Origin of the photoactivity in boron-doped anatase and rutile TiO2 calculated from first principles

The electronic and optical properties of several possible B-doped models in both anatase and rutile polymorphs of TiO2 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 TiO2 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 TiO2 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 TiO2.