PT-100

In-situ observation of Pt oxides on the low index planes of Pt with surface enhanced Raman spectroscopy

Fumiya Sugimuraa, Nanami Sakaia, Tetsuya Nakamuraa, Masashi Nakamuraa, Katsuyoshi Ikedab, Toshio Sakaic, Nagahiro Hoshi*a

1. Introduction

In-situ vibrational spectra of Pt oxides that cannot be measured with IR spectroscopy have been studied on the low index planes of Pt using surface enhanced Raman spectroscopy by bare Au nanoparticles (NPSERS). Two bands appear around 570 and 340 cm-1 at higher potentials in 0.1 M HClO4 saturated with Ar, which are assigned to stretching vibration of Pt- O(H) and libration vibration of Pt-O, respectively. NPSERS spectra are measured in O2 saturated solution for the first time. The band intensities of Pt-O(H) and Pt-O in O2 saturated solution are enhanced significantly compared with those in Ar saturated solution. The onset potentials of Pt-O and Pt-O(H) formation are 1.15 V(RHE) on Pt(100) and 1.2 V(RHE) on Pt(111) and Pt(110). The onset potential of Pt-O and Pt-O(H) and band shape differ from the results using shell isolated surface enhances Raman spectroscopy (SHINERS). The Pt-O and Pt-O(H) band intensities are normalized using COad as an internal standard. Pt-O(H) band intensity depends on surface structures as Pt(110) < Pt(111) << Pt(100), whereas Pt-O band gives different intensity order for Pt(111) and Pt(110) as Pt(111) ≤ Pt(110) << Pt(100) in O2 saturated solution. studied using voltammograms in 0.1 M HClO .5 The reversible redox peaks between 0.6 and 0.85 V(RHE) are assigned to the Polymer electrolyte fuel cells (PEFCs) attract attention as promising energy conversion system. However, there are various problems that must be solved for the wide spread of PEFCs. One of the major issues is the high cost of PEFCs due to the use of Pt in the electrocatalysts. For the wide spread of PEFCs, it is necessary to reduce the loading amount of Pt by the development of electrocatalysts that have high activity for the oxygen reduction reaction (ORR). The activity for the ORR strongly depends on the surface structure according to the studies using Pt single crystal electrodes.1-9 Previous papers report that Pt oxides prevent the ORR1-4,10 elucidation of the formation and reduction of PtOH, and an irreversible anodic peak around 1.06 V(RHE) is assigned to the formation of PtO. Pt oxides were also studied in ultrahigh vacuum (UHV): PtOH and PtO were found using electron energy loss spectroscopy (EELS)13 and X-ray photoelectron spectroscopy (XPS) on the low index planes of Pt.14,15 Vibrational spectroscopy has been used for the study of Pt oxides in electrochemical environments. Surface enhanced Raman spectroscopy (SERS) observed two bands around 555 and 335 cm-1 above 1.1 V(RHE) on roughened Pt film electrode in 0.1 M HClO .16 Deuterium isotope effect is found in 555 cm-1 band, whereas 335 cm-1 band does not give significant nature of Pt oxides is also one of the most important subjects for the development of electrocatalysts with high activity for the ORR. Nature of Pt oxides was studied on Pt nanoparticles using X-ray absorption fine structure (XAFS) in electrochemical environments.11 Magnussen et al. reported the oxide formation on Pt(111) using in-situ surface X-ray diffraction (SXD) recently.12 However, nature of Pt oxides has not been fully elucidated on single crystal electrodes of Pt under potential control. The oxide formation on Pt(111) single crystal electrode was deuterium isotope effect. They assigned the 555 and 335 cm-1 bands to Pt-OH stretching vibration (νPt-OH) and libration vibration of Pt-O (δPt-O), respectively. Similar SERS bands were also observed using sphere segment void of Au covered by thin Pt film.17,18 Infrared reflection absorption spectroscopy (IRAS) found the band of bending vibration