CN111634905A - Method for preparing graphene quantum dots under coupling effect of magnetic field and ultrasonic field - Google Patents
Method for preparing graphene quantum dots under coupling effect of magnetic field and ultrasonic field Download PDFInfo
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- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 claims abstract description 15
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- 238000002360 preparation method Methods 0.000 claims abstract description 11
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- QAOWNCQODCNURD-UHFFFAOYSA-L Sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 claims description 12
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Abstract
The invention belongs to the technical field of graphene preparation and manufacturing, and discloses a method for preparing graphene quantum dots under the coupling action of a magnetic field and an ultrasonic field, which comprises the steps of preparing graphene oxide, and adding deionized water to obtain a graphene oxide aqueous solution; adding a nitric acid solution and concentrated sulfuric acid, mixing, adding into a sealed container containing polytetrafluoroethylene, and carrying out microwave reaction; adding sodium carbonate powder, carrying out ultrasonic treatment, simultaneously applying an alternating-current magnetic field, and influencing the reaction process of the graphene quantum dots by controlling the application of the magnetic field and the ultrasonic field until the reaction process is finished; and (3) carrying out decompression and suction filtration by using a microporous membrane to obtain a crude quantum dot solution, taking the supernatant, and carrying out ultrafiltration in deionized water by using an ultrafiltration membrane to obtain a high-concentration dark brown graphene quantum dot solution. The graphene quantum dots are prepared by using graphene oxide, so that the preparation method is simple and the reaction time is short; the graphene quantum dots are prepared under the coupling effect of the magnetic field and the ultrasonic field, and the obtained graphene quantum dots are excellent in electrochemical performance.
Description
Technical Field
The invention belongs to the technical field of graphene preparation and manufacturing, and particularly relates to a method for preparing graphene quantum dots under the coupling action of a magnetic field and an ultrasonic field.
Background
At present, Graphene Quantum Dots (GQDs) generally have a transverse dimension below 100nm and a longitudinal dimension below several nanometers, and have a Graphene structure with one layer, two layers or several layers, that is, special very small Graphene fragments. The characteristics of the graphene/carbon dot composite material are derived from graphene and carbon dots, and the graphene/carbon dot composite material shows low biological toxicity, excellent water solubility, chemical inertness, stable photoluminescence and good surface modification. The existing preparation methods of the graphene quantum dots are divided into a top-down method and a bottom-up method, the most commonly used method is a hydrothermal method in the top-down method, and the synthesized graphene quantum dots can be stably dispersed in water, but the reaction time and the reaction conditions are harsh; and the purity of the prepared graphene quantum dots is poor.
In summary, the problems of the prior art are as follows: the reaction time and the reaction conditions of the method for synthesizing the graphene quantum dots by the hydrothermal method are harsh; and the purity of the prepared graphene quantum dots is poor.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a method for preparing graphene quantum dots under the coupling action of a magnetic field and an ultrasonic field.
The invention is realized in such a way that a method for preparing graphene quantum dots under the coupling action of a magnetic field and an ultrasonic field comprises the following steps:
preparing graphene oxide, adding the graphene oxide into deionized water, and treating by using a high-speed shearing machine and an ultrasonic dispersion treatment instrument to obtain a graphene oxide aqueous solution.
And step two, adding a nitric acid solution and concentrated sulfuric acid into the graphene oxide aqueous solution obtained in the step one, stirring and mixing uniformly, adding into a sealed container containing 100ml of polytetrafluoroethylene, and carrying out microwave reaction.
And step three, cooling to room temperature after the reaction is finished, adding sodium carbonate powder, performing ultrasonic treatment by using an ultrasonic analyzer, simultaneously applying an alternating-current magnetic field, and influencing the reaction process of the graphene quantum dots by controlling the application of the magnetic field and the ultrasonic field until the reaction is finished.
And step four, carrying out decompression and suction filtration by using a microporous membrane to obtain a crude quantum dot solution, taking the supernatant, and carrying out ultrafiltration in deionized water by using an ultrafiltration membrane for 24-28 h to obtain a high-concentration dark brown graphene quantum dot solution.
