Graphene quantum dot preparation method based on steam coal and plasma reaction system

Abstract

This invention provides a method for preparing graphene quantum dots based on thermal coal and a plasma reaction system, belonging to the field of graphene quantum dot preparation technology. The method includes the following steps: using thermal coal as raw material, it undergoes electrochemical oxidation and KOH ball milling sequentially to obtain pretreated coal; the pretreated coal is introduced into an axial magnetic rotating microwave plasma reactor for segmented temperature-controlled dissociation and recombination reactions; during the reaction, a porous alumina template is used for confined growth, and after the reaction, liquid nitrogen quenching is employed to obtain graphene quantum dots. This invention uses thermal coal to replace high-purity graphite as the carbon source, reducing raw material costs by more than 99%; the pretreatment process, through the synergistic effect of electrochemical oxidation and KOH ball milling, eliminates the need for strong acids and alkalis throughout, completely avoiding the pollution problems of large amounts of acidic and heavy metal-containing wastewater and waste gas generated by the traditional Hummers method, thus aligning with the concepts of green chemistry and sustainable development.

Description

Technical Field

[0001] This invention relates to the field of graphene quantum dot preparation technology, and in particular to a method for preparing graphene quantum dots based on thermal coal and a plasma reaction system. Background Technology

[0002] Graphene quantum dots (GQDs) have shown great potential for application in fields such as bioimaging, optoelectronic devices, sensors, and catalysis due to their unique optoelectronic properties, biocompatibility, and size dependence. However, their large-scale fabrication and commercial application still face a series of key technological bottlenecks:

[0003] 1. Currently, mainstream preparation methods (such as chemical exfoliation) primarily rely on high-purity graphite as raw material. Graphite (especially flake graphite) is an important strategic resource, and its market price is far higher than that of ordinary carbon source materials. In contrast, thermal coal (at pithead price) is a widely available and abundant carbon source. This huge cost difference severely restricts the low-cost, large-scale production of GQDs, hindering their widespread application.

[0004] 2. The Hummers process and its improved versions are currently widely used methods for preparing graphene oxide (GO) and subsequently deriving GQDs. This method involves the use of large amounts of strong oxidants (such as potassium permanganate) and strong acids such as concentrated sulfuric acid and concentrated nitric acid, resulting in harsh reaction conditions. Its fatal flaw lies in the generation of massive amounts of acidic wastewater and waste gas containing heavy metals (manganese). These waste liquids are highly acidic, complex in composition, difficult to treat, and costly, causing serious environmental pollution and failing to meet the requirements of green chemistry and sustainable development. There is an urgent need to develop environmentally friendly alternative processes.

[0005] 3. Pyrolysis methods (including high-temperature pyrolysis and hydrothermal / solvothermal methods) are another important method for preparing GQDs, generally considered relatively "green." However, during pyrolysis, especially in the later stages or under specific conditions, Ostwald ripening is a common phenomenon. In this phenomenon, smaller crystal or sol particles in the solute dissolve and redeposit onto larger crystal or sol particles. The direct consequence is an extremely wide particle size distribution in the prepared GQDs (e.g., fluctuations up to ±50 nm), resulting in severe inhomogeneity. This size inhomogeneity significantly affects the optical properties (such as fluorescence wavelength and intensity) and other performance characteristics of GQDs, leading to large batch-to-batch variations, unstable product performance, and difficulty in meeting the requirements of high-precision applications.

[0006] 4. To overcome the pollution and size issues of traditional methods, some emerging technologies, such as plasma methods, have been explored for GQDs preparation. This method utilizes high-energy plasma to vaporize and recombine carbon sources to form quantum dots. Although plasma methods avoid the use of strong acids and bases, are theoretically cleaner, and have some potential for particle size control, they suffer from significant energy efficiency bottlenecks. The generation and maintenance of high-energy plasma themselves consume enormous amounts of energy, and the energy conversion and utilization efficiency during the process is low. It has been reported that the electron energy utilization rate of such methods is typically less than 30%, resulting in extremely high overall energy consumption (>20 kWh / g GQDs). This extremely high energy cost makes this technical route completely uneconomical for large-scale production, making it difficult to achieve industrial application.

