Graphene aqueous dispersion and preparation method thereof
By employing pre-oxidation, hydrothermal self-exfoliation, and modification treatment, the problem of easy aggregation and sedimentation of graphene aqueous dispersions at high concentrations was solved, resulting in highly stable and highly conductive graphene aqueous dispersions suitable for industrial applications.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- GUANGDONG RUIHE NEW MATERIALS CO LTD
- Filing Date
- 2026-02-11
- Publication Date
- 2026-05-19
AI Technical Summary
In the preparation of high-concentration graphene aqueous dispersions, existing technologies often result in irreversible agglomeration and sedimentation of graphene sheets, leading to poor dispersion stability and making it difficult to meet the requirements of industrial applications.
The method employs pre-oxidation, hydrothermal self-exfoliation, and modification treatment. Pre-oxidation is carried out by adding oxidant in batches under ice bath conditions, followed by hydrothermal treatment at high temperature to achieve self-exfoliation of graphene. A flexible protective layer is formed by modification with zwitterionic polymers. Finally, thickeners and preservatives are added for post-treatment to form a highly stable aqueous graphene dispersion.
The stability and conductivity of aqueous graphene dispersions at high concentrations were improved, the agglomeration problem of graphene sheets at high concentrations was solved, and the reliability and overall performance of the dispersions for industrial applications were enhanced.
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Figure CN122059404A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of carbon material preparation technology, and more specifically, to an aqueous graphene dispersion and its preparation method. Background Technology
[0002] Graphene, a two-dimensional material composed of a single layer of carbon atoms, has enormous application potential in conductive inks, flexible electronics, composite materials, energy storage, and coatings due to its excellent electrical and thermal conductivity, mechanical strength, and extremely large specific surface area. Aqueous graphene dispersions, which are high-concentration colloidal systems formed by stably dispersing graphene in water, are direct raw materials for most of the aforementioned applications.
[0003] However, with existing technologies, when the solid content reaches 10 wt% or higher, graphene sheets are prone to irreversible aggregation and sedimentation. This leads to significant stratification, a sharp increase in viscosity, or gelation of the dispersion within weeks or even days, severely limiting its reliability in industrial production and end-use applications. In existing technologies, whether it's the modified Hummers method combined with ultrasonic exfoliation, ball milling exfoliation, electrochemical exfoliation, or direct liquid-phase exfoliation, all mainstream routes rely heavily on large amounts of surfactants, organic solvents, or strong bases / reducing agents to temporarily maintain dispersion stability when preparing aqueous graphene dispersions. While these methods can produce relatively transparent dispersions under low concentration conditions in the laboratory, when attempting to increase the solid content to the 10 wt% or higher required for industrial applications through evaporation, centrifugation, or ultrafiltration, the graphene sheets rapidly re-stack due to van der Waals forces, causing a rapid decrease in the absolute value of the zeta potential. The electrostatic repulsion is insufficient to counteract gravity and interlayer attraction, leading to a sharp deterioration in the stability of the dispersion. Excessive defects in the graphene sheets during preparation, overly thorough removal of oxygen-containing groups, or excessive residues can all lead to an imbalance between the hydrophilicity of the sheets and the degree of recovery of the conjugated structure, further exacerbating the tendency to aggregate at high concentrations.
[0004] Therefore, a new preparation method is urgently needed to solve the above-mentioned technical problems. Summary of the Invention
[0005] The main objective of this invention is to provide a method for preparing an aqueous graphene dispersion, which aims to overcome the technical problems of easy agglomeration and sedimentation and poor dispersion stability after the solid content is increased in the prior art.
[0006] To address the above problems, this invention proposes a method for preparing an aqueous graphene dispersion, the method comprising: A carbon source is added to a mixed acid to obtain a mixed acid solution. An oxidant is added in batches to the mixed acid solution under ice bath conditions for pre-oxidation treatment to obtain graphite oxide. The graphene oxide was dispersed with deionized water to obtain an initial dispersion, and a reducing agent was added to the initial dispersion to obtain graphene colloid. The graphene colloid is modified to form a modified graphene dispersion, and the modified graphene dispersion is subjected to low-frequency exfoliation treatment to obtain a graphene dispersion. After filtering the graphene dispersion, thickeners and preservatives were added for post-treatment to obtain an aqueous graphene dispersion.
[0007] Furthermore, the carbon source is any one of flake graphite, artificial graphite, mesophase carbon microspheres, pyrolytic graphite, or expandable graphite, and the mixed acid is a mixed solution of concentrated sulfuric acid, sodium nitrate, and potassium persulfate, wherein the mass ratio of concentrated sulfuric acid, sodium nitrate, and potassium persulfate is 80-82%:8:10.
[0008] Furthermore, the reducing agent is at least one of citric acid, ascorbic acid, gallic acid, tea polyphenols, or glucose, and the mass of the reducing agent in the graphene colloid is 0.8 to 0.9% of the initial dispersion.
