Method for manufacturing a self-standing graphene oxide film or a self-standing reduced graphene oxide film

Abstract

The present invention relates to a method for manufacturing a free-standing graphene oxide or reduced graphene oxide film with a thickness of 0.4 μm to 4.0 μm, the method comprising the following sequential steps: A) preparing an aqueous dispersion of graphene oxide or reduced graphene oxide containing 0.1 g.L ‑1 to 30 g.L ‑1 , B) depositing the aqueous dispersion on a flat substrate coated with a polymer film that is soluble in an organic solvent and insoluble in water to form a wet film with a thickness of 1 μm to 3.5 mm, C) drying the wet film, D) separating the polymer film from the flat substrate, E) placing the polymer film in a support frame, F) spraying the polymer film with an organic solvent to dissolve the polymer film, G) separating the graphene oxide or reduced graphene oxide film from the frame to obtain a free-standing graphene oxide or reduced graphene oxide film.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing thin self-standing graphene oxide or reduced graphene oxide films. The method is particularly well-suited for the electronics industry, nuclear industry, medical applications, energy industry, oil and gas industry, water treatment applications, chemical industry, or steelmaking. Background Art

[0002] It is known to produce carbon films as thin film coatings, which mainly consist of the chemical element carbon. These include plasma polymer films, amorphous carbon films (diamond-like carbon, DLC), CVD diamond films, and graphite films.

[0003] Generally, carbon films are produced by chemical vapor deposition (CVD) or physical vapor deposition (PVD). They are deposited in the form of thin films with a film thickness of only a few micrometers.

[0004] It is also known from the paper "Self-supporting graphene oxide films preparation and characterization methods", L. Torrisi et al., Vacuum 160 (2019) 1-11 to prepare self-standing graphene oxide films by depositing a graphene oxide aqueous dispersion on a substrate made of a polytetrafluoroethylene or polycarbonate polymer, drying the film, and then separating and floating the graphene oxide film by immersing the coated substrate in acetone.

[0005] However, pulling the film out of the acetone bath is very difficult and the film tends to wrinkle or even break. This limits the size of the film to a few millimeters. Summary of the Invention

[0006] Therefore, the object of the present invention is to provide a method for manufacturing thin self-standing graphene oxide or reduced graphene oxide films, which enables larger films to be easily obtained.

[0007] For this purpose, the first subject of the present invention includes a method for manufacturing a self-standing graphene oxide or reduced graphene oxide film with a thickness of 0.4 μm to 4.0 μm, the method comprising the following sequential steps:

[0008] A. Prepare an aqueous dispersion containing 0.1 g·L -1 to 30 g·L -1 of graphene oxide or reduced graphene oxide,

[0009] B. Deposit the aqueous dispersion on a flat substrate coated with a polymer film that is soluble in an organic solvent and insoluble in water to form a wet film with a thickness of 1 μm to 3.5 mm,

[0010] C. Dry the wet film to form a graphene oxide or reduced graphene oxide film on the polymer film.

[0011] D. Separate the polymer film coated with the graphene oxide or reduced graphene oxide film from the flat substrate.

[0012] E. Place the polymer film coated with the graphene oxide or reduced graphene oxide film in a support frame.

[0013] F. Spray the polymer film with an organic solvent to dissolve the polymer film and obtain a framed graphene oxide or reduced graphene oxide film.

[0014] G. Separate the graphene oxide or reduced graphene oxide film from the frame to obtain a free-standing graphene oxide or reduced graphene oxide film.

[0015] The method according to the present invention may also have optional features listed below, considered individually or in combination:

[0016] - The graphene oxide, or reduced graphene oxide, is in the form of nanosheets with a thickness of 0.9 nm to 10 nm, or 0.3 nm to 5 nm.

[0017] - The graphene oxide contains 30 wt% to 60 wt% oxygen.

[0018] - The reduced graphene oxide contains less than 30 wt% oxygen.

