Method for preparing highly oriented and highly reflective graphene film by vapor pressure assisted stretching and reduction

Abstract

The present invention belongs to the field of materials science and engineering technology, and relates to a method for preparing a highly oriented and highly reflective graphene film by steam pressure-assisted stretching reduction. At the same time, isotropic stretching and steam treatment are carried out on the graphene oxide film. The steam treatment is to heat a mixed solution of an aqueous hydrogen iodide solution and ethanol to vaporization and then keep it at 90-120 °C for 6-24 h, and the volume ratio of the aqueous hydrogen iodide solution to ethanol is 1:2-5. In the reduction process of the present invention, the graphene film is subjected to isotropic tensile stress, effectively eliminating the wrinkles and ripples caused by thermal fluctuations and changes in layer spacing, and improving the flatness of the graphene sheets.

Description

Technical Field

[0001] The invention belongs to the field of material science and engineering technology, and relates to a method for preparing a high-oriented and high-reflectivity graphene film by steam pressure-assisted stretching and reduction. Background Art

[0002] As a new type of material, graphene macro-assembly film has shown great application potential in electronic devices, energy catalysis, thermal radiation management, aerospace and other fields due to its unique physical and chemical properties, such as light weight, high electrical conductivity, high thermal conductivity and good mechanical properties.

[0003] The flatness of graphene sheets, the close stacking between sheets, and the order of interlayer stacking in graphene macroscopic assembled films have a crucial impact on their performance. Specifically, the flatness of the sheets directly affects the in-plane transmission efficiency of quasiparticles (such as electrons and phonons) on the sheets, including the ballistic transmission and scattering transmission of electrons, and the ballistic fluctuation and scattering process of phonons. The close stacking between sheets has a significant impact on the inter-sheet transmission of quasiparticles, mainly involving the tunneling effect and thermal excitation transmission mechanism of electrons, as well as the coupled transmission and interface scattering process between phonons. If the stacking domain is disordered or there are a large number of dislocations, electrons and phonons will encounter frequent interface scattering and defect scattering during transmission, which will not only hinder the effective transmission of quasiparticles, but also introduce potential barriers in the material, thereby reducing the electrical and thermal conductivity of the graphene macroscopic assembled film. Therefore, achieving high flatness of graphene sheets, close stacking between sheets, and ordered orientation of stacking domains is the key to improving the mechanical properties, electrical conductivity, and thermal conductivity of graphene macroscopic assembled films.

[0004] Traditional methods for preparing graphene macroscopic assembled films, such as vacuum filtration, pouring evaporation, scraping, spraying, and layer-by-layer self-assembly, usually face many challenges, such as wrinkles between graphene sheets. When wrinkles exist in graphene sheets, the order of the sheet stacking domain will be destroyed, which will make it difficult for the sheets to be tightly stacked, resulting in disordered orientation of the sheets, which ultimately manifests as insufficient surface flatness of the film.

[0005] In order to solve the defects of traditional technology, patent application CN111807355B first obtains a self-supporting GO film by vacuum filtration of a GO (graphene oxide) solution, and then immerses the GO film in ethanol for orientation under orthogonal biaxial external force traction, and then places it in a mixed solution of hydroiodic acid and ethanol for chemical reduction, thereby obtaining a graphene macroscopic assembly film. Patent application CN112174120A discloses a method for preparing a high-strength and high-conductivity graphene film, which adds a chemical cross-linking step on the basis of patent application CN111807355B to obtain a high-strength and high-conductivity graphene macroscopic assembly film. However, the flatness of the graphene sheets of the graphene macroscopic assembly films of the above two patents needs to be further improved. Summary of the Invention

[0006] The object of the present invention is to solve the problems existing in the prior art and provide a method for preparing a highly oriented and highly reflective graphene film by steam pressure-assisted stretching and reduction.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing a highly oriented and highly reflective graphene film by steam pressure-assisted stretching and reduction, which simultaneously performs isotropic stretching and steam treatment on a graphene oxide film. The steam treatment is to heat a mixture of an aqueous hydrogen iodide solution and ethanol to vaporization and then keep it at 90 - 120 °C for 6 - 24 h. The volume ratio of the aqueous hydrogen iodide solution to ethanol is 1:2 - 5;

[0009] The thickness of the graphene oxide film is 5 - 100 μm;

[0010] When the thickness of the graphene oxide film ≤ 10 μm, the device for simultaneously performing isotropic stretching and steam treatment on the graphene oxide film includes a solvent pool a and a heating device a. The top of the solvent pool a is a vertically arranged cylinder a, which is divided into upper and lower layers and is detachably connected by 2m membrane fixing screws a, m > 2, and the 2m membrane fixing screws a are circumferentially and evenly distributed around the central axis of the cylinder a; the heating device a is used to heat the solvent pool a;

[0011] When the thickness of the graphene oxide film > 10 μm, the device for simultaneously performing isotropic stretching and steam treatment on the graphene oxide film includes a solvent pool b, a heating device b, a deformable tube, and a regulator; the bottom of the deformable tube is hermetically connected to the top of the solvent pool b, the top of the deformable tube is a vertically arranged cylinder b, which is divided into upper and lower layers and is detachably connected by 2m membrane fixing screws b, m > 2, and the 2m membrane fixing screws b are circumferentially and evenly distributed around the central axis of the cylinder b; the regulator is used to control the radial outward expansion of the cylinder b; the heating device b is used to heat the solvent pool b.

[0012] As Figure 8 shown, during the heat preservation process, hydrogen iodide vapor performs nucleophilic reduction on the graphene oxide film fixed above, while the vapor pressure generated by ethanol vapor exerts an outward pressure on the graphene oxide film. The vapor pressure acts uniformly on the surface of the graphene oxide film. The graphene oxide film is subjected to isotropic tensile stress while being reduced. As the reduction reaction proceeds, the wrinkles and ripples generated due to thermal fluctuations and changes in layer spacing are eliminated, and gases such as carbon dioxide generated due to deoxidation are extruded during the stretching process. At the same time, ethanol molecules enter the film and play a plasticizing role, further releasing some wrinkles and folds. Therefore, the stacking orderliness between graphene sheets increases, the size and packing density of the lamellar stacking domain increase, thereby improving the flatness and orientation of the graphene film.

