Utrafast photoinduced phase-changes in the photocatalyst BiVO4

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.