Lamellar Graphene Oxide Membranes Research Articles

Comparative Retention Analysis of Intercalated Cations Inside the Interlayer Gallery of Lamellar and Nonlamellar Graphene Oxide Membranes in Reverse Osmosis Process: A Molecular Dynamics Study.

Over the past decade, multilayered graphene oxide (GO) membranes have emerged as promising candidates for desalination applications. Despite their potential, a comprehensive understanding of separation mechanisms remains elusive due to the intricate morphology and structural arrangement of interlayer galleries. Moreover, a critical concern of multilayered GO membranes is their susceptibility to swelling within aqueous environments, which hinders their practical implementation. Therefore, this study introduces cation intercalation within GO laminates to elucidate the underlying factors governing swelling behavior and subsequently mitigate it. Moreover, this study performed nonequilibrium molecular dynamics simulations on the cation (Mg2+ or K+)-intercalated lamellar and nonlamellar GO membranes to understand the effect of the arrangement of GO sheets on the retention time of intercalated cations within GO layers, water permeance, and salt rejection mechanism in the reverse osmosis process using cation-intercalated GO membranes. Our results highlight that lamellar GO membranes exhibit higher water permeance, attributed to their well-defined interlayer gallery structure. On the other hand, nonlamellar GO membranes display superior salt rejection due to their complex interlayer gallery structure that impedes salt permeation. Moreover, the structural complexity of nonlamellar GO membranes contributes to greater stability by retention of the more intercalated cations for a longer time within the layers. Furthermore, it is observed that a higher percentage of Mg2+ cations remained inside the GO laminates as compared to K+ cations, hence resulting in the greater stability of the Mg2+-intercalated GO membrane in the aqueous environment.

Tunable nano-wrinked channels of reduced graphene oxide membranes for molecular sieving gas separation

Graphene oxide (GO) lamellar membranes have exhibited excellent performance in watertreatment, however there has been far less progress in the application of gas purification. Actually, exposure of the underlying graphitic framework of GO basically determines the gas permeableness of the GO lamellar membranes, which is usually neglected in the fabrication of GO lamellar membranes. Herein, we report nano-wrinkled lamellar reduced GO (rGO) sheets as gas separation membranes with oxygen-rich domains as face-to-face spacer for CO2 blockage and oxygen-poor domains for fast H2 permeation. Thanks to the excellent size-sieving mechanism of the nano-wrinkled channels, the rGO membranes exhibit high H2 permeance of 6.12 × 10−7 mol·m−2·s−1·pa−1 with exceptional selectivity of 529.9 for H2/CO2 gas mixture, surpassing the state-of-the-art ones. The membranes also show excellent stability in separation performance for H2/CO2 gas mixture under a long-term operation. Therefore, such lamellar membranes with well-tuned reduced GO nanosheets hold great potential in gas separation applications.

Functionalized Graphene Oxide-Based Lamellar Membranes with Tunable Nanochannels for Ionic and Molecular Separation.

Graphene oxide (GO)-based membranes with tunable microstructure and controlled nanochannels have attracted an increasing interest for various applications in wastewater treatment, desalination, gas separation, organic nanofiltration, etc. However, they showed limited use in water desalination due to their lower stability and separation efficiency. In this work, a class of two-dimensional (2D) GO lamellar membranes have been prepared with controlled pores for efficient and fast separation of ions and dye molecules. The GO membranes are fucntionalized with a star-like 6-armed poly(ethylene oxide) using the simple amidation route under mild conditions. The as-prepared covalently cross-linked networks are chemically steady in aqueous medium and show remarkable selectivity (∼100%) for several probe molecules and 10–100 higher permeance than those of the reported GO-based membranes. Further, such membranes are also used for salt separation and show more than 80% rejection for Pb2+ and Ni2+ salts. Moreover, a 1360 nm-thick membrane shows >99% rejection for NaCl with a good water permeance of up to 120 L m–2 h–1 bar–1. Additionally, these membranes are stable for more than 20 days under different conditions.

Two-Dimensional Membranes with Highly Charged Nanochannels for Osmotic Energy Conversion.