[0013] The volume ratio of the aqueous hydrogen iodide solution to ethanol is 1:2 - 5. As the volume ratio of the aqueous hydrogen iodide solution to ethanol changes, the ratio of the reducing hydrogen iodide vapor and ethanol vapor generated during the reduction process changes. The change in the content of hydrogen iodide vapor can adjust the reduction degree of the graphene oxide film, and the change in the ethanol vapor ratio can adjust the stress applied to the film surface. If the volume fraction of ethanol is too high, it will lead to a decrease in the relative content of hydrogen iodide, thereby reducing the reduction efficiency. It will also increase the vapor pressure during the reduction process, which may cause excessive expansion or contraction on the surface and inside of the film, and ultimately may lead to cracking or other deformations of the film. In addition, it may also cause uneven distribution of iodide ions on the film surface, affecting the uniformity of reduction inside the film, thereby damaging the overall quality of the film. If the volume fraction of ethanol is too low, it will lead to insufficient vapor pressure generated, the stress on the film surface will decrease, and it is difficult to fully inhibit the shrinkage caused by the reduction of the film.

[0014] The heat preservation temperature is 90 - 120 °C. As the heat preservation temperature increases, the reaction rate and reduction uniformity of the reduction of hydrogen iodide vapor increase, and the vapor pressure of ethanol vapor and the uniformity of the stress distribution generated on the film plane increase. If the heat preservation temperature is too high, it will accelerate the evaporation rate of the vapor, resulting in uneven stress distribution generated by the vapor inside the film, which may cause thermal stress damage and ultimately cause deformation or even cracking of the film. In addition, in a high-temperature environment, adverse phenomena such as sintering and cracking may also occur on the graphene film. If the heat preservation temperature is too low, it will lead to a decrease in the concentration of iodide ions escaping from the film, affecting the full reduction of the film. At the same time, the vapor pressure of ethanol will also decrease correspondingly, unable to apply sufficient stress to the film surface, thereby affecting the reduction effect and final quality of the film.

[0015] The heat preservation time is 6 - 24 h. Adjusting the heat preservation time can adjust the degree of vapor stretching reduction of the graphene oxide film. If the heat preservation time is too long, the stress of the film at high temperature will gradually accumulate, which may cause cracks or other deformations inside or on the surface of the film, thereby affecting the mechanical properties of the film. At the same time, a long reaction time will increase the chance of side reactions involving oxygen or other gas components in the use environment, thereby causing deterioration of the film. In addition, too long a heat preservation time will also increase the process cost. If the heat preservation time is too short, it will lead to uneven and insufficient reduction of the film, and the regulating effect of the ethanol vapor stress cannot be fully exerted, resulting in uneven stress distribution on the film surface and easy generation of micro-cracks and uneven deformations.

[0016] As a preferred technical solution:

[0017] For a method for preparing a highly oriented and highly reflective graphene film by vapor pressure-assisted stretching reduction as described above, the regulator includes a displacement bracket, a pressing plate, an upper bracket, a limiting structure, and a lower bracket;

[0018] The displacement support is a conical cap-shaped structure with the tip facing upward, including 2n inclined rods I, where n > 2. The upper ends of the 2n inclined rods I are simultaneously connected to the center of the lower surface of the pressing plate, and the lower ends are evenly distributed in a circular pattern around a point o, which is located on the central axis of the pressing plate. The pressing plate is coaxial with the cylinder b;

[0019] The upper-layer support includes 2n horizontal rods, which are radially distributed around the point o. The two ends of the horizontal rods are respectively denoted as the a-end and the b-end, and the a-end is closer to the point o than the b-end. The a-ends of the 2n horizontal rods are respectively hinged to the lower ends of the 2n inclined rods I in a one-to-one correspondence;

[0020] The limiting structure restricts the 2n horizontal rods to move only along their own length directions;

[0021] The b-ends of adjacent 2 horizontal rods are each connected by 1 arc-shaped bar. At least one group of opposite arc-shaped bars consists of three sections: left, middle, and right. The middle section consists of a separated inner layer and outer layer. The outer layer of the middle section is fixedly connected to the left section, and the inner layer of the middle section is fixedly connected to the right section; all the arc-shaped bars enclose a ring, and the ring extends vertically downward to form a circular tube;

[0022] The lower-layer support includes 2n inclined rods II, which are distributed in an umbrella shape around the central axis of the cylinder b. The angle between the inclined rods II and the central axis of the cylinder b is 100 - 105°. The lower layer of the cylinder b extends radially outward and then is connected to the upper ends of the 2n inclined rods II. The lower ends of the 2n inclined rods II are connected to the circular tube.

[0023] For a method for preparing a highly oriented and highly reflective graphene film by steam pressure-assisted stretching reduction as described above, the regulator further includes a height measurement support, which is vertically arranged on one side of the pressing plate and fixedly connected to the upper-layer support.

[0024] For a method for preparing a highly oriented and highly reflective graphene film by steam pressure-assisted stretching reduction as described above, the graphene oxide film is prepared by a suction filtration method, a casting method, or a spin coating method.

[0025] For a method for preparing a highly oriented and highly reflective graphene film by steam pressure-assisted stretching reduction as described above, the grain size height (i.e., the height of the ordered stacking domain) of the highly oriented and highly reflective graphene film is 3.42 - 6 nm, the proportion of the ordered stacking domain exceeds 70%, the full width at half maximum of the XRD diffraction peak is less than 3°, Herman's orientation parameter S is greater than 0.6, the surface roughness (within 30 μm) is lower than 15 nm, the surface reflectance (within the range of 480 - 1050 nm) is not less than 25%, and the conductivity is 2×10 4 -2.5×10 5 S·m -1 , and the tensile strength exceeds 330 MPa.

