Modified Graphene Oxide Membranes Research Articles

Construction of robust one-dimensional nanowire-regulated graphene oxide membranes for efficient dye/salt separation

Loose nanofiltration (NF) for graphene oxide (GO) membranes is an emerging efficient strategy for separating dye/salt mixtures in textile wastewater to realize sustainable resource recovery. This study proposes a novel approach for constructing a laminated GO membrane with interlayer water-transporting nanochannels by intercalating tannic acid nanowires (TANs). The interlayer spacing of the GO membrane can be expanded by one-dimensional TANs while cross-linking between hydrophilic TANs and GO nanosheets enhance the mechanical stability of this sheet-wire structure and interlayer channels. The membrane is able to achieve excellent water permeability without sacrificing selectivity due to the dual driving force resulting from the ultrafast water transport ability of primitive graphene planes and interlayer hydrophilic nanowires. The modified GO membrane exhibits a high water permeability of 52.1 L m−2 h−1 bar−1, which is twice that of the pure GO membrane and excellent selectivity of 85.5 to the methyl blue/NaCl mixed solution. Overall, this work provides a feasible strategy for constructing a mixed-dimensional GO-based membrane with a robust structure and multi-pathways to achieve excellent perm-selectivity for dye/salt separation.

A high-efficient and salt-rejecting 2D film for photothermal evaporation

The solar-driven desalination is seen as a sustainable way to combat water scarcity. However, the solar steam generation efficiency has long been restricted by the high vaporization enthalpy of water and low energy density of natural sunlight. We introduced graphene oxide (GO) cross-linked with polyethyleneimine (PEI) as the photothermal material, with the enriched ammonic functional groups in modified GO membrane (GPM) activating water molecules to evaporate with much lower energy consumption. The vaporization enthalpy at the air-film interface is reduced up to 42% in GPM film by tuning the thermodynamic states of water. Consequently, GPM film enables a high evaporation rate of 2.48kg m-2 h-1 with 95.7% energy conversion efficiency under 1 sun. With the aid of positive charges introduced by hydrolysis of PEI, the GPM exhibits excellent salt resistance and delivers an evaporation rate around 1.8kg m-2 h-1 when treating 20 wt% NaCl solution.

Surface engineering of graphene oxide membranes for selective separation of perfluorooctanoic acids

Perfluoroalkyl compounds (PFCs) are environmental toxicants and their widespread detection and accumulation in the environment can be detrimental to the ecosystem. In this study, surface charge of GO membranes was engineered to enhance selectivity of graphene oxide (GO) membranes and for the removal of perfluorooctanoic acid (PFOA ∼400 Da) in real concentration ranges in wastewater streams. The structure and physicochemical properties of the GO and modified GO nanosheets and their corresponding membranes were assessed using several characterization techniques. The modified GO membranes showed a water flux of 83.1 ± 1 L m−2 h−1 together with maximum rejection efficiency of 98.4% for 50 ppm PFOA at pressure of 0.45 MPa, which is the highest value reported to date. Compared to the GO membrane, the modified GO membrane exhibited higher water flux and PFOA rejection efficiency by 300% and 70%, respectively. The high surface charged and electron rich GO nanosheets reduce the interlayer spacing in the modified membrane and simultaneously enhance the surface hydrophilicity which leads to increased PFOA retention and enhanced water permeation. These results point to the potential of the modified GO membranes for wastewater treatment.

Using Al3+ to Tailor Graphene Oxide Nanochannels: Impact on Membrane Stability and Permeability.

