G-C3N4 composite separation membrane, preparation method and application of g-C3N4 composite separation membrane in filtration of organic matter-containing solution

Abstract

The invention relates to a g-C3N4 composite separation membrane, a preparation method and application of the g-C3N4 composite separation membrane in filtration of an organic matter-containing solution, and belongs to the technical field of membrane separation. The MoS2 / g-C3N4 film is prepared by compounding MoS2 and g-C3N4 nanosheets, and compared with a g-C3N4 pure film and a MoS2 pure film, the interception performance of the composite film on phenol is improved, and the stability of the composite film is relatively good.

Description

Technical Field

[0001] The present invention relates to a g-C3N4 composite separation membrane, a preparation method and an application in filtering an organic matter-containing solution, belonging to the technical field of membrane separation. Background Art

[0002] Two-dimensional graphitic carbon nitride (g-C3N4) material [[1 ,2] has been explored as a very promising candidate in the fields of photocatalytic chemistry and separation membranes. The g-C3N4 nanosheets are metal-free, non-toxic and easy to prepare on a large scale in the laboratory. Importantly, g-C3N4 is a material with a two-dimensional sheet-like structure based on tri-s-triazine rings as basic structural units. There are van der Waals forces between the sheets of g-C3N4, and it has a π-π conjugate structure. This material has special electronic and photocatalytic properties and has a wider absorption spectral range than traditional TiO2 photocatalysts, and can exert photocatalytic effects only under ordinary visible light without ultraviolet light. The unique lattice defects and layered structure in g-C3N4 nanosheets [3] are very suitable for the formation of membrane channels for selective water transport. Zhao et al. [4] studied the permeability of composite g-C3N4 photocatalytic membranes. The results showed that due to the high photocatalytic efficiency of g-C3N4, both the removal efficiency of rhodamine B and the permeation flux were improved. These high separation performance and high permeation performance are all attributed to the role of g-C3N4 nanosheets. However, due to the small specific surface area of bulk g-C3N4 and the easy recombination of photo-generated electron-hole pairs, the transport speed of photo-generated carriers is slow, which limits its photocatalytic activity. Therefore, it is of great significance to improve the photocatalytic activity of g-C3N4.

[0003] In recent years, researchers have found that by a certain preparation method, bulk g-C3N4 can be exfoliated to obtain 2D g-C3N4 nanosheets with good photocatalytic activity. Commonly used methods include thermal oxidation corrosion exfoliation method, chemical intercalation exfoliation method and liquid exfoliation method. However, there are many defects at the interface of 2D g-C3N4 nanosheets prepared by the thermal oxidation corrosion exfoliation method. During the chemical intercalation exfoliation process, chemical intercalation will destroy the structure of single-layer two-dimensional g-C3N4 nanosheets, and the process is relatively complex. The liquid exfoliation method is one of the most commonly used methods for preparing two-dimensional g-C3N4 nanosheets, and its photocatalytic activity and mass yield are higher than those of the thermal oxidation, etching exfoliation method and chemical intercalation exfoliation method. However, this method still has some problems. Organic solvents are used during the exfoliation process and long-time ultrasonic assistance is required. Therefore, this method may produce a large amount of organic waste liquid and is accompanied by a large amount of energy consumption. Therefore, it is particularly important to find a stripping method that is both green and environmentally friendly and can meet high yield and photocatalytic activity.

[0004] In recent decades, free-standing ultrathin films have been a hot topic in research and industrial applications because such free-standing films have a width of several centimeters and a thickness of several nanometers, that is, they possess the characteristics of both macroscopic materials and individual molecules. Ran et al. [5] By inserting Zn-BTC nanowires into two-dimensional MoS2 films, a Zn-BTC / MoS2 composite film was successfully prepared. The results showed that the organic solvent flux of the Zn-BTC / MoS2 composite film was increased by 6 times compared with that of the MoS2 film. At the same time, the composite film maintained excellent sieving ability and could completely retain dye molecules with a size larger than 0.42 nm.

[0005] Although g-C3N4 has been applied in membrane materials, there are still many problems in its self-forming film applications. First, the film-forming performance of g-C3N4 itself is poor. Once the size of the prepared pure g-C3N4 film is too large, there will be many obvious defect cracks on the film surface. At the same time, during the self-forming film process, more cavities will be formed inside the film. The existence of these cavities greatly affects the separation performance of the g-C3N4 film. Second, g-C3N4 itself has certain photocatalytic properties. How to make good use of this property is also a hot topic in current research.

[0006] [1]S Li, L Zhang, X Zhong, et al. Nano-subsidence-assisted precise integration of patterned two-dimensional materials for high-performance photodetector arrays[J]. ACS Nano, 2019.

