Ultrafast Nanofiltration through Large-Area Single-Layered Graphene Membranes

Defect-Healed Carbon Nanomembranes for Enhanced Salt Separation: Scalable Synthesis and Performance

Carbon nanomembranes (CNMs), with a high density of subnanometer channels, enable superior salt separation performance compared to conventional membranes. However, defects that occur during the synthesis and transfer processes impede their technical realization on a macroscopic scale. Here, we introduce a practical and scalable interfacial polymerization method to effectively heal defects while preserving the subnanometer pores within CNMs. The defect-healed freestanding CNMs show an exceptional performance in forward osmosis (FO), achieving a water flux of 105 L m-2 h-1 and a specific reverse salt flux of 0.1 g L-1 when measured with 1 M NaCl as draw solution. This water flux is 10 times higher than that of commercially available FO membranes, and the reverse salt flux is 70% lower. Through successful implementation of the defect-healing method and support optimization, we demonstrate the synthesis of fully functional, centimeter-scale CNM-based composite membranes showing high water permeance and a high salt rejection. Our defect-healing method presents a promising pathway to overcome limitations in CNM synthesis, advancing their potential for practical salt separation applications.

In Situ Nucleation-Decoupled and Site-Specific Incorporation of Å-Scale Pores in Graphene Via Epoxidation

Generating pores in graphene by decoupled nucleation and expansion is desired to achieve a fine control over the porosity, and is desired to advance several applications. Herein, epoxidation is introduced, which is the formation of nanosized epoxy clusters on the graphitic lattice as nucleation sites without forming pores. In situ gasification of clusters inside a transmission electron microscope shows that pores are generated precisely at the site of the clusters by surpassing an energy barrier of 1.3 eV. Binding energy predictions using ab initio calculations combined with the cluster nucleation theory reveal the structure of the epoxy clusters and indicate that the critical cluster is an epoxy dimer. Finally, it is shown that the cluster gasification can be manipulated to form Å-scale pores which then effectively sieve gas molecules based on their size. This decoupled cluster nucleation and pore formation will likely pave the way for an independent control of pore size and density.

Ultrathin Membranes for Separations: A New Era Driven by Advanced Nanotechnology

Ultrathin membranes are at the forefront of membrane research, offering great opportunities in revolutionizing separations with ultrafast transport. Driven by advanced nanomaterials and manufacturing technology, tremendous progresses are made over the last 15 years in the fabrications and applications of sub-50 nm membranes. Here, an overview of state-of-the-art ultrathin membranes is first introduced, followed by a summary of the fabrication techniques with an emphasis on how to realize such extremely low thickness. Then, different types of ultrathin membranes, categorized based on their structures, that is, network, laminar, or framework structures, are discussed with a focus on the interplays among structure, fabrication methods, and separation performances. Recent research and development trends are highlighted. Meanwhile, the performances and applications of current ultrathin membranes for representative separations (gas separation and liquid separation) are thoroughly analyzed and compared. Last, the challenges in material design, structure construction, and coordination are given, in order to fully realize the potential of ultrathin membranes and facilitate the translation from scientific achievements to industrial productions.

Molecular Self-Assembly Enables Tuning of Nanopores in Atomically Thin Graphene Membranes for Highly Selective Transport

Atomically thin membranes comprising nanopores in a 2D material promise to surpass the performance of polymeric membranes in several critical applications, including water purification, chemical and gas separations, and energy harvesting. However, fabrication of membranes with precise pore size distributions that provide exceptionally high selectivity and permeance in a scalable framework remains an outstanding challenge. Circumventing these constraints, here, a platform technology is developed that harnesses the ability of oppositely charged polyelectrolytes to self-assemble preferentially across larger, relatively leaky atomically thin nanopores by exploiting the lower steric hindrance of such larger pores to molecular interactions across the pores. By selectively tightening the pore size distribution in this manner, self-assembly of oppositely charged polyelectrolytes simultaneously introduced on opposite sides of nanoporous graphene membranes is demonstrated to discriminate between nanopores to seal non-selective transport channels, while minimally compromising smaller, water-selective pores, thereby remarkably attenuating solute leakage. This improved membrane selectivity enables desalination across centimeter-scale nanoporous graphene with 99.7% and >90% rejection of MgSO₄ and NaCl, respectively, under forward osmosis. These findings provide a versatile strategy to augment the performance of nanoporous atomically thin membranes and present intriguing possibilities of controlling reactions across 2D materials via exclusive exploitation of pore size-dependent intermolecular interactions.