Graphene oxide (GO) membranes, which form from the lamination of GO sheets, attract much attention due to their unique nanochannels. There is much interest in controlling the nanochannel structures and improving the aqueous stability of GO membranes so they can be effectively used in separation and filtration applications. This study employed a simple yet effective method of introducing trivalent aluminum cations to a GO sheet solution through the oxidation of aluminum foil, which modifies the nanochannels in the self-assembled GO membrane by increasing the inter-sheet distance while decreasing intra-sheet spacing. The Al3+ modification resulted in an increase in membrane stability in water, methanol, ethanol, and propanol, yet decreased membrane permeability to water and propanol. These changes were attributed to strong interactions between Al3+ and the membrane oxygenated functional groups, which resulted in an increase in membrane hydrophobicity and a decrease in the intra-sheet spacing as supported by surface tension, contact angle, atomic force microscopy, and X-ray photoelectron spectroscopy measurements. Our approach for forming Al3+ modified GO membranes provides a method for improving the aqueous stability and tailoring the permeation selectivity of GO membranes, which have the potential to be implemented in vapor separation and fuel purification applications.

In-plane and through-plane dielectric properties of graphene oxide membrane: Effect of Al3+ modification

A graphene oxide (GO) membrane can be used in structural dielectric capacitors and planar micro-supercapacitors as a dielectric separator material due to its excellent through-plane and in-plane dielectric properties. The ability to tailor the dielectric properties of the GO membrane is crucial for dielectric applications. We believe this to be the first demonstration describing how Al3+ ions impact the dielectric properties of the GO membrane through cation to oxygenated functional groups interactions. In this study, the Al3+-modification to the GO membrane was realized through a facile pre-coordination method followed by layer-by-layer assembling. The influence of Al3+ on the GO membranes’ through-plane and in-plane dielectric properties (dielectric constant and dielectric loss) was investigated in the frequency range from 0.01 Hz to 105 Hz. Plane-to-plane and edge-to-edge Al3+coordination with the functional group on the basal plane or edge of the GO sheets was proposed to understand the metal ion's impact on the dielectric properties. The results demonstrated that the Al3+ modification was an effective strategy to tailoring the GO membranes’ dielectric properties in a low frequency regime and further suggested the possibility of designing dielectric GO membranes with controllable dielectric performance. At a low frequency range (0.01–1 Hz), the addition of Al3+ increased the through-plane dielectric constant and decreased the in-plane dielectric constant. In terms of dielectric loss, unmodified GO membranes exhibited a higher in-plane dielectric loss than the through-plane dielectric loss. Al3+-modified GO membranes exhibited increased through-plane dielectric loss and decreased in-plane dielectric loss compared to the unmodified GO membranes.

Retarded transport properties of graphene oxide based chiral separation membranes modified with dipeptide

The separation of racemates is still a great challenge, since enantiomers commonly exhibit identical physicochemical properties in achiral environments, but differ in biological responses in chiral surroundings. Recently, graphene oxide (GO) based enantionseparation membranes had been proven to follow the facilitated or retarded transport mechanism, dependent on the choice of chiral selectors. Herein, a dipeptide was selected to act as the chiral selector for separating racemates, since it has a longer molecular length than single amino acids and two chiral centers as well, thus more potentiallypenetrating into the interlayer space in GO membranes. Results show that dipeptide-modified GO membranes exhibit a maximum separation factor of 1.85 with a 1–3 orders of magnitude improved flux in comparison to conventional chiral separation membranes, particularly a reversal of the transport mechanism from facilitated to retarded with respect to GO membranes previously reported. This is a likely consequence of the L-isomeric dipeptide that possesses a much stronger affinity toward the D-isomeric chiral analyte than the L-isomeric one due to the occurrence of extra nonstereoselective interaction between probes and the opposite GO substrate, so that such the dipeptide modified GO membrane exhibits a retarded transport mechanism, thereby offering a promise to achieve the continuous operation. These findings demonstrate that the extra nonstereoselective interaction plays a key role in the chiral recognition, especially in the confined space, and provides new opportunities for controlling the conversion from facilitated to retarded transport mechanisms and designing the novel enantioseparation membranes, aiming at the large-scale production of diversely pure enantiomers.

