Theoretical predictions--motivated by recent advances in epitaxial engineering--indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.

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Heterostructured interfaces of oxides, which can exhibit transport and reactivity characteristics remarkably different from those of bulk oxides, are interesting systems to explore in search of highly active cathodes for the oxygen reduction reaction (ORR). Here, we show that the ORR of ∼85 nm thick La 0.8 Sr 0.2 -CoO 3-δ (LSC 113 ) films prepared by pulsed laser deposition on (001)-oriented yttriastabilized zirconia (YSZ) substrates is dramatically enhanced (∼3-4 orders of magnitude above bulk LSC 113 ) by surface decorations of (La 0.5 Sr 0.5 ) 2 CoO 4(δ (LSC 214 ) with coverage in the range from ∼0.1 to ∼15 nm. Their surface and atomic structures were characterized by atomic force, scanning electron, and scanning transmission electron microscopy, and the ORR kinetics were determined by electrochemical impedance spectroscopy. Although the mechanism for ORR enhancement is not yet fully understood, our results to date show that the observed ORR enhancement can be attributed to highly active interfacial LSC 113 /LSC 214 regions, which were shown to be atomically sharp.

This study reports and compares the adsorption and dissociation of water on oxidized and reduced CeO 2 (100) and CeO 2 (111) thin films. Water adsorbs dissociatively on both surfaces. On fully oxidized CeO 2 (100) the resulting surface hydroxyls are relatively stable and recombine and desorb as water over a range from 200 to 600 K. The hydroxyls are much less stable on oxidized CeO 2 (111), recombining and desorbing between 200 and 300 K. Water produces 30% more hydroxyls on reduced CeO 1.7 (100) than on oxidized CeO 2 (100). The hydroxyl concentration increases by 160% on reduced CeO 1.7 (111) compared to oxidized CeO 2 (111). On reduced CeO 1.7 (100) most of the hydroxyls still recombine and desorb as water between 200 and 750 K. Most of the hydroxyls on reduced CeO 1.7 (111) react to produce H 2 at 560 K, leaving O on the surface. A relatively small amount of H 2 is produced from reduced CeO 1.7 (100) between 450 and 730 K. The differences in the adsorption and reaction of water on CeO X (100) and CeO X (111) are attributed to different adsorption sites on the two surfaces. The adsorption site on CeO 2 (100) is a bridging site between two Ce cations. This adsorption site does not change when the ceria is reduced. The adsorption site on CeO 2 ( 111) is atop a single Ce cation, and the proton is transferred to a surface O in a site between three Ce cations. When the CeO X (111) is reduced, vacancy sites are produced which allows the water to adsorb and dissociate on the 3-fold Ce cation sites. Recently, Molinari et al. 9 calculated that dissociation is favored