Anhydrous Etching of MAX Phases for Faster and More Efficient Synthesis of MXenes

Anhydrous synthesis of MXene facilitates the control over the surface functionalization and the final physicochemical properties. Here, we propose a novel anhydrous etching solution, consisting of high-boiling-point dimethylsulfoxide as organic medium, NH₄HF₂ as etchant, CH₃SO₃H as acid, and NH₄PF₆ as intercalant. This anhydrous solution does not only enable one to increase the reaction temperature up to 100 °C for accelerating the etching and delamination of Ti₃AlC₂ MAX crystals, but also suppresses the destructive side reaction of the produced Ti₃C₂T<i>_x</i> MXene flakes. Consequently, the etching reaction is completed in 4 h at 100 °C and produces high-quality monolayer Ti₃C₂T<i>_x</i> MXene with an electrical conductivity of 8,200 S cm^-1 and a yield of over 70%. The synthesized Ti₃C₂T<i>_x</i> exhibits more F-terminations than conventional aqueous Ti₃C₂T<i>_x</i>. The atypical surface structure of Ti₃C₂T<i>_x</i> MXene leads to an exceptionally high ultimate tensile strength (167 ± 8 MPa), which is about 5 times larger than the ones synthesized in aqueous HF solution (31.7 ± 7.8 MPa).