Search

Abstract: High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors. [Copyright &y& Elsevier]

Abstract: Hierarchical and hollow oxide nanostructures are very promising gas sensor materials due to their high surface area and well-aligned nanoporous structures with a less agglomerated configurations. Various synthetic strategies to prepare such hierarchical and hollow structures for gas sensor applications are reviewed and the principle parameters and mechanisms to enhance the gas sensing characteristics are investigated. The literature data clearly show that hierarchical and hollow nanostructures increase both the gas response and response speed simultaneously and substantially. This can be explained by the rapid and effective gas diffusion toward the entire sensing surfaces via the porous structures. Finally, the impact of highly sensitive and fast responding gas sensors using hierarchical and hollow nanostructures on future research directions is discussed. [Copyright &y& Elsevier]

Abstract: Sn-precursor nanowires (NWs) were prepared by refluxing a solution containing tin oxalate (SnC2O4) and ethylene glycol. The precursor NWs were heat-treated at 600°C, which successfully transformed them into SnO2 NWs without undergoing any morphological change. The morphology of the SnO2 NWs could be manipulated either by controlling the SnC2O4 concentration or by adding easily reducible noble metal salts, such as PdCl3, AgNO3 or RuCl3·H2O. The thicker and shorter SnO2 NWs, which resulted from the use of a high SnC2O4 concentration during the preparation, showed lower gas sensitivity due to their decreased gas sensing surface area. The addition of catalytically active Pd, Ag, or Ru not only changed the gas sensitivity but also significantly shortened the gas recovery times. The gas sensing characteristics of the SnO2 NWs were related to the NW morphology and, when applicable, the catalyst used. [Copyright &y& Elsevier]

An amperometric CO2 sensor was fabricated using a Pt|NASICON (Na+ conductor, Na3Zr2Si2PO12)|Pt cell and a porous Na2CO3–BaCO3 (1:1.7 in molar ratio) auxiliary layer. The sensor with the Na2CO3–BaCO3 layer showed a faster response than that with the Na2CO3 layer, and the sensor signal was barely influenced by the humidity. When operated in an amperometric mode with an applied external electric field, the sensor could detect the CO2 concentration stably and precisely without the need for a reference gas or a reference electrode material, and the sensor signal was constant for extended test periods (60 days). The sensing mechanism and the long-term stability is discussed in terms of the amperometric operation of the sensor. [Copyright &y& Elsevier]

A solid-state amperometric CO2 sensor was fabricated by combining a Li+-ion conductor (Li2CO3–Li3PO4–Al2O3) with a porous layer and a Li2CO3 auxiliary layer deposited on sensing and counter electrodes, respectively. The Li+-ion pumping current increased linearly with increasing CO2 concentration ranging from 200 to 2000 ppm at 400 °C and the 90% response time was approximately 55 s. This sensor was able to reliably detect a small change in the CO2 concentration. [Copyright &y& Elsevier]

Pristine SnO 2 nanowires (NWs), Cr 2 O 3 -decorated SnO 2 NWs, and SnO 2 –Cr 2 O 3 core-shell nanocables (NCs) were prepared by thermal evaporation, and their gas-sensing characteristics were investigated. The decoration of discrete p-type Cr 2 O 3 nanoclusters on SnO 2 NWs increased their response to trimethylamine (TMA). Highly sensitive and selective detection of TMA was attributed to the chemical affinity and catalytic activity of Cr 2 O 3 toward TMA. In contrast, the SnO 2 –Cr 2 O 3 NCs, formed by the coating of a continuous p-type Cr 2 O 3 overlayer on SnO 2 NWs, showed negligibly low responses to all analyte gases used. The variations in gas-sensing characteristics due to configuration changes in one-dimensional SnO 2 –Cr 2 O 3 hetero-nanostructures are explained and discussed in relation to the gas-sensing mechanisms of n- and p-type oxide semiconductors. [ABSTRACT FROM AUTHOR]

Abstract: Three different compositions of ZnO–In2O3 composite nanofibers were prepared by electrospinning, and their gas sensing characteristics were compared to those of pure ZnO and In2O3 nanofibers. All ZnO–In2O3 composite nanofibers showed high response to trimethylamine (TMA), relatively low cross-response to C2H5OH, and negligible cross-responses to CO and H2; pure ZnO or In2O3 nanofibers did not show selective detection of TMA. The maximum responses to 5ppm TMA of the ZnO–In2O3 composite nanofibers with the compositions of [Zn]:[In]=67:33, 50:50, and 33:67 by at% were 133.9 at 300°C, 82.9 at 350°C, and 119.4 at 375°C, respectively. Considering all the sensing characteristics such as gas response, selectivity, and sensing/recovering kinetics together, the operation of the ZnO–In2O3 nanofiber sensor with the composition of [Zn]:[In]=33:67 by at% at 375°C was determined to be in optimal condition to detect TMA. The significant enhancement of gas response and selectivity by the formation of composite nanofibers is discussed in relation to the variation of particle size, the formation of hetero-interfaces between ZnO and In2O3, and the combination of two sensing materials with different catalytic properties, gas adsorption behaviors, and acid–base properties. [Copyright &y& Elsevier]

