Graphene Membranes Research Articles

Raman Scattering Measurement of Suspended Graphene under Extreme Strain Induced by Nanoindentation.

Graphene is known as a superstiff and extremely strong material. Hence, applying strains greater than 1% to graphene and simultaneously measuring changes in its physical properties has been challenging because of the limited methodologies for measuring both high strain and other physical properties. Here, Raman scattering measurement of suspended graphene under extremely high biaxial strain as large as 6.1% using an atomic force microscopy (AFM)-Raman spectroscopy measurement tool is reported. Nanoindentation is performed using AFM tips machined to have a flat top and a hole shape, resulting in a strained graphene area sufficiently large to enable the acquisition of a Raman signal. At the same time, the laser light is focused on the strained flat area of the graphene membrane. The Raman signals of the G and 2D bands of graphene are redshifted by 282 and 684 cm-1 , respectively, which is unprecedented for graphene. This measurement technique provides an effective methodology to measure variations in the physical properties of atomically thin materials under superhigh strain.

Highly efficient and selective H2/CH4 separation by graphene membranes with embedded crown ethers

Designing a highly selective and fast separation membrane is critical to realize H2-sieving with efficient performance, which is still a challenge by far. In this study, we combine molecular dynamics (MD) and first-principle approaches to investigate the H2 separation property of graphene crown ether. Our MD results demonstrate that graphene crown ether exhibits a superior H2-sieving performance under various temperatures and external pressures, allowing fast passage for H2 while completely rejecting CH4. The H2 permeability exceeds 105 GPU at selectivities exceeding 1500. These results highlight an outstanding performance of crown ether pores for separating H2 from mixtures with CH4. Observed from MD simulation trajectories, this highly selective separation is caused by the intrinsic dimensional difference between the two types of gases and the pore. The first-principle calculations further confirm that it is energetically more favorable for H2 to transit the graphene crown ether than for CH4. Our findings suggest a novel application of the graphene crown ether in H2-sieving with great promise for future membrane designs.

On the Performance of Vertically Aligned Graphene Array Membranes for Desalination.

In this paper, we perform molecular dynamics simulations to investigate the performance of multilayer graphene slit membranes. Graphene slit membranes at a critical slit size have been found to be promising desalination membranes. In this contribution, it is shown that multilayer slit membranes have the potential to provide significantly better permeability while retaining outstanding salt rejection. Improved permeability of the membrane is achieved by using slits of widths larger than the critical slit size required to reject salt through size exclusion, and desalination of sea water is performed by increased resistance to salt passage through the multilayering. To facilitate the design process of future multilayer membranes, we analyze the flow resistance of the membrane as a combination of electrical resistors in series and show that this analogy works for membranes where the layers possess the same slit size, as well as membranes with layers of different slit sizes. Comparing with single layer graphene membranes, it was shown that it is possible to obtain 55% improvement in permeability without loss in salt rejection capabilities through multilayering. This opens up possibilities for membrane designers to be free from the restrictions of using a single layer graphene slit membrane with a fixed slit width.

Microfabrication of sealable microcell array with ultrathin metal-graphene membrane

A significant challenge in quasi-two dimensional (2D) materials applications is the fabrication of large, suspended layers. This issue is particularly critical when the fabrication of closed cells is required, for example in in-operando x-ray photoemission spectroscopy (XPS) or transmission electron microscopy (TEM), or the design of Golay cell-like detectors. In the quasi-totality of working protocols, the 2D layer transfer is performed in a liquid environment, and the large surface tension forces that are developed during the drying steps often result in the layer damage. This effect becomes dramatic when the fabrication of sealed cells is required. Here we propose an alternative route that involves the sealing in a two-step process, with, first, the fabrication of a large and suspended membrane and then a sealing step performed with the help of a microfabricated self-aligned sealing cork. As a 2D suspended membrane, we choose an ultrathin membrane of single-layer Graphene coupled with a 7.5 nm thick film of evaporated titanium which was transferred over a large array of micrometric holes fabricated on a 30 μm thick silicon membrane. As a cork, we used a silicone structure obtained by pouring liquid silicone in a microfabricated silicon well. The throughput of the whole fabrication process was assessed either by electronic or optical microscopy, to obtain the total yield of integer cells, and by generating a pressure difference within each cell to test the sealing efficiency. A fabrication and sealing yield larger than 95% on arrays as large as 10 mm 2 were demonstrated. • Micro- and nano-meter scale sealed cells are an emerging field for XPS and TEM imaging in-operando. • Graphene and other 2D materials are being studied as suspended membranes for cells sealing. • The freestanding area is limited by defects and the grain size of the 2D crystalline materials. • Graphene-metal nanomembrane and secondary sealing were used to create arrays of sealed cells over more than 10 mm 2 .

