Gas-sensing Enhancement Research Articles

NO2 gas-sensing enhancement by selective laser surface treatment of SnO2 nanorods

A new surface engineering technology has been introduced that can dramatically change the morphology and composition of existing SnO2 nanorods (NRs) to SnOx nanobeads (NBs) + NRs with the addition of a 1–6s laser + convex lens pair (LCP) process. Unlike the existing flame chemical vapour deposition (FCVD) method, which has a two-dimensional effect on a sample, LCP can control a relatively local area with a radius of several microns, making it a more sophisticated and advanced process. Notably, the LCP process at each second was directly linked to the change in the composition of the surface of the SnO2 NRs, and the response to NO2 gas in the SnO2 NBs + NRs with 1second LCP process was improved by approximately three times compared to that of bare SnO2 NRs. Therefore, a new gas-sensing mechanism was proposed through comparing the types of oxygen defect on the SnO2 NRs surface during a slight morphological transition and thermodynamic data of the composition before and after gas sensing.

In situ synthesis of Zn-doped In2S3/In2O3 composites for the monitoring of trace ethanol at low temperature

In this paper, Zn-doped In2S3 flowers grow on the surface of the ceramic tube, and the surface was locally oxidized by injecting oxygen on the surface, so as to prepare Zn-doped In2S3/In2O3 composites with good sensing to ethanol gas. The characterization results showed that the composite has a hierarchical flower structure with uniform size (5–6 μm). Gas sensing tests exhibit that the sensor has outstanding response for ethanol gas. Zn doping and local oxidation significantly reduced the operating temperature of In2S3 from the original 240 °C–100 °C, and the gas sensing response of the sensor was also significantly improved. When the temperature is 100 °C, the response value of pure In2S3 sensor for 10 ppm ethanol gas is 3.55 and Zn/In2S3/In2O3 sensor's response value is increased to 22.82, which can be attributed to the increase of oxygen vacancy caused by Zn doping and the construction of In2S3/In2O3 heterojunction. Finally, density functional theory (DFT), as the theoretical basis of first-principle calculation, is used to calculate and analyze the gas-sensing enhancement mechanism of composite materials.

Enhanced acetone sensing of MOFs derived Co3O4-ZIF hierarchical structure under the strategy of internal construction and external modification

Co3O4-ZIF-x (x = 0,1,2,3) composites have been prepared by in-situ growth of ZIF membrane on MOFs-derived hierarchical porous Co3O4 by the solvothermal method with adjusting the ratio of dimethylimidazole. The effects of ZIF membrane on Co3O4 structure and acetone-sensing properties are investigated by the techniques of XRD, Raman, SEM, TEM, BET, XPS and performance testing, and the possible gas-sensing enhancement mechanism is analyzed. The results show that the typical Co3O4-ZIF-1 sensor has the maximum response of 77.8–50 ppm acetone at 180 °C, the short response/recovery time of 7/35 s, and the low detection limit of 0.1 ppm (3.28). It also has the reliable acetone selectivity against to other interfering gases, as well as good repeatability and high stability. Its enhanced acetone-sensing property may be attributed to the synergistic effect of the improved surface state, high specific surface area, adjustable pore size and rich reaction activation sites, which caused by the hierarchical porous structure of MOFs-derived Co3O4 and the gas adsorption and enrichment effect of ZIF membrane, promoting the further reaction and diffusion of acetone. The “internal construction and external modification” role of MOFs playing in Co3O4 provides a new idea for designing high performance sensing materials.

ZnO Nanorod-Immobilized Pt Single-Atoms as an Ultrasensitive Sensor for Triethylamine Detection.

