The crystallization of anatase and the conversion to bronze-type TiO₂ under hydrothermal conditions

When the anatase form of TiO₂ was heated at a constant rate of 6°C/min to 450°C it crystallized from hydrated amorphous TiO₂ gel at 170°C in pure water or at <150°C in NaOH solutions. The uptake of Na⁺ ions into crystallized anatase affected the reactions subsequent to this initial crystallization while only anatase crystals continued to grow with increasing temperature in pure water. Immediately after the nearly amorphous second stage at 325°C, conversion from colloidal anatase particles to square sheet-shaped bronze-type TiO₂ crystals began at 350°C and was complete at 425°C in 0.5 M NaOH. This conversion was considered to proceed via crystallographic shear rather than via dissolution and precipitation since this also happened with thermal treatment to 700°C in air.