Vacancy formation energy as a descriptor of the exfoliability of MAX phases to MXenes

Abstract A thermodynamic descriptor for chemical exfoliation of MXenes remains elusive, primarily because the process is dynamic and MXenes are inherently metastable. Here we show that vacancy formation energies provide such a descriptor. Synthesized MXenes exhibit positive vacancy formation energies for M and X elements, while non-synthesized MXenes show negative energies for M or X elements, or both. Similarly, exfoliable precursor MAX phases are characterized by negative vacancy formation energies for A elements. Using this correlation, we identify seven hitherto unreported MXenes as potentially synthesizable in HF at pH = 2. At higher pH, additional unknown MXenes are identified as locally stable together with their exfoliable MAX phases but remain inaccessible in HF, highlighting the need for alternative alkaline etchants. The exfoliable precursor MAX phases are predominantly Al-based. The proposed descriptor is transferable to other 2D materials such as MBenes and may be adapted to other acidic or alkaline etchants.