Direct Evidence of Surface Reduction in Monoclinic BiVO4
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 similar to 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.