[0026] Beneficial effects:

[0027] (1) The present invention prepares a graphene film by a method of steam pressure-assisted stretching and reduction. During the reduction process, the graphene film is subjected to isotropic tensile stress, effectively eliminating the wrinkles and ripples caused by thermal fluctuations and changes in layer spacing, improving the flatness of the graphene sheets. At the same time, gases such as carbon dioxide generated by deoxidation are extruded during the stretching process, increasing the stacking order between the graphene sheets, and improving the size and packing density of the lamellar stacking domain.

[0028] (2) The graphene film prepared by the present invention has the advantages of high orientation, high reflectivity, high conductivity, high tensile strength and low surface roughness, and has a broader application prospect in the fields of electronic devices, energy catalysis, thermal radiation management, aerospace and so on. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 X-ray diffraction data of the graphene films of Example B1 and Comparative Examples 2-4 (corresponding Figure 1 to a in Figure 1 and the data of a are obtained by testing with a D8 Advance type X-ray diffractometer, where the tube voltage is set to 40 kV and the tube current is set to 40 mA) and a comparison diagram of the ordered stacking domain area, layer spacing, and ordered stacking domain height (corresponding Figure 1 to b in

[0030] Figure 2 A SEM comparison diagram of the graphene films of Example B1 and Comparative Examples 2-4; among them, Figure 2 a in Figure 2 is the surface of the graphene film of Comparative Example 2, Figure 2 e in Figure 2 is the cross-section of the graphene film of Comparative Example 2, Figure 2 b in Figure 2 is the surface of the graphene film of Comparative Example 3, Figure 2 f in Figure 2 is the cross-section of the graphene film of Comparative Example 3,

[0031] Figure 3 A comparison diagram of the visible-near infrared band reflectivity of the graphene films of Example B1 and Comparative Examples 2-4;

[0032] Figure 4 A top view of the isotropic stretching and steam treatment device in Example A1 of the present invention;

[0033] Figure 5 Three-dimensional comparison diagrams of the surface morphologies of the graphene films of Example B1 and Comparative Examples 2 to 4;

[0034] Figure 6 Root mean square data comparison diagrams of the graphene films of Example B1 and Comparative Examples 2 to 4;

[0035] Figure 7 Image standard deviation curve comparison diagrams of the graphene films of Example B1 and Comparative Examples 2 to 4;

[0036] Figure 8 Schematic diagram of the topological structure change of the graphene oxide film lamellae before and after the steam pressure-assisted stretching and reduction process;

[0037] Figure 9 Schematic diagram of the isotropic stretching and steam treatment device in Example A1 of the present invention;

[0038] Figure 10 Schematic diagram of the isotropic stretching and steam treatment device in Example A2 of the present invention; In the figure, 45° is the initial angle; S is the length of the inclined rod I of the displacement bracket; H is the initial height of the displacement bracket, that is, the vertical distance between the pressing disc and the upper bracket; △H is the maximum pressing displacement; △L is the maximum stretching displacement;

[0039] Figure 11 Top view of the regulator in Example A2 of the present invention;

[0040] Figure 12 Side view of the middle section of the circular arc bar in the upper bracket of the present invention;

[0041] Among them, 1 - solvent pool a, 2 - cylinder a, 3 - film fixing screw a, 4 - circular arc bar, 5 - solvent pool b, 6 - height measuring bracket, 7 - limiting structure, 8 - film fixing screw b, 9 - deformable tube, 10 - inclined rod I, 11 - pressing disc, 12 - horizontal rod, 13 - inclined rod II, 14 - cylinder b. Specific embodiments

[0042] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0043] The following are the test methods for the relevant performance indicators in each example and comparative example:

[0044] Grain size height: According to the Scherrer formula (L c= Kλ / (βcosθ)), substitute the Scherrer constant K (the value of K is taken as 0.9), the X-ray wavelength λ, the angle θ of the diffraction peak of the ordered stacking domain, and the full-width at half-maximum β of the XRD diffraction peak to calculate the grain size height L c .

[0045] Proportion of ordered stacking domain: Fit the diffraction peak to obtain the diffraction peak of the ordered stacking domain and the diffraction peak of the disordered stacking domain, and calculate the proportion of the area of the diffraction peak of the ordered stacking domain in the entire diffraction peak to obtain the proportion of the ordered stacking domain.

[0046] Full-width at half-maximum of the XRD diffraction peak: According to the Bragg's law equation (nλ = 2dsinθ), substitute the X-ray wavelength λ, the diffraction angle θ, and the diffraction order n (usually 1) to calculate the interlayer spacing d (between 0.36 - 0.37 nm); the difference in the diffraction angle 2θ at half of the diffraction peak peak value is the full-width at half-maximum of the XRD diffraction peak.

[0047] Herman's orientation parameter S: Obtain the scattering image from the small-angle X-ray scattering instrument (SAXS) at the BL16B1 beamline of the Shanghai Synchrotron Radiation Facility, and obtain the scattering intensity distribution of the diffraction spot. After processing the scattering intensity distribution with the azimuth angle φ as the variable, obtain the scattering intensity I(φ), and obtain the weighted average <cos 2 (φ)> of the scattering intensity at different azimuth angles through integration or summation. According to the Herman's orientation parameter calculation formula (S = (3<cos 2 φ> - 1) / 2), obtain the orientation parameter S.

[0048] Surface roughness: Obtain the three-dimensional surface topography map of the graphene film by testing with an atomic force microscope (Bruker Corporation, model: Dimension FastScan / Icon), and obtain the root mean square roughness (i.e., surface roughness) data and the image standard deviation curve through software analysis and processing. The entire testing process is carried out within a range of 30 μm.

[0049] Surface reflectance: Test the surface reflectance of the graphene film with a microscopic spectroscopy system (Shanghai Optics Co., Ltd., spectrometer detection module: PG2000-Pro spectrometer), and the test wavelength range is 480 - 1050 nm.

[0050] Conductivity: Measure the conductivity of the graphene film with a four-probe conductivity tester.

[0051] Tensile strength: Test the tensile strength of the graphene film with an electronic universal material testing machine (INSTRON 5969).

[0052] The preparation process of the graphene oxide aqueous solution used in each of the following examples is as follows: Graphene oxide (purity ≥ 99%, carbon-oxygen ratio of 2 - 3:1, thickness < 5 nm) is uniformly dispersed in water by stirring and ultrasonic methods to obtain a graphene oxide aqueous solution with a specific concentration.