[0019] - The aqueous dispersion in step A contains 0.5 g·L -1 to 15 g·L -1 of graphene oxide or reduced graphene oxide, and the wet film in step B has a thickness of 1.0 μm to 2.5 mm.

[0020] - The flat substrate is a glass substrate, a metal substrate, a ceramic substrate, or a plastic substrate.

[0021] - The polymer film is selected from polyacrylates, polyvinyl esters, polyvinyl alcohols, polyurethanes, or mixtures thereof.

[0022] - The drying in step C is carried out at a temperature below 150 °C for a time of 1 minute to 100 minutes.

[0023] - The separation in step D is carried out by peeling the polymer film coated with the graphene oxide or reduced graphene oxide film from the flat substrate.

[0024] - The peeling is completed by wetting the interface between the polymer film and the substrate.

[0025] - The support frame is positioned on the edge of the graphene oxide or reduced graphene oxide film.

[0026] - The organic solvent in step F is selected from acetone, ethanol, isopropanol or a mixture thereof.

[0027] - The aqueous dispersion in step A contains graphene oxide, and the free-standing graphene oxide film is transparent.

[0028] - The thickness of the free-standing graphene oxide or reduced graphene oxide film is 0.7 μm to 2.5 μm.

[0029] - The free-standing graphene oxide or reduced graphene oxide film has a length of 5 mm to 500 mm and a width of 5 mm to 500 mm.

[0030] - The areal density of the free-standing graphene oxide or reduced graphene oxide film is 0.01 g·m -2 to 20 g·m -2 ,

[0031] - The method further includes step H, during which the graphene oxide of the free-standing graphene oxide film, or the reduced graphene oxide of the free-standing reduced graphene oxide film is reduced to obtain a free-standing reduced graphene oxide film, or further reduced.

[0032] Other features and advantages of the present invention will be described in more detail in the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be better understood by referring to the following drawings (which are provided for illustrative purposes only and are in no way limiting):

[0034] - Figure 1 , which shows an example of the nanosheet crystals of (reduced) graphene oxide according to the present invention.

[0035] - Figure 2 , which shows a flat substrate coated with a polymer film and a (reduced) graphene oxide film according to the present invention.

[0036] - Figure 3 , which shows the polymer film coated with a (reduced) graphene oxide film according to the present invention after removing the substrate.

[0037] - Figure 4 , which shows a free-standing (reduced) graphene oxide film obtained by the method according to the present invention. DETAILED DESCRIPTION

[0038] In the following description, the term "(reduced) graphene oxide" generally refers to graphene oxide or reduced graphene oxide.

[0039] Without wishing to be bound by any theory, it is believed that the method according to the present invention allows for the production of large free-standing (reduced) graphene oxide films with a specific thickness and high quality.

[0040] In the first step (step A) of the method, a water mixture is prepared by dispersing 0.1 g·L -1 to 30 g·L -1 of (reduced) graphene oxide in water.

[0041] Graphene oxide contains 30 wt% to 60 wt% oxygen. Due to the oxygen functional groups corresponding to this oxygen content (e.g., carboxyl (-COOH), carbonyl (-C=O), and hydroxyl (-OH)), graphene oxide can be easily dispersed in water. In addition, the oxygen-containing functional groups contribute to the formation of a film due to the interaction between the functional groups of adjacent graphene oxide sheets.

[0042] Reduced graphene oxide contains less than 30 wt% oxygen. Due to the reduced oxygen content, the electrical conductivity and thermal conductivity of the free-standing film are further improved.

[0043] Preferably, the (reduced) graphene oxide has the form of nanosheet crystals, i.e., a nano-object with one external dimension on the nanoscale and the other two external dimensions significantly larger and not necessarily on the nanoscale. Figure 1 An example of a (reduced) graphene oxide flake according to the present invention is shown. In this example, the lateral dimension means the maximum length of the (reduced) graphene oxide passing through the X-axis and the thickness means the height of the (reduced) graphene oxide passing through the Z-axis. The width of the nanosheet crystal passing through the Y-axis is shown.

