In Situ X-Ray Diffraction Unravels Etching Processes of Nb ₂ AlC in Lewis Acidic Molten Salts

The Lewis acidic etching route is an efficient method for preparing MXenes with tunable properties through redox reaction between Lewis acidic salts (such as CuCl2 and CoCl2) and the A-layer elements (such as Al, Ga, Sn, and Si) in the MAX phase. However, the etching pathways and phase transition are still lacking in-depth exploration. This work employs in situ time-resolved synchrotron radiation X-ray diffraction (SRXRD) to unveil the contrasted etching processes of Nb2AlC in different molten salts. It is demonstrated that CuCl2 etching leads to a transformation from Nb2AlC to Nb2CuC and ultimately yields Nb2CTx MXene; no intercalation of molten salts into the MXene interlayers is found in the process, while CoCl2 drives Lewis acidic corrosion, resulting from Nb2AlC to Nb2CoC, subsequently decomposing into NbC. Similar NbC layers form during etching with FeCl2/NiCl2, indicating that the redox potential of molten salts alone cannot predict the final Lewis acidic etching outcomes. Therefore, two distinct Lewis acidic reactions, including MXene formation etching and carbide decomposition, are identified. This study provides fundamental insights into the selectivity of Lewis acidic reaction routes via in situ X-ray techniques, which is of great significance for the controllable synthesis and specific applications of MXenes.