A method for evaluating the hydrophilicity of graphene films

Abstract

The present invention provides a method for evaluating the hydrophilicity of a graphene film, comprising the following steps: (1) placing a graphene film on a porous grid to obtain a graphene film / porous grid; (2) placing a water droplet on the surface of the graphene film of the graphene film / porous grid; (3) obtaining a morphology image of the water droplet on the surface of the graphene film through an environmental scanning electron microscope; and (4) calculating or directly measuring the contact angle θ of the water droplet through the morphology image. The method of one embodiment of the present invention uses a graphene film that is not disturbed by a substrate as an evaluation object, thereby eliminating the influence of the substrate on the measurement of the hydrophilicity of the graphene; and by using an environmental scanning electron microscope, a clearer morphology image of the water droplet on the surface of the graphene film can be obtained.

Description

Technical Field

[0001] The invention relates to a graphene film, in particular to a method for evaluating the intrinsic hydrophilicity of a graphene film. Background Art

[0002] Graphene is composed of a single layer of sp 2 Honeycomb-shaped two-dimensional atomic crystals composed of hybridized carbon atoms have many excellent physical and chemical properties, such as extremely high carrier mobility, excellent electrical conductivity, thermal conductivity, light transmittance, mechanical strength and good chemical stability, etc., and show broad application prospects in electronic devices, energy storage, thermal management, aerospace and biomedicine.

[0003] The detection and evaluation of the properties of graphene films have always been the focus of research in the field of graphene. The reason is that, on the one hand, the properties of graphene films will reflect the intrinsic information of the graphene structure, thereby providing an experimental reference for the study of the growth method of graphene films; on the other hand, different application fields will put forward specific requirements for the properties of graphene films, such as electrical conductivity, thermal conductivity, light transmittance, etc., which requires accurate characterization of the properties of graphene films. Among the many physical and chemical properties of graphene films, the study of hydrophilicity has always received widespread attention. The hydrophilicity of graphene films directly reflects the magnitude of the interaction potential energy between graphene and water molecules. According to the evaluation results of hydrophilicity, relevant structural information such as the defect density, doping degree and number of layers of graphene films can be obtained, thereby providing data support for the study of the intrinsic properties of graphene. More importantly, hydrophilicity has an important influence on the application of graphene films. Specifically, in the field of energy storage, it will affect the active material loading of graphene electrodes in lithium-ion batteries; in the field of thermal management, it will affect the heat exchange efficiency of graphene coatings in heat exchange devices; in the field of functional films, it will affect the ion permeability selectivity of graphene sieving membranes, etc. Therefore, the evaluation and regulation of the hydrophilicity of graphene films is the key to the transition of graphene films from laboratories to practical applications.

[0004] However, the intrinsic hydrophilicity of graphene films has always been controversial, and the measurement results of different literatures vary greatly, which shows that the evaluation method of the hydrophilicity of graphene films still needs to be improved. At present, static contact angle measurement is the main evaluation method of the hydrophilicity of graphene films. Specifically, a water drop is first dropped on the graphene surface, and the shape image of the water drop is obtained through an optical microscope. Then, digital image processing and related algorithms are used to obtain the contact angle of the water drop. The size of the contact angle reflects the hydrophilicity of the graphene film.

[0005] However, the above method has certain limitations. Generally speaking, the growth of graphene film needs to be carried out on a specific substrate, such as a metal substrate such as copper, nickel, or an insulating substrate such as glass, sapphire, etc. However, the above contact angle measurement method does not exclude the influence of the substrate on the hydrophilicity measurement of graphene. Specifically, the substrate will dope the graphene through charge transfer and other methods, thereby changing the hydrophilicity of the graphene itself. In addition, the ups and downs of the substrate itself will also directly affect the measurement results of the contact angle. Therefore, the construction of suspended graphene without substrate interference to measure the contact angle can truly evaluate the intrinsic hydrophilicity of the graphene film.