[0044] Advantageously, the lateral dimension of the (reduced) graphene oxide nanosheet crystals is 0.1 μm to 100 μm, and more preferably 0.5 μm to 20 μm.

[0045] Preferably, the width dimension of the (reduced) graphene oxide nanosheet crystals is 0.1 nm to 100 μm.

[0046] Advantageously, the thickness of the graphene oxide nanosheet crystals is 0.9 nm to 10 nm. Advantageously, the thickness of the reduced graphene oxide nanosheet crystals is 0.3 nm to 5 nm.

[0047] Such dimensions are favorable for the formation of a film with a thickness below 4 μm.

[0048] The concentration of (reduced) graphene oxide in the aqueous dispersion is preferably from 0.1 g·L -1 to 20 g·L-1 and advantageously is 0.5 g·L -1 to 15 g·L -1 . Such a concentration is favorable for forming a film with a thickness of less than 4 μm.

[0049] According to one variant, the water mixture consists of (reduced) graphene oxide and water. According to another variant, the water mixture contains additional compounds such as nanoparticles, surfactants or dispersants. Several types of nanoparticles (such as in particular ceramics, metals, metal oxides, salts, organic compounds) can be added in particular to the water mixture to impart new or improved properties to the free-standing (reduced) graphene oxide film. For electrochemical applications, for example, Pt, Au, Ru, TiO2, ZnO, SnO2, Cu2O, MnO2, Mn3O4, NiO and SiO2 can be added. Surfactants can be added to the dispersion in water, especially when adding reduced graphene oxide and / or nanoparticles. Examples of possible surfactants / dispersants are: sodium dodecylbenzenesulfonate (SDBS), DISPERBYK®-2010 which is an aqueous emulsion of a structured acrylate copolymer supplied by BYK® and having a pigment affinity group, DISPERBYK®-2012 which is a solution of a copolymer supplied by BYK® and having a pigment affinity group.

[0050] Preferably, the water mixture does not contain any binder. In fact, it is preferred not to change the film structure with a binder, especially for applications where the properties of the carbon film are sought. The binder can significantly change the electrical conductivity and thermal conductivity.

[0051] Optionally, the aqueous dispersions are mixed to improve the dispersion of (reduced) graphene oxide and optional compounds in water. The mixing can be done by mechanical agitation, ultrasonic bath, high shear mixing. Mixing is particularly preferred when the aqueous dispersion is prepared with reduced graphene oxide containing less than 20 wt% oxygen.

[0052] In the second step (step B) of the method, the aqueous dispersion is deposited on a flat substrate coated with a polymer film soluble in an organic solvent to form a wet film with a thickness of 1.0 μm to 3.5 mm. The aqueous dispersion at least partially covers the flat substrate.

[0053] Due to such a thickness of the wet film formed from an aqueous dispersion of (reduced) graphene oxide containing 0.1 g·L -1 to 30 g·L -1 , a free-standing (reduced) graphene oxide film with a thickness of 0.4 μm to 4.0 μm can be obtained.

[0054] Preferably, the wet film with a thickness of 1.0 μm to 2.5 mm is formed from a dispersion containing 0.5 g·L-1 to 15 g·L -1 The formation of an aqueous dispersion of (reduced) graphene oxide enables the obtaining of a free-standing (reduced) graphene oxide film having a thickness of from 0.4 μm to 4.0 μm.

[0055] Preferably, a wet film having a thickness of from 50 μm to 400 μm is formed from an aqueous dispersion comprising 1 g·L -1 to 10 g·L -1 of (reduced) graphene oxide enables the obtaining of a free-standing (reduced) graphene oxide film having a thickness of from 0.6 μm to 3.0 μm.

[0056] The substrate is flat, i.e. it has a horizontal surface without raised areas or depressions. Preferably, the flat substrate is a glass substrate, a metal substrate, a ceramic substrate or a plastic substrate.