Thickness-Varied Carbon Nanomembranes from Polycyclic Aromatic Hydrocarbons

Despite the prospects of intrinsically porous planar nanomaterials in separation applications, their synthesis on a large scale remains challenging. In particular, preparing water-selective carbon nanomembranes (CNMs) from self-assembled monolayers (SAMs) is limited by the cost of epitaxial metal substrates and molecular precursors with specific chemical functionalities. In this work, we present a facile fabrication of CNMs from polycyclic aromatic hydrocarbons (PAHs) that are drop-cast onto arbitrary supports, including foils and metalized films. The electron-induced carbonization is shown to result in continuous membranes of variable thickness, and the material is characterized with a number of spectroscopic and microscopic techniques. Permeation measurements with freestanding membranes reveal a high degree of porosity, but the selectivity is found to strongly depend on the thickness. While the permeance of helium remains almost the same for 6.5 and 3.0 nm thick CNMs, water permeance increases by 2 orders of magnitude. We rationalize the membrane performance with the help of kinetic modeling and vapor adsorption experiments.

Recent Developments in Nanoporous Graphene Membranes for Organic Solvent Nanofiltration: A Short Review

Graphene-based membranes are promising candidates for efficient organic solvent nanofiltration (OSN) processes because of their unique structural characteristics, such as mechanical/chemical stability and precise molecular sieving. Recently, to improve organic solvent permeance and selectivity, nanopores have been fabricated on graphene planes via chemical and physical methods. The nanopores serve as an additional channel for facilitating ultrafast solvent permeation while filtering organic molecules by size exclusion. This review summarizes the recent developments in nanoporous graphene (NG)-based membranes for OSN applications. The membranes are categorized depending on the membrane structure: single-layer NG, multilayer NG, and graphene-based composite membranes hybridized with other porous materials. Techniques for nanopore generation on graphene, as well as the challenges faced and the perspectives required for the commercialization of NG membranes, are also discussed.

Monolayer graphene membranes for molecular separation in high-temperature harsh organic solvents

The excellent thermal and chemical stability of monolayer graphene makes it an ideal material for separations at high temperatures and in harsh organic solvents. Here, based on understanding of solvent permeation through nanoporous graphene via molecular dynamics simulation, a resistance model was established to guide the design of a defect-tolerant graphene composite membrane consisting of monolayer graphene on a porous supporting substrate. Guided by the model, we experimentally engineered polyimide (PI) supporting substrates with appropriate pore size, permeance, and excellent solvent resistance and investigated transport across the resulting graphene-covered membranes. The cross-linked PI substrate could effectively mitigate the impacts of leakage through defects across graphene to allow selective transport without defect sealing. The graphene-covered membrane showed pure solvent permeance of 24.1 L m^-2 h^-1 bar^-1 and stable rejection (∼90%) of Allura Red AC (496.42 g mol^-1) in a harsh polar solvent, dimethylformamide (DMF), at 100 °C for 10 d.

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.

Advantages, limitations, and future suggestions in studying graphene-based desalination membranes

The potential of novel 2D carbon materials such as nanoporous single-layer graphene and multilayer graphene oxide membranes is based on their possible advantages such as high water permeability, high selectivity capable of rejecting monovalent ions, with high salt rejection, reduced fouling, and high chemical and physical stability. Here we review how the field has advanced in the study of their performances in various desalination approaches such as reverse osmosis, forward osmosis, nanofiltration, membrane distillation, and solar water purification. The research on making high-performance graphene membranes which started with reverse osmosis applications is seemingly evolving towards other directions.