Further, in the first step, the preparation method of the graphene oxide comprises the following steps:
(1) adding 1g of graphite powder into 1mL of concentrated sulfuric acid, and carrying out heat bath for 12-24 h at 100-120 ℃; and cooling, placing in a centrifuge tube for centrifugation, washing with deionized water, and drying to obtain the pre-oxidized graphite.
(2) Keeping the ice bath temperature at 3-5 ℃, mixing pre-oxidized graphite with 2g of sodium nitrate, and adding 50mL of concentrated sulfuric acid under magnetic stirring; the ice bath temperature was controlled to 20 ℃ or lower, and 6g of potassium permanganate was added in three portions with stirring.
(3) Removing the ice bath, placing for 24h, transferring to a beaker, adding 100mL of deionized water, and heating at 100-120 ℃ for 10-15 min; adding 300mL of deionized water and 10mL of 30% sodium peroxide solution, stirring, and centrifuging for 15-20 min.
(4) And (4) washing the centrifuged supernatant by using hydrochloric acid according to the ratio of 1:10 to remove sulfate ions, and then washing the supernatant to be neutral by using deionized water.
(5) Washing with absolute ethyl alcohol, centrifuging at 10000-11000 rpm, and vacuum drying at 60-80 ℃ for 24-28 hours to obtain the graphene oxide.
Further, the detection method for removing the sulfate ions comprises the following steps:
and (3) putting the washing liquid into a test tube, and adding a barium chloride solution, wherein if white precipitates are generated, the sulfate ions are not removed completely, and if no white precipitates are generated, the sulfate ions are removed completely.
Further, in the first step, the preparation method of the graphene oxide aqueous solution comprises the following steps:
(I) dispersing the prepared graphite oxide solid in water to form graphite oxide turbid liquid with the mass concentration of 2-7.0 mg/ml.
(II) respectively adopting a high-speed shearing machine and an ultrasonic dispersion treatment instrument to treat for 2-3 h, centrifuging for 10-20 min at 500-700 rpm/min by using a low-speed centrifuge, discarding the lower-layer precipitate, and taking the upper-layer suspension.
(III) adding 1/10 HCL solution 100ml into the upper layer suspension, soaking for 0.5-1.0 h, then centrifuging at 12000-13000 rpm/min, 10-20 min each time, discarding the supernatant until the pH is 7, adding a proper amount of deionized water into the lower layer precipitate to dissolve, and then obtaining the graphene oxide aqueous solution, and storing.
Further, the concentration of the graphene oxide aqueous solution is 500mg/mL-1。
Further, in the second step, the condition of the oxidation reaction of the graphene oxide aqueous solution, the nitric acid solution and the concentrated sulfuric acid comprises:
and mixing 10mL of graphene oxide aqueous solution, 5mL of 65% nitric acid solution and 5mL of 98% concentrated sulfuric acid, and carrying out microwave reaction for 5-10 min.
Further, the radiation conditions of the microwave reaction are as follows: the temperature is 200-250 ℃, the radiation power is 800-1000W, and the pressure is 30-40 Mpa.
Further, in the third step, the power of the ultrasonic analyzer is 0-3 kW, the ultrasonic frequency is 40-80 KHz, and the ultrasonic time is 5-10 minutes.
Furthermore, in the third step, the intensity of the alternating current magnetic field is 0.001-0.3T, and the frequency range is 5-50 Hz.
Further, in step four, the ultrafiltration membrane used in the ultrafiltration has a molecular weight cut-off of 100kDa, 50kDa, 30kDa, 10kDa or 3 kDa.
In summary, the advantages and positive effects of the invention are: according to the invention, the graphene quantum dots are prepared by using graphene oxide, the preparation method is simple and rapid, and the reaction time is short; the graphene quantum dots are prepared under the coupling effect of the magnetic field and the ultrasonic field, and the obtained graphene quantum dots are excellent in electrochemical performance. Experiments show that the fluorescence quantum yield of the graphene quantum dots prepared by the method is higher than 10%, the fluorescence emission wavelength basically does not change along with the change of the excitation light wavelength, the fluorescence emission peak positions excited by light with the wavelength of 330-500 nm are all located between 500-600 nm, and the method has wide application prospects in the fields of fluorescence labeling imaging, drug delivery, disease diagnosis, analysis and detection and the like.
