A Janus-type cellulose composite membrane, its preparation method and application

Abstract

This invention belongs to the field of solar-powered seawater desalination and discloses a Janus-type cellulose composite membrane, its preparation method, and its applications. The membrane comprises hydrophilic cellulose nanofibers, modified MXene, and a hydrophobic PTFE membrane. The preparation method includes adding CTAC to an MXene dispersion to obtain a modified MXene dispersion; mixing the modified MXene dispersion with a cellulose nanofiber suspension; adding CaCl2 solution for crosslinking; preparing a cellulose nanofiber / MXene photothermal conversion membrane by vacuum filtration; and finally, growing a hydrophobic PTFE membrane on the membrane to obtain the Janus-type cellulose composite membrane. The preparation method of this invention is low-cost, and the prepared membrane is superhydrophilic on one side and superhydrophobic on the other, exhibiting good salt resistance, high mechanical strength, excellent photothermal performance, and high water evaporation efficiency.

Description

Technical Field

[0001] This invention relates to the field of solar-powered seawater desalination, and more specifically, to a Janus-type cellulose composite membrane, its preparation method, and its application. Background Technology

[0002] Solar-powered seawater evaporation technology has advantages such as low cost, simple operation, and high efficiency. In this technology, heat-insulating, water-absorbing sponge foam floats above the seawater; a photothermal conversion film is attached to the foam, and the heat converted from the absorbed light is confined within the photothermal conversion film.

[0003] Photothermal conversion films should possess excellent light absorption capabilities to ensure high photothermal conversion performance. Typically, light-absorbing nanofillers are embedded in the film matrix, responsible for absorbing sunlight and converting it into heat. MXene materials, with their excellent light absorption capabilities, enable efficient photothermal conversion and are considered excellent light-absorbing nanofillers. The more MXene nanofillers added, the stronger the light absorption capability of the matrix / MXene photothermal conversion film; however, a high proportion of nanofillers can lead to a deterioration in the mechanical properties of the matrix / MXene photothermal conversion film.

[0004] Photothermal conversion membranes should possess good hydrophilicity to drive water molecules upwards to the surface. Cellulose nanofiber membranes, with their porous structure, large specific surface area, and hydrophilic properties, can be used as a supporting matrix for light-absorbing nanofillers. The hydrophilic, porous cellulose can act as a channel for the capillary upward transport of water molecules. However, cellulose nanofiber membranes are hydrophilic on both sides; salts tend to accumulate on the membrane surface. The precipitated salts reduce light absorption and clog the pores between nanofibers, leading to a sharp decline in solar evaporation performance.

[0005] The prior art (CN 113603935 A) discloses a composite aerogel with Janus properties, its preparation method and application. The composite aerogel includes a composite aerogel body, which comprises a hydrophobic upper layer and a hydrophilic lower layer. The upper layer is a silane-modified cellulose nanofiber / Ti3C2TxMXene aerogel, and the lower layer is a cellulose nanofiber aerogel. The interface between the upper and lower layers is chemically cross-linked to form a whole.

[0006] The prior art (CN 115725112 A) discloses a Janus bilayer aerogel, its preparation method and application. The Janus bilayer aerogel is made of two-dimensional MXene nanosheets, trimethoxysilane, and narrow bandgap transition metal sulfides as the main raw materials. First, the two-dimensional MXene nanosheets are chemically crosslinked with chitosan and acetic acid and then freeze-dried to obtain MXene-based aerogel. Then, a mixed solution of trimethoxysilane, n-heptane and narrow bandgap transition metal sulfides is sprayed onto the surface of the MXene-based aerogel to obtain a three-dimensional photothermal material with internal vertical channels and different physical properties on the upper and lower surfaces.

[0007] The prior art (CN 115055063 A) discloses a method for preparing a Janus bifunctional photothermal membrane for distillation using composite nanofiber membranes. The composite membrane consists of three nanofiber layers: a polytetrafluoroethylene (PTFE) nanofiber membrane as a hydrophobic base layer, a polyvinylidene fluoride (PVDF) nanofiber membrane as an intermediate binder layer, and a carbon nanotube-modified dendritic cellulose nanofiber membrane as a hydrophilic photothermal and dye adsorption functional layer. The preparation method includes the following steps:

[0008] (1) Preparation of hydrophobic polytetrafluoroethylene nanofiber membrane; (2) Preparation of polyvinylidene fluoride nanofiber bonding layer; (3) Preparation of carbon nanotube-modified dendritic cellulose nanofiber membrane; (4) Hot pressing composite.

