Single-nanoscale (<10 nm) inorganic materials exhibit performance superior to that of bulk materials (>100 nm); however, most nanomaterials are yet to gain industrial applications due to their low durability, dispersibility, and difficulties in handling. Inorganic nanomaterials with hierarchical nanostructures, such as flower-, sheaf-, and leaf-like structures, are promising for various applications owing to their high surface activity, structural stability, good dispersibility, and recoverability, attributed to their submicrometer to microscale overall diameter and single-nanoscale open surface structures. In liquid-phase syntheses, additives function as ligands. For example, organic molecules and inorganic ions play an important role in the formation of hierarchical nanoarchitectures. Herein, I review ligand-assisted synthesis techniques based on the non-classical crystallization theory for fabricating nanoarchitectures and the high functionalities for catalytic, electrochemical, and gas sensing applications of the resulting nanostructures. I further discuss in detail the mesocrystal structures, orientation, functionalization, and formation mechanisms with tiny organic molecules and inorganic ions for spiky-shaped Nb₂O₅ particles and raspberry-shaped Co₃O₄ nanoparticles.

This paper investigated the effect of two-step ultrasound irradiation (miniaturization and oxidation) on various gallium-based liquefied alloys in ethanol and hydrazine monohydrate at 60 °C. The miniaturization and oxidation behavior of the liquefied alloys were analyzed. In the Ga-based alloy system, the miniaturization process was not as efficient as that of pure gallium due to changes in corrosive properties, surface tension, and viscosity caused by the addition of other metals. In the oxidation step, nanosized crystal grains were abundantly generated at the initial stage, and metal-doped γ-Ga₂O₃ was finally obtained in the Ga–Al and Ga–Mg systems. However, in the Ga–Ag system, the alloy’s oxidation rate was slow, and Ag-doped γ-Ga₂O₃ was not obtained. These results suggest that the redox potential of added metals significantly affects the oxidation behavior. By utilizing this phenomenon, it is possible to synthesize γ-Ga₂O₃ nanoparticles with controlled composition and morphology.

The collaborative research between first-principles calculations and atomic resolution electron microscope is powerful tool for materials research to understand the origin of material property in an atomic scale and further improvement of materials properties. In this review, our recent achievements regarding collaborative research between first-principles calculations and atomic resolution electron microscope on Li-ion battery materials were explained. Two examples were presented. One is influence of grain boundary in cathode electrode material LiCoO₂ (Ref. 11) and the other is influence of domain boundary in solid-state electrolyte materials (La,Li)TiO₂ (Ref. 15) on the battery materials properties.

The development of nanocarbon-integrated ceramic composites is highly desired to improve the functionality of oxide-based materials, such as conductivity, catalytic activity, and optical and mechanical properties. Among nanocarbons, fluorescent nanocarbon materials with unique chemical/physical properties, known as carbon dots (C-dots), have received considerable attention in recent years. C-dots can be structurally controlled by surface modification and defect introduction in the carbon core, making them suitable for the synthesis of nanocomposite materials with ceramics. This review provides an overview of the synthetic design of nanocomposite materials with C-dots in semiconductor oxides and glass matrices using solution processes. In particular, interactions due to the microstructure of the matrix, the bonding-induced immobilization and dispersion of C-dots in the matrix, and physicochemical approaches to understanding the fluorescence factors of C-dots are described based on the changes in fluorescence properties.

Sintering is a crucial process in the production of ceramics, where particles are bonded and interstitial pore space is reduced. The morphological evolution during sintering was studied by X-ray microtomography, which reveals a topological model described by Euler characteristics as a function of relative density. Domain coarsening was observed, where the characteristic length increased with densification. Sintering stress and bulk viscosity were derived from microtomographic images and compared to values obtained through discontinuous sinter forging. Results showed good agreement, suggesting that X-ray microtomography can provide reliable measurements of sintering parameters.

Red luminescence phosphor materials are important in practical applications because they are one of the three primary colors of light and are advantageous for improving color rendering in white light emitting diodes (LEDs). In particular, for red phosphor materials for white LED applications, Eu²⁺- or Ce³⁺-doped oxide materials are desired instead of non-oxides, such as nitride and fluorides, from the viewpoints of cost and safety in synthesis. However, blue-light excitable red luminescent oxide phosphor materials are extremely limited; thus, there are insufficient design concepts as to what type of matrix crystal is suitable for expression. Therefore, this review focuses on the coordination environment of the luminescent centers of Eu²⁺ and Ce³⁺ in the oxide lattice, which can be described as a crystal field. Specifically, luminescence properties and local structures in phosphors composed of oxide hosts with strong crystal fields, such as NaMgPO₄:Eu²⁺, M₃Sc₄O₉:Ce³⁺ (M = Ba and Sr), and BaCa₂Y₆O₁₂:Ce³⁺, are discussed. Finally, based on the description in this review, a concept that is useful for realizing red luminescence in oxide hosts is presented.

