GO Membranes Research Articles

Graphene Oxide and its Derivatives for Gas Separation Membranes

AbstractGraphene oxide (GO), a derivative of graphene with a hydrophilic 2D crystalline nanostructure and being an exfoliated form of graphite, is designed with several oxygen‐functional groups. Due to its characteristics it has attracted much interest for applications for gas separation membranes. This review focuses on GO‐containing membranes for gas separation. A basic look at gas separation membranes is presented, and the properties of graphene, GO, reduced GO (RGO) as well as their preparation approaches are covered. Different methods for fabrication of GO membranes in the style of mixed‐matrix membranes, thin‐film composites, layered GO membranes etc. are investigated. The gas separation performance of fabricated GO membranes is evaluated, highlighting recent developments and prospectives. Finally, future trends of GO membranes are described.

Designing GO Channels with High Selectivity for CO2 /N2 Separation via Incorporating Metal Ions.

Graphene oxide (GO) membranes holds great potential for high-performance CO2 capture. Aiming at enhancing the CO2 separation performance and structural stability of GO membranes, functionalizing GO channels with metal ions confers a promising strategy. In this study, we reported the fabrication of metal ion-incorporated GO membranes with remarkably improved CO2 /N2 separation performance. The metal ions within GO channels contribute to facilitating CO2 transport, decreasing N2 solubility, hindering N2 diffusion, and form multiple interactions with GO nanosheets. After introducing Mg2+ ions, the CO2 /N2 separation factor of GO membrane is remarkably increased from 4 to 48.8 with the CO2 permeance increases 1.5 times. Moreover, the separation performance of the GO-Mg2+ membranes shows an excellent long-term stability owing to the structural robustness. This study could provide insights into the regulation of the microstructure of metal ion-functionalized GO membranes for highly selective transport of specific molecules.

Surface modified and functionalized graphene oxide membranes for separation of strontium from aqueous solutions

Graphene oxide-strontium (GO-Sr) composites prepared under different ambient conditions were characterized using morphological and spectroscopic techniques to optimize the uptake of Sr from aqueous solutions. These studies indicated that interactions among GO and Sr2+ ions are highly sensitive to size and aging of GO sheets, as well as pH of the ambience. Further, the extent of Sr uptake on GO sheets was found to be largely influenced by relative fractions of the associated –COOH, –OH, C–O–C functional groups and sp2-C domains. Membranes prepared using various forms of GO were evaluated for their Sr separation ability and, a window of parameters for optimum separation of Sr has been proposed. Among the variety of membranes studied, those made up of fresh and large GO sheets were found to exhibit superior Sr adsorption capacity (~296 mg/g) at limited GO mass. Further, adsorption efficiency of these GO membranes was observed to deteriorate with aging of GO sheets and rise of GO mass on membrane. The membrane based filtration procedure introduced in present work facilitates to provide a lamellar structure of GO sheets with abundant surface area, diverse and accessible sites for Sr2+ ion uptake and offer high Sr adsorption efficiencies.

Chitosan-Graphene Oxide Composite Membranes for Solid-Phase Extraction of Pesticides.

Solid-phase extraction (SPE) coupled to LC/MS/MS analysis is a valid approach for the determination of organic micropollutants (OMPs) in liquid samples. To remove the greatest number of OMPs from environmental matrices, the development of innovative sorbent materials is crucial. Recently, much attention has been paid to inorganic nanosystems such as graphite-derived materials. Graphene oxide has been employed in water-purification processes, including the removal of several micropollutants such as dyes, flame retardants, or pharmaceutical products. Polysaccharides have also been widely used as convenient media for the dispersion of sorbent materials, thanks to their unique properties such as biodegradability, biocompatibility, nontoxicity, and low cost. In this work, chitosan–graphene oxide (CS_GO) composite membranes containing different amounts of GO were prepared and used as sorbents for the SPE of pesticides. To improve their dimensional stability in aqueous medium, the CS_GO membranes were surface crosslinked with glutaraldehyde. The composite systems were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, swelling degree, contact angle, and mechanical measurements. As the GO content increased, a decrease in surface homogeneity, an improvement of mechanical properties, and a reduction of thermal stability of the CS-based membranes were observed. The increased dimensional stability in water, together with the presence of high GO amounts, made the prepared composite membranes more efficacious than the ones based just on CS in isolating and preconcentrating different hydrophilic/hydrophobic pollutants.

