Nitrogen-doped TiO$_2$ for visible light photocatalysis

Using first-principles calculations we investigate the effects of nitrogen doping on the photocatalytic properties of rutile TiO$_2$. We find that N can be incorporated into the bulk with appreciable solubility in the form of a substitutional or N$_2$ split-interstitial defect. Substitutional N gives rise to a deep-acceptor state, while the split-interstitial behaves as a donor. We confirm that substitutional N on the O site is found to effectively lower the absorption of light to the visible-range by approximately 0.6 eV with respect to the bandgap of 3.1 eV, in good agreement with recent experiments. Our results indicate that the N$_2$ split-interstitial defects do not to contribute to the observed onset of visible-light absorption yet can potentially play a role in the pinning of the Fermi Level. We conclude that N is a good dopant choice for TiO$_2$, increasing the versatility of TiO$_2$ for use as a photocatalyst for sub-UV illumination. This work was supported by the NSF MRSEC Program under Grant No. DMR05-20415.