•An effective design has been realized for self-heated sensors using network nanowires.•The density of nanowires was optimized via controlling of the gap size of heat-to-heat electrodes.•The dense of ...network nanowires consumes a high power but is effective for detection of reducing gas.
Developing metal oxide gas sensors for internet-of-things (IoT) and portable applications require low-power consumption because of the limited battery in devices. This requirement is challenging because metal oxide sensors generally need high working temperatures, especially for reducing gases. Herein, we present an effective design and fabrication method of a SnO2 nanowire (NW) sensor for reducing gases by using the Joule heating effect at NW nanojunctions without needing an external or integrated heater. The sensor’s low-power consumption at around 4 mW was controlled by the size and nanojunction density of the device. The sensor has a simple design and is easy to fabricate. A proof-of-concept of a portable gas sensor module can be realised for monitoring highly toxic reducing gases, such as H2S, NH3 and C2H5OH, by using the developed self-heated NWs.
•Porous network Fe2O3 effectively prepared from Fe3O4/rGO.•The porous network Fe2O3 exhibited good performance to ethanol gas.•The developed strategy can employed for preparation of other porous ...network metal oxide.
Nanoporous network metal oxides are potential candidates for various applications such as filtration, biomaterials devices, and sensing materials. The present work focused on the simple and scalable fabrication of the α-Fe2O3 nanoporous network for ethanol gas sensor using Fe3O4/reduced graphene oxide (rGO) as a precursor. The analyzed morphology and crystal structure indicated that the α-Fe2O3 nanoporous network was formed due to some factors during thermal procedures such as the phase transformation from magnetite to hematite, nanoparticle agglomeration, and combustion of rGO. The ethanol gas-sensing properties of the α-Fe2O3 nanoporous network were investigated. The response to 100ppm ethanol gas was as high as 9.5, while the cross-gas responses to 100ppm NH3, H2, and CO gases were all lower than 2.0. These values indicated a good selectivity of the sensors. Furthermore, the 90% response times to ethanol gas were less than 5s at 400°–450°C. The proposed strategy has potential in the preparation of other porous network metal oxides to achieve high-performance gas sensors.
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•Facile on-chip electrospinning has been developed for preparing ZnFe2O4 nanofibers.•ZnFe2O4 nanofiber sensors can detect H2S gas down to ppb level.•Nanograin size and crystallinity ...have correlative effect on gas sensing performance.
ZnFe2O4 nanofiber gas sensors are cost-effectively fabricated by direct electrospinning on microelectrode chip with Pt interdigitated electrodes and subsequent calcination under different conditions to maximize their response to H2S gas. The synthesized nanofibers of approximately 30–100 nm in diameter show typical spider-net-like morphology of the electrospun nanofibers. The ZnFe2O4 nanofibers comprise many 10–25 nm nanograins, which results in multi-porous structures. Moreover, the nanofibers exhibit the single phase of cubic-spinel-structure ZnFe2O4. The density, crystallinity and grain size of ZnFe2O4 nanofiber that strongly affect gas-sensing properties can be optimized by controlling electrospun time, annealing temperature, annealing time and heating rate. Under optimal conditions, the ZnFe2O4 nanofiber sensors exhibit high sensitivity and selectivity to H2S at sub-ppm levels. Excellent gas-sensing performances are attributed to effects of multi-porous structure, nanograin size and crystallinity, which is explained by the sensing mechanisms of ZnFe2O4 nanofiber sensors to H2S gas.
•rGO-loaded ZnFe2O4 nanofibers have simply prepared by electrospinning.•rGO-loaded ZnFe2O4 nanofiber sensors can detect H2S gas down to ppb level.•The loading rGO can result in an enhancement of H2S ...gas ensing performance.
