3D Membrane Research Articles

Facile Fabrication of Defect-Controlled Graphene Oxide Membrane through Shear-Induced Alignment for Regulating Ion Transport.

Two-dimensional (2D) membranes enable ion-sieving through well-defined subnanoscale channels. In particular, graphene oxide (GO), a representative 2D material with a flexible structure, can be manufactured into various types of membranes, while defects such as pores and wrinkles are readily formed through self-aggregation and self-folding during membrane fabrication. Such defects provide a path for small ionic or molecular species to be easily penetrated between the layers, which deteriorates membrane performance. Here, we demonstrate the effect of shear-induced alignment with continuous agitation on GO membrane structure during pressure-assisted filtration. The shear stress exerted on the GO layers during deposition is controlled by varying the agitation rate and solution viscosity. The well-stacked 2D membrane is obtained via the facile shear-controlled process, leading to an improved salt rejection performance without additional physicochemical modifications. This simple approach can be extensively utilized to prepare the well-ordered structure of other 2D materials in various fields where the defect control is required.

Stress wave response in a two-dimensional membrane subjected to hypervelocity impact of a micro-flyer

The stress wave propagation in 2D membrane subjected to hypervelocity impact (HVI) is studied in this paper. A triangular-pulse stress wave model is developed and the complete solution is derived to characterize the far-field stress wave initiated by HVI. The developed model is verified by the explicit dynamics finite element, and the predictive capability for the HVI event is evaluated by the smoothed particle hydrodynamics. Based on this theoretical model, the influences of the system parameters (the time ratio, the pulse width, and the propagation distance) on the dynamic behaviors of the far-field stress wave are analyzed. A waveform distortion phenomenon during the far-field stress wave propagation is observed and the associated distortion criterion is proposed. When the distortion criterion is satisfied, the waveform of the circumferential stress wave is transformed from the normal-triangle shape to the inverted-triangle one, leading to the consequent change of the stress state. It indicates that the waveform distortion phenomenon should be considered when studying the dynamic responses and the related damage behaviors of 2D membranes.

Controlled Synthesis of Perforated Oxide Nanosheets with High Density Nanopores Showing Superior Water Purification Performance.

A method for creating genuine nanopores in high area density on monolayer two-dimensional (2D) metallic oxides has been developed. By use of the strong reduction capability of hydroiodic acid, active metal ions, such as FeIII and CoIII, in 2D oxide nanosheets can be reduced to a divalent charge state (2+). The selective removal of FeO2 and CoO2 metal oxide units from the framework can be tuned to produce pores in a range of 1-4 nm. By monitoring of the redox reaction kinetics, the pore area density can be also tuned from ∼0.9 × 104 to ∼3.3 × 105 μm-2. The universality of this method to produce much smaller pores and higher area density than the previously reported ones has been proven in different oxide nanosheets. To demonstrate their potential applications, ultrasmall metal organic framework particles were grown inside the pores of perforated titania oxide nanosheets. The optimized hybrid film showed ∼100% rejection of methylene blue (MB) from the water. Its water permeance reached 4260 L m-2 h-1 bar-1, which is 1-3 orders of that for reported 2D membranes with good MB rejections.

A self-cleaning photocatalytic composite membrane based on g-C3N4@MXene nanosheets for the removal of dyes and antibiotics from wastewater

The present paper deals with the coupling the advantages of photocatalysis and membrane technology in order to resolve the issue of membrane fouling without any compromise on separation characteristics. The g-C3N4 (CN) nanosheets have been used to modify two-dimensional (2D) MXene membrane, to obtain novel g-C3N4@MXene/PES (CN-MX) photocatalytic composite membrane through vacuum filtration for wastewater treatment. The 2D materials and membranes have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) to evidence successful modification. The CN-MX composite membrane exhibited higher permeability than that of pure MXene membrane, which achieved a maximum permeability of 1790 L·m−2·h−1·bar−1. The composite membrane also showed a fabulous removal ratio for different organic pollutants: 98 % for Congo red (CR), 96 % for Trypan blue (TB) and 86 % for Tetracycline hydrochloride (TC), respectively. The best performed membrane (M3) was found to sustain three consecutive separation cycles without any performance deterioration and thereby, confirming a significant improvement in anti-fouling characteristic of the membrane.

