With a bandgap of 2.4 eV, BiVO₄ is an interesting material for photo-catalytic applications in the visible part of the spectrum.¹ We report on the photo-electrochemical performance of doctor-bladed films of 0.5 µm BiVO₄ particles. Addition of methanol to the solution results in a significant increase of the anodic photocurrent (Figure 1), and values of up to 20 µA/cm² were observed under AM1.5 illumination. When annealing BiVO₄ pellets in air at 700°C, sintering of the particles occurs and the color of the material changes from bright yellow to reddish/pink. This can be explained by evaporation of Bi or V₂O₅ from the BiVO₄ lattice, since both these species have fairly high vapor pressures (>10^-4 torr at 500°C). This results in the formation of oxygen vacancies which form energy levels deep in the bandgap. The reddish/pink color is attributed to optical excitations involving these energy levels. When crushing the fired pellet in a mortar, the material reverts back to its original yellow color. This is consistent with oxygen vacancies being re-filled by atmospheric oxygen, since BiVO₄ is also known for its high oxygen ion conductivity. Electrical conductivity measurements show an activation energy of 0.23 eV, indicating that electron transport occurs via a hopping mechanism. The conductivity remains constant as a function of p(O₂), indicating that oxygen vacancies are not the main defect in this material.

[1] K. Sayama, A. Nomura, T. Arai, T. Sugita, R. Abe, M. Yanagida, T. Oi, Y. Iwasaki, Y. Abe, and H. Sugihara, J.Phys.Chem.B 110 (2006) 11352.