Delamination of MXene Flakes Using Simple Inorganic Bases

Since the discovery of the first MXene, Ti3C2Tz, in 2011, more than 30 new MXene chemistries have been reported. A universal method via which all MXenes can be synthesized is by etching the "A" layer from the MAX phase using hydrofluoric acid, HF. After etching, to delaminate the resulting MXene multilayers, MLs, they are typically treated with a surfactant like tetrabutylammonium hydroxide, TBAOH, tetramethylammonium hydroxide, TMAOH, dimethyl sulfoxide, DMSO, etc. In our earlier work, we found that treating HF-etched Ti3C2Tz MLs with a base like NaOH renders them ion-exchangeable, similar to the earlier reported treatments using expensive and more toxic bases like TMAOH or TBAOH. In this work, we study the effects of sodium hydroxide, NaOH, treatment on Nb2CTz and Mo2Ti2C3Tz MXene MLs. We show that, similar to Ti3C2Tz, Na ions also intercalate in these two MXenes. This was clearly confirmed by X-ray diffraction analysis, which showed an increase in the interlayer spacing post treatment of HF-etched MXenes with NaOH. This increase in the interlayer space reduces the van der Waals interaction between the flakes, allowing for their facile delamination to single or few layers. It was also observed that NaOH treatment reduces the flakes' surface oxidation during processing, compared to when TBAOH is used. The surface chemistry and morphology of the MXenes post NaOH and TBAOH treatments were also analyzed using X-ray photoelectron spectroscopy and scanning electron microscopy, respectively, and compared. Finally, we conclude that inexpensive NaOH can be used to substitute for TBAOH or TMAOH, conventionally used for MXene delamination.