Short hydrogen bonds at the water/TiO2 (anatase) interface

The nature of peculiar, short H bonds formed by water molecules in contact with the (101) anatase surface and their effects on the structural and vibrational properties of the first water layers adsorbed on the same surface have been investigated by performing density functional theory (DFT) total energy calculations and ab initio molecular dynamics (AIMD) simulations at different temperatures. Present results show that these short H bonds originate from a water/anatase interface effect related to an electronic charge transfer from surface Ti atoms to surface O atoms, mediated by water molecules. Further, AIMD simulations per-formed at low temperature indicate that such short H bonds are at the ground of both the atomic arrangements of the water layers and the peculiar features appearing in the corresponding vibrational spectra. The same interface effect significantly influences also the atomic arrangements and the vibrational properties of intermediates of the O-2 photoreduction reaction, which turn out to be involved in similar charge transfer processes as well as in the formation of short H bonds. AIMD simulations show that these short H bonds are still present at room temperature and give estimates of the vibrational frequencies of the same intermediates, which are in a quite good agreement with the experimental findings. Such an agreement supports the unifying theoretical picture proposed here for water molecules and O-2 photoreduction intermediates in contact with the anatase surface.