Preparation And Characterization Of Ti₂CT_x Two-dimensional Materials

Ti₂CT_x is one of the two-dimensional materials MXene,which is a new transition metal carbide or nitride with the general formula M_n+1X_nT_x（n=1-3）,where M is an early transition metal and X is C or N,T is a surface termination（F,O,or OH）.MXene has a two-dimensional structure similar to graphene.Rich and unique physicochemical properties make it suitable for many different fields,including energy storage,composite materials,chemical catalysis of lubricating materials.At present,the understanding of the many properties of MXene materials is not perfect.Effective preparation is critical,and HF etching has many disadvantages.In this paper,Ti₂CT_x was prepared by chemical liquid phase method and molten salt etching method,and the similarities and differences between the two preparation methods were investigated.Two-dimensional Ti₂CT_x was prepared by etching Ti₂AlC with molten salt and chemical liquid phase method respectively.Among them,for molten salt etching,Ti₂CT_x was prepared by etching LiF-NaF-KF and NaF-KF two molten salt systems under flowing argon for 40 min at different melting temperature,the product was then intercalated with tetrabutylammonium hydroxide to produce few-layered Ti2CTx.Ti₂CT_x was prepared by etching NaH and HCl mixed solution,LiF and HCl mixed solution and KF and HCl mixed solution at different fluoride ion concentration and temperature for 24 h,the product is then sonicated to produce few-layered Ti₂CT_x.The composition,structure and properties of the two methods were compared using XRD,TEM,infrared spectroscopy and cyclic voltammetry.The results show that after molten salt etching,the product transforms from the MAX phase of the layered structure to the two-dimensional sheet MXene phase material.As the melting temperature increases,the sheet distance of the product increases gradually.For the ternary LiF-NaF-KF system and the binary NaF-KF system,suitable melting temperatures for complete etching of Ti₂AlC are 600°C and850°C,respectively.After intercalate with TBAOH,few-layered Ti₂CT_x can be obtained,but the number of lamellar stacks is large,and the few layers of the product maintain the single crystal property,belonging to the hexagonal lattice;the Ti₂CT_x prepared by the molten salt etching method contains hydroxyl groups and Oxygen functional group,no fluorine functional group.The etching results of the chemical liquid phase method show that Ti₂AlC gradually changes to Ti₂CT_x as the etching temperature increases,and the increase of the fluoride ion concentration is also favorable for etching.For NaF+HCl etchant and KF+HCl,a suitable etching temperature is 60°C,and for LiF+HCl etchant,a suitable etching temperature is70°C.As the etching progresses,the layer spacing of the sample increases,and this change first appears at the edge of the Ti₂AlC sheet and gradually develops toward the middle portion.The products of the LiF+HCl and KF+HCl etchants have larger interlayer distances than NaF+HCl.The multi-layered Ti₂CT_x is separated into few-layered Ti₂CT_x with only a few layers stacked through the intercalation layer,and the layering effect is significantly better than that of the molten salt etching method.The liquid phase product contains three functional groups of fluorine,oxygen and hydroxyl groups,and the increase of the F^-concentration of the etching solution can increase the content of the fluorine functional group.The results of cyclic voltammetry and electrochemical impedance spectroscopy show that the products of both preparation methods have good electrochemical stability and electrical conductivity,but the capacitance of chemical liquid phase products is better than that of molten salt.