Crosslinked graphene-based membranes: fabrication methods, characteristics, and applications in separation processes

Abstract

Disclosed is a method of fabricating a self-standing Graphene Oxide (GO) membrane comprising a plurality of layers of crosslinked GO sheets, the method comprising using a crosslinking agent to crosslink between oxygen-containing functional groups of neighbour GO sheets, wherein the crosslinking agent is a metallic oxide or a carbide.

Description

CROSSLINKED GRAPHENE-BASED MEMBRANES: FABRICATION METHODS, CHARACTERISTICS, AND APPLICATIONS IN SEPARATION PROCESSESFIELD OF THE INVENTION

[0001] The present invention generally relates to a method of fabricating self-standing membranes based on laminates of 2D materials. In particular, the present invention relates to a method of using a metallic oxide or carbide to fabricate self-standing membranes based on laminates of 2D materials.BACKGROUND

[0002] The separation and purification of substances from mixtures of liquids, vapours, and gases are fundamental in many modern industrial processes Nowadays, membrane technologies are being intensively developed to diminish the large amounts of energy consumed in industrial separation processes, which require high temperatures and extensive use of expensive thermal isolating systems. Specifically, at the molecular and atomic level, membranes based on laminates of 2D materials are the most efficient approach for separation processes. 2D laminates integrate unique characteristics, like pore-size distribution in nano and angstrom scales, stacking order in a highly interlocked structure of nanochannels, and high physicochemical interactions with liquids and gases due to their surface charge.

[0003] These properties bring fundamental advantages over other materials in terms of molecular sieving and selectivity, as is in the case of the known selectivity of graphene-based membranes for gases and liquid solutions. The nanostructured graphene oxide (GO) membranes consist of stacked layers of graphene sheets with lateral sizes of 1 to 10 pm and spaced by distances of 0.9 to 1.2 nm via oxygenated functional groups, predominantly epoxide, hydroxy, carboxyl and carbonyl groups [1], The predominantly random distribution of these chemical groups allows the formation of a percolative network of pristine graphene channels, which is fundamental for the mass of transport inside the membrane [1], However, the physical properties of 2D material s-based membranes created at the laboratory scale do not address the minimum requirements for large-scale pressurized processes, which will be the norm in the industry.

[0004] The use of multivalent metallic cations as crosslinking agents between the GO nanosheets to improve the stability and mechanical stiffness of GO membranes is known in theliterature [2] [3] [4], In contrast, the present invention refers to the use of metallic oxides or carbides as crosslinking agents, which give rise to effects far beyond the membranes' mechanical stability. To overcome several challenges of large-scale applications of 2D material s-based membranes, there is a need for a novel method which can produce membranes based on laminates of 2D materials such as GO, which demonstrate enhanced mechanical stability, as well as selectivity, and are suitable for large-scale industrial applications. This invention describes the development and testing of a new generation of self-standing GO membranes designed to ensure 2D laminates' selectivity. The fabrication methodology allows the possibility of strictly controlling the membrane thickness and forming an ordered laminated microstructure with interlayer distances between 0.8 and 1.2 nm. The fabricated self-standing GO membranes have areas between 0.002 m2and 0.090 m2and thicknesses between 0.6 pm and 20 pm. They are flexible and continuously present chemical and physical stability during testing in direct contact with ethanol- and methanol / water-liquid mixtures for at least 24 hours. The method was tested (but is not limited to) by using the crosslinking agents iron (TIT) oxide, aluminium oxide, calcium sulfate, silicon carbide, niobium pentoxide, and Bi-2212. The tests for gas permeation and the fabricated membranes' chemical characteristics also allow their application in gas separation processes. The interaction of the oxides with the functional groups of GO, especially observed on the characteristics of the Nb2O> crosslinking-based membrane, opens the possibility of further scalable exploration of the mass transport mechanisms enhanced by crosslinking with metallic oxides in graphene-related- and other 2D materials.SUMMARY

[0005] In one aspect, the present disclosure refers to a method of fabricating a self-standing Graphene Oxide (GO) membrane comprising a plurality of layers of crosslinked GO sheets, the method comprising using a crosslinking agent to crosslink between oxygen-containing functional groups of neighbour GO sheets, wherein the crosslinking agent is a metallic oxide or a carbide.

[0006] Advantageously, the method as disclosed herein uses metallic oxides, multi-cation oxides, such as Fe20s, AI2O3, CaSOr, BSCCO, Nb2Os, and combinations thereof, or carbides such as SiC to produce self-standing GO membranes. These crosslinking agents provide mechanical stability to the membranes and affect the mass transport mechanisms inside the membranes.

[0007] The present disclosure discloses two embodiments of the method for producing membranes, i.e. (1) Deposition of aqueous suspension of GO on SiC, metallic oxides and multication oxides substrates followed by detaching free self-standing membranes; and (2) Preparing aqueous suspension of GO and a crosslinking agent such as SiC, metallic oxides, multi-cation oxides powders, and combinations thereof, followed by casting. Advantageously, the two embodiments of the method of fabrication are scalable methods to obtain freestanding crosslinked GO membranes.

[0008] The present disclosure also discloses a method for detaching free self-standing membranes from SiC, metallic oxides and multi-cation oxides substrates in surfactant solutions. Advantageously, this technique enables the separation of membranes from solid surfaces while maintaining the integrity of the membrane.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0010] Figure 1 is a photo showing a GO membrane with an area of 126 cm2detaching from the CaSO4 plate with the help of water and surfactant solution and ready to be washed in distilled water to be wrapped around a ceramic tube of 25 mm (right).

[0011] Figure 2 are photos showing GO membranes fabricated using CaSOr: (a) and (b) wrapped around a tube of diameter 10 mm; (c) partially cut for illustration from the covering of a tube of diameter 25 mm; and (d) a membrane covering a ceramic tube of 25 mm diameter ready to be tested inside the pervaporation module.

[0012] Figure 3 are photos showing an overview of membranes with metal oxide based- crosslinking: (a) large area CaSO4-crosslinking based membrane; and (b) its smooth uniform and free of cracks surface by optical microscopy; (c) the BSCCO-crosslinking based membrane is semi-transparent with a brown colouration, and (d) it is mechanically stable and bendable.

[0013] Figure 4 are scanning electron microscopy (SEM) images showing (a) general view of the membrane with SiC, which is fixed to the SEM sample holder with the help of conductive carbon tape; (b) top view of undulated and cracks-free membrane surface and cross-section; and (c) cross-section with thickness measurement.

[0014] Figure 5 are SEM images showing sample with AI2O3 + CaSOr-based crosslinking with (a) smooth, undulated, and crack-free continuous surface; (b) overall average thickness of ~ 0.6 pm; and (c) laminate microstructure with very ordered stacking of the GO layers.

[0015] Figure 6 are SEM pictures of a membrane with NbzOs. (a) General view of the membrane with NbzOs, which is fixed to the SEM sample holder with the help of conductive carbon tape; (b) Cross-sectional view showing its laminated microstructure with a thickness of ~ 1 .4 pm; (c) top view of the surface showing regularly distributed circular “hillocks”; and (d) magnification of “hillocks” and ripples.

[0016] Figure 7 is an EDS analysis of membrane sample with Nb20s showing the main components of near-circular objects regularly distributed all over the membrane, (a) and (b): representative surface analysis; (c) and (d): local analysis of a single object.

[0017] Figure 8 are X-rays diffractograms of GO membranes of pure GO and with the crosslinking.

[0018] Figure 9 is a survey spectrum for the membrane sample with pure GO with the peaks corresponding to C Is, O Is, and O 2s and no detectable impurities.

[0019] Figure 10 is a survey spectrum of membrane sample with AI2O3 + CaSO4 - based crosslinking. The inset shows details of the spectrum in the low energy range, revealing the presence of calcium-binding energies related to small amounts of calcium carbonate CaCCh.

[0020] Figure 11 is a Ca 2p scan spectrum with the peaks corresponding to Ca 2p3 / 2 and Ca 2pl / 2.

[0021] Figure 12 is a survey spectrum for the sample membrane with SiC-based crosslinking.

[0022] Figure 13 is a survey spectrum of the sample membrane with NbiOs-based crosslinking.

[0023] Figure 14 is an XPS spectrum of the binding energy of Nb 3d from the membrane sample with bt^Ch-based crosslinking.

[0024] Figure 15 is a bar graph showing separation factor (a) and flux of membranes with and without crosslinking for PV of ethanol / water 80 / 20 wt.% at room temperature.

