A method for preparing reduced graphene oxide by acid-free microwave-assisted and plasma treatment

Abstract

The application relates to the technical field of graphene material preparation and functional composite material, and discloses a preparation method of acid-free microwave-assisted and plasma-treated reduced graphene oxide, which comprises six core steps of sequentially performed electrochemical stripping, hydrogen peroxide post-functionalization, washing and purification, freeze drying, microwave expansion and oxygen plasma treatment, and the specific steps are as follows: step one, first electrochemical stripping, high-purity graphite raw materials are selected, a polypropylene material electrochemical reactor is taken as a reaction container, and 1.2 kg of graphite raw materials are fixed as an anode assembly. In the application, ammonium bisulfate (NH4HSO4) electrolyte is used to replace strong inorganic acid, the pH value of the system is controlled to be 2-3, no toxic gas is generated in the reaction process, the wastewater only contains ammonium salt and sulfate salt, and the wastewater can be discharged after simple neutralization treatment (the treatment cost is reduced by 70%); the corrosion of strong acid to equipment and the safety risk of personnel are avoided, the service life of the equipment is prolonged by 50%, and the safety production coefficient is significantly improved.

Description

Technical Field

[0001] This invention relates to the field of graphene material preparation and functional composite material technology, specifically a method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide. Background Technology

[0002] Traditional methods for preparing reduced graphene oxide (rGO) suffer from four major drawbacks, severely restricting its large-scale application and environmental friendliness: Dependence on strong acids poses high environmental and safety risks: Traditional processes such as the mainstream Hummers process require the use of strong inorganic acids such as concentrated sulfuric acid and nitric acid. During the reaction, a large amount of acidic wastewater and toxic gases are generated, resulting in high wastewater treatment costs (accounting for more than 30% of the total production cost). Furthermore, strong acids can easily corrode equipment and cause safety accidents. At the same time, residual acid radicals are difficult to remove completely, affecting the purity of the product and its compatibility with subsequent applications.

[0003] Insufficient product performance and poor application adaptability: rGO prepared by traditional methods has low density of oxygen-containing functional groups (such as hydroxyl and epoxy groups) on the surface (O / C ratio mostly ≤0.35), small specific surface area (mostly ≤300m² / g), serious interlayer aggregation, and poor dispersion stability (Zeta potential absolute value ≤25mV). When used as a filler, it has weak interfacial bonding with the matrix such as resin, making it difficult to significantly improve the performance of composite materials (such as tensile strength improvement rate mostly ≤15%).

[0004] Low stripping efficiency and long process cycle: Traditional electrochemical stripping often uses a single electrolyte system, resulting in low intercalation efficiency and graphite stripping time of 8-12 hours; washing and purification rely on single filtration or centrifugation, which does not completely remove impurities (the conductivity of the filtrate is often ≥100μS / cm) and takes more than 24 hours. The overall process cycle is long and difficult to scale up.

[0005] Uneven functionalization and poor product consistency: Traditional functionalization processes lack precise temperature and pH control, resulting in unstable generation of hydroxyl radicals, leading to uneven distribution and severe aggregation of functional groups on the rGO surface; subsequent drying and expansion processes easily cause secondary aggregation of the lamellar sheets, with the product having ≥8 layers, which is difficult to meet the requirements of high-performance composite materials for fillers with low layer number and high uniformity. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide, in order to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide, comprising six core steps performed sequentially: electrochemical exfoliation, post-functionalization with hydrogen peroxide, washing and purification, freeze-drying, microwave expansion, and oxygen plasma treatment. The specific steps are as follows: Step 1: First, perform electrochemical stripping. Select high-purity graphite raw material and use an electrochemical reactor made of polypropylene as the reaction vessel. Fix 1.2 kg of graphite raw material as the anode assembly and use a large-area platinum-coated titanium mesh or stainless steel plate as the cathode. The distance between the anode and the cathode in the reactor is controlled to be 2~3 cm. To prepare the electrolyte solution, add 2-5 L of deionized water to the reactor. Under a top-mounted mechanical stirrer with a power of 300-500 W, slowly add 2-4 kg of ammonium bisulfate (NH₄HSO₄) and stir for 1-2 hours until completely dissolved, forming an electrolyte solution with a concentration of 0.5-1.0 M and a pH of 2-3. The HSO₄⁻ produced by the dissociation of ammonium bisulfate in this solution... - The ion acts as an excellent intercalating agent, and its dissociation formula is NH4HSO4 → NH4 + +HSO4 - ; Connect the anode assembly to the positive terminal of an industrial-grade DC power supply, and the cathode to the negative terminal. Turn on the stirrer for gentle stirring and apply a constant DC voltage of +5~+10V. Graphite oxidation and exfoliation reactions occur at the anode. The main anode reaction is C(s) + 2H₂O → CO₂↑ + 4H₂O. + +4e - C(s) + H₂O → CO↑ + 2H₂O + +2e - ; The CO2, CO, and O2 gases generated between the layers create pressure, which assists in the exfoliation of the graphite layers. This leads to the hydrogen evolution reaction at the cathode, with the reaction equation being 2H₂O. + +2e - →H2↑, continue the reaction for 2~6h until the graphite anode is basically consumed, and a black electrochemically exfoliated graphene oxide (EEGO) dispersion is obtained; Step 2: Next, perform hydrogen peroxide post-functionalization. Transfer the above EEGO dispersion to a glass-lined or polypropylene reactor equipped with a heater and stirrer. While continuously stirring, slowly add 2-3 L of 30% hydrogen peroxide (H₂O₂) at a rate of 50-100 mL / min to avoid excessive exothermic reaction. Then heat the mixture to 60-80°C and maintain this temperature for 6-12 hours. During this process, residual metal ions or carbon defects catalyze a Fenton-like reaction in H₂O₂, decomposing it to produce highly reactive hydroxyl radicals (·OH). The reaction formula is: H₂O₂ → H₂O + ·OH + ·O -Hydroxyl radicals violently attack the sp² carbon network of the graphene lattice, introducing a large number of hydroxyl (-OH) and epoxy (COC) groups, thereby enhancing the functionalization of the material. Step 3: Then, wash and purify the mixture after the reaction. Filter the mixture through a large vacuum filtration system consisting of 20L Buchner funnels, collect the filter cake and redisperse it in deionized water. Repeat the "dispersion-filtration" washing cycle 3 to 5 times until the conductivity of the filtrate is ≤50μS / cm to ensure that impurities such as ammonium salts and sulfates are completely removed. Step 4: After washing, the filter cake is made into a thick water-based slurry with a mass fraction of 10%~20%, and dried by freeze drying. The slurry is frozen with liquid nitrogen and then placed in a freeze dryer to dry for 48~72 hours to obtain fluffy aerogel-like GO foam. Step 5: Next, microwave expansion is performed. The dried GO foam is placed in a large glass tray at a rate of 20-30g / batch and placed in an industrial microwave oven with a power of ≥1500W for 30-60s of high-power microwave treatment. Under the action of microwaves, the GO foam expands violently, increasing in volume by 100-300 times, forming black and fluffy rGO powder. During the microwave treatment, the oxygen-containing functional groups between the GO layers decompose to generate gas, further peeling off the layers and forming a porous structure. Step Six: Finally, oxygen plasma treatment is performed. The microwave-expanded rGO powder is placed in an oxygen plasma processing device, and oxygen with a purity ≥99.99% is introduced. The oxygen flow rate is controlled at 50~100 sccm, the plasma power is 50~100W, and the treatment time is 30~60s. The content of -OH functional groups on the rGO surface is further enhanced by plasma bombardment, and the target product is finally obtained. The product has an O / C ratio of 0.4~0.55, a specific surface area of ​​300~600m² / g, a Zeta potential of -30~-50mV, and 2~10 layers, making it suitable as a nanofiller for sheet molding compounds (SMC).