of Pt-O-H around 1080 cm-1 on the low and high index planes of Pt in 0.1 M HF.19,20 The order of the integrated band intensity of the bending vibration at 0.90 V(RHE) is opposite to that of the ORR activity. This fact shows that PtOH is one of the blocking species of the ORR. However, PtO cannot be measured using IRAS because no IR window can observe bands below 600 cm-1 in aqueous solutions. SERS can measure Pt-O (~500 cm-1) because it uses visible laser and optical window. However, conventional SERS cannot be applied to well-defined single crystal electrodes, because the electrode surface must be roughened in nm scale for the excitation of plasmon. Ikeda et al. and Tian et al. applied SERS to single crystal electrodes using Au nanoparticles for the plasmon excitation. Ikeda et al. uses bare gold nanoparticles (gap-mode plasmon excitation), 21-24 and Tian et al. covers Au nanoparticles with electrically inert shell of SiO2 (shell-isolated nanoparticle- enhanced Raman spectroscopy (SHINERS)).25,26 SHINERS has successfully observed many adsorbed species on well-defined single crystal electrodes.27-30 Recently, Koper et al. reported platinum oxide during electrochemical oxidation of Pt(111) and Pt(100) with SHINERS.31 They detected a single broad Raman band at 590 cm-1 above 1.3 V(RHE) and assigned it to amorphous PtO at the subsurface. However, voltammograms and XPS studies show that PtO is formed above 1.1 V(RHE) on Pt(111), and conventional SERS also detected PtO above 1.1 V(RHE).16 However, no PtO band is found at this potential using SHINERS. In this paper, we have measured PtO that cannot be detected by IRAS on the low index planes of Pt with in-situ surface enhanced Raman spectroscopy using bare Au nanoparticles (NPSERS). We adopted bare Au nanoparticles to obtain higher enhancement than SHINERS where electric field of plasmon is attenuated by silica layer. SERS also enhances the signals of impurities remarkably; we synthesized gold nanoparticles using ultrasonic method32 where no capping reagent is used. It is difficult for SHINERS and NPSERS to compare band intensities between different samples because the band intensity strongly depends on the corrugation of Au nanoparticles. Therefore, we normalized NPSERS band intensity using COad band as internal standard for quantitative surfaces in Ar atmosphere. The solution was dried in Ar atmosphere for the nanoparticles to be fixed on the surfaces with the coverage of 0.3-0.4. Voltammograms and Raman spectra were measured in 0.1 M HClO4 saturated with Ar or O2. The solution was prepared with 60% HClO4 (Ultrapur, Kanto Kagaku) and ultrapure water treated with Milli-Q Advantage A10 (Millipore). D2O (99.9%) was purchased from Wako Pure Chemical Industries Ltd. and used as received. The Raman spectra were measured with RamanRxn1Analyzer (Kaiser Optical Systems, Inc). The wavelength of He-Ne laser was 632.8 nm with 1.0 mW power on the sample. Laser beam was focused by an objective lens (×50 magnification, OFN22 DIC N1, Nikon). Quartz glass was used for the optical window. During Raman measurements, a single crystal surface was separated from the window about 0.5 mm so that mass transport to the surface is not inhibited. We compared the band intensity of NPSERS spectra between deferent samples using adsorbed CO (COad) as an internal standard. In this method, the band of stretching vibration of Pt-CO (ν ) at 480 cm-1 was used to normalize NPSERS band intensity. Advantages of COad as an internal standard are as follows: saturated solution. Band intensities of Pt-O and Pt-O(H) in O2 saturated solution are compared with those in Ar saturated solution on the low index planes of Pt. This is the first vibrational spectroscopy measurement on well-defined single crystal surfaces of Pt during the ORR. 2. Experimental Single