In the case of Fe-doped sample, the peak shifts to 450 nm
and consequently, shows inferior visible light activity compared
to the Ni-doped TiO2. Whereas in the Nb-doped films, there is no
detectable peak shift in the visible light region. Such peak shift may
be due to the formation of new localized electronic states (surface
states) close to the valance band edge. The reduction in absorbance
may be influenced by the grain size effect as light instead of passing
through the film to the detector gets scattered at an angle which
is a function of the grain size.