Abstract: WO3 hollow spheres were prepared by ultrasonic spray pyrolysis of a citric-acid-containing precursor solution, and their trimethylamine-sensing characteristics were investigated. The WO3 hollow spheres showed a high response to 5ppm of trimethylamine (R a/R g =56.9, R a: resistance in air, R g: resistance in gas) with low cross-responses to 5ppm C2H5OH, CO, CH4, C3H8, H2, NH3, and NO2. The high gas response observed was explained by the effective electron depletion of small primary particles and the enhanced gas accessibility of hollow structures with thin and porous shells. The enhanced trimethylamine selectivity was discussed in terms of the interaction between basic analyte gas and acidic sensing materials. [Copyright &y& Elsevier]

Abstract: Punched ZnO nanobelt (NB) network sensors with strong adhesion to Au electrodes were fabricated by the vapor-phase growth of ZnS NB networks and subsequent thermal annealing. The punched ZnO NB networks showed sensitive and selective detection of trimethylamine (TMA), while no significant sensitivity or selectivity to a specific gas was observed in dense ZnO NB networks. This high TMA response can be explained by abundant chemoresistive contacts, effective electron depletion, and high surface area to volume ratio of punched ZnO NB networks. [Copyright &y& Elsevier]

Abstract: The Co3O4 nanofibers were prepared by electrospinning and their gas sensing characteristics were investigated. The Co3O4 sensors prepared by heat treatment of as-spun precursor fibers at 500 and 600°C showed well-developed one-dimensional morphologies and exhibited high responses to 100ppm C2H5OH (R g/R a =51.2 and 45.3; R g, resistance in gas; R a, resistance in air) at 301°C with negligible cross-responses to 100ppm CO, C3H8, and H2 (R g/R a =1.02–2.7). In contrast, the most of one-dimensional morphology of the Co3O4 specimen was lost and the response to 100ppm C2H5OH became significantly lower when the heat treatment temperature was increased to 700°C or when the nanofibers were ultrasonically disintegrated into primary particles. The significant decrease of the gas response was explained and discussed in relation to the gas sensing mechanism of a p-type semiconductor, the morphology of specimens, and the connecting configuration between nanoparticles and nanofibers. [Copyright &y& Elsevier]

Abstract: Hierarchical and hollow In2O3 microspheres were prepared by a solvothermal self-assembly reaction and their CO sensing characteristics were compared to those of an agglomerated counterpart. Upon exposure to 10–50ppm CO at 400°C, the gas responses (R a /R g ; R a : resistance in air, R g : resistance in gas) of the hierarchical and hollow structures were 1.6–2.7 times higher than that of the agglomerated sample. In addition, their 90% responses and recovery speeds were 23 times and 7.7 times higher, respectively. Their significantly improved gas responses and sensing/recovery kinetics were concluded to be related to the nanoporous and well-aligned nanostructures, which allow effective and rapid gas diffusion toward the sensing surfaces. [Copyright &y& Elsevier]

Abstract: The Sn3O4 hierarchical microspheres self-assembled from nanosheets were prepared by a hydrothermal reaction of aqueous solution containing SnCl2·2H2O, H2C2O4, HCl, and N2H4·H2O, which were successfully transformed into nano-porous SnO2 hierarchical microspheres for gas sensor applications by heat treatment at 600°C. The morphology of building blocks within the SnO2 hierarchical spheres could be manipulated from 0-dimensional nanoparticles to 2-dimensional nanosheets by varying the amount of H2C2O4 and N2H4·H2O. The SnO2 hierarchical spheres showed both the ultra-fast response (∼1s) and high sensitivity to 50ppm C2H5OH. The dramatic improvement in gas sensing characteristics was explained by the rapid diffusion of the target gas onto the entire sensing materials through the nano-porous network of nanosheets. [Copyright &y& Elsevier]

Abstract: The CO response time of SnO2 acicular-particle-type gas sensors was shortened to a great extent using rapid microwave calcination. When SnO2 acicular particles were prepared by the slow heating (heating rate: 4.2°C/min) of a SnC2O4 precursor to the calcination temperature (500°C) using a conventional furnace, the 90% response time to 30ppm CO (t 90%(air-to-CO)) was 76s at a sensing temperature of 450°C. However, the t 90%(air-to-CO) decreased markedly down to 5–27s when the precursor was heated rapidly (heating rate: 25–100°C/min) using a microwave furnace. The significant shortening of the response time was attributed to the formation of a mesoporous structure in the SnO2 acicular particles due to rapid gas evolution during the decomposition of the SnC2O4 precursor. [Copyright &y& Elsevier]

Abstract: Porous TiO2 films were prepared by the micro-arc oxidation (MAO) of Ti plates, and their CO sensing properties at low concentrations (5–100ppm) were investigated as an application for an air quality control sensor. The obtained rutile films exhibited the maximum CO gas response at 350°C, which is typical for semiconductor-type gas sensors, and the R a/R g was ∼1.6 for 10ppm CO (R a: resistance in air, R g: resistance in a sample gas). The magnitude of the gas response increased almost linearly with increasing the CO concentration from 5 to 100ppm. The CO sensing performance of the TiO2 sensor in the presence of humidity was investigated and compared with that of a SnO2 sensor. [Copyright &y& Elsevier]

Abstract: A highly sensitive H2 gas sensor was prepared by the thermal oxidation of a Ti plate at 600–1000°C. The H2 sensitivity (the ratio of the resistances between N2 and 1.0% H2 balanced with N2) of the oxidized TiO2 increased exponentially with increasing oxidation temperature up to 900°C but decreased drastically at 1000°C. The maximum sensitivity to 1.0% H2 was 1.2×106, which is the highest value reported in the literature. The mechanism for the enhanced H2 sensing was examined by investigating the phase, thickness and morphology of the oxidized TiO2 layer and the sensor response time. The thermally oxidized specimens exhibited an approximately linear dependence of the sensitivity on the H2 concentrations from 50 to 10,000ppm, and excellent sensitivity (∼103) was still obtained in the sensor operated at temperatures as low as 150°C. [Copyright &y& Elsevier]

