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 can be reached. Finally, calculations of migration paths strongly

suggested that hydrogen prefers 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.

P.M.Kowalski, B.Meyer, D.Marx: Physical Review B, 2009, 79[11], 115410