[0007] [2]Y Zhe, J D Benck, Y Eatmon, et al. Stable, temperature-dependent gas mixture permeation and separation through suspended nanoporous single-layer graphene membranes[J]. Nano Letters, 2018, 18(8): 5057-5069.

[0008] [3] Y Wang, L Li, Y Wei, et al. Water transport with ultralow friction through partially exfoliated g-C3N4 nanosheet membranes with self-supporting spacers[J]. Angewandte Chemie International Edition, 2017, 56(31): 8974-8980.

[0009] [4] H Zhao, S Chen, X Quan, et al. Integration of microfiltration and visible-light-driven photocatalysis on g-C3N4 nanosheet / reduced graphene oxide membrane for enhanced water treatment[J]. Applied Catalysis B Environmental, 2016, 194: 134-140.

[0010] [5] Ran Jin, Huang Qiang, Ai Xinyu, et al. Fabrication of Zn-BTC / MoS2 composite two-dimensional membranes and their performance in organic solvent nanofiltration[J]. CIESC Journal, 2021, 04: 2148-2155. Summary of the Invention

[0011] The first technical problem to be solved by the present invention is that the existing g-C3N4 has poor film-forming performance. Once the size of the prepared pure g-C3N4 membrane is too large, there will be many obvious defect cracks on the membrane surface. At the same time, during the film-forming process, more cavities will be formed inside the membrane. The existence of these cavities greatly affects the separation performance of the g-C3N4 membrane. By compounding MoS2 and nano-ZnO with g-C3N4 nanosheets, MoS2 / g-C3N4 membranes and g-C3N4@ZnO membranes are prepared, and the performance of the prepared membranes is studied, effectively improving the retention of phenol and visible-light photocatalytic degradation performance of the composite membranes.

[0012] The second technical problem to be solved by the present invention is to solve the problems of low yield and high energy consumption in the existing methods for preparing 2D g-C3N4 nanosheets. A method for preparing g-C3N4 nanosheets by gas-phase exfoliation is proposed. The exfoliated g-C3N4 nanosheets are thinner, and the thickness of some sheets can reach single-layer. The structure is complete and the size is larger.

[0013] The technical solution is as follows:

[0014] The first object of the present invention is to provide:

[0015] A two-dimensional g-C3N4 nanosheet, which is prepared by a gas-phase exfoliation method.

[0016] The second object of the present invention is to provide:

[0017] The preparation method of the above two-dimensional g-C3N4 nanosheet includes the following steps:

[0018] Step 1: Heat the g-C3N4 powder and then add the product to liquid nitrogen.

[0019] Step 2: Disperse the product of Step 1 in an ethanol aqueous solution, stir evenly and then perform the first centrifugation to obtain a supernatant.

[0020] Step 3: Perform a second centrifugation on the supernatant obtained in Step 2 to obtain a precipitate, and after drying, obtain a two-dimensional g-C3N4 nanosheet.

[0021] The heating in Step 1 means heating to 300-500 °C, and Step 1 is repeated 1-20 times.

[0022] The first centrifugation in Step 2 is repeated 1-10 times at a rotation speed of 4000-5500 rpm.

[0023] The centrifugation speed of the second centrifugation in Step 3 is 10000-15000 rpm.

[0024] The g-C3N4 powder is prepared by a thermal polymerization method.

[0025] The steps of the thermal polymerization method include: calcining melamine, and obtaining the g-C3N4 powder after grinding, washing and drying the product.

[0026] The calcination process is calcination at 500-600 °C for 1-10 h.

[0027] The washing process is to wash successively with deionized water and absolute ethanol.

[0028] The drying is drying at 60-100 °C for 1-48 h.

[0029] The third object of the present invention is to provide:

[0030] The use of the above two-dimensional g-C3N4 nanosheet in the photocatalytic degradation of organic substances.

[0031] The organic substance is phenol.

[0032] In the photocatalytic degradation described above, a visible light source is used for irradiation.

[0033] The fourth object of the present invention provides:

[0034] A two-dimensional material composite separation membrane is composed of a two-dimensional material and a nanomaterial supported on the two-dimensional material. The two-dimensional material is g-C3N4 nanosheets, and the nanomaterial is MoS2 nanosheets or nano-ZnO.

[0035] In one embodiment, the weight ratio range of the two-dimensional material to the nanomaterial is 1:9 - 9:1.

[0036] The fifth object of the present invention provides:

[0037] A preparation method of a two-dimensional material composite separation membrane includes the following steps:

[0038] Step 1, prepare a dispersion of two-dimensional g-C3N4 nanosheets and a dispersion of two-dimensional MoS2 nanosheets;

[0039] Step 2, after mixing the dispersion of two-dimensional g-C3N4 nanosheets and the dispersion of two-dimensional MoS2 nanosheets, perform suction filtration to obtain a membrane layer.

