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In the electronic nose (E-nose) systems, gas type recognition and accurate concentration prediction are some of the most challenging issues. This study introduced an innovative pattern recognition method of time–frequency attention convolutional neural network (TFA-CNN). A time–frequency attention block was designed in the network, aiming to excavate and effectively integrate the temporal and frequency domain information in the E-nose signals to enhance the performance of gas classification and concentration prediction tasks. Additionally, a novel data augmentation strategy was developed, manipulating the feature channels and time dimensions to reduce the interference of sensor drift and redundant information, thereby enhancing the model's robustness and adaptability. Utilizing two types of metal-oxide-semiconductor gas sensors, this research conducted qualitative and quantitative analysis on five target gases. The evaluation results showed that the classification accuracy could reach 100%, and the coefficient of the determination (R2) score of the regression task was up to 0.99. The Pearson correlation coefficient (r) was 0.99, and the mean absolute error (MAE) was 1.54 ppm. The experimental test results were almost consistent with the system predictions, and the MAE was 1.39 ppm. This study provides a method of network learning that combines time–frequency domain information, exhibiting high performance in gas classification and concentration prediction within the E-nose system. [ABSTRACT FROM AUTHOR]

Zinc hydroxyfluoride (ZnOHF) is a newly found resistive semiconductor used as a gas-sensing material with excellent selectivity to NO2 because of its unique energy band structure. In this paper, Al3+ doping and UV radiation were used to further improve the gas-sensing performance of ZnOHF. The optimized 0.5 at.% Al-ZnOHF sample exhibits improved sensitivity to 10 ppm NO2 at a lower temperature (100 °C) under UV assistance, as well as a short response/recovery time (35 s/96 s). The gas-sensing mechanism demonstrates that Al3+ doping increases electron concentration and promotes electron transfer of the nanorods by reducing the bandgap of ZnOHF, and the photogenerated electrons and holes with high activity under UV irradiation provide new reaction routes in the gas adsorption and desorption process, effectively promoting the gas-sensing process. The synergistic effect of Al3+ and UV radiation contribute to the enhanced performance of Al-ZnOHF. [ABSTRACT FROM AUTHOR]

Ga‐doped ZnO nanoparticles are successfully prepared by a facile coprecipitation method and then are activated by H2 annealing. The as‐prepared ZnO powders are characterized by X‐ray diffractometer (XRD), Brunauer–Emmett–Teller (BET), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies. Gas sensing property using NO2 as probe shows that the 1 at.% Ga‐ZnO powders activated in H2 at 540 °C showed high NO2 response of ≈50/ppm, high selectivity toward typical interferences such as ethanol, acetone, toluene, NO, SO2, C2H6, and C2H4 and high stability as measured at 200 °C. On the contrary, the pure ZnO and the Ga‐ZnO without H2 activation show far lower response, which illustrates the effect of Ga dopant activation by H2 annealing aimed for the high performance gas sensing materials. [ABSTRACT FROM AUTHOR]

Nowadays, xylene is not only one major air pollutant which threats human health even if its concentration is lower than the human olfactory threshold of 470 ppb, but also one of the typical gases exhaled by the lung cancer patients with a criterion of 10–20 ppb. However, in situ detection of the ppb-level xylene for air quality monitoring and breath analysis remains challenging using the easily fabricated and low cost metal oxide semiconductor gas sensors. Herein, a synergetic p+n field effect transistor (FET) amplification circuit is designed to detect the ppb level xylene. By optimizing the load resistor ( RL ) and the p- and n-FET coupling effect, a magnification factor (~7.5) is obtained. This amplification circuit decreases the detection limit of TGS2602 sensor to ~10 ppb xylene with apparent response of about 2.3 and voltage change of >0.5 V, promising for air quality monitoring (the highest permissive limit of 42 ppb) and breath disease analysis (threshold of lung cancer 10–20 ppb). The mechanism is that the matched couple of p + n FETs work synchronically when their ( R\mathrm {L} + R\mathrm {FET}) - I curves nearly coincide with each other. All those results show the prospect of ppb level gas detection with MOX sensors using the synergetic p+n FET amplification circuit. [ABSTRACT FROM PUBLISHER]

