Liu J F, Yin S, Wu H P, Zeng Y W, Hu X R, Wang Y W, Lv G L, Jiang J Z
Laboratory of New-Structured Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China.
J Phys Chem B. 2006 Nov 2;110(43):21588-92. doi: 10.1021/jp0648238.
Hexagonal CoO nanocrystals are coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation and stability property. Structural stability and phase transformation of the hexagonal CoO phase have been investigated by X-ray powder diffraction with Rietveld refinement, transmission electron microscopy, X-ray absorption fine structure, and differential scanning calorimeter. It is found that the hexagonal CoO phase is a metastable phase, which increases its grain size from 50 to 250 nm for refluxing times from 1 to 6 h at 200 degrees C. After 12 h, cubic-structured CoO grains with an average grain size of 20 nm are observed, which spread around big hexagonal CoO grains. After about 24 h, only the cubic CoO phase with an average grain size of 25 nm is detected. The onset temperature of hexagonal-to-cubic phase transformation in CoO is estimated to be 378 degrees C by DSC, using a heating rate of 20 deg/min. The results obtained indicate that the hexagonal-to-cubic phase transformation in nanocrystalline CoO is by nucleation and growth mechanism, starting from the surface to the center of the hexagonal grains.
在水热条件下对六方CoO纳米晶体进行粗化处理，以研究粒径对相变和稳定性的影响。通过采用Rietveld精修的X射线粉末衍射、透射电子显微镜、X射线吸收精细结构和差示扫描量热仪，对六方CoO相的结构稳定性和相变进行了研究。结果发现，六方CoO相是一种亚稳相，在200℃下回流1至6小时，其晶粒尺寸从50纳米增加到250纳米。12小时后，观察到平均粒径为20纳米的立方结构CoO晶粒，它们分布在大的六方CoO晶粒周围。约24小时后，仅检测到平均粒径为25纳米的立方CoO相。使用20℃/分钟的升温速率，通过差示扫描量热法估计CoO中六方相向立方相转变的起始温度为378℃。所得结果表明，纳米晶CoO中的六方相向立方相转变是通过成核和生长机制进行的，从六方晶粒的表面向中心开始。
