2D MATERIAL MEMBRANE WITH IONIC SELECTIVITY

Abstract

There is provided a multi-layered membrane comprising a top layer, a bottom layer, and a spacer layer; wherein said spacer layer is interposed between said top layer and said bottom layer; wherein said top layer, said bottom layer and said spacer layer are each independently composed of one or more selective layers, each selective layer comprising a 2D material; wherein said spacer layer comprises at least one channel for receiving a fluid; wherein said bottom layer comprises a hole with an area in the range of 1 μm.sup.2 to 1 mm.sup.2; and wherein said hole is capable of being in fluid communication with said at least one channels of said spacer layer.

There is also provided a method to synthesize the top layer of a multi-layered membrane as disclosed herein, methods for separating a plurality of ions or molecules in a fluid stream, a device comprising a multi-layered membrane as disclosed herein, and use of the method or the device as disclosed herein in osmotic power generation.

Claims

1. A multi-layered membrane comprising a top layer, a bottom layer, and a spacer layer; wherein said spacer layer is interposed between said top layer and said bottom layer; wherein said top layer, said bottom layer and said spacer layer are each independently composed of one or more selective layers, each selective layer comprising a 2D material; wherein said spacer layer comprises at least one channel for receiving a fluid; wherein said bottom layer comprises a hole with an area in the range of 1 μm.sup.2 to 1 mm.sup.2; and wherein said hole is capable of being in fluid communication with said at least one channel of said spacer layer.

2. The multi-layered membrane of claim 1, wherein width of said hole is in the range of 20 nm to 2 μm, and length of said hole is in the range of 300 nm to 1 mm, or wherein said 2D material is a nanoparticle, or wherein said 2D material is selected from the group consisting of graphene, graphite, hexagonal boron nitride, transition metal dichalcogenide, phosphorene, xene, transitional metal-xene, and combinations thereof.

3. (canceled)

4. (canceled)

5. The multi-layered membrane of claim 2, wherein said transition metal dichalcogenide has a chemical formula MX2, wherein M is a transition metal selected from the group consisting of titanium, vanadium, chromium, manganese, zirconium, niobium, molybdenum, technetium, hafnium, tantalum, tungsten and rhenium; and wherein X is a chalcogen selected from the group consisting of sulfur, selenium and tellurium, or wherein said xene is selected from the group consisting of borophene, silicene, germanene, stanene, phosphorene, arsenene, antimonene, bismuthene, and tellurene.

6. (canceled)

7. The multi-layered membrane of claim 1, wherein said bottom layer comprises one or more layers of substrate; wherein said substrate is independently selected from the group consisting of silicon, silicon nitride (SiN.sub.X), silicon oxide (SiO.sub.2), alumina (Al.sub.2O.sub.3), anodic aluminium oxide, aluminium oxide, titanium dioxide, hafnium dioxide, nylon, polymer, polyether sulfone, polyvinyl alcohol (PVA), polycarbonate (PC), and polyvinylidene fluoride.

8. The multi-layered membrane of claim 7, wherein said substrate is a mechanical support for said bottom layer.

9. The multi-layered membrane of claim 1, wherein said top layer, bottom layer or spacer layer is independently surface-functionalized.

10. The multi-layered membrane of claim 9, wherein said surface-functionalized top layer, surface-functionalized bottom layer or surface-functionalized spacer layer has a different hydrophilicity or hydrophobicity as compared to a non surface-functionalized top layer, bottom layer or spacer layer.

11. The multi-layered membrane of claim 7, wherein when said bottom layer comprises a layer of silicon nitride (SiN.sub.X) substrate, and the area of said bottom layer is in the range of 25 μm.sup.2 to 10 mm.sup.2, the thickness of said silicon nitride substrate is in the range of 10 nm to 500 nm.

12. The multi-layered membrane of claim 11, wherein said bottom layer comprises a layer of silicon substrate beneath said layer of silicon nitride substrate.

13. The multi-layered membrane of claim 1 wherein the height of each selective layer in said spacer layer is in the range of 0.3 nm to 250 nm.

14. (canceled)

15. The multi-layered membrane of claim 1, wherein said top layer comprises a masked graphitic layer comprising a metal layer or metal oxide layer thereon.

