Insight into the Nanoscale Mechanism of Rapid H2O Transport within a Graphene Oxide Membrane: Impact of Oxygen Functional Group Clustering

Multilayer Fluorine-Free MoBTx MBene with Hydrophilic Structural-Modulating for the Fabrication of a Low-Resistance and High-Resolution Humidity Sensor

2D transition metal borides (MBenes) with abundant surface terminals hold great promise in molecular sensing applications. However, MBenes from etching with fluorine-containing reagents present inert -fluorine groups on the surface, which hinders their sensing capability. Herein, the multilayer fluorine-free MoBTx MBene (where Tx represents O, OH, and Cl) with hydrophilic structure is prepared by a hydrothermal-assisted hydrochloric acid etching strategy based on guidance from the first-principle calculations. Significantly, the fluorine-free MoBTx-based humidity sensor is fabricated and demonstrates low resistance and excellent humidity performance, achieving a response of 90% to 98%RH and a high resolution of 1%RH at room temperature. By combining the experimental results with the first-principles calculations, the interactions between MoBTx and H2O, including the adsorption and intercalation of H2O, are understood first in depth. Finally, the portable humidity early warning system for real-time monitoring and early warning of infant enuresis and back sweating illustrates its potential for humidity sensing applications. This work not only provides guidance for preparation of fluorine-free MBenes, but also contributes to advancing their exploration in sensing applications.

Liquid-like and solid-like acetonitrile intercalated into graphite oxide as studied by the spin probe technique

The molecular mobility of acetonitrile intercalated into the inter-plane space of graphite oxide was studied using the spin probe technique. It was revealed that two types of intercalated substance - liquid-like and solid-like - are simultaneously present in between the oxidized graphene planes, and their ratio depends on temperature. The micro-viscosity of liquid-like intercalated acetonitrile was found to be higher than that of bulk acetonitrile and depends on the amount of intercalated liquid.

Magnetically Recyclable Graphene Oxide Demulsifier Adapting Wide pH Conditions on Detachment of Oil in the Crude Oil-in-Water Emulsion

In the present work, an amphiphilic and magnetically recyclable graphene oxide (MR-GO) demulsifier was devised and synthesized by graft of magnetic nanoparticles (Fe₃O₄@SiO₂-APTES) and ethylenediamine on the GO surface. The wettability and surface charges of MR-GO under various pH conditions can be regulated via adjusting the contents and species of surface functional groups (such as amino, carboxyl, and hydroxyl). In the demulsificaition test, MR-GO displayed favorable demulsification performance for crude oil-in-water (O/W) emulsion under pH of 2.0-10.0, thusly greatly improving the application scope of common demulsifier. The optimal dosage of MR-GO was 200 mg/L and the demulsification efficiency attained a maximum value of 99.7% for crude O/W emulsion with pH of 6.0. What's more, owing to its magnetic response performance, the MR-GO can be reused and the demulsification efficiency remained above 91.0% after six cycles. Based on the strong interfacial activity, MR-GO can arrive to the crude oil-water interface. With the synergy effects of interfacial adsorption (π-π/n-π) interactions and electrostatic attraction of demulsifier and interfacial films, and the aid of external mechanical forces, the interfacial films stabilized the emulsion were disrupted. Therefore, the oil droplets coated on the water droplets were gathered rapidly to form oily flocs and then migrated to the water surface to accomplish the demulsification of crude O/W emulsion.

