Structure and Photocatalytic Activity of PdCrOx Cocatalyst on SrTiO3 for Overall Water Splitting

1 Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan 2 Japan Society for the Promotion of Science, Kojimachi Business Center Building, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 1020083 3 Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 3050801, Japan 4 Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan *Correspondence: maedak@chem.titech.ac.jp; Tel.: +813-5734-2239


Introduction
Photocatalytic decomposition of H 2 O into H 2 and O 2 , using a semiconductor powder, is one of the target reactions of artificial photosynthesis in producing a clean energy source [1][2][3][4].Many strategies have been proposed to improve the efficiency of this reaction, such as doping [5], morphological modification [6], application of visible-light-absorbing materials [7][8][9][10], cocatalyst loading [11][12][13][14][15][16][17] and surface protection [18,19].With respect to surface modification, the loading of metal or metal-oxide nanoparticles, which are referred to as cocatalysts, onto the surface of a semiconductor photocatalyst has been widely employed to enhance the activity toward photocatalytic water splitting.The cocatalyst nanoparticle reduces the activation energy for reduction/oxidation reactions and suppresses charge recombination of electron and hole pairs in a semiconductor, thereby enhancing the activity.
Several cocatalysts that consist of more than two metal species have recently been reported for water splitting, and some of them have been more efficient than their single component counterparts [9][10][11][12][13][14].Among these multi-component cocatalysts, mixed oxides of Cr and a paired transition metal have been well studied for overall water splitting.The metal-Cr mixed compounds were reported as promoters for not only the hydrogen evolution reaction (HER) but also the oxygen evolution reaction (OER).Whereas RhCrO x was reported to work as a HER cocatalyst on the surface of various semiconductor photocatalysts [14,15], FeCrO x was recently reported to function as an OER cocatalyst on SrTiO 3 [17].Therefore, metal-Cr mixed compounds are considered to be promising cocatalyst materials for water splitting.
Our group recently reported PdCrO x nanoparticles as a HER cocatalyst [16].However, it was difficult to synthesize an active PdCrO x cocatalyst using a conventional impregnation method, and thus required a photochemical method instead (see Scheme 1).The PdCrO x /semiconductor photocatalyst system is interesting from a synthetic inorganic chemistry and functional materials chemistry perspective.However, compared to a similar mixed oxide cocatalyst such as RhCrO x , the structural characterization of photochemically-synthesized PdCrO x has yet to be completed, and thus the impact of the preparation conditions on the formation of PdCrO x remains unknown.semiconductor photocatalysts [14,15], FeCrOx was recently reported to function as an OER cocatalyst on SrTiO3 [17].Therefore, metal-Cr mixed compounds are considered to be promising cocatalyst materials for water splitting.
Our group recently reported PdCrOx nanoparticles as a HER cocatalyst [16].However, it was difficult to synthesize an active PdCrOx cocatalyst using a conventional impregnation method, and thus required a photochemical method instead (see Scheme 1).The PdCrOx/semiconductor photocatalyst system is interesting from a synthetic inorganic chemistry and functional materials chemistry perspective.However, compared to a similar mixed oxide cocatalyst such as RhCrOx, the structural characterization of photochemically-synthesized PdCrOx has yet to be completed, and thus the impact of the preparation conditions on the formation of PdCrOx remains unknown.In this work, the structure of PdCrOx formed on the surface of SrTiO3 with various irradiation times was characterized using high resolution transmission electron microscopy (HR-TEM), extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS).