Chiral separation of β-cyclodextrin modified graphene oxide membranes with a complete enantioseparation performance

The separation of racemates remains a great challenge as they are enantiomers with similar chemical structures and physicochemical properties in achiral environments. Recently, graphene oxide (GO) based membranes modified with various chiral selectors had been proved to allow for both high permeation flux and enantioselectivity. Herein, mono(6-amino-6-deoxy)-beta-cyclodextrin (β-CD) was selected to serve as chiral selector in GO-based membranes for separating racemates, since it potentially forms specific inclusion complexes with guests. Results show that β-CD displays a much stronger enantioselective affinity toward l-enantiomer than d-enantiomer, so that such β-CD modified GO membrane (β-CD-GOM) exhibits a retarded transport mechanism, having exceptional enantioselectivities with the enantiomeric excess value of nearly 100%. Moreover, β-CD-GOMs are 1-2 orders of magnitude higher in flux than traditional enantioseparation membranes. These findings demonstrate that β-CD-GOMs could provide new opportunities for improving enantioseparation performances with both high permeability and high permselectivity and for the large-scale production of various enantiomers.

Surfactant-modified graphene oxide membranes with tunable structure for gas separation

As one type of novel molecular sieving membranes, graphene oxide (GO) membrane shows significant promise in gas separation. However, the precise control of its laminate structures for molecular sieving is still challenging and much more work is still required to establish the synthesis method in a facile, reproducible and saleable manner. Herein, we report the facile preparation of a sodium dodecylbenzenesulfonate (SDBS) modified GO membrane supported on porous Al2O3 hollow fibers for efficient H2/N2 and H2/CO2 separation. Due to the π-π and hydrophobic interactions between SDBS and GO sheets, SDBS can effectively regulate the interlayer-spacing distance, and surface wettability of the resultant GO membrane. Consequently, the gas separation factor of the SDBS modified GO membrane is significantly increased, which is 323 and 28 (at room temperature) for H2/CO2 and H2/N2 separation, respectively, with hydrogen permeance in the order of magnitude of 0.81 ×10−7 mol m−2 s−1 Pa−1. The SDBS modified GO membrane coated on the industrial applicable Al2O3 hollow fiber will provide a facile and reliable solution for practical application of the GO membrane in advanced gas purification technologies.

Improving the proton conductivity of graphene oxide membranes by intercalating cations

Graphene oxide (GO) membrane has gained increasing attention because of its extraordinary physical and chemical properties and high proton conductivity. GO is rich in oxygenated functional groups, which can support proton transportation. However, pristine GO is unstable at high temperatures due to the removal of oxygen functional groups, resulting in a decrease in the interlayer distance of stacked GO nanosheets. Hence, we propose a modification of GO membranes via the intercalation of cations to enhance the proton conductivity. Modified self-standing GO membranes with Al3+ and La3+ were fabricated by a vacuum filtration method. They exhibited a larger distance of the interlayer that serves as proton hopping pathways. Furthermore, the modified GO membrane showed a higher proton conductivity than a pristine GO membrane even at 80 °C, as confirmed by Electrochemical Impedance Spectroscopy. The results demonstrate that intercalating cations in between GO nanosheets is effective in improving the practical feasibility of proton conducting GO membranes.

Controlling covalent functionalization of graphene oxide membranes to improve enantioseparation performances

At present, the separation of racemates is still a great challenge due to their similarity in chemical structures and physicochemical properties. Here, we proposed a new strategy to improve the enantioseparation performances of graphene oxide (GO) based membranes via controlling the degree of functionalization (DF) of GO by the aid of an epoxide ring-opening reaction with a carboxyl-terminated ionic liquid (IL-COOH) as a spacer, followed by an amidation reaction of the l-glutamic acid (GO-IL-Glu) as chiral selectors. Results show that: i) these membranes are superior in the enantioselectivity and 1–3 orders of magnitude higher in the flux than traditional chiral separation membranes; ii) compared with l-glutamic acid modified GO membranes and their complex membranes with polypeptides in our previous work, they afford a ca. 40–80% increase in enantioselectivities and an order of magnitude increase in fluxes. Herein, IL-COOH groups can not only serve as a spacer, which is propitious to expanding the interlayer spacing of GO-IL-Glu nanosheets, thereby achieving high throughput of enantiomers, but also function as an active site to improve the grafting amount of chiral selectors (l-Glu), helpful for enhancing enantioselectivities. In particular, the flux is not compromised by the improved enantioselectivity in such membranes; on the contrary, it has an order of magnitude increase. Our findings suggest that the strategy used in this work, i.e., combining more chiral selectors with a wider interlayer spacing, could provide new opportunities for simultaneously facilitating high-flux and high-selectivity and a potential application in a great many enantio- and bio-separations.

