Fast Structure Determination of Electrode Surfaces for Investigating

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

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Fast Structure Determination of Electrode Surfaces for Investigating Electrochemical Dynamics Using Wavelength-Dispersive X‑ray Crystal Truncation Rod Measurements Tetsuroh Shirasawa,*,†,‡ Takuya Masuda,§,∇ Wolfgang Voegeli,∥ Etsuo Arakawa,∥ Chika Kamezawa,∥,# Toshio Takahashi,∥ Kohei Uosaki,§,○ and Tadashi Matsushita⊥ †

National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan ‡ PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan § Global Research Center for Environment and Energy Based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan ∥ Department of Physics, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan ⊥ Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan ∇ Research Center for Advanced Measurement and Characterization, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan ○ International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *

ABSTRACT: Determining the atomic structure across electrolyte−electrode interfaces with a sufficient temporal resolution is crucial to understanding how electrochemical reactions proceed. Surface X-ray diffraction is a well-established method for determining interface structures at the atomic scale. However, existing measurement methods are often incapable of quantifying timedependent structural changes during electrochemical processes because acquiring a diffraction rod profile sufficient for structure determination usually takes a longer time than the rate of the structural changes. This report demonstrates that the wavelength-dispersive method, which can acquire a range of the diffraction rod profile at once, is capable of the time-resolved analysis of electrochemical dynamics on a time scale of seconds and less, using electrochemical reactions on Pt(111) electrode surface as examples. In the case of the electrochemical oxidation of methanol, the quantitative analysis of the transient vertical displacement of the Pt(111) surface atomic layer gives evidence for a structural relaxation of the CO poisoning layer during its oxidative stripping. Present limitations and future prospects of the method are also discussed.

1. INTRODUCTION

Surface X-ray diffraction (SXRD) is used routinely as a tool for determining the structure of electrochemical interfaces.5−11 X-ray crystal truncation rod (CTR) scattering profiles appearing in the direction perpendicular to the interface can provide the depth profile of the electron density across the interface nondestructively with an accuracy of 10−2−10−1 Å, which is an unique, invaluable ability. However, conventional CTR measurements, which use monochromatic X-rays of 10− 30 keV and a point or two-dimensional (2D) detector, are not suited to time-resolved quantitative measurements because the acquisition of a CTR profile data set requires the rotation of the sample and detector, resulting in a measurement time of several tens of minutes or more. This is in most cases longer than the relaxation time of the structural changes. In previous studies,

Methods for characterizing solid−liquid interfaces on the appropriate time and length scale have been developed for decades, motivated by scientifically interesting and technologically important phenomena occurring at interfaces, such as wetting/dewetting, corrosion, electrodeposition, and electrochemical energy conversion.1 For electrochemical interfaces, considerable insights into atomic-scale processes have been provided by utilizing in situ techniques, such as scanning probe microscopy, optical spectroscopy, and synchrotron-based X-ray techniques, on atomically clean and well-ordered crystalline electrode surfaces.2−4 However, many dynamical aspects of these processes remain to be clarified, mainly due to the difficulty in observing the dynamical processes through the electrolyte layer with a sufficient spatial and temporal resolution. © XXXX American Chemical Society

Received: October 3, 2017

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DOI: 10.1021/acs.jpcc.7b09784 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

In this paper, we demonstrate the capability of the wavelength-dispersive method for time-resolved quantitative structure determination, using electrochemical reactions on Pt(111) single-crystal electrodes as examples. We used two kinds of electrolyte solutions: 0.1 M HClO4 and 0.1 M HClO4/ 0.5 M CH3OH, which are hereafter referred to as HClO4 solution and methanol solution, respectively. The electrochemical processes on Pt(111) electrodes in HClO4 solutions have been well-established,2,4 and thus we employed this system to examine the validity of the fast structure determination. The electrochemical oxidation of methanol on the Pt(111) was used to show the capability for investigating electrochemical dynamics. The analysis provides information on the dynamics of the COads poisoning layer (the subscript “ads” indicates that the species is adsorbed on the surface), which is the representative inactive reaction intermediate of the methanol decomposition.23−38

