Reduced Holey Graphene Oxide Membranes for Desalination with Improved Water Permeance

Scalable Graphene Oxide Hollow Fiber Membranes for Dye Desalination Enabled by Multi-Purpose Polyamine Functionalization

2D nanosheets such as graphene oxide (GO) can be stacked to construct membranes with fine-tuned nanochannels to achieve molecular sieving ability. These membranes are often thin to achieve high water permeance, but their fabrication with consistent nanostructures on a large scale presents an enormous challenge. Herein, GO-based hollow fiber membranes (HFMs) are developed for dye desalination by synergistically combining chemical etching to form in-plane nanopores (10-30 nm) to increase water permeance and polyamine functionalization to improve underwater stability and enable facile large-scale production using existing membrane manufacturing processes. HFM modules with areas of 88 cm2 and GO layer thicknesses of ≈500 nm are fabricated, and they exhibited a stable dye water permeance of 75 L m-2 h-1 bar-1, rejection of >99.5% for Direct red and Congo red, and Na2SO4/dye separation factor of 300-500, superior to state-of-the-art commercial membranes. The versatility of this approach is also demonstrated using different short polyamines and porous substrates. This study reveals a scalable way of designing 2D materials into high-performance robust membranes for practical applications.

Nanochannel Stability of Chemically Converted Graphene Oxide Membranes

Chemically converted graphene oxide laminate membranes, which exhibit stable interlayered nanochannels in aqueous environments, are receiving increasing attention owing to their potential for selective water and ion permeation. However, how the molecular properties of conversion agents influence the stabilization of nanochannels and how effectively nanochannels are stabilized have rarely been studied. In this study, mono-, di-, and tri-saccharide molecules of glucose (Glu), maltose (Glu2), and maltotriose (Glu3) are utilized, respectively, to chemically modify graphene oxide (GO). The aim is to create nanochannels with different levels of stability and investigate how these functional conversion agents affect the separation performance. The effects of the property differences between different conversion agents on nanochannel stabilization are demonstrated. An agent with efficient chemical reduction of GO and limited intercalation in the resulting nanochannel ensures satisfactory nanochannel stability during desalination. The stabilized membrane nanochannel exhibits a permeance of 0.69 L m-2 h-1 bar-1 and excellent Na2SO4 rejection of 96.42%. Furthermore, this optimized membrane nanochannel demonstrates enhanced stability under varying external conditions compared to the original GO. This study provides useful information for the design of chemical conversion agents for GO nanochannel stabilization and the development of nanochannel membranes for precise separation.

Glycine-Modified Co-MOF Pervaporation Membrane to Enhance Water Transporting

Cobalt-based metal-organic frameworks (Co-MOFs) with a two-dimensional layered morphology have received increasing attention for pervaporation due to their stability and hydrophilic properties. Using amino glycine (Gly) as a cross-linking agent, the Co-MOF ultrathin two-dimensional membrane doped with organic filler sodium alginate (SA) with the "brick-mixed-sand" structure was proposed. Polyacrylonitrile (PAN) was selected as the support layer of the hybrid membrane. The introduction of Gly efficiently solved the nanomaterial stacking problem and controllably adjusted the interlayer spacing between the nanosheets, which demonstrated good performance for ethanol dehydration. The results of this experimental research showed that the total flux of alcohol/water (9:1) separation by Gly-Co-MOF-SA/PAN hybrid membranes reached 1902 g m-2 h-1, which was 67% higher than that of the pure SA membranes. The "brick-mixed-sand" lamellar dense morphology of Gly-Co-MOF not only enhances membrane hydrophilicity but also provides effective channels for the rapid transport of water, which is expected to be used for the dehydration of organic solvents.