[0009] Further, the step of adding a carbon source to a mixed acid to obtain a mixed acid solution, and then adding an oxidant in batches to the mixed acid solution under ice bath conditions for pre-oxidation treatment to obtain graphite oxide includes: A carbon source is added to a mixed acid to form a mixed acid solution. An oxidant is added to the mixed acid solution in batches under ice bath conditions of 0–5°C to obtain a pre-oxidized mixture. The pre-oxidized mixture is heated and stirred continuously, and deionized water is added to cause the pre-oxidized mixture to expand, thereby obtaining an expanded product. The expanded product was treated with hydrogen peroxide to terminate the reaction, and the terminated product was washed and dried to obtain the graphite oxide.
[0010] Further, the step of dispersing the graphite oxide with deionized water to obtain an initial dispersion includes: The graphite oxide is added to deionized water to obtain a mixed slurry containing graphite oxide, wherein the mass ratio of graphite oxide to deionized water is 1:80-120. The mixed slurry was sheared and dispersed and then placed in a reaction vessel. The reaction vessel was heated to 120-180°C to vaporize the residual water between the layers and obtain a graphene-containing slurry. The graphene-containing slurry was naturally cooled to room temperature to obtain an initial dispersion with a solid content of 0.8–1.2 wt%.
[0011] Further, the step of adding a reducing agent to the initial dispersion to obtain graphene colloid includes: A reducing agent is added to the initial dispersion to obtain a reducing agent-containing mixture. The reducing agent-containing mixture is then stirred at a constant temperature of 30–50°C to obtain a preliminary reduced mixture. The preliminary reduced mixture was heated to 80-95°C and stirred to obtain a reduced colloid. The reduced adhesive was subjected to natural cooling to obtain the graphene colloid.
[0012] Further, the step of modifying the graphene colloid to form a modified graphene dispersion includes: A polymer is added to the graphene colloid for adsorption treatment to obtain a graphene colloid containing polymer adsorption, wherein the polymer is one of 2-methacryloyloxyethyl phosphocholine, diallyl dimethyl ammonium chloride, 3-chloro-2-hydroxypropyltrimethylammonium chloride, or a polymer of ethyleneimine. At 50–55°C, the graphene colloid containing polymer adsorption is stirred to fix the polymer on the surface of the graphene colloid to form a flexible protective layer, thereby obtaining the modified graphene dispersion.
[0013] Further, the step of subjecting the modified graphene dispersion to low-frequency exfoliation treatment to obtain the graphene dispersion includes: The modified graphene dispersion was treated with ultrasound at a frequency of 20-30 kHz and a power density of 0.3-0.6 W / mL. The ultrasound frequency and power density were then increased to 40-45 kHz and 0.8-1 W / mL, respectively, and the ultrasound treatment was continued to obtain the graphene dispersion.
[0014] Further, the step of filtering the graphene dispersion and then adding a thickener and preservative for post-treatment to obtain an aqueous graphene dispersion includes: The graphene dispersion was filtered using a filter membrane, and deionized water was added during the filtration process for multiple washes to obtain the filtered dispersion. Thickener and preservative are added to the filtered dispersion for post-treatment to form a formulated graphene dispersion. A trace amount of ammonia is added to the formulated graphene dispersion to adjust the pH to 7.2-7.8 and the mixture is stirred continuously to obtain the aqueous graphene dispersion.
[0015] This application also discloses an aqueous graphene dispersion, which is prepared by any of the above-described methods for preparing aqueous graphene dispersions.
[0016] Beneficial effects: This application proposes a method for preparing an aqueous graphene dispersion, which solves the problems of irreversible agglomeration, rapid sedimentation, and gelation of graphene when the solid content is above 10wt%. The process is mild, energy-efficient, and simple in equipment. The exfoliation process causes minimal damage to the graphene sheets. It combines excellent conductivity with excellent water dispersibility, resolves the contradiction between hydrophilicity and conjugated structure recovery at high concentrations, improves the reliability and comprehensive performance of the dispersion in industrial applications in various fields, and realizes the preparation of high-concentration, highly stable, and highly conductive aqueous graphene dispersions. Attached Figure Description
[0017] Figure 1 This is a schematic diagram of the preparation steps of an aqueous graphene dispersion according to an embodiment of the present invention.
[0018] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation
[0019] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.
[0020] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this specification means the presence of features, integers, steps, operations, elements, modules, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, modules, components, and / or groups thereof. It should be understood that when an element is referred to as “connected” or “coupled” to another element, it may be directly connected or coupled to the other element, or there may be intermediate elements. Furthermore, “connected” or “coupled” as used herein may include wireless connection or wireless coupling. The term “and / or” as used herein includes all or any modules and all combinations of one or more associated listed items.
[0021] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless specifically defined as herein.
[0022] Unless otherwise specified, the preparation methods and materials used in the following examples are conventional methods; unless otherwise specified, the parts of the materials used in the following examples are calculated by mass, and all materials used are new materials purchased from the market.
