Microstructure and crystallization behaviour of TiO2 precursor prepared by the sol-gel method using metal alkoxide

TiO₂ precursors were prepared by the sol-gel method using titanium tetraisopropoxide. The effects of water and hydrochloric acid addition for the hydrolysis and polymerization reaction on the crystal lattice or the molecular structure, and on the crystallization behaviour of prepared precursors, were examined by using Raman, and infrared spectroscopy, X-ray diffraction, DTA-TG and carbon analysis. The quantity of unhydrolysed alkyls in precursors decreased with increasing amounts of H₂O and HCl addition, but the alkyls always remained. Raman spectroscopy revealed that the molecular structure of prepared amorphous precursors resembled that of the anatase phase with increased H₂O and HCl addition. On the crystallization process of these precursors, the ratio of anatase formed at 310 °C was influenced by residual unhydrolysed alkyls. Steric hindrance by the residual alkyls preventing crystallization to crystalline anatase was observed, and there was estimated to be 2.5 mol of octahedral coordination of Ti-O per 1 mol of residual alkyls. It is noted that crystallization of the anatase phase and the transformation from anatase to the rutile phase were also observed on long-term ageing of precursors at room temperature.