Layered Graphene Oxide Membranes Research Articles

Reduction-enhanced water flux through layered graphene oxide (GO) membranes stabilized with H3O+ and OH- ions.

Graphene oxide (GO) is one of the most promising candidates for next generation of atomically thin membranes. Nevertheless, one of the major issues for real world application of GO membranes is their undesirable swelling in an aqueous environment. Recently, we demonstrated that generation of H3O+ and OH- ions (e.g., with an external electric field) in the interlayer gallery could impart aqueous stability to the layered GO membranes (A. Gogoi, ACS Appl. Mater. Interfaces, 2022, 14, 34946). This, however, compromises the water flux through the membrane. In this study, we report on reducing the GO nanosheets as a solution to this issue. With the reduction of the GO nanosheets, the water flux through the layered GO membrane initially increases and then decreases again beyond a certain degree of reduction. Here, two key factors are at play. Firstly, the instability of the H-bond network between water molecules and the GO nanosheets, which increases the water flux. Secondly, the pore size reduction in the interlayer gallery of the membranes, which decreases the water flux. We also observe a significant improvement in the salt rejection of the membranes, due to the dissociation of water molecules in the interlayer gallery. In particular, for the case of 10% water dissociation, the water flux through the membranes can be enhanced without altering its selectivity. This is an encouraging observation as it breaks the traditional tradeoff between water flux and salt rejection of a membrane.

Cationic stabilized layered graphene oxide (GO) membrane for shale gas wastewater treatment: An atomistic insight

In recent years graphene oxide (GO) membranes have emerged as one of the most promising candidates for wastewater treatment. However, one major issue with layered GO membranes is their tendency to swell in an aqueous environment. Cation intercalation inside the interlayer gallery of layered GO membrane is one of the promising solutions to mitigate this problem. However, the effect of cationic concentration inside membrane channels on the mechanism of water permeance and salt rejection is yet to be investigated in detail. This study investigates shale gas wastewater (NaCl, CaCl2, and crude oil) treatment via nonequilibrium molecular dynamics (MD) simulations with cation (K+, Mg2+) intercalated GO membranes. The effect of cationic concentration on water permeance and salt rejection has been presented in detail. This study reveals that the presence of a higher concentration of cations inside the GO membrane induces higher water resistance in the membrane. In addition to the concentration of intercalated ions, interlayer distance also plays a significant role in determining water permeance. We observed that Mg2+ ion intercalated and K+ ion intercalated GO membranes maintain an interlayer distance of around 13.6 Å and 11.4 Å, respectively. As a result, Mg2+ ion intercalated GO membranes exhibit superior water permeance than K+ ion intercalated GO membranes. We also observed a difference in the crude oil aggregate formation (a composition of alkanes, cycloalkanes, and aromatics) in the two systems (more stable aggregate in K+ ion intercalated GO membrane system as compared to Mg2+ ion intercalated GO membrane system) during the forward osmosis (FO) process. This can be attributed to weaker interactions of K+ ions with the crude oil and lower water permeance of K+ ion intercalated GO membranes than Mg2+ ion intercalated GO membranes. This aggregate formation is an important factor for crude oil rejection. Moreover, aromatics are more concentrated at the crude oil-water interface than other hydrocarbons. On the other hand, more cations inside the GO membrane layers results in better Na+ rejection but decreased Cl− rejection.

Bioinspired graphene oxide nanofiltration membranes with ultrafast water transport and selectivity for water treatment

The weak interlayer interactions and poor mechanical stability of graphene oxide (GO) membranes pose significant challenges in any practical application environment. Moreover, the layered stacked GO membranes have low water permeability and low selectivity for small molecules due to their inherent nano-interlayer channels. In order to solve the problems of mechanical properties and size control of interlayer nanochannels, we synthesized GO-chitosan (GO-CS) composites with covalent bonds by a one-step hydrothermal method, and prepared a series of composite membranes by vacuum filtration. The GO-CS composite membrane resulted in a widened and defined two-dimensional (2D) channel due to the intercalation of GO-CS. The prepared GO-CS composite membrane has the highest water permeability of 107.4 L·m−2·h−1·bar−1, which is nearly 4 times that of the GO membrane. Almost complete of methylene blue (MnB), Congo red CR), rhodamine B (RB) dye molecules, with excellent molecular sieving ability. Furthermore, the presence of covalent bonds between GO and CS resulted in significantly enhanced membrane stability. We aim to provide potential applications for next-generation nanofiltration membranes by tuning the microstructure of the membranes.

