Ultrathin graphene-based membrane with precise molecular sieving and ultrafast solvent permeation (1710.00047v1)

Abstract: Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation rates. However, the membranes' use has been limited mostly to aqueous solutions because GO membranes appear to be impermeable to organic solvents, a phenomenon not fully understood yet. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from flakes with large sizes of ~ 10-20 micron. Without sacrificing their sieving characteristics, such membranes can be made exceptionally thin, down to ~ 10 nm, which translates into fast permeation of not only water but also organic solvents. We attribute the organic solvent permeation and sieving properties of ultrathin GO laminates to the presence of randomly distributed pinholes that are interconnected by short graphene channels with a width of 1 nm. With increasing the membrane thickness, the organic solvent permeation rates decay exponentially but water continues to permeate fast, in agreement with previous reports. The application potential of our ultrathin laminates for organic-solvent nanofiltration is demonstrated by showing >99.9% rejection of various organic dyes with small molecular weights dissolved in methanol. Our work significantly expands possibilities for the use of GO membranes in purification, filtration and related technologies.

• The paper introduces highly laminated graphene oxide membranes fabricated from large GO flakes (10–20 µm) that ensure precise molecular sieving in ultrathin (8–10 nm) configurations.
• It demonstrates exceptional methanol permeance of ~7 L·m⁻²·h⁻¹·bar⁻¹ with over 99.9% rejection of organic dyes, outperforming conventional membranes.
• The study reveals that membrane thickness critically influences performance, and suggests cation-crosslinking as a promising strategy to further enhance solvent transport.

Ultrathin Graphene-Based Membranes for Organic Solvent Nanofiltration

This paper presents a paper focused on the fabrication and evaluation of ultrathin graphene oxide (GO) membranes, which exhibit precise molecular sieving properties coupled with ultrafast organic solvent permeation. The research addresses the challenge of organic solvent impermeability in conventional GO membranes by innovating a new membrane structure utilizing large graphene flakes. This novel laminar structure offers promising potential for applications in organic solvent nanofiltration (OSN).

The researchers introduce the concept of highly laminated graphene oxide (HLGO) membranes prepared from large GO flakes with lateral dimensions resembling 10-20 µm, compared to the conventional GO (CGO) membranes which utilize significantly smaller flakes. This alteration in flake size appears pivotal, contributing to a highly laminated structure which is crucial in maintaining an effective and smooth two-dimensional (2D) capillary network. These membranes demonstrate exceptional thinness—approximately 8-10 nm—enabling rapid permeation rates while retaining molecular sieving precision.

The paper provides strong numerical evidence of the permeance enhancement, notably highlighting the ability of HLGO membranes to achieve a methanol permeance of approximately 7 L·m^-2·h^-1·bar^-1 with over 99.9% rejection of organic dyes. This is notably higher compared to the existing polymeric membranes. The permeance is convincingly shown to correlate linearly with the inverse viscosity of the solvents, which underscores the viscosity-driven nature of solvent transport through the laminates.

A critical observation is that these ultrathin HLGO membranes only preserve effective molecular sieving attributes and high permeance rates up to a thickness of approximately 70 nm. For layers beyond this thickness, organic solvent permeation decreases exponentially, suggesting a shift in the dominant permeation pathway from through-pinholes to interlayer diffusion. In contrast, water exhibits an exponential decay transitioning to a linear dependence with thickness, implying possible slip-enhanced flow through graphene capillaries, a phenomenon less pronounced for organic solvents—and consistent with known differences in graphene’s interaction with hydrocarbons versus water.

Theoretical and practical implications of this work are profound. Firstly, the demonstrated capabilities of HLGO membranes significantly expand the operational regimen of graphene-based membranes into organic systems, overcoming previous limitations of organic solvent impermeability. Secondly, this work provides an improved understanding of molecular transport in graphene-based systems, positing pinholes organized into networks as key routes in ultrathin configurations.

The paper further proposes the use of cation-crosslinking as a strategy to enhance membrane performance without sacrificing solute rejection. Experimental attempts using magnesium ions to introduce randomness in the laminar structure suggest potential directions for future research. The noted stability of HLGO membranes in various solvent environments also adds to their appeal for durable, long-term applications.

The paper’s findings are evidently promising for advancing OSN technology, and its potential cross-industry applications cannot be overstated, notably in pharmaceuticals and petrochemicals where efficient separation processes are critical. Future research may explore optimizing crosslinking strategies, exploring other exfoliation techniques, and evaluating membrane performance under industrial conditions. The exploration of these ultrathin GO membranes signals a meaningful progression in membrane technology, particularly within the field of molecular separation applications.