Effects of Alkali Cations and Sulfate/Chloride Anions on the Flux Growth of {001}-Faceted β‑Li₂TiO₃ Crystals

The β-Li₂TiO₃ crystal is an important material in several energy-related applications, and the control of its morphology and exposed facets is an important issue. Herein, we comprehensively studied the flux growth of β-Li₂TiO₃ crystals under different conditions and demonstrated the efficient anisotropic growth of {001}-faceted β-Li₂TiO₃ single crystals from the Na₂SO₄ flux. By examining the effects of the cation and anion in the alkali metal salt-based flux, we found that Na₂SO₄ flux is best for growing large, faceted β-Li₂TiO₃ crystals. In this flux at 1000 °C, the optimal solute concentration is 20 mol % for growing large (∼15.0 μm in lateral size), platy, and faceted β-Li₂TiO₃ crystals. Observations from varying the holding time and cooling rate indicated that these crystals were anisotropically grown. Transmission electron microscopy images with clear electron diffraction spots revealed that the flux-grown platy β-Li₂TiO₃ crystals are single crystalline solids with the {001} plane being the dominant facet. This anisotropic crystal growth could be attributed to the concerted effects of the preferential attachment of sodium cations on the {001} faces, and efficient dissolution of β-Li₂TiO₃ crystals as well as the solvation of the resulting O^2– ions in the sulfate anion-based flux

Fabrication of La₂Ti₂O₇ Crystals Using an Alkali-Metal Molybdate Flux Growth Method and Their Nitridability To Form LaTiO₂N Crystals under a High-Temperature NH₃ Atmosphere

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 (La₂Ti₂O₇) crystals with different morphologies were grown using the Na₂MoO₄, K₂MoO₄, NaCl, and mixed NaCl + K₂MoO₄ (molar ratio = 3:7) fluxes, and their nitridability to form LaTiO₂N crystals under a high-temperature NH₃ atmosphere was also investigated. The effects of the solute concentration and cooling rate on the growth of the La₂Ti₂O₇ crystals were also studied. The X-ray diffraction results revealed that the {100} plane was dominant in the La₂Ti₂O₇ 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 La₂Ti₂O₇ crystals with the preferred ⟨010⟩ and ⟨001⟩ growth directions along the <i>b</i> and <i>c</i> axes were grown using the Na₂MoO₄ and K₂MoO₄ fluxes, respectively. Compared to the Na₂MoO₄ flux, the K₂MoO₄ flux did not show a cooling-rate-dependent effect on the growth of the La₂Ti₂O₇ crystals. It was found that conversion of the La₂Ti₂O₇ crystals to the LaTiO₂N crystals was strongly dependent on the flux used to grow the precursor La₂Ti₂O₇ crystals. That is, the La₂Ti₂O₇ crystals grown using the K₂MoO₄ and NaCl fluxes were nearly completely converted into the LaTiO₂N crystals, while conversion of the La₂Ti₂O₇ crystals grown using the Na₂MoO₄ and mixed NaCl + K₂MoO₄ fluxes to the LaTiO₂N crystals seemed to be not completed yet even after nitridation at 950 °C for 15 h using NH₃ because of the larger crystal size and the presence of unintentional impurities (sodium and molybdenum from the flux) in the La₂Ti₂O₇ crystal lattice. Nevertheless, the LaTiO₂N crystals fabricated by nitriding the La₂Ti₂O₇ crystals grown using the K₂MoO₄ and NaCl fluxes should be suitable for direct solar water splitting

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

Protonated Oxide, Nitrided, and Reoxidized K₂La₂Ti₃O₁₀ Crystals: Visible-Light-Induced Photocatalytic Water Oxidation and Fabrication of Their Nanosheets

