Sodium doping in brookite TiO 2 enhances its photocatalytic activity.

We report in this work that sodium doping of brookite TiO 2 effectively enhances its photocatalytic activity, which becomes three times higher than that of the quasi-spherical brookite TiO 2 . The results demonstrated that the sodium-doped brookite Na x Ti 1- x O 2 can be stable up to 500 °C. At 600°C, the sodium in the brookite precipitates in the form of Na 2 CO 3 , and above 700 °C, the brookite Na x Ti 1- x O 2 transforms into Na 2 Ti 6 O 13 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.