Deposition of PdCrOx nanoparticles
Figure 1 shows a typical time course for H2 evolution observed during the photodeposition of PdCrOx.After an induction period (ca. 1 h), stable H2 evolution (ca.63.3 μmol h -1 ) was observed.The induction period between 0-1 h suggests that PdCrOx cocatalyst deposition was accomplished within 1 h.In the absence of the Pd and Cr sources, H2 production was observed at a moderate rate (0.2 μmol h -1 ).The amount of Cr deposited in the product (PdCrOx/SrTiO3) was quantified by monitoring the concentration of the Pd and Cr sources in the solution using UV-visible absorption spectroscopy.Figure 2 shows the amount of Cr deposited on SrTiO3 as a function of the photoirradiation time.It is noted that all of the Pd precursor added (0.5 wt% Pd) underwent inclusion on the surface of SrTiO3, regardless of the photoirradiation time.Therefore, the amount of Pd in deposited PdCrOx/SrTiO3 was identical.The amount of deposited Cr was increased with the irradiation time.Even without photoirradiation, 0.2 wt% portion of Cr disappeared from the solution phase, which suggests that the Cr species is adsorbed onto the surface of SrTiO3.After 1-5 h irradiation, the amount of loaded Cr reached almost a constant value of 0.7-0.8wt%, which indicated that Cr had been photodeposited.
Figure 3 shows TEM images of the deposited PdCrOx nanoparticles.These observations indicated that the deposited PdCrOx nanoparticles are not only core/shell type particles (core: Metallic Pd, shell: Cr2O3) but also aggregates and isolated metallic Pd nanoparticles.The ratio of the deposited Pd and Cr was also different in each particle.In general, the photodeposition rate of metal nanoparticles (in other words, the rate of photoreduction of metal ions) on the semiconductor surface depends on the surface property of the semiconductor (e.g., crystal faces), as reported by Ohno et al.In this work, the structure of PdCrO x formed on the surface of SrTiO 3 with various irradiation times was characterized using high resolution transmission electron microscopy (HR-TEM), extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS).

Deposition of PdCrO x Nanoparticles
Figure 1 shows a typical time course for H 2 evolution observed during the photodeposition of PdCrO x .After an induction period (ca. 1 h), stable H 2 evolution (ca.63.3 µmol h −1 ) was observed.The induction period between 0-1 h suggests that PdCrO x cocatalyst deposition was accomplished within 1 h.In the absence of the Pd and Cr sources, H 2 production was observed at a moderate rate (0.2 µmol h −1 ).The amount of Cr deposited in the product (PdCrO x /SrTiO 3 ) was quantified by monitoring the concentration of the Pd and Cr sources in the solution using UV-visible absorption spectroscopy.Figure 2 shows the amount of Cr deposited on SrTiO 3 as a function of the photoirradiation time.It is noted that all of the Pd precursor added (0.5 wt% Pd) underwent inclusion on the surface of SrTiO 3 , regardless of the photoirradiation time.Therefore, the amount of Pd in deposited PdCrO x /SrTiO 3 was identical.The amount of deposited Cr was increased with the irradiation time.Even without photoirradiation, 0.2 wt% portion of Cr disappeared from the solution phase, which suggests that the Cr species is adsorbed onto the surface of SrTiO 3 .After 1-5 h irradiation, the amount of loaded Cr reached almost a constant value of 0.7-0.8wt%, which indicated that Cr had been photodeposited.
Metallic and oxidized Pd species were loaded on SrTiO3, as evidenced by XPS measurements (discussed later).The lattice fringe attributable to metallic Pd was observed in the 5 h sample (see 5 h (1)).Therefore, the central part of the nanoparticle consisted of metallic Pd.However, the location of the oxidized Pd species in the TEM images could not be detected.(discussed later).The lattice fringe attributable to metallic Pd was observed in the 5 h sample (see 5 h (1)).Therefore, the central part of the nanoparticle consisted of metallic Pd.However, the location of the oxidized Pd species in the TEM images could not be detected.Figure 3 shows TEM images of the deposited PdCrO x nanoparticles.These observations indicated that the deposited PdCrO x nanoparticles are not only core/shell type particles (core: Metallic Pd, shell: Cr 2 O 3 ) but also aggregates and isolated metallic Pd nanoparticles.The ratio of the deposited Pd and Cr was also different in each particle.In general, the photodeposition rate of metal nanoparticles (in other words, the rate of photoreduction of metal ions) on the semiconductor surface depends on the surface property of the semiconductor (e.g., crystal faces), as reported by Ohno et al. [20].Because SrTiO 3 particles used in this work have featureless morphology, the precise control of the Pd 0 photodeposition would be difficult, leading to the compositional deviation in the Pd/Cr ratio.Metallic and oxidized Pd species were loaded on SrTiO 3 , as evidenced by XPS measurements (discussed later).The lattice fringe attributable to metallic Pd was observed in the 5 h sample (see 5 h (1)).Therefore, the central part of the nanoparticle consisted of metallic Pd.However, the location of the oxidized Pd species in the TEM images could not be detected.