crystal electrodes of the low index planes of Pt were prepared by the method reported by Clavilier et al.33 The orientation of the crystal were determined using the reflected light of a He-Ne laser from (111) and (100) facets within an error of 0.1°. The crystal surface was mechanically polished using diamond slurries. Before the measurements, polished surfaces were annealed in H2/O2 flame about 1300°C, and then cooled in Ar/H2 atmosphere (Ar/H2 = 7/3). The annealed surface was immersed in ultrapure water, and used for the following experiments. NPSERS measurements were carried out using bare Au nanoparticles (about 50 nm in diameter) as plasmonic antennas. The nanoparticles were prepared from 0.5 mM hydrogen tetrachloroaurate (III) tetrahydrate (HAuCl4∙4H2O) aqueous solution by surfactant- and reducer-free synthesis using high-frequency ultrasonic (950 kHz).32 Colloidal solution of the nanoparticles was dropped uniformly on single crystal The following shows the procedure of NPSERS measurement using COad as an internal standard: (1) Au nanoparticles are dispersed on annealed Pt single crystal surface in Ar atmosphere. (2) CO is adsorbed at 0.1 V(RHE) for 15 min in CO saturated solution, and dissolved CO is purged by Ar at 0.1 V(RHE) for 15 min. (3) The amount of COad is determined by anode stripping voltammogram in hanging meniscus configuration. (4) Procedure (2) is repeated. (5) Single crystal surface is approached to ~0.5 mm from the window. (6) Raman spectrum of COad is measured at 0.1 V(RHE). (7) COad is oxidized at 0.8 V(RHE) for 10 min. (8) Raman spectra are measured from 0.1 to 1.4 V(RHE). The single crystal electrode is kept exactly at the same position from procedures (5) to (8). In the measurements of O2 atmosphere, the solution is saturated by O2 at 0.5 V(RHE) for 15 min after the procedure (7). All the potentials are referred to reversible hydrogen electrode (RHE). 3. Results and Discussion 3.1 Voltammograms and adsorbed CO coverage of Pt(hkl) covered by Au nanoparticles 0.1 M HClO4 saturated with Ar with and without Au nanoparticles. These voltammograms were measured before the measurement of anode stripping of adsorbed CO. The crystal orientations are verified on the basis of the total anodic charge of adsorbed hydrogen between 0.05 and 0.4 V(RHE). Table 1 shows the anodic charges in the adsorbed hydrogen region as well as the theoretical values. The observed charges are similar to those calculated on the assumption that one electron transfer occurs on one Pt atom in the first layer; the surfaces are well defined. Voltammograms of the electrodes with bare Au nanoparticles are almost identical with those of the electrodes without Au nanoparticles. This fact shows Au nanoparticles do not affect exposed surface area of the Pt surface, indicating that Au nanoparticles are adsorbed on Pt In order to normalize the band intensity of NPSERS, the number of COad was obtained by anode stripping voltammograms of COad on the low index planes of Pt covered with bare Au nanoparticles. Since two electrons transfer during the oxidation of COad, the number of COad and surface coverage of CO (θCO) are calculated as follows: surface nearly at point. There is enough space below the Au nanoparticles for the adsorption of hydrogen and oxide species, although precise distance between Au nanoparticles and Pt surface is unknown. Metal nanoparticles will be strongly adsorbed on metal surfaces by attractive force due to the DLVO theory that explains dispersion and aggregation of colloids. The attractive force may be strong enough not to be affected by the adsorption of hydrogen, oxide species and carbon monoxide. The amount of Au nanoparticles does not change before and after CO adsorption as shown in Fig. S1. 