[0008] Preferably, in the electrochemical stripping step, when using pre-expanded graphite as raw material, the pre-expanded graphite needs to be compressed into a rod-shaped structure first; 200-500 mL of 30% hydrogen peroxide can also be added to the electrolyte solution to introduce some oxide groups in advance, accelerating the intercalation and stripping between graphite layers. At this time, the composition of the electrolyte solution is 0.5-1.0M ammonium bisulfate + 5%-10% hydrogen peroxide, and the reaction temperature is controlled at 25-30℃ by a water bath, which can shorten the electrochemical stripping time to 1-3 hours; the anode assembly adopts a parallel structure of multiple graphite rods, which are evenly distributed in the central area of ​​the reactor; at the same time, a peristaltic pump is set outside the reactor to circulate the electrolyte solution at a flow rate of 950 mL / h, continuously delivering fresh electrolyte and removing the stripped graphene sheets, maintaining a uniform ion concentration in the reactor, and reducing local ion loss.

[0009] Preferably, in the post-functionalization step of hydrogen peroxide, a segmented temperature control strategy is adopted to optimize the reaction effect, specifically as follows: First, the mixture is heated from room temperature to 60°C and kept at that temperature for 2-4 hours. Then, it is heated to 70-80°C and kept at that temperature for 4-8 hours. By slowly heating the mixture, the rapid decomposition of hydrogen peroxide is avoided, and the oxidative functionalization effect of hydroxyl radicals is fully utilized. During the reaction, the pH value of the solution is monitored in real time by an online pH meter. When the pH value drops to 1-2, 50-100 mL of deionized water is slowly added to maintain the pH value in the range of 1-3, ensuring the continuous generation of hydroxyl radicals. The amount of hydrogen peroxide added is precisely controlled according to the mass of EEGO. The mass-volume ratio of EEGO to hydrogen peroxide is 1g:2~3mL. During the addition process, the temperature of the reaction system is controlled not to exceed 85℃ by a cooling jacket to prevent the exothermic reaction from getting out of control. In addition, after the reaction is completed, the mixture is stirred for 30~60min while cooling to room temperature to ensure that the generated hydroxyl functional groups are evenly distributed on the graphene surface and reduce the aggregation of functional groups.

[0010] Preferably, the washing and purification step employs a combination of "vacuum filtration + centrifugal washing", specifically: First, the solid-liquid mixture is rapidly separated by vacuum filtration. After collecting the filter cake, it is dispersed in deionized water to form a dispersion with a mass fraction of 5%~10%. The dispersion is then placed in a high-speed centrifuge and centrifuged at 8000~10000 rpm for 10~15 min. The precipitate is collected. This centrifugation and washing step is repeated 2~3 times, followed by vacuum filtration. This can significantly shorten the washing time and improve the washing effect, making the conductivity of the filtrate ≤30μS / cm. During the washing process, the pH value of the filtrate is monitored by a pH meter. Washing is stopped when the pH value reaches 6-7 to avoid excessive washing that could lead to the loss of hydroxyl functional groups. For the precipitate after centrifugation, nitrogen is used to purge and assist dispersion. The nitrogen flow rate is 1-2 L / min and the purging time is 10-20 min to prevent the precipitate from agglomerating and to lay the foundation for subsequent drying and microwave expansion steps.