[0006] However, the commonly used contact angle measurement method is not suitable for evaluating the hydrophilicity of suspended graphene. The main reason is that the self-supporting area of ​​a single-layer graphene is limited, and the diameter of suspended graphene is only in the micrometer range. In the above contact angle measurement method, on the one hand, water droplets are usually added drop by drop using a pipette, and the diameter of the water droplets is generally in the millimeter range; on the other hand, the resolution of an optical microscope is relatively limited, and the method of optically photographing and measuring water droplets will have large errors. Summary of the invention

[0007] A main purpose of the present invention is to provide a method for evaluating the hydrophilicity of a graphene film, comprising the following steps:

[0008] (1) placing a graphene film on a porous grid to obtain a graphene film / porous grid;

[0009] (2) placing water droplets on the surface of the graphene film of the graphene film / porous support mesh;

[0010] (3) obtaining a morphology image of water droplets on the surface of the graphene film by an environmental scanning electron microscope; and

[0011] (4) The contact angle θ of the water droplet is obtained through the topography image.

[0012] The method of one embodiment of the present invention uses a graphene film that is not disturbed by a substrate as an evaluation object, thereby eliminating the influence of the substrate on the measurement of the hydrophilicity of graphene; at the same time, through the use of an environmental scanning electron microscope, a clearer morphology of water droplets on the surface of the graphene film can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1A A schematic diagram of a water droplet located on the surface of a suspended region of a graphene film according to an embodiment of the present invention;

[0014] Figure 1B This is a schematic diagram of a graphene film / porous grid covered with water droplets arranged obliquely according to one embodiment of the present invention;

[0015] Figure 1CFor the electron beam Figure 1B The vertical direction of the upper Figure 1B Projection of water droplets;

[0016] Figure 1D For Figure 1C Schematic diagram of the derivation of relevant trigonometric formulas;

[0017] Figure 2A This is a physical picture of a transmission electron microscope grid carrying a graphene film according to Example 1 of the present invention;

[0018] Figure 2B for Figure 2A Scanning electron microscope photo of graphene film;

[0019] Figure 2C is a Raman spectrum of the graphene film of Example 1 of the present invention;

[0020] Figure 3 This is an environmental scanning electron microscope photograph of a water drop on the graphene film of Example 1 of the present invention;

[0021] Figure 4 This is an environmental scanning electron microscope photograph of a large area of ​​water droplets on the graphene film of Example 1 of the present invention;

[0022] Figures 5A to 5C and Figures 5A1 to 5C1 Environmental scanning electron microscope photographs of water droplets on the graphene films of Examples 2-4 of the present invention. DETAILED DESCRIPTION

[0023] Typical embodiments that embody the features and advantages of the present invention will be described in detail in the following description. It should be understood that the present invention can have various changes in different embodiments without departing from the scope of the present invention, and the descriptions and illustrations therein are essentially used for illustration purposes rather than for limiting the present invention.

[0024] An embodiment of the present invention provides a method for evaluating the intrinsic hydrophilicity of a graphene film, comprising the following steps:

[0025] (1) placing a graphene film on a porous support mesh to obtain a porous support mesh carrying a graphene film (hereinafter referred to as "graphene film / porous support mesh");

[0026] (2) placing water droplets on the surface of the graphene film / porous support mesh;

[0027] (3) obtaining a morphology image of water droplets on the surface of the graphene film by environmental scanning electron microscopy; and

[0028] (4) The contact angle θ of the water droplet is calculated from the topography or directly measured. The hydrophilicity of the graphene film can be evaluated based on the value of the contact angle θ.

[0029] In one embodiment, the graphene film of step (1) does not contain a substrate (such as copper foil). For example, a graphene film with a substrate can be prepared by chemical vapor deposition, and then the graphene film is separated from its growth substrate by a non-adhesive transfer method to obtain a graphene film.

[0030] In one embodiment, the method of glue-free transfer includes: laminating a porous carrier mesh on the graphene / copper surface, dripping isopropyl alcohol on the porous carrier mesh so that the carrier mesh and graphene are tightly fitted, and then placing the carrier mesh / graphene / copper in an etching solution to etch the copper foil, and after etching, placing the carrier mesh / graphene in deionized water for cleaning.

[0031] In one embodiment, the porous support mesh for supporting the graphene film has a certain flexibility, and can be a porous membrane or a porous metal mesh.

[0032] In one embodiment, the maximum pore size of the porous mesh may be 50 μm.

[0033] In one embodiment, the pore size of the porous mesh can be 2 to 50 μm, such as 3 μm, 4 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 40 μm; the pore spacing of the porous mesh can be up to 2 μm, such as 1 to 2 μm.