structural changes of electrode surfaces, such as the creation/ destruction of superstructures11−14 and oxidation15,16 and etching17 of electrodes, were studied by monitoring a specific point of a diffraction rod. Such a single-point monitoring can observe the occurrence of structural changes, but it is incapable of determining the atomic positions, which often represent the interaction between the electrode and reactants. Recent advances in high-brilliance synchrotron radiation sources, X-ray optics, and position-sensitive detectors have made the development of the high-speed SXRD methods possible.18,19 Gustafson et al. presented a grazing-incidence reflection geometry using high-energy monochromatic X-rays (∼85 keV) and a large 2D detector, which enables a number of sections of CTRs to be measured at once, just like reflection high-energy electron diffraction (RHEED).18 Magnussen et al. adopted a normal-incidence transmission geometry20,21 to electrochemical interfaces using high-energy monochromatic X-rays (40−70 keV) and a 2D detector to obtain an in-plane SXRD pattern at once.19 These methods can observe in-plane structures during interface processes with a typical time resolution of 1 s. However, the acquisition of CTR profiles for the out-of-plane structure determination still requires a sample rotation. The resulting total acquisition time can be on the order of 100 s or more.18 The present authors have developed a method that can measure a CTR profile in a single acquisition with a time resolution of 1 s.22 This method uses a wavelength-dispersive, convergent X-ray beam and a 2D detector (Figure 1a). A CTR profile can be acquired simultaneously without moving the sample or detector. Its potential for fast structure determination in the depth direction is an advantage as compared to the high-energy monochromatic SXRD techniques.

2. EXPERIMENTAL SECTION 2.1. CTR Measurements in the Wavelength-Dispersive Mode. The experimental layout of the wavelength-dispersive CTR measurement is shown in the Supporting Information (Figure S1). The measurements were performed at beamline NW2A of the Photon Factory Advanced Ring (PF-AR) at KEK. The heart of this method is the use of a wavelength-dispersive (polychromatic), convergent X-ray beam generated from a white synchrotron X-ray beam by using a curved crystal polychromator.22 The polychromatic beam has a one-to-one relationship between each wavelength component and its traveling direction. Using this beam, a range of a CTR profile can be observed simultaneously as a function of wavelength. The polychromator was a double-side-polished Si(110) wafer with a thickness of 0.1 mm, and the Si 111 reflection was used in the transmission geometry. Upstream of the polychromator, two flat mirrors coated with Rh were used to eliminate higherorder harmonics, and a flat-bent Rh-coated mirror was used for vertical focusing. The energy range of the polychromatic X-ray beam was 16−23 keV. The high-energy cutoff is determined by the absorption edge of Rh. The beam size at the sample position was 0.15 mm (fwhm) in both the vertical and horizontal directions. The scattering intensity of each wavelength component was recorded at different pixels on a 2D detector (PILATUS-100 K, DECTRIS Ltd.). The integrated intensity along the CTR can be obtained from a single image.22 The pixel size is 172 × 172 μm2, and the number of pixel is 487 × 195. The distance between the sample and detector was 650 mm. The flux of the polychromatic X-rays, measured at the detector position, was about 109 photons/s in 0.01 keV bandwidth. A Pt(111) single-crystal disk (10 mm in diameter and 2 mm in thickness) was used as the working electrode. The clean (111) surface was prepared by heating the Pt disk at 1600 °C under Ar/H2 (97:3 v/v%) flow using an induction heater.10 A Pt wire was used as the counter electrode, and a Ag/AgCl electrode was used as the reference electrode. For the in situ CTR measurements, a thin-layer-type electrochemical cell made of PCTFE, similar to the one in ref 5, was used. A 6 μm thick Mylar film was used as the X-ray window. After the surface preparation, the sample surface was dipped in deaerated ultrapure water (Millipore Direct-Q) under Ar/H2 atmosphere. The sample was transferred to the in situ electrochemical cell in air, with the surface protected by the water droplet.

