Carbon quantum dot electrolyte solution for photocathode reduction system

Abstract

The present invention discloses a carbon quantum dot electrolyte solution for a photoelectrochemical cathode reduction system, which is synthesized by a hydrothermal method using a cationic surfactant as a precursor, and the raw materials used are low-cost, simple and easy to obtain, and the synthesis method is simple. The synthesized carbon quantum dot aqueous solution can be directly used as an electrolyte solution in a photoelectrochemical cathode reduction system. Compared with traditional electrolyte solutions, the carbon quantum dot electrolyte solution of the present invention significantly promotes the photoelectrochemical activity of the photoelectrochemical cathode reduction system, and different precursors can be selected to synthesize different carbon quantum dot aqueous solutions, realizing multiple functions such as water decomposition to produce hydrogen, oxygen reduction to prepare hydrogen peroxide, and the system stability is relatively high.

Description

Technical Field

[0001] The invention belongs to the technical field of carbon quantum dot synthesis and photoelectrochemical cathode reduction, and specifically relates to a carbon quantum dot electrolyte solution for a photoelectrochemical cathode reduction system. Background Art

[0002] Hydrogen energy is an ideal clean energy with high energy conversion efficiency and high combustion calorific value, which is 3 times that of gasoline and 4.5 times that of coke. The combustion product is water, which is clean and pollution-free. At present, the main methods of hydrogen production include water electrolysis, methane steam reforming in natural gas, and coal-to-hydrogen production. Among them, water electrolysis has a high cost and is difficult to be widely used; both natural gas and coal-to-hydrogen production use non-renewable energy, which is not in line with the concept of sustainable development, and the hydrogen produced by coal-to-hydrogen production contains trace impurities that are difficult to purify. In recent years, photoelectrochemical water decomposition has attracted widespread attention from researchers. Solar energy is a widely available, clean, and sustainable green primary energy source. The use of solar energy photoelectrocatalytic water decomposition to produce hydrogen is considered to be a very promising green new energy preparation method. Photoelectrocatalytic water decomposition is mainly based on the research of water decomposition hydrogen and oxygen evolution (hydrogen peroxide) in semiconductor photoelectrodes and electrolyte systems. The anode undergoes water oxidation oxygen evolution (hydrogen peroxide) reaction and the cathode undergoes water reduction hydrogen evolution reaction. Semiconductor electrode materials have certain defects such as narrow band gap, energy level mismatch of redox reaction, fast electron-hole recombination rate and poor interface charge transfer ability. In order to improve semiconductor electrodes, numerous methods have been proposed, including composite materials, nanostructures, crystal plane engineering, composition regulation, doping and nanocrystal embedding. However, there are few reports on the research of electrolyte solution, another key to photoelectrocatalytic water splitting system.

[0003] In addition to being used for water decomposition to produce hydrogen, the photoelectrocatalytic cathode reduction system can also be used in other fields, and oxygen reduction to produce hydrogen peroxide is one of them. Hydrogen peroxide and its aqueous solution have strong oxidizing properties and are widely used in papermaking, sewage treatment and disinfection. The global demand for hydrogen peroxide is increasing day by day, but the traditional anthraquinone method is complex, costly and inefficient, and the direct hydrogen and oxygen synthesis method has great safety hazards. The electrolysis method for producing hydrogen peroxide has high current efficiency, short process flow and high product quality, but it consumes a lot of electricity and has high production costs, which is not suitable for large-scale industrial production. Photoelectrocatalytic oxygen reduction to produce hydrogen peroxide is a promising method for preparing hydrogen peroxide. It has high current efficiency, simple process flow, high product purity, and solar energy as a primary energy source consumes less electricity. Compared with electrocatalysis, the cost is lower and it is expected to be industrialized. Therefore, it is urgent to construct a high-performance photoelectrocatalytic cathode oxygen reduction system to produce hydrogen peroxide.

