The un-doped TiO2 spectrum shows an absorption onset at 390 nm, which is in good agreement
with the intrinsic energy band gap of titania at 3.2 eV. The absorption spectra for all doped catalysts are shifted to larger wavelength (red shift). The higher absorbance in the range of 400–600 nm (visible light region) is observed for (0.5 wt.% Fe)–TiO2 catalyst, which confirms its superior photocatalytic activity on NB degradation in aqueous solution under UV–VIS irradiation.