Black phosphorus and chromium-manganese-sulfur heterojunction photocatalytic material and preparation method thereof

Abstract

The invention discloses a black phosphorus and chromium-manganese-sulfur heterojunction photocatalytic material which comprises a Cd0. 3Mn0. 7S solid solution and black scales loaded on the Cd0. 3Mn0. 7S solid solution, and the black scales account for 1% of the mass of the Cd0. 3Mn0. 7S solid solution. According to the invention, wide spectrum response is realized through loading of black phosphorus, charge separation efficiency is improved, and Cd0. 3Mn0. 7S photocatalytic hydrogen production is enhanced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of photocatalytic materials, and particularly relates to a black phosphorus@chromium-manganese-sulfur heterojunction photocatalytic material and a preparation method of the black phosphorus@chromium-manganese-sulfur heterojunction photocatalytic material. BACKGROUND

[0002] Chromium-manganese-sulfur (Cd x Mn 1-x S) solid solution has attracted extensive attention as a photocatalytic hydrogen production catalyst with adjustable energy band structure and wide light absorption range. However, Cd x Mn 1-x S faces problems such as high photo-generated charge recombination rate, insufficient active sites, and slow hydrogen evolution kinetics, and still exhibits low photocatalytic activity without a cocatalyst. Loading a suitable cocatalyst on the Mn 1-x Cd x S solid solution is an effective improvement method. This can delay the recombination of electrons and holes, reduce photo-corrosion, and thus improve photocatalytic hydrogen production. Black phosphorus (BP) is a representative material of two-dimensional materials 2D-xenes, and has been widely applied in energy storage, catalysis, and optics since it was first reported in 2014 to be prepared in a single-atom thickness on a field effect transistor. BP can be used as an effective material for solar energy conversion under visible light and infrared irradiation, has a high carrier mobility of 1000 cm, and has bandgap adjustability. However, due to the rapid recombination of photo-generated carriers, BP is easily oxidized. Oxygen vacancies and surface sub-oxides exist on the surface of black phosphorus, which can capture charges and cause recombination, so the photocatalytic hydrogen production performance of black phosphorus alone is not satisfactory.

[0003] In view of the problems of Mn x Cd 1-x S and BP, MCS is synthesized by a simple hydrothermal synthesis method, and 1% BP is loaded thereon. A 2D / 2D type BP / Mn x Cd 1-x S (BPM) heterojunction system is constructed, which retains the inherent properties of the two materials, shortens the diffusion length of photo-generated carriers, effectively promotes charge separation, and realizes the synergistic effect of light absorption, corrosion inhibition, and surface reaction. SUMMARY

[0004] A first object of the application is to provide a black phosphorus@chromium-manganese-sulfur heterojunction photocatalytic material, which has a bandgap adjustable to about 2.0-2.2 eV, is widened to a wider visible light range (even near the infrared edge), significantly improves light capture ability, realizes wide spectrum response through loading of black phosphorus, enhances charge separation efficiency, and enhances Cd 0.3 Mn 0.7 S photocatalytic hydrogen production.

[0005] In order to achieve the above object, the technical scheme adopted by the present application is: a black phosphorus@chromium manganese sulfur heterojunction photocatalytic material, comprising Cd 0.3 Mn 0.7 S solid solution and black scales loaded thereon, the black scales being Cd 0.3 Mn 0.7 S solid solution.

[0006] The technical scheme of the present application also has the following characteristics:

[0007] The Cd 0.3 Mn 0.7 S solid solution is synthesized from CdS and MnS in a mass ratio of 3:7.

[0008] The second object of the present application is to provide a preparation method of a black phosphorus@chromium manganese sulfur heterojunction photocatalytic material, which has a band gap adjustable to about 2.0-2.2eV, is widened to a wider visible light range (even near-infrared edge), significantly improves the light capture ability, realizes wide spectrum response through the loading of black phosphorus, enhances the charge separation efficiency, and enhances the Cd 0.3 Mn 0.7 S photocatalytic hydrogen production.

