First Principles Study On Electromagnetic Properties And Gas Sensing Performance Of Surface Functionalized 2D Sc-based MXenes

In 2011,Professor Gogotsi’s group successfully obtained two-dimensional material Ti₃C₂T₂（T=-OH,-F）,and then named this kind of material MXenes,which further expanded the large family of two-dimensional materials.MXenes exhibit many excellent physical,chemical properties,etc.due to its rich chemical composition.For example,some MXenes（Cr₂C,Mn₂C,etc.）exhibit intrinsic magnetism,which leads to their important applications in the field of spintronics.At present,Sc-based MXenes,as the thinnest member among all MXenes,have good application potential in the field of energy storage and gas sensing due to their large specific surface area.Meanwhile,Sc atoms only have one d electron,resulting in no intrinsic magnetism of Sc-based MXenes.The application of Sc-based MXenes in the field of spintronics is limited.In recent years,some researchers have designed Sc-based MXenes with intrinsic magnetism,but their intrinsic mechanism has not been deeply studied.Based on this,we should further study the intrinsic mechanism of Sc-based MXenes displaying intrinsic magnetism and the sensing performance of Janus Sc₂CF（OH）as a sensing material for toxic gases,so as to promote its development in the field of spintronics and gas sensing.In this thesis,the influence of surface functional groups（-H,-F,-OH,-O-）on electromagnetic properties and gas sensing performance of Sc₂C is systematically investigated by means of density functional theory.Firstly,the electromagnetic properties of pure Sc₂C and Sc₂C under different surface functional groups（symmetric surface functionalization and asymmetric surface functionalization）were investigated.Secondly,the gas sensing performance of OH side and F side of Sc₂CF（OH）to toxic gases（HCHO,NO₂,NO,SO₂,CO）was specifically explored.The research content and main conclusions of this thesis are described as follows.1.Effect of surface functionalization on electromagnetic properties of two-dimensional Sc₂C.For pure Sc₂C,the results show that it is an antiferromagnetic（AFM）metal.Further analysis shows that its magnetism mainly comes from the d orbital of Sc cation,and the Néel temperature is 16.4 K.Under the conditions of symmetric surface functionalization and univalent asymmetric surface functionalization,that is,for Sc₂CT₂（T=-H,-F,-OH,-O-）system and Sc₂CHF,Sc₂CH（OH）,Sc₂CF（OH）system,they are all non-magnetic semiconductors,and the band gap is adjustable according to the surface functional groups.By comparing the projected density of states（PDOS）of Sc₂C,it can be projected that the fermi energy level（E_F）of Sc₂C is projected to decline from metal to semiconductor after the introduction of symmetric and unsymmetrical surface functional groups.Meanwhile,after introducing surface functional groups with different valence states,that is,for Sc₂CTO（T=-H,-F,OH）system,they are ferromagnetic（ferromagnetic,FM）half-metals.Further analysis shows that their magnetic moments are mainly contributed by p orbitals of C anions.Their Curie temperatures are 397.9 K,244.3 K and 307.9 K,respectively.The results not only explain the source of magnetism in Janus Sc₂C MXenes,but also provide theoretical guidance for the further design of new magnetic Sc₂C MXenes.2.Gas sensing performance of Sc₂CF（OH）MXene to noxious gases.The calculation results of the gas sensing performance of Sc₂CF（OH）harmful gases show that the OH side and F side of Sc₂CF（OH）have different sensing performance for the studied harmful gases.Compared with the adsorption systems of HCHO and CO,the OH side of Sc₂CF（OH）has high selectivity for NO₂,SO₂ and NO molecules,and we found that the adsorption systems of NO₂,SO₂ and NO exhibit metallic properties,so the OH side of Sc₂CF（OH）is more conducive to the detection of NO₂,NO and SO₂ molecules.These results indicate that the OH side of Sc₂CF（OH）can be used as gas sensing material and trapping material for NO₂,SO₂ and NO molecules.In contrast,the F side of Sc₂CF（OH）does not show high selectivity to the harmful gases studied,but its conductive property changes from semiconductor to semimetal after adsorption of NO₂ molecules,indicating that the F side of Sc₂CF（OH）is suitable as a sensor material for NO₂ molecules.The sensing performance of different harmful gases on both sides of Sc₂CF（OH）indicates that Sc₂CF（OH）is a potential sensor material for multi-target gas sensors（NO₂ molecules can be detected by OH side and F side,and NO and SO₂ molecules can be detected by OH side）.This thesis consists of four chapters:the first chapter is the introduction,which mainly introduces the new two-dimensional material MXenes,gas sensors and the research progress of MXenes gas sensors.Chapter 2 and Chapter 3 introduce the specific research content and the discussion of the results.In Chapter 2,the effect of surface functionalization on the electromagnetic properties of two-dimensional Sc₂C is studied.In Chapter 3,the gas sensing performance of asymmetric surface functionalized Sc₂CF（OH）MXene to harmful gases is studied.In Chapter 4,the work of this thesis is summarized comprehensively and the future research direction of two-dimensional Sc-based MXenes is looked forward.