Humidity-induced significant microstructural reordering in partially reduced graphene oxide: Insights on water permeation mechanism
TL;DR: In this paper, the authors study the evolution of the structure of GO in a partially reduced form, before and after being hydrated in high humidity conditions, and they reconcile the gravimetry outcomes by suggesting the possibilities of both super-permeating channels and void assisted permeation.
Abstract: Interaction of water and water-based solvents with graphene oxide (GO) has attracted much attention, due to the ability of GO to serve as a highly effective water filtration membrane. In this work, we study the evolution of the structure of GO in a partially reduced form, before and after being hydrated in high humidity conditions. X-ray diffraction (XRD) studies reveal that progressive thermal reduction leads to the increase in the microstructural disorder in the stacking of GO flakes. However, upon hydration of partially reduced GO, microstructural ordering is revealed. This ordered state is characterized by two XRD peaks with substantially smaller full-width-at-half-maximum (FWHM), when compared to the pre-hydration state. The peak corresponding to the sp3 regions has larger d-spacing of ∼9.7 A and an FWHM ∼6 times smaller compared to pre-hydration state, while the other peak corresponds to the ordered sp2 regions with a d-spacing of ∼3.3 A, observed at the characteristic graphitic peak position. Gravimetry studies on suspended films reveal both accelerated and diminished water permeation rates upon annealing when compared to unreduced GO films, which can be attributed to void-assisted permeation in the microstructurally disordered films. The hydrated films in a similar way show a permeation behavior that involves either the increase or decrease in water permeation rates in comparison with pre-hydrated samples. We reconcile to the gravimetry outcomes by suggesting the possibilities of both super-permeating channels and void assisted permeation, and the contribution of each of the mechanisms to the permeation flux.Interaction of water and water-based solvents with graphene oxide (GO) has attracted much attention, due to the ability of GO to serve as a highly effective water filtration membrane. In this work, we study the evolution of the structure of GO in a partially reduced form, before and after being hydrated in high humidity conditions. X-ray diffraction (XRD) studies reveal that progressive thermal reduction leads to the increase in the microstructural disorder in the stacking of GO flakes. However, upon hydration of partially reduced GO, microstructural ordering is revealed. This ordered state is characterized by two XRD peaks with substantially smaller full-width-at-half-maximum (FWHM), when compared to the pre-hydration state. The peak corresponding to the sp3 regions has larger d-spacing of ∼9.7 A and an FWHM ∼6 times smaller compared to pre-hydration state, while the other peak corresponds to the ordered sp2 regions with a d-spacing of ∼3.3 A, observed at the characteristic graphitic peak position. Gravi...
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TL;DR: In this article, the relative contributions of nanochannels and voids, gaps between the lamellae, using electro-impedance spectroscopy, were analyzed with support of supported GO films annealed at different temperatures and the results were fitted to equivalent circuits.
Abstract: Graphene oxide (GO), an ionic and molecular sieve, is an important material for wide-spectrum filtration, since its properties can be tuned by controlling the structure and dimensions of nanochannels between GO nanosheets. In the literature, mechanisms of ion percolation have been proposed assuming GO to be a uniform structure of vertically stacked graphene sheets decorated with functional groups, termed as lamellae. However, in practice, GO is known to have a hierarchical microstructure. In the present work, supported GO films annealed at different temperatures have been studied with the aim of discerning the relative contributions of nanochannels and voids, gaps between the lamellae, using electro-impedance spectroscopy, and the results were fitted to equivalent circuits. Monotonous decrease in the charge transfer resistance Rct and an increase in the percolation resistance RP were observed for GO films annealed up to 160 °C. Increase in RP, taken in perspective with a gradual loss of ordering in nanosheets as observed from X-ray diffraction spectra, enables the conclusion that nanochannels are the dominant pathways of percolation. This was further confirmed by the response of GO films annealed at 180 °C and 200 °C, where a strong dynamic is observed. For these annealed GO films, charge transfer happens both in the conducting films and at the fluorine-doped tin oxide interface. The two processes of ion percolation and charge transfer are, however, interdependent, and are not separated in the impedance response.Graphene oxide (GO), an ionic and molecular sieve, is an important material for wide-spectrum filtration, since its properties can be tuned by controlling the structure and dimensions of nanochannels between GO nanosheets. In the literature, mechanisms of ion percolation have been proposed assuming GO to be a uniform structure of vertically stacked graphene sheets decorated with functional groups, termed as lamellae. However, in practice, GO is known to have a hierarchical microstructure. In the present work, supported GO films annealed at different temperatures have been studied with the aim of discerning the relative contributions of nanochannels and voids, gaps between the lamellae, using electro-impedance spectroscopy, and the results were fitted to equivalent circuits. Monotonous decrease in the charge transfer resistance Rct and an increase in the percolation resistance RP were observed for GO films annealed up to 160 °C. Increase in RP, taken in perspective with a gradual loss of ordering in nanosh...
