The role of surface O-vacancies in the photocatalytic oxidation of Methylene Blue by Zn-doped TiO2: A Mechanistic approach

The nano-sized Ti1-xZnxO2 system (0.01 <= x <= 1.0) was synthesized by the sol-gel route. A combination of techniques such as X-ray diffraction, electron microscopy, vibrational spectroscopy, diffuse reflectance, and X-ray photoelectron spectroscopy (XPS) were used to establish that Zn gets substitutionally doped in the Ti sites of a single phase, anatase structure and hence forth possess O-vacancies in order to maintain the lattice charge neutrality. The photocatalytic efficiency of these photocatalysts were studied by the oxidation of Methylene Blue (MB) dye; the presence of just 1% Zn as dopant increases the reaction rate constant by almost 76%. Under optimal conditions, the MB dye was found to be completely mineralized in 30 min. XPS data indicates a strong correlation between the high photo-oxidation rate of MB with the concentration of surface O-vacancies (whose presence is caused by aleovalent doping). The O-vacancies may be expected to influence the photocatalytic activity by directly altering the surface charge and the oxidation potential of the valence band as observed from the computational studies. Oxygen-vacancies in the surface and in the bulk could be separately identified by electron paramagnetic resonance. The probable intermediates involved in the mineralization of the MB-dye were extensively investigated over all the photocatalysts using ex situ mass spectroscopic studies. A correlation with the mechanistic understanding with that of the structural parameters is attempted. The stable aromatic and aliphatic intermediates produced during the complete mineralization process were identified and were correlated with the surface basicity which in its own term is altered by the presence of O-vacancies on the surface. (C) 2017 Elsevier B.V. All rights reserved.