[0057] The substrate is coated with a polymer film, i.e. the substrate is at least partially covered with a polymer film. The adhesion properties between the substrate and the polymer film are not particularly limited. It can be in particular a chemical adhesion or a physical adhesion. According to a preferred variant of the invention, water is used for adhesion. Before applying the polymer film on top of the substrate, the polymer film is wetted or the substrate is wetted. Then preferably a squeegee is used to press the polymer film against the substrate to remove wrinkles and air bubbles. After drying at the interface, the polymer film adheres sufficiently to the substrate to use the coated substrate as a support for the aqueous dispersion.

[0058] The polymer film is soluble in an organic solvent but not in water and is thus compatible with the aqueous dispersion. The polymer film is preferably a thermoplastic. Preferably, the polymer film is selected from: polyacrylates (e.g. poly(methyl methacrylate), poly(ethyl methacrylate), poly(propyl methacrylate), poly(isobutyl methacrylate)), polyvinyl esters, polyvinyl alcohol, polyurethanes or mixtures thereof.

[0059] The thickness of the polymer film is preferably from 5 μm to 100 μm, more preferably from 15 μm to 50 μm, which represents a good compromise between the ease of dissolution of the polymer film and the support of the (reduced) graphene oxide film.

[0060] Preferably, the aqueous dispersion is deposited by spraying, roll coating, brush coating, screen printing, dip coating, spin coating, knife coating, bar coating or drop casting.

[0061] In the third step (step C) of the method, the wet film of the aqueous dispersion is dried to form a (reduced) graphene oxide film on the polymer film.

[0062] According to a variant, the wet film is dried in air.

[0063] In a preferred variant, as opposed to natural drying in air, the coating is dried forcibly, i.e., actively dried. Forced drying is considered to be beneficial for forming a more uniform (reduced) graphene oxide film due to better control of water removal. In a preferred variant, the drying is carried out at a temperature below 150 °C, more preferably 50 °C to 150 °C, and advantageously 80 °C to 120 °C. The drying can be carried out with pressurized air, an inert gas (N2 or Ar), or under vacuum.

[0064] Advantageously, the drying is carried out for 1 minute to 100 minutes, for example 10 minutes to 60 minutes.

[0065] As Figure 2 shown, after drying, the substrate 1 is coated with a polymer film 2 and a (reduced) graphene oxide film 3. In particular, the thickness of the (reduced) graphene oxide film is 0.4 μm to 4.0 μm.

[0066] In the fourth step (step D) of the method, the polymer film coated with the (reduced) graphene oxide film is separated from the flat substrate.

[0067] The separation is carried out by peeling the polymer film coated with the (reduced) graphene oxide film from the flat substrate. It can be carried out in different ways depending on the nature of the polymer and the way the polymer adheres to the substrate. According to a preferred variant of the invention in which the polymer film adheres to the substrate after being wetted, the polymer film is separated from the substrate by wetting the interface between the polymer film and the substrate. In particular, water can be added near the edge of the polymer film so that the edge is separated from the substrate.

[0068] As Figure 3 shown, after step D, the (reduced) graphene oxide film 3 remains on the polymer film 2.

[0069] In the fifth step (step E) of the method, the polymer film coated with the (reduced) graphene oxide film is placed in a support frame to hold the film in place and facilitate the dissolution of the polymer film.

[0070] The support frame is preferably positioned on the edge of the (reduced) graphene oxide film, and the edge of the (reduced) graphene oxide film can merge with the edge of the polymer film. Thus, the edge is well held and the polymer film is horizontal and tight.

[0071] For this purpose, the polymer film can optionally be cut to the size of the support frame.

[0072] In the sixth step (step F) of the method, the polymer film is dissolved by spraying with an organic solvent. It goes without saying that the spraying is done on the surface of the polymer film that is not coated with the (reduced) graphene oxide film. Due to the spraying, the polymer film is carefully removed without exposing the (reduced) graphene oxide film to the organic solvent. In addition, compared with the method in which the polymer film is immersed in a solvent bath, spraying limits the amount of solvent. Further, the solvent evaporates in air without forced drying or heating, and a free-standing (reduced) graphene oxide film is directly obtained.