[0007] In summary, current graphene quantum dot (GQD) preparation technologies generally suffer from core bottlenecks such as high raw material costs, severe environmental pollution, uneven product size, and excessive energy consumption. These defects seriously hinder the performance optimization, cost reduction, and large-scale production and application of GQDs. Therefore, this invention proposes a graphene quantum dot preparation method based on thermal coal and a plasma reaction system to solve the problems existing in the prior art. Summary of the Invention

[0008] To address the aforementioned issues, this invention proposes a method for preparing graphene quantum dots based on thermal coal and a plasma reaction system. The raw materials are inexpensive and readily available, the process is green and environmentally friendly, the product size is uniform and controllable, and the energy consumption is low. It has extremely important scientific significance and huge industrialization value.

[0009] To achieve the objectives of this invention, the following technical solution is employed: a method for preparing graphene quantum dots based on thermal coal, comprising the following steps:

[0010] S1: Using thermal coal as raw material, electrochemical oxidation treatment and KOH ball milling treatment are carried out sequentially to obtain pretreated coal;

[0011] S2: The pretreated coal is introduced into the axial magnetic rotating microwave plasma reactor for dissociation and recombination reactions under segmented temperature control.

[0012] S3: Graphene quantum dots were obtained by confined growth using a porous alumina template during the reaction and by liquid nitrogen quenching after the reaction.

[0013] S4: The graphene quantum dots are subjected to graded purification.

[0014] Further improvements are made in the following aspects: In S1, the power coal selected is anthracite or lean coal with a fixed carbon content >60% and ash content <25%; the electrochemical oxidation treatment is specifically as follows: using an electrode plate made of refined washed power coal and 5% asphalt as the anode, electrolysis is carried out in pure water, the electrolysis voltage is 12V, the current density is 0.25A / cm², and the electrolysis time is 300min.

[0015] A further improvement is made in that: in step S1, the KOH ball milling process includes the following steps:

[0016] The electrochemically oxidized coal powder and KOH were ball-milled at a mass ratio of 1:3.

[0017] The ball mill speed was 400 rpm, and the ball milling time was 12 hours.

[0018] A further improvement is made in S2, where the microwave source frequency of the axial magnetic rotating microwave plasma reactor is 2.45 GHz and the power is in the range of 0-1500 W; the carrier gas used in the reaction is an Ar / H2 mixture, wherein the volume ratio of Ar to H2 is 95:5 and the reaction pressure is 10-50 Pa.

[0019] A further improvement is that, in S2, the segmented temperature control is specifically as follows: in the temperature range of 300-500℃, a 300W microwave power is used for the deconstruction reaction; in the temperature range of 600-800℃, an 800W microwave power is used for the recombination reaction.

[0020] A further improvement is that S2 also includes an in-situ doping step: injecting NH3 or SO2 gas into the plasma tail flame region at an injection rate of 10 sccm to achieve N doping or S doping.

[0021] Further improvements are made in the following aspects: in S3, the pore size of the porous alumina template is 20-100nm, and the relationship between the diameter d of the prepared graphene quantum dots and the pore size of the template satisfies d≈0.8×pore size; the cooling rate of the liquid nitrogen quenching treatment is >100℃ / s.

[0022] A further improvement is made in S4, where the fractional purification process sequentially includes gradient centrifugation, membrane filtration, and electrophoretic purification; wherein the gradient centrifugation speeds are 8000 rpm and 15000 rpm respectively, the membrane filtration uses a 0.22 μm membrane, and the electrophoretic purification conditions are 50 V voltage and Tris buffer at pH=8.

[0023] A graphene quantum dot plasma reaction system based on thermal coal includes a quartz reaction chamber, a microwave source, an axially adjustable magnetic field device, a porous template support, a segmented temperature control module, and a quenching device.