16. The multi-layered membrane of claim 15, wherein the metal of said metal layer or said metal oxide layer is selected from the group consisting of gold, platinum, copper, aluminium, silver, titanium, hafnium and silicon dioxide.

17. A method to synthesize a top layer of a multi-layered membrane as defined in claim 15, comprising the steps of: (a) providing a spacer layer/bottom layer assembly; (b) dry transferring a selective layer comprising a 2D material on top of the spacer layer of the spacer layer/bottom layer assembly; (c) depositing a metal layer or metal oxide layer on top of said selective layer of step (b) to form a mask; and (d) subjecting said metal layer or metal oxide layer to an etching process.

18. A method for separating a plurality of ions or molecules in a fluid stream comprising the steps of: a) providing a multi-layered membrane; b) contacting a first surface of said multi-layered membrane with said fluid stream under a driving force to selectively allow desired ions or molecules to pass through to a second surface; wherein first surface of said multi-layered membrane is optionally charged; and wherein said multi-layered membrane comprises a top layer, a bottom layer, and a spacer layer; wherein said spacer layer is interposed between said top layer and said bottom layer; wherein said top layer, said bottom layer and said spacer layer are each independently composed of one or more selective layers, each selective layer comprising a 2D material; wherein said spacer layer comprises at least one channel for receiving a fluid; and wherein said bottom layer comprises a hole that is capable of being in fluid communication with said at least one channel of said spacer layer, said hole optionally having an area in the range of 1 μm.sup.2 to 1 mm.sup.2.

19. (canceled)

20. The method of claim 18, wherein when said driving force is the saline concentration gradient of said fluid stream across said multi-layered membrane, the said saline concentration gradient of said fluid stream is in the range of 3 to 1000, or wherein the average saline concentration of said fluid stream is in the range of 2 mM to 1.5 mM.

21. (canceled)

22. A device comprising a multi-layered membrane comprising a top layer, a bottom layer, and a spacer layer; wherein said spacer layer is interposed between said top layer and said bottom layer; wherein said top layer, said bottom layer and said spacer layer are each independently composed of one or more selective layers, each selective layer comprising a 2D material; wherein said spacer layer comprises at least one channel for receiving a fluid; wherein said bottom layer comprises a hole with an area in the range of 1 μm.sup.2 to 1 mm.sup.2; and wherein said hole is capable of being in fluid communication said at least one channel of with said spacer layer.

23. The device of claim 22, further comprising two or more chambers, wherein said membrane is placed between two chambers.

24. The device of claim 22, wherein the saline concentration gradient of said fluid is in the range of 3 to 1000, or wherein the average saline concentration of said fluid is in the range of 2 mM to 1.5 M.

25. (canceled)

26. (canceled)

Description

BRIEF DESCRIPTION OF DRAWINGS

[0114] The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

[0115] FIG. 1 is a schematic illustration of the structure of a device comprising a multi-layered membrane (600) comprising a top layer, a bottom layer, and a spacer layer, wherein each chamber (400, 500) is intended for receiving an electrolyte solution with a given chemical potential, wherein each electrolyte solution is in direct contact with one electrode (200, 300), and the electrodes are configured to be connected to a generator load (100). In this case, the arrows show the movement of the ions due to diffusion from the higher saline concentration chamber (400) to the lower saline concentration chamber (500).

[0116] FIG. 2A is an illustration of the structure of a multi-layered membrane (600) used in the device of FIG. 1. The multi-layered membrane (600) comprises of a bottom layer (700), a spacer layer (800) and a top layer (900), where the spacer layer (800) comprises an array of stacks of selective layers, such that each stack of selective layers (1000) is separated from the next by a channel (1100). The top layer (900) comprises of a top graphitic layer (2000) and a metal or metal oxide layer (2100). The directions of the arrows show the movement of the ions in the solution through the hole in the bottom layer and passing out from the channels (1100) of the spacer layer (800).

[0117] FIG. 2B is an illustration of the cross sectional view of the bottom layer (700) of the multi-layered membrane (600) used in the device of FIG. 1, which comprises of a bottom graphitic layer (1600) supported by a silicon nitride substrate layer (1200) and a silicon layer (1900).