Controlling interlayer spacing and organic solvent permeation in laminar graphene oxide membranes modified with crosslinker

Two-dimensional (2D) laminar membranes are attracting much attention in organic solvent nanofiltration owing to the promising permeation and selective performance derived from their 2D ultrathin separation layer. However, owing to easy swelling (enlargement of interlayer spacing), controlling the interlayer spacing in organic solvents and stable solvent permeation are major issues for their application to organic solvent systems. In this study, laminar GO membranes were fabricated on a porous polyketone (PK) support membrane using triethanolamine (TEOA) as crosslinker (GO-TEOA/PK). The interlayer spacing (d) between GO sheets was characterized by X-ray diffraction. The TEOA crosslinker effectively suppressed swelling of the GO laminar structure. For GO-TEOA/PK, the variation in d corresponded well with the affinity between TEOA and each solvent, Ra(TEOA–solvent), as estimated using the Hansen solubility parameters. Furthermore, common linear correlation between the permeance of alcohols and the parameter combining affinity, viscosity, and equivalent molar diameter was clearly observed. Therefore, the affinity between crosslinker and solvent was an important factor in controlling interlayer spacing and solvent permeation in the laminar GO membranes containing crosslinker.

Electrostatic self-assembly method to prepare intercalated graphene oxide composite membrane to improve hydrophilicity and flux

In this study, a graphene oxide layered double hydroxide (GO/Co-Al LDH) composite was prepared by electrostatic self-assembly method. Then GO and Co-Al LDH were assembled on the CA membrane by vacuum filtration to construct a high-efficiency composite membrane. The prepared CA/GO/Co-Al LDH composite membrane has excellent pollutant separation performance and hydrophilicity that the fluxes and the rejection rates for rhodamine b (RhB), methylene blue (MB), Congo red (CR), petroleum ether, n-hexane and mesitylene were excellent, even rejection rates reached more than 90%. The separation performance and hydrophilicity of GO membranes can be improved by changing the content of Co-Al LDH.

Synergetic effects of dodecylamine-functionalized graphene oxide nanoparticles on antifouling and antibacterial properties of polysulfone ultrafiltration membranes

The potential use of dodecylamine-functionalized graphene oxide (rGO-DDA) nanoparticles as an antifoulant and antibacterial nanofiller was investigated. GO and rGO-DDA were analyzed using Raman spectroscopy, FTIR, SEM, TEM, and TGA that all confirmed the successful functionalization of GO structure. Polysulfone (PSF) ultrafiltration membranes incorporating varied loadings of GO and rGO-DDA were then fabricated via phase inversion approach. All membranes were characterized in terms of chemical structure, morphology, hydrophilicity, porosity and mean pore size. Cross-section SEM images showed the distribution of GO and rGO-DDA between the pores and on the polymer walls. AFM results demonstrated that GO addition increased the roughness of membrane; while with rGO-DDA addition the surface became smoother. Pristine PSF and rGO-DDA based membranes exhibited similar hydrophilicity, while GO-based membranes exhibited higher hydrophilicity as revealed by contact angle measurements. Permeability, separation and antifouling experiments were performed in a cross-flow membrane setup and showed that flux decreases with the increase in GO and rGO-DDA concentration. rGO-DDA membranes showed higher antifouling and antibacterial performance compared to the pristine PSF and GO membranes. Neat PSF exhibited 65.4 % flux recovery ratio (FRR) against BSA that was increased to 86.9 % and 89.1 % with GO-0.1 and rGO-DDA-0.1, respectively. Against HA, FRR was improved from 87.8 % for neat PSF to 95.6 % and 99.3 % with GO-0.1 and rGO-DDA-0.1, respectively. Additionally, rGO-DDA membrane exhibited higher bacteriostatis rate (83.6 %) against H. aquamarina than GO membrane (62.9 %). Moreover, rGO-DDA nanoparticles exhibited excellent dispersibility in several solvents making them promising nanofillers for various membranes with high antifouling and antibacterial performance.