A Near-Isotropic Proton-Conducting Porous Graphene Oxide Membrane

A graphene oxide (GO) membrane is an ideal separator for multiple applications due to its morphology, selectivity, controllable oxidation, and high aspect ratio of the 2D nanosheet. However, the anisotropic ion conducting nature caused by its morphology is not favorable toward through-plane conductivity, which is vital for solid-state electrolytes in electrochemical devices. Here, we present a strategy to selectively enhance the through-plane proton conductivity of a GO membrane by reducing its degree of anisotropy with pore formation on the nanosheets through the sonication-assisted Fenton reaction. The obtained porous GO (pGO) membrane is a near-isotropic, proton-conducting GO membrane, showing a degree of anisotropy as low as 2.77 and 47% enhancement of through-plane proton conductivity as opposed to the pristine GO membrane at 25 °C and 100% relative humidity. The anisotropic behavior shows an Arrhenius relationship with temperature, while the water interlayer formation between nanosheets plays a pivotal role in the anisotropic behavior under different values of relative humidity (RH); that is, as low RH increases, water molecules tend to orient in a bimodal distribution clinching the nanosheets and forming a subnanometer, high-aspect-ratio, water interlayer, resulting in its peak anisotropy. Further increase in RH fills the interlayer gap, resulting in behaviors akin to near-isotropic, bulk water. Lastly, implementation of the pGO membrane, as the solid proton-conductive electrolyte, in an alcohol fuel cell sensor has been demonstrated, showcasing the excellent selectivity and response, exceptional linearity, and ethanol detection limits as low as 25 ppm. The amalgamation of excellent performance, high customizability, facile scalability, low cost, and environmental friendliness in the present method holds considerable potential for transforming anisotropic GO membranes into near-isotropic ion conductors to further membrane development and sensing applications.

Swelling properties of graphite oxides and graphene oxide multilayered materials

Graphite oxide (GtO) and graphene oxide (GO) multilayered laminates are hydrophilic materials easily intercalated by water and other polar solvents. By definition, an increase in the volume of a material connected to the uptake of a liquid or vapour is named swelling. Swelling is a property which defines graphite oxides and graphene oxides. Less oxidized materials not capable of swelling should be named oxidized graphene. The infinite swelling of graphite oxide yields graphene oxide in aqueous dispersions. Graphene oxide sheets dispersed in a polar solvent can be re-assembled into multilayered structures and named depending on applications as films, papers or membranes. The multilayered GO materials exhibit swelling properties which are mostly similar to those of graphite oxides but not identical and in some cases surprisingly different. Swelling is a key property of GO materials in all applications which involve the sorption of water/solvents from vapours, immersion of GO into liquid water/solvents and solution based chemical reactions. These applications include sensors, sorption/removal of pollutants from waste waters, separation of liquid and gas mixtures, nanofiltration, water desalination, water-permeable protective coatings, etc. Swelling defines the distance between graphene oxide sheets in solution-immersed GO materials and the possibility for penetration of ions and molecules inside of interlayers. A high sorption capacity of GO towards many molecules and cations is defined by swelling which makes the very high surface area of GO accessible. GtO and GO swelling is a surprisingly complex phenomenon which is manifested in a variety of different ways. Swelling is strongly different for materials produced using the most common Brodie and Hummers oxidation procedures; it depends on the degree of oxidation, ad temperature and pressure conditions. The value of the GO interlayer distance is especially important in membrane applications. Diffusion of solvent molecules and ions is defined by the size of "permeation channels" provided by the swelled GO structure. According to extensive studies performed over the last decade the exact value of the inter-layer distance in swelled GO depends on the nature of solvent, temperature and pressure conditions, and the pH and concentration of solutions and exhibits pronounced aging effects. This review provides insight into the fundamental swelling properties of multilayered GO and demonstrates links to advanced applications of these materials.

Mobility of liquids intercalated into the interplane space of graphite oxide as revealed by a combination of 19F NMR, 1H NMR and EPR spin probe methods

A combination of ¹⁹F and ¹H NMR with the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence was used to examine the mobility of liquids in the interplane space of graphite oxide (GO) for the first time.