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TL;DR: In this paper , the authors systematically studied various properties of Graphene oxide (GO) membranes exposed to controlled humidity levels ranging from 0% to 90% RH and found that most water gets absorbed in interlayer spaces at low humidity (<25% RH), the fraction of water in the void spaces increases with RH, and the lower bound for the dielectric constant of confined water is estimated to be εwater > 17.
Abstract: Graphene oxide (GO) membranes possess a hierarchical microstructure, with well-ordered crystalline lamellae combining to form a macroscopic membrane. Water can intercalate in GO either in the sub-nanometer interlayer spaces or in the gaps between the lamellae known as voids; distinguishing the contribution of these two has been challenging. Addressing this challenge, we systematically study various properties of GO membranes exposed to controlled humidity levels ranging from 0% to 90% RH. Thickness-dependent dynamic vapor sorption is used to quantify the water content under different humidity environments. Complementing the vapor sorption studies, the AC impedance response of the GO membrane is determined at different humidity values. Our findings suggest that (a) most water gets absorbed in interlayer spaces at low humidity (<25% RH), (b) the fraction of water in the void spaces increases with RH%, (c) the lower bound for the dielectric constant of confined water is estimated to be εwater > 17, and (d) the conductivity increases by 5 to 6 orders of magnitude over a narrow range of water content (13 wt% to 31 wt%). The rapid increase in conductivity over a narrow range of water content suggests a percolative process for the protons. The dielectric constant estimates suggest that confined water behaves distinctly differently in a hydrophilic environment than in a hydrophobic one.
1 citations
TL;DR: In this article, the authors examined the influence of confined water on the mechanical and electromechanical response of graphene oxide films, prepared with variable oxidative states, casted on polydimethylsiloxane (PDMS) substrates.
Abstract: The confinement of water between sub-nanometer bounding walls of layered two-dimensional materials has generated tremendous interest. Here, we examined the influence of confined water on the mechanical and electromechanical response of graphene oxide films, prepared with variable oxidative states, casted on polydimethylsiloxane (PDMS) substrates. These films were subjected to uniaxial strain under controlled humid environments (5 to 90 % RH), while dc transport studies were performed in tandem. Straining resulted in the formation of quasi-periodic linear crack arrays. The extent of water intercalation determined the density of cracks formed in the system thereby, governing the electrical conductance of the films under strain. The crack density at 5 % strain, varied from 0 to 3.5 cracks/mm for hydrated films and 8 to 22 cracks/mm for dry films, across films with different high oxidative states. Correspondingly, the overall change in the electrical conductance at 5 % strain was observed to be ~ 5 to 20 folds for hydrated films and ~ 20 to 35 folds for the dry films. The results were modeled with a decrease in the in-plane elastic modulus of the film upon water intercalation, which was attributed to the variation in the nature of hydrogen bonding network in graphene oxide lamellae.
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TL;DR: Submicrometer-thick membranes made from graphene oxide can be completely impermeable to liquids, vapors, and gases, including helium, but these membranes allow unimpeded permeation of water (H2O permeates through the membranes at least 1010 times faster than He).
Abstract: Permeation through nanometer pores is important in the design of materials for filtration and separation techniques and because of unusual fundamental behavior arising at the molecular scale. We found that submicrometer-thick membranes made from graphene oxide can be completely impermeable to liquids, vapors, and gases, including helium, but these membranes allow unimpeded permeation of water (H 2 O permeates through the membranes at least 10 10 times faster than He). We attribute these seemingly incompatible observations to a low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. Diffusion of other molecules is blocked by reversible narrowing of the capillaries in low humidity and/or by their clogging with water.
2,602 citations
TL;DR: This work investigates permeation through micrometer-thick laminates prepared by means of vacuum filtration of graphene oxide suspensions, which reveal that the GO membrane can attract a high concentration of small ions into the membrane, which may explain the fast ion transport.