Scalable synthesis of nanoporous atomically thin graphene membranes for dialysis and molecular separations via facile isopropanol-assisted hot lamination

Scalable graphene synthesis and facile large-area membrane fabrication are imperative to advance nanoporous atomically thin membranes (NATMs) for molecular separations. Although chemical vapor deposition (CVD) allows for roll-to-roll high-quality monolayer graphene synthesis, facile transfer with atomically clean interfaces to porous supports for large-area NATM fabrication remains extremely challenging. Sacrificial polymer scaffolds commonly used for graphene transfer typically leave polymer residues detrimental to membrane performance and transfers without polymer scaffolds suffer from low yield resulting in high non-selective leakage through NATMs. Here, we systematically study the factors influencing graphene NATM fabrication and report on a novel roll-to-roll manufacturing compatible isopropanol-assisted hot lamination (IHL) process that enables scalable, facile and clean transfer of CVD graphene on to polycarbonate track etched (PCTE) supports with coverage ≥99.2%, while preserving support integrity/porosity. We demonstrate fully functional centimeter-scale graphene NATMs that show record high permeances (∼2-3 orders of magnitude higher) and better selectivity than commercially available state-of-the-art polymeric dialysis membranes, specifically in the 0-1000 Da range. Our work highlights a scalable approach to fabricate graphene NATMs for practical applications and is fully compatible with roll-to-roll manufacturing processes.

Super-Nernstian pH Sensor Based on Anomalous Charge Transfer Doping of Defect-Engineered Graphene

The conventional pH sensor based on the graphene ion-sensitive field-effect transistor (Gr-ISFET), which operates with an electrostatic gating at the solution-graphene interface, cannot have a pH sensitivity above the Nernst limit (∼59 mV/pH). However, for accurate detection of the pH levels of an aqueous solution, an ultrasensitive pH sensor that can exceed the theoretical limit is required. In this study, a novel Gr-ISFET-based pH sensor is fabricated using proton-permeable defect-engineered graphene. The nanocrystalline graphene (nc-Gr) with numerous grain boundaries allows protons to penetrate the graphene layer and interact with the underlying pH-dependent charge-transfer dopant layer. We analyze the pH sensitivity of nc-Gr ISFETs by adjusting the grain boundary density of graphene and the functional group (OH-, NH₂-, CH₃-) on the SiO₂ surface, confirming an unusual negative shift of the charge-neutral point (CNP) as the pH of the solution increases and a super-Nernstian pH response (approximately -140 mV/pH) under optimized conditions.

Correction: Direct growth of a porous substrate on high-quality graphene via in situ phase inversion of a polymeric solution

Correction for 'Direct growth of a porous substrate on high-quality graphene via in situ phase inversion of a polymeric solution' by Yanzhe Qin et al., Nanoscale, 2020, 12, 4953-4958, DOI: 10.1039/C9NR09693K.

Direct growth of a porous substrate on high-quality graphene via in situ phase inversion of a polymeric solution

The key for graphene applications is the successful transfer of graphene from a growth metal substrate to a substrate for application without compromising its high quality. However, state-of-the-art polymethyl methacrylate (PMMA) assisted transfer methods introduce wrinkles, folds and cracks, which are exacerbated for porous substrates. Here we report a novel in situ technique to transfer graphene onto a porous substrate which resolves these issues. Using phase-inversion a porous substrate is grown onto a graphene film with strong adhesion that perfectly matches graphene's topography, and the growth metal substrate is subsequently etched away. We achieve 63 cm2 high-quality single-layered graphene with almost 100% coverage over the pores of the substrate and pore ratios up to 35%. Our study resolves the three main challenges of transferring graphene to porous substrates, which are matching the topographies between the graphene and the porous substrate, achieving high pore ratios and minimizing the stresses on the suspended graphene; this approach may therefore serve as a general guide for attaching graphene or other 2D materials to scaffold structures.

