Chemistry and atomic distortion at the surface of an epitaxial BaTiO ₃ thin film after dissociative adsorption of water

We present a study of the atomic and chemical structure of the surface of a fully strained, TiO ₂-terminated, ferroelectric BaTiO ₃ (BTO) (001) epitaxial film on a SrTiO ₃ substrate after controlled exposure to water. The epitaxial quality was checked by atomic force microscopy and X-ray diffraction. Quantitative low-energy electron diffraction compared with multiple scattering simulations was used to measure the structure of the first few atomic layers of BTO surface. The surface chemistry was investigated using high-resolution X-ray photoelectron spectroscopy. Finally, temperature-programmed desorption measured the desorption energies. We find that water undergoes mainly dissociative adsorption on the polarized BTO(001) surface. There are two competing sites for dissociative adsorption: oxygen vacancies and on-top Ti surface lattice atoms. The Ti on-top site is the dominant site for OH ^- chemisorption. One fifth of the surface Ti atoms bind to OH ^-. The concentration of surface oxygen vacancies acts mainly to favor initial physisorption. Before exposure to water, the outward pointing polarization in the BTO film is stabilized by atomic rumpling in the TiO ₂ termination layer. After exposure to water, the chemisorbed OH ^- species provide the screening, inverting the surface dipole layer and stabilizing the bulk polarization. Molecular adsorption is observed only for high water coverage.