Facile Morphological Modification of Ba₅Nb₄O₁₅ Crystals Using Chloride Flux and in Situ Growth Investigation

The cation-deficient layered perovskite oxide Ba₅Nb₄O₁₅ is one of the functional materials that exhibits a microwave-responsive dielectric property and an ultraviolet-active photocatalytic property. Although systematic control of the morphology of Ba₅Nb₄O₁₅ is beneficial for improving these properties, synthesized Ba₅Nb₄O₁₅ usually has a plate-like shape owing to its crystal structure, with a particle size less than 5 μm. For systematic morphological control of Ba₅Nb₄O₁₅, the crystal growth was studied by using a chloride-based flux method. Idiomorphic plate-like Ba₅Nb₄O₁₅ crystals up to 50 μm in size and polyhedron ones ∼10 μm in size were obtained using a BaCl₂ flux by changing the solute concentration to 5–20 mol % and 50 mol %, respectively. The growth of the Ba₅Nb₄O₁₅ crystals was investigated by thermogravimetric and differential thermal analysis and in situ X-ray diffraction analysis. These analyses revealed the flux-growth manner of Ba₅Nb₄O₁₅ as follows: (I) Ba₅Nb₄O₁₅ was formed by a solid-state reaction above ∼650 °C. (II) After the melting of BaCl₂ above ∼962 °C, the Ba₅Nb₄O₁₅ crystals became larger and assumed idiomorphic shapes, indicating that they were somewhat dissolved in the flux and that the crystal growth was promoted. Increasing the holding time yielded an increased number of crystals larger than 28 μm. This indicates that Ostwald ripening effectively yields Ba₅Nb₄O₁₅ crystals up to 50 μm in size. Chloride fluxes with different alkaline or alkaline earth cation fluxes did not produce such large crystals. It is assumed that the common ion effect of Ba²⁺ in the solute and flux provides an effective reaction field to facilitate Ostwald ripening

Template-Assisted Size Control of Polycrystalline BaNbO₂N Particles and Effects of Their Characteristics on Photocatalytic Water Oxidation Performances

The photocatalytic water oxidation using solar irradiation is a sustainable way to convert a natural source to energy. The perovskite-type oxynitride BaNbO₂N is a candidate photocatalyst for this process because its long-range light absorbance of up to ca. 740 nm leads to the high ability of energy conversion. However, it is necessary to improve its poor performance by optimizing the crystallographic characteristics, chemical formula, depositions of cocatalysts, and so on. In this study, we aimed to identify the dominant factors of the photocatalytic performance of BaNbO₂N. We controlled the particle characteristics by nitriding size-controlled Ba₅Nb₄O₁₅ crystals in sizes of 0.2–50 μm as sacrificial templates. Porous BaNbO₂N secondary particles of different sizes were achieved, and they exhibited distinctive photocatalytic performances for O₂ evolution with rates between 14.1 and 113.9 μmol·h^–1, depending on the precursor size and nitriding time. By correlating the performance with the basal particle characteristics, we assume that the crystallinity and anion deficiency are the two dominant factors that competitively affect the photocatalytic performance of BaNbO₂N