Properties of hydrogen and hydrogen-vacancy complexes in the rutile phase of titanium dioxide

The interaction of atomic H with host atoms and oxygen vacancies (VO) in the rutile phase of the TiO2 metal oxide has been investigated by using density-functional theory-local spin density (DFT-LSD) and DFT-LSD+U theoretical methods. The achieved results show that H in rutile presents quite different and peculiar properties with respect to other semiconductors and metal oxides. It behaves indeed neither as an amphoteric impurity, as it does in Si and GaAs, nor as a shallow donor, as it has been proposed in ZnO. Moreover, H in rutile represents a failure of a theoretical model proposing a universal alignment of the H-induced electronic level in the energy gaps of semiconductors, which predicts a shallow donor behavior of H in ZnO and TiO2. Present results show indeed that H behaves as a deep donor in rutile and always forms an OH+ complex, independent of the position of the Fermi energy. This very unusual behavior of H can be accounted for by a peculiar property of TiO2 regarding its capability of localizing extra electrons at Ti+3 sites. The electron lost by H can be accommodated indeed by a Ti+4 atom which evolves in a Ti+3 defect. This accounts for the deep behavior of H and implies that the electronic level it induces in the TiO2 energy gap has, actually, a Ti+3 character quite similar to that characterizing an O vacancy (VO), thus distinguishing H in rutile from H in other semiconductors. Finally, H can form stable H–VO complexes where it takes the place of the missing O atom by forming a bond with a prevailing ionic character, at variance with a multicenter bond model proposed for the same complexes in ZnO.