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Feature selection plays an important role in improving the performance of classification or reducing the dimensionality of high-dimensional datasets, such as high-throughput genomics/proteomics data in bioinformatics. As a popular approach with computational efficiency and scalability, information theory has been widely incorporated into feature selection. In this study, we propose a unique weight-based feature selection (WBFS) algorithm that assesses selected features and candidate features to identify the key protein biomarkers for classifying lung cancer subtypes from The Cancer Proteome Atlas (TCPA) database and we further explored the survival analysis between selected biomarkers and subtypes of lung cancer. Results show good performance of the combination of our WBFS method and Bayesian network for mining potential biomarkers. These candidate signatures have valuable biological significance in tumor classification and patient survival analysis. Taken together, this study proposes the WBFS method that helps to explore candidate biomarkers from biomedical datasets and provides useful information for tumor diagnosis or therapy strategies.

Compositional action recognition is a novel challenge in the computer vision community and focuses on revealing the different combinations of verbs and nouns instead of treating subject-object interactions in videos as individual instances only. Existing methods tackle this challenging task by simply ignoring appearance information or fusing object appearances with dynamic instance tracklets. However, those strategies usually do not perform well for unseen action instances. For that, in this work we propose a novel learning framework called Counterfactual Debiasing Network (CDN) to improve the model generalization ability by removing the interference introduced by visual appearances of objects/subjects. It explicitly learns the appearance information in action representations and later removes the effect of such information in a causal inference manner. Specifically, we use tracklets and video content to model the factual inference by considering both appearance information and structure information. In contrast, only video content with appearance information is leveraged in the counterfactual inference. With the two inferences, we conduct a causal graph which captures and removes the bias introduced by the appearance information by subtracting the result of the counterfactual inference from that of the factual inference. By doing that, our proposed CDN method can better recognize unseen action instances by debiasing the effect of appearances. Extensive experiments on the Something-Else dataset clearly show the effectiveness of our proposed CDN over existing state-of-the-art methods.

Despite both hydronium and hydroxide ions exhibiting anomalously high mobilities in aqueous solutions, OH− conduction in solid electrolytes is considerably slower than that of H+, impeding a paradigm shift to many renewable energy and bioelectronic applications in an alkaline environment. Herein, we present that the mobility/diffusivity of hydroxide ions along a 2D ordered hydroxyl network can reach a level comparable to that of proton/hydroxide ion conduction in diluted acids/bases and some artificial or biomolecular proton conductors. Such a superionic conduction characteristic was quantified via direct measurements on individual single-layer nanosheets of layered double hydroxides (LDHs). A combined simulations study revealed that the superior hydroxide ion conduction was directly correlated with the hydroxyl network on an LDH sheet, which served as both a free hydroxide ion donor and 2D hopping lane in the presence of water. It was also revealed that the surface conduction was generally ∼1 order of magnitude faster than that along the gallery between nanosheets. This work provides insights into the origin and intrinsic mechanism of hydroxide ion conduction along ordered hydrogen-bonded nanostructures, which might inspire the creation of superionic functional materials in other systems.

Capillary condensation of water is ubiquitous in nature and technology. It routinely occurs in granular and porous media, can strongly alter such properties as adhesion, lubrication, friction and corrosion, and is important in many processes used by microelectronics, pharmaceutical, food and other industries1–4. The century-old Kelvin equation5 is frequently used to describe condensation phenomena and has been shown to hold well for liquid menisci with diameters as small as several nanometres1–4,6–14. For even smaller capillaries that are involved in condensation under ambient humidity and so of particular practical interest, the Kelvin equation is expected to break down because the required confinement becomes comparable to the size of water molecules1–22. Here we use van der Waals assembly of two-dimensional crystals to create atomic-scale capillaries and study condensation within them. Our smallest capillaries are less than four angstroms in height and can accommodate just a monolayer of water. Surprisingly, even at this scale, we find that the macroscopic Kelvin equation using the characteristics of bulk water describes the condensation transition accurately in strongly hydrophilic (mica) capillaries and remains qualitatively valid for weakly hydrophilic (graphite) ones. We show that this agreement is fortuitous and can be attributed to elastic deformation of capillary walls23–25, which suppresses the giant oscillatory behaviour expected from the commensurability between the atomic-scale capillaries and water molecules20,21. Our work provides a basis for an improved understanding of capillary effects at the smallest scale possible, which is important in many realistic situations. In the tiniest of capillaries, barely larger than a water molecule, condensation is surprisingly predictable from the macroscopic Kelvin condensation equation, a coincidence partially owing to elastic deformation of the capillary walls.