Low concentration acetone gas detection is significantly important for diabetes diagnosis as 1.8–10 ppm of acetone exists in exhaled breath from diabetes patients. A new interlocking p+n field-effect transistor (FET) circuit has been proposed for Mn-doped ZnO nanoparticles (MZO) to detect the acetone gas at low concentration, especially close to 1.8 ppm. It is noteworthy that MZO in this interlocking amplification circuit shows a low voltage signal of <0.3 V to the acetone <2 ppm while it displays a transilient response with voltage signal >4.0 V to >2 ppm acetone. In other words, the response to acetone from 1 ppm to 2 ppm increases by ~1233%, which is competent to separate diabetic patients from healthy people. Moreover, the response to 2 ppm acetone is hardly influenced by high relative humidity of 85%. In the meanwhile, MZO in this interlocking circuit possesses a high acetone selectivity compared to formaldehyde, acetaldehyde, toluene and ethanol, suggesting a promising technology for the widespread qualitative screening of diabetes. Importantly, this interlocking circuit is also applicable to other types of metal oxide semiconductor gas sensors. The resistance jump of p- and n-FETs induced by the change of their gate voltages is deemed to make this interlocking circuit produce the transilient response. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • SnO 2 /MO x (M=Zn, Ga, W) nanotubes and nanofibers were fabricated by one-step electrospinning and subsequent calcination. • Selectivity was effectively changed and controlled by designing various SnO 2 /MO x heterostructures. • SnO 2 /MO x sensors were firstly used to compose sensor arrays to detect VOCs mixtures by means of matrix manipulation. Abstract It is a challenge to effectively detect the components of a gas mixture using metal oxide sensors which are easily fabricated and inexpensive. In this work, heterostructured SnO 2 /MO x (i.e. M = Zn, Ga and W) nanotubes (NTs) and nanofibers (NFs) are synthesized via a one-step electrospinning technology and subsequent calcination for use in gas sensors. In specific, compared with pure SnO 2 NTs, SnO 2 /ZnO NTs show an enhanced response to 100 ppm ethanol and acetone, while SnO 2 /Ga 2 O 3 NTs show an obviously higher response to 100 ppm ethanol and SnO 2 /WO 3 NFs show an optimal response to 100 ppm xylene. All these sensors are selective in the presence of typically interfering gases such as formaldehyde, benzene and toluene. As a proof-of-concept, sensor arrays constructed using these three sensors precisely detected the gas mixtures of ethanol, acetone and xylene by means of matrix manipulation, delivering a superior accuracy of <9% deviation for the gas concentration of 10 ppm and 20 ppm and <38% for the concentration of 5 ppm. These results show the possibility of improved selectivity in detecting gas mixtures using heterostructured gas-sensing materials for sensor arrays. [ABSTRACT FROM AUTHOR]

Highlights • MEMS compatible Au/SnO 2 :NiO films were fabricated using sputtering and self-assembly techniques. • The Au/SnO 2 :NiO film showed high sensitivity, selectivity and stability, as well as good consistency to NO 2. • The enhanced sensing property was attributed to catalytic role of the AuNPs and p-n junctions between Au, SnO 2 and NiO. Abstract Here, heterostructured Au/SnO 2 :NiO thin films are fabricated by sputtering SnO 2 :NiO target onto the self-assembled Au nanoparticle (NPs, diameter ∼ 9.5 nm) arrays and then activated by H 2 annealing. Compared with the SnO 2 and SnO 2 :NiO thin film counterparts, the 20 nm thick Au/SnO 2 :NiO film shows the highest NO 2 sensing performance after activated at optimized temperature of 500 °C, with high response of ∼185 to 5 ppm NO 2 , low detection limit of ∼50 ppb, high selectivity, good stability and also low sensor-to-sensor variation of <15%. The gas sensing property enhancement could not only be attributed to catalytic role of the AuNPs, but also to the effective Schottky barrier and p-n junction formation between Au, SnO 2 and NiO, which is verified by the relatively higher Fermi level of ∼4.56 eV compared with those of 4.1–4.3 eV as measured by the ultraviolet photoelectron spectroscopy. Furthermore, this method is of generality to effectively enhance the electron transfer in other sensing materials like pure SnO 2 and WO 3 , showing the promising potency of this Micro-Electrical-Mechanical System (MEMS) compatible heterostucture fabrication method in wafer-scale gas sensor production with high sensitivity, selectivity and good consistency towards low concentration gas detection such as ppb-level NO 2. [ABSTRACT FROM AUTHOR]

