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• WO 3 nanosheets were fabricated and the surface was loaded by Ru using an impregnation method. • A great sensitization effect is obtained by loading of Ru for WO 3 , unde oxygen and non-oxygen condition. • The sensitization mechanism of Ru is due to the electronic interaction and chemical activity of Ru. • The sensitization of Ru for WO 3 is a chemical sensitization which governs the sensing process in oxygen free condition. Gas sensors based on tungsten trioxide (WO 3) have been greatly investigated and successfully commercialized. Generally, pristine WO 3 is quite sensitive to oxidizing gases such as nitrogen dioxide and ozone. However, when noble metals dispersed on the surface, WO 3 could become very sensitive to reducing gases, such as hydrogen (H 2) and acetone. Notably, the sensitization mechanisms are varied with different noble metals. Recently, ruthenium (Ru) has been found to be a very effective sensitizer, which enable WO 3 to be sensitive to reducing gases. In the present study, Ru loaded WO 3 (Ru-WO 3) nanosheets were prepared and the sensitization mechanism was investigated under both dry and humid conditions. The basic sensing mechanism was revealed by the oxygen adsorption and surface catalytic activities. According to our results, it was found that Ru loading can greatly promote the sensor response of WO 3 to H 2 , even with a very small amount. Ru-WO 3 could also respond to H 2 in both oxygen containing and non-oxygen background, nevertheless, the basic sensing processes were significantly different. With presence of humidity, the sensitization mechanism of Ru become different from that under a dry condition. [ABSTRACT FROM AUTHOR]

• The performance of Pd-In 2 O 3 to methane is significantly improved with Pd loading, with low operation temperatures. • A discrimination behavior over NO 2 for methane detection was found and the mechanism was investigated. • The high catalytic activity of the printed catalyst film is responsible for the enhanced selectivity. • The power-law response behavior indicates that the printed catalyst film did not change the basic sensing mechanism. Gas sensors based on In 2 O 3 were prepared and were found to be very sensitive to methane with Pd loading (Pd-In 2 O 3), even at a low temperature of 300 °C. However, Pd-In 2 O 3 sensors suffer from a high cross-sensitive response to NO 2 , CO and ethanol, which are typical interfering gases for the detection of CH 4. In order to promote the selective response to CH 4 , a filter based on Pt-Al 2 O 3 catalyst film was printed onto the top of Pd-In 2 O 3 sensor layer. It was found that catalytic film coated sensors could selectively respond to methane even in the mixture of methane with CO, NO 2 , and ethanol. The cross-sensitive responses to the interference of VOCs, CO, NO 2 were almost eliminated. According to the catalytic conversion measurement, it was indicated that CO and ethanol were fully and selectively oxidized to CO 2 and H 2 O. In case of NO 2 , it was proposed that nitrogen dioxide molecules may be blocked on the surface of the catalyst film. Based on the oxygen adsorption behavior, it was suggested the printed catalyst film has no change on the basic sensing mechanism of In 2 O 3 based sensors following the general model that is the adsorption of oxygen and interaction with methane molecule and the resulted electron transfer process. Our investigation demonstrated that Pt-Al 2 O 3 printed Pd-In 2 O 3 sensor could be used for the detection of CH 4 leakage and monitor. [ABSTRACT FROM AUTHOR]