[0023] Reference Figure 1 This invention provides a method for preparing an aqueous graphene dispersion, the method comprising: S1: Add a carbon source to a mixed acid to obtain a mixed acid solution. Under ice bath conditions, add an oxidant in batches to the mixed acid solution for pre-oxidation treatment to obtain graphite oxide. In step S1, the carbon source refers to a carbon-containing raw material that can be oxidized into graphite oxide. It can be any of the following: natural flake graphite, artificial graphite, mesophase carbon microspheres, pyrolytic graphite, or expandable graphite with an average flake diameter of 50–200 micrometers. Natural flake graphite is the most commonly used due to its wide availability and high crystallinity. Take 10 parts by weight of the above carbon source and slowly add it to a mixed acid system that has been pre-cooled to 0–5°C. The mixed acid specifically consists of 120 parts by weight of 98% concentrated sulfuric acid, 12 parts by weight of sodium nitrate, and 15 parts by weight of potassium persulfate. The concentrated sulfuric acid acts as the main intercalating agent and dehydrating agent, rapidly entering the graphite interlayer to form graphite interlayer compounds. Sodium nitrate provides a strong oxidizing environment and assists in generating nitrocellulose ions to further expand the interlayer spacing. Potassium persulfate acts as an auxiliary oxidant, initiating a partial oxidation reaction at low temperatures. After adding the carbon source, continue stirring to fully wet and disperse the graphite, avoiding localized agglomeration. Under ice bath conditions controlled at 0–8°C, add the main oxidant in batches. The oxidant can be any one of potassium permanganate, potassium dichromate, potassium chlorate, ammonium persulfate, or benzoyl peroxide, preferably potassium permanganate. The total amount added is 18 parts by weight, with each addition not exceeding 6 parts by weight, and the interval between two consecutive additions is at least 8 minutes to prevent excessively vigorous local reactions that could cause a rapid rise in system temperature. Potassium permanganate in a concentrated sulfuric acid environment slowly generates active intermediates such as Mn₂O₇, which gently oxidizes graphite edges and defect sites, inserting oxygen-containing groups such as epoxy, hydroxyl, and carboxyl groups, while avoiding over-oxidation in the main reaction stage. After all the oxidant has been added, the system is removed from the ice bath and slowly heated to 35°C, and stirred continuously at this temperature for 40 minutes. This medium-temperature holding stage allows the pre-oxidation reaction to proceed fully, further widening the interlayer spacing of the graphite while maintaining a relatively intact sp₂ carbon network. 80 parts by weight of deionized water are added to the reaction system at once, and the temperature is rapidly raised to 90°C and maintained for 20 minutes. The addition of a large amount of deionized water dilutes the concentrated sulfuric acid, releasing enormous heat. Simultaneously, the sulfuric acid molecules inserted between the layers undergo intense hydration with water, generating strong internal pressure. Combined with the partial decomposition of oxygen-containing groups due to high temperature, releasing gases such as CO2 and CO, the system expands dramatically, increasing in volume by more than 200 times, forming a fluffy, worm-like graphene oxide structure. After this extremely vigorous expansion reaction, 40 parts by weight of 30% hydrogen peroxide are added. The hydrogen peroxide rapidly reacts with residual potassium permanganate and manganese dioxide, generating soluble manganese sulfate and releasing oxygen. The system color changes from dark purple to bright yellow, indicating the reaction has terminated and the residual oxidant has been removed. The resulting worm-like graphene oxide is diluted in a large amount of deionized water and repeatedly filtered or centrifuged until the pH of the filtrate is greater than 5 to remove soluble impurities such as sulfate and manganese ions. The washed graphene oxide is then dried in a 50°C forced-air drying oven to obtain graphene oxide. The oxygen-to-carbon atom ratio of graphite oxide is controlled at 0.6 to 0.8, the expansion ratio is greater than 200 times, and the interlayer contains a large number of active epoxy groups, hydroxyl groups and carboxyl groups.
[0024] S2: The graphene oxide is dispersed with deionized water to obtain an initial dispersion, and a reducing agent is added to the initial dispersion to obtain graphene colloid; In step S2, 10 parts (by mass) of the graphite oxide obtained in step S1 are added to 990 parts of deionized water (i.e., a mass ratio of approximately 1:99), with the solid content controlled at approximately 1 wt%. The graphite oxide and deionized water are added to a high-speed disperser and pre-dispersed at a high speed of 4000–6000 rpm for 15–20 minutes. The high-speed disperser rapidly disperses the dry, fluffy, worm-like graphite oxide through strong mechanical shearing and turbulent flow, allowing it to be initially wetted, unbound, and form a coarse dispersion system in water. After pre-dispersion, the coarse dispersion system is transferred to a high-pressure reactor (i.e., a hydrothermal reactor) with a polytetrafluoroethylene lining or a stainless steel inner liner. After sealing, the temperature is raised to 160–180°C at a rate of 2–3°C / min and maintained at this temperature for 30–50 minutes. The bound water and oxygen-containing functional groups remaining between the layers of highly active graphene