Vol. 36, Supplement A (2020)

1.

YSZ-based potentiometric gas sensors using a Au or CeO₂-added Au sensing electrode (SE, CeO₂ content: 4‒16 wt%, thickness: 30‒100 nm) were fabricated by a spin-coating method, and their sensing properties to toluene were examined at 500°C. The magnitude of response (ΔE) to 50 ppm toluene of the sensor attached with 4 wt% CeO₂-added Au SE increased with a decrease in the SE thickness. On the other hand, ΔE of the sensors using 16 wt% CeO₂-added Au SE increased with an increase in the SE thickness, and the sensor attached with the thickest 16 wt% CeO₂-added Au SE showed the largest response (ca. 199 mV). This is probably due to an inreased number of the interfaces between CeO₂-added Au and gas and the increased electrochemical oxidation of toluene at these sites.

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2.

Potentiometric sensors aiming at detecting H₂ were examined. YSZ based sensor elements, H₂, Pt/YSZ/Pt, air, responded to an ppm level of H₂ contamination in air with the value of EMF=30 mV at 350oC. Coating the Pt sensing electrode with a powder of hexagonal boron nitride (h-BN) increased the EMF response, and the sensor response further increased by the milling treatment of the h-BN powder in ethanol. However milling treatment over 24 h reversed the response to decreasing. Specific surface area of the h-BN powder monotonously increased with the ball-milling which induced exfoliation of the BN layer. While, FT-IR analysis revealed that point defects existing in the BN plane react with solvent by the milling for > 48 h. It was concluded that the point defects in h-BN play a decisive role for the property of h-BN auxiliary phase in sensor response to H₂.

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3.

Limiting current type oxygen sensor is compact and detects in a wide oxygen concentration range. However, selectivity of this sensor is not high because almost all gases can be co-diffused in diffusion hole simultaneously with O₂. The objective of this study is to develop limiting current type sensor with the high sensitivity and selectivity in wide oxygen partial pressure. The limiting current type sensor was prepared by using LaGaO₃ based oxide for electrolyte as a fast ion conductor. For electrode, Co based oxide was mainly prepared by screen printing method. Instead of pin hole, mixed conductor of La(Sr)Ga(Fe)O₃ which is stable hole and oxide ion mixed conductor was used for oxygen diffusion layer. Limmiting current was observed on I-V curves of the prepared sensor using LaGaO₃ electrolyte at 600℃. Limiting current was linearly increased with increasing PO₂ and so similar with conventional limiting current type sensor using hole layer, oxygen partial pressure was detect by limiting current. It was found that high selectivity to oxygen was achieved against H₂, CO, and CH₄ which shows high influence on detection in the conventional sensor.

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4.

In this study, we proposed a novel potentiometric CO₂ gas sensor based on a new solid electrolyte of Y-doped La_9.66Si_5.3B_0.7O_26.14 (Y-LSBO) that show the high oxide ionic conductivity even at 300°C, attached with Li₂CO₃-CeO₂-Au composite and Pt as sensing and reference electrode, respectively. As the results of optimizing the composition of sensing electrode, the EMF exhibits the linear relationship with the logarithm of CO₂ concentration in the range of 400-4000 ppm at 400, 450 and 500°C. In addition, the optimized sensor exhibits an excellent Nernstian response to CO₂ gas in not only dry but also humid atmospheres. These results strongly indicate that Y-LSBO-based solid electrolyte CO₂ sensor is a promising candidate for CO₂ monitoring with high sensor response and high stability.

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5.

Tungsten oxide is a useful material for NO₂ sensing. It has been reported that the crystalline phase of tungsten oxide provides gas selectivity. In this study, the NO₂ sensing properties of monoclinic and hexagonal tungsten oxides prepared by hydrothermal synthesis were investigated. Their morphologie and crystal structure were investigated by field emission scanning electron microscope (FE-SEM), X-ray diffraction patterns (XRD). The NO₂ gas sensing properties of monoclinic and hexagonal tungsten oxides were investigated at various operating temperatures from 50 to 200 ° C with 0.3 ppm NO₂ concentration. As the result, at all temperatures, the sensor response of monoclinic tungsten oxide was higher than that of hexagonal tungsten oxide. The optimal operating temperature was 50 °C for monoclinic tungsten oxide and 100 °C for hexagonal tungsten oxide. The optimal operating temperatures of both sensors were lower than those of typical tungsten oxide sensors.

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6.

In this study, the relationship between the CO₂ response of (La, Y)₂O₃/SnO₂ stacked devices and the thickness of sputtered (La, Y)₂O₃ films was investigated. The device had a 100-nm-thick SnO₂ and a (La, Y)₂O₃ film with a thickness in the range of 15 – 90 nm. The CO₂ responses both in dry and humid environments (20%RH) increased with increasing the (La, Y)₂O₃ thickness. Additionally, a larger response was observed in the device with a (La, Y)₂O₃ film deposited by sputtering followed by spin-coating deposition. This two-step deposition of the (La, Y)₂O₃ film may result in an increase in the surface area, leading to the response increase.

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7.

Metal oxide semiconductor shows fairly good sensing performance to ppm concentration of various gases. It’s well-known that the formation of pn junction is very effective for improving their performance. This experimental evidence is based on the concept that the large amount of adsorbed oxygen is caused by the expansion of the depletion layer. However, nobody had succeeded to visualize the expansion of the depletion layer on oxide semiconductor thin film. In this study, the gas sensors with pn junction were fabricated by using vacuum evaporation and heat-treatment to investigate the pn junction effect on the sensor response to 3 ppm H₂S. We also tried to visualize the depletion layer as the potential barrier using Kelvin Probe Force Microscope (KFM).

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8.

