An asymmetric superwetting janus membrane, and a preparation method and application thereof

Abstract

The application discloses an asymmetric super-wetting Janus film and a preparation method and application thereof, and belongs to the technical field of photo-thermal conversion materials and seawater desalination. The Janus film has a three-dimensional porous structure of a foam copper base; one side surface of the foam copper base has an exposed super-hydrophilic layer, and the super-hydrophilic layer comprises needle-shaped nano-structured copper oxide; the other side surface of the foam copper base has an exposed photo-thermal super-hydrophobic layer, and the photo-thermal super-hydrophobic layer is a fluorinated ethylene propylene-laser-induced graphene composite layer. The structure solves the problems of lack of through holes and easy salt deposition of a traditional laser-induced graphene film, realizes the cooperation of efficient water supply from the bottom and efficient photo-thermal evaporation at the top, and exhibits excellent salt deposition resistance and various wastewater purification capabilities, and the treated water quality meets the drinking water standard, so that the Janus film has great application value in the field of solar interfacial evaporation.

Description

Technical Field

[0001] This invention relates to the fields of photothermal conversion materials and seawater desalination technology, specifically to an asymmetric superwetting Janus membrane, its preparation method, and its applications. Background Technology

[0002] Traditional seawater desalination methods, such as multi-stage flash distillation, multi-effect distillation, electrodialysis, and vapor compression, while relatively mature technologies, rely on fossil fuels for power, resulting in high energy consumption, high costs, and greenhouse gas emissions. Interfacial solar evaporation technology, as a novel approach, achieves localized solar thermal conversion by placing photothermal materials at the water-air interface. This heats only the surface water to promote evaporation, significantly reducing heat loss and improving energy efficiency.

[0003] In interfacial solar evaporation systems, carbon-based materials have attracted considerable attention due to their unique optical and thermal properties. Graphene, as a typical example, exhibits great potential due to its high theoretical specific surface area, excellent thermal conductivity, and tunable surface chemistry. However, traditional graphene preparation methods (such as chemical vapor deposition and redox methods) suffer from drawbacks such as complex processes, high costs, low yields, or the use of toxic reagents, which limit its application and promotion in large-scale seawater desalination. Laser-induced graphene (LIG) technology provides a simple, rapid, low-cost, and patternable preparation method that can generate three-dimensional porous graphene networks on the surface of polymer precursors using carbon dioxide or ultraviolet lasers. However, LIG ​​prepared by traditional methods has significant limitations: LIGs mainly form on the material surface, lacking sufficient vertical pores, which limits the rapid transport of water; the inherent hydrophobicity of graphene leads to insufficient water absorption, making it difficult to continuously transport water to the evaporation interface through capillary action, thus limiting evaporation efficiency.

[0004] Furthermore, during long-term seawater desalination, as water continues to evaporate, salt gradually crystallizes and deposits on the evaporator surface, blocking light absorption channels and causing a continuous decline in evaporation performance. The Janus structure has proven to be an effective solution. This is because the superhydrophobic surface, with its low surface energy and micro / nano structure, effectively blocks the diffusion of salt ions from the water body to the evaporation interface; while the superhydrophilic surface, with its strong water absorption and capillary force, continuously draws brine from the solution and delivers it to the evaporation zone, while the high-concentration brine automatically flows back into the solution under the influence of the concentration difference. The synergistic effect of these two mechanisms ensures efficient water supply and evaporation while preventing salt accumulation at the photothermal interface, thus achieving long-term stable anti-salt deposition performance. Zhao et al. developed a Janus evaporator assembled from hydrophilic polyester fabric and hydrophobic wood. By incorporating graphene oxide and titanium dioxide as light-absorbing materials into the hydrogel and modifying it hydrophobically with polydimethylsiloxane, the resulting Janus structure exhibits asymmetric wetting properties, thereby preventing salt ion accumulation at the photothermal interface. However, traditional LIGs do not inherently possess superhydrophobicity. Therefore, superhydrophobic modification of LIGs to enhance their salt tolerance is crucial for improving their practicality and durability in seawater desalination.

[0005] In interfacial evaporation systems, ensuring efficient and continuous water supply is one of the key factors determining evaporation efficiency. If the water supply is insufficient, the surface of the photothermal layer is prone to drying out, which will not only reduce the evaporation rate but also accelerate salt precipitation, leading to channel blockage. Copper foam (CF) has attracted much attention due to its inherent three-dimensional interconnected porous structure, which provides abundant capillary transport channels, enabling rapid rise and uniform diffusion of water. More importantly, the CF surface can be processed in situ to generate nanostructured metal oxides, thereby obtaining superhydrophilicity and further enhancing the rapid wetting and continuous transport capabilities of water. Based on these advantages, previous studies have used copper foam CF as a water supply substrate. For example, He et al. used three-dimensional disordered porous CF as a substrate for a light absorber and converted it into superhydrophilic copper oxide foam through thermal oxidation. Subsequently, a metal-organic framework was grown on the copper oxide foam and carbonized at high temperature, thereby obtaining excellent light absorption capacity, good evaporation performance and salt resistance

[31] . Therefore, copper foam is an ideal hydrophilic material for water supply layers.

