A method for preparing hollow TiO2 nanospheres for PVDF films and their applications

Abstract

The application discloses a preparation method of hollow TiO2 nanospheres for PVDF films and application of the hollow TiO2 nanospheres, and relates to the technical field of energy storage film capacitors. The preparation method of the hollow TiO2 nanospheres for PVDF films comprises the following steps: (1) SiO2 nanospheres are added into ammonia water and stirred uniformly, and then tetrabutyl titanate is added dropwise; after reaction, the obtained product is centrifuged, washed and dried to obtain TiO2 coated SiO2; (2) the TiO2 coated SiO2 is added into a hot strong alkali solution, and after reaction, the product is centrifuged, washed, dried and calcined to obtain the hollow TiO2 nanospheres. The hollow TiO2 nanospheres have a very thin wall thickness, a very high specific surface area and a porous characteristic, so that PVDF molecules can enter the inside of the nanospheres, the long-range ordered structure of the PVDF is effectively limited, and the PVDF film has high energy storage density and high energy storage efficiency.

Description

Technical Field

[0001] This invention relates to the technical field of energy storage thin-film capacitors, and more particularly to a method for preparing hollow TiO2 nanospheres for PVDF thin films and their application. Background Technology

[0002] Polymer-based film capacitors offer advantages such as ultra-high power density, long cycle life, and lightweight design, making them suitable as energy storage devices. They can provide instantaneous energy to meet peak current demands of devices, and also store energy during device hibernation or standby modes, extending battery life. They have broad application prospects in electric vehicles, clean energy systems, and medical devices. However, the energy density of currently widely used biaxially oriented polypropylene (BOPP) film energy storage media remains relatively low (<3 J / cm²). 3 Thin-film dielectrics suitable for future high-energy-density storage and long-term service remain scarce. Developing high-energy-density and high-efficiency electrostatic dielectrics is crucial for reducing the size and weight of energy storage devices. Polyvinylidene fluoride (PVDF) is a polar polymer with excellent polarization properties, capable of generating polarization effects in an electric field, and possesses extremely high energy density (>10 J / cm²). 3 It has become one of the most promising candidates for the next generation of energy storage films.

[0003] While PVDF ferroelectric energy storage films exhibit superior energy density, they also suffer from several drawbacks, with low charge-discharge efficiency (energy storage efficiency) being a key challenge limiting their applications. Under an electric field, PVDF undergoes a ferroelectric phase transition, resulting in significant energy loss and reduced energy storage efficiency. Furthermore, prolonged charge-discharge cycles lead to fatigue and degradation, further degrading performance and severely limiting their lifespan. Therefore, as an emerging energy storage technology, PVDF ferroelectric energy storage films urgently need to address these shortcomings to enhance their application potential in the energy storage field. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for preparing hollow TiO2 nanospheres for PVDF thin films and their applications. The hollow TiO2 nanospheres prepared by this invention have very thin walls and extremely high specific surface area, while also possessing porous characteristics, allowing PVDF molecules to enter the interior of the nanospheres. This effectively restricts the long-range ordered structure of PVDF, resulting in a relaxor ferroelectric material with an irregular electric dipole moment distribution, thereby enabling the PVDF-based thin film to have high energy density and high energy storage efficiency.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] In a first aspect, the present invention provides a method for preparing hollow TiO2 nanospheres for PVDF films, comprising the following steps:

[0007] (1) Add SiO2 nanospheres to ammonia water and stir evenly, then add titanate dropwise, controlling the rate of titanate dropwise addition to 1.5-4.6 mL / h. After the reaction, centrifuge, wash and dry the product to obtain TiO2 coated SiO2. The mass ratio of SiO2 nanospheres, ammonia water and titanate is 1:(3-52):(3.3-50).

[0008] (2) TiO2 coated SiO2 is uniformly dispersed in a hot strong alkali solution or hydrofluoric acid for etching. After constant temperature reaction, the resulting product is centrifuged, washed, dried, and finally calcined under an inert atmosphere to obtain hollow TiO2 nanospheres.

[0009] This invention achieves uniform coating of SiO2 nanospheres with TiO2 by controlling the ratio of SiO2 nanospheres, ammonia, and titanate, as well as the rate of titanate addition. By controlling the concentration of the hot strong alkali solution or hydrofluoric acid solution, the SiO2 nanospheres are completely etched away, resulting in hollow TiO2 nanospheres with very thin walls and extremely high specific surface area, while also possessing porous characteristics. This allows PVDF molecules to enter the interior of the nanospheres, effectively restricting the long-range ordered structure of PVDF.

