A double built-in electric field ternary heterojunction photocatalyst and a preparation method thereof

Abstract

The application discloses a kind of double built-in electric field ternary heterojunction photocatalyst and preparation method thereof.The catalyst is composed of bismuth vanadate, manganese sesquioxide and nickel carbide;The mass ratio of bismuth vanadate, manganese sesquioxide and nickel carbide is 10-5:2-1:1;Bismuth vanadate, manganese sesquioxide and nickel carbide in the photocatalyst form Z-type heterojunction structure.The catalyst is prepared by hydrothermal method and electrostatic self-assembly method, by strictly controlling the proportion of each component and reaction parameter, realize the Z-type heterojunction structure of bismuth vanadate, manganese sesquioxide and nickel carbide, greatly improve the visible light absorption and the performance of carrier recombination inhibition of catalyst.The catalyst based on the synergistic effect of double built-in electric field, Z-type inhibition junction structure and catalyst magnetism, so that the catalyst has excellent photocatalytic activity, by testing, using the catalyst to degrade BP-3, 30min within degradation rate is 60%, 60min degradation rate is 90%.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of bismuth vanadate photocatalyst, specifically to a kind of preparation method of ternary heterostructure photocatalyst of double built-in electric field, belong to photocatalyst material synthesis field. BACKGROUND

[0002] Photocatalysis is a kind of low energy consumption, easy operation, mild reaction condition green technology, has a wide range of applications in the treatment of drug and personal care products (PPCPs) wastewater and photocatalytic hydrolysis hydrogen production.

[0003] Titanium dioxide (TiO2) photocatalyst is widely used in the field of environment and energy due to its excellent photocatalytic activity, non-toxicity and photo-stability. However, TiO2 is only sensitive to ultraviolet light and only utilizes <5% of solar energy, which greatly limits its potential applications. Therefore, it is necessary to explore new strategies to improve the utilization of solar energy. Developing efficient visible light driven catalysts is an effective way, which has become a research hotspot in the field of photocatalysis in recent years. Currently, there are more than 150 kinds of semiconductor materials that can be used for environmental purification, including metal oxides, sulfides, carbides, halides, sulfides, halides and hydroxides. Among them, semiconductor metal oxides, especially bismuth-based metal oxides such as BiVO4, Bi2WO6, Bi2MoO6, Bi4Ti2O7, BiOX (X = Cl, Br, I), Bi5O7I, etc., due to their good band gap, can respond in the visible light range, low toxicity or even non-toxicity, stable physical and chemical properties, have attracted the interest of many researchers. Among them, BiVO4 has attracted widespread attention from researchers due to its suitable band gap, good dispersibility, non-toxicity, corrosion resistance, good stability, environmental friendliness and other characteristics, making it an important candidate material for absorbing solar energy.

[0004] New bismuth vanadate (BiVO4) catalyst is a kind of visible light photocatalyst with potential, but the photocatalytic performance of BiVO4 needs to be improved in practical application. Building heterojunction is considered an effective modification method, as it can effectively improve the light conversion efficiency, Z-type heterostructure is widely studied for pollutant degradation and hydrogen production by water splitting. However, precise control of charge transfer in Z-type heterojunction is still a great challenge. SUMMARY

[0005] In view of the problems existing in the prior art, the first object of the present application is to provide a ternary heterojunction photocatalyst with double built-in electric field, which is based on the synergistic effect between the components, and two built-in electric fields are constructed by bismuth vanadate, manganese sesquioxide and nickel carbide. The double built-in electric field effectively promotes the effective separation and transfer of interface charge, prolongs the lifetime of charge separation, and thus improves the photocatalytic activity.

[0006] The second object of the present application is to provide a preparation method of the dual built-in electric field ternary heterojunction photocatalyst.

[0007] To achieve the above technical objects, the present application provides a dual built-in electric field ternary heterojunction photocatalyst, which is characterized in that it is composed of bismuth vanadate, manganese sesquioxide and nickel carbide; and the mass ratio of the bismuth vanadate, manganese sesquioxide and nickel carbide is 10-5:2-1:1.

[0008] The bismuth vanadate, manganese sesquioxide and nickel carbide in the photocatalyst form a Z-type heterojunction structure.

