Vol. 41, Supplement B (2025)

1.

Nitrous oxide (N₂O) is a harmful greenhouse gas with a global warming potential of approximately 300 times that of carbon dioxide and contributes to ozone layer depletion. While monitoring N₂O emissions is essential, existing detection technologies are often large and costly. Metal oxide semiconductor (MOS) sensors offer a promising alternative due to their low cost, compactness, and long operational life. In this study, SnO₂ and In₂O₃ were used as base materials, with Rhodium (Rh), known for its N₂O decomposition activity, and Strontium oxide (SrO), which is reported to enhance N₂O sensitivity, as catalytic additives. Nanoparticles of Rh-SnO₂, Rh-In₂O₃ (0–5 wt%), and SrO-SnO₂ (0.5–1 wt%) were synthesized via the hot-soap method and fabricated into thin films on alumina substrates using screen-printing. Gas sensing properties were evaluated by monitoring resistance changes under various temperatures (150–400 °C) and N₂O concentrations (100–500 ppm). The sensors demonstrated a significant response at low temperatures (<200 °C). Particularly, 0.1 wt% Rh improved sensitivity, while higher Rh loadings reduced it. The addition of SrO also enhanced the N₂O response. These findings suggest that low-concentration Rh and SrO additives are effective in improving the N₂O sensing performance of MOS-based sensors.

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2.

We investigated Ce and Co substitution in SmFeO₃ perovskites for VOC sensing. The oxides were synthesized via the pyrolysis of the cyano-complex. Gas sensor performance for ethanol and toluene was evaluated across temperatures and concentrations. Co doping enhanced oxidation activity and reduced electrical resistance, enabling lower operating temperatures. The sensor with x = 0.1 exhibited the highest ethanol selectivity, balancing conductivity and surface reactivity. DFT calculations of adsorption energies and electronic interactions supported the observed selectivity trends. Co‑substituted SmFeO₃ demonstrates improved VOC differentiation and energy‑efficient operation. These insights guide the development of advanced perovskite‑based gas sensors for selective ethanol detection.

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3.

Organophosphorus compounds have been used in chemical warfare and terrorist attacks due to their high toxicity, and some are also used as pesticides and industrial raw materials. In particular, sarin is odorless and colorless, making it impossible to detect with the human senses. Therefore, technology for detecting the release and leakage of organophosphorus gases, including sarin, is extremely important. Semiconductor metal oxides (such as SnO₂) are widely used as gas sensor materials because their electrical resistance changes sensitively even at ppm levels of the target gas. In this study, we evaluated the detection potential of organophosphorus gases by using P-doped SnO₂ nanocrystals to detect DMMP, a simulant of sarin. Hexamethylphosphoric triamide (HMPT) was used as the P precursor, and P-SnO₂ nanocrystals were synthesized using the hot soap method. Sensor devices were fabricated using the obtained nanocrystals, and the response of both SnO₂ and P-SnO₂ devices to DMMP gas was measured. The P-SnO₂ devices exhibited superior sensitivity compared to the SnO₂ devices. Additionally, the P-SnO₂ devices demonstrated higher stability in repeated measurements. XRF measurements of the SnO₂ material after gas exposure confirmed the presence of excess P. This is considered to be due to an irreversible reaction between SnO₂ and DMMP.

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4.

The macrostructure of laboratory-fabricated gas sensors was not thoroughly characterized, despite its known influence on gas-sensing performance. In this study, two key parameters were varied: the aggregate size and the electrode gap. All sensor samples with varied aggregates were within the same size range. As a result, aggregate size was found to negatively affect the gas-sensing response, while the electrode gap exhibited a positive effect. Therefore, factors such as grain boundaries, suitable film thickness, and porosity are crucial for achieving optimal gas sensor performance.

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5.