[0034] In one embodiment, the porous membrane may be a carbon membrane, such as an amorphous carbon membrane; the pore size of the porous membrane may be 2 μm, and the pore spacing may be 1 μm or 2 μm.

[0035] In one embodiment, the porous metal mesh may be a porous gold mesh or a copper mesh; the diameter of the porous metal mesh may be 3 mm, the pore size may be 2 μm, and the pore spacing may be 1 μm or 2 μm.

[0036] In one embodiment, the porous grid may be an existing transmission electron microscope grid, such as a commercial transmission electron microscope grid.

[0037] In one embodiment, step (1) comprises:

[0038] Providing a "graphene film / substrate" structure; and

[0039] The graphene film is transferred from the growth substrate to the transmission electron microscope grid by a glue-free transfer method to obtain a graphene film / porous grid;

[0040] The surface of the graphene film on the porous grid is clean, free from substrate interference, and includes a suspended area.

[0041] In one embodiment, the diameter of the water droplets on the surface of the graphene film is 1-20 μm.

[0042] In one embodiment, the diameter of the water drop is smaller than the diameter of the suspended area of ​​the graphene film to avoid the influence of the non-suspended area on the contact angle θ measurement; wherein the suspended area refers to the area of ​​the graphene film corresponding to the holes of the porous mesh, where the graphene film is in a suspended state; the non-suspended area refers to the area of ​​the graphene film corresponding to the connecting part between the holes on the porous mesh. In other words, the diameter of the water drop is smaller than the pore size of the porous mesh.

[0043] In one embodiment, the water droplet is disposed on the suspended region of the graphene film.

[0044] In one embodiment, step (2) includes: placing the graphene film / porous grid in a chamber of an environmental scanning electron microscope, and forming water droplets on the surface of the graphene film by liquefying water vapor.

[0045] In one embodiment, the graphene film / porous grid is tilted in the chamber of the environmental scanning electron microscope to facilitate the measurement of the contact angle θ; wherein the tilt angle of the graphene film / porous grid can be 50 to 90°, for example, 60°, 70°, 75°, 80° or 85°. The tilt angle refers to the angle between the graphene film and the horizontal plane.

[0046] In one embodiment, the pressure of water vapor in the chamber of the environmental scanning electron microscope can be 0 to 6 torr, such as 0.5 torr, 1 torr, 2 torr, 4 torr or 5 torr. By adjusting the pressure of water vapor in the chamber of the environmental scanning electron microscope, water droplets can be uniformly enriched over a large area on the surface of the suspended graphene film.

[0047] In one embodiment, a graphene film / porous grid is fixed to a sample stage having a certain tilt angle β, and the sample stage is placed in a chamber of an environmental scanning electron microscope; wherein the tilt angle β of the sample stage can be 50 to 90°, for example, 60°, 70°, 75°, 80° or 85°. The tilt angle β refers to the angle between the graphene film and the horizontal plane.

[0048] In one embodiment, the sample stage is made of metal.

[0049] In one embodiment, the temperature of the sample stage affects the measurement of the contact angle θ of the water droplet. Too low a temperature will cause the water droplet to freeze, and too high a temperature will affect the liquefaction of water vapor and the resolution of the water droplet. The temperature of the sample stage or the temperature of the graphene film / porous grid on the sample stage can be -6 to 6°C, such as -4°C, -2°C, 0°C, 2°C or 4°C. By adjusting the temperature of the sample stage, the water vapor can be liquefied to achieve the enrichment of micrometer-scale water droplets on the graphene surface.

[0050] In one embodiment, step (2) includes: in order to facilitate the measurement of the contact angle θ, the graphene film / porous grid is fixed to a sample stage with a certain tilt angle β, and the sample stage is placed in a vacuum chamber of an environmental scanning electron microscope. The vacuum chamber is evacuated to a low vacuum, and then water vapor with controllable pressure is introduced into the vacuum chamber through a specific probe, and the temperature of the sample stage is adjusted to achieve large-area uniform enrichment of water droplets on the surface of the suspended area of ​​the graphene film.

[0051] In one embodiment, step (3) includes: by adjusting parameters such as the acceleration voltage and working distance of the environmental scanning electron microscope, a relatively clear morphology of liquid water droplets on the surface of the suspended area of ​​the graphene film is obtained, and the diameter of the water droplets is in the micrometer range.

