Porous Graphene Membranes Research Articles

Permeability of gas molecules through sub-nanometer nitrogen-terminated porous graphene membranes: A DFT study

We present a systematic theoretical investigation of gas permeability through single-layer nanoporous graphene membranes with N-terminated (pyridinic and mixed pyridinic/pyrrolic) pores for He, H2, N2, O2, CO, CO2, H2O, and CH4. Our study is based on density functional theory transition-state calculations and the kinetic theory of gasses. We systematically evaluate pores of varying sizes and shapes up to 5.7 Å in diameter. According to our findings, membranes with pores in the size regime (4.5 Å - 5.0 Å) may exhibit industrially acceptable permeance to several gas molecules and advanced selectivity. Notably, we found that, among a few others, a pore with the same topological features at the pore boundary as those of the characteristic pore of g-C3N4 carbon nitride, is particularly promising. In addition, membranes with larger pores, ∼5.5 Å - 5.7 Å, can effectively separate CH4 from the other molecules. Our findings support the advanced potential of single-layer graphene membranes with nitrogen-terminated sub-nanometer pores, for gas separation applications.

2D Carbonaceous Materials for Molecular Transport and Functional Interfaces: Simulations and Insights.

ConspectusCarbon-based two-dimensional (2D) functional materials exhibit potential across a wide spectrum of applications from chemical separations to catalysis and energy storage and conversion. In this Account, we focus on recent advances in the manipulation of 2D carbonaceous materials and their composites through computational design and simulations to address how the precise control over material structure at the atomic level correlates with enhanced functional properties such as gas permeation, selectivity, membrane transport, and charge storage. We highlight several key concepts in the computational design and tuning of 2D structures, such as controlled stacking, ion gating, interlayer pillaring, and heterostructure charge transfer.The process of creating and adjusting pores within graphene sheets is vital for effective molecular separation. Simulations show the power of controlling the offset distance between layers of porous graphene in precisely regulating the pore size to enhance gas separation and entropic selectivity. This strategy of controlled stacking extends beyond graphene to include covalent organic frameworks (COFs) such as covalent triazine frameworks (CTFs). Experimental assembly of the layers has been achieved through electrostatic interactions, thermal transformation, and control of side chain interactions.Graphene can interface with ionic liquids in various forms to enhance its functionality. A computational proof-of-concept showcases an ion-gating concept in which the interaction of anions with the pores in graphene allows the anions to dynamically gate the pores for selective gas transport. Realization of the concept has been achieved in both porous graphene and carbon molecular sieve membranes. Ionic liquids can also intercalate between graphene layers to form interlayer pillaring structures, opening the slit space. Grand canonical Monte Carlo simulations show that these structures can be used for efficient gas capture and separation. Experiments have demonstrated that the interlayer space can be tuned by the density of the pillars and that, when fully filled with ionic liquids and forming a confined interface structure, the graphene oxide membrane achieves much higher selectivity for gas separations. Moreover, graphene can interface with other 2D materials to form heterostructures where interfacial charge transfers take place and impact the function. Both ion transport and charge storage are influenced by both the local electric field and chemical interactions.Fullerene can be used as a building block and covalently linked together to construct a new type of 2D carbon material beyond a one-atom-thin layer that also has long-range-ordered subnanometer pores. The interstitial sites among fullerenes form funnel-shaped pores of 2.0-3.3 Å depending on the crystalline phase. The quasi-tetragonal phases are shown by molecular dynamics simulations to be efficient for H2 separation. In addition, defects such as fullerene vacancies can be introduced to create larger pores for the separation of organic solvents.In conclusion, the key to imputing functions to 2D carbonaceous materials is to create new interactions and interfaces and to go beyond a single-atom layer. First-principles and molecular simulations can further guide the discovery of new 2D carbonaceous materials and interfaces and provide atomistic insights into their functions.