[0006] However, existing Janus-structured evaporators still suffer from problems such as complex fabrication processes and difficulty in simultaneously achieving photothermal performance and salt resistance. How to provide a Janus membrane that combines excellent photothermal evaporation performance with long-term salt resistance, and whose fabrication process is simple and reliable, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the problems of insufficient water supply and easy salt deposition in laser-induced graphene evaporators in the prior art, and to provide an asymmetric superwetting Janus membrane, its preparation method and application.

[0008] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows: An asymmetric superwetting Janus membrane includes: a foamed copper substrate with a three-dimensional porous structure; an exposed superhydrophilic layer on one side surface of the foamed copper substrate, the superhydrophilic layer comprising copper oxide with needle-like nanostructures; and an exposed photothermal superhydrophobic layer on the other side surface of the foamed copper substrate, the photothermal superhydrophobic layer being a fluorinated ethylene propylene-laser-induced graphene composite layer.

[0009] Preferably, in the aforementioned asymmetric superwetting Janus membrane, the water contact angle of the superhydrophilic layer is 0°.

[0010] Preferably, the water contact angle of the photothermal superhydrophobic layer is greater than 150°.

[0011] Preferably, the fluorinated ethylene propylene-laser-induced graphene composite layer has a three-dimensional porous structure.

[0012] Preferably, the carbon content in the fluorinated ethylene propylene-laser-induced graphene composite layer is greater than 95%.

[0013] Based on a general inventive concept, the present invention also provides a method for preparing an asymmetric superwetting Janus membrane, comprising the following steps: (1) Provide a foamed copper substrate; (2) The foamed copper substrate is immersed in a mixed solution containing sodium hydroxide and ammonium persulfate for chemical oxidation treatment to form a superhydrophilic layer on the surface of the foamed copper substrate. (3) A polyimide solution is coated on the chemically oxidized side of the foamed copper substrate, and after drying, a first laser-induced treatment is performed to form a laser-induced graphene layer. (4) Fluorinated ethylene propylene is coated on the surface of the laser-induced graphene layer, and after drying, a second laser-induced treatment is performed to form a photothermal superhydrophobic layer on this side surface of the copper foam substrate.

[0014] In the above-described preparation method, preferably, the first laser-induced treatment and / or the second laser-induced treatment employ a CO2 laser with a laser energy density of 16 J / cm². 2 Up to 28 J / cm 2 .

[0015] Preferably, in the chemical oxidation treatment step, the mass ratio of sodium hydroxide, ammonium persulfate and water in the mixed solution is (2.5~3.5):1:(9~11), and the treatment time is 15-25 min.

[0016] Preferably, the pore size of the foamed copper substrate is 180-220 μm and the thickness is 1-4 mm.

[0017] Based on a general inventive concept, the present invention also provides an application of the asymmetric superwetting Janus membrane described in any of the above claims in solar interfacial evaporation seawater desalination or wastewater purification.

[0018] This invention involves coating the upper surface of a copper foam (CF) with polyimide (PI) and fluorinated ethylene propylene (FEP), followed by in-situ generation of a superhydrophobic liquid ether (LIG) using laser irradiation. This process endows the evaporator surface with excellent photothermal conversion performance and resistance to salt crystallization. Simultaneously, the bottom surface of the copper foam undergoes chemical oxidation with ammonium persulfate and sodium hydroxide, transforming it into a superhydrophilic surface with a needle-like structure, thereby achieving efficient and continuous bottom-up water supply. This asymmetric superwetting Janus membrane provides a novel strategy for achieving efficient photothermal evaporation and long-lasting salt-resistant solar desalination technology.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention achieves efficient and continuous bottom-up water supply by chemically oxidizing the bottom surface of a copper foam substrate to form a superhydrophilic copper oxide layer with a needle-like nanostructure. Simultaneously, a polyimide-graphene layer is formed on the top surface of the copper foam using laser induction, and fluorinated ethylene propylene is combined with graphene using a secondary laser to construct a superhydrophobic photothermal layer with a three-dimensional porous structure and low surface energy. This asymmetric superwetting Janus structure allows the superhydrophilic layer to continuously supply water through capillary force, diluting the salt concentration at the evaporation interface and creating a concentration gradient that drives the reverse diffusion of salt ions. The superhydrophobic layer, with its low surface energy and micro / nano structure, effectively blocks the contact and deposition of salt ions at the evaporation interface. The synergy of these two elements ensures both efficient photothermal evaporation and long-term stable salt resistance. Furthermore, this Janus membrane exhibits excellent purification effects on heavy metal wastewater, dye wastewater, and acid / alkali wastewater, showing broad application prospects. Attached Figure Description

