Photo-induced monomer/dimer kinetics in methylene blue degradation over doped and phase controlled nano-TiO2 films

Abstract

Transparent TiO₂ thin films containing 1 to 6 wt% of Ni, Fe and Nb doping were synthesized via a hybrid non-aqua sol–gel dip-coating technique. Metal-doping is found to affect the phase formation, polymorphic transition, visible light absorbance and optical transparency of the TiO₂ films. Physico-chemical characterization revealed that, in the given doping range, addition of Ni and Fe enhances the visible light absorbance, while Nb doping does not alter the electronic structure of TiO₂. Higher doping concentrations (4 and 6 wt% of Ni and Fe, respectively) lead to the crystallization of a MTiO₃ (M: Ni or Fe) phase along with anatase and rutile TiO₂. On the other hand, higher Nb doping favors the formation of pure anatase TiO₂. 6 wt% of Ni and Fe doping induces the reduction in band gap (3.1 and 2.9 eV), whereas Nb does not alter the TiO₂ band gap (3.3 eV). In contrast to the low efficiency of undoped TiO₂ films, Ni-doped films yields 71% (for monomer) and 55% (for dimer) photocatalytic degradation efficiency towards methylene blue (MB) under visible light irradiation (λ ∼ 420 nm). This demonstrates the selective and preferential photo-dynamics of the MB monomer during the photocatalytic process. The doped films show a significant Incident photon to current conversion efficiency for oxidation of water under wavelength λ ≥ 400 nm. Such superior improvement in the visible light activity of Ni doped TiO₂ is attributed to the presence of nanosized NiTiO₃ in the nanostructure of the TiO₂ film.