[0004] In order to improve the performance of the photoelectrocatalytic cathode reduction system, exploring suitable new electrolyte solutions is a convenient and effective new idea. The new zero-dimensional carbon material, carbon quantum dots, is a new type of nanomaterial with a carbon core as the center and many different functional groups (such as -COOH, -COOR and -OH groups) connected to the outside. It has excellent conductivity, photoluminescence, modifiability, stability, water solubility, adjustable band gap and unique charge transfer and storage capabilities. The average particle size of this type of emerging low-cost carbon nanomaterial with fluorescent properties is usually less than 10nm. Depending on the selected raw materials, carbon quantum dots can have different structures, and the functional groups contained in the precursors are different, and the performance of the prepared carbon quantum dots is also different. Because the surface groups are charged, carbon quantum dots can produce directional movement during the electrolysis process, so the carbon quantum dot aqueous solution has the function of an electrolyte. In addition, carbon quantum dots can also adjust the band gap of the photoelectrode, which plays a very important role in the transfer of photogenerated charges. At the same time, the semiconductor properties of carbon quantum dots can also form a heterojunction with the photoelectrode, increasing the charge separation efficiency and thus promoting the photoelectrocatalytic efficiency (Applied Catalysis B: Environmental, 2021, 296, 120378). Summary of the invention

[0005] The purpose of the present invention is to provide a carbon quantum dot aqueous solution prepared with a cationic surfactant as a precursor, which can be directly used as an electrolyte solution for a photocathode reduction system, so that different photocathode systems have the efficiency of decomposing water to produce hydrogen and reducing oxygen to hydrogen peroxide.

[0006] In order to achieve the above-mentioned purpose, the carbon quantum dot electrolyte solution for the photocathode reduction system provided by the present invention is prepared by adding a cationic surfactant into deionized water, ultrasonically dissolving it completely, and then transferring it into the lining of a reactor, and hydrothermally reacting it at 180 to 220° C. for 5 to 10 hours. After the reaction is completed, the solution is centrifuged and filtered to obtain a yellow aqueous solution, i.e., a carbon quantum dot aqueous solution.

[0007] The above-mentioned cationic surfactant is any one of tetramethylammonium chloride, tetrapropylammonium chloride, tetrabutylammonium chloride, tetrabutylammonium bromide, tetraethylammonium chloride, tetraethylammonium bromide, dodecyltrimethylammonium chloride, tributylmethylammonium chloride, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, triethylbenzylammonium chloride, trimethylbenzylammonium chloride, 1-butyl-3-methylimidazole chloride, 1-ethyl-3-methylimidazole chloride, 1-allyl-3-methylimidazole chloride, 1-hexyl-3-methylimidazole chloride, and 1-hexyl-3-methylimidazole chloride. Among them, when the cationic surfactant is any one of tetramethylammonium chloride, tetrapropylammonium chloride, tetrabutylammonium chloride, tetrabutylammonium bromide, tetraethylammonium chloride, tetraethylammonium bromide, dodecyltrimethylammonium chloride, tributylmethylammonium chloride, triethylbenzylammonium chloride, and trimethylbenzylammonium chloride, the obtained carbon quantum dot aqueous solution is acidic and is used for photoelectrochemical cathode oxygen reduction to produce hydrogen peroxide; when the cationic surfactant is any one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetrabutylammonium hydroxide, the obtained carbon quantum dot aqueous solution is alkaline and is used for photoelectrochemical cathode water decomposition; when the cationic surfactant is any one of 1-butyl-3-methylimidazole chloride, 1-ethyl-3-methylimidazole chloride, 1-allyl-3-methylimidazole chloride, 1-hexyl-3-methylimidazole chloride, and 1-hexyl-3-methylimidazole chloride, the obtained carbon quantum dot aqueous solution is neutral and is used for photoelectrochemical cathode oxygen reduction to produce hydrogen peroxide.

[0008] The carbon quantum dot electrolyte solution of the present invention preferably controls the concentration of the cationic surfactant in deionized water to be 1-6 mol / L.

[0009] The carbon quantum dot electrolyte solution of the present invention is further preferably subjected to a hydrothermal reaction at 200 to 220° C. for 5 to 8 hours.

