High Pressure Phase Transformation Study of Different Sized Anatase TiO_2 Nanocrystals

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 transformation of anatase nanocrystals 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.