Highlights • A simple process to fabricate flexible and highly sensitive mechanical sensors based on RGO/synergetic fiber bundles with various sensing functions including stretching, pressing, bending, vibration and torsion. • The mechanical sensor exhibited a high sensitivity (GF = −8.9), low limit of detection (0.05%) and wide sensing range (up to 75%) for tensile strain. • The sensor can be employed to detect full-range human actions from subtle physiological signals e.g. phonation, respiration, and real-time pulse wave to human physical activities e.g. finger joint bending, typing or grasping. • Large-area sensor arrays are successfully fabricated to spatially map pressure stimuli. Abstract We propose a flexible, multifunctional and highly sensitive mechanical sensor that can monitor various mechanical strains induced via stretching, pressing, bending, vibration and torsion. The sensor demonstrates a well-designed conductive fiber alignment based on synergetic fiber bundles coated with a thin reduced graphene oxide (RGO) layer (SF/RGO). When stretched, the fiber bundles fracture into gaps, islands, and bundles bridging the gaps; thus, allowing the conductive fiber bundles to serve as mechanical sensors capable of detecting trace tensile strain down to 0.05% with a high sensitivity (Gauge Factor = −8.9). Furthermore, the sensor also demonstrated high durability, wide sensing range (75% for strain, and 36 kPa for pressure). These remarkable features endow our sensing devices to monitor full-range human motions from subtle bio-signals e.g. pulse wave, phonation and respiration to human physical actions e.g. finger typing, bending or grasping. Furthermore, large-area sensor arrays are successfully fabricated to spatially map pressure stimuli, demonstrating great potential for applications in wearable electronics and healthcare. [ABSTRACT FROM AUTHOR]

Graphical abstract Development of high-performance nitrogen dioxide sensors is extremely important not only for industrial applications but also for environmental monitoring and health protection. For practical applications, gas sensors with high sensitivity for low concentrations of nitrogen dioxide are desired. Here, a simple process to fabricate high-performance gas sensors based on porous reduced graphene oxide (PRGO) through a simple drop-casting technique is presented. The PRGO used as active sensing material is prepared through a scalable solution etching approach. This sensor exhibited a high sensitivity in the sub-0.5 ppm range with good selectivity and reversibility, as well as a low experimental detection limit of 20 ppb, which is much lower than the threshold exposure limit proposed by the National Ambient Air Quality Standard (53 ppb). Furthermore, it can be performed at room temperature without UV/IR light illumination or thermal assistance. This superior performance could be attributed to the high adsorption site density due to the edge sites of porous graphene. The relationship between the sensing performance and the microstructure or chemical composition of porous graphene were systematically investigated. This work paves the way for a facile tailoring of the sensor behavior as a function of graphene micromorphology. Highlights • A simple process to fabricate chemiresistor NO 2 gas sensors based on porous reduced graphene oxide with various edge defects is presented. • The sensor exhibited a high sensitivity in the sub-0.5 ppm range with good selectivity and reversibility. • The sensor can be performed at room temperature without UV/IR light illumination or thermal assistance. • Experimental LOD was found to be as low as 20 ppb. Abstract Development of high-performance nitrogen dioxide sensors is extremely important not only for industrial applications but also for environmental monitoring and health protection. For practical applications, gas sensors with high sensitivity for low concentrations of nitrogen dioxide are desired. Here, a simple process to fabricate high-performance gas sensors based on porous reduced graphene oxide (PRGO) through a simple drop-casting technique is presented. The PRGO used as active sensing material is prepared through a scalable solution etching approach. This sensor exhibited a high sensitivity in the sub-0.5 ppm range with good selectivity and reversibility, as well as a low experimental detection limit of 20 ppb, which is much lower than the threshold exposure limit proposed by the National Ambient Air Quality Standard (53 ppb). Furthermore, it can be performed at room temperature without UV/IR light illumination or thermal assistance. This superior performance could be attributed to the high adsorption site density due to the edge sites of porous graphene. The relationship between the sensing performance and the microstructure or chemical composition of porous graphene were systematically investigated. This work paves the way for a facile tailoring of the sensor behavior as a function of graphene micromorphology. [ABSTRACT FROM AUTHOR]