XANES
Figure 4 shows Pd K-edge XANES spectra of PdCrOx/SrTiO3 prepared for various time periods (0-5 h).Data for Pd foil, PdO, and Pd/SrTiO3 (prepared without Cr) are also shown for comparison.The spectral shapes for Pd foil and Pd/SrTiO3 were very similar.Compared to metallic Pd (Pd foil and Pd/SrTiO3), the absorption edges of PdCrOx/SrTiO3 were varied with respect to the irradiation time (Figure 4A).The Pd K-edge positions are located at higher energies, which indicates that their electronic density is lower than metallic Pd. Figure 4B shows an enlarged view that shows that the K-edge positions are shifted to higher energies in the following order: Pd foil ≈ Pd/STO < 15 min < 5 h ≈ 1 h ≈ 30 min < 0 h < PdO.The K-edge of the 0 h sample was located at the highest energy position among all the samples examined.The K-edge position was shifted to the lower energy side with an increase of the irradiation time to 15 min, which indicates that the adsorbed Pd precursor on SrTiO3 was reduced by photoexcited electrons supplied from SrTiO3 within 15 min.After 15 min, the K-edge position was shifted slightly to the higher energy side, which indicates that the electron density of the PdCrOx nanoparticles is slightly decreased after 15 min, i.e., the Pd species were oxidized.These results indicate that the electronic state of the Pd species in the PdCrOx nanoparticles is dependent on the photoirradiation time.
Cr K-edge XANES spectra of PdCrOx/SrTiO3 were also investigated, and the results are shown in Figure S1.In contrast to the Pd K-edge spectra, the Cr K-edge positions were similar, regardless of the photoirradiation time.Therefore, it is concluded that the electronic state of only Pd showed a significant change during photoirradiation.

XANES
Figure 4 shows Pd K-edge XANES spectra of PdCrO x /SrTiO 3 prepared for various time periods (0-5 h).Data for Pd foil, PdO, and Pd/SrTiO 3 (prepared without Cr) are also shown for comparison.The spectral shapes for Pd foil and Pd/SrTiO 3 were very similar.Compared to metallic Pd (Pd foil and Pd/SrTiO 3 ), the absorption edges of PdCrO x /SrTiO 3 were varied with respect to the irradiation time (Figure 4A).The Pd K-edge positions are located at higher energies, which indicates that their electronic density is lower than metallic Pd. Figure 4B shows an enlarged view that shows that the K-edge positions are shifted to higher energies in the following order: Pd foil ≈ Pd/STO < 15 min < 5 h ≈ 1 h ≈ 30 min < 0 h < PdO.The K-edge of the 0 h sample was located at the highest energy position among all the samples examined.The K-edge position was shifted to the lower energy side with an increase of the irradiation time to 15 min, which indicates that the adsorbed Pd precursor on SrTiO 3 was reduced by photoexcited electrons supplied from SrTiO 3 within 15 min.After 15 min, the K-edge position was shifted slightly to the higher energy side, which indicates that the electron density of the PdCrO x nanoparticles is slightly decreased after 15 min, i.e., the Pd species were oxidized.These results indicate that the electronic state of the Pd species in the PdCrO x nanoparticles is dependent on the photoirradiation time.
Cr K-edge XANES spectra of PdCrO x /SrTiO 3 were also investigated, and the results are shown in Figure S1.In contrast to the Pd K-edge spectra, the Cr K-edge positions were similar, regardless of the photoirradiation time.Therefore, it is concluded that the electronic state of only Pd showed a significant change during photoirradiation.

EXAFS
Figure 5 shows the Fourier transforms of k 3 -weighted Pd K-edge EXAFS spectra.The peak of Pd foil at 2.5 Å is assigned to Pd-Pd bonding, and the peak of PdO at 1.5 Å is assigned to Pd-O.The 0 h sample showed a peak at 1.5 Å, which indicates the presence of Pd-O bonding.In the cases of the 0-5 h samples, the intensity of Pd-O bonding became relatively weak with an increase in the photoirradiation time.On the other hand, the intensity of the Pd-Pd bonding peak was increased.Finally, the 5 h sample showed peaks for both Pd-O and Pd-Pd, which indicates that the Pd species in PdCrOx/SrTiO3 transforms from an oxide-like state to a mixture of Pd 0 and Pd oxide as photoirradiation progressed.This trend does not contradict the observation in the XANES analysis (Figure 4).Therefore, it is concluded that the Pd species is reduced from oxide to the metallic state, although not completely, during the photoirradiation process.Similar to the Cr K-edge XANES results, no noticeable change could be identified in the fine structure of Cr (Figure S1).

