Adsorption and dissociation of H2O on in-plane-polarized BaTiO3(001) surfaces and their relation to ferroelectricity

Density-functional calculations have been performed on BaTiO3(001) bare, with adsorbed molecular water and hydroxylated surfaces in the case of surfaces having in-plane polarization. We show that in-plane-polarized BaTiO3(001) flat surfaces have very strong interaction energies with water and that H2O is chemisorbed on both BaO and TiO2 terminations in that case. The adsorption results into surface hydroxyl groups OH with electric dipoles that couple to ferroelectricity in the materials (imprint effect) and point preferentially in the same direction as the polarization in the slab or into molecular water strongly bound to the surface. Moreover we find that the in-plane polarization in the last TiO2 layer is pinned by the dipoles of the OH groups adsorbed for both terminations (especially in the case of the TiO2 one). The influence of strain on hydroxylation is also examined in the case of TiO2 terminations, and tensile strain is found to increase the reactivity of TiO2-terminated BaTiO3(001) surfaces with respect to water dissociation. The evidence of spontaneous water chemisorption on in-plane-polarized BaTiO3 surfaces in ambient moisture conditions might partly explain screening mechanisms on ferroelectric films.