CN113354018B - Solar evaporation hierarchical structure and preparation method thereof - Google Patents

Abstract

The invention discloses a solar evaporation hierarchical structure and a preparation method thereof. The light-heat conversion layer comprises a polymer film layer and a light-heat conversion layer; the polymer film layer is provided with pore channels (namely water and/or water vapor conveying channels) penetrating through the polymer film, and the upper surface and the lower surface of the polymer film layer are provided with irregular cone structures; the light-heat conversion layer is positioned on one side of the polymer film layer. The invention uses the heavy ion irradiation and chemical etching method to obtain the straight hole vertical to the film surface, which not only can be used as the water vapor escape channel of the photo-thermal conversion layer, but also can be used as the continuous water transmission channel of the upper surface evaporation. The integral structure and the hierarchical structure of the invention reduce the dependence of the evaporation system on the incident light angle, and the salt particles are not accumulated in the high-concentration salt solution (30 wt% NaCl solution) by circular evaporation, when the incident light is inclined at a certain angle, the light absorption rate is increased, the evaporation efficiency is increased, and the conditions and requirements of most of the time of the sunlight in practical application are fully met.

Description

A solar evaporation hierarchical structure and its preparation method

Technical field

The invention belongs to the technical fields of new functional membrane materials and high-salt wastewater treatment, and relates to a hierarchical structure for solar evaporation and a preparation method thereof. More specifically, it relates to a photothermal conversion evaporation structure with a rough interface and containing two different hierarchical structures. Preparation method, and its application in solar-driven high-salt wastewater treatment.

Background technique

Energy and water shortages are one of the biggest challenges currently facing humanity. Solar energy is one of the most abundant energy sources on earth. Solar evaporation technology uses solar energy to solve pressing global water problems and is considered one of the most promising green and sustainable technologies in solar energy technology. Solar evaporators have unique advantages such as energy saving, environmental protection, and high efficiency, making them of great significance in many engineering applications, such as generating steam and clean water from wastewater or seawater, industrial solid waste treatment and other basic applications.

The traditional solar evaporation method is to place the absorber at the bottom of the water source. Its photothermal conversion efficiency is low, only 30%-45%. Due to the poor absorption effect of solar energy and the large heat loss caused by placing the absorber at the bottom of the water source, limit its practical application. Later, a volume heating system was developed that dispersed light absorbers throughout the liquid reservoir, which greatly improved the light absorption effect of this type of design, but heated the entire system's liquid reservoir during the evaporation process. The heat loss will still be large. The recently developed interfacial solar evaporation system places the light absorber at the air-water interface, so that only the air-liquid interface is heated during the evaporation process, thus improving the photothermal efficiency.

In recent years, the rapid development of new photothermal materials and various photothermal evaporation structures has effectively improved the interfacial solar evaporation efficiency. At present, the main photothermal materials include plasmon absorbers for local heating of plasma, semiconductors for electron hole generation and relaxation heat generation, and carbonaceous or polymer materials based on molecular vibration. The absorption spectrum of these photothermal materials covers the complete solar spectrum, and their low cost and long-term stability satisfy current application prospects. At present, the most common typical interface solar evaporation systems mainly include two categories. One is a solar evaporation device formed by assembling multiple components such as light absorbers/photothermal materials, water delivery channels, support layers/insulation layers, etc. However, due to the water-absorbing materials The separate existence of support or insulation materials results in a lack of integrity in the design of the solar evaporator, which complicates the actual operation and limits the scope of use of the equipment. Moreover, the support/insulation materials used occupy a large area and are inconvenient to carry. Another type of interface solar evaporation system is an integrated structure solar evaporator. Its simple overall design makes it have a wider scope of application. In practical applications, the integrated structure solar evaporator has unique advantages. Some common thin film integrated evaporation structures currently known include graphene film structure, carbon nanotube film structure, porous polymer film structure, etc. However, the evaporation effects of these structures are not ideal, especially the evaporation efficiency of salt solutions is very low. Currently, various structures are designed for the evaporation of concentrated brine in many studies. The ultimate evaporation concentration of these structures for concentrated brine is 20wt%. However, in practical applications, there are many industrial wastewaters with concentrations greater than 20wt%. How to make solar evaporation structures capable of evaporating concentrated brine? Applicability to a wider and more practical application range is another major challenge for solar evaporation technology. In addition, in practical applications, the sun rises in the east and sets in the west, causing a very large difference in the incident angle of sunlight in the morning and evening. Therefore, how to reduce the dependence of the evaporation structure and device on the angle of incident light while taking into account the requirements of evaporation efficiency is a challenge for current technology. huge challenge.

