不同尺寸锐钛矿结构TiO_2纳米晶的高压相变研究

High Pressure Phase Transformation Study of Different Sized Anatase TiO_2 Nanocrystals

本文利用溶胶-凝胶法合成出5.6 nm、8.6 nm和19.4 nm三种尺寸的锐钛矿结构TiO2纳米晶,并利用金刚石对顶砧压机结合Raman光谱技术研究了尺寸效应对其在23 GPa高压下结构相变的影响。结果发现,对于5.6 nm和8.6 nm的样品,加压到23.2 GPa和22.5 GPa的压力时没有发生结构相变,而是转变为无定型结构,卸除压力后,依然保持为无定型;而19.4 nm的样品在约11.3 GPa时逐渐转变为单斜斜锆石结构,在卸压后转变为α-PbO2结构。相对于块材,尺寸越小,相变压力越大。基于吉布斯自由能理论,尺寸减小引起的表面能增大被认为是相变压力变大的主要原因。

In this paper, the anatase TiO2 nanocrystals with average diameters of 5.6 nm, 8.6 nm and 19.4 nm are synthesized by the sol--gel method. A Raman spectroscopic investigation was carried out to study the size effect on phase trans/ormation of anatase nanoerystals up to about 23 GPa using a diamond anvil cell. With the increase of pressure, the 5.6 nm and 8.6 nm nano--anatase phase remains stable to pressures as high as 23.2 GPa and 22.5 GPa, respectively, and then transformed to an amorphous structure. The new amorphous structure is unquenchable upon release of pressure to ambient pressure. On the other hand, the 19.4 nm nanocrystal undergoes a phase transformation from anatase phase to the monoclinic baddeleyite structure at an onset pressure of ~ 11.3 GPa. When decompressed to ambient pressure, this sample transforms to another phase--α-- PbO2 sructure. The metastability of anatase nanocrystals as a function of pressure is demonstrated to be size dependent, the smaller sample with the higher stability. The increased surface energy induced by decrease of size., is proposed as the main reason for the enhancement of phase transformation pressure based on the Gibbs free energy theory.