Supercritical CO2-assisted solid-phase etching preparation of MXenes for high-efficiency alkaline hydrogen evolution reaction

Two-dimensional (2D) transition metal carbides, nitrides, or carbonitrides, regarded as MXenes, hold great promise for an extensive range of applications such as efficient catalysis and energy conversion and storage materials. Up to now, various strategies for synthesizing MXenes have been explored, typically including an F-containing aqueous etching process, a molten salt etching method, etc. In this work, for the first time, we have designed a novel solid-phase etching approach to synthesize two-dimensional titanium carbide (MXenes) sheets with the assistance of supercritical CO2 (SC CO2), and the solid-phase etching mechanism behind the efficient fabrication process has also been revealed from a series of characterization methods. It is demonstrated that SC CO2-coated FeF3·3H2O can successfully help realize the delamination of multilayer MXenes into single-layer or few-layer MXenes, and at the same time, the as-prepared MXene sheets are demonstrated to have large surface area and size, high quality and rich fluorine termination groups, which make them efficient in the alkaline hydrogen evolution reaction (HER).