This paper investigates the factors governing the successful chemical exfoliation of MAX phases into MXenes, focusing on the selective removal of the A element using hydrofluoric acid (HF). The study combines density functional theory (DFT) calculations with experimental data, including previously unpublished results, to understand why some MAX phases exfoliate while others do not. The research reveals that while complete exfoliation is thermodynamically favorable for all MAX phases considered, the initial etching steps are crucial. The authors propose that successful exfoliation requires a negative free energy change for A element removal and a positive free energy change for M element removal, with the crucial constraint that oxygen-containing species are not formed in the acidic HF environment. This improved understanding is vital for computational screening and discovery of novel two-dimensional materials.

Jonas Björk, Joseph Halim, Jie Zhou, Johanna Rosen