[0009] The composite membranes prepared by the above-mentioned existing technologies have a three-layer structure, the process is relatively complex, and it is not suitable for large-scale production; there are problems with the polyvinylidene fluoride nanofiber bonding layer and the loose bonding between the polytetrafluoroethylene nanocellulose film and the carbon nanotube-modified dendritic cellulose nanofiber film.

[0010] Existing technology (Flexible MXene-based Janus porous fibrous membranes for sustainable solar-driven desalination and emulsions separation) utilizes MXene / poly(dimethylsiloxane) and polylactic acid / titanium dioxide nanofluidic porous fiber membranes. MXene and PDMS solutions are thoroughly mixed and sprayed onto the surface of a PLA / TiO2 porous membrane. When the dried sample is used in seawater desalination, the solar absorption efficiency is 93%, and the seawater evaporation rate at one solar irradiance is 1 kg m³ / s. -2 h -1The evaporation efficiency is 60%. Additionally, the existing technology (MXene Ti3C2: An Effective 2D Light-to-Heat Conversion Material) uses an MXene / polyvinylidene fluoride membrane. The MXene film and PVDF substrate are surface modified by immersing them in a PDMS hexane solution, followed by heat treatment. The measured solar absorptivity is 93%, and the seawater evaporation rate at one solar irradiance is 1 kg m³ / s. -2 h -1 The evaporation efficiency is 84%.

[0011] The aforementioned existing technologies modify MXene using different solutions and apply it to solar-powered seawater desalination. However, there is still significant room for improvement in its light absorption efficiency, seawater evaporation rate, and evaporation efficiency. Summary of the Invention

[0012] To address the problem of salt accumulation on the surface of a double-sided hydrophilic membrane during seawater desalination, which leads to a sharp decline in solar evaporation performance, this invention provides a Janus-type cellulose composite membrane with one hydrophobic side and the other hydrophilic side, exhibiting high efficiency, high mechanical strength, and salt resistance, as well as its preparation method.

[0013] Another objective of this invention is to provide a cellulose nanofiber / MXene photothermal conversion film with high light absorption capacity and high mechanical properties;

[0014] Another object of the present invention is to provide an application of a Janus-type cellulose composite membrane.

[0015] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0016] A Janus-type cellulose composite membrane is prepared mainly from hydrophilic cellulose nanofibers as the matrix, modified MXene as the nanofiller, and hydrophobic polytetrafluoroethylene (PTFE) membrane.

[0017] Furthermore, the modified MXene is obtained by modifying MXene nanosheets with hexadecyltrimethylammonium chloride (CTAC).

[0018] A cellulose nanofiber / MXene photothermal conversion membrane is prepared mainly from hydrophilic cellulose nanofibers as a matrix and modified MXene.

[0019] A method for preparing the above-mentioned Janus-type cellulose composite membrane includes the following steps:

[0020] 1) Add MXene nanosheets to CTAC solution and stir thoroughly to obtain modified MXene dispersion;

[0021] 2) Add nanocellulose suspension to the modified MXene dispersion and stir thoroughly to obtain a mixture;

[0022] 3) Add CaCl2 solution to mixture 1, utilizing Ca... 2+ As a crosslinking agent, stir thoroughly;

[0023] 4) Pour the mixture 2 into a filter dish lined with a filter membrane, turn on the vacuum pump to filter and form a membrane, and dry it to obtain a cellulose nanofiber / MXene photothermal conversion membrane.

[0024] 5) Grow PTFE membranes on cellulose nanofibers / MXene photothermal conversion membranes.

[0025] Further, the concentration of the CTAC solution in step 1) is 0.08-0.2 wt%; and the concentration of MXene nanosheets in the modified MXene dispersion is 3-5 g / L.

[0026] Preferably, the concentration of the CTAC solution is 0.11 wt%, and the concentration of the MXene nanosheets is 4 g / L.

[0027] Further, the concentration of the nanocellulose suspension in step 2) is 0.5 to 1.0 wt%; the volume ratio of the modified MXene dispersion to the nanocellulose suspension is 1:5 to 1:10.

