Toward sustainable energy : photocatalysis of Cr-doped TiO2: 2. effect of defect disorder

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 versus bulk properties. In this work, we show that the effect of chromium on photocatalytic performance of TiO 2 depends on its elemental content and the related defect disorder that is determined by oxygen activity in the oxide lattice. At high oxygen activity, chromium leads to enhanced photocatalytic performance only for dilute solid solutions (up to 0.04-0.043 at.% Cr). Higher chromium content results in a decrease of photocatalytic activity below that for pure TiO 2, despite the observed substantial decrease of the band gap. The photocatalytic performance of Cr-doped TiO 2 annealed in reducing conditions is low within the entire studied range of compositions. The obtained results led to derivation of a theoretical model representing the mechanism of the light-induced reactivity of TiO 2 with water and the related charge transfer. The photocatalytic performance is considered in terms of a competitive effect of several key performance-related properties. The performance is predominantly influenced by the concentration of titanium vacancies acting as reactive surface sites related to anodic charge transfer. [Figure not available: see fulltext.].