The adsorption of phosphonic acid on the TiO2 anatase (101) and rutile (110) surfaces have been investigated by means of efficient density-functional-based tight-binding calculations. We studied the geometries and adsorption energies of several adsorption models to achieve clarification of the discrepancy in the experimental finding of a preferred binding state. In this paper we show that there are several adsorption structures likely to be present on the specific TiO2 surfaces. Those structures have exclusively a bidentate configuration. They have similar adsorption energies but different geometries. For the monodentate complexes, we find a strong trend of the adsorption geometry relaxing toward the bidentate coordination. Also, they have significantly smaller adsorption energies. Furthermore, we extensively demonstrate the reliability of the SCC-DFTB method for this chemical system, which opens the way for studies of adsorption on more complex titania materials.

No abstract

Summary: The infrared absorption (IR) spectrum of alkyl phosphonic acid adsorbed on the α‐Al2O3 (0001) surface has been calculated by means of a density‐functional based tight‐binding method. Thereby mono‐dentate, bi‐dentate and tri‐dentate bonding of the acid to the surface have been considered. In addition, experimentally obtained Fourier Transform Infrared Spectra (FTIR) of octadecylphosphonic acid (ODPA) on the natural surface of aluminium have been included. The absence of the PO band in the experimental surface spectrum and in the calculated spectrum of the tridentate adsorption complexes showed that adsorption of (alkyl)phosphonic acids on aluminium favours tridentate bonding, where the acid is bound to the surface via three symmetric POAl bonds.

. Self-assembled monolayers of alkylphosphonic acids on aluminum oxide surfaces -A theoretical study. Zeitschrift für Anorganische und Allgemeine Chemie, 2010, 636 (8) (001) and boehmite γ-AlOOH (010) surface models. Mono-, bi-, and tridentate adsorption modes have been considered. In addition, the organization of single adsorbed molecules has been compared to the organization at full surface coverage. The height (thickness) of the self-assembled monolayers is always shorter than the length of the phosphonic acid molecules due to tilting of the alkyl chains. Tilt angles at full surface coverage are very similar to the tilt angle of a single adsorbed molecule, indicating that the density of the self-assembled monolayers is limited by the density of adsorption sites. The lateral interactions between alkyl chains are evidenced by small torsions of the adsorbed molecules, which may serve to minimize the repulsion forces between interchain hydrogen atoms. Similar tilt angles have been obtained for mono-, bi-, and tridentate adsorptions. Hence, the coordination mode cannot be characterized by the molecule tilting.____________ [a]