Two-Dimensional-Material Membranes: Manipulating the Transport Pathway for Molecular Separation

Conspectus The discovery of graphene triggers a new era of two-dimensional (2D) materials, which exhibit great potential in condensed matter physics, chemistry, and materials science. Meanwhile, the booming of 2D materials brings new opportunities for the next generation of high-performance (high permeability, selectivity, and stability) separation membranes. Two-dimensional materials with atomic thinness can serve as new building blocks for fabricating ultrathin membranes possessing the ultimate permeation rate. The plane structure with micrometer lateral dimensions provides an excellent platform for the orderly alignment of the nanosheets. Moreover, the apertures of two-dimensional-material membranes (2DMMs), including the in-plane nanopores and interlayer channels, can contribute to the fast and selective transport of small molecules/ions related to molecular separation. Therefore, the emerging 2D materials with various nanostructures, including graphene oxide (GO), zeolite nanosheets, metal-organic framework (MOF) nanosheets, and transition-metal carbides/carbonitrides (MXene), can be assembled into high-performance membranes. Various assembly methods such as filtration, spin coating, and hot dropping have been employed to fabricate 2DMMs, while the processes for separating small molecules/ions tend to demand higher precision, especially in water desalination and gas separation. The nanostructures of 2DMMs and the physicochemical properties of transport pathway need to be finely tuned to meet the requirement. In addition, the stability of 2DMMs, which is critical to the large-scale implementation, must be taken into consideration as well.In this Account, we discuss our recent progress in manipulating molecular transport pathways in 2DMMs by optimizing the assembly behavior of 2D nanosheets, tuning the microstructure of interlayer channels, and controlling the physicochemical properties of the membrane surface. Assembly methods, including vacuum suction assembly, polymer-induced assembly, and external force-driven assembly, have been proposed to construct ordered laminates for molecular transport. The size and chemical structure of interlayer channels were further tailored by strategies such as nanoparticle intercalation, cationic control, and chemical modification. Interestingly, the manipulation of surface properties of 2DMMs was proven to contribute to fast molecular transport through interlayer channels. Moreover, the issues concerning 2DMMs toward practical applications are discussed with an emphasis on the substrate effect, molecular bridge strategy, and preliminary progress in large-scale fabrication. Finally, we conclude this Account with an overview of the remaining challenges and the new opportunities that will be opened up for 2DMMs in molecular separation.