[0024] The quartz reaction chamber is used to contain reactants and provide reaction space; the microwave source is connected to the quartz reaction chamber to provide microwave energy; the axially adjustable magnetic field device is located outside the quartz reaction chamber, and the magnetic field strength is adjustable within the range of 0.1-0.5T to confine the plasma; the porous template support is located inside the quartz reaction chamber to place the porous alumina template; the segmented temperature control module is used to control the temperature at different stages inside the quartz reaction chamber; the quenching device is connected to the quartz reaction chamber to introduce liquid nitrogen for rapid cooling.

[0025] Further improvements include: a drum-type continuous feeding system and a pre-electrostatic precipitator. The drum-type continuous feeding system is used to feed raw materials, and the pre-electrostatic precipitator is used to remove dust from the raw material coal. The voltage of the pre-electrostatic precipitator is 30kV. The microwave source is a multi-magnetron array and adopts phase synchronization technology.

[0026] The beneficial effects of this invention are as follows:

[0027] 1. This invention uses thermal coal to replace high-purity graphite as a carbon source, reducing raw material costs by more than 99%. The pretreatment process uses electrochemical oxidation and KOH ball milling in synergy, eliminating the need for strong acids and alkalis throughout the process. This completely avoids the pollution problems of large amounts of acidic and heavy metal-containing wastewater and waste gas generated by the traditional Hummers process, which is in line with the concepts of green chemistry and sustainable development.

[0028] 2. This invention employs a combination of porous alumina template confined growth and liquid nitrogen rapid quenching to effectively suppress Ostwald ripening, controlling the particle size deviation of graphene quantum dots within ±1nm, thus solving the uniformity problem of particle size fluctuation reaching ±50nm in traditional pyrolysis methods. At the same time, in-situ doping of NH3 / SO2 in the plasma tail flame region increases the fluorescence quantum yield to 40%, and eliminates the need for post-modification processes, significantly improving the batch stability and performance consistency of the product.

[0029] 3. This invention utilizes axial magnetic rotation microwave plasma technology to increase electron utilization to 65% and reduce energy consumption to <5kWh / g, which is only 1 / 4 of that of conventional plasma methods. It is equipped with a drum-type continuous feeding and multi-magnetron array system to increase production capacity. Combined with pre-electrostatic dust removal to solve the problem of coal ash blockage, it takes into account the requirements of low cost, low pollution, high performance and large-scale production, and provides an economically feasible technical path for the industrial application of graphene quantum dots. Attached Figure Description

[0030] Figure 1 This is a process diagram for preparing coal-based graphene quantum dots according to the present invention;

[0031] Figure 2 This is a schematic diagram of the TEM image representation of the present invention;

[0032] Figure 3 This is a schematic diagram of the Raman spectral representation of the present invention;

[0033] Figure 4 This is a schematic diagram of the cracking of coal-based graphene quantum dots according to the present invention;

[0034] Figure 5 This is a schematic diagram of the ash content-yield relationship curve of the present invention. Detailed Implementation

[0035] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0036] Example 1

[0037] according to Figure 1 , 2 As shown in Figures 3, 4, and 5, this embodiment proposes a method for preparing graphene quantum dots based on thermal coal and a plasma reaction system:

[0038] Coal-based precursor directional dissociation process

[0039] The coal used is thermal coal (anthracite / lean coal) with a fixed carbon content >60% and ash content <25%. The refined coal is mixed with 5% pitch and pressed into electrode plates. The pitch forms a three-dimensional conductive network (resistivity...). To ensure uniform current distribution, electrolysis was performed in pure water using anodizing (voltage 12V, current density 0.25A / cm³). 2 (300 min), introduce carboxyl / hydroxyl functional groups to weaken interlayer van der Waals forces;

[0040] KOH ball milling exfoliation: Coal oxidizing powder and KOH were ball milled at a mass ratio of 1:3 (400 rpm, 12 h) to obtain submicron carbon flakes (thickness < 200 nm).

[0041] Effect: The coal seam dissociation energy is reduced by 40% after pretreatment, thus reducing subsequent plasma energy consumption.

[0042] The reaction that occurs at the anode during the electrolysis of pure water is as follows: The carboxyl / hydroxyl groups are embedded in the aromatic interlayer of coal, causing the interlayer spacing to expand from 0.34 nm to 0.52 nm (XRD verification), and the van der Waals forces are reduced by 40%.