Drawings
Fig. 1 is a flowchart of a method for preparing graphene quantum dots under the coupling effect of a magnetic field and an ultrasonic field according to an embodiment of the present invention.
Fig. 2 is a picture of a graphene quantum dot solution under visible light and irradiation of a 365nm ultraviolet lamp according to an embodiment of the present invention;
in the figure: the picture (A) is a picture of a graphene quantum dot solution under the irradiation of a visible light lamp; and (B) is a picture of the graphene quantum dot solution under the irradiation of an ultraviolet lamp.
Fig. 3 is a spectrum characterization diagram of graphene quantum dots provided by an embodiment of the present invention.
Fig. 4 is a fluorescence spectrum of the graphene quantum dot provided in the embodiment of the present invention.
Fig. 5 is a TEM image of the graphene quantum dot provided by the embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Aiming at the problems in the prior art, the invention provides a method for preparing graphene quantum dots under the coupling action of a magnetic field and an ultrasonic field, and the invention is described in detail below with reference to the accompanying drawings.
As shown in fig. 1, the method for preparing graphene quantum dots under the coupling effect of a magnetic field and an ultrasonic field according to the embodiment of the present invention includes the following steps:
s101, preparing graphene oxide, adding the graphene oxide into deionized water, and processing by using a high-speed shearing machine and an ultrasonic dispersion processor to obtain a graphene oxide aqueous solution.
And S102, adding a nitric acid solution and concentrated sulfuric acid into the graphene oxide aqueous solution obtained in the S101, stirring and mixing uniformly, adding into a sealed container containing 100ml of polytetrafluoroethylene, and carrying out microwave reaction.
And S103, cooling to room temperature after the reaction is finished, adding sodium carbonate powder, performing ultrasonic treatment by using an ultrasonic analyzer, simultaneously applying an alternating-current magnetic field, and influencing the reaction process of the graphene quantum dots by controlling the application of the magnetic field and the ultrasonic field until the reaction is finished.
And S104, carrying out pressure reduction and suction filtration by using a microporous membrane to obtain a crude quantum dot solution, taking the supernatant, and carrying out ultrafiltration in deionized water by using an ultrafiltration membrane for 24-28 h to obtain a high-concentration dark brown graphene quantum dot solution.
In step S101 provided in the embodiment of the present invention, the preparation method of graphene oxide includes:
(1) adding 1g of graphite powder into 1mL of concentrated sulfuric acid, and carrying out heat bath for 12-24 h at 100-120 ℃; and cooling, placing in a centrifuge tube for centrifugation, washing with deionized water, and drying to obtain the pre-oxidized graphite.
(2) Keeping the ice bath temperature at 3-5 ℃, mixing pre-oxidized graphite with 2g of sodium nitrate, and adding 50mL of concentrated sulfuric acid under magnetic stirring; the ice bath temperature was controlled to 20 ℃ or lower, and 6g of potassium permanganate was added in three portions with stirring.
(3) Removing the ice bath, placing for 24h, transferring to a beaker, adding 100mL of deionized water, and heating at 100-120 ℃ for 10-15 min; adding 300mL of deionized water and 10mL of 30% sodium peroxide solution, stirring, and centrifuging for 15-20 min.
(4) And (4) washing the centrifuged supernatant by using hydrochloric acid according to the ratio of 1:10 to remove sulfate ions, and then washing the supernatant to be neutral by using deionized water.
(5) Washing with absolute ethyl alcohol, centrifuging at 10000-11000 rpm, and vacuum drying at 60-80 ℃ for 24-28 hours to obtain the graphene oxide.
The detection method for removing the sulfate ions provided by the embodiment of the invention comprises the following steps: and (3) putting the washing liquid into a test tube, and adding a barium chloride solution, wherein if white precipitates are generated, the sulfate ions are not removed completely, and if no white precipitates are generated, the sulfate ions are removed completely.
