Tunable\nFerroionic\nProperties in CeO<sub>2</sub>/BaTiO<sub>3</sub> Heterostructures

Ferroionic materials combine ferroelectric\nproperties\nand spontaneous\npolarization with ionic phenomena of fast charge recombination and\nelectrodic functionalities. In this paper, we propose the concept\nof tunable polarization in CeO<sub>2−δ</sub> (ceria)\nthin (5 nm) films induced by built-in remnant polarization of a BaTiO<sub>3</sub> (BTO) ferroelectric thin film interface, which is buried\nunder the ceria layer. Upward and downward fixed polarizations at\nthe BTO thin film (10 nm) are achieved by the lattice termination\nengineering of the SrO or TiO<sub>2</sub> terminated Nb:SrTiO<sub>3</sub> (NSTO or STN) substrate. We find that the ceria layer punctually\nreplicates the polarization of the BTO interface via a dynamic reconfiguration\nof its intrinsic defects, i.e., oxygen vacancies and small polarons.\nTunable oxidative or reducing properties (redox) also arise at the\nsurface from the built-in polarization. Opposite polarities at the\nceria termination tune the chemo-physical dynamics toward water molecule\nadsorbates. The inversion of the surface potential leads to a modulation\nof the water adsorption–desorption equilibrium and water ionization\n(splitting) redox overpotentials within ±400 mV at room temperature,\ndepending on the ceria termination’s charges. Such tunability\nopens up the perspectives of using ferroionics for wireless electrochemically\nenhanced catalysis.