A PVDF photocatalytic self-cleaning film containing a Z-type 2D / 3D heterojunction and a preparation method and application thereof

Abstract

The application belongs to the technical field of membrane separation, and discloses a PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunction as well as a preparation method and application thereof. The Z-type 2D / 3D g-C3N4 / alpha-Fe2O3 heterojunction photocatalyst is prepared through a hydrothermal reaction, and then is mixed with polyvinylidene fluoride to prepare a casting solution, and a polyvinylidene fluoride photocatalytic self-cleaning membrane is prepared through a non-solvent induced phase separation method. The heterojunction photocatalyst is added into the polyvinylidene fluoride casting solution in the form of an additive, and the PVDF membrane is modified through the introduction of the heterojunction photocatalyst and the NIPS method, so that the membrane is endowed with photocatalytic self-cleaning performance, the mechanical strength of the composite membrane is improved, the hydrophilicity is greatly enhanced, and the composite membrane has better anti-pollution ability and interception performance.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and in particular to a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, its preparation method, and its application. Background Technology

[0002] Environmental pollution has become a global problem affecting human survival and sustainable development. With the rapid development of industry, water pollution has become increasingly serious. Dye wastewater is complex in composition, high in concentration, high in chroma, and difficult to degrade. Dye pollutants accumulated in the environment, such as azo dyes, can produce aromatic amine intermediates under the action of microorganisms, which have strong carcinogenic, teratogenic, and mutagenic effects and potential environmental risks, causing significant negative impacts on the ecological environment.

[0003] Photocatalysis is an attractive and sustainable method for decomposing pollutants in water because it is environmentally friendly, using only oxygen and water as raw materials, and even potentially using sunlight as an energy source. Furthermore, photocatalytic oxidation technology can thoroughly mineralize and decompose most organic substances and some inorganic substances that are harmless to humans and the environment, ultimately producing small inorganic molecules such as H2O and CO2, thus avoiding resource waste and secondary pollution.

[0004] Two-dimensional layered materials, due to their large specific surface area and superior electronic properties, shorten the transmission distance of photoinduced charges, promoting charge transfer and separation. g-C3N4 is a two-dimensional metal-free polymer semiconductor material with good visible light response and high specific surface area; its CN atoms form a highly delocalized π-conjugated system with sp2 hybridization. Three-dimensional nanostructures are considered a promising method for effectively separating photogenerated electron-hole pairs and improving solar energy conversion efficiency. Hematite α-Fe2O3 is a highly promising n-type semiconductor photocatalyst because its band gap is approximately 2.0–2.2 eV, allowing it to absorb most of the visible light spectrum energy. It also exhibits chemical stability in acidic and alkaline solutions, is non-toxic, has multiple synthesis methods, and low preparation costs. Furthermore, its growth morphology can be adjusted by controlling the raw materials and reaction conditions, and it shows high activity in photocatalytic degradation of dyes. However, low light absorption efficiency, rapid photogenerated carrier recombination, and limited conductivity severely inhibit single-component semiconductor photocatalysts from reaching their theoretical photocatalytic efficiency. Researchers have discovered that developing heterojunction systems based on multiple semiconductor materials is an important means to overcome the aforementioned drawbacks simultaneously. Typical photocatalyst (PC) heterojunctions can be classified into type I, type II, and type Z based on the transfer pathways of photogenerated electron-hole pairs. Among them, type Z heterojunctions, due to their unique photogenerated carrier transfer pathway, retain stronger redox and carrier recombination suppression capabilities, thus attracting widespread research in related fields.

