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Embedding nanofillers into polyamide (PA) matrices is considered as a simple yet effective strategy for boosting the performances of thin-film nanocomposite (TFN) nanofiltration (NF) membranes. However, the NF performances are usually compromised by the inadequate interface compatibility between nanofillers and PA matrices as well as the anisotropic orientation of nanofillers. Herein, we propose a distinctive fabrication strategy for TFN membranes featuring excellent compatibility and highly-aligned microstructures. The reactive covalent organic framework (COF) nanosheets are chemically stitched at oil-water interfaces to form continuous nanofilms, and the PA matrices are then in-situ formed to encapsulate them, giving the TFN membranes. The stitched nanofilms contain two types of perpendicularly oriented pores: intrinsic apertures in COF nanosheets, and gaps among adjacent nanosheets. Together with the abundant reactive sites, the stitched nanofilms not only mediate the controllable synthesis of PA matrices, but also are covalently fused into the matrices. The aligned pores of the stitched nanosheets are capable of sieving salt ions, endowing TFN membranes with high separation precision and fast water permeation. This work demonstrates a strategy to control the orientation of COF nanosheets, which is highly desired in diverse fields not limited to membrane separation. Image 1 • Highly-aligned COF nanosheets are embedded into the PA matrices. • COF nanosheets are chemically stitched to form ultrathin nanofilms. • The stitched nanosheets function as a reaction mediator and a performance enabler. • The composite membranes exhibit a boosted water permeance and enhanced selectivity. [ABSTRACT FROM AUTHOR]

Processing two-dimensional (2D) covalent organic frameworks (COFs) into nanofilms has gained widespread attention in water treatment. However, the designed synthesis of COF-based membranes for ion separations still remains a huge challenge though the strategies to produce COF molecular separation membranes have been greatly developed. Herein, we report the construction of bi-layered COF nanofilms for efficient desalination through a simple yet effective secondary growth. The suggested strategy is capable of easily regulating the growth of another COF layer on the pre-synthesized first layer, thus producing crystalline and defect-free bi-layered COF nanofilms with distinct laminated structures. Most importantly, the offset channels are involved at the interface of two nanofilms obtained from the first and secondary growth, which leads to a constricted effective aperture that significantly enhances the ion separation performance. Specifically, the bi-layered COF nanofilms composited with macroporous supports exhibit high rejection rates (~95.8%) to Na 2 SO 4 , observably surpassing the desalination performance of single-layered COF nanofilms (rejection<10%). This work not only develops an ingenious way to fabricate COF-on-COF nanofilms with narrowed channels by tailoring the laminated structure of 2D COFs at the film interface, but also starts an avenue for the adoption of COF platforms for membrane separations with an improved precision. Image 1 • The secondary growth strategy is developed to build bi-layered COF nanofilms. • Nanofilm structures are controllable by adjusting the synthesis conditions. • Offset structures at the film interface enable a narrowed aperture down to ~0.71 nm. • The bi-layered COF nanofilms exhibit a greatly boosted desalination performance with a reasonable flux. [ABSTRACT FROM AUTHOR]