[0028] Preferably, the concentration of the nanocellulose suspension is 0.8 wt%, and the volume ratio of the modified MXene dispersion to the nanocellulose suspension is 1:8.4.

[0029] Further, in step 3), the concentration of CaCl2 is 0.5 to 2.5 wt%; the volume ratio of CaCl2 solution to mixed solution is 1:37 to 1:50.

[0030] Preferably, the concentration of CaCl2 is 2.0 wt%, and the volume ratio of CaCl2 solution to the mixed solution is 1:47.

[0031] Furthermore, step 4) drying methods include natural air drying and drying at 30–60°C.

[0032] Further, in step 5), a PTFE film is grown on the cellulose nanofiber / MXene photothermal conversion film using an electron beam evaporation device or a magnetron sputtering device.

[0033] An application of the above-mentioned Janus-type cellulose composite membrane is used for solar seawater evaporation and solar seawater desalination.

[0034] Cellulose nanofiber membranes are hydrophilic on both sides, and salt tends to accumulate on the upper surface. The precipitated salt reduces light absorption and clogs the pores between nanofibers, leading to a sharp decline in solar evaporation performance. However, the Janus-type cellulose composite membrane (cellulose nanofiber / MXene@PTFE Janus-type membrane) of this invention has a hydrophobic side and a hydrophilic side, which allows it to effectively prevent salt accumulation.

[0035] The interlayer spacing of MXene nanofillers significantly affects the water evaporation performance of MXene composites. This invention innovatively modifies MXene nanofillers, increasing their interlayer spacing, making them more suitable for water molecule flow between the nanolayers of the MXene nanofillers, and significantly improving the seawater evaporation performance of cellulose nanofiber / MXene@PTFE Janus type membranes.

[0036] This invention innovatively uses CTAC as a cationic surfactant. When MXene is immersed in a cationic surfactant solution, the cationic surfactant self-assembles and intercalates into the negatively charged MXene interlayer through electrostatic interaction. The long chain (hydrophobic) and large volume of CTAC increase the interlayer spacing of MXene.

[0037] This invention innovatively adds an appropriate amount of CaCl2 solution during the preparation process, utilizing Ca... 2+ As a crosslinking agent, Ca 2+ It reacts chemically with oxide groups on the surface of two-dimensional materials such as cellulose nanofibers and MXene to form a cross-linked structure, which forms a tight connection between cellulose nanoparticles and MXene nanosheets, increases the interfacial interaction force, and improves the mechanical strength of the cellulose nanofiber / MXene photothermal conversion film.

[0038] Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

[0039] (1) The cellulose nanofiber / MXene@PTFE Janus type membrane of the present invention is hydrophilic on one side and hydrophobic on the other side, which can prevent salt from accumulating on the membrane surface during seawater evaporation and enhance the working stability of seawater evaporation products.

[0040] (2) This invention innovatively employs CTAC-modified MXene light-absorbing nanofiller, increasing the interlayer spacing of the MXene filler and significantly improving the seawater evaporation performance of the cellulose nanofiber / MXene@PTFE Janus membrane; the light absorption rate of this invention reaches as high as 94.7%, and the water evaporation rate obtained at one solar irradiance reaches as high as 1.25 kg m³ / s. -2 h -1 The water evaporation efficiency is as high as 87.8%.

[0041] (3) This invention innovatively uses Ca 2+ As a crosslinking agent, it improves the mechanical strength of cellulose nanofiber / MXene photothermal conversion membrane, with a mechanical strength of up to 73.6 MPa, which can achieve better durability and stability, enabling it to exert greater application potential in the field of seawater desalination.

[0042] (4) The preparation method of the cellulose nanofiber / MXene@PTFE Janus type membrane of the present invention is relatively simple and low in cost, and can be used for large-scale industrial production. Attached Figure Description

[0043] Figure 1 The water contact angle of the cellulose nanofiber / MXene@PTFE Janus type membrane;

[0044] Figure 2 The water contact angle of the PTFE side of the cellulose nanofiber / MXene@PTFE Janus type membrane;

[0045] Figure 3 XRD diffraction patterns of MXene nanofiller and CTAC-modified MXene nanofiller;

[0046] Figure 4 A schematic diagram of a cellulose nanofiber / MXene@PTFE Janus-type membrane applied to a seawater evaporation system;

[0047] Figure 5 Tensile strain diagram of cellulose nanofiber / MXene@PTFE Janus type membrane. Detailed Implementation

[0048] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in this technical field.