Mechanical, Elastic, and Adhesive Properties of Two‐Dimensional Materials: From Straining Techniques to State‐of‐the‐Art Local Probe Measurements

Abstract2D materials, such as graphene, hexagonal boron nitride (hBN), and transition‐metal dichalcogenides (TMDs), are intrinsically flexible, can withstand very large strains (>10% lattice deformations), and their optoelectronic properties display a clear and distinctive response to an applied stress. As such, they are uniquely positioned both for the investigation of the effects of mechanical deformations on solid‐state systems and for the exploitation of these effects in innovative devices. For example, 2D materials can be easily employed to transduce nanometric mechanical deformations into, e.g., clearly detectable electrical signals, thus enabling the fabrication of high‐performance sensors; just as easily, however, external stresses can be used as a “knob” to dynamically control the properties of 2D materials, thereby leading to the realization of strain‐tuneable, fully reconfigurable devices. Here, the main methods are reviewed to induce and characterize, at the nm level, mechanical deformations in 2D materials. After presenting the latest results concerning the mechanical, elastic, and adhesive properties of these unique systems, one of their most promising applications is briefly discussed: the realization of nano‐electromechanical systems based on vibrating 2D membranes, potentially capable of operating at high frequencies (>100 MHz) and over a large dynamic range.

Carbon ene-yne working in oxygenator: A theoretical study

Traditional hollow-fiber membranes (HFM) in the oxygenator are in short supply due to their complicated manufacturing process, high price, and tight production capacity. To address these challenges, herein we designed a simplified molecular dynamics (MD) model that mimics the working procedure of HFM to explore whether carbon ene-yne (CEY) and other two carbon-based 2-dimensional (2D) materials: γ-graphyne and graphdiyne (GDY) could be used in an oxygenator. With the aid of the Density Functional based Tight Binding (DFTB) method, Density Functional Theory (DFT), Adaptive Steered Molecular Dynamics (ASMD), and NVT molecular dynamics (MD) simulations, the penetration barriers of the O2, CO2, and H2O molecules through these three 2D membranes are estimated. Furthermore, their permeance under various differential pressures are also discussed. According to our simulations, the CEY membrane can provide a much better performance for O2 and CO2 exchange than the most commonly used HFM material. CEY should be a promising candidate as O2 and CO2 exchange membrane working in the oxygenator.

ZIF-8 channeled and coordination-bridging two-dimensional WS2 membrane for efficient organic solvent nanofiltration

Zeolitic imidazolate framework-8 (ZIF-8)/WS2 composite 2D membranes were constructed by in-situ growth of ZIF-8 in the interlayer of WS2 laminar membrane. The growth of ZIF-8 nanoparticles in WS2 membrane expanded the interlayer spacing, and endowed WS2 membrane with fixed interlayer spacing by coordination-bridges between WS2 and ZIF-8, which made ZIF-8/WS2 membrane possess not only high solvent permeance but also excellent stability in organic solvents. Isopropanol (IPA) permeance through ZIF-8/WS2 membrane reached 272 L/m2·h·bar, which was about 1.6 times higher than that through pristine WS2 membrane, and the rejection rates for Direct Black 38 (DB38), Congo Red (CR) and Rose Bengal (RosB) were 99%, 99% and 30% respectively. Furthermore, the IPA permeance through ZIF-8/WS2 membrane was almost unchanged after soaking in IPA for 192 h, and IPA permeance and rejection rate for DB 38 changed slightly during 100 h of cross-flow filtration, demonstrating remarkable stability of ZIF-8/WS2 membranes in organic solvents and a promising application potential in organic solvent nanofiltration.

Two-dimensional nanoporous and lamellar membranes for water purification: Reality or a myth?

• Progress in 2D nanoporous and lamellar membranes is critically reviewed. • Sub-nanoporous membranes can achieve high water permeance and rejection. • Lamellar membranes provide effective rejection of dye molecules at high water flux. • Enhanced ion rejection by lamellar membranes requires 2D material functionalisation. • Research is required on scalability, stability, fouling and pore size engineering. Wastewater contains only ∼ 1% impurities that are required to be separated precisely for water recycling and reuse applications, particularly in water scare regions. Membrane-based separation has been recognised as an environmentally-friendly and energy-efficient process for effective pollutant removal and clean water production. To further improve the selectivity and water flux of membranes, two-dimensional (2D) materials, such as graphene and those exhibiting graphene-like properties, have been intensively studied for nanoporous and lamellar membrane fabrication in the last decade. Herein, we critically review the progress in 2D membranes for water purification and uniquely discuss the performance governing factors and research gaps from the standpoint of environmental and material chemists, which are critical to realize 2D membrane commercialization. The major points of this review are: (1) among the six types of reviewed 2D membranes, graphene, graphene oxide and reduced graphene oxide membrane have been predominantly studied for water treatment. (2) atomically thin nanoporous membranes with a defined pore size can achieve excellent water permeance and selectivity but large-scale fabrication of nanoporous membranes with uniform shape and size of nanopores remains a challenge. (3) salt separation performance of the pressurized lamellar membranes may not be comparable to commercial nanofiltration membranes. (4) functionalization or crosslinking of 2D materials is inevitable to stabilize 2D membrane pore structure and/or improve molecule separation. (5) membranes fabricated using emerging 2D materials such as born nitride and MXene appear to show better stability than graphene oxide membranes. (6) several aspects, particularly membrane stability, long-term performance, and membrane fouling propensity need attention to realize commercial application of 2D membranes in water treatment.