[0005] Membrane treatment technology stands out among traditional water treatment technologies due to its strong pollutant removal capacity, environmental friendliness, high efficiency, and energy saving. Membrane separation can reach the molecular level, typically without phase change or additives, significantly reducing overall energy consumption and allowing for relatively simple scale-up. Polyvinylidene fluoride (PVDF), a homopolymer of vinylidene fluoride or a copolymer of vinylidene fluoride with other small amounts of fluorinated vinyl monomers, possesses excellent chemical resistance, high-temperature resistance, radiation resistance, and ease of membrane formation. As a typical ultrafiltration membrane material, it is widely used in various water treatment fields, such as domestic sewage treatment and industrial wastewater treatment. However, PVDF membrane materials have low surface energy and poor affinity for water. Hydrophobic organic pollutants such as proteins or oils are easily adsorbed onto the membrane surface, leading to membrane fouling, which affects the economics and reliability of the membrane, thus restricting its development, application, and promotion. Therefore, combining photocatalysis technology with membrane modification technology to form a composite photocatalytic separation modified membrane can effectively improve the membrane's self-cleaning ability, hydrophilicity, and retention characteristics.

[0006] Based on the above analysis, a novel photocatalytic self-cleaning membrane with comprehensive performance, including structural stability, high permeation flux, and sustainable recyclability, is urgently needed in the industry. Summary of the Invention

[0007] The purpose of this invention is to provide a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, its preparation method, and its application, thereby solving the problems of membrane fouling of existing polyvinylidene fluoride membranes and the insufficient photocatalytic efficiency of photocatalytic membranes prepared by existing methods due to low mass transfer rates and insufficient exposure of active sites, as well as the unsatisfactory lifespan of photocatalytic membranes caused by photocatalyst loss under long-term operating conditions.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0009] This invention provides a method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, comprising the following steps:

[0010] (1) Calcining an organic nitrogen-containing compound yields two-dimensional sheet-like g-C3N4;

[0011] (2) Mix the aqueous solution of K4[Fe(CN)6]·3H2O with two-dimensional sheet g-C3N4 to obtain a precursor solution; add an oxidant to the precursor solution and carry out a hydrothermal reaction to obtain a Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst.

[0012] (3) Mix Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst, polyvinylpyrrolidone, polyvinylidene fluoride and solvent to obtain casting solution; after the casting solution is coated onto the substrate, phase separation is performed in a gel bath to obtain PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunction.

[0013] Preferably, in the above-mentioned method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the organic nitrogen-containing compound in step (1) is urea or melamine;

[0014] In step (1), the calcination temperature is 400-600℃; the calcination time is 3-6h; and the calcination heating rate is 1-5℃ / min.

[0015] Preferably, in the above-mentioned method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the concentration of the aqueous solution of K4[Fe(CN)6]·3H2O in step (2) is 0.02-0.08 mol / L;

[0016] In step (2), the oxidant is 30% hydrogen peroxide;

[0017] In step (2), the mass-to-volume ratio of the aqueous solution of K4[Fe(CN)6]·3H2O, the two-dimensional sheet-like g-C3N4, and the oxidant is 20-40 mL: 0.1-0.3 g: 5-10 mL.

[0018] Preferably, in the above-mentioned method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the temperature of the hydrothermal reaction in step (2) is 160-200℃; and the time of the hydrothermal reaction is 1-4h.

[0019] Preferably, in the above-mentioned method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the mass ratio of the Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst, polyvinylpyrrolidone, and polyvinylidene fluoride in step (3) is 0.1-1.6:1:14-20.

[0020] Preferably, in the above-mentioned method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the mixing method in step (3) is stirring; the stirring temperature is 30-80℃, and the stirring time is 8-18h.

[0021] Preferably, in the above-mentioned method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the thickness of the coating in step (3) is 100-300 μm.

[0022] Preferably, in the above-mentioned method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the gel bath in step (3) is a mixed solution of anhydrous ethanol and water with a volume ratio of 0.5-1.5:0.8-1.3; the temperature of the gel bath is 14-30℃.

[0023] The present invention also provides a method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, thereby obtaining a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction.

[0024] The present invention also provides an application of a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction in the degradation of organic pollutants.

