Potential-Dependent Structures and Potential-Induced Structure

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Potential-Dependent Structures and Potential-Induced Structure Changes at Pt(111) Single-Crystal Electrode/Sulfuric and Perchloric Acids Interfaces in the Potential Region between Hydrogen Underpotential Deposition and Surface Oxide Formation by In situ Surface X-ray Scattering Toshihiro Kondo, Takuya Masuda, Nana Aoki, and Kohei Uosaki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12766 • Publication Date (Web): 18 Apr 2016 Downloaded from http://pubs.acs.org on April 24, 2016

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The Journal of Physical Chemistry

Potential-Dependent Structures and PotentialInduced Structure Changes at Pt(111) Single-Crystal Electrode/Sulfuric and Perchloric Acids Interfaces in the Potential Region between Hydrogen Underpotential Deposition and Surface Oxide Formation by In situ Surface X-ray Scattering Toshihiro Kondo,

†

*,†,‡

Takuya Masuda,

§,⊥

Nana Aoki,

†,‡

and Kohei Uosaki

‡,⊥

Division of Chemistry, 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 305-0044, Japan §

Advanced Key Technologies Division, National Institute for Materials Science (NIMS),

Namiki, Tsukuba 305-0044, Japan

ACS Paragon Plus Environment

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⊥

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International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for

Materials Science (NIMS), Namiki, Tsukuba 305-0044, Japan

KEYWORDS: Pt(111) single-crystal electrode, electrode/electrolyte interfaces, sulfuric and perchloric acids, in situ surface x-ray scattering

ABSTRACT

Potential-dependent structures and potential-induced structure changes of the Pt(111) electrode in perchloric and sulfuric acid electrolyte solutions were investigated in the potential region between hydrogen the underpotential deposition (UPD) (0.05 V vs. RHE) and surface oxide formation (0.95 V) by in situ surface x-ray scattering (SXS) using the electrolyte thickness controllable spectroelectrochemical cell. In both solutions, the interfacial structures, including not only the surface arrangements of the adsorbates, but also the Pt(111) surface atomic arrangements at various potentials, were accurately determined and compared with those previously reported in the literature. Several differences and new results when compared to the previously reported results were found by in situ potential-dependent structure measurements as follows: At 0.90 V, while oxygen species, such as an adsorbed hydroxyl group (OHad), H2O, and/or H3O+ with a total coverage of 1 monolayer (ML), are adsorbed on the atop site of the Pt(111)-(1×1) surface with the (1×1) structure in the HClO4, SO42- (or HSO42-), H2O (or H3O+), and/or OHad with a total coverage of 1 ML are co-adsorbed on the atop site of the Pt(111)-(1×1) surface also with the (1×1) structure in the H2SO4. The interlayer expansion (d12) between the


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first and second outermost Pt layers is partially relaxed due to the UPD hydrogen desorption and mainly due to the adsorption of the oxygen species (OHad, H2O, and/or H3O+) in the HClO4 and due to the SO42- (or HSO4-) and H2O (or H3O+) adsorption in the H2SO4. These potential-induced structure changes were confirmed by in situ x-ray scattering intensity measurements while maintaining a certain scattering point as a function of the potential.

1. INTRODUCTION Platinum is one of the most important electrode materials not only for basic surface science and electrochemistry, but also for various industrial applications such as electrocatalysts for the oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) for the polymer electrolyte membrane fuel cell (PEMFC), and the structures and properties of low index surfaces of single-crystal platinum, especially Pt(111), have been extensively studied in electrochemical environments.1-84 The surface structures of the Pt(111) single-crystal electrode in electrolyte solutions, particularly in acidic media, have been investigated using various techniques, including conventional electrochemical methods (EC) such as thermodynamic analyses and impedance measurements,10-40 scanning tunneling microscopy (STM),41-44 atomic force microscopy (AFM),45,46 several spectroscopic techniques including infrared (IR) spectroscopy,3,5-7,47-65 second harmonic generation (SHG),66-68 sum frequency generation (SFG),69,70 reflectance spectroscopy (RFS) using UV-visible light,71 low energy electron diffraction (LEED),72,73 and x-ray photoelectron spectroscopy (XPS),13,74 and surface x-ray scattering (SXS),1,2,4,75-84 and were found to be strongly dependent on the potentials and adsorption of anion and/or oxygen species. It is noted that we concentrated here these


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dependences on the structures at the Pt(111) single-crystal electrode/electrolyte solution interfaces in an acidic medium, although a huge number of studies about methanol and formaldehyde oxidation reactions including CO adsorption/desorption and electrochemical metal deposition including underpotential deposition (UPD) on the Pt(111) electrode has been reported. In order to understand the fundamental electrochemical reactions of the Pt(111) electrode and to apply them to industrial applications, such as fuel cells and sensors, we have to clarify the potential dependent structures, including anion and/or oxygen species adsorptions, at the Pt(111) electrode/electrolyte interfaces over the entire potential range. Voltammetric profiles of the Pt(111) electrode measured in (a) 0.1 M HClO4 and (b) 0.05 M H2SO4, of which the anions are known to be non-specifically and specifically, respectively, adsorbed on the Pt(111) surface, are well established. Cyclic voltammograms (CVs) in both acidic media can be divided in 5 potential regions (shown in Fig. 1). In both acids, the potential regions of A and A’ (E (electrode potential) < 0.05 V vs. reversible hydrogen electrode (RHE)), and E and E’ (0.95 V < E) are assigned as hydrogen evolution and surface oxide formation/reduction, respectively. Briefly, in the HClO4 solution, it was found by SXS2,4,29,80 in region B (0.05 < E < 0.4 V), which is assigned as the hydrogen underpotential adsorption/desorption (H-UPD), that the interlayer spacing between the first and second outermost Pt layers (d12) expands ca. 0.004 nm (ca. 2 %) in bulk lattice spacing, found by IR3,61,63 in region C, which is assigned as the double layer charging/discharging, that a small amount of oxygen species (H2O, OH, and/or O) and/or perchlorate anions (ClO4-) are adsorbed on the Pt(111) surface, found by EC,4,13,19,30,31,36,40 IR,28,58,61 AFM,46 XPS,13 and SXS29,81 in region D (0.6 V < E < 0.95 V), which is assigned as the