Intelligent graphene oxide membranes with pH tunable channels for water treatment

• The pH tunable two dimensional graphene oxide channels were achieved. • The nano-gating ratios of smart graphene oxide membranes reach ∼ 5. • The smart graphene oxide membranes can reject different sized species. • The smart graphene oxide membranes exhibit promising acid (or alkaline) recovery performance. Adaptive membranes that can intelligently control their permeability or selectivity depending on the surrounding environments are of extraordinary importance in the separation technology area. Herein, we report the pH responsive membranes through covalently grafting weak Polyelectrolytes (poly(acrylic acid) (PAA) or poly(4-vinylpyridine) (P4VP)) onto the regular two dimensional (2D) channels stacked by Graphene oxide (GO) nanosheets. Attributed to the reversible shrinking/stretching function of PAA (or P4VP) chains with the variation of external pH, the switchable 2D GO channels are thus established successfully. Correspondingly, the resulting PAA@GO and P4VP@GO 2D membranes show promising nano-gating ratios of ∼ 5 accompanied by satisfactory water permeances. Furthermore, the pH responsive 2D channels render these two types of membranes with the adaptive ability of rejecting different sized species according to separation requirements. For disposing industrial alkaline (or acid) effluents, PAA@GO (or P4VP@GO) membranes exhibit outstanding alkaline (or acid) recovery performance as well.

Machine Learning Assisted Screening of Two-Dimensional Materials for Water Desalination.

There exists a vast expanse of data in the literature which can be harnessed for accelerated design and discovery of advanced materials for various applications of importance ─ for example, desalination of seawater. Here, we develop a machine learning (ML) model, training it with ∼260 molecular dynamics (MD) computation results, to predict the desalination performance of 2D membranes that exist in the literature. The desalination performance variables of water flux and salt rejection rates are correlated to 49 material features related to the chemistry of the pores and the membranes along with applied pressure, salt concentration, partial charges on the atoms, geometry of the pore, the mechanical properties of the membranes, and the properties of water for the water model used. We used the ML model to screen 3814 structurally optimized 2D materials for maximum water flux and salt rejection rates from the literature. We found some candidates that perform ∼4 times better than the more popularly known 2D materials such as graphene and MoS2. This result is verified using data obtained from MD simulations performed on several representative 2D membranes for different classes. Such validated statistical frameworks using literature data can be very useful in guiding experiments in the field of functional materials for varied applications.

Molecular Permeation in Freestanding Bilayer Silica.

Graphene and other single-layer structures are pursued as high-flux separation membranes, although imparting porosity endangers their crystalline integrity. In contrast, bilayer silica composed of corner-sharing (SiO4) units is foreseen to be permeable for small molecules due to its intrinsic lattice openings. This study sheds light on the mass transport properties of freestanding 2D SiO2 upon using atomic layer deposition (ALD) to grow large-area films on Au/mica substrates followed by transfer onto Si3N4 windows. Permeation experiments with gaseous and vaporous substances reveal the suspended material to be porous, but the membrane selectivity appears to diverge from the size exclusion principle. Whereas the passage of inert gas molecules is hindered with a permeance below 10-7 mol·s-1·m-2·Pa-1, condensable species like water are found to cross vitreous bilayer silica a thousand times faster in accordance with their superficial affinity. This work paves the way for bilayer oxides to be addressed as inherent 2D membranes.

