Growth Manner of Octahedral-Shaped Li(Ni_1/3Co_1/3Mn_1/3)O₂ Single Crystals in Molten Na₂SO₄

The synthesis of shape-controlled crystals has been a highly attractive research topic in modern materials chemistry. In this work, the growth of Li­(Ni_1/3Co_1/3Mn_1/3)­O₂ (NCM) crystals in molten sulfate or carbonate salts (flux) at 1000 °C was systematically studied under various conditions. In situ X-ray diffraction during the growth and thermogravimetry-differential thermal analysis revealed that the growth of NCM crystals in the flux was controlled by liquid-phase sintering according to the Ostwald ripening principle. We studied the effect of Na⁺ in the flux on the crystal shapes and found that Na⁺ was critical in forming octahedral crystals with well-developed facets. Single crystals with well-developed facets were obtained homogeneously from Na₂SO₄, while truncated polyhedral crystals of smaller size were obtained from Li₂SO₄. The shape-controlled NCM crystals showed discharge capacities approaching 160 mAh g^–1 in the operating voltage range of 2.8–4.4 V vs Li/Li⁺ under a low current density of 0.1 C, independent of flux composition. This suggests that the Li⁺ and transition-metal ions in the individual NCM crystals were highly ordered into hexagonal arrangements belonging to the <i>R</i>3̅<i>m</i> space group, without cation mixing

Effect of Side-Plane Width on Lithium-Ion Transportation in Additive-Free LiCoO₂ Crystal Layer-Based Cathodes for Rechargeable Lithium-Ion Batteries

Rechargeable lithium-ion batteries (LIBs) with improved performance, including higher power, higher energy density, and superior cycling performance, are in demand for highly specialized applications. The design and synthesis of cathode materials with effective shapes surrounded by suitable faces for Li-ion transportation are essential for high-performance LIBs. In the present work, we address the effects of side-plane width on the Li-ion diffusion coefficient and charge transfer resistance of additive-free LiCoO₂ electrodes composed of highly controlled crystals with exposed {101}, {012}, and {104} faces. The changes in the shape and order of orientation in the LiCoO₂ crystals were evaluated using electron microscopic observations and X-ray diffraction measurements. The discharge capacity drastically increased from 87.9 to 131.5 mA·h·g^–1 on increasing the width of the side plane in the LiCoO₂ crystal from ca. 11 to ca. 59 nm. Further, electrochemical impedance spectroscopy revealed that the Li-ion diffusion coefficient and charge transfer resistance of the electrodes were related to the dimensions of the exposed {101}, {012}, and {104} faces. Thus, increasing the crystal faces available for Li-ion transfer provides layer-based cathode materials with increased discharge capacities

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

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

Photon Upconverted Emission Based on Dye-Sensitized Triplet–Triplet Annihilation in Silica Sol–Gel System

Photon upconverted emission based on dye-sensitized triplet–triplet annihilation was observed in silica gel systems containing Pt­(II) octaethylporphyrin (triplet sensitizer) and 9,10-diphenylanthracene (singlet emitter). The triplet sensitizer was encapsulated and highly dispersed in the silica gels prepared by the sol–gel method. The singlet emitter was adsorbed on the silica gel pores accessible to the outside. Phosphorescence of the triplet sensitizer was partially quenched, and the singlet emission was enhanced with an increase in the concentration of the singlet emitter. The emission intensity increased in proportion to the approximate square of the irradiation power. The triplet energy transfer from some of the encapsulated triplet sensitizer molecules to the adsorbed singlet emitter molecules was observed in the silica gels followed by the triplet–triplet annihilation and upconverted singlet emission. The phosphorescence quenching and upconverted singlet emission were more significantly observed in the gel dried at a lower temperature (wetter gel). The wetter gel contained higher amounts of solvent and water molecules, in which the triplet sensitizer and singlet emitter should collide and then the sensitized emitters should collide between themselves during their excited-state lifetime. The photon upconversion process required the triplet sensitizer and singlet emitter molecules to be in an environment similar to the solvents

Formation Process of Eosin Y‑Adsorbing ZnO Particles by Electroless Deposition and Their Photoelectric Conversion Properties

The thin films consisting of crystalline ZnO particles were prepared on fluorine-doped tin oxide electrodes by electroless deposition. The particles were deposited from an aqueous solution containing zinc nitrate, dimethyamine–borane, and eosin Y at 328 K. As the Pd particles were adsorbed on the substrate, not only the eosin Y monomer but also the dimer and debrominated species were rapidly adsorbed on the spherical ZnO particles, which were aggregated and formed secondary particles. On the other hand, in the absence of the Pd particles, the monomer was adsorbed on the flake-shaped ZnO particles, which vertically grew on the substrate surface and had a high crystallinity. The photoelectric conversion efficiency was higher for the ZnO electrodes containing a higher amount of the monomer during light irradiation

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

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

Titanium Complex Formation of Organic Ligands in Titania Gels

Thin films of organic ligand-dispersing titania gels were prepared from titanium alkoxide sols containing ligand molecules by steam treatment without heating. The formation of the ligand–titanium complex and the photoinduced electron transfer process in the systems were investigated by photoelectrochemical measurements. The complex was formed between the 8-hydroxyquinoline (HQ) and titanium species, such as the titanium ion, on the titania nanoparticle surface through the oxygen and nitrogen atoms of the quinolate. A photocurrent was observed in the electrodes containing the complex due to the electron injection from the LUMO of the complex into the titania conduction band. A bidentate ligand, 2,3-dihydroxynaphthalene (DHN), formed the complex on the titania surface through dehydration between its two hydroxyl groups of DHN and two TiOH groups of the titania. The electron injection from the HOMO of DHN to the titania conduction band was observed during light irradiation. This direct electron injection was more effective than the two-step electron injection