Understanding the swelling behavior of Ti₃C₂T_x MXene membranes in aqueous media

Two-dimensional (2D) lamellar MXene membranes have demonstrated ultrafast water permeance and outstanding ion rejection performance, thus showing great potential for water purification. However, as typical 2D lamellar structures, MXene membranes tend to swell in aqueous media caused by increased d-spacing, leading to deteriorated mechanical stability and reduced ion sieving efficiency. Despite several chemical and physical confinement attempts to obtain stable ion sieving performance of the membranes, there is still limited knowledge of the main cause of this swelling problem. In this systematic study, the interlayer spacing of the MXene membrane lamellar sheets was altered by intercalating different valence ions (Na⁺, Ca²⁺, and Al³⁺), then we used simultaneous in situ environmental scanning electron microscopy and in situ XRD to investigate the root cause of the swelling phenomenon of pristine and ion-intercalated Ti₃C₂T_x MXene membranes under different environmental conditions. Molecular dynamics simulations were used to fundamentally understand the structure and mobility of water in the MXene channel. As predicted using the theoretical model, the d-space decreases by increasing the charge of the ions in the solution. Trivalent cation intercalated membranes were found to collapse more easily at high temperatures, which could make such membranes suitable for water desalination membranes and temperature sensor applications. Ca and Al intercalation in the MXene membrane have provided more stability to interlayer spacing, hence causing less swelling and improved rejection of ions and other molecules. On the other hand, monovalent cation intercalated membranes were substantially more sensitive to relative humidity increase, making them less suitable for water treatment but rather attractive for humidity sensor applications. This work contributes to the rational design of stable 2D membranes for water purification and sensing applications.