3.2 NPSERS spectra in Ar saturated solution Fig. 2(a) shows the wide range absolute NPSERS spectra on Pt(111) in 0.1 M HClO4. The band of bending vibration of Pt- O-H at 1080 cm-1, which is observed in IRAS spectra, is not found in NPSERS. This fact indicates that bending vibration of Pt-O-H is Raman inactive. Two bands at 3000-3800 and 1630 cm-1 are assigned to the stretching of OH (νOH) and bending vibrations of HOH (δHOH) of water molecule, respectively. In order to investigate whether these bands originate from adsorbed water, we subtracted NPSERS spectrum at 0.1 V(RHE) from those at higher potentials as shown in Fig. 2(b). Trace of the bands of νOH and δHOH is found in Fig. 2(b). However, the peak positions of the bands are independent of the applied potentials. These facts indicate that NPSERS cannot detect the adsorbed water. Fig. 3 shows low wavenumber region of NPSERS spectra on the low index planes of Pt in 0.1 M HClO4 saturated with Ar. The band around 930 cm-1 is the symmetric stretching mode of ClO - (ν (ClO -)). The wavenumber of this band shows no potential dependence; this band originates from perchlorate anion in solution phase. The spectrum at 0.1 V-CO shows the band of ν around 480 cm-1. This band intensity is used as an internal standard of NPSERS band as shown in Experimental section. The band intensity of COad did not change between 0.1 and 0.5 V(RHE) at which the coverage of COad is constant, showing that Au nanoparticles are not removed from the surface during potential application and laser irradiation. COad was oxidized by holding the potential at 0.8 V(RHE) for 10 min. After this procedure, potential was positively stepped from 0.1 to 1.4 V(RHE) with 0.05 V intervals. NPSERS spectra give new bands around 500 cm-1 above 1.1 V(RHE). In order to remove the backgrounds, the spectra were subtracted by a spectrum at 1.0 V(RHE) at which no characteristic band is observed. As shown in Fig. 4, two bands appear clearly around 570 and 340 cm-1 above 1.15 V(RHE). The band positions are almost identical with those of νPt-OH and δ on Pt film using conventional SERS.16 A small band is found between 570 and 340 cm-1 even after the oxidation of COad (Fig. 3). This band still survives at 1.4 V(RHE) at which COad is completely oxidized. This small band disappears in the subtracted spectra in Fig. 4. These facts support that this small band is not due to COad but species in solution phase, although we cannot assign the band at present. We investigated deuterium isotope effect on the two bands around 570 cm-1 and 340 cm-1 to assign the two bands. Fig. S2 shows voltammograms in 0.1 M HClO4/D2O saturated with Ar. Fig. 5 shows the NPSERS spectra on the low index planes of Pt in 0.1 M HClO4/H2O and 0.1 M HClO4/D2O solutions saturated with Ar. If the bands at 570 and 340 cm-1 originate from the formation of PtOH, these bands will shift to lower wavenumber due to the deuterium isotope effect. The band at higher wavenumber shifts to the lower wavenumber (15 cm- 1), whereas no deuterium isotope effect is observed in the band at lower wavenumber, as is the case of the SERS results of Weaver et al.16 Raman spectra of spattered PtO give two broad bands at 438 and 657 cm-1 in solid-air interface.36 Thus we assign the band around 570 cm-1 to the overlapped band of the stretching vibration of Pt-OH and Pt-O (νPt-O(H)), whereas the band at 340 cm-1 is assigned to the libration vibration of Pt- O (δPt-O). In the case of NPSERS where bare Au nanoparticles are used for the enhancement, Au oxides may be also formed on bare Au nanoparticles. In order to prove that the Raman bands of NPSERS spectra does not originate from adsorbed species on Au nanoparticles but Pt electrodes, we measured NPSERS spectra on a gold single crystal electrode. Fig. 6(a) shows the comparison of NPSERS spectrum of Pt(111) with that of Au(111) in 0.1 