This review briefly describes recent advances on the synthesis of metal hydroxide nanomaterials and their assembly into 3D architectures toward applications of catalysis, adsorption, and energy conversion. The discussion is mainly dedicated to the synthesis strategies toward nanohydroxide-based functional materials. A special attention is focused on two reaction schemes for the preparations of: (1) macroporous (hydr)oxides monoliths and (2) monodispersed hydroxide nanoclusters and mesoporous hydroxides.

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Controlling the dispersion stability of functional fine/nano-particles without forming strong uncontrollable aggregates and controlling their assembled structure during material processing is a powerful method to improve the properties of composite materials, such as ceramics and polymer nanocomposites. In this article, surface modification techniques for functional fine/nano-particles toward their homogeneous dispersion in solvents will be briefly reviewed. A suitable combination of surface-engineering protocols and the selection of surface modifiers comprising segments that effectively attach to the particle surface and possess high affinities toward solvents is one of the keys to achieve homogeneous dispersion of fine/nano-particles. Then, a concept using a series of polyethyleneimine (PEI) and fatty acid complexes as surface modifiers to control the dispersion stability of multi-component slurries will be reviewed. The dispersion stability of slurries can be tuned by simply controlling the structures of the PEI-fatty acid complex. Furthermore, processing methods to control the particle-assembly structures using the PEI-fatty acid complex will also be introduced.

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Using the flexible valence states of transition-metal-oxides (TMOs), the optical, electronic, and magnetic properties can be simultaneously controlled by electrochemical oxidation and reduction. Herein we review our recent works on the electrochemically switchable functional thin-film device, which has a three-terminal thin-film-transistor (TFT) structure with the TMO as an active channel layer and the liquid-leakage-free electrolyte as a gate insulator. Thin films of vanadium dioxide, tungsten trioxide, and strontium cobaltite were selected as the channel layer. By applying the gate voltage at room temperature in air, the protonation/deprotonation or oxidation/deoxidation occurs in the TMO channels and their opto-electronic and electro-magnetic properties are reversibly switched by using the mobile ions in the solid TFT structure. The present device with liquid-leakage-free electrolyte provides a novel design concept for the development of future multifunctional switching devices based on TMOs.

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The basic physico-chemical behaviors and laws of etching an alumina-based ceramic core out of investment castings still puzzle the process designer. To investigate the basis, ceramic removal experiments were performed at normal pressure by varying several main factors. The de-coring rate increased as the etching temperature increased. A higher alkali concentration was beneficial to the removal efficiency but had an upper limit because of the metal alkaline corrosion. The de-coring rate underwent a convex-curve variation as the etching time increased since the deposition of a validated insoluble product gradually hinders the etching reaction. The solute additive KCl and solvent additive ethanol have the positive effects of improving the reactivity of the base solution and increasing the permeability of the etching liquid, respectively. The de-coring rate was up to 10.46 wt %·h⁻¹ in a steady process with etching in a 70 wt % KOH solution supplemented with 10 wt % KCl and ethanol.

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Cr₂O₃ nanoparticles (NPs) were successfully prepared by a facile surfactant-modified precipitation method, and their phase composition and microstructure were characterized by X-ray diffraction, field emission-scanning electron microscopy and transmission electron microscopy. The effects of various processing parameters including surfactants type and amount, pH value and firing temperature on the synthesis of Cr₂O₃ NPs were investigated. Highly dispersed Cr₂O₃ NPs with an average crystalline size of 17 nm were obtained under the following optimal conditions: [Cr(NO₃)₃·9H₂O] = 0.0316 mol/L, PEG/Cr₂O₃ = 5 wt %, pH = 5.2, calcination temperature = 500°C and holding time = 2 h. Cr₂O₃ NPs showed a significant accelerating effect on the nitridation of Si powder, the conversion rate of Si to Si₃N₄ after 2 h at 1300°C reached 100 wt % upon using 3 wt % Cr₂O₃ NPs as catalysts, but only 40 wt % in the case without using any catalysts.

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Porous tubular TiO₂ powders were prepared first via a novel facile and non-template route of the nonhydrolytic sol–gel method. The as-prepared TiO₂ powders could then be directly converted into porous tubular TiN powders through an ammonia reduction nitridation process. The results indicated that the obtained TiO₂ powders consisted of a pure, well-crystallized anatase TiO₂ phase of porous tubular morphology, with a Brunauer–Emmett–Teller (BET) specific surface area of 39 m²/g and an average pore size of 11 nm and pore volume of 0.012 cm³/g. After the porous tubular TiO₂ powders were calcined at 800°C for 2 h under an ammonia (NH₃) gas atmosphere, TiN powders maintaining their porous tubular architecture were obtained, with a BET specific surface area of 35 m²/g and an average pore size and pore volume of 12 nm and 0.096 cm³/g, respectively. The electrochemical performance of the porous tubular TiN powders demonstrated that TiN powders could be a promising electrode material for supercapacitors.

