Bismuth vanadate (BiVO4) is a key prototypical photocatalyst for water splitting, due to efficient collection of sunlight with an absorption onset at 2.5 eV, close to the maximum flux of the solar spectrum, and high solar to hydrogen conversion efficiency of up to 9.2%. Despite these promising characteristics, the fundamental nature and dynamics of photoexcitations in BiVO4 remain unclear. We now use advances in x-ray pump-probe techniques at sub-picosecond timescales to study the interactions of photo-excitations with the crystal lattice and connected changes of the atomic valency state in BiVO4 thin films. We measure pump-probe X-Ray Diffraction (XRD), X-ray Diffuse Scattering (XDS) and X-ray Absorption Near-Edge (XANES) at EuXFEL and APS to resolve structural and electronic dynamics. We find an unexpected ultrafast photoinduced structure change from monoclinic to tetragonal phases. From dynamics of related electronic valency changes and lattice strain fields, we draw up a detailed mechanistic model of our observations.
The formation of local strain fields is a key aspect in understanding light-induced processes in semiconductors: For instance, electric conductivity is influenced by the formation of polarons, quasiparticles that evolve from the interaction of a charge carrier with the lattice. We performed pump-probe experiments with an X-ray Free-Electron Laser (XFEL) to measure the photoinduced X-ray scattering dynamics of epitaxial BiVO4 with femtosecond time resolution. We then compared this data to simulations of different localized strain fields in a regular quadratic lattice. While the material shows little diffuse scattering, comparison with simulations of an acoustic strain wave indicates that the material is contracting in a concerted motion.
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