Galvanic Replacement and Etching of MAX-Related Phases in Molten Salts toward MXenes: An <i>In Situ</i> Study
Emile Defoy, M. Baron, Amandine Séné, Anissa Ghoridi, Dominique Thiaudière, Stéphane Celerier, Patrick Chartier, Florian Brette, Vincent Mauchamp, David Portehault
Abstract
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.