Unusual crystallization behaviors of anatase nanocrystallites from a molecularly thin titania nanosheet and its stacked forms: Increase in nucleation temperature and oriented growth
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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DOI：
10.1021/ja0668116
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发表时间：
2007-01-10
影响因子：
15
通讯作者：
Sasaki, Takayoshi
中科院分区：
文献类型：
--
作者：
Fukuda, Katsutoshi;Ebina, Yasuo;Sasaki, Takayoshi
通过对有序薄膜加热过程的监测，研究了纳米TiO 2在二维反应体系中的晶化行为，薄膜厚度可以从分子薄的单层薄膜控制到近似为1 nm的层叠多层结构.通过全反射荧光、X射线吸收、近边结构分析和使用同步辐射源的面内X射线衍射测量来鉴定加热产物。由五层或更多层堆叠的纳米片组成的膜在400-500摄氏度下转化为纳米晶，这是纳米晶从本体反应物的正常结晶温度。随着通过将纳米片层的数量减少到五层或更少而使膜变得更薄，发现结晶温度增加，并且对于单层膜最终达到800 ° C。有趣的是，优先生长的碳沿着的c轴强烈促进这些碳膜。这些不寻常的行为可以理解的结晶从二维系统的几乎没有分布的反应物。二氧化钛纳米片微晶比TiO 2的晶胞尺寸薄得多，因此，特别是对于具有临界数量（2-3）的堆叠纳米片层的TiO 2膜，需要广泛的原子扩散进行转化。TiO_2纳米片和TiO_2纳米片在结构上有一定的相似性，这可能是TiO_2纳米片取向生长的原因。
Crystallization behaviors of anatase nanocrystallites from an ultrathin two-dimensional reactant composed of exfoliated titania nanosheets have been studied by monitoring the heating process of their well-organized films, with which the film thickness can be controlled from a molecularly thin monolayer to a stacked multilayer structure with a stepwise increment of similar to 1 nm. The heated products were identified by means of total reflection fluorescence X-ray absorption near-edge structure analysis and in-plane X-ray diffraction measurements using a synchrotron radiation source. The films composed of five or more layers of stacked nanosheets were transformed into anatase at 400-500 degrees C, which is a normal crystallization temperature of anatase from bulk reactants. As the film became thinner by decreasing the number of nanosheet layers to five or less, the crystallization temperature was found to increase and finally reached 800 degrees C for the monolayer film. Interestingly, preferential growth of anatase along the c-axis was strongly promoted for these ultrathin films. These unusual behaviors may be understood in terms of crystallization from the two-dimensional system of scarcely distributed reactants. The titania nanosheet crystallite is much thinner than the unit cell dimensions of anatase, and therefore, extensive atomic diffusion is required for the transformation particularly for the ultrathin films with a critical number (2-3) of stacked nanosheet layers. There is some structural similarity between anatase and titania nanosheet, which may account for the oriented growth of anatase nanocrystallites.