Electrochemical Layer-by-Layer Deposition of Pseudomorphic Pt

Article pubs.acs.org/JPCC

Electrochemical Layer-by-Layer Deposition of Pseudomorphic Pt Layers on Au(111) Electrode Surface Confirmed by Electrochemical and In Situ Resonance Surface X‑ray Scattering Measurements Masayo Shibata,† Naoko Hayashi,† Takara Sakurai,† Ayumi Kurokawa,† Hitoshi Fukumitsu,‡,§ Takuya Masuda,‡ Kohei Uosaki,‡,§,∥,* and Toshihiro Kondo†,* †

Division of Science, Graduate School of Humanities and Sciences, Ochanomizu University, Ohtsuka, Bunkyo-ku, Tokyo 112-8610, Japan ‡ Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Namiki, Tsukuba, Ibaraki 305-0044, Japan § Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan ∥ International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT: Conditions for electrochemical deposition of atomically flat, pseudomorphic Pt layer on Au(111) electrode surface were established based on electrochemical and in situ resonance surface X-ray scattering (RSXS) studies on potential dependence of Pt deposition. When Pt was deposited at relatively large overpotential, a three-dimensionally grown rough Pt layer was formed on a Au(111) surface. When the overpotential was very small, Pt nuclei was formed but did not grow any more. At appropriate overpotential between these two potentials, atomically flat, pseudomorphic Pt layer was grown with a layer-by-layer growth mode.

1. INTRODUCTION Ultrathin metal layers on foreign metal substrates have characteristics different from both substrate metal and deposited metal1,2 and recently attract much interest because they often have electrocatalytic activities higher than both metals.3−27 Such high electrocatalytic activities reflect the modulation of surface electronic energy partially due to the atomic arrangement change of the epitaxially deposited metal layers.12−27 The epitaxial growth of a well-defined metal layer has been achieved by vapor deposition, molecular beam epitaxy (MBE), and metalorganic chemical vapor deposition (MOCVD) under ultrahigh vacuum (UHV) condition.28−30 As compared to the metal deposition by these techniques in UHV, electrochemical metal deposition is economical and easy because expensive equipment is not necessary for this process. Electrochemical deposition process of many epitaxial metal layers on different metal substrates have been investigated at an atomic level using in situ techniques such as scanning tunneling microscopy (STM) and surface X-ray scattering (SXS).4−8,17,19,24,25,31−52 There are three growth modes at the initial stage of the deposition process depending on how nucleation and growth process proceeds, namely Frank-van der Merwe (FM), Volmer−Weber (VW), and Stranski−Krastanow (SK) modes.53−57 They can be simply described as layer-by-layer growth (2D), island growth (3D), and layer-by-layer plus island © 2012 American Chemical Society

growth (2D + 3D), respectively. The growth process depends on the growth rate, i.e., current density, which in turn depends on the overpotential and the concentration of the metal ion in the solution. Large numbers of nuclei are formed and nuclei grow three-dimensionally with a higher growth rate when the overpotential is relatively large but smaller numbers of nuclei are formed and nuclei grow two-dimensionally with a lower growth rate when the overpotential is relatively small. Thus, in order to prepare the epitaxially deposited precious metal layer on the different metal single crystal surface, 2D nuclei should be formed by applying the relatively small overpotential in the solution containing metal ion with a relatively low concentration. Platinum is one of the most powerful catalysts for a wide range of catalytic reactions and therefore, epitaxially deposited Pt monolayer on a single crystal of foreign metal is expected to be a good electrocatalyst with high activity and, moreover, leads to the reduction of the loading amount of the precious Pt. Adzic et al. prepared Pt monolayer on various single crystal metal surfaces using galvanostatic replacement58−61 but the surface atomic arrangement of Pt is not really well ordered in an atomic dimension. Structural studies of electrochemically Received: October 13, 2012 Revised: November 19, 2012 Published: November 20, 2012 26464