Figure 1. (a) Schematic illustration of the in situ wavelength-dispersive X-ray CTR scattering measurement of the Pt(111) electrode surface. (b) The (00L) rod intensities measured in the methanol solution during the potential scan of 5 mV/s (symbols) and the intensities calculated for the optimized surface structure models (solid lines). (c) The scattering intensities at L = 2.8 and 3.2 of the (00L) rod (the positions are indicated by arrows in (b)) during the potential scan in the HClO4 solution and (d) in the methanol solution. The reciprocal space index L is based on the lattice constant of 6.797 Å in the [111] direction, which corresponds to the length of the body diagonal of the bulk fcc lattice. B


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thus optimized only Δz1 and the occupancy, with Δz2 and u fixed at known values. According to the simulations shown in the Supporting Information (Figure S3), the change of the CTR profile is dominated by Δz1. We found that the optimized value of Δz1 is slightly affected by the chosen values of Δz2, but the difference is within the uncertainty (Figure S4). For the HClO4 solution system (0.1 M HClO4), Δz2 was fixed at 0.0 according to the previous CTR studies.10,15 For the methanol solution system (0.1 M HClO4/0.5 M CH3OH), Δz2 was fixed at 0.02 Å, according to the analysis of static CTR data with a wider profile range (Figure S2). In the present CTR range, the CTR is insensitive to u (Figure S3), and thus it was fixed at 0.1 Å in both systems. It should be noted that neither the timeresolved CTR data nor the static CTR data could determine whether adsorbates such as H, OH, or CO exist on the surface, similarly to the previous reports,12,13 due probably to their much weaker X-ray scattering and likely to their structural incoherency as compared to Pt.

The CTR profile and electrochemical data were simultaneously recorded. To ensure the mass transport near the sample surface in the thin-layer-type cell, the thickness of the electrolyte layer above the sample surface was set to more than 1 mm. The recorded CTR intensities were normalized by the spectrum of the incident polychromatic X-ray beam to obtain the quantitative CTR profile to be analyzed. In order to eliminate the energy dependence of the X-ray absorption of the electrolytes, the incident spectrum was measured with the incident beam passing through the electrolyte layer above the sample surface. The difference in the X-ray path length in the electrolyte layer was estimated to be ∼1% between the CTR measurements and the incident spectrum measurements, and thus the energy-dependent X-ray absorption effect can be expected to be almost completely eliminated by the normalization. Each data shown below is a single acquisition data. We did not integrate the data of repetitive measurements because the CTR intensity gradually decreased during successive measurements, probably due to a deterioration of surface quality. 2.2. Data Analysis. In the structure analysis, we optimized a surface structure model to reproduce the measured (00L) rod (specular reflection) profile by least-squares fitting. We used a three-layer model that preserves the number of surface Pt atoms (see Figure 2d), considering that the Pt atoms do not

3. RESULTS AND DISCUSSION A typical detector image with an acquisition time of 1 s is shown at the lower part of Figure 1a. The (00L) rod profiles are seen on both sides of the Pt 111 Bragg peak (masked with a tungsten wire). Figure 1b shows the CTR intensity profiles (symbols) at the electrode potential of 0.0 V (vs reversible hydrogen electrode, RHE) and 1.0 V in the methanol solution, which were recorded simultaneously with cyclic voltammetry (CV). During the potential scans, the change of the intensity was nearly opposite on the lower and upper side of the Bragg peak, as displayed in Figure 1c and 1d. Such an asymmetric change is mainly caused by the vertical displacement of the surface atomic layer Δz1 (see Figure S3). In the present systems, the displacement is induced by electrode−adsorbate interactions. The intensity change in the HClO4 solution does not show a clear hysteresis in the potential loop. In stark contrast, in the methanol solution the change is hysteretic in the potential region of ca. 0.5−0.8 V. Figure 2 displays the optimized values of Δz1 (a positive value means an outward displacement) and the occupancy with the simultaneously measured CV in the HClO4 solution. We mention that the CV measured in the in situ thin-layer-type electrochemical cell contains additional currents from the illdefined side surface of the sample, and thus it is rather different from that only from the (111) surface (dashed line in Figure 2c), which was measured in a hanging meniscus cell prior to the CTR measurement. The difference does not affect the following discussion. The values of Δz1 show a clear potential dependence (Figures 2a and 2d), while the changes of occupancy are within the uncertainty (Figures 2b and 2e). The reason for the relatively large uncertainty of the occupancy is that the CTR is rather insensitive to the occupancy in the profile range (Figure S3). The parameter becomes more important in the region further away from the bulk Bragg peak, particularly near the so-called anti-Bragg points such as L = 1.5 and 4.5, the midpoints between two bulk Bragg peaks. Similar results were obtained in the methanol solution system. Hereafter we discuss the results of Δz1. We compare the values of Δz1 with the previous static CTR analysis10 to show the validity of the time-resolved analysis. For the potential scan rate of 50 mV/s (Figure 2a), in the potential region of 0.0−0.4 V the change of Δz1 is caused by the underpotential deposition of hydrogen (negative-going scan) and its desorption (positive-going scan). The average value at

