Atomistic mechanisms of Ti₃AlC₂ etching: oxidation, surface stability, and selectivity

Mechanism of MAX phase etching determines the properties and stability of produced MXenes, yet the details of this complex process remain poorly understood. Herein, we present a computational study into the etching mechanism of the Ti₃AlC₂ MAX phase in fluoride-containing aqueous solution using density functional theory. Our results reveal the tendency of Ti₃AlC₂ to undergo partial oxidation with oxygen occupying the interlayer space between Al and Ti. Upon Al removal, the oxidized structure exothermically transforms into configurations resembling O-terminated Ti₃C₂ MXenes, suggesting that MXene-like fragments form even before the etching process is complete. A systematic analysis of Ti₃AlC₂ surface chemistry across varying electric potentials reveals that OH-rich terminations at MAX phase edges promote selective Al extraction while suppressing Ti dissolution, whereas O-terminated edges can compromise the selectivity of the etching reaction. Using molecular dynamics simulations, we show that Al extraction proceeds with low energy barriers (~0.25 eV) in both fluorine-free and fluorine-containing aqueous environments. We identify Al diffusion within the MAX phase as the rate-limiting step of the etching process and show that tensile stress facilitates intralayer Al migration. The developed computational framework offers a valuable tool for exploring the critical stages of MXene formation and optimizing synthesis conditions.