dx.doi.org/10.1021/jp3101283 | J. Phys. Chem. C 2012, 116, 26464−26474

The Journal of Physical Chemistry C

Article

The deposition was carried out in a deaerated 0.1 M HClO4 solution containing 0.05 mM H2PtCl6.51 The electrode potential was negatively scanned from open circuit potential (OCP), ca. 1.1 V (vs RHE) to 0.77 V, 0.90 V, and 0.92 V at a scan rate of 2 mV s−1, kept at each potential for a certain period (200−3500 s) and then the Au(111) disk was removed from the electrochemical cell. After the Au(111) disk was washed with conc. H2SO4 and ultrapure water to remove the adsorbed PtCl62− ion, the disk was set on a conventional onecompartment electrochemical cell and cyclic voltammogram (CV) was recorded in a deaerated 0.05 M H2SO4 solution by scanning the potentials negatively from +0.4 V with a scan rate of 20 mV s−1. Potential was cycled only once as it is known that the oxide formation/reduction cycles of Pt(111) electrode leads to the progressive disordering of the surface. 2.3. RSXS Measurements. In situ RSXS measurements were carried out at a bending-magnet beamline BL4C at Photon Factory. After the annealing and quenching of the Au(111) disk as the same procedure as the above, the Au(111) disk was set to the SXS cell.69,70 X-ray radiation was monochromated by a Si(111) double-crystal system and its energy was calibrated using the absorption edge energy of a Pt foil.51 In order to prevent the fluorescence X-ray from the deposited Pt and Pt complex in the solution, the energy range of the detector (NaI scintillation counter) was narrowed down as possible. The energy range of the incident X-ray was selected to be between 11.40 and 11.75 keV in order to contain the PtLIII absorption edge (11.565 keV). It was confirmed that the beam position was not out of position during changing the incident X-ray energy between 11.40 and 11.75 keV. All of the in situ RSXS measurements were carried out in a 0.1 M HClO4 solution containing 0.05 mM PtCl62 at OCP before and after the Pt deposition so that Pt deposition during the RSXS measurements was avoided. A reciprocal coordinate system (H, K, L) with two components (H and K) lying parallel to the surface and the other one (L) along the surface normal was used in this study. Structures along the direction normal to the surface were quantitatively determined from the least-squares fitting to the (00) rod and energy dependence data, measured at the (0 0 L) position, with a kinematic calculation based on a specific interfacial model69,70 consisting of three layers on top of the Au(111)-(1 × 1) substrate. Surface atomic/molecular arrangements were also quantitatively determined from the leastsquares fitting to the (0 1) rod and energy dependence data, measured at the (0 1 L) position, with the same calculation based on the same model as above. In the fitting, Pt, Au, PtCl62−, and O were considered. Coverage in each layer is described using ML as a unit where 1 ML corresponds to 1.39 × 1015 atoms (or molecules) cm−2.

deposited Pt ultrathin layers on the Au(111) single crystal electrode surface have been carried out by several groups using STM34,39,62,63 but the results were different from each other. While we have reported that Pt grows on an atomically flat Au(111) surface in FM mode,34 Waibel et al.39 and Strbac et al.62 reported that Pt grows on a Au(111) surface in VW mode. This may be caused by the difference in the electrochemical conditions employed for the Pt deposition. Both the concentration of Pt complex in the solution and the deposition overpotential were lower and, consequently, the growth rate was slower in the case of layer-by-layer deposition than in the case of island deposition. Recently, we have confirmed that while the electrochemically deposited Pt monolayer prepared by the Naohara’s procedure34 is pseudomorphic on the Au(111) surface, a rough Pt surface was formed when the deposition overpotential was larger and the concentration of Pt complex was higher using ex situ resonance SXS (RSXS) technique in our preliminary report.51 In this work, detailed investigation on the electrochemical deposition process of Pt on a Au(111) single crystal electrode surface was carried out by conventional electrochemical measurements and in situ RSXS measurements. Although nucleation and growth processes can be independently controlled by double potential method,64−67 it is too complicated because there are too many parameters such as applied potential, pulse duration, and the concentration of metal ion. We employed a novel method that potential was slowly scanned to a preset potential and kept at that potential for a certain period51 so that the deposition condition can be controlled easily and conditions for the formation of atomically flat, pseudomorphic Pt layer on Au(111) surface were established.

2. EXPERIMENTAL SECTION 2.1. Materials. Au(111) single crystal disks (diameter: 10 mm, thickness: 5 mm) were purchased from Surface Preparation Laboratory (The Netherlands) and used after the previously described pretreatments.50,51,68,69 A Pt wire (diameter: 0.3 mm) used as a counter electrode and Pt foil (thickness: 0.01 mm) used as a reference of PtLIII edge energy were purchased from Nilaco. A Hg/Hg2SO4 (sat. Na2SO4) electrode [mercury(I) sulfate electrode; MSE] used as a reference electrode was purchased from Autolab. Ultrapure reagent grade H2SO4 and HClO4 and reagent grade H2PtCl6 were purchased from Wako Pure Chemicals, Aldrich, and Sigma, respectively, and were used without further purification. Water was purified using a Milli-Q system (Yamato, WQ-500). Ultrapure N2 (99.9995%) was purchased from Kotobuki Sangyo. A 6.0-μm thick Mylar film was purchased from Chemplex. 2.2. Electrochemical Measurements. Before the Pt deposition, the Au(111) disk was annealed using a Bunsen burner or a hydrogen flame, cooled in a quartz vessel for a few minutes and then quenched in ultrapure water.69,70 A Pt wire and MSE were used as the counter and reference electrodes, respectively. The potential values were referred to a reversible hydrogen electrode (RHE). The electrode potential was controlled by a potentiostat/galvanostat (Hokuto Denko, HA-151) and an external potential was provided by a function generator (Hokuto Denko, HB-111). Potential and current were recorded on a personal computer connected data logger (GRAPHTECH, GL-200A).

