Oxygen vacancies and OH species in rutile and anatase TiO2 polymorphs

Ab initio density functional calculations in the beyond-LSD (Local Spin Density), LSD + U approximation have been performed to investigate the properties of bridging hydroxyls (OHb) at the rutile (1 1 0) and anatase (1 0 1) surfaces as well as of bulk oxygen vacancies (VOx) and Ti-OH defects in the same two polymorphs. The aim of this study is to deepen our understanding of the properties of these defects both in rutile and anatase as well as to reveal similar behaviours of VOx and OH defects. In the case of rutile OHbs, a good agreement is found between present results and those of a recent STM study. Anatase OHbs show properties similar to those of rutile OHbs. On the contrary, a remarkable difference is found between the structural and electronic properties of bulk VOxs in rutile and anatase, which could explain the higher electron mobility and photocatalytic activity observed for the latter compound. Bulk Ti-OH defects are predicted to easily form through insertion of atomic H in TiO2 lattices. Moreover, in the case of rutile, these defects should present properties quite similar to those of VOxs. Finally, the results achieved here stress the importance of beyond-LSD methods when dealing with defects in TiO2.