A Resonant Graphene NEMS Vibrometer.

Measuring vibrations is essential to ensuring building structural safety and machine stability. Predictive maintenance is a central internet of things (IoT) application within the new industrial revolution, where sustainability and performance increase over time are going to be paramount. To reduce the footprint and cost of vibration sensors while improving their performance, new sensor concepts are needed. Here, double-layer graphene membranes are utilized with a suspended silicon proof demonstrating their operation as resonant vibration sensors that show outstanding performance for a given footprint and proof mass. The unveiled sensing effect is based on resonant transduction and has important implications for experimental studies involving thin nano and micro mechanical resonators that are excited by an external shaker.

Array of Graphene Variable Capacitors on 100 mm Silicon Wafers for Vibration-Based Applications.

Highly flexible, electrically conductive freestanding graphene membranes hold great promise for vibration-based applications. This study focuses on their integration into mainstream semiconductor manufacturing methods. We designed a two-mask lithography process that creates an array of freestanding graphene-based variable capacitors on 100 mm silicon wafers. The first mask forms long trenches terminated by square wells featuring cone-shaped tips at their centers. The second mask fabricates metal traces from each tip to its contact pad along the trench and a second contact pad opposite the square well. A graphene membrane is then suspended over the square well to form a variable capacitor. The same capacitor structures were also built on 5 mm by 5 mm bare dies containing an integrated circuit underneath. We used atomic force microscopy, optical microscopy, and capacitance measurements in time to characterize the samples.

Graphene Membranes for Multi‐Dimensional Electron Microscopy Imaging: Preparation, Application, and Prospect

AbstractTechnological breakthrough in electron microscopy (EM) has started a resolution revolution in EM imaging. Nowadays, the promotion of resolution demands a robust background‐noise‐free EM support for specimen preparation, which is a primary bottleneck of high‐resolution EM imaging. Owing to the atomic thickness and excellent physical properties, graphene has attracted widespread attention in the EM field to achieve high‐resolution multi‐dimensional imaging. However, it is still challenging to prepare high‐quality suspended graphene membranes. Problems like breakage, contamination and wrinkling reduce the quality of suspended graphene membranes, which inhibit their wide and killer applications in EM imaging. In this review, suspended graphene membranes are looked deeply into for multi‐dimensional EM imaging. This study begins with a brief introduction to EM development, followed by a discussion of the synthesis of high‐quality graphene. Then it summarizes various approaches to produce suspended graphene membranes and their killer applications in multi‐dimensional EM characterization, including high‐resolution 2D imaging, cryo‐EM 3D reconstruction, and 4D in situ liquid EM. Based on current achievements, the prospects of graphene membranes for more cutting‐edge applications are finally proposed.

Electrically conductive carbon nanotube/graphene composite membrane for self-cleaning of biofouling via bubble generation

Biofouling is a major operational problem in membrane-based filtration processes, owing to the formation of intractable biofilms. Recently, electrically conductive membranes have attracted significant interest for fouling mitigation by membrane self-cleaning. However, water flux has not been completely recovered by electrochemical self-cleaning using bubble generation on the cathodic membrane surface, with the cause of insufficient water flux recovery not clearly identified. In this study, a carbon nanotube (CNT)/graphene membrane that exhibits high stability even under high voltage (i.e., 15 V) was fabricated. The self-cleaning effect was investigated under different applied voltages and self-cleaning times and evaluated based on water flux recovery and via quantitative biofilm analysis. The developed self-cleaning membrane achieved more than 4 log viable cell removal and 95.2% total extracellular polymeric substance removal on the membrane surface in 10 min. Through filtration and self-cleaning cycles, 100% water flux recovery was achieved, and the stability of the CNT/graphene membrane was verified. It was confirmed that the insufficient recovery of self-cleaning through conventional water electrolysis occurred due to the residual biofilm matrix, and the water flux recovery efficiency was improved using high voltage, suggesting that sufficient recovery can be achieved by a combination of physical and chemical/biological cleaning methods.