[0010] The beneficial effects of the present invention are as follows:

[0011] The present invention uses a cationic surfactant as a precursor and adopts a simple hydrothermal method to synthesize a carbon quantum dot aqueous solution. The raw material cost is low, the synthesis method is simple and easy to promote, and the prepared carbon quantum dot aqueous solution can be directly used as an electrolyte solution for a photocathode system. Compared with traditional electrolyte solutions, the photocathode system composed of carbon quantum dots as electrolytes not only has excellent activity and good stability, but also increases the photocurrent density and improves the photocathode reduction efficiency. The photocathode system composed of a carbon quantum dot aqueous solution as an electrolyte solution and a semiconductor material photocathode is worthy of further in-depth research and is expected to achieve practical production applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is the XRD pattern of carbon quantum dots synthesized using 1-butyl-3-methylimidazole chloride as a precursor in Example 1.

[0013] Figure 2 This is the XRD pattern of carbon quantum dots synthesized using tetrabutylammonium chloride as a precursor in Example 4.

[0014] Figure 3 This is the XRD pattern of carbon quantum dots synthesized using tetraethylammonium hydroxide as a precursor in Example 6.

[0015] Figure 4 This is a fluorescence test graph of carbon quantum dots synthesized using 1-butyl-3-methylimidazole chloride as a precursor in Example 1.

[0016] Figure 5 This is a graph of the oxygen reduction electrochemical LSV curve test of the carbon quantum dot aqueous solution synthesized using 1-butyl-3-methylimidazole chloride as a precursor in Example 1.

[0017] Figure 6 It is a comparison chart of the electrochemical LSV curve test of oxygen reduction in the traditional Na2SO4 electrolyte solution under neutral conditions and the carbon quantum dot aqueous solution synthesized using 1-butyl-3-methylimidazole chloride as a precursor in Example 1.

[0018] Figure 7 This is a comparison chart of oxygen reduction electrochemical IT curve tests of the carbon quantum dot aqueous solution synthesized using 1-butyl-3-methylimidazole chloride as a precursor in Example 1 and the traditional neutral Na2SO4 electrolyte solution.

[0019] Figure 8 This is a comparison chart of ultraviolet absorption of hydrogen peroxide detected by the DPD / POD method after electrolysis of the carbon quantum dot aqueous solution synthesized using 1-butyl-3-methylimidazole chloride as a precursor in Example 1 and the traditional neutral Na2SO4 electrolyte solution for 10 minutes.

[0020] Fig. 9 This is a test graph of the oxygen reduction electrochemical LSV curve of the carbon quantum dot aqueous solution synthesized using tetrabutylammonium chloride as a precursor in Example 4.

[0021] Fig.10 It is a comparison chart of the electrochemical LSV curve test of oxygen reduction in a traditional HClO4 electrolyte solution under acidic conditions and in an aqueous solution of carbon quantum dots synthesized using tetrabutylammonium chloride as a precursor in Example 4.

[0022] Fig.11 This is a comparison chart of oxygen reduction electrochemical IT curve tests of the carbon quantum dot aqueous solution synthesized using tetrabutylammonium chloride as a precursor in Example 4 and the traditional acidic HClO4 electrolyte solution.

[0023] Fig.12This is a comparison chart of ultraviolet absorption of hydrogen peroxide detected by the DPD / POD method after electrolysis of the carbon quantum dot aqueous solution synthesized with tetrabutylammonium chloride as a precursor in Example 4 and the traditional acidic HClO4 electrolyte solution for 10 minutes.

[0024] Fig.13 This is an electrochemical LSV curve test diagram of water decomposition of the carbon quantum dot aqueous solution synthesized using tetraethylammonium hydroxide as a precursor in Example 6.

[0025] Fig.14 It is a comparison diagram of the electrochemical LSV curves of water decomposition in a traditional NaOH electrolyte solution under alkaline conditions and in an aqueous solution of carbon quantum dots synthesized using tetraethylammonium hydroxide as a precursor in Example 6.

[0026] Fig.15 It is a comparison chart of electrochemical IT curve tests of water decomposition between the carbon quantum dot aqueous solution synthesized using tetraethylammonium hydroxide as a precursor in Example 6 and the traditional alkaline NaOH electrolyte solution. DETAILED DESCRIPTION

[0027] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited to these embodiments.

