Photoexcited Hole Transfer to a MnOx

Article pubs.acs.org/JPCC

Photoexcited Hole Transfer to a MnOx Cocatalyst on a SrTiO3 Photoelectrode during Oxygen Evolution Studied by In Situ X‑ray Absorption Spectroscopy Masaaki Yoshida,*,†,‡ Takumi Yomogida,† Takehiro Mineo,† Kiyofumi Nitta,§ Kazuo Kato,§ Takuya Masuda,∥ Hiroaki Nitani,#,∇ Hitoshi Abe,#,∇ Satoru Takakusagi,○ Tomoya Uruga,§ Kiyotaka Asakura,○ Kohei Uosaki,∥,⊥,◆ and Hiroshi Kondoh† †

Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan Cooperative Research Fellow of Catalysis Research Center, Hokkaido University, Sapporo, Hokkaido 001-0021, Japan § Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ∥ Global Research Center for Environment and Energy Based on Nanomaterials Science and ⊥International Center for Materials Nanoarchitechtonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan # Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ∇ Department of Materials Structure Science, School of High Energy Accelerator Science, Graduate University for Advanced Studies, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ○ Catalysis Research Center, Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan ◆ Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan ‡

S Supporting Information *

ABSTRACT: Photoexcited hole transfer to MnOx cocatalysts on SrTiO3 photoelectrodes was examined under controlled potential conditions during UV irradiation using in situ Mn Kedge X-ray absorption fine structure (XAFS) spectroscopy. The absorption edges of spectra were found to shift to higher energies during irradiation, indicating that MnOx cocatalysts were oxidized by the migration of photoexcited holes accompanied by a positive potential shift of the MnOx cocatalysts. This oxidation process was promoted by the application of a positive applied potential, suggesting that the photoexcited hole transfer was enhanced by upward band bending at the cocatalyst−photoelectrode interface. Structural changes of the MnOx cocatalyst were found to depend on the UV photon intensity; thus, the observations of photoexcited electron transfer by XAFS are associated with the photoelectrochemical activity during water splitting.

1. INTRODUCTION Photoelectrochemical overall water splitting is an attractive means of sustainable hydrogen production.1−16 Extensive efforts have been directed toward the development of efficient photoelectrodes since Honda and Fujishima first reported a photoelectrochemical water splitting system using TiO2-treated platinum wire.14 Recently, Brillet et al. reported a solar-tohydrogen conversion efficiency as high as 3.1% using various photovoltaic devices.15 Unfortunately, this level of efficiency is insufficient for commercial applications; therefore, there is a requirement to increase the hydrogen production efficiency by improving the photoelectrochemical activity. The photoelectrochemical activity for water splitting is dependent on the migration of photoexcited carriers.17−22 Enhancement of the upward band bending increases the lifetime of photoexcited holes by promoting carrier separation, © XXXX American Chemical Society

as reported in the studies by transient absorption spectroscopy (TAS).17−21 In addition, a time-resolved IR absorption study has suggested that band bending in the space charge layer controls the charge separation between photogenerated electrons and holes.22 The associated slow recombination between photoexcited electrons and holes leads to enhancement of the photoelectrochemical activity. Therefore, one strategy for efficient charge separation is to apply a potential bias to the photoelectrodes to promote the migration of photoexcited carriers. The modification of photoelectrode surfaces by metal oxide catalysts (called cocatalysts) is also an effective means of Received: July 29, 2014 Revised: September 24, 2014

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dx.doi.org/10.1021/jp507657p | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. AFM images of the SrTiO3 substrate surface (a) before and (b) after MnOx photodeposition. (c) Cross sectional SEM image of the SrTiO3 substrate after MnOx photodeposition.