EXAFS
Figure 5 shows the Fourier transforms of k 3 -weighted Pd K-edge EXAFS spectra.The peak of Pd foil at 2.5 Å is assigned to Pd-Pd bonding, and the peak of PdO at 1.5 Å is assigned to Pd-O.The 0 h sample showed a peak at 1.5 Å, which indicates the presence of Pd-O bonding.In the cases of the 0-5 h samples, the intensity of Pd-O bonding became relatively weak with an increase in the photoirradiation time.On the other hand, the intensity of the Pd-Pd bonding peak was increased.Finally, the 5 h sample showed peaks for both Pd-O and Pd-Pd, which indicates that the Pd species in PdCrO x /SrTiO 3 transforms from an oxide-like state to a mixture of Pd 0 and Pd oxide as photoirradiation progressed.This trend does not contradict the observation in the XANES analysis (Figure 4).Therefore, it is concluded that the Pd species is reduced from oxide to the metallic state, although not completely, during the photoirradiation process.Similar to the Cr K-edge XANES results, no noticeable change could be identified in the fine structure of Cr (Figure S1).

XPS
Figure 6 shows XPS Pd 3d spectra for the PdCrOx/SrTiO3 samples.All of the Pd 3d peaks can be deconvoluted into two peaks with binding energies of 337.1 and 335.2 eV, which are assigned to oxidized and metallic Pd species, respectively, although the binding energies are slightly different

XPS
Figure 6 shows XPS Pd 3d spectra for the PdCrO x /SrTiO 3 samples.All of the Pd 3d peaks can be deconvoluted into two peaks with binding energies of 337.1 and 335.2 eV, which are assigned to oxidized and metallic Pd species, respectively, although the binding energies are slightly different from those in the references (PdO: 336.5 eV; metallic Pd in Pd/SrTiO 3 : 334.8 eV) [21].The intensity ratio of the 335.2 eV peak becomes stronger relative to the 337.1 eV peak with an increase in the photoirradiation time, which is consistent with the trend observed in the EXAFS spectra (Figure 5).The details of the surface electronic states of PdCrO x were previously investigated using XPS [16].The XPS Pd 3d peak position of Pd species co-existing with Cr 3+ was thereby determined to be higher than that of bulk PdO(II).In addition, the Pd 3d peak position and structure of PdCrO x was different from the PdO(II) single oxide, and the surface absorbed Pd precursor.Therefore, the 337.1 eV peak is not a single Pd oxide or the Pd precursor but another Pd species that interacted strongly with Cr.

XPS
Figure 6 shows XPS Pd 3d spectra for the PdCrOx/SrTiO3 samples.All of the Pd 3d peaks can be deconvoluted into two peaks with binding energies of 337.1 and 335.2 eV, which are assigned to oxidized and metallic Pd species, respectively, although the binding energies are slightly different from those in the references (PdO: 336.5 eV; metallic Pd in Pd/SrTiO3: 334.8 eV) [21].The intensity ratio of the 335.2 eV peak becomes stronger relative to the 337.1 eV peak with an increase in the photoirradiation time, which is consistent with the trend observed in the EXAFS spectra (Figure 5).The details of the surface electronic states of PdCrOx were previously investigated using XPS [16].The XPS Pd 3d peak position of Pd species co-existing with Cr 3+ was thereby determined to be higher than that of bulk PdO(II).In addition, the Pd 3d peak position and structure of PdCrOx was different from the PdO(II) single oxide, and the surface absorbed Pd precursor.Therefore, the 337.1 eV peak is not a single Pd oxide or the Pd precursor but another Pd species that interacted strongly with Cr.In the case of Cr, the Cr 2p peak in various PdCrO x /SrTiO 3 samples was located in a position similar to Cr 2 O 3 , as shown in Figure S2.Therefore, the valence state of Cr on the surface of PdCrO x /SrTiO 3 is independent of the photoirradiation conditions.
On the basis of the XAFS and XPS measurements, it is thus likely that the deposited core/shell nanostructures on SrTiO 3 consist of metallic Pd, oxidized Pd, and Cr III (i.e., Cr 2 O 3 ), where the core and shell are mainly formed by metallic Pd and Cr 2 O 3 , respectively, with oxidized Pd species being located at the core/shell interface.However, we could not observe the oxidized Pd species in the TEM images shown in Figure 3, due to the compositional inhomogeneity of the deposited nanoparticles.