In summary, it is difficult for existing solar evaporation structures and devices to be applied to a wider range and scenarios while ensuring evaporation efficiency. In addition, in real application scenarios, the sun rises in the east and sets in the west, and the incident angle of sunlight changes greatly, which will reduce the light absorption efficiency and thus the evaporation efficiency. Therefore, in view of the above existing problems, it is of great application value to develop an integrated solar evaporation structure and its preparation method that meet the above conditions.

Contents of the invention

The object of the present invention is to provide a solar evaporation hierarchical structure and a preparation method thereof.

In order to achieve the above objects, the present invention provides the following technical solutions:

A solar evaporation hierarchical structure, which includes a polymer film layer and a photothermal conversion layer; the polymer film layer has a straight hole channel (ie, a water and water vapor transport channel) penetrating the polymer film, and the polymerization Both the upper and lower surfaces of the polymer film layer have irregular conical structures; the photothermal conversion layer is located on one side of the polymer film layer.

Further, the cone-shaped structure is an irregular cone-shaped structure with a large aspect ratio.

After the first heavy ion irradiation and chemical etching, the polymer film forms straight holes perpendicular to the film surface, namely water and water vapor transport channels. The water and water vapor transport channels transport water to the upper surface for evaporation, and the lower surface is illuminated The water vapor generated by the thermal conversion layer is transported to the upper surface. The two surfaces of the polymer film with straight holes formed an irregular cone-shaped structure with a large aspect ratio after the second heavy ion irradiation and chemical over-etching. A layer of light-dissipating material was deposited on one surface using coating technology to form the structure. The photothermal conversion layer evaporates water, and the other surface uses heat conduction to evaporate surface water.

In the present invention, the polymer film can be made of, but is not limited to, polyethylene terephthalate (PET) or polycarbonate (PC). The required thickness must be enough to ensure that straight holes and irregular conical structures are obtained. Under the premise that the membrane material still has self-sustaining mechanical strength, and can only float on the liquid surface under its own buoyancy. For example, for a PET film with a irradiation dose of 1×10 ⁵ ions/cm ² for the first time and a PET film with a irradiation dose of 1×10 ⁹ ions/cm ² for the second time, the film thickness can be above 30 μm.

The material of the photothermal conversion layer can be gold, silver, copper, aluminum, palladium, cobalt, chromium, iron, indium, molybdenum, niobium, nickel, lead, platinum, tin, tantalum, vanadium, tungsten, zinc, manganese, Metals such as antimony, bismuth, and germanium can also be alloys such as nickel-chromium, nickel-iron, titanium-aluminum, etc., can also be inorganic non-metallic materials such as graphite, or can be a combination of multiple materials. The thickness of the photothermal conversion layer is not less than 50nm, and the preferred thickness is 100nm.

The invention also provides a method for preparing the above solar evaporation hierarchical structure.

The preparation method of the solar evaporation hierarchical structure provided by the invention includes the following steps:

1) Perform the first heavy ion irradiation and the first chemical etching on the polymer film in sequence;

2) The polymer treated in step 1) is then subjected to a second heavy ion irradiation and a second chemical etching in sequence;

3) Deposit photothermal conversion materials on either side of the etched polymer film to obtain the solar evaporation hierarchical structure.