oxide rapidly vaporize at high temperatures, generating enormous internal pressure. Simultaneously, the high temperature triggers partial pyrolysis of the oxygen-containing functional groups, producing gases such as CO and CO2. These gases, along with the vapor, exert a strong thrust on the interlayer, achieving efficient self-exfoliation of the graphene oxide. After the reaction, the mixture is naturally cooled to room temperature, and the reactor is opened to obtain a primary dispersion of graphene oxide. At this point, the average thickness of the graphene oxide is less than 1 nanometer, the monolayer ratio is greater than 95%, and the absolute value of the Zeta potential is greater than 50 mV, indicating that the sheet surface carries a large number of negative charges. Take 1000 parts of the above primary dispersion of graphene oxide (containing approximately 10 parts of solid graphene oxide), and add 8–12 parts of a green reducing agent. The green reducing agent can be selected from food-grade or environmentally friendly weak reducing agents such as citric acid, ascorbic acid (i.e., vitamin C), tea polyphenols, gallic acid, and glucose, with L-ascorbic acid being preferred. First, the graphene sheets are gently stirred in a constant-temperature water bath at 55–65°C for 20–40 minutes to remove epoxy groups and some hydroxyl groups from the surface. Then, the temperature is raised to 80–90°C and the reaction continues for another 20–30 minutes to further remove edge carboxyl groups and remaining hydroxyl groups. The entire reduction process is carried out in two temperature stages, maintaining an oxygen-to-carbon ratio between 0.18 and 0.22, retaining approximately 18%–22% of oxygen-containing functional groups, increasing the conductivity to over 8000–10000 S / m, and preserving sufficient hydrophilic groups (mainly edge carboxyl groups and a small amount of phenolic hydroxyl groups) to prevent irreversible stacking in water over a long period. After the reduction reaction is complete, the sheets are naturally cooled to room temperature to obtain a partially reduced graphene colloid (rGO colloid or graphene aqueous dispersion) with a solid content of approximately 1 wt% and significant fluidity.
[0025] S3: Modify the graphene colloid to form a modified graphene dispersion, and perform low-frequency exfoliation treatment on the modified graphene dispersion to obtain a graphene dispersion. In step S3, the graphene colloid is modified to form a modified graphene dispersion, and a zwitterionic polymer is added for non-covalent functionalization modification. Zwitterionic polymers are polymers whose molecular chains simultaneously contain positively charged centers (such as quaternary ammonium salt groups) and negatively charged centers (such as phosphate, sulfonate, or carboxylate groups), including poly(2-methacryloyloxyethyl phosphocholine) (MPC polymer), poly(diallyl dimethyl ammonium chloride) (PDDA), poly(3-chloro-2-hydroxypropyltrimethylammonium chloride), branched polyethyleneimine (PEI), or chitosan, etc., with molecular weights between 30,000 and 80,000. These polymers exhibit electroneutrality or weakly cationic properties in water, but the distance between their positive and negative charge centers is very close (usually less than 1 nm), enabling them to form extremely strong multi-point electrostatic adsorption on the graphene surface, while simultaneously generating steric hindrance effects through their long-chain hydrophobic framework. The partially reduced graphene colloid was heated to approximately 55°C, and about 1.2 parts (relative to the initial 10 parts of the original carbon source) of the selected zwitterionic polymer were added. The mixture was reacted for about 35 minutes with stirring at a moderate speed. During the reaction, the polymer molecules were electrostatically attracted to the positively charged centers of the polymer through residual carboxyl and hydroxyl groups; the negatively charged centers of the polymer interacted with the cation-π interaction of the graphene π electron cloud; and the hydrophobic segments of the polymer interacted with the graphene sp... 2 The hydrophobic interactions between the regions allow the polymer to form a flexible protective layer approximately 5–8 nanometers thick on the graphene surface, providing electrostatic repulsion (the absolute value of the zeta potential increases to over 60 mV after modification) and steric repulsion, making it virtually impossible for the sheets to approach each other even at a high concentration of 20 wt%. The modified dispersion was placed in a 35°C constant temperature water bath and initially tested at a frequency of 28 kHz and a power density of 0.5 W / cm². 2 The device was treated with a standard ultrasonic cleaner for 20 minutes, then switched to a 40kHz frequency and a power density of 0.9W / cm². 2 After 25 minutes of further processing, the cavitation bubbles at higher frequencies became smaller and more numerous, generating denser microjets. These, combined with the loosened interlayer forces, separated the remaining 2-3 layers. The alternation of the two frequencies created a multi-scale synergistic cavitation effect, reducing the average number of layers to below 1.1, with a single-layer ratio approaching 100%. After processing, the dispersion was a mirror-like deep black. Even after centrifugation for 30 minutes, the supernatant remained completely transparent and free of any precipitate, yielding a graphene dispersion.
[0026] S4: After filtering the graphene dispersion, thickener and preservative are added for post-treatment to obtain an aqueous graphene dispersion.
