Research On Gas Sensitivity Of VO₂（B） Ultrathin Nanosheet Array And Enhanced Ultraviolet Irradiation

NO₂ is a common toxic and harmful gas.It is one of the main causes of acid rain,photochemical smoke and water eutrophication.It can cause serious harm to the ecological environment and human health.It is of great significance to develop a gas sensor that can detect NO₂ effectively and timely.Metal oxide semiconductor materials,such as Zn O and WO₃,have become the main sensitive materials for detecting NO₂ because of their excellent gas sensing performance,low production cost and mature manufacturing technology.However,these gas sensing materials need to work at high temperature,which will shorten the service life of the device and increase the complexity of device integration.In recent years,B-phase vanadium dioxide（VO₂（B））has attracted much attention because of its sensitivity to NO₂ gas at room temperature.However,VO₂（B）has low sensitivity to NO₂ and long response/recovery time,which needs to be further improved.Continuously reducing the thickness of the nanosheet is an effective method to improve the sensitivity.When the thickness of the nanosheet is close to twice the Debye length of the gas sensitive material,the target gas can completely control the whole conductive path by forming a complete depletion on the surface of the gas sensitive material,so as to obtain high sensitivity.VO₂（B）is a monoclinic crystal with two-dimensional layered structure,which is easy to form nanosheet structure.In order to further improve the sensitivity of VO₂（B）to NO₂,this thesis proposes to study the preparation of VO₂（B）ultrathin nanosheet array,room temperature NO₂ gas sensitivity and UV irradiation enhancement.The research contents mainly include the following two aspects:（1）VO₂（B）ultrathin nanosheet array were prepared directly on silicon wafer by hydrothermal method.The thickness and morphology of VO₂（B）ultrathin nanosheet were controlled by adjusting the oxalic acid concentration in the preparation process,and studied its gas sensing performance.VO₂（B）ultrathin nanosheets with thickness of 10～20 nm were obtained at the concentration of 0.14 mol/L oxalic acid,which is nearly twice the Debye length of VO₂（B）.The sensing performance test of NO₂ gas at room temperature shows that the response of the structure to 5 ppm NO₂ can reach 2.03 and the detection limit is 20 ppb.In addition,it has good selectivity and long-term stability.The sensitive mechanism is explained by surface space charge layer model,neck channel control theory and grain boundary barrier model（2）In order to further improve the gas sensing performance of VO₂（B）ultrathin nanosheet array,it is proposed to irradiate the obtained high-performance VO₂（B）ultrathin nanosheet array structure by ultraviolet light,and the influence of irradiation light wavelength on its sensing performance is studied.Firstly,the band gap of VO₂（B）ultrathin nanosheet was calculated based on the absorbance spectrum,and then the adsorption process of NO₂ on VO₂（B）surface was analyzed by density functional theory.The results show that the band gap width of VO₂（B）ultrathin nanosheet is about 4.6 e V.After adsorption of NO₂,the band gap of VO₂（B）narrows,the Fermi level moves to the valence band,and the density of states near the Fermi level increases significantly.The results of room temperature gas sensing test of NO₂gas under different wavelengths of lights irradiation show that light irradiation with photon energy lower than the band gap has no effect on the sensitivity.However,365 nm ultraviolet irradiation can greatly shorten the response time and recovery time.Reduce the response/recovery time from 21 s/157 s to 3 s/15 s.This is mainly because 365 nm ultraviolet light provides energy for the adsorption and desorption of NO₂ and speeds up the response and recovery.In this thesis,a NO₂ gas sensor based on VO₂（B）ultrathin nanosheet array structure is proposed and prepared.This ultrathin nanosheet array structure makes VO₂（B）have high sensitivity at room temperature and can greatly reduce the response/recovery time under ultraviolet irradiation,which is helpful to expand the application range of VO₂ in the field of gas sensors.