In the present invention, the heavy ion irradiation can be used but is not limited to Kr, Xe, Ta, Bi plasma. The ion energy depends on the type and thickness of the membrane, and the radiation fluence depends on the type and design structure size of the membrane. The first irradiation fluence required under the conditions involved in the embodiments of the present invention is generally 1×10 ⁴ ions/cm ² -1×10 ⁸ ions/cm ² , and the second irradiation fluence is generally 1× 10 ⁸ ions/cm ² -1×10 ¹⁰ ions/cm ² , ions are injected vertically into the polymer film. A wider fluence range is also possible, but a lower irradiation fluence for the first time will reduce the final evaporation efficiency, a lower irradiation fluence for the second time will reduce the final light absorption performance, and a higher fluence will reduce the final evaporation efficiency. mechanical strength.

In the present invention, the chemical etching is performed twice. In the first etching, the etching liquid uses 5-9M NaOH aqueous solution and is etched under heating in a water bath of 45-65°C. The etching time is 30min-15h (preferably 3.5h-6h). Specifically The etching time is related to the film thickness, radiation fluence, and required structure size. During the second etching, the etching solution uses 2.5-9M NaOH solution. The solvent is a mixture of methanol and water, with a methanol volume content of 50%-95%. The etching time is 15-60 minutes at room temperature, and both sides of the film are etched simultaneously. For PET films, the preferred second etching conditions are 2.5M NaOH solution, methanol volume content 50%, and etching time 40 minutes.

The photothermal conversion layer coating can be deposited by, but not limited to, ion sputtering, vacuum evaporation, vacuum ion plating, chemical reaction deposition, electroplating, etc. The photothermal conversion layer can be deposited on either side of the polymer film. Material.

Compared with the prior art, the present invention has the following beneficial effects:

(1) The present invention is an integrated structure of light absorber/photothermal material, water absorber, support layer/heat insulation layer. The thickness of the evaporation system is on the order of microns, which is easy to float on the liquid surface and can withstand the evaporation under different conditions of the liquid surface. oscillation.

(2) The present invention utilizes heavy ion irradiation and chemical etching to obtain straight holes perpendicular to the film surface, which not only serve as escape channels for water vapor in the photothermal conversion layer, but also serve as water transmission channels for continuous evaporation on the upper surface.

(3) The hierarchical structure of the present invention has efficient absorption in the ultraviolet-visible-near-infrared band, and the average light absorption rate in the band range of 250nm-2300nm can reach a maximum of 96%.

(4) The integrated structure and hierarchical structure of the present invention reduce the dependence of the evaporation system on the angle of incident light. When the angle of incident light is tilted within a certain range, the light absorption rate increases and the evaporation efficiency increases, which fully meets the needs of large sunlight in practical applications. Part-time tilt conditions.

(5) The irregular conical structure on the upper surface of the polymer film of the present invention can quickly diffuse and infiltrate the liquid, increase the surface evaporation area, improve the evaporation efficiency, and perform ion exchange with the liquid surface through the water delivery channel. At 30wt% There is no accumulation of salt particles in the NaCl solution during cyclic evaporation for 10 days (10 hours of light exposure per day).

(6) The photothermal conversion material of the present invention can be used not only with metals, but also with non-metals such as graphite, which can meet the requirements of different application scenarios.

(7) The preparation method of the present invention is simple and can achieve large-scale preparation. The irradiation, etching, and deposition of the photothermal conversion layer of the polymer film included in this method have all been prepared on a large scale.

(8) The preparation method described in the present invention has a greater tolerance for key parameters such as radiation ion type, fluence, etching liquid ratio, etching time, photothermal conversion layer thickness, etc., thereby reducing the preparation accuracy requirements. The yield rate is guaranteed.

Description of the drawings

Figure 1 is a schematic diagram of the solar evaporation hierarchical structure of the present invention.

Figure 2 is a scanning electron microscope image of the surface and cross-section of the solar evaporation hierarchical structure in Embodiment 1 of the present invention.

Figure 3 shows the water evaporation rate of the solar evaporation hierarchical structure during the solar water evaporation process in Embodiment 2 of the present invention.

Figure 4 shows the solar water evaporation rate at different incident angles of incident light in the solar evaporation hierarchical structure in Embodiment 3 of the present invention.