[0027] In step S4, over 99% of the graphene in the dispersion has achieved monolayer or few-layer stability. Cross-flow ultrafiltration is employed using ceramic membranes or polyethersulfone (PES) hollow fiber ultrafiltration membranes with a molecular weight cutoff (MWCO) of 100kDa–300kDa, with dynamic circulation filtration at a low transmembrane pressure of 0.1–0.2 MPa. This cross-flow mode reduces membrane fouling, and continuous deionized water replenishment during dialysis washing thoroughly removes free zwitterionic polymers, small-molecule residual reducing agents, and their byproducts. Generally, 3–5 washes are performed until the conductivity of the filtrate is below 10 μS / cm, removing small-molecule impurities that could compromise long-term stability. After filtration, the process proceeds to a post-treatment stage involving concentration and the addition of thickeners and preservatives. The solid content is increased from approximately 1 wt% to 18–22 wt%. Concentration also utilizes the same ceramic ultrafiltration membrane system as filtration, maintaining the cross-flow ultrafiltration mode until the volume is concentrated approximately 20 times to achieve the target solid content. During this process, the temperature is controlled at 30–40°C and gentle stirring is maintained to prevent localized over-concentration leading to gelation. After concentration to approximately 20 wt% solids, the interlayer spacing of the graphene sheets decreases sharply. A thickener is then added to construct a weak gel structure. Thickeners such as xanthan gum, sodium carboxymethyl cellulose (CMC-Na), hydroxyethyl cellulose (HEC), gum arabic, or locust bean gum can be used, with an addition amount of 0.1%–0.3% (relative to the final dispersion mass). A preservative is added after the thickener. Preservatives such as potassium sorbate, sodium benzoate, sodium dehydroacetate, calcium propionate, or nisin can be used, with an addition amount of 0.05%–0.1% to achieve long-lasting antibacterial effects. Potassium sorbate remains highly effective within a pH range of 3–8, making it particularly suitable for this system. To ensure sufficient swelling and hydration of the thickener and uniform dispersion of the preservative, the mixture was stirred at a medium-high speed of 2500–3500 rpm for 10–15 minutes. Simultaneously, a trace amount of ammonia or sodium hydroxide solution was added dropwise to precisely adjust the pH of the system to 7.2–7.8, maximizing the thickening efficiency of thickeners such as xanthan gum and maintaining the highest activity of preservatives such as potassium sorbate. After the above-mentioned filtration, ultrafiltration concentration, and synergistic post-treatment of thickener and preservative, the resulting aqueous graphene dispersion had a stable solid content of 18–22 wt%, an average number of layers of less than 1.3, a conductivity exceeding 12000 S / m, a viscosity of 1500–4000 mPa·s, and a pH of 7.2–7.8. Under accelerated testing at 60℃, no precipitation, stratification, or mold growth was observed, and the centrifugal stability (15000 rpm, 30 minutes) was also demonstrated.
[0028] It is worth noting that although the above embodiments do not list all possible raw material ratios and process parameter ranges, those skilled in the art can adjust key parameters such as molecular weight, amount added, and pH value within the recommended range according to actual needs.
[0029] In another embodiment, referring to Table 1, comparative examples were selected for performance testing. All samples were tested under the same conditions. The solid content was controlled at 20wt%±0.5wt%, the test temperature was 25±0.5℃, and the testing instruments were uniformly an aeroluminescence microscope (AFM) to count the number of layers and the monolayer rate. The conductivity was measured using a four-probe tester for the dried film, and the zeta potential was measured using a Malvern Zetasizer Nano-ZS. The accelerated aging test was conducted in a 60℃ constant temperature forced-air drying oven, and the centrifugal stability was tested in a 15000rpm high-speed centrifuge. Long-term static storage at room temperature was carried out under conditions of no light and no vibration. Comparative Example 1 is a preparation process using the existing modified Hummers method combined with a large amount of ionic surfactants (such as sodium dodecyl sulfate SDS or CTAB). This process does not perform pre-oxidation and hydrothermal self-exfoliation. The exfoliation mainly relies on long-term high-power ultrasound, which leads to severe layer defects, excessive reduction, low conductivity, and extremely poor stability at high concentrations. Obvious delamination appears after 9 days, and complete gelation occurs after 21 days. Comparative Example 2 is an existing 20wt% graphene aqueous slurry. Although it can barely maintain its properties without sedimentation for 3 months, the number of layers is relatively thick, the conductivity is only about 6000 S / m, and there is obvious precipitation after centrifugation. The core difference between this example and the two comparative examples is that: through pre-oxidation, high-temperature expansion, and hydrothermal self-exfoliation processes, a single-layer ratio of >98% is achieved. Combined with a green weak reducing agent, 18-22% of oxygen-containing groups are precisely retained. Even at high concentrations, it maintains extremely high Zeta potential and steric hindrance effect, conductivity exceeding 12000 S / m, and moderate viscosity. After 30 days at 60℃ and 6 months at room temperature, there is still no precipitation or thickening. After centrifugation at 15000 rpm for 30 minutes, the supernatant is clear and transparent. This example achieves an industrial-grade aqueous graphene dispersion with high solid content (20wt%), high conductivity, and high stability.
[0030] Table 1:
[0031] In one embodiment, the carbon source is any one of flake graphite, artificial graphite, mesophase carbon microspheres, pyrolytic graphite, or expandable graphite, and the mixed acid is a mixed solution of concentrated sulfuric acid, sodium nitrate, and potassium persulfate, wherein the mass ratio of concentrated sulfuric acid, sodium nitrate, and potassium persulfate is 80-82%:8:10.
[0032] In one embodiment, the reducing agent is at least one of citric acid, ascorbic acid, gallic acid, tea polyphenols, or glucose, and the mass of the reducing agent in the graphene colloid is 0.8 to 0.9% of the initial dispersion.
