15 research outputs found

    Fabrication of La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> Crystals Using an Alkali-Metal Molybdate Flux Growth Method and Their Nitridability To Form LaTiO<sub>2</sub>N Crystals under a High-Temperature NH<sub>3</sub> Atmosphere

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    Flux growth is a promising method that allows one to control over the crystalline phase, crystal shape, crystal size, and crystal surface through the selection of a suitable flux. In this work, lanthanum titanate (La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub>) crystals with different morphologies were grown using the Na<sub>2</sub>MoO<sub>4</sub>, K<sub>2</sub>MoO<sub>4</sub>, NaCl, and mixed NaCl + K<sub>2</sub>MoO<sub>4</sub> (molar ratio = 3:7) fluxes, and their nitridability to form LaTiO<sub>2</sub>N crystals under a high-temperature NH<sub>3</sub> atmosphere was also investigated. The effects of the solute concentration and cooling rate on the growth of the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals were also studied. The X-ray diffraction results revealed that the {100} plane was dominant in the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> platelet crystals grown using the alkali-metal molybdate fluxes. When the solute concentration was increased from 1 to 20 mol %, the average size of the crystals decreased without considerable alteration of the overall crystal morphology. The La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals with the preferred ⟨010⟩ and ⟨001⟩ growth directions along the <i>b</i> and <i>c</i> axes were grown using the Na<sub>2</sub>MoO<sub>4</sub> and K<sub>2</sub>MoO<sub>4</sub> fluxes, respectively. Compared to the Na<sub>2</sub>MoO<sub>4</sub> flux, the K<sub>2</sub>MoO<sub>4</sub> flux did not show a cooling-rate-dependent effect on the growth of the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals. It was found that conversion of the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals to the LaTiO<sub>2</sub>N crystals was strongly dependent on the flux used to grow the precursor La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals. That is, the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals grown using the K<sub>2</sub>MoO<sub>4</sub> and NaCl fluxes were nearly completely converted into the LaTiO<sub>2</sub>N crystals, while conversion of the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals grown using the Na<sub>2</sub>MoO<sub>4</sub> and mixed NaCl + K<sub>2</sub>MoO<sub>4</sub> fluxes to the LaTiO<sub>2</sub>N crystals seemed to be not completed yet even after nitridation at 950 °C for 15 h using NH<sub>3</sub> because of the larger crystal size and the presence of unintentional impurities (sodium and molybdenum from the flux) in the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystal lattice. Nevertheless, the LaTiO<sub>2</sub>N crystals fabricated by nitriding the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals grown using the K<sub>2</sub>MoO<sub>4</sub> and NaCl fluxes should be suitable for direct solar water splitting

    Template-Assisted Size Control of Polycrystalline BaNbO<sub>2</sub>N Particles and Effects of Their Characteristics on Photocatalytic Water Oxidation Performances

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    The photocatalytic water oxidation using solar irradiation is a sustainable way to convert a natural source to energy. The perovskite-type oxynitride BaNbO<sub>2</sub>N 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 BaNbO<sub>2</sub>N. We controlled the particle characteristics by nitriding size-controlled Ba<sub>5</sub>Nb<sub>4</sub>O<sub>15</sub> crystals in sizes of 0.2–50 μm as sacrificial templates. Porous BaNbO<sub>2</sub>N secondary particles of different sizes were achieved, and they exhibited distinctive photocatalytic performances for O<sub>2</sub> evolution with rates between 14.1 and 113.9 μmol·h<sup>–1</sup>, 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 BaNbO<sub>2</sub>N

    Fabrication of NIR-Vis Upconversion YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Ln (Ln = Yb, Er, Ho, and Tm) Crystal Layers by Flux Coating and Investigation of Growth Manner

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    High-quality upconversion YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>: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 LiNO<sub>3</sub>–KNO<sub>3</sub> flux effectively promoted crystal growth. Additionally, the near-infrared-to-visible (NIR-vis) upconversion fluorescence properties of the YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Ln crystal layers could be tuned by varying the type of dopant (Ln element). YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Er, YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Ho, and YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Tm crystal layers showed red, green, and blue fluorescence, respectively, under 980 nm laser irradiation

