In order to understand the origin of the visible light degradation
of Ni doped TiO2, the optical energy band gap, as estimated
from UV visible absorbance spectrum were considered. The energy
band value for 4 wt.% Ni-doped TiO2 (3.2 eV) was found to be higher
than that of Fe-doped TiO2 (3.0 eV).