[0052] In one embodiment, the acceleration voltage of the environmental scanning electron microscope may be 5-30V, such as 10V, 15V, 20V, 25V or 30V; the working distance may be 4-20mm, such as 6mm, 8mm, 10mm or 15mm.

[0053] Figures 1A to 1C Schematic diagram of the principle of measuring the contact angle θ of a water droplet on a suspended graphene film using an environmental scanning electron microscope. Figure 1A Schematic diagram of the state of a water drop on a horizontally placed suspended graphene film. Figure 1B Schematic diagram of the state of a water droplet when the graphene film / porous grid is set at an inclined angle β. Figure 1C for Figure 1B Projection of water droplets; Figure 1D For Figure 1C Schematic diagram of the derivation of relevant trigonometric function formulas.

[0054] In one embodiment, step (4) includes: directly measuring the contact angle θ of the water droplet in the environmental scanning electron microscope photograph to obtain the value of θ.

[0055] In another embodiment, step (4) comprises: measuring Figure 1C The characteristic lengths a, b, c of the water drop in the projection are calculated by combining the value of the tilt angle β and the trigonometric formula Calculate the contact angle θ.

[0056] according to Figure 1D From the results of a, b, and c, we can deduce that a / b=(1-cosθsinβ) / (sinθcosβ), b / c=cosβ / 2, thus obtaining the above trigonometric function formula: (ab) / c=(1-sin(θ+β)) / sinθ.

[0057] Reference Figure 1CAs shown, the projection of the water drop can be regarded as the splicing of two coaxial semi-ellipses, a and b are the short semi-axes of the two semi-ellipses, and c is the common long axis of the two semi-ellipses.

[0058] In one embodiment, the characteristic lengths a, b, and c of the water droplets in the environmental scanning electron microscope photograph may be measured by digital image processing software.

[0059] In step (4) of one embodiment, environmental scanning electron microscope photos of different magnifications may be taken at different regions to facilitate subsequent statistical analysis of the water drop contact angle θ.

[0060] In one embodiment, step (4) includes: taking environmental scanning electron microscope photos of the distribution of water droplets at different positions of the suspended area of ​​the graphene film, combining digital image processing software and related formulas to calculate the contact angle θ of the water droplets, and performing statistical analysis on the contact angle θ values ​​of different areas to obtain the intrinsic hydrophilicity of the graphene film.

[0061] In one embodiment, the statistical analysis includes calculating the contact angles of all water droplets in a certain area and calculating an average value to obtain a statistical analysis value of the contact angle θ.

[0062] A method for evaluating the intrinsic hydrophilicity of a graphene film according to an embodiment of the present invention comprises step (5), wherein step (5) comprises removing water droplets on the surface of the suspended graphene film.

[0063] In one embodiment, step (5) includes: stopping the introduction of water vapor into the environmental scanning electron microscope chamber to increase the vacuum degree of the chamber, thereby removing water droplets on the surface of the suspended area of ​​the graphene film; wherein the vacuum degree in the chamber can be increased to, for example, less than 10 -4 Pa.

[0064] The method for evaluating the hydrophilicity of a graphene film according to one embodiment of the present invention uses an environmental scanning electron microscope to obtain a morphology image of water droplets on the surface of the graphene film. The advantage of this method is that, on the one hand, water vapor can be introduced into the vacuum chamber through a probe, and the water vapor can be liquefied by adjusting the temperature of the sample stage to achieve the enrichment of micrometer-sized water droplets on the graphene surface; on the other hand, compared with an optical microscope, the method of an environmental scanning electron microscope has a larger magnification and a higher resolution, and can achieve clear imaging of micrometer-sized water droplets, thereby effectively solving the problem of difficulty in measuring the contact angle of micrometer-sized water droplets.

[0065] A method for evaluating the hydrophilicity of a graphene film according to an embodiment of the present invention uses a suspended graphene film that is not disturbed by a substrate as an evaluation object, and achieves large-area uniform enrichment of water droplets on the surface of the suspended graphene by regulating the water vapor pressure in the vacuum chamber of an environmental scanning electron microscope; with the help of high-resolution image acquisition by an environmental scanning electron microscope, statistical results of the contact angle of water droplets on the surface of the suspended area of ​​the graphene film in a certain area can be obtained based on digital image processing, thereby achieving an evaluation of the intrinsic hydrophilicity of the graphene film.