[0009] The improvement of the catalytic performance of the ternary catalyst of the present application is attributed to the existence of the special dual built-in electric field and the unique Z-type heterojunction structure between the bismuth vanadate / manganese sesquioxide and the bismuth vanadate / nickel carbide. Under the synergistic effect of the two, the redox potential of the catalyst is expanded, the separation of the fast carriers is realized, and thus the photocatalyst is endowed with excellent photocatalytic performance.

[0010] As a preferred scheme, the mass ratio of the bismuth vanadate, manganese sesquioxide and nickel carbide is 9-8:1.5-1:1.

[0011] As a preferred scheme, the photocatalyst is in the shape of a four-pointed star, and the particle size is 0.5-2.5 μm. The bismuth vanadate is in the shape of a four-pointed star, and the loading of the manganese sesquioxide and the nickel carbide does not change the initial morphology of the bismuth vanadate.

[0012] The present application also provides a preparation method of the dual built-in electric field ternary heterojunction photocatalyst. The bismuth source and the alum source are used to crystallize the bismuth vanadate by a high-pressure hydrothermal method; the bismuth vanadate is dissolved in water, and then a mixed solution of PVP ethanol solution and manganese salt solution is added to the bismuth vanadate solution to perform a hydrothermal reaction. The powder obtained after the hydrothermal reaction is calcined to obtain a Mn2O3 / BiVO4 composite material; the Mn2O3 / BiVO4 composite material and Ni3C are dispersed in a solvent, and then electrostatic self-assembly and drying are sequentially performed to obtain the dual built-in electric field ternary heterojunction photocatalyst.

[0013] As a preferred scheme, the bismuth source is at least one of bismuth nitrate, bismuth sulfate, bismuth phosphate and hydrates thereof. Further preferably, the bismuth source is bismuth nitrate pentahydrate.

[0014] As a preferred scheme, the vanadium source is at least one of sodium vanadate, potassium vanadate, ammonium vanadate and hydrates thereof. Further preferably, the alum source is sodium vanadate dodecahydrate.

[0015] As a preferred solution, the manganese salt is at least one of potassium permanganate, potassium manganate and hydrates thereof. Further preferably, the manganese salt is potassium permanganate.

[0016] The selection of raw materials is one of the key factors to maintain the morphology, structure and purity of the catalyst. Within the above preferred range, the morphology of the obtained catalyst is optimal, and the catalytic performance is also optimal. If raw materials outside the above range are selected, the four-star shape of the catalyst will be irregularly curved, and in severe cases, random crystals will be produced, resulting in a decrease in catalytic activity or even no catalytic activity.

[0017] As a preferred solution, the conditions of the hydrothermal reaction are as follows: the mixture of PVP ethanol solution and manganese salt solution is stirred uniformly with bismuth vanadate, and then reacted at 0.1-5 MPa and 100-200℃ for 1-4 h. Further preferably, the PVP is K90 type PVP with a molecular weight of ≥1.3 million, the stirring time is 20-30 min, the pressure is 0.5-2 MPa, and the temperature is 140-160℃. The stirring time must be strictly in accordance with the above requirements. If the stirring time is too low, the dispersion effect cannot be achieved, and if the stirring time is too long, the bismuth vanadate will agglomerate and precipitate.

[0018] As a preferred solution, the sintering conditions are as follows: the temperature is 100-500℃, and the time is 1-5 h. Further preferably, the sintering temperature is 250-350℃, and the time is 3.5-4.5 h.

[0019] As a preferred solution, in the Mn2O3 / BiVO4 composite material, the mass ratio of BiVO4 to Mn2O3 is 20-5:1. Further preferably, the mass ratio of BiVO4 to Mn2O3 is 12-8:1, and most preferably, the mass ratio of BiVO4 to Mn2O3 is 9-10:1. BiVO4 is the base material, and the photocatalytic performance of the heterojunction structure formed by BiVO4 and Mn2O3 increases first and then decreases with the addition of Mn2O3, and reaches the highest when the mass ratio of BiVO4 to Mn2O3 is 9-10:1.

[0020] As a preferred solution, the preparation process of Ni3C is as follows: organic nickel salt and solvent amine are heated at 200-400℃ for 1-10 h in a protective atmosphere, and then washed by ultrasonic washing and centrifugal washing in sequence after cooling. Further preferably, the temperature is 250-350℃, and the time is 2-4 h. Too large or too small temperature and time will result in a decrease in product conversion rate, producing Ni2C or NiC as by-products, which cannot meet the requirements of subsequent electrostatic self-assembly.