In step S101 provided in the embodiment of the present invention, the preparation method of the graphene oxide aqueous solution is as follows:
(I) dispersing the prepared graphite oxide solid in water to form graphite oxide turbid liquid with the mass concentration of 2-7.0 mg/ml.
(II) respectively adopting a high-speed shearing machine and an ultrasonic dispersion treatment instrument to treat for 2-3 h, centrifuging for 10-20 min at 500-700 rpm/min by using a low-speed centrifuge, discarding the lower-layer precipitate, and taking the upper-layer suspension.
(III) adding 1/10 HCL solution 100ml into the upper layer suspension, soaking for 0.5-1.0 h, then centrifuging at 12000-13000 rpm/min, 10-20 min each time, discarding the supernatant until the pH is 7, adding a proper amount of deionized water into the lower layer precipitate to dissolve, and then obtaining the graphene oxide aqueous solution, and storing.
The concentration of the graphene oxide aqueous solution provided by the embodiment of the invention is 500mg/mL-1。
In step S102 provided in the embodiment of the present invention, the conditions for performing an oxidation reaction on the graphene oxide aqueous solution, the nitric acid solution, and the concentrated sulfuric acid include: and mixing 10mL of graphene oxide aqueous solution, 5mL of 65% nitric acid solution and 5mL of 98% concentrated sulfuric acid, and carrying out microwave reaction for 5-10 min.
The radiation conditions of the microwave reaction provided by the embodiment of the invention are as follows: the temperature is 200-250 ℃, the radiation power is 800-1000W, and the pressure is 30-40 Mpa.
In step S103 provided by the embodiment of the present invention, the power of the ultrasonic analyzer is 0 to 3kW, the ultrasonic frequency is 40 to 80KHz, and the ultrasonic time is 5 to 10 minutes.
In step S103 provided by the embodiment of the present invention, the alternating-current magnetic field intensity is 0.001 to 0.3T, and the frequency range is 5 to 50 Hz.
In step S104 provided by the embodiment of the present invention, the cut-off molecular weight of the ultrafiltration membrane used in the ultrafiltration is 100kDa, 50kDa, 30kDa, 10kDa, or 3 kDa.
The invention is further described with reference to specific examples.
Example 1
The method for preparing the graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field, which is provided by the embodiment 1 of the invention, comprises the following steps:
(1) preparing graphene oxide, adding the graphene oxide into deionized water, and treating for 2 hours by using a high-speed shearing machine and an ultrasonic dispersion treatment instrument to obtain the graphene oxide with the concentration of 500mg/mL-1The aqueous solution of graphene oxide of (1).
(2) Mixing 10mL of graphene oxide aqueous solution, 5mL of 65% nitric acid solution and 5mL of 98% concentrated sulfuric acid, adding the mixture into a sealed container containing 100mL of polytetrafluoroethylene, and carrying out microwave reaction for 5min at the temperature of 200 ℃, the radiation power of 800W and the pressure of 30 Mpa.
(3) And after the reaction is finished, cooling to room temperature, adding sodium carbonate powder, carrying out ultrasonic treatment for 5 minutes by using an ultrasonic analyzer with the power of 0kW and the ultrasonic frequency of 40KHz, simultaneously applying an alternating-current magnetic field with the intensity of 0.001T and the frequency range of 5Hz, and influencing the reaction process of the graphene quantum dots by controlling the application of the magnetic field and the ultrasonic field until the reaction is finished.
(4) And (3) carrying out decompression and suction filtration by using a microporous membrane to obtain a crude quantum dot solution, taking the supernatant, and carrying out ultrafiltration in deionized water for 24 hours by using an ultrafiltration membrane with the molecular weight cutoff of 3kDa to obtain a high-concentration dark brown graphene quantum dot solution.
Example 2
The method for preparing the graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field, which is provided by the embodiment 2 of the invention, comprises the following steps:
(1) preparing graphene oxide, adding the graphene oxide into deionized water, and treating for 2.2h by using a high-speed shearing machine and an ultrasonic dispersion treatment instrument to obtain the graphene oxide with the concentration of 500mg/mL-1The aqueous solution of graphene oxide of (1).