thin film. Although the SrTiO3 material is not the visible-lightresponse photoelectrode such as WO3, BiVO4, and Fe2O3, the MnOx cocatalyst on the SrTiO3 photoelectrode can substantially improve the photoelectrochemical activity during water splitting, as well as in the case of visible-light-response photoelectrodes.23,24 Therefore, in the present study, the relationship between photoexcited hole transfers and photoelectrochemical activity was investigated in detail using in situ XAFS spectroscopy to study a SrTiO3 photoelectrode modified with MnOx thin-film cocatalysts as a model photoelectrochemical system. The data demonstrated that photoexcited hole transfer from the SrTiO3 photoelectrode to the MnO x cocatalyst is dependent on both the electrode potential and the UV light photon intensity.

improving the photoelectrochemical activity because metal oxides, such as MnOx,23−26 IrO2,27 cobalt phosphate,28 and nickel borate29 can function as reaction sites for oxygen evolution.16 Among these, MnOx is known to substantially improve the photocatalytic or photoelectrochemical activity during water splitting because MnOx coatings on semiconductors generate efficient reaction sites for oxygen evolution because of the formation of various crystal structures and chemical states.23−26 The surface modification of semiconductor devices with MnOx cocatalysts is therefore currently of significant interest with regard to constructing efficient photoelectrochemical systems for water splitting. To date, studies concerning the application of cocatalysts on photoelectrodes have been carried out using various methods. The Fermi level of a Pt cocatalyst on a GaN photocatalyst electrode was observed by in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATRSEIRAS),30 and Ye et al. examined the effects of various cocatalysts on a W-doped BiVO4 photocatalyst by optical fibermodified scanning electrochemical microscopy.31 Research concerning the dynamics of photoexcited carrier transfer at the cocatalyst−photoelectrode interface has been limited, however, because of the difficulty in directly probing the chemical states of cocatalysts during the photoelectrochemical reaction. X-ray absorption fine structure (XAFS) spectroscopy is a powerful method for the investigation of local structures in aqueous solutions.32−38 Recently, we observed the transfer of photoexcited carriers from SrTiO3 photoelectrodes to MnOx cocatalysts in photoelectrochemical water splitting by in situ Mn K-edge XAFS spectroscopy39 and found differing photoexcited carrier transfers between the MnOx particulates and the

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The photodeposition of MnOx was performed using a procedure reported in our previous study.39 An n-type SrTiO3 substrate with a polished surface (Nb: 0.1 wt %, Furuuchi Chemical Co.) was immersed in an aqueous solution of Mn(NO3)2·6H2O (2.2 × 10−2 M) (98%, Kanto Chemicals) and irradiated using a Xe lamp (300 W, Cermax). The surface morphology of the resulting MnOx/ SrTiO3 was observed by atomic force microscopy (AFM; SPM9500J3, Shimadzu) and scanning electron microscopy (SEM; S4700, Hitachi High Technologies). The prepared samples were attached to a Cu wire with Ag paste and inserted into a Teflon electrochemical rod to form the working electrode. 2.2. Photoelectrochemical Properties. The working electrode was placed in a Teflon electrochemical cell along with a Pt counter electrode and a Ag/AgCl (saturated KCl) B



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Figure 2. (a) Linear sweep voltammograms of bare SrTiO3 (blue) and MnOx/SrTiO3 (red) in a 0.1 M Na2SO4 aqueous solution without (dotted line) and with (solid line) UV irradiation. (b) Linear sweep voltammograms of MnOx/SrTiO3 in a 0.1 M Na2SO4 aqueous solution under UV irradiation with a photon flux of 8.3 (red line), 1.1 (blue line), and 0.2 (black line) × 1015 photons·cm−2·s−1 at 340 ± 20 nm. Scan rate: 50 mV·s−1. Labels A−E indicate the conditions under which in situ XAFS spectra were acquired, as described in the text.

