Heat-stimulated crystallization and phase transformation of titania nanoparticles

• Amorphous state of titania is stabilized by water molecules in its structure. • After water removal amorphous titania transforms to anatase of the same particle size. • Anatase-to-rutile transition occurs when the crystallite size becomes above 35–45 nm. • Crystallite size distribution explains gap in anatase and rutile crystallite size. To better understand the fundamental aspects of nanoparticle formation and transformation in the TiO 2 -H 2 O system the amorphous titania was synthesized as a precursor to studying crystal genesis, evolution, and transformation under heat treatment in air. The necessary depth of study was provided by comprehensive analysis using methods of PXRD, HT-PXRD, DSC-TG, TEM, BET adsorption, and helium pycnometry. The smallest crystallite size of the anatase phase is shown to be defined by the size of the initial amorphous titania nanoparticles. The amorphous state of initial titania nanoparticles is stabilized by water molecules incorporated into their structure. The anatase-to-rutile phase transition occurs when the average crystallite size of the anatase reaches the value of 35–45 nm. An explanation for a drastic change of the average crystallite size during the anatase-to-rutile transition repeatedly found in researches is in the difference in shapes of curves of crystallite size distribution.