3. RESULTS AND DISCUSSION 3.1. Electrochemical Characteristics during Pt Deposition on Au(111). Figure 1 shows a linear sweep voltammogram (LSV) of the Au(111) electrode measured in a deaerated 0.1 M HClO4 solution containing 0.05 mM H2PtCl6 scanned negatively from OCP (ca. 1.1 V) with a scan rate of 2 mV s−1. Cathodic current due to the reduction reaction shown by the reaction 1 was started to flow around 0.95 V and a cathodic peak was observed at 0.69 V. PtCl 6 2 − + 4e− → Pt(0) + 6Cl− 26465

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constant current of ca. −1.2 μA cm−2 as shown in Figure 2(iii). These behaviors at 0.90 and 0.77 V show that the deposition proceeds in nucleation and growth mode.53,54 3.2. Electrochemical Characteristics of Pt/Au(111) Electrodes in a H2SO4 Solution. Figure 3 shows CVs of the Au(111) electrodes recorded in a deaerated 0.05 M H2SO4 with a scan rate of 50 mV s−1 after Pt was deposited potentiostatically at (i) 0.92 V, (ii) 0.90 V, and (iii) 0.77 V for various durations as indicated by arrows A−H in Figure 2 and being rinsed with conc. H2SO4 and then ultrapure water. At the Pt/Au(111) electrode prepared at 0.92 V (i), i.e., small overpotential, CVs did not change much with deposition time. A broad cathodic peak for Pt oxide reduction at around 0.7 V and peaks for hydrogen UPD at potentials more negative than 0.4 V prove the deposition of Pt but they did not grew with time. The presence of a pair of peaks corresponding to the Au oxide formation (1.6 V)/reduction (1.1 V) indicates that free, i.e., uncovered by Pt, Au surface was still exposed to the solution even after 3500 s deposition. At the Pt/Au(111) electrode prepared at 0.90 V (ii), i.e., medium overpotential, the peaks due to the Pt oxide formation/reduction and hydrogen UPD increased with deposition time and became constant when the deposition time was longer than 2000 s, confirming the deposition of Pt. The peak due to Au oxide reduction decreased with deposition time and became invisible when the deposition time was longer than 2000 s, indicating the Au surface was fully covered by Pt. Anodic current at potentials more positive than 1.55 V increased with deposition time because of efficient oxygen evolution at Pt surface. Results obtained at the Pt/Au(111) electrode prepared at 0.77 V (iii), i.e., large overpotential, were similar to those obtained at the Pt/Au(111) electrode prepared at 0.90 V except for the continuous increase of the peaks due to the Pt oxide formation/reduction and hydrogen UPD with deposition time. The peak due to Au oxide reduction decreased with deposition time but did not become invisible, indicating the small fraction of the Au surface was still exposed to the solution. Shapes of these CVs, especially in hydrogen UPD region, are slightly different from that of a Pt(111) single crystal electrode. Similar results were obtained at pseudomorphic Pd thin layers deposited on Au(111) and Au(100) substrates,5,7,15,36,37,47 suggesting the growth of pseudomorphic Pt layer.

Figure 1. Linear sweep voltammogram of a Au(111) electrode measured in deaerated 0.1 M HClO4 containing 0.05 mM H2PtCl6 with a scan rate of 2 mV s−1. Electrode potential scan was negatively started from OCP (ca. 1.1 V). Arrows (i), (ii), and (iii) show the deposition potentials of 0.92, 0.90, and 0.77 V, respectively.

No cathodic peaks for underpotential deposition (UPD) of Pt were observed. We have already reported that the rough Pt layers grew on the Au(111) surface49 when the negative going scan was stopped at a potential, which is more negative than 0.69 V. In this study, potential scan was stopped at (i) 0.92 V, (ii) 0.90 V, and (iii) 0.77 V, which are more positive than 0.69 V, shown by arrows in Figure 1 and the potential was kept at these three potentials for certain periods of time. Figure 2 shows time dependencies of the electrode potential and current density when the electrode potential was scanned to and kept at (i) 0.92 V, (ii) 0.90 V, and (iii) 0.77 V. Arrows of A (200 s), B (400 s), C (1000 s), D (1500 s), E (2000 s), F (2500 s), G (3000 s), and H (3500 s) in Figure 2 represent the deposition time selected in the present study. When the potential scan was stopped at 0.92 V, cathodic current decreased immediately and became constant at ca. −0.1 μA cm−2 as shown in Figure 2(i). When the potential scan was stopped at 0.90 V, however, the cathodic current kept increasing, reached maxima of ca. −1.7 μA cm−2 at 200 s after the potential scan was stopped, and then gradually decreased and became constant at ca. −1.0 μA cm−2 as shown in Figure 2(ii). A similar trend was observed when the potential scan was stopped at 0.77 V with the maximum current of ca. −3.8 μA cm−2 at 350 s after the potential scan was stopped and