[0025] In this invention, the PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction can activate the Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst on its surface under visible light irradiation, generating reactive oxygen free radicals with oxidizing properties. These reactive oxygen free radicals can degrade organic pollutants, mineralizing them into CO2 and H2O. The preparation method of this invention involves adding the already prepared Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst as an additive to the polyvinylidene fluoride casting solution. Modifying the PVDF membrane through nanoparticle blending and NIPS methods not only improves the mechanical strength of the composite membrane but also significantly enhances its hydrophilicity, resulting in better antifouling ability and retention performance. Blending is the simplest and most commonly used membrane modification method. Compared to other methods, blending modification has the following advantages: modification and film formation are carried out simultaneously, the process is simple, and no cumbersome post-processing steps are required; the additive can simultaneously cover the membrane surface and the inner wall of the membrane pores without causing damage to the membrane structure.

[0026] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) This invention endows the composite membrane with photocatalytic capabilities. Membrane separation technology cannot fundamentally remove pollutants, which easily deposit on the membrane surface and micropores, forming membrane fouling and thus reducing the membrane's separation efficiency. This invention couples photocatalysis technology and membrane separation technology to design and construct a photocatalytic composite membrane that combines separation and photodegradation capabilities.

[0028] (2) The PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunction provided by the present invention has higher hydrophilicity and significant photocatalytic performance compared with traditional PVDF ultrafiltration membrane; In the Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst, the combination of 2D / 3D g-C3N4 / α-Fe2O3 can form a Z-type heterojunction structure. Due to its unique photogenerated carrier transfer pathway, the Z-type heterojunction has stronger redox and carrier recombination suppression capabilities. While improving the photoresponse performance of both, it solves the disadvantage of fast electron-hole recombination rate of α-Fe2O3 and g-C3N4, improves its electron transport rate, effectively promotes the catalytic efficiency of 2D / 3D g-C3N4 / α-Fe2O3, effectively reduces membrane fouling and slows down the rate of membrane flux decline.

[0029] (3) The PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunction provided by the present invention can activate the photocatalyst under visible light to generate reactive oxygen free radicals with oxidizing properties, thereby degrading organic pollutants. Compared with PVDF membranes modified with ultraviolet photocatalysts (such as TiO2), it significantly reduces energy consumption and cost, and prevents ultraviolet rays from causing potential harm to the membrane, laboratory equipment and operators.

[0030] (4) The method of preparing PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunction is a blending modification method. The photocatalyst 2D / 3Dg-C3N4 / α-Fe2O3 in the membrane is not easily dissolved with water flow during use, which solves the problem of photocatalyst being difficult to recover and easy to be lost, avoids potential secondary pollution to water bodies, and at the same time ensures the durability and stability of the membrane structure. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0032] Figure 1 This is a scanning electron microscope image of the Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst of Example 1;

[0033] Figure 2 This is a cross-sectional scanning electron microscope image of the M4 ultrafiltration membrane in Example 4;

[0034] Figure 3 The graph shows a performance comparison of the M4 to M8 ultrafiltration membranes in Examples 4 to 8 and the M0 ultrafiltration membrane in Comparative Example 1. Detailed Implementation

[0035] This invention provides a method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, comprising the following steps:

[0036] (1) Calcining an organic nitrogen-containing compound yields two-dimensional sheet-like g-C3N4;

[0037] (2) Mix the aqueous solution of K4[Fe(CN)6]·3H2O with two-dimensional sheet g-C3N4 to obtain a precursor solution; add an oxidant to the precursor solution and carry out a hydrothermal reaction to obtain a Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst.

[0038] (3) Mix Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst, polyvinylpyrrolidone, polyvinylidene fluoride and solvent to obtain casting solution; after the casting solution is coated onto the substrate, phase separation is performed in a gel bath to obtain PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunction.