Conformational interlocking induced construction of hierarchically porous graphene membrane for ultrahigh-pressure sensing

Based on microstructure engineering, graphene oxide (GO)-based piezoresistive pressure sensors have attracted increasing attention due to its amazing sensitivities, but the point-to-point contact mechanism among nanosheets compromises their high-pressure monitoring capabilities. Inspired by the multiple conformations of GO nanosheets, we utilize π-π conjugated interlayer interactions to lock the 3D crumpled conformation of nanosheets, resulting in controllable assembly into a flexible graphene membrane with hierarchically porous structures (PGM). These designed structures include microscale pores formed by the loose stacking of 3D crumpled nanosheets and nanoscale pores supported by densely packed crumples, reaching up to 100 nm in height. The PGM flexible sensor exhibits an unprecedented ultrahigh-pressure response of up to 2000 kPa, with high sensitivities of 1.1 and 0.7 kPa−1 in the pressure ranges of 1–600 kPa and 600–2000 kPa, respectively, and demonstrates over 10,000 cycles of high-pressure stability. Moreover, the PGM flexible sensors enable real-time monitoring of pressure vessel inflation/deflation processes and human activities such as finger, elbow, and knee bending, as well as walking, running, and jumping. This hierarchically porous structure provides a promising avenue for ultra-wide-range and ultra-high-pressure sensing devices, with potential applications in structural health monitoring, personal healthcare, and medical diagnostics.

Molecular dynamics investigation on seawater desalination mechanism driven by external pressure through porous graphene membranes

The seawater desalination through porous graphene membrane is considered as promising candidate technology for water purification, but the desalination mechanism is seldom concerned at nanoscale. In this study, the microstructures of pressure driven seawater desalination based on porous graphene membrane was established, and the desalination performance was studied by means of molecular dynamics simulations. Based on the observed dynamic evolution of the water molecules transportation behaviors and microstructures, the salt rejection and water permeation mechanism were proposed. The effects of graphene pore diameter, external pressure and temperature on the desalination performance were investigated. The results indicate that the water flux can be increased by 67.20% when the external pressure increases from 100 MPa to 200 MPa. Under the external pressure of 300 MPa, when the diameter of graphene pore increased from 1 nm to 2 nm and 3 nm, the number of filtered water molecule is increased by 48.83% and 67.33%, respectively. The filtered water molecules can be increased by 4.06% with the increase of temperature from 300 K to 360 K. The possible desalination mechanism was explored and revealed by us, which is expected to be helpful for efficient seawater desalination.

Ultralarge suspended and perforated graphene membranes for cell culture applications.

With its high mechanical strength and its remarkable thermal and electrical properties, suspended graphene has long been expected to find revolutionary applications in optoelectronics or as a membrane in nano-devices. However, the lack of efficient transfer and patterning processes still limits its potential. In this work, we report an optimized anthracene-based transfer process to suspend few layers of graphene (1-, 2- and 4-layers) in the millimeter range (up to 3 mm) with high reproducibility. We have explored the advantages and limitations for patterning of these membranes with micrometer-resolution by focused ion beam (FIB) and picosecond pulsed laser ablation techniques. The FIB approach offers higher patterning resolution but suffers from the low throughput. We demonstrate that cold laser ablation is a fast and flexible method for micro-structuring of suspended graphene. One promising field of application of ultimately thin, microporous graphene membranes is their use as next-generation cell culture supports as alternative to track-etched polymer membranes, which often exhibit poor permeability and limited cell-to-cell communication across the membranes. To this end, we confirmed good adhesion and high viability of placental trophoblast cells cultivated on suspended porous graphene membranes without rupturing of the membranes. Overall, there is high potential for the further development of ultrathin suspended graphene membranes for many future applications, including their use for biobarrier cell culture models to enable predictive transport and toxicity assessment of drugs, environmental pollutants, and nanoparticles.

On the Choice of Different Water Model in Molecular Dynamics Simulations of Nanopore Transport Phenomena.

