DFT-study of the adsorption of organic molecules on low-index titanium dioxide surfaces

The adsorption of small (bio-)organic molecules on clean, low-index TiO2 surfaces has been investigated by density-functional based tightbinding calculations with the goal to rationalise the propensity of such molecules to self-assemble into row-like structures or two-dimensionally ordered patches. The systems studied range from phosphonic acids to the nucleotide cytidin monophosphate adsorbed on TiO2 anatase(101) and rutile(110) surfaces. We studied the geometries and adsorption energies of several adsorption models and obtain several possible adsorption structures that can be present on the specific TiO2 surfaces with comparable probability. For the pure phosphonic acids the preferable coordination is exclusively bidentate with similar adsorption energies but several different geometries. Monodentate and tridentate arrangements have significantly smaller adsorption energies and tend to relax towards the bidentate coordination. Despite the different steric situation, cytidin monophosphate exhibits exactly the same trends as phosphonic acids with bidentate coordination via a combination of oxo, alkoxy and hydroxyl groups.