventional route. All the STM images along conventional route are displayed in Figure S3 in Supporting Information. After the first deposition cycle, Au(111) surface is partially covered with randomly distributed Pt islands as shown in Figure 2(a). The third deposition cycle results in a Au(111) surface almost covered with Pt islands (Figure 2(b)), and further repetition brings about a quite rough surface (Figure 2(c)). The heights of the Pt islands span from ~0.08 nm to ~0.98 nm, implying that the number of stacked Pt layers ranges from one to four. Here, the height of uncovered single-layered islands is noted to be ~0.08 nm as discussed in our previous work54 (See also below). On the other hand, the diameters of the Pt islands in Figure 2(c) (10.5±3.2 nm) are certainly larger than those (8.5±2.7 nm) in Figure 2(a), indicating widening pre-existing Pt islands as well as growing to multiple-layered islands. Figure 2(d) presents the variations in area occupied by each layer in multiple-layered Pt islands prepared using conventional route as a function of deposition cycle. The estimation procedure of the occupied areas is fully described in Supporting Information. After the first deposition, ~40% of Au(111) surface is covered with single-layered Pt islands. The second deposition leads to an increase in area occupied by Pt islands up to ~75%, and roughly a half of the islands are double-layered ones. Upon the third deposition, the populations of doubleand triple-layered islands increase significantly, while the occupied portion of Au(111) by Pt islands (i.e., the area covered by the first layer) increases slightly. Such a trend in the area change is persistent with further depositions. After the fifth exposure, ~90% of Au(111) surface is covered with Pt islands, and most of the islands are double-layered ones with considerable amounts of higher layers.

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Figure 2. Pt islands on Au(111) prepared using conventional route. STM images after deposition cycle of (a) 1, (b) 3, and (c) 5. (d) Variations in area occupied by each layer in Pt islands. The numbers in (d) correspond to stacked orders in Pt islands. Scan size: 500×500 nm.

Figure 3 represents the STM images of Pt islands on a Au(111) surface prepared using CO route. All the STM images along CO route are displayed in Figure S4 in Supporting Information. The STM image obtained after the first deposition, Figure 3(a), is trivially identical to Figure 2(a), because CO is not involved yet in the particular deposition process. However, the third deposition of CO route induces a clear difference: bare Au sites are rarely 10


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visible in CO route (Figure 3(b)), while void spaces are certainly discernible in conventional route (Figure 2(b)). The fifth depositions using the two routes enhance the morphological differences further as contrasted in Figures 3(c) and 2(c). Notable is that the sizes of Pt islands remained 6.97±0.08 nm regardless of the number of deposition cycle. The variations in area occupied by each layer of Pt islands formed via CO route (Figure 3(d)) quite differ from that obtained using conventional route (Figure 2(d)). CO route quickly covers ~90% of Au(111) surface with Pt islands after the second deposition, while conventional route slowly occupies ~90% of Au surface after the fifth deposition. Furthermore single-layered Pt islands are most popular in CO route, while double-layered ones are most prevalent in conventional route. Such morphological differences between the two routes are also confirmed by the differences in roughness calculated from the observed STM images as shown in Figure S5 of Supporting Information. Therefore, CO route is uniquely effective in formation of singlelayered Pt islands on Au(111). One should note that the observation of double-layered Pt islands along with CO route is due to an uncontrollable oxidative removal of a small portion of protecting CO during a transfer from a CO-saturated solution to a Pt precursor solution through the air60. Figure 4 presents the high-resolution STM images of Pt islands on Au(111). In singlelayered Pt islands obtained after the third deposition cycle of CO route (Figure 4(a)), there are ordered domains where the spot periodicities are 0.51±0.03 nm. Considering that one Pt-Pt distance is 0.277 nm, the unit cell is assignable to a (√3×√3)R30° pattern. On the other hand, the image of the top layers of double-layered Pt islands obtained using conventional route (Figure 4(b)) reveals that the distance between the spots in a hexagonal symmetry is 0.30±0.02 nm, indicating that the Pt atoms in the top layer are arrayed in a pattern of (1×1).

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These observations are consistent with our previous work42.

Figure 3. Pt islands on Au(111) prepared using CO route. STM images after deposition cycle of (a) 1, (b) 3, and (c) 5. (d) Variations in area occupied by each layer in Pt islands. The numbers in (d) correspond to stacked orders in Pt islands. Scan size: 500×500 nm.

