Near-IR photoluminescence and structural properties of TiO$_2$ powders with nanocrystalline anatase/brookite matrix

The properties are studied for near-IR photoluminescence (PL) observed at 865–870 nm for precipitated TiO$_{2}$ powders having nanocrystalline anatase/brookite structure: the dependence of NIR PL on annealing in oxygen, on content and crystallinity quality of brookite, and on excitation energy $(E_{ex})$. NIR PL in bi-phase powders was found to demonstrate a behavior characteristic of NIR PL in brookite. NIR emission intensity grows with increasing brookite content and at annealing in oxygen, when oxygen vacancies content decreases and crystallinity quality of brookite improves. The results indicate that in the bi-phase powders, brookite contains deep-level defect traps which are regarded responsible for NIR-PL in brookite and are favorable for photocatalytic reactions. NIR emission is observed for the band $(E_{ex}\approx E_{b})$ and below band-gap green $(E_{ex}<E_{b})$ excitation. A mechanism underlying NIR-PL in brookite with green excitation is suggested to be similar to that for the red PL known for anatase at below band-gap excitation.