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.