Freeze-sonication delamination method of high crystalline-quality MXene with high yield

MXene has attracted broad attention for its unique physicochemical properties and has flourished in many research areas. Since MXene obtained by high crystalline-quality MAX phase has excellent stability and properties, the etching of high crystalline-quality MAX phase and delamination into monolayer will become an inevitable trend. However, the delamination of the high crystalline-quality MXene is significantly more challenging than conventional MXene because of the more vital van der Waals forces of the more regular interlayer structure. Here, we propose a freeze-sonication delamination (FSD) strategy that can solve the difficult problem of efficiently exfoliation of high crystalline-quality MXene. The principle is to utilize the synergistic effect of ultrasound and ionic intercalation to allow a large number of water molecules to penetrate into the interlayer, which results in volume expansion, and then ultrasound to obtain monolayer MXene in a frozen state. This method not only increases the yield of high crystalline-quality monolayer MXene but also prevents the size reduction and the nanosheet destruction. The yield of monolayer MXene reached 74.2 % with a concentration of 19.8 mg/mL after six FSD cycles (the time of ultrasound treatment is 5 min after per freeze). Meanwhile, under the protection of ice, the obtained monolayer MXene has a larger size with more complete nanosheets than ultrasound-delaminated MXene. Supercapacitors made of high crystalline-quality MXene exhibits excellent gravimetric capacitance of 261.1 F/g and satisfactory cycling stability.