[0033] In one embodiment, the step of adding a carbon source to a mixed acid to obtain a mixed acid solution, and then adding an oxidant in batches to the mixed acid solution under ice bath conditions for pre-oxidation treatment to obtain graphite oxide includes: A carbon source is added to a mixed acid to form a mixed acid solution. An oxidant is added to the mixed acid solution in batches under ice bath conditions of 0–5°C to obtain a pre-oxidized mixture. The pre-oxidized mixture is heated and stirred continuously, and deionized water is added to cause the pre-oxidized mixture to expand, thereby obtaining an expanded product. The expanded product was treated with hydrogen peroxide to terminate the reaction, and the terminated product was washed and dried to obtain the graphite oxide.
[0034] In the above embodiments, any one of the following—natural flake graphite, artificial graphite, mesophase carbon microspheres, pyrolytic graphite, or expandable graphite with an average flake diameter of 50–200 micrometers—is selected as a carbon source. Ten parts of this carbon source are added to a mixed acid system consisting of 120 parts of concentrated sulfuric acid with a mass concentration of 98%, 12 parts of sodium nitrate, and 15 parts of potassium persulfate. The mixture is stirred thoroughly to form a mixed acid solution. The system is placed in an ice bath at 0–5°C and stirred vigorously. 18 parts of potassium permanganate (which can also be replaced by potassium dichromate, potassium chlorate, etc.) are slowly added in batches as the main oxidant. Each addition shall not exceed 6 parts and shall be spaced at least 8 minutes apart to ensure that the system temperature remains below 20°C and to avoid the risk of explosion due to local overheating. After adding the oxidant, the system was heated to 35°C and stirred for 40 minutes to complete the low-temperature pre-oxidation. Then, 80 parts of deionized water were added quickly at once, and the temperature was rapidly raised to 90°C and maintained for 20 minutes. At this time, the sulfuric acid molecules inserted in the interlayer reacted violently with the water to produce a large amount of gas, and the system expanded rapidly to form a fluffy worm-like structure with an expansion ratio of more than 200 times. Immediately, 40 parts of 30% hydrogen peroxide were added to terminate the reaction. The hydrogen peroxide can not only quickly consume the residual potassium permanganate to generate oxygen to further assist the expansion, but also reduce divalent manganese to soluble divalent manganese ions for easy subsequent washing. The obtained product was repeatedly centrifuged or filtered with deionized water until the pH of the washing solution was >5, and then dried at 50°C for 2 hours to obtain highly active graphite oxide with an oxygen-to-carbon ratio of 0.6-0.8 and a highly porous structure.
[0035] In one embodiment, the step of dispersing the graphite oxide with deionized water to obtain an initial dispersion includes: The graphite oxide is added to deionized water to obtain a mixed slurry containing graphite oxide, wherein the mass ratio of graphite oxide to deionized water is 1:80-120. The mixed slurry was sheared and dispersed and then placed in a reaction vessel. The reaction vessel was heated to 120-180°C to vaporize the residual water between the layers and obtain a graphene-containing slurry. The graphene-containing slurry was naturally cooled to room temperature to obtain an initial dispersion with a solid content of 0.8–1.2 wt%.
[0036] In the above embodiment, 10 parts of highly active graphene oxide were added to 990 parts of deionized water (mass ratio 1:99). The mixture was first pre-dispersed by shearing at 4000-6000 rpm in a disperser for 15-20 minutes to fully wet the graphene oxide and initially dissociate it into larger blocks. The slurry was then transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene. After sealing, the temperature was raised to 120-180°C (preferably 180°C) at a rate of 3-5°C / min and held for 30-60 minutes. At this time, the bound water and oxygen-containing functional groups remaining between the layers rapidly vaporized due to the heat, generating huge internal pressure. Together with thermal stress, this pressure spread the graphene oxide between the layers, achieving near-single-layer or few-layer self-exfoliation. After the holding period, the reactor was naturally cooled to room temperature to obtain an initial dispersion of brownish-yellow transparent graphene oxide with a solid content of 0.8-1.2 wt%.
[0037] In one embodiment, the step of adding a reducing agent to the initial dispersion to obtain graphene colloid includes: A reducing agent is added to the initial dispersion to obtain a reducing agent-containing mixture. The reducing agent-containing mixture is then stirred at a constant temperature of 30–50°C to obtain a preliminary reduced mixture. The preliminary reduced mixture was heated to 80-95°C and stirred to obtain a reduced colloid. The reduced adhesive was subjected to natural cooling to obtain the graphene colloid.
[0038] In the above embodiments, 8-9 parts of any one or more of citric acid, ascorbic acid, gallic acid, tea polyphenols, or glucose (0.8-0.9% by mass) were added to 1000 parts of the initial dispersion. The mixture was gently stirred in a constant temperature water bath at 30-50°C (ideally 60°C) for 30-40 minutes to remove highly reactive epoxy groups and some hydroxyl groups from the graphene sheets. The system was then heated to 80-95°C (preferably 85°C) and stirred for another 20-30 minutes to remove edge carboxyl groups and remaining hydroxyl groups. During the reaction, the color of the dispersion gradually changed from brownish-yellow to dark black. At this point, the graphene sp... 2 The conjugated structure is largely restored, and after the reaction is completed, it is naturally cooled to room temperature to obtain partially reduced graphene colloid.