[0020] 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. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] In the following figure descriptions, CF represents copper foam, PI represents polyimide, LIG ​​represents laser-induced graphene, FEP-LIG represents fluorinated ethylene propylene-laser-induced graphene, and FEP-LIG-CF represents fluorinated ethylene propylene-laser-induced graphene-copper foam. Figure 1 This is a schematic diagram illustrating the preparation process and application of the asymmetric superwetting Janus membrane according to an embodiment of the present invention. Figure 2 The images show a comparison of the microstructure and phase analysis of CF before and after oxidation in an embodiment of the present invention. (a) is a scanning electron microscope image before oxidation, (b) is a scanning electron microscope image after oxidation, (c) is an X-ray diffraction pattern before oxidation, and (d) is an X-ray diffraction pattern after oxidation. Figure 3 These are scanning electron microscope images of polyimide ablation at different laser energy densities according to embodiments of the present invention, where (a) is 0 J / cm². 2 (b) is 16 J / cm 2 (c) is 20 J / cm 2 (d) is 24 J / cm 2 (e) is 28 J / cm 2 ; Figure 4 The following are characterization diagrams of PI and LIG in embodiments of the present invention, wherein (a) is the energy dispersive spectrum of PI, (b) is the energy dispersive spectrum of LIG, (c) is the graph showing the change in carbon content under different laser energy densities, (d) is the Raman spectrum under different laser energy densities, (e) is the transmission electron microscope image of LIG, and (f) is a magnified view of (e). Figure 5 The following diagrams show the molecular dynamics simulation results of the PI to LIG transformation according to an embodiment of the present invention: (a) is the molecular structure of PI, (b) is the structure of 35 PI molecules in one unit cell, (c) is the molecular dynamics simulation result at 2400℃, (d) is the molecular dynamics simulation result at 2600℃, (e) is the molecular dynamics simulation result at 2800℃, (f) is the honeycomb structure diagram of LIG, and (g) is a schematic diagram of the gaseous products. Figure 6 The figures shown are the characterization results of FEP-LIG in this embodiment of the invention, where (a) is a scanning electron microscope image, (b) is a magnified view, (c) is a full energy dispersive spectrum, (d), (e), and (f) are energy dispersive spectrum surface scans of carbon, nitrogen, and fluorine elements, respectively, (g) is a microscopic morphology of the superhydrophilic surface of the Janus membrane; (h) and (i) are magnified views. Figure 7The following is a schematic diagram of the asymmetric superwetting Janus membrane and the wettability test results of an embodiment of the present invention, wherein (a) is a schematic diagram and photograph of the Janus membrane, (b) is the water absorption time of the superhydrophilic surface, (c) is the WCA value variation graph under different laser energy densities, and (d) is the surface adhesion force variation graph before and after hydrophobic modification. Figure 8 The figures shown are the photothermal performance test results of the asymmetric superwetting Janus film in this embodiment of the invention. (a) is the ultraviolet-visible-near-infrared absorption spectrum of CF and FEP-LIG-CF, (b) is a schematic diagram of the photothermal performance test device, (c) is the infrared thermal image of CF under different irradiance, (d) is the infrared thermal image of FEP-LIG-CF under different irradiance, and (e) is a comparison of the temperature distribution curves of the two samples under different irradiance. Figure 9 The following are the evaporation efficiency test results of the asymmetric superwetting Janus membrane according to an embodiment of the present invention: (a) is a schematic diagram of the evaporation efficiency testing device; (b) is a comparison diagram of the surface temperature of different samples under one solar radiation intensity; (c) is a comparison diagram of the water evaporation mass of different samples; (d) is a comparison diagram of the evaporation rate of different samples; (e) is a diagram of the surface temperature of FEP-LIG-CF under different light intensities; (f) is a diagram of the water evaporation mass of the membrane under different light intensities; and (g) is a diagram of the evaporation rate of the membrane under different light intensities. Figure 10 The following are the test results of the seawater desalination performance of the asymmetric superwetting Janus membrane in the embodiment of the present invention. (a) is a diagram showing the seawater desalination and anti-salt deposition effect, (b) is a comparison diagram of the concentration of major ions in the water before and after desalination, (c) is a test diagram of the stability of seawater desalination evaporation efficiency after 5 hours, and (d) is a test diagram of the stability of seawater desalination evaporation efficiency after 5 cycles. Figure 11 The figures shown are test results of the wastewater purification performance of the asymmetric superwetting Janus membrane in the embodiments of the present invention. (a) is a comparison of the results before and after the treatment of heavy metal wastewater, (b) is a graph showing the change in heavy metal ion concentration, (c) is a graph showing the results of the treatment of dye wastewater, (d) is a graph showing the results of the treatment of acid and alkali wastewater, and (e) is a graph showing the results of the treatment of Yellow River water.

[0022] Figure 12 The diagram shows the wettability and seawater desalination structure of LIG-CF in Comparative Example 1 of this invention, where (a) is the WCA of LIG-CF and (b) is the salt deposition results before and after desalination.

[0023] Figure 13 This is the WCA diagram of the membrane of Comparative Example 2 of the present invention.