Continuous silicon carbide fiber-reinforced silicon carbide (SiC_f/SiC) composites have been expected as next-generation highly reliable heat resistant materials. It is well-known that the interface between fiber and matrix acts as an important role for toughening and strengthening SiC_f/SiC composites, and it should be optimally controlled to achieve high performance SiC_f/SiC composites with excellent fracture tolerance. In this study, polypyrrole (Ppy) as an electric conductive polymer was coated on amorphous SiC fibers to increase their surface electric conductivity so that electrophoretic deposition (EPD) method can be applied to form the interphase on the SiC fibers, and carbon interphase was formed on the SiC fibers by EPD method. In addition, unidirectional SiC_f/SiC composites were fabricated by polymer impregnation and pyrolysis (PIP) process, and their mechanical properties were evaluated. Thin Ppy coating with the thickness of around 200 nm drastically increased the surface electric conductivity of the amorphous SiC fibers, and the surface of Ppy-coated SiC fibers was wholly and uniformly coated with flaky graphite particles by EPD. The average thickness of the carbon interphase formed on the Ppy-coated SiC fibers by EPD became thicker with an increase in EPD voltage, and it was revealed that the SiC_f/SiC composites with carbon interphase formed by EPD showed pseudo-ductile fracture behavior and excellent mechanical properties, and the formation of the thick carbon interphase (average thickness; 0.7 ± 0.4 µm, EPD voltage; 5 V) provided the higher bending strength and fracture energy under the experimental condition in present study. These results demonstrated that Ppy coating on the low-conductive SiC fibers was very effective to improve their surface electric conductivity, and uniform and sufficient carbon coating was successfully formed on the SiC fibers by EPD for achieving high performance SiC_f/SiC composites.

Ceramics with starting compositions of P_1−xGe_xNb₉O₂₅ (x = 0–1) were prepared by conventional solid-state reaction method. Crystal phases, microstructure and dielectric properties were investigated. The formed crystal phases for the compositions with x = 0 and 1 were identified as PNb₉O₂₅ and GeNb₁₈O₄₇, respectively, and a continuous solid solution was formed for the compositions of 0 < x < 1. As x increased, a decrease of the grain size, a denser microstructure and an increase of dielectric constant were observed. The composition with x = 1 exhibited the best properties with high dielectric constant of approximately 1000, good temperature stability (<15 % deviation) and relatively low dielectric loss of 0.02–0.07 in the temperature range of 25–200 °C.

InN, one of III–V group compounds, has attracted attention owing to its optical properties in the infrared region. A high quality crystal growth for InN, however, is a key issue because InN is unstable at high temperature due to its thermal instability. Moreover, a microstructure control of InN by controlling a growth condition is desired for various optical applications. Atmospheric pressure halide chemical vapor deposition (APHCVD) method enables a single-crystal growth of InN at moderate growth temperature. In the present study, influence of the growth temperature and time on microstructure of hexagonal-pillar-structure InN single crystals by the APHCVD method on sapphire (1120) substrate is investigated using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The growing temperature of the InN influences significantly to the microstructure of the InN crystal; flower like structures at low temperature of 560 °C, pillar structures at moderate temperature of 590 to 610 °C, and coalesced pillar structure at 640 °C.

In this study, co-substitution of Ni and Ta was performed on SnP₂O₇, and their crystal structure and electrical conductivity were investigated. X-ray powder diffraction patterns showed that the compounds had a cubic structure identical to that of SnP₂O₇, but a secondary phase containing SnO₂ and Ni(PO₃)₂ was observed for x = 0.20. Rietveld analysis was used to refine the crystal structures, revealing that the lattice parameters increased with an increase in x, up to 0.15, and then reached saturation for x = 0.20. The increase in lattice parameter is considered contradictory, but it is suggested that it may be related to the introduction of oxygen vacancies caused by the substitution of lower valence cations, resulting in an increase in P–O bond length. As a result, the co-substitution of Ni and Ta for Sn increases the interstitial proton concentration in the lattice and the electrical conductivity of the compounds was effectively improved when increasing the value of composition x; the highest electrical conductivity of 1.7 × 10⁻² S cm⁻¹ at 250 °C exhibited for Sn_0.85(Ni_2/3Ta_1/3)_0.15P₂O₇.

Green Ba₃Si₆O₁₂N₂:Eu and Ba₃Si₆O₉N₄:Eu phosphors were synthesized below 1000 °C using Eu-doped BaCN₂ as a nitrogen and barium source in conjunction with SiO₂. The development of low-temperature synthesis methods is necessary to reduce energy consumption during the solid-state syntheses of inorganic materials. However, low-temperature reactions tend to provide oxynitride phosphors showing poor crystallinity and low photoluminescence (PL) intensity. In this study, the effect of a BaCl₂ flux on the low-temperature synthesis of Ba₃Si₆O₁₂N₂:Eu phosphors from BaCN₂ and SiO₂ was investigated. The crystallinity and phase purity of the Ba₃Si₆O₁₂N₂:Eu phosphors increased with increasing amount of BaCl₂ employed. The maximum PL intensity obtained from an oxynitride phosphor prepared with a BaCl₂ flux was almost twice that of material prepared without BaCl₂.