Physicochemical Treatments of Graphene Oxide to Improve Water Vapor/Gas Separation Performance of Supported Laminar Membranes: Sonication and H2O2 Oxidation

We investigated the previously unexplored domain of water vapor/gas separation using graphene oxide (GO) membranes, expecting future applications, including gas dehumidifiers and superior humidity controllers. While the importance of manipulation of GO nanosheet size and surface chemistry in traditional water purification and gas separation has been acknowledged, their potential impact on water vapor/gas separation remained unexplored until now. We applied sonication and hydrogen peroxide treatments to GO water dispersions and systematically evaluated the size and surface chemistry of each GO nanosheet. Both treatments reduced the GO nanosheet size to shorten the diffusion length, which improved water permeance. In addition, hydrogen peroxide treatment improved the hydrophilicity of the nanosheet. Our novel findings demonstrate that optimization of GO nanosheet size and the increase in their hydrophilicity via hydrogen peroxide treatments for 5 h significantly enhance water permeance, leading to a remarkable water vapor permeance of 4.6 × 10-6 mol/(m2 s Pa) at 80 °C, a 3.1-fold improvement over original GO membranes, while maintaining a water vapor/nitrogen permeance ratio exceeding 10,000. These results not only provide important insights into the nature of water vapor/gas separation but also suggest innovative methods for optimizing the GO membrane structure.

Nanowire-assisted electrochemical perforation of graphene oxide nanosheets for molecular separation

Two-dimensional nanosheets, e.g., graphene oxide (GO), have been widely used to fabricate efficient membranes for molecular separation. However, because of poor transport across nanosheets and high width-to-thickness ratio, the permeation pathway length and tortuosity of these membranes are extremely large, which limit their separation performance. Here we report a facile, scalable, and controllable nanowire electrochemical concept for perforating and modifying nanosheets to shorten permeation pathway and adjust transport property. It is found that confinement effects with locally enhanced charge density, electric field, and hydroxyl radical generation over nanowire tips on anode can be executed under low voltage, thereby inducing confined direct electron loss and indirect oxidation to reform configuration and composition of GO nanosheets. We demonstrate that the porous GO nanosheets with a lot of holes are suitable for assembling separation membranes with tuned accessibility, tortuosity, interlayer space, electronegativity, and hydrophilicity. For molecular separation, the prepared membranes exhibit quadruple water permeance and higher rejections for salts (>91%) and small molecules (>96%) as/than original ones. This nanowire electrochemical perforation concept offers a feasible strategy to reconstruct two-dimensional materials and tune their transport property for separation.

Multistage Filtration Desalination via Ion Self-Rejection Effect in Cation-Controlled Graphene Oxide Membrane under 1 Bar Operating Pressure

By building a thin graphene oxide membrane with Na+ self-rejection ability, high permeability, and multistage filtration strategy, we obtained fresh water from a saline solution under 1 bar of operating pressure. After five and 11 cycles of the multistage filtration, the Na+ concentration decreased from 0.6 to 0.123 mol/L (below physiological concentration) and 0.015 mol/L (fresh water), respectively. In comparison with the performance of commercial reverse osmosis membranes, energy consumption was only 10% and water flux was higher by a factor of 10. Interestingly, the energy consumption of this multistage filtration strategy is close to the theoretical lowest energy consumption. Theoretical calculations showed that such Na+ self-rejection is attributed to the lower transportation rate of the Na+ than that of water within the graphene oxide membrane for the hydrated cation-π interaction. Our findings present a viable desalination strategy for graphene-based membranes and improve the mechanistic understanding of water/ion transportation behaviors in confined spaces.

Tuning of interfacial HGO@CLS nanohybrid S-scheme heterojunction with improved carrier separation and photocatalytic activity towards RhB degradation

Herein, we premeditated and invented the innovative hybrid photocatalyst 2D/2D CuLa2S4 on holey graphene oxide (HGO) (HGO@CLS) via the hydrothermal method. Electrochemical techniques demonstrate the action of HGO in the HGO@CLS photocatalyst as an effective medium for electron transfer. Combining bimetallic sulfides on porous HGO synergistically provides a higher negative conduction band edge (-0.141 V), greater photo response (10.8 mA/cm2), smaller charge transfer resistance (Rct = 1.79Ω), and lower photoluminescence (PL) spectral intensity. According to our research, the catalytic recitals are sped up when HGO is assimilated into CLS photocatalyst hetero-junction. Additionally, it lowers the reassimilation rate due to the combined mesh nanostructures and functionality of CLS and HGO. UV-Vis DRS, Mott-Schottky, PL, and Electrochemical impedance spectra (EIS) results manifested that the CuLa2S4/HGO makes the spatial separation competent and transference of charge carriers due to the photon irradiation and exhibits superior photocatalytic ability. Electron spin resonance (ESR) analysis confirmed that •OH and h+ were the predominant radical species responsible for Rhodamine B(RhB) degradation. Moreover, conceivable degradation ways of RhB were deduced according to the identified intermediates which are responsible for the degradation of recalcitrant products. To check the stability of the photocatalyst, revival tests were also carried out. Similarly, the oxidative byproducts created in the deprivation courses were looked at, and a thorough explanation for the mechanism of degradation was given.