[0021] As a preferred solution, the organic nickel salt is at least one of nickel acetate, nickel oxalate, nickel formate and hydrates thereof. Further preferably, the organic nickel salt is nickel acetate tetrahydrate.

[0022] As a preferred scheme, the solvent amine is at least one of oleylamine, oleic acid, octadecylamine and hydrates thereof. Further preferably, the solvent amine is oleylamine.

[0023] As a preferred scheme, the protective atmosphere is an argon and / or nitrogen atmosphere.

[0024] As a preferred scheme, the solvent for the ultrasonic washing is ethanol.

[0025] As a preferred scheme, the solvent for the centrifugal washing is a mixture of n-hexane and ethanol in a mass ratio of 10-1:1. Further preferably, the solvent for the centrifugal washing is a mixture of n-hexane and ethanol in a mass ratio of 5-1:1.

[0026] As a preferred scheme, the solvent is n-hexane; the electrostatic self-assembly is carried out under strong stirring at a stirring speed of 200-500 r / min for 1-5 h. Further preferably, the stirring speed is 300-400 r / min for 2-3 h.

[0027] As a preferred scheme, the drying condition is a temperature of 20-80℃ for 1-8 h. Further preferably, the drying temperature is 40-60℃ for 3-4 h.

[0028] Compared with the prior art, the present application has the following beneficial technical effects:

[0029] 1) The photocatalyst provided by the present application is based on the synergistic effect between components, two built-in electric fields are formed by bismuth vanadate, manganese sesquioxide and nickel carbide, the double built-in electric fields effectively promote the effective separation and transfer of interface charges, realize the spatial separation of electron (Ni3C)-hole (BiVO4) enrichment area, expand the oxidation-reduction potential of the catalyst, prolong the life of charge separation, and thus improve the photocatalytic activity.

[0030] 2) In the preparation method provided by the present application, in-situ hydrothermal and electrostatic self-assembly are adopted, by strictly controlling the proportion and reaction parameters of each component, a Z-type heterojunction structure of bismuth vanadate, manganese sesquioxide and nickel carbide is formed, after Mn2O3 and Ni3C are loaded on the surface of BiVO4, the electric field strength in the catalyst is greatly improved, and the visible light absorption capacity is enhanced.

[0031] 3) In the technical scheme provided by the present application, based on the synergistic effect of double built-in electric field, Z-type inhibition junction structure and catalyst magnetism, the catalyst has excellent photocatalytic activity, and tests show that the degradation rate of BP-3 is 60% within 30 min and the degradation rate is 90% within 60 min by using the catalyst. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 Degradation rate comparison chart of Example 1 and Comparative Examples 1-4;

[0033] Figure 2 SEM chart of ternary heterojunction photocatalyst obtained in Example 1. DETAILED DESCRIPTION

[0034] The following is a preferred embodiment of the present application, which is further described in detail by way of specific examples. It should be noted that, for those skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, which are also considered within the scope of protection of the present application.

[0035] Example 1

[0036] The preparation method of the ternary heterojunction photocatalyst with double built-in electric field in this embodiment comprises the following steps:

[0037] (1) Preparation of BiVO4

[0038] Dissolve 0.24 g of Bi(NO3)3·5H2O in 80 mL of deionized water, ultrasonic treatment for 5 min, add 0.4 g of Na3VO4·12H2O to obtain an opaque yellow suspension, and transfer the suspension into a 100 mL hydrothermal reactor to crystallize at 180℃ for 12 h. Then, naturally cool to room temperature, and the precipitate is separated by centrifugation, washed and dried (60℃ for 3 h) to obtain BiVO4 powder.

[0039] (2) Preparation of Mn2O3 / BiVO4

[0040] Put 0.3 g of BiVO4 into a 100 mL polytetrafluoroethylene bottle, add 50 mL of deionized water, ultrasonic dispersion and stir uniformly. Dissolve PVP (1.2 g) in 10 mL of ethanol, and dissolve potassium permanganate (0.072 g) in 20 mL of deionized water. Mix the above two solutions and transfer them into the polytetrafluoroethylene bottle containing the BiVO4 solution, stir for 10 min, then raise the temperature to 160℃ and maintain for 2 h. The obtained powder is washed with deionized water and ethanol for 3 times respectively, and then dried in a vacuum drying oven at a temperature of 60℃. Heat at 300℃ for 4 h to obtain Mn2O3 / BiVO4 composite material, which is abbreviated as Mn / BVO.

