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In situ observations of particle size evolution during the hydrothermal crystallization of TiO2 A time-resolved synchrotron SAXS and WAXS study

In situ observations of particle size evolution during the hydrothermal crystallization of TiO2 A time-resolved synchrotron SAXS and WAXS study

Journal of Crystal Growth 344(1): 0-58

We have monitored the homogeneous crystallization of TiO2phases from aqueous TiCl4solutions at 1 C using both time-resolved small-angle X-ray scattering (TR-SAXS) and time-resolved wide-angle X-ray scattering (TR-WAXS). The WAXS data revealed that metastable anatase was the first phase to crystallize, but it began transforming to rutile within the first hour of reaction. Scherrer analysis of WAXS peak widths indicated that anatase particles grew to an average size near 7 nm on a time scale of minutes, after which their size remained static, while rutile grew rapidly from <3 nm to ~1 nm within one hour, followed by slower growth at a rate of ~.15 nm/hr.Using a novel sample cell designed to withstand temperatures up to 15 C and hydrothermal vapor pressure, we collectedin situSAXS data for the same conditions as the WAXS experiments. Fourier analyses of the SAXS images revealed the rapid emergence of a broad, Gaussian-type particle size distribution ranging in diameter from 1 to ~2 nm with a mean near 7 nm, in excellent agreement with our WAXS analysis. At .5 h, a second population of <3 nm particles emerged, and these crystals grew to ~9 nm after 7.5 h, which we interpret as the nucleation and growth of rutile (as was also observed in the WAXS patterns). In contrast to studies that presume a solid-state transformation from anatase to rutile during growth, our results suggest that anatase and rutile co-precipitate during the early stages of crystallization, but once anatase crystals grow to a critical size of 7 nm, they dissolve and the solution species re-precipitate onto primary rutile nanocrystals.We observed thein situcrystallization of TiO2using time-resolved SAXS and WAXS. Anatase is the first phase to crystallize, but growth stops at a mean size of 7 nm. Anatase and rutile co-precipitate during the initial stage of crystallization. Anatase transforms to rutile through a dissolution/re-precipitation mechanism..

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Accession: 036325787

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DOI: 10.1016/j.jcrysgro.2012.01.044

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