A method for preparing cross-section samples of self-supporting ionic COFs membranes

Abstract

The present invention discloses a method for preparing a cross-section sample of a self-supporting ionic COFs membrane, comprising the following steps: (1) prefabricating a fracture groove; (2) surface modification; (3) laminating; and (4) fracture. The present invention uses a single-crystal silicon wafer as a support carrier for the COFs membrane, transfers the COFs membrane onto the single-crystal silicon wafer, and makes the membrane and the single-crystal silicon wafer closely fit together. The single-crystal cleavage fracture mechanism of the single-crystal silicon wafer (under the action of external force, the membrane breaks along the surface of the COFs membrane) is utilized. <111> The cleavage fracture of the crystal plane drives the fracture of the COFs film, thereby obtaining a large range of flat film cross-section area, and facilitating the flat transfer of the COFs film to the SEM cross-section stage.

Description

Technical Field

[0001] The present invention relates to the technical field of COFs membrane characterization, in particular to a method for preparing a cross-section sample of a self-supporting ionic COFs membrane. Background Art

[0002] Two-dimensional ionic covalent organic frameworks (COFs) membranes have high charge density and unobstructed one-dimensional straight channels, and can be used in the manufacture of nanocurrent reverse electrodialysis (NRED) technology and high-sensitivity biomimetic thermometers. Their thickness and density are important factors determining membrane separation performance, and scanning electron microscopy (SEM) is an important technical means to characterize the density and thickness of membrane cross-section layers.

[0003] Currently, the methods for preparing membrane cross-section samples include direct tearing or low-temperature breaking. However, COFs, as crystalline polymer materials, are prone to brittle fracture. In addition, the thickness of effective self-supporting ionic COFs membranes is usually at the nanometer level. Even if the membrane is successfully broken, it is difficult to adhere the membrane neatly to the scanning electron microscope cross-section sample stage due to its low mechanical strength, which brings great difficulties to characterization. Summary of the Invention

[0004] The present invention aims to solve the problems in the prior art of the membrane cross-section sampling method of self-supporting ionic COFs membranes that easily cause COFs membrane breakage and the COFs membrane is difficult to adhere neatly to the scanning electron microscope cross-section sample stage. The present invention provides a self-supporting ionic COFs membrane cross-section sampling method, which can obtain a large range of flat membrane cross-section areas and is easy to transfer to the SEM sample stage.

[0005] In order to achieve the above object, the present invention adopts the following technical solution: a method for preparing a cross-section sample of a self-supporting ionic COFs membrane, comprising the following steps:

[0006] (1) Prefabricated breaking groove

[0007] Take a rectangular single-crystal silicon wafer and set a number of fracture grooves on the back of the single-crystal silicon wafer. The fracture grooves are parallel to the cleavage plane of the single-crystal silicon wafer. <111> The COFs film is transferred to the single-crystalline silicon wafer using a single-crystalline silicon wafer as a support carrier for the COFs film, so that the film and the single-crystalline silicon wafer are tightly fitted together. This not only drives the COFs film to break synchronously through the cleavage fracture of the single-crystalline silicon wafer, thereby obtaining a large range of flat film cross-section areas, but also facilitates the flat transfer of the COFs film to the SEM cross-section table through the support of the single-crystalline silicon wafer. The breaking groove can be formed by cutting with a diamond knife. The breaking groove facilitates the breaking of the single-crystalline silicon wafer. The breaking groove is parallel to the cleavage plane of the single-crystalline silicon wafer, forcing the single-crystalline silicon wafer and the COFs film to break along the cleavage plane to form and obtain a flat COFs film cross-section. The spacing between the breaking grooves can be set according to actual needs (i.e., the area of ​​the sample).

[0008] (2) Surface modification

[0009] (1) Rinse the single-crystal silicon wafer in step (1) with deionized water, ethyl acetate, and alcohol continuously, and then let it air-dry. Rinse the single-crystal silicon wafer with deionized water, ethyl acetate, and alcohol continuously to remove dust, tiny oil droplets, stains, and other impurities on the surface of the single-crystal silicon wafer. The order and number of rinses are not limited.

[0010] (2) Place the single-crystal silicon wafer prepared in step (1) in a mixed solution of concentrated sulfuric acid and hydrogen peroxide, heat to 100°C, cool, remove the single-crystal silicon wafer, rinse with deionized water, and allow to air dry. This step aims to hydroxylate the surface of the single-crystal silicon wafer to obtain a single-crystal silicon wafer with a super-hydrophilic surface, thereby making the COFs film and the single-crystal silicon wafer more firmly bonded; heating is used to accelerate the hydroxylation reaction on the silicon wafer surface.