Graphene-Based Functional Hybrid Membranes for Antimicrobial Applications: A Review

Graphene-based nanomaterials have shown wide applications in antimicrobial fields due to their accelerated rate of pathogen resistance and good antimicrobial properties. To apply graphene materials in the antimicrobial test, the graphene materials are usually fabricated as two-dimensional (2D) membranes. In addition, to improve the antimicrobial efficiency, graphene membranes are modified with various functional nanomaterials, such as nanoparticles, biomolecules, polymers, etc. In this review, we present recent advances in the fabrication, functional tailoring, and antimicrobial applications of graphene-based membranes. To implement this goal, we first introduce the synthesis of graphene materials and then the fabrication of 2D graphene-based membranes with potential techniques such as chemical vapor deposition, vacuum filtration, spin-coating, casting, and layer-by-layer self-assembly. Then, we present the functional tailoring of graphene membranes by adding metal and metal oxide nanoparticles, polymers, biopolymers, metal–organic frameworks, etc., with graphene. Finally, we focus on the antimicrobial mechanisms of graphene membranes, and demonstrate typical studies on the use of graphene membranes for antibacterial, antiviral, and antifungal applications. It is expected that this work will help readers to understand the antimicrobial mechanism of various graphene-based membranes and, further, to inspire the design and fabrication of functional graphene membranes/films for biomedical applications.

Microscopic Insight into Water Desalination through Nanoporous Graphene: The Influence of the Dipole Moment.

Nanoporous graphene membranes with controllable pore size and chemical functionality may be one of the most desirable materials for water desalination. Herein, we investigate desalination performance of hydrogen-functionalized nanoporous graphene membranes. The charge values on hydrogen atoms (qH) and carbon atoms at the pore rim are systematically adjusted. For qH > 0, the flow rate decreases as qH increases, whereas for qH < 0, the flow rate tends to increase first and then decrease with increasing qH, yielding a peak at ∼ -0.2 e. Moreover, nanopores with large dipole moments at the rim have little effect on the salt rejection. The calculated oxygen and hydrogen density maps, the potential of mean force for water molecule and salt ion passage through the nanopores, and the coordination number unveil the mechanisms underlying water desalination in nanoporous graphene. This work may inspire the design and improvement of two-dimensional membranes for water desalination.

Sensitive Transfer-Free Wafer-Scale Graphene Microphones.

During the past decades micro-electromechanical microphones have largely taken over the market for portable devices, being produced in volumes of billions yearly. Because performance of current devices is near the physical limits, further miniaturization and improvement of microphones for mobile devices poses a major challenge that requires breakthrough device concepts, geometries, and materials. Graphene is an attractive material for enabling these breakthroughs due to its flexibility, strength, nanometer thinness, and high electrical conductivity. Here, we demonstrate that transfer-free 7 nm thick multilayer graphene (MLGr) membranes with diameters ranging from 85–155 to 300 μm can be used to detect sound and show a mechanical compliance up to 92 nm Pa–1, thus outperforming commercially available MEMS microphones of 950 μm with compliances around 3 nm Pa–1. The feasibility of realizing larger membranes with diameters of 300 μm and even higher compliances is shown, although these have lower yields. We present a process for locally growing graphene on a silicon wafer and realizing suspended membranes of patterned graphene across through-silicon holes by bulk micromachining and sacrificial layer etching, such that no transfer is required. This transfer-free method results in a 100% yield for membranes with diameters up to 155 μm on 132 fabricated drums. The device-to-device variations in the mechanical compliance in the audible range (20–20000 Hz) are significantly smaller than those in transferred membranes. With this work, we demonstrate a transfer-free method for realizing wafer-scale multilayer graphene membranes that is compatible with high-volume manufacturing. Thus, limitations of transfer-based methods for graphene microphone fabrication such as polymer contamination, crack formation, wrinkling, folding, delamination, and low-tension reproducibility are largely circumvented, setting a significant step on the route toward high-volume production of graphene microphones.