The heights of uncovered single-layered islands differ from those of the first layers in multiple-layered islands. The single-layered islands of (√3×√3)R30° arrangement is ~0.08 nm high, which is consistent with the uncovered single-layered Pt islands prepared with 10-5 M Pt precursor solution54. Such uncovered single-layered Pt islands are most likely to be Pt islands alloyed with Au, in which the Pt atoms probably submerge into the top Au layer in a 12


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(√3×√3)R30° pattern as depicted at the left-hand side of Figure 4(c). An STM work30 disclosed that islands of Pt-Au surface alloy covered with iodine during galvanic replacement of Cu with Pt were ~0.08 nm high, and a density functional investigation61 revealed that the Pt atoms alloyed with Au(111) and Au(332) strongly prefer for subsurface lattice sites. These observations including ours would denote that alloying adsorbed Pt with Au, probably via partial submersion of Pt into Au surface, is preferred. On the other hand, the ~0.56 nm high islands of (1×1) array should be Pt islands of double layers on the top of Au(111) surface (as illustrated at the right-hand side of Figure 4(c)); if the second layer in a double-layered island were on the top of the alloyed first layer (~0.08 nm high), the height of the double-layered island should be ~0.36 nm, which is not the case. The previously cited STM and theoretical works30,61 disclosed, respectively, that the Pt-Au surface alloyed islands on Au(111) evolved to a Pt monolayer on the top of the Au surface in the presence of adsorbed iodine when the Pt coverage reached to 1 (i.e., completion of galvanic replacement), and that the Pt atoms at subsurface sites segregated to the topmost surface layer upon CO adsorption, due to a strong interaction between Pt and CO. Here again, segregation of Pt atoms alloyed with Au to the top of Au surface may take place when an adsorbate binds strongly to Pt. In this work, the Pt atoms being deposited on alloyed single-layered islands may play a role of the strong adsorbate in segregation of the partially submerged Pt atoms. In other words, the first Pt layers in uncovered single-layered Pt islands are the surface Pt-Au alloy of ~0.08 nm height in (√3×√3)R30°, while the first layers in multiple-layered Pt islands become layers of ~0.28 nm height in (1×1) array on the top of Au surface. Thus, CO route is effective in producing uncovered single-layered Pt islands alloyed with Au.

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Figure 4. High-resolution STM images of Pt islands on Au(111): (a) a (√3×√3)R30° image of alloyed uncovered single-layered Pt islands obtained after the third deposition cycle of CO route, and (b) a (1×1) image of double-layered Pt islands obtained after the fourth deposition cycle of conventional route. (c) Schematic presentation of an uncovered single-layered Pt island submerged into Au surface (the left-hand side), and a double-layered Pt island on the top of Au surface (the right-hand side). 14


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2. Spectroscopic investigations: XPS Figure 5(a) shows a typical XPS spectrum in the binding energy region of Pt 4f after the third deposition cycle on Au(poly) along CO route. In the specific spectrum, there are five spectral components: four peaks assignable to Pt 4f7/2 and 4f5/2 in metallic Pt (71.3 and 74.6 eV) and Pt oxide (72.5 and 75.7 eV), and one peak ascribable to Au 5p (73.8 eV). The presence of the Pt oxide phase is due to a transfer of Pt islands on Au through the air into an ultrahigh vacuum environment. Regardless of the deposition routes, the XPS spectra obtained after various deposition cycles are qualitatively similar to the spectrum in Figure 5(a), but quantitatively different. Figure 5(b) presents the atomic ratios of Pt/Au (described in Experimental) corresponding to the total amount of deposited Pt. When deposition cycle number is smaller than 3, the atomic ratio increases slowly. With higher cycle numbers, however, conventional route abruptly increases the atomic ratio, while CO route does slightly. After the fifth deposition, the atomic ratio obtained using conventional route is roughly three times higher than that acquired using CO route. The coverages of deposited Pt on Au(poly) derived from the XPS atomic ratios are compared with those estimated using STM results on Au(111). Because XPS measures only Pt/Au atomic ratio, a reference Pt layer of a known coverage on Au is needed to convert the XPS atomic ratios to Pt coverages (number ratios of deposited Pt atoms to surface Au atoms). The reference utilized in this work is a full monolayer of Pt on Au, prepared via galvanic replacement of underpotentially deposited Cu monatomic layer on Au with PtCl42- ions (detailed in Supporting Information). Using the obtained XPS atomic ratio of the reference Pt monolayer on Au (0.21±0.01), the converted coverages of deposited Pt on Au(poly) (hereafter, XPS coverage) are displayed in Figure 5(b). Independently from the XPS measurements, on