Figure 3. (a) In situ Mn K-edge XAFS spectra of MnOx/SrTiO3 samples at +1.0 V vs RHE in Ar-saturated 0.1 M Na2SO4 aqueous solution under UV irradiation. Mn foil, MnO, Mn3O4, Mn2O3, MnO2, and KMnO4 were analyzed as reference samples. (b) Average valence states of MnOx cocatalysts using the first inflection point of the Mn reference before and after irradiation at +1.5 V. (c) XAFS spectrum after 30 min irradiation (black dots) and linear combination fitting (red line) by the spectra of the reduced (green line) and oxidized (blue line) states. (d) Fraction of the reduced (red) and oxidized (black) states based on the linear combination fitting of XAFS spectra.

2.3. In Situ XAFS. The in situ Mn K-edge XAFS measurements were carried out at the BL12C line of the Photon Factory (PF), operating at an electron energy of 2.5 GeV with an average current of 450 mA and at the BL01B1 line of the SPring-8, operating at an electron energy of 8.0 GeV with an average current of 100 mA together with a Si(111) doublecrystal monochromator. Data obtained from the prepared samples were collected in the fluorescence mode with 19 element Ge detectors (Ortec) equipped with a Cr filter and solar slits to remove elastic scattering. The photon energy was calibrated using Mn K-edge energy (6537.0 eV), applying the Mn foil first-derivative peak maximum. XAFS data analysis was performed using the Athena and Artemis programs.40−42 The Teflon electrochemical cell was equipped with a Pt wire

reference electrode. This apparatus was then applied to the O2evolving photoelectrochemical reaction of an Ar-saturated aqueous solution of 0.1 M Na2SO4 (99.99%, Merck) using a potentiostat (HA-151B, Hokuto Denko Co.) to control the electrode potential. Milli-Q water (total organic carbon, 18 MΩ·cm) was used in all experiments. Linear sweep voltammetry was carried out under dark conditions and with Xe lamp irradiation at a scan rate of 50 mV·s−1. Neutral density filters were used to limit the photon density. The time sequences of the potential or current were obtained with an AD converter (USB-6211, National Instruments) and recorded on a computer after averaging 100 data points collected at a sampling rate of 100 kHz using the LabVIEW program (National Instruments). C



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Figure 4. In situ Mn K-edge XAFS spectra in Ar-saturated 0.1 M Na2SO4 aqueous solution at (a) +0.5 V and (b) 0.0 V vs RHE under UV irradiation. Insets show the time course of the ratio between the reduced and oxidized states of MnOx cocatalysts.

3.2. In Situ XAFS. We first examined the structural changes of MnOx cocatalysts on SrTiO3 photoelectrodes during UV irradiation. Figure 3a provides the in situ Mn K-edge XAFS spectra of a MnOx/SrTiO3 photoelectrode acquired at +1.0 V under irradiation with a neutral density filter (corresponding to line C in Figure 2b). Prior to irradiation, the MnOx peak is observed at 6558.2 eV, which is essentially the same as the Mn2O3 reference sample peak (6557.0 eV), indicating that Mn3+ is present. During irradiation, this peak gradually shifts to 6559.8 eV, which corresponds to the spectrum of the MnO2 reference (6560.5 eV) and suggests that Mn3+ species were oxidized to Mn4+ because of the migration of photoexcited holes. Two isosbestic points were observed in the XAFS spectra, at 6557.5 and 6587.2 eV (Figure S1 of Supporting Information), and these indicated that this system consisted of both Mn3+ and Mn4+ species, as previously reported.33,45,46 The MnOx thus had two chemical states: the electrochemically reduced Mn3+ (the reduced state) and the photoelectorochemically oxidized Mn4+ (the oxidized state). It should be noted that Mn3+ was not regenerated from Mn4+ following UV irradiation (Figure S2 of Supporting Information); hence, electron migration from the photoelectrode to the MnOx cocatalyst was likely to be restricted by band bending at the cocatalyst−photoelectrode interface because of the characteristics of an n-type semiconductor. However, when the electrode potential was changed to 0.0 V under dark conditions, the Mn4+ was electrochemically reduced to Mn3+. The XAFS spectra did not exhibit any changes under visible light (λ > 380 nm) irradiation, as shown in Figure S3 of Supporting Information, suggesting that the oxidation of MnOx under UV irradiation results from the migration of holes photogenerated by bandgap absorption of the SrTiO3 semiconductor. It is well-known that the edge position is closely associated with the Mn valence state.46−49 Figure 3b shows the edge energies of the initial inflections of MnO, Mn3O4, Mn2O3, and MnO2 and demonstrates good correlation between the edge energy and the average Mn valence state. The average Mn valence state of the MnOx cocatalysts was determined to be +3.1 prior to irradiation (corresponding to the reduced state) and +4.0 after irradiation at +1.5 V (the oxidized state) by comparing the first inflection points of reference samples.

