Origin of the Visible Light Absorption of Boron/Nitrogen Co-doped Anatase TiO2

Very recently boron/nitrogen codoped titanium dioxide (B/N@TiO2) has been synthesized and the crystals are red in color with excellent absorption over the full visible spectrum. In this report, the geometries and electronic states of B/N@TiO2 have been investigated using density functional theory plus Hubbard U contributions. It is found that (i) the net charges introduced by the dopants can be fully compensated when the B/N ratio is 1/3; (ii) when B-doping and B/N-codoping coexist, the electronic states associated with unbalanced B-dopants can form impurity bands; and (iii) the electron excitation from the impurity bands to conduction bands can lead to the optical absorption at similar to 2.0 eV, which has been proposed as the origin of the excellent absorption of visible light offered by red TiO2.