Realization Of Diversity Of MXene Surface Groups: Selective Molten Salt Etching

MXene's unique two-dimensional layered structure,adjustable surface chemistry,hydrophilicity,and excellent metal conductivity make it widely applicable in many fields.MXenes are usually obtained by selective etching of the A atomic layer of the M_n+1AX_n phase precursor.Its chemical formula is M_n+1X_nT_x(where M is a transition metal element,X is C and/or N,n is usually 1-3,and T_-is surface group).Surface groups have a decisive influence on the physical and chemical properties and functional applications of MXene,and only four surface groups have been found and reported(-OH,-O,-F and-Cl).Therefore,exploring new surface groups is of great significance for expanding the MXene family members,structural diversity,property control,and functional applications.The main research contents of this article are as follows:In this paper,a series of MAX phases with different configurations are selected as precursors,and the Lewis acid molten salt Cu Cl₂/Cu Br₂/Cu I is used as an etchant.The reaction rules between different configuration MAX phases and Lewis acid molten salts are investigated.The effects of M-site elements,A-site elements,n-values,and molten salt composition on the reaction behavior were investigated.The results show that for the MAX phases with different M sites(Ti₂Al C,V₂Al C,Nb₂Al C,and Ta₂Al C),Nb₂Al C and Ti₂Al C can be etched to obtain MXene with a surface group of-Cl/-Br/-I.The products of V₂Al C and Ta₂Al C after reaction with molten salts are their carbides and oxides,which may be related to the stability of their corresponding MXene;for the two MAX phases(Ti₃Al C₂,Ti₃Si C₂)with different A sites,Ti₃Si C₂ is more difficult to etch than Ti₃Al C₂,mainly because Ti-Si bonds are stronger than Ti-Al bonds;for MAX phases(Ti₃Al C₂,Ti₂Al C)with different n values,312 and 211 are easily etched into MXene by three types of molten salt systems,indicating that the effect of n value on the etching of Ti_n+1Al C_n into MXene by molten salt is small.The ability of molten salt to etch the MAX phase decreases with the electronegativity of the halogen atom(electronegativity order:Cl>Br>I),MAX phases is harder to be etched.Ti₃C₂Cl₂,Ti₃C₂Br₂and Ti₃C₂I₂ MXene obtained by molten salt etching and Ti₃C₂(OF)obtained by HF etching were assembled into the positive electrode of a zinc ion battery,and the influence of surface groups on the electrochemical performance of Ti₃C₂ MXene was investigated.Ti₃C₂Br₂and Ti₃C₂I₂ MXene show excellent performance as positive electrodes of aqueous zinc ion batteries.Ti₃C₂Br₂and Ti₃C₂I₂cathodes have obvious charge-discharge platforms at 1.6 V and 1.1 V,with specific capacities of 97.6 m Ah·g^-1 and 135 m Ah·g^-1,respectively.This is the first time charge-discharge platform behavior has been observed in MXene.In contrast,the specific capacities of Ti₃C₂Cl₂and Ti₃C₂(OF)are only 46.5 m Ah·g^-1 and 51.7 m Ah·g^-1,and there is no charge-discharge platform.In addition,the energy density of Ti₃C₂Br₂and Ti₃C₂I₂is also about 200%of Ti₃C₂Cl₂and Ti₃C₂(OF).Further research found that the different electrochemical behaviors of the above MXene materials are mainly caused by the different redox behaviors of surface groups during the charge and discharge process.This study shows that the surface groups of MXene are of great significance to the regulation and application of their surface chemistry.