Effect of PdCrO x Nanoparticles deposited on SrTiO 3 upon the Photocatalytic Activity toward Overall Water Splitting
The as-prepared PdCrO x /SrTiO 3 samples were used to conduct photocatalytic reactions.Table 1 shows the overall water-splitting activities of PdCrO x /SrTiO 3 prepared for various time periods (0-5 h).The 0 h sample produced H 2 and O 2 ; however, the H 2 /O 2 ratio was not stoichiometric (entry 1).The total amount of H 2 and O 2 was increased, and the stoichiometry was improved with an increase of the irradiation time (entries 2-5).Therefore, longer irradiation times were effective to maximize the promotional effect of PdCrO x nanoparticles on photocatalytic water splitting.Bare and only Pd-loaded SrTiO 3 were also used for comparison.Only H 2 was evolved at a very slow rate with the unmodified SrTiO 3 (entry 6); therefore, cocatalyst loading was necessary for overall water splitting.Pd-loaded SrTiO 3 produced H 2 and O 2 upon UV irradiation, although the H 2 /O 2 ratio was far from stoichiometric (entry 7).As reported previously, molecular O 2 present in the reaction system can prevent stoichiometric water splitting on a semiconductor photocatalyst, because it can cause backward reactions [16].Therefore, a suitable modification method such as Cr 2 O 3 -shell modification [13,19] is essential to suppress O 2 -related backward reactions and efficiently allow overall water splitting to proceed.Both the stoichiometry and the amount of evolved gases were improved by the loading of a thin Cr 2 O 3 shell on the Pd particles (entry 8).However, the activity of the Pd-core/Cr 2 O 3 -shell system was much lower than that of the optimized PdCrO x /SrTiO 3 .This provides clear evidence of the superior functionality of the optimally-synthesized PdCrO x nanoparticles as cocatalysts.Our previous work has also shown that the photocatalytic activity of PdCrO x /SrTiO 3 (equivalent to the present 5 h sample) for overall water splitting is recoverable, with negligible changes in the valence state of Pd and Cr [16].
In Cr-containing mixed metal (oxide) cocatalysts, the key factors to enhance the promotional effect on photocatalytic water splitting are improvement of the H 2 evolution activity and the suppression of backward reactions (including H 2 /O 2 recombination and O 2 photoreduction) [14].For example, Cr(III) oxide shells formed on noble metal nanoparticles (cores) prevents these backward reactions [19].We have previously reported that PdCrO x nanoparticles with a similar core/shell type structure had the same functionality [17].
The promotional effect of PdCrO x nanoparticles on photocatalytic H 2 evolution was investigated using methanol as an electron donor.Note that, in this reaction, methanol is irreversibly oxidized, with no O 2 evolution [17].As listed in Table 2, all of the tested samples produced H 2 .The 15 min sample, which exhibited relatively low activity for overall water splitting (see Table 1), gave the highest activity among the tested samples (entry 2), although the difference in activity among the PdCrO x /SrTiO 3 samples was not very large.The Pd metal and Pd/Cr 2 O 3 (core/shell) type samples exhibited lower activity (entries 6 and 7).A Cr 2 O 3 shell formed on the surface of noble metal nanoparticles has been reported to prevent not only back reaction but also proton reduction to produce H 2 to a certain extent [19].The H 2 evolution activity using Pd/SrTiO 3 was decreased by the stepwise photodeposition of Cr 2 O 3 thin shell, as shown in Figure S3.The H 2 evolution activity of PdCrO x /SrTiO 3 should be reduced with an increase of the irradiation time due to an increase of the amount of loaded Cr, because the reaction sites are covered with the Cr 2 O 3 shells.However, the observed H 2 evolution activities were not simply decreased by Cr co-loading, as evident in Table 2. Therefore, it is suggested that interaction between Pd and Cr enhanced the intrinsic H 2 evolution activity, although the mechanistic detail is still under investigation.Nevertheless, the H 2 evolution activity of PdCrO x is not significantly changed by the photoirradiation time.Therefore, it is worth considering that there is another factor that influences the overall water-splitting activity of PdCrO x /SrTiO 3 .We also considered that backward reaction caused by evolved O 2 could influence the water-splitting activity.Therefore, two types of backward reaction (H 2 /O 2 recombination and O 2 photoreduction) were examined.Table 3 compares the rates of H 2 /O 2 recombination using the 15 min and 5 h samples, as well as Pd-loaded SrTiO 3 .In contrast to Pd/SrTiO 3 , which exhibited very fast H 2 /O 2 recombination, the PdCrO x loaded samples exhibited much slower recombination rates.Oxygen photoreduction activity was also investigated during photocatalytic H 2 evolution in the presence or absence of O 2 .The introduction of O 2 had a negative impact on the H 2 evolution activity.As detailed in Table 4, almost 97% of the initial activity was lost in the 15 min sample.Nevertheless, the 5 h sample still maintained approximately 20% of the initial activity, even in the presence of O 2 .The difference in the H 2 evolution activity between the two samples is in qualitative agreement with the difference in the O 2 consumption activity.It is thus clear that O 2 photoreduction occurred more actively on the 15 min sample than on the 5 h sample.Therefore, more effective suppression of O 2 photoreduction could be realized by longer time photoirradiation during the preparation of PdCrO x .

