Grain size dependence of anatase-to-rutile structural transformation in gel-derived nanocrystalline titania powders

Nanocrystalline materials, which are composed of ultrafine crystals with dimensions ranging from several to tens of nanometres, are currently receiving much attention by virtue of their special chemical, physical and mechanical properties, which are suited for various applications. For the past two decades, the sol-gel process has become one of the successful techniques for preparing nanocrystalline metallic oxide materials. Among these oxides, titania is a very useful material for its gas sensitivity [1, 2] and excellent dielectric properties [3], as well as having catalysis applications [4-7]. Titania has three naturally occurring polymorphs: brookite (orthorhombic, Pcab), anatase (tetragonal, I41/amd), and rutile (tetragonal, P42/mnm). Rutile is the only stable phase, whereas anatase and brookite are both metastable at all temperatures. It has been widely demonstrated that some properties of titania are very sensitive to its microstructure. For example, the anatase phase is fotmd to be a superior support of V205/TiO2 catalyst for the selective i~artial oxidation reaction relative to the rutile phase [5], while rutile phase has proved to be rauch efficient to catalyse the decomposition of H2S gas than anatase phase in nanocrystalline titania powders with approximately the same specific surface area [6]. Recently [7], it has also been found that a 70%/30% anatase/rutile mix makes the best photocatalyst for the oxidation of organics when titania was applied to the treatment of wastewater. Therefore, it is of importance to investigate, in detail, the structural transformation of the titania system. In our previous papers, we have found that the transformation behaviour of anatase-to-rutile (A ~ R) in gel-derived nanocrystalline titania powders can be affected by many factors, such as impurities [8, 9], preparation conditions [10, 11] and so on. In this letter, the grain size dependence of the A--+ R transformation is reported. Nanocrystalline titania powders were prepared by a sol-gel method [8]. Tetrabutyl titanate was used as the precursor, ethanol as the solvent and hydrochloric acid as the hydrolysis catalyst. The molar ratio of these reactants was: Ti(Osu)4:EtOH:HC1 :H20 = 1:15:0.3:1. The experimental procedure has been described in detail previously. The as-prepared dry gel was confirmed to be of amorphous structure, and then crystallized into an anatase structure after annealing at 523K for 2h. The A + R phase