Highlights • Fabrication of a sensor array using pure and Fe-doped In 2 O 3 nanofibers. • Discriminative detection of benzene, xylene, toluene, ethanol, and formaldehyde. • Distinction between aromatic and non-aromatic indoor pollutants using sensor array. • Gas sensing mechanism underlying Fe-induced change in response and selectivity. Abstract Representative indoor volatile organic compounds (VOCs) such as benzene, xylene, toluene, formaldehyde, and ethanol need to be detected in a highly sensitive and discriminative manner because of their different impact on human health. In this study, pure and 0.05, 0.1, 0.3, and 0.5 at% Fe-doped In 2 O 3 nanofibers were prepared by electrospinning and their gas sensing characteristics toward the aforementioned VOCs were investigated. The doping of In 2 O 3 nanofiber sensor with 0.05 and 0.1 at% Fe shifted the temperature to show the maximum responses to benzene, xylene, and toluene, and reduced responses to ethanol and formaldehyde, thus demonstrating changed gas selectivity. The gas sensing characteristics of 0.5 at% Fe-doped In 2 O 3 nanofiber sensor were substantially different from those of the other sensors. Significantly different gas sensing patterns of pure and Fe-doped In 2 O 3 sensors could be used to discriminate between the five different VOCs at 375 °C and to distinguish between the aromatic and non-aromatic gases at all sensing temperatures. The mechanism underlying the Fe-induced change in gas sensing characteristics has been discussed in relation to the variation of catalytic activity, morphology, oxygen adsorption, and charge carrier concentration. [ABSTRACT FROM AUTHOR]

Highlights • The synthesis of high performance gas sensors based on hematite nanotubes with Au catalyst has been demonstrated. • The Au decorated hematite gas sensor exhibits highly sensitive detection to acetone. • The highly improved sensitivity and selectivity in wet air are attributed to the spill-over effect by the Au catalyst. Abstract Vertically ordered hematite nanotubes decorated with Au nanoparticles are synthesized by a facile electrochemical anodization method and self-agglomeration of Au films. Au decoration significantly enhances the response of hematite nanotubes to acetone at 350 °C and the response to acetone is 2.4 times higher than the response to ethanol, the counterpart target gas. The Au nanoparticles decorated hematite nanotubes exhibit unprecedentedly high and selective responses to acetone at 350 °C with 50% relative humidity condition (calibrated at room temperature, 25 °C). The enhanced gas sensing properties in a humid condition were attributed to the spillover effect by Au nanoparticles and high porosity of hematite nanotubes with a large surface-to-volume ratio. These results indicate that Au nanoparticles decorated hematite nanotubes are promising for use in high quality sensor materials for breath analyzers to diagnose diabetes mellitus. [ABSTRACT FROM AUTHOR]

The lack of gas selectivity in oxide semiconductor chemiresistors has long been an obstacle to realizing discriminative detection of indoor volatile organic compounds (VOCs) with different health impacts. A simple and reliable algorithm to discriminate between critically harmful aromatic VOCs (benzene, xylene, and toluene) and less harmful ethanol is suggested by the simple combination of sensor signals from pure In 2 O 3 and Au-loaded In 2 O 3 2D inverse opal (IO) thin films prepared by heat-treating the precursor-dipped self-assembled polystyrene templates and Au deposition. The Au-loaded In 2 O 3 IO sensor showed unprecedentedly high responses to 5 ppm ethanol (resistance ratio = 1640.2) and comparably high responses to 5 ppm benzene, p- xylene, and toluene (resistance ratio range of 674.5–1012.9). Such high gas responses were attributed to the periodically porous and thus highly gas-accessible structures, while the clear discrimination between aromatic VOCs and ethanol was achieved by tuning gas selectivity through systematic control of the size, morphology, and loading concentration of Au nano-catalysts. The results of this study can be used for reliable and precise monitoring of indoor air pollutants. [ABSTRACT FROM AUTHOR]

A Highly ordered CdS nanoflake array was fabricated by CVD, and its gas sensing characteristics were investigated. The sensor exhibited high response (resistance ratio) of 89% to 5 part per million (ppm) nitrogen dioxide (NO 2 ) under green LED illumination (wavelength 500–540 nm, irradiance 21 W/m 2 ) with excellent selectivity and little interference by humidity. Moreover, the sensor showed promising potential for operating under fluorescent lamp and natural solar light, which can be used for medical diagnosis and indoor/outdoor environment monitoring. This performance is attributed to the low band gap energy (2.4 eV) of CdS and the unique morphology of nanoflake array which can enhance both the light absorption and conductivity. [ABSTRACT FROM AUTHOR]

Reduced graphene oxide (rGO)-In 2 O 3 hybrid materials were prepared by a solvothermal reaction of an In precursor containing rGO sheets, which were coated onto flexible substrates for gas sensors. The rGO-In 2 O 3 flexible sensors showed a high and reversible response (resistance ratio = 22.3) to 500 ppb NO 2 at 150 °C and negligible cross-responses to C 2 H 5 OH, CO, NH 3 , toluene, H 2 , and HCHO. The ultrahigh response and selectivity of rGO-In 2 O 3 hybrid materials to NO 2 were attributed to the chemical affinity of rGO to NO 2 and the extension of the electron depletion layer in n -type In 2 O 3 forming a p - n junction with the p -type rGO. [ABSTRACT FROM AUTHOR]