Exclusive and fast water channels in zwitterionic graphene oxide membrane for efficient water–ethanol separation

AbstractGraphene oxide (GO) membranes have shown great prospects as the next‐generation membranes to tackle many challenging separation issues. However, the employment of GO membranes remains difficult for the precise separation of molecules with strong coupling effect and small size discrepancy such as water–ethanol. Herein, a new strategy of constructing exclusive and fast water channels in GO membrane was proposed to achieve high‐performance water–ethanol separation via the synergy between zwitterion‐functionalized GO and hydrophilic polyelectrolyte. The as‐formed ordered and stable channels possess high‐density ionic hydrophilic groups, which benefit from inhibiting the strong coupling between water and ethanol, facilitating the fast permeation of water molecules while suppressing ethanol molecules. As a result, the ultrathin GO‐based membrane acquires exceptionally high separation performance with a flux of 3.23 kg/m2 h and water–ethanol separation factor of 2,248 when separating water–ethanol (10 wt%/90 wt%) mixture at 343 K. This work paves a feasible way to construct 2D channels for the high‐efficiency separation of strong‐coupling mixtures.

Superoleophobic graphene oxide/halloysite nanotube composite membranes for oil-water separation

In order to obtain separation membrane materials with high permeation flux and excellent oil/water emulsion separation performance, various strategies for inserting nano-second phases have been proposed to expand the membrane spacing and thereby improve water permeability. In this work, a series of nanocomposite membranes graphene oxide/halloysite nanotubes (GO/HNTs) composed of HNTs and GO nanosheets were successfully prepared by vacuum filtration of HNTs/GO suspensions prepared by a modified Hummers' method. The HNTs intercalate and grow around the GO nanosheets to increase the interlayer spacing of the composite membrane, which is beneficial to increase the permeation flux. In addition, thanks to the doping of hydrophilic HNTs and the uneven surface morphology due to the increase in surface roughness, the membrane permeation flux is improved. The GO/HNTs composite membrane was used for oil-water separation experiments with an endless separation device. The GO/HNTs composite membrane with underwater super-oleophobicity has a water flux of 1470 L m−2h−1 when the GO/HNTs content ratio is 1/3, which is 3 times that of pure GO membranes, while maintaining a high rejection of over 99.5%. The composite membrane maintains high water flux and high rejection rate under different pH conditions, and has acid and alkali resistance. After 10 times of repeated use, the high water flux was maintained as expected, indicating that the composite membrane is reusable.

Self-cleaning loose nanofiltration membranes enabled by photocatalytic Cu-triazolate MOFs for dye/salt separation

Although membrane separations have evinced an enormous potential in the treatment of dye wastewater, the ubiquitous membrane fouling leads to a dramatic decline of separation efficacy and life span, impeding the progress of membrane-based applications. The copper-triazolate MOFs (CuTz-1) exhibit robust thermal and chemical stability with a unique feature of degrading organic molecules under the visible light, rendering them highly promising for construction of self-cleaning membrane. This study presents the utilization of photocatalytic MOFs (CuTz-1) for construction of the CuTz-1/graphene oxide (CuTz-1/GO) composite membranes via filtration-assisted assembly followed by chemical crosslinking. The presence of CuTz-1 among the GO nanosheets promotes an enhancement of the GO interlayer distance, thereby promoting a higher water flux without compromising the dye rejections. The best-performing CuTz-1/GO membranes exhibits the high water permeance of 40.2 L m−2 h−1 bar−1 and superior dye rejections (Congo red: 99.4%, direct red: 98.2%, methyl blue: 94.9%), low salt rejections (e.g., NaCl: 0.3% and Na2SO4: 19.6%), as well as long-term nanofiltration stability. Furthermore, after visible light irradiation, the treated membrane was nearly recovered to have the original separation efficacy due to effective photocatalytic removal of the attached dyes from the membrane surface. The CuTz-1/GO membrane also exhibits an ultra-high bacteriostasis rate of 100%, probably to be ascribed to the combined antimicrobial effect of CuTz-1 and GO. This work is expected to give a useful guideline for developing loose nanofiltration membranes with favorable photocatalytic and antimicrobial properties, offering an appropriate solution of using self-cleaning membranes for the treatment of dye wastewater.