Figure 2. (a, d) Displacement and (b, e) occupancy of the surface atomic layer of the Pt(111) electrode during the potential scan in the HClO4 solution and (c, f) the simultaneously measured CV. The potential scan rate is 50 mV in (a)−(c) and 100 mV/s in (d)−(f). The surface structure model is shown in (d), where the dashed line indicates the atomic layer position in the bulk. The dashed curve in (c) is a CV in a hanging meniscus electrochemical cell, which was measured prior to the CTR measurement.

dissolve in the solutions in the present potential range.6,16 The possible fitting parameters are the displacement from the bulk position in the depth direction for the surface layer (Δz1) and the second layer (Δz2), the occupancy of the surface layer, and positional fluctuation u (Debye−Waller effect) of the surface layer. We found that the single acquisition CTR profile data (Figure 1b) is not sufficient to optimize all the parameters: generally, the full optimization requires a wider range of CTR such as shown in Figure S2. In the time-resolved analysis, we C


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The Journal of Physical Chemistry C 0.2 V over the positive- and negative-going scans is 0.04 ± 0.02 Å, which agrees with the previous study within the uncertainty.10 In the so-called double-layer region of ca. 0.4− 0.6 V, the surface is almost free from specific adsorbates. The average value at 0.55 V is 0.02 ± 0.01 Å, which also agrees with the previous value at 0.57 V.10 In the potential range of 0.6−0.9 V, the change of Δz1 is caused by the adsorption (positivegoing scan) and desorption (negative-going scan) of the oxygen species, mostly OHads.10 The current peak at ca. 0.8 V corresponds to the specific adsorption/desorption of OHads. The average value at 0.9 V is 0.03 ± 0.01 Å as in the previous study.10 Essentially the same results were obtained for a faster scan rate of 100 mV/s (Figure 2d). The good agreement with the previous static analysis indicates the validity of the timeresolved analysis for the determination of the most dominant structural parameter Δz1. In the methanol solution, a hysteretic structural change is observed as shown in Figure 3a, which is correlated with the

oxidative stripping of the COads layer, which is known as COads + OHads → CO2 + H+ + e−.32 It is consistent with the decrease in COads population observed by static infrared absorption spectroscopy (IRAS) experiments.30,31 At a more positive potential, the coverage of oxygen species, mostly OHads, is known to increase. The average value of Δz1 at 0.8 V is 0.03 ± 0.02 Å, which coincides with the value in the HClO4 solution at 0.8 V where the surface is mostly covered with OHads (cf. Figures 2a and 2d). In the negative-going scans, the value of Δz1 increases in the potential region of 0.8−0.4 V through a pathway different from the positive-going scan, which corresponds to the reductive desorption of OHads and the creation of COads on the free Pt sites. The hysteretic behavior is more apparent in the faster scan rate of 50 mV/s (Figures 3c and 3d). In the positive-going scan, the decrease in Δz1 is delayed until ca. 0.8 V, and the COads stripping is not completed even at ca. 1.0 V as indicated by the larger value of Δz1 than that in the slower scan rate (cf. Figure 3a). The results convincingly demonstrate the widely accepted idea that the slow stripping of the COads layer is the major rate-determining step of the methanol oxidation in the present potential range. Such a change in COads population during the potential scan was reported on Pt polycrystalline films deposited on a silicon prism by using time-resolved surface-enhanced IRAS in the attenuated total reflection mode (ATR-SEIRAS)25,26 but has not been reported on single-crystal electrodes due to the technical difficulties.3 The behavior of COads is more clearly shown by potential step experiments with a time resolution of 0.5 s. In Figure 4a,

Figure 3. (a, c) Displacement of the surface atomic layer of the Pt(111) electrode during the potential scans in the methanol solution and (b, d) the simultaneously measured CV. The potential scan rate is 5 mV in (a, b) and 50 mV/s in (b, d).