[0011] (3) The single-polished single-crystalline silicon wafer in step (2) is placed in an acetone solution of 3-aminopropyltrimethoxysilane, soaked in an inert gas atmosphere, taken out, washed with deionized water and ethanol in sequence, and dried to obtain a surface-modified single-crystalline silicon wafer. Insufficient bonding between the single-polished single-crystalline silicon wafer and the COFs film will result in loose adhesion between the single-polished single-crystalline silicon wafer and the COFs film, resulting in an uneven COFs film, which will affect the selection of the COFs film observation area and the accuracy of the COFs film thickness measurement. In the present invention, the surface of the single-polished single-crystalline silicon wafer is modified (aminoated) by 3-aminopropyltrimethoxysilane to form a hydrogen bond between the single-polished single-crystalline silicon wafer and the COFs film, thereby improving the bonding strength and adhesion between the single-polished single-crystalline silicon wafer and the COFs film and ensuring the flatness of the COFs film. The inert gas can be nitrogen, argon, or helium, etc., in order to prevent the oxygen in the air from affecting the amination.

[0012] (3) Fitting

[0013] The cleaned COFs film is immersed in a liquid medium, and the single-polished single-crystal silicon wafer obtained in step (3) is placed with the front side facing up. The COFs film is then removed from the solvent and dried. The COFs film can be prepared by methods known in the art, including but not limited to the method disclosed in patent CN113234326 A. The COFs film is immersed in a liquid medium so that the COFs film can be fully unfolded so that it can be evenly laminated to the single-polished single-crystal silicon wafer to avoid wrinkles.

[0014] (4) Fracture

[0015] Place the single-crystal silicon wafer obtained in step (3) facing up. Place a force-bearing object within the break groove. Then, press down on the single-crystal silicon wafer on either side of the break groove, causing both the single-crystal silicon wafer and the COF film to break along the break groove. This procedure yields a rectangular COF film cross-section sample. The force-bearing object primarily serves to provide a force point, facilitating the break of the single-crystal silicon wafer along the break groove.

[0016] Preferably, in step (1), the thickness of the single-polished single-crystal silicon wafer is 0.2-0.8 mm. The thickness of the single-polished single-crystal silicon wafer affects the fracture process. If the thickness is too small, small defects such as microcracks and chipping will form during the fracture process, and the fracture may occur along these small defects rather than along the cleavage plane. If the silicon wafer is too thick, it is not easy to fracture. Taking all factors into consideration, the thickness of the single-polished single-crystal silicon wafer in the present invention is controlled to be 0.2-0.8 mm.

[0017] Preferably, in step (1), the thickness of the single-polished monocrystalline silicon wafer is 0.4-0.5 mm.

[0018] Preferably, in step (1), the single-crystal silicon wafer is rinsed three times continuously with deionized water, ethyl acetate and ethanol.

[0019] Preferably, in step (2), the volume ratio of concentrated sulfuric acid to hydrogen peroxide is 2: 1. When the volume ratio of concentrated sulfuric acid to hydrogen peroxide is 2: 1, the surface hydroxylation of the single-polished single-crystal silicon wafer is most complete.

[0020] Preferably, in step (2), the heating temperature is 100°C and the heating time is 5 hours. The higher the heating temperature, the faster the reaction. However, when the temperature exceeds 100°C, it exceeds the boiling point of hydrogen peroxide, causing hydrogen peroxide to vaporize, which is not conducive to the reaction. When the heating temperature is lower than 100°C, the reaction speed is slow and the reaction is incomplete. The heating time affects the degree of reaction. Therefore, in the present invention, the heating temperature is controlled to 100°C and the heating time is controlled to 5 hours.

[0021] Preferably, in step (3), the volume ratio of 3-aminopropyltrimethoxysilane to acetone is 1:10.

[0022] Preferably, in step (3), the immersion is carried out for 3 days in an inert gas atmosphere, and the inert gas is nitrogen, argon or helium.

[0023] Preferably, in step (iii), the cleaned COFs membrane is obtained by washing the COFs membrane with ethanol, methanol, and water in sequence to remove impurities such as residual monomers, catalysts, and organic solvents.

[0024] Preferably, in step (3), the liquid medium is ethanol, water, or methanol. Ethanol is safer than methanol; compared to pure water, ethanol is highly volatile, and the drying rate at the center and edge of the membrane does not differ significantly, ensuring uniform membrane drying. Therefore, ethanol is the most preferred liquid medium.

