Interfacial water on TiO$_2$ anatase (101) and (001) surfaces: First-principles study with TiO$_2$ slabs dipped in bulk water*

Density functional molecular dynamics simulations using supercells with ``bulk'' water between the TiO$_2$ anatase (101) and (001) surfaces were carried out to elucidate the behaviour of water molecules and hydrogen bond networks on the interfaces of representative photocatalysts. It is demonstrated that the adsorption manners (molecular or dissociative) of water molecules on the vacuum surfaces still hold in the presence of ``bulk'' water on the interfaces. We also showed explicit atomistic structures of strong and weak hydrogen bonds on the TiO$_2$/water interfaces, which had been proposed experimentally so far. We then suggested a two-layer model for the interfacial water on both surfaces investigated. Our results also give insights into the H$_2$O or OH adsorption coverage on the interfaces and their hydrophobicity- hydrophilicity, which is important to understand the photocatalytic reaction mechanisms microscopically.