An overview of recent advances in measuring and understanding the exceptionally high and anisotropic H+/OH– ion conductivities of representative 2D materials., Ion conducting membranes/electrolytes have been employed extensively in some important industrial and biological systems, especially in fuel cells, water electrolyzers, gas separation, sensors and biological selective ion transport, acting as one of the core components and sometimes directly determining the device performance. However, the traditional polymeric proton exchange membranes (PEMs)/anion exchange membranes (AEMs) suffer from highly toxic preparation procedures, poor thermal and chemical stabilities, and unsatisfactory ion conductivities. This has triggered researchers worldwide to explore alternative inorganic building blocks with high ion conductivities and stabilities from the new materials library, hoping to solve the above long-lasting problems. The recent burgeoning research on two-dimensional (2D) materials has unveiled exceptionally high ionic conductivities, which raises the feasibility of fabricating high-performance nanosheet-based ion conductors/membranes. In this perspective, the recent advances in measuring and understanding the exceptionally high and anisotropic H+/OH– ion conductivities of representative 2D materials, e.g. graphene oxide (GO), vermiculite and layered double hydroxide (LDH) nanosheets, are reviewed. In particular, regarding the anisotropic ionic conduction in 2D nanosheets, possible design strategies and technological innovations for fabricating macroscopic nanosheet-based ionic conductors/membranes are proposed for maximizing the high in-plane conduction, which may serve to guide future development of high-performance industrial and biological systems relying on H+/OH– conducting membranes.

A themed collection on advanced memory technologies, guest edited by Materials Horizons Community Board Member Ye Zhou and Nanoscale Horizons Community Board Member Pengzhan Sun.

Ni(2+)Mn(3+) layered double hydroxide (LDH) nanoplatelets have been hydrothermally synthesized in a homogeneous precipitation of mixed Ni(2+)/Mn(2+) salts at a molar ratio of 2 : 1 via the hydrolysis of hexamethylenetetramine (HMT) and in situ oxidation with H2O2. After anion-exchange, NiMn LDH was exfoliated into unilamellar nanosheets. Subsequent flocculation of NiMn LDH nanosheets with (reduced) graphene oxide (GO/rGO) into superlattice composites was achieved and further tested as electrocatalysts for oxygen evolution reaction (OER). The face-to-face heteroassembly of NiMn LDH nanosheets with conductive rGO at an alternating sequence resulted in a small overpotential of 0.26 V and a Tafel slope of 46 mV per decade, which is much superior to as-exfoliated nanosheets. The analyses of electrochemical activity surface area (ECSA) and impedance spectra clearly indicated that the superlattice structure was ideal in facilitating the migration/transfer of the charge and reactants, revealing the electrochemical energetics and mechanism behind the synergistic effect arising from molecular hybridization. The proof of concept toward total water splitting using the newly developed hybrid electrocatalyst was demonstrated by an electrolysis cell powered by a single AA battery.