    Fabrication of NIR-Vis Upconversion YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Ln (Ln = Yb, Er, Ho, and Tm) Crystal Layers by Flux Coating and Investigation of Growth Manner

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    High-quality upconversion YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>: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 LiNO<sub>3</sub>–KNO<sub>3</sub> flux effectively promoted crystal growth. Additionally, the near-infrared-to-visible (NIR-vis) upconversion fluorescence properties of the YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Ln crystal layers could be tuned by varying the type of dopant (Ln element). YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Er, YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Ho, and YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Tm crystal layers showed red, green, and blue fluorescence, respectively, under 980 nm laser irradiation

    Flux-Assisted Fabrication of Vertically Aligned Layered Double Hydroxide Plates on in Situ Formed Alumina Particles

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    Layered double hydroxide (LDH) is an environmentally benign anion exchanger that can adsorb various toxic anions. In this work, we demonstrate the fabrication of plate-like Mg–Al-type LDH crystals on in situ formed alumina particles using a flux method at a relatively low temperature (∼350 °C). At or below 300 °C, the melted Al source crystallized to form AlOOH or γ-alumina particles in KNO<sub>3</sub>–NaNO<sub>3</sub> flux. However, LDH crystals did not form due to the inferior crystallization properties of the Mg precursor. Increasing the holding temperature up to 350 °C and above facilitated crystallization of the dissolved Mg and Al species in flux to yield plate-like LDH crystals on the preformed alumina particles. Top-surface and cross-sectional FE-SEM and EPMA analyses revealed the vertical alignment of the crystalline LDH plates on the surface of the alumina particles. On the other hand, solid-state reactions did not yield these well-grown, plate-like LDH crystals. The TG-DTA profile of the LDH precursors with flux depicted the decomposition and crystallization events that the Al and Mg precursors undergo. On the basis of the results from these characterization studies, we propose a mechanism in which LDH crystals sequentially form on the surface of the alumina particles

    Environmentally Friendly Flux Growth of High-Quality, Idiomorphic Li<sub>5</sub>La<sub>3</sub>Nb<sub>2</sub>O<sub>12</sub> Crystals

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    High-quality, idiomorphic, single-phase Li<sub>5</sub>La<sub>3</sub>Nb<sub>2</sub>O<sub>12</sub> crystals were successfully grown using a LiOH flux cooling method at the relatively low temperature of 500 °C at a solute concentration of 5 mol %. The grown Li<sub>5</sub>La<sub>3</sub>Nb<sub>2</sub>O<sub>12</sub> crystals had polyhedral shapes with well-developed, flat {211} and {110} faces. Their shapes were relatively uniform, and the average crystal size was approximately 59.2 μm. No aggregation was observed in scanning electron microscopy images. The high crystallinity of the Li<sub>5</sub>La<sub>3</sub>Nb<sub>2</sub>O<sub>12</sub> crystals was confirmed by transmission electron microscopy images. Their lattice parameter was determined from the X-ray diffraction pattern to be <i>a</i> = 1.281 nm, which is consistent with the literature value (<i>a</i> = 1.282 nm). Furthermore, the crystal phase, form, size, and crystallinity of the flux-grown Li<sub>5</sub>La<sub>3</sub>Nb<sub>2</sub>O<sub>12</sub> crystals were obviously dependent on the growth conditions including the solute concentration and holding temperature

    Chloride Flux Growth of La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> Crystals and Subsequent Nitridation To Form LaTiO<sub>2</sub>N Crystals

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    Highly crystalline, platelike La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> were grown from a NaCl flux, and LaTiO<sub>2</sub>N crystals were obtained by subsequent nitridation under NH<sub>3</sub> flow. The TEM analysis indicated that the flux-grown platelike La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals are single-crystalline growing along the <i>a</i> axis. The shapes and sizes of the LaTiO<sub>2</sub>N crystals were almost unchanged from the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> precursor. In addition, LaTiO<sub>2</sub>N crystals remained single-crystalline with a porous nanostructure. The optical absorption edges of the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> and LaTiO<sub>2</sub>N crystals were approximately 320 and 600 nm

    Low-Temperature Flux Growth and Upconversion Fluorescence of the Idiomorphic Hexagonal-System NaYF<sub>4</sub> and NaYF<sub>4</sub>:Ln (Ln = Yb, Er, Tm) Crystals