[0025] Therefore, the present invention has the following beneficial effects:

[0026] (1) Using a single-crystal silicon wafer as the support carrier of the COFs film, the COFs film is transferred to the single-crystal silicon wafer so that the film and the single-crystal silicon wafer are closely attached. The single-crystal cleavage fracture mechanism of the single-crystal silicon wafer (under the action of external force) is used to fracture the COFs film. <111> The cleavage fracture of the crystal plane drives the fracture of the COFs film, thereby obtaining a large range of flat film cross-section area, and at the same time facilitating the flat transfer of the COFs film to the SEM cross-section stage;

[0027] (2) The surface of the single-polished single-crystalline silicon wafer is modified (amino) by 3-aminopropyltrimethoxysilane to form a hydrogen bond between the single-polished single-crystalline silicon wafer and the COFs film, thereby improving the bonding strength and fit between the single-polished single-crystalline silicon wafer and the COFs film and ensuring the flatness of the COFs film. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 These are scanning electron microscope images of the COFs membrane cross section in the COFs membrane cross section sample obtained in Example 1 at different magnifications.

[0029] Figure 2 This is a scanning electron microscope image of the cross section of the COFs film in Example 2.

[0030] Figure 3 These are scanning electron microscope images of the COFs film cross section in Example 3 at different magnifications. DETAILED DESCRIPTION

[0031] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0032] Example 1

[0033] (1) Prefabricated breaking groove

[0034] Take a 0.4mm thick, rectangular single-polished single-crystal silicon wafer, and use a diamond knife to separate several breaking grooves on the back of the single-polished single-crystal silicon wafer. The breaking grooves are parallel to the cleavage surface of the single-polished single-crystal silicon wafer, and the spacing between adjacent breaking grooves is 5mm.

[0035] (2) Surface modification

[0036] (1) Rinse the single-crystal silicon wafer prepared in step (1) three times with deionized water, ethyl acetate, and ethanol, and then let it air dry.

[0037] (2) Place the single-crystal silicon wafer prepared in step (1) in a mixed solution of concentrated sulfuric acid and hydrogen peroxide, heat at 100°C for 5 hours, cool, take out the single-crystal silicon wafer, wash it with deionized water, and let it air-dry; the volume ratio of concentrated sulfuric acid to hydrogen peroxide is 2:1.

[0038] (3) The single-polished single-crystal silicon wafer prepared in step (2) is placed in an acetone solution of 3-aminopropyltrimethoxysilane, with the volume ratio of 3-aminopropyltrimethoxysilane to acetone being 1:10. After soaking in an argon atmosphere for 3 days, the wafer is taken out and washed with deionized water and ethanol in sequence, and dried to obtain a surface-modified single-crystal silicon wafer.

[0039] (3) Fitting

[0040] The washed COFs film is immersed in ethanol, and the COFs film is laminated to the front of the single-polished single-crystalline silicon wafer obtained in step (3) with the front side facing up. The COFs film is then removed from the solvent and dried. The washed COFs film is obtained by the following method: the COFs film is washed with ethanol, methanol and water in sequence.

[0041] (4) Fracture

[0042] Place the single-crystal silicon wafer obtained in step (3) with the front side facing up, press a force-bearing object (syringe needle) into the breaking groove, and then press down the single-crystal silicon wafers on both sides of the breaking groove to make the single-crystal silicon wafer and the COFs film break along the breaking groove together. By following this operation, a rectangular COFs film cross-section sample can be obtained.

[0043] With the COFs membrane in the COFs membrane cross-section sample facing the conductive adhesive on the SEM cross-section stage, the COFs membrane is attached to the conductive adhesive. At this time, the scanning electron microscope images of the COFs membrane cross-section in the COFs membrane cross-section sample at different magnifications are as follows: Figure 1 shown.

[0044] from Figure 1 It can be seen that the COFs film is at the same level as the edge of the silicon wafer in a large area; as the magnification increases further, the COFs film and the silicon wafer are tightly attached; as the magnification increases further, it can be found that the conductive adhesive, COFs film, and the edge of the single-polished single-crystal silicon wafer are basically at the same level; the cross-section of the COFs film is flat and dense, and its cross-sectional thickness can be clearly measured.

[0045] Comparative Example 1

[0046] The washed (cleaning method is the same as in Example 1) COFs membrane (COFs membrane is the same as in Example 1) was immersed in liquid nitrogen at -196°C for 5 seconds, and then both sides of the COFs membrane were clamped with tweezers and the membrane was broken in liquid nitrogen.

[0047] The broken COFs film was carefully attached to the conductive glue on the SEM cross-section stage with tweezers, and the SEM image was obtained as shown below. Figure 2 shown.

[0048] Since COFs films are very thin and difficult to handle, Figure 2 It can be seen that the membrane is in a bent or damaged state when observed under an electron microscope, and it is difficult to find a flat cross-section position for observation.