In Silico Design and Characterization of Graphene Oxide Membranes with Variable Water Content and Flake Oxygen Content

Graphene oxide (GO) membranes offer exceptional promise for certain aqueous separation challenges, such as desalination. Central to unlocking this promise and optimizing performance for a given separation is the establishment of a detailed molecular-level understanding of how the membrane's composition affects its structural and transport properties. This understanding is currently lacking, in part due to the fact that, until recently, molecular models with a realistic distribution of oxygen functionalities and interlayer flake structure were unavailable. To understand the effect of composition on the properties of GO membranes, models with water contents and oxygen contents, varying between 0% and 40% by weight, were prepared in this work using classical molecular dynamics simulations. The change in membrane interlayer distance distribution, water connectivity, and water diffusivity with water and oxygen content was quantified. Interlayer distance distribution analysis showed that the swelling of GO membranes could be controlled by separately tuning both the flake oxygen content and the membrane water content. Water-molecule cluster analysis showed that a continuous and fully connected network of water nanopores is not formed until the water content reaches ∼20%. The diffusivity of water in the membrane was also found to strongly depend on both the water and the oxygen content. These insights help understand the structure and transport properties of GO membranes with sub-nanometer interlayer distances and could be exploited to enhance the performance of GO membranes for aqueous separation applications. More broadly, the high-throughput in silico approach adopted could be applied to other nanomaterials with intrinsic non-stoichiometry and structural heterogeneity.

How Pore Hydrophilicity Influences Water Permeability?

Membrane separation is playing increasingly important role in providing clean water. Simulations predict that membrane pores with strong hydrophobicity produce ultrahigh water permeability as a result of low friction. However, experiments demonstrate that hydrophilic pores favor higher permeability. Herein we simulate water molecules transporting through interlayers of two-dimensional nanosheets with various hydrophilicities using nonequilibrium molecular dynamics. We reveal that there is a threshold pressure drop (ΔP _T), exceeding which stable water permeability appears. Strongly hydrophobic pores exhibit extremely high ΔP _T, prohibiting the achievement of ultrahigh water permeability under the experimentally accessible pressures. Under pressures < ΔP _T, water flows in hydrophobic pores in a running-stop mode because of alternative wetting and nonwetting, thus leading to significantly reduced permeability. We discover that hydrophilic modification to one surface of the nanosheet can remarkably reduce ΔP _T by > 99%, indicating a promising strategy to experimentally realize ultrafast membranes.

Insight into hydrogen bonds and characterization of interlayer spacing of hydrated graphene oxide

The number of hydrogen bonds and detailed information on the interlayer spacing of graphene oxide (GO) confined water molecules were calculated through experiments and molecular dynamics simulations. Experiments play a crucial role in the modeling strategy and verification of the simulation results. The binding of GO and water molecules is essentially controlled by hydrogen bond networks involving functional groups and water molecules confined in the GO layers. With the increase in the water content, the clusters of water molecules are more evident. The water molecules bounding to GO layers are transformed to a free state, making the removal of water molecules from the system difficult at low water contents. The diffuse behaviors of the water molecules are more evident at high water contents. With an increase in the water content, the functional groups are surrounded by fewer water molecules, and the distance between the functional groups and water molecules increases. As a result, the water molecules adsorbed into the GO interlamination will enlarge the interlayer spacing. The interlayer spacing is also affected by the number of GO layers. These results were confirmed by the calculations of number of hydrogen bonds, water state, mean square displacement, radial distribution function, and interlayer spacing of hydrated GO. Graphical Abstract This work research the interaction between GO functional groups and confined water molecules. The state of water molecules and interlayer spacing of graphene oxide were proved to be related to the number of hydrogen bonds.

Computational characterisation of dried and hydrated graphene oxide membranes

A multi-step molecular dynamics procedure was developed to construct fully flexible atomistic models of graphene oxide (GO) membranes. The method of preparation replicates the experimental synthesis of the material; i.e. the flow-directed self-assembly of individual flakes onto a substrate or filter. A total of 180 GO membrane models were prepared with water contents varying between 0 and 20%, providing an insight into changes in the membrane's interlayer distance with swelling. Membranes with 15% water content have an average interlayer distance (0.80 nm), bulk density (1.77 g cm^-3) and tensile modulus (18.1 GPa) in excellent agreement with the experimental literature, demonstrating that air-dried membranes have 15% water content. The models reveal the intrinsic structural heterogeneity and complex morphology of GO membranes. This feature has previously been unaccounted for in both experimental interpretations and GO nanopore models, which often use pre-defined and idealised 2D geometries. Completely dried membranes have considerable free pore volume. This observation explains the modest change in interlayer distance (0.02 nm) as the membrane's water content is increased from 0% to 10% compared to a much more significant change (0.12 nm) as the water content is increased from 10% to 20%. Combined with this new understanding of membrane swelling, the availability of such representative models opens the possibility of the molecular-level design of GO membranes for a variety of applications, such as gaseous and aqueous separations.

