Graphene-based Membranes Research Articles

A Novel Fabrication Approach for Multifunctional Graphene-based Thin Film Nano-composite Membranes with Enhanced Desalination and Antibacterial Characteristics

A practical fabrication technique is presented to tackle the trade-off between the water flux and salt rejection of thin film composite (TFC) reverse osmosis (RO) membranes through controlled creation of a thinner active selective polyamide (PA) layer. The new thin film nano-composite (TFNC) RO membranes were synthesized with multifunctional poly tannic acid-functionalized graphene oxide nanosheets (pTA-f-GO) embedded in its PA thin active layer, which is produced through interfacial polymerization. The incorporation of pTA-f-GOL into the fabricated TFNC membranes resulted in a thinner PA layer with lower roughness and higher hydrophilicity compared to pristine membrane. These properties enhanced both the membrane water flux (improved by 40%) and salt rejection (increased by 8%) of the TFNC membrane. Furthermore, the incorporation of biocidal pTA-f-GO nanosheets into the PA active layer contributed to improving the antibacterial properties by 80%, compared to pristine membrane. The fabrication of the pTA-f-GO nanosheets embedded in the PA layer presented in this study is a very practical, scalable and generic process that can potentially be applied in different types of separation membranes resulting in less energy consumption, increased cost-efficiency and improved performance.

3D nanoporous crystals enabled 2D channels in graphene membrane with enhanced water purification performance

Two-dimensional (2D) materials have been received increasing attention in various fields such as physics, material science, chemistry and engineering. In particular, the graphene-based membrane is an emerging subject mainly due to the atomic thickness, simple processing and the compatibility with other materials. In the case of water treatment, stable graphene-based laminar structures as well as high separation performance are pursued. In this work, nanoporous crystals embedded graphene laminate membranes was reported for water purification. Reduced graphene oxide (rGO) nanosheets obtained from a solution chemical process act as building blocks to construct the 2D channels through a pressure-driven filtration process. By incorporating three-dimensional (3D) nanoporous crystals with sub-nano sized aperture size into the 2D graphene laminates, both the inter-layer spacing and numbers of nanofluidic channels are increased, leading to greatly-enhanced water separation performance. The optimized 3D/2D membranes exhibit 15 times higher water permeability than that of the rGO membrane with similar high dye retention rate. The significance of such 3D nanoporous structure and transport mechanism through the 3D/2D membranes is systematically studied. This general approach of enhancing the molecular transport through 2D nanofluidic channels proposed here may also find application in gas separation and battery membranes.

İyonik Polimer Eyleyicilerde Kullanılan Ag katkılı Grafen-esaslı Elektrot Membranların Özelliklerine İndirgenme Yönteminin Etkisi

İyonik Polimer Eyleyicilerde Kullanılan Ag katkılı Grafen-esaslı Elektrot Membranların Özelliklerine İndirgenme Yönteminin Etkisi

A porous graphene composite membrane intercalated by halloysite nanotubes for efficient dye desalination

Porous reduced graphene oxide (PRGO) was obtained by making pores on graphene oxide layers. Halloysite nanotubes, modified by Poly (sodium-p-styrenesulfonate) (PSS), were used to enlarge the interlayer spacing of PRGO. These two materials were immobilized on membrane surface through a time-saving, facile solvent evaporation method. The sandwich structure, formed through solvent evaporation process, provided additionally continuous pathway for water and salts, thus improving the water permeability of the composite membranes. Comprehensive characterizations of the materials were characterized by FT-IR, TEM, Raman, EDS and XRD. The composite membranes were characterized by SEM, FT-IR and EDS. The composite membranes exhibited a higher separation effect for salts and dye (i.e. 4.7% for MgSO4, 4.7% for MgCl2, 6.8% for NaCl, and 14.3% for Na2SO4; up to 97.9% for Reactive Black 5). The pure water permeation of composite membranes could reach as high as 8.8L/(m2hbar). Hence, the graphene-based hybrid membranes presented a potential application in separation for salts and dyes.