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the other hand, the coverage values of deposited Pt on Au(111) (hereafter, deposited Pt STM coverage) could be estimated using STM results, based on the atomic arrangements of Pt (i.e., the (√3×√3)R30° structure of uncovered single-layered Pt island, and the (1×1) structure of covered ones) and occupied areas. The estimation procedure of deposited Pt STM coverages is detailed in Supporting Information. The variations in deposited Pt STM coverages of conventional and CO routes (faint thick red and gray lines, respectively) are slightly higher than the corresponding XPS coverages in general, although the deposited Pt STM coverages after the second and third depositions, especially using conventional route, are notably higher. Considering the differences in surface crystallographic orientations of the employed Au surfaces (i.e., (111) versus (poly)) and in the coverage-measuring methods (i.e., STM versus XPS), however, the difference between the two Pt coverages measured independently would not be significant in general. In fact, Figures S6 and S7 in Supporting Information demonstrates that the charges of CO stripping and hydrogen on Pt islands on Au(111) and Au(poly) produced using conventional and CO routes coincide with each other. Therefore, the spectroscopic and electrochemical Pt coverages on Au(poly) are comparable to deposited Pt STM coverages on Au(111). The comparison of deposited Pt STM and XPS coverages would justify the existence of alloyed Pt islands of ~0.08 nm height in (√3×√3)R30° structure. For example, the fact that the structure of uncovered single-layered Pt island is (√3×√3)R30°, while the structure of covered ones is (1×1) leads to the STM coverages of ~0.6 and ~1.2 after the fifth cycles of CO and conventional routes, respectively, fairly close to the XPS coverages (~0.5 and ~1.6, respectively). If the structure of uncovered single-layered island were (1×1), the deposited Pt STM coverages would be ~1.2 for CO route and ~2.3 for conventional route, irrationally

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higher than the corresponding XPS coverages. Although the reason for the formation of the first monatomic layer of (√3×√3)R30° structure is not clearly understood, the deposited Pt STM and XPS coverages are reasonably consistent with each other.

Figure 5. (a) Typical XPS spectrum of Pt islands after the second deposition cycle of CO route. (b) Variations in Pt/Au atomic ratio as a function of deposition cycle. Squares and circles correspond to conventional and CO routes, respectively. The atomic ratios were converted to deposited Pt coverages, using the atomic ratio of a reference Pt layer on Au of coverage ~1 (triangle). Red and gray thick faint lines represent deposited Pt STM coverages of conventional and CO routes, respectively. 17



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3. Electrochemical investigations: adsorption of hydrogen and CO Figure 6 shows the CO stripping and subsequent voltammograms of Pt islands on Au(poly) prepared using conventional and CO routes. As exemplified with Figure 6(d), the voltammograms in Figure 6 present sequential occurrences of CO stripping and surface oxidation in the initial anodic scan, surface oxide reductions and hydrogen adsorption in the following cathodic scan, and hydrogen desorption in the last anodic scan (Note that the curves for surface oxidation without CO are presented with red dashed lines.). Figure 6 demonstrates the changes in voltammograms depending on the number of deposition cycle along the two deposition routes. After the first deposition (Figures 6(a) and (f)), there is a small peak of CO stripping at 0.78 V, followed by a modified current profile of Au oxidation due to the presence of Pt. In the subsequent cathodic scan, two surface oxide reduction peaks of Au and Pt appear at ~0.82 and ~0.35 V, respectively (referring to the voltammograms of Au(poly) and Pt(poly) at the bottom of Figure 6). Furthermore, the charges of hydrogen underpotential deposition are barely observable at the potential region below 0.1 V. As deposition number increases, however, the charges of the electrochemical processes related to Pt (i.e., CO stripping, Pt surface oxidation/reduction and hydrogen underpotential deposition) become enhanced. Concomitantly, the surface oxidation/reduction characteristics of Au fade out. The CO stripping behavior needs to be more specifically addressed. There are two CO oxidation peaks at high and low potentials on Pt islands on Au. After the first deposition cycle, adsorbed CO is oxidized at trivially identical potential of 0.78 V as shown in Figures 6(a) and (f). After the second cycle, two oxidation peaks at 0.59 and 0.74 V are apparent (Figures 6(b) and (g)). Along further deposition cycles, the CO stripping peaks at the higher potential

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Figure 6. CO-stripping and subsequent voltammograms of Pt islands on Au(poly) in 0.05 M H2SO4 solution. The left-hand side corresponds to the voltammograms after conventional route deposition cycle of (a) 1, (b) 2, (c) 3, (d) 4, and 5; the right-hand side does to the voltammograms after CO route deposition cycle of (f) 1, (g) 2, (h) 3, (i) 4,and (j) 5. Surface oxidation curves are presented with red dashed lines. At the bottom, the voltammograms of Au(poly) and Pt(poly) with adsorbed CO in 0.05 M H2SO4 solution are displayed as references. Scan rate: 50 mV/sec. 19