Figure 2. Time dependences of the electrode potential and current density during electrochemical deposition of Pt measured in deaerated 0.1 M HClO4 containing 0.05 mM H2PtCl6. Electrode potential scan was negatively started from OCP with a scan rate of 2 mV s−1, stopped at (i) 0.92 V, (ii) 0.90 V, and (iii) 0.77 V, and then kept at each potential for a certain period. Arrows A−H show the deposition periods; A: 200 s (black), B: 400 s (pink), C: 1000 s (light-blue), D: 1500 s (red), E: 2000 s (green), F: 2500 s (orange), G: 3000 s (purple), and H: 3500 s (blue). 26466



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Figure 3. CVs of the Pt deposited Au(111) electrodes, which were prepared at (i) 0.92 V, (ii) 0.90 V, and (iii) 0.77 V for several deposition periods; (i) A: 200 s (black dotted line), B: 400 s (pink solid line), C: 1000 s (light-blue solid line), and H: 3500 s (blue dotted line); (ii) A: 200 s (black dotted line), B: 400 s (pink solid line), D: 1500 s (red dotted line), and E: 2000 s (green solid line); (iii) A: 200 s (black dotted line), B: 400 s (pink solid line), D: 1500 s (red dotted line), E: 2000 s (green solid line), and H: 3500 s (blue dotted line). Insets: CVs with expanded current and potential scales of (i) and (ii).

Figure 4. Deposition time dependences of (a) charge density for the reduction of Au oxide (closed squares), which corresponded to amount of Au surface without Pt deposition, (b) charge density for the hydrogen UPD (closed triangles), which corresponded to amount of deposited Pt, (c) charge density for the reduction of Pt oxide (closed circles), which corresponded to amount of deposited Pt, and (d) charge density (closed diamonds), which were measured during the Pt deposition. Green, red, and blue colors present the data obtained from the Pt deposited Au(111) electrodes, which were prepared at 0.92, 0.90, and 0.77 V, respectively, for several deposition periods.

the coverages of the deposited Pt, θPt, obtained from the charges of the hydrogen UPD (open symbols) and of the Pt oxide reduction (closed symbols) as a function of (1 − θAu), in which θAu was obtained from the charge for the Au oxide reduction . When the deposition was carried out at 0.92 V [Figure 2(i)], i.e., small overpotential, the charge for the reduction of Au

3.3. Deposition Process of Pt on Au(111) Surface. Figure 4 shows the deposition time dependencies of charges (a) for the reduction of Au oxide, (b) for the hydrogen UPD, and (c) for the reduction of Pt oxide, all of which are obtained from the CVs shown in Figure 3, and (d) the charges flowed during Pt deposition obtained from Figure 2, for the deposition at 0.92 V (green), 0.90 V (red) and 0.77 V (blue) and Figure 5 shows 26467



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Figure 5. Relation between θPt, which was obtained from the charge for hydrogen UPD (open symbols) and the reduction of Pt oxide (closed symbols), and (1 − θAu), in which θAu was obtained from the charge for the reduction of Au oxide at the Pt deposited Au(111) electrodes, which were prepared at (i) 0.92, (b) 0.90, and (c) 0.77 V. Dotted lines represent θPt = (1 − θAu).

