Preparation Method of ZnO / Lignin Two-Dimensional Nanosheet Composite Material and Its Application in Ultraviolet Shielding

Abstract

The present invention relates to the technical field of ultraviolet shielding materials, specifically to a preparation method and application of ZnO / lignin two-dimensional nanosheet composites. First, the lignin deionized water dispersion is first frozen with liquid nitrogen and then freeze-dried under vacuum to obtain flaky lignin; then it is dispersed in deionized water and mixed with a zinc-containing inorganic salt solution, stirred and dispersed at room temperature, the pH value is adjusted to 9-12, the solid is separated and washed to neutrality, and finally dried under vacuum to obtain zinc oxide / lignin two-dimensional nanosheet composites. The present invention introduces flaky lignin and simultaneously induces the formation of two-dimensional zinc oxide. On the one hand, the uniform dispersion of zinc oxide and the combination of lignin significantly increase the solid-solid interface area, solving the problem of nano-toxicity and improving the ultraviolet absorption performance. On the other hand, the carboxyl / hydroxyl groups contained in lignin form hydrogen bonds with zinc oxide, which has stability, and at the same time scavenges the free radicals generated by zinc oxide, reducing cytotoxicity.

Description

Technical Field

[0001] The present invention relates to the technical field of ultraviolet shielding materials, and particularly relates to a preparation method and application of a zinc oxide / lignin two-dimensional nanosheet composite material. Background Art

[0002] In recent years, the amount of ultraviolet radiation has increased with the destruction of the ozone layer, resulting in a variety of harmful effects, including even skin cancer. To protect the body from excessive ultraviolet radiation, ultraviolet (UV) absorbing (shielding) agents have been widely used in sunscreens. However, the photoinstability, toxicity, and damage to the marine ecosystem of most artificial sunscreen ingredients have hindered their efficacy and safety. Adding or introducing natural ingredients into sunscreens to reduce cell damage has become a research direction in the field of anti-ultraviolet.

[0003] As an amphoteric oxide, nano-zinc oxide is a new type of semiconductor functional material with a wide bandgap energy of 3.37 eV. When irradiated with ultraviolet light, electrons in the valence band absorb ultraviolet light and are excited to the conduction band, generating electron-hole pairs, thereby absorbing ultraviolet light. Zinc oxide is widely studied because of its simple manufacturing process, non-toxic and odorless nature, high chemical / physical stability, and low cost. In addition, the high photocatalytic activity of zinc oxide may generate free radicals and reactive oxygen species (ROS), accelerating human skin aging. The morphology of nano-ZnO has a significant impact on its photocatalytic performance. Among them, zinc oxide nanoparticles are prone to aggregation, which will limit the ultraviolet shielding performance. At the same time, the relatively low transmittance of these nanoparticles in the visible light region results in an unnatural white appearance on the skin. Compared with ZnO particles, ZnO nanosheets have a high surface-to-volume ratio, uniform substance distribution, stable properties, and retain semiconductor characteristics, a short photogenerated carrier migration path, and effectively prevent the recombination of e- / h+.

[0004] Lignin (AL) is the second most abundant renewable resource in the world. It is a heterogeneous aromatic polymer mainly composed of phenylpropane units (guaiacyl, syringyl, and p-hydroxyphenyl units) and various functional groups. Lignin shows antioxidant activity because the presence of phenolic pharmacophores can scavenge reactive free radical species and form highly stable intermediate forms. The phenolic compounds in lignin play an important role in the antibacterial properties of lignin, especially its side chain structure and functional groups. At the same time, lignin itself contains a large number of phenols, ketones, and intramolecular hydrogen bonds, and has the ability to absorb ultraviolet light. In addition, lignins from various sources have been proven to be safe, and lignin has no cytotoxicity to the human body. Lignin is freeze-dried to form a flaky morphology, which significantly increases the interfacial area, improves the dispersibility, enhances the antioxidant effect of lignin, and slows down ultraviolet induction.