[0039] In this invention, the organic nitrogen-containing compound in step (1) is preferably urea or melamine, and more preferably melamine;

[0040] The calcination temperature in step (1) is preferably 400-600℃, more preferably 480-570℃, and even more preferably 550℃; the calcination time is preferably 3-6h, more preferably 4-6h, and even more preferably 5h; the calcination heating rate is preferably 1-5℃ / min, more preferably 2-4℃ / min, and even more preferably 3℃ / min.

[0041] In this invention, the concentration of the aqueous solution of K4[Fe(CN)6]·3H2O in step (2) is preferably 0.02-0.08 mol / L, more preferably 0.04-0.06 mol / L, and even more preferably 0.05 mol / L;

[0042] In step (2), the oxidant is preferably 30% hydrogen peroxide;

[0043] In step (2), the preferred mass-to-volume ratio of the aqueous solution of K4[Fe(CN)6]·3H2O, the two-dimensional sheet-like g-C3N4, and the oxidant is 20-40 mL: 0.1-0.3 g: 5-10 mL, more preferably 23-37 mL: 0.14-0.26 g: 6-8 mL, and even more preferably 30 mL: 0.22 g: 7 mL.

[0044] In this invention, the mixing method in step (2) is preferably ultrasonication and / or stirring; the ultrasonication time is preferably 30-90 min, more preferably 45-80 min, and even more preferably 50 min; the stirring time is preferably 30-90 min, more preferably 35-60 min, and even more preferably 40 min.

[0045] In this invention, the temperature of the hydrothermal reaction in step (2) is preferably 160-200°C, more preferably 170-190°C, and even more preferably 180°C; the time of the hydrothermal reaction is preferably 1-4 hours, more preferably 1.5-3 hours, and even more preferably 2 hours.

[0046] In this invention, step (2) after the hydrothermal reaction further includes centrifugation, washing, and drying; washing is preferably done by alternating washing with anhydrous ethanol and water 3 to 6 times, more preferably 4 times; the drying temperature is preferably 40 to 80°C, more preferably 45 to 70°C, and more preferably 60°C; the drying time is preferably 6 to 10 hours, more preferably 7 to 9 hours, and more preferably 8.5 hours.

[0047] In this invention, the mass ratio of Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst, polyvinylpyrrolidone, and polyvinylidene fluoride in step (3) is preferably 0.1-1.6:1:14-20, more preferably 0.6-1.3:1:15-18, and even more preferably 0.9:1:16.

[0048] In this invention, the solvent in step (3) is preferably N,N-dimethylformamide; the mass ratio of polyvinylidene fluoride to solvent is preferably 10-20:80-100, more preferably 13-18:81-92, and even more preferably 16:82.

[0049] In this invention, the mixing method in step (3) is preferably stirring; the stirring temperature is preferably 30-80°C, more preferably 37-64°C, more preferably 52°C, and the stirring time is preferably 8-18h, more preferably 9-15h, more preferably 13h.

[0050] In this invention, the thickness of the coating in step (3) is preferably 100-300 μm, more preferably 170-270 μm, and even more preferably 230 μm.

[0051] In this invention, the gel bath in step (3) is preferably a mixed solution of anhydrous ethanol and water with a volume ratio of 0.5-1.5:0.8-1.3, more preferably 0.7-1.4:0.9-1.1, and even more preferably 1.1:1; the temperature of the gel bath is preferably 14-30°C, more preferably 17-25°C, and even more preferably 22°C.

[0052] The present invention also provides a method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, thereby obtaining a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction.

[0053] The present invention also provides an application of a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction in the degradation of organic pollutants.

[0054] In this invention, the organic pollutant is preferably methylene blue.

[0055] In this invention, the reaction conditions for degrading organic pollutants further include visible light irradiation; the wavelength range of visible light is preferably 300-700 nm, more preferably 360-680 nm, and even more preferably 460 nm.