[0118] FIG. 2C is an illustration of the SiNx substrate layer (1200) used as the support for the bottom layer (700), where the 300 nm thick SiNx substrate (1200) has a rectangular hole (1300) of about 10 μm.sup.2 in size, where length (1400) is about 10 μm and width (1500) is about 1 μm.

[0119] FIG. 2D is an illustration of the cross-sectional view of the spacer layer (800) of the multi-layered membrane (600) used in the device of FIG. 1, comprising of stacks of selective layers, where each stack of selective layers (1000) has a height (1700) and is separated from the next stack of selective layers by a channel (1100) of a distance (1800). The distance (1800) is referred to as the width of the channel. The height of a channel is also represented by 1700, similar to the height of a stack of selective layers (1000).

[0120] FIG. 3A is a plot showing the relationship between the ionic conductance of the device normalized to the ionic conductivity of the solution (G/G.sub.Bulk), in the absence of concentration gradient where C.sub.i=C.sub.0=1 M, and the height (1700, in angstrom, Å) of a channel (1100) of FIG. 2D. C.sub.i and C.sub.0 being the saline concentration in each of the chambers (500 and 400) of FIG. 1.

[0121] FIG. 3B is a plot showing the relationship between the ionic mobility of the device for the cations (K.sup.+) and anions (Cl.sup.−) normalized to the ionic mobility of the solution (μ/μ.sub.Bulk), in the presence of a concentration gradient of 3:1 where C.sub.i=0.3 M and C.sub.0=0.1 M, and the height (1700, in angstrom, Å) of a channel (1100) of FIG. 2D. C.sub.i and C.sub.0 being the saline concentration in each of the chambers (500 and 400) of FIG. 1.

[0122] FIG. 3C is a plot showing the relationship between the transference number of the cation (IC) of the device at different electrolyte concentrations but fixed diffusion potential (i.e. fixed concentration gradient of 3:1), at heights of 7 Å and 30 Å of a channel (1100) of FIG. 2D. C.sub.avg being the average value of the saline concentration in each of the chambers (400 and 500) of FIG. 1 (i.e. (C.sub.o+C.sub.i)/2).

[0123] FIG. 4A is a plot showing the ionic mobility of the device for the cations (K.sup.+) and anions (Cl.sup.−) normalized to the ionic mobility of the solution (μ/μ.sub.Bulk) as a function of concentration gradient, while the inset plot show the ratio of the ionic mobility of the cations ((K.sup.+) to anions (Cl.sup.−) as a function of concentration gradient. C.sub.i and C.sub.o represents the saline concentration in each of the chambers (500 and 400) of FIG. 1.

[0124] FIG. 4B is a plot showing the ionic mobility of the device for the cations (K.sup.+ or Na.sup.+) and anions (Cl.sup.−) normalized to the ionic mobility of the solution (μ/μ.sub.Bulk) as a function of different electrolyte concentrations and different concentration gradient. C.sub.i and C.sub.0 represents the saline concentration in each of the chambers (400 and 500) of FIG. 1, and C.sub.avg represents the average value of the saline concentration in each of the chambers (400 and 500) of FIG. 1 (i.e. (C.sub.o+C.sub.i)/2).

[0125] FIG. 5A is a plot showing the maximum osmotic power generated (kW/m.sup.2) in the device as a function of different salinity gradient and concentration when the height (1700, in angstrom, Å) of a channel (1100) of FIG. 2D), is 7 Å.

[0126] FIG. 5B is a plot showing the maximum energy efficiency (%) in the device as a function of different salinity gradient and concentration when the height (1700, in angstrom, Å) of a channel (1100) of FIG. 2D, is 7 Å.

[0127] FIG. 5C is a plot showing the relationship between the transference number of the cation (K.sup.+) of the device at different electrolyte concentrations where the height (1700, in angstrom, Å) of a channel (1100) of FIG. 2D, is at 7 Å and 30 Å, and the concentration gradient of varies from 3:1 to 1000:1.