M HClO4 saturated with Ar. No band is found on Au(111) at 1.2 V(RHE) at which bands of νPt-O(H) and δPt-O are observed on Pt(111). Unlike the bands of νPt-O(H) and δPt-O on the Pt(111), the band of stretching vibration of Au-O (νAuO) is observed around 590 cm-1 above 1.4 V(RHE). No distinct band is found around 330 cm-1 on Au(111). In addition, band shift (dν/dE) of ν and ν around 570-590 cm-1 are 55 and 30 cm-1 V-1, respectively (Fig. 6(b)). These results verify that NPSERS band around 570 and 330 cm-1 on the Pt electrodes is not due to Au-O on bare Au nanoparticles. NPSERS spectra are similar to those of conventional SERS of Pt film except the onset potential of ν and δ bands.16 However, the spectra differ from those reported by SHINERS where a single band of amorphous PtO2 is observed around 590 cm-1 above 1.3 V(RHE).31 Raman spectrum of crystalline PtO gives two bands at 504 and 545 cm-1.37 However, only single band of crystalline PtO is found on SiO support.37 Interaction between Pt oxides and SiO2 shell around Au nanoparticles might change the band shape of Pt oxides in SHINERS. As described above, NPSERS gives two bands of νPt-O(H) and δPt-O on the low index planes of Pt above 1.15 V(RHE) at which Pt oxides are formed at the subsurface according to X-ray absorption fine structure (XAFS)11 and surface X-ray diffraction (SXD).12 However, the onset potential of PtO formation is 1.06 V(RHE) according to the voltammogram of Pt(111).5 In other spectroscopic methods, PtO was detected above 0.90 V(RHE) by XPS14,15 and 1.0 V(RHE) by conventional SERS.16 The onset potential of PtO and PtO(H) formation is higher than these potentials in NPSERS. Au nanoparticles do not affect the formation of Pt oxides according to the voltammograms in Figs. 1 where He-Ne laser is not irradiated. The difference between previous reports and our NPSERS measurements may be attributed to the presence of strong electric field of plasmon around Au nanoparticles when the laser is irradiated. The strong electric field might expel PtO and PtOH in the neighborhood of Au nanoparticles at lower potentials where coverages of PtO and PtOH are low (Fig. 7(b)(c)). The coverage of PtO and PtOH reaches full coverage at 1.15 V(RHE) according to the voltammograms in Fig. 1. Pt oxides are also formed at the subsurface at this potential12 and there is no room for Pt oxides to be expelled from the neighborhood of Au nanoparticles. PtO and PtOH are located in the range of strong electric field of the plasmon of Au nanoparticles as shown in Fig. 7(d); SERS signals can be obtained. Therefore NPSERS may observe both PtO and PtOH at the topmost layer and PtO at the subsurface. When the adsorbate is strongly adsorbed between Au nanoparticles and electrode surface, Au nanoparticles can enhance Raman signals of the adsorbates at any potential as is the case of gap mode Raman. However, the Pt oxides are not adsorbed directly to Au nanoparticles in NPSERS and SHINERS configuration. NPSERS and SHINERS can observe Pt oxides only at the potentials where Pt oxides approach to Au nanoparticles. That is why PtO and PtOH are found at higher potentials than previous reports. 3.3 Structural effects on NPSERS spectra in Ar saturated and O2 saturated solutions Figs. 8(a) and 8(b) show structural effects on the band intensity of ν around 570 cm-1 and δ around 340 cm-1 plotted against the potential in Ar saturated solution, respectively. The band intensity of νPt-O(H) increases as Pt(110) ≤ Pt(111) < Pt(100). Structural effects on the band intensity of δPt-O (Fig. 8(b)) are not so clear as those of νPt-O(H) because the band intensity of δPt-O is too small to be measured precisely. NPSERS measurements in O2 saturated solution are necessary for the study of correlation between the ORR activity and Pt oxides (PtO, PtOH). IRAS