Molecular dynamics study of the pore formation in single layer graphene oxide by a thermal reduction process.

Nanoporous graphene is considered the next-generation material for reverse osmosis water desalination providing both high water permeability and almost complete salt rejection. The main problem with graphene is the difficulty of synthesizing membranes with a consistent subnanometer pore size distribution. A recently proposed solution involves processing as-grown graphene oxide (GO) monolayers via a mild temperature annealing pre-treatment causing GO functional groups to cluster into small oxidized islands. A following harsh thermal reduction process creates pores only in the small oxidized regions. However, a suitable relationship between the area of the GO islands and the pore dimension is still missing. Here, we study in detail the effects of such a thermal reduction process on the graphene oxide sheet by means of molecular dynamics simulations, particularly highlighting and analysing the process parameters affecting the final pore area. Besides proving that epoxides represent the most suitable functional group to induce carbon removal and, thus, pore generation in reduced GO, we find a twofold way to achieve control over the pore size: tuning the dimension and shape of the initial clustered GO areas or changing the harsh reduction process temperature. An accurate balance of these parameters consistently gives rise to targeted pore dimensions in graphene membranes.

Stable cation-controlled reduced graphene oxide membranes for improved NaCl rejection

Reduced graphene oxide (rGO) membranes have drawn intensive attention in applications of desalination and molecular sieving, owing to their stable and selective two-dimensional (2D) channels. However, their performance, particularly the selectivity for water and ions separation, is still under the theoretical limit. In this research, we incorporated potassium cations (K+) into the laminar channels of rGO membranes to form stable and selective K-controlled rGO (K-rGO) membranes for water and NaCl separation. These K-rGO membranes can be prepared by either thermally reducing K-controlled GO (K-GO) membranes in air or by soaking rGO membranes thermal-reduced in air (Air-rGO) in KCl solutions. Compared with reported K-GO and Air-rGO membranes, as-prepared K-rGO membranes exhibit narrower interlayer spacing in NaCl solutions due to the strong K+-π interaction between K+ and π-electron cloud on rGO nanosheets. This strong K+-π interaction can stabilize and confine 2D nanochannels and prevent the permeation of other cations, such as Na+, into rGO laminates, contributing to the enhanced NaCl rejection (91%) of K-rGO membranes. This work advances the understanding of the mass transport in rGO membranes and provides new insights into the design of selective 2D membranes for desalination, ionic sieving and other environmental applications.

Etching and acidifying graphene oxide membranes to increase gas permeance while retaining molecular sieving ability

AbstractGraphene oxide (GO) nanosheets stacked in parallel with subnanometer channels can exhibit an excellent size‐sieving ability for membrane‐based gas separation. However, gas molecules have to diffuse through the tortuous nanochannels, leading to low permeability. Herein we demonstrate two versatile approaches to modify the GO (before membrane fabrication by vacuum‐filtration) to collectively increase gas permeability, etching using hydrogen peroxide to generate in‐plane nanopores and acidifying using hydrochloric acid. For example, a membrane prepared at a pH of 5.0 using the 4‐h‐etched GO (HGO‐4h) shows He permeability of 5.3 Barrer and He/CH4 selectivity of 800, which are 5 times and 1.5 times those of the GO membranes, respectively. Decreasing the pH from 5.0 to 2.0 for HGO‐4h enhances He permeability to 57 Barrer and He/CH4 selectivity to 1,800. The HGO‐4h prepared at the pH of 2.0 exhibits separation properties of H2/CO2, H2/N2, He/N2, and He/CH4 surpassing their corresponding upper bounds.