Synthesis of SrTiO 3
The SrTiO 3 was prepared using the polymerized complex method [6].Titanium tetraisopropoxide (95.0+%,FUJIFILM Wako Pure Chemical, Osaka, Japan) was dissolved in methanol.Anhydrous citric acid (FUJIFILM Wako Pure Chemical, Osaka, Japan) was then added to the solution and heated at 353 K. SrCO 3 (99.9%,Kanto Chemical, Tokyo, Japan) and ethylene glycol (>99.5%,FUJIFILM Wako Pure Chemical, Osaka, Japan) were added to the solution.After heating at 423 K overnight for polymerization to proceed, the solution was heated in air at 573 K, and then at 823 K for 2 h to remove organic residues.The precursor was finally calcined at 1423 K for 2 h.

Cocatalyst Loading
The PdCrO x cocatalyst was deposited on the surface of SrTiO 3 by a simultaneous photodeposition method, as reported previously [16].An amount of 0.2 g of SrTiO 3 was dispersed in 100 mL of 10 vol% methanol aqueous solution containing 1.0 wt% Cr (vs.SrTiO 3 ) from K 2 CrO 4 and 0.5 wt% Pd (vs.SrTiO 3 ) from Na 2 PdCl 4 •3H 2 O (FUJIFILM Wako Pure Chemicals Co., Osaka, Japan) as precursors.The solution was completely degassed and irradiated at room temperature for various time periods of 0-5 h.Evolved gases were detected using gas chromatography (GC-3200, Shimadzu Co., Kyoto, Japan) with a thermal conductivity detector (TCD), an MS-5A column and argon as a carrier gas (GL Science Co., Tokyo, Japan).
The amount of K 2 CrO 4 (i.e., deposited Cr) remaining in solution was quantified using UV-visible absorption spectroscopy.First, the PdCrO x /SrTiO 3 powder was collected by filtration and dried overnight at 343 K.After the addition of 0.27 mmol of EDTA•2Na (99.5%, Dojin Chem., Tokyo, Japan) to the filtrate, the mixture was boiled for 0.5 h.The solution was analyzed using UV-visible absorption spectroscopy.

Characterization
Prepared samples were studied using TEM, XAFS and XPS.TEM observations and the combined EDX were conducted using a JEM-2010F apparatus (JEOL, Tokyo, Japan).The Pd and Cr K-edge XAFS spectra were collected at AR-NW10A of the Photon Factory Advanced Ring and BL-9A of the Photon Factory, respectively (Proposal no.2014S2-006, Photon factory, Tsukuba, Japan).Standard samples (Pd foil, PdO, Cr foil, Cr 2 O 3 ) were measured by the transmission method.PdO and Cr 2 O 3 were diluted in boron nitride and compressed to form pellets. Various PdCrO x /SrTiO 3 samples prepared for various photoirradiation times (0-5 h) were measured by the fluorescence method using a multichannel solid state detector.The photon energies were calibrated according to the X-ray absorption edge of Pd foil (24350 eV) and Cr foil (5989 eV).Analysis of the raw XAFS spectra was conducted using the Athena and Artemis programs [22].The Fourier transforms of the k 3 -weighted EXAFS spectra were typically in the 3.0-11 Å −1 region.The XPS spectra were obtained using an ESCA 3400 (Shimadzu, Kyoto, Japan) with a Mg anode.The binding energy of the C 1s peak (285.0 eV) that originated from adventitious carbon was used as the XPS reference.