Pressure-driven water permeation through multilayer graphene nanosheets

Graphene-based membranes have been shown to exhibit excellent potential in transporting water and separating ions and contaminants. The pressure-driven water permeation through a graphene (GE) multilayer containing slit-like gaps in each layer is investigated using molecular dynamics simulations in the present study. The effects of the driving pressure magnitude, as well as the multilayer configuration parameters including the number of GE layers, their porous ratio, the gap width and offset, the interlayer distance, and the hydrogenation of the gaps, are studied. The results show that the permeate flux is highly dependent on the number and the porous ratio of the GE layers, but is almost insensitive to the number of gaps at the same porous ratio as long as the gap width is large enough to allow water permeation. A counter-intuitive dependence of the water flow rate on the offset distance is found that the former increases as the later increases. When the gaps are hydrogenated, the energy barrier for the water molecules entering the nanochannel is lowered, leading to much higher water flow rate across the membrane. This study is helpful for the application of GE-based membranes for desalination and nanofiltration.

Graphene-based membranes for molecular and ionic separations in aqueous environments

Abstract Graphene-based laminar materials open up to new applications for molecular and ionic separations in aqueous environments due to the atomic thickness, mechanical strength, chemical stability and other fantastic properties. Recent advances on controlling the structure and chemical functionality of graphene-based membranes can potentially lead to new classes of tools for desalination, dehydration, toxicant rejection, specific ionic separation and so on. The recent developments of graphene-based membranes prepared by using a concept to form interlayer space between graphene sheets and creating nanoscale or sub-nanoscale pores in a graphene lattice, together with their mass-transfer mechanisms and potential applications in aqueous environments are reviewed. A summary and outlook is further provided on the opportunities and challenges in this arising field. This article is expected to address the intricate details of mass transport through two distinct graphene-based membranes in aqueous environment and to optimize the fabrication of graphene-based membranes as a fascinating separation system for a wide range of applications.

Current Status and Perspectives of Graphene-based Membranes for Gas Separation

Current Status and Perspectives of Graphene-based Membranes for Gas Separation

Breathable Vapor Toxicant Barriers Based on Multilayer Graphene Oxide.

There is tremendous interest in graphene-based membranes as protective molecular barriers or molecular sieves for separation technologies. Graphene oxide (GO) films in the dry state are known to be effective barriers for molecular transport and to expand in the presence of moisture to create enlarged intersheet gallery spaces that allow rapid water permeation. Here we explore an application for GO membranes as water-breathable barrier layers for personal protective equipment, which are designed to allow outward perspiration while protecting the wearer from chemical toxicants or biochemical agents in the local environment. A device was developed to measure permeation rates of small-molecular toxicants in the presence of counter-current water flow simulating active perspiration. The technique was applied to trichloroethylene (TCE) and benzene, which are important environmental toxicants, and ethanol as a limiting case to model very small, highly water-soluble organic molecules. Submicron GO membranes are shown to be effective TCE barriers, both in the presence and absence of simulated perspiration flux, and to outperform current barrier technologies. A molecular transport model is developed, which suggests the limited toxicant back-permeation observed occurs not by diffusion against the convective perspiration flow in hydrophobic channels, but rather through oxidized domains where hydrogen-bonding produces a near-stagnant water phase. Benzene and ethanol permeation fluxes are higher than those for TCE, likely reflecting the effects of higher water solubility and smaller minimum molecular dimension. Overall, GO films have high water breathability relative to competing technologies and are known to exclude most classes of target toxicants, including particles, bacteria, viruses, and macromolecules. The present results show good barrier performance for some very small-molecule species, but not others, with permeation being favored by high water solubility and small minimum molecular dimension.

Impact of Surface Ionization on Water Transport and Salt Leakage through Graphene Oxide Membranes

Nanopores in graphene-based membranes have shown great potential in enabling highly efficient water desalination. The effective engineering of these membranes necessitates a fundamental understanding of how the physical structure and chemical details of their nanopores govern their desalination performance. Using molecular dynamics simulations, we investigate how the ionization of the functional groups along the perimeter of the nanopores in single-layer graphene oxide membranes affects water transport and salt leakage. Moderate ionization enhances water transport and leads to higher salt leakage. With further ionization of the functionalization groups, water flux through the pore is reduced while the salt leakage shows little change. The significant role of functionalization groups’ ionization in modulating the water flux and salt leakage through the nanopores is attributed to the different organizations of water molecules and ions in and near the pore as the pore becomes charged due to ionization.