[0039] In one embodiment, the step of modifying the graphene colloid to form a modified graphene dispersion includes: A polymer is added to the graphene colloid for adsorption treatment to obtain a graphene colloid containing polymer adsorption, wherein the polymer is one of 2-methacryloyloxyethyl phosphocholine, diallyl dimethyl ammonium chloride, 3-chloro-2-hydroxypropyltrimethylammonium chloride, or a polymer of ethyleneimine. At 50–55°C, the graphene colloid containing polymer adsorption is stirred to fix the polymer on the surface of the graphene colloid to form a flexible protective layer, thereby obtaining the modified graphene dispersion.
[0040] In the above embodiments, 1 to 1.5 parts of any one of the following polymers with a molecular weight of 30,000 to 80,000—poly(2-methacryloyloxyethyl)phosphocholine (MPC), poly(diallyldimethylammonium chloride) (PDDA), poly(3-chloro-2-hydroxypropyltrimethylammonium chloride), or polyethyleneimine—are added to the graphene colloid. These polymers all possess positively and negatively charged groups or strongly polar groups, enabling them to rapidly attach to the partially reduced graphene surface through multiple electrostatic attraction, hydrogen bonding, and hydrophobic interactions. The mixture is stirred at a constant temperature of 50–55°C for 30–40 minutes to allow the polymer chains to fully extend and firmly fix, forming a flexible protective layer with a thickness of approximately 5–8 nm. This protective layer simultaneously provides strong electrostatic repulsion (the absolute value of the Zeta potential increases to over 60 mV) and steric hindrance repulsion, blocking π-π stacking and van der Waals attraction between graphene sheets, ensuring that the dispersion maintains fluidity and long-term stability even when concentrated to 20 wt%.
[0041] In one embodiment, the step of subjecting the modified graphene dispersion to low-frequency exfoliation to obtain a graphene dispersion includes: The modified graphene dispersion was treated with ultrasound at a frequency of 20-30 kHz and a power density of 0.3-0.6 W / mL. The ultrasound frequency and power density were then increased to 40-45 kHz and 0.8-1 W / mL, respectively, and the ultrasound treatment was continued to obtain the graphene dispersion.
[0042] In this embodiment, the modified graphene dispersion was placed in a constant temperature water bath at 30–35°C and treated for 15–25 minutes with an ultrasonic cleaner at a frequency of 20–30 kHz (optimal 28 kHz) and a power density of 0.3–0.6 W / mL to generate large cavitation bubbles that initially loosen the remaining low-layer regions. The frequency was then increased to 40–45 kHz and the power density was increased to 0.8–1 W / mL for another 20–30 minutes. The high-frequency small cavitation bubbles, lubricated by the flexible polymer protective layer, completely peeled off the remaining multilayer regions to an average number of layers <1.1. The entire process alternated between the two frequencies to generate a multi-scale cavitation effect, resulting in the graphene dispersion.
[0043] In one embodiment, the step of filtering the graphene dispersion and then adding a thickener and preservative for post-treatment to obtain an aqueous graphene dispersion includes: The graphene dispersion was filtered using a filter membrane, and deionized water was added during the filtration process for multiple washes to obtain the filtered dispersion. Thickener and preservative are added to the filtered dispersion for post-treatment to form a formulated graphene dispersion. A trace amount of ammonia is added to the formulated graphene dispersion to adjust the pH to 7.2-7.8 and the mixture is stirred continuously to obtain the aqueous graphene dispersion.
[0044] In the above embodiments, the graphene dispersion was subjected to cross-flow ultrafiltration at 0.1–0.2 MPa using a ceramic or organic membrane with a molecular weight cutoff of 100 kDa, while continuously adding deionized water for 3–5 dialysis washes to remove free polymers and small molecule impurities, resulting in a pure dispersion. After further ultrafiltration and concentration to a solid content of 18–22 wt%, 0.1–0.3 parts of thickeners such as xanthan gum, sodium carboxymethyl cellulose, or hydroxyethyl cellulose, and 0.05–0.08 parts of preservatives such as potassium sorbate or sodium benzoate were added. A trace amount of ammonia was then added to adjust the pH to 7.2–7.8. Finally, the mixture was stirred at 2000–3000 rpm for 10–15 minutes to form a three-dimensional weak gel structure cross-linked by thickener molecules, thus obtaining an aqueous graphene dispersion.
[0045] This application also discloses an aqueous graphene dispersion, which is prepared by any of the above-described methods for preparing aqueous graphene dispersions, and includes the corresponding technical features of the above-described preparation methods, which will not be repeated here.
[0046] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.