[0010] In the preparation of TiO2-coated SiO2, if too much ammonia is used, the water content in the solution increases, leading to a vigorous reaction between the titanate ester and water. After hydrolysis, the titanate ester itself forms titanium dioxide nanospheres, which are difficult to coat with SiO2 nanospheres. If the SiO2 nanosphere content is too high, SiO2 tends to clump together and is difficult to disperse; if the SiO2 nanosphere content is too low, the titanate ester itself forms spheres, making it difficult to form TiO2 coatings on SiO2. If the titanate ester is added too quickly, it easily forms TiO2 nanospheres, making it difficult to coat SiO2 nanospheres; if the titanate ester is added too slowly, the reaction time is too long. Therefore, this invention, by controlling the ratio of SiO2 nanospheres, ammonia, and titanate ester, as well as the rate of titanate ester addition, facilitates the uniform coating of TiO2 onto the surface of SiO2 nanospheres.

[0011] Preferably, in step (1), the titanate is at least one of dibutyl titanate, tributyl titanate, and tetrabutyl titanate, and is preferably tetrabutyl titanate.

[0012] Preferably, in step (1), the centrifugation speed is 4000-8000 rpm and the centrifugation time is 5-8 min.

[0013] Preferably, the drying temperature in step (1) is 30-50℃.

[0014] Preferably, in step (1), the particle size of TiO2 coated on the surface of SiO2 nanospheres is 90-150 nm.

[0015] Preferably, in step (2), the concentration of the hot strong alkali solution is 1-10 mol / L, and the concentration of the hydrofluoric acid is 8.5-55%.

[0016] When TiO2-coated SiO2 is dispersed in a hot strong alkali solution or hydrofluoric acid, if the concentration of the solution is too high, the etching rate is too fast, which can easily lead to significant damage to the hollow structure of the TiO2. Conversely, if the concentration is too low, the etching rate is too slow. Therefore, this invention controls the concentration of the solution within the aforementioned range, ensuring that the SiO2 nanospheres are completely etched away without easily damaging the hollow structure of the TiO2 nanospheres.

[0017] Preferably, the temperature of the hot strong alkali solution in step (2) is 55-85℃.

[0018] More preferably, the strong alkaline solution in step (2) is a sodium hydroxide solution or an ammonium hydroxide solution.

[0019] Preferably, the synthesis method of the SiO2 nanospheres is as follows: first, anhydrous ethanol, deionized water and ammonia are mixed evenly to obtain a mixed solution; then, tetraethyl orthosilicate is added to the mixed solution and stirred evenly; finally, the obtained product is centrifuged, washed and dried to obtain SiO2 nanospheres.

[0020] Preferably, the volume ratio of anhydrous ethanol, deionized water, ammonia, and tetraethyl orthosilicate is 3:1:(0.05-0.5):(0.035-0.25).

[0021] Preferably, the concentration of the ammonia water is 10-40%.

[0022] Preferably, in step (2), the centrifugation speed is 4000-8000 rpm and the centrifugation time is 5-8 min.

[0023] Preferably, the drying temperature in step (2) is 60-80℃.

[0024] Preferably, the calcination temperature in step (2) is 240-650℃ and the calcination time is 3-4h.

[0025] Preferably, the hollow TiO2 nanospheres in step (2) have a particle size of 100-200 nm and a specific surface area of ​​550-1050 m². 2 g -1 .

[0026] Secondly, the present invention also provides a method for preparing a PVDF thin film, comprising the following steps:

[0027] (1) Hollow TiO2 nanospheres and PVDF are added to an organic solvent and mixed evenly to obtain a mixed solution of hollow TiO2 nanospheres, wherein the mass ratio of hollow TiO2 nanospheres to PVDF is (0.01-0.15):1;

[0028] (2) The mixed solution is then poured onto the substrate, dried at a constant temperature and then dried in a vacuum. Finally, the film is peeled off the substrate to obtain the PVDF film.

[0029] This invention addresses the large crystal domains formed during the natural growth of PVDF films by introducing hollow porous nanospheres to disrupt the long-range ordered structure during the growth process. This prevents the formation of large crystal domains with irregular electric dipole moment distribution in the film during natural growth, resulting in a significant improvement in the energy storage efficiency of PVDF.

[0030] If the doping concentration of hollow TiO2 nanospheres is too low, it cannot achieve a good modification effect; if the doping concentration is too high, it is easy to form discharge channels, leading to a decrease in the insulation performance of the PVDF film. Therefore, this invention controls the mass ratio of hollow TiO2 nanospheres to PVDF, which helps the hollow TiO2 nanospheres to better block the formation of large crystal domains, resulting in short-range ordered regions in the PVDF, thereby obtaining a PVDF film with high energy density and high energy storage efficiency.

