Adjustable interlayer spacing of ultrathin MXene-derived membranes for ion rejection

Robust lamellar membranes with adjustable interlayer spacing have exhibited exceptional permeability and play a key role in many applications. Herein, ultrathin MXene-derived membranes (thickness: 100 nm) supported on α-Al₂O₃ tubular supports were designed and the interlayer spacing between stacked neighboring nanosheets was finely tuned via sintering-temperature regulation. Below 400 °C, the spacing decreased from 3.71 (60 °C) to 2.68 (400 °C) and the membranes with a well-packed layer nanoarchitecture exhibited regulable ion rejection properties. The space reduction can be attributed to the moisture loss and de-functionalization (-OH) occurring within the MXene film at elevated temperatures. Along with the rise of sintering temperature, the dark green MXene layer transformed progressively into a white reflective membrane above 400 °C, owing to the oxidation of Ti₃C₂T_x nanosheets into TiO₂ nanoparticles. This resulted in a filtration-mode change from interlayer transport pathways to longitudinal-lateral nanochannels and, hence, a reduction in ion retention. The optimal MXene-derived membranes (T400) exhibited favorable ion-rejection rate (99.2% for VO²⁺). Therefore, the method provides a facile and straightforward strategy for adjusting the interlayer spacing of the membranes fabricated from two-dimensional nanosheets for achieving satisfactory ion rejection.