The electronic structure and optical properties of aluminium-doped rutile and anatase TiO₂ have been investigated using density functional theory with plane wave basis sets and pseudo-potentials. This was done using the periodic supercell method as implemented within the CASTEP software package with Al concentrations approaching the very low levels present in industrial samples of rutile TiO₂. Defect states involving substitution of a titanium atom for an aluminium atom were studied along with the more stable configuration of two adjacent aluminium substitutions with an oxygen vacancy in between. In the latter case aluminium does not introduce band gap states but leads to an increase in the band gap in both anatase and rutile. This suggests that aluminium doping pushes the absorption edge further into the UV and therefore reduces the photo-catalytic activity. Single oxygen vacancies in anatase were also studied. These defect states in anatase do stabilise the crystal but less so than in rutile. It is therefore likely that Al determines crystal phase during nucleation as opposed to
increasing the transformation rate as has been suggested.