• Journal of Sensor Science and Technology
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• v.30 no.2
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• pp.94-98
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• 2021

In this study, pure and Co3O4/CoFe2O4-loaded Indium oxide (In2O3) nanofibers were synthesized by the electrospinning of an Indium/Polyvinylpyrrolidone precursor solution containing cobalt and iron bimetallic zeolitic imidazolate frameworks and subsequent heat treatment. The ethanol, toluene, p-xylene, benzene, carbon monodxide, and hydrogen gas sensing characteristics of the solution were measured at 250-400 ℃. 0.5 at%-Co3O4/CoFe2O4-loaded In2O3 nanofibers exhibited extreme response (resistance ratio - 1) to 5 ppm of ethanol (210.5) at 250 ℃ and excellent selectivity over the interfering gases. In contrast, pure In2O3 nanofibers exhibited relatively low responses to all the analyte gases and low selectivity above 250-400 ℃. The superior response and selectivity toward ethanol is explained by the catalytic roles of Co3O4 and CoFe2O4 in gas sensing reaction and the electronic sensitization induced by the formation of p (Co3O4/CoFe2O4)-n (In2O3) junctions.

• Journal of Sensor Science and Technology
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• v.25 no.3
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• pp.218-222
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• 2016

Pure and V-doped NiO porous structures were prepared by the evaporation-induced surfactant assembly and subsequent pyrolysis of assembled structures, and their gas sensing characteristics were investigated. Pure NiO porous structures showed negligible gas responses (S=$R_g/R_a$, $R_g$: sensor resistance in analytic gas; $R_a$: sensor resistance in air) to 5 ppm trimethylamine (S=1.17) as well as other interfering gases such as ethanol, p-xylene, toluene, benzene and formaldehyde (S=1.02-1.13). In contrast, the V-doped NiO porous structures exhibited a high response and selectivity to 5 ppm trimethylamine (S=14.5) with low cross-responses to other interfering gases (S=4.0-8.7) at $350^{\circ}C$. The high gas response of V-doped NiO porous structures to trimethylamine was explained by electronic sensitization, that is, the increase in the chemoresistive variation due to the decrease in the hole concentration. The enhanced selectivity to trimethylamine was discussed in relation to the interaction between basic trimethylamine gas and acidic V catalysts.

• Ceramist
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• v.22 no.1
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• pp.70-81
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• 2019

Breath analysis is rapidly evolving as a non-invasive disease recognition and diagnosis method. Metal oxide gas sensors are one of the most ideal platforms for realizing portable, hand-held breath analysis devices in the near future. This paper reviewed the recent developments in metal oxide gas sensors detecting exhaled biomarker gases such as nitric oxides, acetone, ammonia, hydrogen sulfide, and hydrocarbons. Emphasis was placed on strategies to tailor sensing materials/films capable of highly selective and sensitive detection of biomarker gases with negligible cross-response to ethanol, the major interfering breath gas. Specific examples were given to highlight the validity of the strategies, which include optimization of sensing temperature, doping additives, utilizing acid-base interaction, loading catalysts, and controlling gas reforming reaction. In addition, we briefly discussed the design and optimization method of gas sensor arrays for implementing the simultaneous assessment of multiple diseases. Breath analysis using high-performance metal oxide gas sensors/arrays will open new roads for point-of-care diagnosis of diseases such as asthma, diabetes, kidney dysfunction, halitosis, and lung cancer.

• Journal of Sensor Science and Technology
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• v.28 no.5
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• pp.271-276
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• 2019

Pure and 0.5, 1, 2, 5, and 10 at% of Mo-doped $Co_3O_4$ yolk-shell spheres were synthesized by ultrasonic spray pyrolysis of droplets containing Co nitrate, ammonium molybdate, and sucrose and their gas sensing characteristics to 5 ppm trimethylamine (TMA), ethanol, p-xylene, toluene, ammonia, carbon monoxide, and benzene were measured at $225-325^{\circ}C$. The sensor using pure $Co_3O_4$ yolk-shell spheres showed the highest response to p-xylene and very low response to TMA at $250^{\circ}C$, while the doping of Mo into $Co_3O_4$ tended to increase the overall responses of gas sensors. In particular, the sensor using 5 at% Mo-doped $Co_3O_4$ yolk-shell spheres exhibited the high response to TMA with low cross-responses to other interfering gases. The high response and selectivity of Mo-doped $Co_3O_4$ yolk-shell spheres to TMA are attributed to the electronic sensitization by higher valent Mo doping and acid-base interaction between TMA and Mo components.

• Journal of Sensor Science and Technology
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• v.30 no.4
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• pp.191-195
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• 2021

Pure ZnFe₂O₄ and Fe₂O₃-ZnFe₂O₄ hetero-composite spheres were prepared by ultrasonic spray pyrolysis of a solution containing Zn- and Fe-nitrates. Additionally, the sensing characteristics of these spheres in the presence of 5 ppm ethanol, benzene, p-xylene, toluene, and CO (within the temperature range of 275-350 ℃) were investigated. The Fe₂O₃-ZnFe₂O₄ hetero-composite sensor with a cation ratio of [Zn]:[Fe]=1:3 exhibited a high response (resistance ratio = 140.2) and selectivity (response to p-xylene/response to ethanol = 3.4) to 5 ppm p-xylene at 300 ℃, whereas the pure ZnFe₂O₄ sensor showed a comparatively lower gas response and selectivity. The reasons for the superior response and selectivity to p-xylene in Fe₂O₃-ZnFe₂O₄ hetero-composite sensor were discussed in relation to the electronic sensitization due to charge transfer at Fe₂O₃-ZnFe₂O₄ interface and Fe₂O₃-induced catalytic promotion of gas sensing reaction. The sensor can be used to monitor harmful volatile organic compounds and indoor air pollutants.