Remarkable efforts have been devoted for enhancing gas selectivity of chemiresistors via tuning the sensing materials. However, the selective detection of low reactive gases remains challenging. Herein, we report a new strategy for selective detection of low reactive gases by patterning nanofibers and tuning the catalytic property of electrode. In this approach, straight single In 2 O 3 nanofibers are patterned onto Au, Pt, and ITO interdigitated electrodes (IDEs) via direct-write near-field electrospinning; the resulting low coverage of the sensing materials (∼0.12%) exposes the electrode to analyte gases. The gas sensing characteristics of the sensors are determined by the catalytic activity of each electrode. Furthermore, the functionalization of Pt IDE with Au nanoparticles could achieve extremely high selectivity and response toward xylene. The sensing properties and mechanisms of the nanopatterned sensors are investigated regarding electrode composition, degree of electrode exposure, and catalyst location on the electrode and/or sensing materials. The key strategies for achieving high selectivity are the conversion of low reactive xylene gas into more reactive intermediate species while highly reactive interference gases are completely oxidized at open catalytic electrodes. Catalyst functionalization and exposure of electrodes can provide new guidelines for designing high performance gas sensors for new applications. [Display omitted] • 1D metal oxide single nanofiber patterns with exposed electrodes are prepared via near-field electrospinning methods. • The effect of the catalytic activity of the electrode on gas sensitivity and selectivity are described. • Au functionalized Pt electrodes improved responses and selectivity of In 2 O 3 single nanofibers to xylene gas. • A potential of patterned sensors as indoor air quality monitoring application is presented. [ABSTRACT FROM AUTHOR]

An ultrasensitive and selective trimethylamine (TMA) sensor was fabricated by the urea-assisted uniform loading of Au nanoparticles (∼5 nm) on Cr 2 O 3 yolk-shell spheres prepared by ultrasonic spray pyrolysis. The response (resistance ratio) of the Cr 2 O 3 yolk-shell spheres uniformly loaded with fine Au nanoparticles to 5 ppm TMA was 200.9 at 225 °C, which was significantly higher than those of pure Cr 2 O 3 yolk-shell spheres (24.6) and Cr 2 O 3 yolk-shell spheres loaded with heavily agglomerated Au particles (9.2). The detection limit of Cr 2 O 3 yolk-shell spheres uniformly loaded with Au nanoparticles was as low as 4.3 ppb. The unprecedentedly high TMA response is explained and discussed in relation to the gas-accessible morphology of the yolk-shell spheres, and the catalytic and electronic promotion of the gas sensing reaction by both Cr 2 O 3 and well-dispersed Au nanoparticles. [ABSTRACT FROM AUTHOR]

Cr 2 O 3 /ZnCr 2 O 4 nanocomposite powders and phase-pure ZnCr 2 O 4 powders were prepared by the galvanic replacement of Zn in ZnO hollow spheres by Cr and subsequent heat treatment. The Cr 2 O 3 /ZnCr 2 O 4 nanocomposite powders prepared by galvanic replacement consisted of nanocrystalline Cr 2 O 3 and ZnCr 2 O 4 particles, which showed a high response (resistance ratio) of 69.2 to 5 ppm xylene at 275 °C and excellent xylene selectivity. In contrast, both Cr 2 O 3 and ZnO powders showed selectivity to ethanol, and no significant selectivity to a specific gas was found for the ZnCr 2 O 4 powders. The Cr 2 O 3 /ZnCr 2 O 4 composite powders consisting of coarse particles prepared by solid-state reaction showed relatively low response and selectivity to xylene. The high selectivity and response to xylene of the Cr 2 O 3 /ZnCr 2 O 4 nanocomposite powders were attributed to the synergistic promotion of the methylbenzene-sensing reaction by two sensing materials with different catalytic activities and the high chemiresistive variation of the small particles, respectively, both of which result from the intimate and uniform mixing of nanocrystalline Cr 2 O 3 and ZnCr 2 O 4 particles. [ABSTRACT FROM AUTHOR]

The pure and 0.3 wt% Pt-doped SnO 2 hollow nanospheres were prepared by the oxidation of pure and Pt-doped Sn nanoscrystals embedded in carbon matrix and their gas sensing characteristics were investigated. The formation of hollow morphology was attributed to the nanoscale Kirkendall effect due to rapid outward diffusion of Sn ions and relatively slow inward diffusion of oxygen. Pure SnO 2 hollow nanospheres showed a high response (resistance ratio) of 93.3 when exposed to 5 ppm ethanol. The response to 5 ppm ethanol was significantly increased to 1399.9 with doping 0.3 wt% Pt. In addition, selectivity to ethanol was also enhanced by Pt doping. Ultrasensitive and selective detection of ethanol in pure and Pt-doped SnO 2 nanospheres is explained by the effective electron depletion in hollow structures and catalytic promotion of gas sensing reaction. [ABSTRACT FROM AUTHOR]

