Origin of the different photoactivity ofN-doped anatase and rutileTiO2

We have investigated the origin of the experimentally observed change in photoactivity of anatase and rutile ${\mathrm{TiO}}_{2}$ induced by substitutional $\mathrm{N}$-doping using state-of-the-art density functional theory calculations. Our results show that in both polymorphs $\mathrm{N}\phantom{\rule{0.3em}{0ex}}2p$ localized states just above the top of the $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ 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 $\mathrm{N}$-induced contraction of the $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ band, resulting in an overall increase of the optical transition energy. Experimental trends are well described by these results.