Triethylamine (TEA) is a flammable and highly toxic gas, and the fast, accurate, and sensitive detection of gas TEA remains greatly challenging. Herein, we report a ZnO nanorod anchored with a single-atom Pt catalyst (Pt1/ZnO) as a gas sensor for TEA detection. The sensor shows high selectivity and high response to gas TEA with a response value of 4170 at 200 °C, which is 92 times higher than that of pure ZnO. Moreover, the Pt1/ZnO sensor has very short response and recovery times of only 34 and 76 s, respectively, and also has a high response to ppb-level TEA gas (100 ppb-21.6). The gas-sensing enhancement mechanism of the Pt1/ZnO sensor to gas TEA was systematically investigated using band structure analysis, in situ diffuse reflectance infrared Fourier transformation spectroscopy, and density functional theory calculations. The results show that the oxygen vacancies on Pt1/ZnO can effectively activate the adsorbed oxygen. Moreover, chemical bonds can be formed between Pt single atoms and N atoms in TEA to achieve effective adsorption and activation of TEA molecules, facilitating the reaction between TEA and the adsorbed oxygen on Pt1/ZnO, and thereby obtaining high gas-sensing performance. This work highlights the crucial role of Pt single-atom in improving the sensing performance for gas TEA detection, paving the way for developing more advanced gas sensors.

A novel room-temperature formaldehyde gas sensor based on walnut-like WO3 modification on Ni-graphene composites.

Ternary composite with great modulation of electron transfers has attracted a lot of attention from the field of high-performance room-temperature (RT) gas sensing. Herein, walnut-like WO3-Ni–graphene ternary composites were successfully synthesized by the hydrothermal method for formaldehyde (HCHO) sensing at RT. The structural and morphological analyses were carried out by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). SEM and TEM studies confirmed that walnut-like WO3 nanostructures with an average size of 53 ± 23 nm were functionalized. The Raman and XPS results revealed that, due to the deformation of the O-W-O lattice, surface oxygen vacancies Ov and surface-adsorbed oxygen species Oc were present. The gas-sensing measurement shows that the response of the WO3-Ni-Gr composite (86.8%) was higher than that of the Ni-Gr composite (22.7%) for 500 ppm HCHO at RT. Gas-sensing enhancement can be attributed to a p-n heterojunction formation between WO3 and Ni-Gr, Oc, spill-over effect of Ni decoration, and a special walnut-like structure. Moreover, long term stability (%R = 61.41 ± 1.66) for 30 days and high selectivity in the presence of other gases against HCHO suggested that the proposed sensor could be an ideal candidate for future commercial HCHO-sensing in a real environment.

Theoretical approach to boosting NH3-sensing based on GeSe/SnS vdW heterostructure with strain modulation

Stacking multiple two-dimensional materials to form a heterostructure is emerging as an effective way for tuning gas-sensitive properties. In this work, the GeSe/SnS van der Waals (vdW) heterostructure with Janus structure was constructed and investigated via first-principles calculations. The formation of the GeSe/SnS vdW heterostructure leads to spontaneous symmetry breaking and energy level splitting of the system, bring about new electronic properties. The GeSe/SnS heterostructure shows a smaller band gap of 1.272 eV, with an electron-rich layer formed by interlayer charge transfer (0.171 e) due to the built-in electric field, which promotes the NH3 adsorption energy and charge transfer ability. Further investigate was performed for the effect of compressive strain on the band gap and interface structure of GeSe/SnS heterostructure. The application of −6% compressive strain causes the largest work function change (0.423 eV) of the GeSe/SnS heterostructure before and after NH3 adsorption on the GeSe layer, with maximum adsorption energy (0.805 eV) and charge transfer (0.089 e), which implies the enhanced interaction between NH3 and heterostructure. Our work theoretically predicts that constructing a heterostructure and applying strain can synergistically enhance the NH3 response of SnS and GeSe, which paves the way for gas-sensing enhancement studies of black phosphorus-like materials.