Water transport confined in graphene oxide channels through the rarefied effect

This work combines classic hydrodynamics with kinetic theory to provide a molecular insight into water transport between graphene oxide sheets.

Study of Graphene Oxide Structural Features for Catalytic, Antibacterial, Gas Sensing, and Metals Decontamination Environmental Applications

This study represents a comprehensive review about the structural features of graphene oxide (GO) and its significance in environmental applications. Two dimensional (2D) GO is tremendously focused in advanced carbon-based nanomaterials for environmental applications due to its tunable physicochemical characteristics. Herein, we report foundational structural models of GO and explore the chemical bonding of oxygen moieties, with graphite basal plane using various characterization tools. Moreover, the impact of these oxygen moieties and the morphology of GO for environmental applications such as removal of metal ions and catalytic, antibacterial, and gas sensing abilities have here been critically reviewed for the first time. Environmental applications of GO are highly significant because, in the recent era, the fast progress of industries, even in the countryside, results in air and water pollution. GO has been widely investigated by researchers to eradicate such environmental issues and for potential industrial and clinical applications due to its 2D structural features, large surface area, presence of oxygen moieties, nonconductive nature, intense mechanical strength, excellent water dispersibility, and tunable optoelectronic properties. Thence, particular emphasis is directed toward the modification of GO by varying the number of its oxygen functional groups and by coupling it with other exotic nanomaterials to induce unique properties in GO for potential environmental remediation purposes.

Molecular Insight into Water Desalination across Multilayer Graphene Oxide Membranes

Transport of ionic solutions through graphene oxide (GO) membranes is a complicated issue because the complex and tortuous structure inside makes it very hard to clarify. Using molecular dynamics (MD) simulations, we investigated the mechanism of water transport and ion movement across multilayer GO. The significant flow rate is considerably influenced by the structural parameters of GO membranes. Because of the size effect on a shrunken real flow area, there is disagreement between the classical continuum model and nanoscaled flow. To eliminate the variance, we obtained modified geometrical parameters from density analysis and used them in the developed hydrodynamic model to give a precise depiction of water flow. Four kinds of solutions (i.e., NaCl, KCl, MgCl₂, and CaCl₂) and different configurational GO sheets were considered to clarify the influence on salt permeation. It is found that the abilities of permeation to ions are not totally up to the hydration radius. Even though the ionic hydration shell is greater than the opening space, the ions can also pass through the split because of the special double-deck hydration structure. In the structure of GO, a smaller layer separation with greater offsetting gaps could substantially enhance the membrane's ability to reject salt. This work establishes molecular insight into the effects of configurational structures and salt species on desalination performance, providing useful guidelines for the design of multilayer GO membranes.

Carbon membranes for efficient water-ethanol separation

We demonstrate, on the basis of molecular dynamics simulations, the possibility of an efficient water-ethanol separation using nanoporous carbon membranes, namely, carbon nanotube membranes, nanoporous graphene sheets, and multilayer graphene membranes. While these carbon membranes are in general permeable to both pure liquids, they exhibit a counter-intuitive "self-semi-permeability" to water in the presence of water-ethanol mixtures. This originates in a preferred ethanol adsorption in nanoconfinement that prevents water molecules from entering the carbon nanopores. An osmotic pressure is accordingly expressed across the carbon membranes for the water-ethanol mixture, which agrees with the classic van't Hoff type expression. This suggests a robust and versatile membrane-based separation, built on a pressure-driven reverse-osmosis process across these carbon-based membranes. In particular, the recent development of large-scale "graphene-oxide" like membranes then opens an avenue for a versatile and efficient ethanol dehydration using this separation process, with possible application for bio-ethanol fabrication.