[0024] Figure 14 This is the WCA diagram of the membrane of Comparative Example 3 of the present invention. Detailed Implementation

[0025] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0026] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0027] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0028] Copper foam (CF) was purchased from Kunshan Guangjiayuan New Materials Co., Ltd.; sodium hydroxide (NaOH) and ammonium persulfate (APS) were purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd.; polyimide (PI) solution was provided by Shantou Yijia Plastic Products Co., Ltd.; fluorinated ethylene propylene (FEP) was purchased from Dongguan Zhanyang Polymer Materials Co., Ltd.; glacial acetic acid and FeCl3 were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; (NH4)2CuCl4·2H2O, Ni(ClO4)2·6H2O, [Co(NH3)6]Cl3, methyl orange, and methylene blue were all purchased from Shanghai Maclean Biochemical Technology Co., Ltd.; seawater and river water were collected from the Yellow Sea and the Yellow River, respectively.

[0029] Example This embodiment provides an asymmetric superwetting Janus membrane, the structure of which includes a copper foam substrate, a superhydrophilic layer on the bottom surface, and a photothermal superhydrophobic layer on the top surface. The superhydrophilic layer is composed of in-situ grown needle-like copper oxide nanoparticles, and the photothermal superhydrophobic layer is a composite layer of fluorinated ethylene propylene and laser-induced graphene.

[0030] The method for preparing this Janus membrane includes the following steps: (1) Preparation of superhydrophilic layer Copper foam (CF) with a pore size of 195 μm and a thickness of 2 mm was selected as the substrate. The CF was ultrasonically cleaned in anhydrous ethanol to remove surface impurities and oil, and then dried in an oven at 80 °C. The dried CF was then immersed in a mixed solution of sodium hydroxide (NaOH), ammonium persulfate (APS), and distilled water in a mass ratio of 3:1:10 for 20 min at room temperature for chemical oxidation treatment. Drying in an oven at 80 °C resulted in the formation of a needle-like nanostructured copper oxide layer on the surface of the CF framework, thus obtaining a superhydrophilic layer.

[0031] (2) Preparation of photothermal superhydrophobic layer A layer of polyimide (PI) solution was uniformly coated on one side of the CF after step (1) treatment, and then dried in an oven at 200°C for 2 hours to solidify into a film. Then, a CO2 laser with a wavelength of 10.6 μm was used, with a scanning speed of 100 mm / s, a scanning spacing of 0.01 mm, and a laser energy density range of 0 to 28 J / cm². 2 Under specific parameters, the PI film underwent a first laser-induced treatment, transforming it in situ into a laser-induced graphene (LIG) layer with a three-dimensional porous structure. Subsequently, a fluorinated ethylene propylene (FEP) dispersion was uniformly coated onto the surface of this LIG layer and dried at 60°C. Finally, a second laser-induced treatment was performed using a CO2 laser with the same parameters, causing the FEP and LIG to recombine under laser light, forming a fluorinated ethylene propylene-laser-induced graphene (FEP-LIG) composite layer with both excellent photothermal and superhydrophobic properties.

[0032] Figure 1 The preparation process and applications of Janus membranes are demonstrated.

[0033] According to the test, the Janus membrane prepared in this embodiment has a water contact angle of 0° for the bottom superhydrophilic layer and a water contact angle of 153° for the top photothermal superhydrophobic layer.

[0034] Experimental methods for physical and chemical properties: The microstructure and elemental composition of the material surface were analyzed using a scanning electron microscope (SEM) (Quanta 250 FEG) equipped with energy-dispersive spectroscopy (EDS) and a transmission electron microscope (TEM) (FEI Talos F200X G2). The transformation of copper to copper oxide was confirmed by X-ray diffraction (XRD) (D8 ADVANCE). The evolution of the PI to LIG transformation was analyzed using Raman spectroscopy (HR Evolution). The water contact angle (WCA) and surface adhesion of the material surface were measured using a contact angle meter (JC2000D1) and a surface adhesion tester (DC AT 25).

[0035] Photothermal effect experimental method: A xenon lamp (CEL-S500) was used to simulate sunlight and verify the photothermal conversion performance of FEP-LIG-CF. An optical power meter (SM-206) was used to calibrate the irradiance. The surface temperature distribution of FEP-LIG-CF under illumination was recorded using an infrared thermal imager (UTI260B). The light absorption capacity of FEP-LIG-CF was measured using an ultraviolet-visible-near-infrared spectrometer (UV-3600i Plus).

[0036] Water evaporation experimental method: In this embodiment, a sponge was placed at the bottom of the asymmetric superwetting Janus membrane to absorb water and increase buoyancy; the edges of the membrane were covered with foam to prevent water evaporation and further enhance buoyancy. The luminous intensity of the xenon lamp was between 0.5 and 2 kW·m. 2 Adjustments were made within a specified range. Changes in water mass were recorded using a balance with a resolution of 0.01 mg. Condensed water vapor was collected at the bottom of the glass jar. Ion concentrations in seawater, heavy metal wastewater, dye wastewater, acidic wastewater, and the evaporated water were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES) (Agilent 5800). pH test strips and a TDS meter were used to test acidic wastewater, alkaline wastewater, and river water, respectively.