Figure 5 shows the cyclic evaporation rate of the solar water evaporation hierarchical structure in 30wt% NaCl solution for 10 days (10 hours of light exposure per day, 100 hours in total) in Example 4 of the present invention.

Detailed ways

The present invention will be further described in detail below with reference to specific embodiments and corresponding drawings. The structural schematic diagram of the present invention is only to clearly show the structure of the device involved in the present invention. The arrangement of the conical structure is idealized and the cone angle is appropriately enlarged. In a real structure, the aspect ratio of the cone structure is much larger than that shown in the schematic diagram, so this schematic diagram should not be considered to strictly reflect the proportional relationship of the geometric dimensions of the device involved in the present invention. In addition, the embodiments described in this specification are only some of the embodiments of the present invention and are intended to further illustrate the content of the present invention and should not be considered to limit the specific scope of the present invention. Based on the embodiments described in this specification, other embodiments obtained by those skilled in the art without making creative efforts should all fall within the scope of the present invention.

Figure 1 is a schematic diagram of the solar evaporation hierarchical structure of the present invention. This structure includes a polymer film layer, a photothermal conversion layer and a transport channel. The specific preparation process is divided into three steps:

1) Preparation of straight holes (i.e., transport channels for water and water vapor)

The purpose of this step is to form cylindrical straight holes penetrating the polymer film in the polymer film. The first is the irradiation of the polymer film. The purpose is to form an etchable latent track in the polymer film that runs through the entire film. Therefore, any electron energy loss in the irradiated polymer film is greater than the threshold required for track etching. ions can be used for polymer film irradiation. The required ion energy is related to the thickness of the polymer membrane and the geometric size of the required straight holes. It is enough to ensure that the ions can completely penetrate the polymer membrane. The required irradiation fluence is related to the type of irradiated material and the required structural size. Then there is chemical etching. After the polymer film undergoes the above-mentioned irradiation process, etched latent tracks are generated in the film. Under the action of the etching liquid, etching will be performed along the track direction to form a cylindrical straight hole penetrating the polymer film. , forming a transport channel for water and water vapor. The size of the straight hole involved in the present invention can be appropriately adjusted according to the irradiation fluence and etching time.

2) Preparation of double-sided irregular pyramidal structure

The purpose of this step is to etch an irregular cone-shaped structure with a large aspect ratio on the surface of the polymer film. The first is a second irradiation of the polymer film with straight holes, which is the same as the first irradiation process described above. This is followed by a second chemical etching of the polymer membrane with straight holes. After the second irradiation process, etchable latent tracks are created within the membrane. Under the action of the etching liquid, the track etching rate along the track direction is much greater than the bulk etching rate parallel to the film surface, so a biconical structure with a large aspect ratio with opposite cone tips is formed in the film. As the etching progresses As further progress is made, the diameter of the cone base at the membrane surface becomes larger and larger, and eventually adjacent cone bases overlap each other, and the unetched polymer between the cone holes forms an irregular cone-like convex structure. There are various types and proportions of etching liquids. For different polymer films, the composition and proportions of etching liquids are also different. This specification cannot list them all. Any method that is not mentioned in the embodiments of this specification is used. The polymer film and etching solution, but using the irradiation and etching methods described in this specification to obtain a hierarchical structure similar to that of the present invention, are considered to be simple equivalent replacements and should be covered by the protection scope of the present invention. Regarding the etching time, just ensure that the cone bottoms overlap each other.

3) Deposition of photothermal conversion layer

The purpose of this step is to provide a photothermal conversion layer of sufficient thickness. When the photothermal conversion material is deposited on one side of the etched polymer film in step 2, it covers the entire inner wall of the cone hole and etches the remaining convex portion. When light is incident from the other side, the entire photothermal conversion layer is equivalent to a hollow cone array. There is no strict requirement for the number of all photothermal conversion materials in the photothermal conversion layer. They can be one type or multiple types, as long as the hierarchical structure is stable. In principle, there are no strict requirements on thickness, as long as it can float on the liquid surface using its own buoyancy. There are no special requirements for the deposition coating equipment used. The deposited photothermal conversion material can cover the inner wall of the cone hole and etch the remaining convex parts.

