First principles study of the electronic structure and magnetism of oxygen-deficient anatase TiO2 (0 0 1) surface

Abstract

We have studied the electronic structure and magnetic properties of oxygen vacancy on anatase TiO2 (001) surface using density functional theory (DFT) calculations. We find that the oxygen vacancy on the surface may be responsible for the unexpected ferromagnetism in anatase TiO2 films without doping. The formation energies of surface vacancies are calculated. Results of our first-principles GGA+U calculations show that the oxygen vacancy at the ridge has the lowest formation energy. The spins induced by the local oxygen vacancy form a stable ferromagnetic state, and the anion vacancy can result in a magnetic moment of 1.63μB. The localized state of spin polarization will be formed in the band gap. The magnetic moment which is produced by the oxygen vacancy mainly comes from the d orbitals of two low-charge-state Ti ions converted from Ti4+ ions adjacent to the surface oxygen vacancy. We calculate the magnetic coupling between the two oxygen vacancies at the ridge in different distances. Although the spin dimerization will be formed along with the decrease of Ti–Ti distance, the results also show that the two oxygen vacancies on anatase TiO2 (001) surface are mainly coupled ferromagnetically. In addition, the other kind of oxygen vacancy at the valley can also induce the ferromagnetism on the surface. The experimentally observed d0-ferromagnetism behavior in TiO2 system is in good agreement with our calculated results.