Thermally Stable Nanocrystalline TiO2 Photocatalysts Synthesized via Sol−Gel Methods Modified with Ionic Liquid and Surfactant Molecules

Abstract

Recently, sol−gel methods employing ionic liquids (ILs) have shown significant implications for the synthesis of well-defined nanostructured inorganic materials. Herein, we synthesized nanocrystalline TiO2 particles via an alkoxide sol−gel method employing a water-immiscible room temperature IL (1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]) as a new solvent medium and further modified with nonionic surfactant (polyoxyethylenesorbitan monooleate) as a pore templating material. Detailed information on the preparative method, synthesis route and mechanism, crystallographic and structural properties, and photocatalytic activity of the TiO2 particles is described. The possible rationale for the formation of nanocrystalline TiO2 particles with high surface area and activity is discussed with respect to the special characteristics of [bmim][PF6] as well as the role of the surfactant self-assembly in the sol−gel network. Due to its capping effect and water immiscibility, the use of [bmim][PF6] in sol−gel synthesis of TiO2 induces controlled hydrolysis of titanium alkoxide precursor, resulting in a stable sol−gel network with an ordered array, and localized water-poor conditions, resulting in the formation of completely condensed and directly crystalline systems at ambient condition. The low surface energy and adaptability of [bmim][PF6] facilitate the generation of very small nanocrystalline TiO2 particles and then it also acts as a particles aggregation inhibitor. The ensuing TiO2 particles have good thermal stability to resist pore collapse and anatase-to-rutile crystal phase transformation during thermal treatment.