[0073] Preferably, the organic solvent is an alcohol or a ketone. For example, the organic solvent is selected from: acetone, ethanol, isopropanol or a mixture thereof.

[0074] The spraying preferably lasts from 1 minute to 20 minutes.

[0075] As Figure 4 shown, after dissolving the polymer film, a framed (reduced) graphene oxide film according to the present invention is obtained.

[0076] In the seventh step (step G) of the method, the (reduced) graphene oxide film is separated from the frame to obtain a free-standing (reduced) graphene oxide film. <m

[0077] This can be done by removing the support frame or by cutting the (reduced) graphene oxide film.

[0078] The thickness of such a free-standing (reduced) graphene oxide film is from 0.4 μm to 4.0 μm. In the case of graphene oxide, the latter contains from 30% to 60% by weight of oxygen. In the case of reduced graphene oxide, the latter contains less than 30% by weight of oxygen.

[0079] Without wishing to be bound by any theory, it seems that such a free-standing (reduced) graphene oxide film has high quality. In fact, it is considered that such a (reduced) graphene oxide film has no cracks and is uniform. Further, it is considered that due to the method according to the present invention, the (reduced) graphene oxide dispersion in the film is uniform. Finally, it is considered that compared with the prior art, such a free-standing (reduced) graphene oxide layer has greater mechanical resistance.

[0080] When the thickness of the free-standing (reduced) graphene oxide film is less than 0.4 μm, the film is considered to be very brittle. In fact, there is a risk of film rupture. When the thickness of the free-standing (reduced) graphene oxide film is greater than 4.0 μm, the (reduced) graphene oxide film is considered to be too thick for some purposes.

[0081] Preferably, the thickness is from 0.4 μm to 3.5 μm, more preferably from 0.6 μm to 3.0 μm, and advantageously from 0.7 μm to 2.5 μm.

[0082] Preferably, the free-standing (reduced) graphene oxide film has a length of 5 mm to 500 mm and a width of 5 nm to 500 nm.

[0083] Preferably, the free-standing (reduced) graphene oxide film does not contain a binder.

[0084] According to one variant, the film consists of (reduced) graphene oxide. According to another variant, the film contains additional compounds such as nanoparticles, surfactants or dispersants, as described above.

[0085] Due to the method according to the invention, the surface density of the free-standing (reduced) graphene oxide film is 0.01 g·m -2 to 20 g·m -2 , more preferably 0.05 g·m -2 to 15 g·m -2 , and advantageously 0.1 g·m -2 to 6 g·m -2 . It is believed that this specific surface density further improves the properties of the free-standing (reduced) graphene oxide film.

[0086] In the case of graphene oxide, due to its thickness, its composition and its manufacturing process, the free-standing graphene oxide film has the additional advantage of being transparent in the visible spectrum, which makes it suitable for applications seeking transparency.

[0087] The method may optionally include an eighth step (step H), during which:

[0088] - reducing the graphene oxide of the free-standing graphene oxide film to obtain a free-standing reduced graphene oxide film, or

[0089] - further reducing the reduced graphene oxide of the free-standing reduced graphene oxide film.

[0090] The method of reducing graphene oxide is well known.

[0091] Depending on the process conditions, the final oxygen content in the reduced graphene oxide can vary between 0.1 wt% and 25 wt%. Additionally, step H does not change the other properties of the free-standing film described above.

[0092] According to the first variant of step H, the free-standing (reduced) graphene oxide film is heated in an inert atmosphere, a reducing atmosphere (e.g., argon / hydrogen 95%:5% v / v), or under vacuum at a temperature above 150 °C (and up to 3000 °C). The heat treatment preferably lasts from 10 minutes to 2 hours. In this variant, the (reduced) graphene oxide film is preferably placed between two rigid sheets to avoid forming wrinkles during the heat treatment and to keep the film flat. The sheets can be made in particular of graphite or alumina.

[0093] According to the second variant of step H, the (reduced) graphene oxide can be reduced at room temperature with a chemical reagent such as hydrazine vapor or hydroiodic acid, or by moderate heating.