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hydroxyl group (OHad) adsorption/desorption, where OHad adsorption/desorption reversibly takes place. Briefly, in the H2SO4 solution, it was found by SXS4,29,77,81,84 in region B’ (0.05 < E < 0.35 V), which is assigned as H-UPD, that d12 expands ca. 2 % as well as in the case of HClO4, found by EC,14,22,23,25,26,32-34,36 STM,41,43 IR,5,49,51-53,55-57,59,60,62-65 and SHG66,68 in region C’ (0.35 V < E < 0.6 V), which is assigned as the sulfate anion adsorption/desorption, such that the sulfate (SO42-) or bisulfate (HSO4-) anion is co-adsorbed with oxygen species such as H2O and/or H3O+ on the Pt(111) surface with a (√3×√7)R19.1° structure, found by EC,14,16,39,40 and IR28,52,61 in region D’ (0.6 V < E < 0.95 V), which is assigned as the SO42- (or HSO4-) and/or OHad adsorption/reduction, such that the SO42- (or HSO4-) anion further co-adsorbs with oxygen species such as H2O, H3O+, and/or OHad on the Pt(111) surface. In each potential region, as mentioned above, the interfacial structure analyses were carried out by measurement methods utilizing their respective advantages, but there is nothing about comprehensive structural studies in the potential region between 0.05 V and 0.95 V, i.e., in the potential region between the H-UPD and surface oxide formation. STM measurements provide structural information at an atomic resolution but only of the outermost layer, therefore, the surface structure of the electrode cannot be determined by STM when the surface is covered with adsorbates such as anions and/or water molecules. Information obtained by vibrational spectroscopy, such as IR and SFG, is only about the adsorbed species with a high time resolution, but not about the electrode surface atoms. Information obtained by other optical techniques, such as SHG and RFS, contains contributions not only from the electrode surface, but also from the bulk. From the ex situ measurements, such as XPS and LEED, we cannot obtain any information under potential control, i.e., an electrochemical environment.


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By combining the crystal truncation rod (CTR) and surface x-ray diffraction (SXRD) measurements, on the other hand, SXS is one of the best methods to investigate the threedimensional (3D) interfacial structure at an atomic level.1,2,4,75-96 For studying the Pt(111) electrode,2,4,75-84 several groups performed SXS measurements but the studied potential region was limited. Furthermore, the interfacial structure in the OHad adsorption/desorption and surface oxide formation potential regions has not yet clearly been investigated by SXS. Previously, using this in situ technique, we accurately determined the potential-dependent structures, such as not only the surface atomic arrangements of the Au(111) electrode, but also the adsorbed structures of the adsorbed anion and/or oxygen species (H2O, H3O+, and/or OHad), at the Au(111) singlecrystal electrode/sulfuric acid solution interface at various potentials.93 Moreover, their potentialinduced structure change has been recently investigated.87 In addition to these measurements, using the electrolyte thickness controllable spectroelectrochemical cell, we succeeded in accurately monitoring the potential-induced structure change at the electrode/electrolyte interface.89 In this study, the potential-dependent structures at the Pt(111) single-crystal electrode/sulfuric and perchloric acid solution interfaces were accurately determined over the entire potential region from H-UPD to surface oxide formation and their potential-induced structure change was comprehensively investigated by in situ SXS measurements.

2. EXPERIMENTAL SECTION 2.1. Materials.

The Pt(111) single-crystal disk (diameter: 10 mm, thickness: 5 mm) was

purchased from Surface Preparation Laboratory. The disk was annealed at 1600°C for more than


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10 hours under Ar and H2 (97:3 v/v%) flow using an induction heater (HOTSHOT-2 kW, Amerithem).46,75 Ultrapure reagent-grade H2SO4 and HClO4 and reagent-grade NaCl were purchased from Wako Pure Chemicals and were used without further purification. Water was purified using a Milli-Q system (Yamato, WQ-500). Ultrapure N2 (99.9995%) and Ar/H2 mixed gases (97 (99.9995%) : 3 (99.999%)) were purchased from Tomoe Shokai. A 6.0 µm thick Mylar film (Chemplex, D) was used as the window of the spectroelectrochemical cell, which was specially designed for the in situ SXS measurements and was made of Kel-F.89,94 2.2. Electrochemical and In situ SXS Measurements.

Both the cyclic voltammetry and the

in situ SXS measurements were carried out using the spectroelectrochemical cell.89,94 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). The cyclic voltammograms (CVs) were recorded on a personal computer through a data-logger (Graphtech, GL-900). A Pt wire and Ag/AgCl (saturated NaCl) electrode were used as the counter and reference electrodes, respectively. All electrode potentials were quoted vs. RHE. The spectroelectrochemical cell was set on a six-circle diffractometer (HUBER, 5020) installed in an undulator beamline BL3A at the Photon Factory. X-ray radiation was monochromated by a Si(111) double-crystal system and was focused by a Rh-coated bending mirror. The beam size of the incident x-ray was 0.5 mm (vertical) × 0.2-0.5 mm (horizontal), which was adjusted by a slit placed in front of the cell. Intensity of the incident x-ray was measured by the ion chamber, which was placed in front of the sample, in order to normalize the data with the incident x-ray intensity. The incident x-ray energy of 18.00 keV (wavelength: 0.6889 Å) was selected to avoid any fluorescence from the Pt substrate. For the SXS measurements, a hexagonal coordinate system in which H and K are parallel to the surface and L


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is normal to the surface was used. H, K, and L have units of a*, b*, and c*, respectively, which are defined as |a*| = |b*| = 4π/√3a = 2.614 Å-1 and |c*| = 2π/√6a = 0.9244 Å-1, where a is the nearest neighbor distance of 2.775 Å. Reflectivity measurements, i.e., (00) rod measurements, were carried out along the (00L) direction that is normal to the surface, and each reflectivity was a rocking-curve integrated intensity, which was normalized with the incident x-ray intensity. The SXRD profiles were measured at L = 0.2 (incident angle: ca. 0.6 °). An energy sensitive silicon device detector (SDD) was used to detect the scattered and diffracted x-rays. 2.3. Procedures.

