  Understanding the interaction of organic molecules with TiO2 surfaces is
important for a wide range of technological applications. While density
functional theory (DFT) calculations can provide valuable insight about these
interactions, traditional DFT approaches with local exchange-correlation
functionals suffer from a poor description of non-bonding van der Waals (vdW)
interactions. We examine here the contribution of vdW forces to the interaction
of small organic molecules (methane, methanol, formic acid and glycine) with
the TiO2 (110) surface, based on DFT calculations with the optB88-vdW
functional. The adsorption geometries and energies at different configurations
were also obtained in the standard generalized gradient approximation (GGA-PBE)
for comparison. We find that the optB88-vdW consistently gives shorter surface
adsorbate-to-surface distances and slightly stronger interactions than PBE for
the weak (physisorbed) modes of adsorption. In the case of strongly adsorbed
(chemisorbed) molecules both functionals give similar results for the
adsorption geometries, and also similar values of the relative energies between
different chemisorption modes for each molecule. In particular both functionals
predict that dissociative adsorption is more favourable than molecular
adsorption for methanol, formic acid and glycine, in general agreement with
experiment. The dissociation energies obtained from both functionals are also
very similar, indicating that vdW interactions do not affect the thermodynamics
of surface deprotonation. However, the optB88-vdW always predicts stronger
adsorption than PBE. The comparison of the methanol adsorption energies with
values obtained from a Redhead analysis of temperature programmed desorption
data suggests that optB88-vdW significantly overestimates the adsorption
strength, although we warn about the uncertainties involved in such
comparisons.