[0094] Finally, the present invention relates to the use of the free-standing (reduced) graphene oxide film according to the invention for manufacturing electronic devices, power generation devices, heat exchanger blocks for chemical processing plants, and electrodes for electric arc furnaces.

[0095] The present invention will now be illustrated in tests carried out for reference only. The tests are not restrictive.

[0096] Example:

[0097] For Test 1, graphene oxide in powder form was pressed until a film was obtained. The oxygen content of the graphene oxide was 45 wt%.

[0098] For Test 2, a poly(isobutyl methacrylate) film was first wetted with water and applied on a flat glass substrate and dried.

[0099] Then, a dispersion of graphene oxide with an oxygen content of 45 wt% and containing 5 g·L -1 in water was prepared. The dispersion was deposited on the flat glass substrate coated with poly(isobutyl methacrylate) by doctor blading to obtain a wet film with a thickness of 100 μm. Then the wet film was dried at a temperature of 80 °C for 20 minutes. After drying, the poly(isobutyl methacrylate) film coated with the graphene oxide film was peeled off from the flat substrate by introducing water between the substrate and the polymer film. The polymer coated with graphene oxide was placed in a support frame. Then, acetone was sprayed on the polymer film for 5 minutes to dissolve the polymer film and obtain a graphene oxide film. Then the graphene oxide film was removed from the support frame.

[0100] For Test 3, a poly(isobutyl methacrylate) film was first wetted with water and applied on a flat glass substrate and dried.

[0101] Then, a dispersion containing 5 g·L -1A dispersion of graphene oxide with an oxygen content of 45 wt%. The dispersion was deposited on a flat glass substrate coated with poly(isobutyl methacrylate) by doctor blading to obtain a wet film with a thickness of 200 μm. Then the wet film was dried at a temperature of 80 °C for 20 minutes. After drying, the poly(isobutyl methacrylate) film coated with the graphene oxide film was peeled off from the flat substrate by introducing water between the substrate and the polymer film. The polymer coated with graphene oxide was placed in a support frame. Then, acetone was sprayed on the polymer film for 5 minutes to dissolve the polymer film and obtain a graphene oxide film. Then the graphene oxide film was removed from the support frame.

[0102] For Experiment 4, the free-standing graphene oxide film obtained by the method of Experiment 2 was further reduced by placing it between two graphite sheets in an oven under vacuum conditions (P < 400 mbar) at 300 °C for 1 hour.

[0103] For Experiment 5, the free-standing graphene oxide film obtained by the method of Experiment 2 was further reduced by placing it between two graphite sheets in an oven under N2 atmosphere at 900 °C for 1 hour.

[0104] For Experiment 6, the poly(isobutyl methacrylate) film was first wetted with water and applied on a flat glass substrate and dried.

[0105] Then a high-shear mixer (6000 rpm for 1 hour) was used to prepare an aqueous dispersion containing 5 g·L -1 Reduced graphene oxide (with an oxygen content of 17 wt%) and 0.1 vol% of DISPERBYK®-2010 based on the volume of the aqueous dispersion. The dispersion was deposited on a flat glass substrate coated with poly(isobutyl methacrylate) by doctor blading to obtain a wet film with a thickness of 200 μm. Then the wet film was dried at a temperature of 80 °C for 20 minutes. After drying, the poly(isobutyl methacrylate) film coated with the reduced graphene oxide film was peeled off from the flat substrate by introducing water between the substrate and the polymer film. The polymer coated with reduced graphene oxide was placed in a support frame. Then, acetone was sprayed on the polymer film for 5 minutes to dissolve the polymer film and obtain a reduced graphene oxide film. Then the reduced graphene oxide film was removed from the support frame.

[0106] The percentage of oxygen in the (reduced) graphene oxide of the film was measured by thermogravimetric analysis, thermal programmed desorption, and X-ray photoelectron spectroscopy. The thickness of the film was measured by atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and micrometer. The areal density of the graphene oxide film was measured using an accurate analytical balance. Transmittance measurements were completed using the device Haze-gard i from BYK-Gardner in the visible light range following ASTM D1003.

