Interface Water on TiO2 Anatase (101) and (001) Surfaces: First-Principles Study with TiO2 Slabs Dipped in Bulk Water

We investigated the TiO2 anatase (101) and (001) interfaces dipped in bulk water on the atomic scale by first-principles density-functional molecular dynamics simulations. We verified that the water adsorption models proposed in the previous studies with less than a couple of water layers on the vacuum surfaces still hold. On the contrary, novel adsorption structures of interfacial water molecules are also found. Our results indicate that water molecules around the interface between the TiO2 and bulk water can be described by the following two-layer model: The first water layer can be defined as the water molecules adsorbed at Ti-5C sites moleculary on the anatase (101) surfaces and dissociatively on the (001). The second layer on the anatase (101) surface can be defined as the water molecules adsorbed to O-2C or adsorbed water to Ti-5C via strong HB. Second layer on the (001) consists of water molecules bound to the first-layer water molecules via the strong and weak HBs. Contour maps of the atomic densities show that water molecules in the second layers are relatively mobile. This two-layer model well accounts for the experimental results of solid-state H-1 NMR.