[0040] In one embodiment, in Step 2, the weight ratio of two-dimensional g-C3N4 nanosheets to two-dimensional MoS2 nanosheets is 1:9 - 9:1.

[0041] In one embodiment, in Step 1, the preparation method of two-dimensional g-C3N4 nanosheets or two-dimensional MoS2 nanosheets includes the following steps:

[0042] Step a, heat-treat the g-C3N4 or MoS2 powder, and then add the product to liquid nitrogen;

[0043] Step b, disperse the product of Step a in an ethanol aqueous solution, stir evenly and then perform the first centrifugation to obtain a supernatant;

[0044] Step c, perform a second centrifugation on the supernatant obtained in Step b to obtain a precipitate, and after drying, obtain two-dimensional g-C3N4 nanosheets or two-dimensional MoS2 nanosheets.

[0045] The heating in Step a means heating to 300 - 500 °C, and Step 1 is repeated 1 - 20 times.

[0046] The first centrifugation in Step b is repeated 1 - 10 times, and the rotation speed is 4000 - 5500 rpm.

[0047] The centrifugation speed of the second centrifugation in Step c is 10000 - 15000 rpm.

[0048] The sixth object of the present invention provides:

[0049] A method for preparing a two-dimensional material composite separation membrane, comprising the following steps:

[0050] Step 1, preparing a dispersion of two-dimensional g-C3N4 nanosheets;

[0051] Step 2, adding Zn(NO3)2 to the dispersion and filtering to form a membrane;

[0052] Step 3, soaking the membrane layer obtained in Step 2 in a Zn(NO3)2 solution, then soaking it in a NaOH solution, taking it out and calcining to obtain a composite separation membrane.

[0053] In one embodiment, the weight ratio of Zn(NO3)2 to g-C3N4 nanosheets is 5-15:1.

[0054] In one embodiment, in Step 3, the concentration of the Zn(NO3)2 solution is 1-50 mg / ml, and the concentration of the NaOH solution is 0.1-2 mol / L.

[0055] The seventh object of the present invention provides:

[0056] Use of the above two-dimensional material composite separation membrane for filtering and / or photocatalytic decomposition of an organic matter solution.

[0057] The eighth object of the present invention provides:

[0058] Use of ZnO and / or MoS2 nanosheets for repairing surface defects of a g-C3N4 membrane.

[0059] The ninth object of the present invention provides:

[0060] Use of ZnO and / or MoS2 nanosheets for improving the rejection rate of a g-C3N4 membrane for organic matters in a solution.

[0061] Beneficial effects

[0062] (1) In the method for preparing g-C3N4 nanosheets, the gas-phase exfoliation method requires heating the material. High temperature can accelerate the vibration of the material's sheets, thereby increasing the distance between the sheets. When the supercooled liquid gas enters between the sheets, it will violently vaporize and rapidly expand. The gas molecules collide with the sheets, generating a force to separate the sheets. Liquid nitrogen is usually used as the vaporization medium. Because liquid nitrogen has a very low temperature and is inexpensive. The gas-phase exfoliation method for preparing nanosheets is simpler and more environmentally friendly.

[0063] (2) The exfoliated g-C3N4 nanosheets are relatively thin, and the thickness of some nanosheets can reach a single layer. The structure is complete and the size is relatively large. Compared with traditional exfoliation methods, the gas-phase exfoliation method has significantly improved the structural properties of g-C3N4 nanosheets.

[0064] (3) Different heating temperatures have different effects on the results of preparing g-C3N4 nanosheets by the gas-phase exfoliation method. The higher the heating temperature, the better the exfoliation effect and the higher the yield. When the temperature reaches 500 °C, the exfoliation effect and the yield increase significantly.

[0065] (4) g-C3N4 can photocatalytically degrade phenol at room temperature. The catalytic rate of g-C3N4 nanosheets is 2-3 times that of non-exfoliated g-C3N4. Preparing g-C3N4 nanosheets by the gas-phase exfoliation method can effectively improve the photocatalytic performance of g-C3N4.

[0066] (5) Compared with the pure g-C3N4 membrane and the pure MoS2 membrane, the rejection performance and visible-light photocatalytic degradation performance of the composite membrane for phenol have both been improved, and the stability of the composite membrane is better.

[0067] (6) When the mass ratio of g-C3N4 to MoS2 is 1:1, the performance of the g-C3N4 / MoS2 composite membrane is improved the most. The rejection rate has been greatly increased compared with the pure membrane, and the rejection rate of phenol is 42.1%. At the same time, the visible-light photocatalytic performance is 1.5 times that of the pure g-C3N4 membrane and 3 times that of the pure MoS2 membrane compared with the pure g-C3N4 membrane.