[0066] The method of one embodiment of the present invention is of great significance for clarifying the intrinsic properties of graphene films, regulating the surface properties of graphene films, and promoting the commercial application of graphene films. At the same time, the method can also be extended to the measurement of the intrinsic hydrophilicity of other two-dimensional materials, which is helpful to promote the characterization and evaluation of the properties of two-dimensional materials.

[0067] The following is a further description of the method for evaluating the hydrophilicity of a graphene film according to one embodiment of the present invention in conjunction with the accompanying drawings and specific examples. The methods used are conventional methods unless otherwise specified, and the raw materials involved can be obtained from public commercial channels unless otherwise specified.

[0068] Example 1

[0069] The intrinsic hydrophilicity of graphene films was evaluated by the following steps:

[0070] Step 1: Take a sample of graphene film grown on a copper metal substrate and transfer the graphene film from the copper substrate to a commercial transmission electron microscope grid (aperture 4 μm) by a non-adhesive transfer method. See the actual picture. Figure 2A , the graphene film transferred to the transmission electron microscope grid was characterized, see Figure 2B , 2C ; Among them, the method of glue-free transfer is: stick the electron microscope grid to the surface of graphene / copper foil, flatten it with a glass sheet, and then add a drop of isopropyl alcohol on the grid. Due to the surface tension of isopropyl alcohol, it will make the grid and graphene fit tightly after evaporation, and then place the grid / graphene / copper in 0.1mol / L Na2S2O8 etching solution to etch the copper foil. After etching, the grid / graphene is placed in deionized water for cleaning 3 times and then dried.

[0071] Step 2: Fix the transmission electron microscope grid with the transferred graphene film on a sample stage with a certain tilt angle β with copper tape, and fix the sample stage on the test table of the environmental scanning electron microscope sample chamber with conductive silver glue; wherein β is 70°.

[0072] Step 3: Pump down the sample chamber of the environmental scanning electron microscope to a pressure of <10 -4Pa, set the working parameters of the environmental scanning electron microscope, specifically, the working voltage is adjusted to 25V, the working distance is adjusted to 6mm, the sample stage temperature is set to 0℃, and after the temperature of the sample stage drops to the set temperature, a certain amount of water vapor is introduced into the electron microscope chamber through a specific probe, and the water vapor pressure is set to 6torr.

[0073] Step 4: Obtain a clearer electron microscope photo of the water droplet on the surface of the suspended area of ​​the graphene film by focusing, and take photos of different magnifications in different areas to facilitate subsequent statistical analysis of the water droplet contact angle.

[0074] Step 5: After the shooting is finished, stop introducing water vapor into the chamber and set the vacuum degree of the chamber to a pressure of <10 -4 Pa, and after the water droplets on the graphene film are completely evaporated, the vacuum is broken to take samples.

[0075] Step 6: Use digital image processing software to measure the characteristic lengths a, b, and c of water droplets in environmental scanning electron microscope photos, and use relevant trigonometric functions to calculate the characteristic lengths a, b, and c of water droplets in environmental scanning electron microscope photos. The contact angle θ of the water drop can be calculated by combining the tilt angle β of the sample stage, and the intrinsic hydrophilicity of the graphene film can be obtained by statistically analyzing the contact angles of water droplets in different regions. Among them, A = 1.08 μm, B = 0.40 μm, C = 4.34 μm, β = 70°.

[0076] Figure 4 This is the distribution result of large-area water droplets on the suspended area of ​​the graphene film. There are 9 water droplets that can be counted in this area (completely in the suspended area of ​​the graphene, some large water droplets falling in the non-suspended area are not counted), and the contact angles are 49°, 45°, 49°, 50°, 51°, 47°, 48°, 45°, and 46°, respectively. The average value is 48°±2. It can be seen that the shapes and contact angles of water droplets at different positions on the graphene film in this area are relatively close, and the water droplets are evenly distributed, indicating that the graphene film in this area is hydrophilic.

[0077] pass Figure 2B From the scanning electron microscope photos, we can see that the integrity of the graphene film after transfer is high, and the damage to the graphene film during the transfer process is small; Figure 2C The Raman spectroscopy characterization results show that the graphene film has no obvious defects.

