Anatase TiO2 Surface Functionalization by Alkylphosphonic Acid: A DFT+D Study

We report on a DFT study of the adsorption of nbutylphosphonic acid [CH3(CH2)3PO(OH)2] on the anatase (101) TiO2 model surface. The pure GGA (PBE and rPBE) approach is compared to a hybrid functional (B3LYP). Dispersion forces are taken into account in the Grimme framework correcting both energies and gradients. The surface coverage is varied from 0.25 ML (one phosphonic unit every four surface Ti5c) to 1 ML (one phosphonic unit/Ti5c). The overall picture resulting from these three approaches is qualitatively the same, although quantitative details are different. At low coverage, the alkylphosphonic acid adsorbs in a monodissociated bidentate mode, forming two PO−Ti5c bonds with one additional OH−Os bond, where Ti5c and Os refer to surface titanium and oxygen atoms. At full coverage, the molecules adsorb in a monodentate mode and reorient in order to minimize lateral repulsion and maximize H-bonds. The contribution of dispersion forces to the adsorption energy increases with the coverage. The 0.75 and 1 ML coverages, which are the most favored, correspond nicely to the reported experimental coverages. The results suggest that for entropic reasons the surface could be rather disordered, with coexisting domains of different coverages 0.75 and 1 ML. No additional states in the band gap of the semiconductors are formed for this hybrid organic−inorganic system. The workfunction decreases by 0.7 eV when a full self-assembled monolayer is supported on the surface.