Molecular simulation of layered GO membranes with amorphous structure for heavy metal ions separation

Currently, layered graphene oxide (GO) membranes have gained considerable attention due to their high separation efficiency in water treatment. However, a molecular-level separation mechanism remains elusive due to the complex irregular stacking feature of GO nanoscale flakes in actual GO membranes. Most existing molecular simulation studies using idealized GO membrane models might largely ignore the membrane structural heterogeneity. In this work, a molecular simulation protocol was applied to construct a non-idealized amorphous GO membrane model, and its structural morphology and property fit well with the realistic GO membranes. For the first time, the actual structure-oriented GO membrane model was used to study the separation behavior of heavy metal ions from aqueous solutions. The effects of the stacking structures and morphologies for the amorphous membranes on the water permeation and ion rejection were investigated. The constituent GO flakes with varied sheet sizes and oxidization degrees were considered. Our simulations demonstrate that the GO-based amorphous membranes with different-sized GO sheets can exhibit distinct ion rejection mechanisms and water penetration behavior. This finding has not been observed in previous idealized membrane models. Our simulation study provides novel insight into the separation mechanisms and improves the microscopic understanding of layered GO membranes.

Arresting Aqueous Swelling of Layered Graphene-Oxide Membranes with H3O+ and OH- Ions.

Over the past decade, graphene oxide (GO) has emerged as a promising membrane material with superior separation performance and intriguing mechanical/chemical stability. However, its practical implementation remains very challenging primarily because of its undesirable swelling in an aqueous environment. Here, we demonstrated that dissociation of water molecules into H3O+ and OH- ions inside the interlayer gallery of a layered GO membrane can strongly affect its stability and performance. We reveal that H3O+ and OH- ions form clusters inside the GO laminates that impede the permeance of water and salt ions through the membrane. Dynamics of those clusters is sensitive to an external ac electric field, which can be used to tailor the membrane performance. The presence of H3O+ and OH- ions also leads to increased stability of the hydrogen bond (H-bond) network among the water molecules and the GO layers, which further reduces water permeance through the membrane, while crucially imparting stability to the layered GO membrane against undesirable swelling.

Effect of oxygen-containing functional groups of layered graphene oxide membrane on the removal of amoxicillin: a molecular dynamics study

ABSTRACT The removal of amoxicillin (AMX) from the pharmaceutical industrial wastewater with a non-lamellar graphene oxide (GO) membrane has been investigated using non-equilibrium molecular dynamics (MD) simulations. The influence of oxygen-containing functional groups of the GO nanosheets on the separation of AMX molecules is investigated by considering three different membranes. These membranes are GO, reduced GO, and extended reduced GO which differ in the number of oxygen-containing functional groups on the GO nanosheets. AMX solution is used as the feed solution, and a mixture of and solution is used as a draw solution. This study reveals that the water and ion permeance depend on the number of oxygen-containing functional groups and pore sizes of the membranes. Reducing the number of oxygen-containing functional groups of the membranes leads to smaller pore sizes and lower water permeance. The retention and dynamics of AMX molecules reveal that the AMX molecules are likely to be retained on the basal plane of the membrane. This can be attributed to a strong interaction between hydroxyl and epoxy functional groups of the GO nanosheets with AMX molecules.

Layered graphene oxide membranes functioned by amino acids for efficient separation of metal ions

As a kind of heavy metal, lead ions (Pb2+) are a great threat to human health and the ecological environment. Due to their excellent properties, graphene and its derivatives are expected to be used for water purification. Amino acid functional groups were designed for graphene oxide membranes via dehydration condensation. The effects of the amino acid groups on the membrane performance for separating metal ions were investigated by the molecular dynamics. Under the premise that the water permeability is basically the same, the retention rate for Pb2+ by alanine groups with negatively charged terminals is 1.6 times of that by the carboxyl groups. The number-density map and the radial distribution function indicate that a large area of local high-density regions of Pb2+ will be formed around the negatively charged alanine groups. At the same time, the diffusion coefficient for Pb2+ and Na+ can be effectively reduced by the electrostatic attraction of the negative groups. It is concluded that functioned by the negatively charged alanine, the retention rate for metal ions by the layered graphene oxide membranes can be significantly improved at the cost of a little loss in water permeability.