cannot be adopted in O2 saturated solution because the electrode is pushed to the window resulting in the inhibition of transfer of O2 to the electrode surface. NPSERS measurement has an advantage that the electrode is separated from the window about 0.5 mm, so that the diffusion of O2 to the electrode is not inhibited. Au nanoparticles may be adsorbed on Pt surface at point as discussed in section 3.1. The space between Au nanoparticles and Pt surface will be long enough not to prevent mass transfer of O2 to Pt surface, because the distance between side-on O2 molecule and Pt surface will be as small as that of oxide species. Therefore, NPSERS can study the Pt oxides formation in O2 saturated solution. Fig. 9 shows the subtracted NPSERS spectra of the low index planes of Pt in 0.1 M HClO4 saturated with O2. Bands of νPt-O(H) and δ are found around 570 and 340 cm-1 above 1.15 V(RHE) more intensely than those in Ar saturated solution. Figs. 10(a) and 10(b) depict the normalized band intensity of νPt-O(H) and δPt-O plotted against E in O2 saturated solution. The band intensity of νPt-O(H) in O2 saturated solution is about 1.5 times as high as that of Ar at 1.4 V(RHE). The enhancement of the intensity of δPt-O is higher than that of νPt-O(H): the intensity of δPt-O of Pt(100) in O2 saturated solution is twice as high as that in Ar saturated solution. Structural effects on the νPt-O(H) band intensity are Pt(110) ≤ Pt(111) << Pt(100), as is the case of Ar saturated solution. However, the δPt-O band intensity gives different structural effects: Pt(111) < Pt(110) << Pt(100). There is no dissolved gas effect on the onset potential of νPt- O(H) and δPt-O. We assume the order of the coverages of PtO and PtO(H) above 1.15 V(RHE) are the same as those at 0.90 V(RHE) without laser irradiation. Onset potentials of PtO(H) and PtO formation on Pt(100) are the lowest in the low index planes, showing that PtO is formed on Pt(100) most easily. The order of the ORR activity is Pt(100) < Pt(111) < Pt(110) in 0.1 M HClO4. The band intensity of νPt-O(H) has inverse correlation with the ORR activity, as is the case of PtOH in the IRAS measurement. The band intensity of δPt-O is the highest on Pt(100) of which ORR activity is the lowest. However, the order of band intensity of δPt-O on Pt(111) and Pt(110) contradicts to the order of the ORR activity. These facts indicate that PtO as well as PtOH inhibits the ORR on Pt(100). On Pt(110) and Pt(111), however, PtOH may inhibit the ORR activity more strongly than PtO on Pt(110) and Pt(111). Coverage of PtO may be too small to affect the ORR on Pt(111). Higher activity of Pt(110) compared with Pt(111) may be attributed to the possibility that PtO is formed at the inactive site of the ORR on reconstructed Pt(110). 3.4 Possible cause of enhancement of NPSERS band intensities in O2 saturated solution There are three possible factors for the enhancement of NPSERS band intensities in O2 saturated solution: (1) Increase of surface area in O2 (2) Increase of the formation of PtO and PtOH by O2 (3) Dissociative adsorption of O2 without electron transfer. In order to elucidate the cause of enhancement of NPSERS signal, we conducted three experiments. Firstly, we anticipated that the enhancement of νPt-O(H) and δPt-O band intensities is due to the increase of surface area in O2 atmosphere; we measured voltammograms after NPSERS measurements. Fig. 11 shows the voltammograms of Pt(111) before and after NPSERS measurement. The anodic charges of the adsorbed hydrogen region after NPSERS measurement in O2 saturated solution are approximately equal with that of Ar. This fact shows that increase of νPt-O(H) and δPt-O bands in O2 is not due to the increase of the surface area. Secondly, we anticipated the increase of PtO formation (Pt + electrolysis of