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Modeling water adsorption on rutile (110) using van der Waals density functional and DFT+U methods

N. Kumar, P. R. C. Kent, D. J. Wesolowski, and J. D. Kubicki

Journal of Physical Chemistry C 117 23638 (2013)

We study the energetics and structure of water absorption on the ideal rutile TiO2 (110) surface using dispersion-corrected periodic density functional theory (DFT) calculations and on-site Coulomb potential (DFT+U) corrections. Conventional (PBE) and self-consistent dispersion-corrected DFT methods (vdw-DF1 and vdw-DF2) both suggest that molecular adsorption of intact water molecules on the rutile (110) surface is increasingly preferred with increasing simulation slab thickness. However, empirical dispersion corrections indicate a mix of molecular and dissociated water may coexist at room temperature, with less dependence on slab thickness. This same behavior is seen for DFT+U with U = 3 eV in combination with or without self-consistent dispersion corrected DFT. We find that the preference for the occurrence of dissociated water increases with increasing U. When compared with experimental bond-length data for the adsorbed water species, none of the methods and slab thicknesses correctly predict all bond lengths simultaneously. However, of the methods that energetically-favor coexisting associated and dissociated water species on the surface, the three-layer slab with conventional DFT (PBE) and the empirically dispersion-corrected DFT methods come closest to correctly reproducing all of the experimentally-observed bond lengths. We conclude that the current level of DFT is insufficient to definitively distinguish between the fully associated and partially-dissociated states of water adsorbed on the pristine rutile (110) surface, due to the very small (~0.1 eV) total energy differences between these states.

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