Prior to each measurement, the Pt(111) disk was annealed at 1600°C for 2

hours under the flowing Ar and H2 mixed gas.46,75 After cooling under the Ar and H2 flow for 7 min., the surface was quenched by ultrapure water saturated with the Ar and H2 mixed gas. It was then transferred to the spectroelectrochemical cell with a drop of ultrapure water on the surface to avoid any surface contamination. Measurements were carried out in a 0.05 M H2SO4 or 0.1 M HClO4 electrolyte solution, which was deaerated by passing ultrapure N2 gas through the solution for more than 30 min before it was injected into the cell. The Pt(111) electrode made contact with the electrolyte solution while keeping the potential at 0 V (vs. Ag/AgCl), where the electrode surface was covered with HUPD. Only the (111) face was in contact with the electrolyte solution during the electrochemical measurements. For the potential-dependent structure measurements, electrode potential was scanned in the positive or negative direction from 0 V, then stopped at a certain potential at which the SXS measurement was going to be carried out, while keeping the thickness of the solution layer between the electrode and Mylar window at ca. 5 mm. After holding the potential for more than 10 min, the SXS measurements were carried out with the electrolyte thickness of ca. 30 µm, using the electrolyte thickness controllable spectroelectrochemical cell,89 so that x-ray scattering


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by the electrolyte layer becomes minimal. The electrolyte thickness was estimated from the reflectivity with and without the electrolyte solution on the electrode. The cell was turned over and then the in situ SXS measurements were carried out while flowing ultrapure N2 gas in the cell cover. For the potential-induced structure change measurements, the cell was turned over before the potential scan from 0 V and the Mylar window pushed to the electrode with the electrolyte thickness of ca. 100 µm, using the electrolyte thickness controllable spectroelectrochemical cell,86 while keeping the potential of 0 V. The potential was then scanned during the monitoring of the scattered and/or diffracted x-ray at a certain scattering position. 2.4. Data Analysis.

In the reflectivity curves, several peaks of the scattering x-ray intensity

near L = 0, 3, and 6 (H = K = 0) were observed. The peaks around L = 3 and 6 for Pt(111) correspond to the cubic (1,1,1) and (2,2,2), respectively. Between these peaks, the reflectivity was low and depended on the detailed structure of the interface. Structures along the direction normal to the electrode surface were quantitatively determined from the least-square fitting to the reflectivity data with a kinematical calculation based on a specific interfacial model. All the fittings were carried out using models of the three layers on top of the Pt(111)-(1×1) surface. The amounts of various atoms, ions, and molecules, such as Pt, oxygen species (O), perchlorate anion (ClO4-), sulfate anion (SO42-), and sulfur (S), in each layer were estimated by considering that one monolayer (ML) corresponds to 1.50 × 1015 cm-2. Using the estimated electrolyte thickness as described above, in the fitting, a three-layer structure model, where each layer consists of platinum, oxygen, sulfur, chlorine, sulfate anion, or perchlorate anion, was assumed. Fittings with all possible structure models were carried out, and the fitting result, which gave the lowest


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χ2 value among all the fitting results, was taken as a best-fit result. For the platinum oxide layers formed at the potential more positive than 0.90 V, the contribution of oxygen was neglected, because the electron density of the oxygen atom is only 10% that of the platinum atom. Although oxygen atoms adsorbed on the Pt(111) surface were observed in the XP spectra which were measured in the ultra-high vacuum (UHV),74 we considered that the oxygen atoms do not exist in the electrochemical environment, then the observed oxygen species were postulated to be H2O (or H3O+) and/or OHad in the discussion.

3. RESULTS AND DISCUSSIONS 3.1. Potential-Dependent Structure Measurements in 0.1 M HClO4. Figure 2 shows a typical CV of a Pt(111) single-crystal electrode in deaerated 0.1 M HClO4 between 0.05 V – 0.95 V, obtained at the scan rate of 1 mV s-1 in the spectroelectrochemical cell with the electrolyte thickness of ca. 100 µm. When the potential was scanned to a potential more positive than 0.95 V, a high anodic current due to the surface oxide formation was observed.11,12,65,73 Because once the surface oxide film forms, the surface atomic arrangement of Pt(111) is collapsed and it is no longer returned back to the original, then the shape of the CV is changed, thus the limit of the positive scanning was 0.95 V, where the OHad is completely adsorbed. This CV shape well matched the one in Fig. 1(a), confirming that the cell resistance under the present electrochemical conditions with the electrolyte thickness of ca. 100 µm is negligible at least at the scan rate of 1 mV s-1.


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The in situ SXS measurements for the potential-dependent structure analysis were carried out at the potentials indicated by arrows a – e in Fig. 2, i.e., a at 0.22 V in the potential region B, b at 0.39 V where the UPD hydrogen is desorbed, c at 0.57 V, which was more negative than the potential where the oxidation current due to the OHad adsorption was observed, d at 0.75 V which was a more negative potential than the sharp spike due to the observed OHad adsorption, and e at 0.90 V which was a more positive potential than that the current observed due to the OHad adsorption. The reflectivity profiles measured at these potentials are shown in Fig. 3(a). The standard error values of the scattered x-ray intensity are indicated in the data circles. The structure parameters obtained from the least-square fitting with a kinematical calculation at various potentials are listed in Table 1. Based on these values obtained from the fitting, the structures along the direction normal to the surface at the Pt(111) electrode/HClO4 interface were determined as described below. a. At 0.22 V.

The best-fit data at this potential showed that only the interlayer distance,

d12, between the first and second outermost Pt layers is expanded by ca. 2.2 % of the bulk lattice spacing, as schematically illustrated in Fig. 4a. This ca. 2.2 % expansion normal to the surface was reported from the results of the SXS measurements.2,4,29,80 Lateral expansion was not observed from this measurement, because the reflectivity shows only the surface structure normal to the surface. However, the SXRD peak intensity corresponding to the surface (1×1) structure at this potential was slightly lower than those observed at the more positive potential, as described below, revealing that the surface atomic arrangement of the Pt atoms is also expanded. No oxygen species were observed on this Pt(111) surface, although some amount of adsorbed oxygen species (maybe H2O molecule) was observed on the reconstructed Au(111) surface.86,93


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b. At 0.39 V.

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Figure 3(a)b shows the reflectivity profile measured at 0.39 V, which was

just more positive than the potential of the H-UPD current (Fig. 2). The best-fit data at this potential showed that an ca. 1.3 % expansion of d12 still remained, as schematically shown in Fig. 4b. Based on the SXS results, Marković et al. reported that the d12 expansion was relaxed after the UPD hydrogen was completely desorbed in the HClO4.2,4,29,80 This discrepancy is probably caused by the difference in the experimental conditions. By using the electrolyte thickness controllable spectroelectrochemical cell,89 in the present study, the electrolyte thickness was accurately controlled for all the measurements. For the potential-dependent structure measurements, the electrode potential was scanned while keeping the electrolyte thickness at ca. 5 mm. After the potential scan was stopped at certain potentials (at 0.39 V in this case), the electrolyte thickness was reduced to ca. 30 µm and then the CTR measurements were carried out. For the potential-induced structure change measurements as described below, on the other hand, the electrolyte thickness was kept at ca. 100 µm, where it was confirmed to be able to monitor the interfacial structures with the potential scan rate of 1 mV s-1.89 In these measurements, however, both the potential-dependent structure measurement and potentialinduced structure change measurement, which was the so-called x-ray voltammetry (XRV), were carried out in the same configuration, i.e., the same electrolyte thickness, which was not clearly shown. In addition, they used 2 – 5 mV s-1, which was slightly higher than that in the present study, as the scan rates in the XRV measurements. Moreover, the number of data points in the CTR curves in the present study was more than that by them. Therefore, the present results seemed to be more reliable. c. At 0.57 V.

