Research On Novel Two-dimensional Transition Metal Disulfide Gas Sensors: WSe₂ And WS₂ Material

Recently,the application of gas sensors in daily life and industry scene has increased greatly.Metal oxide materials,as the most traditional and widely studied gas sensing materials,have some shortcomings including high operating temperature and poor selectivity because of their generally good sensitivity to most gases.These can not meet the requirements of high sensitivity and high selectivity at room temperature.Two-dimensional transition metal dichalcogenides（TMDs）,due to its fold structure and large specific surface area,can provide a large number of active sites,and thus TMDs show a rich and excellent performance and has attracted extensive attention in various fields such as catalytic and energy storage.TMDs monolayer has different properties from bulk TMDs,and is expected to meet the requirements of high sensitivity and micro or even small gas sensors.In addition,defect engineering,as an important method of controlling material performance,is often used to improve the sensitivity and selectivity of materials.Therefore,in this work,density functional theory（DFT）method was used to investigate the gas sensing behavior between NO₂and WSe₂monolayers,including perfect and defective monolayers.Meanwhile,the selective sensing ability of NH₃and NO₂gas on the WSe₂monolayer with different selenium vacancy concentrations was investigated.Finally,this work also explored the adsorption sensing behavior of NO₂on WS₂monolayer,and conducted a comparative study with WSe₂monolayer.（1）Based on density functional theory,the oriented adsorption behavior of NO₂on WSe₂monolayer was revealed by a large-scale sampling study.It is found that NO₂maintains anisotropic oriented adsorption behavior on perfect monolayer and three defective monolayers（selenium vacancy,tungsten vacancy and anti-site defect of selenium replacing tungsten）.Although symmetry of monolayer is decreases from triangular symmetry to bilateral symmetry after defects appear,the NO₂molecules adsorbed on the four monolayers extend mainly along the Se-W bond and Se-W-Se trough.Molecular dynamics simulations reproduce these trends well.This anisotropic adsorption behavior can be attributed to the heter-atomic bonds of WSe₂monolayer and the polarity of NO₂molecule itself.Out-of-plane polarization of monolayer is thought to play a key role in the vertical and oriented adsorption behaviors.NO₂has suitable adsorption energy（0.35,0.42,0.95 and 0.78 e V）and considerable charge transfer number（-0.205,-0.208,-0.164 and-0.160 e）on four WSe₂monolayers（perfect,selenium vacancy,tungsten vacancy and anti-site defect of selenium replaced tungsten）.And the degree of increase in adsorption energy and charge transfer varies with defects.These wonderful discoveries may provide a solid foundation for a wide range of applications of materials beyond imagination.（2）Density functional theory was used to investigate the adsorption and sensing behavior of NH₃and NO₂on perfect and selenium-vacancy WSe₂monolayer.The effect of selenium vacancy concentration on gas sensing behavior was investigated.The calculation results show that perfect monolayer can realize effective sensing of NH₃and NO₂respectively.However,selectivity is inhibited due to close adsorption energy in the mixture of the two gases.Furthermore,it was found that the presence of selenium vacancy increased the adsorption energy and charge transfer number of NH₃and NO₂in various degrees,which provided an opportunity for selective sensing of the two gases.On monolayers containing different selenium vacancy concentrations,NH₃（vacancy concentration about 2.000～5.556%）and NO₂（vacancy concentration about>8.333%）can be selectively detected,respectively.Results expresses that WSe₂monolayer is a two-dimensional selective sensor material with adjustable defects for NH₃and NO₂.（3）Based on density functional theory,the adsorption sensing behavior of NO₂on WS₂monolayer（perfect monolayer and defective monolayer）was investigated,and was compared with the WSe₂monolayer.It is found that NO₂has appropriate adsorption energy（0.28,0.28,1.67 and 0.90 e V）and charge transfer number（-0.097,-0.125,-0.147,and-0.071e）on four WS₂monolayers（perfect,sulfur vacancy,tungsten vacancy,and anti-site defect monolayers of sulfur replaced tungsten）to ensure the effective sensing of NO₂on WS₂monolayer.The partial adsorption energy and charge transfer number of WS₂monolayer are smaller than those of WSe₂monolayer,which may be due to the different anions between two materials.In addition,compared with the WSe₂monolayer,the lower proportion of NO₂oriented adsorption on WS₂monolayer may be related to the poor net charge and out-of-plane polarity ability of WS₂monolayer.This result verifies the reasons for NO₂oriented adsorption behavior on WSe₂monolayer further,and provides a reasonable solution to understand the inherent differences between WS₂and WSe₂materials.