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slightly shift in the negative potential direction and ultimately disappears, while those at the lower potential shift to 0.52 V and become sharp as much as that on Pt(poly) (the bottom of Figure 6). The observation of two CO oxidation peaks implies that there are two adsorption states: strongly adsorbed CO and weakly adsorbed CO. A strongly bound CO to Pt monolayer on Au was demonstrated experimentally39 and theoretically47 so that a significant influence of Au on Pt at a low Pt coverage is certainly responsible for the CO stripping peak at 0.78 V. On the other hand, because the weakly adsorbed CO become prevalent at higher amount of Pt and because its potential is close to that on Pt(poly) (0.50 V), a decrease in the influence of Au turns the weakly adsorbed CO more popular. The oxidation peak potential shifts of the two COs on Pt islands may be relevant to the higher surface oxidation inertness of Au than Pt regarding (revealed by the voltammograms of Au(poly) and Pt(poly) at the bottom of Figure 6) leading to a high formation overpotential of surface oxygen species (such as adsorbed OH) required for CO oxidative removal. As exemplified by higher surface oxidation onset potential after the first deposition cycle (the red dashed lines in Figures 6(a) and (f)), the inertness of Au would increase the formation overpotential of surface oxygen species to result in a high CO oxidation potential especially at low Pt coverage (equivalently under a strong influence of Au). However, further increases in coverage of Pt (or decreases in the influence of Au) cause surface oxidation potential shifts to lower potentials followed by CO oxidation potential shifts in the same direction. Therefore, a strong influence of Au on Pt would be a reason for the appearance of the CO oxidation at 0.78 V caused by a strong binding of CO to Pt and a high formation overpotential of surface oxygen species. In the development of CO stripping peaks, conventional and CO routes slightly differ from each other. For example, Figures 6(b) and (g) demonstrate that the stripping charge for

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strongly adsorbed CO on Pt islands of CO route is higher than that on those of conventional route. Correlation of this difference to the occupied areas by the first and second layers (Figures 2(d) and 3(d)) implies that a higher population of double-layered islands in Figure 2(d) may be related to a higher charge of weakly adsorbed CO oxidation in Figure 6(b). However, another correlation of Figures 6(e) and (j) observed after the fifth deposition cycle to their occupied area analysis (Figures 2(d) and 3(d)) demonstrates that the presence of multiple-layered islands are not strongly related to the appearance of weakly adsorbed CO stripping. Namely, CO is stripped off at the lower potential on single-layered Pt islands of CO route not having a significant amount of multiple layers. Therefore, an increase in Pt coverage in lateral (CO route) and vertical (conventional route) directions, suppresses the formation of strongly adsorbing CO. Figure 7 compares the coverages of electrochemically active Pt atoms exposed to solution phase, estimated using three different ways: coulometries of CO and hydrogen, and structural analysis of STM images. It is important to recognize that the coverages of CO and hydrogen reflect simultaneously the number of Pt atoms available for adsorption (equivalent to a Pt coverage) and the adsorption stoichiometries of the adsorbing species to the available Pt atoms. Therefore, if Pt coverages are available independently from the coverages of CO and hydrogen, comparison of the Pt coverages with those of the adsorbing species would lead to estimation of their adsorption stoichiometries. Furthermore, any differences in the adsorption stoichiometries of CO and hydrogen on the investigated Pt surfaces (i.e., singleand multiple-layered Pt islands on Au and Pt(poly)) would reveal distinguishing adsorptive behavior of CO and hydrogen on the structurally different Pt surfaces referring to well-known Pt(poly). In this particular regard, Figure 7 deserves a detailed analysis as described below.

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Figure 7 clearly demonstrates that the curves of hydrogen coverage are significantly and consistently lower than those of CO coverage. On single- and multiple-layered Pt islands on Au(poly), the coverage ratio of CO to hydrogen after the first deposition is ~4 and decreases to ~1.2 after the fifth deposition. A coverage ratio of CO to hydrogen obtained on an identical Pt surface corresponds to their adsorption stoichiometry ratio. For example, the coverage ratio of CO to hydrogen in the CO stripping voltammogram on Pt(poly) (~0.85, the bottom of Figure 6) is interpretable on the basis that one hydrogen atom adsorbs on one Pt atom, while CO adsorbs on atop, bridge and hollow sites. Then the observed higher coverage ratios of CO

Figure 7. Variations in coverages of CO, hydrogen and electrochemically active Pt as a function of deposition cycle. Squares and triangles correspond to the coverages of hydrogen and CO, respectively. Open and filled symbols represent CO and conventional routes, respectively. Red and black thick faint lines represent electrochemically active Pt STM coverages of conventional and CO routes, respectively. The error bars represent the standard deviations calculated using four or five measurements for each point. 22


