Elementary processes governing V₂AlC chemical etching in HF

The literature on MXenes, an important class of 2D materials discovered in 2011, is now abundant. Yet, the lack of well-defined structures, with definite crystal orientations, has so far hindered our capability to identify some key aspects ruling MXene's chemical exfoliation from their parent MAX phase. Herein the chemical exfoliation of V₂AlC is studied by using well-defined square pillars with lateral sizes from 7 μm up to 500 μm, processed from centimeter-sized V₂AlC single crystals. The MXene conversion kinetics are assessed with μm spatial resolution by combining Raman spectroscopy with scanning electron and optical microscopies. HF penetration, and the loss of the Al species, take place through the edges. At room temperature, and on a reasonable time scale, no etching can takes place by HF penetration through the basal planes, viz. normal to the basal planes. In defect-free pillars, etching through the edges is isotropic. Initially the etching rate is linear with a rate of 2.2 ± 0.3 μm h^-1 at 25 °C. At a distance of ≈45 μm, the etching rate is greatly diminished.