Tailoring Monovalent Ion Sieving in Graphene-Oxide Membranes with High Flux by Rationally Intercalating Crown Ethers

Two-dimensional membranes have shown promising potential for ion-selective separation due to their well-defined interlayer channels. However, the typical "trade-off" effect of throughput and selectivity limits their developments. Herein, we report a precise tailoring of monovalent cation sieving technology with enhanced water throughput via the intercalation of graphene-oxide membranes with selective crown ethers. By tuning the lamellar spacing of graphene oxide, a critical interlayer distance (∼11.04 Å) is revealed to maximize water flux (53.4 mol m-2 h-2 bar-1) without sacrificing ion selectivity. As a result, the elaborately enlarged interlayer distance offers improved water permeance. Meanwhile, various specific cations with remarkably high selectivity can be separated in mixed solutions because of the strong chelation with crown ethers. This work opens up a new avenue for high-throughput and precise regulation of ion separations for various application scenarios.

A critical review on graphene oxide membrane for industrial wastewater treatment

An important way to promote the environmental industry's goal of carbon reduction is to promote the recycling of resources. Membrane separation technology has unique advantages in resource recovery and advanced treatment of industrial wastewater. However, the great promise of traditional organic membrane is hampered by challenges associated with organic solvent tolerance, lack of oxidation resistance, and serious membrane fouling control. Moreover, the high concentrations of organic matter and inorganic salts in the membrane filtration concentrate also hinder the wider application of the membrane separation technology. The emerging cost-effective graphene oxide (GO)-based membrane with excellent resistance to organic solvents and oxidants, more hydrophilicity, lower membrane fouling, better separation performance has been expected to contribute more in industrial wastewater treatment. Herein, we provide comprehensive insights into the preparation and characteristic of GO membranes, as well as current research status and problems related to its future application in industrial wastewater treatment. Finally, concluding remarks and future perspectives have been deduced and recommended for the GO membrane separation technology application for industrial wastewater treatment, which leads to realizing sustainable wastewater recycling and a nearly "zero discharge" water treatment process.

From nanohole to ultralong straight nanochannel fabrication in graphene oxide with swift heavy ions

Porous architectures based on graphene oxide with precisely tailored nm-sized pores are attractive for biofluidic applications such as molecular sieving, DNA sequencing, and recognition-based sensing. However, the existing pore fabrication methods are complex, suffer from insufficient control over the pore density and uniformity, or are not scalable to large areas. Notably, creating vertical pores in multilayer films appears to be particularly difficult. Here, we show that uniform 6-7 nm-sized holes and straight, vertical nanochannels can be formed by simply irradiating graphene oxide (GO) films with high-energy heavy ions. Long penetration depths of energetic ions in combination with localized energy deposition and effective self-etching processes enable the creation of through pores even in 10 µm-thick GO films. This fully scalable fabrication provides a promising possibility for obtaining innovative GO track membranes.