Cost-effective fabrication of sensors and detection of ultralow concentrations of toxic gases are important concerns for environmental monitoring. In this study, the reduced graphene oxide (RGO)-loaded ZnFe2O4 nanofibers (ZFO-NFs) were fabricated by facile on-chip electrospinning method and subsequent heat treatment. The multi-porous NFs with single-phase cubic spinel structure and typical spider-net morphology were directly assembled on Pt-interdigitated electrodes. The diameters of the RGO-loaded ZFO-NFs were approximately 50–100 nm with many nanograins. The responses to H2S gas showed a bell-shaped behaviour with respect to RGO contents and annealing temperatures. The optimal values of the RGO contents and the annealing temperatures were found to be about 1.0 wt% and 600 °C, respectively. The response of the RGO-loaded ZnFe2O4 NFs to 1 ppm H2S gas was as high as 147 at 350°C while their cross-gas responses to SO2 (10 ppm), NH3 (100 ppm), H2 (250 ppm), C3H6O (1000 ppm), and C2H5OH (1000 ppm) were rather low (1.8−5.6). The high sensor response was attributed to formation of a heterojunction between RGO and ZnFe2O4 and due to the fact that NFs consisted of many nanograins which resulted in multi-porous structure and formation of potential barriers at grain boundaries.
•The Pt–ZTO hollow octahedra were synthesised for acetone gas sensor.•The Pt–ZTO sensor could detect acetone at ppb level with excellent selectivity and stability.•The Pt–ZTO sensor exhibited a ...36.9-fold enhancement in response to acetone compared with the pristine ZTO.•The Pt–ZTO sensor can be applied for diabetes diagnosis applications.
Acetone in exhaled breath can be used as a biomarker for diabetes diagnosis, but its concentration is extremely low. Thus, a gas sensor with high sensitivity and a low detection limit (sub-ppm level) is vital in practical application. Here, ultrafine Pt nanoparticles were prepared and used to decorate the surface of Zn2SnO4 (ZTO) hollow octahedra for enhanced acetone gas-sensing performance towards breath analysis. The Pt nanoparticles were prepared by a simple wet chemical method, whereas the decoration on the surface of ZTO was conducted by mixing different amounts of Pt nanoparticles. The synergistic effect of the high catalytic activity of Pt nanoparticles and the large surface area of ZTO hollow octahedra endowed the fabricated sensor with excellent acetone-sensing performance at the optimum temperature of 350 °C. Compared with pristine ZTO, the 1 wt% Pt–ZTO device showed the best acetone sensing behaviour at 350 °C with an excellent selectivity, repeatability, and stability. It was able to detect acetone at ppb level, which was satisfactory for breath analysis in diabetes diagnosis. Notably, the 1 wt% Pt–ZTO sensor exhibited a 36.9-fold enhancement in response to acetone compared with the pristine ZTO. Moreover, the effect of Pt decoration on gas-sensing behaviours and the gas-sensing efficiency enhancement mechanism of ZTO hollow octahedra were discussed.
Fabrication of a high-performance room-temperature (RT) gas sensor is important for the future integration of sensors into smart, portable and Internet-of-Things (IoT)-based devices. Herein, we ...developed a NO2 gas sensor based on ultrathin MoS2 nanoflowers with high sensitivity at RT. The MoS2 flower-like nanostructures were synthesised via a simple hydrothermal method with different growth times of 24, 36, 48, and 60 h. The synthesised MoS2 nanoflowers were subsequently characterised by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, energy-dispersive X-ray spectroscopy and transmission electron microscopy. The petal-like nanosheets in pure MoS2 agglomerated to form a flower-like structure with Raman vibrational modes at 378 and 403 cm−1 and crystallisation in the hexagonal phase. The specific surface areas of the MoS2 grown at different times were measured by using the Brunauer–Emmett–Teller method. The largest specific surface area of 56.57 m2 g−1 was obtained for the MoS2 nanoflowers grown for 48 h. This sample also possessed the smallest activation energy of 0.08 eV. The gas-sensing characteristics of sensors based on the synthesised MoS2 nanostructures were investigated using oxidising and reducing gases, such as NO2, SO2, H2, CH4, CO and NH3, at different concentrations and at working temperatures ranging from RT to 150 °C. The sensor based on the MoS2 nanoflowers grown for 48 h showed a high gas response of 67.4% and high selectivity to 10 ppm NO2 at RT. This finding can be ascribed to the synergistic effects of largest specific surface area, smallest crystallite size and lowest activation energy of the MoS2-48 h sample among the samples. The sensors also exhibited a relative humidity-independent sensing characteristic at RT and a low detection limit of 84 ppb, thereby allowing their practical application to portable IoT-based devices.
•Nanoporous and crystal evolution in nickel oxide nanosheets was investigated.•Correlation between the porous size, crystal size, defect level and gas sensing properties was clarified.•Effect of ...carrier gases on sensing characteristics was studied to determine the sensing mechanism.•The nanoporous NiO nanosheets are promising for H2 and H2S monitoring.