[0009] In order to achieve the above object, the technical scheme adopted by the present application is: a preparation method of a black phosphorus@chromium manganese sulfur heterojunction photocatalytic material, which is implemented according to the following steps:

[0010] Step 1: weigh (CH3COO)2Cd and (CH3COO)2Mn, mix them in H2O, continuously stir, and ensure that all solid substances are completely dissolved in water; 40mL of H2O corresponds to 3mmol of (CH3COO)2Cd and 7mmol of (CH3COO)2Mn;

[0011] Step 2: add thioacetamide (TAA) to the solution obtained in step 1 and continue stirring; the molar ratio of (CH3COO)2Cd to thioacetamide (TAA) is 3:10;

[0012] Step 3: transfer the mixed solution obtained in step 2 to a reaction kettle and continuously heat;

[0013] Step 4: after the reaction in step 3 is completed, wait for the reaction kettle to naturally cool to room temperature, and then use a high-speed centrifuge to separate the precipitate in the solution;

[0014] Step 5: after the precipitate is washed with deionized water and ethanol three times, it is placed in a vacuum drying oven for complete drying;

[0015] Step 6: grind the dried orange powder to obtain the MCS-7 photocatalytic material;

[0016] Step 7, MCS-7 photocatalytic material is weighed and dissolved in H2O, and ultrasonic is used until completely dissolved; 30 mL of H2O corresponds to 100 mg of MCS-7;

[0017] Step 8, the solution obtained in step 7 is added to a three-necked flask, Ar gas is introduced, and the interference of oxygen is excluded;

[0018] Step 9, black phosphorus is taken and added to the three-necked flask to prepare a composite material; the volume ratio of black phosphorus to H2O is 1:30;

[0019] Step 10, the mixture of step 9 is stirred and the water bath is maintained, and after 3.5h-4h of Ar gas introduction, centrifugal treatment is performed;

[0020] Step 11, ethanol and water are used for cleaning, and a vacuum drying box is used for drying, and the obtained yellow powder is black phosphorus@chromium manganese sulfur heterojunction photocatalytic material.

[0021] The technical scheme of the present application also has the following characteristics:

[0022] In the step 1: continuous stirring for 9min-10min.

[0023] In the step 2, stirring for 25min-30min.

[0024] In the step 3: continuous heating at 140℃~160℃ for 23h~24h.

[0025] In the step 5: the temperature of the vacuum drying box is set to 50℃~60℃ and maintained for 7h~8h.

[0026] In the step 7: ultrasonic for 25min~30min.

[0027] In the step 8: Ar is introduced for 25min-30min.

[0028] In the step 10: the water bath temperature is 50℃-60℃.

[0029] The beneficial effects of the present application are: the black phosphorus@chromium manganese sulfur heterojunction photocatalytic material and the preparation method of the present application, the MCS-7 solid solution photocatalytic material prepared by one-step hydrothermal method, the preparation process is simple, and the photocatalytic hydrogen production activity of the prepared composite photocatalytic material is significantly improved compared with MnS and CdS photocatalysts, the light absorption range is significantly widened, and the surface reactivity is optimized. Then, by loading BP, the transfer of photo-generated electrons in MCS is accelerated, and the photocatalytic hydrogen production performance is further improved. The prepared 1%BPM shows excellent photocatalytic hydrogen production performance (167.39mmol g -1 h-1). BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 are XRD contrast diagrams of Comparative Example MCS (1, 3, 5, 7, 8, 9) and XRD contrast diagrams of 2%, 1%, 0.5% BP loaded MCS-7;

[0031] Figure 2 are SEM and TEM diagrams of the BPM heterojunction composite photocatalytic material;

[0032] Figure 3 are XPS diagrams of Comparative BP, MCS-7, and the BPM heterojunction composite photocatalytic material of Example 1.

[0033] Figure 4 are solid UV-Vis absorption spectrum diagrams of CdS, MnS, MCS, MCS-n (n = 1-7) of Comparative Examples 1-4, and the BPM heterojunction composite photocatalytic material of Example 1.

[0034] Figure 5 are performance diagrams of photocatalytic hydrogen production of CdS, MnS, MCS, MCS-n (n = 1-7) of Comparative Examples 1-4, and the BPM heterojunction composite photocatalytic material of Example 1. DETAILED DESCRIPTION

[0035] The technical solutions of the present application will be further described in detail below in combination with the description and specific examples of the accompanying drawings.

[0036] The black phosphorus@chromium-manganese-sulfur heterojunction composite photocatalytic material of the present application is composed of black phosphorus BP and solid solution Cd 0.3 Mn 0.7 S, wherein the loading amount of black phosphorus BP is 1%.

