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

Abstract

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–Ti5C/O1–Ti4C bond with a distance of 2.0–2.1 Å and a weaker HC1CO1O2–H···O2C/O2···H–O2C 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.