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to hydrogen imply that hydrogen adsorption stoichiometry on Pt islands on Au is lower than that on Pt(poly), especially when deposited amount of Pt is small, and that as more Pt deposits on Au, hydrogen adsorption stoichiometry increases. In other words, Pt islands on Au are not effective in hydrogen adsorption so much as on Pt(poly), ascribed to a strong influence of Au substrate not permitting hydrogen adsorption in our previous work54. A comparison of electrochemically active Pt coverages with the coulometrically determined coverages of CO and hydrogen would lead to estimation of their adsorption stoichiometries on Pt islands. The numbers of electrochemically active Pt atoms at surfaces relative to the number of Au atoms are possible to be extracted using STM results as described in Supporting Information (hereafter, the extracted Pt coverages are termed electrochemically active Pt STM coverage). Figure 7 clearly demonstrates that the red faint thick line representing electrochemically active Pt STM coverage of conventional route coincides with that of CO coverage, while the black faint thick line of CO route is far below. Thus the adsorption stoichiometries of CO on Pt islands on Au prepared using the two different routes should differ from each other. As deposition cycle is repeated, the stoichiometry of CO adsorbed on multiple-layered Pt islands decreases from ~1.7 to ~1, while that on single-layered Pt islands fairly remains at ~1.8. Application of the same analysis to hydrogen coverages discloses that the adsorption stoichiometry of hydrogen is in the range of 0.4~0.6 at low Pt coverages regardless of deposition routes, and that it continuously increases ultimately to ~0.8 and ~1.7 after the fifth deposition cycle of conventional and CO routes, respectively. Thus, it is reasonably concluded that single-layered Pt islands differ from multiple-layered Pt islands in terms of adsorption stoichiometries of CO and hydrogen. The higher adsorption stoichiometries of CO and hydrogen on single-layered Pt islands

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of (√3×√3)R30° would be understood in terms of influence of Pt to Au. Au surfaces do not permit adsorption of CO and hydrogen. In a domain of (√3×√3)R30° as presented in Figure 4(c), one Au atom is encircled by three Pt atoms, so that the Au and Pt atoms are quite probable to interact with each other perhaps electronically. As a consequence, the Au atoms modified by the Pt atoms would have features of Pt to allow adsorption of CO and hydrogen to increase their adsorption stoichiometries on single-layered Pt islands. To account for the observed adsorption stoichiometries of CO and hydrogen on single-layered Pt islands, approximately two CO molecules and two hydrogen atoms are suggested to be accommodated in the unit cell of (√3×√3)R30°. In addition, the increase in hydrogen adsorption stoichiometry and the decrease in CO adsorption stoichiometry along conventional route strongly indicate that the interaction between substrate Au and electrochemically active Pt at the top layers decreases for multiple-layered Pt islands to become similar to bulk Pt because of a larger distance between the Au and Pt atoms. One example is that the hydrogen adsorption stoichiometry on multiple-layered islands (~0.8) is still low but close to the generally accepted value on Pt(poly) (1.0). For another instance, one may argue that the adsorption stoichiometry of CO on multiple-layered Pt islands prepared via conventional route (~1) is rather high, considering that the CO coverage on Pt(poly) (Figure 6) is ~0.85. However, Figure 2(d) (also Table S1 in Supporting Information) indicates that the population of single-layered Pt islands (> 20%) is still significant. A calculation of CO coverage after the fifth deposition cycle of conventional route using occupied areas and CO adsorption stoichiometries obtained on single-layered islands and bulk Pt returns a value of 0.73 (=0.23×0.33×1.8 + 0.70×0.85) which is close enough to the experimentally observed value (0.78). Thus, the Pt atoms of (1×1) array in multiple-layered Pt islands on Au are influenced

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rarely by Au to be close to those of Pt(poly). The differences between the Pt islands prepared using conventional and CO routes in the adsorption behavior of CO and hydrogen definitely propose that CO and hydrogen are able to adsorb on the Au atoms in single-layered Pt islands. The observations regarding the adsorption of CO and hydrogen on Pt islands on Au imply a mutual interaction between Pt and Au. The lower hydrogen stoichiometry on Pt islands on Au referring to Pt(poly) supports the modification of Pt by Au. On the other hand, the higher stoichiometries of CO and hydrogen on single-layered Pt islands indicate the modification of Au by Pt. Thus, it becomes clear that the electrochemical property of Pt deposited on Au is possible to be manipulated by tuning the morphologies of Pt deposits.