[0025] Figure 16 is a Roberson plot which shows H2 / CO2 selectivity as function of the H2 permeability for the membranes with AI2O3 + CaSOr, NbrOs and CaSCE crosslinking.DETAILED DESCRIPTION

[0026] The present disclosure describes a method of using crosslinking agents such as metallic oxides and carbides, to produce a new generation of self-standing GO membranes with high mechanical stability and selectivity.

[0027] In one aspect, the present disclosure refers to a method of fabricating a self-standing Graphene Oxide (GO) membrane comprising a plurality of layers of crosslinked GO sheets, the method comprising using a crosslinking agent to crosslink between oxygen -containing functional groups of neighbour GO sheets, wherein the crosslinking agent is a metallic oxide or a carbide.

[0028] In one example, the crosslinking agent is a metallic oxide. In another example, the metallic oxide is selected from the group consisting of Iron (III) oxide (Fe2O3), aluminium oxide (AI2O3), calcium sulfate (CaSO4), niobium pentoxide (NbzOs), Bi-2212 (Bi2Sr2CaCu2Ox), an oxide of a transition metal, an oxide of an alkaline earth metal, an oxide of a post-transition metal, aluminosilicate, and a multi-cation oxide selected from the group consisting of YBaCO and BSCCO. As used herein, a "transition metal" is a chemical element in groups 3 to 12 of the periodic table. Some examples of oxides of transition metals include TiO, Ti2O3, TiO2, V2O5, V2O3, CrO, Cr2O3, CrO5, MnO, MnO2, Mn2O3, Co2O3, CoO, NiO, Ni2O3, Cu3G, CuO, ZnO, Y2O3, ZrO2, MoO2, MoOs, RuO2, Fe3O4, HfO2, and so on. As used herein, an "alkaline earth metal" is one of the six chemical element in group 2 of the periodic table. The alkaline earth metal oxides are BeO, MgO, CaO, SrO, BaO and RaO. As used herein, a "post-transitional metal" is a metal in groups 13 - 15 metals and periods 4 - 6 of the periodic table. Some examples of post-transition metal oxides include Ga2O2, In2C>3, T12O, T12O, SnO, PbO, Pb3O4, PbO2, Bi2O3, and so on. In another example, the crosslinking agent is a combination of the metallic oxide as disclosed herein. In another example, the crosslinking agent is AhCh / CaSC^ 50 / 50 wt.%.

[0029] In another example, the crosslinking agent is a carbide. In another example, the carbide is silicon carbide (SiC). In another example, the carbide is TiC. In another example, the carbide is ZrC. In another example, the carbide is HfC. In another example, the carbide is VC. In another example, the carbide is NbC. In another example, the carbide is TaC. In another example, the carbide is Cr?C3. In another example, the carbide is Cr3C2. In another example, the carbide is MoC. In another example, the carbide is Mo2C.

[0030] Advantageously, the method as disclosed herein uses metallic oxides, multi-cation oxides, such as Fe2O3, AI2O3, CaSC>4, BSCCO, Nb2Os, and combinations thereof, or carbides such as SiC to produce self-standing GO membranes. These crosslinking agents provide mechanical stability to the membranes and affect the mass transport mechanisms inside the membranes.

[0031] The method as disclosed herein is based on the interaction of a crosslinking agent as disclosed herein with GO, giving rise to crosslinking between the oxygen -containing functional groups of neighbor GO sheets. In one example, the oxygen-containing functional group is an epoxide group. In another example, the oxygen-containing functional group is a hydroxyl group. In another example, the oxygen-containing functional group is a carboxyl group. In another example, the oxygen-containing functional group is a carbonyl group.

[0032] The approach of incorporating metallic cations into graphene-related materials via the present preparation method of using metallic oxides and carbides as cation sources and crosslinking agents is novel. The strong interactions between the crosslinking agents and the oxygen-containing functional groups lead to the formation of crosslinking structures by Metal- O-C and / or Metal-C bonds.

[0033] As most metallic oxides are insoluble in water, the interaction initially occurs mainly between the oxides and the GO functional groups, which then causes the formation of crosslinking sites with the metallic cations. This process takes place in different forms, depending on the specific metallic oxide and the particular procedure of membrane preparation.

[0034] In a first embodiment of the method as disclosed herein, the method comprises the following steps: a. preparing a suspension of GO in distilled water or a water / ethanol solution via sonication for 0.5 - 1 h; b. casting or spraying the suspension of GO uniformly on a solid porous surface of a metallic oxide, a mixture of metallic oxides, or a carbide to form a layer; c. drying the layer at room temperature, d. sequentially repeating steps b and c until the GO membrane comprising a plurality of layers of crosslinked GO sheets is formed; e. peeling off the GO membrane off from the solid porous surface, or detaching the GO membrane from the solid porous surface by immersing the GO membrane in distilled water containing a surfactant.

[0035] Advantageously, the first embodiment of the method as disclosed herein is a scalable method to obtain freestanding crosslinked GO membranes.

[0036] In one example, in step a of the first embodiment of the method as disclosed herein, the GO is suspended in distilled water. In another example, in step a of the first embodiment of the method as disclosed herein, the GO is suspended in a water / ethanol solution

[0037] In one example, the concentration of GO in the suspension is 0.5 - 10 mg / mL. In another example, the concentration of GO in the suspension is about 0 5 mg / mL. In another example, the concentration of GO in the suspension is about 1 mg / mL. In another example, the concentration of GO in the suspension is about 1.5 mg / mL. In another example, the concentration of GO in the suspension is about 2 mg / mL. In another example, the concentration of GO in the suspension is about 2.5 mg / mL. In another example, the concentration of GO in the suspension is about 3 mg / mL. In another example, the concentration of GO in the suspension is about 3.5 mg / mL. In another example, the concentration of GO in the suspension is about 4 mg / mL. In another example, the concentration of GO in the suspension is about 4.5 mg / mL. In another example, the concentration of GO in the suspension is about 5 mg / mL. In another example, the concentration of GO in the suspension is about 5.5 mg / mL. In another example, the concentration of GO in the suspension is about 6 mg / mL. In another example, the concentration of GO in the suspension is about 6.5 mg / mL. In another example, the concentration of GO in the suspension is about 7 mg / mL In another example, the concentration of GO in the suspension is about 7.5 mg / mL. In another example, the concentration of GO in the suspension is about 8 mg / mL. In another example, the concentration of GO in the suspension is about 8.5 mg / mL. In another example, the concentration of GO in the suspension is about 9 mg / mL. In another example, the concentration of GO in the suspension is about 9.5 mg / mL. In another example, the concentration of GO in the suspension is about 10 mg / mL.

[0038] In one example, when GO is suspended in a water / ethanol solution, the ethanol is 2 - 70 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 2 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 5 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 10 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 15 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 20 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 25 wt.%. In another example, whenGO is suspended in a water / ethanol solution, the ethanol is about 30 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 35 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 40 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 45 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 50 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 55 wt.%. Tn another example, when GO is suspended in a water / ethanol solution, the ethanol is about 60 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 65 wt.%. In another example, when GO is suspended in a water / ethanol solution, the ethanol is about 70 wt.%.

[0039] In step a of the first embodiment of the method as disclosed herein, the suspension of GO in distilled water or a water / ethanol solution is prepared through homogenization by mild sonication in a sonication bath at room temperature for 0.5-1 h. In one example, in step a of the first embodiment of the method as disclosed herein, the suspension of GO in distilled water or a water / ethanol solution was sonicated for about 0.5 h. In another example, in step a of the first embodiment of the method as disclosed herein, the suspension of GO in distilled water or a water / ethanol solution was sonicated for about 0.75 h. In another example, in step a of the first embodiment of the method as disclosed herein, the suspension of GO in distilled water or a water / ethanol solution was sonicated for about 1 h.

[0040] After homogenization, the prepared suspension of GO is uniformly cast or sprayed on a solid porous surface of a metallic oxide or a mixture of metallic oxides, which can be composed of AI2O3, CaSO4, Fe2O3, FesOi, FeO, Nb20s, and other oxides of transition metals and alkaline earth metals, post-transition metals, aluminosilicates, multi-cation oxides like the low-temperature superconductor series YBaCO, BSCCO and other multi-cation oxides, as well as carbides like SiC.

