Tracking the Local Structure Change during the Photoabsorption Processes of Photocatalysts by the Ultrafast Pump-Probe XAFS Method

Yohei Uemura,Shunsuke Nozawa,Tetsuo Katayama,Kiyotaka Asakura,Toshihiko Yokoyama

doi:10.3390/app10217818

Abstract

The birth of synchrotron radiation (SR) facilities and X-ray free electron lasers (XFELs) has led to the development of new characterization tools that use X-rays and opened frontiers in science and technology. Ultrafast X-ray absorption fine structure (XAFS) spectroscopy for photocatalysts is one such significant research technique. Although carrier behavior in photocatalysts has been discussed in terms of the band theory and their energy levels in reciprocal space (k-space) based on optical spectroscopic results, it has rarely been discussed where photocarriers are located in real-space (r-space) based on direct observation of the excited states. XAFS provides information on the local electronic and geometrical structures around an X-ray-absorbing atom and can address photocarrier dynamics in the r-space observed from the X-ray-absorbing atom. In this article, we discuss the time dependent structure change of tungsten trioxide (WO3) and bismuth vanadate (BiVO4) photocatalysts studied by the ultrafast pump-probe XAFS method in the femtosecond to nanosecond time scale with the Photon Factory Advanced Ring (PF-AR) and the SPring-8 Angstrom Compact free-electron LAser (SACLA). WO3 shows a femtosecond decay process of photoexcited electrons followed by a structural change to a metastable state with a hundred picosecond speed, which is relaxed to the ground-state structure with a nanosecond time constant. The Bi L3 edge of BiVO4 shows little contribution of the Bi 6s electron to the photoabsorption process; however, it is sensitive to the structural change induced by the photoexcited electron. Time-resolved XAFS measurements in a wide range time domain and with varied wavelengths of the excitation pump laser facilitate understanding of the overall details regarding the photocarrier dynamics that have a significant influence on the photocatalytic performance.

Highlights

The X-ray absorption fine structure (XAFS) method has been developed over the past half century according to the progress of X-ray sources
We describe our studies on the photoexcitation processes of the tungsten trioxide (WO3 ) and bismuth vanadate (BiVO4 ) photocatalysts by pump-probe XAFS in synchrotron radiation (SR) and X-ray free electron lasers (XFELs) [19,20,21,22] over the last decade
WO3 is known as a wide-bandgap semiconductor, of which the bandgap is ca. 2.4 eV

Summary

Introduction

The X-ray absorption fine structure (XAFS) method has been developed over the past half century according to the progress of X-ray sources. Its time resolution has been improved significantly. In the early days of XAFS experiments (i.e., in the 1970s before the birth of synchrotron radiation (SR) facilities), it took a couple of days or more than a week to obtain a XAFS spectrum with a good signal-to-noise (S/N) ratio due to the low X-ray flux of standard X-ray tubes. SR facilities were the first game-changer to make the measurement time of XAFS much shorter. It became possible to measure an XAFS spectrum with a good S/N ratio in less than 1 h in the 1980s using SR facilities. Further progress was achieved by the development of fast data acquisition techniques such as quick

Methods

Results

Conclusion