[0005] Chinese patent CN109468845A uses alkali lignin and 3-chloro-2-hydroxypropyltrimethylammonium chloride solution to mix to obtain lignin quaternary ammonium salt, and then adds zinc nitrate and hexamethylenetetramine to form anti-ultraviolet particles, which improves stability and specific surface area. However, a large amount of toxic organic solvents are used in the preparation process. Chinese patent CN113332170A obtains water-soluble lignin by adding lysine and glyoxal to enzymatic lignin, and composites it with titanium dioxide with chitosan, and freeze-dries it to obtain lignin / TiO2 nanoparticle sunscreen, but the production process is complicated and involves toxic and polluting organic solvents. According to the above patents, the green, sustainable and pollution-free production process of lignin zinc oxide anti-ultraviolet composite materials needs to be improved. Summary of the invention

[0006] In order to reduce the harm caused by organic reagents and inorganic sunscreen nanoparticles to the human body and nature, the present invention provides a method for preparing a zinc oxide / lignin two-dimensional nanosheet composite material, and the prepared composite material can be applied to anti-ultraviolet fields such as sunscreen, film and coating.

[0007] In order to achieve the purpose of the present invention, the following technical scheme is adopted: a zinc oxide / lignin two-dimensional nanosheet composite material is prepared by the following steps:

[0008] (1) Lignin is mixed with deionized water at a solid-liquid mass ratio of 1:10 to 40, fully dispersed (generally by ultrasonic dispersion for about 20 minutes), frozen with liquid nitrogen, and then freeze-dried in a vacuum at -35 to -55°C (generally for about 3 days) to obtain flaky lignin.

[0009] (2) Weigh the flaky lignin obtained in step (1), disperse it in deionized water, and then mix it with a zinc-containing inorganic salt solution, wherein the concentration of the zinc-containing inorganic salt solution is 1 mol / L, and the mass ratio of the flaky lignin to the total mass of the zinc-containing inorganic salt solution and deionized water is 1:34-126, and disperse it by stirring at room temperature, and then adjust the pH value to 9-12 (sodium hydroxide solution and / or potassium hydroxide solution can be added for pH adjustment, and the concentration is preferably 0.5 mol / L), and stir the reaction to fully convert the zinc inorganic salt into Zn(OH)2 or Zn(OH)4 2- (Preferably stirring at 30°C for 2 hours), centrifuging to separate the solid, washing with distilled water and ethanol until neutral, and vacuum drying (preferably drying at 90°C for 4 hours) to obtain a zinc oxide / lignin two-dimensional nanosheet composite material.

[0010] In the present invention, ZnO is in-situ grown on the surface of freeze-dried flaky lignin through an alkalized zinc solution. On the one hand, the growth of ZnO is restricted. The hydroxyl and carboxyl groups in lignin can form highly dense hydrogen bonds with the surface of zinc oxide. The high specific surface area of the flaky lignin will provide a larger interfacial area, effectively solving the problems of easy agglomeration, difficult dispersion, poor compatibility and nano-toxicity of zinc oxide, and having excellent ultraviolet absorption performance. On the other hand, the significant increase in the interfacial area is conducive to improving the antioxidant effect of lignin to scavenge the free radicals induced by zinc oxide photocatalysis and ultraviolet rays, so that lignin can more effectively act as a free radical scavenger to maintain the stability of the composite material. In addition, the synergistic effect between the rich phenolic, carboxyl and hydroxyl groups in lignin and zinc oxide further improves the ultraviolet shielding ability of the composite material.

[0011] The zinc-containing inorganic salt solution described in step (2) can be any one of zinc chloride, zinc nitrate or zinc sulfate.

[0012] The zinc oxide / lignin two-dimensional nanosheet composite material prepared by the above method of the present invention forms flakes with a high specific surface area, which is conducive to the uniform dispersion of zinc oxide, reduces agglomeration, eliminates the harm of cytotoxicity to the human body, and improves the antioxidant and ultraviolet shielding abilities. At the same time, zinc oxide has a wide source and low price; lignin is widely distributed, rich in resources and low in pollution, and is a renewable green resource.

[0013] Compared with the existing technology, the advantages of the present invention are as follows:

[0014] 1. In the present invention, the oxygen-containing functional groups of lignin form chemical bonds with zinc oxide, and the existing two-dimensional structure of lignin is used to induce the growth of zinc oxide crystals, control the uniform distribution, improve the stability and bond energy of the chemical bonds. At the same time, zinc oxide and lignin cover each other to enhance the ultraviolet shielding effect.

[0015] 2. The high specific surface area of the flaky lignin will provide a larger interfacial area, effectively solving the problems of easy agglomeration, difficult dispersion, poor compatibility and nano-toxicity of zinc oxide. Better dispersion can enhance the interaction between the composite materials, and it has excellent ultraviolet absorption performance.

