Structural Modification of TiO₂ Surfaces in Bulk Water and Binding Motifs of a Functionalized C₆₀ on TiO₂ Anatase and Rutile Surfaces in Vacuo and in Water: Molecular Dynamics Studies

The nature of several TiO₂ surfaces in liquid water, as well as the adsorption of a functionalized C₆₀, L*C₆₀ (where L is a carboxylic acid), on TiO₂ anatase and rutile low index surfaces in vacuo and in liquid water have been studied at the self-consistent charge density functional tight-binding (SCC-DFTB) level of theory. It is shown that the SCC-DFTB method provides very good agreement with the high-level DFT data. The typical binding motif of L*C₆₀@TiO₂ is found to be the formation of a strong HC¹C­(O²H)­O¹–Ti_5C/O¹–Ti_4C bond with a distance of 2.0–2.1 Å and a weaker HC¹CO¹O²–H···O_2C/O²···H–O_2C hydrogen bond. In some cases, a terminal OH of the linking group coordinates with a Ti–O–Ti bridging oxygen and loses the H to the surface. The adsorption energies in vacuum range between 21 and 82 kcal/mol depending on the surface. The density of states of these species reveals the presence of peaks below the surface conduction band upon ligand adsorption, which is due to the low-lying lowest unoccupied molecular orbitals (LUMOs) of the L*C₆₀. Electron transfer from the surface to the ligand is thus possible via the initial UV photoexcitation of the surface followed by nonradiative relaxation of the excited electron to the LUMO of the ligand. This pattern was observed for all six surfaces considered in the present work. Solvation of C₆₀@TiO₂ in liquid water does not change the qualitative character of surface–ligand binding. In all cases, the ligand remains bound to the surface in the presence of water. The interaction of water molecules with the surface shows various patterns depending on the surface index. Anatase (101) and (100) surfaces favor nondissociative water adsorption, while anatase (001) and rutile (001), (110), and (101) surfaces show dissociative water adsorption which results in OH/OH₂-terminated TiO₂ surfaces. The latter finding is in agreement with several previous DFT studies reported by others.