Triple-shelled, double-shelled, and dense WO 3 spheres were prepared by ultrasonic spray pyrolysis and their NO 2 gas sensing characteristics were investigated. Triple-shelled WO 3 yolk–shell spheres showed an unprecedentedly high response (resistance ratio = 100–49.0) to 50 ppb NO 2 at 100–200 °C with negligible cross-responses to 5 ppm acetone, CO, NH 3 , ethanol, and toluene, whereas dense WO 3 spheres showed low NO 2 response (8.9–3.1). The response and selectivity to NO 2 were maintained in a highly humid (relative humidity 80%) atmosphere. The ultrahigh response and selectivity of triple-shelled WO 3 spheres to several tens ppb levels NO 2 was attributed to the enhanced gas accessibility of the yolk–shell morphology with thin and permeable multiple shells. [ABSTRACT FROM AUTHOR]

Urchin-like Mg-doped ZnO nanowire networks were prepared by MgO-seeded vapor-phase growth of ZnO nanowires, and their potential as gas-sensing materials was investigated. The response (resistance ratio) of the urchin-like Mg-doped ZnO nanowire networks to 5 ppm C 2 H 5 OH at 350 °C was as high as 343, which is significantly higher than that of pure ZnO nanowire networks (7.0). In addition, the Mg-doped ZnO nanowire network sensors showed excellent selectivity to C 2 H 5 OH and an unprecedentedly high response (28.8) even to 0.25 ppm C 2 H 5 OH. The enhancement of the gas response and selectivity to C 2 H 5 OH was attributed to Mg-doping-induced decrease of the charge carrier concentration, the change of nanowire thickness/morphology, and the catalytic promotion of the C 2 H 5 OH sensing reaction. [ABSTRACT FROM AUTHOR]

Machine learning (ML) methodologies were applied to detect and discriminate five indoor volatile organic compounds (VOCs) such as benzene, xylene, toluene, formaldehyde, and ethanol using a sensor array constructed of five In 2 O 3 -based semiconducting metal oxide (SMO) gas sensors. The sensor array was evaluated using principal component analysis (PCA) and neural network-based classification in terms of the gas sensor data type/amount, neural network algorithms, sensor combinations, and environmental factors. The PCA analyses indicated the limitations on the discrimination of VOCs under temperature- and/or humidity-interfered gas sensing environments. Gas detection/discrimination could be improved significantly by using three supervised algorithms, i.e., artificial neural networks (ANNs), deep neural networks (DNNs), and 1-dimensional convolutional neural networks (1D CNNs). The neural network algorithm prediction based on the entire gas sensing/purge transient data outperforms deep learning-assisted predictions based on partial gas sensing transients. Compared to 1D CNNs, DNNs are more appropriate in terms of training/validation/test datasets. The effects due to humidity variation are more significant than those due to temperature fluctuation. A 2-sensor mode combination can be exploited to replace the 5-sensor operation in ML-based applications. The indoor pollutants can be successfully discriminated even under the variation of ambient humidity and temperature by ML-based approaches. • Gas interference features of In 2 O 3 -based sensors were combined with deep learning. • Numeric/image data formats were used for input into neural network models. • Humidity interference is proven to be more influential than temperature variations. • Gas detection/discrimination was improved significantly using neural networks. • Neural networks provide gas sensors high endurance against environmental effects. [ABSTRACT FROM AUTHOR]

Branched ZnO nanowires (NWs) doped with Ni were grown by a three-step vapor phase method for the sensitive and selective detection of p -xylene. ZnO NWs were directly grown on sensor substrates with Au electrodes, which were transformed into NiO NWs by the thermal evaporation of NiCl 2 powder at 700 °C. ZnO branches doped with Ni were grown from NiO NWs by the thermal evaporation of Zn metal powder at 500 °C. The stem NiO NWs played the role of catalyst for the growth of ZnO branches through vapor–liquid–solid mechanism. The Ni-doped branched ZnO NWs showed enhanced gas response ( S = resistance ratio) to methyl benzenes, especially to 5 ppm p -xylene ( S = 42.44) at 400 °C. This value is 1.7 and 2.5 times higher than the responses to 5 ppm toluene ( S = 25.73) and C 2 H 5 OH ( S = 16.72), respectively, and significantly higher than the cross-responses to other interfering gases such as benzene, HCHO, trimethylamine, H 2 , and CO. The selective detection of xylene was attributed to the catalytic role of the Ni component. This novel method to form catalyst-doped hierarchical ZnO NWs provides a promising approach to accomplish superior gas sensing characteristics by the synergetic combination of enhanced chemiresistive variation due to the increased number of branch-to-branch Schottky barrier contacts and the catalytic function of the Ni dopant. [ABSTRACT FROM AUTHOR]

Pure and CuO-loaded In 2 O 3 nanofibers were prepared by electrospinning and their H 2 S sensing characteristics were investigated. The loading of CuO on In 2 O 3 nanofibers significantly enhanced the gas response (ratio of the resistance in air to that in gas) toward 5 ppm H 2 S from 515 to 1.16 × 10 5 at 150 °C. The CuO-loaded In 2 O 3 nanofibers also exhibited high gas response (9.17 × 10 3 toward 5 ppm H 2 S) at room temperature. The CuO-loaded In 2 O 3 nanofibers showed ultrahigh selectivity to H 2 S concerning interferences with NO 2 , H 2 , CO, NH 3 , C 2 H 5 OH, C 3 H 6 O, TMA, C 7 H 8 , and C 8 H 10 at room temperature and 150 °C. The operation of the sensor using pulse heating was suggested reliable H 2 S sensing with complete recovery. The ultrasensitivie and ultraselective H 2 S sensing characteristics are explained in terms of the creation and disruption of p–n junctions in the presence and absence of H 2 S, respectively, the high specific surface area provided by the networks of one-dimensional polycrystalline nanofibers, and the abundance of p–n junctions due to the uniform mixing between p-CuO and n-In 2 O 3 nanograins within the nanofibers. [ABSTRACT FROM AUTHOR]