Interface-Confined Channels Facilitating Water Transport through an IL-Enriched Nanocomposite Membrane

Improving the permeance of the polyamide (PA) membrane while maintaining the rejection is crucial for promoting the development of membrane separation technology in the practical water-treatment industry. Herein, a novel metal-ionic liquid (Zn-IL) coordination compound was synthesized by in situ growth to improve the water permeance of PA nanofiltration membranes, using an amine-functionalized IL (1-aminopropyl-3-methylimidazolium chloride, [AEMIm][Cl]) as a ligand to react with Zn(NO₃)₂·6H₂O. Piperazine (PIP) and trimesoyl chloride (TMC) were adopted to prepare the PA layer covering the Zn-IL complex. Due to the unique property of the Zn-IL complex, the Zn-IL/PIP-TMC absorbing force to water was increased, enabling the fast transport of water molecules through the membrane pore channels in the form of free water. The resulting Zn-IL/PIP-TMC nanocomposite membrane exhibited a high permeance of up to 26.5 L m^-2 h^-1 bar^-1, which is 3 times that of the PIP-TMC membrane (8.8 L m^-2 h^-1 bar^-1), combined with rejection above 99% for dyes such as methyl blue.

Graphene-Based Membranes for Water Desalination: A Literature Review and Content Analysis

Graphene-based membranes have unique nanochannels and can offer advantageous properties for the water desalination process. Although tremendous efforts have been devoted to heightening membrane performance and broadening their application, there is still lack of a systematic literature review on the development and future directions of graphene-based membranes for desalination. In this mini-review, literature published between 2011 and 2022 were analyzed by using the bibliometric method. We found that the major contributors to these publications and the highest citations were from China and the USA. Nearly 80% of author keywords in this analysis were used less than twice, showing the broad interest and great dispersion in this field. The recent advances, remaining gaps, and strategies for future research, were discussed. The development of new multifunctional nanocomposite materials, heat-driven/solar-driven seawater desalination, and large-scale industrial applications, will be important research directions in the future. This literature analysis summarized the recent development of the graphene-based membranes for desalination application, and will be useful for researchers in gaining new insights into this field.

Scalable Dry-Pressed Electrodes Based on Holey Graphene

ConspectusHoley graphene (hG) is a structural derivative of graphene with arrays of through-thickness holes of a few to tens of nanometers in diameter, randomly distributed across the nanosheet surfaces. In most bulk preparation methods, the holes on hG sheets are preferentially generated from the pre-existing defects on graphene. Therefore, contrary to intuitive belief, hG is not necessarily more defective than the intact graphene. Instead, it retains essential parent properties, including high electrical conductivity, high surface area, mechanical robustness, and chemical inertness. Furthermore, the added holey structural motif imparts unique properties that are not present in unmodified graphene, making hG advantageous in numerous applications such as sensing, membranes, reinforcements, and electrochemical energy storage. In particular, the presence of holes enhances the mass transport through the nanosheet plane and thus significantly reduces tortuosity. This difference is a key advantage for using hG in energy storage applications where the transport of ions through the thickness becomes more hindered as the electrode thickness increases to meet practical energy density requirements.An unexpected discovery is that the holes of the hG sheets enable the dry hG powder to be directly compressed into robust monoliths. hG not only can be pressed into monoliths by itself but also can host other electrochemically active materials as a compressible matrix. This important yet unique property, which is not available for other carbon materials including intact graphene, significantly broadens the application horizon in energy storage applications. With the dry compressibility, electrodes with ultrahigh mass loading and thus ultrahigh areal capacity may be conveniently fabricated without toxic solvents or parasitic binders, which are required in conventional slurry-based approaches for electrode fabrication. The dry-press electrode preparation process can be completed within minutes regardless of mass loading. In comparison, high-mass-loading electrodes for advanced battery chemistries using conventional fabrication methods often need stringent and time-consuming process control. hG can also be combined with electrochemically active battery materials while maintaining dry compressibility. This has allowed the unprecedented, convenient manipulation of a wide variety of thick electrode compositions and architectures, which provides not only outstanding performance but also new physical insights for various battery chemistries.In this Account, we first present some basic observations on the dry compressibility of hG as well as the mechanistic investigations from atomistic modeling rationalizing this unique property. We then showcase the applications of neat and composite dry-pressed hG electrodes for various energy storage platforms including supercapacitors, lithium (Li) ion batteries, Li-O₂ batteries, and Li-S/Se batteries. The preparation and performance of thick electrodes with practical mass loadings and unique electrode architecture manipulation, both enabled by the dry compressibility of hG, are highlighted and discussed.

