Fluoride-free molten salt hydrate-assisted synthesis of MXene in air down to 150 °C

Abstract

The conventional Lewis acid molten salt etching approach for synthesizing MXenes typically necessitates elevated temperatures exceeding 550 °C, in addition to the use of inert gas protection to prevent oxidation. Additionally, delamination of molten salt-etched MXenes typically requires hazardous intercalating agents. Herein, a scalable and non-toxic low-temperature shielded salt (LSS) approach for synthesizing MXene in air is reported, with the use of only a small portion of salts and a low reaction temperature down to 150 °C. Especially, the synergistic effect of the increased diffusion rate by Li+ ions and the phase change of magnesium chloride hexahydrate (MgCl2·6H2O) enables a redox-controlled A-site etching of the parent MAX phase, which even facilitates the synthesis of hard-to-etch MXenes. As a proof-of-concept demonstration, several theoretically hard-to-synthesize MXenes including Cr2CTx and Nb2CTx are successfully prepared through the LSS technique, where Cr2CTx is not achieved by Lewis acidic molten salt yet. Compared to conventional techniques, this low-temperature shielded salt etching method exhibits unconventional molten behavior while offering several advantages, including a non-oxidizing environment, shorter processing time, and elimination of highly corrosive washing agents and organic intercalants. These advances render the LSS approach a promising route for synthesizing MXenes with applications toward diverse practical applications. Graphical abstract: [Figure not available: see fulltext.]