Dehydration of acetic acid using layered graphene oxide (GO) membrane through forward osmosis (FO) process: a molecular dynamics study

ABSTRACT The applicability of layered graphene oxide (GO) membrane for dehydration of acetic acid using forward osmosis (FO) process is investigated with non-equilibrium molecular dynamics (MD) simulations. The performance of layered GO membrane with different pore offset distance (W) and interlayer distance (H) is investigated. The parameter W is a measure of the size of the constituting GO nanosheets. With the increase in W, the water permeance through layered GO membrane decreases and salt rejection increases. On the other hand, as H increases from 8 Å to 12 Å, the water permeance of the layered GO membrane increases and the corresponding salt rejection decreases. But as H increases beyond 12 Å, the water permeance through layered GO membrane decreases and salt rejection increases. The presence of pinhole defects on the GO nanosheets of layered GO membrane provides shorter route to the water molecules for trans-sheet flow which increases the water permeance of the membranes. With the increase in draw solution concentration the water permeance through the membrane increases but the corresponding variation in the draw solute rejection is much less (<2.0%). With the appropriate tuning of sizes of the GO nanosheets and interlayer spacings, layered GO membrane could be a suitable candidate for dehydration of acetic acid using FO process.

Layer-by-layer assembly of bio-inspired borate/graphene oxide membranes for dye removal

Dye wastewater is harmful to the ecological environment because of its potential biological toxicity, teratogenicity, carcinogenicity, and mutagenicity. We fabricated a layered graphene oxide (GO) membrane through layer-by-layer (LBL) self-assembly. We used borate to crosslink with GO on a polyethyleneimine (PEI)-coated hydrolyzed polyacrylonitrile (hPAN) support. Fourier transform infrared (FTIR) spectrometry, Raman spectra, and x-ray photoelectron spectroscopy (XPS) confirmed the presence of a crosslinking reaction. The dynamic thermomechanical analysis (DMA) results indicated that the introduction of borate can significantly improve the mechanical properties of the membrane. The Young’s modulus, ultimate tensile strength, and proportional limit of borate that was assembled twice as the outermost layer were increased by 110.81%, 62.37%, and 53.72%, respectively, as compared to those of a single-layered GO membrane. Moreover, the pure water fluxes of the layered GO membrane did not obviously decrease with an increase in the number of layers. The flux of the membrane with an outermost layer of borate was greater than that of the previous GO layer. The salt and dye rejection of the membranes was augmented with an increase in the number of layers. For the GO membrane assembled three times, rejection to methyl orange (MO), methylene blue (MB), NaCl, MgCl2, and MgSO4 reached 74.02%, 88.56%, 14.55%, 27.50%, and 41.95%, respectively. The use of borate as an inorganic crosslinker can avoid the environmental pollution caused by organic agents, and improve the mechanical properties as well as the filter capability of the layered GO membrane. Therefore, this study presents a novel method of membrane preparation for dye removal.

Molecular dynamics study on applying layered graphene oxide membranes for separating cadmium ions from water

Because cadmium can seriously damage human organs, we explored the prospect of using layered graphene oxide membranes to separate cadmium ions from water by molecular dynamics. The effects of three geometric parameters on membrane performance were investigated. The results indicate that the effects of the interlayer spacing and offset value on the salt rejection rate are much greater than the corresponding effects on water permeability. By contrast, for the gap size, comparable effects on the salt rejection rate and water permeability were observed. Then, the mechanism of water transport and ion migration were investigated. The number-density map revealed why the gap size has a more significant effect on water permeability. The radial distribution function and coordinate number indicate that Cd2+ is surrounded by two hydration shells. The second shell is more flexible so that it allows the loss of several water molecules to enable Cd2+ to pass through the membrane. Potential of mean force indicates the energy barriers for the migration of Cd2+ and Cl- on the surface of nanosheets are much larger than that for water molecules. The water permeability can reach up to 458 L/(m2⋅h⋅bar), which is much higher than the value for commercial membranes.

Electric-Field-Induced Ionic Sieving at Planar Graphene Oxide Heterojunctions for Miniaturized Water Desalination.

Layered graphene oxide membranes (GOMs) offer a unique platform for precise sieving of small ions and molecules due to controlled sub-nanometer-wide interlayer distance and versatile surface chemistry. Pristine and chemically modified GOMs effectively block organic dyes and nanoparticles, but fail to exclude smaller ions with hydrated diameters less than 9 Å. Toward sieving of small inorganic salt ions, a number of strategies are proposed by reducing the interlayer spacing down to merely several angstroms. However, one critical challenge for such compressed GOMs is the extremely low water flux (<0.1 Lm-2 h-1 bar-1 ) that prevents these innovative nanomaterials from being used in real-world applications. Here, a planar heterogeneous graphene oxide membrane (PHGOM) with both nearly perfect salt rejection and high water flux is reported. Horizontal ion transport through oppositely charged GO multilayer lateral heterojunction exhibits bi-unipolar transport behavior, blocking the conduction of both cations and anions. Assisted by a forward electric field, salt concentration is depleted in the near-neutral transition area of the PHGOM. In this situation, deionized water can be extracted from the depletion zone. Following this mechanism, a high rejection rate of 97.0% for NaCl and water flux of 1529 Lm-2 h-1 bar-1 at the outlet via an inverted T-shaped water extraction mode are achieved.

