Probing excited states of photocatalysts by x-ray spectroscopy in XFELs
Yohei Uemura
European X-ray Free Electron Laser GmbH, Schenenfeld, Germany
yohei.uemura@xfel.eu

Developing a clean and long-standing energy resource is a key issue to establishing anenvironmentally sustainable society. Producing electricity from the water-splitting reaction
using sunlight is an ideal approach to generate electricity without using fossil fuels andproducing any harmful by-products. Photocatalysts and photoelectrodes have been developed
for decades since they play a central role to promote the water-splitting reaction. Among various
kinds of photosensitive materials, metal oxides are good candidates for photocatalysts(ex. TiO2,a-Fe2O3, BiVO4, WO3). In order to improve their catalytic performances, modifying the energy
levels of the conduction band (CB) and the valence band (VB) is an important subject.
Fundamental studies such as theoretical calculations, electrochemical methods andspectroscopic studies have been conducted to understand the basic properties of photocatalytic
materials to develop new materials. X-ray absorption spectroscopy (XAS) is a newly developed
methodology to observe the excited states of such materials. Owing to establishing x-ray freeelectron lasers (XFELs) in several countries, very fast processes inside photocatalysts afterexcitation can be observed even within a picosecond.
In this presentation, I would like to introduce recent studies on photocatalytic materials usingtransient XAS at XFELs. We have measured transient XAS of several different materials(WO3 1-3,Fe2O3 4 and CuWO4 5) at different facilities. For WO3, we found that a metastable state is formedafter photoexcitation and it includes a significant structural change. In addition, a fast decay
process was observed just after the photoexcitation (< 1 ps) owing to the arrival timing monitor
which corrects the timing jitter between each x-ray pulse and laser pulse. For a-Fe2O3, we
employed soft x-ray XAS (Fe L3 edge) at PAL-XFEL (South Korea) to observe the changes in thevalence of Fe atoms after photoexcitation directly. The transient changes of the photoexcited
Fe atoms were successfully observed by the delay of 10 ps. From the result of the transient FeL3 XAS, two different kinetic processes were discovered. We also performed resonant inelastic
scattering (RIXS) experiments for the photoexcited a-Fe2O3 and the modulation of the local
structure of Fe was observed during the charge recombination process by the transient RIXS. For
CuWO4, the electron density modulation was observed at Cu L3 XAS. This electron modulation
does not come from the change of the valence state of Cu but the modulation reflects the charge
density changes in neighbouring atoms. We also measured the transient O K-edge XAS of aFe2O3 and CuWO4. I would also like to present the results of O Kedge XAS.
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Ismail, A. S. M.; Uemura, Y., et al., Phys. Chem. Chem. Phys. 2020, 22, 2685-2692.
Uemura, Y.; Ismail, A. S. M., et al., J. Phys. Chem. C 2021, 125, 7329-7336.