counter electrode, a Ag/AgCl (saturated KCl) reference electrode, and the sample working electrode. The X-ray beam was applied at an incident angle of 4° through a polypropylene window (12.0 μm thick). Milli-Q water was used in all experiments. XAFS spectra of the prepared samples were measured at various potentials under dark conditions and under UV irradiation with a Xe lamp equipped with an IR-cut filter and neutral density filters. Potentials versus Ag/AgCl were converted to the reversible hydrogen electrode (RHE) scale by the equation ERHE = EAg/AgCl + 0.197 V + (0.059 V) × pH.

3. RESULTS AND DISCUSSION 3.1. Surface Morphologies and Photoelectrochemical Properties. Panels a and b of Figure 1 show AFM images of the SrTiO3 substrate before and after photodeposition of the MnOx cocatalyst, respectively, indicating that the MnOx species completely covered the SrTiO3 substrate. The thickness of the MnOx thin film was estimated at approximately 300 nm or less from the cross-sectional SEM image, as shown in Figure 1c. Figure 2a presents the photoelectrochemical properties of the bare and MnOx-photodeposited SrTiO3. In the case of the bare SrTiO3, the photooxidation current onset potential was observed at ca. +0.5 V but was shifted to a more negative value (ca. 0.0 V) for the MnOx/SrTiO3. The photocurrent obtained from the MnOx/SrTiO3 was increased by 0.2 mA·cm−2 at +1.5 V compared with that of the bare SrTiO3. The negative shift of the onset potential for water splitting and the increased photocurrent demonstrate that MnOx indeed functioned as an oxygen evolution catalyst, as previously reported.23−26 The onset potential at 0.0 V can be regarded as the approximate flat band potential of the MnOx/SrTiO3, and at potentials above 0.0 V, upward band bending increases because of the characteristics of an n-type semiconductor, as has been established in prior studies.19,30,43,44 Linear sweep voltammograms (LSV) were obtained for the MnOx/SrTiO3 samples during UV irradiation at photon fluxes of 8.3, 1.1, and 0.2 × 1015 photons·cm−2·s−1 at 340 ± 20 nm, as shown in Figure 2b. At an applied bias of +1.5 V, the photocurrent densities at various light intensities were approximately proportional to the light intensity. D



