A comprehensive phase diagram of lowest-energy structures and compositions of the rutile TiO2(110) surface in equilibrium with a surrounding gas phase at finite temperatures and pressures was determined using density-functional theory in combination with a thermodynamic formalism. The exchange of oxygen, hydrogen, and water molecules with the gas phase was considered. Particular attention was given to the convergence of all calculations with respect to lateral system size and slab thickness. In addition, the reliability of semi-local density functionals in describing the energetics of the reduced surfaces was critically evaluated. For ambient conditions the surface was found to be fully covered by molecularly adsorbed water. At low coverages, in the limit of single isolated water molecules, molecular and dissociative adsorption modes become energetically degenerate. Oxygen vacancies form in strongly reducing, oxygen-poor environments. However, already at slightly more moderate conditions it was shown that removing full TiO2 units from the surface was thermodynamically preferred. In agreement with recent experimental observations it was furthermore confirmed that even under extremely hydrogen-rich environments the surface could not be fully hydroxylated, but only a maximum coverage with hydrogen of about 0.6–0.7 monolayer could be reached. Finally, calculations of migration paths strongly suggested that hydrogen preferred diffusing into the bulk over desorbing from the surface into the gas phase.

Composition, Structure, and Stability of the Rutile TiO2(110) Surface: Oxygen Depletion, Hydroxylation, Hydrogen Migration, and Water Adsorption. Kowalski, P.M., Meyer, B., Marx, D.: Physical Review B, 2009, 79[11], 115410