Isothermal Crystallization Behavior of Poly(vinylidene fluoride) Based Membrane with Graphene Oxide

Abstract Graphene oxide (GO) sheets were obtained via an improved Hummer's method, and the Poly(vinylidene fluoride) (PVDF)/GO membranes employing GO as nucleating agent were prepared by solution casting. The crystalline behaviors and the crystallization kinetics of PVDF/GO membranes with varying GO concentration were investigated after isothermal crystallization at different temperatures. The results showed that the addition of GO induced the β-phase formation of PVDF. The spherulite diameter reached the peak value when the content of GO was 0.1 wt%, then reduced to the minimum value at 1 wt% GO within the tested temperature range of isothermal crystallization. It is worth mentioning that the relative fraction of β phase F(β) was dramatically improved to 0.85 for the PVDF/GO membrane with 0.5 wt% GO at 155°C. Furthermore, an enhancement of the crystallization rate was observed as the contents of GO increased in the system.

Principal Component Analysis of the Effect of Batch Variation, TiO2 Content and Reduction Temperature on the Surface Energy of TiO2/Graphene Oxide Membranes upon UV-C Activation

In this work principal component analysis (PCA) was used to study the effect of batch, reduction temperature, TiO2 content and UV-C irradiation on the surface energy, -polarity and interlayer spacing of photoactive TiO2/GO composite membranes. Two PCA models were successfully developed. The first PCA model shows a negative correlation between reduction temperature and d-spacing. Less hydrophilic TiO2/GO membranes were obtained at 160 °C compared to membranes reduced at 140 °C. Also, the surface polarity and surface energy were significantly enhanced by the addition of higher TiO2 content. The second model explain the effect of UV-C activation on the surface energy of the TiO2/GO composite membranes. For the GO membranes containing TiO2 a significant increase in surface energy and -polarity was observed after UV-C activation. Moreover, a positive correlation between the TiO2 content and surface energy after UV-C activation was observed. Higher TiO2 content results in higher surface energy. GO batches prepared by different groups was not found to significantly affect the properties of the TiO2/GO composite membranes suggesting that the preparation method is relative robust.

Preparation of graphene oxide/silica hybrid composite membranes and performance studies in water treatment

Water shortages have become a major problem facing the world today. Membrane separation technology is commonly used in water treatment applications. The development of new materials for water treatment can reduce the energy required for water treatment, reduce cost, and improve efficiency. The unique structure and properties of graphene and silica make them attractive materials for preparation of nanofiltration membranes for water treatment. We have successfully prepared a graphene oxide/silica (GO/SiO2) hybrid composite materials by in situ hydrolysis, using tetraethyl orthosilicate (TEOS) as a silicon source in an alkaline environment. The chemistry and structure of these materials are characterized by TEM, FTIR, Raman, and XPS of GO and the GO/SiO2 composite. BET porosimetry reveals that the total pore volume of the composite is 2.84 cm3 g−1, the specific surface area is 2897 m2 g−1, and the average pore diameter is 3.97 nm. We prepared GO membranes and GO/SiO2 composite membranes by vacuum suction filtration. The morphology of the membrane was observed by FESEM and AFM. The composite membrane has a larger surface roughness (Rms = 9.39). We also tested the thermal stability by TGA, and hydrophilicity by water contact angle measurements. The water permeance of the composite membrane is up to 229.15 L m−2 h−1 bar−1, and the rejection of the rhodamine B dye molecules is as high as 99%. The GO/SiO2 hybrid composite membrane has good hydrophilicity and thermal stability, high water permeance and rejection, and can be developed as a high-performance material for water treatment.

Ion transport through graphene oxide fibers as promising candidate for blue energy harvesting

Nanostructured graphene based membranes demonstrated excellent capabilities in various applications in nanofiltration and energy conversion due to unique atomically smooth surfaces and adjustable pore size or interlayers spacing at Angstrom scales. There are some reports on the osmotic power generation using physical confinements and electrostatic interactions between ions and GO membranes. However, the results indicated insufficient power densities (˂1 W/m2) can be achieved because of swelling of interlayer spacing of the GO membranes upon exposure to aqueous solutions which results in reducing the influence of confinement on ionic motilities. Here, the GO fibers is presented as one dimensional macrostructures including abundant aligned 2D nanochannels to produce electricity from salt concentration gradient. We used the GO fibers intercalated via cations to control the interlayer’s spacing depending on the cation’s hydrated size and consequently enhancing a stable confinement on ionic transport. Remarkable surface charge on nanochannel walls as well as cation pining of interlayer distances, provide a high mobility discrimination and the osmotic power density can reach to a value of 38 W/m2. This study introduces the GO fibers as new scalable structures for nanofluidics systems which could find range of applications in energy harvesting and molecular sieving.