[0041] (3) Preparation of Ni3C / Mn2O3 / BiVO4

[0042] Preparation of Ni3C: 422 mg of nickel acetate tetrahydrate and 20 mL of oleylamine were poured into a three-necked flask, then magnetic stirring was carried out under a continuous argon flow, the mixture was heated and kept at 300℃ for 3 h. Then the sample was cooled to room temperature, a certain amount of ethanol was put into a round-bottom flask, after stirring and ultrasonic treatment, a black Ni3C sample was obtained by centrifugal washing with n-hexane and ethanol at a ratio of 2:1, and dried in a vacuum drying oven at 60℃.

[0043] 0.25 g of Mn / BVO and 0.0198 g of Ni3C were dispersed into 20 mL of n-hexane respectively, and stirred vigorously for 2 h. Mn / BVO and Ni3C were compounded by electrostatic self-assembly method. Then the mixture was vacuum dried at 60℃ for 2 h to form Ni3C / Mn2O3 / BiVO4 (denoted as NC / Mn / BVO) ternary heterostructure.

[0044] A photochemical reaction instrument equipped with a 300W xenon lamp and a 420nm filter was used to degrade BP-3 with visible light source λ≥420nm. The specific operation is as follows: 25 mg of catalyst was added into a quartz test tube containing 50 mL of 10 mg / L BP-3 solution, and the visible light catalytic activity of the prepared catalyst was tested by degrading BP-3, 60% was degraded in 30 min, 90% was degraded in 60 min, and the degradation rate reached the highest 95% in 90 min.

[0045] Comparative Example 1

[0046] The preparation method of a photocatalyst in the present comparative example comprises the following steps:

[0047] (1) Preparation of BiVO4

[0048] 0.24 g of Bi(NO3)3·5H2O was dissolved in 80 mL of deionized water, ultrasonic treatment for 5 min, 0.4 g of Na3VO4·12H2O was added to obtain an opaque yellow suspension, and the suspension was transferred into a 100 mL hydrothermal reactor and crystallized at 180℃ for 12 h. Then, it was naturally cooled to room temperature, the precipitate was separated by centrifugation, washed and dried (60℃ drying for 3 h) to obtain BiVO4 powder.

[0049] (2) Preparation of Mn2O3 / BiVO4

[0050] Into a 100 mL Teflon bottle, 0.3 g of BiVO4 was added with 50 mL of deionized water, ultrasonically dispersed and stirred uniformly. PVP (1.2 g) was dissolved in 10 mL of ethanol, and potassium permanganate (0.072 g) was dissolved in 20 mL of deionized water. The above two solutions were mixed and transferred into the Teflon bottle containing the BiVO4 solution, stirred for 10 min, and then the temperature was raised to 160°C for 2 h. The resulting powder was washed with deionized water and ethanol three times, respectively, and then dried in a vacuum drying oven at a temperature of 60°C. The composite material of Mn2O3 / BiVO4 (denoted as Mn / BVO) was obtained by heating at 300°C for 4 h.

[0051] The visible light catalytic activity of the prepared Mn / BVO catalyst was tested by degrading BP-3 using a photochemical reaction instrument under the same conditions as above, 38% degradation in 30 min, 51% degradation in 60 min, and the degradation rate reached a maximum of 59% in 90 min.

[0052] Comparative Example 2

[0053] The preparation method of a photocatalyst in the present comparative example comprises the following steps:

[0054] (1) Preparation of BiVO4

[0055] 0.24 g of Bi(NO3)3·5H2O was dissolved in 80 mL of deionized water, ultrasonically treated for 5 min, and 0.4 g of Na3VO4·12H2O was added to obtain an opaque yellow suspension. The suspension was transferred into a 100 mL hydrothermal reactor and crystallized at 180°C for 12 h. Then, it was naturally cooled to room temperature, and the precipitate was separated by centrifugation, washed, and dried (60°C for 3 h) to obtain BiVO4 powder.