[0053] Example A1

[0054] The device for simultaneously performing isotropic stretching and steam treatment on a graphene oxide film with a thickness ≤ 10 μm, as Figure 9 、 Figure 4 shown, includes a solvent pool a1 and a heating device a;

[0055] The top of the solvent pool a1 is a vertically arranged cylinder a2. The cylinder a2 is divided into upper and lower layers and is detachably connected by 2m membrane fixing screws a3, where m > 2, and the 2m membrane fixing screws a3 are circumferentially and uniformly distributed around the central axis of the cylinder a2;

[0056] The heating device a is used to heat the solvent pool a1.

[0057] Example A2

[0058] The device for simultaneously performing isotropic stretching and steam treatment on a graphene oxide film with a thickness > 10 μm, as Figure 10 、 Figure 11 shown, includes a solvent pool b5, a heating device b, a deformable tube 9, and a regulator;

[0059] The bottom of the deformable tube 9 is hermetically connected to the top of the solvent pool b5. The top of the deformable tube 9 is a vertically arranged cylinder b14. The cylinder b14 is divided into upper and lower layers and is detachably connected by 2m membrane fixing screws b8, where m > 2, and the 2m membrane fixing screws b8 are circumferentially and uniformly distributed around the central axis of the cylinder b14;

[0060] The heating device b is used to heat the solvent pool b5;

[0061] The regulator is used to control the radial outward expansion of the cylinder b14 along the cylinder b14;

[0062] The regulator includes a displacement bracket, a pressing disk 11, an upper bracket, a limiting structure 7, a lower bracket, and a height measuring bracket 6;

[0063] The displacement bracket is a conical cap-shaped structure with the tip upward, including 2n inclined rods I10, where n > 2. The upper ends of the 2n inclined rods I10 are simultaneously connected to the center of the lower surface of the pressing disk 11, and the lower ends are circumferentially and uniformly distributed around a point o, where the point o is located on the central axis of the pressing disk 11, and the pressing disk 11 is coaxial with the cylinder b;

[0064] The upper bracket includes 2n horizontal rods 12. The 2n horizontal rods 12 are radially distributed around point o. The two ends of the horizontal rod 12 are respectively denoted as the a end and the b end. The a end is closer to point o than the b end. The a ends of the 2n horizontal rods 12 are respectively and hinged to the lower ends of the 2n inclined rods I10 one by one;

[0065] The limiting structure 7 restricts the 2n horizontal rods 12 to move only along their own length directions;

[0066] Such as Figure 11 、 Figure 12 As shown, the b ends of adjacent 2 horizontal rods 12 are each connected by 1 arc bar 4. At least one group of opposite arc bars 4 consists of left, middle, and right sections. The middle section consists of a separated inner layer and outer layer. The outer layer of the middle section is fixedly connected to the left section, and the inner layer of the middle section is fixedly connected to the right section; all the arc bars 4 enclose a ring, and the ring extends vertically downward to form a circular tube;

[0067] The lower bracket includes 2n inclined rods II 13. The 2n inclined rods II 13 are distributed in an umbrella shape around the central axis of the cylinder b. The included angle between the inclined rod II 13 and the central axis of the cylinder b is 100 - 105°. The lower layer of the cylinder b extends radially outward and is connected to the upper ends of the 2n inclined rods II 13. The lower ends of the 2n inclined rods II 13 are connected to the circular tube.

[0068] Example B1

[0069] A method for preparing a highly oriented and highly reflective graphene film by vapor pressure assisted stretching and reduction, the specific steps are as follows:

[0070] (1) Preparation of raw materials;

[0071] Graphene oxide aqueous solution: the concentration is 1mg·g -1 ;

[0072] A mixed solution of hydrogen iodide aqueous solution and ethanol: the concentration of the hydrogen iodide aqueous solution (i.e., the mass percentage of hydrogen iodide in the total mass of hydrogen iodide and water) is 45wt%, and the volume ratio of the hydrogen iodide aqueous solution to ethanol is 1:3;

[0073] The device used for isotropic stretching and vapor treatment: the device described in Example A2, n is 4, m is 3, the height of the solvent pool b is 10cm, the volume is 96mL, and the included angle between the inclined rod II and the central axis of the cylinder b is 102°;

[0074] (2) Preparation of graphene oxide film;

[0075] After filtering the graphene oxide aqueous solution with a Celgard 3501 membrane by suction filtration, it is dried at 40°C for 24h, the filter membrane is removed, and a graphene oxide film with a thickness of 15μm is obtained;

[0076] (3) Prepare a highly oriented and highly reflective graphene film;

[0077] When performing isotropic stretching and steam treatment on the graphene oxide film prepared in step (2), first add 25 mL of a mixed solution of hydrogen iodide aqueous solution and ethanol to the solvent pool b, and then fix the graphene oxide film between the upper and lower cylinders b through the film fixing screw b and adjust the film fixing screw b to keep the graphene oxide film in a taut state. Then heat it through the heating device b until both hydrogen iodide and ethanol are vaporized. At the same time, press down the pressing plate and keep it at 90 °C for 24 h (the maximum tensile displacement of the graphene oxide film after heat preservation is 2 mm). After post-treatment (taking out the film, rinsing with ethanol, and drying in a vacuum oven at 60 °C), a highly oriented and highly reflective graphene film is obtained.

[0078] The height of the grain size of the finally prepared highly oriented and highly reflective graphene film is 5.83 nm, the proportion of the ordered stacking domain is 87%, the full width at half maximum of the XRD diffraction peak is 1.38°, the Herman's orientation parameter S is 0.7, the surface roughness is 13.9 nm, the surface reflectivity is 30%, and the conductivity is 2×10 5 S·m -1 , and the tensile strength is 580 MPa.

[0079] Comparative Example 1

[0080] A method for preparing a graphene film is basically the same as that in Example B1, except that: the mixed solution of hydrogen iodide aqueous solution and ethanol is replaced with an equal volume of hydrogen iodide aqueous solution (concentration: 45 wt%).