well-known hysteretic CV shown in Figure 3b. The structural change is reasonably interpreted as being caused by the reaction intermediate, COads. In the potential region of 0.0−0.4 V, it is known that most of the surface is covered with the COads.25−31 The average value of Δz1 over the potential region is 0.11 ± 0.01 Å. The large displacement is probably caused by the strong interaction between COads and Pt.39,40 Since other possible intermediates such as HCHO and HCOOH are only weakly adsorbed on the surface or even partially dissolve,24 their contribution to the structural change would be fairly small. The value of Δz1 coincides with that of the Pt(111)-(2 × 2)-3CO ordered structure formed in methanol-free CO-dissolved HClO4 solutions, 0.09 ± 0.02 Å, as determined by the static CTR analysis.13 The agreement implies the formation of a similar adsorption structure even in the methanol solution. We note that the half-order diffraction spots from the (2 × 2) structure were not observed in the methanol solution likely because the long-range ordering is prevented by other adsorbates. In Figure 3a, the value of Δz1 decreases abruptly at ca. 0.6 V in the positive-going scan, associated with the steep increase in anodic current due to the methanol oxidation at the COads-free Pt site.35−37 Thus, the decrease in Δz1 is probably due to the

Figure 4. Time evolutions of the displacement of the surface atomic layer of the Pt(111) electrode in the methanol solution and the simultaneously measured anodic current after a potential step (a) from 0.0 to 0.8 V and (b) from 0.9 to 0.0 V.

after the potential step from 0.0 to 0.8 V, the decrease in Δz1 due to the COads stripping proceeds gradually in several tens of seconds. In contrast, in Figure 4b, after the reversal potential step from 0.9 to 0.0 V the growth of the COads layer almost finishes in a few seconds. It is notable that Δz1 does not decrease for about 5 s after the positive-going potential step, as highlighted in Figure 4a. In the induction time Δz1 increases slightly, instead of decreasing, with a slight increase in anodic current. A similar behavior is observed in the potential scan experiments (Figure 3): in the positive-going scans Δz1 begins to increase slightly at ca. 0.4 V with the slight increases in anodic current, and then it begins to decrease at ca. 0.6 and 0.8 V in Figure 3a and 3c, respectively. This change, first Δz1 increases and then decreases during the COads stripping, does D


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is a function of wavelength and glancing angle; in the case of the (00L) rod, L = 2d sin α/λ, where d is the lattice spacing. Such an approach has been used to measure X-ray reflectivity data with the same X-ray optics as in the present experiment.59 When the sample is rotated 45° from the horizontal plane around the direction of the X-rays with the minimum energy, the L range can be extended from 2.5−3.5 to 2.5−5.0. Such an extended (00L) rod data, which includes the anti-Bragg point of L = 4.5, would allow a more accurate structure determination along the depth direction. At present, however, the X-ray flux is insufficient to measure the CTR near the anti-Bragg point, which is more than 10 times weaker than the CTR near the Bragg peak. The upcoming even brighter X-ray sources such as diffraction-limited storage rings and optics adapted to these sources would greatly advance the wider CTR profile measurements. Furthermore, nonspecular CTR data is necessary for the in-plane structure analysis. The dispersive method can also measure a nonspecular CTR profile,22 but at present the 2D detector can measure only one of either the specular CTR or a nonspecular CTR. Using a sufficiently large 2D detector, the simultaneous acquisition of the specular and a nonspecular CTRs would be possible in the wavelength- and glancing angle-dispersive setup, although brighter X-ray sources might be required.