A Two-Dimensional Lamellar Vermiculite Membrane for Precise Molecular Separation and Ion Sieving

Lamellar membranes constructed from two-dimensional (2D) nanosheets have exhibited exceptional permselective characteristics. However, their complex and contaminative nanosheet synthesis, low structural stability, and low chemical resistance severely limit industrial-scale production and practical applications. Herein, the stability, molecular separation, ion-sieving properties, and broad applicability were evaluated to demonstrate the application potential of 2D vermiculite (VMT) nanomaterials in high-performance membrane development. First, the large-scale 2D VMT nanosheets with average lateral sizes of ∼12 μm were prepared from the widely occurring natural clay via a facile procedure, and the 2D lamellar VMT membrane showed excellent long-term stability in harsh environments, even in an ultrasonic bath. Furthermore, the VMT membrane showed fast solvent permeance with a favorable retention rate of dye molecules and a surface charge-governed ionic transport behavior because of the negatively charged nanochannel, indicating the potential in both molecule separation and ion sieving. Moreover, the broad applicability of the VMT membrane was also confirmed using a simple intercalation optimizing strategy developed for other 2D membranes. Building on these findings, our work shows a possible route to the development of advanced membranes for energy and environmental applications.

Anomalously low electrostatic bending stiffness of graphene oxide 2D membranes regulates their environmental fate in aquatic ecosystems

Low electrostatic bending stiffness of graphene oxide sheets determines the conformation and consequently their environmental fate in aquatic environments.

One-Atom-Thick Crystals as Emerging Proton Sieves.

Two-dimensional (2D) crystals, despite their atomic thickness, have long been considered as impermeable membranes to all molecules and atoms under ambient conditions: even the smallest of atoms, hydrogen, is expected to take billions of years to penetrate the 2D lattice covered with dense electron clouds. Recently it has been found that monolayer graphene, hexagonal boron nitride, and some other one-atom-thick crystals are highly permeable to protons, raising fundamental questions about the details of the transport process. In this Perspective, we review the mechanism of proton transport through 2D crystals and the related room-temperature quantum effects; the potential applications of 2D membranes in proton-related separation and sieving techniques, including proton exchange membranes and hydrogen isotope separation; and factors that enhance proton permeation and in turn influence 2D membrane design.

Tunable Strong Coupling of Mechanical Resonance between Spatially Separated FePS3 Nanodrums.

Coupled nanomechanical resonators made of two-dimensional materials are promising for processing information with mechanical modes. However, the challenge for these systems is to control the coupling. Here, we demonstrate strong coupling of motion between two suspended membranes of the magnetic 2D material FePS3. We describe a tunable electromechanical mechanism for control over both the resonance frequency and the coupling strength using a gate voltage electrode under each membrane. We show that the coupling can be utilized for transferring data between drums by amplitude modulation. Finally, we also study the temperature dependence of the coupling and how it is affected by the antiferromagnetic phase transition characteristic of this material. The presented electrical coupling of resonant magnetic 2D membranes holds the promise of transferring mechanical energy over a distance at low electrical power, thus enabling novel data readout and information processing technologies.

Three-dimensional magnetic fibrous scaffold with icariin expanded by supercritical CO2 for bone tissue engineering under static magnetic field

The electrospun fibrous scaffold has shown a great potential due to an extracellular matrix-mimicking structure of nanofibers, however, a three-dimensional (3D) fibrous scaffold, much similar to in vivo environment, still remains challenging in fabrication. The magnetic nanoparticles (MNPs) have been explored to promote bone-related cells activity under static magnetic field (SMF). Herein, via electrospinning, Fe 3 O 4 MNPs and icariin (ICA) from traditional Chinese medicine were introduced into polycaprolactone (PCL) fibers to manufacture two-dimensional (2D) membranes (PCL/Fe 3 O 4 /ICA), which were then expanded to 3D scaffold by depressurization of subcritical CO 2 fluid. The expanding behavior was more remarkable for the membrane collected from rotary collector than that on plate collector, especially after the addition of Fe 3 O 4 MNPs. Co-cultured with pre-osteoblasts, PCL/Fe 3 O 4 /ICA 3D scaffold induced a higher cell proliferation viability than that in 2D membrane in later period, and the combined utilization with SMF group showed the highest cell viability. When implanted subcutaneously, 3D scaffold exhibited better cell infiltration, internal collagen deposition and angiogenesis due to the enhanced porosity and the action of ICA. This highly porous magnetic PCL/Fe 3 O 4 /ICA 3D scaffold provided a new idea for the design and application of magnetic scaffold in the bone tissue engineering. • The polycaprolactone/Fe 3 O 4 /icariin (PCL/Fe 3 O 4 /ICA) 2D magnetic fibrous membrane was fabricated via electrospinning. • The supercritical CO 2 -expanded 3D magnetic scaffold was only fabricated by using 2D membrane collected from rotary collector. • PCL/Fe 3 O 4 /ICA 3D magnetic composite scaffold induced the highest MC3T3-E1 cells proliferationunder static magnetic field. • PCL/Fe 3 O 4 /ICA 3D magnetic composite scaffold showed better cell infiltration, internal collagen deposition and angiogenesis.

