In the search for efficient photo-catalysts for solar water splitting, the past few years have shown an increasing interest in complex metal oxides. When dispersed as a powder in an aqueous solution, several of these materials (e.g. In_1-xNi_xTaO₄, InVO₄, BiVO₄, NaTaO₃, etc.) can evolve oxygen and/or hydrogen. However, with typical evolution rates of 1 to 100 µmole.g^-1.h^-1, efficiencies are still too low for practical applications.
To investigate the relevant materials properties, thin dense films of InVO₄ were made by spray pyrolysis. The optical absorption spectra reveal a bandgap of 3.2 eV, which seems to contradict earlier reports on visible light absorption by InVO₄. Detailed impedance analysis shows a high donor density (6x10¹⁹ cm^-3) and a very low conductivity (3x10^-8 Scm^-1). This can be explained by the presence of deep donors in the material (cf. Figure 1A). The presence of ionized deep donors in the depletion layer gives rise to an optical transition from the valence band to the donor level, and we believe that this transition is responsible for the visible light photo-activity observed for InVO₄ powders. A similar effect is observed for BiVO₄ powders, which change color from yellow to reddish when annealed in air at 700°C (Figure 1B). No changes in the crystal structure occur, and the change in color is attributed to oxygen vacancies that form deep donor states in BiVO₄. Experimental evidence for the deep donor model will be presented, and the consequences for the design of efficient photocatalysts based on complex oxides will be discussed.