Despite being only one-atom thick, defect-free graphene is considered to be completely impermeable to all gases and liquids1,2,3,4,5,6,7,8,9,10. This conclusion is based on theory3,4,5,6,7,8 and supported by experiments1,9,10 that could not detect gas permeation through micrometre-size membranes within a detection limit of 105 to 106 atoms per second. Here, using small monocrystalline containers tightly sealed with graphene, we show that defect-free graphene is impermeable with an accuracy of eight to nine orders of magnitude higher than in the previous experiments. We are capable of discerning (but did not observe) permeation of just a few helium atoms per hour, and this detection limit is also valid for all other gases tested (neon, nitrogen, oxygen, argon, krypton and xenon), except for hydrogen. Hydrogen shows noticeable permeation, even though its molecule is larger than helium and should experience a higher energy barrier. This puzzling observation is attributed to a two-stage process that involves dissociation of molecular hydrogen at catalytically active graphene ripples, followed by adsorbed atoms flipping to the other side of the graphene sheet with a relatively low activation energy of about 1.0 electronvolt, a value close to that previously reported for proton transport11,12. Our work provides a key reference for the impermeability of two-dimensional materials and is important from a fundamental perspective and for their potential applications.

Graphene oxide (GO) features a unique structure of sp2 hybridized carbon clusters and rich oxygen-containing functional groups co-existing within a two-dimensional (2D) lattice. Due to energetic favorability, the oxygen functional groups tend to cluster together, leaving the empty sp2 aromatic nanoclusters to form an interconnected percolation pathway. Upon stacking numerous GO nanosheets together to form a macroscopic lamellar membrane, the oxygen functional groups attached on GO basal planes act as pillars to support the unoxidized sp2 percolation pathway on individual layers to form a nanocapillary network by connecting with each other across the whole lamellar structure, which is presumably responsible for transmembrane mass transport. In recent years, the research on mass transport properties of GO-based membranes has received intense interest and significant advancements have been achieved both experimentally and theoretically. In this chapter, we review the recent developments in this burgeoning research field by emphasizing the transport properties of water, dissolved ions and molecules, and gaseous atoms and molecules through GO membranes. Finally, a perspective has been given for future developments.

Graphene-based nanomaterials have been used as separation membranes and adsorbents due to its unique two-dimensional single atomic layer structure, large specific surface area and excellent properties. In this paper, we first review gas, water and ions mass transport phenomenon through nanoporous graphene membranes and layered structures of graphene oxides. The development of nanoporous graphene has been limited by the current preparation technique and immature hole-drilling technology; while the laminated graphene oxide membranes have a great application potential in the field of water purification due to its simple, low-cost preparation methods, high permeability and selectivity. Then, we review the adsorption behaviors of heavy metal ions, dyes and organic pollutants in aqueous solution with graphene-based nanomaterials. The interaction mechanism between functional groups on graphene-based materials and pollutants has been analyzed. Finally, we prospect the opportunities and challenges of graphene-based nanomaterials in environmental applications, such as membrane separation, water desalination and pollutants removal.

We report a voltage generator based on a graphene network (GN). In response to the movement of a droplet of ionic solution over a GN strip, a voltage of several hundred millivolts is observed under ambient conditions. In the voltage-generation process, the unique structure of GN plays an important role in improving the rate of electron transfer. Given their excellent mechanical properties, GNs may find applications for harvesting vibrational energy in various places such as raincoats, umbrellas, windows, and other surfaces that are exposed to rain.

Interfacial shear strength (IFSS) between particle and matrix in particulate polymer composites is a critical property in determining the mechanical behaviors since it is directly related to not only their Young’s modulus or specific strength, but also energy absorbing capability. However, the conventional techniques often present a technical challenge to accurately measure the IFSS between fillers and matrix in the composites. This is more apparent in graphene particulate composites due to their nano-scale dimensions as well as the platelet-shaped geometry. Here, the focus of this study is to use a semi-empirical approach to determine the IFSS of graphene particulate composites by combining experiments with finite element (FE) modeling. The materials of interest are reduced graphene oxide (RGO) and polycarbonate (PC). The tensile testing was performed to characterize the mechanical properties, while simultaneously monitoring the acoustic emission events in order to measure the global debonding stress (GDS) in the composites. By coupling thermal stress analysis and deformation analysis with the GDS as input to a FE model, the IFSS of the RGO particulate PC composites was successfully estimated by about 136 MPa, avoiding unnecessary assumptions and uncertainties which are seem to be inevitable with the conventional techniques for the IFSS measurement.