• Journal of Sensor Science and Technology
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• v.29 no.2
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• pp.112-117
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• 2020

Xylene is a hazardous volatile organic compound that should be precisely measured to monitor indoor air quality. However, the selective and sensitive detection of ppm-level xylene using oxide-semiconductor gas sensors remains a challenge. In this study, pure and Cr-doped Co₃O₄ nanoparticles (NPs) were prepared using flame spray pyrolysis, and their gas-sensing characteristics to 5-ppm xylene at 250 ℃ were investigated. The 4 at% Cr-doped Co₃O₄ NPs exhibited a high gas response to 5-ppm xylene (resistance ratio to gas and air = 39.1) and negligible cross-responses to other representative and ubiquitous indoor pollutants such as ethanol, benzene, formaldehyde, carbon monoxide, and ammonia. In this paper, the enhancement of the gas response and selectivity of Co₃O₄ NPs to xylene by Cr doping was discussed in relation to the catalytic promotion of the gas-sensing reaction. This sensor can be used to monitor indoor xylene.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.21 no.9
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• pp.783-788
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• 2008

We suggest a CNT-based gas sensor for breath alcohol measurement. The sensor was composed of single-walled carbon nanotubes (SWCNTs) thin film on flexible PES (polyethersulfone) substrate, and the SWCNTs thin film was formed by multiple spray-coating with SWCNTs solution which was well-dispersed, highly controlled and functionalized in ethanol solvent. In this work, three types of SWCNTs thin films were deposited with changes in the number of spray-coatings to 20, 40 and 60 times in order to compare electrical response properties of the SWCNTs thin films. from the fabricated sensors, conductance and capacitance responses were measured and discussed. Alcohol gas sensors have been commercialized widely as gauge for breath alcohol measurement which is applicable to checking whether car drivers are drinking-driving or not. Our alcohol gas sensors showed good sensitivity and linearity even at room temperature.

• Journal of the Korean Physical Society
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• v.73 no.10
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• pp.1437-1443
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• 2018

In this work, $SnO_2$ modified with reduced graphene oxide (rGO) and carbon nanotubes (CNTs) separately and combined sensitized by using the co-precipitation method and their sensing behavior toward ethanol vapor at room temperature were investigated. An interdigitated electrode (IDE) gold substrate is very expensive compared to a fluorine doped tin oxide (FTO) substrate; hence, we used the latter to reduce the fabrication cost. The structure and the morphology of the studied materials were characterized by using differential thermal analyses (DTA) and thermogravimetric analysis (TGA), transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller surface area and Barrett-Joyner-Halenda (BJH) pore size measurements. The studied composites were subjected to ethanol in its gas phase at concentrations from 10 to 200 ppm. The present composites showed high-performance sensitivity for many reasons: the incorporation of $SnO_2$ and CNTs which prevents the agglomeration of rGO sheets, the formation of a 3D mesopourus structure and an increase in the surface area. The decoration with rGO and CNTs led to more active sites, such as vacancies, which increased the adsorption of ethanol gas. In addition, the mesopore structure and the nano size of the $SnO_2$ particles allowed an efficient diffusion of gases to the active sites. Based on these results, the present composites should be considered as efficient and low-cost sensors for alcohol.

• Journal of Sensor Science and Technology
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• v.24 no.6
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• pp.419-422
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• 2015

Pure and Mo-doped ZnO hollow nanofibers were prepared by single capillary electrospinning and their gas sensing characteristics toward 5 ppm ethanol, trimethylamine (TMA), CO and $H_2$ were investigated. The gas responses and responding kinetics were dependent upon sensing temperature and Mo doping. Mo-doped ZnO hollow nanofibers showed high response to 5 ppm TMA ($R_a/R_g=111.7$, $R_a$: resistance in air, $R_g$: resistance in gas) at $400^{\circ}C$, while the responses of pure ZnO hollow nanofibers was low ($R_a/R_g=47.1$). In addition, the doping of Mo enhanced selectivity toward TMA. The enhancement of gas response and selectivity to TMA by Mo doping to ZnO nanofibers was discussed in relation to the interaction between basic analyte gas and acidic additive materials.

• Journal of Sensor Science and Technology
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• v.28 no.4
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• pp.231-235
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• 2019

We investigated the facile and effective strategy for sensitive and selective $C_2H_5OH$ sensors based on the In-decorated NiO nanoigloos. The In-decorated NiO nanoigloos is fabricated by RF sputtering using 750 nm-diameter polystyrene beads using a soft-template. The morphological evolution based on the Van der Drift model was generated through a heterojunction between In metal and NiO, resulting in a pyramidal rough surface. Upon decorating the In on the NiO surface, high sensitivity and selectivity to $C_2H_5OH$ were observed, and gas sensing mechanism was demonstrated by a high surface-to-volume and double Schottky barrier. We are confident that the method presented in this study will have a significant impact on the fabrication of effective nanostructures and their application for the gas sensors.