A method for preparing a two-dimensional nanofluidic unipolar ion rectifier film
By preparing and constructing asymmetric two-dimensional nanofluidic unipolar ion rectifier membranes, the problems of insufficient mechanical strength and nanochannel regulation in existing ion rectifier membranes are solved, achieving efficient ion directional transport and simplifying the preparation process, making it suitable for large-scale production.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- BEIJING UNIV OF TECH
- Filing Date
- 2026-04-17
- Publication Date
- 2026-06-02
AI Technical Summary
Existing ion rectifier membranes suffer from problems in industrial applications, such as interlayer delamination, insufficient mechanical strength, small specific surface area, difficulty in precisely controlling nanochannel size, and complex preparation processes, which limit their performance improvement and large-scale production.
By preparing nanosheets with relatively uniform scale distribution and constructing multi-scale nanosheets using chemical etching or physical fragmentation methods, and combining them with polymer crosslinking agents to form an asymmetric two-dimensional nanofluidic unipolar ion rectifier membrane, an ion rectifier membrane with gradient changes or composite asymmetric structures can be prepared using vacuum-assisted filtration technology.
It achieves efficient ion-directed transport, improves mechanical strength and specific surface area, simplifies the preparation process, is suitable for large-scale production, and has unidirectional ion transport characteristics with high rectification ratio.
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Abstract
Description
Technical Field
[0001] This invention relates to a method for preparing a two-dimensional nanofluidic unipolar ion rectifying membrane. This ion rectifying membrane possesses a unique asymmetric interlayer nanochannel structure, enabling the regulation of directional ion transmembrane transport. It belongs to the field of functional materials and membrane chemistry technology. Background Technology
[0002] Ion rectifier membranes, as functional membrane materials with selective unidirectional ion transport characteristics, can achieve directional ion sieving and rectification control. Their potential applications cover multiple fields such as energy, water resources, biomedicine, and environmental governance, and they are of significant strategic importance in promoting the development and industrialization of new energy sources, including efficient energy conversion, wastewater resource recovery and utilization, and high-end ion separation and chemical engineering. A key characteristic of ion rectifier membranes is charge asymmetry or structural asymmetry. Currently, research on ion rectifier membranes mainly focuses on charge-asymmetric bipolar ion rectifier membranes, but existing technologies still have many shortcomings, severely limiting their industrial application and performance improvement. On the one hand, the interface compatibility of charge heterojunctions is poor, easily leading to problems such as interlayer delamination and insufficient mechanical strength, affecting long-term stability. On the other hand, traditional polymer or inorganic material ion rectifier membranes have small specific surface areas, making it difficult to precisely control the size of nanochannels and challenging to functionalize their surfaces, thus failing to meet the practical requirements of efficient ion rectification. Furthermore, existing methods for fabricating rectifier films rely on complex nanofabrication processes (such as precision photolithography, template etching, and ion or electron beam irradiation). These processes are cumbersome, costly, and difficult to scale up for industrial production. They are also prone to channel blockage during fabrication, leading to uneven pore size distribution in the film. Therefore, there is an urgent need to develop a unipolar ion rectifier film fabrication technology that combines high rectification ratio with a simple, environmentally friendly, and scalable fabrication process. Two-dimensional nanofluids are constructed by stacking two-dimensional nanosheets, allowing ions and fluids to transport within the confined space between the two-dimensional layers. They offer advantages such as precisely adjustable nanochannel sizes, ultra-high specific surface area, and high-density channel arrays. Based on this, this invention proposes a method for fabricating a two-dimensional nanofluidic unipolar ion rectifier film, aiming to overcome the shortcomings of existing technologies and achieve dual optimization of unipolar ion rectifier film performance and fabrication process. Summary of the Invention
[0003] The main objective of this invention is to develop a novel method for a two-dimensional nanofluidic unipolar ion rectifying membrane. By precisely controlling the size of the nanosheet unit, an asymmetric layered ion rectifying membrane with a gradient change from narrow to wide, or a combination of narrow and wide interlayer spacing characteristics, is constructed to achieve a highly efficient ion rectification effect and promote directional ion transmembrane transport.
