Formation mechanism of fine anatase crystals from amorphous titania under hydrothermal conditions
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Formation mechanism of fine anatase crystals from amorphous titania under hydrothermal conditions Kazumichi Yanagisawa, Yuichi Yamamoto, Qi Feng, and Nakamichi Yamasaki Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Kochi 780, Japan (Received 25 August 1997; accepted 5 January 1998)
Crystallization of amorphous titania prepared by hydrolysis of ethoxide was accelerated even by a small amount of water in the vapor phase. The existence of water promoted the change of localized structure of the amorphous titania to anatase structure, which resulted in acceleration of anatase nucleation. The anatase crystals grew in steam by solid-state epitaxial growth, but stopped growing in a short time. The growth of anatase crystals under hydrothermal conditions could be divided into the following two stages: the first stage with fast growth rate by the solid-state epitaxial growth and the second stage with slow growth rate by the dissolution and deposition process.
I. INTRODUCTION
Most researchers have not paid attention to the role of water on preparing materials in air at high temperatures over 100 ±C. As concerns anatase, it is formed by heating amorphous hydrous titania in air at temperatures above 300 ±C, followed by dehydration.1–4 It is surprising that anatase is crystallized under hydrothermal conditions4–8 and even by refluxing6 where water exists to prevent dehydration, at much lower temperatures in comparison with heating in air. When the amorphous titania powder derived from ethoxide was treated in a special autoclave for hydrothermal hotpressing, the amount of water in the autoclave greatly affected the size of the anatase crystals that crystallized from the amorphous titania by the treatment.9 Water must have some effect on the crystallization. We report here the role of water to accelerate the crystallization of the amorphous titania prepared from ethoxide and its crystallization mechanism. II. EXPERIMENTAL
The starting material was prepared by hydrolysis of titanium tetraethoxide using the procedure developed by Barringer and Bowen;10 0.2 M titanium tetraethoxide/ethanol solution was poured with agitation into a 0.6 M water/ethanol solution with equivalent volume to prepare amorphous hydrous titania. The resultant precipitate was washed with ethanol and dried in vacuum at room temperature. The as-prepared powder was amorphous and consisted of spherical particles with average diameter of 0.7 mm. The surface area of the powder was about 300 m2yg, which suggested that the amorphous titania contained fine primary particles. When the as-prepared powder was placed in moisture, organic materials that remained in it were gradually removed. Its amorphous structure gradually changed J. Mater. Res., Vol. 13, No. 4, Apr 1998
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to anatase structure while keeping it in moisture. It crystallized to anatase even at 30 ±C by keeping it in moisture with relative humidity of 75% for 150 days,
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