Figure 3(a)c shows the reflectivity profile measured at 0.57 V, which was

more negative than the potential where the OHad current just started to flow (Fig. 2). The best-fit


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data showed at this potential that the 1.3 % surface expansion of d12 still remained and a small amount of ClO4- and/or oxygen species was adsorbed on the Pt(111)-(1×1) surface, as schematically shown in Fig. 4c. It is known that ClO4- is non-specifically adsorbed on the Pt(111) surface. Actually, Hirota et al. reported that the peak intensity of the absorption band corresponding to ClO4- was lower than the detector limit for the IR measurement,61 but Ogasawara et al. observed the band corresponding to ClO4- in the IR spectrum at this potential.64 Tripkovic et al. observed the ClO4- adsorption at this potential and they interpreted that this is due to Cl-, which is produced by the decomposition of ClO4- or is a comtaminant in the HClO4 solution.3,30 On the other hand, it was reported that a small amount of ClO4- was co-adsorbed with water molecules on the poly-crystalline Pt surface by the electrochemical,97 electrochemical quartz crystal microbalance (EQCM),98,99 and ellipsometry100 measurements. The fitting did not match well when the adsorbate was only oxygen species (H2O and/or H3O+) and was only Cl-. Only when the co-adsorbate was ClO4- and oxygen species did the fitting match well in the present study, suggesting that a small amount of ClO4- is co-adsorbed with oxygen species on the Pt(111) surface, but not form the ordered structure, at this potential. d. At 0.75 V.

Figure 3(a)d shows the reflectivity profile measured at 0.75 V, which was

just more negative than the potential where the OHad sharp spike was observed at 0.79 V (Fig. 2). The best-fit data showed at this potential that the surface expansion of d12 was relaxed, and about half a monolayer of oxygen species (OHad, H2O, and/or H3O+) was adsorbed on the Pt(111)(1×1) surface, as schematically shown in Fig. 4d. It seemed that the stronger oxygen species adsorption induces a larger relaxation of the first outermost Pt layer than the weaker adsorption of water and ClO4-. A p(2×2)_3O structure was observed in the SXRD profiles at this potential (Fig. 5). In the UHV, an oxygen atom was adsorbed on the Pt(111)-(1×1) surface with the


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p(2×2)_O structure at a coverage of 0.25 ML,4,101 but there were no reports about this p(2×2)_3O structure both in the UHV and electrochemical environments. Because the peak intensities in these SXRD profiles were not so high that a part of the surface was covered with these adsorbates. Interfacial structure at this potential was discussed more detail below. e. At 0.90 V.

Figure 3(a)e shows the reflectivity profile measured at 0.90 V, which was

more positive than the potential of the OHad current (Fig. 2). The best-fit data showed at this potential that just 1 ML of oxygen species (OHad, H2O, and/or H3O+) was adsorbed on an atop site of the Pt(111)-(1×1) structure, as schematically shown in Fig. 4e. From charge of the OHad current, Gómez-Marin et al.11,12 and Marković et al.29,31 proposed that 1/2 ML of OHad is adsorbed on the Pt(111)-(1×1) surface at this potential. From the density functional theory (DFT) calculation and XPS results, on the other hand, Bondarenko et al. proposed that 1/3 ML of OHad is co-adsorbed with 2/3 ML of H2O on the atop site with a (3×1) structure.13 Their proposed structure matched well the present result. However, we cannot conclude from the present result whether 1 ML of oxygen species is 1/2 ML of OHad and 1/2 ML of H2O or 1/3 ML of OHad and 2/3 ML of H2O. In any case, it was found that 1 ML of oxygen species is adsorbed on the atop site with the (1×1) structure at this potential. As described in the section below, the increase in the scattering intensity at (0 1 0.2), which indicates the formation of the surface (1×1) structure, supported this conclusion. Actually, Michaelides and Hu reported the results of the DFT calculations that OHad is adsorbed at the atop site with a coverage of 1 ML.102 3.2. Potential-Induced Structure Change Measurements in 0.1 M HClO4. In the top views in Fig. 4, the thin white rhomboids correspond to the surface (1×1) structure, showing that formation and deformation of these (1×1) arrangements denote the


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increase and decrease, respectively, of the intensity at the scattering point of (0 1 0.2). It is noted that the intensity at (0 1 0.2) represents not only the surface (1×1) structure, but also the (1×1) bulk of Pt. As in Fig. 3(b), the most typical difference between the outermost Pt layer expansion and its relaxing was observed at L = 5.1. Thus, we employed (0 1 0.2) and (0 0 5.1) scattering points as the monitoring points, which reflect the d12 value and the surface (1×1) structure, respectively. Figure 6 shows the potential dependences of (a) the current density, (b) the scattering intensity at (0 0 5.1), and (c) the scattering intensity at (0 1 0.2), measured in the deaerated 0.1 M HClO4 at the scan rate of 1 mV s-1. In the positive scan, the (0 0 5.1) intensity gradually decreased from 0.05 V to 0.39 V, which was associated with the desorption of UPD hydrogen, remained constant from 0.39 V to ca. 0.60 V, steeply decreased from ca. 0.6 V to ca. 0.9 V, which was associated with the adsorption of oxygen species (OHad, H2O, and/or H3O+) then became constant. In the negative scan, the (0 0 5.1) intensity remained constant from 0.95 V to ca. 0.85 V, steeply increased from ca. 0.85 V to ca. 0.55 V, which was associated with the desorption of oxygen species (OHad, H2O, and/or H3O+), remained constant from ca. 0.55 V to 0.39 V, then gradually increased from 0.39 V to 0.05 V, which was associated with the H-UPD. These results indicate that the surface relaxation of d12 partially and mainly takes place due to UPD hydrogen desorption and the oxygen species (OHad, H2O, and/or H3O+) adsorption, respectively. In the positive scan, the (0 1 0.2) intensity gradually increased from 0.05 V to 0.39 V, which was associated with the desorption of UPD hydrogen, remained constant between 0.39 V and ca. 0.5 V, and steeply increased with two-steps from ca. 0.65 V to ca. 0.9 V, which was associated with the adsorption of oxygen species (OHad, H2O, and/or H3O+). In the negative scan,