[0016] 3. The zinc oxide / lignin two-dimensional nanosheet composite material is an effective organic / inorganic ultraviolet protection material, which improves the antioxidant property, scavenges the free radicals induced by zinc oxide photocatalysis and ultraviolet rays, and reduces the harm of cytotoxicity to the human body.

[0017] 4. The production process of the present invention is simple. Lignin is a green renewable material, rich in resources and low in price. Description of the Drawings

[0018] Figure 1XRD patterns of the zinc oxide / lignin two-dimensional nanosheet composite prepared in Example 1 and Comparative Example 2;

[0019] Figure 2 TEM pattern of the zinc oxide / lignin two-dimensional nanosheet composite sample prepared in Example 1;

[0020] Figure 3 UV-Vis spectra of the zinc oxide / lignin two-dimensional nanosheet composites prepared in Examples 1-8 and Comparative Examples 1-3;

[0021] Figure 4 Summary of the comparison of SPF values of the zinc oxide / lignin two-dimensional nanosheet composites prepared in Examples 1-8 and Comparative Examples 1-3. Detailed Description of the Invention

[0022] The present invention will be further described in detail below with reference to examples:

[0023] Example 1

[0024] (1) Weigh 0.5 g of lignin powder, add it to 20 ml of deionized water (the solid-liquid ratio is 1:40), and ultrasonicate for 20 min. Freeze it with liquid nitrogen and vacuum freeze-dry at -35 °C for 3 days to obtain flaky lignin (ALⅠ).

[0025] (2) Disperse 0.05 g of the two-dimensional lignin obtained in step (1) into 40 ml of deionized water, ultrasonicate for 10 min, add 60 ml of a zinc chloride solution with a concentration of 1 mol / L (the solid-liquid mass ratio is 1:90, that is, the mass ratio of flaky lignin to the total mass of the zinc-containing inorganic salt solution and deionized water is 1:90, the same below), and stir magnetically for 30 min.

[0026] (3) Add 0.5 mol / L sodium hydroxide solution to the solution in step (2), adjust the pH value to 12, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and vacuum dry at 90 °C for 4 h to obtain the zinc oxide / lignin two-dimensional nanosheet (ZnO-90 / AL-12) composite.

[0027] The TEM pattern of the ZnO-90 / AL-12 composite is as shown in Figure 2 As can be seen from the figure, zinc oxide and lignin are stacked in a sheet-to-sheet manner, providing a large interfacial area, reducing aggregation, eliminating the problem of nano-toxicity, and at the same time improving the ultraviolet shielding ability.

[0028] The UV-Vis spectrum of the ZnO-90 / AL-12 composite is as shown in Figure 3As shown, the solid powder is pressed into tablets and placed in an ultraviolet-visible-near-infrared spectrophotometer tester to scan the transmittance in the range of 200-500 nm. The smaller the transmittance value, the better the shielding effect of the powder. It indicates that there is a synergistic effect between zinc oxide and lignin, and the ultraviolet shielding region becomes wider.

[0029] The SPF numerical pattern of the ZnO-90 / AL-12 composite material is as Figure 4 shown. 0.4 g of the powder is mixed evenly with 3.6 g of vaseline in an agate mortar to prepare a simulated sunscreen with a powder mass fraction of 10%. 32.5 mg of the sample to be tested is weighed on a PMMA test plate using an analytical balance and evenly applied with a gloved finger, and the sun protection index of the sample is tested with an SPF instrument. The higher the sun protection index, the better the ultraviolet shielding ability. The value in this example is 7.

[0030] Example 2

[0031] (1) Weigh 1 g of lignin powder and add it to 20 ml of deionized water (solid-liquid ratio of 1:20), and ultrasonicate for 20 min. Use liquid nitrogen for freezing and vacuum freeze-dry at -55 °C for 3 days to obtain flaky lignin (ALⅡ).

[0032] (2) Disperse 0.05 g of the two-dimensional lignin obtained in step (1) into 40 ml of deionized water, ultrasonicate for 10 min, add 60 ml of a zinc nitrate solution with a concentration of 1 mol / L (solid-liquid mass ratio of 1:126), and stir magnetically for 30 min.