The water transport through nanoporous multilayered graphene at 300k is investigated using molecular dynamics (MD) simulation with different water models in this study. We used functionalized and non-functionalized membranes along with five different 3-point rigid water models: SPC (simple point charge), SPC/E (extended simple point charge), TIP3P-FB (transferable intermolecular potential with 3 points-Force Balance), TIP3P-EW (transferable intermolecular potential with 3 points with Ewald summation) and OPC3 (3-point optimal point charge) water models. Based on our simulations with two water reservoirs and a porous multilayered graphene membrane in-between them, it is evident that the water transport varies significantly depending on the water model used, which is in good agreement with previous works. This study contributes to the selection of a water model for molecular dynamics simulations of water transport through multilayered porous graphene.

Ionic-liquid-gated porous graphene membranes for efficient CO2/CH4 separation

Ion-gated graphene membranes coated with monolayer ionic liquids (ILs) on porous graphene have shown good separation performance. However, it is necessary to match the IL with the appropriate pore size of porous graphene for practical applications. In this study, we investigated the effects of different ILs and graphene pore sizes on the CO2/CH4 separation performance using non-equilibrium molecular dynamics simulations. When the same anion was used as a gating ion, the different cation sizes influenced the separation performance of the membrane. The selectivity of [BMIM][BF4] for CO2 reached 104.2, which was the highest among the [EMIM][BF4], [BMIM][BF4], and [HMIM][BF4] systems for 6 Å pores. With increasing pore size, larger anions were required as the gating ions. [EMIM][PF6] was found to be a suitable gating ion for 7 Å pores and led to higher selectivity and CO2 membrane permeability. We also studied the effects of the number of IL molecules and the charges of the pore atoms for [EMIM][PF6] with 7 Å pore systems. The results showed that increasing the charge of the pore atoms could significantly improve the selectivity for CO2, reaching as high as 231.82. Although increasing the amount of IL molecules can enhance selectivity, it may decrease the CO2 membrane permeability. Further, non-spherical symmetric anion gating was studied. When [EMIM][NTF2] was coated on porous graphene with 6 Å pores, neither CO2 nor CH4 could pass through the membrane pores. For 7 Å pores, the CO2 separation selectivity reached 88, and the permeation number of CO2 was 22. [EMIM][PF6] and [EMIM][NTF2] were determined to be unsuitable gating ions for 8.5 Å pores. They showed poor selectivity and significant permeation of both CO2 and CH4 for porous graphene with 8.5 Å pores.

Temporal reverse osmotic salt filtration mechanism of multi-layered porous graphene

Reverse osmosis (RO) technology is currently the most progressive, energy-saving and efficient membrane separation technology . Meanwhile, graphene becomes a promising candidate for fabricating the RO membranes in water desalination due to its high salt rejection and water flux. The concept of “temporal selectivity” is first proposed in our previous work in terms of the time difference between the penetration time of an ion passing through the pore and the tangential slipping time for the ion sliding across the pore. Nevertheless, the temporal selectivity mechanism of multilayered graphene membrane remains ambiguous. In this paper, the RO process of saltwater through porous graphene column RO membrane is studied by using molecular dynamics (MD) simulations method, and the effects of rotating angular velocity and the thickness of RO membrane on desalination performance of seawater are considered first. The MD results show that the salt rejection increases with the rotation speed of porous membrane increasing while the water flux initially increases and then decreases . Meanwhile, the interfacial slip velocity increases linearly with angular velocity increasing. On the other hand, the increasing thickness of porous graphene membrane can enhance the selectivity and reduce the permeability of water molecules. As expected, the tri-layered porous graphene RO membrane can achieve high salt rejection at low interfacial slip velocity. In order to ensure high selectivity and energy conservation and efficient, the pore structure of the porous graphene RO membrane is optimized. The results show that the optimized nanopores can increase the water flux significantly, whereas the salt rejection is not changed appreciably. It is found that the pore size of the innermost layer membrane near the feed region has the most significant effect on the water flux. The water flux increases sharply with the increase of pore diameter and the salt rejection remains totally higher than 80%. Moreover, the RO membrane with a special Type 3 structure exhibits excellent performance in seawater desalination, specifically, the ultrahigh water flux reaches 20029 L·cm<sup>–2</sup>·d<sup>–1</sup> and the super salt rejection arrives at 94%. The research results further clarify and verify the mechanism of the temporal selectivity in RO process, and improve the water flux under the condition of the same membrane thickness by designing gradient hole. The findings can conduce to the in-depth theoretical understanding of porous graphene-based membranes and designing and developing the large-scale seawater desalination devices and water filtration equipment.

