Stability and improvement of two-dimensional transition metal carbides and/or carbonitrides (MXene)

In recent years, a new family of two-dimensional (2D) transition metal carbides and/or carbonitrides, labeled MXenes, has attracted immense attention from researchers. Due to unusual hydrophilicity, electrical conductivity, flexibility, and pseudocapacitance, MXenes have great potential application in energy storage, water desalination, catalysis, electromagnetic interference shielding, transparent conductive films, and so on. However, MXenes exhibit poor stability because of their structural defects, active transition metals, and termination groups. These greatly destroy the sheet structure and decrease their conductivity, thereby restricting their application fields. In this review, the structure and synthesis methods of MXenes are briefly introduced. Then, we focus on current research studies regarding the stability of MXenes. The mechanism of oxidation is also discussed. Ti vacancies and the edges are the preferential oxidation sites in MXene sheets. Based on this, the methods to improve the MXene stability, including controlling the storage environment, improving the synthesis method, annealing in an atmosphere, modification based on the surface electric state, and doping impurities, are further discussed. First, the optimal requirements for MXenes storage are low temperature, desiccation, and oxygen isolation. Second, soft etching methods must be applied to synthesize MXenes to reduce the defect density of their sheet surface. Then, annealing MXenes in an atmosphere can enable the tailoring of the surface structure and functional groups for enhanced MXene stability. Lastly, more methods have been applied to improve the stability of MXenes based on their surface electric state. Since the MXene sheet surface is electronegative, their oxidation can be impeded by loading cations into the sheets. Similarly, since the edge of these sheets is electropositive, polyanions can be absorbed onto the edge to protect the MXene sheets. Moreover, compositing metal oxides, organic macromolecules, and nanocarbon on their surface can also improve the stability of MXenes. Finally, doping with impurities can also improve the band energy of MXenes. Meanwhile, our idea to improve the stability of MXenes is also briefly introduced.