Dependence between Ionic Liquid Structure and Mechanism of Visible-Light-Induced Activity of TiO₂ Obtained by Ionic-Liquid-Assisted Solvothermal Synthesis

Because of the tremendous structural diversity of ionic liquids (ILs), simple transfer of observations performed for one IL used for IL-TiO₂ preparation on different samples is not possible. Therefore, four ionic liquids, all containing distinct nitrogen-bearing organic cations (pyridinium, pyrrolidinium, ammonium, imidazolium), were used for the first time for the preparation of IL-TiO₂ composites. The role of the individual IL cation in the synthesis of TiO₂ microspheres, as well as the effect of the IL structure on the mechanism of the visible-light (Vis)-induced photoactivity of IL-TiO₂ was presented and discussed in regard to structure, morphology, absorption properties, elemental composition, and reactive species involved in the photocatalytic reaction of phenol degradation. The successful modification of the TiO₂ with organic IL species including possible interactions between IL and TiO₂ surface, as well as the TiO₂ matrix (doping with N), were confirmed. The sample that exhibited the highest photoactivity under Vis irradiation (58%) was TiO₂ prepared in a presence of 1-butylpyridinium chloride with a IL:precursor molar ratio of 1:3. For this sample, the highest partial decomposition of cationic species of IL was observed resulting in interaction of N atoms with deeper sites of TiO₂ (Ti-N_<i>x</i>) as well as the highest surface defects in a form of Ti³⁺. The superoxide radical species O₂^• – were found to be main active species responsible for high efficiency of degradation under Vis irradiation