Structure and Reactivity of Water Layers on Defect-Free and Defective Anatase TiO₂(101) Surfaces

The adsorption of thin water overlayers on the (101) surface of TiO ₂ anatase has been studied through Car-Parrinello molecular dynamics (CPMD) simulations. We compared the structural and dynamic properties of one and two H₂O monolayers adsorbed on defect-free and partially reduced surfaces at 160 K. As for an isolated adsorbed H₂O molecule, we found that water dissociation is possible only on the defective surface. With respect to a single H₂O, the interaction between H₂O molecules reduces the dissociation barrier: a water molecule incorporated in a monolayer (ML) or a bilayer (BL) over a defected surface dissociates spontaneously at 160 K. On the perfect surface, a water ML forms a regular array of Ti-coordinated molecules, whereas on the defective surface some molecules shift coordination from Ti to a bridging (O_2c) oxygen, leading to a mixed ML state with molecular water adsorbed on two different sites and two hydroxyls resulting from the dissociation. Only after adsorption of a second water layer O _2c-coordinated molecules are formed also on the regular surface, leading to a more disordered state. Although most structural differences between water overlayers on defect-free and defective surfaces disappear at BL coverage, water molecules are more mobile in the presence of the defect. The presence of H₂O-O_2c only above ML coverage and the lack of water dissociation on the defect-free surface are in agreement with the findings of recent temperature-programmed desorption experiments, which have been further analyzed on the basis of the calculated adsorption energies.