[0011] Preferably, in the microwave expansion step, a gradient power microwave processing strategy is adopted, specifically as follows: First, pre-treat with 500~800W power for 10~15s to make the internal temperature of GO foam rise evenly. Then, increase the power to 1500~2000W and treat for 20~45s to avoid local overheating or structural damage caused by single high-power treatment. During microwave treatment, the surface temperature of GO foam is monitored in real time using an infrared thermometer, and the temperature is controlled within the range of 200~300℃ to ensure that the oxygen-containing functional groups in the interlayer are fully decomposed and do not undergo graphitization reduction. For the dense GO film formed by oven drying, it is necessary to mechanically crush it before microwave treatment to break it into particles with a particle size ≤5mm, and then microwave expansion is performed, which can increase the expansion volume by 200~250 times. The microwave-treated rGO powder was lightly ground using a low-shear ball mill at a speed of 100-200 rpm for 5-10 minutes to break up large agglomerates and make the powder particle size uniformly distributed in the range of 1-5 μm with a bulk density of ≤0.1 g / cm³, which makes it easy to incorporate into SMC slurry.

[0012] Preferably, in the oxygen plasma treatment step, a vacuum chamber of a plasma treatment device is used to uniformly spread rGO powder on the sample holder to a thickness of 1-3 mm, ensuring sufficient contact between the plasma and the rGO powder. Specifically: First, evacuate the chamber to a vacuum level ≤10Pa, then introduce oxygen, controlling the oxygen flow rate at 50~100 sccm to maintain the chamber pressure at 50~100Pa. Next, turn on the plasma power supply, adjusting the power to 50~100W, and process for 30~60 seconds. The plasma bombardment generates active oxygen species (such as O2). + The reaction of hydroxyl groups (·O) with the surface of rGO further introduces hydroxyl functional groups, resulting in a -OH functional group density ≥2×10⁻⁶ on the rGO surface. 14 pcs / cm²; During plasma processing, the processing parameters are adjusted in real time through power feedback of the radio frequency power supply to avoid uneven distribution of functional groups caused by power fluctuations; After processing, keep oxygen flowing in and allow the chamber to cool naturally to room temperature before removing the rGO powder to prevent the hydroxyl functional groups on the surface of rGO from undergoing a dehydration reaction at high temperatures.

[0013] Preferably, in the electrochemical stripping step, a flow reaction system can also be used, in which the electrolyte solution is circulated in the reactor by a peristaltic pump at a circulation rate of 950 mL / h. The circulation path is set to draw out from the bottom of the reactor, pass through an external heat exchanger, and then return from the top. The heat exchanger controls the solution temperature at 25~30℃. The anode assembly adopts a rotatable structure with a rotation speed of 10~20 rpm, so that the anode surface is in full contact with the electrolyte solution, reducing ion diffusion restriction. When the graphite raw material is pre-expanded graphite, the mass ratio of ammonium bisulfate to pre-expanded graphite in the electrolyte solution is 3:1~5:1, which can further improve the intercalation efficiency and shorten the stripping time to 1~2 h. At the same time, during the electrochemical stripping process, the absorbance of the solution is monitored by an online UV-Vis spectrophotometer. When the absorbance reaches 1.5~2.0 and remains stable, the stripping reaction is determined to be complete.

[0014] Preferably, it also includes product post-processing and application adaptation steps, specifically: The rGO powder treated with oxygen plasma was dispersed. Unsaturated polyester resin was selected as the dispersion medium. 0.5% to 2.0% of the resin weight of rGO powder was weighed and added to the resin. The mixture was mixed using a Dispermat high-shear mixer at a speed of 3000 to 5000 rpm for 30 to 45 minutes to form a uniform, non-agglomerated rGO-resin dispersion. Subsequently, initiator, release agent, and thickener were added to the dispersion in sequence, and finally glass fiber was added. After stirring evenly, SMC slurry was obtained. In addition, rGO powder can be dispersed in deionized water, and 0.1% to 0.5% (mass fraction) of dispersant can be added. After ultrasonic dispersion, a stable rGO aqueous dispersion can be obtained, which can be used in energy storage, sensors and other fields.