(2) Mixing 10mL of graphene oxide aqueous solution, 5mL of 65% nitric acid solution and 5mL of 98% concentrated sulfuric acid, adding the mixture into a sealed container containing 100mL of polytetrafluoroethylene, and carrying out microwave reaction for 7min at the temperature of 210 ℃, the radiation power of 850W and the pressure of 32 Mpa.
(3) And after the reaction is finished, cooling to room temperature, adding sodium carbonate powder, carrying out ultrasonic treatment for 6 minutes by using an ultrasonic analyzer with the power of 1kW and the ultrasonic frequency of 50KHz, simultaneously applying an alternating-current magnetic field with the intensity of 0.05T and the frequency range of 10Hz, and influencing the reaction process of the graphene quantum dots by controlling the application of the magnetic field and the ultrasonic field until the reaction is finished.
(4) And (3) carrying out decompression and suction filtration by using a microporous membrane to obtain a crude quantum dot solution, taking supernate, and carrying out ultrafiltration for 25h in deionized water by using an ultrafiltration membrane with the molecular weight cutoff of 10kDa to obtain a high-concentration dark brown graphene quantum dot solution.
Example 3
The method for preparing the graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field, provided by the embodiment 3 of the invention, comprises the following steps:
(1) preparing graphene oxide, adding the graphene oxide into deionized water, and treating for 2.4 hours by using a high-speed shearing machine and an ultrasonic dispersion treatment instrument to obtain the graphene oxide with the concentration of 500mg/mL-1The aqueous solution of graphene oxide of (1).
(2) Mixing 10mL of graphene oxide aqueous solution, 5mL of 65% nitric acid solution and 5mL of 98% concentrated sulfuric acid, adding the mixture into a sealed container containing 100mL of polytetrafluoroethylene, and carrying out microwave reaction for 7min at the temperature of 220 ℃, the radiation power of 900W and the pressure of 34 Mpa.
(3) And after the reaction is finished, cooling to room temperature, adding sodium carbonate powder, carrying out ultrasonic treatment for 7 minutes by using an ultrasonic analyzer with the power of 1.5kW and the ultrasonic frequency of 60KHz, simultaneously applying an alternating-current magnetic field with the intensity of 0.1T and the frequency range of 20Hz, and influencing the reaction process of the graphene quantum dots by controlling the application of the magnetic field and the ultrasonic field until the reaction is finished.
(4) And (3) carrying out decompression and suction filtration by using a microporous membrane to obtain a crude quantum dot solution, taking the supernatant, and carrying out ultrafiltration for 26h in deionized water by using an ultrafiltration membrane with the molecular weight cutoff of 30kDa to obtain a high-concentration dark brown graphene quantum dot solution.
Example 4
The method for preparing the graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field, which is provided by the embodiment 4 of the invention, comprises the following steps:
(1) preparing graphene oxide, adding the graphene oxide into deionized water, and treating for 2.8 hours by using a high-speed shearing machine and an ultrasonic dispersion treatment instrument to obtain the graphene oxide with the concentration of 500mg/mL-1The aqueous solution of graphene oxide of (1).
(2) Mixing 10mL of graphene oxide aqueous solution, 5mL of 65% nitric acid solution and 5mL of 98% concentrated sulfuric acid, adding the mixture into a sealed container containing 100mL of polytetrafluoroethylene, and carrying out microwave reaction for 8min at the temperature of 240 ℃, the radiation power of 950W and the pressure of 35 Mpa.
(3) And after the reaction is finished, cooling to room temperature, adding sodium carbonate powder, carrying out ultrasonic treatment for 8 minutes by using an ultrasonic analyzer with the power of 2.5kW and the ultrasonic frequency of 70KHz, simultaneously applying an alternating-current magnetic field with the intensity of 0.2T and the frequency range of 45Hz, and influencing the reaction process of the graphene quantum dots by controlling the application of the magnetic field and the ultrasonic field until the reaction is finished.
