Review of functional titanium oxides. I: TiO2 and its modifications

An extensive and wide-ranging literature about the polymorphs of titanium dioxide (TiO2) has accumulated during the last few decades, providing a very large resource of data on its properties, functionality and many present and potential industrial uses. This review focuses on the structural, kinetic, thermodynamic and electrical properties of TiO2 from the viewpoint of the relationship between the crystal structure and its present or potential useful functionality, via the electronic structure. The reason for this focus is the fundamental relationship between the electronic band structure of this wide band-gap semiconductor and its interaction with light and chemical species. Intense interest in the photoactivity of TiO2 followed the demonstration by Fujishima and Honda in 1972 of its ability to dissociate water using sunlight. Approaches to band gap engineering via chemical modifications are surveyed and correlated with band-structure calculations using Density Functional Theory and Hartree Fock methods. In the last section, progress in TiO2 applications and prospects for new applications of this material are summarised.