4. Electrocatalysis : ethanol oxidation Figure 8 compares the cyclic voltammograms of ethanol oxidation on Pt islands on Au(poly) prepared using conventional and CO routes. For a comparative purpose, the voltammograms of ethanol oxidation on Au(poly) and Pt(poly) are presented at the bottom of Figure 8. In general, the behavior of ethanol oxidation on Pt islands on Au is qualitatively similar to that on Pt(poly)62-66. Specifically, Pt surfaces are passivated at low potential (< 0.2 V), signaling that C1 intermediates coming from C-C bond breakage (CH3CH2OH → (CH3CHO → -CHx →) CO) passivate or poison Pt surfaces. The adsorbed C1 intermediates start being oxidized in anodic scan approximately at the potential above 0.2 V (C1-pathway, CH3CH2OH → (CH3CHO → -CHx →) CO → CO2). As potential increases more, ethanol oxidation on poison-free metallic surfaces proceeds further via C2-pathway (CH3CH2OH →

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Figure 8. Ethanol oxidation voltammograms of Pt islands on Au(poly) in 2.0 M ethanol + 0. 5 M H2SO4 solution. The voltammograms in the left-hand side corresponds to those after conventional route deposition cycle of (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5; the voltammograms in the right-hand side corresponds to those after CO route deposition cycle of (f) 1, (g) 2, (h) 3, (i) 4, and (j) 5. At the bottom, the voltammograms of Au(poly) and Pt(poly) in 2.0 M ethanol + 0.5 M H2SO4 solution are displayed as references. Scan rate: 50 mV/sec. 26


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CH3CHO → CH3COOH) to show an oxidation peak at ~0.6 V as the surface become oxidized. With anodic scan continued, ethanol oxidation resumes on oxidized surfaces of Pt above 0.8 V via C2-pathway also. Upon reversal of the scan direction, ethanol oxidation current increases again as oxidized surfaces become reduced and peaks at ~0.4 V due to gradual building-up of poisoning species (i.e., C1 intermediates). Ethanol oxidation on Au(poly) merely takes place only at the potential higher than 0.8 (the bottom of Figure 8). Despite the qualitative similarity, quantitative details concerning ethanol oxidation on Pt islands on Au prepared using the two different routes and Pt(poly) are different. The onset oxidation potential of C1 intermediates is consistently ~0.2 V on Pt islands of conventional route, while it increases from ~0.3 to ~0.4 V on Pt islands of CO route. Comparing the onset potential on Pt(poly) (~0.4 V), Pt islands on Au are more effective in removal of C1 intermediates. Concerning ethanol oxidation via C2-pathway (i.e. the peaks at ~0.4 V in cathodic scan and at ~0.6 V in anodic scan), the three Pt surfaces are distinguishing. Specifically, on Pt islands of conventional route the peak current in anodic scan is higher than that in cathodic scan, the opposite of which is true for Pt islands of CO route and Pt(poly). Along conventional route, the peak potentials in anodic and cathodic scans decrease respectively from 0.69±0.02 to 0.59±0.01 V, and from 0.39±0.03 to 0.36±0.01 V, and the currents observed on both scans increases gradually as shown in Figure 9. (Note that the potential values reported here and the current values appearing in Figure 9 are average values obtained with more than four independent experiments whose standard deviations are presented with error bars. Thus, it should be bore in mind that the average values may differ slightly from the corresponding voltammograms in Figure 8.) After the fourth deposition cycle of conventional route, in addition, the oxidation currents in anodic scan is higher than

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Figure 9. Variations in ethanol oxidation current on Pt islands on Au(poly) as a function of deposition cycle during (a) anodic and (b) cathodic scans. Circles and squares represent conventional and CO routes, respectively. Faint thick lines are ethanol oxidation currents on Pt(poly).

on Pt(poly) (thick faint lines in Figure 9) by a factor of 2, while those in cathodic scan are more or less similar. Recalling that C2-pathway contributes mainly to ethanol oxidation on Pt surfaces, Pt islands of conventional route is slightly more efficient than Pt(poly) despite their lower overpotentials for oxidative removal of C1-intermediates. On the other hand, Figure 9 28


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clearly demonstrates that ethanol oxidation on Pt islands of CO route via C2-pathway is much more effective than on Pt(poly). The most efficient one (with enhancement factors of roughly 5 and 2.5 in anodic scan and cathodic scan, respectively) is Pt islands obtained after the second deposition cycle of CO route, which results in alloyed single-layered Pt islands of (√3×√3)R30° covering ~90% of Au surface with scattered (1×1) second layers (~15%) (Figure 3(d)). As deposition cycle of CO route continues, the oxidation current in cathodic scan reaches to a constant level higher than that on Pt islands of conventional route, while that in anodic scan decreases to that on Pt islands of conventional route. The ethanol oxidation peak potentials on Pt islands of CO route decreases in a way similar to those on Pt islands of conventional route. Moreover, ethanol oxidation current at the potential above 0.8 V is higher on Pt islands of CO route than on Pt(poly), while that on Pt islands of conventional route is comparable. Therefore, it is clear that single-layered Pt islands on Au is much more effective in ethanol oxidation than multiple-layered ones and Pt(poly). Ethanol oxidation enhancement on single-layered Pt islands on Au would be a higher reaction rate on the particular surface. Because Au does not permit ethanol oxidation at the potential below 0.8 V at all (the bottom of Figure 8), the ethanol oxidation currents via C1and C2-pathways certainly come from deposited Pt. However, details of the oxidation currents differ from those on Pt(poly), reflecting different influences of Au on deposited Pt depending on morphology. The observed ethanol oxidation enhancement on Pt islands, especially on single-layered islands, would be due to an increase in the kinetics of C2pathway. Because oxidation current via C2-pathway in anodic scan reduces at a potential where surface oxidation starts, any reason lowering surface oxidation potential will cause a potential shift of C2-pathway in the negative direction. Indeed, the peak potential of C229