[0033] (3) Add a 0.5 mol / L sodium hydroxide solution to the solution in step (2) to adjust the pH value to 11, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and vacuum dry at 90 °C for 4 h to obtain a zinc oxide / lignin two-dimensional nanosheet (ZnO-126 / AL-11) composite material.

[0034] The SPF numerical pattern of the ZnO-126 / AL-11 composite material is as Figure 4 shown, and the value is 8.

[0035] Example 3

[0036] (1) Weigh 2 g of lignin powder and add it to 20 ml of deionized water (solid-liquid ratio of 1:10), and ultrasonicate for 20 min. Use liquid nitrogen for freezing and vacuum freeze-dry at -45 °C for 3 days to obtain flaky lignin (ALⅢ).

[0037] (2) Disperse 0.05 g of the two-dimensional lignin obtained in step (1) into 40 ml of deionized water, ultrasonicate for 10 min, add 60 ml of a zinc sulfate solution with a concentration of 1 mol / L (solid-liquid ratio of 1:108), and stir magnetically for 30 min.

[0038] (3) Add 0.5 mol / L sodium hydroxide solution to the solution in step (2), adjust the pH value to 10, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and dry it in vacuum at 90 °C for 4 h to obtain zinc oxide / lignin two-dimensional nanosheet (ZnO-108 / AL-10) composite material.

[0039] The SPF numerical spectrum of ZnO-108 / AL-10 composite material is as Figure 4 shown, and the value is 8.

[0040] Example 4

[0041] (1) Weigh 0.5 g of lignin powder, add it to 20 ml of deionized water (the solid-liquid ratio is 1:40), and ultrasonicate for 20 min. Use liquid nitrogen to freeze and freeze-dry in vacuum at -35 °C for 3 days to obtain flaky lignin (ALⅠ).

[0042] (2) Disperse 0.2 g of the two-dimensional lignin obtained in step (1) into 40 ml of deionized water, ultrasonicate for 10 min, add 60 ml of zinc sulfate solution with a concentration of 1 mol / L (the solid-liquid ratio is 1:40), and stir magnetically for 30 min.

[0043] (3) Add 0.5 mol / L sodium hydroxide solution to the solution in step (2), adjust the pH value to 9, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and dry it in vacuum at 90 °C for 4 h to obtain zinc oxide / lignin two-dimensional nanosheet (ZnO-40 / AL-9) composite material.

[0044] The SPF numerical spectrum of ZnO-40 / AL-9 composite material is as Figure 4 shown, and the value is 7.

[0045] Example 5

[0046] (1) Weigh 1 g of lignin powder, add it to 20 ml of deionized water (the solid-liquid ratio is 1:20), and ultrasonicate for 20 min. Use liquid nitrogen to freeze and freeze-dry in vacuum at -55 °C for 3 days to obtain flaky lignin (ALⅡ).

[0047] (2) Disperse 0.2 g of the two-dimensional lignin obtained in step (1) into 40 ml of deionized water, ultrasonicate for 10 min, add 60 ml of zinc nitrate solution with a concentration of 1 mol / L (the solid-liquid ratio is 1:47), and stir magnetically for 30 min.

[0048] (3) Add 0.5 mol / L sodium hydroxide solution to the solution in step (2), adjust the pH value to 11, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and dry it in vacuum at 90 °C for 4 h to obtain the zinc oxide / lignin two-dimensional nanosheet (ZnO-47 / AL-11) composite material.

[0049] The SPF numerical spectrum of the ZnO-47 / AL-11 composite material is as Figure 4 shown, and the value is 7.

[0050] Example 6

[0051] (1) Weigh 2 g of lignin powder, add it to 20 ml of deionized water (solid-liquid ratio is 1:10), and ultrasonicate for 20 min. Use liquid nitrogen to freeze and freeze-dry in vacuum at -45 °C for 3 days to obtain flaky lignin (ALⅢ).

[0052] (2) Disperse 0.2 g of the two-dimensional lignin obtained in step (1) into 40 ml of deionized water, ultrasonicate for 10 min, add 60 ml of zinc chloride solution with a concentration of 1 mol / L (solid-liquid ratio is 1:34), and stir magnetically for 30 min.

[0053] (3) Add 0.5 mol / L sodium hydroxide solution to the solution in step (2), adjust the pH value = 12, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and dry it in vacuum at 90 °C for 4 h to obtain the zinc oxide / lignin two-dimensional nanosheet (ZnO-34 / AL-12) composite material.

