Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: Insights from TiO2

To understand the impact of particle size on phase stability and phase tran sformation during growth of nanocrystalline aggregates we conducted experim ents using titania (TiO2) samples consisting of nanocrystalline anatase (46 .7 wt %, 5.1 nm) and brookite (53.3 wt %, 8.1 nm). Reactions were studied i sochronally at reaction times of 2 h in the temperature range 598-1023 K an d isothermally at 723, 853, and 973 K by X-ray diffraction (XRD). A numeric al deconvolution method was developed to separate overlapping XRD peaks, an d an analytical method for determining phase contents of anatase, brookite, and rutile from XRD data was established. Results show that, in contrast t o previous studies, anatase in our samples transforms to brookite and/or ru tile before brookite transforms to rutile. Thermodynamic and kinetic analys es further support this conclusion. For general titania samples, the transf ormation sequence among anatase and brookite depends on the initial particl e sizes of anatase and brookite, since particle sizes determine the thermod ynamic phase stability at ultrafine sizes. These results highlight extremel y important size-dependent behavior that may be expected in other nanocryst alline systems where multiple polymorphs are possible.