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3.1 to 3.4 during UV irradiation, whereas at +1.0 V, MnOx on the photoelectrode was oxidized more rapidly and the average Mn valence state shifted from 3.1 to 4.0 during UV irradiation. It should be noted that the photocurrent is affected not only by the oxygen evolution reaction from water but also by the oxidation of MnOx cocatalyst. In fact, the time course of photocurrent during in situ XAFS measurements at 0.5 V (Figure S4 of Supporting Information) indicates that the current of the initial 30 min includes that of oxidation reaction of MnOx cocatalyst because the photocurrent exhibited larger values, consistent with the result of XAFS measurements at 0.5 V indicating that the Mn valence of MnOx cocatalyst shifted within 30 min (Figure 5). The observation of constant photocurrent after 30 min suggests that oxygen evolution reaction proceeds even after the oxidation of MnOx cocatalyst. On the other hand, from this result, we concluded that both Mn3+ and Mn4+ oxide species can enhance the photoelectrochemical activity as oxygen evolution cocatalysts on the SrTiO3 photoelectrode, in good agreement with the previous reports that Mn3+ and Mn4+ oxide species function as efficient water oxidation electrocatalysts.32−38,50 3.4. Light Intensity Dependence of the XAFS Spectra. To examine the light intensity dependence of the photoexcited hole transfer, XAFS spectra were obtained while varying the light intensity (lines D and E in Figure 2b). Figure 6a shows the in situ Mn K-edge XAFS spectra of the MnOx cocatalyst on a SrTiO3 photoelectrode at +1.5 V under UV irradiation with a light intensity of 8.3 × 1015 photons·cm−2·s−1 (340 ± 20 nm). After UV irradiation, the peak position at 6557.5 eV is shifted to 6559.6 eV. The result of the linear combination fitting shows that the MnOx cocatalyst is approximately 80% oxidized within 15 min and the average Mn valence state plateaus at the 60 min mark. In contrast, in the XAFS spectra acquired at a photon intensity of 0.2 × 1015 photons·cm−2·s−1 (Figure 6b), the initial peak position at 6557.4 eV gradually shifts to 6558.9 eV during the UV irradiation. Over the course of the 135 min irradiation, the proportion of the oxidized state gradually increases and eventually accounts for ca. 80% of the MnOx. Figure 7 shows the time course of the Mn valence in MnOx cocatalysts on SrTiO3 photoelectrodes at +1.5 V under UV irradiation with photon intensities of 8.3 and 0.2 × 1015 photons·cm−2·s−1. When the weaker light intensity (0.2 × 1015 photons·cm−2·s−1) was applied, the MnOx cocatalysts were gradually oxidized within 135 min because of the lower quantity of photogenerated holes resulting from reduced absorption of UV light. Meanwhile, at the higher light intensity (8.3 × 1015 photons·cm−2·s−1), the cocatalysts were oxidized to the +4.0 valence within 60 min and maintained that valence state, indicating that an abundance of photoexcited holes was generated by the absorption of copious amounts of UV light and that these holes were efficiently transferred to the MnOx. Hence, it is evident that differences in the incident photon intensity affect the rate of Mn valence change because of variations in the number of photogenerated holes in the SrTiO3 photoelectrode. 3.5. Correlation between Photoelectrochemical Activity and Photoexcited Hole Transfer to MnO x Cocatalysts. On the basis of the above results, the extent of peak shift and the rate of change of Mn valence demonstrated by XAFS spectra depend on the voltage applied to the electrode and the photon intensity of the UV lamp, respectively. Variations in the Mn valence are likely to be associated with photoexcited hole transfer to the MnOx cocatalyst, which

