Understanding of Homogeneous Spontaneous Precipitation for Monodispersed TiO2 Ultrafine Powders with Rutile Phase around Room Temperature

Monodispersed TiO2 ultrafine particles were obtained from aqueous TiOCl2 solution with a 0.67 M Ti4+ concentration prepared by diluting TiCl4 with the homogeneous precipitation process in the range 17–230°C. With the spontaneous hydrolysis of TiOCl2, which means the natural decrease of the pH value in the aqueous solution, all monodispersed precipitates were crystallized with the anatase or rutile TiO2 phase during the reactions. The TiO2 precipitate with the pure rutile phase was fully formed at temperatures below 65°C, which did not involve the evaporation of H2O, and above 155°C, which were available by suppressing it. The TiO2 precipitate with the rutile phase, including a small amount of the anatase phase, started to be formed at intermediate temperatures above 70°C, showing the full formation of the anatase phase above 95°C under the free evaporation of H2O. However, in the case of completely suppressing H2O evaporation at temperatures above 70°C, the TiO2 precipitate with the anatase phase that had already been formed by rapid reaction was fully transformed with the reaction time into the precipitate with the rutile phase by the vapor pressure of H2O. Therefore, the formation of TiO2 precipitates with the rutile phase around room temperature would be caused by the existence of capillary pressure between the agglomerated needle-shaped particles or the ultrafine clusters, together with the slow reaction rate.