Takaaki Tomai

Supercritical fluid processing has developed into a versatile and environmentally responsible approach for the controlled fabrication of nanomaterials, distinguished by low density, viscosity, high diffusivity, and negligible surface tension. These intrinsic features enhance nucleation, efficient mass transport, and accurate control over particle dimensions, morphology, and compositional uniformity. This review comprehensively examines the fundamental principles of supercritical fluids, encompassing phase behavior, thermophysical characteristics, and the mechanisms governing nanoparticle formation, along with important engineering aspects related to both batch and continuous‐flow reactor configurations. The preparation of metal, metal oxide, and composite nanostructures through supercritical techniques is critically evaluated, emphasizing their benefits compared with traditional solid‐state and solution‐based synthesis routes. Special attention is given to the evolution toward high‐entropy oxides, where supercritical conditions enable homogeneous multication distribution, enhanced phase development, and defect modulation under kinetically advantageous environments. By establishing clear relationships between processing conditions, structural characteristics, and electrochemical performance, this review highlights the promise of SCF‐enabled methodologies as scalable and sustainable pathways for designing advanced multicomponent oxide materials suited for energy storage and catalytic applications.

Hetero-anion doping has emerged as an effective approach to modulate the physicochemical and catalytic properties of perovskite-type oxides. In this study, nanosized CeO2 synthesized by a continuous flow-type supercritical hydrothermal synthesis (SHS) process was employed as a precursor for a solid-state reaction with Ba(NH2)2, enabling the low-temperature synthesis of single-phase perovskite oxynitride–hydride BaCeO3–xNyHz (BaCeO3–xNyHz–SHS) with a high surface area and abundant N3– and H– species. Ru-loaded BaCeO3–xNyHz–SHS exhibited a maximum ammonia synthesis rate of 9.0 mmol g–1 h–1 at 300 °C and 0.9 MPa and showed up to a 6-fold enhancement in activity compared with the corresponding Ru/BaCeO3–xNyHz prepared from commercial CeO2, owing to the enhanced generation of anion vacancies at the Ru–support interface. Moreover, BaCeO3–xNyHz–SHS acted as a highly efficient support for various transition metals in ammonia synthesis and for Ni in ammonia decomposition, achieving >99% conversion at 560 °C with a low apparent activation energy (75 kJ mol–1). These findings demonstrate that enhancing the solid-state reactivity of oxide precursors through nanoscale engineering provides a general strategy for the low-temperature synthesis of highly active hetero-anion perovskite catalysts for ammonia synthesis and decomposition.

High-entropy oxides (HEOs) composed of more than five transition metal elements are promising noble-metal-free electrocatalysts for sustainable water oxidation. However, the low-temperature synthesis of size-controlled, single-phase, and crystalline HEO nanoparticles remains challenging because multicomponent oxide formation requires simultaneous nucleation and crystallization of multiple metal species with different hydrolysis behaviors. Herein, we report an alcohol-assisted subcritical hydrothermal strategy for synthesizing size-controlled HEO nanoparticles composed of Mn, Fe, Co, Ni, and Zn. By tuning the water–alcohol co-solvent composition, the average particle size decreased from 17.2 to 10.0 nm without significant changes in the overall metal composition. Structural characterizations confirmed that the obtained nanoparticles retained a single-phase spinel oxide structure with homogeneous elemental distribution and similar local coordination environments. The size reduction was considered to be attributed to changes in solvent properties, particularly the lowered dielectric constant of the water–alcohol co-solvent, which promotes rapid nucleation and suppresses particle coarsening. The resulting smaller HEO nanoparticles exhibited enhanced alkaline oxygen evolution reaction activity. The nearly unchanged Tafel slopes indicate that the improved activity mainly originates from an increased number of accessible active sites rather than a change in the reaction mechanism. This work provides a simple solution-based route for nanoscale control of crystalline multicomponent oxide electrocatalysts and offers a sustainable design strategy for noble-metal-free water oxidation catalysts.

Precise control over the particle size of metal–organic frameworks (MOFs) is pivotal for optimizing their performance in catalysis, separation, and drug delivery. However, conventional synthetic strategies largely depend on empirical trial-and-error, which lacks predictive power and fails to decode the complex interplay between nucleation and growth kinetics. Herein, we report a materials process informatics framework for the predictive control of MOF particle size, using zeolitic imidazolate framework-8 (ZIF-8) as a representative model system. A comprehensive database was constructed through systematic curation from the literature, with seven process descriptors employed as input features. Multiple machine-learning algorithms were benchmarked, among which the Categorical Boosting (CB) model achieved the best predictive performance after hyperparameter optimization, with a coefficient of determination (R²) of 0.90 on the test set. Furthermore, SHapley Additive exPlanations (SHAP) analysis identified precursor concentration ratio and reaction time as the most influential parameters governing particle size. Experimental validation using an automated synthesis platform showed excellent agreement between predicted and measured particle sizes, confirming the model's robustness and predictive reliability. Overall, the proposed framework enables intelligent synthesis optimization and targeted experimental design, thereby providing a practical route toward controllable MOF synthesis. This work demonstrates how materials process informatics can shift MOF particle-size engineering from empirical optimization toward data-driven design, offering a broadly applicable strategy for advanced materials synthesis.

Subcritical hydrothermal synthesis with organic modifiers enables the synthesis of single-phase high-entropy oxide nanoparticles with reduced size and homogeneous multication structures for efficient oxygen evolution catalysis.

High-entropy oxides (HEOs) are promising electrocatalysts for the oxygen evolution reaction (OER) due to their unique electronic structure caused by the interactions of the multiple constituent metal elements. The particle size of the catalysts is one of important determining parameters of the catalysts’ OER activity. However, synthesizing size-controlled single-phase HEO nanoparticles (NPs) remain challenging issue. Herein, we report an alcohol-assisted subcritical hydrothermal synthesis strategy to synthesize spinel-based HEO NPs with average diameter of approx. 10 nm composed of five first transition metal elements. Mixing methanol to water as solvent reduces the bulk dielectric constant, decreasing precursor solubility and increasing the degree of supersaturation. This change of the property of the solvent induces simultaneous burst nucleation and suppresses Ostwald ripening, thereby preventing particle coarsening. The crystallite size was systematically tuned from 13.3 nm down to 7.5 nm while maintaining their compositions. The OER activity of the catalysts synthesized by different alcohol content was enhanced significantly by reducing the particle size, which causes an increase of the specific surface area of the catalysts. This work establishes a promising route for designing crystalized HEO NPs with high catalytic activity.