Theoretical Study On S-Terminated And O-Doped MXene Materials As Gas Sensors

Since graphene was successfully mechanically exfoliated,a large number of two-dimensional materials have emerged,among which two-dimensional transition metal carbides or nitrides（MXene）have received a lot of attention from researchers due to their unique structure and excellent physicochemical properties.Experimentally synthesized MXene materials primarily use O,F or OH as the terminated group,and now these materials have been widely used in various fields,particularly as gas sensors MXene shows excellent application prospects.Recently,several experimental works have discovered some new MXexe materials,such as S as the termination,the substitution of C by O,and so on.In this thesis,the adsorption behaviors of various gas molecules on the surface of S-terminated and O-doped MXene materials were investigated by theoretical calculations,which are divided into three main parts as follows:1.Among the numerous MXene materials,the atomic arrangement of M₂CS₂（M=Sc or Y）differs from all known MXene,and they are both semiconductor materials,which is very favorable for their applications in the field of gas sensors.To investigate the application of these two MXene materials with unique structures in the field of gas sensing,we studied the adsorption behavior of atmospheric molecules（NH₃,NO,NO₂,CH₄,H₂S,O₂,H₂O,CO,N₂,SO₂ and CO₂）,volatile organic compounds（formaldehyde,acetone,ethanol,methanol and toluene）and nicotine on the surface of M₂CS₂ by theoretical calculations.The results demonstrate that gases containing N/O are easily adsorbed on the M₂CS₂surface.The adsorption energy calculation showed that Y₂CS₂could capture NH₃,nicotine,NO_x,ethanol,acetone and methanol,while Sc₂CS₂ could only capture NH₃ and nicotine.In addition,we found that Sc₂CS₂ can effectively detect NH₃/NO/nicotine/ethanol by calculating the charge transfer,density of states and current-voltage relationship for the M₂CS₂-gas system.However,Y₂CS₂ only shows extremely high sensitivity and selectivity for NO.More importantly,the recovery time of both materials after sensing at room temperature is short（<0.04 s）and thus reusable after detection of gas molecules.This work will contribute to our understanding of the chemistry of this unique MXene phase（M₂CS₂）and extend their potential applications.2.Recently,Kamysbayev et al.’s experimental work（Science,2020,369,979）successfully synthesized S-terminated MXene materials for the first time,since S-and O-terminated MXenes have analogous atomic arrangement and electronic properties,we predict that S-terminated MXene materials may be the potential gas sensors.Accordingly,we first explored the adsorption behavior of different gas molecules（including NH₃,NO,NO₂,CH₄,H₂S,O₂,H₂O,CO₂,and N₂）on the surface of M₂CS₂（M=Ti,Zr,or Hf）.The results showed that M₂CS₂ exhibits high adsorption selectivity towards NO_x gas,while the metallic property of M₂CS₂ suppress its application as NO_x sensor.To solve this problem,we adjusted the sensitivity of M₂CS₂ towards NO_x by applying strains on M₂CS₂.The results showed that Hf₂CS₂ and Zr₂CS₂ changed from conductors to semiconductors under 5%and8%strains,respectively,however,no band gap appeared in the band structure when strain was applied on the Ti₂CS₂ system until its structure was disrupted.Interestingly,the sensitivity of Hf₂CS₂ towards NO and NO₂ molecules increased from 15%and 21%（without strain）to 106%and 329%when 5%biaxial strain was applied.Furthermore,the moderate adsorption strength of NO and NO₂ on the surface of Hf₂CS₂ indicates that it can be reused after detecting NO and NO₂.Based on these results,we proposed that Hf₂CS₂ is a NO_x gas sensor with high selectivity,high sensitivity and reusability under strain.In addition,this work provides an effective method to tune the band structure of S-terminated MXene materials,which will further extend their applications.3.Very recently,Michaowski et al.’s experiment（Nature Nanotechnology,2022,17,1192）first revealed the presence of O-doped MXene,（partial replacement of C atoms by O atoms in the MXene lattice）,and quantitative assay analysis showed that the content of O elements could be as high as 30 at.%.Therefore,to investigate the effect of O-doping on the surface chemistry of MXene materials,we systematically studied the adsorption behavior of common gas molecules（including CH₄,CO,CO₂,H₂O,H₂S,N₂,NH₃,NO,NO₂,and O₂）on M₃C₂O₂（M=Ti,Zr,and Hf）and O-doped M₃C₂O₂（O@M₃C₂O₂）surfaces.The results demonstrated that NH₃ molecules could strongly interact with M₃C₂O₂ and O@M₃C₂O₂,together with the obvious charge transfers in the systems.Since a large amount of charge transfer causes a significant change in the resistivity of the adsorption system,we predicted that both M₃C₂O₂ and O@M₃C₂O₂ can effectively detect NH₃.In addition,the adsorption strengths of NO₂ molecules on O@Zr₃C₂O₂ and O@Hf₃C₂O₂ surfaces are significantly enhanced compared to its adsorption on Zr₃C₂O₂and Hf₃C₂O₂.The Bader charge analysis showed that a substantial amount of charge transfer was present in the NO₂/O@Zr₃C₂O₂ and NO_2/O@Hf₃C₂O₂ systems,and thus the O@Zr₃C₂O₂ and O@Hf₃C₂O₂ could be the potential NO₂ gas sensors.