[0037] The present application first adopts a one-step hydrothermal method to synthesize a chromium-manganese-sulfur solid solution, and then hydrothermally synthesizes a BP / Cd 0.3 Mn 0.7 S heterojunction under an argon Ar environment. This method has simple preparation process and high efficiency, and the introduction of BP significantly improves the transfer and separation efficiency of photo-generated electron-hole pairs, inhibits the recombination of photo-generated electron-hole pairs, and improves the hydrogen production efficiency.

[0038] Example 1

[0039] The MCS-7 chromium-manganese-sulfur solid solution photocatalytic material is prepared according to the following steps

[0040] Step 1: 7 mmol of (CH3COO)2Mn·4H2O and 3 mmol of (CH3COO)2Cd·2H2O are weighed and added to 40 ml of ultrapure water, and stirring is continued for 10 min to ensure that all solid substances are completely dissolved in water;

[0041] Step 2: 10 mmol of thioacetamide (TAA) was added into the solution, and stirring was continued for 30 min;

[0042] Step 3: The obtained mixed solution was transferred into a 100 ml stainless steel autoclave (hereinafter referred to as a reaction kettle) with a Teflon lining, and heating was continued at 160°C for 24 h;

[0043] Step 4: After the reaction was completed, the reaction kettle was naturally cooled to room temperature, and a high-speed centrifuge was used to separate the precipitate in the solution;

[0044] Step 5: After the precipitate was washed with deionized water and ethanol three times, it was placed in a vacuum drying oven, and the temperature was set to 60°C and maintained for 8 h to achieve complete drying effect;

[0045] Step 6: The dried orange powder was ground, and the obtained MCS-7 chromium manganese sulfur solid solution photocatalytic material was obtained.

[0046] Comparative Example 1

[0047] CdS material was prepared according to the following steps:

[0048] Step 1: 10 mmol of (CH3COO)2Cd·2H2O was weighed and added to 40 mL of deionized water, and stirring was continued for 10 min to ensure that all solid substances were completely dissolved in water;

[0049] Step 2: 10 mmol of thioacetamide (TAA) was added into the solution, and stirring was continued for 30 min;

[0050] Step 3: The obtained mixed solution was transferred into a 100 ml stainless steel autoclave (hereinafter referred to as a reaction kettle) with a Teflon lining, and heating was continued at 160°C for 24 h;

[0051] Step 4: After the reaction was completed, the reaction kettle was naturally cooled to room temperature, and a high-speed centrifuge was used to separate the precipitate in the solution;

[0052] Step 5: After the precipitate was washed with deionized water and ethanol three times, it was placed in a vacuum drying oven, and the temperature was set to 60°C and maintained for 8 h to achieve complete drying effect;

[0053] Step 6: The dried orange powder was ground, and the obtained CdS material was obtained.

[0054] Comparative Example 2

[0055] MnS material was prepared according to the following steps:

[0056] Step 1: Take 10 mmol of (CH3COO)2Mn-4H2O and add it to 40 mL of deionized water, stir for 10 min, make sure all solid substances are completely dissolved in water;

[0057] Step 2: Add 10 mmol of thioacetamide (TAA) to the solution and continue stirring for 30 min;

[0058] Step 3: Transfer the resulting mixed solution to a 100 ml stainless steel autoclave with a Teflon lining (hereinafter referred to as a reaction kettle), and continue to heat at 160°C for 24 h;

[0059] Step 4: After the reaction is completed, wait for the reaction kettle to naturally cool to room temperature, and then use a high-speed centrifuge to separate the precipitate in the solution;

[0060] Step 5: After the precipitate is washed with deionized water and ethanol three times, it is placed in a vacuum drying oven, set to 60°C and kept for 8 h to achieve complete drying effect;

[0061] Step 6: Grind the dried dark brown powder, and the resulting MnS material is obtained.

