Abstract
Introduction The solar energy is a sustainable and clean energy candidate to achieve a carbon-free society. Among all the possible approaches for solar energy conversion, the photocatalytic water splitting is a promising way to harvest solar energy and produce hydrogen. Recently, Domen and co-workers developed a photocatalyst, Rh/Cr2O3/CoOOH/SrTiO3(STO):Al, which has an external quantum efficiency of 96 per cent at wavelengths between 350 and 360 nm.1 Although it is assumed that CoOOH acts as a OER site and Rh/Cr2O3 acts as a HER site, actual observation of photo-induced carrier transfer to the co-catalysts has not been achieved yet. In this work, we developed a new in situ soft X-ray absorption spectroscopy (SXAS) system with the conversion electron yield (CEY) mode which provides surface sensitive chemical information to study the photoexcited carriers transfer from bulk STO to the co-catalysts especially CoOOH, the OER site.
Experiment The STO:Al photocatalyst system with co-catalysts loaded was prepared with two steps 1; (1) preparation of STO:Al nanoparticles. Using the flux method, STO and SrCl2 mixed with molar ratio of 1:10, and heated in an alumina crucible at 1373 K for 10 hours. (2) photodepostition for the HER and OER co-catalysts loading on the STO surface. One-hundred mg of STO:Al was dispersed in 50 ml water and performed 3 steps photodeposition in a series for Rh, Cr2O3, and CoOOH.1 In situ SXAS measurements were performed at BL-13A in the Photon Factory (KEK-PF, Tsukuba). The SXAS measurement cell based on the CEY mode2 was modified in the present study such that in situ XAS measurements can be conducted under irradiation of UV-Vis lights onto sample surfaces with different gas environments.
Results and Discussion The standard SXA spectra for Co L-edge were measured using both the total electron yield (TEY) method and the CEY method under dark conditions. From Co L-edge CEY-SXAS measurements, we confirmed that the Co species in the photocatalyst is associated with CoOOH. However, we found from TEY-SXAS measurements under a high vacuum condition that the Co species undergoes auto-reduction with showing a strong pre-edge peak intensity compared with the results obtained from the CEY mode. Therefore, the CEY mode measurement can give more reliable and X-ray-induced-reduction-free SXAS data. Using the CEY mode with UV-Vis irradiation, the Co L-edge XAS gives slight reduction. When the UV-Vis light was delivered on the sample surface, a main peak at 780 eV, slightly shifts to the lower energy side. As the UV-Vis irradiation time increases, the pre-edge peaks are slightly enhanced with reduction in intensity of the main peak, showing that a small amount of Co3+ was reduced to Co2+. This result indicates that the Co is not receiving photoexcited holes but electrons when the photocatalyst is irradiated by the UV-Vis light without water vapor. After saturated water vapor was introduced into the cell, the pre-edge peaks A and B originating from the Co2+ state drastically diminished with a slightly higher energy shift of the main peak C, showing oxidation from Co2+ to Co3+ and further towards Co4+ in the OER site as shown in Fig. 1. These spectral changes indicate that the Co is receiving the photoexcited holes from the bulk STO, and the photoexcited holes are used to oxidize the Co2+ to the Co3+ in the OER sites.
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