[0056] (2) Preparation of Ni3C / BiVO4

[0057] Preparation of Ni3C: 422 mg of nickel acetate tetrahydrate and 20 mL of oleylamine were poured into a three-necked flask, then magnetic stirring was carried out under continuous argon flow, the mixture was heated and kept at 300°C for 3 h. Then the sample was cooled to room temperature, a certain amount of ethanol was put into a round-bottom flask, stirred and ultrasonically treated, then washed with n-hexane and ethanol at a ratio of 2:1 to obtain a black Ni3C sample, which was dried in a vacuum drying oven at 60°C.

[0058] 0.21 g of BiVO4 and 0.0198 g of Ni3C were dispersed into 20 mL of n-hexane, respectively, and stirred vigorously for 2 h. BiVO4 and Ni3C were compounded by electrostatic self-assembly method. Then the mixture was vacuum dried at 60°C for 2 h to form a composite material of Ni3C / BiVO4 (denoted as NC / BVO).

[0059] The visible light catalytic activity of the prepared NC / BVO catalyst was tested by degrading BP-3 using a photochemical reaction instrument under the above conditions, 42% was degraded in 30 min, 61% was degraded in 60 min, and the degradation rate reached a maximum of 71% in 90 min.

[0060] Comparative Example 3

[0061] The preparation method of a photocatalyst in the present comparative example comprises the following steps:

[0062] (1) Preparation of BiVO4

[0063] 0.24 g of Bi(NO3)3·5H2O was dissolved in 80 mL of deionized water, ultrasonic treatment for 5 min, 0.4 g of Na3VO4·12H2O was added to obtain an opaque yellow suspension, and the suspension was transferred into a 100 mL hydrothermal reactor at 180°C for 12 h of crystallization reaction. Then, it was naturally cooled to room temperature, the precipitate was separated by centrifugation, washed and dried (60°C drying for 3 h) to obtain BiVO4 powder (denoted as BiVO4).

[0064] The visible light catalytic activity of the prepared BiVO4 catalyst was tested by degrading BP-3 using a photochemical reaction instrument under the above conditions, 28% was degraded in 30 min, 35% was degraded in 60 min, and the degradation rate reached a maximum of 45% in 90 min.

[0065] Comparative Example 4

[0066] The preparation method of a photocatalyst in the present comparative example comprises the following steps:

[0067] (1) Preparation of Mn2O3

[0068] PVP (1.2 g) was dissolved in 10 mL of ethanol, and potassium permanganate (0.072 g) was dissolved in 20 mL of deionized water. The above two solutions were mixed and transferred into a polytetrafluoroethylene bottle, stirred for 10 min, then the temperature was raised to 160°C and kept for 2 h. The obtained powder was washed with deionized water and ethanol for 3 times respectively, and then dried in a vacuum drying oven at a temperature of 60°C. Mn2O3 material (denoted as Mn2O3) was obtained by heating at 300°C for 4 h.

[0069] The visible light catalytic activity of the prepared catalyst was tested by degrading BP-3 using a photochemical reaction instrument under the above conditions, 16% was degraded in 30 min, 22% was degraded in 60 min, and the degradation rate reached a maximum of 28% in 90 min.

[0070] Example 1 is a ternary heterostructure photocatalyst with double built-in electric field prepared by a standard method, Comparative Example 1 is a binary heterostructure Mn2O3 / BiVO4 composite material, Comparative Example 2 is a binary heterostructure Ni3C / BiVO4 composite material, Comparative Example 3 is a single-component BiVO4 material, and Comparative Example 4 is a single-component Mn2O3 material.