[0015] This invention provides a method for preparing acid-free, microwave-assisted, and plasma-treated reduced graphene oxide. It has the following beneficial effects: 1. This invention replaces strong inorganic acids with ammonium bisulfate (NH4HSO4) electrolyte, controls the pH value of the system at 2-3, produces no toxic gases during the reaction process, and the wastewater contains only ammonium salts and sulfates, which can meet the discharge standards after simple neutralization treatment (reducing treatment costs by 70%). It avoids the corrosion of equipment and personnel safety risks caused by strong acids, extends the service life of equipment by 50%, and significantly improves the safety production factor. 2. This invention employs a "flow reaction system + multiple graphite rods in parallel anode" in the electrochemical stripping stage. The circulating electrolyte ensures uniform ion concentration, reducing the stripping time from the traditional 8-12 hours to 2-6 hours (which can be further reduced to 1-2 hours for pre-expanded graphite raw materials). The washing and purification process uses a combination of "vacuum filtration + centrifugal washing," resulting in a filtrate conductivity ≤30μS / cm, thorough impurity removal, and a 60% reduction in washing time. The entire process cycle (including drying and expansion) is controlled within 72-96 hours, which is 40% shorter than the traditional process, demonstrating potential for large-scale production. 3. This invention employs segmented temperature control (60℃ for 2-4 hours → 70-80℃ for 4-8 hours) and real-time pH adjustment during hydrogen peroxide post-functionalization to ensure continuous generation of hydroxyl radicals, resulting in an O / C ratio of 0.4-0.55 and a hydroxyl functional group density ≥2×10⁻⁶. 14 The product volume expands by 100-300 times with microwave expansion (gradient power 1500-2000W), resulting in a specific surface area of ​​300-600m² / g, a Zeta potential of -30 to -50mV, and only 2-10 layers. Its dispersion stability is significantly better than that of traditional products (no sedimentation after 30 days of storage). 4. When this invention is used as an SMC nanofiller, adding it at 0.5%-2.0% of the resin weight can increase the tensile strength of SMC composite materials by 28%-35%, the flexural strength by 22%-28%, and the impact strength by 20%-22%, far exceeding the modification effect of traditional rGO. The product can also be used to prepare stable aqueous dispersions, which are suitable for energy storage electrode materials, sensor sensitive layers, and other fields, expanding the application scenarios by more than 50% compared with traditional products. Attached Figure Description

[0016] Figure 1 This is a flowchart of the preparation process of the present invention; Figure 2 This is a schematic diagram of the electrochemical stripping optimized structure of the present invention; Figure 3 This is a schematic diagram of the segmented temperature control and pH regulation of hydrogen peroxide post-functionalization in this invention; Figure 4 This is a schematic diagram of the microwave expansion gradient power processing of the present invention. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, and should not be construed as limiting the invention.

[0019] Example 1 This embodiment provides a method for preparing acid-free microwave-assisted plasma-treated graphene oxide reduction, which does not use strong inorganic acids such as sulfuric acid and nitric acid throughout the process. High hydroxyl functionalization rGO is prepared through a six-step synergistic process, the specific steps of which are as follows: 1. Preparation of raw materials and equipment Raw material specifications: The high-purity graphite raw materials are pre-expanded graphite (325 mesh, expansion ratio 600 times) with a purity of 99.95%, ammonium bisulfate (NH4HSO4, ACS grade, purity 99.5%), hydrogen peroxide (H2O2, 30% w / w, analytical grade), and deionized water (conductivity ≤10μS / cm); the cathode material is a platinum-coated titanium mesh (area 0.6m²).

[0020] Equipment Model: 40L polypropylene electrochemical reactor, 0-20V / 100A industrial DC power supply (model HD-100A), top-mounted mechanical stirrer (power 400W, model RW20), 20L Buchner funnel vacuum filtration system (vacuum degree -0.095MPa), freeze dryer (model FD-1A-50), 1800W industrial microwave oven (model M1-211A), oxygen plasma treatment equipment (model PT-100, chamber volume 8L), online pH meter (model PHS-3C), conductivity meter (model DDS-307).

[0021] 2. Electrochemical stripping First, 1.2 kg of pre-expanded graphite is compressed into a rod-shaped structure with a diameter of 3 cm and a length of 15 cm, and fixed to the anode assembly as the anode; a platinum-coated titanium mesh is installed along the inner wall of a 40 L polypropylene reactor as the cathode, and the distance between the anode and the cathode is controlled at 2.5 cm to ensure no contact.

[0022] Preparation of electrolyte solution: Add 3L of deionized water to the reactor, turn on the stirrer (80rpm), slowly add 3kg of ammonium bisulfate, and continue stirring for 1.5h until completely dissolved, forming a 0.8M electrolyte solution. The pH value was measured to be 2.5. The dissociation of ammonium bisulfate follows the formula: NH4HSO4 → NH4 + +HSO4 - .

[0023] Electrical Connection: Connect the anode assembly to the positive terminal of the DC power supply and the cathode to the negative terminal. Turn on the stirrer to maintain gentle stirring and apply a constant +8V DC voltage. Graphite oxidation and exfoliation reactions occur at the anode. The main reaction formula is: C(s) + 2H₂O → CO₂↑ + 4H₂O + +4e - C(s) + H₂O → CO↑ + 2H₂O + +2e - The CO2, CO, and O2 gases generated between the layers form pressure gases, which assist in pressure-assisted stripping; hydrogen evolution reaction occurs at the cathode: 2H + +2e - →H2↑.

[0024] During the reaction, a peristaltic pump installed outside the reactor circulates the electrolyte solution at a rate of 950 mL / h, continuously delivering fresh electrolyte and removing the exfoliated graphene sheets. After the reaction lasts for 4 hours, the graphite anode is basically consumed, resulting in a black electrochemically exfoliated graphene oxide (EEGO) dispersion.

[0025] 3. Post-functionalization of hydrogen peroxide The entire EEGO dispersion was transferred to a 50L glass-lined reactor. While maintaining the stirrer speed at 100 rpm, 2.5L of 30% hydrogen peroxide was slowly added at a rate of 80mL / min. During the addition process, the temperature of the reaction system was controlled to not exceed 80℃ by using a cooling jacket.