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the (0 1 0.2) intensity steeply decreased with two-steps from ca. 0.9 V to ca. 0.55 V, which was associated with the desorption of oxygen species (OHad, H2O, and/or H3O+), remained constant between ca. 0.55 V and 0.39 V, and gradually decreased from 0.39 V to 0.05 V, which was associated with the H-UPD. The (0 1 0.2) intensity change in the H-UPD potential region B indicates that the lateral surface relaxation takes place during the desorption of the UPD hydrogen. The two-step (0 1 0.2) intensity change in the OHad adsorption/desorption potential region D indicates that the adsorption/desorption of oxygen species (OHad, H2O, and/or H3O+) takes place in two modes; one is the 2D nucleation/growth mode of the (1×1) structure and the other is the phase change from random to (1×1) through the p(2×2) structure. Gómez-Marin et al. interpreted the two types of adsorption processes;11,12 one is a random adsorption followed by a disorder-order phase transition in the OHad28 and the other is two dissociative adsorptions of OHad from two different kinds of water, such as from an adsorbed water molecule on the Pt(111) surface and from a water molecule in the solution.19 The intensity change at both the (0 0 5.1) and (0 1 0.2) scattering points had a hysteresis in the adsorption/desorption of oxygen species (OHad, H2O, and/or H3O+), but no hysteresis in the H-UPD region, indicating that the former process is rather slow as compared to the latter, even though a symmetric oxidation/reduction current wave due to the adsorption/desorption of oxygen species (OHad, H2O, and/or H3O+) was observed. When the positive scan was not stopped at 0.95 V and continued, the (0 1 0.2) intensity steeply decreased as associated with the high oxidation current for the surface oxide formation, indicating that the place-exchange reaction of oxygen species (OHad, H2O, and/or H3O+) with the surface Pt atoms takes place at a potential more positive than 0.95 V, as well as in the case of


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Au(111).87,93 However, the surface atomic arrangement of Pt(111) is not returned back to the original state when the surface oxide is reduced, although that of Au(111) is returned back, suggesting that the place-exchange reaction of Pt(111) proceeds not only at the outermost Pt layer but also in the bulk. 3.3. Potential-Dependent Structure Measurements in 0.05 M H2SO4. Figure 7 shows a typical CV of a Pt(111) single-crystal electrode in the deaerated 0.05 M H2SO4 between 0.05 V and 0.95 V obtained at the scan rate of 1 mV s-1 in the spectroelectrochemical cell with the electrolyte thickness of ca. 100 µm. When the potential was scanned to a potential more positive than 0.95 V, an anodic current due to the surface oxide formation was observed.36,37,39,40 Because once the surface oxide film forms, the surface atomic arrangement of Pt(111) has collapsed and it is no longer returned back to the original, then the shape of the CV is changed, thus limiting the positive scanning to 0.95 V. This shape of the CV well match the one in Fig. 1(b), confirming that the cell resistance under the present electrochemical condition with the electrolyte thickness of ca. 100 µm is negligible at least at the scan rate of 1 mV s-1. The SXS measurements for the potential-dependent structure analysis were carried out at potentials indicated by arrows a’ – d’ in Fig. 7, i.e., a’ at 0.22 V in the potential region B, b’ at 0.36 V just between the adsorption/desorption potential regions of the H-UPD (B) and SO42(HSO4-) (C), c’ at 0.59 V, which was more negative than the potential where the adsorption/desorption current was observed, and d’ at 0.90 V which was a more positive potential than when the small oxidation/reduction current was observed. The reflectivity profiles measured at these potentials are shown in Fig. 8(a). The standard error values of the scattered x-


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ray intensity are denoted in the data circles. The structure parameters obtained from the leastsquare fitting with the kinematical calculation at various potentials are listed in Table 2. Based on these values obtained from the fitting, the structures along the direction normal to the surface at the Pt(111) electrode/H2SO4 interface were determined as described below. a’. At 0.22 V.

The best-fit data at this potential showed that only the interlayer distance,

d12, between the first and second outermost Pt layers has expanded by ca. 3.1 % of the bulk lattice spacing, as schematically illustrated in Fig. 9a’, as well as in the case of HClO4. A slightly greater expansion in the H2SO4 was observed than in the HClO4, suggesting that these measurements were carried out after the potential scan between 0.05 V and 0.59 V including the stronger SO42- (or HSO42-) adsorption potential region in the H2SO4 while not including the stronger OHad adsorption potential region in the HClO4. This expansion normal to the surface was also reported based on the results of the SXS measurements.4,29,77,81,84 Lateral expansion was observed because the SXRD peak intensity corresponding to the surface (1×1) structure at this potential was slightly lower than those observed at the more positive potential, as described below, indicating that the surface has also expanded in the lateral direction. No oxygen species were observed on this Pt(111) surface, although some amount of adsorbed oxygen species (maybe H2O molecule) was observed on the reconstructed Au(111) surface.86,93 b’. At 0.36 V.

Figure 8(a)b’ shows the reflectivity profile measured at 0.36 V, which was

just between the adsorption/desorption potential regions of the H-UPD (B’) and SO42- (HSO4-) (C’). The best-fit data at this potential showed that the surface relaxation did not take place and the ca. 1.8 % expansion of d12 still remained, as schematically shown in Fig. 9b’. Based on the SXS results, Marković et al. reported that the d12 expansion was relaxed after the UPD hydrogen


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was completely desorbed.4,29,77,81,84 This discrepancy is probably caused by the same reasons described above in the HClO4. c’. At 0.59 V.

