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

Abstract

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.

Molecular dynamics simulation of graphene-based thin films for seawater desalination

“…The dependence of R on the interlayer distance h shows that the nanochannel formed by the two GE layers contributes to the molecular sieving by restricting the admittance of the ions into the system. These results are also in agreement with the observations from previously reported experimental studies [252,261].…”

Graphene-PSS/l-DOPA nanocomposite cation exchange membranes for electrodialysis desalination

“…Moreover, the smaller interlayer distance between the rGO nanosheets resulting from chemical reduction alongside with lower swelling may restrict the passage of ionic species through the membrane which is therefore reflected in the higher area resistance of SrGO/PVDF_45. 35 The area resistance of the SGO/PVDF_45 CEM was higher than previously reported. The current efficiency of the SrGO/PVDF_45 was higher by 0.05 than that of the SGO/PVDF_45, and comparable to the reference CEM.…”

“…To prepare chemically reduced GO (rGO), GO was reduced by mixing with 0.1 g L-ascorbic acid and stirring continuously for 24 h (Figure 3a). 35 L-ascorbic acid has been reported as a green and mild reducing agent used for reducing GO. 36,37 The solution turned from brown to black in color.…”

“…In producing rGO, the oxygen containing functional groups were removed from GO; resulting in a reduction in the interlayer distance between the nanosheets and a higher tendency for restacking. 35 This reduced interlayer distance resulted in SrGO/PVDF CEMs with a lower thickness than their SGO counterparts.…”

2D Material Nanofiltration Membranes: From Fundamental Understandings to Rational Design

“…Recently, considerable research efforts have been taken on the development of 2D material NF membranes for water purification, due to their tunable pore size, geometry and chemistry, good chemical resistance, sufficient mechanical strength, and superior antifouling properties. [ 13 ] The attempted 2D materials include graphene, [ 14 ] graphene oxides (GOs), [ 15 ] reduced graphene oxides (rGOs), [ 16 ] MXenes, [ 17 ] layered transition metal dichalcogenide (TMDCs), [ 18 ] 2D metal‐organic frameworks (MOFs), [ 19 ] 2D covalent organic frameworks (COFs), [ 20 ] etc. More importantly, the quickly expanding 2D material categories [ 21 ] provide us with a diversified materials platform for the maneuver of sub‐2‐nanometer pores to optimize the balance mentioned above for NF membranes.…”