Adsorption of organic molecules on titania: The advantages of using bond orders to gauge interaction strength

The adsorption of six derivatives of methane with different functional groups on the anatase (101) facet of titania (TiO2) is studied using quantum chemical calculations at the DFT/PBC level of theory. The strength of the resulting interactions is evaluated using (1) the adsorption energy, (2) an analysis of the electron density using the Quantum Theory of Atoms In Molecules (QTAIM), and (3) Hirshfeld bond orders. It is shown that the latter provide a straightforward way to gain internally consistent information on the relative strengths of the various interactions, which is much more detailed than the adsorption energies on the one hand and far less dependent on the bond type than the results of a QTAIM analysis on the other. The results show that amines, thiols, silanols and carboxylic acids adsorb rather poorly, while phosphonic and sulfonic acids bind strongly to the titania surface, in agreement with experimental observations.