Regulating molecules/ions sieving channels of MXene-based membranes by edge-capping strategy

Abstract

Regulating molecules/ions transport within two-dimensional (2D) sub-nanometer channels to achieve a higher separation efficiency is of considerable interest for MXene membranes in sustainable water treatment. Herein, sodium tripolyphosphate (STPP) was introduced into Ti3C2Tx nanosheets to construct a series of modified STPP-MXene membranes based on the edge-capping strategy. The critical role of edge-capping in modulating selective molecules/ions separation within 2D sub-nanometer channels was highlighted. The edge-capping method was demonstrated to moderately narrow the shoulder spacing between adjacent Ti3C2Tx nanosheets and impaired their positive electric field intensity. In addition, the DFT results showed that the cations exhibited a much lower relative energy with STPP-Ti3C2Tx (-0.0024 eV) compared to Ti3C2Tx nanosheets (-0.2092 eV), causing cations to permeate more readily through the STPP-MXene membrane. As a result, the appropriately designed and prepared STPP-MXene membrane demonstrated an excellent molecules/ions selectivity (CR/NaCl selectivity = 52.6, CR/Na2SO4 selectivity = 30.3), much higher than that of the pristine MXene membrane. Moreover, by protecting the Ti at the edges of Ti3C2Tx nanosheets, the STPP-MXene membrane retained its original two-dimensional structure to maintain a stable separation performance, even when exposed to a water environment for up to 60 days. In contrast, the pristine MXene membrane was completely oxidized due to its weak stability. This study provides a theoretical basis for the feasibility of edge-capping strategy as a broad modification alternative to design high-efficiency molecular/ionic sieving MXene membranes.