Structure Of Graphene Oxide Membranes Research Articles

“Ion-cage” structure of graphene oxide membranes with stable interlayer spacings towards efficient desalination

Multilayered graphene oxide (GO) membrane with sub-nanometer channels is an ideal candidate for desalination. However, excluding small ions using GO-based membranes in the pressured filtration processes remains a great challenge due to the tortuous transport paths of laminates and their water-swelling characteristic. In our previous work, we developed the cation-controlled theory to precisely regulate the interlayer spacing of GO membranes in aqueous solutions. In this work, we proposed an “ion-cage” strategy to enhance the stability (anti-swelling) of the K-controlled GO (icGO-K) membrane for achieving efficient desalination. The icGO-K membrane, coated with positively charged polyethyleneimine (PEI) on its upper and lower surfaces, immobilized intercalated cation K+ between the laminates by electrostatic repulsion to maintain the stability of the confined interlayer spacing. The icGO-K membrane demonstrated superior rejection of 95.3 %, 83.2 %, 81.8 %, and 70.7 % for MgCl2, MgSO4, NaCl, and Na2SO4 while maintaining competitive permeance of ∼ 3.5 L m-2h−1 bar−1 in the desalination process. More importantly, the icGO-K membrane exhibited ultra-long desalination stability up to 720 h. The enhanced performance can be attributed to the synergistic effect of the confined interlayer spacing and surface electrostatic repulsion. Overall, this work provides attractive strategy to fabricate fine-tuning 2D laminate nanochannels for high efficiency of desalination application.

Tailoring the interlayer channel structure of graphene oxide membrane with conjugated cationic dyes for butanol dehydration

For the design of high-performance GO membrane, the structure of intercalation material is a key consideration due to its decisive impacts on nanosheet assembly and mass transfer channel structure. Herein, conjugated cationic dyes were incorporated into GO interlayer channels. The large π-conjugated structure and delocalized positive charge can form π-π and electrostatic interactions with the non-oxidized region and negatively-charged group on GO nanosheet, respectively. Through interlayer cross-linking, the nanosheet orientation and interlayer bonding can be improved, conferring the membrane with higher anti-swelling capacity, more ordered channel structure, and controllably tailored interlayer d-spacing. The electron cloud density and planar structure of cationic dye molecules are evidenced to be critical in influencing membrane structure and separation performance. After incorporating conjugated cationic dyes, the separation factor for water/butanol separation can reach 5.3 times of that of pristine GO membrane, while the permeate flux also increases by 20%. This work provides a category of promising intercalation materials for GO membranes with good potential application in butanol dehydration.

Synergistically tailoring the hierarchical channel structure of graphene oxide membrane through co-assembly strategy for high-performance butanol dehydration

A significant boost in permeate flux along with the desirable separation factor remains a challenging issue for graphene oxide (GO) membranes in pervaporation applications. Herein, a facile approach of co-assembling pristine and etched GO nanosheets was proposed to tailor the hierarchical channel structure (involving the slits, defects and interlayer galleries) of GO membrane and achieve remarkable improvements in both the permeate flux and water/butanol separation factor. Pristine GO nanosheets guarantee a defect-free membrane structure, while etched GO (eGO) provides abundant defects/slits for molecular transport, thus reducing molecular diffusion resistance and doubling permeate flux. The introduction of eGO confers a more ordered nanosheet assembly and narrower interlayer spacing, thus strengthening the size-exclusion effect of the interlayer channels towards butanol molecules and realizing a 3-times enhancement in the separation factor. The subsequent intercalation of Congo red (CR) molecules ulteriorly optimizes the interlayer channels and results in the superior water/butanol separation factor of 5705 with the permeate flux of 4.81 kg/(m2h). This work provides a facile and feasible method for constructing high-performance GO-based membranes with great potential for practical butanol dehydration process.

Novel Orientation-Sensitive Spin Probes for Graphene Oxide Membranes Study.

Spin probe EPR spectroscopy is currently the only method to quantitatively report on the orientational ordering of graphene oxide membranes. This technique is based on the analysis of EPR spectra of a membrane containing stable radicals sorbed on oxidized graphene planes. The efficiency of the method depends on the spin probe structure; therefore, it is important to find stable paramagnetic substances that are most sensitive to the alignment of graphene oxide membranes. In the present work, three novel stable nitroxide radicals containing aromatic fragments with two nitrogen atoms were tested as spin probes to study graphene oxide membranes. The spin-Hamiltonian parameters of the radicals in graphite oxide powder and orientational order parameters of the probes inside graphene oxide membrane were determined. The sensitivity of one of these radicals to membrane orientational ordering was found to be higher than for any of spin probes used previously. A likely reason for this higher sensitivity is the presence of heteroatoms which can facilitate interaction between paramagnetic molecules and oxygen-containing groups on the inner surface of the membrane. The new high-sensitivity spin probe may significantly increase the potential of EPR spectroscopy for studying the internal structure of graphene oxide membranes.

