Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes

Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesizing a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition-metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this replacement reaction between the Zn element from molten ZnCl₂ and the Al element in MAX phase precursors (Ti₃AlC₂, Ti₂AlC, Ti₂AlN, and V₂AlC), novel MAX phases Ti₃ZnC₂, Ti₂ZnC, Ti₂ZnN, and V₂ZnC were synthesized. When employing excess ZnCl₂, Cl-terminated MXenes (such as Ti₃C₂Cl₂ and Ti₂CCl₂) were derived by a subsequent exfoliation of Ti₃ZnC₂ and Ti₂ZnC due to the strong Lewis acidity of molten ZnCl₂. These results indicate that A-site element replacement in traditional MAX phases by late transition-metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach. In addition, this is the first time that exclusively Cl-terminated MXenes were obtained, and the etching effect of Lewis acid in molten salts provides a green and viable route to preparing MXenes through an HF-free chemical approach.