[0068] (7) Nano-ZnO can effectively fill the cracks and cavities in the g-C3N4 membrane, and the performance has been greatly improved. The rejection rate of the composite membrane exceeds 88%, and the photocatalytic performance is 2 times that of the g-C3N4 / MoS2 composite membrane. Description of the Drawings

[0069] Figure 1 Schematic diagram of the principle for preparing g-C3N4 nanosheets by the gas-phase exfoliation method.

[0070] Figure 2 Infrared spectra of bulk g-C3N4 and g-C3N4 nanosheets.

[0071] Figure 3 X-ray diffraction analysis of the crystal structures of bulk g-C3N4 and g-C3N4 nanosheets

[0072] Figure 4 Particle size distribution diagram.

[0073] Figure 5 Static comparison diagram of bulk g-C3N4 and g-C3N4 nanosheets (60 days)

[0074] Figure 6 It is an ultraviolet fluorescence test diagram (left: bulk g-C3N4; right: CN-500)

[0075] Figure 7 Figures (a) and (b) are SEM images of bulk g-C3N4; (c) is a TEM image of bulk g-C3N4; (d) is a TEM image of g-C3N4 nanosheets

[0076] Figure 8 It shows the effect of illumination time on the photocatalytic degradation of phenol by bulk g-C3N4 and CN-500

[0077] Figure 9 : Flow chart for the preparation of g-C3N4 / MoS2 composite membrane

[0078] Figure 10 : Electrostatic distribution diagram of g-C3N4

[0079] Figure 11 : Flow chart for the preparation of g-C3N4 / MoS2 composite membrane

[0080] Figure 12 : Process flow chart of pervaporation

[0081] Figure 13 : SEM images and EDS elemental analysis of the surface and cross-section of g-C3N4 / MoS2 composite membrane

[0082] Figure 14 : SEM images and EDS elemental analysis of the surface and cross-section of g-C3N4@ZnO composite membrane

[0083] Figure 15 : Effect of g-C3N4 / MoS2 composite membranes with different MoS2 contents on the phenol rejection performance

[0084] Figure 16 : Cyclic test diagram of the phenol rejection performance of the g-C3N4 / MoS2 composite membrane with a mass ratio of g-C3N4 to MoS2 of 1:1 Figure 17 : Cyclic test diagram of the phenol rejection performance of the g-C3N4@ZnO composite membrane

[0085] Figure 18 : Performance diagram of visible-light photocatalytic degradation of phenol

[0086] Figure 19 : Preparation mechanism diagram of the composite membrane. (a): g-C3N4 / MoS2 composite membrane; (b): g-C3N4@ZnO composite membrane Specific implementation manners

[0087] The reagents and instruments used in the experiment for preparing g-C3N4 nanosheets by gas-phase exfoliation method are shown in Table 1 and Table 2 respectively:

[0088] Table 1 Experimental reagents

[0089]

[0090] Table 2 Experimental instruments

[0091]

[0092] Preparation of g-C3N4 by thermal polymerization method

[0093] A large amount of g-C3N4 powder was prepared by thermal polymerization method in the experiment. The specific experimental steps are as follows: First, weigh 10 g of melamine and place it in a covered alumina crucible; then place the crucible in a tube furnace and calcine it at 550 °C (heating rate is 5 °C / min) for 4 h; the calcined product is a light yellow block solid, which is ground with a mortar and then washed three times with deionized water and absolute ethanol to remove unreacted impurities; then it is dried at 80 °C for 24 h to obtain g-C3N4 powder.

[0094] Preparation of g-C3N4 nanosheets by gas-phase exfoliation method

[0095] Weigh 5 g of the prepared g-C3N4 powder and place it in a crucible. Then heat the tube furnace to 300 °C. Place the crucible in the preheated tube furnace and heat it for 10 min. Pour 100 ml of liquid nitrogen into a 500 ml polytetrafluoroethylene beaker, and quickly add the heated g-C3N4 powder into the liquid nitrogen. Shake the beaker until the liquid nitrogen is completely vaporized. This is a complete exfoliation process. Repeat the above experimental steps until the 10th exfoliation is completed. Add 500 ml of ethanol / water solution (volume ratio is 1:1) to the polytetrafluoroethylene beaker. Stir with a glass rod for 5 min and then ultrasonically disperse for 30 min. Use an electric precipitation centrifuge to centrifuge the dispersion. The rotation speed is 4800 rpm. Take the supernatant and centrifuge it under the same conditions and repeat 5 times to obtain a g-C3N4 nanosheet dispersion. Take the g-C3N4 nanosheet dispersion and centrifuge it at a high speed. The rotation speed is 12000 rpm. Take the precipitate and vacuum dry it at 80 °C for 12 h to obtain the powder of g-C3N4 nanosheets for subsequent sample characterization. The sample is denoted as CN-300. Change the heating temperature to 400 °C and 500 °C, and the samples are denoted as CN-400 and CN-500 respectively. Figure 1 Schematic diagram of the principle for preparing g-C3N4 nanosheets by gas-phase exfoliation method.