Tuning interlayer spacing of graphene oxide membrane to enhance its separation performance of hydrogen isotopic water in membrane distillation

Separation of light water/tritiated water (H2O/HTO) is an important process in the nuclear energy sector to treat tritium-containing radioactive wastewater, for which H2O/D2O separation is usually used as a research model to minimize safety concerns. Recently Graphene oxide (GO)-based membrane distillation processes have demonstrated high separation factor, however, the permeation flux is low to affect the overall separation performance. In this study, the approach of tuning interlayer spacing and structure of GO membranes was explored to enhance the separation performance through two ways: (i) GO membrane was intercalated using cations (Na+, or Ca2+), or cationic surfactant-cetyltrimethylammonium bromide (CTAB) to vary interlayer spacing, and the resulting GO-Ca membrane gave the highest permeation flux of 0.87 kg·m−2·h−1 among the intercalated GO membranes with a separation factor of 4.6% to deliver higher separation performance than the pristine GO membrane; (ii) GO-framework (GOF) membrane was prepared by cross-linking GO nanosheets with cations (Ca2+, or Al3+) to prepare GOF@Ca and GOF@Al membrane, and GOF@Al membrane achieved a separation factor of 4.9% and a permeation flux of 0.9 kg·m−2·h−1, both higher than that of the pristine GO membrane. Furthermore, the up-to-date H2O/D2O separation data from different GO-based membrane distillation processes were summarized to understand the correlation between the permeation flux and the separation factor of processes. The results of this study suggested that tuning interlayer spacing and structure of GO membranes can be a good approach to enhance the overall separation performance by simultaneously improving permeation flux and separation factor for the separation of H2O/D2O, which can guide for future development of low-cost and efficient H2O/HTO separation processes needed in the nuclear energy field.

Graphene Oxide Membranes for Trace Hydrocarbon Contaminant Removal from Aqueous Solution.

The aim of this paper is to shed light on the application of graphene oxide (GO) membranes for the selective removal of benzene, toluene, and xylene (BTX) from wastewater. These molecules are present in traces in the water produced from oil and gas plants and are treated now with complex filtration systems. GO membranes are obtained by a simple, fast, and scalable method. The focus of this work is to prove the possibility of employing GO membranes for the filtration of organic contaminants present in traces in oil and gas wastewater, which has never been reported. The stability of GO membranes is analyzed in water solutions with different pH and salinity. Details of the membrane preparation are provided, resulting in a crucial step to achieve a good filtration performance. Material characterization techniques such as electron microscopy, x-ray diffraction, and infrared spectroscopy are employed to study the physical and chemical structure of GO membranes, while gas chromatography, UV-visible spectroscopy, and gravimetric techniques allow the quantification of their filtration performance. An impressive rejection of about 90% was achieved for 1 ppm of toluene and other pollutants in water, demonstrating the excellent performance of GO membranes in the oil and gas field.

Competition of co-existing cations to eliminate negative effect of Na+ on graphene oxide membrane structure and stabilize the separation performance

The impact of inorganic ions present in aqueous environment on the structural stability of graphene oxide (GO) membrane has raised increasing attention. This study investigated how the competition of co-existing cations with Na+ affected GO membrane structure and separation performance. Results showed that the presence of Na+ alone (NaCl solution) led to significantly increased membrane flux (400% of the ultrapure water flux) and decreased rejection (from 17.2% to 3.5%). However, the flux and rejection of GO membrane were more stable when the solutions contained co-existing K+ or Ca2+. The separation performance variation was attributed to the structural variation of the GO membrane. Characterized by X-ray diffraction, the orderly laminar structure of GO membrane completely disappeared in NaCl solution but was well maintained in NaCl/KCl or NaCl/CaCl2 solution. The concentration required for stabilization was 1 mM for co-existing Ca2+, which was lower than that of K+ (5 mM). The ability of co-existing K+ or Ca2+ to eliminate the negative effect of Na+ was ascribed to their priority to enter nano-capillary channels via stronger attractive interactions with GO nanosheets. Owing to the small size of K+ or bridging effect of Ca2+ with GO nanosheets, interlayer interactions within GO membrane were enhanced, and thus, its structure was stabilized.