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    Idiomorphic NaYF<sub>4</sub> and NaYF<sub>4</sub>:Ln (Ln = Yb, Er, Tm) crystals with upconversion fluorescence were successfully grown by the NaNO<sub>3</sub> flux cooling method at a relatively low holding temperature. The grown NaYF<sub>4</sub> and NaYF<sub>4</sub>:Ln crystals had a hexagonal prismatic form, and their well-developed surfaces were relatively flat. TEM images indicated that the NaYF<sub>4</sub> 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 (Yb<sup>3+</sup>, Er<sup>3+</sup>, and Tm<sup>3+</sup>) did not affect the shape, morphology, or crystallinity of the flux-grown NaYF<sub>4</sub>:Ln crystals. Additionally, the upconversion fluorescence properties of NaYF<sub>4</sub>: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 NaYF<sub>4</sub>:10%Yb,1%Er, NaYF<sub>4</sub>:50%Yb,1%Er, and NaYF<sub>4</sub>:10%Yb,1%Tm crystals, respectively, were observed under 980 nm laser irradiation via two- or three-photon upconversion processes

    NH<sub>3</sub>‑Assisted Flux Growth of Cube-like BaTaO<sub>2</sub>N Submicron Crystals in a Completely Ionized Nonaqueous High-Temperature Solution and Their Water Splitting Activity

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    As the 600 nm-class photocatalyst, BaTaO<sub>2</sub>N is one of the promising candidates of the perovskite-type oxynitride family for photocatalytic water splitting under visible light. The oxynitrides are routinely synthesized by nitriding corresponding oxide precursors under a high-temperature NH<sub>3</sub> atmosphere, causing an increase in the defect density and a decrease in photocatalytic activity. To improve the photocatalytic activity by reducing the defect density and improving the crystallinity, we here demonstrate an NH<sub>3</sub>-assisted KCl flux growth approach for the direct synthesis of BaTaO<sub>2</sub>N crystals. The effects of various fluxes, solute concentration, and reaction time and temperature on the phase evolution and morphology transformation of the BaTaO<sub>2</sub>N crystals were systematically investigated. By changing the solute concentration from 10 to 50 mol %, it was found that phase-pure BaTaO<sub>2</sub>N crystals could only be grown with the solute concentrations of ≥10 mol % using the KCl flux, and the solute concentration of 10 mol % was solely favorable to directly grow cube-like BaTaO<sub>2</sub>N crystals with an average size of about 125 nm and exposed {100} and {110} faces at 950 °C for 10 h. The time- and temperature-dependent experiments were also performed to postulate the direct growth mechanisms of cube-like BaTaO<sub>2</sub>N submicron crystals. The BaTaO<sub>2</sub>N crystals modified with Pt and CoO<sub><i>x</i></sub> nanoparticles showed a reasonable H<sub>2</sub> and O<sub>2</sub> evolution, respectively, due to a lower defect density and higher crystallinity achieved by an NH<sub>3</sub>-assisted KCl flux method

    High-Quality Ultralong Hydroxyapatite Nanowhiskers Grown Directly on Titanium Surfaces by Novel Low-Temperature Flux Coating Method

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    Idiomorphic, one-dimensional (1-D), and high-quality hydroxyapatite (HAp) nanocrystals were successfully, directly, and densely grown on a Ti substrate at the relatively low temperature of 300 °C using a KNO<sub>3</sub>–LiNO<sub>3</sub> flux coating method. The grown HAp crystals have a 1-D shape with a very high aspect ratio (much larger than 100) and an average size of 3250 × 25 nm (length × width). The ultralong 1-D crystals grown at 300 °C were identified as highly crystalline HAp by their X-ray diffraction (XRD) patterns, which clearly displayed the four characteristic lines of HAp between 31.5° and 34.5°. Additionally, high-resolution transmission electron microscopy (HRTEM) images demonstrated that these ultralong whiskers were high-quality because point and line defects were not observed. From the energy-dispersive X-ray spectroscopy (EDS) analysis, major components were homogeneously distributed in the HAp whiskers. In addition, the effects of holding temperature and starting composition on the forms and average sizes of the grown HAp whiskers were investigated
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