[0056] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0057] Example 1

[0058] This embodiment provides a Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst, the preparation method of which includes the following steps:

[0059] (1) Weigh 10g of urea into a crucible and calcine it at 550℃ for 4h at a rate of 2℃ / min to obtain two-dimensional sheet g-C3N4;

[0060] (2) Take 0.2675g of the two-dimensional sheet g-C3N4 prepared above, add it to 32mL of K4[Fe(CN)6]·3H2O aqueous solution (0.04mol / L), ultrasonically disperse for 1h, stir for 1h, add 8mL of hydrogen peroxide solution (30%), transfer to 50mL of Teflon-lined high pressure vessel, and hydrothermally react at 180℃ for 4h, cool to room temperature in the environment; collect the product, wash it three times alternately with anhydrous ethanol and deionized water, and vacuum dry it at 60℃ for 6h to obtain Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst.

[0061] The Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst prepared above was characterized by scanning electron microscopy, and the results are as follows: Figure 1 As shown. By Figure 1 It can be seen that 3D leaf-shaped α-Fe2O3 grows on 2D sheet-like g-C3N4.

[0062] Example 2

[0063] This embodiment provides a Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst, specifically referring to Example 1, except that the mass of the two-dimensional sheet g-C3N4 in step (2) is 0.1337g.

[0064] Example 3

[0065] This embodiment provides a Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst, specifically referring to Example 1, except that the concentration of the aqueous solution of K4[Fe(CN)6]·3H2O in step (2) is 0.02mol / L.

[0066] Example 4

[0067] This embodiment provides a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the preparation method of which includes the following steps:

[0068] (1) The Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst of Example 1, polyvinylpyrrolidone, and polyvinylidene fluoride were dissolved in DMF at a mass ratio of 0.5:1:16 (the mass ratio of polyvinylidene fluoride to DMF was 16:82). The mixture was stirred at 60°C for 12 hours until fully dissolved, and then allowed to stand for 12 hours to remove bubbles, thus obtaining the casting solution.

[0069] (2) The casting solution was coated onto a glass plate with a thickness of 250 μm. Then, it was immersed in a gel bath at 20 °C consisting of anhydrous ethanol and deionized water in a volume ratio of 1.0:1.0 for phase separation. The separated membrane was then transferred to deionized water for soaking to remove excess solvent. Finally, it was stored in clean deionized water to obtain a PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunctions, which was designated as M4 ultrafiltration membrane.

[0070] The cross-section of the M4 ultrafiltration membrane prepared above was characterized by scanning electron microscopy, and the results are as follows: Figure 2 As shown. By Figure 2 It can be seen that the cross-sectional surface of the ultrafiltration membrane is dense, and the lower half consists of larger finger-shaped pores.

[0071] Example 5

[0072] This embodiment provides a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the preparation method of which includes the following steps:

[0073] (1) The Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst of Example 1, polyvinylpyrrolidone, and polyvinylidene fluoride were dissolved in DMF at a mass ratio of 0.2:1:15 (the mass ratio of polyvinylidene fluoride to DMF was 16:82). The mixture was stirred at 70°C for 12 hours until fully dissolved, and then allowed to stand for 12 hours to remove bubbles, thus obtaining the casting solution.

[0074] (2) The casting solution was coated onto a glass plate with a thickness of 150 μm. Then, it was immersed in a gel bath at 20 °C consisting of anhydrous ethanol and deionized water in a volume ratio of 0.5:1.0 for phase separation. The phase-separated membrane was transferred to deionized water for soaking to remove excess solvent, and then stored in clean deionized water to obtain a PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunctions, denoted as M5 ultrafiltration membrane.

[0075] Example 6

[0076] This embodiment provides a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the preparation method of which includes the following steps:

[0077] (1) The Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst of Example 1, polyvinylpyrrolidone, and polyvinylidene fluoride were dissolved in DMF at a mass ratio of 1:1:15 (the mass ratio of polyvinylidene fluoride to DMF was 16:82). The mixture was stirred at 50°C for 12 hours until fully dissolved, and then allowed to stand for 10 hours to remove bubbles, thus obtaining the casting solution.