[0078] Figure 3 This is a typical environmental scanning electron microscope photo of a water drop on the suspended area of ​​the graphene film. It can be seen that under appropriate working parameters, a clear water drop image can be seen on the suspended area of ​​the graphene film, and the contact angle of the water drop is 48° (calculated according to the aforementioned trigonometric formula, A = 1.08μm, B = 0.40μm, C = 4.34μm, β = 70°).

[0079] In 2018, Schneider et al. of Leiden University measured the contact angle of suspended graphene by liquid bubbling method (Nat. Commun. 9, 4185 (2018)), and the measurement result was 43°±2. It can be seen that the contact angle of the suspended area of ​​the graphene film measured by the embodiment of the present invention is close to the result of the literature, but the method of the present invention is more convenient in operation, and can realize the imaging of multiple water droplets in a large area and the analysis and calculation of the contact angle.

[0080] In addition, experiments show that when β is between 50 and 90°, the measured contact angle of the graphene film does not change much.

[0081] Embodiment 2-4

[0082] The same steps, equipment, parameters, etc. as those in Example 1 were used, except that the temperatures of the sample stage were -2°C, 0°C, and 2°C, respectively. The comparison results are shown in FIG. Figures 5A to 5C as well as Figures 5A1 to 5C1 As shown, it can be seen that a clear water drop image can be formed within this temperature range. According to the aforementioned trigonometric function formula, the contact angles of the suspended areas of the graphene film are calculated to be 52°, 51°, and 51°, respectively.

[0083] Unless otherwise defined, the terms used in the present invention have the meanings commonly understood by those skilled in the art.

[0084] The embodiments described in the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Those skilled in the art may make various other substitutions, changes and improvements within the scope of the present invention. Therefore, the present invention is not limited to the above embodiments, but is only limited by the claims.

Claims

1. A method for evaluating the hydrophilicity of a graphene film, comprising the following steps: (1) placing a graphene film on a porous grid to obtain a graphene film / porous grid; (2) placing water droplets on the surface of the graphene film of the graphene film / porous support mesh; (3) obtaining a morphology image of water droplets on the surface of the graphene film by an environmental scanning electron microscope; and (4) calculating or directly measuring the contact angle θ of the water droplet through the topography image; in, The step (2) comprises: placing the graphene film / porous grid in a chamber of an environmental scanning electron microscope, and forming water droplets on the surface of the graphene film by liquefying water vapor; the graphene film / porous grid is tiltedly arranged in the chamber of the environmental scanning electron microscope, and its tilt angle β is 50 to 90°, and the tilt angle β refers to the angle between the graphene film and the horizontal plane; The electron beam is vertically irradiated above the inclined graphene film to obtain a projection image of the water droplet, wherein the projection image is formed by splicing two coaxial semi-ellipses, a and b are the short semi-axes of the two coaxial semi-ellipses, and c is their common long axis; Measure the lengths of a, b, and c, and use the formula The value of the contact angle θ is obtained by calculation.

2. The evaluation method according to claim 1, wherein: The porous carrier mesh is a porous membrane or a porous metal mesh, and the pore size of the porous carrier mesh is less than or equal to 50 μm.

3. The evaluation method according to claim 1, wherein: The graphene film includes a suspended area, and the suspended area is arranged corresponding to the holes of the porous carrier mesh.

4. The evaluation method according to claim 1, wherein: In the chamber, the pressure of water vapor is 0-6 torr.

5. The evaluation method according to claim 3, wherein: The water drop is disposed in the suspended area.

6. The evaluation method according to claim 1, wherein: The graphene film / porous grid is located on a sample stage, and the temperature of the sample stage is -6 to 6°C; And / or, the diameter of the water droplet is smaller than the pore size of the porous carrier mesh.

7. The evaluation method according to claim 1, wherein: The diameter of the water droplets is 1 to 20 μm.

8. The evaluation method according to claim 1, wherein: The acceleration voltage of the environmental scanning electron microscope is 5-30V, and the working distance is 4-20mm.

9. The evaluation method according to claim 1, wherein: The contact angles θ of multiple water droplets in a certain area of ​​the graphene film are calculated, and the average value is calculated to obtain a statistical analysis value of the contact angle θ.

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