[0026] A segmented temperature control strategy was adopted: the mixture was first heated from room temperature to 60°C at a rate of 5°C / h and held for 3 hours; then heated to 75°C and held for 6 hours to ensure that hydrogen peroxide slowly decomposed to generate hydroxyl radicals.

[0027] During the reaction, the pH was monitored in real time using an online pH meter. When the pH dropped to 1.5, 80 mL of deionized water was slowly added to maintain the pH in the range of 1.5 to 2.5. After the reaction was completed, stirring was continued for 45 min, and the mixture was allowed to cool naturally to room temperature to ensure that the hydroxyl functional groups were evenly distributed.

[0028] 4. Washing and purification The reaction mixture was filtered using a combination of vacuum filtration and centrifugal washing: the reaction mixture was first filtered through a 20L Buchner funnel vacuum filtration system (vacuum degree -0.095MPa) to collect the black filter cake; the filter cake was then dispersed in 10L of deionized water to form a dispersion with a mass fraction of 8%, which was then placed in a high-speed centrifuge (model TGL-20M) and centrifuged at 9000rpm for 12min to collect the precipitate.

[0029] Repeat the centrifugation and washing steps twice, and then perform vacuum filtration. During this process, monitor the pH value of the filtrate with a pH meter and the conductivity with a conductivity meter. Stop washing when the pH value of the filtrate reaches 6.5 and the conductivity drops to 28 μS / cm. Purge the precipitate after centrifugation with nitrogen gas at a flow rate of 1.5 L / min for 15 min to prevent agglomeration.

[0030] 5. Freeze-drying The washed filter cake was mixed with an appropriate amount of deionized water to make a thick aqueous slurry with a mass fraction of 15%. The slurry was poured into liquid nitrogen and quickly frozen into a block. The block was then placed in a freeze dryer, and the cold trap temperature was set to -50℃ and the vacuum degree to 10Pa. The block was dried for 60 hours to obtain a fluffy aerogel-like GO foam with a density of 0.08 g / cm³.

[0031] 6. Microwave expansion GO foam was placed in a large glass tray with a diameter of 20cm in batches of 25g, and then placed in an 1800W industrial microwave oven. A gradient power processing strategy was adopted: first, it was pre-treated at 600W for 12s, and then the power was increased to 1800W for 35s. During this period, the surface temperature of GO foam was monitored by an infrared thermometer and controlled within the range of 250~280℃.

[0032] After microwave treatment, the GO foam expands dramatically, increasing in volume by 250 times, forming a black, fluffy rGO powder. The powder is then placed in a low-shear ball mill (model XQM-1) and ground at 150 rpm for 8 minutes to obtain a uniform powder with a particle size of 2~4 μm and a bulk density of 0.09 g / cm³.

[0033] 7. Oxygen plasma treatment The microwave-expanded rGO powder was evenly spread on the sample holder of the plasma processing equipment to a thickness of 2 mm. The chamber was closed and evacuated to 8 Pa. Oxygen with a purity of 99.995% was introduced and the oxygen flow rate was controlled at 80 sccm to maintain the pressure in the chamber at 80 Pa.

[0034] The plasma power supply was turned on and adjusted to 80W, with a processing time of 45s. Parameters were adjusted in real time via RF power feedback to avoid power fluctuations. After processing, oxygen was continuously introduced, and the rGO powder was removed after the chamber naturally cooled to room temperature (approximately 30 minutes). At this point, the density of -OH functional groups on the rGO surface was 2.5 × 10⁻⁶. 14 pcs / cm²

[0035] 8. Product performance verification The final product was characterized as follows: the O / C ratio was 0.52 as determined by XPS analysis; the specific surface area was 520 m² / g as determined by BET method; the Zeta potential was -42 mV as determined by Zeta potentiometer; and the number of layers was observed to be 3 to 5 as determined by TEM, which meets the product characteristics of claim 1 and can be directly used as SMC nanofiller.

[0036] Example 2 Please see Figures 1-4 Furthermore, based on Example 1, this example further optimizes the electrochemical stripping step using a flow reaction system, while the remaining steps are the same as in Example 1, with the following specific improvements: 1. Construction of a Flow Electrochemical Stripping Device A 50L glass flow electrochemical reactor was selected, and the circulation path was set: the electrolyte solution was drawn from the bottom of the reactor by a peristaltic pump (model BT100-1L), cooled by an external plate heat exchanger (model BR0.1-1.0), and then returned from the top of the reactor. The heat exchanger stabilized the solution temperature at 28℃.

[0037] The anode assembly adopts a rotatable structure, which connects and fixes six pre-expanded graphite rods (compressed and molded) with a diameter of 1.5cm in parallel at a rotation speed of 15rpm to ensure that the anode surface is in full contact with the electrolyte solution; the cathode is still a platinum-coated titanium mesh, with a distance of 2.5cm between it and the anode.

[0038] 2. Electrochemical stripping optimization operation Electrolyte solution preparation: Add 3L of deionized water to the reactor, add 3kg of ammonium bisulfate and 300mL of 30% hydrogen peroxide, stir and dissolve to form a mixed electrolyte of 0.8M ammonium bisulfate + 8% (v / v) hydrogen peroxide, and the mass ratio of pre-expanded graphite to ammonium bisulfate is 1:5.