[0128] FIG. 5D is a plot showing the maximum osmotic power generated (kW/m.sup.2) in the device as a function of the height (1700, in angstrom, Å) of a channel (1100) of FIG. 2D, at a fixed concentration gradient of 100.

EXAMPLES

[0129] Non-limiting examples of the invention 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 invention.

[0130] Materials

[0131] All the reagents were obtained from commercial suppliers and used without further purification. Commercially available solvents like acetone and isopropyl alcohol were purchased from Sigma-Aldrich (USA). Graphite and other 2D materials were purchased from HQ Graphene (Netherlands). The purchased silicon nitride (SiN.sub.X) substrate was grown on top of a 400 μm thick silicon (Si) layer. Both the SiN.sub.X and Si are in the shape of a circle with a 10.16 cm (4 inches) diameter wafer and are double-side polished.

Example 1: Synthesis of a Multi-Layered Membrane

[0132] To demonstrate the concept of blue energy generation, a device as shown in FIG. 1 will be used and it requires a suitable active multi-layered membrane (600) to be placed in between the two chambers (400, 500), each chamber containing its own electrolyte solution of concentration C.sub.o and C.sub.i respectively. As shown in FIG. 2A, the multi-layered membrane comprises a plurality of multi-layered structures stacked upon one another, wherein the spacer layer comprises an array of stacks of selective layers such that each stack of selective layer (1000) is separated from the next by a channel (1100). The multi-layered membrane has channels for passing the ions and molecules through the multi-layered membrane. Because of the difference in chemical potential of the electrolyte solutions between the two chambers (400, 500) of the device in FIG. 1, ions or molecules will pass through the active multi-layered membrane (600). Ionic species with different charge or valence will have an enhanced mobility within the channels of the active multi-layered membrane and will diffuse through the channels at different speeds. This will cause an imbalance in the charge neutrality of the system, resulting in an osmotic current and osmotic potential. These osmotic potential and current generate an electrical power that is collected by the generator load (100) through the electrodes (200, 300).

[0133] To prepare a prototype multi-layered membrane of trilayer structure as shown in FIG. 2A, three graphitic crystal layers were isolated by mechanical exfoliation. To obtain the graphitic crystal layers, a thick graphite crystal is laid on a low-residue tape. The thick graphite crystal is then pressed between another piece of the same tape to cleave the graphite crystal into two pieces. The process is repeated for several times till the piece of tape is fully covered with such crystals. The final thickness of those flakes is between 0.3 nm to 500 μm, with an area size of few microns square to millimetre square.

[0134] Firstly, to prepare the bottom layer (700), a graphitic crystal layer (1600) prepared by mechanical exfoliation as described above was transferred onto a substrate (1200) as shown in FIG. 2B, where the substrate (1200) is a 300 nm thick silicon nitride (SiN.sub.X) substrate (1200) with a rectangular hole (1300) of about 1×10 μm.sup.2 as shown in FIG. 2C. This layer of SiN.sub.X substrate is supported by a layer of silicon (Si) substrate (1900). This graphitic layer (1600) is then etched by reactive ion etching using the rectangular hole (1300) in the SiN.sub.X substrate (1200) as an etch mask. Therefore, the dimension of the hole in the graphitic layer (1600) is identical to that in the substrate (1200). Reference to the “hole” in the context of this disclosure is thus the hole that is present in the graphitic layer (1600).

[0135] Next, graphitic layers as selective layers, each with a height ranging from 0.7 nm to 35 nm were exfoliated onto a 300 nm thick SiO.sub.2 substrate to form a stack of selective layers. Approximately 2 to 117 graphene layers as selective layers was used. The height (1700) of each stack of selective layers was confirmed by measuring using atomic force microscopy (Dimension FastScan, Bruker, USA) in tapping mode. This stack of selective layers is then patterned by electron beam lithography and dry etching onto the bottom graphitic layer, in an array of ribbons which are now stacks of selective layers, where each stack of selective layers (1000) is of several microns in width, 150 nm in length and spaced at a distance (1800) of 100 nm from each other, to form the second (spacer) graphitic layer as shown in FIG. 2D prior to stacking on top of the bottom layer. This spacer graphitic layer contains nanometer-sized channels (1100) wherein the height (1700) of each channel (1100) is equivalent to the height of a stack of selective layers (1000) in the spacer layer.