[0004] This invention prepares nanosheets with relatively uniform size distribution through chemical or physical methods, and constructs multi-scale nanosheets using chemical etching or physical fragmentation techniques. Subsequently, the nanosheet units for film formation are classified according to size, and nanosheets of different sizes are cross-linked with one or more polymer solutions to form a film-forming precursor solution. Finally, the film-forming precursor solution is placed on a substrate film for vacuum-assisted filtration. After drying, the two-dimensional film naturally detaches from the substrate film, thus obtaining a two-dimensional nanofluid ion rectifying membrane with an asymmetric structure.
[0005] This invention provides a method for preparing an asymmetric two-dimensional nanofluidic unipolar ion rectifying film, the specific steps of which include the following two steps:
[0006] The first method involves an asymmetric membrane with a gradient in interlayer spacing, from narrow to wide, and specifically includes the following steps:
[0007] The first step is to prepare nanosheet units;
[0008] Nanosheet units are prepared from layered precursor materials by physical exfoliation, chemical etching, or chemical intercalation exfoliation; preferably, the nanosheets are relatively uniform in size with a size fluctuation range of less than 20%.
[0009] The second step involves precisely controlling the size gradation to prepare a multi-scale nanosheet precursor solution containing a larger range of sheet size fluctuations (e.g., greater than or equal to 1000%).
[0010] (1) The nanosheets prepared in the first step are etched at 40℃~60℃ for 4 hours to 6 hours with a 0.1%~1% mass percentage acid, alkali or oxidant solution to obtain nanosheets with multi-size distribution;
[0011] (2) Disperse the nanosheets obtained in step (1) in water or organic solvent, wash the nanosheets by centrifugation at 3000 to 8000 speed, take the bottom precipitate, and repeat the centrifugation washing 0 to multiple times until the pH of the nanosheets dispersed in water or organic solvent is equal to 6 to 8, to obtain multi-scale nanosheet precursor solution.
[0012] The multi-scale nanosheet precursor liquid contains two or more nanosheets of different sizes, and the size difference between at least two nanosheets is greater than or equal to 500 nm.
[0013] The third step is to prepare a polymer crosslinking agent solution with a mass percentage concentration of 0.05%~0.5%, and to crosslink the multi-scale nanosheet precursor solution from step (2) with the polymer crosslinking agent solution at room temperature for 8 to 12 hours to obtain the film-forming precursor solution.
[0014] The fourth step involves placing the membrane pretreatment solution onto the polymer substrate membrane and preparing the membrane using vacuum-assisted filtration (vacuum range -0.04 MPa to -0.1 MPa). After natural drying or immersion in a polar organic solvent, the resulting membrane detaches from the substrate membrane, yielding a two-dimensional ion rectifier membrane with an asymmetric structure.
[0015] The second method involves an asymmetric membrane structure with a composite of narrow and wide interlayer spacing, and includes the following steps:
[0016] The first step is to prepare nanosheet units;
[0017] Nanosheets are prepared from layered precursor materials by physical exfoliation, chemical etching or chemical intercalation exfoliation, and then purified by gradient centrifugation or filtration to obtain nanosheet units with relatively uniform size distribution; preferably, nanosheet units with relatively uniform size and sheet-oriented size fluctuation range of less than 20% are obtained.
[0018] The second step involves precisely controlling the size grading to prepare precursor solutions for large-size and small-size nanosheets, with the preferred size fluctuation range being greater than or equal to 1000%.
[0019] (1) The nanosheets obtained in the first step are etched at 40 ℃ to 60 ℃ for 4 hours to 6 hours with an acid, alkali or oxidant solution of 0.1% to 1% by mass percentage, or ultrasonically crushed for 5 minutes to 60 minutes, or physically ground for 10 hours to 24 hours to obtain nanosheets with multi-size distribution.
[0020] (2) Disperse the nanosheets obtained in step (1) in water or organic solvent, wash the nanosheets by centrifugation at 3000 to 8000 speed, take the bottom precipitate, and repeat the centrifugation washing 0 to multiple times until the pH of the nanosheets dispersed in water or organic solvent is equal to 6 to 8, and obtain a nanosheet dispersion with multiple sizes.
[0021] (3) Centrifuge the nanosheet dispersion with multiple size distribution obtained in step (2) at 1000 to 4000 speeds for 5 to 10 minutes, collect the precipitate and redisperse it in water, and stir it evenly to obtain the precursor solution of large-size nanosheets.
