Fig. 1: Exfoliation/etching process in HF.
From: Vacancy formation energy as a descriptor of the exfoliability of MAX phases to MXenes
a Schematic representation of an exfoliation reaction pathway on a representative potential energy surface (PES). The reaction begins with the removal of the A-layer leading to the formation of 2D MXene layers, which is thermodynamically favored (in HF). However, the reaction can further proceed exothermically, forming other products that reside at a global minimum on the PES. Having MXene formation as an intermediary step, vacancy formation energies are used as a suitable measure for the local stability of MXenes and the potential for further reactions. b For the initial step of exfoliation, i.e., MXene formation, the vacancy formation energy (\(\Delta {E}_{f}^{{\rm{A}}}\)) of the element must be negative to enable its removal from the precursor MAX phase. To assess if the resultant MXene is locally stable, the vacancy formation energies for the M (\(\Delta {E}_{f}^{{\rm{M}}}\)) and X (\(\Delta {E}_{f}^{{\rm{X}}}\)) elements must be positive.
