Theoretical Study Of MXenes And Their Potential Application As A Gas Sensor

Gas sensors play a significant role in our daily life and industry manufacturing.Metal oxides are stable and often used as gas-sensitive materials,however suffer from high temperatures,low sensitivity,and susceptibility to poisoning.Two-dimensional（2D）materials provide a better choice for high-performance gas sensors due to their special atomic structure and excellent physical properties.The physical principle of 2D materials to detect gas is that the charge transfer happens when molecules are adsorbed on 2D materials,which in turn causes changes in resistivity and achieves the detection of gas molecules.MXenes is a novel 2D material with the chemical formula of M₂X,M₃X₂,or M₄X₃,M for metallic elements,X for C or N.Previous studies have shown that MXenes have the advantages of a large specific surface area,a large number of reaction sites,and high conductivity.Thus,MXenes semiconductor materials are candidates for gas-sensing materials for high-performance gas sensors.The Janus structure is polar,which can enhance the adsorption of gas molecules and the sensitivity of gas detection.Therefore,we conducted a global search on MXenes and their Janus structures for semiconductor materials with first-principles calculations,and proposed a monolayer Sc₂CF₂semiconductor material and a monolayer Janus Sc₂CFCl semiconductor material as gas molecule detection materials and analyzed their charge transfer and physical properties.The main research contents of this thesis are as follows:1.MXenes are promising candidates for high-performance gas sensors due to their special electronic structure and excellent properties.We constructed 80 M₂XT₂（M=Sc,Ti,V,Cr,Zr,Nb,Mo,Hf,Ta,W;X=C,N;T=O,F,OH,Cl）structures,and found that 10semiconductor structures belong to the semiconductor.Then,21 types of Janus MXenes structures were constructed,and the first-principles calculation results show that 4 types of these Janus MXenes were semiconductor materials,including Sc₂CF（OH）,Sc₂CFCl,Sc₂CCl（OH）and Cr₂CF（OH）.The molecular dynamics（MD）simulation showed that these four Janus MXenes semiconductor structures were stable at room temperature.2.The detection performance of Sc₂CF₂as a gas sensor and the influence of common gas molecular adsorption on its electronic structure were explored.It is found that the most significant amount of charge transfer occurred between the monolayer Sc₂CF₂and NO₂,indicating that Sc₂CF₂has good selectivity for NO₂.Electronic structure calculations show that the apparent charge transfer results from the lowest unoccupied molecular orbital（LUMO）of NO₂being lower than the valence band maximum（VBM）of Sc₂CF₂.Moreover,MD simulations show that NO₂can adsorb on the 2D material surface stably at room temperature.The influence of biaxial strain on the adsorption system was further discussed,and the results show that the biaxial strain could increase the adsorption energy and charge transfer of the NO₂adsorption system,thereby improving the selectivity and sensitivity of the detection of NO₂molecules by monolayer Sc₂CF₂.Additionally,the adsorption behavior and charge transfer of polar polyatomic molecules on the surface of monolayer Sc₂CF₂with h-BN as a substrate were studied,and the results showed that the h-BN substrate could hardly alter the main results,indicating that h-BN is a potential substrate material in experiments.3.The detection performance of Janus Sc₂CFCl as a gas sensor and the influence of common gas molecular adsorption on its electronic structure were explored.It is found that the Fermi level of Janus Sc₂CFCl was higher than the lowest unoccupied molecular orbital（LUMO）of NO₂,and the Fermi level of Janus Sc₂CFCl had a large energy difference with the LUMO of NO₂,resulting in a large amount of charge transfer between NO₂and the monolayer Janus Sc₂CFCl.Therefore,the monolayer Janus Sc₂CFCl highly selective for was NO₂molecules.Furthermore,the effects of biaxial strain on the adsorption energy and charge transfer of the adsorption system were further discussed,and the results indicated that the biaxial strain could enhance the adsorption energy and charge transfer of the NO₂adsorption system,and improve the selectivity and sensitivity of the detection of NO₂molecules by the monolayer Janus Sc₂CFCl.