[0022] (4) Centrifuge the nanosheet dispersion with multiple sizes in step (2) at 3000 to 8000 speeds for 5 to 10 minutes, collect the supernatant, and obtain the precursor solution of small-sized nanosheets;
[0023] Furthermore, the centrifugation speed of the nanosheet dispersion in step (4) is greater than that in step (3).
[0024] The multi-size distributed nanosheet precursor liquid contains two or more nanosheet units of different sizes, and includes at least two nanosheet units with a size difference of 500 nm or more.
[0025] The third step is to prepare a polymer crosslinking agent with a mass percentage concentration of 0.05%~0.5%, and mix the precursor liquid of large-size nanosheets in step (3) and the precursor liquid of small-size nanosheets in step (4) with the polymer crosslinking agent at room temperature and stir for 8 to 12 hours to crosslink them, thereby obtaining film-forming precursor liquid one and film-forming precursor liquid two.
[0026] In the fourth step, the membrane pretreatment solution 1 from the third step is placed on the polymer substrate membrane and a membrane is prepared by vacuum-assisted filtration. Then, the membrane pretreatment solution 2 is prepared on the obtained membrane by vacuum-assisted filtration. After natural drying or soaking in a polar organic solvent, the obtained membrane is detached from the substrate membrane, and a two-dimensional ion rectifier membrane with an asymmetric structure is obtained.
[0027] The acid, alkaline solution or oxidizing agent mentioned in the above two methods includes hydrochloric acid, sulfuric acid, sodium hydroxide, potassium hydroxide or hydrogen peroxide.
[0028] The polymer crosslinking agent is one or more of polyvinyl alcohol, polyacrylamide, carboxymethyl cellulose, chitosan, dialdehyde starch, glutaraldehyde, or glyoxal.
[0029] The polar organic solvents include N,N-dimethylformamide, dimethyl sulfoxide, anhydrous ethanol, and isopropanol.
[0030] The nanosheets include one or more of sodium montmorillonite (MMT) nanosheets, lithium montmorillonite (Laponite) nanosheets, MXene nanosheets, boron nitride nanosheets (BNNS), layered double hydroxide nanosheets (LDH), or graphene oxide (GO) nanosheets.
[0031] The base membrane includes cellulose acetate, polytetrafluoroethylene, polyvinylidene fluoride, polyethersulfone, or nylon, with a pore size of 0.2 micrometers to 0.5 micrometers.
[0032] The concentration of all nanosheet precursor solutions is 2-8 mg / mL, preferably 4 mg / mL; when the precursor solution or dispersion is mixed with the polymer crosslinking agent solution, the mass ratio of the precursor solution or dispersion to the polymer crosslinking agent solution is generally 5-50:1.
[0033] Applications, for cations (e.g., K) + Rectification. It can precisely control the direction of ion transport, achieving unidirectional ion transport behavior that promotes forward transport and inhibits reverse transport.
[0034] The innovation of this invention lies in:
[0035] 1. This invention provides a novel method for preparing two-dimensional asymmetric unipolar ion rectifier films by controlling the size of nanosheets.
[0036] 2. The unipolar ion rectifier membrane prepared by this invention has a typical rectification effect, which promotes unidirectional ion transport.
[0037] 3. The ion transport rectification can be altered by controlling the asymmetric morphology of the nanochannel structure. Attached Figure Description
[0038] Figure 1 . Transmission electron microscopy (TEM) surface morphology of the nanosheets of this invention (illustration);
[0039] Figure 2 Scanning electron microscope (SEM) cross-sectional image of the ion rectifier film of the present invention (illustration);
[0040] Figure 3 Small-angle X-ray diffraction (XRD) spectrum of the ion rectifier film of the present invention (illustration);
[0041] Figure 4 Time-of-flight secondary ion mass spectrometry of the ion rectifier membrane of this invention (illustration).
[0042] Figure 5 . Illustration of the ion current testing device of the present invention;
[0043] Figure 6 The voltammetric curves of the ion-rectifying membrane of this invention tested in KCl solutions of different concentrations (illustrated).