[0049] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0050] Example 1

[0051] 0.1 g of MXene nanosheets were added to 25 mL of 0.11 wt% CTAC solution and stirred thoroughly for 1 hour to obtain a modified MXene dispersion. Next, the modified MXene dispersion was added to 210 mL of 0.8 wt% nanocellulose suspension and stirred thoroughly for 1 hour to obtain a mixture. Then, 5 mL of 2.0 wt% CaCl2 solution was added to the mixture and stirred thoroughly for 20 minutes. The mixture was poured into a filter dish lined with a filter membrane, and vacuum-filtered to form a membrane. After air-drying, a cellulose nanofiber / MXene photothermal conversion membrane was obtained. A PTFE membrane was then grown as a hydrophobic layer using magnetron sputtering. Finally, a cellulose nanofiber / MXene@PTFE Janus-type membrane was obtained.

[0052] Figure 1 The water contact angle of the cellulose nanofiber / MXene side of the prepared cellulose nanofiber / MXene@PTFE Janus type membrane indicates that the cellulose nanofiber / MXene is hydrophilic. Figure 2 The water contact angle of the PTFE side of the prepared cellulose nanofiber / MXene@PTFE Janus type membrane indicates that the grown PTFE side is hydrophobic. Figure 3 The XRD diffraction patterns are shown in the comparison diagrams of MXene nanofiller and CTAC-modified MXene nanofiller. The comparison reveals that the diffraction peaks of the CTAC-modified MXene nanosheets shift towards smaller angles, indicating that CTAC modification increases the interlayer spacing of the MXene nanosheets.

[0053] Example 2

[0054] 0.15 g of MXene nanosheets were added to 50 mL of 0.2 wt% CTAC solution and stirred thoroughly for 1 hour to obtain a modified MXene dispersion. Next, the modified MXene dispersion was added to 250 mL of 1.0 wt% nanocellulose suspension and stirred thoroughly for 1 hour to obtain a mixture. Then, 8 mL of 0.5 wt% CaCl2 solution was added to the mixture and stirred thoroughly for 20 minutes. The mixture was poured into a filter dish lined with a filter membrane, and vacuum filtration was performed to form a membrane. The membrane was dried at 30 °C to obtain a cellulose nanofiber / MXene photothermal conversion membrane. A PTFE membrane was then grown as a hydrophobic layer using electron beam evaporation. Finally, a cellulose nanofiber / MXene@PTFE Janus-type membrane was obtained.

[0055] Example 3

[0056] 0.18 g of MXene nanosheets were added to 36 mL of 0.08 wt% CTAC solution and stirred thoroughly for 1 hour to obtain a modified MXene dispersion. Next, the modified MXene dispersion was added to 360 mL of 0.5 wt% nanocellulose suspension and stirred thoroughly for 1 hour to obtain a mixture. Then, 8 mL of 2.5 wt% CaCl2 solution was added to the mixture and stirred thoroughly for 20 minutes. The mixture was poured into a filter dish lined with a filter membrane, and vacuum-filtered to form a membrane. The membrane was dried at 40 °C to obtain a cellulose nanofiber / MXene photothermal conversion membrane. A PTFE membrane was then grown as a hydrophobic layer using magnetron sputtering. Finally, a cellulose nanofiber / MXene@PTFE Janus-type membrane was obtained.

[0057] Example 4

[0058] 0.2 g of MXene nanosheets were added to 57 mL of 0.15 wt% CTAC solution and stirred thoroughly for 1 hour to obtain a modified MXene dispersion. Next, the modified MXene dispersion was added to 356 mL of 0.9 wt% nanocellulose suspension and stirred thoroughly for 1 hour to obtain a mixture. Then, 10 mL of 1.5 wt% CaCl2 solution was added to the mixture and stirred thoroughly for 20 minutes. The mixture was poured into a filter dish lined with a filter membrane, and vacuum-filtered to form a membrane. The membrane was dried at 45 °C to obtain a cellulose nanofiber / MXene photothermal conversion membrane. A PTFE membrane was then grown as a hydrophobic layer using magnetron sputtering. Finally, a cellulose nanofiber / MXene@PTFE Janus-type membrane was obtained.