[0043] Dual function of asphalt: 5% asphalt forms a three-dimensional conductive network (resistivity) This ensures that the current is distributed evenly.

[0044] Microwave plasma reaction system

[0045] Magnetic confinement plasma source: 2.45GHz microwave source (0-1500W adjustable) coupled to a quartz reaction cavity ( 50mm);

[0046] An axial magnetic rotation device (adjustable magnetic field of 0.1-0.5T) extends the electron movement path, raising the electron temperature to 5-10eV;

[0047] The carrier gas is Ar / H2 (95:5) at a pressure of 10-50 Pa.

[0048] Segmented temperature control and recombination mechanism:

[0049]

[0050] Results: Ionization efficiency increased by 50%, carbon conversion rate >90%.

[0051] Quantum dot controllable synthesis technology

[0052] Size-confined growth: A porous alumina template (pore size 20-100nm) is placed inside the reaction chamber, and the quantum dot diameter d satisfies: d≈0.8×pore size (e.g., pore size 50nm → quantum dot 40nm).

[0053] Gradient quenching to prevent agglomeration: Liquid nitrogen is introduced immediately after the reaction ends, with a cooling rate >100℃ / s, to inhibit Ostwald ripening.

[0054] In-situ doping functionalization: NH3 / SO2 (10 sccm) was injected into the plasma tail flame region to achieve N / S doping, and the fluorescence quantum yield was increased to 40% (undoped only 15%).

[0055] Equipment scale-up plan: Continuous drum feeding system + multi-magnetron array (phase synchronization technology) → production capacity 5kg / day. Coal ash blockage: pre-electrostatic precipitator (30kV) to remove submicron particles; Quantum dot agglomeration: add 1wt% PVP in-situ dispersant.

[0056] Post-processing and characterization

[0057] Fractional purification: gradient centrifugation (8000→15000rpm) → 0.22μm filter membrane → electrophoretic purification (50V, pH=8 Tris buffer).

[0058] Structural verification: HRTEM: lattice stripe spacing 0.24 nm (corresponding to graphene crystal plane); XPS: sp 2 Carbon content > 80%, O / C ratio < 0.2.

[0059] Table 1: Comparison of Advantages

[0060]

[0061] Example 2

[0062] according to Figure 1 , 2 As shown in Figures 3, 4, and 5, this embodiment proposes a method for preparing graphene quantum dots based on thermal coal and a plasma reaction system:

[0063] Raw material: Anthracite (68% fixed carbon, 18% ash);

[0064] Results: Nitrogen-doped quantum dots with a particle size of 5 nm were obtained, QY=42%, and carbon yield was 92%.

[0065] Comparative example:

[0066] Hummers process: QY=35%, energy consumption 22kWh / g, contains sulfuric acid waste liquid;

[0067] Conventional plasma method: particle size distribution ±4nm, energy consumption 18kWh / g.

[0068] Example 3

[0069] according to Figure 1 , 2 As shown in Figures 3, 4, and 5, this embodiment proposes a method for preparing graphene quantum dots based on thermal coal and a plasma reaction system:

[0070] Using lean coal with 25% ash content, after reducing the ash content to 18% through pre-electrostatic precipitator (30kV): the quantum dot yield increased to 79% and the particle size distribution narrowed to ±1.6nm;

[0071] It is proven that ash content ≤20% is a necessary condition for achieving high yield (>80%).

[0072] Validation data:

[0073] Table 2: Comparison of the effects of different ash content coal quality on quantum dot yield

[0074]

