The hydrothermal analogy role of ionic liquid in transforming amorphous TiO2 to anatase TiO2: elucidating effects of ionic liquids and heating method

In this study, the specific coexistence of water and ionic liquid being the lower temperature thermal annealing condition for anatase crystallization of amorphous titania at ambient pressure was found. The test ionic liquids were 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium tetrafluoroborate. After deep investigation, we found that there existed an analogy between our lower temperature thermal annealing treatment system (LTTAT) and hydrothermal treatment system. In LTTAT system, the ionic liquid played an important role in driving surface crystallization of amorphous TiO₂ to the anatase phase by retaining a suitable amount of water through a dissolution–crystallization mechanism. We could observe higher hydroxyl group ratio of hydroxylated titanium compound from X-ray photoelectron spectroscopy (XPS) data during initial thermal annealing period. The self-assembly ability of ionic liquid then lead to kinetical dehydration and crystallization of hydroxylated titanium compound around it during the following annealing process. Based upon this proposition, the effects of different types of ionic liquid and its amount, temperature effect, and heating method on anatase crystallinity, characterized by X-ray diffraction (XRD), were investigated. It was found that different temperatures and microwave heating effect were observed for different types of ionic liquid. From these observations, it was pointed out that we could get better anatase crystallinity and good photodegradation performance by using the system containing ionic liquid having higher water-adsorbing ability and microwave heating annealing.