Ab initio studies of hydrogen and acceptor defects in rutile TiO2

The behaviour of hydrogen in both defect free and acceptor doped bulk rutile TiO2 is investigated through defect calculations performed within the density functional theory formalism. Both interstitial and substitutional hydrogen defects are shown to behave as shallow donors in the material, thereby existing as effectively positive hydroxide defects, OHO center dot, and substitutional hydrogen defects, H-O(center dot). Of the investigated isolated hydrogen defects, the OHO center dot defect is shown to have the lowest formation energy over the whole Fermi level range under both oxidising and reducing conditions. However, the formation energy of H-O(center dot) is only 0.35 eV higher under conditions where TiO2 is in equilibrium with Ti2O3, indicating that it exists as a minority defect in rutile TiO2 under highly reducing conditions such as under growth of TiO2 on a Ti metal surface. The enthalpy of hydration of oxygen vacancies is calculated to be -1.64 and -1.60 eV for the 2 x 2 x 3 and 3 x 3 x 3 supercells, indicating that OHO center dot defects will prevail in wet atmospheres, even at high temperatures. Hydrogen incorporated in the material can furthermore be expected to associate significantly with various acceptor centres (e. g., substitutional N and Al acceptors and Ti vacancies). The binding energies of the substitutional NHOx ( imide) defect and of the association complexes between H and fully ionised substitutional Al, (Al-Ti' x OHO center dot x OHO center dot)(x), and Ti vacancies, (v(Ti)"" x OHO center dot)'" and (v(Ti)"" x 4OH(O)(center dot))(x) (i.e., Ruetschi type defects), are calculated to be -0.36, -0.12, -0.47 and -0.82 eV, respectively.