Fig. 2: Evidence of correlation.
From: Vacancy formation energy as a descriptor of the exfoliability of MAX phases to MXenes
a Vacancy formation energies of M elements(\(\Delta {E}_{f}^{{\rm{M}}}\);y coordinate) vs. X elements (\(\Delta {E}_{f}^{{\rm{X}}}\); x coordinate) in MXenes, and (b) of the A elements (\(\Delta {E}_{f}^{{\rm{A}}}\)) in the corresponding parent MAX phases. c Schematic crystal structures of a MXene (M2AX) with F, O, and OH surface terminations (T). A correlation for the metastability of MXenes and the exfoliability (etching of the A element) of MAX phases is identified. Synthesized MXenes in HF (green symbols) are located in the yellow-shaded region (x > 0, y > 0), while unsynthesized MXenes (red symbols) are situated in the turquoise-shaded region (x < 0, y < 0, or both). Simultaneously, A-etched MAX phases (green) are present in the yellow-shaded region (\(\Delta {E}_{f}^{{\rm{A}}} < 0\)), while non-etchable are positioned in the turquoise-shaded region (\(\Delta {E}_{f}^{{\rm{A}}} > 0\)). All values are calculated at a pH of 2, with an ionic concentration of 0.001 mol/l at room temperature.