Inadequate mass transportation of semipermeable membranes causes poor osmotic energy conversion from salinity-gradient. Here, the lamellar graphene oxide membranes (GOMs) constructed with numerous fusiform-like nanochannels, that are pre-filled with negatively charged polyanion electrolytes, to both enhance the ion permeability and ion selectivity of the membrane for energy harvest from the salinty gradient, were developed. The as-prepared membrane achieved the maximum output power density of ∼4.94 W m-2 under a 50 fold salinity gradient, which is 3.5 fold higher than that of pristine GOM. The enhancement could be ascribed to the synergistic impact of the expanded nanochannels and the enhanced space charge density. Via feeding with the artificial salinity water and monovalent cation electrolytes, the system could realise the power output up to 14.7 W m-2 and 34.1 W m-2 , respectively. Overall, this material design strategy could provide an alternative concept to effectively enhance ion transport of other two-dimensional (2D) membranes for specific purposes.

Understanding the effect of hydroxyl/epoxy group on water desalination through lamellar graphene oxide membranes via molecular dynamics simulation

The lamellar graphene oxide (GO) membranes could offer well-defined 2D nanochannels for fast water permeation and excellent ions rejection. In this work, lamellar GO membranes were constructed by stacking GO nanosheets layer-by-layer via molecular simulation. The effect of hydroxyl and epoxy groups as well as their content on the desalination performance in lamellar GO membranes were investigated by molecular dynamics simulation. The transport and separation mechanisms of water molecules and ions passing through the lamellar GO membranes with different functional group were also revealed from thermodynamic and hydrogen bond network. It is found that water flux in GO membranes would be influenced by the order of water molecules and the hydrogen network formed between functional groups and water molecules. GOOH membranes showed a higher Na+ rejection rate than Cl− at high hydroxyl contents.

Non-Continuum Intercalated Water Diffusion Explains Fast Permeation through Graphene Oxide Membranes

Recent experimental studies have revealed unconventional phase and transport behaviors of water confined within lamellar graphene oxide membranes, which hold great promise not only in improving our current understanding of nanoconfined water but also in developing high-performance filtration and separation applications. In this work, we explore molecular structures and diffusive dynamics of water intercalated between graphene or graphene oxide sheets. We identify the monolayer structured water between graphene sheets at temperature T below Tc = ∼315 K and an interlayer distance d = 0.65 nm, which is absent as the sheets are oxidized. The non-continuum collective diffusion of water intercalation between graphene layers facilitates fast molecular transport due to reduced wall friction. This solid-like structural order of intercalated water is disturbed as T or d increases to a critical value, with abnormal declines in the coefficients of collective diffusion. Based on a patched model of graphene oxide sheets consisting of spatially distributed pristine and oxidized regions, we conclude that the non-continuum collective diffusion of intercalated water can explain fast water permeation through graphene oxide membranes as reported in recent experimental studies, in stark contrast to the conventional picture of pressure-driven continuum flow with boundary slip, which has been widely adopted in literature but may apply only at high humidity or in the fully hydrated conditions.

Intrinsic high water/ion selectivity of graphene oxide lamellar membranes in concentration gradient-driven diffusion.

Although graphene oxide lamellar membranes (GOLMs) are effective in blocking large organic molecules and nanoparticles for nanofiltration and ultrafiltration, water desalination with GOLM is challenging, with seriously controversial results. Here, a combined experimental and molecular dynamics simulation study shows that intrinsic high water/ion selectivity of GOLM was achieved in concentration gradient-driven diffusion, showing great promise in water desalination. However, in pressure-driven filtration the salt rejection was poor. This study unveils a long-overlooked reason behind the controversy in water desalination with GOLM and further provides a fundamental understanding on the in-depth mechanism concerning the strong correlation of water/ion selectivity with the applied pressure and GO nanochannel length. Our calculations and experiments show that the applied pressure weakened the water-ion interactions in GO nanochannels and reduced their permeation selectivity, while the length of nanochannels dominated the mass transport processes and the ion selectivity. The new insights presented here may open up new opportunities for the optimization of GOLMs in this challenging area.

Salt concentration, pH and pressure controlled separation of small molecules through lamellar graphene oxide membranes

For the first time, pressure, salt concentration and pH demonstrated advantages for tuning the nanochannels within lamellar graphene oxide (LGO) membranes to control the separation of small molecules. This provides a new avenue for designing and engineering efficient LGO membranes for molecular separation.