Ionic Liquid-Reduced Graphene Oxide Membrane with Enhanced Stability for Water Purification

There has been a growing interest in water purification by graphene oxide (GO) laminate membranes due to their exceptional hydrophilicity, high throughput, and extraordinary separation performance originating from their two-dimensional and well-defined nanostructure. However, the swelling and stability in an aqueous environment are areas of concern for the GO laminate membranes. Here, a novel methylimidazolium ionic liquid-reduced GO (mimG)-assembled GO laminate membrane (mimG-GO) with remarkable stability was fabricated by a vacuum-assisted strategy for water purification. Methylimidazolium-based ionic liquid-reduced graphene oxide (mimG) was prepared by a facile nucleophilic ring-opening mechanism. Fabricated membranes were thoroughly characterized for stability, structural, permeance, and rejection properties in an aqueous environment. A combination of cationic mimG and GO nanosheets improves membrane stability in the aqueous environment via cation-π interactions and creates nanofluidic channels for facile water transport while yielding significant enhancement in the salt and dye separation performance. The pore size and the number of nanofluidic channels were precisely controlled via material deposition and laminate thickness to remove salts from water. The mimG-GO laminate membrane containing 72.2 mg m^-2 deposition showed a permeance of 14.9 LMH bar^-1, 50% higher than 9.7 LMH bar^-1 of the neat GO laminate membrane, in addition to an increase in Na₂SO₄ salt rejection from 46.6 to 77.4%, overcoming the flux-rejection trade-off. The mimG-GO laminate membrane also rejected various anionic dyes (i.e., 99.9% for direct red 80 (DR 80), 96.8% for reactive black 5 (RB 5), and 91.4% for methyl orange (MO)). The mimG-GO laminate membrane containing 361.0 mg m^-2 deposition showed the highest rejection for Na₂SO₄ (92.1%) and 99.9% rejection for DR 80, 99.0% rejection for RB 5, and 98.1% rejection for MO dyes keeping a flux of 2.6 LMH bar^-1. Partial reduction and covalent grafting of ionic liquid moieties on GO helped to enhance the cation-π interaction between GO laminates, which showed enhanced stability, frictionless water transport, with high salt and dye rejection. Moreover, a simultaneous improvement in water permeance and solute rejection reveals the great potential of ionic liquid-functionalized GO laminate membranes for water-based applications.

Atomic-Level Pt Electrocatalyst Synthesized via Iced Photochemical Method for Hydrogen Evolution Reaction with High Efficiency

In heterogeneous catalysis, metal particle morphology and size can influence markedly the activity. It is of great significance to rationally design and control the synthesis of Pt at the atomic level to demonstrate the structure-activity relationship toward electrocatalysis. Herein, a powerful strategy is reported to synthesize graphene-supported platinum-based electrocatalyst, that is, nanocatalysts with controllable size can be prepared by iced photochemical method, including single atoms (Pt-SA@HG), nanoclusters (Pt-Clu@HG), and nanocrystalline (Pt-Nc@HG). The Pt-SA@HG exhibits unexpected electrocatalytic hydrogen evolution reaction (HER) performances with 13 mV overpotential at 10 mA cm^-2 current densities which surpass Pt-Clu@HG and Pt-Nc@HG. The in situ X-ray absorption fine structure spectroscopy (XAFS) and density functional theory (DFT) calculations determine the Pt-C₃ active site is linchpin to the excellent HER performance of Pt-SA@HG. Compared with the traditional Pt-N_x coordination structure, the pure carbon coordinated Pt-C₃ site is more favorable for HER. This work opens up a new way to adjust the metal particle size and catalytic performance of graphene at a multiscale level.

Mass Transport via In-Plane Nanopores in Graphene Oxide Membranes

Angstrom-confined solvents in 2D laminates can travel through interlayer spacings, through gaps between adjacent sheets, and via in-plane pores. Among these, experimental access to investigate the mass transport through in-plane pores is lacking. Our experiments allow an understanding of this mass transport via the controlled variation of oxygen functionalities, size and density of in-plane pores in graphene oxide membranes. Contrary to expectations, our transport experiments show that higher in-plane pore densities may not necessarily lead to higher water permeability. We observed that membranes with a high in-plane pore density but a low amount of oxygen functionalities exhibit a complete blockage of water. However, when water-ethanol mixtures with a weaker hydrogen network are used, these membranes show an enhanced permeation. Our combined experimental and computational results suggest that the transport mechanism is governed by the attraction of the solvents toward the pores with functional groups and hindered by the strong hydrogen network of water formed under angstrom confinement.