[0096] Infrared analysis

[0097] By detecting the FT-IR spectra of bulk g-C3N4 powder and g-C3N4 nanosheets. The peak positions of the samples are basically the same, indicating that the preparation of g-C3N4 nanosheets by gas-phase exfoliation does not change the chemical microstructure of g-C3N4. There are a large number of primary amines and secondary amines at the edge of g-C3N4, resulting in the stretching vibration of the N-H bond, which appears in the broad peak region of 3600-3100 cm -1 . There are a large number of 3-s-triazine ring structures inside g-C3N4, and the peak at 813 cm -1 is caused by the vibration of this structure. The peaks between 1650-1240 cm -1 correspond to the stretching vibrations of C-N bonds and C-NH-C bonds respectively. Figure 2 are the infrared spectra of bulk g-C3N4 and g-C3N4 nanosheets.

[0098] XRD analysis

[0099] The crystal structures of bulk g-C3N4 and g-C3N4 nanosheets were analyzed by X-ray diffraction ( Figure 3 ). It can be clearly observed from the figure that there are two characteristic peaks (003) and (100) in the XRD spectrum of g-C3N4. A strong peak will appear at about 28° for the stacking structure of the conjugated aromatic plane similar to graphite. And a strong peak appears at the 13° position for the repeating unit inside the sheet. As the temperature increases, the intensities of the (003) and (100) peaks decrease, indicating that g-C3N4 nanosheets can be prepared. The peak area at 500 °C is significantly smaller than that at 300 °C and 400 °C, indicating that the exfoliation effect is better when the heating temperature is 500 °C.

[0100] Particle size analysis

[0101] It can be clearly seen from the detection results that the particle size of the exfoliated g-C3N4 nanosheets is smaller than that of the unexfoliated g-C3N4. At the same time, it can also be clearly seen that the particle size of CN-500 is significantly smaller than that of CN-300 and CN-400. This shows that the heating temperature has a significant effect on the exfoliation effect. At the same time, it can be seen that the particle size of CN-500 reaches 290 nm, which is much smaller than the particle size of g-C3N4 nanosheets prepared by traditional exfoliation processes (about 300 nm). Figure 4 is the particle size distribution diagram.

[0102] Dispersibility and fluorescence properties of g-C3N4 nanosheets

[0103] Comparing the dispersion liquid after standing for 2 months ( Figure 5)It can be clearly observed that only CN-500 is well-dispersed in the dispersion liquid, while CN-300, CN-400 and the unexfoliated g-C3N4 powder have settled to the bottom of the reagent bottle. The results show that during the preparation of g-C3N4 nanosheets by the gas-phase exfoliation method, good exfoliation effect can only be achieved when the heating temperature is higher than a certain level. From the above experimental results, it can be seen that CN-500 has the best structural properties, so the subsequent characterization tests mainly detect CN-500. It can be seen from the fluorescence function test that under ultraviolet light irradiation, CN-500 has an obvious fluorescence effect( Figure 6 ).

[0104] SEM and TEM Analyses

[0105] By observing the structure of g-C3N4 powder through SEM, it can be found that there is an obvious lamellar stacking structure. The unexfoliated g-C3N4 presents a large block stacking state, which seriously affects the performance of g-C3N4. It can be seen from TEM that the lamellae of the exfoliated g-C3N4 nanosheets are thinner, and some can reach the single-layer state. The exfoliated g-C3N4 nanosheets have a complete structure and a relatively large lateral size. The results of electron microscopy tests show that the gas-phase exfoliation method can prepare g-C3N4 nanosheets, and the structural properties of the prepared nanosheets have been greatly improved.

[0106] Photocatalytic Performance of g-C3N4 and g-C3N4 Nanosheets

[0107] First, prepare an aqueous phenol solution with a concentration of 10 mg / ml. Weigh 0.1 g of unexfoliated g-C3N4 powder and add it to the aqueous phenol solution. First, disperse it ultrasonically in the dark for 5 min and then stir it in the dark for 25 min to make the g-C3N4 particles reach the adsorption equilibrium. Then irradiate the solution with a visible light source. The color of the light source is white and the power is 100 w. Take samples every 0.5 h and use a high-speed centrifuge with a speed of 12,000 rpm to remove the g-C3N4 in the solution. Use a UV-visible spectrophotometer to detect the phenol concentration, the test wavelength is 213 nm, and the solution is an ethanol / water solution (volume ratio 1:1). The phenol standard curve is measured before the experiment starts. The whole experiment is carried out at room temperature (25 °C). The photocatalytic performance of CN-500 is operated using the same steps.