not match with a simple model that the area of the COads layer simply decreases without a relocation of COads throughout the stripping. In such a model, Δz1 should start to decrease from the beginning. The initial increase in Δz1 indicates that the COads layer is relaxed to a more strongly adsorbed, less dense COads layer simultaneously with the stripping, which can be achieved if the COads is mobile on the surface during the stripping. This idea is reasonable, considering that the COads is reported to be mobile on the surface terrace41,42 and that the adsorption energy of COads increases with decreasing COads coverage due to the suppression of the repulsive intermolecular interaction.43 An example of such a structural change is the (2 × 2)-3CO (0.75 ML) → (√19 × √19)-13CO (13/19 ML) phase transition occurring in the methanol-free CO-dissolved HClO4 and H2SO4 solutions, which is caused by the decrease in COads coverage by a positive-going potential step.44−49 The averaged value of Δz1, 0.13 Å, in the (√19 × √19) structure46 is much larger than the value, 0.09 Å, in the (2 × 2) structure,13 probably due to the stronger CO−Pt interaction in the (√19 × √19) structure.50,51 The idea of the structural relaxation of COads layer was proposed by Markovic et al. for the COads oxidation of the (2 × 2) structure,52 based on the single-point monitoring of SXRD spots. Osawa et al. also suggested its occurrence on polycrystalline film electrodes in the COdissolved H2SO4 solution, based on time-resolved ATRSEIRAS measurements.41 However, firm structural evidence for the formation of a strongly adsorbed COads layer has not been provided. The present quantitative time-resolved analysis demonstrates the structural relaxation more clearly and its occurrence even in the methanol oxidation. The occurrence of the structural relaxation indicates the process of COads stripping which was proposed for the COads layer formed in the methanol-free CO-dissolved solutions.41,55,56 In this idea, the COads oxidation is initiated in the vicinity of the steps,53,54 where the OHads is preferentially formed even at lower potentials than that needed on the (111) terrace (ca. 0.6 V).55−58 The COads on the terrace diffuses to the steps, so as to compensate the concentration gradient near the steps, and the less dense COads layer is simultaneously formed. When the COads layer becomes so sparse and the potential is so high that the OHads can be formed on the terrace, the COads oxidation can occur even at the terrace and accelerate the COads stripping, as indicated by the abrupt decrease in Δz1. Finally, we discuss the present limitations of the wavelengthdispersive method and possible solutions. In the present work, owing to the limited L range of the single acquisition CTR data, only the most dominant structural parameter Δz1 could be successfully determined while the other minor parameters Δz2 and u were fixed at the plausible values. For a full optimization of unknown surface structures, a wider range of CTR data is required. In the (00L) profile measurements, the ratio of the maximum L value to the minimum L value is almost equal to the ratio of the maximum X-ray energy and minimum X-ray energy, which is 23 keV/16 keV ∼ 1.4 in the present work. Therefore, the expected L range is from 2.5 to 3.5. We note that the data below L = 2.6 and above L = 3.4 were omitted for the analysis (see Figure 2b) because of their weak CTR signal. One way of extending the L range is to utilize the convergence angle of the fan-shaped polychromatic X-ray beam (see Figure 1a) to change the glancing angle in addition to the X-ray wavelength.59 Such a wavelength-dependent glancing angle α(λ)can further extend the range of CTR, since the momentum transfer

4. CONCLUSIONS In conclusion, we demonstrated the capability of the simultaneous CTR profile measurement in the wavelengthdispersive mode to study the structural change of electrode surface layers, using the electrochemical processes on Pt(111) electrodes as examples. In the HClO4 solution, the results of the fast structure determination of the surface atomic layer were consistent with the well-established electrochemical models. During the electrochemical oxidation of methanol, the structural changes induced by the reaction intermediate COads layer were observed. The structural change due to the oxidative stripping of the COads layer is well correlated with the increase in anodic current, illustrating the so-called COads poisoning effect. The analysis demonstrated the transient formation of a strongly adsorbed, less dense COads layer in the beginning of the COads stripping. This indicates that the reaction steps proposed for COads oxidation in CO-dissolved solutions, that is the initial COads oxidation near active step sites and the following diffusion of terrace COads to the step sites, occur also in the methanol solution. In the present status, the dispersive CTR method is very useful for investigating structural changes during electrochemical reactions, but the analysis depends to some degree on prior knowledge of initial and/or final structures. Possible improvements that would help to move toward the capability for full determination of unknown surface structures were discussed.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09784. Details on the experimental layout and data analysis (PDF)

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Tetsuroh Shirasawa: 0000-0001-5519-6977 Kohei Uosaki: 0000-0001-8886-3270 Present Address #

Department of Materials Structure Science, SOKENDAI (The Graduate University for Advanced Studies), Tsukuba, Ibaraki, 305-0801, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by JST, PRESTO Grant Number JPMJPR13C5, and by JSPS KAKENHI Grant Number 26105008. This study was also supported by and partly conducted at the Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN). The synchrotron radiation experiments were performed at PF-AR and PF with the approval of Photon Factory Program Advisory Committee Proposal Numbers 2013S2-001 and 2015S2-009.

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Fast Structure Determination of Electrode Surfaces for Investigating

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