[0059] Example 5

[0060] 0.25 g of MXene nanosheets were added to 70 mL of 0.18 wt% CTAC solution and stirred thoroughly for 1 hour to obtain a modified MXene dispersion. Next, the modified MXene dispersion was added to 380 mL of 0.95 wt% nanocellulose suspension and stirred thoroughly for 1 hour to obtain a mixture. Then, 9 mL of 1.8 wt% CaCl2 solution was added to the mixture and stirred thoroughly for 20 minutes. The mixture was poured into a filter dish lined with a filter membrane, and vacuum filtration was performed to form a membrane. The membrane was dried at 50 °C to obtain a cellulose nanofiber / MXene photothermal conversion membrane. A PTFE membrane was then grown as a hydrophobic layer using electron beam evaporation. Finally, a cellulose nanofiber / MXene@PTFE Janus-type membrane was obtained.

[0061] Example 6

[0062] 0.35 g of MXene nanosheets were added to 113 mL of 0.13 wt% CTAC solution and stirred thoroughly for 1 hour to obtain a modified MXene dispersion. Next, the modified MXene dispersion was added to 627 mL of 0.92 wt% nanocellulose suspension and stirred thoroughly for 1 hour to obtain a mixture. Then, 20 mL of 0.6 wt% CaCl2 solution was added to the mixture and stirred thoroughly for 20 minutes. The mixture was poured into a filter dish lined with a filter membrane, and vacuum filtration was performed to form a membrane. The membrane was dried at 60 °C to obtain a cellulose nanofiber / MXene photothermal conversion membrane. A PTFE membrane was then grown as a hydrophobic layer using electron beam evaporation. Finally, a cellulose nanofiber / MXene@PTFE Janus-type membrane was obtained.

[0063] Comparative Example 1

[0064] 0.1 g of MXene nanosheets were added to 25 mL of 0.10 wt% CTAC solution and stirred thoroughly for 1 hour to obtain a modified MXene dispersion. Next, the modified MXene dispersion was added to 210 mL of 0.8 wt% nanocellulose suspension and stirred thoroughly for 1 hour to obtain a mixture. Then, 5 mL of 2.1 wt% CaCl2 solution was added to the mixture and stirred thoroughly for 20 minutes. The mixture was poured into a filter dish lined with a filter membrane, and vacuum-filtered to form a membrane. After air-drying, a cellulose nanofiber / MXene photothermal conversion membrane was obtained.

[0065] Comparative Example 2

[0066] 0.1 g of MXene nanosheets were added to 210 mL of a 1.5 wt% nanocellulose suspension and stirred thoroughly for 1 hour to obtain a mixture. Then, 5 mL of a 0.3 wt% CaCl2 solution was added to the mixture and stirred thoroughly for 20 minutes. The mixture was poured into a filter dish lined with a filter membrane, and the membrane was formed by vacuum filtration. After air drying, a cellulose nanofiber / MXene photothermal conversion membrane was obtained. A PTFE membrane was then grown as a hydrophobic layer using magnetron sputtering. Finally, a cellulose nanofiber / MXene@PTFE Janus-type membrane was obtained.

[0067] Comparative Example 3

[0068] 0.1 g of MXene nanosheets were added to 20 mL of 0.23 wt% CTAC solution and stirred thoroughly for 1 hour to obtain a modified MXene dispersion. Next, the modified MXene dispersion was added to 210 mL of 1.6 wt% nanocellulose suspension and stirred thoroughly for 1 hour to obtain a mixture. The mixture was then poured into a filter dish lined with a filter membrane, and vacuum-filtered to form a membrane. After air drying, a cellulose nanofiber / MXene photothermal conversion membrane was obtained. A PTFE membrane was then grown as a hydrophobic layer using magnetron sputtering. Finally, a cellulose nanofiber / MXene@PTFE Janus-type membrane was obtained.

[0069] Comparative Example 4

[0070] 0.1 g of MXene nanosheets were added to 15 mL of 0.24 wt% CTAC solution and stirred thoroughly for 1 hour to obtain a modified MXene dispersion. Next, the modified MXene dispersion was added to 60 mL of 1.3 wt% nanocellulose suspension and stirred thoroughly for 1 hour to obtain a mixture. Then, 5 mL of 2.8 wt% CaCl2 solution was added to the mixture and stirred thoroughly for 20 minutes. The mixture was poured into a filter dish lined with a filter membrane, and vacuum-filtered to form a membrane. After air-drying, a cellulose nanofiber / MXene photothermal conversion membrane was obtained. A PTFE membrane was then grown as a hydrophobic layer using magnetron sputtering. Finally, a cellulose nanofiber / MXene@PTFE Janus-type membrane was obtained.