[0108] From Figure 8 it can be seen that both bulk g-C3N4 and g-C3N4 nanosheets have certain photocatalytic performance. The catalytic rate of g-C3N4 nanosheets is greater than that of bulk g-C3N4. The results show that the g-C3N4 nanosheets prepared by the gas-phase exfoliation method effectively improve the photocatalytic performance. The experimental results also show that under certain light conditions, g-C3N4 can catalyze the degradation of VOCs at room temperature. Preparation of g-C3N4 / MoS2 Composite Membrane

[0109] First, two-dimensional MoS2 nanosheets were prepared by the gas-phase exfoliation method. The experimental operation steps are as follows: Weigh 5 g of the prepared g-C3N4 powder and place it in a crucible. Then heat the tube furnace to 500 °C. Place the crucible in the pre-heated tube furnace and heat for 10 min. Pour 100 ml of L-N2 into a 500-ml polytetrafluoroethylene beaker. Quickly add the heated g-C3N4 powder into the L-N2. Shake the beaker until the L-N2 is completely vaporized. This is a complete exfoliation process. Repeat the above experimental steps until the 10th exfoliation is completed. Add 500 ml of ethanol / water solution (volume ratio 1:1) to the polytetrafluoroethylene beaker. Stir with a glass rod for 5 min and then ultrasonically disperse for 30 min. Use an electric precipitation centrifuge to centrifuge the dispersion. The rotation speed is 4800 rpm. Take the supernatant and continue to centrifuge under the same conditions and repeat 5 times to obtain the g-C3N4 nanosheet dispersion. Take the g-C3N4 nanosheet dispersion and perform high-speed centrifugation. The rotation speed is 12000 rpm. Take the precipitate and vacuum dry it at 80 °C for 12 h to obtain the powder of g-C3N4 nanosheets. The sample is denoted as CN-500.

[0110] In addition, the purchased MoS2 powder also needs to be exfoliated into MoS2 nanosheets, also using the gas-phase exfoliation method. The steps are as follows: Weigh 5 g of the prepared MoS2 powder and place it in a crucible. Then heat the tube furnace to 400 °C. Place the crucible in the pre-heated tube furnace and heat for 15 min. Pour 100 ml of L-N2 into a 500-ml polytetrafluoroethylene beaker. Quickly add the heated MoS2 powder into the L-N2. Shake the beaker until the L-N2 is completely vaporized. This is a complete exfoliation process. Repeat the above experimental steps until the 8th exfoliation is completed. Add 500 ml of ethanol / water solution (volume ratio 1:1) to the polytetrafluoroethylene beaker. Stir with a glass rod for 5 min and then ultrasonically disperse for 30 min. Use an electric precipitation centrifuge to centrifuge the dispersion. The rotation speed is 4000 rpm. Take the supernatant and continue to centrifuge under the same conditions and repeat 5 times to obtain the MoS2 nanosheet dispersion. Take the MoS2 nanosheet dispersion and perform high-speed centrifugation. The rotation speed is 10000 rpm. Take the precipitate and vacuum dry it at 85 °C for 10 h to obtain the powder of MoS2 nanosheets.

[0111] Disperse the above-mentioned g-C3N4 nanosheets and MoS2 nanosheets in deionized water respectively to obtain dispersions.

[0112] Next, the composite film was prepared as follows: The dispersion of CN-500 and the dispersion of MoS2 nanosheets were mixed in a certain mass ratio and ultrasonically dispersed for 30 min. Vacuum filtration was carried out to form a film. During the filtration process, a 0.22 μm PVDF membrane was used. After the nanosheets were retained, the PVDF membrane served as the support layer at the same time to form the composite film. The total content of g-C3N4 nanosheets (CN-500) and MoS2 nanosheets for preparing the film material was 10 mg. The mass ratio of g-C3N4 nanosheets to MoS2 nanosheets was set as 10:0, 9:1, 8:2, 6:4, 5:5, 4:6, 2:8, 1:9, 0:10. Figure 9 It is the preparation flow chart of the g-C3N4 / MoS2 composite film.

[0113] The films prepared at the ratios of 10:0 and 0:10 are pure g-C3N4 film and pure MoS2 film respectively.

[0114] Preparation of g-C3N4@ZnO composite film

[0115] There are many methods for preparing nano-ZnO, and a suitable method needs to be selected to prepare a g-C3N4@ZnO composite film with good performance. How to introduce the zinc source is extremely important. Through molecular simulation, the charge distribution and electrostatic potential distribution of g-C3N4 were calculated, and it was found that a negative electrostatic potential presented around the unique crystal plane defect holes of g-C3N4. By introducing a zinc source with Zn 2+ , Zn 2+ was adsorbed through the electrostatic potential, and then nano-ZnO was formed through subsequent reactions. In addition, Zn 2+ can also be adsorbed through the multi-channel adsorption property of the film itself. Figure 10 It is the electrostatic distribution diagram, 1 is the positive electrostatic potential, 2 is the negative electrostatic potential, and 3 is the neutral. Zn(NO3)2 was selected as the zinc source because Zn(NO3)2 has the highest dissociation degree in ethanol / water solution.

