Effect of Base/Nucleophile Treatment on Interlayer Ion Intercalation, Surface Terminations, and Osmotic Swelling of Ti₃C₂T<i>_z</i> MXene Multilayers

The first MXene discovered, Ti 3 C 2 T z, was synthesized by etching aluminum, Al, from the nanolaminated MAX phase, Ti 3 AlC 2, using hydrofluoric acid, HF. To delaminate the resulting MXene multilayers, MLs, it was necessary to increase the interlayer spacing, by first treating them with relatively large organic cations such as tetrabutylammonium hydroxide, TBAOH, dimethyl sulfoxide, DMSO, etc. When etched with a combination of LiF and HCl on the other hand, the Li cations spontaneously intercalated and no extra delamination step was needed. Herein, we attempt to understand why some molecules intercalate into the HF-etched MXene, while others do not. We find that treating HF-etched Ti 3 C 2 T z MLs with a base, like NaOH, renders them ion exchangeable. This base treatment was found to reduce the −F terminations on the MXene surfaces, which most likely weakens the interlayer hydrogen bonding and therefore allows for ion exchange and concomitant hydration. We exploit this nucleophilic dehalogenation to functionalize the Ti 3 C 2 T z surfaces using several different nucleophiles like sodium stearate, lithium ethoxide, and diisopropyl xanthogen polysulfide. We also demonstrate the effect of interlayer ions and other functional terminations on the electrochemical performance of Ti 3 C 2 T z in sodium ion and lithium sulfur batteries. Finally, we find that the interlayer spacing between MXene sheets derived using LiF + HCl increases dramatically when exposed to low-concentration salt solutions; this was attributed to osmotic swelling. This phenomenon was earlier observed in clays but is shown for the first time in the case of MXenes.