Unusual Crystallization Behaviors of Anatase Nanocrystallites from a Molecularly Thin Titania Nanosheet and Its Stacked Forms: Increase in Nucleation Temperature and Oriented Growth
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- 作者：Katsutoshi Fukuda ; Yasuo Ebina ; Tatsuo Shibata ; Takashi Aizawa ; Izumi Nakai ; Takayoshi Sasaki
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：January 10, 2007
- 年：2007
- 卷：129
- 期：1
- 页码：202 - 209
- 全文大小：346K
- 年卷期：v.129,no.1(January 10, 2007)
- ISSN：1520-5126
文摘
Crystallization behaviors of anatase nanocrystallites from an ultrathin two-dimensional reactantcomposed of exfoliated titania nanosheets have been studied by monitoring the heating process of theirwell-organized films, with which the film thickness can be controlled from a molecularly thin monolayer toa stacked multilayer structure with a stepwise increment of ~1 nm. The heated products were identified bymeans of total reflection fluorescence X-ray absorption near-edge structure analysis and in-plane X-raydiffraction measurements using a synchrotron radiation source. The films composed of five or more layersof stacked nanosheets were transformed into anatase at 400-500 
C, which is a normal crystallizationtemperature of anatase from bulk reactants. As the film became thinner by decreasing the number ofnanosheet layers to five or less, the crystallization temperature was found to increase and finally reached800 C for the monolayer film. Interestingly, preferential growth of anatase along the c-axis was stronglypromoted for these ultrathin films. These unusual behaviors may be understood in terms of crystallizationfrom the two-dimensional system of scarcely distributed reactants. The titania nanosheet crystallite is muchthinner than the unit cell dimensions of anatase, and therefore, extensive atomic diffusion is required forthe transformation particularly for the ultrathin films with a critical number (2-3) of stacked nanosheetlayers. There is some structural similarity between anatase and titania nanosheet, which may account forthe oriented growth of anatase nanocrystallites.
C, which is a normal crystallizationtemperature of anatase from bulk reactants. As the film became thinner by decreasing the number ofnanosheet layers to five or less, the crystallization temperature was found to increase and finally reached800 C for the monolayer film. Interestingly, preferential growth of anatase along the c-axis was stronglypromoted for these ultrathin films. These unusual behaviors may be understood in terms of crystallizationfrom the two-dimensional system of scarcely distributed reactants. The titania nanosheet crystallite is muchthinner than the unit cell dimensions of anatase, and therefore, extensive atomic diffusion is required forthe transformation particularly for the ultrathin films with a critical number (2-3) of stacked nanosheetlayers. There is some structural similarity between anatase and titania nanosheet, which may account forthe oriented growth of anatase nanocrystallites.