The magnetic transitions in graphene oxide (GO) have been investigated experimentally. Micron-sized GO flakes exhibit dominant diamagnetism accompanied by weak ferromagnetism at room temperature. However, when the lateral dimensions of GO flakes are reduced from micron-size to nano-size, a clear transition from dominant diamagnetism to ferromagnetism is observed. After reducing the GO chemically or thermally, the dominant magnetic properties are not altered markedly except for the gradual enhancement of ferromagnetic components. In contrast, at 2 K, significant paramagnetism is present in both the micron-sized and nano-sized GO sheets. The effects of different functional groups on magnetic transitions in graphene derivatives have been further investigated using on hydroxyl-, carboxyl-, amino- and thiolfunctionalized graphene. The results reveal that significant diamagnetism with weak ferromagnetism is present at room temperature in all of these functionalized graphene derivatives and the ability of different functional groups to introduce magnetic moments follows the order -SH > -OH > -COOH, -NH2. Notably, at 5 K, diamagnetism, paramagnetism and ferromagnetism coexist in thiol-, hydroxyland carboxyl-functionalized graphene, while amino-graphene exhibits dominant paramagnetism, analogous to the low-temperature magnetism in GO. These results indicate that diamagnetism, paramagnetism and ferromagnetism can coexist in graphene derivatives and magnetic transitions among the three states can be achieved which depend on edge states, vacancies, chemical doping and the attached functional groups. The results obtained may help settle the current controversy about the magnetism of graphene-related materials.

With the combination of surfactant and freeze-drying, we have developed two kinds of graphene spongy structures. On the one hand, using foams of soap bubbles as templates, three-dimensional porous graphene sponges with rich hierarchical pores have been synthesized. Pores of the material contain three levels of length scales, including millimeter, micrometer and nanometer. The structure can be tuned by changing the freezing media, adjusting the stirring rate or adding functional additives. On the other hand, by direct freeze-drying of a graphene oxide/surfactant suspension, a porous framework with directionally aligned pores is prepared. The surfactant gives a better dispersion of graphene oxide sheets, resulting in a high specific surface area. Both of the obtained materials exhibit excellent absorption capacity and good compression performance, providing a broad range of possible applications, such as absorbents, storage media, and carriers.

Recently, graphene oxide (GO) membranes have been reported with the ability to separate different solutes in aqueous suspensions by a molecular sieving effect. On the other hand, we propose that the chemical interactions between ions and GO membranes might also take effect in selective ion transmembrane transportation. In this paper, on the basis of the permeation of Cu2+ and Mg2+ sources through hydroxyl-, carboxyl-, and amino-functionalized graphene membranes, the delicate ion–graphene interactions which might be responsible for the selective ion permeation are investigated. We demonstrate experimentally that the coordination between transition-metal cations and carboxyl functionalities and the cation−π interactions between main-group cations and sp2 regions are responsible for the selective transport of small ions through graphene-based membranes, which is beyond the scope of molecular sieving effect proposed previously. Notably, by grafting amino groups onto the graphene basal planes, the permeations ...