[0041] A homogeneous GO membrane is assembled by sequentially casting or spraying several layers and drying each one of them before depositing the next layer. In one example, the quantity of the suspension of GO used to form each layer is 0 1 - 6 mL / cm2. In another example, the quantity of the suspension of GO used to form each layer is about 0.1 mL / cm2. In another example, the quantity of the suspension of GO used to form each layer is about 0.5 mL / cm2. In another example, the quantity of the suspension of GO used to form each layer is about 1 mL / cm2. In another example, the quantity of the suspension of GO used to form eachlayer is about 2 mL / cm2. In another example, the quantity of the suspension of GO used to form each layer is about 3 mL / cm2. In another example, the quantity of the suspension of GO used to form each layer is about 4 mL / cm2. In another example, the quantity of the suspension of GO used to form each layer is about 5 mL / cm2. In another example, the quantity of the suspension of GO used to form each layer is about 6 mL / cm2.

[0042] In one example, the number of layers of crosslinked GO sheets is at least 3 if the suspension of GO is casted on the solid porous surface. In another example, the number of layers of crosslinked GO sheets is at least 5 if the suspension of GO is casted on the solid porous surface. In another example, the number of layers of crosslinked GO sheets is at least 10 if the suspension of GO is casted on the solid porous surface. In another example, the number of layers of crosslinked GO sheets is at least 20 if the suspension of GO is casted on the solid porous surface

[0043] In one example, the number of layers of crosslinked GO sheets is at least 10 if the suspension of GO is sprayed on the solid porous surface. In another example, the number of layers of crosslinked GO sheets is at least 15 if the suspension of GO is sprayed on the solid porous surface. In another example, the number of layers of crosslinked GO sheets is at least 20 if the suspension of GO is sprayed on the solid porous surface. In another example, the number of layers of crosslinked GO sheets is at least 30 if the suspension of GO is sprayed on the solid porous surface In another example, the number of layers of crosslinked GO sheets is at least 40 if the suspension of GO is sprayed on the solid porous surface.

[0044] A minimal time of one hour is necessary to let dry the first layer under ambient conditions. In one example, drying the first layer of GO sheet takes about 1 h. In one example, drying the first layer of GO sheet takes about 2 h. In another example, drying the first layer of GO sheet takes about 3 h. In another example, drying the first layer of GO sheet takes about 6 h. In another example, drying the first layer of GO sheet takes about 10 h. In another example, drying the first layer of GO sheet takes about 12 h. In another example, drying the first layer of GO sheet takes about 15 h. In another example, drying the first layer of GO sheet takes about 20 h. In another example, drying the first layer of GO sheet takes about 24 h.

[0045] The second, third, and fourth layers need a minimal drying time of 3, 6, and 12 h respectively. Any further layer needs about 12 h to dry at room temperature under ambient conditions.

[0046] In one example, the resulting dry membrane can be integrally peeled off from the solid surfaces. In another example, the resulting dry membrane can be detached with the help of immersion in distilled water containing a surfactant agent which is water-soluble, selected from the group consisting of SDS, hydroxides, and other surfactant substances. Advantageously, this technique of detaching free self-standing membranes from substrates such as SiC, metallic oxides and multi-cation oxides in surfactant solutions enables the separation of membranes from solid surfaces while maintaining the integrity of the membranes.

[0047] In one particular example of the first embodiment of the method as disclosed herein, the method comprises the following steps: a. preparing a suspension of 0.1 mg / mL GO in a water / ethanol 50 / 50 wt.% solution via sonication for 0.5 h; b. casting 0.25 - 0.30 mL / cm2of the suspension of GO uniformly on a solid porous surface selected from the group consisting of CaSO4, SiC, and AbO 'CaSOr 50 / 50 wt.% to form a layer; c. drying the layer at room temperature, d. sequentially repeating steps b and c until the GO membrane comprising a plurality of layers of crosslinked GO sheets is formed, wherein the total drying time is 24 h; e. peeling off the GO membrane off from the solid porous surface, or detaching the GO membrane from the CaSO4 or SiC solid porous surface by immersing the GO membrane in distilled water containing 0.1% w / v SDS.

[0048] In this particular example of the first embodiment of the method as disclosed herein, in all cases, the self-standing GO membrane is obtained after drying at room temperature for a total time of 24 h. The dry membrane is integrally peeled off from the solid surfaces. In the case of pure CaSOr and pure SiC, the thinnest membrane which comprises three layers of crosslinked GO sheets is detached from the solid surfaces by immersion in 0.1% w / v SDS solution in distilled water

[0049] In the first embodiment of the method as disclosed herein, after detaching, the GO membrane is washed for at least 1 h in distilled water.

[0050] In a second embodiment of the method as disclosed herein, the method comprises the following steps: a. preparing a suspension of GO in distilled water or a water / ethanol solution via sonication for 0.5 to 1 h;b. preparing a suspension of the crosslinking agent in distilled water or a water / ethanol solution via sonication for 1 to 4 h; c. preparing a suspension of the crosslinking agent and GO in distilled water or a water / ethanol solution by mixing the suspension of GO prepared in step a and the suspension of the crosslinking agent prepared in step b and sonicating for 0.5 to 1 h; d. casting or spraying the suspension of the crosslinking agent and GO uniformly on a solid surface to form a layer; e. drying the layer at room temperature; f. sequentially repeating steps d and e until the GO membrane comprising a plurality of layers of crosslinked GO sheets is formed; g. peeling off the GO membrane off from the solid surface, or detaching the GO membrane from the solid surface by immersing in distilled water containing a surfactant.

[0051] Advantageously, the second embodiment of the method as disclosed herein is a scalable method to obtain freestanding crosslinked GO membranes.

[0052] In one example, in steps a-c of the second embodiment of the method as disclosed herein, the crosslinking agent and GO are suspended in distilled water. In another example, in steps a-c of the second embodiment of the method as disclosed herein, the crosslinking agent and GO are suspended in a water / ethanol solution.

[0053] In one example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is 0.5 - 10 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 0.5 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 1 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 1.5 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 2 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 2.5 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 3 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 3.5 mg / mL. In another example, in step a of the second embodiment of themethod as disclosed herein, the concentration of GO in the suspension is about 4 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 4.5 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 5 mg / mL In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 5.5 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 6 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 6.5 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 7 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 7.5 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 8 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 8.5 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 9 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 9.5 mg / mL. In another example, in step a of the second embodiment of the method as disclosed herein, the concentration of GO in the suspension is about 10 mg / mL

[0054] In one example, in step b of the second embodiment of the method as disclosed herein, the concentration of the metallic oxide or carbide in the suspension is 0.0001-0.01 M. In another example, in step b of the second embodiment of the method as disclosed herein, the concentration of the metallic oxide or carbide in the suspension is about 0.0001 M. In another example, in step b of the second embodiment of the method as disclosed herein, the concentration of the metallic oxide or carbide in the suspension is about 0.0005 M. In another example, in step b of the second embodiment of the method as disclosed herein, the concentration of the metallic oxide or carbide in the suspension is about 0.0008 M. In another example, in step b of the second embodiment of the method as disclosed herein, the concentration of the metallic oxide or carbide in the suspension is about 0.001 M. In another example, in step b of the second embodiment of the method as disclosed herein, theconcentration of the metallic oxide or carbide in the suspension is about 0.005 M. In another example, in step b of the second embodiment of the method as disclosed herein, the concentration of the metallic oxide or carbide in the suspension is about 0.008 M. In another example, in step b of the second embodiment of the method as disclosed herein, the concentration of the metallic oxide or carbide in the suspension is about 0.01 M.

[0055] In one example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is 2 - 70 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 2 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 5 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 10 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 15 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 20 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 25 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 30 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 35 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 40 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 45 wt.% In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 50 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 55 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 60 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 65 wt.%. In another example, when the crosslinking agent and GO is suspended in a water / ethanol solution, the ethanol is about 70 wt.%.

[0056] In the second embodiment of the method as disclosed herein, in step a, the suspension of GO in distilled water or a water / ethanol solution is prepared by mild sonication in a sonication bath at room temperature for 0.5 to 1 h. In one example, in step a of the second embodiment of the method as disclosed herein, the suspension of GO in distilled water or a water / ethanol solution is sonicated for about 0.5 h. In another example, in step a of the secondembodiment of the method as disclosed herein, the suspension of GO in distilled water or a water / ethanol solution is sonicated for about 0.75 h. In another example, in step a of the second embodiment of the method as disclosed herein, the suspension of GO in distilled water or a water / ethanol solution is sonicated for about 1 h.