hydrogen UPD and Pt oxide reduction and (1 − θAu) with slope of 1 [Figure 5(ii)] suggest that the atomically flat Pt monolayer was epitaxially formed on the Au(111) surface in a two-dimensional, i.e., FM, growth mode, at least up to 1 ML, confirming the previously reported ex situ RSXS result that atomically flat Pt monolayer was grown on the Au(111) surface at the same overpotential after 2000 s deposition.51 In the case of two-dimensional growth of monoatomic Pt layer on Au(111) by the reaction 1, i.e., 4 electron process, the charge required to deposit a full monolayer is 888 μC cm−2. However, ca. 1800 μC cm−2 flowed by the monolayer was completed at ca. 2000 s, as shown in Figure 4(d), indicating that the cathodic current flowed during the Pt deposition was not only due to the reduction of PtCl62− (reaction 1) but about 50% of the current was due to other reduction reactions. The origin of the excess current will be discussed below. Even after the completion of the monolayer, almost constant reduction current flowed [Figure 2(ii), Figure 4(d)], suggesting the further deposition of Pt layer by two-dimensional growth mode, as the charges for hydrogen UPD and Pt oxide reduction did not change. When the deposition potential was (iii) 0.77 V [Figure 2(iii)], i.e., relatively large overpotential, charge for the reduction of Au oxide decreased with the deposition time but did not became 0 (ca. 25 μC cm−2) [Figure 4(a)], showing that about 5% of the Au surface was still uncovered by Pt even after more than 3000 s deposition. Charges for hydrogen UPD [Figure 4(b)] and Pt oxide reduction [Figure 4(c)] continuously increased with the deposition time and exceeded the values expected for atomically flat monolayer, i.e., 222 μC cm−2 for hydrogen UPD and 444 μC cm−2 for Pt oxide reduction, indicating that the surface roughness increased with increasing the deposition time. Actual values of θPt obtained by the charges both for hydrogen UPD and Pt oxide reduction are larger than those expected from the linear relations with (1 − θAu) with slope of 1 and the deviation became larger as (1 − θAu), i.e., deposition time, increased. These results suggest that the Pt layers grew in the three-dimensional, i.e., VW, mode. The comparison between the charges due to hydrogen UPD and Pt oxide reduction and that flown during Pt deposition [Figure 4(d)] showed excess current flowed at this potential than at 0.90 V. Almost constant reduction current flowed continuously after ca. 1500 s at this potential [Figure 2(iii), Figure 4(d)]. As mentioned above, the charge flown during the Pt deposition was always larger than that required to deposit a given Pt layer, the cathodic current during the Pt deposition

Figure 6. Schematically shown deposition processes of Pt on the Au(111) surface when the overpotentials are (i) relatively small, (ii) intermediate, and (iii) relatively large.

oxide decreased [Figure 4(a)] and those for the hydrogen UPD [Figure 4(b)] and Pt oxide reduction [Figure 4(c)] increased with the deposition time initially as mentioned before, showing the deposition of Pt on the Au surface. The Pt deposition, however, seemed to stop at around 400 s with the coverage of 0.25. Moreover, the charge for the reduction of Au oxide gradually increased and those for the hydrogen UPD and Pt oxide reduction gradually decreased with deposition time, suggesting the slow dissolution of deposited Pt. This is because the reversible potential for the deposition shifted negatively as the deposition reaction proceeded due to the release of Cl− ion (c.f. Reaction 1). It must be noted that only negligible current flowed at this potential after ca. 400 s [Figure 2(i), Figure 4(d)]. Linear relations between θPt obtained by the charges both for hydrogen UPD and Pt oxide reduction and (1 − θAu) with slope of 1 [Figure 5(i)] shows the two-dimensional, i.e., FM mode, growth of the Pt layer. When the deposition was carried out at 0.90 V [Figure 2(ii)], i.e., intermediate overpotential, charge for the reduction of Au oxide [Figure 4(a)] and those for the hydrogen UPD [Figure 4(b)] and Pt oxide reduction [Figure 4(c)] initially decreased and increased, respectively, with the deposition time and finally became constant at around 2000 s to be 0 [Figure 4(a)] and ca. 220 μC cm−2 [Figure 4(b)] and ca. 444 μC cm−2 [Figure 4(c)], respectively. The values of constant charges for hydrogen UPD, ca. 220 μC cm−2, and Pt oxide reduction were in a good agreement with the values expected for the epitaxial Pt monolayer on a Au(111)-(1 × 1) surface with the (1 × 1) structure, 222 and 444 μC cm−2. These results and the linear relations between θPt obtained by the charges both for 26468



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Figure 7. In situ RSXS profiles of (a) (00) rod, incident energy dependences of the scattering X-ray intensity at (b) (0 0 0.8) and (c) (0 0 2.7), (d) (01) rod, and incident energy dependence of the scattering X-ray intensity at (e) (0 1 1.7) of the Pt deposited Au(111), which were prepared at OCP just after dipping (red), at 0.90 V for 400 s (blue), for 2000 s (green), and for 3000 s (black). Closed circles and solid lines represent experimental data points and fitting curves, respectively. Vertical scales of (a) and (d) are adjusted to present all data in one figure by multiplying the intensity by numbers shown in the figures.

was not only due to the reduction of PtCl62− (reaction 1) but also due to other reduction reactions. There are two possible reactions. One is the oxygen reduction reaction (ORR) at the

deposited Pt surface. Small amounts of oxygen may dissolve during the long Pt deposition time. However, since the concentration of oxygen should be minimal and the deposition 26469



