In Situ X-ray Diffraction Study of MXene Synthesis by the Reaction of Ti3AlC2 with Molten Zinc and Tin Chlorides

Using molten salts for etching aluminum (Al) away from the MAX phase for MXene synthesis is an attractive alternative method that allows one to avoid the use of toxic hydrofluoric acid (HF) solutions. However, the mechanism of the MAX phase reaction with molten salts remains to date unclear due to the lack of in situ data. Here, we present a detailed in situ time-resolved synchrotron radiation X-ray diffraction study of the MAX phase annealing in molten ZnCl2 and SnCl2. The reaction of salts with the MAX phase is found to occur in two stages. The initial period of annealing results in the delamination of two-dimensional (2D) Ti3C2 layers, vigorous evolution of AlCl3 bubbles, and dissolution of Zn in a ZnCl2 melt. The chlorine-terminated Ti3C2 sheets formed in the delaminated state are restacked into a relatively well-ordered MXene structure (P63/mmc, a = 3.071 Å and c = 18.577 Å) during the prolonged annealing in molten salts. Surprisingly, the data recorded directly in molten salts at temperatures up to 873 K demonstrate that Ti3C2Clx MXene shows no swelling in both liquid ZnCl2 and SnCl2. The structure of MXene studied directly in the molten salts is found to be the same as in ex situ experiments performed after cooling and water washing under ambient conditions. The absence of the “pristine” melt-swollen phase indicates a rather different mechanism of MXene formation compared to HF-based solution methods. Formation of MXene by gradually removing Al from the MAX phase starting at the edges of flakes and propagating into the deeper parts of interlayers is not possible, since the molten salt is not capable of penetrating between Cl-terminated Ti3C2 layers.