Abstract: Graphene-based materials can have well-defined nanometer pores and can exhibit low frictional water flow inside them, making their properties of interest for filtration and separation. We investigate permeation through micrometer-thick laminates prepared by means of vacuum filtration of graphene oxide suspensions. The laminates are vacuum-tight in the dry state but, if immersed in water, act as molecular sieves, blocking all solutes with hydrated radii larger than 4.5 angstroms. Smaller ions permeate through the membranes at rates thousands of times faster than what is expected for simple diffusion. We believe that this behavior is caused by a network of nanocapillaries that open up in the hydrated state and accept only species that fit in. The anomalously fast permeation is attributed to a capillary-like high pressure acting on ions inside graphene capillaries.
2,055 citations
TL;DR: A simple scalable method is demonstrated to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl and decrease exponentially with decreasing sieve size, but water transport is weakly affected.
Abstract: Ion permeation and selectivity of graphene oxide membranes with sub-nm channels dramatically alters with the change in interlayer distance due to dehydration effects whereas permeation of water molecules remains largely unaffected. Graphene oxide membranes show exceptional molecular permeation properties, with promise for many applications1,2,3,4,5. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of ∼9 A (ref. 4), which is larger than the diameters of hydrated ions of common salts4,6. The cutoff is determined by the interlayer spacing (d) of ∼13.5 A, typical for graphene oxide laminates that swell in water2,4. Achieving smaller d for the laminates immersed in water has proved to be a challenge. Here, we describe how to control d by physical confinement and achieve accurate and tunable ion sieving. Membranes with d from ∼9.8 A to 6.4 A are demonstrated, providing a sieve size smaller than the diameters of hydrated ions. In this regime, ion permeation is found to be thermally activated with energy barriers of ∼10–100 kJ mol–1 depending on d. Importantly, permeation rates decrease exponentially with decreasing sieve size but water transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for the entry of water molecules and large slip lengths inside graphene capillaries. Building on these findings, we demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl.
1,297 citations
TL;DR: In this article, a combination of high-resolution in situ X-ray photoemission and Xray absorption spectroscopies was used to monitor the deoxygenation process and comprehensively evaluate the electronic structure of graphene oxide thin films at different stages of the thermal reduction process.
Abstract: Despite the recent developments in graphene oxide due to its importance as a host precursor of graphene, the detailed electronic structure and its evolution during the thermal reduction remain largely unknown, hindering its potential applications. We show that a combination of high-resolution in situ X-ray photoemission and X-ray absorption spectroscopies offer a powerful approach to monitor the deoxygenation process and comprehensively evaluate the electronic structure of graphene oxide thin films at different stages of the thermal reduction process. It is established that the edge plane carboxyl groups are highly unstable, whereas carbonyl groups are more difficult to remove. The results consistently support the formation of phenol groups through reaction of basal plane epoxide groups with adjacent hydroxyl groups at moderate degrees of thermal activation (∼400 °C). The phenol groups are predominant over carbonyl groups and survive even at a temperature of 1000 °C. For the first time, a drastic increase...
1,265 citations
TL;DR: Here, cationic control of the interlayer spacing of graphene oxide membranes with ångström precision is demonstrated using K+, Na+, Ca2+, Li+ or Mg2+ ions, suggesting that other ions could be used to produce a wider range of interlayer spacings.
Abstract: Graphene oxide membranes-partially oxidized, stacked sheets of graphene-can provide ultrathin, high-flux and energy-efficient membranes for precise ionic and molecular sieving in aqueous solution. These materials have shown potential in a variety of applications, including water desalination and purification, gas and ion separation, biosensors, proton conductors, lithium-based batteries and super-capacitors. Unlike the pores of carbon nanotube membranes, which have fixed sizes, the pores of graphene oxide membranes-that is, the interlayer spacing between graphene oxide sheets (a sheet is a single flake inside the membrane)-are of variable size. Furthermore, it is difficult to reduce the interlayer spacing sufficiently to exclude small ions and to maintain this spacing against the tendency of graphene oxide membranes to swell when immersed in aqueous solution. These challenges hinder the potential ion filtration applications of graphene oxide membranes. Here we demonstrate cationic control of the interlayer spacing of graphene oxide membranes with angstrom precision using K+, Na+, Ca2+, Li+ or Mg2+ ions. Moreover, membrane spacings controlled by one type of cation can efficiently and selectively exclude other cations that have larger hydrated volumes. First-principles calculations and ultraviolet absorption spectroscopy reveal that the location of the most stable cation adsorption is where oxide groups and aromatic rings coexist. Previous density functional theory computations show that other cations (Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+) should have a much stronger cation-π interaction with the graphene sheet than Na+ has, suggesting that other ions could be used to produce a wider range of interlayer spacings.
1,082 citations