In this study, the impact of chitosan (CS) on Mg²⁺ incorporation into hydroxyapatite (HAp) was investigated. Initial Mg/(Ca+Mg) ratios of 0–0.15 resulted in Mg-incorporated HAp containing CS (CMgHAp), while a ratio of 0.2 yielded amorphous calcium phosphate containing CS. Mg/(Ca+Mg) and (Ca+Mg)/P molar ratios of the obtained CMgHAp particles were lower than those of Mg-incorporated HAp particles without CS, suggesting that Mg incorporation into HAp was suppressed by CS. Initial Mg/(Ca+Mg) ratio of 0.025 led to CMgHAp particles with lower specific surface area than HAp particles containing CS, while ratios of 0.05–0.15 led to particles with higher specific surface area. Transmission electron microscopy showed aggregated particles of CMgHAp, since the observed particle sizes were larger than the crystallite sizes of CMgHAp calculated using X-ray diffraction patterns. Analyses of Fourier transform infrared spectroscopy confirms the interaction of CS with the phosphate group in Mg-incorporated HAp. Calcination of the CMgHAp particles decreased the particle size and increased the specific surface area due to decomposition of CS, indicating that CS aggregated Mg-incorporated HAp particles.

Carbon-supported niobium oxide nanoparticle catalysts were synthesized for the oxygen reduction reaction (ORR) in acidic media. These catalysts were prepared using a ⁶⁰Co gamma-ray and heat-treated with polyacrylonitrile. Carbon black powder or multi-walled carbon nanotube (CNT) was used as the support for the niobium nanoparticle. Nb₂O₅ nanoparticles with low crystallinity were fine and well dispersed on the carbon support in the as-irradiated samples. The loading amount of niobium increased with the absorbed dose, which indicated that niobium nanoparticles were produced by the irradiation. The as-irradiated composite nanoparticles were heat-treated under an H₂ atmosphere (5 %) or a mixed atmosphere of H₂ (2 %) and O₂ (0.05 %) to introduce active sites. The ORR activities were significantly enhanced by the heat treatment. Furthermore, the ORR activities were improved by using multi-walled CNT as the support.

Large-scale production of nano-sized LTA-type zeolite particles by beads-milling and recrystallization method were demonstrated. The particles were finely milled, and their crystallinity decreased with increasing milling time. The beads-milled particles retained a zeolitic local structure of Al and Si but cause partial amorphization and loss of gas adsorption ability. Recrystallization recovered the long-range order of the zeolite crystal structure and gas adsorption ability. Smaller beads lead to finer particles and inhibit amorphization of the zeolite.

(Na₂O)_35.7(MO)_7.2(SiO₂)_57.1 and (Na₂O)_33.7(MO)_13.3(SiO₂)_53.4 glass materials that feature alkaline earth metals (M = Mg, Ca, Sr, Ba) were evaluated as Na⁺ conducting materials. Following glass synthesis from the melt under air at 1350 °C, Na environments, conductivity, and water leaching were investigated. ²³Na NMR spectrometry indicated Na–O distances decreased in the order M = Mg > Ca > Sr > Ba. The conductivities and activation energies of conduction respectively decreased and increased slightly in the order M = Mg, Ca, Sr, and Ba. Regardless of M, the conductivities and activation energies of conduction of (Na₂O)_33.7(MO)_13.3(SiO₂)_53.4 were lower and higher, respectively, than those of (Na₂O)_35.7(MO)_7.2(SiO₂)_57.1. Na leaching into water decreased in the order Ca < Mg < Sr < Ba. For each M, (Na₂O)_33.7(MO)_13.3(SiO₂)_53.4 was more resistant to leaching than (Na₂O)_35.7(MO)_7.2(SiO₂)_57.1.

CeO₂ powders co-doped with Bi and La (Ce_0.8,Bi_0.2−y,La_y)O_2−δ (CBLO) were prepared using hydrothermal processing. The entire doping molar amount of Bi and La against Ce [(Bi + La)/Ce] was fixed as 20 %. The sample surfaces were modified with CoO_x using the chemisorption calcination cycle method. Only the peaks of CeO₂ were detected by X-ray diffraction (XRD) in the obtained powder samples. The amounts of impregnated CoO_x were almost equal for all samples. The 2-naphthol decomposition activity and the antiviral activity of obtained sample powders were evaluated in water in the dark, revealing the 2-naphthol decomposition activity as higher in samples with a higher Bi ratio, but the antiviral activity was highest in the sample synthesized with a Bi:La ratio of 2:8. These findings suggest that both the affinity and the decomposition power against the viruses affected the antiviral activity. The activity exhibited different compositional dependence from that of the 2-naphthol decomposition activity.