Understanding the formation mechanism and control of crystal evolution of nanoporous materials is important in the synthesis of nanoporous materials for enhanced gas-sensing performance. Herein, single crystal nickel hydroxide nanosheets were prepared by hydrothermal reaction between nickel chlorine and ammonium hydroxide. Then, the products were subsequently calcined at 400, 500, 600, and 700 °C in air to form nanoporous NiO nanosheets. Evolution of the nanopores and crystal size of the nanoporous NiO nanosheets were investigated X-ray diffraction, thermogravimetric and differential thermal analysis, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption/desorption isotherm and photoluminescence excitation spectroscopy, towards gas-sensing applications. Results showed that the calcination temperature altered the crystallinity, morphology, specific surface area, and the porous structure of the NiO nanosheets. Gas-sensing properties of the synthesized NiO nanosheets towards H2 and H2S were investigated to clarify the effect of material characteristics on gas-sensing performance. The NiO nanosheets calcined at 700 °C for 2 h exhibited the highest response to H2 despite having the largest crystal size and the lowest specific surface area. The response of the porous NiO nanosheet device to H2S gas in air and in nitrogen as carrier was also studied to determine the sensing mechanism.
The morphology and crystalline size of metal oxide-sensing materials are believed to have a strong influence on the performance of gas sensors. In this paper, we report a comparative study on the ...ethanol-sensing characteristics of ZnO nanorods, nanowires, and porous nanoparticles. The porous ZnO nanoparticles were prepared using a simple thermal decomposition of a sheet-like hydrozincite, whereas the nanorods and nanowires were grown by hydrothermal and chemical vapor deposition methods, respectively. The morphology and crystal structure of the synthesized materials were characterized by field-emission scanning electron microscopy and x-ray diffraction. Ethanol gas-sensing characteristics were systematically studied at different temperatures. Our findings show that for ethanol gas-sensing applications, ZnO porous nanoparticles exhibited the best sensitivity, followed by the nanowires and nanorods. Gas-sensing properties were also examined with respect to the role of crystal growth orientation, crystal size, and porosity.
•Facile method has employed to prepare mesoporous Co3O4 nanochains with high specific surface area.•The Co3O4 nanochains has highly sensitive and selective toward H2S gas with rapid response.•Such a ...potential gas sensing strategy can be easily extended to other metal oxides.
In this paper, cobalt carbonate hydroxide (Co(CO3)0.5(OH)..11H2O) nanowires were successfully fabricated by a simple hydrothermal route without using surfactants and by subsequent heat treatment in air at 600 °C for 5 h to obtain mesoporous Co3O4 nanochains. As-synthesized nanochains with length of several micrometers consisted of well-linked Co3O4 nanoparticles with an average size of 50 nm. The sensor based on mesoporous Co3O4 nanochains was used to detect flammable and toxic gases, including H2S, NH3, CO, and H2. Results showed potential of mesoporous Co3O4 nanochains as sensor material for detection of hydrogen sulfides at low concentration with rapid response.
ZnO nanostructures can be synthesized using different techniques for gas sensor applications, but different synthesis methods produce different morphologies, specific surface areas, crystal sizes, ...and physical properties, which consequently influence the gas-sensing properties of materials. Many parameters such as morphology, specific surface areas, crystal sizes, and defect level can influence the gas-sensing properties of ZnO nanostructures. However, it is not clear which parameter dominates the gas-sensing performance. This study clarified the correlation between crystal size, defect level, and gas-sensing properties of ZnO nanostructures prepared from hydrozincite counterparts by means of field emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction and photoluminescence spectra. Results showed that the average crystal size of the ZnO nanoparticles increased with thermal decomposition temperatures from 500 °C to 700 °C. However, the sample treated at 600 °C, which has the lowest visible-to-ultraviolet band intensity ratio showed the highest response to ethanol and NO
2
. These results suggested that defect level but not size is the main parameter dominating the sensor performance. The gas sensing mechanism was also elucidated on the basis of the correlation among decomposition temperatures, crystal size, defect level, and gas sensitivity.
ZnO nanostructures were synthesized for ethanol and nitrogen dioxide gas-sensing applications. Results pointed out that the defect levels dominating the gas-sensing performance but not the morphology, specific surface area or crystal size.
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