Advancing Non-Aqueous Etching Strategy for Swift and High-Yield Synthesis of 2D Molybdenum Carbides (MXenes)

Abstract

Aqueous hydrofluoric acid (HF)-based solutions are widely used for etching MAX phases to synthesize high-purity 2D molybdenum carbides (MXenes). However, their applicability is limited to selected MAX phases, and the production of certain MXenes, such as Mo-based MXenes, remains challenging owing to low quality, low yield, and the time-intensive process, often requiring several days to weeks. In this study, a non-aqueous etchant for faster and more efficient synthesis of high-purity Mo-based MXenes is introduced. This etchant, containing Cl⁻ and F⁻ ions, is adequately effective to etch the MAX phase using the F⁻ ions of moderate concentration regenerated from GaF₆³⁻ byproducts but only mildly caustic to prevent damage to the resulting MXene. Using this approach, the rapid production of Mo₂CT_x is demonstrated within 24 h at 100 °C, achieving up to 90% multilayer and 45% monolayer yields. Furthermore, the resulting monolayer Mo₂CT_x flake exhibits larger sizes and fewer defects, with an electrical conductivity of 5.9 S cm⁻¹, 6.5 times higher than that (0.9 S cm⁻¹) of aqueous HF-Mo₂CT_x. This enhancement results in improved electrocatalytic activity of high-purity Mo₂CT_x for hydrogen evolution reactions. These findings highlight the potential of non-aqueous etching solutions to address the limitations of HF-based MXene synthesis.