Understanding Polymorphic Phase Transformation Behavior during Growth of Nanocrystalline Aggregates:  Insights from TiO₂

Abstract

To understand the impact of particle size on phase stability and phase transformation during growth of nanocrystalline aggregates we conducted experiments 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 isochronally at reaction times 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 an analytical 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 rutile before 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 initial particle sizes of anatase and brookite, since particle sizes determine the thermodynamic phase stability at ultrafine sizes. These results highlight extremely important size-dependent behavior that may be expected in other nanocrystalline systems where multiple polymorphs are possible.

Potential scenarios for nanomaterial release and subsequent alteration in the environment

“…Combustion (process 6) is the process of reactions with air at an elevated temperature and generally implies oxidation of the elemental components of the ENM or even phase transformation [43]. If the former occurs, then it is a special case of oxidation (process 2) facilitated by heat.…”

Flame-Sprayed Pure and Ce-Doped TiO2 Photocatalysts

“…Particle size is one of the important factors for controlling phase stability. Zhang and Bandfiled [21] reported that if the particle sizes of the three nanocrystalline phases are equal, rutile has the most stable sizes greater than 35 nm. Gouma [22] had already confirmed this finding using in-situ high TEM analysis of the anatase to rutile transformation in nanocrystals and showed the critical particle size for the onset of rutile nucleation to be close to 30 nm.…”

Magnetic recyclable microcomposite silica-steel core with TiO2 nanocomposite shell photocatalysts for sustainable water purification

“…102 This can be attributed to the lower surface energy of anatase driving its preferential formation due to the large surface area to bulk volume ratio of the small particles typically formed during sol-gel processing. 103,104,105,106 Additional factors, such as the precursors employed and reaction conditions can also influence TiO 2 crystal growth. 107,108 The predominance of anatase TiO 2 in the sol-gel coatings employed in the present study (only a small amount of rutile polymorph found in P25(5)) is consistent with previous sol-gel TiO 2 calcination studies carried out at below 700 °C, 45 and TiO 2 -P25 nanocomposite coatings deposited onto glass and flat steel substrates.…”