[0062] Comparative Example 3

[0063] Prepare MCS-n (n = 1-6) material, according to the following steps:

[0064] Step 1: Take x mmol of (CH3COO)2Mn-4H2O (x = 0.1-0.6) and (1-x) mmol of (CH3COO)2Cd-2H2O (x = 0.1-0.6) and add them to 40 ml of ultrapure water, continue to stir for 10 min, make sure all solid substances are completely dissolved in water;

[0065] Step 2: Add 10 mmol of thioacetamide (TAA) to the solution and continue stirring for 30 min;

[0066] Step 3: Transfer the resulting mixed solution to a 100 ml stainless steel autoclave with a Teflon lining (hereinafter referred to as a reaction kettle), and continue to heat at 160°C for 24 h;

[0067] Step 4: After the reaction is completed, wait for the reaction kettle to naturally cool to room temperature, and then use a high-speed centrifuge to separate the precipitate in the solution;

[0068] Step 5: After the precipitate is washed with deionized water and ethanol three times, it is placed in a vacuum drying oven, set to 60°C and kept for 8 h to achieve complete drying effect;

[0069] Step 6: Grind the dried orange powder, the obtained is X%BPM composite photocatalytic hydrogen production material.

[0070] Comparative Example 4

[0071] The X%BPM composite photocatalytic hydrogen production material is prepared according to the following steps:

[0072] Step 1: Weigh the synthesized MCS-7 in Example 1 and dissolve it in 30 mL of H2O, and ultrasonic for 30 min to ensure complete dissolution.

[0073] Step 2: Add to a 50 mL three-necked flask, and pass Ar gas for 30 min. Exclude the interference of oxygen.

[0074] Step 3: Take X mL (X = 1, 0.5, 2) black phosphorus (BP) and add it to the three-necked flask to prepare the composite material.

[0075] Step 4: Stir and maintain in a water bath at 60°C, pass Ar gas for 4 h, and then centrifuge.

[0076] Step 5: Clean with ethanol and water, and dry in a vacuum drying box. The obtained yellow powder is X%BPM composite photocatalytic hydrogen production material.

[0077] Figure 1 The XRD patterns of CdS, MnS, MCS, MCS-n (n = 1-9) of Comparative Examples 1-4 and X%BP / MCS-7 (X = 0.5, 1, 2) composite photocatalytic material of Example 1.

[0078] Figure 1The diffraction peaks of MCS-X (1 < X < 9) at 26.46°, 51.95°, 72.43° can be observed, which are consistent with the (111), (311), (420) crystal planes of cubic CdS (PDF #89-0440). Meanwhile, the diffraction peak of MCS-X (1 < X < 7) at 43.88° of cubic CdS gradually shifts to a high angle (yellow area) as the proportion of Mn increases. Such changes indicate that Mn is incorporated into the CdS lattice and forms a manganese-cadmium-sulfur solid solution. However, when the proportion of Mn increases to 8 and 9, there is an excessive shift compared to the characteristic peaks of CdS (PDF #89-0440). When MCS-7 appears diffraction peaks at 25.81° and 29.35°, these two diffraction peaks correspond to the (100), (101) crystal planes of cubic MnS (PDF #40-1289). This indicates that MCS-7, 8, 9 at this time may be composed of a mixed phase of manganese-cadmium-sulfur solid solution and cadmium sulfide. In addition, MCS-8, 9 appears obvious diffraction peaks at 34.3°, 49.29°, 61.39°, which correspond to the (200), (220), (222) crystal planes of cubic α-MnS (PDF #06-0518) (pink area), which proves that the samples of MCS-8, 9 are mainly a mixed phase of MnS and CdS rather than a solid solution. Therefore, in the subsequent control experiment, we only explore the hydrogen production performance of MCS-X (1-7). Figure 1 b shows the XRD patterns of MCS-7 monomer and BPM loaded with different mass ratios of phosphorus. Due to the small mass fraction of P, the characteristic peaks of P cannot be clearly identified. The XRD pattern of BPM matches well with cubic CdS (PDF #89-0440), and the characteristic peaks at 26.54°, 44.04° and 52.16° correspond to the (111), (220) and (311) crystal planes of CdS (PDF #80-0019).

[0079] Figure 2 are SEM and TEM images of CdS, MnS, MCS, MCS-n (n = 1-9) of Comparative Examples 1-4 and X%BP / MCS-7 (X = 0.5, 1, 2) composite photocatalytic materials of Example Sub-1. As shown in Figure 2 a MCS-7 synthesized by one-step hydrothermal method presents a sheet structure, and these sheet layers are stacked on each other. Figure 2 b In b, the content of black phosphorus (P) in BPM is relatively low, and its morphology has no significant difference compared with MCS-7. The synthesized BPM by transmission electron microscopy (TEM) presents a regular sheet morphology, and the surface is loaded with a small amount of BP.