H-passivated nanoporous graphene membranes for CO2/N2 separation: A reactive molecular dynamic simulation

We used a reactive force-field (ReaxFF) to investigate the capability of H-passivated nanoporous graphene as a membrane for the separation of carbon dioxide from nitrogen. Unlike traditional atomistic force-fields, the reactive force field considers chemical bond-forming/braking and estimates the polarisation effect more accurately. We examined the effect of pore size and pore shape in several gas mixtures with different mole ratios. We assessed the selectivity of six H-passivated pores by counting the number of transmitted molecules from them. The simulation shows that the H-modified porous graphene membrane with pore diameter (short side) of about 2.4 Å only allow N2 molecules to permeate through and have the best separation selectivity. The CO2 molecules can pass through pores greater than 4.2 Å diameters. However, they tend to accumulate near the graphene membranes due to non-bonded interactions and do not diffuse in the vacuum until they saturate the graphene walls.

Tuning the Transport Properties of Gases in Porous Graphene Membranes with Controlled Pore Size and Thickness

AbstractPorous graphene membranes have emerged as promising alternatives for gas‐separation applications due to their atomic thickness enabling ultrahigh permeance, but they suffer from low gas selectivity. Whereas decreasing the pore size below 3 nm is expected to increase the gas selectivity due to molecular sieving, it is rather challenging to generate a large number of uniform small pores on the graphene surface. Here, a pore‐narrowing approach via gold deposition onto porous graphene surface is introduced to tune the pore size and thickness of the membrane to achieve a large number of small pores. Through the systematic approach, the ideal combination is determined as pore size below 3 nm, obtained at the thickness of 100 nm, to attain high selectivity and high permeance. The resulting membrane shows a H2/CO2 separation factor of 31.3 at H2 permeance of 2.23 × 105 GPU (1 GPU = 3.35 × 10−10 mol s−1 m−2 Pa−1), which is the highest value reported to date in the 105 GPU permeance range. This result is explained by comparing the predicted binding energies of gas molecules with the Au surface, −5.3 versus −21 kJ mol−1 for H2 and CO2, respectively, increased surface–gas interactions and molecular‐sieving effect with decreasing pore size.

Tuning the transport properties of gases in porous graphene membranes with controlled pore size and thickness

Tuning the transport properties of gases in porous graphene membranes with controlled pore size and thickness

Porous graphene membranes under small tensile strains exhibit higher percolation barriers to the passing molecules

We use first-principles calculations to explore the change of the separation properties of strained porous graphene membranes. Small to moderate lateral strain, biaxial (1% - 2%) or uniaxial (1% to 7%, depending on the direction), always increases the percolation barriers of the same passing molecules (e.g. methane, cyclopropane, neopentane) regardless of the pore size, shape and functionalization, when compared with their strain-free values. The pore size expansion under such strain is no match for the electronic structure redistribution, which leads to more rigid pore plane (against the out-of-pore-plane bending). Larger strain will eventually make the geometric enlargement dominant over the electronic effect and the barrier falls. The energy barrier in response to biaxial strain can rise by 0.2 - 0.3 eV, corresponding to a substantial drop of gas transport rate by three to five orders of magnitude. The barrier-to-strain response is highly anisotropic for uniaxial strain. Strain (6% - 7%) in the direction of the longest pore diameter can cause the barrier to rise even more by 0.3 - 0.4 eV (gas percolation rate slashes by five to seven orders of magnitude), which can effectively shut down the molecules’ percolation. Our results demonstrate that there is plenty space for strain engineering the separation performance of porous graphene membranes.