Photocatalytic Reaction
Water-splitting reactions were conducted in a top-irradiation cell connected to a closed gas circulation system.An amount of 0.1 g of PdCrO x /SrTiO 3 was dispersed in 140 mL of distilled water or 10 vol% aqueous methanol solution.The reactant solutions were degassed completely, followed by photoirradiation (Xe lamp, 300 W, 20 A, λ ≥ 300 nm).Evolved gases were analyzed using gas chromatography.The reproducibility of the gas evolution rate was within ~20%.

Backward Reactions
Water formation from H 2 and O 2 was conducted under similar conditions to that for the photocatalytic reaction.An amount of 0.025 g of PdCrO x /SrTiO 3 was dispersed in pure water.The reactant solution and gas phase were degassed completely and purged with hydrogen and oxygen.The reduction of gases was observed under dark conditions.Photocatalytic O 2 reduction activity was investigated under similar experimental conditions to those used for the photocatalytic hydrogen evolution reaction with O 2 .

Conclusions
Photochemical preparation of PdCrO x multi-component nanoparticles on SrTiO 3 was studied to develop a new cocatalyst for overall water splitting.The valence state and fine structure of the Pd species in PdCrO x were strongly dependent on the photoirradiation time during PdCrO x deposition.The Pd species in PdCrO x was changed from oxide to oxide/metal mixed phase with an increase of the photoirradiation time, which contributed to enhanced activity for overall water splitting.The results of photocatalytic reactions indicated that the effect of activity enhancement was due to the suppression of O 2 -photoreduction, a backward reaction of overall water splitting.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2073-4344/9/1/59/s1:Funding: This work was also partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas "Mixed Anion" (Project JP16H06441) and "I 4 LEC" (Project JP17H06438) from the Japan Society for the Promotion of Science (JSPS).T.K. acknowledges financial support in the form of a JSPS Fellowship for Young Scientists (Project JP18J10548).

Scheme 1 .
Scheme 1. Photochemical preparation of PdCrOx nanoparticles on the surface of SrTiO3.

Scheme 1 .
Scheme 1. Photochemical preparation of PdCrO x nanoparticles on the surface of SrTiO 3 .

Figure 2 .
Figure 2. Amount of Cr contained in PdCrO x /SrTiO 3 at various photodeposition times.

Figure 3 .
Figure 3. Transmission electron microscopy (TEM) images and Pd/Cr weight ratios of PdCrOx/SrTiO3 prepared for various time periods (15 min, 1 and 5 h).The weight ratio was calculated from energy dispersive X-ray spectroscopy (EDX) measurements.

Figure 3 .
Figure 3. Transmission electron microscopy (TEM) images and Pd/Cr weight ratios of PdCrO x /SrTiO 3 prepared for various time periods (15 min, 1 and 5 h).The weight ratio was calculated from energy dispersive X-ray spectroscopy (EDX) measurements.

Figure 5 .
Figure 5. Fourier transforms of k 3 -weighted Pd K-edge EXAFS spectra for PdCrO x /SrTiO 3 prepared for various time periods (0-5 h).Data for Pd foil, PdO and Pd/SrTiO 3 are shown as references.

Figure 5 .
Figure 5. Fourier transforms of k 3 -weighted Pd K-edge EXAFS spectra for PdCrOx/SrTiO3 prepared for various time periods (0-5 h).Data for Pd foil, PdO and Pd/SrTiO3 are shown as references.
Figure S1: (A) XANES spectra and (B) Fourier transforms of k 3 -weighted Cr K-edge of PdCrO x nanoparticles (0-5 h) on SrTiO 3 ; Figure S2: XPS Cr 2p spectra of PdCrO x /SrTiO 3 prepared for various time periods (0-5 h).Data for K 2 CrO 4 and Cr 2 O 3 are shown as references; Figure S3: Time course of H 2 evolution over Pd or Cr 2 O 3 /Pd nanoparticle loaded SrTiO 3 .Author Contributions: T.K. conducted most of the experiments and analysis.T.K. and K.M. designed the experiments; T.K., S.N. and D.L. performed the experiments and analyzed the data; T.K. and K.M. wrote the manuscript.All authors discussed and provided comments on the experiments and the manuscript during preparation.

Table 4 .
Rates of H 2 evolution and O 2 consumption over PdCrO x /SrTiO 3 samples (15 min and 5 h) in the presence or absence of O 2 a .