Graphene and graphene oxide (G-O) have been demonstrated to be excellent filters for various gases and liquids, showing potential applications in areas such as molecular sieving and water desalination. In this paper, the selective trans-membrane transport properties of alkali and alkaline earth cations through a membrane composed of stacked and overlapped G-O sheets ("G-O membrane") are investigated. The thermodynamics of the ion transport process reveal that the competition between the generated thermal motions and the interactions of cations with the G-O sheets results in the different penetration behaviors to temperature variations for the considered cations (K(+), Mg(2+), Ca(2+), and Ba(2+)). The interactions between the studied metal atoms and graphene are quantified by first-principles calculations based on the plane-wave-basis-set density functional theory (DFT) approach. The mechanism of the selective ion trans-membrane transportation is discussed further and found to be consistent with the concept of cation-π interactions involved in biological systems. The balance between cation-π interactions of the cations considered with the sp(2) clusters of G-O membranes and the desolvation effect of the ions is responsible for the selectivity of G-O membranes toward the penetration of different ions. These results help us better understand the ion transport process through G-O membranes, from which the possibility of modeling the ion transport behavior of cellular membrane using G-O can be discussed further. The selectivity toward different ions also makes G-O membrane a promising candidate in areas of membrane separations.

The efficient recovery of acids from iron-based electrolytes using graphene oxide (GO) membranes was demonstrated for the first time. The results revealed that the amount of H+ permeating the GO membranes and reaching drains was two orders of magnitude larger than that of Fe3+. Notably, when the FeCl3 source concentration was reduced to certain extent, Fe3+ could be completely blocked by GO membranes. The mechanism for the effective separation of H+ from Fe3+ was studied, suggesting that the molecular sieving effect of GO nanocapillaries and the coordination between Fe3+ and GO were responsible for the effective blockage of Fe3+ while the rapid propagation of H+ through hydrogen-bonding networks along water layers within the interlayer spacing was responsible for the fast migration of H+. These properties made GO membranes promising cation-exchange membranes for applications of wastewater reuse and membrane separation.

We explored supercapacitor devices by utilizing 3-D highly porous (with average pore size of ∼80 nm) carbon nanotube sponges as a conductive substrate for polyaniline deposition. The porous structure of the sponge is beneficial for precursor penetration and uniform deposition of polyaniline on to the nanotubes. The self-supported, free-standing and flexible carbon nanotube polyaniline composite structure does not require any conductive additives or mechanical binders and delivers excellent areal capacitance (1.85–1.62 F/cm2 in the 4.9–49 mA/cm2 current density range) coupled with high rate capability and cycle stability.

Oil spills in the sea have caused many serious environmental problems worldwide. In this study, carbon nanotube (CNT) sponges were used to cleanup oil slicks on sea waters. This method was compared with two traditional representative sorbents, including polypropylene fiber fabric and woolen felt. The CNT sponges had a larger oil sorption capacity than the other two sorbents. The maximum oil sorption capacity (Q m) of the CNT sponge was 92.30 g/g, which was 12 to 13.5 times larger than the Q m of the other two sorbents (the Q m of the polypropylene fiber fabric and woolen felt were 7.45 and 6.74 g/g, respectively). In addition, unlike the other two sorbents, the CNT sponge was superhydrophobic and did not adsorb any water during oil spill cleanup. CNT sponges are potentially very useful for cleaning up oil spills from sea water.

Nanosheets exhibit high and anisotropic hydroxyl ion conductivity, promising potentials in energy or environment applications., When the dimensionality of layered materials is reduced to the physical limit, an ultimate two-dimensional (2D) anisotropy and/or confinement effect may bring about extraordinary physical and chemical properties. Layered double hydroxides (LDHs), bearing abundant hydroxyl groups covalently bonded within 2D host layers, have been proposed as inorganic anion conductors. However, typical hydroxyl ion conductivities for bulk or lamellar LDHs, generally up to 10−3 S cm−1, are considered not high enough for practical applications. We show that single-layer LDH nanosheets exhibited exceptionally high in-plane conductivities approaching 10−1 S cm−1, which were the highest among anion conductors and comparable to proton conductivities in commercial proton exchange membranes (for example, Nafion). The in-plane conductivities were four to five orders of magnitude higher than the cross-plane or cross-membrane values of restacked LDH nanosheets. This 2D superionic transport characteristic might have great promises in a variety of applications including alkaline fuel cells and water electrolysis.

