Sodium doping in brookite TiO₂ enhances its photocatalytic activity

We report in this work that sodium doping of brookite TiO₂ effectively enhances its photocatalytic activity, which becomes three times higher than that of the quasi-spherical brookite TiO₂. The results demonstrated that the sodium-doped brookite Na Ti_1- O₂ can be stable up to 500 °C. At 600°C, the sodium in the brookite precipitates in the form of Na₂CO₃, and above 700 °C, the brookite Na Ti_1- O₂ transforms into Na₂Ti₆O₁₃ by a twinning process with the orientation relationship of [1-2-3]_Matrix//[1-23]_Twins and (-2-10)_Matrix//(1-1-1)_Twins. The differences in the ionic radius and the electronegativity between Na and Ti destroy the local atomic arrangement of the brookite structure and produce microstructures such as the core-shell structure, local lattice distortion, interstitial atoms, and atomic vacancies, which are critical to its excellent photocatalytic activity.