Influence of the presence of cations on the water and salt dynamics inside layered graphene oxide (GO) membranes

Although over the past few years, graphene oxide (GO) has emerged as a promising membrane material, the applicability of layered GO membranes in water purification/seawater desalination is still a challenging issue because of the undesirable swelling of GO laminates in the aqueous environment. One of the ways to tune the interlayer spacing and to arrest the undesirable swelling of layered GO membranes in the aqueous environment is to intercalate the interlayer spacing of the GO laminates with cations. Although the cation intercalation imparts stabilization to GO laminates in the aqueous environment, their effect on the performance of the membrane is yet to be addressed in detail. In the present study we have investigated the effect of cation intercalation on the performance of layered GO membranes using molecular dynamics simulation. For the same interlayer spacing, the cation intercalated layered GO membranes have a higher water flux as compared to the corresponding pristine layered GO membranes. In the presence of the cations, the water molecules inside the interlayer gallery get more compactly packed. The presence of the cations also increases the stability of the hydrogen bond network among the water molecules inside the membrane. This can be attributed to slow water reorientation dynamics inside the interlayer gallery in the presence of the cations. The synergistic effect of all these changes is that the water permeability through the cation intercalated layered GO membranes is higher as compared to that through the corresponding pristine layered GO membranes. On the other hand, the intercalation of the cations (K+, Mg2+) leads to higher rejection of Na+ ions whereas the rejection of Cl- ions slightly decreases.

Photo-induced ultrafast active ion transport through graphene oxide membranes

Layered graphene oxide membranes (GOM) with densely packed sub-nanometer-wide lamellar channels show exceptional ionic and molecular transport properties. Mass and charge transport in existing materials follows their concentration gradient, whereas attaining anti-gradient transport, also called active transport, remains a great challenge. Here, we demonstrate a coupled photon-electron-ion transport phenomenon through the GOM. Upon asymmetric light illumination, cations are able to move thermodynamically uphill over a broad range of concentrations, at rates much faster than that via simple diffusion. We propose, as a plausible mechanism, that light irradiation reduces the local electric potential on the GOM following a carrier diffusion mechanism. When the illumination is applied to an off-center position, an electric potential difference is built that can drive the transport of ionic species. We further develop photonic ion switches, photonic ion diodes, and photonic ion transistors as the fundamental elements for active ion sieving and artificial photosynthesis on synthetic nanofluidic circuits.

Accessing of graphene oxide (GO) nanofiltration membranes for microbial and fouling resistance

GO is of great interesting for membrane materials due to its unique properties including antimicrobial activity and antifouling property. However, the differences between GO nanosheets and layered GO membranes gained little attention. Herein, we fabricated GO membranes that have a thickness of around 30 nm and a void d-spacing of 0.67 nm (wet state) with single layer GO nanosheets of 10–75 nm in lateral size and investigated their antimicrobial and antifouling potential for practical use. It exhibits typical nanofiltration performance, with the rejections of 65.23%, 17.79%, 42.10% and 12.47% to Na2SO4, NaCl, Mg2SO4 and MgCl2, respectively. The antimicrobial test demonstrated that GO nanosheets lose their antimicrobial activity after they stacked together. A 14-day test showing that GO membrane has excellent resistance for organic fouling, with reversible fouling of more than 98%. But when it comes to tapwater, the proportion of irreversible fouling is surging to more than fifty percent even after cleaning with 2000 mg/L of sodium hypochlorite. Thus, we suggested that there are more details should be investigated before put layered GO membrane into practical use. Surprisingly, GO membrane exhibits excellent mechanical stability during the 14-day test, showing its exciting potential in water treatment.

Kinetic study of water transport in layered graphene oxide membranes

ABSTRACTLayered graphene oxide (GO) membranes are considered promising separation membranes because of their lamellar structure, strong mechanical strength and strong hydrophilic properties. However, there are a limited number of studies on the mechanism of water transport through a layered GO membrane. In this paper, by using a self-synthesized layered GO membrane, we developed and validated the first dynamic equation to describe water transport through a layered GO membrane. The results showed that the relationship between membrane flux and GO loading is an exponential function and that the water flux decreases with increasing GO loading. Then, the dynamic equation was applied to previously reported layered GO membranes to further verify its applicability. This kinetic study on layered GO membranes provides insight into the development of high-performance membranes with high permeability and selectivity.

