Ultrathin two-dimensional membranes by assembling graphene and MXene nanosheets for high-performance precise separation

Membrane technology has attracted significant attention in the field of separation and purification due to its high efficiency and low energy consumption. In this case, to overcome the "trade-off" limitation between high permeability and selectivity, tremendous efforts have been devoted to designing and exploiting ultrathin membranes. Consequently, graphenes and MXenes, which have the advantages of single-atom layer thickness, large specific surface area, ease of fabrication, and controllable layer-by-layer assembly, have emerged as ideal candidates for the fabrication of ultrathin membranes. Compared to conventional polymeric membrane materials, 2D separation membranes assembled using graphene and MXene nanosheets, leveraging their unique laminar structure and ultrathin thickness, significantly shorten the transport path of guest molecules, minimizing the mass transfer resistance while maintaining or even enhancing separation selectivity. Furthermore, precise sieving of guest molecules can be achieved by tuning the interlayer spacing or pore sizes of these 2D nanosheets. This review comprehensively summarizes the state-of-the-art advancements in ultrathin 2D membranes assembled by graphene and MXene nanosheets, including the synthesis of nanosheets, the fabrication principles, structure design, and mass transfer mechanism of ultrathin 2D membranes, and their related precise separation applications. We focus on advanced ultrathin 2D membranes that simultaneously exhibit high permeability and selectivity and discuss the suitable strategies and mechanisms to develop ultrathin 2D membranes that can overcome the inherent "trade-off" effect in traditional separation membranes. Finally, a critical consideration of the opportunities and challenges associated with ultrathin 2D membranes assembled by graphene and MXene nanosheets towards high-performance precise separation is also presented.