Fabrication of hollow porous ZnO@ZnS heterostructures via hydrothermal method and enhanced gas-sensing performance for ethanol

Heterostructures with great modulation of electron transfer afford a great opportunity for gas-sensing applications. Herein, a hollow porous ZnO@ZnS core-shell structure with high gas-sensing performance has been successfully hydrothermally synthesized. The phase composition and the morphology of the as-prepared products were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope and energy dispersive X-ray spectroscopy. The results show that the pure ZnO and the ZnO@ZnS appear a mesoporous structure and a hollow porous structure, respectively. Gas-sensing measurements show that the gas-sensing performance of the ZnO@ZnS based sensor is obviously higher than that of the pure ZnO, and it has a high selectivity, quick response-recovery and stability to ethanol. Its response reaches 114.2 to 100 ppm ethanol, and the response and recovery time is 3 s and 8 s to ethanol, respectively. The gas-sensing enhancement of the ZnO@ZnS can be attributed to the Schottky barrier at the interface of the ZnO@ZnS and the special hollow porous structure. The strategy put forward is generally applicable to design efficient ethanol sensors with optimized sensing performances.

Surface Functionalization of Ti3C2Tx MXene with Highly Reliable Superhydrophobic Protection for Volatile Organic Compounds Sensing.

Two-dimensional (2D) transition-metal carbides (Ti3C2Tx MXene) have received a great deal of attention for potential use in gas sensing showing the highest sensitivity among 2D materials and good gas selectivity. However, one of the long-standing challenges of the MXenes is their poor stability against hydration and oxidation in a humid environment, limiting their long-term storage and applications. Integration of an effective protection layer with MXenes shows promise for overcoming this major drawback. Herein, we demonstrate a surface functionalization strategy for Ti3C2Tx with fluoroalkylsilane (FOTS) molecules through surface treatment, providing not only a superhydrophobic surface, mechanical/environmental stability but also enhanced sensing performance. The experimental results show that high sensitivity, good repeatability, long-term stability, and selectivity and faster response/recovery property were achieved by the FOTS-functionalized when Ti3C2Tx was integrated into chemoresistive sensors sensitive to oxygen-containing volatile organic compounds (ethanol, acetone). FOTS functionalization provided protection to sensing response when the dynamic response of the Ti3C2Tx-F sensor to 30 ppm of ethanol was measured over in the 5 to 80% relative humidity range. Density functional theory simulations suggested that the strong adsorption energy of ethanol on Ti3C2Tx-F and the local structure deformation induced by ethanol adsorption, contributing to the gas-sensing enhancement. This study offers a facile and practical solution for developing highly reliable MXene based gas-sensing devices with response that is stable in air and in the presence of water.

Toward agricultural ammonia volatilization monitoring: A flexible polyaniline/Ti3C2Tx hybrid sensitive films based gas sensor

The monitoring of ammonia (NH3) produced in agricultural fields is of great importance due to its huge threat to ecological environment and human health, yet routine monitoring technologies rely on manual completion and complex agricultural environment brings great challenges to the application feasibility of gas sensors. Herein, a flexible chemiresistive gas sensor based on polyaniline (PANI)/Ti3C2Tx hybrid sensitive films is developed toward agricultural NH3 volatilization monitoring. The hybrid film was deposited on the flexible polyimide substrate by an in-situ self-assembly method. The sensor shows high sensitivity, low detection limit, excellent repeatability, high selectivity and good air stability, which could be attributed to the gas-sensing enhancement effects of PANI/Ti3C2Tx Schottky junction and improved protonation degree of PANI in the hybrids. Especially, the sensor exhibits excellent NH3-sensing properties to 20–80 % relative humidity (RH) environments at a temperature range of 10–40 ℃, making it promising for practical agricultural applications. Furthermore, the application feasibility of the sensor to ammonia volatilization monitoring is verified through agricultural simulation experiments. This work provides a fast and accurate gas sensor methodology toward unattended agricultural application, which is an important supplement to the existing technical methods and may greatly pushes forward the development of modernized intelligent agriculture.

Surface Functionalization of Layered Molybdenum Disulfide for the Selective Detection of Volatile Organic Compounds at Room Temperature.

