Density Functional Comparative Study Of NO₂ Oriented Adsorption And High Efficiency Sensing Density Functional:Novel Two-dimensional MoX₂（X=S,Se,Te） Monolayer

Nitrogen dioxide（NO₂）is the main harmful gas from automobile exhaust,which is the main environmental factor causing acid rain and ozone formation.Long-term exposure to low concentration of NO₂is easy to cause respiratory diseases.Therefore,real-time monitoring and detection of NO₂concentration is particularly important.In recent years,transition metal dichalcogenides（TMDs）is a new kind of special two-dimensional material,which has good geometrical space structure and large specific surface area.Due to their structural properties,TMDs materials have rich surface chemical properties and higher adsorption site sensitivity than graphene and conventional metal oxides.Therefore,it is of fundamental significance to study the interaction and chemical reaction between gaseous substances and the single layer surface of TMDs two-dimensional semiconductor materials.Despite some successful preliminary work,many details of gas-solid interactions are still poorly understood.In this paper,large-scale sampling density functional theory（DFT）was used to investigate the adsorption and sensing of NO₂on TMDs monolayer.On the basis of previous studies on WSe₂monolayer,adsorption performance of NO₂on MoSe₂monolayer was first explored and compared with previous studies on WSe₂monolayer.Secondly,the gas-solid interaction of NO₂/MoS₂was explored and compared with MoSe₂and Mo Te₂with the same cation.Finally,in order to explore whether other molecules have the same unique oriented adsorption behavior on Mo X₂monolayer as NO₂molecules,O₃molecules are selected for extended exploration.（1）The adsorption and sensing behavior of NO₂on MoSe₂perfect and defective monolayers including selenium vacancy(V_Se),molybdenum vacancy(V_Mo),and antisite defect of selenium on molybdenum site(Se _Mo)by using a large-scale sampling density functional method.The adsorption energy can reach 0.29 e V with 0.182 e charge transfer on the perfect monolayer.The adsorption ability is improved largely on V_Se,V_Mo,and Se _Momonolayers（3.27,0.93,and 1.20 e V）with considerable charge transfer（0.583,0.152,and 0.139 e,respectively）,demonstrating the potential of MoSe₂monolayers as NO₂sensing materials.Meanwhile,similar to WSe₂monolayer,oriented adsorption of NO₂is disclosed.NO₂molecule extends mainly along the Se-Mo-Se trough and the Se-Mo bond on all the four MoSe₂monolayers.Whereas,the oriented adsorption ability is weaker than the WSe₂monolayer,which can be confirmed by the relatively small proportion of oriented adsorption conformations.This kind of double oriented adsorption behavior（vertical adsorption and adsorption along specific direction of the two dimensional material）was considered to be due to the good polarity match between molecule and monolayer.Molecular dipole moment and material bond population are believed to be two important parameters that determine the oriented adsorption capacity of molecular-material systems.The successive discovery of the unique oriented adsorption and the deepening of understanding provide chance for molecular design of oriented adsorption of inorganic small molecules on two-dimensional layered materials.The strong dissociation ability of the anionic vacancy monolayer to NO₂molecule indicates that MoSe₂monolayer has a very high adsorption,capture,and activation potential for nitrogen oxides.Comparative study proved that the Mo site activity was less than the W site activity on the perfect monolayer,and defects could help the Mo site activity overtake.These findings provide pivotal guidance for creating advanced surface regulated electronic systems,such as sensors.（2）The gas-solid interaction in NO₂/MoS₂system was investigated by using a large-scale sampling density functional method.Confirm the optimal adsorption configuration of NO₂on MoS₂monolayer.The source of NO₂dissociation activity was explained.By means of cross analysis of band structure and DOS,the impurity states on the top of valence band of S-vacancy monolayer（MoS₂-V_S）were discovered,which had been neglected for a long time.This provides a reasonable explanation for the dissociation activity of NO₂.Further comparison of the atomic structure reveals that impurity states originate from not fully occupied valence orbitals.It is also consistent with the fact that Mo material has dissociation activity while W material has no dissociation activity.The NO₂sensing response of perfect Mo X₂（X=S,Se,Te）monolayer increases with the increase of X atomic number.Interestingly,NO₂dissociation did not occur on the Mo Te₂-V_Temonolayer.It is preliminarily speculated that it may be related to the inert electron pair effect of 6s of Te atom.In addition,NO₂double oriented adsorption behavior is revealed again on MoS₂monolayer.Different from MoSe₂and Mo Te₂monolayers,NO₂orientation adsorption on MoS₂perfect monolayer deviates obviously.It is speculated that it is related to space limitation and larger electronegativity of S atom.The oriented adsorption capacity of Mo X₂monolayer is Mo Te₂（64.4%）>MoSe₂（44.8%）>MoS₂（42.7%）.New insights into NO₂dissociation phenomena and oriented adsorption on MoS₂and Mo X₂monolayers open a wide space for future expansion of surface-related material electronic device design.（3）The adsorption and sensing of O₃molecules on four monolayers Mo X₂（X=S,Se,Te）was investigated by density functional theory of large-scale sampling and compared with NO₂molecule.The adsorption behavior of O₃molecules on the perfect Mo X₂monolayer is similar to that of NO₂.The adsorption energy and charge transfer increase with the increase of the anion atomic number.O₃molecule has higher adsorption energy（0.37,0.53,0.88 e V）and charge transfer（0.139,0.223,0.335 e）than NO₂molecule on perfect Mo X₂monolayer.The adsorption sensing potential of Mo X₂monolayer for O₃molecules was demonstrated.The appearance of defects improves the O₃adsorption sensing of Mo X₂to different degrees.Interestingly,a large number of direct dissociation phenomena of O₃molecules occur on Mo X₂monolayer.The dissociation ability of Mo X₂monolayer to O₃molecules is greater than that to NO₂molecules.In addition,the double oriented adsorption behavior of O₃molecule on Mo X₂monolayer still exists,while the orientation adsorption capacity is much smaller than that of NO₂molecule.The oriented adsorption ratio was Mo Te₂（14.6%）>MoSe₂（7.3%）>MoS₂（1.0%）.The revelation of the oriented adsorption behavior of O₃molecules brings a broader vision for the design of inorganic small molecule orientation molecular devices on two-dimensional thin layer materials.