Etching Mechanism of Monoatomic Aluminum Layers during MXene Synthesis

Understanding the etching mechanisms of MXene and obtaining direct insights into the influence of etchants on structural features and defects are of critical importance for improving MXene properties, optimization of etching protocols, and exploring new MXene compositions. Despite their importance, such studies have been challenging because of the monoatomic thickness of the A-element layers being etched and aggressive etchants that hinder in situ studies. Here, we visualize the etching behavior of the Ti3AlC2 MAX phase in different etchants at the atomic scale using focused ion beam and electron microscopy. We also report on the structural changes in the Ti3AlC2 phase as a function of etching time and etchant type (LiF/HCl, HF, or NH4HF2) to reveal the etching mechanism for the first time. Interestingly, direct visualization reveals an unexpected stepwise etching where the edge Al atoms at the middle layers of the MAX particle are not etched despite contact with the acidic etchant counterintuitively. Also, while the propagation of the etching front occurs in the direction normal to the inner basal plane for all etchants, we reveal that HF and NH4HF2 etch the grain boundaries of polycrystal MAX particles to expose more edge sites to the etchant, which is not observed for LiF/HCl.