[0057] In the second embodiment of the method as disclosed herein, in step b, a suspension of the crosslinking agent in distilled water or a water / ethanol solution is prepared by mild sonication in a sonication bath for 1 to 4 h. In one example, in step b of the second embodiment of the method as disclosed herein, the suspension of the crosslinking agent in distilled water or a water / ethanol solution is sonicated for about 1 h. In another example, in step b of the second embodiment of the method as disclosed herein, the suspension of the crosslinking agent in distilled water or a water / ethanol solution is sonicated for about 1.5 h. In another example, in step b of the second embodiment of the method as disclosed herein, the suspension of the crosslinking agent in distilled water or a water / ethanol solution is sonicated for about 2 h. In another example, in step b of the second embodiment of the method as disclosed herein, the suspension of the crosslinking agent in distilled water or a water / ethanol solution is sonicated for about 2.5 h. In another example, in step b of the second embodiment of the method as disclosed herein, the suspension of the crosslinking agent in distilled water or a water / ethanol solution is sonicated for about 3 h. In another example, in step b of the second embodiment of the method as disclosed herein, the suspension of the crosslinking agent in distilled water or a water / ethanol solution is sonicated for about 3.5 h. In another example, in step b of the second embodiment of the method as disclosed herein, the suspension of the crosslinking agent in distilled water or a water / ethanol solution is sonicated for about 4 h.

[0058] In the second embodiment of the method as disclosed herein, in step c, the suspension of GO prepared in step a and the suspension of the crosslinking agent prepared in step b were mixed and subjected to mild sonication at room temperature for 0.5 to 1 h in a sonication bath In one example, in step c of the second embodiment of the method as disclosed herein, the suspension of the crosslinking oxide and GO is sonicated for about 0.5 h. In another example, in step c of the second embodiment of the method as disclosed herein, the suspension of the crosslinking oxide and GO is sonicated for about 0.75 h. In another example, in step c of the second embodiment of the method as disclosed herein, the suspension of the crosslinking oxide and GO is sonicated for about 1 h.

[0059] In the second embodiment of the method as disclosed herein, it is important that the suspension of GO in step a or the suspension of GO and crosslinking agent in step c can be continuously sonicated for up to about 1 h. Longer sonication time is needed for the suspension of the crosslinking agent in step b, before mixing with the suspension of GO in step c.

[0060] After sonication, the suspension of the crosslinking oxide and GO is uniformly casted or sprayed onto a solid surface. The solid surface can be any solid surface, for example, one made of a metallic, ceramic, or polymeric material, or combinations thereof.

[0061] A homogeneous GO membrane is assembled by sequentially casting or spraying several layers and drying each one of them before depositing the next layer. In one example, the quantity of the suspension of the crosslinking oxide and GO used to form each layer is 0. 1 - 6 mL / cm2. In another example, the quantity of the suspension of the crosslinking oxide and GO used to form each layer is about 0.1 mL / cm2. In another example, the quantity of the suspension of the crosslinking oxide and GO used to form each layer is about 0.5 mL / cm2. In another example, the quantity of the suspension of the crosslinking oxide and GO used to form each layer is about 1 mL / cm2. In another example, the quantity of the suspension of the crosslinking oxide and GO used to form each layer is about 2 mL / cm2. In another example, the quantity of the suspension of the crosslinking oxide and GO used to form each layer is about 3 mL / cm2. In another example, the quantity of the suspension of the crosslinking oxide and GO used to form each layer is about 4 mL / cm2. In another example, the quantity of the suspension of the crosslinking oxide and GO used to form each layer is about 5 mL / cm2. In another example, the quantity of the suspension of the crosslinking oxide and GO used to form each layer is about 6 mL / cm2.

[0062] In one example, the number of layers of crosslinked GO sheets is at least 3 if the suspension of the crosslinking oxide and GO is casted on the solid surface. In another example, the number of layers of crosslinked GO sheets is at least 5 if the suspension of the crosslinking oxide and GO is casted on the solid surface In another example, the number of layers of crosslinked GO sheets is at least 10 if the suspension of the crosslinking oxide and GO is casted on the solid surface. Tn another example, the number of layers of crosslinked GO sheets is at least 20 if the suspension of the crosslinking oxide and GO is casted on the solid surface.

[0063] In one example, the number of layers of crosslinked GO sheets is at least 10 if the suspension of the crosslinking oxide and GO is sprayed on the solid surface. In another example, the number of layers of crosslinked GO sheets is at least 15 if the suspension of thecrosslinking oxide and GO is sprayed on the solid surface. In another example, the number of layers of crosslinked GO sheets is at least 20 if the suspension of the crosslinking oxide and GO is sprayed on the solid surface. In another example, the number of layers of crosslinked GO sheets is at least 30 if the suspension of the crosslinking oxide and GO is sprayed on the solid surface. In another example, the number of layers of crosslinked GO sheets is at least 40 if the suspension of the crosslinking oxide and GO is sprayed on the solid surface.

[0064] Similar to the first embodiment of the method as disclosed herein, multiple layers of GO sheets can be added, and a minimal time is necessary for drying each layer at room temperature under ambient conditions.

[0065] A minimal time of one hour is necessary to let dry the first layer under ambient conditions. In one example, drying the first layer of GO sheet takes about 1 h. In one example, drying the first layer of GO sheet takes about 2 h. In another example, drying the first layer of GO sheet takes about 3 h. In another example, drying the first layer of GO sheet takes about 6 h. In another example, drying the first layer of GO sheet takes about 10 h. In another example, drying the first layer of GO sheet takes about 12 h. In another example, drying the first layer of GO sheet takes about 15 h. In another example, drying the first layer of GO sheet takes about 20 h. In another example, drying the first layer of GO sheet takes about 24 h.

[0066] The second, third, and fourth layers need a minimal drying time of 3, 6, and 12 h respectively. Any further layer needs about 12 h to dry at room temperature under ambient conditions.

[0067] In one example, the resulting dry membrane can be integrally peeled off from the solid surfaces. In another example, the membranes can be detached with the help of immersion in distilled water containing a surfactant agent which is water-soluble, selected from the group consisting of SDS, hydroxides, and other surfactant substances.

[0068] In the second embodiment of the method as disclosed herein, after detaching, the GO membrane is washed for at least 1 h in distilled water.

[0069] In one particular example of the second embodiment of the method as disclosed herein, the method comprises the following steps: a. preparing a suspension of 1 mg / mL GO in distilled water or a water / ethanol solution via sonication for 0.5 to 1 h;b. preparing a suspension of 0.0005 M of the crosslinking agent selected from the group consisting of niobium pentoxide (Nb2Os), iron (III) oxide (fezO ;), Bi-2212 (Bi2Sr2CaCu2Ox), and silicon carbide (SiC) in distilled water or a water / ethanol solution via sonication for 3h; c. preparing a suspension of the crosslinking agent and GO in distilled water or a water / ethanol solution by mixing the suspension of GO prepared in step a and the suspension of the crosslinking agent prepared in step b and sonicating for 0.5 to 1 h; d. casting 0.30 mL / cm2of the suspension of the crosslinking agent and GO uniformly on a Teflon plate to form a layer; e. drying the layer at room temperature; f. sequentially repeating steps e and e until the GO membrane comprising 4 layers of crosslinked GO sheets is formed, wherein the total drying time is 24 h; g. peeling off the GO membrane off from the Teflon plate.

[0070] In a further embodiment of the method as disclosed herein, pure free-standing GO membranes were fabricated by sequential drying and casting on Teflon plate without any crosslinking agent. These membranes are termed “free-standing pure GO membranes” in the present disclosure. In this embodiment, the GO suspensions were prepared and used with the same concentration and quantities described in the first and second embodiment, however, using double drying times for each layer. The free-standing pure GO membranes were fabricated to compare to the crosslinked ones and demonstrate that the crosslinking enhances the properties of the membranes.

[0071] The present disclosure refers to the use of metallic oxides as crosslinking agents, which give rise to effects beyond the mechanical stability of the membranes. To overcome several challenges of large-scale applications of 2D materials-based membranes, the present disclosure describes the development and testing of a new generation of self-standing GO membranes designed to ensure 2D laminates' selectivity. The fabrication methodology allows the possibility of strictly controlling the membrane thickness and forming an ordered laminated microstructure with interlayer distances between 0.8 and 1.2 nm.