Claims
1. A method for preparing an aqueous dispersion of graphene, characterized in that, The method includes: A carbon source is added to a mixed acid to obtain a mixed acid solution. An oxidant is added in batches to the mixed acid solution under ice bath conditions for pre-oxidation treatment to obtain graphite oxide. The graphene oxide was dispersed with deionized water to obtain an initial dispersion, and a reducing agent was added to the initial dispersion to obtain graphene colloid. The graphene colloid is modified to form a modified graphene dispersion, and the modified graphene dispersion is subjected to low-frequency exfoliation treatment to obtain a graphene dispersion. After filtering the graphene dispersion, thickeners and preservatives were added for post-treatment to obtain an aqueous graphene dispersion.
2. The method for preparing the aqueous graphene dispersion according to claim 1, characterized in that, The carbon source is any one of flake graphite, artificial graphite, mesophase carbon microspheres, pyrolytic graphite, or expandable graphite, and the mixed acid is a mixed solution of concentrated sulfuric acid, sodium nitrate, and potassium persulfate, wherein the mass ratio of concentrated sulfuric acid, sodium nitrate, and potassium persulfate is 80-82%:8:
10.
3. The method for preparing the aqueous graphene dispersion according to claim 1, characterized in that, The reducing agent is at least one of citric acid, ascorbic acid, gallic acid, tea polyphenols, or glucose, and the mass of the reducing agent in the graphene colloid is 0.8 to 0.9% of the initial dispersion.
4. The method for preparing the aqueous graphene dispersion according to claim 1, characterized in that, The steps of adding a carbon source to a mixed acid to obtain a mixed acid solution, and then adding an oxidant in batches to the mixed acid solution under ice bath conditions for pre-oxidation treatment to obtain graphite oxide include: A carbon source is added to a mixed acid to form a mixed acid solution. An oxidant is added to the mixed acid solution in batches under ice bath conditions of 0–5°C to obtain a pre-oxidized mixture. The pre-oxidized mixture is heated and stirred continuously, and deionized water is added to cause the pre-oxidized mixture to expand, thereby obtaining an expanded product. The expanded product was treated with hydrogen peroxide to terminate the reaction, and the terminated product was washed and dried to obtain the graphite oxide.
5. The method for preparing the aqueous graphene dispersion according to claim 1, characterized in that, The step of dispersing the graphite oxide with deionized water to obtain an initial dispersion includes: The graphite oxide is added to deionized water to obtain a mixed slurry containing graphite oxide, wherein the mass ratio of graphite oxide to deionized water is 1:80-120. The mixed slurry was sheared and dispersed and then placed in a reaction vessel. The reaction vessel was heated to 120-180°C to vaporize the residual water between the layers and obtain a graphene-containing slurry. The graphene-containing slurry was naturally cooled to room temperature to obtain an initial dispersion with a solid content of 0.8–1.2 wt%.
6. The method for preparing the aqueous graphene dispersion according to claim 1, characterized in that, The step of adding a reducing agent to the initial dispersion to obtain graphene colloid includes: A reducing agent is added to the initial dispersion to obtain a reducing agent-containing mixture. The reducing agent-containing mixture is then stirred at a constant temperature of 30–50°C to obtain a preliminary reduced mixture. The preliminary reduced mixture was heated to 80-95°C and stirred to obtain a reduced colloid. The reduced adhesive was subjected to natural cooling to obtain the graphene colloid.
7. The method for preparing the aqueous graphene dispersion according to claim 1, characterized in that, The step of modifying the graphene colloid to form a modified graphene dispersion includes: A polymer is added to the graphene colloid for adsorption treatment to obtain a graphene colloid containing polymer adsorption, wherein the polymer is one of 2-methacryloyloxyethyl phosphocholine, diallyl dimethyl ammonium chloride, 3-chloro-2-hydroxypropyltrimethylammonium chloride, or a polymer of ethyleneimine. At 50–55°C, the graphene colloid containing polymer adsorption is stirred to fix the polymer on the surface of the graphene colloid to form a flexible protective layer, thereby obtaining the modified graphene dispersion.
8. The method for preparing the aqueous graphene dispersion according to claim 1, characterized in that, The step of subjecting the modified graphene dispersion to low-frequency exfoliation treatment to obtain a graphene dispersion includes: The modified graphene dispersion was treated with ultrasound at a frequency of 20-30 kHz and a power density of 0.3-0.6 W / mL. The ultrasound frequency and power density were then increased to 40-45 kHz and 0.8-1 W / mL, respectively, and the ultrasound treatment was continued to obtain the graphene dispersion.
9. The method for preparing the aqueous graphene dispersion according to claim 1, characterized in that, The step of filtering the graphene dispersion and then adding a thickener and preservative for post-treatment to obtain an aqueous graphene dispersion includes: The graphene dispersion was filtered using a filter membrane, and deionized water was added during the filtration process for multiple washes to obtain the filtered dispersion. Thickener and preservative are added to the filtered dispersion for post-treatment to form a formulated graphene dispersion. A trace amount of ammonia is added to the formulated graphene dispersion to adjust the pH to 7.2-7.8 and the mixture is stirred continuously to obtain the aqueous graphene dispersion.
10. An aqueous dispersion of graphene, characterized in that, It is prepared by the method for preparing the aqueous graphene dispersion as described in any one of claims 1-9.