Atomistic Mechanisms of Ti₃AlC₂ Etching: Oxidation, Surface Stability, and Selectivity

Mechanisms governing MAX-phase etching set the properties and stability of resulting MXenes, yet the details of this complex process remain poorly understood. Combining thermodynamic analysis of surface chemistries with ab initio molecular dynamics and enhanced free-energy sampling, we directly model elementary steps at the MAX/etching solution interface. We find that purely thermodynamic analysis overestimates etching selectivity, which is governed by the kinetics of elementary reaction steps. Our calculations reveal that initial exposure to water drives localized, thermodynamically favored edge oxidation that seeds MXene-like terminations into the uppermost layers of Ti₃AlC₂ with oxygen occupying the interlayer space between Al and Ti. Subsequent Al extraction proceeds with low energy barriers (∼0.25 eV) in both fluorine-free and fluorine-containing aqueous environments, whereas Ti extraction is kinetically hindered with barriers spanning 0.70-2.12 eV depending on the surface chemistry. Al diffusion in the MAX lattice is identified as a rate-limiting step, but tensile stress arising from interior-layer oxidation and the formation of MXene-like terminations substantially lowers the intralayer diffusion barriers, facilitating Al transport. The developed computational framework offers a valuable tool for exploring the critical stages of MXene formation, optimizing synthesis conditions, and discovering novel MXene chemistries.