| Abstract
| - To understand the impact of particle size on phase stability and phase transformation during growth ofnanocrystalline aggregates we conducted experiments using titania (TiO2) samples consisting of nanocrystallineanatase (46.7 wt %, 5.1 nm) and brookite (53.3 wt %, 8.1 nm). Reactions were studied isochronally at reactiontimes of 2 h in the temperature range 598−1023 K and isothermally at 723, 853, and 973 K by X-ray diffraction(XRD). A numerical deconvolution method was developed to separate overlapping XRD peaks, and ananalytical method for determining phase contents of anatase, brookite, and rutile from XRD data was established.Results show that, in contrast to previous studies, anatase in our samples transforms to brookite and/or rutilebefore brookite transforms to rutile. Thermodynamic and kinetic analyses further support this conclusion.For general titania samples, the transformation sequence among anatase and brookite depends on the initialparticle sizes of anatase and brookite, since particle sizes determine the thermodynamic phase stability atultrafine sizes. These results highlight extremely important size-dependent behavior that may be expected inother nanocrystalline systems where multiple polymorphs are possible.
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