La-doped MgFe₂O₄ (MgFe₂-xLaxO₄) powders were prepared using a malic acid complex and their CO-sensing properties examined across a temperature range from 300 to 500 °C under humid conditions. The addition of La considerably enhanced the MgFe₂O₄ sensor’s CO sensitivity. Of the La-doped sensors investigated, the sensor with 7.5 mol% La-doped MgFe₂O₄ showed the highest sensitivity in the operating temperature between 300 and 350 °C. At 300 °C, the sensitivity to CO gas was estimated to be 11.2 times higher than that of the sensor fabricated using pure MgFe₂O₄ powder. At 350 °C, the electrical resistance of the MgFe_1.85La_0.15O₄ sensor to 0 or 100 ppm CO increased with relative humidity (RH%). With constant humidity, the gas sensitivity (ΔR) of the MgFe_1.85La_0.15O₄ sensor varied linearly with the logarithm of the CO concentration from 100 to 500 ppm range under humid conditions. For the MgFe_1.85La_0.15O₄ sensor, gas sensitivity increased with relative humidity.

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9.

It was clarified, in our previous studies, that MgO-Fe₂O₃ composite oxides showed fairly good sensor response to 10 ppb H₂S, suggesting that MgO behaves as p-type and MgFe₂O₄ as n-type. The increase in the depletion layer with the formation of the p-n junction reflected the increase in sensor response. In this study, we prepared the core-shell spherical particles with Fe₂O₃ spherical particles as a core and MgO as a shell in order to measure those potential barrier heights. It was clarified by HAADF-STEM that the spherical particles had a hollow structure and microcrystals of MgFe₂O₄ were distributed at the particles from outside to inside. The lattice fringes in the microcrystal of HR-TEM image were identified as the (220) and (311) planes of MgFe₂O₄ within an error of 3%. It is considered that MgFe₂O₄ microcrystals were formed by diffusion and solid solution of Mg from the outside of spherical particles to the inside. From measuring the surface potential by KFM, not only the n-n junction by Fe₂O₃ and MgFe₂O₄ but also the p-n junction by MgFe₂O₄ and MgO were formed. Two junctions were appeared as the potential barrier height. It was suggested that MgO behaved as a semiconductor based on the temperature dependence of electrical resistance.

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10.

Sensing properties of adsorption/combustion-type gas sensors utilizing 5 wt% Pt-loaded alumina (5Pt/γ(r)α(t)-Al₂O₃ sensors, r and t: the mixing ratio of γ-Al₂O₃ and α-Al₂O₃ as a catalytic support (wt%)) to volatile organic compounds (VOCs, ethanol and toluene) have been investigated in this study. The magnitude of general responses and the integrated dynamic responses of the 5Pt/γ(50)α(50)-Al₂O₃ sensor were the largest among the series of sensors, which were probably affected by the adsorption property and the oxidation activity of ethanol on their surface and their thermal conductance. On the other hand, their integrated dynamic response gradually increased with an increase in the amount of α-Al₂O₃, probably because their improved thermal conductance and oxidation activities per unit surface area. We will discuss their sensing properties to ethanol as well as toluene in the symposium, on the basis of further detailed results.

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11.

Nitric oxide (NO) contained in human breath has a correlation with the extent of asthma, and clinical tests using exhaled breath have attracted attention in the medical field. We developed an analytical chip for detection NO composed of a porous glass and a 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxyde-1-oxyl (PTIO). In this study, to measure the NO concentration in exhaled air with high sensitivity, we devised a measurement method using two types of analytical chips. PTIO chip converted NO into NO₂, which was found to be 1:1 relationship. Therefore, we devised a method of measuring the converted NO₂ using a NO₂ analytical chip, and two types of chips were simultaneously exposed to the NO atmosphere. The percentage of converted NO gradually increased with time, and that was 93% ± 2% and 99% ± 1% at 12 hours and 24 hours, respectively. There was a linear relationship between the logarithmic change in absorbance at 567 nm of the PTIO chip and exposure time, but the accuracy and sensitivity were found to be low. For the developed method using NO₂ analytical chip, high correlation was obtained between the amount of change in absorbance at 525 nm and the exposed NO concentration. These results suggested that the method using the two types of chips enables high sensitivity measurement of NO in exhaled air.

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Plenary Lecture

This presentation will first introduce why we need the E-nose and how to build the intelligent gas sensing array system based on both CMOS-based resistive type and surface acoustic wave (SAW) type sensing mechanisms. Furthermore, several applications would be developed based on the developed gas sensing system. A plug-in handheld smart gas sensing device for freshness of food and other safety; Beverage manufacture and quality detection including tea, coffee bean, and red wine, etc.; Indoor or outdoor environmental gas sensing system for PM 2.5, second hand smoke detection, and other hazardous gas; Medical diagnostic applications; and other IOT applications, etc.

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Keynote Lecture

In conventional gas sensing material including metal oxide, the high-temperature operation (>200 oC) is necessary for response and recovery, which has a drawback of power consumption. Graphene with superior properties including high surface-to-volume ratio and high conductivities can be a good candidate of the gas sensing material. The most attractive behavior of graphene-based gas sensor is the room-temperature operation. We already demonstrated the acetone sensing of 600 ppb of 1.9% response at room temperature. Due to the good adsorption of gas molecular on graphene, the response and recovery can be further improved by UV illumination for a long-term usage. With a short-time UV illumination of 5 min, the response can be 7-fold (7X) higher and recovery can be fully returned to baseline with a very low power consumption of UV LED. We also observed the spacing of interdigital electrodes from 50 to 400 μm can help on the acetone sensitivity for 4X higher, which may be from the doping effect by glass substrate. Furthermore, a rapid thermal annealing treated on graphene can also help for approximately 4X in NO₂ sensing response with more than 95% recovery by UV illumination.

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