Graphene kirigami as an ultra-permeable water desalination membrane

In the present study, we reveal the potential of graphene kirigami (GK) as an ultra-permeable membrane for water desalination using molecular dynamics simulation. The results demonstrate that the GK membrane has superior performance in water permeance, obtaining over 103 L/m2/h/bar with 100% salt rejection. The theoretical efficiency of water permeance using GK membrane is 2.4–5.8 times higher than that of the nanoporous graphene membrane and 2–4 orders of magnitude higher than reported conventional reverse osmosis membranes. Due to the unique geometry of the kirigami structure, the GK membrane would form the adsorption channels to reduce the energy barrier during the desalination processes and guide water molecules to be filtered more effectively. Moreover, compared with the nanoporous graphene, the GK membranes do not have rigorous pore geometry restrictions for desalination and can be generated via mechanical deformation, which provides significant convenience for future applications in practical projects.

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.

Photocatalytic-triggered nanopores across multilayer graphene for high-permeation membranes

2D nanoporous graphene nanomaterials have been considered for the development of high permeability membranes, compared to dense laminate architectures. Current perforation technologies, however, have struggled to deliver a membrane for practical use due to a lack of scalability and increased related complexity/costs over commercial membranes. Herein, the perforation of ultrathin graphene membranes, with thicknesses ranging from 50 to 200 nm were performed via a triggered and site-selective photocatalytic etching process. The perforated graphene membranes exhibited a narrow distribution of in-plane nanopores with sizes ranging from 20 and up to100 nm, depending on irradiation durations. The surface pore density across porous graphene can be tuned, achieving a maximum surface density of 1011 cm−2, depending on the amounts of pore-mediators i.e. nano-catalysts loaded to multilayer graphitic assemblies. The perforated membranes exhibited a water permeation of 85 LMH/bar, 3.5 times higher compared to unperforated membrane analogues though a decrease in dye removal (∼20% for the methylene blue organic dye) was noted over the extended permeation duration (2-hour). The synergetic characteristics between inherent nanochannels between graphite planes and incorporated nanopores across such ultrathin perforated graphene membranes promise improvements in water treatment using such architectures of high permeability graphene membranes.

Mitigation of hazards and risks of emerging pollutants through innovative treatment techniques of post methanated distillery effluent - A review

Distillery wastewater has high biological and chemical oxygen demand and requires additional treatment before it can be safely discharged into receiving water. It is usually processed through a biomethanation digester and the end product is the post-methanated distillery effluent (PMDE). Research have shown that PMDE released by molasses-based distilleries is a hazardous effluent that can cause harm to the biota and the environment; it contains elevated amount of total dissolved solids (TDS), total suspended solids (TSS) and excess levels of persistent organic compounds (POPs), heavy metals, phenolic compounds, and salts. The practice of wastewater reuse for irrigation in many water scarce countries necessitates the proper treatment of PMDE before it is discharged into receiving water. Convention methods have been in practice for decades, but innovative technologies are needed to enhance the efficiency of PMDE treatment. Advance physical treatment such as membrane separation technology using graphene, ion-exchange and ultrafiltration membranes; chemical treatment such as advanced oxidation methods, electrocoagulation and photocatalytic technologies; biological treatment such as microbial and enzymatic treatment; and hybrid treatment such as microbial-fuel cell (MFC), genetically modified organisms (GMO) and constructed wetland technologies, are promising new methods to improve the quality of PMDE. This review provides insight into current accomplishments evaluates their suitability and discusses future developments in the detoxification of PMDE. The consolidated knowledge will help to develop a better management for the safe disposal and the reuse of PMDE wastewater.

New Structural Insights into Densely Assembled Reduced Graphene Oxide Membranes

AbstractDensely assembled graphene‐based membranes have attracted substantial interest for their widespread applications, such as compact capacitive energy storage, ion/molecular separation, gas barrier films, and flexible electronics. However, the multiscale structure of densely packed graphene membranes remains ambiguously understood. This article combines X‐ray and light scattering techniques as well as dynamic electrosorption analysis to uncover the stacking structure of the densely stacked reduced graphene oxide (rGO) membranes. The membranes are produced by reducing graphene oxide (GO) membranes with hydrazine, during which the colloidal interactions between GO sheets are modulated by the electrolyte solution. In contrast to the common notion that direct reduction of densely assembled GO sheets in parallel tends to result in significant “graphitization”, this article unexpectedly discovers that the resultant densely packed rGO membrane can still retain the interconnected network nanochannels and show good capacitive performances. This inspires the development of a hierarchical structural model to describe the densely packed rGO membranes. This article further shows that the nanochannel network can be fine‐tuned at the sub‐nanometer level by tailoring the salt concentration and the reduction temperature to render exceptional volumetric capacitance and good rate performance for rGO membranes even with increased packing density.

