Study On Synthesis And Gas Sensing Properties Of WO₃ And Its Hydrate Nanomaterials

For the past few years,air pollution problem caused by continuous development of society and industrialization has been becoming more and more serious.A variety of gas sensors were fabricated for the effective detection of toxic and harmful,explosive and flammable gases,along with the increasing concern in environmental pollution monitoring.To meet practical application requirement,much technological effort has been focused on improving sensing performance of gas sensors,including the response,selectivity,repeatability,long term stability,as well as the response and recovery speed.It is well known that sensing material,as the core part of gas sensor,plays a crucial role in the gas sensing performance.SMOX（semiconducting metal oxides）have been widely used in the field of gas sensors due to the simple synthesis route,low cost,easily controllable morphology and etc.The surface morphology and grain size of the material can largely influence the gas sensing proterties because of the formation of the electron depletion layer near the surface.So,the design of nanostructured SMOX gas sensitive materials with tailored characteristics（components,morphology,surface chemistry state,crystal structure,etc.）has attracted extensively attention and has opened up new ideas for the development of gas sensors.Tungesten oxide（WO₃）as a typical n-type semiconductor material,exhibits superior physical and chemical properties and can be applied in the field of gas sensing.Although considerable efforts have been made on the development of WO₃ based gas sensors,the material also suffers from the low sensitivity,weak selectivity and bad interference immunity.In this paper,modification methods,including changing calcination temperature,heterogeneous cationic doping and noble metal loading,were applied to enhance the sensing performances of material.Through the carefully investigation of the phase structure,chemical composition and surface chemical state of the material,the mechanism of enhanced sensing properties was established.Specific contents are summarized as follows:1.Monodispersed WO₃ nanolamella were synthesized by using a simple solvothermal method and a subsequent controlled annealing route.The morphologies and sensing properities were changed by different calcination temperatures,300°C,400°C,500°C and600°C.The gas sensors based on WO₃ material annealed at 300°C exhibited highest response towards 100 ppm xylene,having a response about 46.6 at 280°C.The detection limit could reach as low as 1 ppm for xylene.In addition,the sensor exhibited good selectivity to xylene.The high response was attributed to good permeability,large specific surface area,surface dangling bonds and crystal defect.2.The pure and Cr-doped WO₃ nanofibers with different concentrations were successfully synthesized by electrospinning and their morphology and gas-sensing properties were investigated.The nanofibers showed porous structure,which were constructed by a lot of small grains.This kind of loose structure could provide more active sites for surface reaction and allow fast gas diffusion.The gas sensors based on 4 mol%Cr-doped WO₃ nanofibers exhibited highest response to 100 ppm xylene,having a response about 35.04,which was as 5times as that of pure WO₃ sensor.The detection limit of the Cr-doped WO₃ sensor could reach as low as 5 ppm for xylene.The enhanced gas sensing performance of Cr-doped WO₃ sensor can be attributed to the more oxygen vacancies and surface chemical changed by Cr doping,which was beneficial to the adsorption of oxygen and xylene.3.Pure and Pd-doped WO₃·H₂O with different concentrations were synthesized via a tartaric acid-assisted hydrothermal method,which had high crystalline quality.The gas sensors based 0.6 at%Pd-doped WO·H₂O had the highest gas response of 21.0 to 10 ppm xylene at a lower optimum operating temperature（230°C）.The experimental results also showed 0.6 at%Pd-doped WO·H₂O had fast response/recovery time,lower detection limit（even to ppb-level）,and good selectivity towards xylene compared with the pure one.Meanwhile,the high-performance results were explained by catalysis of Pd and defects produced by the slight distortion of the crystal lattice.4.Pure WO₃·H₂O and Au-doped WO₃·H₂O nanomaterials were prepared by a facile and efficient hydrothermal method with the assistance of HAuCl₄.It can be confirmed that the 0.3at%Au-doped WO₃·H₂O gas sensors exhibit highest response to xylene at a relatively low operating temperature.The material exhibited a liner relationship between the response and concentration in the range of 100 ppb-1000 ppb.The enhanced gas sensing performance of Au-doped WO₃·H₂O sensor was attributed to the synergistic effects of catalysis of Au nanoparticles and the formation of Au/WO₃·H₂O Schottky junction.