(4) And (3) carrying out decompression and suction filtration by using a microporous membrane to obtain a crude quantum dot solution, taking the supernatant, and carrying out ultrafiltration for 27h in deionized water by using an ultrafiltration membrane with the molecular weight cutoff of 50kDa to obtain a high-concentration dark brown graphene quantum dot solution.
Comparative example 1
The method for preparing the graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field, which is provided by the comparative example 1, comprises the following steps:
(1) preparing graphene oxide, adding the graphene oxide into deionized water, and treating for 3 hours by using a high-speed shearing machine and an ultrasonic dispersion treatment instrument to obtain the graphene oxide with the concentration of 500mg/mL-1The aqueous solution of graphene oxide of (1).
(2) Mixing 10mL of graphene oxide aqueous solution, 5mL of 65% nitric acid solution and 5mL of 98% concentrated sulfuric acid, adding the mixture into a sealed container containing 100mL of polytetrafluoroethylene, and carrying out microwave reaction for 10min at the temperature of 250 ℃, the radiation power of 1000W and the pressure of 40 Mpa.
(3) And after the reaction is finished, cooling to room temperature, adding sodium carbonate powder, carrying out ultrasonic treatment for 10 minutes by using an ultrasonic analyzer with the power of 3kW and the ultrasonic frequency of 80KHz, simultaneously applying an alternating-current magnetic field with the intensity of 0.3T and the frequency range of 50Hz, and influencing the reaction process of the graphene quantum dots by controlling the application of the magnetic field and the ultrasonic field until the reaction is finished.
(4) And (3) carrying out decompression and suction filtration by using a microporous membrane to obtain a crude quantum dot solution, taking the supernatant, and carrying out ultrafiltration for 28h in deionized water by using an ultrafiltration membrane with the molecular weight cutoff of 100kDa to obtain a high-concentration dark brown graphene quantum dot solution.
And (4) conclusion: the X-ray photoelectron spectra of the graphene quantum dots obtained in the above examples and comparative examples were measured using the american siemer femier science VGMultilab 2000. The relative content of carbon and oxygen in the graphene quantum dots is obtained by calculating the integral area of the X-ray photoelectron energy spectrograms of C1s and O1s, and the ratio of oxygen to carbon (i.e. relative content of oxygen/relative content of carbon) is obtained, which is shown in table 1.
TABLE 1 relative content of oxygen element/relative content of carbon element
The fluorescence emission of the graphene quantum dots obtained in the above example 2 was measured, and fig. 2 is a picture of aqueous solutions of the graphene quantum dots 2-GQDs @ (50-100k), 2-GQDs @ (30-50k), 2-GQDs @ (10-30k), 2-GQDs @ (3-10k) and 2-GQDs @ (<3k) under visible light and 365nm ultraviolet lamp irradiation; FIGS. 3-4 are normalized fluorescence emission spectra; fig. 5 is a TEM photograph of the graphene quantum dot prepared in this example.
The above description is only for the purpose of illustrating the present invention and the appended claims are not to be construed as limiting the scope of the invention, which is intended to cover all modifications, equivalents and improvements that are within the spirit and scope of the invention as defined by the appended claims.