H O: Pt + H O → PtO + 2H+ + 2e-. In addition to this reaction, it is assumed that PtO is formed by the dissociation of O2 which is adsorbed on Pt surface in O2. In order to confirm this assumption, we measured NPSERS low index planes of Pt in Ar or O2 saturated solutions. The currents of Pt oxides formation in O2 saturated solution are less than those of Ar except Pt(111). Therefore, Pt oxidation is not enhanced by O2 in Pt(100) and Pt(110). The charge of Pt oxides formation in O2 saturated solution increases compared with Ar on Pt(111). However, the increase of the charge is less than the enhancement of SERS signal. Thus enhancement of Pt oxides formation in O2 saturated solution is not the cause of the enhancement of NPSERS signals. Finally, we anticipated dissociative adsorption of O2 without electron transfer: O2 + Pt → 2PtO. In Ar, PtO is formed by detected around 530 and 330 cm-1. Fig. 13 shows NPSERS spectra of Pt(111) in O2 and O2 saturated 0.1 M HClO4. The bands show no significant shift. This fact indicates that O of PtO does not originate from O2, and PtO is produced from H2O. We cannot reveal the cause of the increase of νPt-O(H) and δPt-O band intensities in O2 saturated solution at present. As a possible another factor, dissolved O2 molecules may enhance the optical electric field around Au nanoparticles. Theoretical study is necessary for the elucidation of this phenomenon. 4. Conclusions Surface enhanced Raman spectroscopy by bare Au nanoparticles (NPSERS) detects bands of stretching vibration of Pt-O(H) (ν) at 570 cm-1 and libration vibration of Pt-O (δ) at 340 cm-1 on the low index planes of Pt in both Ar and O saturated 0.1 M HClO4 solutions. The onset potentials of PtO and PtOH formation are 1.15 V(RHE) on Pt(100) and 1.2 V(RHE) on Pt(111) and Pt(110), where surface coverage of PtO and PtOH reaches full coverage and PtO is formed at subsurface. No dissolved gas effect is found on the onset potentials, whereas the band intensities of νPt-O(H) and δPt-O are enhanced in O2 saturated solution significantly. In O2 saturated solution, the integrated band intensity of νPt-O(H) increases as Pt(110) ≤ Pt(111) << Pt(100). However, the band intensity of δPt-O gives different order as Pt(111) < Pt(110) << Pt(100). The band intensities of δPt-O and νPtO(H) are the highest on Pt(100). The ORR activity increases as Pt(100) < Pt(111) < Pt(110). These facts suggest that PtO as well PT-100 as PtOH deactivates the ORR on Pt(100). On Pt(110) and Pt(111), however, PtOH rather than PtO governs deactivation of the ORR.

References

1 N. M. Markovic, R. R. Adzic, B. D. Cahan, E. B. Yeager, J. Electroanal. Chem. 1994, 377, 249.
2 N. M. Markovic, T. J. Schmidt, B. N. Grgur, H. A. Gasteiger, R. J. Behm, P. N. Ross, J. Phys. Chem. B, 1999, 103, 8571.
3 F. Uribe, M. S. Wilson, T. Springer and S. Gottesfeld, Proc. the Workshop in Structural Effects in Electrocatalysis and Oxygen Electrochemistry, 92–11, ed. D. Scherson, D. Tryk, M. Daroux, X. Xing, Electrochem. Soc., Pennington, NJ, 1992, 494.
4 M. R. Tarasevich, Elektrokhimiya, 1973, 599, 9.
5 A. M. Gomez-Marin, J. Clavilier, J. M. Feliu, J. Electroanal Chem. 2013, 688, 360.
6 M. D. Macia, J. M. Campina, E. Herrero, J. M. Feliu, J. Electroanal. Chem. 2004, 564, 141.
7 A. Kuzume, E. Herrero, J. M. Feliu, J. Electroanal. Chem. 2007, 599, 333.
8 A. Hitotsuyanagi, M. Nakamura, N. Hoshi, Electrochim. Acta. 2012, 82, 512.
9 N. Hoshi, M. Nakamura, A. Hitotsuyanagi, Electrochim. Acta. 2013, 112, 899.
10 S. Sugawara, K. Tsujita, S. Mitsushima, K. Shinohara, K. Ota, Electrocatalysis. 2011, 2, 60.
11 H. Imai, K. Izumi, M. Matusmoto, Y. Kubo, K. Kato, Y. Imai, J. Am. Chem. Soc. 2009, 131, 6293.
12 J. Drnec, M. Ruge, F. Reikowski, B. Rahn, F. Carlà, R. Felci, O. M. Magnussen, D. A. Harrington, Electrochim. Acta. 2017, 224, 220.