Protonated lanthanum titanium oxide H₂La₂Ti₃O₁₀ and oxynitride H₂La₂Ti₃O_{10–3/2<i>x</i>}N_<i>x</i> crystals were synthesized from the oxide, nitrided, and reoxidized layered K₂La₂Ti₃O₁₀ crystals prepared by solid-state reaction through proton exchange. Here, we investigated the holding time of nitridation of oxide K₂La₂Ti₃O₁₀ crystals influencing their crystal structure, shape, and absorption wavelength and band gap energy. The XRD and SEM results confirmed that the crystal structure and plate-like shape of the parent oxide were maintained after nitridation at 800 °C for 10 h, and the color of crystals was changed from white to dark green. However, no clear absorption edges were observed in the UV–vis diffuse reflectance spectra of the nitrided crystals due mainly to the reduced titanium species (Ti³⁺), which act as the recombination center of the photogenerated charge carriers. To decrease the amount of the reduced titanium species, the nitrided crystals were further reoxidized at 400 °C for 6 h. After partial reoxidation, the absorption intensity in the longer wavelength region was reduced, and the absorption edges appeared at about 449–460 nm. The photocatalytic activity for the water oxidation half-reaction was evaluated only for the protonated samples. The protonated reoxidized K₂La₂Ti₃O₁₀ crystals showed the O₂ evolution rate of 180 nmol·h^–1 (for the photocatalytic water oxidation) under visible-light irradiation, and the unexpected photocatalytic decomposition of N₂O adsorbed onto the photocatalyst surfaces was observed for the protonated oxide and protonated nitrided layered K₂La₂Ti₃O₁₀ crystals. Furthermore, lanthanum titanium oxide [La₂Ti₃O₁₀]^2– and oxynitride [La₂Ti₃O_{10–3/2<i>x</i>}N_<i>x</i>]^2– nanosheets were successfully fabricated by proton exchange and mechanical exfoliation (sonication) of the oxide, nitrided, and reoxidized K₂La₂Ti₃O₁₀ crystals. The TEM results revealed that the lateral sizes of the fabricated nanosheets grown along the ⟨001⟩ direction are 270–620 nm. Apparently, the colloidal suspensions of the fabricated nanosheets showed a Tyndall effect, implying their good dispersion and stability for several weeks in water

Flux-Mediated Topochemical Growth of Platelet-Shaped Perovskite LiNbO₃ Single Crystals from Layered Potassium Niobate Crystals

Well-defined, platelet-shaped LiNbO₃ single crystals were prepared by a flux-mediated topochemical reaction from platelet K₄Nb₆O₁₇ crystals with a mixture of LiNO₃ solute and alkali metal nitrate (LiNO₃, NaNO₃, and KNO₃) fluxes at 600 °C. Crystallographic structural characterizations revealed that the LiNbO₃ crystals inherited the K₄Nb₆O₁₇ crystal shape with well-developed {012} faces. The topochemical reaction in molten KNO₃ promoted smooth surface formation, in contrast to LiNO₃ and NaNO₃, which formed rough surfaces. We further found that the dissolution and deposition reaction occurs repeatedly in the vicinity of the LiNbO₃ crystal and KNO₃ flux interface. It is considered that the KNO₃ flux hybridized with the LiNO₃ solute provides a moderate solubility and dissolution rate suitable for crystal growth. We also achieved reed-shaped LiNbO₃ crystal growth by applying the same technique to the KNb₃O₈ crystal template

Environmentally Friendly Flux Growth of High-Quality, Idiomorphic Li₅La₃Nb₂O₁₂ Crystals

High-quality, idiomorphic, single-phase Li₅La₃Nb₂O₁₂ 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 Li₅La₃Nb₂O₁₂ 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 Li₅La₃Nb₂O₁₂ 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 Li₅La₃Nb₂O₁₂ crystals were obviously dependent on the growth conditions including the solute concentration and holding temperature

Chloride Flux Growth of La₂Ti₂O₇ Crystals and Subsequent Nitridation To Form LaTiO₂N Crystals

Highly crystalline, platelike La₂Ti₂O₇ were grown from a NaCl flux, and LaTiO₂N crystals were obtained by subsequent nitridation under NH₃ flow. The TEM analysis indicated that the flux-grown platelike La₂Ti₂O₇ crystals are single-crystalline growing along the <i>a</i> axis. The shapes and sizes of the LaTiO₂N crystals were almost unchanged from the La₂Ti₂O₇ precursor. In addition, LaTiO₂N crystals remained single-crystalline with a porous nanostructure. The optical absorption edges of the La₂Ti₂O₇ and LaTiO₂N crystals were approximately 320 and 600 nm

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

NH₃‑Assisted Flux Growth of Cube-like BaTaO₂N Submicron Crystals in a Completely Ionized Nonaqueous High-Temperature Solution and Their Water Splitting Activity

As the 600 nm-class photocatalyst, BaTaO₂N 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 NH₃ 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 NH₃-assisted KCl flux growth approach for the direct synthesis of BaTaO₂N crystals. The effects of various fluxes, solute concentration, and reaction time and temperature on the phase evolution and morphology transformation of the BaTaO₂N crystals were systematically investigated. By changing the solute concentration from 10 to 50 mol %, it was found that phase-pure BaTaO₂N 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 BaTaO₂N 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 BaTaO₂N submicron crystals. The BaTaO₂N crystals modified with Pt and CoO_<i>x</i> nanoparticles showed a reasonable H₂ and O₂ evolution, respectively, due to a lower defect density and higher crystallinity achieved by an NH₃-assisted KCl flux method

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

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 KNO₃–LiNO₃ 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