Fig. 6. The absorbance max at around 420 nm observed in undoped TiO2 sample may be due the interference oscillation.
The max at 420 nm was shifted up to 450 nm in Ni-doped TiO2.
While in case of Fe doped sample the peak shifted to 500 nm.
There is no significant peak shift as was detected in the visible light region for Nb-doped sample compared to Fe or Ni. Along with the visible absorbance a clear band edge shifts toward lower energy side (from 350 to 365 nm) was observed for Fe doped TiO2.
Where as there is no considerable peak shift in Ni or Nb doped TiO2.
The estimated energy band gap using Tauch relation based on the absorbance for un-doped, Ni, Fe and Nb doped are 3.2, 3.2, 3.1 and 3.3 respectively.