Galvanic Replacement and Etching of MAX-Related Phases in Molten Salts toward MXenes: An In Situ Study

International audience ; Galvanic replacement in molten salts has recently been uncovered as a pathway to design new compositions of layered transition metal carbides related to the family of MAX phases, Mn+1AlXn, where M is a transition metal, X is arbon for carbide phases, and A is mainly an element of IIIA and IVA groups of the periodic table. These compounds can be further involved in etching processes by removal of A elements to yield new 2D transition metal carbides n+1XnTz, so-called MXenes, where T stands for surface groups. Although promising, the chemical modification of MAX-related materials in molten salts has been reported only for few compounds and little is known about the orresponding reaction mechanisms. In this work, we question the versatility of galvanic reactions in molten salts for MAX-related phases, by combining for the first time in situ X-ray diffraction and in situ X-ray absorption spectroscopy uring reactions in molten salts for two compounds: Ti3AlC2 and Mo2Ga2C. The first one shows minute-scale ransformation into molten ZnCl2 towards Ti3ZnC2, followed by evolution into Ti3C2Cl2. On the contrary we do not observe replacement, but etching of Mo2Ga2C into Mo2GaC and then orthorhombic Mo2C, with a loss of the layered structure. We highlight the role of the molten salt chemistry in this process and discuss the different behaviors of these two MAX-related phases versus galvanic reactivity in molten salts, to open the way to new MAX compositions.