[0136] The selective layers in the spacer graphitic layer were annealed at 400° C. prior to assembling with the bottom layer. At this annealing temperature, contaminants such as hydrocarbons and polymer residues are removed. The stacks of selective layers are then released from the SiO.sub.2 substrate by a wet etching process and transferred with a polymeric film on top of the bottom layer (700) by a custom-made micromanipulator. The polymeric film was then removed from the stacks of selective layers by dipping the sample in acetone and isopropyl alcohol, followed by another step of annealing at 400° C.

[0137] After assembling the bottom and spacer graphitic layers, a thick graphitic layer (2000) of about 50 to 120 nm was exfoliated on SiO.sub.2 and transferred in a manner like previously done for the spacer layer, on top of the spacer layer of the bottom layer/spacer layer assembly as a top graphitic layer, and a gold mask (2100) was deposited on top of the top graphitic layer (2000). Thereafter, a final dry etching step removes the part of the channels that is not protected by the gold layer (2100), hence defining the final length of the channels. The final dry etching step results in the extremes of the spacer layer (800) being flushed with the top layer (900), which may result in a better fit of the multi-layer membrane (600) to a device.

Example 2: Characterization of Multi-Layered Membrane Performance in a Device

[0138] The multi-layered membrane as synthesized in Example 1 was incorporated into a device as shown in FIG. 1 and subjected to testing to characterize the multi-layered membrane's performance. The multi-layered membrane (600) was integrated perpendicular to the ionic solutions in the two chambers (400 and 500).

[0139] Characterization of the ionic conductance of the multi-layered membrane was done using the Axopatch 200B Patch-Clamp Amplifier (Molecular Devices, USA). A voltage, sweeping at 200 mV to 200 mV, was applied between the two Ag/AgCl electrodes. The resulting current was measured by the Axonpatch 200B Patch-Clamp Amplifier. The multi-layered membrane conductance at each ionic concentration was then extracted from the slope of the measured current vs voltage curve. By using the Henderson and GHK formalism, the individual ionic mobilities were extracted.

[0140] Based on the results of FIG. 3A, an enhanced ionic conductance (i.e. G/G.sub.Bulk>1) was observed in the device when the height of each channel in the spacer layer is at 3 nm or less, and this phenomenon was observed in the absence of a concentration gradient. The smaller the height of each channel in the spacer layer, the greater the ionic conductance achieved, with the highest ionic conductance observed for 7 Å high channels. This enhancement of ionic conductance may be due to different ionic mobilities of ions and molecules inside the channels as shown in FIG. 3B as compared to their bulk values, and their 2D physical confinement with these channels.

[0141] It is well known that water through graphene nanocapillaries shows an increased structural order that leads to fast water flow and slip lengths that can go up to several hundreds of nanometers. For the smallest channels of the membrane in the present disclosure, the height of each channel at 7 Å is comparable to the hydrated ion diameter and the total ionic conductivity is significantly related to the ionic diffusivity at the surface. It has been proven experimentally and by Density Functional Theory (DFT) calculations that graphene interacts with ions in its close vicinity via the delocalized it-electrons, leading to preferential absorption of cations. This phenomenon combined with the preferred alignment of dipolar water molecules in the near-surface region corresponds to an enhanced diffusivity of cations and reduced flow of anions. At high saline concentrations of more than 0.1 M, ion-ion correlations become significant and the enhanced selectivity is disrupted. For the device with height of channels greater than 30 Å, the ionic conductance is comparable to the bulk solution, indicating a complete disruption of the physical confinement effect and a smaller contribution of the surface conductivity to the total ionic conductance.

[0142] Based on the results of FIG. 3B, it was found that the ionic mobility of the ions inside the channels are higher than in a bulk solution, although it was expected that the ions inside a confined and restricted space would move equally or slower than in a bulk solution where they are free to move in any direction. Further, the results of FIG. 3B showed that the cations can move faster than the anions when the height of the channel is below 30 Å, and hence the membrane is exhibiting cation selectivity. This is surprising because the cations K.sup.+ have a similar size to the anions Cl— and it would be expected for the ionic mobilities of the two types of ions to be similar, however the results showed ionic selectivity, which in this case is for the cations K.sup.+.