[0039] Turn on the peristaltic pump, set the circulation flow rate to 950 mL / h, apply a constant DC voltage of +10V, and monitor the absorbance of the solution at a wavelength of 270 nm using an online UV-Vis spectrophotometer (model UV-2600). When the absorbance reaches 1.8 and remains stable for 30 min, the stripping reaction is considered complete. At this time, the reaction time is only 1.5 h, which is 62.5% shorter than that of Example 1.

[0040] The subsequent hydrogen peroxide post-functionalization to oxygen plasma treatment steps are the same as in Example 1. The final product has an O / C ratio of 0.50, a specific surface area of ​​500 m² / g, 4 to 6 layers, and a stripping yield of 92%.

[0041] Example 3 Please see Figures 1-4Furthermore, based on Example 1, this example further describes the post-processing of the rGO powder prepared in Example 1 to adapt it for use in SMC composite materials and aqueous dispersions. The specific steps are as follows: 1. SMC slurry preparation and performance testing Weigh 100 kg of unsaturated polyester resin (model 191#), and weigh 1 kg of rGO powder prepared in Example 1 at 1.0% of the resin weight. Add the powder to the resin and mix at 4000 rpm for 40 min using a Dispermat high-shear mixer (model AE30) to form an agglomerated rGO-resin dispersion. The particle size of the rGO dispersion was measured to be 80~100 nm using a laser particle size analyzer.

[0042] Add 1.5 kg of initiator (MEKP), 0.8 kg of release agent (zinc stearate), and 2.5 kg of thickener (MgO) to the dispersion in sequence. After stirring evenly, add 40 kg of glass fiber (model ECR450-13μm) and continue stirring for 20 min to obtain SMC slurry.

[0043] The SMC slurry was molded using conventional processes (temperature 140℃, pressure 5MPa, time 8min) to prepare standard test specimens. The specimens were tested using a universal testing machine (model CMT5105): the tensile strength was 85MPa, which is 35% higher than that of the SMC composite material without rGO; the flexural strength was 120MPa, which is 28% higher; and the impact strength was 25kJ / m², which is 22% higher, meeting the requirements for high-performance SMC applications.

[0044] 2. Preparation of rGO aqueous dispersion Weigh 5g of the rGO powder prepared in Example 1, add it to 10L of deionized water, add 30g of dispersant (polyvinylpyrrolidone PVP, molecular weight 10000), put it into an ultrasonic cleaner (power 600W, model KQ-600VDE), and ultrasonically disperse for 45min to obtain an rGO aqueous dispersion with a concentration of 0.5mg / mL.

[0045] The dispersion was sealed and left for 30 days without significant sedimentation or stratification. The absorbance was monitored by a UV-Vis spectrophotometer and showed no significant change. The zeta potential was -40mV, indicating good stability. It can be used in energy storage electrode materials, sensor sensitive layers, and other fields.

[0046] Example 4 Please see Figures 1-4Furthermore, based on Example 1, we obtained the following: rGO was prepared using the traditional Hummers method, specifically by mixing 5g of natural graphite powder with 23mL of concentrated sulfuric acid, slowly adding 15g of potassium permanganate under an ice bath, heating to 35℃ and reacting for 2h, adding 40mL of deionized water and continuing to heat to 98℃ and reacting for 30min, adding 100mL of deionized water and 5mL of hydrogen peroxide to terminate the reaction, washing and drying, and then reducing with hydrogen to obtain rGO.

[0047] Characterization of the comparative product: O / C ratio is 0.35, specific surface area is 280 m² / g, Zeta potential is -25 mV, and the number of layers is 8~12. After preparing the SMC composite material, the tensile strength is increased by 12%, the flexural strength is increased by 10%, and the impact strength is increased by 8%. However, a large amount of acidic wastewater is generated during the preparation process, which requires additional treatment. The safety and environmental protection are far lower than the method of this invention.

[0048] In summary, the rGO prepared by the acid-free process of this invention has higher hydroxyl functionality, specific surface area and dispersion stability. When applied to SMC composite materials, its performance is significantly improved. Moreover, the process is environmentally friendly, safe and can be mass-produced, solving many defects of traditional methods.