Highlight: [•] Methyl benzene sensor using Cr-doped Co3O4 nanorods was suggested. [•] Responses to xylene and toluene of pure Co3O4 nanorods were high. [•] Doping of Cr to Co3O4 nanorods significantly enhanced selectivity to methyl benzenes. [Copyright &y& Elsevier]

Abstract: An ultraselective and ultrasensitive trimethylamine (TMA) sensor was achieved using MoO3 nanoplates-prepared by ultrasonic spray pyrolysis followed by a heat treatment at 450°C. The small and thin MoO3 nanoplates with gas-accessible structures showed an unusually high response to 5ppm TMA (ratio of resistance to air and gas=373.74) at 300°C with detection limit as low as 45ppb. Moreover, the ratios of the cross-responses to interfering gases (i.e., 5ppm C2H5OH, CO, CH4, C3H8, H2, and NO2) to the response to 5ppm TMA were extremely low (0.008–0.016). The source of the ultraselective and highly sensitive detection of TMA with negligible interference from other gases is discussed with respect to the acid/base properties, size, and morphology of the MoO3 sensing materials. [Copyright &y& Elsevier]

Abstract: The CuO-loaded SnO2 hollow spheres were prepared by ultrasonic spray pyrolysis and their H2S sensing characteristics in dry and humid atmospheres for the diagnosis of halitosis were investigated. The loading of CuO to SnO2 hollow spheres decreased the humidity dependence of H2S sensing characteristics down to negligible level, increased the gas response (ratio of resistance to air and gas) in highly humid atmosphere (relative humidity 80%) to 1ppm H2S from 3.13 to 22.4, and enhanced the selectivity to H2S over CO, NH3, CH3COCH3, C6H6, CH3C6H5, (CH3)2C6H4, and NO at 300°C. Ultraselective and ultrasensitive detection of H2S under highly humid atmosphere with negligible interferences from other biomarker gases provides a promising and reliable analysis tool for real-time diagnosis of halitosis from exhaled breath. [Copyright &y& Elsevier]

Abstract: The gas sensing characteristics of pure and 0.4–2.0at% Ni-loaded SnO2 nanoparticles have been measured in dry and humid atmospheres. Approximately the same response to 50ppm CO, response/recovery kinetics, and resistance in air regardless of wide range of humidity variation from dry to 80% r.h. have been accomplished by loading 1.0 and 2.0at% Ni to SnO2. The role of Ni related surface species in the decrease of humidity dependence of gas sensing characteristics has been elucidated by diffuse-reflectance Fourier transform IR spectroscopy. The work function values determined from the transient of sensor resistance and contact potential difference revealed that Ni loading to SnO2 determines the appearance of surface electron acceptors responsible for a significant upward energy bands bending even in N2 atmosphere (>0.5eV), and, ultimately, explains the significant increase of the sensors baseline resistance and the decrease of the sensor signals. In this way, the origins of the ambivalent effect of Ni loading are clarified and the way towards a rational optimization of the sensor performance opened. [Copyright &y& Elsevier]

Abstract: NiO and Fe-doped NiO hollow spheres with the shell thickness of ∼12nm have been prepared by applying uniform coatings of Ni- and Fe-precursors onto Ni spheres, partial oxidation of Ni spheres near their surfaces at 300°C, the dissolution of core Ni using dilute HCl aqueous solution, and subsequent heat treatment at 500°C, and their gas sensing characteristics at 300–400°C were compared. The response to 100ppm C2H5OH of Fe-doped NiO hollow spheres at 350°C (R g/R a =172.5; R g, resistance in gas; R a, resistance in air) was 31.4 times higher than that of NiO hollow spheres (R g/R a =5.5). The reasons for the significant enhancement of C2H5OH response by doping Fe into NiO hollow spheres were discussed in relation to the incorporation of Fe components into the NiO lattice and their consequent impact on the gas sensing reaction. [Copyright &y& Elsevier]

Abstract: Pure ZnO and CuO–ZnO composite hollow spheres were prepared by one-pot, glucose-mediated hydrothermal reaction with subsequent heat treatment. The pure ZnO hollow spheres could selectively detect C2H5OH at 475°C. The CuO–ZnO hollow spheres prepared from the solutions with the compositions of [Cu2+]/([Cu2+]+[Zn2+])=0.02 and 0.04 showed high responses to 5ppm H2S (R a /R g =13.3 and 32.4, R a : resistance in air, R g : resistance in gas) with negligible cross responses to 100ppm C2H5OH, C3H8, CO and H2 (R a /R g =1.4–1.8) at 336°C. They also showed selective C2H5OH detection at 475°C. The dual selective detection of H2S and C2H5OH by the composite particles was due to electrochemical interactions with H2S of nano-scale p(CuO)–n(ZnO) junctions within the hollow spheres. [Copyright &y& Elsevier]