[0078] (2) The casting solution was coated onto a glass plate with a thickness of 200 μm. Then, it was immersed in a gel bath at 20 °C consisting of anhydrous ethanol and deionized water in a volume ratio of 1.2:1.0 for phase separation. The separated membrane was then transferred to deionized water for soaking to remove excess solvent. Finally, it was stored in clean deionized water to obtain a PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunctions, which was designated as M6 ultrafiltration membrane.

[0079] Example 7

[0080] This embodiment provides a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the preparation method of which includes the following steps:

[0081] (1) The Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst of Example 1, polyvinylpyrrolidone, and polyvinylidene fluoride were dissolved in DMF at a mass ratio of 0.1:1:16 (the mass ratio of polyvinylidene fluoride to DMF was 16:82). The mixture was stirred at 60°C for 12 hours until fully dissolved, and then allowed to stand for 10 hours to remove bubbles, thus obtaining the casting solution.

[0082] (2) The casting solution was coated onto a glass plate with a thickness of 100 μm. Then, it was immersed in a gel bath at 20 °C consisting of anhydrous ethanol and deionized water in a volume ratio of 1.0:1.2 for phase separation. The separated membrane was then transferred to deionized water for soaking to remove excess solvent. Finally, it was stored in clean deionized water to obtain a PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunctions, which was designated as M7 ultrafiltration membrane.

[0083] Example 8

[0084] This embodiment provides a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, the preparation method of which includes the following steps:

[0085] (1) The Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst of Example 1, polyvinylpyrrolidone, and polyvinylidene fluoride were dissolved in DMF at a mass ratio of 1.5:1:16 (the mass ratio of polyvinylidene fluoride to DMF was 16:82). The mixture was stirred at 80°C for 10 h until fully dissolved, and then allowed to stand for 10 h to remove bubbles, thus obtaining the casting solution.

[0086] (2) The casting solution was coated onto a glass plate with a thickness of 260 μm. Then, it was immersed in a gel bath at 20 °C consisting of anhydrous ethanol and deionized water in a volume ratio of 1.0:1.2 for phase separation. The phase-separated membrane was transferred to deionized water for soaking to remove excess solvent, and then stored in clean deionized water to obtain a PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunctions, denoted as M8 ultrafiltration membrane.

[0087] Comparative Example 1

[0088] This comparative example provides a PVDF matrix membrane, the preparation method of which includes the following steps:

[0089] (1) Dissolve polyvinylpyrrolidone and polyvinylidene fluoride in DMF at a mass ratio of 1:16 (the mass ratio of polyvinylidene fluoride to DMF is 16:82), stir at 60°C for 12 hours until fully dissolved, and then let stand for 12 hours to remove bubbles to obtain casting solution.

[0090] (2) The casting solution was coated onto a glass plate with a thickness of 250 μm. Then it was immersed in a gel bath at 20 °C consisting of anhydrous ethanol and deionized water in a volume ratio of 1.0:1.0 for phase separation. The membrane after phase separation was transferred to deionized water for soaking to remove excess solvent. Then it was stored in clean deionized water to obtain a PVDF matrix membrane, denoted as M0 ultrafiltration membrane.

[0091] Performance Testing: The water flux, methylene blue rejection rate, and photocatalytic performance of the ultrafiltration membranes prepared in Examples 4-8 and Comparative Example 1 were tested. Specifically, 100 mL of deionized water was permeated under 0.1 MPa using a vacuum filtration device, and the required time was recorded. The pure water flux of the ultrafiltration membrane was calculated. The effective area of ​​the ultrafiltration membrane was 12.56 cm². 2Simultaneously, 100 mL of methylene blue solution (100 ppm) was permeated using a vacuum filtration device, and the rejection rate of the ultrafiltration membrane was tested. Then, the ultrafiltration membrane was removed from the vacuum filtration device, rinsed to remove free methylene blue from its surface, and its pure water flux was tested. The contaminated membrane, after being filtered with methylene blue solution, was immersed in hydrogen peroxide solution for photocatalytic degradation experiments, continuously irradiated under visible light (wavelength range 400–700 nm) for 4 hours, and its pure water flux was tested.