[0049] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0050] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide, characterized in that, The process includes six core steps performed sequentially: electrochemical stripping, post-functionalization with hydrogen peroxide, washing and purification, freeze-drying, microwave expansion, and oxygen plasma treatment. The specific steps are as follows: Step 1: First, perform electrochemical stripping. Select high-purity graphite raw material and use an electrochemical reactor made of polypropylene as the reaction vessel. Fix 1.2 kg of graphite raw material as the anode assembly and use a large-area platinum-coated titanium mesh or stainless steel plate as the cathode. The distance between the anode and the cathode in the reactor is controlled to be 2~3 cm. To prepare the electrolyte solution, add 2-5 L of deionized water to the reactor. Under a top-mounted mechanical stirrer with a power of 300-500 W, slowly add 2-4 kg of ammonium bisulfate (NH₄HSO₄) and stir for 1-2 hours until completely dissolved, forming an electrolyte solution with a concentration of 0.5-1.0 M and a pH of 2-3. The HSO₄⁻ produced by the dissociation of ammonium bisulfate in this solution... - Ions, as excellent intercalating agents, have the following dissociation formula: NH4HSO4→NH4 + +HSO4 - Connect the anode assembly to the positive terminal of an industrial-grade DC power supply, and the cathode to the negative terminal. Turn on the stirrer for gentle stirring, and apply a constant DC voltage of +5~+10V. Graphite oxidation and exfoliation reactions occur at the anode. The main anode reaction formula is as follows: C(s)+2H2O→CO2↑+4H + +4e - 、C(s)+H2O→CO↑+2H + +2e - The CO2, CO, and O2 gases generated between the layers create pressure, which assists in the exfoliation of the graphite layers. This leads to the hydrogen evolution reaction at the cathode, with the reaction equation being 2H₂O. + +2e - →H2↑, continue the reaction for 2~6h until the graphite anode is basically consumed, and a black electrochemically exfoliated graphene oxide (EEGO) dispersion is obtained; Step 2: Next, perform hydrogen peroxide post-functionalization. Transfer the above EEGO dispersion to a glass-lined or polypropylene reactor equipped with a heater and stirrer. While continuously stirring, slowly add 2-3 L of 30% hydrogen peroxide (H₂O₂) at a rate of 50-100 mL / min to avoid excessive exothermic reaction. Then heat the mixture to 60-80°C and maintain this temperature for 6-12 hours. During this process, residual metal ions or carbon defects catalyze a Fenton-like reaction in H₂O₂, decomposing it to produce highly reactive hydroxyl radicals (·OH). The reaction formula is: H₂O₂ → H₂O + ·OH + ·O - Hydroxyl radicals violently attack the sp² carbon network of the graphene lattice, introducing a large number of hydroxyl (-OH) and epoxy (COC) groups, thereby enhancing the functionalization of the material. Step 3: Then, wash and purify the mixture after the reaction. Filter the mixture through a large vacuum filtration system consisting of 20L Buchner funnels, collect the filter cake and redisperse it in deionized water. Repeat the "dispersion-filtration" washing cycle 3 to 5 times until the conductivity of the filtrate is ≤50μS / cm to ensure that impurities such as ammonium salts and sulfates are completely removed. Step 4: After washing, the filter cake is made into a thick water-based slurry with a mass fraction of 10%~20%, and dried by freeze drying. The slurry is frozen with liquid nitrogen and then placed in a freeze dryer to dry for 48~72 hours to obtain fluffy aerogel-like GO foam. Step 5: Next, microwave expansion is performed. The dried GO foam is placed in a large glass tray at a rate of 20-30g / batch and placed in an industrial microwave oven with a power of ≥1500W for 30-60s of high-power microwave treatment. Under the action of microwaves, the GO foam expands violently, increasing in volume by 100-300 times, forming black and fluffy rGO powder. During the microwave treatment, the oxygen-containing functional groups between the GO layers decompose to generate gas, further peeling off the layers and forming a porous structure. Step Six: Finally, oxygen plasma treatment is performed. The microwave-expanded rGO powder is placed in an oxygen plasma processing device, and oxygen with a purity ≥99.99% is introduced. The oxygen flow rate is controlled at 50~100 sccm, the plasma power is 50~100W, and the treatment time is 30~60s. The content of -OH functional groups on the rGO surface is further enhanced by plasma bombardment, and the target product is finally obtained. The product has an O / C ratio of 0.4~0.55, a specific surface area of ​​300~600m² / g, a Zeta potential of -30~-50mV, and 2~10 layers, making it suitable as a nanofiller for sheet molding compounds (SMC).

2. The method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide according to claim 1, characterized in that, In the electrochemical stripping step, when using pre-expanded graphite as raw material, the pre-expanded graphite needs to be compressed into a rod-shaped structure first. 200-500 mL of 30% hydrogen peroxide can also be added to the electrolyte solution to introduce some oxide groups in advance, accelerating the intercalation and stripping between graphite layers. At this time, the electrolyte solution consists of 0.5-1.0 M ammonium bisulfate + 5%-10% hydrogen peroxide. The reaction temperature is controlled at 25-30℃ using a water bath, which can shorten the electrochemical stripping time to 1-3 hours. The anode assembly uses a parallel structure of multiple graphite rods, evenly distributed in the central area of ​​the reactor. Simultaneously, a peristaltic pump is installed outside the reactor to circulate the electrolyte solution at a flow rate of 950 mL / h, continuously delivering fresh electrolyte and removing the stripped graphene sheets, maintaining a uniform ion concentration within the reactor and reducing local ion loss.

3. The method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide according to claim 1, characterized in that, In the hydrogen peroxide post-functionalization step, a segmented temperature control strategy is adopted to optimize the reaction effect, specifically: First, the mixture is heated from room temperature to 60°C and kept at that temperature for 2-4 hours. Then, it is heated to 70-80°C and kept at that temperature for 4-8 hours. By slowly heating the mixture, the rapid decomposition of hydrogen peroxide is avoided, and the oxidative functionalization effect of hydroxyl radicals is fully utilized. During the reaction, the pH value of the solution is monitored in real time by an online pH meter. When the pH value drops to 1-2, 50-100 mL of deionized water is slowly added to maintain the pH value in the range of 1-3, ensuring the continuous generation of hydroxyl radicals. The amount of hydrogen peroxide added is precisely controlled according to the mass of EEGO. The mass-volume ratio of EEGO to hydrogen peroxide is 1g:2~3mL. During the addition process, the temperature of the reaction system is controlled not to exceed 85℃ by a cooling jacket to prevent the exothermic reaction from getting out of control. In addition, after the reaction is completed, the mixture is stirred for 30~60min while cooling to room temperature to ensure that the generated hydroxyl functional groups are evenly distributed on the graphene surface and reduce the aggregation of functional groups.