The following are some examples, intended to specifically illustrate the technical solution of the present invention.

Example 1

This embodiment discloses a method for preparing a solar evaporation hierarchical structure, specifically as follows:

1) Use Ta ions provided by a high-energy heavy ion accelerator to irradiate the PET film. The ion energy is 16 MeV/u, the irradiation flux is 6×10 ⁴ ions/cm ² , the film thickness is 38 μm, and the ions completely penetrate the film. Then put the irradiated PET film into 400ml of etching solution to ensure that both sides of the film are in full contact with the etching solution. The etching solution is 5M NaOH aqueous solution, and the etching is heated in a 50°C water bath for 6 hours.

2) Wash the etched PET film in step 1) repeatedly with deionized water several times and then dry it naturally. The second time, use the same ion beam as in step 1) for a second irradiation, with an irradiation dose of 1×10 ⁹ ions/cm ² , ions completely penetrate the film. Then put the PET film after the second irradiation into 400ml of etching solution to ensure that both sides of the film are in full contact with the etching solution. The etching solution is 2.5M NaOH, in which the solvent is a mixture of methanol and water. The volume of methanol and water The ratio is 1:1, etching at room temperature for 40 minutes.

3) Wash the etched PET film repeatedly with deionized water several times and then dry it naturally. Put it into the ion sputtering coating instrument. Select the sputtering target as a gold target, set the sputtering current to 10mA, and the coating time to 3000s. , the required hierarchical structure is obtained after sputtering.

Figure 2 is a scanning electron microscope image of the structure's photothermal conversion surface and cross section in this embodiment. As can be seen from Figure 2, the straight holes are regular in shape, evenly distributed, perpendicular to the membrane surface, and penetrate the film; the conical structures on both sides of the membrane tend to have the same size and are evenly distributed.

Example 2

This embodiment discloses a method for preparing a solar evaporation hierarchical structure, specifically as follows:

1) Use Ta ions provided by a high-energy heavy ion accelerator to irradiate the PET film. The ion energy is 16 MeV/u, the irradiation flux is 6×10 ⁴ ions/cm ² , the film thickness is 38 μm, and the ions completely penetrate the film. Then put the irradiated PET film into 400ml of etching solution to ensure that both sides of the film are in full contact with the etching solution. The etching solution is 5M NaOH solution, and the etching is heated in a 50°C water bath for 3.5 hours.

2) Wash the etched PET film in step 1) repeatedly with deionized water several times and then dry it naturally. The second time, use the same ion beam as in step 1) for a second irradiation, with an irradiation dose of 1×10 ⁹ ions/cm ² , ions completely penetrate the film. Then put the PET film after the second irradiation into 400ml of etching solution to ensure that both sides of the film are in full contact with the etching solution. The etching solution is 2.5M NaOH, in which the solvent is a mixture of methanol and water. The volume of methanol and water The ratio is 1:1, etching at room temperature for 40 minutes.

3) Wash the etched PET film repeatedly with deionized water several times and then dry it naturally. Put it into the ion sputtering coating instrument. Select the sputtering target as a gold target, set the sputtering current to 10mA, and the coating time to 3000s. , the required hierarchical structure is obtained after sputtering.

The prepared hierarchical structure was placed on deionized water and placed under a solar simulator to conduct a water evaporation efficiency test under a solar light intensity.

Figure 3 shows the water evaporation efficiency of the hierarchical structure in the solar water evaporation process in this embodiment. It can be seen from Figure 3 that the hierarchical structure under this condition can achieve an evaporation rate of more than 1.4kg m ^-2 h ^-1 under a certain solar light intensity (the incident light is perpendicular to the film surface), and maintain a stable evaporation rate.

