Authors: Raphael Shirley, Markus Kraft*, and Oliver R. Inderwildi

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 pseudopotentials. 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 photocatalytic activity. Single oxygen vacancies in anatase were also studied. Reactions to form the most stable defects are exothermic for both phases. Finally, migration of aluminium in both phases is investigated. Migration transition states are found to have a significantly lower energy in rutile. At industrially relevant temperatures, overcoming the barrier to migration is probably only possible in rutile.

• This paper draws from preprint 71: Electronic and optical properties of aluminium-doped anatase and rutile TiO₂ from ab initio calculations

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