Designing Carbon/Oxygen Ratios of Graphene Oxide Membranes for Proton Exchange Membrane Fuel Cells

Graphene oxide (GO), which is the oxidized form of graphene, has holes and functional groups on the surface and thus has high potential to be used as an electrochemical transport channel material. In this study, differently modified GO membranes are applied as electrolytes of proton exchange membrane fuel cells (PEMFCs) with controlled carbon/oxygen ratios. The critical and desired properties of the electrolyte, such as electron conductivity, proton conductivity, interfacial reactivity, and cell performance are evaluated in identical platinum-sputtered model electrodes. Among them, with the help of an increased concentration of oxygen-containing groups, a GO membrane with a low carbon/oxygen ratio shows a 2.9-fold improved maximum power density and advanced electrochemical properties compared with the pristine GO membrane. The characterization of GO suggests that the redox state of the membrane is an important factor for controlling the proton conductivity, interfacial reactivity, and maximum power density of PEMFCs.

Graphene oxide composite membranes cross-linked with urea for enhanced desalting properties

Cross-linking reaction is an important method to prepared stable graphene oxide (GO) membrane, however, the cross-linking reaction with common diamine always induces the water flux decline and ion rejection decrease because GO is partly reduced to result in some oxygen functional groups lose and the cross-linking diamine was inserted to enlarge the free spacing of GO layers. Herein, GO was cross-linked with urea (UR) to show high water flux and good ion rejection. XPS results demonstrated that UR was chemically linked with GO sheets and cellulose acetate (CA) membrane support. In comparison with GO/CA membrane, the water contact angle increase of UR modified membrane (GO-UR/CA) is 10°, but the water contact angle increase of ethane diamine (EDA) modified GO membrane (GO-EDA/CA) achieved 40°, which resulted in GO-UR/CA membrane almost achieving the similar water permeation flux with the pristine GO/CA and the much lower water flux of GO-EDA/CA at same GO specific mass deposition. The “d” spacing of GO-UR/CA in wet state is about 9.7 nm, and GO-EDA/CA reaches a 0.95 nm “d” spacing, however, GO-UR/CA membrane yielded a best separation performance with the ion rejections reaching 21.6%, 26.8%, 63.2%, 73.5%, 81.9% for KCl, NaCl, CaCl2, MgSO4, and CuSO4, respectively. The reasons for that was the higher O content, the introduced the polar ketone groups and smaller free “empty” space in GO-UR/CA membrane. This finding may provide a new viewpoint to comprehend the mechanism of molecule and ions permeation in GO membrane.

Role of filled PLGA in improving enantioselectivity of Glu-GO/PLGA composite membranes