[0071] Test methods for evaporation rate and evaporation efficiency:

[0072] Over 3600 seconds, a computer uses an electronic analytical balance to record the changes in the mass of water produced by solar steam.

[0073] The formula for calculating the evaporation efficiency of water is:

[0074]

[0075] Where m represents the evaporation rate, h lv This represents the total enthalpy of vapor-liquid phase transition, including sensible heat and phase transition enthalpy; P represents solar irradiance.

[0076] Figure 4 This is a schematic diagram of applying the cellulose nanofibers / MXene@PTFE Janus-type membranes prepared in Examples 1-6 to a seawater evaporation system.

[0077] The water evaporation rates of Examples 1-6 and Comparative Examples 1-4 were measured and are listed in Table 1. The comparison shows that the water evaporation rate of the seawater evaporation system using the cellulose nanofiber / MXene@PTFE Janus-type membrane prepared in Examples 1-6 was significantly higher than the water evaporation rate (1.14 kg / m³) of the seawater evaporation system using the cellulose nanofiber / MXene membrane prepared in Comparative Example 1. -2 h -1 Cellulose nanofiber / MXene@PTFE Janus type membranes made with hydrophobic PTFE membranes can prevent the sharp decline in solar evaporation performance caused by salt accumulation on the membrane surface, and can effectively improve the water evaporation rate of the system.

[0078] As shown in Table 1, the water evaporation rates in Examples 1-6 are significantly higher than the 0.89 kg / m³ water evaporation rate of the seawater evaporation system using the cellulose nanofiber / MXene@PTFE Janus-type membrane prepared with MXene without CTAC modification in Comparative Example 2. -2 h -1 Modifying MXene nanofillers with CTAC increases the interlayer spacing, resulting in a higher water evaporation rate for the prepared cellulose nanofiber / MXene@PTFE Janus type membrane. The interlayer spacing of MXene nanofillers significantly affects the water evaporation performance of MXene composites. Appropriately increasing the interlayer spacing allows for better flow of water molecules between the nanolayers, thus improving the water evaporation performance of the MXene composite. Therefore, modifying MXene nanofillers to increase their interlayer spacing is an effective way to further improve the seawater evaporation performance of cellulose nanofiber / MXene@PTFE Janus type membranes.

[0079] Figure 5 Tensile-strain curves of the cellulose nanofiber / MXene@PTFE Janus-type membranes prepared in Example 1 and Comparative Example 3. When Ca... 2+ When the content of [component name] was changed from 0 wt% in Comparative Example 3 to 2.0 wt% in Example 1, the tensile strength of the membrane increased from 20.9 MPa to 73.6 MPa, and the strain increased from 3.3% to 3.9%. It is evident that the tensile strain strength of the composite membrane is enhanced.

[0080] The comparison shows that adding an appropriate amount of CaCl2 solution as a crosslinking agent improves the mechanical strength of the cellulose nanofiber / MXene@PTFE Janus type membrane. However, if the content of CaCl2 solution is too high, the composite membrane will become too rigid and brittle, losing its flexibility and reducing its mechanical properties. It may also lead to scaling problems, hindering the transport of water molecules and resulting in excessively low evaporation rates and evaporation efficiencies. As can be seen from Table 1, the mechanical strength of Examples 1-6 is significantly higher than that of Comparative Examples 1-4, while the mechanical strength of Comparative Example 4, which added 2.8 wt% CaCl2 solution, is significantly reduced to 20.5 MPa.

[0081] According to the results in Table 1, the light absorption rates of Examples 1-6 were significantly improved compared to Comparative Examples 1-4. This is because the content of modified MXene and CaCl2 solution in the overall solution was moderate. If the content of MXene is too low, the appearance color of the film will not be deep enough, the light absorption rate will be reduced, and thus the water evaporation rate and water evaporation efficiency will be too low. If the content of modified MXene is too high, scale will form on the surface of the composite film, clogging the pores and hindering the transport of water molecules, which will also result in a low water evaporation rate and water evaporation efficiency.