Two-dimensional MXene has been receiving attention for its outstanding properties of intrinsic metallic conductivity, high specific surface area and abundant surface terminations, showing great potential in gas sensing. However, it suffers from low sensing response and poor chemical stability. To address these issues, we propose a series of surface treatment to anchor interaction sites onto MXene, and to pin its optimized chemical state. Interaction sites including –OH, –NH 2 , and –O– were functionalized through self-polymerization of dopamine and controlled oxidation of the MXene base. These functionalized groups can interact with gas molecules through hydrogen bonding; besides, multiple Schottky barriers are generated within the sensing layer, thus improving formaldehyde sensing response by over 16-fold in comparison with pristine MXene. This optimized sensing state was further pinned through a hydrophobic treatment, and the composite obtained demonstrated a long-term stability (larger than 25 days) as well as a high sensitivity. [Display omitted] • Gas sensing performance of MXene was improved through surface functionalization and optimized sensing state pinning. • Ti–O–Ti terminations were proved to be superior for VOCs adsorption than Ti–OH. • Schottky barriers were generated within the sensing layer, which enhanced the resistance response upon gas adsorption. • The obtained composite features a simultaneous improved sensitivity and long-term stability. [ABSTRACT FROM AUTHOR]

This study presents a theoretical and experimental investigation on the power-law response to oxygen in the presence of reducing gases for metal oxide semiconductor gas sensors. In the presence of reducing gases, the exponents of power-law response to oxygen are derived, which allow us an effective probe to reveal the basic sensing mechanism. In order to obtain the power-law exponent, the transducer and receptor functions have been calculated in the presence of reducing gases. The transducer functions is built on two different conduction mechanisms, namely, Schottky barrier model and grain model. The receptor functions have been calculated by using the law of mass action for oxygen with different concentrations of reducing gases. The power response of SnO 2 to oxygen has been characterized in the presence of CO and found to be well consistent with theoretical expectation. [ABSTRACT FROM AUTHOR]

Black phosphorus nanosheet (BPNS) is considered as a promising gas sensing material due to its unique puckered layer structure and excellent physical/chemical properties. However, previously reported BPNS based gas sensors mostly suffered from low selectivity and/or poor stability. Herein, we propose to modify BPNS through surface chemistry and regulate its gas sensing properties based on surface terminations/decorated nanoparticles. A two-step functionalization was utilized including self-polymerization of dopamine, and secondary reactions to anchor self-assembly monolayers or metal catalysts via the polydopamine (PDA) medium. The modified PDA layer could offer hydrogen bonding interactions with gas analytes, which leads to a 9-fold increase in NO 2 sensitivity compared with bare BPNS (4000% for PDA@BPNS versus 426% for bare BPNS at 1 ppm). Further surface functionalization through secondary reactions yields a variety of modified PDA@BPNS composites with extended sensing properties (e.g., better stability and selectivity). We finally fabricated a sensor array system containing these modified PDA@BPNS composites and successfully identified a variety of gas molecules with good separation efficiency. [Display omitted] • Gas sensing performance of BPNS was regulated through surface chemistry. • PDA decorated on BPNS provided adsorption sites for gas and anchoring sites for secondary reactions. • Secondary reactions yielded extended gas sensing functions. • A sensor array containing various modified BPNS composites was fabricated for gas identification. [ABSTRACT FROM AUTHOR]