Effect of graphene oxide (GO) nanosheet sizes, pinhole defects and non-ideal lamellar stacking on the performance of layered GO membranes: an atomistic investigation.

The effect of non-idealities, namely pinhole defects and non-ideal lamellar stacking of nanosheets, on the performance of size-differentiated graphene oxide (GO) laminates is investigated using equilibrium molecular dynamics (MD) simulations. With the increase in sizes of the constituent GO nanosheets the water permeability of the layered GO membranes decreases and salt rejection increases. But with the inclusion of non-idealities the difference in water permeability between these membranes substantially reduced. The pinholes on the GO nanosheets provide shorter routes for trans-sheet flow, thereby increasing the water permeability of the membranes. The non-ideal stacking of the nanosheets without pinhole defects results in slight reduction in water permeability because of blockage of permeation pathways inside the membranes. However, with pinhole defects non-ideal stacking becomes favorable for water permeation through the layered GO membranes; as this time the non-ideal stacking leads to formation of voids inside the membranes, which act as routes for shorter permeation pathways. The effect of these non-idealities is more significant for layered GO membranes composed of large GO nanosheets. Although the water permeability through the layered GO membrane is greatly enhanced because of these non-idealities (about 10 times), the corresponding variation in the salt rejection is very low (<2%).

Gated ion transport through layered graphene oxide membranes

The gate-induced directional ion transport in 2D layered materials provides a new way for effective control over the transport behaviors in synthetic systems.

Multilayer Graphene Oxide Membrane in Forward Osmosis: Molecular Insights

We investigate the performance of layered graphene oxide (GO) membrane in forward osmosis (FO) process for seawater desalination via nonequilibrium molecular dynamics (MD) simulations. A 0.56 M NaCl solution is used as the feed solution and a mixture of 1.0 M MgCl2 and 0.05 M Al2(SO4)3 solution is used as a draw solution. The effect of internal structure of the layered GO membrane on its performance as a FO membrane is investigated by considering three different membrane configurations which differ in their pore offset distance W. The dynamics of the permeating water and ions through the layered GO membrane is also investigated. Increase in W reduces the water permeability of the layered GO membrane and increases its salt rejection. One of the reason for this could be the blockage of permeate pathways through the layered GO membrane due to the movement of the GO nanosheets. This becomes more prominent with increasing W. Increasing W also leads to increase in distance traversed by the permeating water mole...

Gas permeation through double-layer graphene oxide membranes: The role of interlayer distance and pore offset

Hierarchically stacked graphene oxide (GO) membranes have sparked considerable interest due to their prominent separation performance; however, the underlying separation mechanisms remain elusive. In this study, we conducted molecular dynamics (MD) simulations to explore the role of interlayer distance and pore offset in gas (H2, CH4, N2 and CO2) permeation through double-layer GO membranes. Gas permeance is found to increase with the interlayer distance and pore offset until the interlayer distance exceeds a critical value. With elongating the interlayer distance and pore offset, a sieving effect occurs to overcome preferential adsorption and dominates the transport in mixed H2/CO2, resulting in selective permeation shifting from CO2 to H2. This simulation study provides mechanistic insight into gas permeation through layered GO membranes, and would facilitate the design of new GO membranes for high-performance gas separation.

Water and salt dynamics in multilayer graphene oxide (GO) membrane: Role of lateral sheet dimensions

Dependence of salt rejection efficiency and water permeability of layered graphene oxide (GO) membranes on the lateral dimension of constituting sheets are studied through equilibrium molecular dynamic (MD) simulation and experiments. This study suggests that with increasing sheets dimension permeability of the GO membranes decreases but its selectivity increases. The velocity and permeation time of the water molecules while permeating through the membrane are influenced to a greater extent by the pore offset distance (W) of the membranes. More over the larger pore offset distance increases the path length that the water molecules and ions have to traverse for permeating through the layered GO membranes. Based on the simple technique discussed in this work, one can construct GO membranes of required water permeability and salt rejection without the application of any foreign nanomaterials with the GO membrane, which retains the inherent selectivity of the GO membranes. This work also provides the effect of internal structure of GO membrane on the atomistic level details of the solvation shell of ions while they are permeating through the membrane.