Figure 8(a)c’ shows the reflectivity profile measured at 0.59 V, which was

more negative than the potential of the current for the adsorption/desorption of SO42-. At this potential, the results of the IR and STM measurements indicated that ca. 0.2 ML of SO42- (or HSO4-) co-adsorbed with the oxygen species such as H2O and/or H3O+ on the Pt(111) with the (√3×√7) structure. In order to confirm whether the surface is relaxed or still expanded, the fitting was first carried out when the three-layer model was assuming that the first, second, and third layers are SO42- (or HSO4-) and oxygen species (1 : 1), Pt, and Pt, respectively. In this case, the best-fit data showed that the surface is completely relaxed and 0.22 ML of SO42- (or HSO4-) and oxygen species (1 : 1) co-adsorbed on the Pt(111)-(1×1) surface. This adsorbed amount of SO42(or HSO4-) and oxygen species (1 : 1) well matched that adsorbed by the (√3×√7) structure. The fitting was then carried out when the three-layer model was assumed that the first, second, and third layers were O, S, and O, respectively. The best-fit data at this potential showed that the Pt(111)-(1×1) substrate was covered with the first, second, and third layers composed of 0.20 ML of oxygen, 0.22 ML of sulfur, and 0.83 ML of oxygen species, respectively. The distances between the first and second, the second and third, and the third and Pt(111)-(1×1) substrate layers were 0.151 nm, 0.108 nm, and 0.131 nm, respectively. These numbers are in good agreement with those estimated from the model observed by STM41 and IR.51 They suggested that SO42- (or HSO42-) and H2O (or H3O+) were co-adsorbed on the Pt(111)-(1×1) surface with the (√3×√7)R19.1° structure with a molar ratio of 1:1 through three oxygen atoms and three (or two) hydrogen atoms, respectively, as schematically shown in Fig. 9c’. In the SXRD for the scan along the √3 direction, we also observed the SXRD peak at (1/√3 1/2√3) shown in Fig. 10,


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confirming the existence of this superstructure. Unfortunately, we observed no peaks in the SXRD profile in the scan along the √7 direction, because of its relatively large distance between the adsorbed SO42- (or HSO42-) ions. This interfacial structure was also supported by a Monte Carlo simulation.28 Garcia-Araez et al. indicated by thermodynamic electrochemistry studies that the oxidation and reduction spikes observed around 0.52 V suggest the disorder/order phase transition.14,17 Relatively high root mean square (RMS) values of these layers (Table 2) and relatively high noise in the SXRD profiles (Fig. 10) reflect that the surface structure of the adsorbates has several types of defects and/or the surface is rough. d’. At 0.90 V.

Figure 8(a)d’ shows the reflectivity profile measured at 0.90 V, which was

a more positive potential where the small oxidation/reduction current peaks were observed around 0.77 V (Fig. 7). The best-fit data at this potential showed that the Pt(111)-(1×1) substrate was covered with the first, second, and third layers composed of 0.62 ML of oxygen, 0.33 ML of sulfur, and 2.15 ML of oxygen species, respectively. The distances between the first and second, the second and third, and the third and Pt(111)-(1×1) substrate layers were 0.140 nm, 0.108 nm, and 0.150 nm, respectively. The SXRD peak intensity corresponding to the surface (1×1) structure at this potential was greater than those observed at the more negative potentials, as described below, indicating that the adsorbates existed on the Pt(111)-(1×1) surface with the (1×1) structure, as well as in the case in the HClO4. Higher wavenumber shifts and a peak intensity increase of the IR band for the S-O stretching vibration of the sulfate anion suggested that SO42- (or HSO4-) was co-adsorbed with H2O (or H3O+) on the Pt(111)-(1×1) surface through three (3 fold-symmetry) and two (2 fold-symmetry) oxygen atoms with a molar ratio of 1:1.51,62 Garcia-Araez et al. demonstrated by thermodynamic electrochemistry studies14,17 that the SO42(not HSO42-) was more adsorbed at this potential than at 0.59 V with a small amount of OHad co-


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adsorption. They also suggested that the “hump” shaped oxidation and reduction currents observed around 0.78 V and 0.74 V, respectively (Figs. 1(b) and 7), showed the reaction without a significant phase change.14,17 Actually, no superstructures, except for the surface (1×1) structure, were observed by the present SXRD measurements at this potential. When considering together all of these results, the structure at this potential can be schematically shown in Fig. 9d’. It is noted that each SO42-, OHad, and H2O was not arranged in order but they were adsorbed at the atop site and formed the surface (1×1) structure. The relatively high RMS values of these layers (Table 2) reflect that the surface structure of the adsorbates has several types of defects and/or the surface is rough. 3.4. Potential-Induced Structure Change Measurements in 0.05 M H2SO4. As well as the case in the HClO4, in the top views of Fig. 9, the thin white rhomboids correspond to the surface (1×1) structure, suggesting that the formation and deformation of these (1×1) arrangements mean the increase and decrease, respectively, of the intensity at the scattering point of (0 1 0.2). It is noted that the intensity at (0 1 0.2) represents not only the surface (1×1) structure, but also the (1×1) bulk of Pt. As in Fig. 8(b), as well as the case in the HClO4, the most typical difference between the outermost Pt layer expansion and its relaxing was observed at L = 5.1. Thus, we employed the (0 1 0.2) and (0 0 5.1) scattering points as the monitoring points, which reflect the d12 value and the surface (1×1) structure, respectively, as in the case in the H2SO4. Figure 10 shows the potential dependences of (a) the current density, (b) the scattering intensity at (0 0 5.1), and (c) the scattering intensity at (0 1 0.2), measured in the deaerated 0.05 M H2SO4 at the scan rate of 1 mV s-1.


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In the positive scan, the (0 0 5.1) intensity gradually decreased from 0.05 V to ca. 0.4 V, which was associated with the desorption of UPD hydrogen, steeply decreased from ca. 0.4 V to ca. 0.6 V, which was associated with the co-adsorption of SO42- (or HSO4-) and H2O (or H3O+), then became almost constant. In the negative scan, the (0 0 5.1) intensity remained constant from 0.95 V to ca. 0.55 V, steeply increased from ca. 0.55 V to ca. 0.35 V, which was associated with the co-desorption of SO42- (or HSO4-) and H2O (or H3O+), then gradually increased until 0.05 V, which was associated with the H-UPD. These results indicate that the slight and main surface relaxations of d12 take place due to the UPD hydrogen desorption and the SO42- (or HSO4-) and H2O (or H3O+) co-adsorption, respectively. In the positive scan, the (0 1 0.2) intensity slightly increased from 0.05 V to ca. 0.35 V, which was associated with the desorption of UPD hydrogen, remained constant between ca. 0.35 V and ca. 0.65 V in the co-adsorption potential region C’, steeply increased from ca. 0.65 V to ca. 0.75 V, which was associated with the further co-adsorption of SO42- (or HSO4-) and H2O (or H3O+), then gradually increased to 0.95 V. In the negative scan, the (0 1 0.2) intensity gradually decreased from 0.95 V to ca. 0.8 V, steeply decreased from ca. 0.8 V to ca. 0.6 V, which was associated with the small desorption amount of SO42- (or HSO4-) and H2O (or H3O+), remained constant between ca. 0.6 V and 0.36 V in the co-desorption potential region C’, and gradually decreased from 0.39 V to 0.05 V, which was associated with the H-UPD. The (0 1 0.2) intensity change in the H-UPD potential region B’ indicates that lateral surface relaxation takes place during the desorption of UPD hydrogen, as well as in the case in the HClO4. Based on these results, it is indicated that the surface relaxation mainly takes place by the co-adsorption of SO42- (or HSO4-) and H2O (or H3O+). On the other hand, the surface (1×1)


