Chemical Exfoliation and Delamination Methods of MXenes

Since MXene was prepared by HF in 2011, its outstanding physical/chemical properties had shown great potential on multiple fronts, which also promoted to the exploration of its synthesized methods. And safe and efficient synthesis routes are gradually being proposed. Because of their milder and safer than HF, in-situ HF-forming etching methods are widely studied firstly, such as fluoride salts (e.g., LiF, KF, and NH 4 F) in HCl or H 2 SO 4 and hydrothermal route with low toxicity etching agents (e.g., NaBF 4 in HCl). However, the releases of volatile HF gas formed during mixing are still risky. Bifluoride etchants (e.g., NaHF 2 , KHF 2 , and NH 4 HF 2 ), molten fluorinated salts etchants (e.g., NH 3 F, NaF) and Lewis acidic molten salts etchants (e.g., CuCl 2 , CoCl 2 ) have also been used in the preparation of MXene with good results. Thereafter, non-hazardous synthesis routes or fluorine-free etching methods are gradually being developed to explore efficient preparation methods and enrich MXene performance. Considering the process of electrons transfer for chemical etching, electrochemical etching methods were used to prepare MXene. In addition, alkali etching methods (e.g., KOH, NaOH), halogen-based etching methods (e.g., molecular chlorine, bromine, and iodine), and ionic liquid-based etching methods (e.g., EMIMBF 4 , BMIMPF 6 ) have also been explored with the successful preparation of MXene, also revealing the safe, efficient methods. Besides, diverse attempts have also been applied to organic-soluble etching methods for the preparation and storage of MXene to make up for MXene's weak stability in water. Although the preparation methods of MXenes are being explored extensively, a mature and scalable method is still a challenge.