Functionalized g-C3N4 nanosheet interlayer enables enhanced separation performance of nanofiltration membranes

•Hydrophilized g-C3N4 nanosheets are prepared by tannic acid (TA) assisted exfoliation.•TFNi membranes are fabricated by introducing a TA-C3N4 nanosheet interlayer.•The interlayer makes ultra-thin separation layer possible.•The TFNi membrane exhibits water permeance of 34.25 L m−2 h−1 bar−1 and Na2SO4 rejection of 98.53%.