[0080] Figure 3XPS spectra of CdS, MnS, MCS, MCS-n (n = 1-9) of Comparative Examples 1-4 and X% BP / MCS-7 (X = 0.5, 1, 2) composite photocatalytic materials of Example 1. Figure 3 a) The peaks at 411.9 eV and 405.15 eV correspond to Cd 3d3 / 2 and Cd 3d5 / 2, respectively. Figure 3 b) The main peaks of S 2p are at 162.75 eV and 161.5 eV, corresponding to S 2p1 / 2 and S 2p3 / 2, respectively. Figure 3 c) The two characteristic peaks of Mn 2p are at 652.4 eV and 641.54 eV, corresponding to Mn 2p1 / 2 and Mn 2p3 / 2, respectively. The peaks of P 2p1 / 2 and P 2p3 / 2 are at 129.65 eV and 129.8 eV, respectively. The peak at about 133 eV corresponds to the oxide (PxOy) species of phosphorus. The peak values before and after loading change due to oxidation during sample preparation. In the total spectrum, the characteristic peak of the P element is not obvious compared to the characteristic peaks of Cd, Mn, and S elements. Compared with the original MCS-7, all the peaks of Cd 3d, Mn 2p, and S 2p spectra after loading P slightly shift to the direction of higher binding energy. This may be due to the decrease in the surface electron density of MCS-7. The P 2p peak shifts to the direction of lower binding energy, indicating that BP is successfully loaded on MCS-7 and electrons are transferred from MCS-7 to BP.

[0081] Figure 4 Solid UV-Vis absorption spectra of CdS, MnS, MCS, MCS-n (n = 1-9) of Comparative Examples 1-4 and X% BP / MCS-7 (X = 0.5, 1, 2) composite photocatalytic materials of Example 1. Figure 4 a) The UV-Vis absorption spectrum shows that the loading of phosphorus (P) does not cause a significant shift in the absorption edge of MCS-7, indicating that the P element does not enter the crystal lattice structure of the host material, but exists in the form of surface loading. With the loading of P, the light capture ability of BPM in the long wave region of 560-800 nm is significantly improved compared to the original MCS-7, which indicates that the loading of BP significantly improves the visible light absorption ability of BPM. The loading of 1% BP exhibits the best light capture ability, while excessive P loading (>1%) leads to a decrease in light absorption performance, even lower than the original MCS-7. Figure 4 b) The band gap values of the photocatalysts are calculated.

[0082] Figure 5 Electrochemical series tests were performed on the CdS, MnS, MCS, MCS-n (n = 1–9) comparative examples 1–4 and the X%BP / MCS-7 (X = 0.5, 1, 2) composite photocatalyst material of Example 1. Linear sweep voltammetry (LSV) curves were measured using a three-electrode system in 0.5 M Na₂SO₄ electrolyte. Figure 5 As shown in Figure a, the reduction capability of MCS-7 is significantly improved compared to pure CdS. 1% BPM exhibits the best reduction capability when the current density reaches 2 mA / cm². - At time 2, the overpotential is lower, and the hydrogen evolution activity is significantly enhanced. In electrochemical impedance spectroscopy (EIS) testing ( Figure 5 (b) The 1% BPM exhibits the smallest semicircle diameter, indicating its lowest resistance. Notably, pristine CdS and MCS-7 show low photocurrent response and high EIS resistance; with successful BP loading, the charge kinetic efficiency of the reaction is significantly improved. This can be attributed to the successful construction of a layered structure in a 2D / 2D BP / MCS-7 heterojunction system between BP and BPM. In the photocurrent response comparison experiment, 1% BPM showed the highest photocurrent density, indicating the highest charge transfer efficiency. The recombination process of photogenerated electrons and holes was investigated by testing the photoluminescence (PL) spectrum of the photocatalyst. In contrast, 1% BPM has the longest average carrier lifetime (τav = 7.65 ns). This means that photogenerated carriers (electrons and holes) in 1% BPM can survive for a longer time before recombination, having more time to migrate to the catalyst surface, thus improving photocatalytic efficiency and other properties. Compared with MCS-7, this also indicates that loading BP as a co-catalyst significantly suppresses the recombination process of photogenerated electrons. The above data indicate that the loading of the co-catalyst BP can effectively separate and transport photogenerated electrons from MCS-7, enhance interfacial charge transfer and separation, and thus promote hydrogen generation.