Claims (8)
1. A method for preparing graphene quantum dots under the coupling action of a magnetic field and an ultrasonic field is characterized by comprising the following steps:
preparing graphene oxide, adding the graphene oxide into deionized water, and treating by using a high-speed shearing machine and an ultrasonic dispersion treatment instrument to obtain a graphene oxide aqueous solution; the preparation method of the graphene oxide comprises the following steps:
(1) adding 1g of graphite powder into 1mL of concentrated sulfuric acid, and carrying out heat bath for 12-24 h at 100-120 ℃; cooling, centrifuging in a centrifuge tube, washing with deionized water, and drying to obtain pre-oxidized graphite;
(2) keeping the ice bath temperature at 3-5 ℃, mixing pre-oxidized graphite with 2g of sodium nitrate, and adding 50mL of concentrated sulfuric acid under magnetic stirring; controlling the ice bath temperature to be below 20 ℃, adding 6g of potassium permanganate in three times, and stirring;
(3) removing the ice bath, placing for 24h, transferring to a beaker, adding 100mL of deionized water, and heating at 100-120 ℃ for 10-15 min; adding 300mL of deionized water and 10mL of 30% sodium peroxide solution, stirring, and centrifuging for 15-20 min;
(4) washing the centrifuged supernatant by hydrochloric acid according to a ratio of 1:10 to remove sulfate ions, and washing the supernatant to be neutral by deionized water;
(5) washing with absolute ethyl alcohol, centrifuging at 10000-11000 rpm, and vacuum drying at 60-80 ℃ for 24-28 hours to obtain graphene oxide;
step two, adding a nitric acid solution and concentrated sulfuric acid into the graphene oxide aqueous solution obtained in the step one, stirring and mixing uniformly, adding into a sealed container containing 100ml of polytetrafluoroethylene, and carrying out microwave reaction; the conditions for carrying out oxidation reaction on the graphene oxide aqueous solution, the nitric acid solution and the concentrated sulfuric acid comprise:
mixing 10mL of graphene oxide aqueous solution with 5mL of 65% nitric acid solution and 5mL of 98% concentrated sulfuric acid, and carrying out microwave reaction for 5-10 min;
step three, cooling to room temperature after the reaction is finished, adding sodium carbonate powder, performing ultrasonic treatment by using an ultrasonic analyzer, simultaneously applying an alternating-current magnetic field, and influencing the reaction process of the graphene quantum dots by controlling the application of the magnetic field and the ultrasonic field until the reaction is finished;
and step four, carrying out decompression and suction filtration by using a microporous membrane to obtain a crude quantum dot solution, taking the supernatant, and carrying out ultrafiltration in deionized water by using an ultrafiltration membrane for 24-28 h to obtain a high-concentration dark brown graphene quantum dot solution.
2. The method for preparing graphene oxide according to claim 1, wherein the method for detecting the removal of sulfate ions comprises:
and (3) putting the washing liquid into a test tube, and adding a barium chloride solution, wherein if white precipitates are generated, the sulfate ions are not removed completely, and if no white precipitates are generated, the sulfate ions are removed completely.
3. The method for preparing graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field according to claim 1, wherein in the first step, the preparation method of the graphene oxide aqueous solution comprises the following steps:
(I) dispersing the prepared graphite oxide solid in water to form graphite oxide turbid liquid with the mass concentration of 2-7.0 mg/ml;
(II) respectively processing for 2-3 h by adopting a high-speed shearing machine and an ultrasonic dispersion processor, centrifuging for 10-20 min at 500-700 rpm/min by using a low-speed centrifuge, discarding the lower-layer precipitate, and taking the upper-layer suspension;
(III) adding 1/10 HCL solution 100ml into the upper layer suspension, soaking for 0.5-1.0 h, then centrifuging at 12000-13000 rpm/min, 10-20 min each time, discarding the supernatant until the pH is 7, adding a proper amount of deionized water into the lower layer precipitate to dissolve, and then obtaining the graphene oxide aqueous solution, and storing.
4. The method for preparing graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field according to claim 3, wherein the concentration of the graphene oxide aqueous solution is 500mg/mL-1。
5. The method for preparing graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field according to claim 1, wherein the radiation conditions of the microwave reaction are as follows: the temperature is 200-250 ℃, the radiation power is 800-1000W, and the pressure is 30-40 Mpa.
6. The method for preparing graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field according to claim 1, wherein in the third step, the power of the ultrasonic analyzer is 0-3 kW, the ultrasonic frequency is 40-80 KHz, and the ultrasonic time is 5-10 minutes.
7. The method for preparing the graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field according to claim 1, wherein in the third step, the alternating-current magnetic field intensity is 0.001-0.3T, and the frequency range is 5-50 Hz.
8. The method for preparing graphene quantum dots under the coupling effect of the magnetic field and the ultrasonic field according to claim 1, wherein in the fourth step, an ultrafiltration membrane is used in the ultrafiltration, and the molecular weight cut-off of the ultrafiltration membrane is 100kDa, 50kDa, 30kDa, 10kDa or 3 kDa.
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