We report the fabrication of hybrid films of graphene and monolayer titania using a simple electrostatic self-assembly method. Ultraviolet (UV) responses of the hybrid films based on the graphene–titania structure were investigated. We observed that the resistance of the graphene–titania hybrid increased exponentially with UV irradiation time and decreased exponentially when UV was turned off. Time constants of the order of hundreds of seconds were identified and found to be sensitive to the gas environment of graphene. The UV response as well as the time constant is tunable by varying the number of titania layers. Our results confirmed that UV irradiation played a significant role in the resistance modulation of graphene as well as graphene–titania hybrid films.

The movement of nanoparticles across cell membranes in a controlled manner is important for developing applications related to filtration, penetration, and biological systems. Here, the tunable transport of metal nanoparticle suspensions with well-defined particle sizes and surface groups through a ≈1 mm-thick porous carbon nanotube sponge filter is reported. Au particles with a relatively large diameter (40 nm) alone are easily trapped by the nanotube sponge even in a dilute water flow, but they can quickly pass through the sponges by the addition of much smaller particles (4 nm) to the suspension. A mechanistic study shows that the dominating interaction mode between Au particles and the sponge is physical trapping within the nanotube networks rather than Coulomb attraction. In the nanoparticle mixture, surface groups on the small particles tend to form hydrogen bonds with groups on large particles, thereby increasing interparticle distances, and preventing formation of severe agglomerates; large particles can therefore move across interconnected nanotubes more smoothly. It might be possible to switch the transport of many different nanoparticles across porous media on and off by adding or removing smaller particles of the same type.

Large area (the order of centimeters), flexible thin films of amorphous carbon were prepared on copper foils by an ethanol flame synthesis method. The carbon films exhibit high transmittance in the visible and near infrared range (up to 90% at 550 nm). Schottky diodes formed by combining the carbon films with n-type silicon wafers show high on/off ratio (up to 150) photocurrents at air-mass 1.5 and illumination intensity of 100 mW/cm2.

Freestanding carbon-based composite thin films, consisting of a carbon nanotube network patched with graphene sheets, were assembled by a solid-phase layer-stacking approach with ethanol wetting. The composite films are highly flexible, transparent, and conductive, showing a sheet resistance of 735 Ω/sq at 90% transmittance (at 550 nm). Under AM1.5 illumination, heterojunction solar cells made from the composite films and n-type silicon show a power conversion efficiency of up to 5.2%.

Significant achievements have been made on the development of next-generation filtration and separation membranes using graphene materials, as graphene-based membranes can afford numerous novel mass-transport properties that are not possible in state-of-art commercial membranes, making them promising in areas such as membrane separation, water desalination, proton conductors, energy storage and conversion, etc. The latest developments on understanding mass transport through graphene-based membranes, including perfect graphene lattice, nanoporous graphene and graphene oxide membranes are reviewed here in relation to their potential applications. A summary and outlook is further provided on the opportunities and challenges in this arising field. The aspects discussed may enable researchers to better understand the mass-transport mechanism and to optimize the synthesis of graphene-based membranes toward large-scale production for a wide range of applications.

A synchronous ion separation and electricity generation process has been developed using G-O membranes. In addition to the size effect proposed prevsiouly, the separation of ions can be attributed to the different interactions between ions and G-O membranes; the generation of electricity is due to the confinement of G-O membranes and the mobility difference of ions. Efficient energy transduction has been achieved with G-O membranes, converting magnetic, thermal and osmotic energy to electricity, distinguishing this material from other commercial semi-permeable membranes. Our study indicated that G-O membranes could find potential applications in the purification of wastewater, while producing electricity simultaneously. With G-O membranes, industrial magnetic leakage and waste heat could also be used to produce electricity, affording a superior approach for energy recovery.

