RESUMEN
Understanding the excited state of photocatalysts is significant to improve their activity for water splitting reaction. X-ray absorption fine structure (XAFS) spectroscopy in X-ray free electron lasers (XFEL) is a powerful method to address dynamic changes in electronic states and structures of photocatalysts in the excited state in ultrafast short time scales. The ultrafast atomic-scale local structural change in photoexcited WO3 was observed by W L1 edge XAFS spectroscopy using an XFEL. An anisotropic local distortion around the W atom could reproduce well the spectral features at a delay time of 100 ps after photoexcitation based on full potential multiple scattering calculations. The distortion involved the movement of W to shrink the shortest W-O bonds and elongate the longest one. The movement of the W atom could be explained by the filling of the dxy and dzx orbitals, which were originally located at the bottom of the conduction band with photoexcited electrons.
RESUMEN
Dispersive X-ray absorption fine structure (DXAFS) spectroscopy is a versatile measurement technique for analyzing chemical reactions in real time. We have developed a novel time-resolved DXAFS instrument based on two polychromators and a wide-range position-sensitive detector that permits direct observation of the synergistic effects of two elements. This system enables simultaneous acquisition of the X-ray absorption spectra for two different elements without any mechanical movement of the X-ray optics. The developed system was successfully applied to monitor both the synthesis of a Ni-Cu bimetallic catalyst, which revealed that the reduction of Ni occurred at a higher temperature than that of Cu, and the charge-discharge processes of a LixNi0.5Mn1.5O4-based lithium-ion battery, which demonstrated that the redox reactions of Ni and Mn occurred sequentially at specific electrode potentials.