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Table 1. Structure Parameters Obtained from the Analyses of the RSXS Profiles of the Pt/Au(111) Electrode Measured in a 0.1 M HClO4 Aqueous Solution Containing 0.05 mM H2PtCl6 at OCP Just after Dipping, Based on the Three Layer Model of PtCl62−/Au/Au on the Au(111)-(1 × 1) Surface a

distance, zAus‑Au(1) (Å) distance, zAu(1)−Au(2)(Å) distance, zPtCl62‑‑Au(2)(Å) coverage, ρ Au(1) (ML) coverage, ρAu(2) (ML) coverage, ρPtCl62−(ML) RMS, σAu(1) (Å) RMS, σAu(2) (Å) RMS, σPtCl62− (Å)

from (00) rod

from incident energy dependence at (0 0 0.8)


from (01) rod


2.36 ± 0.03

2.35 ± 0.02

2.35 ± 0.03

2.36 ± 0.02

2.35 ± 0.05

2.36 ± 0.03

2.35 ± 0.03

2.35 ± 0.05

2.36 ± 0.03

2.35 ± 0.07

3.50 ± 0.05

3.50 ± 0.05

3.49 ± 0.08

3.50 ± 0.05

3.49 ± 0.07

1.00 ± 0.01

1.00 ± 0.03

1.00 ± 0.02

1.00 ± 0.03

1.00 ± 0.05

1.00 ± 0.02

1.00 ± 0.03

1.00 ± 0.03

1.00 ± 0.05

1.00 ± 0.06

0.14 ± 0.02

0.14 ± 0.05

0.14 ± 0.06

0.14 ± 0.05

0.14 ± 0.08

0.085 ± 0.030 0.15 ± 0.03 0.21 ± 0.06

0.086 ± 0.006 0.15 ± 0.05 0.21 ± 0.05

0.087 ± 0.008 0.14 ± 0.04 0.20 ± 0.06

0.086 ± 0.003 0.14 ± 0.05 0.20 ± 0.06

0.089 ± 0.008 0.16 ± 0.08 0.23 ± 0.08

a Subscriptions of Aus, Au(1), Au(2), and PtCl62− represent the Au substrate, the second outermost Au layer, the first outermost Au layer, and the adsorbed PtCl62− layer, respectively.

Table 2. Structure Parameters Obtained from the Analyses of the RSXS Profiles of the Pt/Au(111) Electrode Prepared in a 0.1 M HClO4 Aqueous Solution Containing 0.05 mM H2PtCl6 at 0.90 V for 400 s, Based on the Three Layer Model of PtCl62‑/Pt/ Au on the Au(111)-(1 × 1) Surface a

from (00) rod



from (01) rod


distance, zAus‑Au(2) (Å) distance, zAu(2)‑Pt (Å) distance, zPtCl62‑‑Pt (Å) coverage, ρAu(2) (ML) coverage, ρPt (ML) coverage, ρPtCl62− (ML) RMS, σAu(2) (Å) RMS, σPt (Å) RMS, σPtCl62− (Å)

2.36 ± 0.01

2.35 ± 0.03

2.35 ± 0.03

2.36 ± 0.03

2.36 ± 0.03

2.29 ± 0.05

2.29 ± 0.04

2.28 ± 0.02

2.29 ± 0.03

2.29 ± 0.06

1.66 ± 0.06

1.66 ± 0.07

1.66 ± 0.04

1.66 ± 0.05

1.66 ± 0.06

1.00 ± 0.02

1.00 ± 0.03

1.00 ± 0.04

1.00 ± 0.03

1.00 ± 0.07

0.21 ± 0.02 0.14 ± 0.03

0.21 ± 0.03 0.14 ± 0.03

0.21 ± 0.03 0.14 ± 0.06

0.21 ± 0.03 0.14 ± 0.05

0.21 ± 0.04 0.14 ± 0.07

0.097 ± 0.003 0.13 ± 0.07 0.27 ± 0.08

0.097 ± 0.005 0.13 ± 0.08 0.26 ± 0.09

0.099 ± 0.004 0.16 ± 0.04 0.25 ± 0.12

0.099 ± 0.007 0.12 ± 0.03 0.18 ± 0.06

0.097 ± 0.007 0.12 ± 0.07 0.20 ± 0.13

Subscriptions of Aus, Au(2), Pt, and PtCl62− represent the Au substrate, the first outermost Au layer, the deposited Pt layer, and the adsorbed PtCl62− layer, respectively.