[0082] In addition, after desalination via cellulose nanofiber / MXene@PTFE Janus-type membrane, the Na+ in seawater... + K + Ca 2 + Mg 2+ The concentration of [the substance] decreased by 3 to 4 orders of magnitude, while exhibiting a very high rejection rate of over 99.9%, which is far below the drinking water quality testing standards of the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO).

[0083] Table 1. A solar irradiance (1000 W / m²) 2 Water evaporation rate and mechanical strength under )

[0084]

[0085] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A Janus-type cellulose composite membrane, characterized in that, The material was prepared from hydrophilic cellulose nanofibers as a matrix, modified MXene as a nanofiller, and a hydrophobic polytetrafluoroethylene membrane; the modified MXene was obtained by modifying MXene nanosheets with hexadecyltrimethylammonium chloride. The method for preparing the Janus-type cellulose composite membrane includes the following steps: 1) Add MXene nanosheets to a hexadecyltrimethylammonium chloride solution and stir thoroughly to obtain a modified MXene dispersion; the hexadecyltrimethylammonium chloride self-assembles and intercalates into the negatively charged MXene interlayer through electrostatic interaction; 2) Thoroughly mix the modified MXene dispersion with the cellulose nanofiber suspension to obtain a mixture; 3) Add calcium chloride solution and stir thoroughly; 4) A cellulose nanofiber / MXene photothermal conversion membrane is obtained by vacuum filtration and drying. 5) A polytetrafluoroethylene membrane was grown on the cellulose nanofiber / MXene photothermal conversion membrane to obtain a Janus-type cellulose composite membrane; The concentration of the hexadecyltrimethylammonium chloride solution is 0.08~0.2 wt%; the concentration of MXene nanosheets in the modified MXene dispersion is 3~5 g / L. The concentration of the cellulose nanofiber suspension is 0.5~1.0 wt%; the volume ratio of the modified MXene dispersion to the cellulose nanofiber suspension is 1:5~1:

10. The concentration of calcium chloride is 0.5~2.5 wt%; the volume ratio of calcium chloride solution to the mixed solution is 1:37~1:

50. The Janus-type cellulose composite membrane is used for solar-powered seawater evaporation.

2. A method for preparing a Janus-type cellulose composite membrane as described in claim 1, characterized in that, Includes the following steps: 1) Add MXene nanosheets to a hexadecyltrimethylammonium chloride solution and stir thoroughly to obtain a modified MXene dispersion; 2) Thoroughly mix the modified MXene dispersion with the cellulose nanofiber suspension to obtain a mixture; 3) Add calcium chloride solution and stir thoroughly; 4) A cellulose nanofiber / MXene photothermal conversion membrane is obtained by vacuum filtration and drying. 5) A polytetrafluoroethylene membrane was grown on the cellulose nanofiber / MXene photothermal conversion membrane to obtain a Janus-type cellulose composite membrane.

3. The method for preparing the Janus-type cellulose composite membrane according to claim 2, characterized in that, In step 1), the concentration of the hexadecyltrimethylammonium chloride solution is 0.08~0.2 wt%; and the concentration of MXene nanosheets in the modified MXene dispersion is 3~5 g / L.

4. The method for preparing the Janus-type cellulose composite membrane according to claim 2, characterized in that, Step 2) The concentration of the cellulose nanofiber suspension is 0.5~1.0 wt%; the volume ratio of the modified MXene dispersion to the cellulose nanofiber suspension is 1:5~1:

10.

5. The method for preparing the Janus-type cellulose composite membrane according to claim 2, characterized in that, Step 3) The concentration of calcium chloride is 0.5~2.5 wt%; the volume ratio of calcium chloride solution to mixed solution is 1:37~1:

50.

6. The method for preparing the Janus-type cellulose composite membrane according to claim 2, characterized in that, Step 4) Drying methods include natural air drying and drying at 30~60℃.

7. The method for preparing the Janus-type cellulose composite membrane according to claim 2, characterized in that, Step 5) Grow polytetrafluoroethylene film on cellulose nanofiber / MXene photothermal conversion film using electron beam evaporation equipment or magnetron sputtering equipment.

8. An application of the Janus-type cellulose composite membrane according to claim 1, characterized in that, Used for solar-powered seawater evaporation.

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