Abstract: The 0.45mol% Ru-doped SnO2 (Ru–SnO2) hollow spheres, each with a shell thickness of ∼17nm, were prepared by the uniform coating of Ru- and Sn-precursors onto SiO2 microspheres with diameters of ∼300nm, followed by heat treatment at 350°C and subsequent removal of the core SiO2 using a dilute HF solution. The Ru–SnO2 hollow spheres showed high responses to 5ppm trimethylamine (TMA) at 350°C with negligible cross-responses to toluene, benzene, NH3, CO, H2, and C3H8 and decreased cross-responses to C2H5OH, while undoped SnO2 hollow spheres showed similar responses to TMA and C2H5OH. The selective and sensitive detection of TMA that was achieved in Ru-doped SnO2 hollow spheres is explained in relation to the catalytic sensitization of Ru and the gas-accessible hollow morphology of the thin shell configurations. [Copyright &y& Elsevier]

Abstract: The SnO2 nanowire networks were grown on a laser-scribed Al2O3 substrate containing 16 sensor elements with Au electrodes and Pt heaters via a single step vapor phase reaction, and their gas sensing characteristics were studied. The sensor showed a high response to 5ppm NO2 (R g /R a =1909, R g : resistance in gas, R a : resistance in air) at the sensor temperature of 141°C and high and selective responses to 100ppm C2H5OH (R a /R g =301.8–326.4) at 240 and 296°C, which demonstrated that high performance SnO2 nanowire network gas sensors can be fabricated on a large scale through a facile and cost-effective route. [Copyright &y& Elsevier]

Abstract: Gas sensors were designed and fabricated using oxide nanofibers as the sensing materials on micro platforms using micromachining technology. Pure and Pt doped SnO2 nanofibers were prepared by electrospinning and their H2S gas sensing characteristics were subsequently investigated. The sensing temperatures of 300 and 500°C could be attained at the heater powers of 36 and 94mW, respectively, and the sensors showed high and fast responses to H2S. The responses of 0.08wt% Pt doped SnO2 nanofibers to 4–20ppm H2S, were 25.9–40.6 times higher than those of pure SnO2 nanofibers. The gas sensing characteristics were discussed in relation to the catalytic promotion effect of Pt, nano-scale morphology of electrospun nanofibers, and sensor platform using micro heater. [Copyright &y& Elsevier]

Abstract: In2O3 hollow spheres with shell thicknesses of ∼150nm and ∼300nm were prepared by the one-pot synthesis of indium-precursor-coated carbon spheres via hydrothermal reaction and subsequent removal of core carbon by heat treatment. The gas response (R a /R g , R a : resistance in air, R g : resistance in gas) of the thin hollow spheres to 100ppm C2H5OH was 137.2 at 400°C, which was 1.86 and 3.84 times higher than that of the thick hollow spheres and of the nanopowders prepared by precipitation, respectively. The gas sensing characteristics are discussed in relation to the shell configuration of the hollow spheres. The enhanced gas response of the hollow spheres was attributed to the effective diffusion of analyte gas toward the entire sensor surface via very thin and nano-porous shells. [Copyright &y& Elsevier]

Abstract: Highly crystalline ZnO hierarchical nanostructures were prepared at room temperature through the alkaline hydrolysis of zinc salt by the forced mixing of two immiscible solutions: Zn-nitrate aqueous solution and oleic-acid-dissolved n-hexane solution. The oleic acid acted as a surfactant in the room-temperature formation of well-defined ZnO hierarchical nanostructures, which subsequently demonstrated a sensitive and selective detection of C2H5OH. The responses of these hierarchical nanostructures to 10–100ppm C2H5OH ranged from 15.7 to 177.7, which were 7–9 times higher than those of the agglomerated nanoparticles. [Copyright &y& Elsevier]

Abstract: This work presents a simple and versatile route to produce macroporous p-type metal oxide thin films. Two-dimensional arrays of p-type NiO films with a hollow hemisphere structure were fabricated by colloidal templating and RF-sputtering followed by a subsequent heat treatment. The diameter and shell thickness of the NiO hemisphere were 800nm and 20nm, respectively. X-ray diffraction and high-resolution transmission electron microscopy analysis indicate that the pure NiO phase with grain size of 10nm was obtained at calcination temperatures that exceeded 450°C. Close-packed arrays of hollow NiO hemispheres were found to exhibit p-type gas sensing properties against (CO, H2, C3H8, CH4, NO2, and C2H5OH), leading to significantly enhanced responses to C2H5OH (R gas /R air =5.0 at 200ppm). [Copyright &y& Elsevier]

Abstract: The SnO2 nanowires (NWs) network gas sensors were fabricated on a micro-electrode and heater suspended in a cavity. The sensors showed selective detection to C2H5OH at a heater power during sensor operation as low as 30–40mW. The gas response and response speed of the SnO2 NWs sensor to 100ppm C2H5OH were 4.6- and 4.7-fold greater, respectively, than those of the SnO2 nanoparticles (NPs) sensor with the same electrode geometry. The reasons for these enhanced gas sensing characteristics are discussed in relation to the sensing materials and sensor structures. [Copyright &y& Elsevier]

Abstract: An integrated catalytic combustion H2 sensor has been fabricated by using MEMS technology. Both the sensing elements and the reference elements could be integrated into the suspended micro heaters connected in a suitable circuit such as a Wheatstone configuration with low power consumption. Two sensitive elements and two reference sensors were integrated together onto a single chip. The size of chip was 5.76mm2 and the catalytic combustion sensor showed high response to H2 at operating voltage of 1V. The response and recovery times to 1000ppm H2 were 0.36s and 1.29s, respectively. [Copyright &y& Elsevier]