[0028] Example 1

[0029] Weigh 1.7462g (0.01mol) of 1-butyl-3-methyl imidazole chloride and add it to 10mL of deionized water. Ultrasonic treatment was performed for 20min to completely dissolve 1-butyl-3-methyl imidazole chloride. Then the resulting solution was transferred into a polytetrafluoroethylene-lined reactor, and the reactor was sealed. The hydrothermal reaction was carried out at 220°C for 8h. After the hydrothermal reaction was completed, the solution was centrifuged at 10000r / min for 10min, and then filtered with a filter membrane with a pore size of 220nm to obtain a clear and transparent yellow carbon quantum dot aqueous solution. The pH of the carbon quantum dot aqueous solution was measured by a pH meter to be 7.12.

[0030] Example 2

[0031] In this embodiment, the hydrothermal reaction was carried out at 220° C. for 5 h, and the other steps were the same as those in Example 1 to obtain a clear and transparent yellow carbon quantum dot aqueous solution. Compared with Example 1, the carbon quantum dot aqueous solution in this embodiment was lighter in color and had less obvious fluorescence.

[0032] Example 3

[0033] In this example, 2.7792 g (0.01 mol) of tetrabutylammonium chloride was used to replace 1-butyl-3-methylimidazole chloride in Example 1, and the other steps were the same as in Example 1 to obtain a clear and transparent orange-yellow carbon quantum dot aqueous solution. The pH of the carbon quantum dot aqueous solution was measured by a pH meter to be 3.41.

[0034] Example 4

[0035] In this embodiment, the hydrothermal reaction was carried out at 220° C. for 5 h, and the other steps were the same as those in Example 3 to obtain a clear and transparent yellow carbon quantum dot aqueous solution. Compared with Example 3, the properties of the carbon quantum dot aqueous solution in this embodiment were not significantly different, and the reaction time was shorter.

[0036] Example 5

[0037] 5.8mL (0.01mol) of 25% tetraethylammonium hydroxide aqueous solution was diluted with 4.2mL deionized water, and the tetraethylammonium hydroxide was fully mixed by ultrasonic treatment for 20min, and then the obtained solution was transferred into a polytetrafluoroethylene-lined reactor, and the reactor was sealed, and hydrothermally reacted at 200°C for 8h. After the hydrothermal reaction, the solution was centrifuged at 10000r / min for 10min, and then filtered with a filter membrane with a pore size of 220nm to obtain a clear and transparent yellow carbon quantum dot aqueous solution. The pH of the carbon quantum dot aqueous solution was measured by a pH meter to be 12.78.

[0038] Example 6

[0039] In this embodiment, the hydrothermal reaction was carried out at 200° C. for 5 h, and the other steps were the same as those in Example 5 to obtain a clear and transparent yellow carbon quantum dot aqueous solution. Compared with Example 5, the carbon quantum dot aqueous solution obtained in this embodiment has almost no difference in properties, and the reaction time is shorter.

[0040] Example 7

[0041] 2.2778g (0.01mol) of benzyltriethylammonium chloride was added to 10mL of deionized water, and the benzyltriethylammonium chloride was fully dissolved by ultrasonic treatment for 20min, and then the resulting solution was transferred into a polytetrafluoroethylene-lined reactor, and the reactor was sealed, and hydrothermal reaction was carried out at 200°C for 5h. After the hydrothermal reaction, the centrifuge was centrifuged at 10000r / min for 10min, and the supernatant was filtered with a filter membrane with a pore size of 220nm to obtain a clear and transparent light yellow carbon quantum dot aqueous solution. The pH of the carbon quantum dot aqueous solution was measured by a pH meter to be 11.10.

[0042] The carbon quantum dot aqueous solutions obtained in Examples 1, 4, and 6 were freeze-dried to obtain powders, and XRD characterization was performed using an X-ray powder diffractometer. Figures 1 to 3 The results show that a broad diffraction peak appears at 2θ=23.7° in all three images. This is the result of a large number of irregular carbon atoms stacking together. It is these irregular carbon atoms that constitute the core of the carbon quantum dots. Figure 4This is a fluorescence test diagram of the carbon quantum dot aqueous solution obtained in Example 1. The excitation-dependent photoluminescence of the obtained carbon quantum dot aqueous solution indicates that the carbon quantum dots can be excited by visible light in the near-ultraviolet region.

