Direct Evidence of Surface Reduction in Monoclinic BiVO₄

TLDR

The study uses all‑electron DFT WIEN2k calculations to model the electronic structure and EELS near‑edge features of BiVO4. Electron‑energy‑loss spectroscopy shows that BiVO4 surfaces exhibit a 5‑nm shell where vanadium is reduced from +5 to ~+4, implying ~15 % oxygen vacancies that segregate to the surface, a finding corroborated by XPS and supported by DFT calculations.

Abstract

Local measurements of the oxidation state of vanadium in monoclinic BiVO4 particles by means of electron energy-loss spectroscopy in scanning transmission electron microscopy reveal a pronounced surface reduction: within a 5-nm-thick shell, the oxidation state of vanadium is reduced from +5 to about +4. Thus, charge neutrality near the surface demands for ∼15% oxygen vacancies. Our results provide direct evidence for the segregation of oxygen vacancies at the surface of BiVO4. This observation is confirmed by X-ray photoelectron spectroscopy. The experimental findings are complemented with all-electron density functional theory based WIEN2k calculations of the density of electron states and of the electron energy-loss near-edge structure. The theoretical results provide further information on the electronic changes induced by the experimentally verified oxygen vacancies.