Gas-sensing Properties Of Wo₃·H₂O And WO₃ Thin Films Prepared By Chemical Bath Deposition

In the past few decades,the emission of toxic,flammable and explosive gases has caused serious air pollution,which is detrimental to the environment and human health.In order to monitor air quality in real time,gas detection technology has attracted widespread attention in the fields of science and industry.In addition,gas sensors are also widely used in medical and food safety fields.Resistive gas sensors are widely welcomed for their excellent sensing characteristics,as well as the advantages of low cost,easy manufacturing and simple measurement.Its working principle is based on the resistance change caused by the surface reaction of the gas and the sensitive material.Sensitive materials play a vital role in determining sensing performance.As a typical N-type semiconductor material,WO₃has rich nanostructure morphologies.This paper focuses on the in-situ deposition of WO₃?H₂O and WO₃nanostructured thin films on gas sensor chips and the investigation of their gas-sensing properties.The main experimental results are as follows:(1)WO₃·H₂O thin films were grown in situ on the substrate by chemical bath deposition(CBD),and the film samples were characterized by SEM,XRD,UV-Vis,etc.SEM observations show that the WO₃·H₂O film consists of an array of nanosheets growing perpendicular to the substrate,and there are uniform pores between the nanosheets.The gas sensing tests indicate that the optimal working temperature of the WO₃·H₂O nanosheet films is 50?,and the 60-min grown WO₃·H₂O nanosheet films exhibit the best response to NO₂ gas.The response value to1ppm NO₂gas can reach 8.43,and the lowest detectable concentration of NO₂gas is 100ppb.The introduction of ultraviolet light irradiation can greatly improve the gas-sensitive response of WO₃·H₂O nanosheet films to NO₂,and the response value to 1ppm NO₂at room temperature is as high as 1012.(2)The CBD WO₃·H₂O film is further heat-treated at different temperatures to obtain WO₃film.The results of thermogravimetric analysis showed that WO₃·H₂O lost crystal water at about214?.XRD results also show that monoclinic phase WO₃can be obtained by heat treatment above 200?.The gas sensitivity test at different temperatures showed that the optimal working temperature of the WO₃nanosheet films is 150?,and the 300?-annealed WO₃nanosheet films shows the best response to NO₂gas.Ultraviolet light irradiation further improves the gas sensitivity of the WO₃nanosheet films.(3)Amorphous carbon film of different thickness is further deposited on the surface of the WO₃nanosheet films.SEM results showed that the deposition of amorphous carbon film did not change the morphology of the WO₃nanosheet films.Gas sensitivity tests at different temperatures show that the best working temperature of WO₃/C composite film is 100°C.The WO₃/C composite film coated with 3 layers of amorphous carbon showed the best response to NO₂gas,the response value to 1ppm NO₂gas was as high as 82.17,and the lowest detectable NO₂ gas concentration was 100ppb.The WO₃/C composite films have a slower response/recovery speed.Introduction of a 200°C heat pulse during the detection process can accelerate the desorption of the gas,improving its response recovery speed and increasing the response value to low concentrations of NO₂.