Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have recently attracted much attention because of their electronic structure and properties that differ from their bulk counterparts [1]. MXenes were first discovered at Drexel University in 2011 [2] by selective chemical etching of aluminum layers from ternary transition metal carbides known as MAX. The general chemical formula of MXene is written as (M_n+1X_nT_x) [3], where M is an early transition metal (Ti, Nb, Ta, V, Mo, etc.), X is C and/or N, and n = 1- 3 [4]. Due to the chemical synthesis in fluorine-containing solution MXene have surface functional groups (T_x), which can be fluorine (-F), oxygen (-O), or hydroxyl (-OH) groups [3, 4]. Most chemically synthesized MXenes are hydrophilic and some exhibit high electrical conductivity [5, 6].

More than 20 different MXenes have been experimentally synthesized and several more varieties of MXenes have been predicted, rendering MXenes among the most diverse, tunable and fastest growing 2D materials [3]. MXenes have already shown a promising performance in many applications such as energy storage [7], electromagnetic interference shielding [8], water desalination [9], and optoelectronics [10]. However, the moiety and quantity of the surface functional groups have been shown to be dependent on the synthesis conditions [11], and shown to affect the quality and properties of MXene [5, 12].

Here, we systematically study the thermal stability as well as the surface moieties of MXenes, namely (Ti₃C₂T_x, Mo₂CT_x, Mo₂TiC₂T_x and Mo₂Ti₂C₂T_x), using simultaneous thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) from ambient temperature to 1500 °C. We also discuss the effect of MXene synthesis, that is etchant type and concentration, on surface functional groups. For example, the amount of –OH, –F, and structural water is different and dependent on varying the composition of fluorine-containing etchants. Also, the thermal stability of MXenes strongly depends on their chemical composition. This study sheds lights on the importance of synthesis conditions in controlling MXenes compositions, structure and their surfaces for a variety of potential applications.

References

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