[0083] The black phosphorus@chromium-manganese-sulfur heterojunction composite photocatalyst material prepared by the method of this invention was used for photocatalytic water splitting to produce hydrogen, to illustrate its photocatalytic performance.

[0084] The specific photocatalytic performance testing procedure is as follows: 10 mg of photocatalyst, 4.2007 g of sodium sulfide, and 1.5755 g of sodium sulfite from wastewater were dissolved in 50 mL of aqueous solution. After sonication for 15 min, the solution was transferred to the photoreactor. A 300 W xenon lamp (30.72 mW·cm⁻²) was used as a simulated light source for 1 h of irradiation. Simultaneously, the reaction temperature was maintained at room temperature using circulating water. The amount of hydrogen generated was recorded using a GC 7900 gas chromatograph equipped with an MS-5A column and a thermal conductivity detector.

Claims

1. A black phosphorus@chromium-manganese-sulfur heterojunction photocatalytic material, characterized in that, including Cd 0.3 Mn 0.7 S solid solution and black scale, black scale being Cd 0.3 Mn 0.7 1% of the mass of the S solid solution. 2.The black phosphorus@chromium-manganese-sulfur heterojunction photocatalytic material according to claim 1, characterized in that, The Cd 0.3 Mn 0.7 The S solid solution was synthesized from CdS and MnS in a mass ratio of 3:

7.

3. A method for preparing black phosphorus@chromium-manganese-sulfur heterojunction photocatalytic material, characterized in that, The following steps are implemented in detail: Step 1: Weigh (CH3COO)2Cd and (CH3COO)2Mn and mix them in H2O, continuously stir to ensure that all solid substances are completely dissolved in water; 3 mmol (CH3COO)2Cd and 7 mmol (CH3COO)2Mn correspond to 40 mL H2O; Step 2: Add thioacetamide (TAA) to the solution obtained in Step 1 and continue stirring; the molar ratio of (CH3COO)2Cd to thioacetamide (TAA) is 3:10; Step 3: Transfer the mixed solution obtained in Step 2 to a reaction kettle and continue heating; Step 4: After the reaction in Step 3 is complete, wait for the reaction kettle to naturally cool to room temperature, and then use a high-speed centrifuge to separate the precipitate from the solution; Step 5: After washing the precipitate with deionized water and ethanol three times, place it in a vacuum drying oven for complete drying; Step 6: Grind the dried orange powder to obtain the MCS-7 photocatalytic material; Step 7: Weigh the MCS-7 photocatalytic material and dissolve it in H2O, and then ultrasonic it until it is completely dissolved; 100 mg of MCS-7 corresponds to 30 mL of H2O; Step 8: Add the solution obtained in Step 7 to a three-necked flask, and introduce Ar gas to exclude the interference of oxygen; Step 9: Take black phosphorus and add it to the three-necked flask to prepare a composite material; the volume ratio of black phosphorus to H2O is 1:30; Step 10: Stir the mixture in Step 9 and maintain the water bath, and then centrifuge after introducing Ar gas for 3.5-4 hours; Step 11: Clean with ethanol and water, and dry in a vacuum drying oven; the yellow powder obtained is the black phosphorus@chromium-manganese sulfur heterojunction photocatalytic material.

4. The method according to claim 3, wherein the method is characterized by, In Step 1: continuously stir for 9-10 minutes.

5. The method according to claim 4, wherein the method is characterized by, In Step 2: stir for 25-30 minutes.

6. The black phosphorus@chromium-manganese-sulfur heterojunction photocatalytic material according to claim 5, characterized in that, In Step 3: continuously heat at 140-160°C for 23-24 hours.

7. The method according to claim 6, wherein the method is characterized by, In Step 5: set the temperature of the vacuum drying oven to 50-60°C and maintain it for 7-8 hours.

8. The method according to claim 7, wherein the method is characterized by, In Step 7: ultrasonic for 25-30 minutes.

9. The method according to claim 8, wherein the method is characterized by, In Step 8: introduce Ar for 25-30 minutes.

10. The method according to claim 9, wherein the method is characterized by, In Step 10: the water bath temperature is 50-60°C.

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