Lateral-Size-Dependent Ion Transport Behavior of the MXene Membrane Facilitates Efficient Ion Sieving

Two-dimensional (2D) nanofluidic channels have shown great potential for efficient ion separation due to the synergy of the charge microenvironment and confined 2D planar channels. However, current 2D channels mainly rely on postdecoration with ion-specific recognition molecules, while the regulation of the membrane structure and the role of the channel itself have been overlooked. Herein, we constructed two types of membranes to demonstrate the critical role of building units in 2D lamellar membranes. By tuning the lateral size of MXene nanosheets, we prepared a large-lateral-sized MXene membrane (LMM)(∼5.47 μm) and a small-lateral-sized MXene membrane (SMM)(∼524 nm) to systematically study the ion transport behavior. The results showed that the LMM exhibited preferential transport of monovalent ions (Li+, Na+, and K+) while impeding permeation of multivalent ions (Ca2+, Mg2+, and Al3+), contributing to the selectivity of Li+/Mg2+ ≈ 12. In contrast, the SMM allowed for the fast transport of all monovalent/divalent ions, leading to poor selectivity (e.g., Li+/Mg2+ ≈ 2.3), which could be attributed to the abundant nonselective defects in the SMM. Density functional theory calculations reveal that the weakest interaction between MXene and Li+ determines the selective transport of Li+ over other ions. This work highlights the importance of building defect-free 2D membranes for ion sieving using nondecorated 2D nanofluidic channels.