Abstract: Undoped and Pd-doped SnO2 hollow nanofibers were prepared by single capillary electrospinning and their gas sensing characteristics for 100ppm H2, 100ppm CO, 500ppm CH4, and 100ppm C2H5OH were investigated. The gas responses and responding kinetics were closely dependent upon the sensor temperature and Pd doping concentration. The undoped and 0.08wt% Pd-doped SnO2 nanofibers showed selective detection to C2H5OH at 385 and 440°C. In the 0.4wt% Pd-doped SnO2 nanofiber sensor, the selective detection to CH4 and H2 was optimized at 440°C with the minimum cross-sensitivity to C2H5OH. The tuning of gas selectivity via the combinatorial control of Pd doping concentration and sensor temperature in the electrospun hollow nanofibers was discussed in relation to the gas sensing mechanism. [Copyright &y& Elsevier]

Abstract: The ZnO–SnO2 core–shell nanowires (NWs) were synthesized by a continuous two-step vapor growth method at different synthesis temperatures. A crystalline 15–20nm-thick, SnO2 shell layer was pseudo-epitaxially coated on ZnO NWs with a diameter of 50–80nm. The gas response of the ZnO–SnO2 core–shell NW sensor to 10ppm NO2 reached ∼33 times enhancement compared to that of the ZnO NWs at 200°C. In addition, the ZnO–SnO2 core–shell NW sensors showed selective detection to NO2 at 200–300°C and to C2H5OH at 400°C. The enhanced gas responses to NO2 and C2H5OH are discussed in relation to the thin SnO2 shell layer and core–shell configuration of the NWs. [Copyright &y& Elsevier]

Abstract: Various morphologies of Co-containing precursors such as nanorods, nanosheets, and nanocubes were prepared by controlling the solvothermal reaction using cobalt acetate, L(+)-lysine, and oxalic acid, all of which were successfully converted into Co3O4 nanostructures without morphological variation. The gas responses of these Co3O4 nanosheets, nanorods, and nanocubes to 100ppmC2H5OH at 300°C were 10.5, 4.7, and 4.5 times higher than those of the Co3O4 agglomerated nanopowders, respectively. In addition, the selectivity to C2H5OH over CO and H2, as well as the response/recovery kinetics, were significantly improved. These enhanced gas-sensing characteristics were attributed to the less agglomerated nanostructures of the sensing materials. [Copyright &y& Elsevier]

Abstract: Tungsten-coated carbon microspheres were prepared by one-pot hydrothermal reaction of an aqueous solution containing glucose and sodium tungstate. The spheres were converted into WO3 hollow microspheres by the decomposition of their core carbon. The [glucose]/[sodium tungstate] ratio of the stock solution determined not only the morphology of the precursors but also the phase of the powders after calcination. The WO3 hollow microspheres showed a higher gas response and more selective detection of 0.5–2.5ppm NO2 than WO3 solid and nano-porous microspheres did. The enhanced NO2 sensing characteristics are explained in relation to the surface area, pore volume, and hollow morphology. [Copyright &y& Elsevier]

Abstract: The CuO-functionalized SnO2 nanowire (NW) sensors were fabricated by depositing a slurry containing SnO2 NWs on a polydimethylsiloxane (PDMS)-guided substrate and subsequently dropping Cu nitrate aqueous solution. The CuO coating increased the gas responses to 20ppm H2S up to ∼74-fold. The R a /R g value of the CuO-doped SnO2 NWs to 20ppm H2S was as high as 809 at 300°C, while the cross-gas responses to 5ppm NO2, 100ppm CO, 200ppmC2H5OH, and 100ppmC3H8 were negligibly low (1.5–4.0). Moreover, the 90% response times to H2S were as short as 1–2s at 300–400°C. The selective detection of H2S and enhancement of the gas response were attributed to the uniform distribution of the sensitizer (CuO) on the surface of the less agglomerated network of the SnO2 NWs. [Copyright &y& Elsevier]

Abstract: The NO2 gas sensing characteristics of semiconductor type gas sensors with channels composed of necked ZnO nanoparticles (NPs) were investigated in this study. The heat treatment of the NPs at 400°C led to their necking and coarsening. The response of the necked-NP-based sensors was as high as 100 when exposed to 0.2ppm of NO2 at 200°C. As the concentration of NO2 increased to 5ppm, their response was enhanced to approximately 400. During the repeated injection of NO2 gas with a concentration of 0.4ppm, the sensors exhibited stable response characteristics. Furthermore, the 90% response and recovery times of the gas sensor were as fast as 13 and 10s, respectively. These observations indicate that the non-agglomerated necking of the NPs induced by the heat treatment significantly enhances the gas sensing characteristics of the NP-based gas sensors. [Copyright &y& Elsevier]

Abstract: This paper suggests a simple and cost-effective method for fabricating nanowire (NW) gas sensors. Oxide NWs were deposited uniformly over a defined electrode area by coating with a solution containing NWs after patterning with polydimethylsiloxane (PDMS). The hydrophobic PDMS guide wall prevented outstretching of the deposited solution, which allowed the density of the NWs in the sensor to be manipulated conveniently. The high-density NWs sensor enhanced the gas sensitivity toward CO, C3H8, C2H5OH, and NO2 but reduced the gas response and recovery speed. The correlation between the NW density and gas sensing characteristics are discussed in relation to the gas sensing mechanism. [Copyright &y& Elsevier]