[0037] Molecular dynamics simulation method: This paper uses molecular dynamics simulation to study the formation mechanism of LIG. Modeling and optimization of PI were performed using Materials Studio software. 35 PI monomers are arranged within a unit cell (24.2 × 24.2 × 24.2 Å), with a density of approximately 1.4 g / cm³. 3 The simulation procedure is as follows: First, the entire system is relaxed at room temperature (300 K); second, the temperature is increased from 300 K to the target temperature within 25 ps, and then held at that temperature for 1250 ps; finally, the system is cooled back to 300 K within 25 ps. The time step for the entire simulation is 0.25 fs.

[0038] Experiment 1: Microscopic Morphology and Structural Characterization Although CF has a porous structure, its lack of superhydrophilicity limits its water absorption capacity. For example... Figure 2 As shown in Figure a, the original CF structure is loose and porous, with a smooth copper framework. After chemical oxidation treatment, as shown in Figure a... Figure 2 As shown in b, the smooth surface of the copper framework transforms into a disordered pine needle-like structure. This is because, in an alkaline environment, APS acts as a strong oxidizing agent, oxidizing copper atoms into copper ions. Simultaneously, hydroxide ions in the solution immediately combine with copper ions, forming numerous elongated nanorods on the copper surface. Due to the instability of Cu(OH)₂, it eventually dehydrates to form CuO. On the one hand, the CuO surface possesses hydrophilic hydroxyl groups; on the other hand, the pine needle-like nanostructure exhibits a strong capillary effect, rapidly transporting water to the surface for photothermal evaporation. Figure 2c and 2d show the XRD patterns before and after oxidation treatment, respectively. Before treatment, the peaks observed were located at 43.3°, 50.4°, and 74.1°, corresponding to the (111), (200), and (220) crystal planes of Cu, respectively. After treatment, the peaks at 32.7°, 35.5°, 38.7°, 48.9°, and 61.7° corresponded to the (110), (-111), (111), (-202), and (-113) crystal planes of CuO, respectively, confirming the transformation of Cu to CuO.

[0039] CF has poor photothermal performance; this invention employs LIG technology to enhance its photothermal properties. For example... Figure 3 As shown in figure a, PI is deposited on the porous CF surface, completely covering the pores. Figure 3 b, 3c, 3d, and 3e show the values ​​at 16 J / cm². 2 20 J / cm 2 24J / cm 2 and 28 J / cm 2 Scanning electron microscope images at laser energy densities. At lower laser energy densities ( Figure 3 (b) The pore structure on the PI surface is relatively sparse. As the laser energy density increases, the pore structure gradually becomes denser. Figure 3 The image shows that the originally smooth PI surface transformed into a loose, porous carbon framework. This is attributed to the PI material absorbing the photon energy of the laser, causing a sudden increase in internal temperature and generating gases such as CO, CO2, and H2. These gases, upon release, formed a porous structure. This porous carbon framework not only facilitates liquid transport but also enhances photothermal properties.

[0040] The conversion of PI to LIG is essentially a carbonization process. Figure 4 a shows the EDS spectrum of PI, where the contents of C, N, and O are 75.17%, 7.12%, and 17.71%, respectively. After laser ablation treatment at a laser energy density of 28 J / cm² ( Figure 4 (b) The carbon content rose to 95.02%, while the nitrogen and oxygen contents decreased to 3.63% and 1.34%, respectively. Figure 4 c shows the change in carbon content under different laser energy densities. As the laser energy density increases, the carbon content shows an upward trend. Figure 4 d shows the Raman spectra at different laser energy densities. The original PI Raman spectrum did not show characteristic peaks; when the laser energy density increased to 16 J / cm², the characteristic peaks were observed. 2 At that time, 1350 cm -1 Peak D appears at 1580 cm. -1 A G peak appears at 2700 cm, but at 2700 cm. -1The 2D peak has not yet been observed. The D peak indicates defects or edges in the LIG, while the G and 2D peaks reflect the number of layers in the LIG. When the laser energy density is further increased to 28 J / cm²... 2 At this point, PI underwent more thorough carbonization, resulting in the appearance of a 2D peak. Raman spectroscopy results indicate that LIG is composed of multiple layers of graphene. Figure 4 e and 4f show transmission electron microscopy images of LIG with a lattice spacing of 0.34 nm, which is consistent with the distance between adjacent (002) crystal planes in graphite.

[0041] Figure 5 a shows the molecular structure of PI, while Figure 5 b depicts a structure with 35 PI molecules arranged within a single unit cell. Based on this, the effect of different temperatures on graphene formation was investigated, and the results showed that at 2400 K ( Figure 5 c) and 2600 K ( Figure 5 d) No graphene structure was formed at this stage, and honeycomb graphene was not formed until 2800 K. Figure 5 (d), which is consistent with the findings of Vashisth et al. Figure 5 f shows the graphene structure composed of pentagonal, hexaagonal, and heptagonal carbon rings. Figure 5 g indicates that gases such as CO, CO2, and H2 were generated during the carbonization process, which explains the formation of pores in the LIG.