Mechanistic Probing of Encapsulation and Confined Growth of Lithium Crystals in Carbonaceous Nanotubes (Adv. Mater. 51/2021)

Anode Materials In article number 2105228, Ming-Sheng Wang and co-workers reveal the encapsulation mechanism and confined growth kinetics of lithium crystals in carbonaceous nanotubes by in situ transmission electron microscopy. Unusual Li nanostructures such as 2D membranes and multi-crystal nanowires are discovered. Importantly, sufficient nitrogen/oxygen doping or pre-lithiation would be critical for the carbon hosts to reliably encapsulate Li metal in a battery anode.

Vapor Adsorption Measurements with Two-Dimensional Membranes.

Two‐dimensional (2D) membranes display extraordinary mass transfer properties, in particular for the permeation of gaseous substances. Their ultimate thickness not only ensures the shortest diffusion pathways, but also makes the membrane surface play a significant role in accommodating and guiding the permeating molecules. As saturated vapors of water and organic solvents are often observed to pass 2D membranes faster than inert gases, condensation is believed to be responsible for surface‐mediated transport. Here, we present a spectroscopic experiment to probe adsorption of condensable species on 2D membranes under realistic conditions. Polarization‐modulation infrared reflection absorption spectroscopy (PM IRAS) is coupled with a reaction chamber and a vacuum system to control the vaporous environments. The measurements are demonstrated to yield quantitative information on the amount of adsorbates onto supported 2D layers. As a case study, the azeotropic mixture of water and propanol is revealed to maintain its molar composition upon interaction with carbon nanomembranes.

Amorphous TiO2 Bridges Stabilized WS2 Membranes with Excellent Filtration Stability and Photocatalysis-Driving Self-Cleaning Ability.

Two-dimensional (2D) membranes as a new type of water filtration membrane have shown great potential in water separation and purification. However, their long-term stability under cross-flow conditions and their antifouling property are two main concerns for practical separation and purification processes. In this work, a strategy of nanoparticle bridges based on amorphous TiO2 is developed to link adjacent WS2 nanosheets on a WS2 membrane surface, leading to a strong membrane surface with excellent stability during 204 h of continuous cross-flow filtration. Moreover, the amorphous TiO2 bridges also form a TiO2/WS2 heterojunction on the WS2 membrane surface, exhibiting an impressive photocatalysis-driving self-cleaning property by pollutant photodegradation. And the flux recovery ratio (FRR) exceeds 95% after three cycles of separation experiments. The excellent long-term stability and photocatalysis-driving self-cleaning property of the WS2/TiO2 membrane provide a new approach to construct robust 2D membranes.

Ionic surfactants as assembly crosslinkers triggered supramolecular membrane with 2D↔3D conversion under multiple stimulus

HypothesisGeneral strategies leading to 2D assemblies promise a significant step forward in the development of supramolecular materials with diversity and superiority. Considering molecular packing parameter indicates a connection between molecular geometry and aggregate morphology, we predict the introduction of ionic surfactants as assembly crosslinker would be endowed to develop a methodology of 2D supramolecular assembles. ExperimentsIn this work, by introducing ionic surfactants such as sodium dodecylsulfate (SDS), the molecular packing parameter P in bolaamphiphile (A2G) system was increased, which successfully manipulated the transformation of the 3D vesicles into 2D membranes. This 2D membranes further showed excellent light and enzyme response, and thus 2D to 3D morphological conversion can be rationally controlled via UV/Vis light irradiation and alternate addition of β-CD and α-amylase. Significantly, the 2D feature revealed not only a remarkable fluorescence enhancement to luminescent molecules but also the ability to effectively remove pollutants from water through filtration. FindingsWe report a general and facile strategy for the construction of 2D supramolecular membranes, initiated by introducing ionic surfactants as assembly crosslinker to increase P. In the existence of stimulus response factors, 2D↔3D morphological conversion can be further controlled in a flexible manner, which opens up a new paradigm leading to interconvertible supramolecular materials.