Tunable Ferroionic Properties in CeO₂/BaTiO₃ Heterostructures

Ferroionic materials combine ferroelectric properties and spontaneous polarization with ionic phenomena of fast charge recombination and electrodic functionalities. In this paper, we propose the concept of tunable polarization in CeO_2-δ (ceria) thin (5 nm) films induced by built-in remnant polarization of a BaTiO₃ (BTO) ferroelectric thin film interface, which is buried under the ceria layer. Upward and downward fixed polarizations at the BTO thin film (10 nm) are achieved by the lattice termination engineering of the SrO or TiO₂ terminated Nb:SrTiO₃ (NSTO or STN) substrate. We find that the ceria layer punctually replicates the polarization of the BTO interface via a dynamic reconfiguration of its intrinsic defects, i.e., oxygen vacancies and small polarons. Tunable oxidative or reducing properties (redox) also arise at the surface from the built-in polarization. Opposite polarities at the ceria termination tune the chemo-physical dynamics toward water molecule adsorbates. The inversion of the surface potential leads to a modulation of the water adsorption-desorption equilibrium and water ionization (splitting) redox overpotentials within ±400 mV at room temperature, depending on the ceria termination's charges. Such tunability opens up the perspectives of using ferroionics for wireless electrochemically enhanced catalysis.