[0044] Figure 7 The ion rectification ratio of the ion rectification membrane of the present invention under different concentrations of KCl solution (illustration);
[0045] Figure 8 The voltammetric curve of the ion rectifying membrane with asymmetric differences in nanochannels of the present invention in KCl solution (illustration); Detailed Implementation
[0046] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments, but the present invention is not limited to the following embodiments.
[0047] Example 1 (First Method):
[0048] (1) 0.4 g of MMT layered aggregate powder was placed in 100 mL of water to fully swell, stirred for 12 hours to dissociate the interlayer, and then purified by gradient centrifugation to obtain relatively uniform MMT nanosheets with a size distribution range fluctuation of less than 20%.
[0049] (2) MMT nanosheets with relatively uniform size distribution were stirred with 100 mL of 0.6% HCl solution at 40 °C for 4 hours. The resulting MMT nanosheets were then repeatedly centrifuged and washed at 5000 rpm. The bottom precipitate was collected, added back into water and dispersed evenly. The pH was adjusted to 6 to obtain a multi-scale distribution MMT nanosheet solution with a size distribution fluctuation range of more than 1000%.
[0050] (3) Dissolve 0.5 g of polyvinyl alcohol powder in 100 mL of water and stir at 90 °C for 2 hours to obtain the first mixture; 0.5% glutaraldehyde is the second mixture.
[0051] (4) At room temperature, the 4 mg / mL nanosheet solution with multi-scale distribution in step (2) is mixed with the first mixture at a mass ratio of 20:1 and stirred for 12 hours; then the second mixture is added at the same mass ratio and stirred for 12 hours to obtain the membrane preparation solution.
[0052] (5) Place 15 mL of the membrane pretreatment solution on the cellulose acetate base membrane and perform vacuum filtration (-0.06 MPa); after natural drying, the two-dimensional membrane detaches from the base membrane to obtain the two-dimensional MMT-based ion rectifier membrane.
[0053] Surface morphology of multi-scale MMT nanosheets, such as Figure 1 As shown, the nanosheets have a complete layer structure, with the sheet size ranging from small to large, and are evenly dispersed without obvious aggregation.
[0054] Cross-sectional view of MMT-based ion rectifier membrane as shown in Figure Figure 2 As shown, the film is 20 micrometers thick and is a two-dimensional layered structure formed by the stacking of nanosheets layer by layer.
[0055] Small-angle X-ray diffraction pattern of ion rectifier film as follows Figure 3 As shown, the 2θ angle changes from 6.8 degrees to 4.3 degrees. Based on Bragg's equation, the interlayer distance of the ion-rectifying film is calculated to be 1.3 nanometers to 2.1 nanometers. Meanwhile, Figure 4 The time-of-flight secondary ion mass spectrometry (TOF-MS) of the ion rectifier film was obtained by deep etching from one side of the film surface toward the film thickness direction. As the etching depth increases, the content of interlayer organic matter gradually decreases until it disappears, showing a gradient decrease in interlayer spacing from wide to narrow, which proves the asymmetric structural characteristics of the film.
[0056] like Figure 5 This is a schematic diagram of an ion transport device. An ion rectifier membrane is placed in the middle of the test cell, and KCl solutions of the same concentration are added to both sides. Two Ag / AgCl electrodes are used to apply transmembrane potentials, with the positive electrode located on the side of the interlayer spacing of the membrane. The direction of ion transport is controlled by applying a scanning potential.
[0057] like Figure 6 The figures show the voltammetric curves of the MMT-based ion rectifier membrane in 0.1 mM to 200 mM KCl solutions. The nonlinear current-voltage curves demonstrate the nonlinear transport of ions, and the ion current increases with increasing KCl solution concentration. Under positive voltage, K... + Ion transport from the narrow interlayer spacing to the wide interlayer spacing of the rectifier film is promoted, resulting in a stronger ion current. Under negative voltage, K + Ion transport from the wide-interlayer spacing to the narrow-interlayer spacing of the rectifier film is suppressed, resulting in a weaker ion current. This is because the narrow-interlayer spacing of the rectifier film provides uniform coverage of the double layer, exhibiting strong ion selectivity and a higher ion concentration within the nanochannels; conversely, the wide-interlayer spacing provides less coverage of the double layer, resulting in weaker ion selectivity and a lower ion concentration within the nanochannels. Therefore, ion transport is promoted along the concentration gradient and suppressed in the opposite direction.