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pathway in anodic scan shifts in the negative direction (Figure 8) as an increase in the amount of deposited Pt induces a potential shift of surface oxidation in the same direction (Figure 6). However, the lower surface oxidation overpotential would not be the only reason for higher peak current observed on single-layered Pt islands (Figure 8): if it is true, a huge current increase in ethanol oxidation should be observed on multiple-layered Pt islands also. The reason mainly responsible for the observed enhancement would be that the reaction rate of C2-pathway on single-layered Pt islands of (√3×√3)R30° is much higher than on multiplelayered Pt islands of (1×1). As C1-intermediates are removed in anodic scan, ethanol oxidation current on single-layered Pt islands increases at a higher rate and peaks when surface oxidation starts, so that the peak current on single-layered Pt islands is much larger than that on multiple-layered one. An increase in the amount of deposited Pt along CO route induces a cathodic shift in surface oxidation as shown in Figure 6, which in turn results in a cathodic shift in ethanol oxidation in anodic scan with reduced peak currents. Likewise, the higher reaction rate of C2-pathway on single-layered Pt islands is also revealed in cathodic scan. Before building-up of C1-intermediates, the current on single-layered Pt islands reaches to a higher current than that on multiple-layered Pt islands. However, the peak potential of C2-pathway in cathodic scan does not shift significantly, indicating that the potential for formation of C1-intermediates does not change seriously. The above explanation is also applicable to multiple-layered Pt islands of conventional route except a reaction rate of C2pathway on (1×1) surfaces, lower than that on single-layered islands but higher than that on Pt(poly). Regarding C2-pathway at potential higher than 0.8 V, its reaction rate would be much faster on oxidized single-layered Pt islands than on oxidized multiple-layered ones so that at potentials above 0.8 V, ethanol oxidation current observed on oxidized surface of

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single-layered islands is more significant. Then, single-layered Pt islands of (√3×√3)R30° is quite unique in terms of ethanol oxidation. Ethanol oxidation enhancement on single-layered Pt islands on Au is another demonstration of the interaction between substrate Au and deposited Pt. Ethanol oxidation on multiple-layered islands on Au, especially after the fifth deposition cycle of conventional route, is quite comparable to that on Pt(poly) except ethanol oxidation in anodic scan is enhanced (Figure 8). Because the Au surface is mainly covered with double-layered islands after the fifth deposition cycle of conventional route (Figure 2(d)), the longer distance between Au and electrochemically active Pt reduces the influence of Au as discussed in Section 3. On the other hand, electrochemically active Pt in single-layered island of CO route, alloyed with Au, would be influenced more seriously by Au. Thus, more interaction between Au and Pt is concluded to result in a higher ethanol oxidation rate. However, this work is not able to provide further details of ethanol oxidation on alloyed Pt/Au surface; in this particular point of view, a further work would be needed. Comparison of the ethanol oxidation enhancement on single-layered Pt islands with those on other Pt surfaces on Au prepared using methods different from CO route is instructive. The most closest one to ours is Pt monolayer on Au(111) prepared using galvanic replacement of Cu67. On that particular Pt layer on Au(111), the enhancement factor of ethanol oxidation (versus Pt(111)) was ~5 as comparing the currents measured at 0.5 V during anodic scan in 0.1 M HClO4 containing 0.5 M ethanol with scan rate of 1 mV/sec. Here, it should be noted that in ref. 67, the measured currents were normalized to the measured Pt surface area, while the current density in Figure 9 was normalized to the Au area. Using electrochemically active Pt coverage of ~0.3 in Figure 7 after the second cycle of CO