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structure due to the adsorbates mainly undergo the further co-adsorption of SO42- (or HSO4-) and H2O (or H3O+). The intensity change at both the (0 0 5.1) and (0 1 0.2) scattering points had a hysteresis in the adsorption/desorption of SO42- (or HSO4-) and H2O (or H3O+), but no hysteresis in the HUPD region, indicating that the former process is rather slow as compared to the latter, even though the symmetric oxidation/reduction current waves and spikes due to the adsorption/desorption of SO42- (or HSO4-) and H2O (or H3O+) were observed around 0.5 V. When the positive scan was not stopped at 0.95 V and continued, the (0 1 0.2) intensity steeply decreased as associated with the oxidation current for the surface oxide formation, indicating that the place-exchange reaction of the adsorbed oxygen species with the surface Pt atoms takes place at a potential more positive than 0.95 V, as well as in the case of Au(111).87,93 However, the surface atomic arrangement of Pt(111) does not return back to the original state when the surface oxide is reduced, although that of Au(111) does return, suggesting that the place-exchange reaction of Pt(111) proceeds not only at the outermost Pt layer, but also in the bulk, as well as in the case in the HClO4.

4. CONCLUDING REMARKS The potential-dependent structures at the Pt(111) single-crystal electrode/0.1 M HClO4 and /0.05 M H2SO4 electrolyte solution interfaces were accurately determined by in situ surface x-ray scattering (SXS) over the entire potential region between the hydrogen-underpotential adsorption/desorption (H-UPD) and surface oxide formation. In both electrolyte solutions, in the


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H-UPD potential region, expansion of the interlayer spacing (d12) between the first and second outermost Pt layers was observed. In the more positive potential region, the co-adsorptions of OHad, H2O, and/or H3O+, and SO42- (or HSO4-) with H2O and/or OHad were observed with the (1×1) structure in the HClO4 and H2SO4, respectively. It was found that not the desorption of UPD hydrogen, but these adsorptions induced the relaxation of the d12 expansion. The potentialinduced structure changes at these interfaces were also investigated by in situ SXS. The surface expansion is partially relaxed due to the UPD hydrogen desorption in both solutions and mainly due to the oxygen species (OHad, H2O, and/or H3O+) adsorption and due to the SO42- (or HSO4-) and H2O (or H3O+) co-adsorption in the HClO4 and H2SO4, respectively.


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Figure 1. Typical cyclic voltammograms of the Pt(111) single-crystal electrodes measured in (a) 0.1 M HClO4 and (b) 0.05 M H2SO4 with a scan rate of 20 mV s-1 at a meniscus mode. Potential regions of A – E in (a) and A’- E’ in (b) are explained in the text.


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Figure 2. Cyclic voltammogram of the Pt(111) electrode measured in a deaerated 0.1 M HClO4 between 0.05 V – 0.95 V, obtained with a scan rate of 1 mV s-1 in the spectroelectrochemical cell with the electrolyte thickness of ca. 100 µm. Arrows a - e represent the potentials where the SXS measurements were carried out.


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Figure 3. (a) The reflectivity profiles of the Pt(111)/0.1 M HClO4 interface a at 0.22 V (red) in the potential region B, b at 0.39 V (orange) (× 100) at the potential boundary between B and C, c at 0.57 V (purple) (× 10000) at the potential boundary between C and D, d at 0.75 V (light blue) (×1000000) in the potential region D, and e at 0.90 V (blue) (× 100000000) at the potential boundary between D and E. (b) The reflectivity profiles between L = 3 and 6 obtained at 0.22 V (red), at 0.39 V (orange), at 0.57 V (purple), at 0.75 V (light blue), and at 0.90 V (blue). The circles and solid lines are the experimental data and calculated curves, respectively, fitted by the least-squares method with a kinematical calculation using the models with three layers on the Pt(111) substrate as shown in Figure 4.


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Figure 4.

The schematic illustrations of the Pt(111) electrode/HClO4 interface at various

potentials. Side and top views are left and right sides, respectively. In the top views, the thin white rhomboids in a, b, c, d, and e and white thick rhomboid in d represent (1×1) surface unit lattices and p(2×2) unit lattice, respectively.


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Figure 5. SXRD profiles measured at L = 0.2 in the (a) (01) and (b) (11) directions at 0.75 V in the deaerated 0.1 M HClO4.


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Figure 6. Potential dependences of (a) the current density, (b) the (0 0 5.1) intensity, and (c) the (0 1 0.2) intensity of the Pt(111) electrode measured in the deaerated 0.1 M HClO4 with the scan rate of 1 mV s-1. Positive and negative going scans represent red and blue curves, respectively.


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Figure 7. Cyclic voltammogram of the Pt(111) electrode measured in a deaerated 0.05 M H2SO4 between 0.05 V – 0.95 V, obtained with a scan rate of 1 mV s-1 in the spectroelectrochemical cell with the electrolyte thickness of ca. 100 µm. Arrows a’ – d’ represent the potentials where the SXS measurements were carried out.


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Figure 8. (a) The reflectivity profiles of the Pt(111)/0.05 M H2SO4 interface a’ at 0.22 V (red) in the potential region B’, b’ at 0.36 V (orange) at the potential boundary between B’ and C’ (× 100), c’ at 0.59 V (light blue) (× 10000) at the potential boundary between C’ and D’, and d’ at 0.90 V (blue) (×1000000) at the potential boundary between D’ and E’. (b) The reflectivity profiles between L = 3 and 6 obtained at 0.22 V (red), at 0.36 V (orange), at 0.59 V (light blue), and at 0.90 V (blue). The circles and solid lines are the experimental data and calculated curves, respectively, fitted by the least-squares method with a kinematical calculation using the models with three layers on the Pt(111) substrate as shown in Figure 9.


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Figure 9. The schematic illustrations of the Pt(111) electrode/H2SO4 interface at various potentials. Side and top views are left and right sides, respectively. In the top views, the thin white rhomboids in a’, b’, c’, and d’ and white rectangle in c’ represent (1×1) surface unit lattices and (√3×√7)R19.1° lattice, respectively.