The Pt modified SnO 2 porous spheres for NO 2 gas sensor have been synthesized through facile hydrothermal method followed by a chemically reducing process. The porous Pt-SnO 2 spheres with the size of 400–700 nm in diameter exhibit a dominant pore size of ca. 15 nm and specific surface area of 36.6 m 2 g −1 , which can provide substantial chemical adsorption sites and abundant channels for the diffusion of NO 2 molecules. The SnO 2 porous spheres with an optimized loading amount of Pt nanoparticles (0.25 wt%) display superior gas sensing performances including higher response (5770) and selectivity to 5 ppm NO 2 gas, as well as shorter response and recovery times (30 s/90 s) than other reported NO 2 sensing materials at a lower operating temperature (80 °C). The comparison experiments indicate that Pt catalyst loading not only improves the response to NO 2 gas and reduces the operating temperature, but also exhibits high selectivity to NO 2 gas. [ABSTRACT FROM AUTHOR]

We report for the first time using pure cobaltous tungstate (CoWO 4) as an efficient ethanol sensor by controlling the Co3+-Co2+ redox couple in CoWO 4. The prepared Co x WO 4 has a flower-like structure self-assembled by a mesoporous microbelt. Moreover, the Co x WO 4 sensor with a Co3+/Co2+ ratio of 38.9% exhibits a high response (42.9–100 ppm ethanol), fast response/recovery time (11/13 s), low detection limit (0.42 ppm), as well as remarkable short-term and long-term repeatability. Notably, the Co x WO 4 sample maintains a constant response value over a wide range of humidity (30–90 RH%), manifesting its good anti-humidity properties. The outstanding sensing performance is owing to the Co3+-Co2+ redox couple, high surface area and moderate porosity of the obtained samples. The self-transforming of the Co3+-Co2+ redox couple contributes to the formation of absorbed oxygen (O 2 -), oxygen vacancies and the reduction of the band gap, which is a crucial factor in improving the sensing properties of Co x WO 4. • Pure cobalt tungstate was used for the first time as an efficient ethanol sensor. • The sensor shows a fast response/recovery time and low detection limit. • The self-transforming of Co3+-Co2+ redox on the surface is also a crucial factor. [ABSTRACT FROM AUTHOR]

Water vapor greatly interferes with gas sensing characteristics of metal oxide semiconductors (MOS). Consequently, for the first time, Nd 2 O 3 -loaded In 2 O 3 porous nanorods were synthesized which exhibited excellent H 2 S sensitivity along with prominent humidity-independent characteristics, whereas pristine In 2 O 3 without Nd 2 O 3 showed deteriorated H 2 S sensing features under increasing ambient humid conditions in a wide operating temperature range. The mechanism of humidity-independent H 2 S sensing properties was researched via X-ray photoelectrons spectroscopy (XPS), in-situ Fourier Transform infrared spectroscopy (in-situ DRIFTS) and the corresponding gas sensing measurements, which is in close relations with the inhibition of chemisorbed oxygen and scavenging hydroxyl groups on In 2 O 3 surface induced by surficial Nd 2 O 3 component. This strategy poses a cutting-edge comprehending of temperature-dependent humidity-interference on MOS, paves a novel route for fabricating H 2 S sensors with exceptional sensitivity, selectivity and anti-humidity properties, which will be quite conducive for real-time exhaled breath analyses in disease diagnosis. [Display omitted] • Nd 2 O 3 -loaded In 2 O 3 porous nanorods were prepared for anti-humidity H 2 S detections. • Temperature-dependent anti-humidity behaviors of Nd 2 O 3 -loaded In 2 O 3 are identified. • The optimized H 2 S sensor exhibits well selectivity, sensitivity and low detection limit. • In-situ Fourier Transform infrared spectra verifies the anti-humidity mechanism. [ABSTRACT FROM AUTHOR]

Regulation based on the crystalline structure modulations is capable of optimizing the band gap and chemisorbed oxygen components thus adjusting the gas sensing characteristics of sensing materials. Herein, In 2 O 3 hierarchical-assembled microspheres with diverse crystalline structures were synthesized via a simple hydrothermal method combined with calcinating procedure under different temperatures. The gas sensor based on rhombohedral In 2 O 3 (rh-In 2 O 3) microspheres assembled by pine needle shaped nanorods annealed at 350 ℃ (350-In 2 O 3) exhibited superior H 2 S sensing performances at relative low operating temperature of 100 ℃ with low detection limit (50 ppb) and optimized stability compared with cubic In 2 O 3 (c-In 2 O 3). Notably, H 2 S sensing characteristics of In 2 O 3 samples with different crystalline structures certificated that crystal structures can dominate the H 2 S sensing reactions with different mechanisms. This work enlightened a novel prospect for designing rh-In 2 O 3 based sensors and provides an innovative pathway for analyzing the feasible H 2 S sensing mechanisms. [Display omitted] • In 2 O 3 materials with diverse crystalline structures were prepared for H 2 S detections. • Rhombohedral In 2 O 3 microspheres is composed of nanorods (diameter of 8 nm). • Rhombohedral In 2 O 3 based sensor displayed the optimized H 2 S sensitivity. • Crystalline structure controls H 2 S sensing mechanism. [ABSTRACT FROM AUTHOR]