[0058] The ion current rectification ratio of the ion rectifier membrane under different concentrations of KCl solution can be calculated by comparing the ratio of the current value corresponding to the maximum positive voltage (1.5V) to the absolute value of the current corresponding to the maximum negative voltage (-1.5V). The corresponding calculation results are as follows: Figure 7 As shown in the bar chart, the relationship between the rectification ratio and KCl solubility shows that when the KCl solution concentration is less than 10 mM, the ion rectification ratio gradually increases; when it is greater than 10 mM, the ion rectification ratio gradually decreases. At a concentration of 10 mM, the double-layer coverage within the nanochannel is optimal, resulting in the highest ion rectification ratio of approximately 25.1.
[0059] Example 2 (Second Method):
[0060] (1) 0.4 g of MMT layered aggregate powder was placed in 100 mL of water to fully swell, stirred for 12 hours to dissociate the interlayer, and then purified by gradient centrifugation to obtain relatively uniform MMT nanosheets with a size distribution range fluctuation of less than 20%.
[0061] (2) The nanosheets obtained in step (1) are sonicated in an ultrasonic water bath for 30 minutes to obtain a multi-scale MMT nanosheet solution with a size distribution fluctuation range greater than 1000%. The multi-scale nanosheet solution is centrifuged at 1000 rpm for 5 minutes, and the bottom precipitate is taken and dispersed in water. After stirring evenly, a large-size MMT nanosheet solution is obtained. The multi-size nanosheet solution is centrifuged at 3000 rpm for 5 minutes, and the supernatant is taken to obtain a small-size MMT nanosheet solution.
[0062] (3) Dissolve 0.5 g of polyvinyl alcohol powder in 100 mL of water and stir at 90 °C for 2 hours to obtain the first mixture. 0.5% glutaraldehyde is the second mixture.
[0063] (4) At room temperature, a 4 mg / mL large-size MMT nanosheet solution was mixed with the first mixture at a mass ratio of 20:1 and stirred for 12 hours. Then, the second mixture was added at the same mass ratio and stirred for 12 hours to obtain the first film-forming precursor solution. A 2 mg / mL small-size MMT nanosheet solution was mixed with the first mixture at a mass ratio of 40:1 and stirred for 12 hours. Then, the second mixture was added at the same mass ratio and stirred for 12 hours to obtain the second film-forming precursor solution.
[0064] (5) Place 5 mL of the first membrane preparation solution on the cellulose acetate substrate membrane and perform vacuum filtration (-0.06 MPa); after filtration, continuously filter 10 mL of the second membrane preparation solution; after natural drying, the two-dimensional membrane falls off the substrate membrane to obtain the two-dimensional MMT-based ion rectifier membrane.
[0065] Example 3 (Second Method):
[0066] (1) The precursor Ti3AlC2 powder was etched by in-situ generation of HF system using HCl+LiF. After stirring and etching for 24 h, it was washed until neutral. After being peeled by hand shaking, it was centrifuged at 3500 rpm and the supernatant was extracted to obtain relatively uniform MXene nanosheets with a size distribution range fluctuation of less than 20%.
[0067] (2) 100 mL of MXene nanosheet solution with uniform size distribution (1 mg / mL) was sonicated in an ultrasonic water bath (80%) for 60 minutes to obtain a nanosheet solution with a multi-scale distribution with a size distribution fluctuation range greater than 2000%. The obtained nanosheet solution was centrifuged at 4000 rpm for 10 minutes, and the bottom precipitate was taken and dispersed in water. After stirring evenly, a large-size MXene nanosheet solution was obtained. Subsequently, the multi-size nanosheet solution was centrifuged at 6000 rpm for 5 minutes, and the supernatant was taken to obtain a small-size MXene nanosheet solution.
[0068] (3) Dissolve 0.5 g of polyvinyl alcohol powder in 100 mL of water and stir at 90 °C for 2 hours to obtain the first mixture. 0.5% glutaraldehyde is the second mixture.