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route, the enhancement factor is turned out to be ~15. On the other hand, the enhancement factor in ethanol oxidation on Au@Pt core-shell nanocubes prepared galvanic replacement of thick Ag layers on Au nanoparticles was ~8.4 against Pt black in 1.0 M NaOH with 1.0 M ethanol with 50 mV/sec scan rate68. Although slight differences in determination of enhancement factor, single-layered Pt islands on Au would be the best one in ethanol oxidation among the Pt surfaces on Au reported so far. The distinguishing electrochemical behavior of CO, hydrogen and ethanol on singlelayered Pt islands alloyed with Au from that on multiple-layered Pt islands clearly demonstrates the significance of CO route, a powerful tool to tune the electrochemical properties by manipulation of the morphologies of Pt deposits on Au. Brimaud et al.39 demonstrated that smooth Pt monolayer (whose coverage was ~1) electrodeposited on Au in the presence of CO differed from Pt deposits prepared using galvanic replacement of a Cu monolayer on Au and 10 nm Pt(111) octahedrons in terms of adsorbed CO vibrational frequencies. This specific difference of the particular smooth Pt monolayer on Au was ascribed to an electronically modified Pt referring to bulk Pt(111) (i.e., electronic ligand and strain effects), originating from the presence of Au and different morphology from rough Pt agglomerates on Au produced via galvanic replacement of Cu. The single-layered Pt deposits presented in this work are obviously distinctive from the smooth Pt monolayer electrodeposited in the presence of CO in some aspects: deposition method (irreversible adsorption versus electrodeposition), coverage (0.33 versus 1.0), and chemical state (partially submerged into Au versus on the top of Au). Although a direct comparison of the two Pt deposits on Au produced in the presence of CO but via different deposition methods is not possible in this work, the experimental results reported in this work are enough to support the

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significance and uniqueness of the alloyed uncovered single-layered Pt islands on Au produced using CO route.

Conclusions This work demonstrated formation of alloyed single-layered Pt islands on Au using CO route and their distinctive electrochemical properties regarding adsorption of CO and hydrogen, and ethanol oxidation. STM works revealed that CO route produced alloyed single-layered Pt islands of (√3×√3)R30°, while conventional route generated multiplelayered Pt islands of (1×1), mostly double-layered ones, on the top of Au. Quantitative analysis of Pt deposits using XPS confirmed these STM structural characteristics of Pt islands. Detailed comparison of the coverages of CO, hydrogen and electrochemically active Pt on Pt islands reached to two important conclusions: one is that the adsorption stoichiometry of hydrogen is lower than that of CO, and the other is that the adsorption stoichiometries of CO and hydrogen on single-layered Pt deposits of (√3×√3)R30° were higher than those on multiple-layered ones of (1×1). The former was due to the influence of Au (not permitting hydrogen adsorption) to Pt so that hydrogen adsorption on Pt islands took place to a less extent than on Pt(poly); the latter was ascribed to the influence of Pt (strongly adsorbing CO and hydrogen) to Au so that Au were available for CO and hydrogen adsorption. In addition, the electrocatalytic performance of single-layered Pt deposits in ethanol oxidation was much higher than that of multiple-layered one. The most efficient Pt in ethanol oxidation among the investigated Pt deposits using irreversible adsorption method was single-layered Pt islands covering ~90% of Au surface with some scattered (1×1) second layers (~15%). Close inspection of oxidation voltammograms suggested that a higher reaction rate of ethanol

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oxidation via C2-pathway on single-layered Pt islands was responsible for the enhancement. A stronger interaction between Au and Pt in single-layered Pt islands than in multiple-layered ones was an explanation for the observed increase in the ethanol oxidation activity, although further details were not understood clearly due to the complexity of ethanol oxidation. This work demonstrated the uniqueness of single-layered Pt islands alloyed with Au referring to multiple-layered Pt islands and Pt(poly). Thus, CO route would be a promising method to control morphology of Pt deposits on Au and thereby to tune their electrochemical properties.

Acknowledgements This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2013R1A1A2007139) and Chungnam National University (project number: 2014-0696-01).

Supporting Information Galvanic replacement of UPD Cu on Au with Pt (Figure S1); Estimation of area occupied by each layer in multiple-layered Pt islands (Figures S2−S5); Estimation of deposited Pt STM coverage; Estimation of electrochemically active Pt STM coverage; CO stripping and subsequent voltammograms of Pt islands on Au(111) prepared using conventional CO route (Figures S6 and S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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Int. Ed. 2011, 50, 2674-2676. 5. Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Philip, N. R.; Lucas, C. A.; Markovic, N. M. Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability Science, 2007, 315, 493-497. 6. Wang, J. X.; Inada, H.; Wu, L.; Zhu, Y.; Choi, Y. M.; Liu, P.; Zhou, W.-P.; Adzic, R. R. Oxygen Reduction on Well-Defined Core-Shell Nanocatalysts: Particle Size, Facet, and Pt Shell Thickness Effects, J. Am. Chem. Soc. 2009, 131, 1729-17302. 7. Gong, K.; Su, D.; Adzic, R. R. Platinum-Monolayer Shell on AuNi0.5Fe Nanoparticle Core Electrocatalyst with High Activity and Stability for the Oxygen Reduction Reaction,

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Contrasting Electrochemical Behavior of CO, Hydrogen, and Ethanol

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