Correlative operando microscopy of oxygen evolution electrocatalysts.

Transition metal (oxy)hydroxides are promising electrocatalysts for the oxygen evolution reaction1-3. The properties of these materials evolve dynamically and heterogeneously4 with applied voltage through ion insertion redox reactions, converting materials that are inactive under open circuit conditions into active electrocatalysts during operation5. The catalytic state is thus inherently far from equilibrium, which complicates its direct observation. Here, using a suite of correlative operando scanning probe and X-ray microscopy techniques, we establish a link between the oxygen evolution activity and the local operational chemical, physical and electronic nanoscale structure of single-crystalline ß-Co(OH)2 platelet particles. At pre-catalytic voltages, the particles swell to form an α-CoO2H1.5·0.5H2O-like structure-produced through hydroxide intercalation-in which the oxidation state of cobalt is +2.5. Upon increasing the voltage to drive oxygen evolution, interlayer water and protons de-intercalate to form contracted ß-CoOOH particles that contain Co3+ species. Although these transformations manifest heterogeneously through the bulk of the particles, the electrochemical current is primarily restricted to their edge facets. The observed Tafel behaviour is correlated with the local concentration of Co3+ at these reactive edge sites, demonstrating the link between bulk ion-insertion and surface catalytic activity.

Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials.

Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices, and for the coupling of light absorption with other functional properties. In these materials, the strong inversion symmetry breaking that is due to spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the bandgap, which may enable efficiencies beyond the maximum possible in a conventional p-n junction solar cell. Ferroelectric oxides are also stable in a wide range of mechanical, chemical and thermal conditions and can be fabricated using low-cost methods such as sol-gel thin-film deposition and sputtering. Recent work has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and ferroelectric-electrode interfaces can greatly increase the current harvested from ferroelectric absorber materials, increasing the power conversion efficiency from about 10(-4) to about 0.5 per cent. Further improvements in photovoltaic efficiency have been inhibited by the wide bandgaps (2.7-4 electronvolts) of ferroelectric oxides, which allow the use of only 8-20 per cent of the solar spectrum. Here we describe a family of single-phase solid oxide solutions made from low-cost and non-toxic elements using conventional solid-state methods: [KNbO3]1 - x[BaNi1/2Nb1/2O3 - δ]x (KBNNO). These oxides exhibit both ferroelectricity and a wide variation of direct bandgaps in the range 1.1-3.8 electronvolts. In particular, the x = 0.1 composition is polar at room temperature, has a direct bandgap of 1.39 electronvolts and has a photocurrent density approximately 50 times larger than that of the classic ferroelectric (Pb,La)(Zr,Ti)O3 material. The ability of KBNNO to absorb three to six times more solar energy than the current ferroelectric materials suggests a route to viable ferroelectric semiconductor-based cells for solar energy conversion and other applications.

Formation of BiFeO3 from a Binary Oxide Superlattice Grown by Atomic Layer Deposition.

We report on the growth of polycrystalline BiFeO3 thin films on SiO2 /Si(001) and Pt(111) substrates by atomic layer deposition using the precursors ferrocene, triphenyl-bismuth, and ozone. By growing alternating layers of Fe2 O3 and Bi2 O3 , we employ a superlattice approach and demonstrate an efficient control of the cation stoichiometry. The superlattice decay and the resulting formation of polycrystalline BiFeO3 films are studied by in situ X-ray diffraction, in situ X-ray photoelectron spectroscopy, and transmission electron microscopy. No intermediate ternary phases are formed and BiFeO3 crystallization is initiated in the Bi2 O3 layers at 450 °C following the diffusion-driven intermixing of the cations. Our study of the BiFeO3 formation provides an insight into the complex interplay between microstructural evolution, grain growth, and bismuth oxide evaporation, with implications for optimization of ferroelectric properties.

A facile route for producing single-crystalline epitaxial perovskite oxide thin films.

We report how a low vacuum pressure process followed by a few-minute annealing enables epitaxial stabilization, producing high-quality, phase-pure, single-crystalline epitaxial, and misfit dislocation-free BiFeO3(001) thin films on SrTiO3(001) at ∼450 °C less than current routes. These results unambiguously challenge the widely held notion that atomic layer deposition (ALD) is not appropriate for attaining high-quality chemically complex oxide films on perovskite substrates in single-crystalline epitaxial form, demonstrating applicability as an inexpensive, facile, and highly scalable route.

Hollandites as a new class of multiferroics.

Discovery of new complex oxides that exhibit both magnetic and ferroelectric properties is of great interest for the design of functional magnetoelectrics, in which research is driven by the technologically exciting prospect of controlling charges by magnetic fields and spins by applied voltages, for sensors, 4-state logic, and spintronics. Motivated by the notion of a tool-kit for complex oxide design, we developed a chemical synthesis strategy for single-phase multifunctional lattices. Here, we introduce a new class of multiferroic hollandite Ba-Mn-Ti oxides not apparent in nature. BaMn3Ti4O14.25, designated BMT-134, possesses the signature channel-like hollandite structure, contains Mn(4+) and Mn(3+) in a 1:1 ratio, exhibits an antiferromagnetic phase transition (TN ~ 120 K) with a weak ferromagnetic ordering at lower temperatures, ferroelectricity, a giant dielectric constant at low frequency and a stable intrinsic dielectric constant of ~200 (1-100 MHz). With evidence of correlated antiferromagnetic and ferroelectric order, the findings point to an unexplored family of structures belonging to the hollandite supergroup with multifunctional properties, and high potential for developing new magnetoelectric materials.