[0054] The XRD spectrum of the ZnO-34 / AL-12 composite material is as Figure 1 shown, and the peaks of the composite material correspond to the peaks of zinc oxide in sequence, indicating the successful synthesis of the material.

[0055] The SPF numerical spectrum of the ZnO-34 / AL-12 composite material is as Figure 4 shown, and the value is 8.

[0056] Example 7

[0057] (1) Weigh 2 g of lignin powder, add it to 20 ml of deionized water (solid-liquid ratio is 1:10), and ultrasonicate for 20 min. Use liquid nitrogen to freeze and freeze-dry in vacuum at -35 °C for 3 days to obtain flaky lignin (ALⅢ).

[0058] (2) Disperse 0.2 g of the two-dimensional lignin obtained in step (1) into 40 ml of deionized water, ultrasonicate for 10 min, add 60 ml of zinc chloride solution with a concentration of 1 mol / L (solid-liquid ratio is 1:34), and stir magnetically for 30 min.

[0059] (3) Add 0.5 mol / L sodium hydroxide solution to the solution in step (2), adjust the pH value to 9, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and dry it in vacuum at 90 °C for 4 h to obtain a zinc oxide / lignin two-dimensional nanosheet (ZnO-300 / AL-9) composite material.

[0060] The SPF numerical spectrum of the ZnO-34 / AL-9 composite material is as Figure 4 shown, and the value is 7.

[0061] Example 8

[0062] (1) Weigh 1 g of lignin powder, add it to 20 ml of deionized water (the solid-liquid ratio is 1:20), and sonicate for 20 min. Freeze it with liquid nitrogen and freeze-dry it in vacuum at -55 °C for 3 days to obtain flaky lignin (ALⅠ).

[0063] (2) Disperse 0.05 g of the two-dimensional lignin obtained in step (1) into 40 ml of deionized water, sonicate for 10 min, add 60 ml of zinc chloride solution with a concentration of 1 mol / L (the solid-liquid ratio is 1:91), and stir magnetically for 30 min.

[0064] (3) Add 0.5 mol / L sodium hydroxide solution to the solution in step (2), adjust the pH value to 12, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and dry it in vacuum at 90 °C for 4 h to obtain a zinc oxide / lignin two-dimensional nanosheet (ZnO-91 / AL-12) composite material.

[0065] The SPF numerical spectrum of the ZnO-34 / AL-9 composite material is as Figure 4 shown, and the value is 7.

[0066] Comparative Example 1

[0067] (1) Weigh 2 g of lignin powder, add it to 20 ml of deionized water (the solid-liquid ratio is 1:10), sonicate for 20 min, and age for 2 days.

[0068] (2) Disperse 0.2 g of the lignin particles obtained in step (1) into 40 ml of deionized water, sonicate for 10 min, add 60 ml of zinc chloride solution with a concentration of 1 mol / L (the solid-liquid ratio is 1:34), and stir magnetically for 30 min.

[0069] (3) Add 0.5 mol / L sodium hydroxide solution to the solution in step (2), adjust the pH value to 12, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and dry it in vacuum at 90 °C for 4 h to obtain a zinc oxide / lignin (AL-ZnO) composite material.

[0070] The SPF value spectrum of the AL-ZnO material is as Figure 4 shown, and the value is 2. The composite material prepared in this comparative example was not rapidly freeze-dried with liquid nitrogen, so that lignin could not form a flaky structure, and the lignin particles were unevenly distributed on the flaky zinc oxide and easily agglomerated, resulting in weak ultraviolet shielding ability.

[0071] Comparative Example 2

[0072] (1) Weigh 2 g of lignin powder, add it to 20 ml of deionized water (the solid-liquid ratio is 1:10), ultrasonicate for 20 min, freeze it with liquid nitrogen, and vacuum freeze-dry at -55 °C for 3 days to obtain flaky lignin (ALⅢ).

[0073] (2) Disperse 0.2 g of the lignin particles obtained in step (1) into 40 ml of deionized water, ultrasonicate for 10 min, add 60 ml of zinc chloride solution with a concentration of 1 mol / L (the solid-liquid ratio is 1:34), and stir magnetically for 30 min.