[0081] The height of the grain size of the finally prepared graphene film is 3.08 nm, the full width at half maximum of the XRD diffraction peak is 4.5°, the Herman's orientation parameter S is 0.59, the surface roughness is 30.3 nm, and the surface reflectivity is 20%.

[0082] Compared with Example B1, in Comparative Example 1, the flatness of the graphene sheets of the graphene film, the close packing between the sheets, and the ordered orientation of the packing domain are significantly deteriorated. This is mainly because only the hydrogen iodide aqueous solution is used in Comparative Example 1, and the high volatility of ethanol is not utilized to enhance the vapor pressure of the solution. Since hydrogen iodide is a strong acid, it will completely ionize in water, which reduces the evaporation tendency of water, resulting in a relatively low overall vapor pressure. The low vapor pressure cannot eliminate the wrinkles and ripples caused by thermal fluctuations and changes in layer spacing during the reduction process. When there are wrinkles, it will destroy the order of the sheet packing domain, making it difficult for the sheets to be closely packed, thus leading to the disordered orientation of the sheets, and finally resulting in insufficient flatness of the film surface.

[0083] Comparative Example 2

[0084] A method for preparing a graphene film is basically the same as that of Example B1, except that: the graphene oxide film is not fixed between the upper and lower cylindrical bodies b by the film fixing screw b, and the graphene oxide film is only placed on the top of the solvent pool a.

[0085] The height of the grain size of the finally prepared graphene film is 3.33 nm, the proportion of the ordered stacking domain is 65%, the full width at half maximum of the XRD diffraction peak is 2.41°, the Herman's orientation parameter S is 0.55, the surface roughness is 43.9 nm, and the surface reflectivity is 18%.

[0086] Compared with Example B1, in Comparative Example 2, the flatness of the graphene sheets of the graphene film, the close packing between the sheets, and the ordered orientation of the stacking domain are significantly worse. This is because after the reduction of the graphene oxide film without stretching, the shrinkage of the sheets leads to an increase in corrugations, folds, and wrinkles, and there are more wrinkles between the sheets, resulting in disordered stacking between the layers and a lower orientation degree of the graphene film, and finally manifested as insufficient flatness of the film surface.

[0087] Comparative Example 3

[0088] A method for preparing a graphene film is basically the same as that of Example B1, except that: m in the device described in Example A2 is 1.

[0089] The height of the grain size of the finally prepared graphene film is 2.21 nm, the proportion of the ordered stacking domain is 56%, the full width at half maximum of the XRD diffraction peak is 3.64°, the Herman's orientation parameter S is 0.5, the surface roughness is 32 nm, and the surface reflectivity is 20%.

[0090] Comparative Example 4

[0091] A method for preparing a graphene film is basically the same as that of Example B1, except that: m in the device described in Example A2 is 2.

[0092] The height of the grain size of the finally prepared graphene film is 6.03 nm, the proportion of the ordered stacking domain is 62%, the full width at half maximum of the XRD diffraction peak is 1.33°, the Herman's orientation parameter S is 0.6, the surface roughness is 22.7 nm, and the surface reflectivity is 27%.

[0093] Comparing Comparative Example 3 and Comparative Example 4 with Example B1, the flatness of the graphene sheets in the graphene film, the close packing between the sheets, and the ordered orientation of the packing domains are significantly worse. This is because in Comparative Example 3 and Comparative Example 4, the shrinkage of the film in all directions cannot be inhibited during reduction; at the same time, the vapor pressure has uneven distribution of vapor stress on the film, and cannot generate isotropic stress in the film, and cannot uniformly apply an outward pressure to the graphene oxide film, resulting in insufficient isotropic tensile stress on the graphene oxide film during the reduction process. The wrinkles and ripples generated by thermal fluctuations and changes in layer spacing are difficult to be completely eliminated, and gases such as carbon dioxide generated by deoxidation cannot be well extruded during the stretching process.

[0094] The X-ray diffraction data, ordered packing domain area, layer spacing, and ordered packing domain height of the graphene films of Example B1 and Comparative Examples 2-4 are compared as Figure 1 shown, the SEM comparison is as Figure 2 shown, the reflectance in the visible-near infrared band is compared as Figure 3 shown, the three-dimensional surface topography comparison is as Figure 5 shown, the root mean square data comparison is as Figure 6 shown, the image standard deviation curve comparison is as Figure 7 shown; it can be seen from Figure 2 that there are wrinkles, folds or voids in the graphene film sheets of Comparative Examples 2-4, while the sheets of the highly oriented and highly reflective graphene film of Example B1 appear to be tight and flat.

[0095] Comparative Example 5

[0096] A method for preparing a highly oriented and highly reflective graphene film by vapor pressure-assisted stretching reduction is basically the same as Example B1, except that in the mixed solution of aqueous hydrogen iodide and ethanol, the volume ratio of aqueous hydrogen iodide to ethanol is 1:7.

[0097] The height of the grain size of the finally prepared highly oriented and highly reflective graphene film is 2.25 nm, the full width at half maximum of the XRD diffraction peak is 3.57°, the Herman's orientation parameter S is 0.63, the surface roughness is 40.5 nm, and the surface reflectance is 15%.

[0098] Comparing Comparative Example 5 with Example B1, the flatness of the graphene sheets in the graphene film, the close packing between the sheets, and the ordered orientation of the packing domains are significantly worse. This is because in Comparative Example 5, the volume ratio of the aqueous hydrogen iodide solution to ethanol in the mixed solution of the aqueous hydrogen iodide solution and ethanol is 1:7, and the volume fraction of ethanol is too high, resulting in a decrease in the relative content of hydrogen iodide, thereby reducing the reduction efficiency. As a result, the graphene film cannot be fully reduced during the reduction process, affecting the flatness of the sheets and the orderliness of the packing domains. The too high volume fraction of ethanol increases the vapor pressure during the reduction process, causing excessive expansion or contraction on the surface and inside of the film, damaging the close packing between the sheets and the overall structure of the graphene film. In addition, the too high volume fraction of ethanol leads to uneven distribution of iodide ions on the film surface, affecting the uniformity of reduction inside the film and further damaging the overall quality of the film.