Semiconducting two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are considered promising sensing materials due to the high surface-to-volume ratio and active sensing sites. However, the reported strategies for 2D TMDCs toward sensing of volatile organic compounds (VOCs) present with some drawbacks. These include high operation temperatures, low gas response, and complex fabrication, limiting the development of room-temperature gas sensors. In this study, 2D MoS2 nanoflakes were prepared by liquid-phase exfoliation, and their surface was functionalized with Au nanoparticles (NPs) through a facile solution mixing method. MoS2 decorated with Au NPs with an average size of 10 nm was used as a material platform for an electrochemical sensor to detect a wide variety of VOCs at room temperature. Through dynamic sensing tests, the enhancement of gas-sensing performance in terms of response and selectivity, especially in detecting oxygen-based VOCs (acetone, ethanol, and 2-propanol), was demonstrated. After Au functionalization, the response of the gas sensor to acetone improved by 131% (changing from 13.7% for pristine MoS2 to 31.6% for MoS2-Au(0.5)). Sensing tests under various relative humidity values (10-80%), bending or long-term conditions, indicated the sound robustness and flexibility of the sensor. Density functional theory simulations suggested that the adsorption energy of VOC molecules on MoS2-Au is significantly higher than that on pristine MoS2, contributing to the gas-sensing enhancement; a VOC-sensing mechanism for Au-decorated MoS2 nanoflakes was proposed for the first time for the highly sensitive and selective detection of oxygen-based VOCs.

A high-performance flexible gas sensor based on self-assembled PANI-CeO2 nanocomposite thin film for trace-level NH3 detection at room temperature

A resistive-type flexible ammonia (NH3) sensor was proposed and developed in this work, which was prepared by depositing polyaniline-cerium dioxide (PANI-CeO2) nanocomposite thin film on flexible polyimide (PI) substrate through in-situ self-assembly method. The effect of CeO2 nanoparticles on the polymerization of aniline was studied by comparing the morphological, structural and chemical features of the pure PANI and PANI-CeO2 nanocomposite, and the dynamic polymerization processes were also recorded and investigated. In this process, an interesting phenomenon was found that the protonation and oxidation degrees of PANI in PANI-CeO2 nanocomposite were improved significantly according to the XPS spectra analysis, which should be ascribed to the synergetic oxidation of CeO2 nanoparticles and ammonium persulfate (APS). Meanwhile, the NH3-sensing performances of the pure PANI and PANI-CeO2 film sensors were evaluated at room temperature (∼25 °C), which showed that the PANI-CeO2 film sensor possessed enhanced response, reduced recovery time, perfect response-concentration linearity, good reproducibility, splendid selectivity, remarkable long-term stability, ultra-low detectable concentration (16 ppb) and theoretical detection limit (0.274 ppb), and outstanding flexibility without significant response decrease after 500 bending/extending cycles. It was speculated that the excellent sensing performances should probably benefit from the gas-sensing enhancement effect of p-n junction, the improved protonation degree and modified morphology of PANI by the addiction of CeO2 nanoparticles. And, the high flexibility might originate from the flexible structure of PANI chains, and the good adhesion and nano-mechanical performance of PANI-CeO2 film. Besides, the effect of relative humidity on the sensing properties of PANI-CeO2 film sensor was also discussed and analyzed. Therefore, the proposed high-performance flexible PANI-CeO2 thin film sensor holds great promise for application into hand-held or wearable electronic devices for trace-level NH3 detection at room temperature.