• Stretchable gas sensors were developed based on microstructured MXene/PU core-sheath fibers. • The sensor exhibited higher sensitivity and SNR than that of IE-based planar sensors. • Sensitivity and stretchability can be tailored through microcracks/wrinkles engineering. • Workability at both relaxed and stretching states was realized with minimal strain interference. Emerging wearable electronics pose urgent needs for high-performance, wearable gas sensors. However, the state-of-the-art gas sensors based on planer interdigited electrodes (IE) are commonly flexible yet not stretchable, which is not desirable for integration with human skin or clothing. Here, we report the fabrication of a stretchable and wearable gas sensor by using MXene/polyurethane (PU) core-sheath fibers. The synergetic effects of charge transfer and swelling-induced stretching offer the MXene/PU fiber sensor with high sensitivity to acetone (5–325 fold higher than planer MXene sensor deposited on an IE), wide sensing range (from ppb level to saturated vapor), and high signal-to-noise ratio (SNR, 160 % higher than planer MXene sensor). To further improve the sensing performance of the MXene/PU fiber, microstructures including microcracks and microwrinkles were developed into the fiber sheath. The microcracks can amplify the swelling-induced resistance variation of the conductive sheath; and therefore, leading to an enhanced sensing response 40 % higher than the flat core-sheath fiber. Furthermore, by developing microwrinkles onto the conductive sheath, a highly stretchable fiber-based gas sensor has been successfully demonstrated, which can accommodate human skin deformation (30 %) with a minimal sensing interference from tensile strain. The as-reported MXene/PU fibers can be knitted into clothing and serve as wearable gas sensors with good sensing reliability and gas permeability. Our results provide insight in utilizing microstructure engineering to tailor the sensing performance of gas sensors. [ABSTRACT FROM AUTHOR]

• A new gas sensing material was designed based on CPP/T-2DP. • Sensitivity of CPP/T-2DP improved by over 5-fold compared with phosphorene. • An enhancement mechanism was proposed for the highly sensitive NO 2 detection. • The nanospacer intercalation strategy can be extended to other 2D sensing materials. Phosphorene has recently been receiving attention for gas sensing due to its unique geometric and electronic structure. However, several challenges still remain that hinder the potential application of phosphene, especially its ambient instability and sheets restacking during sensing film formation. Herein, we report on a new sensing material design based on incorporating chemically passivated phosphorene (CPP) with porous triazine-based two-dimensional polymer (T-2DP). The T-2DP nanosheets could act as nanospacers to prevent the dense stacking of phosphorene layers whilst providing abundant pathway for penetration and adsorption of gas molecules. Such CPP/T-2DP nanocomposite exhibited extended ambient stability, improved sensing speed, excellent selectivity and good flexibility for NO 2 detection. The sensitivity of the nanocomposite sensor improved by over 5-fold in comparison with pristine phosphorene (2410 % for the nanocomposite vs 400 % for pristine phosphorene at 1 ppm of NO 2). An enhancement mechanism for the CPP/T-2DP nanocomposite was proposed for the highly sensitive NO 2 detection. In addition, this nanospacer intercalation strategy can be extended to other two-dimensional gas sensing nanomaterials, e.g. RGO. These results could provide inspirations in the design of two-dimensional sensing materials/composites and improve their prospects for wearable electronic applications. [ABSTRACT FROM AUTHOR]

• Zeolite films were printed onto top of sensing layers based on Pd-SnO 2 to improve selectivity. • A highly selective response to CH 4 over CO and ethanol was achieved by coating ZSM-5 zeolite film on the surface of Pd-SnO 2. • Zeolite films lead to a highly catalytic conversion of CO and ethanol, responsible for the improvement of selectivity to CH 4. Gas sensors based on Pd-loaded SnO 2 were prepared with printed zeolite films on the top of the sensor layer to enhance selective response to methane. As expected, Pd-loaded SnO 2 has a sensitive response to CH 4 and VOCs, such as ethanol and acetone vapors. Pt catalyst was dispersed onto the surface to promote the catalytic activity of zeolite. According to the catalytic characterizations, it was indicated that interfered gases, CO and ethanol were catalytically oxidized into CO 2 and water vapor. Whereas the catalytic combustion of methane is a negligible and sensitive response to the interested analyte was significantly promoted. Thus, the selective response to CH 4 was greatly improved even in a mixture of CO and VOCs. Based on the power-law response to oxygen, it was found that the resistive responses of all sensors were highly oxygen-dependent, suggesting that oxygen adsorbates on the surface were involved in the sensing process and no difference with zeolite coated sensors. Thus, it can be concluded that zeolite films printed onto the sensing layer surface serve as a catalytic filter and responsible for the enhanced sensitive and selective response of methane. [ABSTRACT FROM AUTHOR]