Defect Chemistry of Titanium Dioxide. Application of Defect Engineering in Processing of TiO2-Based Photocatalysts

The present work brings together the concepts of defect chemistry and photoelectrochemistry in order to consider TiO2-based photosensitive oxide semiconductors as photocatalysts for water purification. This paper reports the most recent progress in the defect chemistry of TiO2 and its solid solutions with aliovalent ions forming donors and acceptors. The relationship between the defect-related properties, such as electrical and photocatalytic properties, are outlined. It is shown that reactivity, photoreactivity, and the related charge transfer of photocatalysts based on TiO2 are determined by defect disorder and the related chemical potential of electrons. Therefore, defect chemistry may be used as a framework for the processing of well-defined TiO2-based photocatalysts. The photoreactivity of TiO2 with water and its solutes is considered in terms of the effect of both collective and local properties. The effect of noble metals attached to TiO2 as a separate phase, such as platinum, on photoelectrochemical properties and the related photocatalytic performance of TiO2 is discussed. The key functional properties, which are responsible for the efficient conversion of solar energy into chemical energy (required for water purification), are outlined. The effect of TiO2 doping with aliovalent ions on properties is considered in terms of the doping mechanisms and the related semiconducting properties. It is argued that comparison of the experimental data reported in the literature on the photocatalytic properties of TiO2 dictates the need to establish standards for photocatalysts, which are well-defined. This paper reports the processing conditions of well-defined TiO2. It is argued that knowledge of the mass transport kinetic data, such as chemical and self-diffusion coefficients, is needed for selecting the optimal processing conditions.