Origin of the different photoactivity of N-doped anatase and rutile TiO₂

We have investigated the origin of the experimentally observed change in photoactivity of anatase and rutile TiO₂ induced by substitutional N-doping using state-of-the-art density functional theory calculations. Our results show that in both polymorphs N 2p localized states just above the top of the O 2p valence are present. In anatase these states cause a redshift of the absorption band edge towards the visible region. In rutile, instead, this effect is offset by the concomitant N-induced contraction of the O 2p band, resulting in an overall increase of the optical transition energy. Experimental trends are well described by these results.