The primary aim of this investigation was to improve the chiral separation performance of graphene oxide (GO) based composite membranes via tuning the interlayer spacing between GO sheets using the association of poly(L-Glutamic acid sodium), PLGA, onto the surface of GO sheets. Results show that: i) compared with conventional chiral separation membranes, these membranes exhibit superior chiral resolution properties, which are 1–2 orders of magnitude higher in the flux and greater in the enantioselectivity; ii) compared with pristine L-Glutamic modified GO membranes, they also afford a significant improvement in enantioselectivity with a slight reduction in flux. In present work, it is demonstrated that in such composite membranes, the incorporated PLGA can not only serve as an additional chiral recognizing agent, facilitating the transport of D-isomer of the chiral probe, but also as a kind of filler, reducing the size of pores or channels between GO sheets. Especially, both roles can significantly contribute to enhancing the enantioselectivitiy of composite membranes. Our findings indicate that the combination of GO-based materials and polypeptides might offer unusual control in the selectivity and the flux of the corresponding composite membranes, achieved by regulating the interlayer spacing between GO sheets, and thus such composite membranes have a potential to simultaneously facilitate high-flux and high-selectivity and a promising application in a great number of enantio- and bio-separations.

Application of graphene oxide in water treatment

Graphene oxide has good hydrophilicity and has been tried to use it into thin films for water treatment in recent years. In this paper, the preparation methods of graphene oxide membrane are reviewed, including vacuum suction filtration, spray coating, spin coating, dip coating and the layer by layer method. Secondly, the mechanism of mass transfer of graphene membrane is introduced in detail. The application of the graphene oxide membrane, modified graphene oxide membrane and graphene hybrid membranes were discussed in RO, vaporization, nanofiltration and other aspects. Finally, the development and application of graphene membrane in water treatment were discussed.

Development of a stable cation modified graphene oxide membrane for water treatment

Membranes prepared from layers of graphene oxide (GO) offer substantial advantages over conventional materials for water treatment (e.g. greater flux), but the stability of GO membranes in water has not been achieved until now. In this study the behavior of GO membranes prepared with different quantities and species of cations has been investigated to establish the feasibility of their application in water treatment. A range of cation-modified GO membranes were prepared and exposed to aqueous solutions containing specific chemical constituents. In pure water, unmodified and Na-modified GO membranes were highly unstable, while GO membranes modified with multivalent cations were stable provided there were sufficient quantities of cations present; their relative capability to achieve GO stability was as follows: Al3+ > Ca2+ > Mg2+ > Na+. It is believed that the mechanism of cross-linking, and membrane stability, is via metal-carboxylate chelates and cation-graphite surface interactions (cation-π interaction), and that the latter appears to increase with increasing cation valency. The instability of cation (Ca or Al)-modified GO membranes by NaCl solutions during permeation occurred as Na+ exchanged with the incorporated multivalent cations, but a high content of Al3+ in the GO membrane impeded Al3+/Na+ exchange and thus retained membrane stability. In solutions containing biopolymers representative of surface waters or seawater (protein and polysaccharide solutions), Ca-GO membranes (even with high Ca2+ content) were not stable, while Al-GO membranes were stable if the Al3+ content was sufficiently high; Al-formed membranes also had a greater flux than Ca-GO membranes.

Exceptional chiral separation of amino acid modified graphene oxide membranes with high-flux

The primary aim of this investigation was to achieve chiral separations via fabricating graphene oxide (GO) based membranes with high separation performances, derived from incorporating a chiral selector (L-Glutamic acid) into GO flakes, based on considerations of GO membranes having the inherently high throughput nature. The GO membrane was fabricated via a simple vacuum filtration method, in which L-Glutamic acid not only provided the stacked GO nanosheets with the necessary stability to overcome their inherent dispensability in water environment but also finely tuned spacing of the GO nanosheets and the resulting performance. The corresponding membrane structure together with GO and amino acid modified GO nanosheets was systematically characterized by SEM, TEM, AFM, XPS, XRD, FT-IR and so on. Finally, enantioseparation performances of amino acid modified GO membranes toward 3, 4-Dihydroxy-D, L-phenylalanine were detected. Results show that such membranes exhibit extraordinary chiral resolution properties, which are 1−2 orders of magnitude higher in the flux and greater in selectivity, compared to common chiral separation membranes. Our findings demonstrate that the modified GO membranes might provide another general approach to simultaneously facilitate high-flux and high-selectivity for a host of enantio- and bio-separations.