Elettra | APE-HE In the race to develop cleaner, smarter, and more sustainable technologies, scientists are exploring new ways to control how materials behave, especially at their surfaces, where many important chemical reactions occur. Our research introduces a novel strategy using ferroionics, an emerging concept that combines the electrical control offered by ferroelectric materials with the chemical reactivity of ionic oxides. This approach enables reversible control of surface catalytic behavior without the need for external power or wiring, relying instead on internal electric fields designed into the material’s structure. We fabricated ultra-thin heterostructures composed of two oxide materials: BaTiO₃ (BTO), a well-known ferroelectric that naturally carries a permanent internal electric field, and CeO₂ (ceria), a widely used catalytic oxide. Each layer was only a few nanometers thick (5-10 nm), yet the interaction between them led to remarkable effects. Specifically, the buried electric field in BaTiO₃ was found to influence the chemical properties of the surface ceria layer, even though the field itself is not directly accessible. By carefully controlling the direction of this polarisation during fabrication, either pointing upward or downward, we were able to reversibly shift the surface of ceria between more oxidizing and more reducing states. In practical terms, this allowed us to preprogram surface chemical reactivity using only internal fields, with no applied voltage. Two nominally identical structures, differing only in polarisation direction, showed clearly distinct surface behaviors in Fig. 1 (a). To understand the origin of this effect, we used synchrotron-based X-ray Absorption Spectroscopy (XAS) and X-ray Linear Dichroism (XLD) at the Elettra synchrotron facility. These advanced techniques provided insight into the local electronic structure of the BaTiO₃/CeO₂ interface. As expected, the Ti atoms in BTO showed a clear shift in orbital occupation depending on polarisation, confirming that the interface experiences strong electronic asymmetry, see Fig. 1 (b). Unexpectedly, we also detected a dichroic signal in the Ce M-edge spectra, an unusual finding for centrosymmetric CeO₂. This result indicated that the built in electric field in BTO layer affects the electronic environment of Ce atoms, likely by inducing changes in the distribution of oxygen vacancies and Ce³⁺ ions. These charged point defects are key players in surface redox reactions and catalytic performance. To probe how these internal fields influence the material at the nanoscale, we employed Piezoresponse Force Microscopy (PFM), which revealed that the CeO₂ surface inherits the polarisation state of the underlying BTO layer (Fig. 1 c). We also used Kelvin Probe Force Microscopy (KPFM) to map surface potential differences. A pronounced shift of around 400 millivolts was observed between upward- and downwardpolarised samples, as shown in Fig. 1 (d), confirming that the built-in polarisation modulates the electrochemical environment at the surface. This potential difference is significant enough to influence charge transfer reactions involving adsorbed molecules like water. To assess the role of the surrounding environment, we compared sample behavior in ambient air and dry argon. In air, where water vapor is present, the CeO₂ surface responded clearly to polarisation direction. But in dry conditions, this response was suppressed, see Fig. 1 (e). This showed that adsorbed water molecules are essential to screening or amplifying the polarisation effect. Water acts not only as a reactant, but also as a dynamic medium that enables or enhances the polarisation-induced surface reconfiguration. This observation helps explain why the same material behaves differently depending on its exposure to the environment. We then complemented our experiments with Density Functional Theory (DFT) simulations, which showed that the presence of oxygen vacancies near the surface significantly enhances water binding energy. These vacancies, influenced by the built-in polarisation, shift the local electronic structure and promote stronger interactions with water molecules. This, in turn, can lower the energy barriers for reactions such as water splitting and hydrogen evolution. Therefore, by manipulating the polarisation state in the ferroelectric layer, we can indirectly tune the reaction energetics on the CeO₂ surface through both defect chemistry and adsorbate interactions. Overall, our findings demonstrate that ferroionic heterostructures offer a powerful, reversible, and energy-free method for controlling surface reactivity. Rather than relying on external electric fields or high temperatures, this method uses the intrinsic polarisation of a ferroelectric to guide the chemical state of a catalytic surface. The modulation is stable, reproducible, and environmentally responsive. This concept opens exciting opportunities in the design of nextgeneration catalytic and sensing platforms that are more efficient, compact, and sustainable. Applications could include hydrogen production, gas sensors, or electrochemical devices where reaction control is critical. Ferroionic materials offer a path toward “intelligent” catalysts, materials whose chemical behavior is not only active but responsive, tunable, and preprogrammed at the atomic level. These are surfaces that don’t need to be powered up by external stimuli, they are already working by design, governed by invisible but deeply embedded electric fields. Reversible surface catalysis through ferroionic design Elettra Highlights 2024–2025 106
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