Due to the intrinsic properties of wide band gap and the 5d shell being vacant, WO 3 still has the disadvantages of low response dynamics, slow response/recovery speed and high detection limit for real-time gas monitoring. To address such issues, the Ni3+ doped WO 3 hierarchical hollow structure composed of irregular nanosheets was achieved and the content of adsorbed oxygen was adjusted by Ni3+ doping to further improve the gas sensing performance of WO 3. In comparison to pure WO 3 , the 3 wt%Ni-WO 3 sensor exhibited remarkable response value (396), short response/recovery speed (13 s/6 s) and low detection limit (510 ppt) at 160 ℃. The impressive gas properties could be attributed to the reduced band gap and oxygen inhibiting effect of Ni3+ doping, which provides more NO 2 adsorption sites and making it easier for NO 2 to capture more electrons in the conduction band by restraining the oxygen ions chemisorbed on the surface. The unique sensing characteristics of the Ni3+ doped WO 3 suggest its wonderful potential applications in environmental pollution gas detection. The mechanism of oxygen inhibiting effect induced by Ni3+ doping was also discussed in detail [Display omitted] • The Ni3+ doped WO 3 was synthesized via soaking and oil bath evaporation. • The sensor based on Ni3+ doped WO 3 presented rapid response/recovery speed and low detection limit. • The superior sensing characteristic can be attributed to the oxygen inhibiting effect. [ABSTRACT FROM AUTHOR]

Triethylamine (TEA) is a typical pollution in chemical industry and Corrupt markers of aquatic products. TEA emissions have a serious impact on human health, industrial production and food safety. In this study, hierarchical flower-like Pt-loaded/Ce-doped In 2 O 3 microspheres are synthesized via simple hydrothermal and in-situ reduction methods. The triethylamine (TEA) sensing measurements display the 0.5Pt-3CeIn gas sensor exhibits high sensitivity (R a /R g = 900–100 ppm triethylamine), ultra-low optimal working temperature (100 ℃), enhanced responding and recovering speed (158/45 s) and excellent selectivity towards different target gases (ethanol, acetone, xylene, methanol, nitrogen dioxide and methane) including amine (response towards 50 ppm NH 3 is lower than 10). The TEA detection limit is as low as 5 ppb and the sensor exhibits a wide TEA detection range. The introduction of Pt and Ce components into In 2 O 3 effectively reduces the operating temperature, promotes the chemisorbed oxygen content (18.14 %) and wildly decreases the band gap (2.97 eV). The gas sensing mechanism of Pt/Ce-In 2 O 3 , which is dominated by electronic and chemical sensitization of Pt, catalytic function of Ce dopants and the functional interface between Pt nanoparticles and Ce-doped In 2 O 3 microspheres, is proposed. This work as well testifies the feasibility for multi-components cooperative optimization of MOS in gas sensing applications. [Display omitted] [ABSTRACT FROM AUTHOR]

In this paper, Ni 0.9 Zn 0.1 O/ZnO composite fibers were prepared by a combined approach of electrospinning and hydrothermal method for enhanced H 2 S gas sensing performance. The composite fibers have a diameter of 2 µm and are densely and vertically covered with irregularly shaped nanosheets with a thickness of about 20 nm. When the operating temperature is 100 °C, the maximum response (Ra/Rg) of the Ni 0.9 Zn 0.1 O/ZnO sample to 5 ppm H 2 S is about 450, which is about 15 times of pure ZnO nanofibers gas sensor (∼ 30). Moreover, Ni 0.9 Zn 0.1 O/ZnO based gas sensors showed good selectivity and low detection limit (10 ppb) to H 2 S. The nanosheets structure of Ni 0.9 Zn 0.1 O/ZnO composite fibers, as well as the proper energy band structure alignment between Ni 0.9 Zn 0.1 O and ZnO, have improved its gas sensing performance. • A novel nanosheets covered fiber with large specific surface area synthesized by electrospinning and hydrothermal method. • Ni 0.9 Zn 0.1 O/ZnO composite show high response and low detection limits for the first time as H 2 S detection materials. • Excellent properties is attributed to the interconversion of p-n heterojunctions and metal-n type semiconductor contacts. [ABSTRACT FROM AUTHOR]