Soft X-ray spectroscopic studies of the electronic structure of M:BiVO4 (M = Mo, W) single crystals

Bismuth vanadate, BiVO₄, is a promising material for use as an anode in photoelectrochemical water splitting. However, its conversion efficiency is limited by poor bulk charge transport, which is via small-polarons. We report here the use of a suite of X-ray spectroscopic probes to determine the electronic structure of 0.3-0.6 at% M:BiVO₄ (M = Mo or W). The results are interpreted in the context of current theories regarding the influence of doping on the existence of inter-band gap small-polaron states and their effect on the conversion efficiency of BiVO₄. Preliminary X-ray absorption and emission measurements reveal that doping widens the band gap from 2.50 to 2.75 eV, whereas the indirect nature of the band gap remains unaffected. X-ray absorption spectroscopy verified that the doping levels did not affect the distorted tetrahedral environment of V⁵⁺ in BiVO₄. For BiVO₄ and W:BiVO₄, V L₃ resonant inelastic X-ray scattering showed energy loss features related to charge transfer from low lying valence metal/oxygen states to unoccupied V e_g conduction band states. A 3.8 eV energy loss feature, coupled with small polaron-like peaks measured in valence band resonant photoelectron spectroscopy of M:BiVO₄, point to the population of inter-band gap V 3d states of e_g symmetry. The data reveals the existence of a band gap state in the absence of an applied bias in M:BiVO₄, linked to small-polaron formation. We tentatively assign it as a deep trap state, which suggests that the improved conversion efficiency of M:BiVO₄ relative to the undoped material is largely due to the increased carrier concentration in spite of increased carrier recombination rates.