Fig. 5: Role of pH in exfoliation.
From: Vacancy formation energy as a descriptor of the exfoliability of MAX phases to MXenes
a Ti vacancy formation energy (ΔEf) in Ti2CF2 MXene as a function of pH in HF. ΔEf reaches zero at the critical pH (pHcr) and changes sign beyond this threshold. The pH range is divided into metastable (yellow-shaded) and unstable (turquoise-shaded) zones by pHcr. The dissolved reservoir species are indicated in the legend. b Bar diagram illustrating critical pH values for M- and A-element vacancies in hypothetical and unsynthesized MXenes with F (top), O (middle), and OH (bottom) surface groups. The yellow-shaded bars highlight the exfoliable range, where \(\Delta {E}_{f}^{{\rm{A}}} < 0\), \(\Delta {E}_{f}^{{\rm{M}}} > 0\), and \(\Delta {E}_{f}^{{\rm{X}}} > 0\). This range is bounded by \({{\rm{pH}}}_{cr}^{{\rm{A}}}\) (upper) and \({{\rm{pH}}}_{cr}^{{\rm{M}}}\) (lower) and disappears if \({{\rm{pH}}}_{cr}^{{\rm{A}}} < {{\rm{pH}}}_{cr}^{{\rm{M}}}\). MXenes with locally unstable ΔEf values are excluded.