a

but also even after the completion of the monolayer because almost constant reduction current flowed [Figure 2(ii), Figure 4(d)] but the charges for hydrogen UPD and Pt oxide reduction did not change as mentioned above. In order to precisely determine the atomic/molecular arrangement of the Pt/Au(111) interphase during the Pt deposition and confirm the two-dimensional growth beyond monolayer at 0.90 V, in situ RSXS measurements were carried out at the Au(111) electrode in a 0.1 M HClO4 solution containing 0.05 mM H2PtCl6 after several Pt deposition periods. Figure 7 shows in situ RSXS profiles of the Au(111) electrode obtained at OCP just after dipping, i.e., 0 s deposition, (red) and after 400 s (blue), 2000 s (green), and 3000 s (black) Pt deposition at 0.90 V; (a) (00) rod profile measured by an incident X-ray with 11.587 keV, (b, c) incident energy dependencies of the scattering X-ray intensity at (b) (0 0 0.8) and (c) (0 0 2.7), (d) (01) rod profile measured by an incident X-ray with 11.587 keV, and (e) incident energy dependence of the scattering X-ray intensity at (0 1 1.7). The atomic/molecular arrangements normal and parallel to the

potentials are relatively positive, the ORR current should not be so large even at the Pt(111) electrode. Another plausible cathodic reaction is the reduction of PtCl62− to PtCl42− as follows: PtCl 6 2 − + 2e− → PtCl4 2 − + 2Cl−

(2)

The difference between the redox potential of the reaction 2 and that of the reaction 1 is less than ca. 0.05 V71−73 and, therefore, both reactions 1 and 2 may take place at the same time. Pt deposition processes at (i) small, (ii) intermediate, and (iii) large overpotentials are schematically summarized in Figure 6 based on above discussions. A similar result about the overpotential dependence of growth mode was observed by in situ STM for the electrochemical deposition of Ru on Au(111).74 3.4. Determination of Surface Atomic Arrangements by in Situ RSXS Measurements. At 0.9 V (intermediate overpotential), Pt layer deposited by two-dimensional growth mode not only up to the completion of the monoatomic layer 26470



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Table 3. Structure Parameters Obtained from the Analyses of the RSXS Profiles of the Pt/Au(111) Electrode Prepared in a 0.1 M HClO4 Aqueous Solution Containing 0.05 mM H2PtCl6 at 0.90 V for 2000 s, Based on the Three Layer Model of PtCl62‑/Pt/ Au on the Au(111)-(1 × 1) Surface a

from (00) rod



from (01) rod


distance, zAus‑Au(2) (Å) distance, zAu(2)‑Pt (Å) distance, zPtCl62‑‑Pt (Å) coverage, ρAu(2) (ML) coverage, ρPt (ML) coverage, ρPtCl62− (ML) RMS, σAu(2) (Å) RMS, σPt (Å) RMS, σPtCl62− (Å)

2.35 ± 0.02

2.35 ± 0.05

2.36 ± 0.02

2.36 ± 0.03

2.35 ± 0.04

2.29 ± 0.02

2.29 ± 0.03

2.28 ± 0.02

2.29 ± 0.06

2.29 ± 0.04

3.34 ± 0.05

3.34 ± 0.05

3.34 ± 0.03

3.34 ± 0.03

3.34 ± 0.06

1.00 ± 0.01

1.00 ± 0.01

1.00 ± 0.03

1.00 ± 0.03

1.00 ± 0.06

1.00 ± 0.03 0.14 ± 0.03

1.00 ± 0.03 0.14 ± 0.05

0.98 ± 0.03 0.14 ± 0.03

1.00 ± 0.07 0.14 ± 0.06

1.00 ± 0.05 0.14 ± 0.06

0.087 ± 0.003 0.12 ± 0.08 0.20 ± 0.13

0.089 ± 0.006 0.13 ± 0.08 0.20 ± 0.08

0.092 ± 0.006 0.14 ± 0.03 0.18 ± 0.06

0.088 ± 0.007 0.13 ± 0.06 0.19 ± 0.06

0.091 ± 0.006 0.11 ± 0.05 0.18 ± 0.06

Subscriptions of Aus, Au(2), Pt, and PtCl62− represent the Au substrate, the first outermost Au layer, the deposited Pt layer, and the adsorbed PtCl62− layer, respectively.

a

Table 4. Structure Parameters Obtained from the Analyses of the RSXS Profiles of the Pt/Au(111) Electrode Prepared in a 0.1 M HClO4 Aqueous Solution Containing 0.05 mM H2PtCl6 at 0.90 V for 3000 s, Based on the Three Layer Model of PtCl62‑/Pt/ Pt on the Au(111)-(1 × 1) Surface a

distance, zAus‑Pt(2) (Å) distance, zPt(2)−Pt(1) (Å) distance, zPtCl62‑‑Pt(1) (Å) coverage, ρPt(2) (ML) coverage, ρPt(1) (ML) coverage, ρPtCl62− (ML) RMS, σPt(2) (Å) RMS, σPt(1) (Å) RMS, σPtCl62− (Å)