Buckling failure of square ice-nanotube arrays constrained in graphene nanocapillaries

Graphene confinement provides a new physical and mechanical environment with ultrahigh van der Waals pressure, resulting in new quasi-two-dimensional phases of few-layer ice. Polymorphic transition can occur in bilayer constrained water/ice system. Here, we perform a comprehensive study of the phase transition of AA-stacked bilayer water constrained within a graphene nanocapillary. The compression-limit and superheating-limit (phase) diagrams are obtained, based on the extensive molecular-dynamics simulations at numerous thermodynamic states. Liquid-to-solid, solid-to-solid, and solid-to-liquid-to-solid phase transitions are observed in the compression and superheating of bilayer water. Interestingly, there is a temperature threshold (∼275 K) in the compression-limit diagram, which indicates that the first-order and continuous-like phase transitions of bilayer water depend on the temperature. Two obviously different physical processes, compression and superheating, display similar structural evolution; that is, square ice-nanotube arrays (BL-VHDI) will bend first and then transform into bilayer triangular AA stacking ice (BL-AAI). The superheating limit of BL-VHDI exhibits local maxima, while that of BL-AAI increases monotonically. More importantly, from a mechanics point of view, we propose a novel mechanism of the transformation from BL-VHDI to BL-AAI, both for the compression and superheating limits. This structural transformation can be regarded as the "buckling failure" of the square-ice-nanotube columns, which is dominated by the lateral pressure.

Molecular Dynamics Simulations Reveal that Water Diffusion between Graphene Oxide Layers is Slow

Membranes made of stacked layers of graphene oxide (GO) hold the tantalizing promise of revolutionizing desalination and water filtration if selective transport of molecules can be controlled. We present the findings of an integrated study that combines experiment and molecular dynamics simulation of water intercalated between GO layers. We simulated a range of hydration levels from 1 wt.% to 23.3 wt.% water. The interlayer spacing increased upon hydration from 0.8 nm to 1.1 nm. We also synthesized GO membranes that showed an increase in layer spacing from about 0.7 nm to 0.8 nm and an increase in mass of about 15% on hydration. Water diffusion through GO layers is an order of magnitude slower than that in bulk water, because of strong hydrogen bonded interactions. Most of the water molecules are bound to OH groups even at the highest hydration level. We observed large water clusters that could span graphitic regions, oxidized regions and holes that have been experimentally observed in GO. Slow interlayer diffusion can be consistent with experimentally observed water transport in GO if holes lead to a shorter path length than previously assumed and sorption serves as a key rate-limiting step.

Graphene Oxide as an Effective Barrier on a Porous Nanofibrous Membrane for Water Treatment

A novel graphene oxide (GO)-based nanofiltration membrane on a highly porous polyacrylonitrile nanofibrous mat (GO@PAN) is prepared for water treatment applications. GO with large lateral size (more than 200 μm) is first synthesized through an improved Hummers method and then assembled on a highly porous nanofibrous mat by vacuum suction method. The prepared GO@PAN membrane is characterized by scanning electron microscopy, transmission electron microscopy, Raman spectrum, X-ray diffraction, and so forth. The results show that graphene oxide can form a barrier on the top of a PAN nanofibrous mat with controllable thickness. The obtained graphene oxide layer exhibits "ideal" pathways (hydrophobic nanochannel) for water molecules between the well-stacked GO nanosheets. Water flux under an extremely low external pressure (1.0 bar) significantly increased due to the unique structure of the GO layer and nanofibrous support. Furthermore, the GO@PAN membrane shows high rejection performance (nearly 100% rejection of Congo red and 56.7% for Na2SO4). A hydrophilic-hydrophobic "gate"-nanochannel model is presented for explaining the water diffusion mechanism through the GO layer. This method for fabrication of the GO membrane on a highly porous support may provide many new opportunities for high performance nanofiltration applications.