Research Of Nitrogen Dioxide Gas Sensor Based On Tungsten Oxide With Hierarchical Structures

The gas sensor is the crucial element for obtaining information on the type and concentration of the monitored gas.It is necessary to develop a gas sensor with excellent characteristics such as high sensitivity,good selectivity,and rapid response recovery for detection gas with low concentration.As the core of the sensing element,sensitive materials are the focus research in the field of gas sensing.In this paper,we aimed to fabricate NO₂ gas sensor with high performance using WO₃ as sensitive material.WO₃ powders with different hierarchical structures were prepared by high-temperature hydrothermal/solvothermal synthesis methods.The relationship between sensitivity of WO₃ materials and the nanostructures were studied.The detection of NO₂ gas with ppb level by the sensor is realized.Firstly,WO₃nanocrystals were prepared by one-step solvothermal method.The limit of detection was 10 ppb.Then WO₃ hollow spheres were prepared by solvothermal method to improve the response and recovery characteristics of the materials.The solvothermal,water bath and acidification methods were used to synthesize WO₃ with similar flower-like nanostructures.After comparing the sensitivity and response/recovery characteristics of the materials,the relationship between the structural parameters and the sensitivity characteristics was studied.Finally,the method of doping modification was studied.The influence of Sn-doping on the gas sensing characteristics of WO₃ to NO₂ was discussed.This method improved the sensitivity and the response/recovery speed of the device.The specific research content is as follows:1.Monodispersed and uniform WO₃ nano-blocks with dimensions of about 300nm and thickness of about 100 nm were prepared by using a simple solvothermal method.The prepared WO₃ material exhibited excellent sensitivity to NO₂ with a detection limit of 10 ppb and a corresponding response of 3.2.The response/recovery time for 40 ppb of NO₂ was 11 min/12 min.In addition,the sensorr had good selectivity to NO₂ and was almost insensitive to other interfering gases.The detection of NO₂ with ppb level was mainly attributed to the good crystallinity of the material.2.In order to improve the response and recovery speed of the material,WO₃hollow microspheres were prepared by using solvothermal method with sodium tungstate as the source material and gas sensors were fabricated.Morphologies of the products at different reaction times were studied through time-evolution experiment,and the formation process and formation mechanism of the hollow spheres were discussed.The prepared hollow spheres which had a diameter of 2?m and a spherical shell thickness of³00 nm showed good repeatability and stability to target gas of NO₂.The response/recovery time of the sensor to 1 ppm NO₂ was 6 min/1.4 min,and the response recovery speed was significantly improved.We attributed the increase of response recovery speed to the hollow structure of the material,allowing target gas to pass quickly.However,the adhesion of the prepared hollow microspheres reduced the specific surface area of the material,resulting no significant enhancement of the sensitivity,which needed to be further improved.3.Dscrete flower-like structures possess space between the discrete nanosheets and the gap between the structures that are conducive to the inflow and outflow of target gases to the sensitive body.In the view of this structure advantage,one-step solvothermal method,dilute concentrated hydrochloric acid solution in water bath,and acidification treatment of hydrothermal precursor were used to prepare three kinds of flake-like WO₃ nanomaterials.The SEM results showed that all the flower-like structures were assembled from nanosheets and the homogeneity was good.Since there were differences in the size and the thickness of the nanostructured unit between the three prepared materials,the characteristics to the target gas of NO₂were also different.The solvothermal method prepared WO₃ had a nanosheet structure unit with thickness between 150 nm-200 nm.By changing the sintering temperature of the material,it was found that the material sintered at 500°C showed the best sensitivity,and the detection limit of NO₂ was 5 ppb,but its response/recovery time was longer.The flower-like structures synthesized by a water bath method had a nanosheet thickness of only about 20 nm,and the response recovery speed has been greatly improved.However,due to the adhesion between the nanostructures,the sensitivity of the material was low and the detection limit was only 40 ppb.Finally,we synthesized the hydrothermalprecursors and acidified the precursors to obtain a nanosheet assembled structure with a diameter of 1?m and a nanosheet thickness of70 nm.The sensors based on this hierarchical structure exhibited the highest sensitivity to NO₂,detection limit was down to 2 ppb.In this part,we compared the sensitivity and response/recovery time of three materials to NO₂.The results showed that the acidified material had the highest sensitivity to NO₂,which was attributed to the basic assembled structure unit of the microstructure:nanosheets.Due to the longer length of the nanosheets of this structure,the formed flower-like structure had better permeability after being stacked,which was favorable for the gas diffusion,achieving the purpose of improvement of the utilization rate of the sensitive body.The sawtooth edge of the sheet also provided more active sites for gas contact reactions.The material synthesized by the water bath method had the shortest response/recovery time.This was due to the pore structure,which enabled the material to exhibit faster response/recovery characteristics to NO₂.4.The addition of the second component is a common method to improve the gas-sensing properties of gas sensors,and there are fewer related reports about WO₃-based NO₂ sensors.Therefore,the influence of Sn doping on the gas sensing properties of WO₃ materials was investigated.With WCl₆ as the tungsten source,different amounts of tin chloride were incorporated during the hydrothermal process to synthesize nanosheets with different sizes and good dispersibility.The characteristics of the doped material to NO₂ gas were studied.Through analysis of XRD?EDX and XPS,it was proved that Sn was successfully incorporated into the WO₃ crystal lattice.However,the incorporation of Sn almost had no influence on the morphology of the material.Therefore,the difference of gas-sensing properties was not directly related to the morphology of the material.Gas-sensitivity tests were carried out on pure phase and doped nanosheets.It was found that when the doping amount of Sn was 2 wt%,the gas sensitivity was best,both the sensitivity and response/recovery speed of the sensor were significantly improved.Compared with the undoped WO₃ material,the response to 100 ppb NO₂ was increased from 22 to 55,and the response/recovery time was reduced from 519 s/277 s to 249 s/181 s.In addition,doping also lowered the detection limit of the material from 10 ppb to 5 ppb,which was another intuitive manifestation of the improvement in sensitivity characteristics.The experimental results showed that the doping of Sn changed the oxygen defect content of the material.The 2 wt%Sn-doped sample had the highest oxygen defect content and the best sensitivity.It can be concluded that the higher oxygen defect content contributed to the better gas sensing properties of the material.Although the morphology was not the main reason for the different properties of each doping level,in the view of the structure of the material,nanoflakes with very small sizes existing between nanoflakes with larger sizes avoided the aggregation of the nanostructures.This kind of structure increased the porosity of the microstructure and facilitated the enhancement of the gas-sensing properties of the material.