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

TL;DR

This paper presents a novel, environmentally friendly method for synthesizing new Zn-based MAX phases and Cl-terminated MXenes via a reaction with Lewis acidic molten salts, expanding the accessible materials space.

Contribution

It introduces a top-down replacement reaction approach using molten ZnCl2 to create new MAX phases and MXenes, including the first Cl-terminated MXenes, without high-temperature or HF-based methods.

Findings

Synthesized new Zn-based MAX phases such as Ti3ZnC2 and V2ZnC.

Produced Cl-terminated MXenes like Ti3C2Cl2 and Ti2CCl2.

Demonstrated a green, HF-free route for MXene synthesis.

Abstract

Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesize 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 Zn element from molten ZnCl2 and 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 prepare MXenes through an HF-free chemical approach.