To investigate the underlying mechanism of ethanol sensing in Pt-ZnO nanocrystals, we synthesized a series of materials with varying platinum contents (0, 0.1, and 5 wt%) using the hot-soap method. To clarify the role of surface and bulk features in the sensing process, we performed operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at 350 °C under ethanol exposure in both Air and N₂ atmospheres. In the presence of air, characteristic peaks corresponding to adsorbed intermediates such as acetate and carbonate species were detected, indicating that ethanol is oxidized via interactions with surface oxygen species. In contrast, DRIFTS spectra acquired under N₂ revealed a pronounced increase in peaks related to oxygen vacancies, particularly in the case of 5 wt% Pt/ZnO, suggesting a significant involvement of defect sites in ethanol adsorption or reaction pathways. Additionally, operando Raman spectroscopy conducted under N₂ atmosphere showed clear shifts and intensity changes in lattice vibrational modes of ZnO upon ethanol exposure, providing further evidence for lattice-level interactions. These combined findings indicate that the sensing mechanism of Pt/ZnO nanocrystals involves not only conventional surface oxygen species, but also deeper lattice oxygen and oxygen vacancy contributions, particularly under reducing conditions where surface oxygen is limited.

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6.

To gain insights into the design of high-sensitivity VOC sensors, ethanol—a representative volatile organic compound (VOC) was selected as the target gas. Platinum-loaded WO₃ nanocrystals were synthesized via a hot-soap method using high-boiling-point solvents, and the resulting nanoparticles were screen-printed onto alumina substrates equipped with gold comb electrodes to fabricate sensor devices. The structure and composition of the synthesized nanocrystals were evaluated using XRD, XRF, SEM, and STEM. Electrical resistance measurements under dry air revealed that Pt-loaded WO₃ exhibited higher ethanol response than pristine WO₃ across the entire temperature range of 200–450 °C, with particularly enhanced sensitivity observed for the 1 wt% Pt-WO₃ sample. To investigate the reaction mechanism at 350 °C, operando DRIFTS and Raman spectroscopy were conducted. DRIFTS analysis showed strong adsorption of acetate species, an intermediate, on 1 wt% Pt-WO₃, and gas analysis confirmed greater CO₂ production, suggesting efficient ethanol oxidation. Furthermore, under N₂ atmosphere, Pt promoted dehydrogenation, and Raman analysis revealed reversible involvement of lattice oxygen in WO₃. These findings indicate that the improved sensitivity and ethanol sensing mechanism are attributed to a synergistic effect of Pt catalytic activity, intermediate adsorption, and the participation of WO₃ lattice oxygen.

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7.

To investigate the influence　of nanostructure and exposed facets on gas sensing properties using thin-film-type sensors, we synthesized dispersions of SnO₂ nanorod and nanoparticle and evaluated the sensor responses. These dispersions were prepared via hydrothermal synthesis under varying temperatures and solvent pH conditions. The resulting suspensions were spin-coated onto alumina substrates equipped with a comb-type gold electrodes to fabricate thin-film sensor devices. The thickness of the films composed of nanorods and nanoparticles were 550 nm and 600 nm, respectively. At 350 °C in synthetic air, the SnO₂ nanorod-based sensor exhibited higher electrical resistance than the nanoparticle-based sensor, likely due to the reduced number of grain boundaries in the nanorod structure. Despite this, the sensor response to H₂ at 350 °C was higher for the nanoparticle-based sensor than for the nanorod-based one. This may be attributed to the fact that the lateral surfaces of the nanorods predominantly expose the {110} crystal facet, which is known to be less reactive, thus resulting in a lower fraction of high-reactivity surface area. Moreover, the reduced number of grain boundaries in the nanorod film may further contribute to its lower sensitivity.

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8.