[0107] The results are shown in Table 1 below:

[0108]

[0109] Tests according to the present invention have shown free-standing (reduced) graphene oxide films with excellent quality.

Claims

1. A method for manufacturing a free-standing graphene oxide film or a free-standing reduced graphene oxide film with a thickness of 0.4 μm to 4.0 μm, comprising the following steps in sequence: A. Prepare a water dispersion of graphene oxide or reduced graphene oxide containing 0.1 g·L -1 to 30 g·L -1 thereof. B. Depositing the aqueous dispersion on a flat substrate coated with a polymer film that can be dissolved in an organic solvent and cannot be dissolved in water to form a wet film with a thickness of 1 μm to 3.5 mm, C. The wet film is dried to form a graphene oxide film or a reduced graphene oxide film on the polymer film, wherein, The drying is carried out at a temperature of 80°C to 150°C for 1 minute to 100 minutes, D. Separating the polymer film coated with the graphene oxide film or the reduced graphene oxide film from the flat substrate, E. Placing the polymer film coated with the graphene oxide film or the reduced graphene oxide film in a support frame, F. Spraying the polymer film with an organic solvent to dissolve the polymer film and obtain a framed graphene oxide film or a framed reduced graphene oxide film, G. Separating the graphene oxide film or the reduced graphene oxide film from the frame to obtain the free-standing graphene oxide film or the free-standing reduced graphene oxide film.

2. The method according to claim 1, wherein the graphene oxide or the reduced graphene oxide is in the form of nanosheet crystals with a thickness of 0.9 nm to 10 nm or 0.3 nm to 5 nm.

3. The method according to any one of claims 1 or 2, wherein the aqueous dispersion in step A comprises 0.5 g·L -1 to 15 g·L -1 of graphene oxide or reduced graphene oxide and the thickness of the wet film in step B is 1.0 μm to 2.5 mm.

4. The method according to any one of claims 1 or 2, wherein the flat substrate is a glass substrate, a metal substrate, a ceramic substrate or a plastic substrate.

5. The method according to any one of claims 1 or 2, wherein the polymer film is selected from polyacrylates, polyvinyl esters, polyvinyl alcohol, polyurethanes or mixtures thereof.

6. The method according to any one of claims 1 or 2, wherein the separation in step D is carried out by peeling the polymer film coated with the graphene oxide film or the reduced graphene oxide film from the flat substrate.

7. The method according to claim 6, wherein the peeling is completed by wetting the interface between the polymer film and the substrate.

8. The method according to any one of claims 1 or 2, wherein the support frame is positioned at the edge of the graphene oxide film or the reduced graphene oxide film.

9. The method according to any one of claims 1 or 2, wherein the organic solvent in step F is selected from acetone, ethanol, isopropanol or mixtures thereof.

10. The method according to any one of claims 1 or 2, wherein the aqueous dispersion in step A contains graphene oxide, and wherein the free-standing graphene oxide film is transparent.

11. The method according to any one of claims 1 or 2, wherein the thickness of the free-standing graphene oxide film or the free-standing reduced graphene oxide film is 0.7 μm to 2.5 μm.

12. The method according to any one of claims 1 or 2, wherein the free-standing graphene oxide film or the free-standing reduced graphene oxide film has a length of 5 mm to 500 mm and a width of 5 mm to 500 mm.

13. The method according to any one of claims 1 or 2, wherein the areal density of the free-standing graphene oxide film or the free-standing reduced graphene oxide film is 0.01 g·m -2 to 20 g·m -2 .

14. The method for manufacturing a free-standing reduced graphene oxide film according to any one of claims 1 or 2 further includes step H, during which the graphene oxide of the free-standing graphene oxide film or the reduced graphene oxide of the free-standing reduced graphene oxide film is reduced to obtain a free-standing reduced graphene oxide film, or further reduced.

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