Oxygen Migration Pathways in Layered LnBaCo2O6-δ (Ln = La - Y) Perovskites.

Layered LnBaCo2O6-δ perovskites are important mixed ionic-electronic conductors, exhibiting outstanding catalytic properties for the oxygen evolution/reduction reaction. These phases exhibit considerable structural complexity, in particular, near room temperature, where a number of oxygen vacancy ordered superstructures are found. This study uses bond valence site energy calculations to demonstrate the key underlying structural features that favor facile ionic migration. BVSE calculations show that the 1D vacancy ordering for Ln = Sm-Tb could be beneficial at low temperatures as new pathways with reduced barriers emerge. By contrast, the 2D vacancy ordering for Ln = Dy and Y is not beneficial for ionic transport with the basic layered parent material having lower migration barriers. Overall, the key criterion for low migration barriers is an expanded ab plane, supported by Ba, coupled to a small Ln size. Hence, Ln = Y should be the best composition, but this is stymied by the low temperature 2D vacancy ordering and moderate temperature stability. The evolution of the oxygen cycling capability of these materials is also reported.

Insights into Oxygen Migration in LaBaCo2O6-δ Perovskites from In Situ Neutron Powder Diffraction and Bond Valence Site Energy Calculations.

Layered cobalt oxide perovskites are important mixed ionic and electronic conductors. Here, we investigate LaBaCo2O6-δ using in situ neutron powder diffraction. This composition is unique because it can be prepared in cubic, layered, and vacancy-ordered forms. Thermogravimetric analysis and diffraction reveal that layered and disordered samples have near-identical oxygen cycling capacities. Migration barriers for oxide ion conduction calculated using the bond valence site energy approach vary from E b ∼ 2.8 eV for the cubic perovskite to E b ∼ 1.5 eV for 2D transport in the layered system. Vacancy-ordered superstructures were observed at low temperatures, 350-400 °C for δ = 0.25 and δ = 0.5. The vacancy ordering at δ = 0.5 is different from the widely reported structure and involves oxygen sites in both CoO2 and LaO planes. Vacancy ordering leads to the emergence of additional migration pathways with low-energy barriers, for example, 1D channels with E b = 0.5 eV and 3D channels with E b = 2.2 eV. The emergence of these channels is caused by the strong orthorhombic distortion of the crystal structure. These results demonstrate that there is potential scope to manipulate ionic transport in vacancy-ordered LnBaCo2O6-δ perovskites with reduced symmetry.

Effect of light scattering on upconversion photoluminescence quantum yield in microscale-to-nanoscale materials.

Scattering affects excitation power density, penetration depth and upconversion emission self-absorption, resulting in particle size -dependent modifications of the external photoluminescence quantum yield (ePLQY) and net emission. Micron-size NaYF4:Yb3+, Er3+ encapsulated phosphors (∼4.2 µm) showed ePLQY enhancements of >402%, with particle-media refractive index disparity (Δn): 0.4969, and net emission increases of >70%. In sub-micron phosphor encapsulants (∼406 nm), self-absorption limited ePLQY and emission as particle concentration increases, while appearing negligible in nanoparticle dispersions (∼31.8 nm). These dependencies are important for standardising PLQY measurements and optimising UC devices, since the encapsulant can drastically enhance UC emission.

Chemically activated high grade nanoporous carbons from low density renewable biomass (Agave sisalana) for the removal of pharmaceuticals.

HYPOTHESIS: Enlarging the range of viable nanoporous carbon precursors, namely by the acid treatment of low density biomass residues, can overcome issues related with the availability and quality of raw materials that have potential impact on cost and quality grade of the final product. EXPERIMENTS: Nanoporous carbons were prepared following a two-step process: H2SO4 digestion/polycondensation of biomass waste (Agave sisalana, sisal) at temperature below 100 °C and atmospheric pressure to obtain acid-chars that were further chemically activated with KOH or K2CO3. Selected synthesized nanoporous carbons were tested for the removal of pharmaceutical compounds - ibuprofen and iopamidol - in aqueous solutions. FINDINGS: The structure and density of the acid-chars are highly dependent on the concentration of H2SO4 used in the digestion and polycondensation steps. An adequate choice of the acid-char synthesis conditions, activating agent and contact method allowed to feature nanoporous carbons with specific surface areas ranging from 600 to 2300 m2 g-1 and apparent densities reaching 600 kg m-3. The adsorption capacity of a sample obtained by KOH-activation for the removal of micropollutants from water was twice higher than the value attained by a golden activated carbon (Cabot-Norit) commercialized for this specific purpose.