First-Principles Study On The Adsorption Of Carbon,Nitrogen And Hydrogen On Ti₃C₂ Modified By Functional Groups

Two-dimensional Ti₃C₂ is one of the most representative materials of the new two-dimensional transition metal carbide and carbonitride family(MXenes).Its unique layered structure and zero band gap characteristics are widely used in carbon,nitrogen and hydrogen gas sensors,catalysts,super capacitors,lithium ion and non-lithium ion batteries,but practical application in carbon,nitrogen and hydrogen gas sensors requires further improvement in adsorption performanceBased on the first-principles calculation method,this paper systematically studied the adsorption performance of the three gases H₂,NH₃ and CH₄ of intrinsic Ti₃C₂ under the two modification conditions through functional group modification and element doping.The main research content and calculation results of this paper are as follows:(1)Based on first-principles density functional theory and materials calculation software Materials Studio,a 3D structure model of intrinsic Ti₃C₂ was built,and the most stable geometric structure was obtained through structural optimization of the Castep module.The calculation results show that the intrinsic Ti₃C₂ has no band gap,it is a conductor,and it has a work function of 4.162 e V,and the Ti atom chemical properties on its surface are very active.It can be seen from its partial-wave state density that the 3d orbital of Ti atoms on the surface of Ti₃C₂ is the fundamental reason why Ti₃C₂ behaves as a conductor.(2)Based on the study of intrinsic Ti₃C₂ geometric structure and electrical properties,four functional groups F,O,OH,and COOH were used to modify Ti₃C₂ by single-sided and double-sided.The calculation results show that when F,O,OH,and COOH are modified on single side,the most stable adsorption positions for the four functional groups are all at the hollow site fcc of Ti atoms on the surface,and the adsorption energy is relatively large.The functional group atoms and Ti atoms on the Ti₃C₂ surface formed a new chemical bond,and the greater the adsorption energy,the greater the number of charge transfers between the functional group and Ti₃C₂.At the same time,only the O functional group with the largest adsorption energy opens the band gap of Ti₃C₂;when double-sided modification,The most stable adsorption positions of the four functional groups(F,O OH,COOH)are still at the hollow site fcc of the Ti atom on the surface.After double-sided modification,none of the four functional groups can open the band gap of Ti₃C₂,but the double-sided adsorption increased the number of bands on both sides of the Fermi level,and the energy bands become denser.(3)Based on the GGA-PBE exchange correlation functional under the Castep module,the adsorption performance of H₂,NH₃,and CH₄ in the Ti₃C₂ system modified by the four functional groups of F,O,OH,and COOH has been studied in depth.The calculation results show that the Ti₃C₂ system modified with F functional groups on both sides has the best adsorption effect on the three gases,and its adsorption energy for H₂,NH₃,CH₄ are respectively the largest-0.405 e V,-2.250 e V,-0.898 e V,and the adsorption performance is better than the current Ti₃C₂ system modified by other functional groups.The optimal adsorption positions of the three gases are at the hollow sites of Ti atoms on the surface,and at the same time it follows the rule that the greater the adsorption energy,the greater the number of charge transfers between the two systems.Among them,H₂ and CH₄ are non-polar molecules,the interaction between the two systems and the Ti₃C₂ system is mainly dispersion force and inductive force.NH₃ is a polar molecule,the interaction force also has orientation force It can be seen from the partial-wave state density that the orbital hybridization between atoms plays an important role in gas adsorption.(4)Based on the GGA-PBE exchange correlation functional under the Castep module,the C atom in the Ti₃C₂T₂(T=F,O,OH,COOH)system is modified by substitutional doping with three elements of N,P,and As,P and As doping is the first time to be used in the Ti₃C₂system modified by functional groups.Based on the dual modified Ti₃C₂ system,we further study its adsorption performance for H₂,NH₃,and CH₄.The calculation results show that the formation energy of the new N-Ti₃C₂T₂ system after N atom doping is negative,and the formation energy of N-Ti₃C₂(OH)₂ is the largest-11.697 e V;and the formation energy of the P atom doped system is positive,and it can exist stably after additional energy provided;the formation energy of the As-Ti₃C₂F₂ system after the doping of As atoms is the largest-7.937e V;three atoms of N,P and As failed to open the energy band of Ti₃C₂,and all doping systems are conductors.However,the doping of N,P,As atoms changed the sensitivity of the original system to the three gases H₂,NH₃,and CH₄.As-Ti₃C₂O₂ has the best adsorption effect on H₂molecules(-0.384 e V),and P-Ti₃C₂(COOH)₂ has the largest adsorption energy for NH₃,but considering that the formation energy of the new system after P doping is positive,the best adsorption effect for NH₃ is the As-Ti₃C₂O₂ system(-2.704 e V),the adsorption energy of As-Ti₃C₂F₂ for CH₄ is also the largest(-0.697 e V).In summary,As atom doping further improves the adsorption performance of Ti₃C₂ system for NH₃,and it is the best doping choice for the preparation of NH₃ gas sensors.