Visible-Light Photocatalytic Activity of Ionic Liquid TiO2 Spheres: Effect of the Ionic Liquid's Anion Structure

The effect of the ionic liquid's (IL) anion type ([BMIM][X], where BMIM=1-butyl-3-methylimidazolium and X=[Br], [PF6], or octylsulfonate [OctSO(4)]) and IL content on the morphology, surface properties, and photoactivity of TiO2 obtained by the solvothermal method has been systematically investigated. Our results revealed that the presence of [Br] and [OctSO(4)] anions favor the formation of anatase phase, whereas the presence of [PF6] causes formation of anatase and Ti(OH)PO4 phase mixtures. The highest photocatalytic activity under visible radiation (approximately four times higher than pristine TiO2) was observed for the TiO2 sample grown in the presence of 1-butyl-3-methylimidazolium bromide for ionic liquid to TiO2 precursor molar ratio 1:2. Visible-light-induced phenol degradation was found to be realized mainly by oxygen radical anions, whereas the contributions of the other processes involving reactions with trapped electrons, holes, and hydroxyl radical, are limited in the overall reaction mechanism. Quantum chemical calculations on a model of anatase vacancy suggest that the charge transfer between the bromide anion and molecular oxygen interacting with the vacancy is a source of photoactivity induced by visible photons.