Structure, Composition, and Functionalization of MXenes

MXene is a two-dimensional (2D) transition metal-based compound of carbides as well as nitrides having unsymmetrical arrangement and different functional groups. The general formula for MXene exists to be M_n+1 X_n T_x, where M delineates a transition metal, X corresponds to carbon or nitrogen, T denotes functional groups like fluorine, hydroxyl, or oxygen, and n is an integer. The crucial parameters governing the environmental application of MXenes are ordered structure and its physical state. Perhaps, MXenes comprising the transition elements doublet have been shown in two different forms: one is ordered solid form, and the other is solid solution form. The first 2: 1 ordering between M elements (Mo_2/3Sc_1/3)₂ AlC was discovered in 2017 in an M₂ AX phase. Hitherto, >20 i-MAX phases seemed to come up, and all were crystallized in more than one space group symmetries. Also, as per the theoretical calculation, they are close to degenerate in energy. This much diversity in MXene’s framework specifies various configurations of MXenes, accompanied by the coveted properties that maybe produced. According to several scale computing approaches, one can learn regarding the compositional as well as distribution trends of the surface functional groups -F, -O, and -OH, upon 2D transition element nitrides as well as carbides (MXenes). In this chapter, we will study about the different structural phases and compositions of MXenes. We will further review different surface compositions and their effects on MXene applications.