Insights on Tuning the Nanostructure of rGO Laminate Membranes for Low Pressure Osmosis Process

In this research, rGO laminates were prepared by a controlled partial reduction step, aimed to avoid aggregation and tune the interlayer spacing (d) between the rGO layers. The mild reducing agent Vitamin C (l-AA) and cross-linker poly(carboxylic acid)s were used to improve the stability of the assembled rGO laminate membranes. AFM was used for the first time to further investigate the statistical size distribution of spacing between rGO layers. Topographical images of the edges of the rGO layers were obtained with an AFM instrument; interlayer spacing profiles were extracted, and then the data was plotted and fitted with Gaussian curves. We confirmed that the differently sized spacing coexisted, and their size distribution was affected by the reduction degree of rGO. At greater levels of reduction, more interlayer spacing was formed in the smaller size range, while few large gaps were still present. The obtained rGO laminate composite membranes were evaluated in a low pressure osmosis process such as forward osmosis (FO). The water permeation was higher in the rGO membrane prepared with a medium reduction degree (1.2-R) than the sample prepared by higher reduction degree (2.0-R) due to well-balanced nanochannels in hydrophilic regions and hydrophobic walls for fast transport of water molecules. The solute flux of the FO membrane was inversely correlated to the reduction degree. These findings helped in developing future strategies for designing high water flux and low reverse solute flux rGO membranes that are ideal for an FO process.