Molecular advection–diffusion through graphene nanopores

Nanoporous graphene promises extremely high flow rates in membrane separation applications owing to its single atom thickness. While sub-nanometer graphene pores are desirable for rejecting ions and small molecules, transport through larger graphene nanopores is relevant in nanofiltration applications and to leakage flow through membrane defects. The permeance of these pores is commonly estimated using continuum transport relationships in graphene membrane modeling, although errors are expected due to sub-continuum behavior that emerges for smaller nanopores. In this paper we perform molecular dynamics simulations to understand the departure of solute advection–diffusion through graphene nanopores from continuum theory. We find that the same diameter dependent effective pore thickness can be used to adjust continuum models for both the net flow rate and solute mass transfer rate to match simulations. The increased resistance to both modes of transport is attributed to reduced molecule mobility in the dense layers of fluid that form on either side of the graphene and within the pore. The results further indicate the extent of deviations from continuum solute mass transfer that can be expected through graphene nanopores, toward more accurate modeling of nanoporous atomically thin membranes.

Control one-dimensional length of rectangular pore on graphene membrane for better desalination performance

At present, there is a general contradiction between permeability and selectivity of reverse osmosis (RO) membranes for desalination; a membrane with higher water permeability will give a lower salt rejection or selectivity, and vice versa. In this work, single-layer nanoporous graphene is used as RO membrane to investigate the effects of pore shape to reduce this contradiction by molecular dynamics simulations. Two kinds of pores (round and rectangular pores) with different sizes are simulated. For round pore, although the water permeability increases with the increase of the pore size, the salt rejection rate drops rapidly. For rectangular pore, reasonable designed pore structure can achieve improved water permeability and high salt rejection of graphene membrane by keeping one-dimensional length (i.e. the width) of the pore less than the size of the hydrated ions and increasing the other dimensional length. The restriction of one dimension can prevent the passage of hydrated ions through the pore effectively. This ‘one-dimensional restriction’ provides a simple strategy for designing RO membrane with variable pore structures to obtain a better desalination performance.

Water desalination by charged multilayer graphene membrane: A molecular dynamics simulation

Graphene, due to its unique excellent properties, is proposed as a developing method with high efficiency compared to classical water desalination methods. In this regard, charging the membrane is considered a promising and effective approach to enhance the performance of the graphene membrane. In this research, by using molecular dynamics simulations, the water desalination by charged multilayer graphene is evaluated and the influence of electric charge amount and geometric parameters, including the pore diameter and the interlayer distance, are investigated. According to the results, the multilayer nanoporous graphene with 16.35 Å pore diameter, in which the electric charge is distributed with the proper amount and particular arrangement on the carbon atoms on the periphery of the pores, shows a significant enhancement of 59.4% in the water flow rate and improves the salt rejection efficiency from 67.27% to the desired value of 94.54%, compared to the uncharged multilayer graphene with 12.6 Å pore diameter. Moreover, it is demonstrated that increasing the interlayer distance of the membrane diminishes the effect of electric charge on improving the water flux.

Confined lamellar channels structured by multilayer graphene for high-efficiency desalination

As a versatile two-dimensional (2D) material, graphene has attracted an increasing amount of attention from different fields, e.g., Geim et al. (Science 2019, 363: 145–148) experimentally reported artificial graphene capillaries with a 2D slit of a few angstroms in height, where water fully moves through these capillaries and all salt ions are excluded. To understand the phenomenon observed experimentally, here we systematically investigate hydrodynamic properties of the lamellar graphene channels with different interlayer height, such as self-diffusion, viscosity and permeability of waters. Molecular dynamics simulations show that water viscosity decreases with the increasing of channel height, while the diffusion coefficient and flux of water enhance. For the channel with a height of 0.78 nm, the water permeability is far larger than the values of commercial reverse osmosis membranes, simultaneously exhibiting a salt rejection of >95%. Besides, the simulated results show that the flexible channels initially filled with water will not collapse when the channels are formed by more than two layers of graphene sheets on each side. This work highlights the stability mechanism of the graphene channel, which will show its importance for future development of graphene membranes.