Adsorption of water on the TiO2 (rutile) (110) surface:: A comparison of periodic and embedded cluster calculations

Periodic density functional theory (DFT) based on plane waves (PW) and Hartree-Fock (HF) based on the linear combination of atomic orbitals (LCAO) calculations using slabs separated by vacuum gaps were carried out to model the H2O-TiO2 (rutile) (110) interface. Positions of all atoms were allowed to relax except atoms in the central layer of the slab. Both associative and dissociative adsorption mechanisms were considered for half-monolayer and monolayer coverages. Five different orientations of H2O molecules on the TiO2, surface were studied to determine the most energetically favorable water positions for the associative mechanism. Two slab thicknesses (three Ti layers and five Ti layers) were chosen to test the effect of slab depth on calculated surface structures and adsorption energies. Results indicate that associative adsorption is favorable by -8 to -20 kJ/mol/H2O depending on the slab thickness for full-monolayer coverage. Embedded cluster HF calculations were also performed for comparison. Adsorption energies of H2O in the embedded cluster case are much more favorable for the associative mechanism. The role of H-bond formation on the adsorption energies and structures is discussed.