The ratios of the reduced and oxidized MnOx states during photoelectrochemical reaction were evaluated quantitatively using least-squares fitting of each normalized XAFS spectrum with the linear combination of the reduced and oxidized states of the MnOx cocatalysts. Figure 3c shows the results obtained from the linear combination fitting of the XAFS spectrum after a 30 min irradiation at +1.0 V, from which it is evident that there are only two MnOx, corresponding to the reduced and the oxidized states. Similar behavior was observed in the other fitting results. Time-dependent changes of the ratio between the reduced and the oxidized states were analyzed, as shown in Figure 3d. Over the course of the irradiation, the MnOx cocatalysts gradually transitioned from the reduced to the oxidized states because of the reactions of photoexcited holes. These results demonstrate that the slow oxidation of MnOx cocatalysts proceeds via the migration of photoexcited holes, as described in our previous publication.39 It is to be noted that the rate of oxidation reaction of MnOx cocatalyst in XAFS spectra is much slower, indicating that the detection of Mn4+ species is not direct evidence to decide the active species for water oxidation. 3.3. Electrode Potential Dependence of XAFS Spectra. To investigate the effect of the applied electrode potential, in situ Mn K-edge XAFS spectra were acquired for a MnOx/ SrTiO3 photoelectrode at different electrode potentials. Figure 4a summarizes the XAFS spectra obtained at +0.5 V (line B in Figure 2b). The peak position at 6557.8 eV is observed to gradually shift to 6559.2 eV over 150 min of irradiation, suggesting that some of the Mn3+ was oxidized to Mn4+ because of the migration of photoexcited holes, just as occurred at +1.0 V. The results of the linear combination fitting (Figure 4a, inset) demonstrate that the proportion of the oxidized state was gradually increased by irradiation. Conversely, the XAFS spectra at 0.0 V (line A in Figure 2b) do not exhibit any changes in peak position on UV irradiation, as shown in Figure 4b, indicating that photoexcited holes were not transferred from the photoelectrode to the MnOx cocatalyst. The average Mn valence states were evaluated from the ratio of the reduced (Mn+3.1) and oxidized (Mn+4.0) states. Figure 5 shows the time course of the average Mn valence states of MnOx/SrTiO3 at +1.0, +0.5, and 0.0 V under UV irradiation. At 0.0 V, the average valence state of the MnOx cocatalysts was unchanged even after UV irradiation. At +0.5 V, MnOx cocatalysts on the photoelectrode were gradually oxidized by photoexcited holes and the average valence state changed from

Figure 5. Time course of Mn valence of the MnOx cocatalyst on a SrTiO3 photoelectrode at +1.0 (black line), +0.5 (red line), and 0.0 (blue line) V vs RHE under irradiation. E



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Figure 6. Time-dependent changes of in situ Mn K-edge XAFS spectra at +1.5 V vs RHE under UV irradiation at (a) 8.3 and (b) 0.2 × 1015 photons· cm−2·s−1. Insets show the changes in the proportions of Mn valence states.

and +1.0 V. When the electrode remains at the flat band potential of 0.0 V (as approximately estimated from Figure 2a), band bending does not occur at the MnOx/SrTiO3 interface, as shown in Figure 8a; thus, photoexcited holes do not immigrate to the MnOx (see Figure 5) because of the absence of a driving force and instead recombine with photoexcited electrons in the photoelectrode. This scenario is consistent with the results demonstrating negligible photoelectrochemical activity in Figure 2b. Conversely, at an electrode bias of +0.5 or +1.0 V, upward band bending results at the MnOx/SrTiO3 interface because of the characteristics of the n-type semiconductor (Figures 8b). This upward band bending promotes charge separation and results in the migration of photoexcited holes toward the MnOx (Figure 5), in good agreement with the photooxidation current evident in Figure 2b. The average Mn valence state after irradiation at +1.0 V was higher (Mn4.0+) than that at +0.5 V (Mn3.3+), which indicates that the upward band bending enhanced the photoexcited hole transfer and the local electrochemical potential for electrons of MnOx cocatalyst was shifted positive over the oxidation−reduction potential of Mn3+/Mn4+. The positive potential shift of MnOx cocatalyst is likely to attain oxygen evolution potential and play an essential role in the photoelectrochemical oxygen evolution reaction. The photooxidation of MnOx cocatalyst was found to be dependent on the incident light intensity (Figure 7). Higher levels of light intensity produced rapid structural changes in the MnOx cocatalyst as compared with weaker irradiation because of associated increases in the photoexcited hole-transfer rate. In addition, the photooxidation current during oxygen evolution increased according to the photon intensity (Figure 1b), indicating that the photooxidation current is related to the amount of photoexcited hole transfer. Taken together, these results indicate that the rate of structural change of the Mn valence correlates with the photoelectrochemical activity. We have demonstrated that upward band bending facilitates photoexcited hole transfer, which in turn causes rapid oxidation of the MnOx cocatalyst. The application of a positive bias to ntype semiconductors suppresses recombination between photoexcited electrons and holes, as reported in a previous work based on IR and TAS studies.17−22 Our results demonstrate that increases in upward band bending at the photoelectrode−

