Ferroelectric nanomaterials offer the promise of switchable electronic properties at the surface, with implications for photo- and electrocatalysis. Studies to date on the effect of ferroelectric surfaces in electrocatalysis have been primarily limited to nanoparticle systems where complex interfaces arise. Here, we use MBE-grown epitaxial BaTiO3 thin films with atomically sharp interfaces as model surfaces to demonstrate the effect of ferroelectric polarization on the electronic structure, intermediate binding energy, and electrochemical activity toward the hydrogen evolution reaction (HER). Surface spectroscopy and ab initio DFT+U calculations of the well-defined (001) surfaces indicate that an upward polarized surface reduces the work function relative to downward polarization and leads to a smaller HER barrier, in agreement with the higher activity observed experimentally. Employing ferroelectric polarization to create multiple adsorbate interactions over a single electrocatalytic surface, as demonstrated in this work, may offer new opportunities for nanoscale catalysis design beyond traditional descriptors.

We report the in situ, direct epitaxial synthesis of (0001)-oriented PdCoO2 thin films on c-plane sapphire using ozone-assisted molecular-beam epitaxy. The resulting films have smoothness, structural perfection, and electrical characteristics that rival the best in situ grown PdCoO2 thin films in the literature. Metallic conductivity is observed in PdCoO2 films as thin as ∼2.0 nm. The PdCoO2 films contain 180° in-plane rotation twins. Scanning transmission electron microscopy reveals that the growth of PdCoO2 on the (0001) surface of Al2O3 begins with the CoO2 layer.

Homologous series are layered phases that can have a range of stoichiometries depending on an index n. Examples of perovskite-related homologous series include (ABO3)nAO Ruddlesden–Popper phases and (Bi2O2) (An−1BnO3n+1) Aurivillius phases. It is challenging to precisely control n because other members of the homologous series have similar stoichiometry and a phase with the desired n is degenerate in energy with syntactic intergrowths among similar n values; this challenge is amplified as n increases. To improve the ability to synthesize a targeted phase with precise control of the atomic layering, we apply the x-ray diffraction (XRD) approach developed for superlattices of III–V semiconductors to measure minute deviations from the ideal structure so that they can be quantitatively eradicated in subsequent films. We demonstrate the precision of this approach by improving the growth of known Ruddlesden–Popper phases and ultimately, by synthesizing an unprecedented n = 20 Ruddlesden–Popper phase, (ATiO3)20AO where the A-site occupancy is Ba0.6Sr0.4. We demonstrate the generality of this method by applying it to Aurivillius phases and the Bi2Sr2Can–1CunO2n+4 series of high-temperature superconducting phases.

When it comes to providing the unusual combination of optical transparency, p-type conductivity, and relatively high mobility, Sn 2+-based oxides are promising candidates. Epitaxial films of the simplest Sn 2+ oxide, SnO, are grown in an adsorption-controlled regime at 380 • C on Al 2 O 3 substrates by molecular-beam epitaxy, where the excess volatile SnO x desorbs from the film surface. A commensurately strained monolayer and an accompanying van der Waals gap is observed near the substrate interface, promoting layers with high structural perfection notwithstanding a large epitaxial lattice mismatch (−12%). The unintentionally doped films exhibit p-type conductivity with carrier concentration 2.5×10 16 cm −3 and mobility 2.4 cm 2 V −1 s −1 at room temperature. Additional physical properties are measured and linked to the Sn 2+ valence state and corresponding lone-pair charge-density distribution.