Tuning the Interlayer Spacings in Dry Graphene Oxide Membranes via Ions.

We show the experimental achievement of dry GO membranes with interlayer spacings in the range from 7.09 Å to 8.72 Å, tuned and fixed by salts. We found that the interlayer spacings were dominated by the anions or the groups with negative charges in between the GO membranes. Density functional theory (DFT) calculations reveal that the highly efficient tuning of the interlayer spacing in dry GO membranes is due to ion-π interactions on the graphene sheets, together with the steric effects of anions in between the GO sheets. The findings are helpful for extending their potential applications including chemical sensors, nanomaterial devices preparation, chemical catalysis and synthesis, and gas separation.

Preparation of glycine mediated graphene oxide/g-C3N4 lamellar membranes for nanofiltration

In this work, we fabricated a series of composite membranes for nanofiltration by filtration-assisted assembly of graphene oxide (GO, with a thickness of ca. 2 nm) and graphitic carbon nitride (g-C3N4, with a thickness of ca. 5.2 nm) nanosheets with the assistance of glycine as a molecular linker to enhance their interactions. The effects of g-C3N4 and glycine concentrations on the lamellar spacing of GO and membrane performance were subsequently investigated. Interestingly, experimental and characterization results showed that both g-C3N4 and glycine could increase the interlayer spacing of pristine GO membranes, but the former reduced the dimensions of nanochannels while the latter led to enlarged channels. As compared with pristine GO membranes, the composite membranes resulted in faster water transportation without sacrificing solute retention. With further functionalization by introducing hyperbranched polyethyleneimine (HPEI) coating, we demonstrated that an integrated Gly-GO/g-C3N4 membrane (ca. 116 nm in thickness) exhibited excellent separation performance for various organic dye solutions under different operational conditions (feed concentration, pH, etc.). Long-term stability experiments showed that this membrane yielded 90%–93% dye rejection with only a slight decline in permeance over a 40 h testing period, indicating acceptable stability. The pure water permeance of the Gly-GO/g-C3N4 membrane reached as high as 207 L h−1 m−2 bar−1. These results indicate that the covalent modification of small molecules can improve the packing of GO nanosheets and lead to outstanding nanofiltration performance which has great potential applications in the field of water purification and separation.

The roles of oxygen-containing functional groups in modulating water purification performance of graphene oxide-based membrane

Due to the excellent mechanical properties and tunable inter-layer distance spacing (D-spacing), graphene oxide based nanofiltration membranes (GONMs) have attracted tremendous attention for wastewater treatment and seawater desalination. However, GONMs with different reduction degrees or originate from different graphite resources present totally distinct purification performance. The principal consideration of the differences between those GONMs is the oxygen-containing functional groups (carboxyl (–COOH), hydroxyl (–OH) and epoxy (–COC-)). While, it is still not fully understood what role do different oxygen-contained functional groups play. Herein, in order to reveal this knowledge gap, we fabricated and systematically studied three oxygen-containing functional groups dominant (C-GO, H-GO and E-GO) GO membranes. Zeta potential was measured to analysis the surface properties of C-GO, H-GO and E-GO nanosheets. Furthermore, XRD was used to observe the D-spacing of GONMs. The results showed that the oxygen-contained functional groups with different hydrophilicity and electricity modulated the D-spacing of GONMs effectively, which resulted in different water permeance and sieving performance of the GONMs. Besides, C-GONMs with abundant negative charges exhibited excellent antifouling capacity for Trypan blue (dye) owing to the electrostatic interaction effect. Such differences between GONMs with three kinds of oxygen-contained functional groups pave a powerful platform to better understand and design the GO based separation membrane.