Highly stable and active low-temperature CO oxidation catalysts without noble metals are desirable to achieve a sustainable society. While zero-dimensional to three-dimensional Co₃O₄ nanoparticles show high catalytic activity, simple-structured nanocrystals easily self-aggregate and become sintered during catalytic reaction. Thus, complex three-dimensional nanostructures with high stability are of considerable interest. However, the controlled synthesis of complex nanoscale shapes remains a great challenge as no synthesis theory has been established. In this study, 100 nm raspberry-shaped nanoparticles composed of 7–8 nm Co₃O₄ nanoparticles were synthesized by hydrothermally treating cobalt glycolate solution with sodium sulfate. Surface single nanometer-scale structures with large surface areas of 89 m²·g⁻¹ and abundant oxygen vacancies were produced. The sulfate ions functioned as bridging ligands to promote self-assembly and suppress particle growth. The Co₃O₄ nano-raspberry was highly stable under catalytic tests at 350 °C and achieved nearly 100% CO conversion at room temperature. The addition of bridging ligands is an effective method to control the formation of complex but ordered three-dimensional nanostructures that possessed extreme thermal and chemical stability and exhibited high performance.

The development of a new synthesis method is still required for very fine oxide nanoparticles. In this study, a single-flow supercritical fluid system has been developed for the synthesis of highly crystalline nanosized oxide particles. Niobium oxide particles were synthesized by single-flow supercritical water treatment, batch-type supercritical water treatment and subcritical water treatment. Niobium pentoxide nanoparticles synthesized by single-flow supercritical water treatment at 673 K, 24.5 MPa, and 15 ml min⁻¹ flow rate had a pseudohexagonal structure. The morphology of the nanoparticle was a rod, and it has a smaller particle size and larger crystallite size than those of the oxide particles synthesized by the other methods, because the particle growth and the decomposition of surfactant were rapidly suppressed in the single-flow supercritical water treatment. The nanosized niobium pentoxide is useful as a catalyst in harsh environments and as a precursor powder of lead-free piezoelectric materials.

Niobium pentoxide particles with a complex three-dimensional (3D) nanostructure consisting of a spiky structure have been developed as recyclable and recoverable Lewis acid catalysts. The morphology of the niobium pentoxide was successfully controlled from 1D to 3D via a bridging-ligand-assisted hydrothermal treatment, without changing the crystal structure. Compared with dispersed one-dimensional (1D) niobium pentoxide nanorods with a major-axis length and minor-axis length of 20 nm and 5–8 nm, respectively, the spiky-shaped niobium pentoxide composed of 300 nm spherical cores and nanorods with a minor-axis length of 5 nm maintained its surface nanostructure even after calcination at 400 °C in air. The 400 °C-calcined spiky particles exhibited the highest production rate of 2-((4-methoxyphenyl)amino)-2-phenylacetonitrile (0.115 mmol m ⁻² ) in a Strecker reaction, resulting in a nanoscale and ordered surface structure of spiky particles that simultaneously exhibit high specific reactivity and high structural stability. Acid site analysis and Raman spectroscopy revealed that stable nanorods that grew in the (001) orientation functioned as Lewis acid catalysts and that the origin of the acidity was a flexible Nb–O polyhedral structure in the single-nanoscale (<10 nm) niobium oxide rods. This study proposes that the spiky-shaped niobium pentoxide exhibits sintering resistivity and high activity and has potential applications as a recoverable and recyclable solid acid catalyst.

Silsesquioxane (SQ) derivatives possessing intramolecular H₂C = CH- groups and Si-H groups were designed as precursors for ternary silicon oxycarbide (SiOC). By using R-Si(OMe)₃, H-Si(OEt)₃ and (H-Si(Me)₂)₂O as starting compounds, SQ derivatives of VH-SQ (R = vinyl) and St-H-SQ (R = stylyl) were successfully synthesized through the conventional sol-gel route. Simultaneous thermogravimetric and mass spectroscopic analyses up to 1000 °C revealed that in situ cross-linking via hydrosilylation and demethanation of VH-SQ suppressed the evolution of gaseous hydrocarbon species to afford amorphous SiOC having a composition close to the desired stoichiometric SiO_2(1−x)C_x (x = ca. 0.3) with a high yield. The effect of carbon content on the phase separation and crystallization of the SQ-derived amorphous SiOC was studied by several spectroscopic analyses and TEM observation. The results were discussed aiming to develop a novel polymer-derived ceramics (PDCs) route for in situ formation of binary β-SiC-amorphous SiO₂ nanocomposites with enhanced thermal and mechanical stability.