Example 3

This embodiment discloses a method for preparing a solar evaporation hierarchical structure, specifically as follows:

1) Use Ta ions provided by a high-energy heavy ion accelerator to irradiate the PET film. The ion energy is 16 MeV/u, the irradiation flux is 6×10 ⁴ ions/cm ² , the film thickness is 38 μm, and the ions completely penetrate the film. Then put the irradiated PET film into 400ml of etching solution to ensure that both sides of the film are in full contact with the etching solution. The etching solution is 5M NaOH solution, and the etching is heated in a 50°C water bath for 3.5 hours.

2) Wash the etched PET film repeatedly with deionized water several times and then dry it naturally. Use the same ion beam as in step 1) for the second time to irradiate for the second time, with an irradiation dose of 1×10 ⁹ ions/ cm ² , the ions completely penetrate the film. Then put the PET film after the second irradiation into 400ml of etching solution to ensure that both sides of the film are in full contact with the etching solution. The etching solution is 2.5MNaOH, in which the solvent is a mixture of methanol and water, and the volume ratio of methanol to water is 1:1, etching at room temperature for 40 minutes.

3) Wash the etched PET film repeatedly with deionized water several times and then dry it naturally. Put it into the ion sputtering coating instrument. Select the titanium target as the sputtering target, set the sputtering current to 150mA, and the coating time to 1000s. ; The second time the sputtering target is selected as a gold target, the sputtering current is set to 10mA, and the coating time is 2000s; the third time the sputtering target is selected as a copper target, the sputtering current is set to 40mA, the coating time is 2000s, and the sputtering time is 2000s. After the injection is completed, the required hierarchical structure is obtained.

The prepared hierarchical structure was placed on deionized water and placed under a solar simulator with strong sunlight to test the water evaporation rate during solar water evaporation at different incident angles of incident light.

Figure 4 shows the water evaporation rate of the hierarchical structure in this embodiment during the solar water evaporation process at different incident light angles. It can be seen from Figure 4 that when the hierarchical structure tilts the incident light angle from 0 to 60°, the evaporation rate increases, and reaches the optimal evaporation rate (1.68kg m ^-2 h ^-1 at a certain angle (here 30°) ).

Example 4

This embodiment discloses a method for preparing a solar evaporation hierarchical structure, specifically as follows:

1) Use Ta ions provided by a high-energy heavy ion accelerator to irradiate the PET film. The ion energy is 16 MeV/u, the irradiation flux is 6×10 ⁴ ions/cm ² , the film thickness is 38 μm, and the ions completely penetrate the film. Then put the irradiated PET film into 400ml of etching solution to ensure that both sides of the film are in full contact with the etching solution. The etching solution is 5M NaOH solution, and the etching is heated in a 50°C water bath for 3.5 hours.

2) Wash the etched PET film repeatedly with deionized water several times and then dry it naturally. For the second time, the PET film is irradiated with Xe ions provided by a high-energy heavy ion accelerator. The ion energy is 19.5MeV/u. The radiation injection The radiation dose is 1×10 ⁹ ions/cm ² and the irradiation flux is 1×10 ⁹ ions/cm ² , and the ions completely penetrate the film. Then put the PET film after the second irradiation into 400ml of etching solution to ensure that both sides of the film are in full contact with the etching solution. The etching solution is 2.5M NaOH, in which the solvent is a mixture of methanol and water. The volume of methanol and water The ratio is 1:1, etching at room temperature for 30 minutes.

3) Wash the etched PET film repeatedly with deionized water several times and then dry it naturally. Put it into the ion sputtering coating instrument. Select the sputtering target as a carbon target, set the sputtering current to 40mA, and the coating time to 1000s. , the required hierarchical structure is obtained after sputtering.

The prepared hierarchical structure was placed in a 30wt% NaCl solution and placed under a solar simulator with strong sunlight to test the cycle evaporation efficiency for 10 days.

Figure 5 shows the cyclic evaporation efficiency of the hierarchical structure in this example in 30wt% NaCl solution for 10 days (10 hours of illumination every day, 100 hours in total). It can be seen from Figure 5 that the hierarchical structure can maintain stable cycle evaporation efficiency for 10 days in 30wt% NaCl solution.