[0092] Test results are as follows Figure 3 As shown, from Figure 3 As can be seen from the results, compared with the PVDF matrix membrane, each of the M4 to M8 ultrafiltration membranes with Z-type 2D / 3Dg-C3N4 / α-Fe2O3 heterojunction photocatalyst exhibits superior permeability and good photocatalytic self-cleaning performance. The increased permeability may be due to the following two factors: (1) the addition of heterojunction photocatalyst nanoparticles will endow the membrane with hydrophilicity, thereby increasing the rate at which water passes through the membrane; (2) compared with the PVDF matrix membrane, the pore size and porosity of the membrane with heterojunction photocatalyst are enlarged, which undoubtedly benefits permeability. Compared with simply washing the contaminated membrane with water, exposing the membrane to visible light can effectively catalyze the decomposition of methylene blue attached to the membrane pores, thereby bringing a higher flux recovery rate. Therefore, this modification scheme significantly improves the hydrophilicity and photocatalytic degradation ability of the membrane, providing a certain reference value for the development of new heterojunction photocatalytic membranes.

[0093] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction, characterized in that, Includes the following steps: (1) Calcining an organic nitrogen-containing compound yields a two-dimensional sheet-like g-C3N4; (2) Mix the aqueous solution of K4[Fe(CN)6]·3H2O with two-dimensional sheet g-C3N4 to obtain a precursor solution; add an oxidant to the precursor solution and carry out a hydrothermal reaction to obtain a Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst; (3) Mix Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst, polyvinylpyrrolidone, polyvinylidene fluoride and solvent to obtain casting solution; after coating the casting solution onto the substrate, perform phase separation in a gel bath to obtain PVDF photocatalytic self-cleaning membrane containing Z-type 2D / 3D heterojunction. The concentration of the aqueous solution of K4[Fe(CN)6]·3H2O in step (2) is 0.02~0.08 mol / L; In step (2), the oxidant is 30% hydrogen peroxide; In step (2), the mass-to-volume ratio of the aqueous solution of K4[Fe(CN)6]·3H2O, the two-dimensional sheet-like g-C3N4, and the oxidant is 20~40mL:0.1~0.3g:5~10mL.

2. The method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction according to claim 1, characterized in that, The organic nitrogen-containing compound in step (1) is urea or melamine; In step (1), the calcination temperature is 400~600℃; the calcination time is 3~6h; and the calcination heating rate is 1~5℃ / min.

3. The method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction according to claim 1, characterized in that, The temperature of the hydrothermal reaction in step (2) is 160~200℃; the time of the hydrothermal reaction is 1~4h.

4. A method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction according to claim 1 or 3, characterized in that, In step (3), the mass ratio of Z-type 2D / 3D g-C3N4 / α-Fe2O3 heterojunction photocatalyst, polyvinylpyrrolidone, and polyvinylidene fluoride is 0.1~1.6:1:14~20.

5. The method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction according to claim 4, characterized in that, The mixing method in step (3) is stirring; the stirring temperature is 30~80℃, and the stirring time is 8~18h.

6. A method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction according to claim 1, 3, or 5, characterized in that, The thickness of the coating in step (3) is 100~300μm.

7. The method for preparing a PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction according to claim 6, characterized in that, In step (3), the gel bath is a mixed solution of anhydrous ethanol and water with a volume ratio of 0.5~1.5:0.8~1.3; the temperature of the gel bath is 14~30℃.

8. A PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction is prepared by the method described in any one of claims 1 to 7.

9. The application of the PVDF photocatalytic self-cleaning membrane containing a Z-type 2D / 3D heterojunction as described in claim 8 in the degradation of organic pollutants.

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