[0043] The carbon quantum dot aqueous solutions synthesized in the above-mentioned Examples 1, 4, and 6 were used as electrolyte solutions, and electrochemical tests were carried out using an electrochemical workstation CHI660E. Ag / AgCl electrode was used as the reference electrode, Pt sheet electrode was used as the counter electrode, and the prepared CuBi2O4 was used as the working electrode. A three-electrode system was constructed to carry out electrode experiments on the carbon quantum dot aqueous solution. The light source used in the experiment was an LED light bulb array with λ=395nm. LSV tests were carried out under dark and light conditions, respectively; the voltage was controlled at -0.4V vs RHE for it tests. At the same time, the same tests were carried out on the corresponding traditional electrolyte solutions. The traditional electrolyte solutions were formulated based on maintaining the same conductivity as the corresponding carbon quantum dot aqueous solutions. Oxygen was introduced as a reactant into Examples 1 and 4 and the traditional electrolyte solutions for comparison to carry out photoelectrochemical oxygen reduction tests. The results are shown in the figure. Figures 5 to 15 .

[0044] contrast Figure 5 , Figure 6 It can be seen from the LSV curve that the photocurrent of the neutral carbon quantum dot aqueous solution obtained in Example 1 is increased by two times compared with the traditional neutral Na2SO4 electrolyte solution. Figure 7 The it curve shows that the neutral carbon quantum dot aqueous solution obtained in Example 1 has better stability and larger current, which preliminarily proves that carbon quantum dots have a positive effect on the activity and stability of the photocathode reduction system. In order to test whether the system produces hydrogen peroxide, the electrolyte solution after electrolysis for 10 minutes was tested using the DPD / POD detection method. Figure 8 The ultraviolet absorption spectrum of H2O2 detected by the DPD / POD method was compared with the standard curve based on the absorption peak at λ=510nm. It was calculated that the carbon quantum dot aqueous solution of Example 1 produced about 8.3mmol / L of hydrogen peroxide after electrolysis for 10min, while the Na2SO4 electrolyte solution produced about 4.2mmol / L of hydrogen peroxide after electrolysis for 10min. The hydrogen peroxide production of the carbon quantum dot system was almost twice that of the traditional neutral Na2SO4 electrolyte solution, which proved that: first, the carbon quantum dot system reduced oxygen to produce hydrogen peroxide; secondly, compared with the traditional electrolyte, the neutral carbon quantum dot system promoted the activity of the photocathode oxygen reduction reaction to produce hydrogen peroxide.

[0045] contrast Fig. 9 , Fig.10 From the LSV curve, it can be seen that the photocurrent of the acidic carbon quantum dot aqueous solution obtained in Example 4 is slightly smaller than that of the traditional acidic HClO4 electrolyte solution. Fig.11The middle it curve also shows that the photocurrent of the acidic carbon quantum dot aqueous solution of Example 4 is smaller than that of the traditional acidic HClO4 electrolyte solution, but the carbon quantum dot system has a photoresponse phenomenon compared with the HClO4 system. The electrolyte solution electrolyzed for 10 minutes was tested using the DPD / POD method. Fig.12 The ultraviolet absorption spectrum of H2O2 detected by DPD / POD method, from the absorption peak at λ=510nm in the figure and the standard curve, it can be calculated that the acidic carbon quantum dot aqueous solution obtained in Example 4 produced about 2.9mmol / L of hydrogen peroxide after electrolysis for 10min, and the acidic HClO4 electrolyte solution produced about 0.9mmol / L of hydrogen peroxide after electrolysis for 10min. This proves that: first, the carbon quantum dot system reduces oxygen to produce hydrogen peroxide; second, compared with the traditional electrolyte HClO4 system, the acidic carbon quantum dot system has a promoting effect on the selectivity of the photocathode oxygen reduction reaction to produce hydrogen peroxide.

[0046] contrast Fig.13 , Fig.14 From the LSV curve, it can be seen that the photocurrent of the alkaline carbon quantum dot aqueous solution of Example 6 is increased by two times compared with the traditional alkaline NaOH electrolyte solution. Fig.15 The it curve also shows that compared with the NaOH system, the carbon quantum dot system has better stability and larger current. The it curve theoretical calculation shows that the hydrogen production of the alkaline carbon quantum dot aqueous solution in Example 6 after 30 minutes of electrolysis is about 0.48 mmol / L, and the hydrogen production of the alkaline NaOH electrolyte solution after 30 minutes of electrolysis is about 0.28 mmol / L; the hydrogen production of the alkaline carbon quantum dot system is 1.7 times that of the NaOH system, which preliminarily proves that the alkaline carbon quantum dots have a promoting effect on the activity of the photocathode water decomposition hydrogen production reaction.