[0074] (3) Add 0.5 mol / L sodium hydroxide solution to the solution in step (2), adjust the pH value = 8, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and vacuum dry at 90 °C for 4 h to obtain the zinc oxide / lignin (ZnO-34 / AL-8) composite material.

[0075] The SPF value spectrum of the ZnO-34 / AL-8 material is as Figure 4 shown, and the value is 3. For the composite material prepared in this comparative example, the pH is neutral, so that zinc oxide cannot form a flaky structure, the size of zinc oxide is too small, there is nano-toxicity, and at the same time, it is unevenly distributed on the flaky lignin and easily agglomerates.

[0076] Comparative Example 3

[0077] (1) Weigh 2 g of lignin powder, add it to 20 ml of deionized water (the solid-liquid ratio is 1:10), ultrasonicate for 20 min, freeze it with liquid nitrogen, and vacuum freeze-dry at -35 °C for 3 days to obtain flaky lignin (ALⅢ).

[0078] (2) Disperse 0.2 g of the lignin particles obtained in step (1) into 40 ml of deionized water, ultrasonicate for 10 min, add 60 ml of zinc chloride solution with a concentration of 1 mol / L (the solid-liquid ratio is 1:34), and stir magnetically for 30 min.

[0079] (3) Add 0.5 mol / L sodium hydroxide solution to the solution in step (2), adjust the pH value = 13, and stir magnetically for 2 h. Centrifuge to separate the solid, wash it with ethanol and water until neutral, and vacuum dry at 90 °C for 4 h to obtain the zinc oxide / lignin (ZnO-34 / AL-13) composite material.

[0080] The SPF numerical atlas of the ZnO-34 / AL-13 material is as Figure 4 shown, and the value is 3. For the composite material prepared in this comparative example, the pH is strongly alkaline, which causes zinc oxide not to form a flaky structure, cannot come into full contact with flaky lignin, and is prone to agglomeration.

[0081] As mentioned above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and its concept of the present invention, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.

Claims

1. A preparation method of ZnO / lignin two-dimensional nanosheet composite material, and the composite material is used for sunscreen, characterized in that: The steps include: (1) Mixing lignin and deionized water at a solid-liquid mass ratio of 1:10 to 40, fully dispersing, freezing with liquid nitrogen, and then freeze-drying in a vacuum to obtain flaky lignin; (2) Weigh the flaky lignin obtained in step (1), disperse it in deionized water, and then mix it with a zinc-containing inorganic salt solution. The concentration of the zinc-containing inorganic salt solution is 1 mol / L. The mass ratio of the flaky lignin to the total mass of the zinc-containing inorganic salt solution and deionized water is 1:34 - 126. Stir and disperse at room temperature, then adjust the pH value to 9 - 12, and stir and react to fully convert the zinc inorganic salt into Zn(OH)₂ and / or Zn(OH)₄ 2- , centrifuge to separate the solid, wash it with distilled water and ethanol until neutral, and vacuum dry to convert Zn(OH)₂ and / or Zn(OH)₄ 2- into zinc oxide to obtain a zinc oxide / lignin two-dimensional nanosheet composite material; The vacuum freeze-drying temperature in step (1) is -35 to -55°C.

2. The preparation method of the ZnO / lignin two-dimensional nanosheet composite material according to claim 1, characterized in that: In step (2), the pH is adjusted using sodium hydroxide solution and / or potassium hydroxide solution.

3. The preparation method of the ZnO / lignin two-dimensional nanosheet composite material according to claim 2, characterized in that: The concentration of the pH adjustment solution in step (2) is 0.5 mol / L.

4. The preparation method of the zinc oxide / lignin two-dimensional nanosheet composite material according to claim 1, wherein: The stirring reaction condition is 30°C for 2 hours.

5. The preparation method of the ZnO / lignin two-dimensional nanosheet composite material according to claim 1, characterized in that: The drying condition in step (2) is vacuum drying at 90°C for 4 hours.

6. The preparation method of the ZnO / lignin two-dimensional nanosheet composite material according to claim 1, wherein: In step (2), the zinc inorganic salt is any one or more of zinc chloride, zinc nitrate or zinc sulfate.

7. A ZnO / lignin two-dimensional nanosheet composite material, characterized in that: Prepared according to the preparation method described in any one of claims 1 to 6.

8. The application of the ZnO / lignin two-dimensional nanosheet composite material according to claim 7, characterized in that: Use in sunscreen.

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