Ferroelectric polarization switching induced from water adsorption in BaTiO3 ultrathin films

The influence of water on the out-of-plane polarization of BaTiO3 (BTO) ultrathin films is studied by means of density functional theory calculations. The adsorption is investigated for different coverages on both terminations of BTO with, for each case, all possible states of polarization, namely, paraelectric, polarized upward, and polarized downward. We thus demonstrate different behavior as a function of the termination. While H2O adsorbs only dissociatively on the BaO termination, with a reinforced interaction compared to BTO without out-of-plane polarization, only molecular adsorption is observed on the TiO2 termination. In addition, the presence of water is able to switch the polarization. Whatever the initial state of polarization is, water induces a downward state on the BaO termination and an upward polarization on the TiO2 one. A detailed analysis of this phenomenon is given.