Ion-Exchange and Cation Solvation Reactions in Ti3C2 MXene

Ti₃C₂ and other two-dimensional transition metal carbides known as MXenes are currently being explored for many applications involving intercalated ions, from electrochemical energy storage, to contaminant sorption from water, to selected ion sieving. We report here a systematic investigation of ion exchange in Ti₃C₂ MXene and its hydration/dehydration behavior. We have investigated the effects of the presence of LiCl during the chemical etching of the MAX phase Ti₃AlC₂ into MXene Ti₃C₂T_x (T stands for surface termination) and found that the resulting MXene has Li⁺ cations in the interlayer space. We successfully exchanged the Li⁺ cations with K⁺, Na⁺, Rb⁺, Mg²⁺, and Ca²⁺ (supported by X-ray photoelectron and energy-dispersive spectroscopy) and found that the exchanged material expands on the unit-cell level in response to changes in humidity, with the nature of expansion dependent on the intercalated cation, similar to behavior of clay minerals; stepwise expansions of the basal spacing were observed, with changes consistent with the size of the H₂O molecule. Thermogravimetric analysis of the dehydration behavior of these materials shows that the amounts of H₂O contained at ambient humidity correlates simply with the hydration enthalpy of the intercalated cation, and that the diffusion of the exiting H₂O proceeds with kinetics similar to clays. Furthermore, these results have implications for understanding, controlling, and exploiting structural changes and H₂O sorption in MXene films and powders utilized in applications involving ions, such as electrochemical capacitors, sensors, reverse osmosis membranes, or contaminant sorbents.