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

Fabrication of NIR-Vis Upconversion YO_1–<i>x</i>F_1+2<i>x</i>:Ln (Ln = Yb, Er, Ho, and Tm) Crystal Layers by Flux Coating and Investigation of Growth Manner

High-quality upconversion YO_1–<i>x</i>F_1+2<i>x</i>:Ln (Ln = Yb, Er, Ho, and Tm) layers with good adhesion, comprising densely packed, plate-shaped idiomorphic crystals, were directly grown on stainless steel (SUS) substrates by flux coating. The LiNO₃–KNO₃ flux effectively promoted crystal growth. Additionally, the near-infrared-to-visible (NIR-vis) upconversion fluorescence properties of the YO_1–<i>x</i>F_1+2<i>x</i>:Ln crystal layers could be tuned by varying the type of dopant (Ln element). YO_1–<i>x</i>F_1+2<i>x</i>:Yb,Er, YO_1–<i>x</i>F_1+2<i>x</i>:Yb,Ho, and YO_1–<i>x</i>F_1+2<i>x</i>:Yb,Tm crystal layers showed red, green, and blue fluorescence, respectively, under 980 nm laser irradiation

Fabrication of NIR-Vis Upconversion YO_1–<i>x</i>F_1+2<i>x</i>:Ln (Ln = Yb, Er, Ho, and Tm) Crystal Layers by Flux Coating and Investigation of Growth Manner

High-quality upconversion YO_1–<i>x</i>F_1+2<i>x</i>:Ln (Ln = Yb, Er, Ho, and Tm) layers with good adhesion, comprising densely packed, plate-shaped idiomorphic crystals, were directly grown on stainless steel (SUS) substrates by flux coating. The LiNO₃–KNO₃ flux effectively promoted crystal growth. Additionally, the near-infrared-to-visible (NIR-vis) upconversion fluorescence properties of the YO_1–<i>x</i>F_1+2<i>x</i>:Ln crystal layers could be tuned by varying the type of dopant (Ln element). YO_1–<i>x</i>F_1+2<i>x</i>:Yb,Er, YO_1–<i>x</i>F_1+2<i>x</i>:Yb,Ho, and YO_1–<i>x</i>F_1+2<i>x</i>:Yb,Tm crystal layers showed red, green, and blue fluorescence, respectively, under 980 nm laser irradiation

Low-Temperature Flux Growth and Upconversion Fluorescence of the Idiomorphic Hexagonal-System NaYF₄ and NaYF₄:Ln (Ln = Yb, Er, Tm) Crystals

Idiomorphic NaYF₄ and NaYF₄:Ln (Ln = Yb, Er, Tm) crystals with upconversion fluorescence were successfully grown by the NaNO₃ flux cooling method at a relatively low holding temperature. The grown NaYF₄ and NaYF₄:Ln crystals had a hexagonal prismatic form, and their well-developed surfaces were relatively flat. TEM images indicated that the NaYF₄ crystals were of good crystallinity. Their size and shape were relatively uniform, and they were poorly aggregated. The crystal phase, form, and size depended on the growth temperature and the solute concentration. In contrast, the addition of dopant ions (Yb³⁺, Er³⁺, and Tm³⁺) did not affect the shape, morphology, or crystallinity of the flux-grown NaYF₄:Ln crystals. Additionally, the upconversion fluorescence properties of NaYF₄:Ln crystals were also dependent on the type and mixture ratio (i.e., starting composition) of the dopants. The green, orange, and blue upconversion fluorescences of NaYF₄:10%Yb,1%Er, NaYF₄:50%Yb,1%Er, and NaYF₄:10%Yb,1%Tm crystals, respectively, were observed under 980 nm laser irradiation via two- or three-photon upconversion processes