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A new synthetic route to prepare yttrium barium copper oxide superconductive fibers using electrospinning in conjunction with the polymerizable complex method was developed. The as-spun fibers exhibited wool-like tactility and were 5–7 µm thick. The fibers therefore exhibited sufficient flexibility to be fabricated into arbitrary shapes. Although the fibers shrunk and exhibited brittle tactility because of the decomposition of organic compounds during heat treatment, the fibers predominantly maintained their fibrous form. Scanning electron microscopy observations revealed the growth of metal oxide grains during sintering. Powder X-ray diffraction pattern of the annealed fibers showed good agreement with the pattern of YBa₂Cu₃O_7−δ. Magnetic property measurements of a ground fiber sample using a superconducting quantum interference device revealed that the superconducting transition temperature of the sample was 91 K.

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Thin films and sintered bodies of Gd₂O₃ were prepared by alkoxide solution coating or uniaxial pressing of an oxide powder, followed by firing in a synthesized air atmosphere. Both samples were initially hydrophilic after firing. However, their water contact angles increased during storage in ambient air. The surface hydrophobicity was non-existent after ozonation, but it recovered during two weeks of ambient air storage. The hydrophobicization rate of Gd₂O₃ film was higher than that of SiO₂ film. It corresponded to the carbon concentration ratio on the surface. Fourier-transform infrared spectroscopy revealed a difference in stretching vibrations of the OH group at around 3400 and 3600 cm⁻¹ between SiO₂ and Gd₂O₃ films. The hydrophobicization rate of the sintered body depended on storage atmosphere and the temperature when the sample was removed from the furnace. Results suggest that this hydrophobicity originated from the adsorption of organic substances from ambient air onto the surface. Surface roughness of the sintered body also affected hydrophobicity. The Cassie mode contribution was inferred for a surface with a water contact angle without roughness of 65°.

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A Ta-based oxynitride photocatalyst, BaTaO₂N, was obtained by an ammonothermal method using supercritical ammonia as a reaction medium. Oxynitrides are generally obtained via multi-step reaction processes requiring high-temperature nitridation of an oxide precursor. In this study, we have successfully synthesized an oxynitride via a simple one-pot method. Moreover, BaTaO₂N was obtained at a temperature approximately 300°C lower than that of the conventional method. NaNH₂ was selected as a basic mineralizer with the aim to improve the solubility of the starting materials in supercritical ammonia. Addition of NaNH₂ led to a BaTaO₂N material with improved crystallinity (i.e., NaNH₂ inhibited lattice defects). Under visible light irradiation, Pt-loaded BaTaO₂N samples reduced water into hydrogen in the presence of a sacrificial reagent.

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Eu-doped strontium aluminates (SrAl₂O₄:Eu²⁺) has been attracting attention as a highly potential mechanoluminescent (ML) material in a wide range of applications such as visualization of stress distribution for the concretes, steel materials, etc. In this study, we examined the single effect by the addition of flux, boric acid (H₃BO₃), in the synthesis process for the improvement of the ML intensity of SrAl₂O₄:Eu²⁺. Crystal structure, photoluminescence and ML properties were examined at various H₃BO₃ concentrations. It was found that the addition of H₃BO₃ provided 2.5 times improvement of ML intensity. This is probably due to the increase in the trap concentration.

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ALnO₃ (A = Ca, Sr, Ba; Ln = Ce, Pr, Tb) were synthesized as environmentally friendly inorganic yellow pigments by a conventional solid state reaction. SrCeO₃, BaCeO₃, BaPrO₃, SrTbO₃ and BaTbO₃ were obtained in a single phase form. Among them, the color of SrTbO₃ and BaTbO₃ were bright yellow, because of the strong optical absorption in the blue light region. The yellowness values of SrTbO₃ and BaTbO₃ expressed in a color coordinate were comparable to those of praseodymium yellow (ZrSiO₄:Pr) and bismuth vanadate (BiVO₄), respectively. Furthermore, the yellow color of SrTbO₃ is more greenish than that of ZrSiO₄:Pr, and BaTbO₃ showed a purer yellow color as compared with BiVO₄.

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CsVO₃ nanoparticles were synthesized by the evaporation of aqueous solutions of peroxo-isopolyvanadic acid and cesium carbonate. The resulting phosphor showed white light Photoluminescence (PL) property with a broad peak in the range of 400–700 nm at an excitation wavelength of 350 nm. The as-prepared CsVO₃ was heat-treated at 200 and 400°C, and an increase in the particle size and PL intensity was observed with increasing heat-treatment temperatures.

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