Fig. 1: Graphene oxide (GO) membrane with quasi-vertically asymmetric channels for ion sieving.

a Schematic of typical GO membrane with 2D horizontal water channels stacked on the substrate (left), and GO membrane with vertical channels based on a substrate with asymmetric pores (right). Scanning electron microscope (SEM) images of small-pore side (b) and large-pore side (c) of a mixed cellulose ester (MCE) substrate. Small-pore side has a pore size of approximately 0.2 ~ 0.5 μm, while the large-pore side has a pore size of approximately 2 μm. d Cross-section and surface of AD-rGO membrane stacked on the small-pore side of MCE substrate. e SEM image of AD-rGO flakes inserted into the large-pore side of a MCE substrate. In the cross-section images of MCE substrate (i-iii), AD-rGO flakes have self-assembled within it. The red-shaded area marks the AD-rGO membranes, illustrating their main distribution within the MCE substrate. f Pure water permeances of the AD-rGO membranes. The membranes are stacked on the small pore side (with horizontal channels) and inserted into the large pore side (with vertical channels) of substrate, respectively. g Water permeances and rejection rates of the AD-rGO membrane with vertical channels for heavy metal ions. h Summary of the filtration performance of state-of-the-art membranes reported in the literature based on water permeances and rejection rates for heavy metal ions. Orange, green, and blue circles represent nanofiltration membranes (NFMs), two-dimensional material membranes (2DMS), and GO-based membranes (GOMs), respectively. The values are mean ± SD (n = 3).