It is worth emphasizing that the above-mentioned embodiments only elaborate the technical solutions of the present invention, and it cannot be concluded that the specific implementation of the present invention is limited to these descriptions. Any simple modification or equivalent replacement of the technical solution of the present invention without departing from the concept of the technical solution of the present invention shall be covered by the protection scope of the present invention.

Claims (5)

1. A solar evaporation hierarchical structure, which includes a polymer film layer and a photothermal conversion layer; the polymer film layer has a straight hole channel penetrating the polymer film, and the upper and lower surfaces of the polymer film layer are uniform It has an irregular conical structure; the photothermal conversion layer is located on one side of the polymer film layer; The cone-shaped structure is an irregular cone-shaped structure with a large aspect ratio; The material constituting the polymer film is polyethylene terephthalate or polycarbonate; The material constituting the photothermal conversion layer is selected from at least one of metals, alloys, and graphite; The metal is specifically selected from the group consisting of gold, silver, copper, aluminum, palladium, cobalt, chromium, iron, indium, molybdenum, niobium, nickel, lead, platinum, tin, tantalum, vanadium, tungsten, zinc, manganese, antimony, bismuth and At least one of germanium; The alloy is nickel-chromium, nickel-iron or titanium-aluminum; The effective thickness of the photothermal conversion layer is not less than 50nm; The preparation method of the solar evaporation hierarchical structure includes the following steps: 1) Perform the first heavy ion irradiation and the first chemical etching on the polymer film in sequence; 2) The polymer treated in step 1) is then subjected to a second heavy ion irradiation and a second chemical etching in sequence; 3) Deposit a photothermal conversion material on any side of the polymer film treated in step 2) to obtain the solar evaporation hierarchical structure; In the step 1) of the heavy ion irradiation step, the electron energy loss of the irradiated ions used in the irradiated polymer film is greater than the threshold required for track etching, specifically Kr, Xe, Ta or Bi; The ion energy depends on the type and thickness of the membrane, and the irradiation fluence depends on the type and design structure size of the membrane; For the first chemical etching, the etching solution uses 5-9M NaOH aqueous solution and is heated in a water bath at 45-65°C for etching, and the etching time is 30min-15h; For the second chemical etching, the etching liquid uses a 2.5-9M NaOH solution, the solvent is a mixture of methanol and water, the methanol volume content is 50%-95%, and the etching time is 15-60 minutes at room temperature. 2. The preparation method of the solar evaporation hierarchical structure according to claim 1, comprising the following steps: 1) Perform the first heavy ion irradiation and the first chemical etching on the polymer film in sequence; 2) The polymer treated in step 1) is then subjected to a second heavy ion irradiation and a second chemical etching in sequence; 3) Deposit a photothermal conversion material on any side of the polymer film treated in step 2) to obtain the solar evaporation hierarchical structure; In the step 1) of the heavy ion irradiation step, the electron energy loss of the irradiated ions used in the irradiated polymer film is greater than the threshold required for track etching, specifically Kr, Xe, Ta or Bi; The ion energy depends on the type and thickness of the membrane, and the irradiation fluence depends on the type and design structure size of the membrane; For the first chemical etching, the etching solution uses 5-9M NaOH aqueous solution and is heated in a water bath at 45-65°C for etching, and the etching time is 30min-15h; For the second chemical etching, the etching liquid uses a 2.5-9M NaOH solution, the solvent is a mixture of methanol and water, the methanol volume content is 50%-95%, and the etching time is 15-60 minutes at room temperature. 3. The preparation method according to claim 2, characterized in that: The first heavy ion irradiation dose is 1×10 ⁴ ions/cm ² -1×10 ⁸ ions/cm ² ; The second heavy ion irradiation dose is 1×10 ⁸ ions/cm ² -1×10 ¹⁰ ions/cm ² . 4. The preparation method according to any one of claims 2, characterized in that: the deposition method in step 3) is ion sputtering, vacuum evaporation, vacuum ion plating, chemical reaction deposition method or electroplating method. 5. Application of the solar evaporation hierarchical structure of claim 1 in photothermal water treatment.