Semiconductor gas sensors for acetone detection face challenges such as high operating temperatures, low selectivity, and unclear sensing mechanisms. Since acetone adsorbs on Lewis acid sites, increasing these sites on p-block metal oxides can enhance sensing performance. We previously showed that amorphous In-Sn-Zn oxide (ITZO) and Ga-In-Sn-Zn oxide (GITZO) nanoparticles have more acetone adsorption sites than single-metal oxides. In this study, we investigated the acetone selectivity of amorphous GITZO nanoparticles by analyzing reaction mechanisms using spectroscopic measurements with acetone and ethanol, a known interfering gas. We also compared GITZO with ITZO nanoparticles. The results suggest that improved sensor response is due to partial oxidative decomposition of acetone on surface Lewis acid sites. The improved sensing performance is attributed to an increased number of Lewis acid sites and higher catalytic activity due to quaternization.

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9.

The carbon nanotube (CNT)–tungsten trioxide (WO₃) sensor is expected to exhibit greatly enhanced gas-detection performance compared to a CNT sensor. The sensor in which the CNT network provides the conductive pathway. Therefore it suppress the increase in sensor resistance caused by depletion‐layer broadening at the p‐n junction. In this study, monoclinic WO₃ (m-WO₃) crystals were hydrothermally synthesized and deposited onto a CNT film―previously formed on an alumina substrate with gold electrodes—at various volume ratios (m-WO₃/CNT = 0.05-1.0) to create p‐n junction semiconductor gas‐sensor devices. In air, the electrical resistance (Ra) of each sensor decreased with increasing operating temperature (50, 150, and 250 °C). The maximum Ra was observed at a volume ratio of 0.9, while the highest sensor response to NO₂ (0.5 and 5 ppm) occurred at a volume ratio of 0.8.

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10.

This paper addresses a chemiresistive hydrogen sensor device using a thin film of platinum-supported tungsten oxide to realize an efficient multi-point hydrogen detection system. The hydrogen-sensitive film was prepared by a sol-gel method and immobilized on various interdigital electrodes on a quartz glass substrate. The sensor devices were fabricated under various fabrication conditions, and the responses were evaluated for determining the optimal fabrication conditions. Then, the performances required for the practical applications, such as sensitivity and reproducibility, were verified. Experimental results revealed excellent response of the hydrogen sensor device and electrical properties such as resistance dependent on the film thickness. From several results, it was also concluded that the catalytic activity of the Pt electrode itself is one of the important factor for obtaining good hydrogen response.

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11.

Breath and skin gases have attracted attention as samples that can be easily collected noninvasively. Among them, acetone gas emitted from the skin is generated during lipid metabolism, and its concentration is an indicator of diabetes and diet, so a real-time monitoring method is required. However, existing acetone gas sensors present challenges, such as the difficulty of real-time monitoring. In this study, we developed a printed acetone gas sensor that can quantify acetone gas at the skin gas level in real time using screen printing technology. The measurement solution was phosphate buffer (pH 7) containing 1-methoxy PMS and NADH, and was evaluated using chronoamperometry. The results showed a concentration correlation in the range of 50-1000 ppb, with a detection limit of 73 ppb. The concentration of acetone gas emitted from the skin ranged from 77 to 205 ppb, suggesting the possibility of monitoring acetone gas in skin gas using this sensor.

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12.

Yttria-stabilized zirconia (YSZ)-based sensors were fabricated using an m wt% Au-added ZnCr₂O₄ sensing electrode and a Pt counter electrode (ZnCr₂O₄ (m = 0) or mAu ZnCr₂O₄ sensors (m = 1, 5, 10)), and their sensing properties to CO balanced with air were examined by measuring the electromotive force (E) and short-circuit current (I) at 550 and 610°C. The mAu-ZnCr₂O₄ sensors (m = 5, 10) showed relatively large current responses (ΔI) to CO among all sensors. The magnitude of ΔI of the sensors monotonically increased with increasing CO concentration in the low concentration range (below 100 ppm) at 550°C, while they tended to saturate in the higher concentration range. Increasing the operating temperature to 610°C improved the linearity between ΔI and CO concentration over the whole concentration range from 10 to 1000 ppm. The ΔI of these sensors at 610°C were larger than that at 550°C in the high concentration range (above 500 ppm), suggesting that elevated temperatures enhanced the electrochemical oxidation of CO at the triple-phase boundaries

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13.