[0069] (4) At room temperature, a 2 mg / mL large-size MXene nanosheet solution was mixed with the first mixture at a mass ratio of 30:1 and stirred for 12 hours. Then, the second mixture was added at the same mass ratio and stirred for 12 hours to obtain the first film-forming precursor solution. A 2 mg / mL small-size MXene nanosheet solution was mixed with the first mixture at a mass ratio of 30:1 and stirred for 12 hours. Then, the second mixture was added at the same mass ratio and stirred for 12 hours to obtain the second film-forming precursor solution.
[0070] (5) Place 10 mL of the first membrane preparation solution on the cellulose acetate substrate membrane and perform vacuum filtration (-0.06 MPa); after filtration, continuously filter 10 mL of the second membrane preparation solution; after natural drying, the two-dimensional membrane falls off the substrate membrane to obtain the two-dimensional MXene-based ion rectifier membrane.
[0071] Example 4 (Second Method):
[0072] (1) Using graphite as raw material, graphite oxide precursor was obtained by modified Hummers method. After ultrasonic physical exfoliation and gradient centrifugation purification, relatively uniform GO nanosheets with size distribution range fluctuation of less than 20% were obtained.
[0073] (2) 100 mL of a uniformly sized GO nanosheet dispersion (2 mg / mL) was sonicated in an ultrasonic water bath (80%) for 60 minutes to obtain a multi-scale nanosheet solution with a size distribution fluctuation range greater than 2000%. The obtained solution was centrifuged at 4000 rpm for 10 minutes, and the bottom precipitate was collected, dispersed in water, and stirred evenly to obtain a large-size GO nanosheet solution. Subsequently, the multi-size nanosheet solution was centrifuged at 8000 rpm for 5 minutes, and the supernatant was collected to obtain a small-size GO nanosheet solution.
[0074] (3) Dissolve 0.05 g of polyvinyl alcohol powder in 100 mL of water and stir at 90 °C for 2 hours to obtain the first mixture. 0.05% glutaraldehyde is the second mixture.
[0075] (4) At room temperature, a 2 mg / mL large-size GO nanosheet solution was mixed with the first mixture at a mass ratio of 20:1 and stirred for 12 hours. Then, the second mixture was added at the same mass ratio and stirred for 12 hours to obtain the first film-forming precursor solution. A 2 mg / mL small-size GO nanosheet solution was mixed with the first mixture at a mass ratio of 20:1 and stirred for 12 hours. Then, the second mixture was added at the same mass ratio and stirred for 12 hours to obtain the second film-forming precursor solution.
[0076] (5) Place 10 mL of the first membrane preparation solution on the cellulose acetate substrate membrane and perform vacuum filtration (-0.06 MPa); after filtration, continuously filter 10 mL of the second membrane preparation solution; after natural drying, the two-dimensional membrane falls off the substrate membrane to obtain the two-dimensional GO-based ion rectifier membrane.
[0077] Example 5 (Second Method):
[0078] (1) 0.8 g of Laponite layered aggregate powder was placed in 100 mL of water to fully swell, stirred for 2 hours to dissociate the interlayer, and then purified by gradient centrifugation to obtain relatively uniform Laponite nanosheets with a size distribution range fluctuation of less than 20%.
[0079] (2) The nanosheet dispersion obtained in step (1) was stirred at room temperature for 12 hours to obtain a multi-scale distribution nanosheet solution with a size distribution fluctuation range greater than 2000%. The obtained solution was centrifuged at 1000 rpm for 5 minutes, and the bottom precipitate was taken and dispersed in water. After stirring evenly, a large-size Laponite nanosheet solution was obtained. Subsequently, the multi-size nanosheet solution was centrifuged at 3000 rpm for 5 minutes, and the supernatant was taken to obtain a small-size Laponite nanosheet solution.
[0080] (3) Dissolve 0.5 g of polyvinyl alcohol powder in 100 mL of water and stir at 90 °C for 2 hours to obtain a mixture.
[0081] (4) At room temperature, the 8 mg / mL large-size Laponite nanosheet solution and the mixed solution were mixed and stirred for 12 hours at a mass ratio of 5:1 to obtain the first film-forming precursor solution; the 8 mg / mL small-size Laponite nanosheet solution and the mixed solution were mixed and stirred for 12 hours at a mass ratio of 5:1 to obtain the second film-forming precursor solution.
