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

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.

A new QEXAFS system on the general XAFS beamline at the Shanghai Synchrotron Radiation Facility

“…Its inherent advantages, including short time scanning periods, low radiation dose, information on kinetics, and the elucidation of chemical-reaction and phase-transition mechanisms, allow QEXAFS to play an important role in the field of time-resolved studies (Eslava et al, 2016;Prestipino et al, 2011;Gaur et al, 2019;Marchionni et al, 2020). Differing from the sophisticated setups of energy-dispersive XAFS (Flank et al, 1982) and pump-probe XAFS (Oyanagi et al, 2001;Bressler et al, 2009;Uemura et al, 2020;Smolentsev et al, 2018), the QEXAFS method can be developed on the general XAFS beamline and shares most of the devices with the conventional XAFS experimental method.…”

The new X-ray absorption fine-structure beamline with sub-second time resolution at the Taiwan Photon Source

“…However, the EDXAS setup does not allow for measurements of the fluorescence signal in samples, and it requires a very flat sample surface. Pump-probe XAS can only be used for samples that can be excited by pulse laser, such as photocatalysts (Uemura et al, 2020;Smolentsev et al, 2018). The QXAFS technique can be more flexible and convenient for in situ/operando XAFS measurements.…”

Metastable structure of photoexcited WO$_{3}$ determined by the pump--probe extended X-ray absorption fine structure spectroscopy and constrained thorough search analysis

“…[3] We have acquired PP X-ray absorption near edge structure (PP-XANES) spectra of WO 3 after pulsed laser irradiation and have found that the MS was created within 150 ps, followed by relaxation to the ground state (GS) with a time constant of 2 ns. [4][5][6][7] We proposed that the photoelectrons were trapped at distorted stoichiometric W sites in the crystal lattice, similar to the trapping of polarons, instead of at surface or bulk defects. [6] The formation time for the MS, 150 ps, was longer than that for the other oxides such as TiO 2 and Fe 2 O 3 , consistent with previouslyreported optical measurement results [8].…”