[0042] LIG formed by direct laser ablation lacks superhydrophobicity; therefore, this invention uses FEP to modify it. However, when FEP is coated on the LIG surface, it completely covers the LIG, resulting in the loss of its photothermal properties. Therefore, it is necessary to use a laser to remove most of the FEP, allowing a small amount of FEP to combine with LIG at the high temperature generated by the laser, thereby forming an FEP-LIG composite material with superhydrophobic properties. Figure 6 a and Figure 6 As shown in b, a layer of FEP can be observed on the LIG surface; Figure 6 The EDS spectrum in c also confirmed the presence of the F element. Figure 6 Images 6d, 6e, and 6f show the EDS plots for C, N, and F elements, respectively. The results indicate that F is uniformly distributed on the LIG surface. Meanwhile, no PI or FEP was detected on the superhydrophilic surface, which maintained a needle-like structure. This suggests that the FEP-LIG composite material exists only in the upper part of the Janus film and does not penetrate into the underlying superhydrophilic layer, thus not affecting the water absorption performance.

[0043] Experiment 2: Wetting and Photothermal Performance Testing Figure 7Image 7a shows a schematic diagram and real-world images of an asymmetric superwetting film. Its upper surface consists of a superhydrophobic FEP-LIG-CF photothermal layer, while the lower surface comprises an oxidized superhydrophilic layer. As shown in the real-world images, water droplets roll off the superhydrophobic surface but spread completely on the superhydrophilic surface. Image 7b demonstrates that water is completely absorbed within milliseconds of contact with the superhydrophilic surface. Figure 7 c shows the water contact angle (WCA) of the FEP-LIG-CF surface under different laser energy densities. Before coating with FEP, the water contact angle of LIG-CF was 79°. After coating with FEP, the water contact angle actually decreased to 44°. This is because FEP blocks the porous structure of LIG, eliminating the micro- and nano-structures on the surface, thus causing the WCA to decrease rather than increase. Laser energy density is positively correlated with WCA; when the laser energy density reaches 28 W / cm², the WCA increases. 2 At this point, the WCA is 153°. This is because the laser removes the FEP coating from the LIG surface, exposing the micro / nanoporous structure. Furthermore, the high temperature of the laser causes a small amount of FEP to bind with the LIG. The combination of the micro / nano structure and the low surface energy gives the surface its superhydrophobic properties. Figure 7 Figure d shows the change in bond strength before and after FEP modification. Before modification, the bond strength of LIG-CF was 80 N / mm². 2 After modification, the bond strength of FEP-LIG-CF decreased to 42 N / mm. 2 This indicates that the FEP-LIG-CF surface has low adhesion performance.

[0044] Highly efficient photothermal conversion materials must possess excellent light absorption properties. Figure 8 This paper presents the UV-Vis-NIR spectra of CF and FEP-LIG-CF in the wavelength range of 200 to 2500 nm, which almost covers the entire spectral range of natural sunlight. It can be seen that CF has low absorbance; however, its photothermal performance is significantly improved when FEP-LIG is coated on the CF surface. The superior photothermal conversion performance of LIG is attributed to its inherent strong broadband absorption, efficient nonradiative relaxation, and three-dimensional porous structure. Figure 8 b shows a schematic diagram of a photothermal conversion performance testing device, which consists of a xenon lamp and an infrared thermal imager. The xenon lamp simulates sunlight, while the infrared thermal imager is used to measure surface temperature. Figure 8 c and 8d show the surface temperatures of CF and FEP-LIG-CF after 10 min of xenon lamp irradiation, respectively. At a solar intensity of 1000 W / m², 2 ), 1.5 solar irradiance (1500 W / m 2 ) and 2 solar irradiance (2000 W / m 2Under irradiation conditions, the surface temperatures of CF were 39 ℃, 42.3 ℃, and 45.7 ℃, respectively. The surface temperature of FEP-LIG-CF increased significantly under 1 solar intensity (1000 W / m²). 2 ), 1.5 solar irradiance (1500 W / m 2 ) and 2 solar irradiance (2000 W / m 2 Under the irradiation conditions, the temperatures reached 77.8 ℃, 91.7 ℃, and 92.7 ℃, respectively. Figure 8 e shows the temperature curves of CF and FEP-LIG-CF over 10 min under different irradiances. The heating rate and final temperature of FEP-LIG-CF are significantly higher than those of CF.

[0045] Experiment 3: Evaporation performance and salt resistance test Figure 9 A schematic diagram of the evaporation efficiency test is shown, in which a balance records the dynamic changes in the weight of water, and an infrared thermal imager monitors the temperature changes. Figure 9 b shows the effect at 1 solar radiation intensity (1000 W / m²). 2 Temperature distribution after 1 hour of irradiation. Under these conditions, the surface temperature of water was 35.5 ℃, CF reached 39 ℃, and FEP-LIG-CF reached 45 ℃. Figure 9 c and 9d demonstrate the effect under 1 solar irradiance (1000 W / m²). 2 The dynamic evaporation curve and evaporation rate of water after 1 hour of irradiation. The evaporation rate of pure water is 0.81 kg·m³. -2 ·h -1 The water evaporation rate on the CF material is 1.17 kg·m. -2 ·h -1 The water evaporation rate on the FEP-LIG-CF was 1.47 kg·m³. -2 ·h -1 This excellent evaporation efficiency stems from the superior photothermal properties and water transport capabilities of FEP-LIG-CF itself. Furthermore, the water evaporation performance of FEP-LIG-CF under different light intensities was investigated. Figure 9 As shown in e, the surface temperature of FEP-LIG-CF is positively correlated with light intensity, especially at two solar light intensities (2000 W / m²). 2 Under these conditions, the temperature can rise to 48.2 ℃. Figure 9 f and 9g respectively illustrate the dynamic changes in water evaporation and evaporation rate after 1 hour of irradiation under different light intensities. Similarly, these parameters are positively correlated with light intensity, showing a positive correlation at two different solar intensities (2000 W / m²). 2 At this temperature, the evaporation rate can reach 1.96 kg·m³. -2 ·h-1 .

