Yoshitake Masuda's research while affiliated with National Institute of Advanced Industrial Science and Technology and other places

Metal oxides have been prepared with high temperature annealing for a long time. However, metal oxides are synthesized in nature at ordinary temperature and atmospheric pressure. In this study, learning from nature, metal oxides were synthesized at room temperature. This study focuses on “Formation and two-dimensional patterning of particle self-assembled structure”, “Synthesis of ceramics in aqueous solution and their two-dimensional patterning”, “Microstructure control and crystal face control of ceramics”, “Gas and odor sensors with highly active crystal faces”, and “Molecular sensors for detection of environmental toxin or cancer marker”. In particular, concept of crystal growth control and nanostructure control of metal oxides was proposed. The two-dimensional patterning of ceramic nanostructured films and particle self-assembled structure was achieved. In addition, a dendritic structure of tin oxide with metastable {101} crystal facets was developed in aqueous solution. They were applied to chemical sensors and gas sensors.

A diabetes sensor was developed to detect low concentrations of acetone gas, which is a diabetes biomarker. A WO3 nanoneedle film was synthesized via an aqueous process for use as a sensitive sensing membrane. Acetone was adsorbed and oxidized on the WO3 nanoneedle film, which changed the sensor resistance. The sensor exhibited a high response of Ra/Rg = 19.72, where Ra is the sensor resistance in air, and Rg is the sensor resistance in air containing 10 ppmv acetone gas. The sensor also exhibited a high response (25.36) to 1 ppmv NO2, which is related to asthma. Furthermore, the sensor responded to various biogases associated with diseases. The sensor responses to 10 ppmv of the lung cancer marker gases acetaldehyde and toluene were 13.54 and 9.49, respectively. The sensor responses to 10 ppmv isoprene, ethanol, para-xylene, hydrogen, and NH3 were 7.93, 6.33, 4.51, 2.08, and 0.90, respectively. Trace amounts of acetone and NO2 gases (25 and 250 ppbv, respectively) were detected. The limits of detection for acetone and NO2 gases were estimated to be 2.4 and 1.5 ppbv, respectively. The sensor exhibited superior ability to detect low concentrations of biomarker gases. The unique characteristics of the WO3 nanoneedle film contributed to its high response rates.

Laminated CuO/Al2CuO4/CuO layers were developed to serve as an interlayer between the conducting oxide CaCu3Ru4O12 + 30 vol.% CuO and nitride substrate Si3N4. Screen-printing was used to fabricate these films. The Al2CuO4 layer impeded the chemical reaction between CaCu3Ru4O12 and Si3N4 and hence suppressed the decomposition of CaCu3Ru4O12. The CuO interlayers improved the adhesion between the Si3N4 substrate, Al2CuO4 layer, and CaCu3Ru4O12 + 30 vol.% CuO film. The main difference between the CaCu3Ru4O12 + 30 vol.% CuO film on the Si3N4 and Al2O3 substrates having the same interlayers was the presence of a small amount of RuO2 (formed from decomposition of CaCu3Ru4O12) and cracks in the film on the Si3N4 substrate. The difference in the coefficients of thermal expansion between the Si3N4 substrate, interlayers, and film caused cracks to appear when the temperature of the film decreased after sintering. The resistivities of the films on the Si3N4 and Al2O3 substrates were 7 and 3 mΩ·cm at 100 °C, respectively. The film on the Si3N4 substrate exhibited higher resistivity than the film on the Al2O3 substrate from room temperature to 600 °C. This is because the cracks on the film on the Si3N4 substrate scattered the charge carriers. A decrease in crack width with increasing temperature was also observed, which in turn decreased the resistivity of the film with temperature. The conducting properties of the film developed in this work are adequate for practical applications. However, the interlayer can be further improved to obtain films with better performance. The major achievement of the present study is the development of an interlayer that suppresses the chemical reactions between the functional oxide and nitride substrate during high-temperature sintering.

Gas sensors with high sensitivity and high selectivity are required in practical applications to distinguish between target molecules in the detection of volatile organic compounds, real-time security alerts, and clinical diagnostics. Semiconducting tin oxide (SnO2) is highly regarded as a gas-sensing material due to its exceptional responsiveness to changes in gaseous environments and outstanding chemical stability. Herein, we successfully synthesized a large-lateral-area SnO2 nanosheet with a loose structure as a gas sensing material by a one-step facile aqueous solution process without a surfactant or template. The SnO2 sensor exhibited a remarkable sensitivity (Ra/Rg = 1.33) at 40 ppt for acetone, with a theoretical limit of detection of 1.37 ppt, which is the lowest among metal oxide semiconductor-based gas sensors. The anti-interference ability of acetone was higher than those of pristine SnO2 and commercial sensors. These sensors also demonstrated perfect reproducibility and long-term stability of 100 days. The ultrasensitive response of the SnO2 nanosheets toward acetone was attributed to the specific loose large lateral area structure, small grain size, and metastable (101) crystal facets. Considering these advantages, SnO2 nanosheets with larger lateral area sensors have great potential for the detection and monitoring of acetone.

Cold crystallization and morphology control of ZnO nanostructures have been achieved. Various ZnO nanostructures, such as ZnO rod arrays, high c‐axis‐oriented stand‐alone ZnO self‐assembled films, ZnO particulate films, ZnO nanorod arrays, semicircular ZnO nanowhisker assemblies, bunched rose‐ and core–shell‐like ZnO particles, twin ZnO nanoarray assemblies, ZnO nanotubes, and ZnO nanoflowers, have been developed using the hexagonal crystal structure of ZnO. These structures have advantages, such as high anisotropy, high specific surface area, high crystallinity, and characteristic crystal facets, and can be applied in devices such as photoelectric conversion sensors, chemical sensors, gas sensors, batteries, and catalysts. Additionally, various nanostructures using other oxide materials are expected to be developed by applying the scientific knowledge obtained from the cold crystallization and morphology control of ZnO. Cold crystallization, which is a long‐standing challenge in the field of ceramics, has been realized, which is a historical turning point in the field of ceramics research.

The detection of low-concentration gases and odors in fields such as healthcare, mobility, and indoor environment control is attracting tremendous attention. The development of high-sensitivity gas sensors capable of detecting low concentrations of gases and molecules is strongly desired. This review focuses on tin oxide nanomaterials, which are substances that are being actively researched as semiconductor-type gas sensors. In particular, the development of novel tin oxide nanomaterials over the last decade and their gas sensing applications are discussed. It is revealed that dimension and morphology affect the sensing performance. Tin oxide nanomaterials having nano-meter size which is similar to size of a depletion layer, and controlled microstructure such as a nanosheet structure shows high sensing performance. The dendritic structure in which 2D nanosheets are connected by crystal growth points the direction for future sensor development. Furthermore, the high gas adsorption performance and reactivity of the metastable crystal plane will be a guide for future sensor development.

Ceramic powders and structures have been synthesized and manufactured by high-temperature firing. However, there are increasing expectations for the development of nanostructures that cannot be obtained by the firing method in search of higher functionality. In addition, cold crystallization is required from the viewpoints of energy saving, low environmental load, low cost, and hybridization with organic and metal materials. In this paper, I would like to introduce the development of powders and nanostructures by cold crystallization and morphological control for tin oxide, titanium dioxide, and zinc oxide. These powders and structures can be applied to various devices. A high spillover effect is expected in various fields. Furthermore, it is expected to contribute to SDGs (Sustainable Development Goals) in environmental fields such as energy saving and low environmental load.