Impact of monovalent cations on the separation performance of graphene oxide membrane for different organic matters.

Graphene oxide (GO) membrane has become a promising membrane due to its advantages of high flux, good anti-pollution performance, and controllable cost. Since wastewater contains inorganic ions and organic substances, the effect of inorganic ions on the removal of organic matters has attracted attention. This study investigated the impact of monovalent cations (Na+ and K+) on the removal of different organic matters by GO membranes. The results showed that the removal by GO membranes of both macromolecular organic compounds (humic acid, sodium alginate, and bull serum albumin) and small molecular organic substances (new coccine, methyl orange, and methylene blue) were high (>75%). The coexisting Na+ caused the increase in flux and the decrease in removal of small molecular organic matters, while the impact on the removal of macromolecular organic matters was weak. The impact of the coexisting K+ was weaker than Na+. With the crosslinking of ethylenediamine, the structure of GO membrane was more stable. The impacts of Na+ on the flux and separation performance of crosslinked membranes were less than that of non-crosslinked membranes. This suggests that crosslinked GO membrane has more potential for practical wastewater treatment.

Efficient Permeance Recovery of Organically Fouled Graphene Oxide Membranes

The emergence of facile approaches for the large-scale production of graphene oxide (GO) membranes necessitates a clearer understanding of their potential to foul and, more importantly, strategies for efficient recovery of membrane performance following fouling. Here, we systematically investigated the feasibility of water, ethanol, and hypochlorite as cleaning agents to remove organic foulants over a GO membrane. Among them, 100 ppm hypochlorite solution showed a remarkable ability to remove bovine serum albumin (BSA) and could recover the membrane flux up to 98% after five cycles of BSA filtration and cleaning. The potential of hypochlorite was also demonstrated for permeance recovery during molecular filtration of tannic acid and methyl blue. Scanning electron microscopy, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray diffraction (XRD) analyses were used to study the oxidative effects of hypochlorite on the GO membrane, and it was determined that exposure to higher concentrations of hypochlorite (>1000 ppm) degrades the structure of GO membrane and deteriorates the membrane performance after three cycles of cleaning. The studies demonstrate that the use of a modest concentration of hypochlorite is effective in restoring permeance of this class of high flux nanofiltration membranes.

Water and salt dynamics in multilayer graphene oxide (GO) membrane: Role of lateral sheet dimensions

Dependence of salt rejection efficiency and water permeability of layered graphene oxide (GO) membranes on the lateral dimension of constituting sheets are studied through equilibrium molecular dynamic (MD) simulation and experiments. This study suggests that with increasing sheets dimension permeability of the GO membranes decreases but its selectivity increases. The velocity and permeation time of the water molecules while permeating through the membrane are influenced to a greater extent by the pore offset distance (W) of the membranes. More over the larger pore offset distance increases the path length that the water molecules and ions have to traverse for permeating through the layered GO membranes. Based on the simple technique discussed in this work, one can construct GO membranes of required water permeability and salt rejection without the application of any foreign nanomaterials with the GO membrane, which retains the inherent selectivity of the GO membranes. This work also provides the effect of internal structure of GO membrane on the atomistic level details of the solvation shell of ions while they are permeating through the membrane.

Dynamic microstructure of graphene oxide membranes and the permeation flux

Graphene oxide (GO) membranes have been reported to be a promising separation barrier that can retain small molecules and multi-valent salts because of the well-defined interlayer space between GO flakes. However, while some studies suggested fast liquid transport through the extremely tortuous transport path, contradictory observations (e.g. low permeation flux) have also been obtained. This paper revealed the dynamic microstructure of GO membranes, which affected the membrane performance significantly. We showed that all GO membranes prepared by varied methods and on different substrates presented a severe reduction in water permeability during filtration, due to the compaction of their original loose microstructure. The water flux could drop continuously from tens of LMH bar−1 to <0.1 LMH bar−1 after more than ten hours. This result demonstrated that the structure of GO membranes prepared by current approaches was far from the ideal laminar structure. The high permeability of GO membranes observed could be contributed by the disordered membrane microstructure. Therefore, the transport mechanisms assuming perfect laminar structure in GO membranes, and the fast transport hypothesis may not fully describe the water transport in GO membranes. Interestingly, the loosely packed microstructure of GO membranes was also found reversible depending on the storage conditions.