Reduced graphene oxide (RGO) has been employed as an electrode for a series of vertically structured photodetectors. Compared with mechanically exfoliated or chemical vapor deposited graphene, RGO possesses more oxygen containing groups and defects, which are proved to be favorable to enhance the performance of photodetectors. As a matter of fact, RGO with different reduction levels can be readily obtained by varying the annealing temperature. The synthesis procedures for the RGO material are suitable for large scale production and its performance can be effectively improved by functionalization or element doping. For RGO-based devices, the Schottky junction properties and photoelectric conversion have been investigated, primarily by analyzing their current–voltage characteristics. Subsequently, the ON/OFF ratio, responsivity and detectivity of the photodetectors were closely examined, proving that the RGO material could be effectively utilized as the electrode material; also, their relationship with the RGO reduction levels has also been explored. By analyzing the response/recovery speed of the RGO-based photodetectors, we have studied the effects of oxygen-containing functional groups and crystalline defects on the photoelectric conversion.

Materials with low temperature coefficient of resistivity (TCR) are of great importance in some areas, for example, highly accurate electronic measurement instruments and microelectronic integrated circuits. In this work, we demonstrated the ultrathin graphene–graphene oxide (GO) hybrid films prepared by layer-by-layer assembly with very small TCR (30–100 °C) in the air. Electrical response of the hybrid films to temperature variation was investigated along with the progressive reduction of GO sheets. The mechanism of electrical response to temperature variation of the hybrid film was discussed, which revealed that the interaction between graphene and GO and the chemical doping effect were responsible for the tunable control of its electrical response to temperature variation. The unique properties of graphene–GO hybrid film made it a promising candidate in many areas, such as high-end film electronic device and sensor applications.

The in situ self-assembly of two types of typical two-dimensional (2D) nanomaterials (i.e., graphene oxide (GO) and monolayer titania (TO)) is realized using a simple drop-casting method. Within the as-prepared hybrid films, the GO and TO nanosheets arrange alternately into a lamellar structure. Notably, the hybridization of GO and TO suppresses the formation of coffee-rings when drop-cast, which is attributed to the strong interactions between the GO and TO nanosheets. Finally, the mechanism for the in situ hybridization of these two types of nanosheets into heterogeneous lamellar films and the suppression of the coffee-ring effect are discussed. These results demonstrate the potential applications of drop-cast hybrid films for high-quality membrane deposition from liquid phases.

The selective ion penetration and water purification properties of freestanding graphene oxide (GO) membranes are demonstrated. Sodium salts permeated through GO membranes quickly, whereas heavy-metal salts infiltrated much more slowly. Interestingly, copper salts were entirely blocked by GO membranes, and organic contaminants also did not infiltrate. The mechanism of the selective ion-penetration properties of the GO membranes is discussed. The nanocapillaries formed within the membranes were responsible for the permeation of metal ions, whereas the coordination between heavy-metal ions with the GO membranes restricted the passage of the ions. Finally, the penetration processes of hybrid aqueous solutions were investigated; the results revealed that sodium salts can be separated effectively from copper salts and organic contaminants. The presented results demonstrate the potential applications of GO in areas such as barrier separation and water purification.