2D Polymer Nanosheets for Membrane Separation

Since the discovery of single-layer graphene in 2004, the family of 2D inorganic nanosheets is considered as ideal membrane materials due to their ultrathin atomic thickness and fascinating physicochemical properties. However, the intrinsically nonporous feature of 2D inorganic nanosheets hinders their potential to achieve a higher flux to some extent. Recently, 2D polymer nanosheets, originated from the regular and periodic covalent connection of the building units in 2D plane, have emerged as promising candidates for preparing ultrafast and highly selective membranes owing to their inherently tunable and ordered pore structure, light weight, and high specific surface. In this review, the synthetic methodologies (including top-down and bottom-up methods) of 2D polymer nanosheets are first introduced, followed by the summary of 2D polymer nanosheets-based membrane fabrication as well as membrane applications in the fields of gas separation, water purification, organic solvent separation, and ion exchange/transport in fuel cells and lithium-sulfur batteries. Finally, based on their current achievements, the authors' personal insights are put forward into the existing challenges and future research directions of 2D polymer nanosheets for membrane separation. The authors believe this comprehensive review on 2D polymer nanosheets-based membrane separation will definitely inspire more studies in this field.

Synthesis of holey graphene for advanced nanotechnological applications

Holey or porous graphene, a structural derivative of graphene, has attracted immense attention due to its unique properties and potential applications in different branches of science and technology. In this review, the synthesis methods of holey or porous graphene/graphene oxide are systematically summarized and their potential applications in different areas are discussed. The process-structure-applications are explained, which helps relate the synthesis approaches to their corresponding key applications. The review paper is anticipated to benefit the readers in understanding the different synthesis methods of holey graphene, their key parameters to control the pore size distribution, advantages and limitations, and their potential applications in various fields.

Zwitterionic Hydrogel-Impregnated Membranes with Polyamide Skin Achieving Superior Water/Salt Separation Properties

Support-free nonporous membranes have emerged as a new material platform for osmotic pressure-driven processes due to its insusceptibility to internal concentration polarization (ICP). Herein, we demonstrate high-performance membranes of zwitterionic hydrogels impregnated in porous membranes with a skin layer of highly cross-linked polyamides on both sides prepared by gel-liquid interfacial polymerization (GLIP). Such a configuration eliminates the pores and thus ICP, while the thin polyamide layer provides high salt rejection but negligible resistance to the water transport compared with the hydrogels. The polyamide skin layers are characterized using scanning electron microscopy and atomic force microscopy. The effect of the hydrogel compositions and polyamide formation conditions on the water/salt separation properties is thoroughly investigated. Example membranes show water permeance and salt rejection comparable to state-of-the-art commercial forward osmosis membranes and essentially no ICP.

Grafting Activated Graphene Oxide Nanosheets onto Ultrafiltration Membranes Using Polydopamine to Enhance Antifouling Properties

Graphene oxide (GO) nanosheets are negatively charged and exhibit excellent antifouling properties. However, their hydrophilicity makes it challenging for their grafting onto membrane surfaces to improve antifouling properties for long-term underwater operation. Herein, we demonstrate a versatile approach to covalently graft GO onto ultrafiltration membrane surfaces in aqueous solutions at ≈22 °C. The membrane surface is first primed using dopamine and then reacted with activated GO (aGO) containing amine-reactive esters. The aGO grafting improves the membrane surface hydrophilicity without decreasing water permeance. When the membranes are challenged with 1.0 g/L sodium alginate in a constant-flux crossflow system, the aGO grafting increases the critical flux by 20% and reduces the fouling rate by 63% compared with the pristine membrane. The modified membranes demonstrate stability for 48 h operation and interval cleanings using NaOH solutions.