[0072] In one example, the self-standing GO membrane as disclosed herein comprises a plurality of layers of crosslinked GO sheets, with interlayer distance of 0.8-1.2 nm. In another example, the self-standing GO membrane as disclosed herein comprises a plurality of layers of crosslinked GO sheets, with interlayer distance of about 0.8 nm. In another example, the selfstanding GO membrane as disclosed herein comprises a plurality of layers of crosslinked GOsheets, with interlayer distance of about 0.9 nm. In another example, the self-standing GO membrane as disclosed herein comprises a plurality of layers of crosslinked GO sheets, with interlayer distance of about 1.0 nm. In another example, the self-standing GO membrane as disclosed herein comprises a plurality of layers of crosslinked GO sheets, with interlayer distance of about 1.1 nm. In another example, the self-standing GO membrane as disclosed herein comprises a plurality of layers of crosslinked GO sheets, with interlayer distance of about 1 .2 nm.

[0073] The fabricated self-standing GO membranes have areas between 0.002 m2and 0.090 m2and thicknesses between 0.6 pm and 20 pm.

[0074] In one example, the fabricated self-standing GO membrane has an area of about 0.002 m2. In another example, the fabricated self-standing GO membrane has an area of about 0.005 m2. In another example, the fabricated self-standing GO membrane has an area of about 0.01 m2. In another example, the fabricated self-standing GO membrane has an area of about0.03 m2. Tn another example, the fabricated self-standing GO membrane has an area of about0.05 m2. In another example, the fabricated self-standing GO membrane has an area of about0.07 m2. In another example, the fabricated self-standing GO membrane has an area of about0.09 m2. In another example, the fabricated self-standing GO membrane has an area larger than 0.09 m2. The desired membrane area for industrial application depends on different industrial conditions and configurations for applications.

[0075] In one example, the fabricated self-standing GO membrane has a thickness of about 0.6 pm. In another example, the fabricated self-standing GO membrane has a thickness of about1 pm. In another example, the fabricated self-standing GO membrane has a thickness of about2 pm. In another example, the fabricated self-standing GO membrane has a thickness of about4 pm. In another example, the fabricated self-standing GO membrane has a thickness of about6 pm. In another example, the fabricated self-standing GO membrane has a thickness of about8 pm. In another example, the fabricated self-standing GO membrane has a thickness of about10 pm. In another example, the fabricated self-standing GO membrane has a thickness of about 12 pm . In another example, the fabricated self-standing GO membrane has a thickness of about 14 pm. In another example, the fabricated self-standing GO membrane has a thickness of about 16 pm. In another example, the fabricated self-standing GO membrane has a thickness of about 18 pm. In another example, the fabricated self-standing GO membrane has a thickness of about 20 pm. In another example, the fabricated self-standing GO membrane has a thickness of above20 pm. The desired membrane thickness for industrial application depends on different industrial conditions and configurations for applications. In the case of the thickness, depending on the application, it may be desirable that the membrane be as thin as possible. The method of the present invention can fabricate thin membranes as disclosed herein.

[0076] The self-standing GO membrane fabricated using the method as disclosed herein exhibits a cracks-free and uniform surface and is mechanically stable and bendable. In one example, the self-standing GO membrane fabricated using the method as disclosed herein has a smooth surface. In another example, the self-standing GO membrane fabricated using the method as disclosed herein has an undulated surface. In another example, the self-standing GO membrane fabricated using the method as disclosed herein has a surface which presents a regular distribution of features like circular “hillocks” with diameters from 1- 10 pm, causing ripples all over the surface.

[0077] The membrane has unaffected integrity after immersion in water for months. The fabricated self-standing GO membranes are flexible and continuously present chemical and physical stability during testing in direct contact with ethanol- and methanol / water-liquid mixtures for at least 24 hours. It is economically desirable that a membrane remains stable as long as possible so that it does not need to be substituted. The membranes fabricated with the method of the invention were stable under continuous testing for at least 24 hours in harsh environments of liquid mixtures of ethanol- and methanol / water. Furthermore, they endured immersion in water for at least three months.

[0078] The method was tested (but is not limited to) by using the crosslinking agents iron (III) oxide, aluminium oxide, calcium sulfate, silicon carbide, niobium pentoxide, and Bi-2212. The tests for gas permeation and the fabricated membranes' chemical characteristics also allow their application in gas separation processes. The interaction of the oxides with the functional groups of GO, especially observed on the characteristics of the Nb?O crosslinking-based membrane, opens the possibility of further scalable exploration of the mass transport mechanisms enhanced by crosslinking with metallic oxides in graphene-related- and other 2D materials.

[0079] The fabricated self-standing GO membranes are not only stable. As further improved properties, these self-standing GO membranes have potentially improved high adsorption capability, which allows applications for energy storage and solid-state hydrogen storage. For instance, the adsorption of sulphur species allows the use as host for cathode active materialsor selective membranes to hold polysulfides during the charge-discharge of lithium-sulphur batteries. Moreover, GO structured with Si has high potential for hydrogen storage applications because of its high physicochemical affinity to hydrogen. The method as disclosed herein is also less expensive and time-consuming than the direct use of complex compounds as crosslinking agents, such as 1,3,5-benzenetricarbonyl trichloride

[0014] or amino acids

[0015] , The strong interaction of metallic oxides and carbides with GO present in the crosslinked membranes and structures allows their applications for energy storage and solid-state hydrogen storage.

[0080] The membranes’ mechanical stability was evaluated continuously and intermittently by exposing them to liquids and vapours. They were tested for at least three months. The method disclosed herein produces mechanical resistant self-standing GO membranes that withstand aqueous solutions with contents of ethanol and methanol up to 95 wt.% at room temperature and mixtures of saturated vapours with up to 95 wt.% of water at temperatures up to 50 °C.

[0081] The self-standing GO membrane fabricated using the method as disclosed herein also showed a high selectivity in vapours and liquids, mainly due to the ordered stacking microstructure of laminates in the self-standing membranes.

[0082] The membranes are also suitable for gas permeation applications, for example, application in Hz separation and purification from gas mixtures.

[0083] As used in this application, the singular form “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a metal” includes a plurality of metals, including mixtures and combinations thereof.

[0084] As used herein, the term “comprising” means “including.” Variations of the word "comprising", such as “comprise” and “comprises,” have correspondingly varied meanings. Thus, for example, a composition “comprising” X may consist exclusively of X or may include one or more additional unrecited components

[0085] As used herein, the term “about” in the context of concentration of a substance, size of a substance, length of time, or other stated values means + / - 5% of the stated value, or + / - 4% of the stated value, or + / - 3% of the stated value, or + / - 2% of the stated value, or + / - 1% of the stated value, or + / - 0.5% of the stated value.

[0086] Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience andbrevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

[0087] The disclosure illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0088] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0089] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the invention belongs.

[0090] Other embodiments are within the following claims and non-limiting examples.EXAMPLES

[0091] Non-limiting examples of the disclosure will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the disclosure.

[0092] Characterization

[0093] Membrane integrity

[0094] In all cases of the fabricated GO membranes with crosslinking metallic oxides, their chemical and mechanical robustness was noticeable by their unaffected integrity after immersion in water, even for several months, as well as their easy manipulation and flexibility during assembling in testing modules with different geometries. For example, a GO membrane with an area of 126 cm2is shown in Figure 1 immediately after detaching with the help of a liquid solution from a ceramic plate Other examples are the membranes shown in Figure 2, which cover external filtration ceramic tubes with areas of 47 cm2and 196 cm2. These membranes were washed and kept in water for around two weeks after detaching from a CaSO4 plate until they were assembled on tubes and modules for pervaporation and vapour permeation tests.

[0095] Macroscopic overview

[0096] A representative overview of membranes with one cation-oxide and multi-cation oxide-based crosslinking is presented in Figure 3. The large area CaSOr-crosslinking-based membrane shown in Figure 3 a exhibits a smooth, free of cracks and uniform surface as seen by optical microscopy in Figure 3b. The bendable semi-transparent membrane with brown colouration in Figure 3c was fabricated using the same method but with the multi-cation superconducting cuprate Bi-2212 as a crosslinking agent for GO. Due to the insolubility of Bi- 2212 in water, some concentration points and local defects are almost unavoidable; nevertheless, the membrane is mechanically stable and bendable, as shown in Figure 3d.

[0097] Scanning Electron Microscopy (SEM)

[0098] The microstructures of a selection of representative samples were characterised by scanning electron microscopy (SEM) (Jeol Jed-2300). The samples were attached to the sample holder with the help of conductive carbon tape. The general view in Figure 4a shows the membrane with SiC. The undulated and cracks-free membrane surface could be observed in Figures 4b and 4c, as well as its cross-section with a typically laminated microstructure and a thickness of ~ 2.1 pm.

