Towards sustainable energy : photocatalysis of Cr-doped TiO2: 3 : effect of oxygen activity

The present chain of five papers considers the concept of solar-to-chemical energy conversion using TiO ₂-based semiconductors. The series reports the effect of chromium on the key performance-related properties of polycrystalline TiO ₂ (rutile), including electronic structure, photocatalytic activity, intrinsic defect disorder, electrochemical coupling and surface vs. bulk properties. The present work reports the effect of oxygen activity in the oxide lattice on photocatalytic activity of pure and Cr-doped TiO ₂ (0.04 at% Cr). Processing of specimens included annealing at 1273 K in the gas phase of controlled oxygen activity in the range 10 ⁻¹² Pa < p(O ₂) < 10 ⁵ Pa. We show that the increase of oxygen activity results initially in a decrease of photocatalytic activity, minimum around the n-p transition point, and then increase assuming maximum at p(O ₂) = 10 ⁵ Pa. The obtained results are considered in terms of a theoretical model that explains the effect of defect disorder on the reactivity of TiO ₂ with water. The minimum of the photocatalytic activity corresponds to the n-p transition point. The maximum of performance at high p(O ₂) is determined by increased concentration of titanium vacancies forming surface active sites.