Water-Dispersible Ti 3C 2T z MXene Nanosheets by Acid-Free, Molten Salt Etching

Molten-salt etching of Ti3AlC2 MAX phase offers a promising route to produce 2D Ti3C2Tz (MXene) nanosheets without the need for hazardous HF. However, molten-salt etching generally results in MXene clays that are not fully exfoliated or water-dispersible, thus preventing nanosheet processing. This occurs because molten-salt etching generally results in a high level of -F terminal functionalities that render the MXene clay hydrophobic. Here, we demonstrate a molten salt (SnF2) etching method that, for the first time, produces water-dispersible Ti3C2Tz nanosheets without the need for HF. In molten salt etching, SnF2 diffuses between the layers during etching to form AlF3 and Sn as byproducts; Sn spheres form internally and separate the layers. The stable, aqueous Ti3C2Tz dispersion yields a ζ potential of -31.7 mV, because of -OH terminal groups introduced by KOH washing. X-ray diffraction and electron microscopy confirm the formation of Ti3C2Tz etched clay with substantial d-spacing as compared to the clay from a traditional HF-process. This work is the first to use molten salt etching to successfully prepare colloidally stable aqueous dispersions of Ti3C2Tz nanosheets. The Ti3C2Tz nanosheet film and clay produced by this method also show excellent specific and areal capacitance and conductivity.