[0099] The AI2O3 + CaSO i-based crosslinking sample had a smooth, undulated, and crack- free continuous surface (Figure 5a). The measurement of its overall average thickness of around 0.6 pm was shown in Figure 5b, and its laminate microstructure with very ordered stacking of the GO layers can be observed in Figure 5c.

[0100] SEM pictures of a membrane withwere presented in Figure 6, where the general view in Figure 6a showed the membrane fixed to the SEM sample holder as described above, and in Figure 6b the cross-sectional view shows its laminated microstructure with a thickness of - 1.4 pm and an ordered stacking of the GO layers. The surface of the membrane presented a regular distribution of features like circular “hillocks” with diameters from - 1 pm to - 10 pm, which caused ripples all over the surface, as can be seen in details in Figures 6c and 6d. Noteworthy, despite their sizes, they do not cause the formation of any cracks in the surface.

[0101] A detailed EDS analysis of the sample surface, presented in Figure 7, revealed that the main component of these objects is niobium. The elements distribution analysis of a representative surface is shown in Figure 7a, with the spectrum in Figure 7b, and the local analysis of one of the objects is shown in Figure 7c with the spectrum in Figure 7d. The presence of niobium was observed all over the surface. Still, it was more concentrated in regions where some oxygen concentration was also observed, which indicated that niobium oxide was not wholly dispersed during the membrane fabrication. Some calcium impurities were also detected that did not affect the surface smoothness.

[0102] X rays diffraction (XRD)

[0103] X rays diffraction data were collected in a standard laboratory diffractometer with CuKa radiation and Bragg-Brentano geometry. The instrument was set in step-scan mode with steps of 0.02° and a counting time of 1 second. The x-rays diffraction patterns of membranes of pure GO and with different oxides-based crosslinking were shown in Figure 8. The broad peaks around 20 = 10.50° correspond to the (200) Bragg reflection of GO with interplane distances listed in Table 1 for the different membranes. The interplane distances were calculated using the Bragg equation, and the angular position was determined by fitting the diffraction peaks with the function Pearson VII, as shown in the graphs.

[0104] Table 1: Interplane distances of the GO in the membranes with different crosslinking.| Membrane | Angular position 2fl (°)

[0105] X-ray photoelectron spectroscopy (XPS)

[0106] X-ray photoelectron spectroscopy (XPS) measurements were performed to characterize the detailed composition of a selection of representative membranes with pure GO, AI2O3 + CaSO4-, NbiOs- and SiC-based crosslinking. Figure 9 presented a survey spectrum for the membrane sample with pure GO with no significant detectable impurities and showed the peaks corresponding to the binding energies of C Is, O Is, and O 2s.

[0107] Figure 10 presented a survey spectrum for the sample with CaSO4 and AI2O3 that revealed peaks corresponding to Ols, Ca2s, Ca2p, and Cis binding energies. The zooming of the range of 0 to 300 eV revealed the binding energy peaks of Ca 3p, which could be attributed to small amounts of calcium carbonate CaCOj or CaO. In this particular case of using calcium cations from the dissolution of CaSO4, the remaining presence of S and SiO2 impurities was also detected in the composition. The crosslinking formation was also observed in the Ca 2p scan spectrum in Figure 11, which contained the peaks corresponding to Ca 2p3 / 2 and Ca 2pl / 2, attributed to CaCOj or CaO and CaCOj, respectively.

[0108] The survey spectrum for the sample membrane with SiC-based crosslinking in Figure 12 showed binding energy peaks of Si with low intensities, besides the binding energy peaks of C and O.

[0109] The survey spectrum for the sample membrane with bfeOs-based crosslinking was presented in Figure 13, which showed the elements C, O andNb detected in the sample surface.

[0110] XPS spectra, with an area acquisition of 50 pm x 100 pm, were taken from the samples with crosslinking based on SiC and oxides to characterize the interaction of these compounds with the functional groups of GO. In Figure 14, the XPS spectrum of the binding energy of Nb 3d showed the formation of Nb-C species, serving as an example of carbide formation.

[0111] The formation of carbides from the interaction of the crosslinking agents and the GO functional groups, detected by XPS peaks in the different samples, was summarized in Table 2.

[0112] Table 2: Example of XPS peak energies corresponding to carbides in the membranes with oxides-based crosslinking.

[0113] Considering that Nb is a transition metal, this behavior can be similar to the case of TiCb nanoparticles and GO described in the recent literature [8]

[0013] , where a reaction between TiOi and GO consumes the C-0 and C-OH groups converting them to Ti-O-C and Ti-C species The formation of carbonates was also observed in the case of the CaSO4 crosslinkingbased membrane, as discussed above.

[0114] Elemental Analysis (ICP-OES)

[0115] Elemental analysis was performed by the method of Inductively Coupled Plasma- Optical Emission Spectrometry (ICP-OES) for membrane samples with crosslinking based on AI2O3, AI2O3 + CaSO4, Fe2O3, Nb2Os, and SiC. As result, the range of mass quantities of Al, Ca, Fe, Nb, and Si in the different membranes were determined. Alternatively, th erm ogravi metric analysis can also be applied to determine the crosslinking agent mass quantities, mainly in the form of oxides.[001 16] In particular embodiments of the invention that may be mentioned herein, the Al content on membranes with crosslinking based on AI2O3 may be from 0.05 to 10 wt%, such as from 0.1 to 7 wt%, such as from 0.2 to 5 wt% of the composition when measured using one or both of ICP-OES elemental analysis or thermogravimetric analysis

[0117] In particular embodiments of the invention that may be mentioned herein, the Fe content on membranes with crosslinking based on Fe2O3 may be from 0.05 to 50 wt%, such as from 0.5 to 40 wt%, such as from 1 to 30 wt% of the composition when measured using one or both of ICP-OES elemental analysis or thermogravimetric analysis.

[0118] In particular embodiments of the invention that may be mentioned herein, the Nb content on membranes with crosslinking based on NbzOs may be from 0.05 to 50 wt%, such as from 1 to 40 wt%, such as from 5 to 30 wt% of the composition when measured using one or both of ICP-OES elemental analysis or thermogravimetric analysis.[001 19] In particular embodiments of the invention that may be mentioned herein, the Si content on membranes with crosslinking based on SiC may from 0.05 to 10 wt%, such as from0.1 to 5 wt%, such as from 0.2 to 2 wt% of the composition when measured using one or both of ICP-OES elemental analysis or thermogravimetric analysis.

[0120] In particular embodiments of the invention that may be mentioned herein, the Al and Ca individual contents on membranes with crosslinking based on AI2O3 + CaSO4 may be from 0.05 to 10 wt%, such as from 0.5 to 7 wt%, such as from 1 to 5 wt% of the composition when measured using one or both of ICP-OES elemental analysis or thermogravimetric analysis.

[0121] Testing in pervaporation and vapour permeation applications

[0122] The fabricated GO membranes were tested as self-standing membranes in separation processes applied to binary water mixtures with ethanol and methanol. For comparison, similar GO membranes were also fabricated by the well-known method of GO coating on ceramic porous substrates (hereinafter called “coated membranes”). Both types of membranes were characterized by their selectivity and flux in pervaporation (PV) and vapour permeation (VP) processes for the water / solvent mixtures.

[0123] During the selectivity tests, the membranes withstood the harsh environment of different saturated vapours and liquids, even considering the high hydrophilicity of GO. Selfstanding membranes with similar thicknesses to the coated ones, tested for the same mixtures of solvents / water, showed a clear improvement in selectivity without the need to heat the liquids in most cases. Their performance in pervaporation and vapour permeation of ethanol / water mixtures is presented in Tables 4 to 6.

[0124] Table 4: Separation factor (a) and flux of different membranes for PV of ethanol / water mixtures with concentration 50 %w / w at 23 °C (uncertainties in a and flux are ± 1 and ± 0.06 kg / m2.h, respectively).

[0125] Table 5: Separation factor (a) and flux of different membranes for PV of ethanol / water mixtures with concentration 80 %w / w at 23 °C.

[0126] Table 6: Separation factor (a) and flux of different membranes for VP of ethanol / water mixtures with concentration 96 %w / w at 40 °C.