4. The method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide according to claim 1, characterized in that, The washing and purification step employs a combination of "vacuum filtration + centrifugal washing", specifically as follows: First, the solid-liquid mixture is rapidly separated by vacuum filtration. After collecting the filter cake, it is dispersed in deionized water to form a dispersion with a mass fraction of 5%~10%. The dispersion is then placed in a high-speed centrifuge and centrifuged at 8000~10000 rpm for 10~15 min. The precipitate is collected. This centrifugation and washing step is repeated 2~3 times, followed by vacuum filtration. This can significantly shorten the washing time and improve the washing effect, making the conductivity of the filtrate ≤30μS / cm. During the washing process, the pH value of the filtrate is monitored by a pH meter. Washing is stopped when the pH value reaches 6-7 to avoid excessive washing that could lead to the loss of hydroxyl functional groups. For the precipitate after centrifugation, nitrogen is used to purge and assist dispersion. The nitrogen flow rate is 1-2 L / min and the purging time is 10-20 min to prevent the precipitate from agglomerating and to lay the foundation for subsequent drying and microwave expansion steps.

5. The method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide according to claim 1, characterized in that, In the microwave expansion step, a gradient power microwave processing strategy is adopted, specifically as follows: First, pre-treat with 500~800W power for 10~15s to make the internal temperature of GO foam rise evenly. Then, increase the power to 1500~2000W and treat for 20~45s to avoid local overheating or structural damage caused by single high-power treatment. During microwave treatment, the surface temperature of GO foam is monitored in real time using an infrared thermometer, and the temperature is controlled within the range of 200~300℃ to ensure that the oxygen-containing functional groups in the interlayer are fully decomposed and do not undergo graphitization reduction. For the dense GO film formed by oven drying, it is necessary to mechanically crush it before microwave treatment to break it into particles with a particle size ≤5mm, and then microwave expansion is performed, which can increase the expansion volume by 200~250 times. The microwave-treated rGO powder was lightly ground using a low-shear ball mill at a speed of 100–200 rpm for 5–10 minutes to break up large agglomerates and achieve a uniform particle size distribution within the range of 1–5 μm and a bulk density ≤0.1 g / cm³. 3 It is easy to incorporate SMC slurry later.

6. The method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide according to claim 1, characterized in that, In the oxygen plasma treatment step, the rGO powder is evenly spread on the sample holder in the vacuum chamber of the plasma treatment equipment, with a spreading thickness of 1-3 mm, to ensure full contact between the plasma and the rGO powder. Specifically: First, evacuate the chamber to a vacuum level ≤10Pa, then introduce oxygen, controlling the oxygen flow rate at 50~100 sccm to maintain the chamber pressure at 50~100Pa. Next, turn on the plasma power supply, adjusting the power to 50~100W, and process for 30~60 seconds. The plasma bombardment generates active oxygen species (such as O2). + The reaction of hydroxyl groups (·O) with the surface of rGO further introduces hydroxyl functional groups, resulting in a -OH functional group density ≥2×10⁻⁶ on the rGO surface. 14 pcs / cm²; During plasma processing, the processing parameters are adjusted in real time through power feedback of the radio frequency power supply to avoid uneven distribution of functional groups caused by power fluctuations; After processing, keep oxygen flowing in and allow the chamber to cool naturally to room temperature before removing the rGO powder to prevent the hydroxyl functional groups on the surface of rGO from undergoing a dehydration reaction at high temperatures.

7. The method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide according to claim 1, characterized in that, In the electrochemical stripping step, a flow reaction system can also be used, in which the electrolyte solution is circulated in the reactor by a peristaltic pump at a circulation rate of 950 mL / h. The circulation path is set to draw out from the bottom of the reactor, pass through an external heat exchanger, and then return from the top. The heat exchanger controls the solution temperature at 25~30℃. The anode assembly adopts a rotatable structure with a rotation speed of 10~20 rpm, which ensures full contact between the anode surface and the electrolyte solution and reduces ion diffusion restriction. When the graphite raw material is pre-expanded graphite, the mass ratio of ammonium bisulfate to pre-expanded graphite in the electrolyte solution is 3:1~5:1, which can further improve the intercalation efficiency and shorten the stripping time to 1~2 h. At the same time, during the electrochemical stripping process, the absorbance of the solution is monitored by an online UV-Vis spectrophotometer. When the absorbance reaches 1.5~2.0 and remains stable, the stripping reaction is considered complete.

8. The method for preparing acid-free microwave-assisted and plasma-treated reduced graphene oxide according to claim 1, characterized in that, It also includes product post-processing and application adaptation steps, specifically: The rGO powder treated with oxygen plasma was dispersed. Unsaturated polyester resin was selected as the dispersion medium. 0.5% to 2.0% of the resin weight of rGO powder was weighed and added to the resin. The mixture was mixed using a Dispermat high-shear mixer at a speed of 3000 to 5000 rpm for 30 to 45 minutes to form a uniform, non-agglomerated rGO-resin dispersion. Subsequently, initiator, release agent, and thickener were added to the dispersion in sequence, and finally glass fiber was added. After stirring evenly, SMC slurry was obtained. In addition, rGO powder can be dispersed in deionized water, and 0.1% to 0.5% (mass fraction) of dispersant can be added. After ultrasonic dispersion, a stable rGO aqueous dispersion can be obtained, which can be used in energy storage, sensors and other fields.

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