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

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, M n +1 AlX n, where M is a transition metal, X is carbon for carbide phases, and A is mainly an element of the IIIA and IVA groups of the periodic table. These compounds can be further involved in etching processes by the removal of A elements to yield new 2D transition-metal carbides, M n +1 X n T z, the 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 a few compounds, and little is known about the corresponding 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 during reactions in molten salts for two compounds, Ti 3 AlC 2 and Mo 2 Ga 2 C. The first one shows minute-scale transformation into molten ZnCl 2 toward Ti 3 ZnC 2, followed by evolution into Ti 3 C 2 Cl 2 . On the contrary, we do not observe replacement but etching of Mo 2 Ga 2 C into Mo 2 GaC and then orthorhombic Mo 2 C, with a loss of the layered structure. We highlight the role of 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.