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Figure 10. SXRD profiles measured at L = 0.2 in the (a) (1 1/2) direction (b) azimuthal scan at 0.59 V in the deaerated 0.05 M H2SO4.


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Figure 11. Potential dependences of (a) the current density, (b) the (0 0 5.1) intensity, and (c) the (0 1 0.2) intensity of the Pt(111) electrode measured in the deaerated 0.05 M H2SO4 with the scan rate of 1 mV s-1. Positive and negative going scans represent red and blue curves, respectively.


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Table 1. Structural Parameters Obtained from the Analyses of the Reflectivity Profiles at Pt(111) Electrode/Perchloric Acid (0.1 M HClO4) Interface at Various Potentials as a ThreeLayer Model on Pt(111)a.

Potential (vs. RHE)

0.22 V

0.39 V

0.57 V

0.75 V

0.90 V

fm1

fPt

fPt

fClO4+O

fO

fO

fm2

fPt

fPt

fPt

fPt

fPt

fm3

fPt

fPt

fPt

fPt

fPt

coverage, ρm1 / ML

0.93 ± 0.06

0.90 ± 0.05

0.26 ± 0.17

0.51 ± 0.09

1.01 ± 0.06

coverage, ρm2 / ML

0.99 ± 0.05

0.97 ± 0.03

0.97 ± 0.07

0.97 ± 0.06

0.99 ± 0.06

coverage, ρm3 / ML

1.00 ± 0.01

0.99 ± 0.03

0.99 ± 0.03

0.98 ± 0.03

1.00 ± 0.03

distance, dm12 / nm

0.232 ± 0.003

0.230 ± 0.003

0.246 ± 0.036

0.134 ± 0.014

0.146 ± 0.005

distance, dm23 / nm

0.228 ± 0.003

0.228 ± 0.002

0.230 ± 0.002

0.227 ± 0.005

0.227 ± 0.004

distance, dm3s / nm

0.227 ± 0.002

0.227 ± 0.002

0.227 ± 0.002

0.227 ± 0.002

0.227 ± 0.002

RMS, σm1 / nm

0.087 ± 0.016

0.091 ± 0.036

0.30 ± 0.02

0.33 ± 0.02

0.088 ± 0.059

RMS, σm2 / nm

0.098 ± 0.035

0.099 ± 0.045

0.18 ± 0.03

0.12 ± 0.04

0.092 ± 0.067

RMS, σm3 / nm

0.091 ± 0.017

0.090 ± 0.037

0.099 ± 0.020

0.089 ± 0.039

0.089 ± 0.039

The atomic form factors, fPt, fClO4+O, and fO are of Pt, ClO4- + O (1:1), and O, respectively. The term fm means the atomic form factor of specie of m. The subscripts of m1, m2, and m3 represent the first layer, second layer, and third layer, respectively. Distances of dm12, dm23, and dm3s represent atomic layer distances between the first and second layers, between the second and third layers, and between the third layer and Pt(111)-(1×1) layer as a substrate, respectively. a


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Table 2. Structural Parameters Obtained from the Analyses of the Reflectivity Profiles at Pt(111) Electrode/Sulfuric Acid (0.05 M H2SO4) Interface at Various Potentials as a Three-Layer Model on Pt(111)a.

0.22 V

0.36 V

0.59 Vb

0.59 Vc

0.90 V

fm1

fPt

fPt

fSO4+O

fO

fO

fm2

fPt

fPt

fPt

fS

fS

fm3

fPt

fPt

fPt

fO

fO

coverage, ρm1 / ML

0.93 ± 0.06

0.90 ± 0.06

0.22 ± 0.09

0.21 ± 0.09

0.62 ± 0.04

coverage, ρm2 / ML

0.98 ± 0.05

0.96 ± 0.04

0.98 ± 0.05

0.22 ± 0.06

0.33 ± 0.06

coverage, ρm3 / ML

1.00 ± 0.01

0.98 ± 0.03

0.99 ± 0.02

0.83 ± 0.03

2.15 ± 0.13

distance, dm12 / nm

0.234 ± 0.002

0.231 ± 0.002

0.342 ± 0.022

0.151 ± 0.014

0.140 ± 0.007

distance, dm23 / nm

0.228 ± 0.002

0.227 ± 0.002

0.227 ± 0.003

0.108 ± 0.005

0.108 ± 0.005

distance, dm3s / nm

0.227 ± 0.001

0.226 ± 0.002

0.227 ± 0.002

0.131 ± 0.002

0.150 ± 0.002

RMS, σm1 / nm

0.11 ± 0.01

0.10 ± 0.03

0.35 ± 0.12

0.43 ± 0.12

0.36 ± 0.06

RMS, σm2 / nm

0.10 ± 0.03

0.11 ± 0.05

0.13 ± 0.03

0.32 ± 0.09

0.33 ± 0.08

RMS, σm3 / nm

0.10 ± 0.01

0.069 ± 0.009

0.087 ± 0.013

0.28 ± 0.04

0.22 ± 0.08

Potential (vs. Ag/AgCl)

The atomic form factors, fPt, fSO4+O, fO, and fS are of Pt, SO42- (HSO4-) + O (1:1), O, and S, respectively. The term fm means the atomic form factor of specie of m. The subscripts of m1, m2, and m3 represent the first layer, second layer, and third layer, respectively. Distances of dm12, dm23, and dm3s represent atomic layer distances between the first and second layers, between the second and third layers, and between the third layer and Pt(111)-(1×1) layer as a substrate, respectively. bThree-layer model was fitted assuming that the first, second, and third layers are SO42- (HSO4-) and O species (1:1), Pt, and Pt layers, respectively. cThree-layer model was fitted assuming that the first, second, and third layers are O, S, and O, respectively. a


41


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Page 42 of 58

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: +81-3-5978-5347. Author Contributions All authors contributed equally.

ACKNOWLEDGMENT This work was partially supported by Grants-in-Aids for Scientific Research (C) (No. 26410008) and for Scientific Research (KAKENHI) in Priority Area of “3D Active Sites” (No. 15H01045) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. TK acknowledges an open-lab program at the Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN) in National Institute for Materials Science (NIMS). We wish to thank Dr. Tamura of the Japan Atomic Energy Agency for his help with the CTR fitting program. The synchrotron radiation experiments were performed as projects approved by the Photon Factory Program Advisory Committee (PAC Nos. 2012G506, 2013G095, 2014G124, and 2015G121).


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BRIEFS Pt(111) Single-Crystal Electrode/HClO4 and /H2SO4 Interfaces


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TOC


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Potential-Dependent Structures and Potential-Induced Structure

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