The visible light response in the
doped materials arises from the
electron transition from the localized N orbitals to the conduction band or to the surface
adsorbed O2.
Doping TiO2 with
carbon or sulfur also is found to
enhance visible light activated
photocatalytic activity. The
visible light response in these
doped materials is thought to
arise from the presence of localized energy levels of the dopant
lying above the valence band or
oxygen vacancies.