Polaron-Induced Deep Defect Levels in Brookite TiO2: A Many-Body Green's Function Theory Study

Brookite is now recognized as an active phase of TiO2 which exhibits superior activities compared with anatase and rutile in some photocatalytic reactions. However, there is still little research and knowledge on its electron properties as well as the crucial role of defects in brookite. Using the ab initio many-body Green's function theory, we examined the quasiparticle structures of defects, including oxygen vacancies, Ti interstitials, and hydroxyl groups, in the bulk and the (210) surface of brookite. We discovered that small polarons may generate a deep defect band and a shallow one which are approximately 0.7 and 0.3 eV below the conduction band minimum (CBM). In brookite bulk, oxygen vacancy can only create a deep defect band which is 1.1 eV below CBM and induced by the sigma bonds formed between Ti 3d orbitals. These features are quite distinct from those in anatase and rutile. We also found that introducing hydroxyl groups into brookite bulk would make the band gap narrow by at least 0.4 eV, which may help to enhance its visible light absorption. The calculated band gap and defect levels of reduced brookite are in excellent agreement with the experiments.