Nanofluidic Transport Theory with Enhancement Factors Approaching One

High performance water transport in nanopores has drawn a great deal of attention in a variety of applications, such as water desalination, power generation, and biosensing. High water transport enhancement factors in carbon-based nanopores have been reported over the classical Hagen-Poiseuille (HP) equation which does not account for the physics of transport at molecular scale. Instead, comparing the experimentally measured transport rates to that of a theory, that accounts for the microscopic physics of transport, would result in enhancement factors approaching unity. Such a theory is currently missing. Here, molecular corrections are introduced into the HP equation by considering the variation of key hydrodynamical properties (viscosity and friction) with thickness and diameter of pores in ultrathin graphene and finite-length carbon nanotubes (CNTs) using Green-Kubo relations and molecular dynamics (MD) simulations. The corrected HP (CHP) theory successfully predicts the permeation rates from nonequilibrium MD pressure driven flows. The previously reported enhancement factors over no-slip HP (of the order of 1000) approach unity when the permeations are normalized by the CHP flow rates. The results of our study will help better understand nanoscale flows in carbon-based pores and tubes.

Ultrafast-Selective Nanofiltration of an Hybrid Membrane Comprising Laminated Reduced Graphene Oxide/Graphene Oxide Nanoribbons

In this study, reduced graphene oxide (rGO) and graphene oxide nanoribbons (GONRs) are used to fabricate a composite membrane that exhibits ultrafast water permeance (312.8 L m^-2 h^-1 bar^-1) and precise molecular separation (molecular weight cutoff: 269 Da), which surpass the upper bound of previously reported polymer and graphene-based nanofiltration membranes. As two-dimensional GONR exhibits a width on the scale of nanometers, its nanochannels can be enlarged without hindering the stacking of rGO. Moreover, abundant oxygen-containing groups on the edge and surface of GONR enhance the electrostatic interactions between the filtered molecules and the membrane nanochannel. By the synergistic effect, rejection and water flux are considerably increased. Owing to the chemically stable nature of rGO, the composite membrane is highly stable in aqueous media (from acidic to alkaline) and is recyclable during repeated filtration tests.

Facile Fabrication of Large-Area Atomically Thin Membranes by Direct Synthesis of Graphene with Nanoscale Porosity

Direct synthesis of graphene with well-defined nanoscale pores over large areas can transform the fabrication of nanoporous atomically thin membranes (NATMs) and greatly enhance their potential for practical applications. However, scalable bottom-up synthesis of continuous sheets of nanoporous graphene that maintain integrity over large areas has not been demonstrated. Here, it is shown that a simple reduction in temperature during chemical vapor deposition (CVD) on Cu induces in-situ formation of nanoscale defects (≤2-3 nm) in the graphene lattice, enabling direct and scalable synthesis of nanoporous monolayer graphene. By solution-casting of hierarchically porous polyether sulfone supports on the as-grown nanoporous CVD graphene, large-area (>5 cm² ) NATMs for dialysis applications are demonstrated. The synthesized NATMs show size-selective diffusive transport and effective separation of small molecules and salts from a model protein, with ≈2-100× increase in permeance along with selectivity better than or comparable to state-of-the-art commercially available polymeric dialysis membranes. The membranes constitute the largest fully functional NATMs fabricated via bottom-up nanopore formation, and can be easily scaled up to larger sizes permitted by CVD synthesis. The results highlight synergistic benefits in blending traditional membrane casting with bottom-up pore creation during graphene CVD for advancing NATMs toward practical applications.

State-of-the-Art and Future Prospects for Atomically Thin Membranes from 2D Materials

Atomically thin 2D materials, such as graphene, hexagonal boron-nitride, and others, offer new possibilities for ultrathin barrier and membrane applications. While the impermeability of pristine 2D materials to gas molecules, such as He, allows the realization of the thinnest physical barrier, nanoscale vacancy defects in the 2D material lattice manifest as nanopores in an atomically thin membrane. Such nanoporous atomically thin membranes (NATMs) present potential for enabling ultrahigh permeance and selectivity in a wide range of novel separation processes. Herein, the transport properties observed in NATMs are described and recent experimental progress achieved in their fabrication is summarized. Some of the challenges in NATM scale-up for practical applications are highlighted and several opportunities are identified, including the possibility of blending traditional membrane-processing approaches. Finally, a technological roadmap is presented with a contextual discussion for NATMs to progress from research to applications.