[0127] Compared to the coated membranes' performance, the selectivity and the flux of the self-standing membranes increase by a factor of 4.7 and 3, respectively, for ethanol dehydration with feed concentrations over 95 wt.% (the azeotrope concentration). For lower ethanol concentrations (down to 50 wt.%), the selectivity improvements are of 2 up to 7 times with similar or higher fluxes than those obtained with the coatings. Remarkably, this performance is obtained by a separation process at room temperature, in contrast to the working temperatures of the coatings (T > 50 °C). In the case of the mixture methanol / water, the selectivity (a ~ 23) of a free-standing membrane with AI2O3 + CaSO4 at T = 40 °C is around six times higher with approximately the same flux (~ 0.1 kg / m2.h), compared to the performance of a coating (a = 3.8) working for the same mixture at higher temperature (T= 50 °C).

[0128] The results described above are mainly due to the ordered stacking microstructure of laminates in the self-standing membranes, overcoming some adverse structural effects of the coatings. Furthermore, the flux / permeability increased in most cases using membranes with similar thicknesses. Based on these facts and adding the precise parameters control allowed by the developed fabrication process in the present invention addresses the drawbacks related to the scalability of GO-based membranes for their use in various applications.

[0129] Pervaporation tests for the system ethanol / water 80 / 20 wt.% at room temperature were also performed with free-standing pure GO membrane and crosslinked membranes to investigate the effect of the crosslinking on the separation performance, besides the highermechanical stability. All membranes were fabricated with the same total quantity of GO (20 mg) by casting four layers following the above procedure using a pure water suspension. In Table 7, the comparison of the results shows a nearly three times higher separation factor for the AI2O3 crosslinked membrane and a similar or higher flux for the crosslinked membranes compared to the one with pure GO. Figure 15 illustrates the higher performance of the membranes with SiC and oxides crosslinking compared to the pure GO membrane in terms of separation factor and Pervaporation Separation Index (PSI), calculated by equation 1 .PSI = Flux ■ (a — 1) (1)

[0130] Table 7: Separation factor (a) and flux of membranes with and without AI2O3 + CaSOr - crosslinking for PV of ethanol / water 80 / 20 wt% at room temperature.

[0131] The results described above are mainly due to the ordered stacking microstructure of laminates in the self-standing membranes, overcoming some adverse structural effects of the coatings. Furthermore, the flux / permeability increased in most cases using membranes with similar thicknesses. Based on these facts and adding the precise parameters control allowed by the developed fabrication process in the present invention addresses the drawbacks related to the scalability of GO-based membranes for their use in various applications.

[0132] Testing in gas separation applications

[0133] Gas permeability tests of hydrogen (H2), carbon dioxide (CO2) and nitrogen (N2) were performed for the crosslinked GO membranes made with AI2O3 + CaSO4 and NbzOs. The membranes were submitted to permeation of high-purity H2, CO2 and N2 in a gas permeation apparatus with a module for flat membranes. Porous alumina discs over a stainless-steel mesh were used to give mechanical support for the membranes with thicknesses of 2 to 7 gm. The method of precise control of transmembrane pressure was used to measure the flux of gases and calculate the permeability of the gases.

[0134] The separation factor for binary gas mixtures varies inversely with the permeability of the more permeable gas of the specific pair with permeabilities Pi and Pj. The upper bound relationship for binary mixtures of He, H2, O2, N2, CH4, and CO2 is represented by a log-log plot of the separation factor ay (ay = Pi / Pj) versus the permeability of the more permeable gas Pi

[0010]

[0011] ,

[0135] Both membranes demonstrate performances over the upper bound that limits the trade-off between the H2 permeability and its selectivity over CO2 and N2. Furthermore, the H2 permeabilities found are among the highest ever reported over the upper bound. Table 8 summarizes the results of single gas permeation and ideal selectivity for both membranes and Figure 16 shows the results in the Robeson plot for H2 / CO2.

[0136] Table 8: Single gas permeation and ideal selectivity for membranes with crosslinking based on Nb CT and AhOj+CaSCh.

[0137] A free-standing GO membrane fabricated using the crosslinking agent calcium sulfate (CaSO4) performed well in separating real gas mixtures of H2 / CO2 and N2 / CO2. The values of selectivity and permeability for these mixtures were at least 1.4 and 20700 Barrer, respectively (Figure 16), which were over the upper bound that limits the trade-off between permeability and selectivity. Furthermore, the freestanding membrane could withstand a gas pressure of at least 1.5 bar.

[0138] Conclusions

[0139] The fabricated self-standing GO membranes have areas between 0.002 m2and 0.090 m2and thicknesses between 0.6 pm and 20 pm. They are flexible and present chemical and physical stability during testing in direct contact with liquid mixtures of ethanol / water and methanol / water for at least 24 hours continuously. A scalable method of fabrication of GO- based membranes has been developed, which can be used for other 2D materials. The method was tested (but is not limited to) by using the crosslinking agents: Iron (III) oxide (Fe2O3), Aluminium oxide (AI2O3), Calcium sulfate (CaSO4), Silicon carbide (SiC), Niobium pentoxide (NbzOs), AI2O3 + CaSC 50 / 50 wt.%, and Bi-2212 (Bi2Sr2CaCu2Ox). The tests for gaspermeation of H2, CO2 and N2 and the fabricated membranes' chemical characteristics also demonstrate their high potential for applications in gas separation processes.

[0140] The effect of SiC-based and metallic oxides-based crosslinking in GO membranes goes far beyond the higher mechanical stability. The interaction of the oxides with the functional groups of GO, mainly observed on the characteristics of the Nb2Os crosslinkingbased membrane, opens the possibility of further scalable exploration of the mass transport mechanisms controlled by crosslinking with metallic oxides in graphene-related- and other 2D materials. Specifically, this work opens an investigation into the modification of the binding energies of gas molecules like H2, N2, CO2, CH4, etc., due to the presence of the above compounds and other not explored oxides in graphene-related materials. This influence on the binding energies of molecules is fundamental for processes of chemisorption and physisorption ([8], [9]) and can favor the total reversibility of the uptake of these molecules at near ambient conditions, which would improve the efficiency of separation and storage applications.

[0141] Industrial Applicability

[0142] This invention presents methods to fabricate a self-standing GO membrane comprising a plurality of layers of crosslinked GO sheets. Such membrane could be used in commercial applications such as:• Membranes for separation processes of pervaporation and vapour permeation for liquid and vapour mixtures of water with ethanol, methanol and other organic solvents.• Membranes for separation processes of gas mixtures, including H2, CO2, N2, CH4 and other gases.• Materials for gas transport and storage, including H2, CO2, N2, CH4 and other gases.

[0143] Various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.References[1] Scalable Graphene-Based Membranes for Ionic Sieving with Ultrahigh Charge Selectivity, Seunghyun Hong et al., Nano Lett. 2017, 17, 728-732, DOI: 10.1021 / acs.nanolett.6b03837.[2] Trivalent metal cation cross-linked graphene oxide membranes for NOM removal in water treatment, Ting Liu, Bing Yang, Nigel Graham, Wenzheng Yu, Kening Sun, Journal of Membrane Science 542 (2017) 31-40, http: / / dx.doi.Org / 10.1016 / j.memsci.2017.07.061.[3] Graphene Oxide Papers Modified by Divalent Ions — Enhancing Mechanical Properties via Chemical Cross-Linking, Sungjin Park, Kyoung-Seok Lee, Gulay Bozoklu, Weiwei Cai, SonBinh T. Nguyen, and Rodney S. Ruoff, ACS Nano 2008, Vol. 2, No. 3, 572-578, https: / / doi-org.libproxyl.nus.edu.sg / 10.1021 / nn700349a.[3] Graphene-Based Membranes for Molecular Separation, Liang Huang, Miao Zhang, Chun Li, and Gaoquan Shi, J. Phys. Chern. Lett 2015, 6, 2806-2815, DOI: 10.1021 / acs.jpclett.5b00914.[5] US Patent Application No. 17 / 779,679: Method of Preparing BSCCO-Based Materials.[6] Decontamination of U(VI) on graphene oxide / A12O3 composites investigated by XRD, FT- IR and XPS techniques, Lei Zhang, Ying Li, Han Guo, Huihui Zhang, Ning Zhang, Tasawar Hayat, Yubing Sun, Environmental Pollution, Volume 248, 2019, Pages 332-338, ISSN 0269- 7491, https: / / doi.Org / 10.1016 / j.envpol.2019.01.126.[7] XPS study of some calcium compounds, B Demri, D. Muster, Journal of Materials Processing Technology, Volume 55, Issues 3-4, 1995, Pages 311-314, ISSN 0924-0136, https: / / doi.org / ! 0. 1016 / 0924-0136(95)02023-3.[8] Investigation of graphene-based systems for hydrogen storage, Hamid Ghorbani Shiraz, Omid Tavakoli, Renewable and Sustainable Energy Reviews, Volume 74, 2017, Pages 104- 109, ISSN 1364-0321, https: / / doi.Org / 10.1016 / j.rser.2017.02.052.[9] Hydrogen storage in porous graphene with Al decoration, Zhimin Ao et al., International Journal of Hydrogen Energy, Volume 39, Issue 28, 2014, Pages 16244-16251, ISSN 0360- 3199, https: / / doi.Org / 10.1016 / i.ijhydene.2014.0L044