Direct Chemical Vapor Deposition Synthesis of Porous Single-Layer Graphene Membranes with High Gas Permeances and Selectivities.

Single-layer graphene containing molecular-sized in-plane pores is regarded as a promising membrane material for high-performance gas separations due to its atomic thickness and low gas transport resistance. However, typical etching-based pore generation methods cannot decouple pore nucleation and pore growth, resulting in a trade-off between high areal pore density and high selectivity. In contrast, intrinsic pores in graphene formed during chemical vapor deposition are not created by etching. Therefore, intrinsically porous graphene can exhibit high pore density while maintaining its gas selectivity. In this work, the density of intrinsic graphene pores is systematically controlled for the first time, while appropriate pore sizes for gas sieving are precisely maintained. As a result, single-layer graphene membranes with the highest H2 /CH4 separation performances recorded to date (H2 permeance >4000 GPU and H2 /CH4 selectivity >2000) are fabricated by manipulating growth temperature, precursor concentration, and non-covalent decoration of the graphene surface. Moreover, it is identified that nanoscale molecular fouling of the graphene surface during gas separation where graphene pores are partially blocked by hydrocarbon contaminants under experimental conditions, controls both selectivity and temperature dependent permeance. Overall, the direct synthesis of porous single-layer graphene exploits its tremendous potential as high-performance gas-sieving membranes.

Highly porous nanofiber-supported monolayer graphene membranes for ultrafast organic solvent nanofiltration.

Scalable fabrication of monolayer graphene membrane on porous supports is key to realizing practical applications of atomically thin membranes, but it is technologically challenging. Here, we demonstrate a facile and versatile electrospinning approach to realize nanoporous graphene membranes on different polymeric supports with high porosity for efficient diffusion- and pressure-driven separations. The conductive graphene works as an excellent receptor for deposition of highly porous nanofibers during electrospinning, thereby enabling direct attachment of graphene to the support. A universal “binder” additive is shown to enhance adhesion between the graphene layer and polymeric supports, resulting in high graphene coverage on nanofibers made from different polymers. After defect sealing and oxygen plasma treatment, the resulting nanoporous membranes demonstrate record-high performances in dialysis and organic solvent nanofiltration, with a pure ethanol permeance of 156.8 liters m−2 hour−1 bar−1 and 94.5% rejection to Rose Bengal (1011 g mol−1) that surpasses the permeability-selectivity trade-off.

Ultrahigh permeance metal coated porous graphene membranes with tunable gas selectivities

<h2>Summary</h2> Membranes play an important role in gas separation on account of their low cost, energy efficiency, and durability. Gas-separation membranes, however, are subject to permeability-selectivity trade-off, i.e., atomically thin 2D materials such as porous graphene can provide ultrahigh permeances in the range of ∼10⁵–10⁷ GPU but suffer from low gas selectivity. Here, we show a new concept to enhance the selectivity of graphene-based membranes by employing adsorptive separation for binary gas mixtures. The deposition of "microislands" of Pd and Ni on the porous double-layer graphene support allowed us to selectively target H₂ in He/H₂ and CO₂ in H₂/CO₂ mixtures, respectively, thus enabling efficient separation of He and also leading to the highest H₂/CO₂ separation factor of 26 within the ∼10⁵ GPU permeance range. Moreover, the selective targeting of individual gases in a membrane setting through adsorptive separation at room temperature can be a promising alternative for economical gas separation.