[0046] Janus membranes prevent salt deposition because they possess a superhydrophobic layer on the top surface and a superhydrophilic layer on the bottom surface. The unique hydrophobic properties of the superhydrophobic layer prevent it from contacting water and salt ions. On one hand, the superhydrophilic layer continuously supplies water through capillary action, thereby diluting the salt concentration at the evaporation interface and preventing supersaturation crystallization; on the other hand, it creates a salt concentration gradient, driving the spontaneous diffusion of salt ions from high-concentration regions to low-concentration regions. Figure 10 As shown in Figure a, the Janus membrane with asymmetric superwetting properties did not show salt deposition after 5 hours of seawater desalination. To verify the desalination capability of the superhydrophobic surface, the Janus membrane with attached salt was placed in water; after 225 seconds, the salt was completely dissolved, and no salt remained on the surface of the Janus membrane. Figure 10 b shows the concentration of metal ions before and after desalting. Na before desalting + K + Ca 2+ and Mg 2+ The concentrations were 7320 mg·L⁻¹. -1 325 mg·L -1 360 mg·L -1 and 768 mg·L -1 After desalination, their concentrations were 3.5 mg·L⁻¹. -1 0.88 mg·L -1 0.75 mg·L -1 ¹ and 0.15 mg·L -1 The resulting desalinated water meets the drinking water standards set by the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA). Figure 10 c shows a 5-hour seawater desalination test in which the evaporation efficiency remained almost constant over time. Figure 10 The results show that the Janus membrane exhibits excellent durability, with its evaporation efficiency remaining almost unchanged after five cycles of testing.

[0047] Experiment 4: Wastewater purification capacity test In addition to seawater desalination, this invention also systematically evaluated the purification capacity of Janus membranes for other types of water bodies, with results as follows: Figure 11 As shown in a and 11b, the Janus membrane exhibits excellent purification efficiency in the treatment of heavy metal wastewater. Before treatment, the Fe in the wastewater... 2+ Cu 2+ Ni 2+ and Co 2+ The concentrations were as high as 9756 mg·L⁻¹ -1 12235 mg·L -110,541 mg·L -1 and 12885 mg·L -1 After treatment with a Janus membrane, these concentrations dropped sharply to 0.14 mg·L⁻¹. -1 0.11 mg·L -1 0.05 mg·L -1 and 0.12 mg·L -1 The removal rates were all close to 99.99%. Furthermore, such as... Figure 11 As shown in Figure c, the Janus membrane also performs exceptionally well in treating dye wastewater. Taking methyl orange solution as an example, after membrane treatment, the water sample changed from deep orange-red to colorless and transparent, directly demonstrating the effective removal of the dye. Simultaneously, UV-Vis spectroscopy analysis showed that the characteristic absorption peak intensity of methyl orange significantly decreased and almost disappeared, indicating that the dye molecules were efficiently removed by the Janus membrane. Furthermore, Figure 11 Figures d and 11e demonstrate the purification effects of Janus membranes on acidic wastewater, alkaline wastewater, and Yellow River water, respectively. For acidic and alkaline wastewater, the Janus membrane effectively adjusts the pH value, transforming the wastewater from a strongly acidic or strongly alkaline state to neutral water after treatment. For Yellow River water, the total dissolved solids (TDS) concentration significantly decreased from 579 ppm to 2 ppm after membrane treatment. Therefore, Janus membranes are not only suitable for seawater desalination but also perform excellently in purifying heavy metal wastewater, dye wastewater, acidic and alkaline wastewater, and naturally occurring high-turbidity water bodies, highlighting their potential as a multifunctional water treatment material.

[0048] Comparative Example 1 This comparative example provides a LIG-CF film without FEP composite. The only difference between its preparation method and the example is that in step two, after the first laser-induced formation of the LIG layer, no subsequent FEP coating and second laser-induced treatment are performed.

[0049] After testing, such as Figure 12 As shown in Figure a, the water contact angle of the top layer of the membrane prepared in this comparative example is 79°, which indicates hydrophobicity, but it does not reach the superhydrophobic level. Therefore, salt will still be deposited on the surface. Figure 12 b).

[0050] Comparative Example 2 This comparative example provides an FEP-coated LIG-CF film without secondary laser treatment. The only difference between its preparation method and the previous example is that in step two, after coating with FEP and drying at 60 °C, a second laser-induced treatment is not performed.