This study aims to construct a new device structure and sensor system for distributed fiber-optic hydrogen sensors using long string-like light sources. This unique technology, which optically couples linear light sources and transmission fibers with hydrogen-sensitive materials, minimizes the impact of optical loss on sensor devices and enables the realization of long string-like sensor structures. In this presentation, we report the results of our investigation into the selection of high-refractive-index oxide thin films applicable to the string-like light source. We fabricated optical fibers coated with various oxide thin films composed of Ni, W, Al, Ti, and Zn, and captured the emitted light images from the thin films as a result of leaking of propagation light using an infrared camera. Analysis of the digitized image data revealed that ZnO exhibited the best optical properties in terms of emitted light intensity, integrated light quantity, and homogeneity. A positive correlation was observed between the emitted light intensity and the refractive index of each material, consistent with the image data results. However, WO3 and TiO2 deviated slightly from this trend, likely due to their lower transmittance.

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14.

The deterioration of oils increases their acidity, which can cause faults in machinery. Therefore, there is a strong demand for new devices capable of real-time monitoring of the oil acidity (total acid number (TAN)). We have already developed diode-type sensors consisting of an anodized titania film and Pt electrodes, which can detect hydrogen gas at the ppb level. Their current-response transients were measured in both fresh oil and deteriorated oils in this study. The magnitude of the currents increased with an increase in TAN. In addition, the magnitude of the currents increased also with a decrease in the applied voltage during the anodization.

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15.

This study investigates the volatile organic compound (VOC) sensing properties of SmFeO₃-based perovskite materials through two enhancement strategies: gold (Au) nanoparticle loading and cobalt (Co) substitution. The introduction of Au nanoparticles (0.5 and 3 wt%) onto SmFeO₃ surfaces significantly increased ethanol sensitivity and selectivity. XPS and SEM analyses confirmed increased adsorbed oxygen and uniform Au dispersion. DFT results demonstrated strong ethanol adsorption at the Au-SmFeO₃ interface and facile oxygen migration on the Au surface, promoting catalytic oxidation. In contrast, toluene showed weaker interactions and reduced response with increased Au loading. In parallel, Co substitution in SmFe_1–xCo_xO₃ (x = 0.0–0.5) improves electrical conductivity via hole doping and reduces the optimal operating temperature for gas detection. Among the compositions, x = 0.1 exhibited the highest selectivity for ethanol over toluene due to a favorable balance between electrical resistance and oxidation activity. Density functional theory (DFT) calculations revealed that Co-induced oxygen vacancies enhance ethanol adsorption, correlating with experimental trends. Both approaches highlight different mechanisms for enhancing VOC sensing performance. Au loading introduces catalytic interfaces, while Co doping improves intrinsic material properties. These findings contribute to the development of advanced, selective, and energy-efficient gas sensors for practical VOC monitoring applications.

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16.

Ammonia (NH₃) has been highly relevant to human activity. In recent years, non-invasive health diagnostics by detecting very low amounts of NH₃ in human exhaled breath have been reported. Thus, there is a significant demand for NH₃ detection using inexpensive sensors. This study presents the facile fabrication of copper(I) bromide (CuBr) for highly sensitive and selective NH₃ sensors by using thick film technology, a method widely employed in the commercial production of MOS sensors. The fabrication method involved painting a CuBr paste followed by heating it in the air. We optimized the organic liquid compound for preparing the CuBr paste by employing the Hansen solubility parameters (HSP) as a guide. Propylene carbonate was found to be preferable because of its ease of practical use. The SEM observation revealed that the CuBr thick films fabricated had porous structures. The sensors exhibited a clear sensor response to NH₃ as low as 50 ppb, which was comparable with those reported in state-of-the-art studies. These findings constitute a significant step toward commercializing CuBr sensors for NH₃ detection.

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