13 Galen B. Fisher, Brett A. Sexton, Phys.Rev.Lett. 1980, 44,
14 M. Wakisaka, H. Suzuki, S. Mitsui, H. Uchida and M. Watanabe, Langmuir, 2009, 25, 1897.
15 M. Wakisaka, Y. Udagawa, H. Suzuki, H. Uchida and M. Watanabe, Energy Environ. Sci. 2011, 4, 1662.
16 Y. Zhang, X. Gao, M. J. Weaver, J. Phys. Chem. 1993, 97, 8656.
17 I. Yagi, K. Inokuma, N. Ohta, ECS Trans. 2013, 45, 103.
18 T. Ikeshoji, Y. Inokuma, Y, Qian, I. Yagi, M. Otani, The 21st FCDIC Fuel Cell Symposium Proceedings, 2014, B11.
19 H. Tanaka, S. Sugawara, K. Shinohara, T. Ueno, S. Suzuki, N. Hoshi, M. Nakamura, Electrocatalysis, 2015, 6, 295.
20 T. Ueno, H. Tanaka, S. Sugawara, K. Shinohara, A. Ohma, N. Hoshi, M. Nakamura, J. Electroanal. Chem. accepted for publication.
21 K. Ikeda, N. Fujimoto, H. Uehara, Chem. Phys. Lett. 2008, 460, 205.
22 K. Ikeda, J. Sato, N. Fujimoto, N. Hayazawa, S. Kawata, K. Uosaki, J. Phys. Chem. C, 2009, 113, 11816.
23 K. Ikeda, N. Fujimoto, K. Uosaki, J. Phys. Chem. C, 2014, 118, 21550.
24 L. Hu, M. Tanabe, J. Sato, K. Uosaki, K. Ikeda, J. Am. Chem. Soc. 2014, 136, 10299.
25 J. F. Li, Y. F. Huang, Y. Ding, Z. L. Yang, S. B. Li, X. S. Zhou, F. R. Fan, W. Zhang, Z. Y. Zhou, D. Y. Wu, B. Rn, Z. L. Wang, Z. Q. Tian, Nature, 2010, 464, 392.
26 J. R. Anema, J. F. Li, Z. L. Yang, B. Ren, Z. Q. Tian, Annu. Rev. Anal. Chem. 2011, 50, 129.
27 U. Zhumaev, A.V. Rudnev, J. F. Li, A. Kuzumea, Thu-Hien Vu, T. Wandlowski, Electrochim. Acta. 2013, 112, 853.
28 C. Y. Li, J. C. Dong, X. Jin, S. Chen, R. Panneerselvam, A. V. Rudnev, Z. L. Yang, J. F. Li, T. Wandlowski, Z. Q. Tian, J. Am. Chem. Soc, 2015, 137, 7648.
29 N. R. Honesty, A. A. Gewirth. J. Raman Spectrosc, 2012, 46, 43.
30 S. R. Smith, S. Zhou, J. Y. Baron, Y. Choi, J. Lipkowski, Electrochim. Acta. 2016, 210, 925.
31 Yi-Fan Huang, Patricia J. Kooyman, Marc T. M. Koper, Nat. Commun. 2016, 7, 12440.
32 T. Sakai, H. Enomoto, H. Sakai, M. Abe, Colloids Surf. A 2009, 347, 18.
33 J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 1980, 107, 205.
34 Si-Chung Chang, Lam-Wing H. Leung, M. J. Weaver, J. Phys. Chem. 1989, 93, 5341.
35 N. Hoshi, M. Tanizaki, O. Koga, Y. Hori, Chem. Phys. Lett, 2001, 336, 13.
36 J. R. McBride, G. W. Graham, C. R. Peters, W. H. Weber, J. Appl. Phys. 1991, 69, 1596.
37 W. Lin, A. A. Herzing, C. J. Keily, I. E. Wachs, J. Phys. Chem. C, 2008, 112, 5942.