Mg-doped ZnS hierarchical spheres have been synthesized via hydrothermal method using mixed solvents of ethylenediamine and DI water without any surface-active agent. The surface morphology and microstructure studies revealed that the hierarchical spheres were consisted of many well-aligned nanosheets with width 10 nm and length about 50 ~ 100 nm. X-Ray diffraction results show that the ZnS:Mg hierarchical spheres have wurtzite structure with high crystallinity. The absorption edge in the diffuse reflection spectra shifts towards lower wavelength with increasing Mg concentration, indicating an expansion in the bandgap energy that is estimated to be in the range of 3.28 to 3.47 eV. Blue-green photoluminescence with tunable intensity and peak position was observed depending on the Mg content. The Mg2+-activated ZnS phosphor can be good candidates for blue-green components in near-UV white light-emitting diodes.
Polycrystalline Ca 2 Fe 2Àx Al x O 5 (x = 0-1.4) samples were prepared by conventional solid-state reactions. Their crystalline/electronic structures and magnetic properties were characterized in detail. Powder X-ray diffraction analyses revealed that the samples crystallized in orthorhombic brownmillerite-type structures with the occurrence of the Pcmn-Ibm2 phase separation in the region between x = 0.4 and 0.6. The results obtained from analyzing Raman scattering and X-ray-absorption fine-structure spectra also indicated this phase separation. Although x in Ca 2 Fe 2Àx Al x O 5 varies in a wide range from 0 to 1.4, the +3 oxidation state of Fe remained almost unchanged. Magnetization measurements revealed that all Ca 2 Fe 2Àx Al x O 5 samples have weak ferromagnetic order, and both the saturation magnetization and coercive force are dependent on the temperature, x, and structure phases.
A phase transformation from insulating brownmillerite SrCoO2.5 to conducting perovskite SrCoO3 through electrochemical oxidation has been demonstrated for thin films of SrCoOx on a SrTiO3 (001) substrate. The cobalt-oxide film strongly favors the brownmillerite phase of SrCoO2.5 instead of the perovskite phase of SrCoO3 on a SrTiO3 (001) substrate due to its low lattice mismatch. Therefore, the phase transformation has its own retention. The alkaline water electrolysis occurs between the copper cathode and the SrCoO2.5 film anode. The H + ions are attracted to the cathode and generate H2 gas. The OH − ions are attracted to the film's surface and generate a rich amount of oxygen to fill the oxygen vacancy channel of brownmillerite SrCoO2.5. The phase transformation was verified from the change in the out-of-plane lattice constant and the change in the resistivity of the electrolyzed film.
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