[0049] Comparative Example 2

[0050] The difference between Comparative Example 2 and Example 1 is that step (ii) surface modification is omitted, and the rest is exactly the same as Example 1.

[0051] The scanning electron microscope images of the COFs film cross section at different magnifications are shown in Figure 2. Figure 3 shown.

[0052] from Figure 3It can be seen that compared with Comparative Example 1, the adhesion between the COFs film and the single-polished single-crystalline silicon wafer has been greatly improved. It is basically flatly attached to the single-polished single-crystalline silicon wafer, and the fracture is flat; but compared with Example 1, the overall adhesion is not as good as Example 1, and some areas are convex (uneven), and cannot be completely adhered to the single-polished single-crystalline silicon wafer, which affects the SEM's selection of the COFs observation area and also affects the accuracy of the COFs thickness measurement.

[0053] The embodiment described above is only a preferred solution of the present invention and does not limit the present invention in any form. Other variations and modifications are possible without exceeding the technical solution described in the claims.

Claims

1. A method for preparing a cross-section sample of a self-supporting ionic COFs membrane, characterized in that: The following steps are involved: (1) Prefabricated breaking groove A rectangular single-polished single-crystal silicon wafer is taken, and a plurality of break grooves are provided at intervals on the back side of the single-polished single-crystal silicon wafer, wherein the break grooves are parallel to the cleavage plane of the single-polished single-crystal silicon wafer; (2) Surface modification (1) Rinse the single-crystal silicon wafer obtained in step (1) with deionized water, ethyl acetate, and ethanol successively, and then allow to air dry. (2) placing the single-crystal silicon wafer prepared in step (1) in a mixed solution of concentrated sulfuric acid and hydrogen peroxide, heating and then cooling, taking out the single-crystal silicon wafer, washing it with deionized water, and letting it stand to air dry; (3) placing the single-polished single-crystal silicon wafer in step (2) in an acetone solution of 3-aminopropyltrimethoxysilane, soaking it in an inert gas atmosphere, taking it out, washing it with deionized water and ethanol in sequence, and drying it to obtain a surface-modified single-polished single-crystal silicon wafer; (3) Fitting The cleaned COFs film is immersed in a liquid medium, and the single-polished single-crystal silicon wafer obtained in step (3) is placed with the front side facing upwards, so that the COFs film and the front side of the single-polished single-crystal silicon wafer are bonded together, and then the COFs film is removed from the solvent and dried; (4) Fracture Place the single-crystal silicon wafer obtained in step (3) with the front side facing up, place a force-bearing object in the breaking groove, and then press down the single-crystal silicon wafers on both sides of the breaking groove to make the single-crystal silicon wafer and the COFs film break along the breaking groove together. By following this operation, a rectangular COFs film cross-section sample can be obtained.

2. A method for preparing a cross-section sample of a self-supporting ionic COFs membrane according to claim 1, characterized in that: In step (1), the thickness of the single-throw monocrystalline silicon wafer is 0.2-0.8 mm.

3. The method for preparing a cross-section sample of a self-supporting ionic COFs membrane according to claim 2, characterized in that: In step (1), the thickness of the single-throw monocrystalline silicon wafer is 0.4-0.5 mm.

4. The method for preparing a cross-section sample of a self-supporting ionic COFs membrane according to claim 1, characterized in that: In step (1), the single-crystal silicon wafer is rinsed three times in sequence with deionized water, ethyl acetate, and ethanol.

5. The method for preparing a cross-section sample of a self-supporting ionic COFs membrane according to claim 1, characterized in that: In step (2), the volume ratio of concentrated sulfuric acid to hydrogen peroxide is 2:

1.

6. The method for preparing a cross-section sample of a self-supporting ionic COFs membrane according to claim 1, characterized in that: In step (2), the heating temperature is 100°C and the heating time is 5 hours.

7. The method for preparing a cross-section sample of a self-supporting ionic COFs membrane according to claim 1, characterized in that: In step (3), the volume ratio of 3-aminopropyltrimethoxysilane to acetone is 1:

10.

8. The method for preparing a cross-section sample of a self-supporting ionic COFs membrane according to claim 1, characterized in that: In step (3), the substrate is immersed in an inert gas atmosphere for 3 days, where the inert gas is nitrogen, argon or helium.

9. The method for preparing a cross-section sample of a self-supporting ionic COFs membrane according to claim 1, characterized in that: In step (iii), the cleaned COFs membrane is obtained by the following method: washing the COFs membrane with ethanol, methanol and water in sequence.

10. The method for preparing a cross-section sample of a self-supporting ionic COFs membrane according to claim 1, characterized in that: In step (3), the liquid medium is ethanol, water or methanol.

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