Study on defect properties of nanocrystalline TiO2 during phase transition by positron annihilation lifetime

The defect properties of nanocrystalline TiO2 were investigated by positron annihilation lifetime spectroscopy (PALS) and X-ray diffraction (XRD) as a function of annealed temperature that ranged from 300 to 850 °C. Below 500 °C, the measured positron lifetimes of τ1 (200–206 ps) and τ2 (378–402 ps) revealed the existence of mono-vacancy and vacancy-clusters at grain surface and in the micro-void of intergranular region. Between 500 and 750 °C, the phase transition from anatase to rutile was probed by the variations of positron lifetime and XRD pattern. With the increasing temperature from 500 to 850 °C, the positron lifetime τ1, τ2 and its intensity I2 sharply decreased from 200 ps, 378 ps, and 60% to 135 ps, 274 ps, and 33%, respectively. The results clearly indicate that the mono-vacancy or vacancy-clusters at grain surface and micro-voids between the grains were annealed out during the phase transition.

► The defect properties of TiO2 with varying annealed temperatures were studied by PALS.
► The results have clearly indicated the places positrons annihilate in.
► Small size defects at grain surface were annealed out during phase transition.
► Even at high temperature, micro-void still exists in intergranular region of rutile.