The development of graphene-based functional membranes with the ability to effectively filter and separate molecules or ions in solutions based on a simple criterion (for example, the size or charge of solutes) is crucial for various engineering-relevant applications, ranging from wastewater purification and reuse to chemical refinement. Here, we report a hybrid membrane consisting of anionic graphene oxide (GO) and cationic Co-Al (or Mg-Al) layered double hydroxide (LDH) nanosheet (NS) superlattice units for high selectivity charge-guided ion transport. The hybrid membrane possesses a series of characteristics, including being easy to access, mechanically robust, freestanding, flexible and semitransparent as well as having a large area. The interlayer spacing of the hybrid membrane is insensitive to humidity variations, ensuring the structural stability in solution-based mass transport applications. The concentration gradient-driven ion transmembrane diffusion experiments show that the cations bearing various valences can be effectively separated strictly according to their charges, independent of the cationic and charge-balancing anionic species. The relative selectivity of the hybrid membranes toward monovalent and trivalent cations is as high as 30, which is not achievable by GO multilayer stacks, LDH-NS multilayer stacks or their bulk-stratified membranes, indicating that a synergistic effect originating from the molecular-level heteroassembly of GO and LDH-NS has a dominant role in the high-performance charge-guided ion filtration and separation processes. These excellent properties of GO/LDH-NS hybrid membranes make them promising candidates in diverse applications, ranging from wastewater treatment and reuse and chemical refinement to biomimetic selective ion transport. A membrane that combines ultrathin carbon films with metallic nanosheets may find use in wastewater treatment and biomimetic ion transport. Graphene oxide is a promising two-dimensional building block for purification technology because it naturally stacks into thin layers on solution processing. By tailoring the interlayer spacing with different functional groups, researchers can create nanochannels that permit water to flow while rejecting molecules above a certain size. Now, Hongwei Zhu of Tsinghua University in Beijing and co-workers have expanded the capabilities of such membranes by sandwiching minerals known as layered double hydroxides (LDHs) into the graphene oxide framework. The cationic metals within LDH nanosheets helped the membrane selectively filter impurities with different charge levels — for example, particles with triple positive charges were rejected at a ratio of 30:1 relative to singly charge ions. We demonstrate our recent progress in the newly emerging and intriguing research field of developing graphene-based functional membranes with the ability to effectively filtrate and separate molecules or ions in solutions based on a simple criterion (for example, the size or charge of solutes) for various engineering-relevant applications ranging from wastewater purification and reuse to chemical refinement.

By intercalating monolayer titania (TO) nanosheets into graphene oxide (GO) laminates with mild ultraviolet (UV) reduction, the as-prepared RGO/TO hybrid membranes exhibit excellent water desalination performances. Without external hydrostatic pressures, the ion permeations through the RGO/TO hybrid membranes can be reduced to

A cation-controlled selective anion transportation through graphene oxide (GO) membranes is demonstrated in this work. The results reveal that the trans-membrane transport of different anions can be modulated by the corresponding cations. The diverse interactions among anions, cations, and the negatively charged GO membranes are responsible for selective anion permeation through GO membranes. During the ion penetration, electrical potential differences can be generated across drain and source as well as across GO membranes; based on this, the ion distributions around GO membranes can be determined. The results indicate that local ion ordering can be achieved by GO membranes. Interestingly, for the cases of KNO3, Ca(NO3)2, and Ba(NO3)2, alternate aggregations of metallic cations and NO3− anions can be formed around GO membranes, demonstrating the fantastic ability of these membranes for ordering the ions locally in solutions. In addition, based on the electrical potential differences generated by different salts, chlorides are demonstrated to be ideal sources for efficient practical electricity production compared to sulfates and nitrates, while the different voltage signals generated can be used to identify different source solutions for liquid sensing applications. These results indicate that GO membranes can find potential applications in membrane separation, energy generation, ion recognition, and local ion organizing.

A mild annealing procedure was recently proposed for the scalable enhancement of graphene oxide (GO) properties with the oxygen content preserved, which was demonstrated to be attributed to the thermally driven phase separation. In this work, the structure evolution of GO with mild annealing is closely investigated. It reveals that in addition to phase separation, the transformation of oxygen functionalities also occurs, which leads to the slight reduction of GO membranes and furthers the enhancement of GO properties. These results are further supported by the density functional theory based calculations. The results also show that the amount of chemically bonded oxygen atoms on graphene decreases gradually and we propose that the strongly physisorbed oxygen species constrained in the holes and vacancies on GO lattice might be responsible for the preserved oxygen content during the mild annealing procedure. The present experimental results and calculations indicate that both the diffusion and transformation of oxygen functional groups might play important roles in the scalable enhancement of GO properties.