[0047] From the above LSV graph, it can be seen that the photocurrent of the synthesized alkaline carbon quantum dot aqueous solution for water decomposition increases under illumination conditions, indicating that the carbon quantum dots are excited under illumination and promote water decomposition; the photocurrent density of the neutral carbon quantum dot aqueous solution for oxygen reduction increases under illumination conditions, indicating that the carbon quantum dot aqueous solution also promotes cathode oxygen reduction; although the photocurrent density of oxygen reduction of the acidic carbon quantum dot aqueous solution did not increase, it produced more H2O2, indicating that the carbon quantum dot aqueous solution also promotes the selectivity of cathode oxygen reduction to produce hydrogen peroxide. From the it test graph, it can be seen that the carbon quantum dot aqueous solution has good stability as an electrolyte solution photoelectrochemical cathode system. According to the DPD / POD method to detect the ultraviolet spectrum, it can be seen that carbon quantum dots have a promoting effect on oxygen reduction to produce hydrogen peroxide. This provides a new strategy for improving the performance of the photoelectric reduction cathode system and lays a certain foundation for further research in the future.

Claims

1. A carbon quantum dot electrolyte solution for a photocathode reduction system, characterized in that: The electrolyte solution is a yellow carbon quantum dot aqueous solution obtained by completely dissolving a cationic surfactant in deionized water and subjecting the solution to a hydrothermal reaction at 180 to 220° C. for 5 to 10 hours. The cationic surfactant is any one of tetramethylammonium chloride, tetrapropylammonium chloride, tetrabutylammonium chloride, tetrabutylammonium bromide, tetraethylammonium chloride, tetraethylammonium bromide, dodecyltrimethylammonium chloride, tributylmethylammonium chloride, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, triethylbenzylammonium chloride, trimethylbenzylammonium chloride, 1-butyl-3-methylimidazole chloride, 1-ethyl-3-methylimidazole chloride, 1-allyl-3-methylimidazole chloride, 1-hexyl-3-methylimidazole chloride, and 1-hexyl-3-methylimidazole chloride.

2. The carbon quantum dot electrolyte solution for a photocathode reduction system according to claim 1, characterized in that: When the cationic surfactant is any one of tetramethylammonium chloride, tetrapropylammonium chloride, tetrabutylammonium chloride, tetrabutylammonium bromide, tetraethylammonium chloride, tetraethylammonium bromide, dodecyltrimethylammonium chloride, tributylmethylammonium chloride, triethylbenzylammonium chloride, and trimethylbenzylammonium chloride, the obtained carbon quantum dot aqueous solution is acidic and is used for photoelectrochemical cathode oxygen reduction to produce hydrogen peroxide.

3. The carbon quantum dot electrolyte solution for a photocathode reduction system according to claim 1, characterized in that: When the cationic surfactant is any one of tetraethylammonium hydroxide, tetrapropylammonium hydroxide and tetrabutylammonium hydroxide, the obtained carbon quantum dot aqueous solution is alkaline and is used for photoelectrochemical cathode water decomposition.

4. The carbon quantum dot electrolyte solution for a photocathode reduction system according to claim 1, characterized in that: When the cationic surfactant is any one of 1-butyl-3-methylimidazole chloride, 1-ethyl-3-methylimidazole chloride, 1-allyl-3-methylimidazole chloride, 1-hexyl-3-methylimidazole chloride, and 1-hexyl-3-methylimidazole chloride, the obtained carbon quantum dot aqueous solution is neutral and is used for photoelectrochemical cathode oxygen reduction to produce hydrogen peroxide.

5. The carbon quantum dot electrolyte solution for a photocathode reduction system according to claim 1, characterized in that: The concentration of the cationic surfactant in the deionized water is 1-6 mol / L.

6. The carbon quantum dot electrolyte solution for a photocathode reduction system according to claim 1, characterized in that: Carry out hydrothermal reaction at 200-220°C for 5-8h.

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