Graphene-based membranes with limited swelling sieve common salts out of salty water

Researchers in the UK at the University of Manchester report that they have developed a simple scalable method for obtaining graphene-based membranes with limited swelling when they are exposed to water. They say the pore size in the membranes can be precisely controlled which enables them to sieve common salts out of salty water – making it safe to drink.

Cross-linking to prepare composite graphene oxide-framework membranes with high-flux for dyes and heavy metal ions removal

Although graphene oxide framework-based membranes separation represent an effective and potential process in water purification and desalination, instability and low flux of the membrane make them still challenging in industrial application. Herein, isophorone diisocyanate (IPDI) was selected as cross-linking for covalent modification of graphene oxide nanosheets to design the newly graphene oxide-framework membranes (GOF) by a facile vacuum-assisted filtration method. The cross-linked GO-IPDI membrane can be used to effectively remove dyes and heavy metals via filtration. Results of filtration experiments and microstructure characterization confirmed that IPDI cross-linking GO not only enhanced structural stability but also enlarged the nano-channels among GO sheets for higher water permeability. The GO-IPDI membrane exhibited a high flux of 80–100Lm−2h−1bar−1 under an extremely low external pressure (1.0bar). It exhibited a high rejection rate (above 96%) for dyes including methylene blue, methylene orange and rhodamine-B and congo red. While the retention was moderate for Pb2+, Cu2+, Cd2+, Cr3+ ranging from 40% to 70%. These results demonstrated the potential application of GO-IPDI in wastewater and provided a novelty way of design and fabrication of high water permeability and excellent separation performance of graphene-based membranes product.

Electrokinetic Energy Conversion in Self-Assembled 2D Nanofluidic Channels with Janus Nanobuilding Blocks.

Inspired by the microstructure of nacre, material design, and large-scale integration of artificial nanofluidic devices step into a completely new stage, termed 2D nanofluidics, in which mass and charge transportation are confined in the interstitial space between reconstructed 2D nanomaterials. However, all the existing 2D nanofluidic systems are reconstituted from homogeneous nanobuilding blocks. Herein, this paper reports the bottom-up construction of 2D nanofluidic materials with kaolinite-based Janus nanobuilding blocks, and demonstrates two types of electrokinetic energy conversion through the network of 2D nanochannels. Being different from previous 2D nanofluidic systems, two distinct types of sub-nanometer- and nanometer-wide fluidic channels of about 6.8 and 13.8 Å are identified in the reconstructed kaolinite membranes (RKM), showing prominent surface-governed ion transport behaviors and nearly perfect cation-selectivity. The RKMs exhibit superior capability in osmotic and hydraulic energy conversion, compared to graphene-based membranes. The mineral-based 2D nanofluidic system opens up a new avenue to self-assemble asymmetric 2D nanomaterials for energy, environmental, and healthcare applications.

Tunable sieving of ions using graphene oxide membranes.

Graphene oxide membranes show exceptional molecular permeation properties, with promise for many applications. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of ∼9 Å (ref.4), which is larger than the diameters of hydrated ions of common salts. The cutoff is determined by the interlayer spacing (d) of ∼13.5 Å, typical for graphene oxide laminates that swell in water. Achieving smaller d for the laminates immersed in water has proved to be a challenge. Here, we describe how to control d by physical confinement and achieve accurate and tunable ion sieving. Membranes with d from ∼9.8 Å to 6.4 Å are demonstrated, providing a sieve size smaller than the diameters of hydrated ions. In this regime, ion permeation is found to be thermally activated with energy barriers of ∼10-100 kJ mol-1 depending on d. Importantly, permeation rates decrease exponentially with decreasing sieve size but water transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for the entry of water molecules and large slip lengths inside graphene capillaries. Building on these findings, we demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl.

