Size-dependent water transport in laminar graphene oxide membranes: An interplay between interlayer spacing versus tortuosity of transport pathway

Graphene oxide membranes (GOMs), consisting of graphene oxide (GO) laminar microstructures, offer great potential as nanofiltration membranes due to their unique laminar nanochannelled galleries created between the stacked GO flakes. Recently, GOMs made from smaller GO flakes (S-GOMs) were favored for the reason that the less tortuous transport pathways could render better water permeability. Here, we found that this may not necessarily be true. We designed experiments, which suggested that the interlayer spacing between GO flakes may be a more important factor affecting the water permeability than the tortuosity of the water pathway, especially when the two-dimensional nanochannels were smaller than expected under a pressurized filtration setting. Notably, our results showed that as the GO flakes were fragmented into smaller pieces by high-power probe sonication, they underwent a thermal reduction that decreased the size of the interlayer spacing. Coupled with a less wrinkled microstructure that caused tighter compaction under a 3-bar transmembrane pressure, the overall water transport through the S-GOM was reduced despite a less tortuous pathway that was 2.5 times shorter based on theoretical calculation. This led to S-GOM showing 3.3 times lower water permeability than that of GOM made from large GO flakes. On the whole, our results unveil the interplay between the two most important parameters governing the water permeability of GOMs, which will help researchers with more instructional support when developing better performing GOMs for nanofiltration application.