TY - JOUR
T1 - In situ observations of particle size evolution during the hydrothermal crystallization of TiO 2
T2 - A time-resolved synchrotron SAXS and WAXS study
AU - Hummer, Daniel R.
AU - Heaney, Peter J.
AU - Post, Jeffrey E.
N1 - Funding Information:
We thank John Pople, Tom Hostettler, Cathie Condron, Kristina Peterson, and Paul Davis for valuable assistance with instrumentation and equipment at beamline 1–4 of SSRL. We also thank Mike Toney, Glenn Waychunas, Young-Shin Jun, and Mark Angelone for helpful conversations and advice regarding data reduction and analysis. We thank Alice Donahlkova for assistance with the TEM imaging. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences , under contract no. DE-AC02-98CH10886 . Portions of this research were carried out at the Stanford Synchrotron Radiation Light Source, a national user facility operated by Stanford University on behalf of the US Department of Energy, Office of Basic Energy Sciences. This work was made possible by National Science Foundation Grant nos. EAR04-17714 and EAR07-45374 , and by the Center for Environmental Kinetics Analysis (CEKA) , an NSF- and DOE-sponsored Environmental Molecular Science Institute (NSF CHE-0431328 ).
PY - 2012/4/1
Y1 - 2012/4/1
N2 - We have monitored the homogeneous crystallization of TiO 2 phases from aqueous TiCl 4 solutions at 100 °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 ∼10 nm within 1 h, followed by slower growth at a rate of ∼0.15 nm/h. Using a novel sample cell designed to withstand temperatures up to 150 °C and hydrothermal vapor pressure, we collected in situ SAXS 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 ∼20 nm with a mean near 7 nm, in excellent agreement with our WAXS analysis. At 0.5 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.
AB - We have monitored the homogeneous crystallization of TiO 2 phases from aqueous TiCl 4 solutions at 100 °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 ∼10 nm within 1 h, followed by slower growth at a rate of ∼0.15 nm/h. Using a novel sample cell designed to withstand temperatures up to 150 °C and hydrothermal vapor pressure, we collected in situ SAXS 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 ∼20 nm with a mean near 7 nm, in excellent agreement with our WAXS analysis. At 0.5 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.
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U2 - 10.1016/j.jcrysgro.2012.01.044
DO - 10.1016/j.jcrysgro.2012.01.044
M3 - Article
AN - SCOPUS:84858078999
SN - 0022-0248
VL - 344
SP - 51
EP - 58
JO - Journal of Crystal Growth
JF - Journal of Crystal Growth
IS - 1
ER -