[0116] Take a certain amount of the dispersion of g-C3N4 nanosheets (CN-500), and the content of CN-500 is 10 mg. Weigh 0.1 g of Zn(NO3)2 and add it into the dispersion of CN-500. Magnetic stirring was carried out for 30 min. Then vacuum filtration was carried out to form a film. During the filtration process, a 0.22 μm PVDF membrane was used. After the nanosheets were retained, the PVDF membrane served as the support layer at the same time to form the composite film. The film prepared by filtration was immersed in a Zn(NO3)2 solution with a concentration of 10 mg / ml for 4 h, and then immersed in a 0.5 mol / L NaOH solution for 2 h. After soaking in NaOH, the film was left for a period of time to let the water evaporate. The dry film was calcined in a tubular furnace. The calcined film is the g-C3N4@ZnO composite film.

[0117] Preparation of the g-C3N4 / ZnO composite film in Comparative Example 1

[0118] Compared with Example 2, the difference is that after directly mixing ZnO powder (average particle size 90 nm) with g-C3N4 nanosheets, it is calcined to form a film. Take a certain amount of the dispersion of g-C3N4 nanosheets (CN-500), and the content of CN-500 is 10 mg. Weigh 0.05 g of ZnO and add it into the dispersion of CN-500. Stir magnetically for 30 min. Then filter it under vacuum to form a film. Place the dry film prepared by filtration in a tube furnace for calcination. The calcined film is the g-C3N4 / ZnO composite film.

[0119] SEM and EDS analysis of the composite film

[0120] The micro-morphology of the g-C3N4 / MoS2 composite film and the g-C3N4@ZnO composite film was observed by scanning electron microscopy. Through EDS elemental analysis, the N, Mo, and Zn element distributions of the g-C3N4 / MoS2 composite film and the g-C3N4@ZnO composite film were observed.

[0121] The surface and cross-section microstructures of the g-C3N4 / MoS2 composite film and the g-C3N4@ZnO composite film were observed by SEM, and EDS elemental analysis was carried out. The results are as Figure 13 and Figure 14 shown.

[0122] From Figure 13 and Figure 14 it can be seen that both the g-C3N4 / MoS2 composite film and the g-C3N4@ZnO composite film effectively repaired the defects of the pure film, and the improvement effect of the g-C3N4@ZnO composite film was better. There are still small defect cracks on the surface of the g-C3N4 / MoS2 composite film. From the EDS analysis, in the g-C3N4 / MoS2 composite film, both g-C3N4 and MoS2 are relatively evenly dispersed. In the g-C3N4@ZnO composite film, although the surface has been repaired relatively completely, it can be seen from the element distribution that nano-ZnO is loaded and evenly dispersed on the surface and inside of the film, but the amount is lower than that of MoS2.

[0123] Study on the separation performance and visible light catalytic performance of the composite film

[0124] The separation performance of the g-C3N4 / MoS2 composite film and the g-C3N4@ZnO composite film was detected. First, prepare a phenol solution with a total amount of 40 g and a mass fraction of 50%. Take a sample to detect the benzene concentration as a benchmark. Through a pervaporation device, the separation performance of the composite film for phenol / water was detected. The total time was 1 h. After the experiment, the concentration of phenol in the cold trap was detected, and the rejection rate of the g-C3N4@ZnO composite film for phenol was calculated.

[0125] To test the stability of the composite film, the g-C3N4 / MoS2 composite film and the g-C3N4@ZnO composite film with a mass ratio of 1:1 were subjected to 5 cyclic tests.

[0126] The g-C3N4 / MoS2 composite film and the g-C3N4@ZnO composite film were used to degrade phenol under visible light to detect the visible light photocatalytic performance of the composite film. First, a phenol solution with a concentration of 10 mg / ml was prepared at room temperature. The composite film was fixed on a support. The support with the film was placed in the phenol / water solution. It was first magnetically stirred in the dark for 30 min, and then irradiated with a white visible light source. Samples were taken every 0.5 h, and the phenol concentration was detected with a spectrophotometer. The total time was still 4.5 h.