[0082] (5) Place 5 mL of the first membrane preparation solution on the nylon substrate membrane and perform vacuum filtration (-0.06 MPa); after filtration, continuously filter 10 mL of the second membrane preparation solution; after natural drying, immerse the membrane in N,N-dimethylformamide solvent, and then take out the membrane and continue to immerse it in anhydrous ethanol; after natural drying, the two-dimensional membrane detaches from the nylon substrate membrane, thus obtaining the two-dimensional Laponite-based ion rectifier membrane.
[0083] like Figure 8 The figures show the current-voltage curves of the ion rectifier membranes in Examples 1-5 in 10 mM KCl solution, obtained based on the differences in their asymmetric structures. The MMT-based ion rectifier membrane in Example 1 exhibits the largest ion current rectification characteristic. In Examples 2-5, the MMT-based, MXene-based, GO-based, or Laponite-based ion rectifier membranes, due to their asymmetric structures combining narrow and wide interlayer spacing, also exhibit the nonlinear ion transport characteristics of unipolar ion rectifier membranes.
[0084] Characterization of the asymmetric two-dimensional nanofluidic unipolar ion rectifier membrane by X-ray diffraction, ion current testing, and calculation of the ion rectification ratio reveal that the present invention achieves the function of promoting unidirectional ion transport and suppressing reverse ion transport by precisely controlling the asymmetric structural characteristics of the ion rectifier membrane, and obtains the typical rectification characteristics of the rectifier membrane; by changing the asymmetric form of the nanochannel, the magnitude of the ion rectification ratio can be changed.
Claims
1. A method for preparing an asymmetric two-dimensional nanofluidic unipolar ion rectifier membrane, characterized in that, The method for preparing asymmetric membranes with a gradient in interlayer spacing from narrow to wide includes the following steps: The first step is to prepare nanosheet units; Nanoparticles are prepared from layered precursor materials using methods such as physical exfoliation, chemical etching, or chemical intercalation exfoliation. Nanosheets; preferably, nanosheets with relatively uniform size and a size fluctuation range of less than 20%; The second step involves precisely controlling the size grading to prepare a multi-scale nanosheet precursor solution with a larger range of sheet size fluctuations (preferably greater than or equal to 1000%). (1) The nanosheets prepared in the first step are etched at 40 ℃ to 60 ℃ for 4 to 6 hours with a mass percentage concentration of 0.1% to 1% acid, alkali or oxidant to obtain nanosheets with multi-size distribution; (2) Disperse the nanosheets obtained in step (1) in water or organic solvent, wash the nanosheets by centrifugation at 3000 to 8000 speed, take the bottom precipitate, and repeat the centrifugation washing 0 to multiple times until the pH of the nanosheets dispersed in water or organic solvent is equal to 6 to 8, to obtain multi-scale nanosheet precursor solution. The multi-scale nanosheet precursor liquid contains two or more nanosheets of different sizes, and the size difference between at least two nanosheets is greater than or equal to 500 nm. The third step is to prepare a polymer crosslinking agent solution with a mass percentage concentration of 0.05%~0.5%, and to crosslink the multi-scale nanosheet precursor solution from step (2) with the polymer crosslinking agent solution at room temperature for 8 to 12 hours to obtain the film-forming precursor solution. The fourth step involves placing the membrane pretreatment solution onto the polymer substrate membrane and preparing the membrane using vacuum-assisted filtration (vacuum range -0.04 MPa to -0.1 MPa). After natural drying or immersion in a polar organic solvent, the resulting membrane detaches from the substrate membrane, yielding a two-dimensional ion rectifier membrane with an asymmetric structure.