Ultrahigh permeance metal coated porous graphene membranes with tunable gas selectivities

Ultrahigh permeance metal coated porous graphene membranes with tunable gas selectivities

H2 and H2S separation by adsorption using graphene and zinc oxide sheets: Molecular dynamic simulations

Using MD simulation, a series of porous graphene and zinc oxide membranes were designed to separate the H2/H2S mixture gas. Then, the effect of pore size and temperature increase on the rate of gas separation was investigated. The diffusion process, which is one of the most important parameters for measuring the separation of H2 and H2S gas molecules in the porous adsorbents of graphene and zinc oxide, was evaluated at different temperatures. The calculations showed that the penetration of H2 gas molecules due to the uniform distribution on the surface of the adsorbents, by spreading on the membrane, can find the pores more effectively. The simulation results showed that the penetrability and selectivity of the gas molecules are directly related to the pore size, due to the size limitation, only a small amount of hydrogen molecules can pass the minimum pore size (4.929 Å). In this study, the highest penetrability for graphene and zinc oxide nanoparticles was 22% and 46%, respectively. At pores smaller than 13.432 Å, a high degree of selectivity was reported. Also, with increasing pore size on graphene and zinc oxide sheets, a decrease in selectivity was observed. Temperature is another parameter that has been tested in the simulation and shows that its increase has a significant effect on the adsorption of gas molecules so that with increasing temperature, the amount of adsorption on the adsorbent surface decreases. This is because the adsorption process is exothermic. The highest adsorption energy was related to H2S gas with −892.81 kcal/mol in graphene and the lowest was −268.31 kcal/mol for H2 gas in ZnO adsorbent. The results of this study can be used to design industrial applications and reduce environmental impact.

Efficient CH4/CO2 Gas Mixture Separation through Nanoporous Graphene Membrane Designs

Nanoporous graphene membranes have drawn special attention in the gas-separation processes due to their unique structure and properties. The complexity of the physical understanding of such membrane designs restricts their widespread use for gas-separation applications. In the present study, we strive to propose promising designs to face this technical challenge. In this regard, we investigated the permeation and separation of the mixture of adsorptive gases CO2 and CH4 through a two-stage bilayer sub-nanometer porous graphene membrane design using molecular dynamics simulation. A CH4/CO2 gashouse mixture with 80 mol% CH4 composition was generated using the benchmarked force-fields and was forced to cross through the porous graphene membrane design by a constant piston velocity. Three chambers are considered to be feeding, transferring, and capturing to examine the permeation and separation of molecules under the effect of the two-stage membrane. The main objective is to investigate the multistage membrane and bilayer effect simultaneously. The permeation and separation of the CO2 and CH4 molecules while crossing through the membrane are significantly influenced by the pore offset distance (W) and the interlayer spacing (H) of the bilayer nanoporous graphene membrane. Linear configurations (W = 0 Å) and those with the offset distance of 10 Å and 20 Å were examined by varying the interlayer spacing between 8 Å, 12 Å, and 16 Å. The inline configuration with an interlayer spacing of 12 Å is the most effective design among the examined configurations in terms of optimum separation performance and high CO2 and CH4 permeability. Furthermore, increasing the interlayer distance to 16 Å results in bulk-like behavior rather than membrane-like behavior, indicating the optimum parameters for high selectivity and permeation. Our findings present an appropriate design for the effective separation of CH4/CO2 gas mixtures by testing novel nanoporous graphene configurations.

Multilayer Graphene Sheet with Conical Nanopores as a Membrane for High-Permeance Molecular Separation

Mitigating leaks through permeable defects by stacking graphene layers would greatly reduce the molecular permeance through porous graphene membranes for gas separation. We propose a multilayer gra...

Towards Catalytically Active Porous Graphene Membranes with Pulsed Laser Deposited Ceria Nanoparticles.

Porous graphene with catalytically active ceria nanometre-size particles were prepared using pulsed laser deposition (PLD) on graphene produced through chemical vapour deposition (CVD). The reported process provided porous graphene containing ceria nanoparticles as confirmed by HR TEM and XPS. Isotopically labelled 13 C graphene was employed to study desorption of the species containing carbon. Methanol adsorption was utilised to probe the nature of the catalytic activity of prepared ceria decorated graphene. The important role of graphene support for the stabilization of reduced ceria nanoparticles was finally confirmed. Increased dehydrogenation activity of graphene with ceria nanoparticles leading to CO and H2 formation was demonstrated.