Deep in-gap states induced by double-oxygen-vacancy clusters in hydrogenated TiO[formula omitted

Abstract

To reveal the origin of the deep in-gap states in hydrogenated TiO2 (H:TiO2), which are located ∼1.5 eV below the conduction band minimum (CBM) and largely enhance the absorption of visible light, we systematically investigate the structural stabilities, electronic states and optical absorption of various defects in hydrogenated rutile and anatase TiO2 using first-principles calculations with the HSE06 hybrid functional. Our results show that some double-oxygen-vacancy cluster configurations can induce occupied deep in-gap states, while the other defects, including substitutional H impurities and single oxygen vacancies, only induce shallow in-gap states. The deep in-gap states can strongly absorb visible light and effectively produce holes in the valence band (VB). The components of the in-gap states and electron transition channels between the in-gap states and the conduction band (CB) or the VB are also presented. Noticeably, the structural stability study shows that substitutional H is more energetically stable than oxygen vacancies.