Solution-assisted ultrafast transfer of graphene-based thin films for solar cells and humidity sensors

Vacuum filtration enables the fabrication of large-area graphene-based membranes (GBMs), possessing a smoother surface than that by spray, spin coating or drop casting. However, due to the strong interaction with substrates, the separation of thin GBMs from the filter is problematic. Conventional stamping separation/transfer of graphene oxide (GO) thin films requires another substrate and pressing for >10 h, which may damage the delicate structure of the transfer substrates. Other methods require GO to be reduced on filters before separation, thus limiting the reduction methods. Inspired by a coagulation bath that enables rapid formation of ultrastrong GO fibers, we present an ultrafast (<1 min) and solution-assisted strategy to fabricate smooth and freestanding GO films. The diverse interfacial energy of hydrogen bonds also demonstrates another reason for the successful separation. The film thickness ranges from 45 nm to several micrometers. When used as a composite of counter electrodes in dye sensitized solar cells, it showed higher (8.58%) power conversion efficiency than its spin-(7.71%) and spray-coated (8.07%) counterparts. It also showed promising performance in capacitive humidity sensors. The capacitance varied by three orders of magnitude in the range of the relative humidity of 15%–95%. Therefore the strategy realizes an ultrafast and high-quality film production which is suitable for various applications.

Recent developments in graphene-based/nanometal composite filter membranes

Significant achievements have been made on the development of next generation filtration and separation membranes using graphene materials, graphene-based membranes are promising in many areas such as membrane separation, water desalination.

Preparation of freestanding graphene-based laminar membrane for clean-water intake via forward osmosis process

Here, a thin freestanding graphene oxide based laminar membrane was prepared for clean-water intake through a forward osmosis process.

Graphene-based antimicrobial polymeric membranes: a review

Graphene-based nanocomposites have been increasingly used for the design of antimicrobial polymeric membranes due to enhanced antibacterial properties.

Fabrication of the reduced preoxidized graphene-based nanofiltration membranes with tunable porosity and good performance

The reduced preoxidized graphene membranes with in-plane porosity were prepared for studying the specific porosity changing with the water flux.

Simulation insight into water transport mechanisms through multilayer graphene-based membrane

Although single-layer nanoporous graphene has proven to be effective as a reverse osmosis desalination membrane, multilayer nanoporous graphene (MNPG) is economically affordable to be synthesized. In this study, water transport through large cylindrical (LC) and small cylindrical (SC), as well as through large hourglass-shaped (LHGS) pore structures constructed by MNPG is investigated via molecular dynamics (MD) simulations. It was found that the number of occupancy states increases with increasing pressure in LC pore, whereas they decrease with increasing pressure in SC pore. At P=2katm, water molecules must overcome a very large free energy barrier in SC pore owing to large entrance effects, suggesting a dramatic reduction in net flux. The LHGS pore structure is suggested to be a more efficient design for achieving higher flux, compared to other structures.It was found that the hydrophilicity effect could nearly double the flux inside LHGS pore, owing to the strong hydrogen bonds. Moreover, the mean square displacement (MSD) profile in a hydrophilic pore shows larger displacement than a hydrophobic one, which facilitates water filling mechanism. It also indicated that the layers with hydrophilicity effect increase water concentration in the area close to the surface of the layers owing to strong hydrogen bonds. It is concluded that osmotic permeability of water molecules increases substantially inside hydrophilic LHGS pore.

Controlled degrees of oxidation of nanoporous graphene filters for water purification using an aqueous arc discharge

Graphene-based nanofiltration membranes with different degrees of oxidation were produced using a novel aqueous arc discharge process for water purification applications. By adjusting the current level during the arc discharge the degree of oxidation was controlled from 28.1% at 1 A (Gr1A) to 53.9% at 4 A (Gr4A). Over 70% of the resulting graphene sheets had less than 10 layers. The effect of the degree of graphene oxidation on water purification was evaluated by monitoring pressure-driven water flux and the self-diffusion of ions across the membrane. The water flux through the graphene membranes was proportionally increased with pressure, which showed the highest water flux (362.3 L h−1 bar−1 m−2) at 31.09 kPa. Membrane thickness affected the self-diffusion rates of ions. Compared to a commercial polyamide membrane, the transported concentration of Na+ decreased by 67.7% (0.58 mM h−1) and 64.5% (0.55 mM h−1) for the Gr1A and Gr4A manufacturing processes, respectively where the membrane was 1120 nm thick. The ability to control the chemical and physical properties of the graphene membranes enhances their water purification performance and demonstrates their potential for use in practical water purification applications.