▎ 摘　　要

Membranes based on graphene or graphene oxide (GO) have received intensive studies recently for various separation processes. However, a great challenge lies in fast and facile fabrication of GO membranes with high salt rejection. Here we report for the first time that ultra-thin GO films with a good salt rejection can be fabricated via contra-diffusion-induced crosslinking based on cation-pi and electrostatic interactions between cations and GO flakes. It only takes down to 5 min to construct a defect-free GO film. Various cations, including Na+, K+, Mg2+, Ca2+ and Fe3+, can be used as "crosslinkers" to endow the as-prepared GO membranes with highly tunable separation performance. The optimized GO membranes exhibit over 90% rejection of Na2SO4 with filtrate permeance of 4.3 L m-2 h(-1) bar(-1) during the nanofiltration process, which reaches to the performance upper limit of previously reported graphene-based membranes. Such a high performance should be attributed to the unique packing structures of the GO flakes crosslinked by cations and the thin and uniform GO selective layers.