[0127] Figure 15 This is the influence of composite films with different g-C3N4 and MoS2 contents on the phenol retention performance. It can be clearly seen from the figure that when the mass ratio is 1:1, the retention rate of the composite film for phenol reaches the highest. At the same time, it is found that the fluxes of the pure g-C3N4 film and the pure MoS2 film are extremely large, and the retention rates are extremely low, indicating that the pure g-C3N4 film and the pure MoS2 film cannot meet the retention performance at this time. Figure 16 This is the cyclic test diagram of the g-C3N4 / MoS2 composite film when the mass ratio of g-C3N4 to MoS2 is 1:1. It can be clearly seen that the retention and flux of the film change little, and the stability of the film is relatively good. The flux of the g-C3N4@ZnO composite film is 22356.42 g·m -2 ·h -1 , and the retention rate is 88.25%; while for the composite film directly obtained by mixing ZnO nanoparticles and g-C3N4 nanosheets in Comparative Example 1, the retention rate for phenol is only 9.2%, which shows no substantial difference compared with the retention rate of the pure g-C3N4 nanosheet film, indicating that mixing ZnO with g-C3N4 nanosheets directly by nanoparticles to form a film cannot improve its retention performance for phenol. The results show that the performance of the g-C3N4@ZnO composite film is more prominent than that of the g-C3N4 / MoS2 composite film. Figure 17 This is the cyclic test diagram of the g-C3N4@ZnO composite film.

[0128] From the above experimental results, it can be seen that the modified g-C3N4 / MoS2 and g-C3N4@ZnO composite films have greatly improved separation performance. From the results of the photocatalytic degradation of phenol, it can be seen that the visible light photocatalytic performance of the modified g-C3N4 / MoS2 and g-C3N4@ZnO composite films has been greatly improved, and the g-C3N4@ZnO composite film has the best visible light photocatalytic performance. Figure 18 This is the performance diagram of the visible light photocatalytic degradation of phenol.

[0129] It can be found from the experimental results that the performance of the pure g-C3N4 film and the pure MoS2 film is poor. Due to the characteristics of the 2D nanosheet material prepared by vacuum filtration, a large number of cavities will be formed in the film, resulting in a decline in the film performance. However, when the contents of g-C3N4 and MoS2 are similar, although the rejection rate of the composite film is low, there is still an improvement. This may be because the sizes of the g-C3N4 nanosheets and the MoS2 nanosheets are different, and the surface charges are also different. Therefore, during the film formation process, there are fewer cavities in the composite film, improving the film performance. It can be seen from the EDS elemental analysis diagram that when the zinc source is introduced, Zn 2+ is mainly adsorbed on the surface and the internal cavities of the film, effectively reducing the defect cracks and greatly improving the film performance. The mechanism is illustrated as Figure 19 shown.

Claims

1. A preparation method of a g-C3N4 composite separation membrane, characterized in that, It includes the following steps: Step 1: Heat-treat the g-C3N4 powder and then add the product to liquid nitrogen. Step 2: Disperse the product of Step 1 in an ethanol aqueous solution, stir evenly and then perform the first centrifugation to obtain the supernatant. Step 3: Perform the second centrifugation on the supernatant obtained in Step 2 to obtain a precipitate, and after drying, obtain two-dimensional g-C3N4 nanosheets. Step 4: Prepare the dispersion of two-dimensional g-C3N4 nanosheets and the dispersion of two-dimensional MoS2 nanosheets. Step 5: Mix the dispersion of two-dimensional g-C3N4 nanosheets and the dispersion of two-dimensional MoS2 nanosheets, and then perform suction filtration to obtain a membrane layer.

2. The preparation method of the g-C3N4 composite separation membrane according to claim 1, wherein, The heating in Step 1 means heating to 300 - 500 °C, and Step 1 is repeated 1 - 20 times.

3. The preparation method of the g-C3N4 composite separation membrane according to claim 1, wherein, The first centrifugation in Step 2 is repeated 1 - 10 times, and the rotation speed is 4000 - 5500 rpm.

4. The preparation method of the g-C3N4 composite separation membrane according to claim 1, characterized in that, The centrifugation speed of the second centrifugation in Step 3 is 10000 - 15000 rpm.

5. The preparation method of the g-C3N4 composite separation membrane according to claim 1, characterized in that, The described g-C3N4 powder is prepared by a thermal polymerization method.

6. The preparation method of the g-C3N4 composite separation membrane according to claim 5, characterized in that, The steps of the thermal polymerization method include: calcining melamine, and after grinding, washing and drying the product, g-C3N4 powder is obtained.

7. The preparation method of the g-C3N4 composite separation membrane according to claim 6, characterized in that, The calcination process is calcining at 500 - 600 °C for 1 - 10 h; the washing process is washing successively with deionized water and absolute ethanol; the drying is drying at 60 - 100 °C for 1 - 48 h.

8. The preparation method of the g-C3N4 composite separation membrane according to claim 6, characterized in that, In Step 2, the weight ratio of two-dimensional g-C3N4 nanosheets to two-dimensional MoS2 nanosheets is 1:9 - 9:

1.

9. The separation membrane obtained by the preparation method of the g-C3N4 composite separation membrane according to any one of claims 1 - 8.

10. The application of the separation membrane according to claim 9 in the filtration of an organic matter-containing solution.

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