[0099] Comparative Example 6

[0100] A method for preparing a highly oriented and highly reflective graphene film by vapor pressure-assisted stretching reduction is basically the same as Example B1, except that: the holding temperature in step (3) is 80 °C.

[0101] The height of the grain size of the finally prepared highly oriented and highly reflective graphene film is 3.08 nm, the proportion of the ordered packing domain is 67%, the full width at half maximum of the XRD diffraction peak is 2.62°, the Herman's orientation parameter S is 0.55, the surface roughness is 20 nm, and the surface reflectivity is 26%.

[0102] Comparing Comparative Example 6 with Example B1, the flatness of the graphene sheets in the graphene film, the close packing between the sheets, and the ordered orientation of the packing domains are significantly worse. This is because the holding temperature in Comparative Example 6 is too low. The too low holding temperature leads to a decrease in the concentration of iodide ions escaping from the film, affecting the full reduction of the film, so that the sheet structure of the graphene film cannot be well repaired and regularized, thereby affecting the flatness of the sheets and the orderliness of the packing domains. The too low holding temperature causes the vapor pressure of ethanol to decrease accordingly, unable to apply sufficient stress to the film surface, thereby affecting the reduction effect of the film, resulting in insufficient close packing between the sheets and poor ordered orientation of the packing domains, and ultimately affecting the overall quality of the graphene film.

[0103] Comparative Example 7

[0104] A method for preparing a highly oriented and highly reflective graphene film by vapor pressure-assisted stretching reduction is basically the same as Example B1, except that: the holding time in step (3) is 4 h.

[0105] The height of the grain size of the finally obtained highly oriented and highly reflective graphene film is 3.59 nm, the proportion of the ordered packing domain is 53.5%, the full width at half maximum of the XRD diffraction peak is 2.24°, the Herman's orientation parameter S is 0.58, the surface roughness is 30.3 nm, and the surface reflectivity is 8%.

[0106] Compared with Comparative Example 6 and Example B1, the flatness of the graphene sheets of the graphene film, the tight packing between the sheets, and the ordered orientation of the packing domains are significantly deteriorated. This is because the heat preservation time in Comparative Example 7 is too short. Too short heat preservation time will lead to uneven and insufficient reduction of the film, making the sheet structure of the graphene film unable to be fully regularized and repaired, affecting the flatness of the sheets and the orderliness of the packing domains. Too short heat preservation time makes the regulation effect of ethanol vapor stress unable to be fully exerted, resulting in uneven stress distribution on the film surface, destroying the tight packing between the sheets and the ordered orientation of the packing domains, and ultimately affecting the overall quality of the graphene film.

[0107] Example B2

[0108] A method for preparing a highly oriented and highly reflective graphene film by vapor pressure-assisted stretching reduction, the specific steps are as follows:

[0109] (1) Preparation of raw materials;

[0110] Graphene oxide aqueous solution: the concentration is 0.5 mg·g -1 ;

[0111] Mixture of hydrogen iodide aqueous solution and ethanol: the concentration of the hydrogen iodide aqueous solution is 45 wt%, and the volume ratio of the hydrogen iodide aqueous solution to ethanol is 1:2;

[0112] Device for isotropic stretching and vapor treatment: the device described in Example A1, m = 3, the height of solvent pool a is 8 cm, and the volume is 25 mL;

[0113] (2) Preparation of graphene oxide film;

[0114] After filtering the graphene oxide aqueous solution with a Celgard 3501 membrane and drying it at 35°C for 12 h, removing the filter membrane, a graphene oxide film with a thickness of 5 μm is obtained;

[0115] (3) Preparation of highly oriented and highly reflective graphene film;

[0116] When performing isotropic stretching and steam treatment on the graphene oxide film prepared in step (2), first add a mixed solution of 15 mL of aqueous hydrogen iodide solution and ethanol to the solvent pool a. Then fix the graphene oxide film between the upper and lower cylinders a through the film fixing screw a and adjust the film fixing screw a to keep the graphene oxide film in a taut state. Then heat it through the heating device a until both hydrogen iodide and ethanol are vaporized and keep it at 90 °C for 6 h. After post-treatment (taking out the film, rinsing with ethanol, and drying in a vacuum oven at 40 °C), a highly oriented and highly reflective graphene film is obtained.

[0117] The grain size height of the finally prepared highly oriented and highly reflective graphene film is 6 nm, the proportion of the ordered stacking domain is 90%, the full width at half maximum of the XRD diffraction peak is 1.2°, the Herman's orientation parameter S is 0.8, the surface roughness is 12.5 nm, the surface reflectivity is 34%, the conductivity is 2.5×10 5 S·m -1 , and the tensile strength is 450 MPa.

[0118] Example B3

[0119] A method for preparing a highly oriented and highly reflective graphene film by vapor pressure-assisted stretching and reduction, the specific steps are as follows:

[0120] (1) Preparation of raw materials;

[0121] Aqueous graphene oxide solution: the concentration is 7 mg·g -1 ;

[0122] A mixed solution of aqueous hydrogen iodide solution and ethanol: the concentration of the aqueous hydrogen iodide solution is 45 wt%, and the volume ratio of the aqueous hydrogen iodide solution to ethanol is 1:3;

[0123] The device used for isotropic stretching and steam treatment: the device described in Example A2, n is 3, m is 4, the height of the solvent pool b is 12 cm, the volume is 167 mL, and the included angle between the inclined rod II and the central axis of the cylinder b is 103°;

[0124] (2) Preparation of graphene oxide film;

[0125] After casting the aqueous graphene oxide solution with a quartz watch glass as the casting substrate, dry it at 45 °C for 18 h, remove the substrate, and obtain a graphene oxide film with a thickness of 50 μm;

[0126] (3) Preparation of highly oriented and highly reflective graphene film;

[0127] When performing isotropic stretching and steam treatment on the prepared graphene oxide film in step (2), first add 32.5 mL of a mixed solution of aqueous hydrogen iodide and ethanol to the solvent pool b. Subsequently, fix the graphene oxide film between the upper and lower cylinders b through the film fixing screw b and adjust the film fixing screw b to keep the graphene oxide film in a taut state. Then, heat it through the heating device b until both hydrogen iodide and ethanol are vaporized. At the same time, press down the pressing plate and keep it at 120 °C for 15 h (the maximum stretching displacement of the graphene oxide film after the heat preservation is 1.5 mm). After post-treatment (taking out the film, rinsing with ethanol, and drying in a vacuum oven at 60 °C), a highly oriented and highly reflective graphene film is obtained.