[0071] As can be seen from the example and Comparative Examples 1 to 4, the ternary heterostructure photocatalyst with double built-in electric field prepared by the application has the best catalytic degradation capacity. As can be seen from Example 1 and Comparative Examples 1 and 2, under the same conditions, the absence of any one-component structure in the ternary heterostructure will destroy the double built-in electric field and result in degradation of the catalytic degradation capacity. When Ni3C is absent in the ternary heterostructure, the highest degradation rate decreases from 95% to 59%, and when Mn2O3 is absent in the ternary heterostructure, the highest degradation rate decreases from 95% to 71%. As can be seen from Comparative Examples 1 and 2 and Comparative Example 3, under the same conditions, the composite material has a stronger catalytic degradation capacity than any single-component material. It should be noted that the catalytic reaction is not a simple linear superposition reaction, and the essence of the catalytic reaction is the reduction of the activation energy of the reaction. Therefore, simply mixing three components cannot produce any enhancement or synergistic effect. Only when the three components form a double built-in electric field and a special Z-type heterojunction structure through a reaction can the catalytic performance be improved. When BiVO4 is added with manganese sesquioxide to form a Mn2O3 / BiVO4 composite material, the highest degradation rate increases from 45% to 59%, and when BiVO4 is added with nickel carbide to form a Ni3C / BiVO4 composite material, the highest degradation rate increases from 45% to 71%.

[0072] In summary, the application prepares a ternary heterostructure photocatalyst with double built-in electric field by comprehensively controlling the reaction condition parameters such as the types of bismuth source, vanadium source, manganese source, organic nickel compound, calcination temperature and time, heating temperature and time, reaction atmosphere, reaction solvent type, and washing solvent type, and by using a unique electrostatic self-assembly method. The ternary heterostructure photocatalyst has a stronger catalytic degradation capacity than other photocatalysts and has a wider practical value.

Claims

1. A ternary heterojunction photocatalyst with dual built-in electric fields, characterized in that: It is composed of bismuth vanadate, manganese trioxide and nickel carbide; the mass ratio of bismuth vanadate, manganese trioxide and nickel carbide is 10~5:2~1:1; The photocatalyst formed by bismuth vanadate, manganese trioxide, and nickel carbide has a Z-type heterojunction structure. The photocatalyst is tetragonal star-shaped with a particle size of 0.5~2.5μm; The photocatalyst is prepared as follows: bismuth source and vanadium source are crystallized by high-pressure hydrothermal method to obtain bismuth vanadate; bismuth vanadate is dissolved in water, and the powder obtained by hydrothermal reaction after adding a mixture of PVP ethanol solution and manganese salt solution is calcined to obtain Mn2O3 / BiVO4 composite material; the Mn2O3 / BiVO4 composite material and Ni3C are dispersed in solvent, and then subjected to electrostatic self-assembly and drying in sequence to obtain the final product. The hydrothermal reaction conditions are as follows: after stirring the mixture of PVP ethanol solution and manganese salt solution with bismuth vanadate until homogeneous, react at 0.1~5MPa and 100~200℃ for 1~4h; the calcination conditions are as follows: temperature 100~500℃ and time 1~5h.

2. The dual-built-in electric field ternary heterojunction photocatalyst according to claim 1, characterized in that: The bismuth source is at least one of bismuth nitrate, bismuth sulfate, bismuth phosphate, and their hydrates; the vanadium source is at least one of sodium vanadate, potassium vanadate, ammonium vanadate, and their hydrates; and the manganese salt is at least one of potassium permanganate, potassium manganate, and their hydrates.

3. The dual-built-in electric field ternary heterojunction photocatalyst according to claim 1, characterized in that: In the Mn2O3 / BiVO4 composite material, the mass ratio of BiVO4 to Mn2O3 is 20~5:

1.

4. The dual-built-in electric field ternary heterojunction photocatalyst according to claim 1, characterized in that: The preparation process of Ni3C is as follows: organic nickel salt and oleylamine are heated at 200~400℃ for 1~10h in a protective atmosphere, and then cooled and washed by ultrasonication and centrifugation in sequence to obtain the product.

5. The dual-built-in electric field ternary heterojunction photocatalyst according to claim 4, characterized in that: The organonickel salt is at least one of nickel acetate, nickel oxalate, nickel formate, and their hydrates; the protective atmosphere is argon and / or nitrogen atmosphere.

6. The dual-built-in electric field ternary heterojunction photocatalyst according to claim 4, characterized in that: The detergent used for ultrasonic washing is ethanol; the detergent used for centrifugal washing is a mixture of n-hexane and ethanol at a mass ratio of 10 to 1:

1.

7. The dual-built-in electric field ternary heterojunction photocatalyst according to claim 1, characterized in that: The solvent is n-hexane; the electrostatic self-assembly is carried out under strong stirring at a speed of 200-500 r / min for 1-5 h; the drying conditions are: temperature of 20-80℃ for 1-8 h.

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