Figure 7. Time-dependent changes of Mn valence in MnOx cocatalysts on SrTiO3 photoelectrodes at +1.5 V vs RHE under UV irradiation at 8.3 (red line) and 0.2 (black line) × 1015 photons·cm−2·s−1.

should be important in discussing the correlation between the photoelectrochemical activity and photoexcited hole transfer. Figure 8 shows schematic illustrations of the MnOx/SrTiO3 interface during photoelectrochemical water oxidation at 0.0

Figure 8. Schematic illustrations for photoexcited carrier transfer from the SrTiO3 photoelectrode to the MnOx cocatalyst during photoelectrochemical water oxidation at (a) 0.0 and (b) +1.0 V vs RHE. F



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Keio Gijuku Academic Development Fund, Keio Kogaku-kai Fund, Keio Gijuku Koizumi Memorial Fund, and the Cooperative Research Program of Catalysis Research Center in Hokkaido University (13B1010 and 14A1004). We thank S. Maeda and T. Hata for conducting the XAFS measurements as well as the Keio University workshop for producing the measurement cells.

cocatalysts interface can be a key factor in enhancing charge separation and improving the photoelectrochemical water splitting.

4. CONCLUSIONS We studied the structural changes of MnOx cocatalysts on SrTiO3 photoelectrodes via photoexcited hole transfer under photoelectrochemical conditions by in situ XAFS measurements. The peak position in the Mn K-edge XAFS spectra of MnOx thin films on SrTiO3 photoelectrodes shifted to higher energy levels during irradiation, suggesting that the chemical state of the MnOx changed because of the migration of photoexcited holes. The average valence state of the MnOx before irradiation was estimated to be approximately +3.1 by the linear combination fitting of the primary inflection points of reduced and oxidized states. At a +1.0 V applied potential, MnOx on a photoelectrode was oxidized from +3.1 to +4.0 during UV irradiation, demonstrating that photoexcited holes migrated from the photoelectrode to the MnOx. However, when the electrode potential remained at +0.5 V, MnOx was gradually oxidized from +3.0 to +3.3 under irradiation. Moreover, the valence state of MnOx cocatalysts did not change under irradiation at 0.0 V. These results show that the application of a positive potential promotes the migration of photoexcited holes and allows efficient utilization of photoexcited holes during photoelectrochemical water oxidation. MnOx was more rapidly oxidized during irradiation with more intense light (8.3 × 1015 photons·cm−2·s−1) compared with the oxidation rate obtained with lower intensity irradiation (0.2 × 1015 photons·cm−2·s−1), a finding which is in good agreement with the results of photoelectrochemical activity measurements. These data indicate that the rate of Mn valence change is related to the photoelectrochemical activity becauseof increasing rates of photoexcited hole transfer. We have therefore found that in situ X-ray absorption spectroscopy represents a powerful tool for the observation of photoexcited hole migration toward MnOx cocatalysts during oxygen evolution and allows estimation of the positive potential shift of MnOx cocatalysts on a photoelectrode surface.

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ASSOCIATED CONTENT

S Supporting Information *

Observation of isosbestic points, in situ Mn K-edge XAFS spectra of MnOx cocatalysts after UV irradiation, stability of MnOx cocatalyst under visible light irradiation, and the time course of photocurrent during in situ XAFS measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-45-566-1592. Fax: +81-45-566-1697. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed at the Photon Factory (2010G677 and 2012G752) and SPring-8 (2011A1976, 2011B1080, 2012A1623, 2012B1229 and 2013A1039) facilities and was supported by Grants-in-Aid (22850015 and 24750134) from the Japan Society for the Promotion of Science (JSPS), the G



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Photoexcited Hole Transfer to a MnOx

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