2. A method for preparing an asymmetric two-dimensional nanofluidic unipolar ion rectifier membrane, characterized in that, A method for preparing an asymmetric structure membrane with a composite of narrow and wide interlayer spacing includes the following steps: The first step is to prepare nanosheet units; Nanosheets are prepared from layered precursor materials by physical exfoliation, chemical etching or chemical intercalation exfoliation, and then purified by gradient centrifugation or filtration to obtain nanosheet units with relatively uniform size distribution; preferably, nanosheet units with relatively uniform size and sheet-oriented size fluctuation range of less than 20% are obtained. The second step involves precisely controlling the size gradation to prepare precursor solutions for large-sized and small-sized nanosheets, with the optimal fluctuation range for both large and small nanosheets being greater than or equal to 1000%. (1) The nanosheets obtained in the first step are etched at 40℃~60℃ for 4 hours~6 hours with acid, alkali or oxidant solution of mass percentage concentration of 0.1%~1%, or ultrasonically crushed for 5 minutes~60 minutes, or physically ground for 10 hours~24 hours to obtain nanosheets with multi-size distribution; (2) Disperse the nanosheets obtained in step (1) in water or organic solvent, wash the nanosheets by centrifugation at 3000 to 8000 speed, take the bottom precipitate, and repeat the centrifugation washing 0 to multiple times until the pH of the nanosheets dispersed in water or organic solvent is equal to 6 to 8, and obtain a nanosheet dispersion with multiple sizes. (3) Centrifuge the nanosheet dispersion with multiple size distribution obtained in step (2) at 1000 to 4000 speeds for 5 to 10 minutes, collect the precipitate and redisperse it in water, and stir it evenly to obtain the precursor solution of large-size nanosheets. (4) Centrifuge the nanosheet dispersion with multiple sizes in step (2) at 3000 to 8000 speeds for 5 to 10 minutes, collect the supernatant, and obtain the precursor solution of small-sized nanosheets; The centrifugation speed of the nanosheet dispersion in step (4) is greater than that in step (3); The multi-size distributed nanosheet precursor liquid contains two or more nanosheet units of different sizes, and includes at least two nanosheet units with a size difference of 500 nm or more. The third step is to prepare a polymer crosslinking agent with a mass percentage concentration of 0.05%~0.5%, and mix the precursor liquid of large-size nanosheets in step (3) and the precursor liquid of small-size nanosheets in step (4) with the polymer crosslinking agent at room temperature and stir for 8 to 12 hours to crosslink them, thereby obtaining film-forming precursor liquid one and film-forming precursor liquid two. In the fourth step, the membrane pretreatment solution 1 from the third step is placed on the polymer substrate membrane and a membrane is prepared by vacuum-assisted filtration. Then, the membrane pretreatment solution 2 is prepared on the obtained membrane by vacuum-assisted filtration. After natural drying or soaking in a polar organic solvent, the obtained membrane is detached from the substrate membrane, and a two-dimensional ion rectifier membrane with an asymmetric structure is obtained.
3. The method according to claim 1 or 2, characterized in that, The acid, alkaline solution or oxidizing agent mentioned includes hydrochloric acid, sulfuric acid, sodium hydroxide, potassium hydroxide or hydrogen peroxide.
4. The method according to claim 1 or 2, characterized in that, The polymer crosslinking agent is one or more of polyvinyl alcohol, polyacrylamide, carboxymethyl cellulose, chitosan, dialdehyde starch, glutaraldehyde, or glyoxal.
5. The method according to claim 1 or 2, characterized in that, The polar organic solvents include one or more of N,N-dimethylformamide, dimethyl sulfoxide, anhydrous ethanol, and isopropanol.
6. The method according to claim 1 or 2, characterized in that, The nanosheets include one or more of sodium montmorillonite (MMT) nanosheets, lithium montmorillonite (Laponite) nanosheets, MXene nanosheets, boron nitride nanosheets (BNNS), layered double hydroxide nanosheets (LDH), or graphene oxide (GO) nanosheets.
7. The method according to claim 1 or 2, characterized in that, The base membrane includes cellulose acetate, polytetrafluoroethylene, polyvinylidene fluoride, polyethersulfone, or nylon, with a pore size of 0.2 micrometers to 0.5 micrometers.
8. The method according to claim 1 or 2, characterized in that, The concentration of all nanosheet precursor solutions is 2-8 mg / mL, preferably 4 mg / mL; when the precursor solution or dispersion is mixed with the polymer crosslinking agent solution, the mass ratio of the precursor solution or dispersion to the polymer crosslinking agent solution is generally 5-50:
1.
9. A two-dimensional nanofluidic unipolar ion rectifier membrane with an asymmetric structure prepared according to the method of claim 1 or 2.
10. The application of the asymmetric two-dimensional nanofluidic unipolar ion rectifier membrane prepared according to the method of claim 1 or 2, for use with cations (e.g., K+). + ) Rectification.