Covalently Modified Graphene Oxide and Polymer of Intrinsic Microporosity (PIM-1) in Mixed Matrix Thin-Film Composite Membranes

In this study, mixed matrix membranes (MMMs) consisting of graphene oxide (GO) and functionalized graphene oxide (FGO) incorporated in a polymer of intrinsic microporosity (PIM-1) serving as a polymer matrix have been fabricated by dip-coating method, and their single gas transport properties were investigated. Successfully surface-modified GOs were characterized by Fourier transform infrared spectroscopy (FTIR), UV-Vis spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The effect of FGO loading on MMM morphology and performance was investigated by varying the FGO content in polymer matrix from 9 to 84 wt.%. Use of high FGO content in the polymer matrix helped to reveal difference in interaction of functionalized fillers with PIM-1 and even to discuss the change of FGO stiffness and filler alignment to the membrane surface depending on functional group nature.

A Scalable Route to Nanoporous Large-Area Atomically Thin Graphene Membranes by Roll-to-Roll Chemical Vapor Deposition and Polymer Support Casting

Scalable, cost-effective synthesis and integration of graphene is imperative to realize large-area applications such as nanoporous atomically thin membranes (NATMs). Here, we report a scalable route to the production of NATMs via high-speed, continuous synthesis of large-area graphene by roll-to-roll chemical vapor deposition (CVD), combined with casting of a hierarchically porous polymer support. To begin, we designed and built a two zone roll-to-roll graphene CVD reactor, which sequentially exposes the moving foil substrate to annealing and growth atmospheres, with a sharp, isothermal transition between the zones. The configurational flexibility of the reactor design allows for a detailed evaluation of key parameters affecting graphene quality and trade-offs to be considered for high-rate roll-to-roll graphene manufacturing. With this system, we achieve synthesis of uniform high-quality monolayer graphene ( I_D/ I_G < 0.065) at speeds ≥5 cm/min. NATMs fabricated from the optimized graphene, via polymer casting and postprocessing, show size-selective molecular transport with performance comparable to that of membranes made from conventionally synthesized graphene. Therefore, this work establishes the feasibility of a scalable manufacturing process of NATMs, for applications including protein desalting and small-molecule separations.

Single- to Few-Layered, Graphene-Based Separation Membranes

Two-dimensional, graphene-based materials have attracted great attention as a new membrane building block, primarily owing to their potential to make the thinnest possible membranes and thus provide the highest permeance for effective sieving, assuming comparable porosity to conventional membranes and uniform molecular-sized pores. However, a great challenge exists to fabricate large-area, single-layered graphene or graphene oxide (GO) membranes that have negligible undesired transport pathways, such as grain boundaries, tears, and cracks. Therefore, model systems, such as a single flake or nanochannels between graphene or GO flakes, have been studied via both simulations and experiments to explore the transport mechanisms and separation potential of graphene-based membranes. This article critically reviews literature related to single- to few-layered graphene and GO membranes, from material synthesis and characteristics, fundamental membrane structures, and transport mechanisms to potential separation applications. Knowledge gaps between science and engineering in this new field and future opportunities for practical separation applications are also discussed.

Water and Solute Transport Governed by Tunable Pore Size Distributions in Nanoporous Graphene Membranes

Nanoporous graphene has the potential to advance membrane separations by offering high selectivity with minimal resistance to flow, but how mass transport depends on the structure of pores in this atomically thin membrane is poorly understood. Here, we investigate the relationship between tunable pore creation using ion bombardment and oxygen plasma etching, the resulting pore size distributions, and the consequent water and solute transport. Through tuning of the pore creation process, we demonstrate nanofiltration membranes that reject small molecules but offer high permeance to water or monovalent ions. Theoretical multiscale modeling of transport across the membranes reveals a disproportionate contribution of large pores to osmotic water flux and diffusive solute transport and captures the observed trends in transport measurements except for the smallest pores. This work provides insights into the effects of graphene pore size distribution and support layer on transport and presents a framework for designing atomically thin membranes.