[0128] The height of the grain size of the finally prepared highly oriented and highly reflective graphene film is 5.78 nm, the proportion of the ordered stacking domain is 74.7%, the full width at half maximum of the XRD diffraction peak is 1.39°, the Herman's orientation parameter S is 0.69, the surface roughness is 12.1 nm, the surface reflectivity is 35%, the conductivity is 4×10 4 S·m -1 , and the tensile strength is 450 MPa.

[0129] Example B4

[0130] A method for preparing a highly oriented and highly reflective graphene film by vapor pressure-assisted stretching and reduction, the specific steps are as follows:

[0131] (1) Preparation of raw materials;

[0132] Aqueous graphene oxide solution: the concentration is 8 mg·g -1 ;

[0133] A mixed solution of aqueous hydrogen iodide and ethanol: the concentration of the aqueous hydrogen iodide solution is 45 wt%, and the volume ratio of the aqueous hydrogen iodide solution to ethanol is 1:5;

[0134] The device used for isotropic stretching and steam treatment: the device described in Example A2, n is 4, m is 3, the height of the solvent pool b is 12, the volume is 167 mL, and the included angle between the inclined rod II and the central axis of the cylinder b is 105°;

[0135] (2) Preparation of graphene oxide film;

[0136] After casting the aqueous graphene oxide solution with a quartz surface dish as the casting substrate, dry it at 40 °C for 24 h, remove the substrate, and obtain a graphene oxide film with a thickness of 100 μm;

[0137] (3) Preparation of a highly oriented and highly reflective graphene film;

[0138] When performing isotropic stretching and steam treatment on the graphene oxide film prepared in step (2), first add 50 mL of a mixed solution of aqueous hydrogen iodide and ethanol to the solvent pool b. Subsequently, fix the graphene oxide film between the upper and lower cylinders b through the film fixing screw b and adjust the film fixing screw b to keep the graphene oxide film in a taut state. Then, heat it to vaporize both hydrogen iodide and ethanol through the heating device b. At the same time, press down the pressing plate and keep it at 105 °C for 24 h (the maximum tensile displacement of the graphene oxide film after heat preservation is 1 mm). After post-treatment (taking out the film, rinsing with ethanol, and drying in a vacuum oven at 50 °C), a highly oriented and highly reflective graphene film is obtained.

[0139] The height of the grain size of the finally prepared highly oriented and highly reflective graphene film is 4.38 nm, the proportion of the ordered stacking domain is 72%, the full width at half maximum of the XRD diffraction peak is 1.84°, the Herman's orientation parameter S is 0.65, the surface roughness is 13 nm, the surface reflectivity is 30%, the conductivity is 3.5×10 4 S·m -1 , and the tensile strength is 380 MPa.

[0140] Example B5

[0141] A method for preparing a highly oriented and highly reflective graphene film by vapor pressure-assisted stretching reduction, the specific steps are as follows:

[0142] (1) Preparation of raw materials;

[0143] Aqueous graphene oxide solution: the concentration is 20 mg·g -1 ;

[0144] A mixed solution of aqueous hydrogen iodide and ethanol: the concentration of the aqueous hydrogen iodide solution is 45 wt%, and the volume ratio of the aqueous hydrogen iodide solution to ethanol is 1:2;

[0145] The device used for isotropic stretching and steam treatment: the device described in Example A2, n is 3, m is 4, the height of the solvent pool b is 12 cm, the volume is 167 mL, and the angle between the inclined rod II and the central axis of the cylinder b is 100°;

[0146] (2) Preparation of graphene oxide film;

[0147] After scraping and coating the aqueous graphene oxide solution on the glass plate substrate with a scraper, dry it at 40 °C for 12 h, remove the substrate, and obtain a graphene oxide film with a thickness of 70 μm;

[0148] (3) Preparation of highly oriented and highly reflective graphene film;

[0149] When performing isotropic stretching and steam treatment on the graphene oxide film prepared in step (2), first add a mixed solution of 45 mL of aqueous hydrogen iodide solution and ethanol to the solvent pool b. Subsequently, fix the graphene oxide film between the upper and lower cylinders b through the film fixing screw b and adjust the film fixing screw b to keep the graphene oxide film in a taut state. Then, heat it through the heating device b until both hydrogen iodide and ethanol are vaporized. At the same time, press down the pressing plate and keep it at 90 °C for 24 h (the maximum tensile displacement of the graphene oxide film after heat preservation is 1 mm). After post-treatment (taking out the film, rinsing with ethanol, and drying in a vacuum oven at 40 °C), a highly oriented and highly reflective graphene film is obtained.

[0150] The height of the grain size of the finally prepared highly oriented and highly reflective graphene film is 3.55 nm, the proportion of the ordered stacking domain is 78%, the full width at half maximum of the XRD diffraction peak is 2.5°, the Herman's orientation parameter S is 0.72, the surface roughness is 13.5 nm, the surface reflectivity is 27%, the conductivity is 3×10 4 S·m -1 , and the tensile strength is 360 MPa.

