Abstract:The dynamics of the local electronic and geometric structures of WO3 following photoexcitation were studied by femtosecond time-resolved X-ray absorption fine structure (XAFS) spectroscopy using an X-ray free electron laser (XFEL). We found that the electronic state was the first to change followed by the local structure, which was affected within 200 ps of photoexcitation.The utilization of solar energy is one of the most fascinating and important research subjects with regard to obtaining a society less dependent on fossil fuels. For this reason, photocatalysts and photoelectrodes [1] have been developed over the last several decades in order to produce hydrogen from water without the generation of harmful pollutants. More recently, tungsten trioxide (WO3) has received much attention, since its band gap is 2.6 to 2.8 eV, [3] meaning that it functions as an active photocatalyst under visible light irradiation. WO3 has shown the potential to allow the complete decomposition of water to hydrogen (H2) and oxygen (O2) when used in combination with TaON in the so-called Z-scheme photocatalyst system. [4] Various fundamental photocatalytic properties of WO3 have been studied by both theoretical and experimental methods. The main goal of such work has been to determine where photocarriers are created and how and when they are consumed. Density functional theory (DFT) calculations concerning the ground state [5] have shown that the valence band of WO3 is primarily composed of O 2p orbitals and that the conduction band consists of W 5d orbitals. Electrons in the valence band are excited to the conduction band to create photocarriers (electrons and holes) by photoabsorption. These DFT results have thus provided insight into the nature of such photocarriers. The lifetimes of photocarriers in WO3 have been studied using spectroscopic techniques with different time scales.[6]Amano et al.[6a] observed long-living photocarriers having lifetimes longer than 100 s in WO3 microparticles and claimed that these photocarriers such as these could contribute to the activity of the oxide for the water splitting reaction. Pesici et al. [6b] reported that more than 90% of electron-hole pairs recombined within 10 s. Bedja et al. also observed the fast trapping of photoelectrons (within 1 ns) in colloidal WO3.[6c]Although previous fundamental studies have generated significant information with regard to photocarriers in WO3, there have been no studies addressing the local electronic and geometric structure changes around W, especially in the case of photoexcited WO3, even though such data are vital to understanding and improving the photocatalytic performance of this material. Recently, [7] we successfully observed the excited state around W in WO3 100 ps after laser excitation, using a pump-probe X-ray absorption fine structure (XAFS) method employing a PF-AR (a single bunch operation storage ring with a time resolution of 100 ps). [7][8] Following a 400 nm laser pulse excitation, the W LIII-edge XAFS white line peak d...

Ultrafast excitation of photocatalytically active BiVO was characterized by femto- and picosecond transient X-ray absorption fine structure spectroscopy. An initial photoexcited state (≪500 fs) changed to a metastable state accompanied by a structural change with a time constant of ∼14 ps. The structural change might stabilize holes on oxygen atoms since the interaction between Bi and O increases.

Ultrafast XANES spectra and full-potential multiple scattering reveal the anisotropic structure change of photoexcited WO3.