[0075] This invention uses thermal coal instead of high-purity graphite as the carbon source, reducing raw material costs by over 99%. The pretreatment process utilizes the synergistic effect of electrochemical oxidation and KOH ball milling, eliminating the need for strong acids and alkalis throughout the process. This completely avoids the pollution problems caused by the large amounts of acidic and heavy metal-containing wastewater and exhaust gas generated by the traditional Hummers process, aligning with the concepts of green chemistry and sustainable development. Furthermore, the combination of porous alumina template confined growth and liquid nitrogen rapid quenching effectively suppresses Ostwald ripening, controlling the graphene quantum dot particle size deviation within ±1nm, solving the uniformity problem of particle size fluctuations reaching ±50nm in the traditional pyrolysis method. Simultaneously, in-situ doping of NH3 / SO2 in the plasma tail flame region increases the fluorescence quantum yield to 40%, and eliminates the need for post-modification processes, significantly improving the batch stability and performance consistency of the product. Meanwhile, with the help of axial magnetic rotation microwave plasma technology, the electron utilization rate is increased to 65% and the energy consumption is reduced to <5kWh / g, which is only 1 / 4 of that of conventional plasma methods. The matching drum-type continuous feeding and multi-magnetron array system increases the production capacity. Combined with the pre-electrostatic dust removal to solve the coal ash blockage problem, it takes into account the needs of low cost, low pollution, high performance and large-scale production, and provides an economically feasible technical path for the industrial application of graphene quantum dots.

[0076] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing graphene quantum dots based on steam coal, characterized in that, The method comprises the following steps: S1: taking power coal as raw material, sequentially performing electrochemical oxidation treatment and KOH ball milling treatment to obtain pretreated coal material; S2: introducing the pretreated coal material into an axial magnetic rotating microwave plasma reactor to perform dissociation and recombination reaction under segmented temperature control, injecting NH3 or SO2 gas into the plasma afterflame zone at an injection rate of 10 sccm to realize N doping or S doping; S3: performing confined growth by using a porous alumina template during the reaction, and performing liquid nitrogen quenching treatment after the reaction to obtain graphene quantum dots; S4: performing grading purification treatment on the graphene quantum dots.

2. The method of claim 1, wherein the method is a method of preparing graphene quantum dots based on steam activated coal. In the S1, the fixed carbon content of the power coal is >60%, and the ash content is <25%; the electrochemical oxidation treatment is as follows: the electrode plate is prepared by pressing the fine washing coal of the power coal and 5% pitch, the electrode plate is used as an anode, and electrolysis is carried out in pure water, the electrolysis voltage is 12V, the current density is 0.25A / cm 2 , and the electrolysis time is 300min.

3. The method of claim 2, wherein the method is a method of preparing graphene quantum dots based on steam activated coal. In the S1, the KOH ball milling treatment comprises the following steps: ball milling the coal powder after electrochemical oxidation with KOH at a mass ratio of 1:3; the ball milling rotation speed is 400 rpm, and the ball milling time is 12 h.

4. The method of claim 1, wherein the method is a method of preparing graphene quantum dots based on steam activated coal. In the S2, the microwave source frequency of the axial magnetic rotating microwave plasma reactor is 2.45 GHz, and the power is in the range of 0-1500 W; the carrier gas used for the reaction is Ar / H2 mixed gas, wherein the volume ratio of Ar to H2 is 95:5, and the reaction pressure is 10-50 Pa.

5. The method of claim 4, wherein the method is a method of preparing graphene quantum dots based on steam activated coal. In the S2, the segmented temperature control is specifically as follows: in the temperature range of 300-500 ℃, a deconstruction reaction is performed by using a microwave power of 300 W; in the temperature range of 600-800 ℃, a recombination reaction is performed by using a microwave power of 800 W.

6. The method of claim 1, wherein the method is a method of preparing graphene quantum dots based on steam activated coal. In the S3, the pore diameter of the porous alumina template is 20-100 nm, and the relationship between the diameter d of the prepared graphene quantum dots and the pore diameter of the template satisfies d≈0.8×pore diameter; the cooling rate of the liquid nitrogen quenching treatment is >100 ℃ / s.

7. The method of claim 1, wherein the method is a method of preparing graphene quantum dots based on steam activated coal. In the S4, the grading purification treatment comprises gradient centrifugation, filter membrane filtration and electrophoresis purification in sequence; wherein the rotation speed of the gradient centrifugation is 8000 rpm and 15000 rpm in sequence, the filter membrane filtration adopts a 0.22 μm filter membrane, and the electrophoresis purification is performed under the condition of a voltage of 50 V and a Tris buffer solution with pH=8.

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