[0151] Example B6

[0152] A method for preparing a highly oriented and highly reflective graphene film by vapor pressure-assisted stretching and reduction, the specific steps are as follows:

[0153] (1) Preparation of raw materials;

[0154] Aqueous graphene oxide solution: the concentration is 40 mg·g -1 ;

[0155] A mixed solution of aqueous hydrogen iodide solution and ethanol: the concentration of the aqueous hydrogen iodide solution is 45 wt%, and the volume ratio of the aqueous hydrogen iodide solution to ethanol is 1:5;

[0156] The device used for isotropic stretching and steam treatment: the device described in Example A2, n is 4, m is 3, the height of the solvent pool b is 12 cm, the volume is 167 mL, and the angle between the inclined rod II and the central axis of the cylinder b is 100°;

[0157] (2) Preparation of graphene oxide film;

[0158] After scraping the aqueous graphene oxide solution on the glass plate substrate with a scraper, dry it at 40 °C for 24 h, remove the substrate, and obtain a graphene oxide film with a thickness of 95 μm;

[0159] (3) Preparation of a highly oriented and highly reflective graphene film;

[0160] When performing isotropic stretching and steam treatment on the graphene oxide film prepared in step (2), first add 45 mL of a mixed solution of hydroiodic acid aqueous solution and ethanol to the solvent pool b. Subsequently, fix the graphene oxide film between the upper and lower cylinders b through the film fixing screw b and adjust the film fixing screw b to keep the graphene oxide film in a taut state. Then, heat it through the heating device b until both hydroiodic acid and ethanol are vaporized. At the same time, press down the pressing plate and keep it at 90 °C for 24 h (the maximum tensile displacement of the graphene oxide film after heat preservation is 5 mm). After post-treatment (taking out the film, rinsing with ethanol, and drying in a vacuum oven at 50 °C), a highly oriented and highly reflective graphene film is obtained.

[0161] The height of the grain size of the finally prepared highly oriented and highly reflective graphene film is 3.42 nm, the proportion of the ordered stacking domain is 75%, the full width at half maximum of the XRD diffraction peak is 2.62°, the Herman's orientation parameter S is 0.7, the surface roughness is 14.1 nm, the surface reflectivity is 25%, the conductivity is 2×10 4 S·m -1 , and the tensile strength is 340 MPa.

Claims

1. A method for preparing a highly oriented and highly reflective graphene film by steam pressure assisted stretching reduction, characterized in that: At the same time, the graphene oxide film is isotropically stretched and steam-treated, wherein the steam treatment is to heat a mixture of hydrogen iodide aqueous solution and ethanol until vaporization and then keep the mixture at 90-120° C. for 6-24 hours, wherein the volume ratio of hydrogen iodide aqueous solution to ethanol is 1:2-5; The thickness of the graphene oxide film is 5-100 μm; When the thickness of the graphene oxide film is ≤10 μm, the device used for isotropically stretching and steam treating the graphene oxide film at the same time includes a solvent pool a and a heating device a; the top of the solvent pool a is a vertically arranged cylinder a, the cylinder a is divided into an upper and lower layer and is detachably connected by 2m membrane fixing screws a, m>2, and the 2m membrane fixing screws a are evenly distributed around the circumference of the central axis of the cylinder a; the heating device a is used to heat the solvent pool a; When the thickness of the graphene oxide film is greater than 10 μm, the device used for simultaneously isotropically stretching and steam treating the graphene oxide film includes a solvent pool b, a heating device b, a deformable tube and a regulator; the bottom of the deformable tube is sealed and connected to the top of the solvent pool b, the top of the deformable tube is a vertically arranged cylinder b, the cylinder b is divided into two layers, the upper and lower layers are detachably connected by 2m membrane fixing screws b, m>2, and the 2m membrane fixing screws b are evenly distributed around the circumference of the central axis of the cylinder b; the regulator is used to control the radial outward expansion of the cylinder b along the cylinder b; the heating device b is used to heat the solvent pool b.

2. The method for preparing a highly oriented and highly reflective graphene film by steam pressure assisted stretching reduction according to claim 1, characterized in that: The regulator includes a displacement bracket, a pressing plate, an upper bracket, a limiting structure and a lower bracket; The displacement bracket is a conical cap-shaped structure with the tip at the top, comprising 2n inclined rods I, n>2, the upper ends of the 2n inclined rods I are simultaneously connected to the center of the lower surface of the pressing plate, and the lower ends are evenly distributed around a point o, the point o is located on the central axis of the pressing plate, and the pressing plate is coaxial with the cylinder b; The upper support includes 2n horizontal rods, which are radially distributed around point o. The two ends of the horizontal rods are respectively marked as end a and end b. End a is closer to point o than end b. The ends a of the 2n horizontal rods are respectively hinged to the lower ends of the 2n oblique rods I in a one-to-one correspondence. The limiting structure restricts the 2n horizontal rods to move only along their own length direction; The b ends of two adjacent horizontal rods are each connected by an arc bar, and at least one set of opposite arc bars is composed of three sections: left, middle and right. The middle section is composed of a separated inner layer and outer layer. The outer layer of the middle section is fixedly connected to the left section, and the inner layer of the middle section is fixedly connected to the right section. All the arc bars form a circular ring, and the circular ring extends vertically downward to form a circular tube. The lower support includes 2n oblique rods II, which are distributed in an umbrella shape around the central axis of the cylinder b. The angle between the oblique rods II and the central axis of the cylinder b is 100-105°. The lower layer of the cylinder b extends radially outward and is connected to the upper ends of the 2n oblique rods II, and the lower ends of the 2n oblique rods II are connected to the circular tube.

3. The method for preparing a highly oriented and highly reflective graphene film by steam pressure assisted stretching reduction according to claim 2, characterized in that: The regulator also includes a height measuring bracket, which is vertically arranged on one side of the pressing plate and fixedly connected to the upper bracket.

4. The method for preparing a highly oriented and highly reflective graphene film by steam pressure assisted stretching reduction according to claim 1, characterized in that: The graphene oxide membrane is prepared by a suction filtration method, a casting method or a scraping method.

5. The method for preparing a highly oriented and highly reflective graphene film by steam pressure assisted stretching reduction according to claim 1, characterized in that: The grain size of the highly oriented and highly reflective graphene film is 3.42-6nm, the proportion of ordered stacking domains exceeds 70%, the half-peak width of the XRD diffraction peak is less than 3°, and the Herman's orientation parameter S The surface roughness is less than 15nm, the surface reflectivity is not less than 25%, and the conductivity is 2×10 4 -2.5×10 5 S·m -1 , tensile strength exceeds 330MPa.

Images