[0143] To obtain more insights on the different ionic diffusivity under physical confinement, cationic transference number was extrapolated from the osmotic potential at different electrolyte concentrations with fixed diffusion potential (where concentration gradient was fixed at 3:1) as depicted in FIG. 3C. Based on the results of FIG. 3C, in the case of highly confined channels of height 7 Å, the enhanced mobility of cations corresponds to a high anionic rejection. This surface-related effect becomes insignificant only at very high saline concentrations (for example more than 1 M) when ion-ion correlations become significant. In less confined systems as seen for channels of height 30 Å, cation selectivity was not observed. In contrary, anions Cl.sup.− ions show a higher diffusivity than cations K.sup.+ because of the chemical interaction of K.sup.+ ions with the graphitic surface of the multi-layered membrane. In addition, for the less confined systems as seen for channels of height 30 Å, at high saline concentrations, the effect of higher anion diffusivity is progressively reduced until it is completely cancelled for concentrations above 0.1 M.

[0144] Further, based on the results of FIG. 4A, which represents the ionic mobility of K.sup.+ cations and Cl.sup.− anions normalized with respect to their bulk values under a saline concentration gradient, by increasing the saline concentration gradient from 3 to 1000, K.sup.+ cations move faster with respect to the Cl.sup.− anions as show in the inset diagram. In addition, FIG. 4A showed that when saline concentration gradients equal or bigger than 3 both Cl.sup.− and K.sup.+ ions show enhanced ionic mobility with respect to the bulk solution.

[0145] Based on the results of FIG. 4B, which represents the ionic mobility of cations and anions normalized with respect to their bulk values under a saline concentration gradient of 3 or 10. For such small saline concentration gradient, regardless of the average saline concentration inside the channels of the multi-layered membrane, anions show minimal variation of ionic mobility with respect to the bulk while cations show an increased mobility for each saline concentration gradient. This cation ionic mobility enhancement (valid both for K.sup.+ and Na.sup.+) is inversely proportional to the average saline concentration inside the channels of the multi-layered membrane.

[0146] Based on the measured osmotic potential in different salinity gradient of the device, the maximum osmotic power generated (FIG. 5A), maximum energy efficiency (FIG. 5B) and cationic transference number (FIG. 5C) were obtained. The maximum osmotic power (P) was calculated based on 25% of the product of the osmotic voltage (V.sub.osm) and osmotic current (I.sub.osm) from the current-voltage curve measured under the conditions of a salinity gradient, that is P=¼×V.sub.osm×I.sub.osm. From FIG. 5A, it was observed that high power densities in the order of kW/m.sup.2 were obtained for the device when the height of the channels is at 7 Å and concentration ratios are high, wherein the power densities are 3 orders of magnitudes higher than commercially available membranes. Due to the increased cationic selectivity in the highly confined channels of 7 Å height, the maximum energy efficiency can reach as high as 16% as observed in FIG. 5B. Based on results of FIG. 5C, under high confinement the cation selectivity is high and close to 75% (0.75/1) while for devices with reduced physical confinement the cation selectivity is very low, corresponding to a cation transference number close to 0.5. As a result, the power density was drastically reduced in channels with higher heights and less physical confinement as shown in FIG. 5D.

INDUSTRIAL APPLICABILITY

[0147] The membrane and multi-layered membrane as disclosed herein may be deposited on a support or embedded in a matrix, to serve as an active membrane when it is incorporated into another membrane or system.

[0148] Since the method as disclosed herein comprises the use of a membrane or multi-layered membrane which has ionic selectivity, an osmotic voltage and/or an osmotic current may be generated and thus, the method may be suitable for blue energy generation and storage where there is salinity gradient, for example in water desalination plants, nanofiltration, ion-exchange, brine-disposal and water filtration operations, and may find many commercial applications in water purification, pharmaceutical, chemical and fuel separation industries.

[0149] The device as disclosed herein may be energy efficient and generate high power densities suitable for commercial blue energy recovery applications.

[0150] It will be apparent that 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.