Origin of the photoactivity in boron-doped anatase and rutileTiO2calculated from first principles

The electronic and optical properties of several possible B-doped models in both anatase and rutile polymorphs of $\mathrm{Ti}{\mathrm{O}}_{2}$ have been investigated systematically using spin-polarized density functional theory calculations. Our calculated results indicate that the experimentally observed reverse shift of the absorption edge in the B-doped $\mathrm{Ti}{\mathrm{O}}_{2}$ originates from the different chemical environments of B ion. The transition of excited electrons from the valence band to the empty gap states above the Fermi level may be responsible for the redshift of the absorption edge in substitutional B- to O-doped anatase, and the redshift of absorption edge may also be expected in substitutional B- to Ti-doped anatase $\mathrm{Ti}{\mathrm{O}}_{2}$ due to the reduction of electron transition energy, resulting from the decline of conduction band. On contrary, the electron transition energy has a little increase in interstitial B-doped anatase due to the well-known ``band-filling mechanism,'' thus resulting in the blueshift of absorption spectra. Similar doping effects also appear in B-doped rutile $\mathrm{Ti}{\mathrm{O}}_{2}$.