Facile synthesis of SnO2 hierarchical porous nanosheets from graphene oxide sacrificial scaffolds for high-performance gas sensors

Abstract Facile synthesis of metal oxide semiconductors with controllable and novel nanostructures has attracted tremendous attentions due to their crucial importance in gas detection. In this work, we report the design and synthesis of tin oxide hierarchical porous nanosheets (SnO 2 HPNSs) via a graphene oxide (GO) assisted hydrothermal route, where the GO flakes act as scaffolds and fluoride ions (F − ) act as etchant to control the attachment growth of SnO 2 nanosheets. Highly wrinkled SnO 2 /reduced graphene oxide nanosheets (such as SnO 2 /rGO-5.0 mg) have been achieved with a proper addition amount of GO powders. The wrinkled ultrathin nanosheets are subsequently converted to the SnO 2 HPNSs after annealing at 500 °C in air, which are composed of the network interconnected SnO 2 nanosheets (or nanoparticles). Moreover, the SnO 2 HPNSs based gas sensors exhibit greatly enhanced ethanol sensing performance, with high response (77.1–100 ppm) and good selectivity at an optimum operating temperature of 250 °C, compared with the bare SnO 2 nanosheets (4.0–100 ppm, at 275 °C). A possible gas-sensing enhancement mechanism based on this novel hierarchical architecture is discussed.

Highly sensitive acetone-sensing properties of Pt-decorated CuFe2O4 nanotubes prepared by electrospinning

Pristine and Pt-decorated copper ferrite nanotubes (Pt-CuFe2O4 NTs, 0.1%, 0.5%, and 1.0%, mole percent) were prepared by a simple electrospinning method. The samples were characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope. Their gas-sensing properties were evaluated by a commercial CGS-4TPs system. Microscopic images showed that all samples consisted of well-defined nanotubes with diameter of 70‒100nm. Gas-sensing measurements revealed that the Pt-CuFe2O4 NTs had an improved acetone-sensing properties compared with pristine CuFe2O4 NTs. In particular, 0.5% Pt-CuFe2O4 NT-based sensor showed a high response (16.5 at 100ppm), good selectivity, and long-term stability for acetone at 300°C. In addition, more Pt dopants would have a greater effect on promoting the sensing properties of the CuFe2O4 NTs at high acetone concentrations. A gas-sensing enhancement mechanism of Pt-CuFe2O4 NT-based sensors was proposed, according to the catalytic oxidation process of acetone molecules, which could be due to the kinetic competition between Pt dopants and CuFe2O4 NTs.

Research on the Gas Sensing Enhancement by Using CNT/ZnO Composites

Multiwall carbon nanotubes (CNT) were added into ZnO matrix to develop a CNTs/ZnO composite gas sensor. The hybrid sensor is used to detect CO in air by measuring resistance changes of thin CNTs/ZnO films at different working temperature. For comparison, pure ZnO and CNT/ZnO sensors are also examined. The gas sensing results reveal that CNTs/ZnO with the weight ratio (9:100) hybrid sensors exhibit much higher sensitivity and fast response-recovery properties towards CO, at 240°C than the blank ZnO nanowires. Hybrid material of ZnO nanowires and CNT composites could potentially display not only the unique properties of nanowires and those of CNTs, but also an additional novel property.

Carbon doped tungsten oxide nanorods NO2 sensor prepared by glancing angle RF sputtering

Metal oxide semiconductor nanostructures offer potential advantages in sensing applications due to their large surface to volume ratio, lower electron recombination rate, and high stability. However, most methods produce nanostructures with random sizes, distribution and orientations, which are not reliable for practical applications because of poor reproducibility. In this work, homogeneous carbon-doped WO3 nanorods are developed based on the glancing angle deposition (GLAD) technique using radio-frequency magnetron sputtering and investigated for NO2 gas sensing application. The carbon doping is achieved by using acetylene gas as a carbon source. Characterization based on scanning electron microscopy, Auger electron spectroscopy and X-ray diffraction confirm the formation of uniform carbon-doped crystalline WO3 nanorods. The gas-sensing results reveal that carbon-doped WO3 nanorods sensor exhibits not only high response and selectivity to NO2 (0.5–5ppm) but also at low operating temperature (150°C) compared with the undoped ones. The observed gas-sensing enhancement may be attributed to the increase of specific surface area and the decrease of activation energy by carbon doping.