[0051] After testing, such as Figure 13 As shown, the water contact angle of the top surface of the membrane prepared in this comparative example is 44°. However, due to the FEP covering the porous structure of LIG, the wettability is actually reduced.

[0052] Comparative Example 3 This comparative example provides an FEP-coated LIG-CF membrane that has not undergone chemical oxidation treatment. The only difference between its preparation method and the example is that in step one, the CF is not chemically oxidized and is directly subjected to subsequent processing.

[0053] After testing, such as Figure 14 As shown, the water contact angle of the membrane substrate prepared in this comparative example is 115°, which does not reach superhydrophilicity. Therefore, the evaporation efficiency is low, only 1.25 kg·m³. -2 ·h -1 .

[0054] Comparative Example 4 This comparative example provides an FEP-CF membrane without PI composite. The only difference between its preparation method and the previous example is that in step two, PI is not coated on the upper side of the CF membrane, and subsequent processing is performed directly.

[0055] Testing revealed that the membrane prepared in this comparative example lacked the photothermal material LIG, resulting in a low evaporation efficiency of only 1.21 kg·m³. -2 ·h -1 .

[0056] Comparative Example 5 This comparative example provides an FEP-PI-CF film without laser treatment of the PI and FEP. The only difference between this preparation method and the previous example is that in step two, neither the PI on the CF layer nor the FEP is laser treated; subsequent processing proceeds directly.

[0057] Testing revealed that the membrane prepared in this comparative example lacked evaporation properties because the PI completely sealed the pores of the CF.

[0058] In summary, this invention proposes a novel method for preparing asymmetric superhydrophobic Janus films using LIG and CF, which are obtained through simple chemical etching and laser ablation. The FEP-LIG-CF film consists of a top superhydrophobic surface and a bottom superhydrophilic surface. The water contact angle (WCA) of the superhydrophobic surface is 153°, while that of the superhydrophilic surface is 0°. FEP-LIG-CF exhibits strong solar absorption capabilities at 1000 W / m². 2 Under such light intensity, its surface can heat up to 77.8 ℃ within 10 minutes, with an evaporation rate of 1.47 kg·m³. -2 ·h -1Furthermore, this material exhibits excellent resistance to salt deposition. Importantly, FEP-LIG-CF can also be used to purify various types of water, including heavy metal-contaminated wastewater, fuel wastewater, and river water, bringing them up to World Health Organization drinking water standards. This asymmetric superwetting Janus membrane has significant practical application value in the fields of solar-powered seawater desalination and wastewater purification.

Claims

1. An asymmetric superwetting Janus membrane, characterized in that, include: A copper foam substrate with a three-dimensional porous structure; one side surface of the copper foam substrate has an exposed superhydrophilic layer comprising copper oxide with needle-like nanostructures; the other side surface of the copper foam substrate has an exposed photothermal superhydrophobic layer, which is a fluorinated ethylene propylene-laser-induced graphene composite layer.

2. The asymmetric superwetting Janus membrane according to claim 1, characterized in that, The water contact angle of the superhydrophilic layer is 0°.

3. The asymmetric superwetting Janus membrane according to claim 1, characterized in that, The water contact angle of the photothermal superhydrophobic layer is greater than 150°.

4. The asymmetric superwetting Janus membrane according to claim 1, characterized in that, The fluorinated ethylene propylene-laser-induced graphene composite layer has a three-dimensional porous structure.

5. The asymmetric superwetting Janus membrane according to claim 1, characterized in that, The carbon content in the fluorinated ethylene propylene-laser-induced graphene composite layer is greater than 95%.

6. A method for preparing an asymmetric superwetting Janus membrane as described in any one of claims 1-5, characterized in that, Includes the following steps: (1) Provide a foamed copper substrate; (2) The foamed copper substrate is completely immersed in a mixed solution containing sodium hydroxide and ammonium persulfate for chemical oxidation treatment to form a superhydrophilic layer on the surface of the foamed copper substrate. (3) A polyimide solution is coated on the chemically oxidized side of the foamed copper substrate, and after drying, a first laser-induced treatment is performed to form a laser-induced graphene layer. (4) Fluorinated ethylene propylene is coated on the surface of the laser-induced graphene layer, and after drying, a second laser-induced treatment is performed to form a photothermal superhydrophobic layer on this side surface of the copper foam substrate.

7. The preparation method according to claim 6, characterized in that, The first and / or second laser-induced treatments employ a CO2 laser with a laser energy density of 16 J / cm². 2 Up to 28 J / cm 2 .

8. The preparation method according to claim 6, characterized in that, In the chemical oxidation treatment step, the mass ratio of sodium hydroxide, ammonium persulfate, and water in the mixed solution is (2.5~3.5):1:(9~11), and the treatment time is 15-25 min.

9. The preparation method according to claim 6, characterized in that, The pore size of the copper foam substrate is 180-220 μm and the thickness is 1-4 mm.

10. An application of the asymmetric superwetting Janus membrane as described in any one of claims 1-5 in solar interfacial evaporation for seawater desalination or wastewater purification.

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