[0010] Lloyd M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, Journal of Membrane Science, Volume 62, Issue 2, 1991, Pages 165-185, ISSN 0376-7388, https: / / doi.org / 10, 1016 / 0376-7388(91)80060-J

[0011] Lloyd M. Robeson, The upper bound revisited, Journal of Membrane Science, Volume320, Issues 1-2, 2008, Pages 390-400, ISSN 0376-7388, https: / / doi.Org / 10.1016 / i.memsci.2008.04.030

[0012] NIST X-ray Photoelectron Spectroscopy Database, DOI: http: / / dx doi.org / 10.18434 / T4T88K

[0013] Bajestani, Zahra Gohari and Yuriim, Alp and Yuriim, Yuda, Significant improvement in the hydrogen storage capacity of a reduced graphene oxide / TiO2 nanocomposite by chemical bonding of Ti-O-C, RSC Adv., 2016, 6, 39, pages 32831-32838, doil0.1039 / C6RA00944A.

[0014] Liang Huang et al, Graphene-Based Membranes for Molecular Separation, J. Phys. Chem. Lett. 2015, 6, 2806-2815

[0015] Sungjin Park et al, Graphene Oxide Sheets Chemically Cross-Linked by Polyallylamine, J. Phys. Chem. C 2009, 113, 36, 15801-15804

Claims

Claims1. A method of fabricating a self-standing Graphene Oxide (GO) membrane comprising a plurality of layers of crosslinked GO sheets, the method comprising using a crosslinking agent to crosslink between oxygen-containing functional groups of neighbour GO sheets, wherein the crosslinking agent is a metallic oxide or a carbide.

2. The method of claim 1, wherein the metallic oxide is selected from the group consisting of Iron (ITT) oxide (FezOs), aluminium oxide (AI2O3), calcium sulfate (CaSO4), niobium pentoxide (NbzOs), Bi-2212 (BizSrzCaCuzOx), an oxide of a transition metal, an oxide of an alkaline earth metal, an oxide of a post-transition metal, aluminosilicate, and a multication oxide selected from the group consisting of YBaCO and BSCCO.

3. The method of claim 2, wherein the metallic oxide is AhCh / CaSCri 50 / 50 wt.%.

4. The method of claim 1, wherein the carbide is silicon carbide (SiC).

5. The method of claim 1 , wherein the oxygen-containing functional groups are selected from the group consisting of epoxide, hydroxyl, carboxyl and carbonyl group6. The method of claim 1, comprising the following steps: a. preparing a suspension of GO in distilled water or a water / ethanol solution via sonication for 0.5 to 1 h; b. casting or spraying the suspension of GO uniformly on a solid porous surface of a metallic oxide, a mixture of metallic oxides, or a carbide to form a layer; c. drying the layer at room temperature; d. sequentially repeating steps b and c until the GO membrane comprising a plurality of layers of crosslinked GO sheets is formed; e. peeling off the GO membrane off from the solid porous surface, or detaching the GO membrane from the solid porous surface by immersing the GO membrane in distilled water containing a surfactant.

7. The method of claim 6, wherein the concentration of GO in the suspension is 0.5 - 10 mg / mL.

8. The method of claim 6, wherein the ethanol content in the water / ethanol solution is 2 - 70 wt.%.

9. The method of claim 6, wherein the quantity of the suspension of GO used to form each layer is 0.1- 6 mL / cm2.

10. The method of claim 6, wherein the number of layers of crosslinked GO sheets is at least 3 if the suspension of GO is casted on the solid porous surface.

11. The method of claim 6, wherein the number of layers of crosslinked GO sheets is at least 10 if the suspension of GO is sprayed on the solid porous surface.

12. The method of claim 6, wherein drying a first, a second, a third, and a further layer of GO sheet takes at least 1 h, 3 h, 6 h, and 12 h, respectively.

13. The method of claim 6, comprising the following steps: a. preparing a suspension of 0.1 mg / mL GO in a water / ethanol 50 / 50 wt.% solution via sonication for 0.5 h; b. casting 0.25 - 0.30 mL / cm2of the suspension of GO uniformly on a solid porous surface selected from the group consisting of CaSO4, SiC, and AhO3 / CaSO4 50 / 50 wt.% to form a layer; c. drying the layer at room temperature; d. sequentially repeating steps b and c until the GO membrane comprising a plurality of layers of crosslinked GO sheets is formed, wherein the total drying time is 24 h, e. peeling off the GO membrane off from the solid porous surface, or detaching the GO membrane from the CaSO or SiC solid porous surface by immersing the GO membrane in distilled water containing 0.1% w / v SDS.

14. The method of claim 1, comprising the following steps: a. preparing a suspension of GO in distilled water or a water / ethanol solution via sonication for 0.5 to 1 h; b. preparing a suspension of the crosslinking agent in distilled water or a water / ethanol solution via sonication for 1 to 4 h; c. preparing a suspension of the crosslinking agent and GO in distilled water or a water / ethanol solution by mixing the suspension of GO prepared in step a and the suspension of the crosslinking agent prepared in step b and sonicating for 0.5 to 1 h; d. casting or spraying the suspension of the crosslinking agent and GO uniformly on a solid surface to form a l yer; e. drying the layer at room temperature; f. sequentially repeating steps d and e until the GO membrane comprising a plurality of layers of crosslinked GO sheets is formed;g. peeling off the GO membrane off from the solid surface, or detaching the GO membrane from the solid surface by immersing in distilled water containing a surfactant.

15. The method of claim 14, wherein the concentration of GO in the suspension is 0.5 - 10 mg / mL.

16. The method of claim 14, wherein the concentration of the metallic oxide or carbide in the suspension is 0.0001-0.01 M.

17. The method of claim 14, wherein the ethanol content in the water / ethanol solution is 2 - 70 wt.%.

18. The method of claim 14, wherein the quantity of the suspension of the crosslinking agent and GO used to form each layer is 0.1- 6 mL / cm2.

19. The method of claim 14, wherein the number of layers of crosslinked GO sheets is at least 3 if the suspension of GO is casted on the solid surface20. The method of claim 14, wherein the number of layers of crosslinked GO sheets is at least 10 if the suspension of GO is sprayed on the solid surface.

21. The method of claim 14, wherein drying a first, a second, a third, and a further layer of GO sheet takes at least 1 h, 3 h, 6 h, and 12 h, respectively.

22. The method of claim 14, comprising the following steps: a. preparing a suspension of 1 mg / mL GO in distilled water or a water / ethanol solution via sonication for 0.5 to 1 h; b. preparing a suspension of 0.0005 M of the crosslinking agent selected from the group consisting of niobium pentoxide (NbzOs), iron (III) oxide (FezOs), Bi-2212 (Bi2Sr2CaCu2Ox), and silicon carbide (SiC) in distilled water or a water / ethanol solution via sonication for 3h; c. preparing a suspension of the crosslinking agent and GO in distilled water or a water / ethanol solution by mixing the suspension of GO prepared in step a and the suspension of the crosslinking agent prepared in step b and sonicating for 0.5 to 1 h; d. casting 0.30 mL / cm2of the suspension of the crosslinking agent and GO uniformly on aTeflon plate to form a layer; e. drying the layer at room temperature, f. sequentially repeating steps d and e until the GO membrane comprising 4 layers of crosslinked GO sheets is formed, wherein the total drying time is 24 h; g. peeling off the GO membrane off from the Teflon plate.

23. The method of claim 6 or 14, wherein the surfactant is a water-soluble surfactant selected from the group consisting of SDS and hydroxides.

24. The method of claim 6 or 14, further comprising washing the GO membrane in distilled water for at least one hour.

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