Doping of Polymorphs for Altered Optical and Photocatalytic Properties
Figure 3
Schematic representation of the electronic structure. (a) Far left: TiO2, (b) left center: with oxygen vacancy, (c) right center: with carbon doping, (d) far right: with B and F codoping. The valance band is principally oxygen in character. The conduction band is principally Ti & in character. In the atomic limit, each TiO2 unit contributes two filled oxygen orbitals, four half-filled oxygen orbitals, and four titanium valance electrons. In the pure material, (far left) the four titanium valance electrons fill the vacant oxygen orbitals. When an oxygen atom is removed, (center left) three oxygen orbitals are removed, but only 4 electrons. The remaining two electrons are forced to reside in Ti orbitals. Upon substitution of C for O, (center right) the carbon bands βfloat upβ into the band gap due to weaker nuclear-electron interaction. Onsets in the photoabsorption spectrum are seen corresponding to transitions from both occupied carbon bands to the conduction band. B-F codoping (far right) fills the half-full F- orbital with a large energy benefit.