Cation-dependent structural instability of graphene oxide membranes and its effect on membrane separation performance

Graphene oxide (GO) membranes formed by stacked GO nanosheets can separate contaminants from water via nanosized capillaries, the properties of which, however, might be altered by intercalation of solutes. This study first reports the impacts of common inorganic ions on the structure and separation performance of non-covalently bonded GO membranes. When filtering NaCl, KCl, MgCl2, CaCl2, Na2SO4, and K2SO4 solutions, the flux of GO membranes, fabricated by vacuum filtration, exhibited significant increase over the filtration time in Na+ solutions, but maintained stable in the other electrolyte solutions. Rejection decline was observed for all the electrolytes with an order of Na+>Mg2+≈Ca2+>K+ for the chlorides. X-ray diffraction indicated the GO membrane structure in K+ solutions was well maintained and slightly expanded in Mg2+/Ca2+ solutions over time. However, the ordered structure of GO membranes disappeared after long exposure to Na+, explaining the significantly increased flux and reduced rejection. The effect of Na+ on the GO membrane structure was ascribed to its relatively large size but weak attraction with GO nanosheets. These findings highlight the importance of introducing strong covalent bonding between GO nanosheets to stabilize the structure and separation performance of GO membranes for water purification.

Structure of graphene oxide membranes in solvents and solutions.

The change of distance between individual graphene oxide sheets due to swelling is the key parameter to explain and predict permeation of multilayered graphene oxide (GO) membranes by various solvents and solutions. In situ synchrotron X-ray diffraction study shows that swelling properties of GO membranes are distinctly different compared to precursor graphite oxide powder samples. Intercalation of liquid dioxolane, acetonitrile, acetone, and chloroform into the GO membrane structure occurs with maximum one monolayer insertion (Type I), in contrast with insertion of 2-3 layers of these solvents into the graphite oxide structure. However, the structure of GO membranes expands in liquid DMSO and DMF solvents similarly to precursor graphite oxide (Type II). It can be expected that Type II solvents will permeate GO membranes significantly faster compared to Type I solvents. The membranes are found to be stable in aqueous solutions of acidic and neutral salts, but dissolve slowly in some basic solutions of certain concentrations, e.g. in NaOH, NaHCO3 and LiF. Some larger organic molecules, alkylamines and alkylammonium cations are found to intercalate and expand the lattice of GO membranes significantly, e.g. up to ∼35 Å in octadecylamine/methanol solution. Intercalation of solutes into the GO structure is one of the limiting factors for nano-filtration of certain molecules but it also allows modification of the inter-layer distance of GO membranes and tuning of their permeation properties. For example, GO membranes functionalized with alkylammonium cations are hydrophobized and they swell in non-polar solvents.

The structure of graphene oxide membranes in liquid water, ethanol and water–ethanol mixtures

The structure of graphene oxide (GO) membranes was studied in situ in liquid solvents using synchrotron radiation X-ray diffraction in a broad temperature interval. GO membranes are hydrated by water similarly to precursor graphite oxide powders but intercalation of alcohols is strongly hindered, which explains why the GO membranes are permeated by water and not by ethanol. Insertion of ethanol into the membrane structure is limited to only one monolayer in the whole studied temperature range, in contrast to precursor graphite oxide powders, which are intercalated with up to two ethanol monolayers (Brodie) and four ethanol monolayers (Hummers). As a result, GO membranes demonstrate the absence of "negative thermal expansion" and phase transitions connected to insertion/de-insertion of alcohols upon temperature variations reported earlier for graphite oxide powders. Therefore, GO membranes are a distinct type of material with unique solvation properties compared to parent graphite oxides even if they are composed of the same graphene oxide flakes.

Electronic properties and atomic structure of graphene oxide membranes

We have performed a near-edge X-ray absorption fine-structure (NEXAFS) and a transmission electron microscopy (TEM) investigation of freely suspended graphene oxide (GO) sheets. We utilized a photoemission electron microscope to identify GO membranes and to acquire C K and O K absorption spectra. The overall line shape of C K-edge spectra demonstrates that the honeycomb carbon network of graphene is the scaffold of GO. However, the intensity ratio of π ∗ and σ ∗ resonances, and a broad feature at about 20 eV from the edge, indicate the presence of new carbon bonds. The O K-edge spectra show that oxidized regions are made of carbonyl, epoxide, and hydroxyl groups attached to the plane of graphene, while carboxyl groups might also be present at the edges. Further, our study indicates the presence of ordered arrangements of oxygen atoms in GO sheets. Our investigation provides a new and efficient route to study the electronic structure of suspended membranes.