Correlation Between Anatase-to-rutile Transformation and Grain Growth in Nanocrystalline Titania Powders
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Correlation between anatase-to-rutile transformation and grain growth in nanocrystalline titania powders Xing-zhao Ding and Xiang-huai Liu Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China (Received 19 June 1997; accepted December 1997)
During the heat treatment of an anatase nanocrystalline powder at a high temperature, the two processes of anatase-to-rutile (A ! R) transformation and grain growth would occur simultaneously and affect each other. With decrease of the original anatase grain size, the A ! R transformation temperature range became extended on both sides, which may be partially attributed to the prevention effect of grain growth on this transformation. On the other hand, the grain growth process could be significantly enhanced by the A ! R transformation, which can be ascribed to the higher atomic mobility because of the bond breakage during the transformation.
I. INTRODUCTION
II. EXPERIMENTAL
In the last decade, nanostructured materials have attracted great attention all over the world for both scientific research and industrial applications based on their structural peculiarity and excellent properties. Titania is a very useful material and has received great attention in recent years for its humidity sensitive1 and gas sensitive behavior,2,3 excellent dielectric property,4 as well as having catalysis applications.5–7 Naturally, titania has three different crystallographic forms: brookite (orthorhombic, Pcab), anatase (tetragonal, I41yamd), and rutile (tetragonal, P42ymnm). Rutile is the only stable phase, whereas anatase and brookite are both metastable at all temperatures. Of these three polymorphs, rutile and anatase are of the most importance. It has been widely demonstrated that some properties of titania are very sensitive to its microstructure. For example, anatase has been found to be much more photoactive than rutile,8 and it was also found to be a superior support of V2 O5yTiO2 catalyst for the selective partial oxidation reaction relative to the rutile phase,5 while the rutile phase has proved to be much more efficient to catalyze the decomposition of H2 S gas than anatase phase in nanocrystalline titania powders with approximately the same specific surface area.6 Recently,7 it has also been found that a 70y30% anataseyrutile mixture makes the best photocatalyst for the oxidation of organics when titania was applied to the treatment of wastewater. Therefore, it is essential to investigate and control the microstructure of the titania system. In this paper, the correlation between grain growth and the anatase-to-rutile (A ! R) transformation in gel-derived nanocrystalline titania powders was investigated.
Nanocrystalline titania powders were prepared by a sol-gel method, and the sample preparation procedure has been described in detail in a previous paper.9 The microstructure and average grain size of the as-prepared titania powders are listed in Table I. The samp
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