Unexpected Self-Assembly of Nanographene Oxide Membranes upon Electron Beam Irradiation for Ultrafast Ion Sieving

Nanographene oxide (nGO) flakes-graphene oxide with a lateral size of ≈100 nm or less-hold great promise for superior flux and energy-efficient nanofiltration membranes for desalination and precise ionic sieving owing to their unique high-density water channels with less tortuousness. However, their potential usage is currently limited by several challenges, including the tricky self-assembly of nano-sized flakes on substrates with micron-sized pores, severe swelling in aqueous solutions, and mechanical instability. Herein, the successful fabrication of a robust membrane stacked with nGO flakes on a substrate with a pore size of 0.22 µm by vacuum filtration is reported. This membrane achieved an unprecedented water permeance above 819.1 LMH bar^-1, with a high rejection rate of 99.7% for multivalent metal ions. The nGO flakes prepared using an electron beam irradiation method, have uniquely pure hydroxyl groups and abundant aromatic regions. The calculations revealed the strong hydrogen bonds between two nGO flakes, which arise from hydroxyl groups, coupled with hydrophobic aromatic regions, greatly enhance the stability of stacked flakes in aqueous solutions and increase their effective lateral size. The research presents a simple yet effective approach toward the fabrication of advanced 2D nanographene membranes with superior performance for ion sieving applications.