from (00) rod



from (01) rod


2.29 ± 0.03

2.30 ± 0.02

2.28 ± 0.04

2.29 ± 0.02

2.30 ± 0.06

2.22 ± 0.05

2.22 ± 0.02

2.21 ± 0.06

2.22 ± 0.02

2.22 ± 0.05

1.96 ± 0.06

1.96 ± 0.03

1.96 ± 0.07

1.96 ± 0.03

1.95 ± 0.05

1.00 ± 0.03

1.00 ± 0.02

1.00 ± 0.06

1.00 ± 0.04

1.01 ± 0.03

0.38 ± 0.06

0.38 ± 0.03

0.38 ± 0.07

0.38 ± 0.06

0.39 ± 0.05

0.14 ± 0.08

0.14 ± 0.03

0.14 ± 0.07

0.14 ± 0.05

0.14 ± 0.05

0.11 ± 0.03 0.18 ± 0.11 0.29 ± 0.16

0.11 ± 0.08 0.19 ± 0.07 0.30 ± 0.14

0.12 ± 0.08 0.18 ± 0.08 0.30 ± 0.08

0.11 ± 0.07 0.16 ± 0.05 0.26 ± 0.11

0.10 ± 0.05 0.16 ± 0.04 0.28 ± 0.14

a Subscriptions of Aus, Pt(2), Pt(1), and PtCl62− represent the Au substrate, the second outermost deposited Pt layer, the first outermost deposited Pt layer, and the adsorbed PtCl62− layer, respectively.

θPt. 0.14 ML of PtCl62− was found to be adsorbed not only on the Au(111)-(1 × 1) surface but also on the deposited Pt(1 × 1) surface even at OCP just after dipping of the Au(111) disk with a (√7 × √7)R19.1° structure. Analyses of (01) rod together with energy dependence data obtained at (0 1 L) position show that Pt atoms deposited only at cubic closest packing (ccp) site of Au(111), indicating that the deposited Pt layers are pseudomorphic to the Au(111) substrate at least up to 1.4 ML. The above results are in agreement with those obtained for Pd deposition at Au(111) surface.5,7,37

electrode surface were precisely determined by the least-squares fitting to the (00) rod together with energy dependence data obtained at (0 0 L) position and to the (01) rod together with energy dependence data obtained at (0 1 L) position, respectively. The structural parameters obtained from the fitting with kinematical calculations of the above-mentioned 4 samples are listed in Tables 1−4 and schematic illustrations of the side views of the obtained interfacial structures of these samples are shown in Figure 8. These results confirmed that Pt was deposited twodimensionally without roughening, i.e., layer-by-layer, on the Au(111) surface at least up to ∼1.4 ML. Figure 9 shows the relationship between θPt obtained from the charge for hydrogen UPD and that obtained from RSXS measurement. When the Pt coverage was less than 1 ML, both values agreed each other but while maximum θPt obtained from hydrogen UPD was 1 ML, θPt obtained from RSXS increased to more than 1 ML. This is because all Pt deposited on Au(111) can be detected by the RSXS measurement but only Pt atoms exposed to solution, i.e., surface Pt atoms, contribute to the electrochemically obtained

4. CONCLUSIONS Electrochemical deposition of ultrathin Pt layers on the Au(111) single crystal surface in a deaerated 0.1 M HClO4 solution containing 0.05 mM H2PtCl6 at small, intermediate, and large overpotentials were investigated. When the overpotential was small, Pt grew two-dimensionally but stopped growing below monolayer, leaving Au(111) surface uncovered by Pt. At intermediate overpotential, electrochemical measure26471



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Figure 8. Pt deposited Au(111) electrode surfaces for several deposition periods and time course of cathodic current density during Pt deposition. Overpotential during the Pt deposition is 0.90 V.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +81-3-5978-5347; fax: +81-3-5978-5347; e-mail: kondo. [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research (C) (No. 20550009) from Japan Society of Promotion of Science (JSPS) and the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics (MANA) and MEXT Program for Development of Environmental Technology using Nanotechnology from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. Synchrotron radiation experiments were performed as projects approved by the Photon Factory Program Advisory Committee (PAC Nos. 2009G038, 2010G051, and 2011G089).

Figure 9. Relation between coverage of Pt obtained from RSXS measuremens, θPt_RSXS, and that obtained from charge for hydrogen UPD, θPt_EC. Dotted line corresponds to θPt_RSXS = θPt_EC.

■

ment showed that Pt grew two-dimensionally to form atomically flat layer, which fully covers Au(111) surface. Three-dimensional growth of Pt on Au(111) was observed at large overpotential. In situ RSXS measurements showed that the electrochemical deposition of Pt at intermediate potential indeed proceeded with two-dimensional growth mode forming an epitaxial layer with pseudomorphic structure at least up to 1.4 ML. This study provides conditions for the electrochemical atomic layer epitaxial growth of Pt on the Au(111) surface.

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Electrochemical Layer-by-Layer Deposition of Pseudomorphic Pt

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