[0031] Preferably, the organic solvent in step (1) is a mixture of DMF and acetone in a mass ratio of 1:1.

[0032] Preferably, the constant temperature drying temperature in step (2) is 35-95℃, and the drying time is 0.5-4h.

[0033] Constant temperature drying is used to evaporate excess solvent. If the temperature is too high, the evaporation rate will be too fast, which will form pores in the film and affect the performance of the film. Controlling the drying temperature during the film growth process is beneficial to improving the performance of PVDF energy storage films.

[0034] More preferably, the vacuum drying temperature in step (2) is 130-145°C and the drying time is 6.5-24h.

[0035] Since the boiling point of DMF is around 150℃, the temperature of vacuum drying needs to be controlled to be close to the boiling point of DMF in order to completely remove excess solvent. At this point, the film has already been formed, and the performance of the film will not be affected by the increase in temperature.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] (1) By controlling the ratio of SiO2 nanospheres, ammonia, and titanate, as well as the rate of titanate addition, this invention enables TiO2 to coat SiO2. By controlling the concentration of hot strong alkali solution or hydrofluoric acid solution, the SiO2 nanospheres are completely etched away, resulting in hollow TiO2 nanospheres with very thin walls and extremely high specific surface area, as well as porous characteristics. This allows PVDF molecules to enter the interior of the nanospheres, effectively restricting the long-range ordered structure of PVDF.

[0038] (2) This invention introduces hollow nanospheres as a domain wall to hinder the formation of large crystal domains during the growth of PVDF, thereby obtaining a relaxor ferroelectric material with an irregular electric dipole moment distribution, and thus a PVDF energy storage film with high energy storage density and high energy storage efficiency. Attached Figure Description

[0039] Figure 1 This is a scanning electron microscope image of the hollow TiO2 nanospheres described in this invention.

[0040] Figure 2 This is a scanning electron microscope image of the pure PVDF thin film described in this invention.

[0041] Figure 3 This is a scanning electron microscope image of the hollow TiO2 nanosphere modified PVDF film described in this invention. Detailed Implementation

[0042] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments, but the scope of protection and implementation of the present invention are not limited thereto.

[0043] Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0044] Example 1

[0045] A method for preparing hollow TiO2 nanospheres for PVDF films includes the following steps:

[0046] (1) Preparation of SiO2 nanospheres: First, add 300 mL of aqueous ethanol, 100 mL of deionized water and 12.5 mL of ammonia water to a 500 mL beaker and stir to mix them evenly to obtain a mixed solution with an ammonia water concentration of 28%; then add 12.5 mL of tetraethyl orthosilicate to the mixed solution and stir for 10 h. Finally, collect the white precipitate by centrifugation, and then wash it with deionized water and anhydrous ethanol until it is neutral. Finally, dry it in vacuum at 60 °C to obtain spherical SiO2 nanospheres.

[0047] (2) Preparation of TiO2-coated SiO2: 100 mg of SiO2 nanospheres were dispersed in 100 mL of anhydrous ethanol and sonicated for 1 h to obtain a SiO2 nanosphere suspension with a concentration of 1 mg / mL. Then, 1 mL of ammonia water was added and stirred for 30 min. The mass concentration of ammonia water was 28%. Subsequently, 3.8 mL of tetrabutyl titanate was added dropwise at a rate of 1.5 mL / h and stirred for 12 h. After the reaction, the product was centrifuged, washed, and dried in a vacuum oven at 80 °C for 12 h to obtain TiO2-coated SiO2.

[0048] (3) Preparation of hollow TiO2 nanospheres: TiO2 coated SiO2 was ultrasonically dispersed in 8.5% hydrofluoric acid for etching, and the reaction was carried out in an oil bath at 55°C for 8 hours. The resulting product was then centrifuged, washed, and dried at 60°C. Finally, it was calcined in a tube furnace under an argon atmosphere at a temperature of 350°C, a heating rate of 3°C / min, and a calcination time of 3 hours to obtain hollow TiO2 nanospheres.

[0049] Figure 1 Here is a scanning electron microscope image of hollow TiO2 nanospheres, from... Figure 1 It is known that the hollow TiO2 nanospheres described in this invention have very thin walls and extremely high specific surface area.

[0050] A method for preparing a PVDF thin film includes the following steps:

[0051] (1) Dissolve 1g of PVDF powder in 20mL of DMF solvent and stir for 2h to obtain a uniform and transparent solution A. The concentration of solution A is 50mg / mL.

[0052] (2) Disperse 80 mg of hollow TiO2 nanospheres in a mixed solvent of DMF and acetone at a volume ratio of 1:1, with a volume of 20 mL. Sonicate for 2 h and stir for 0.5 h to obtain a uniform suspension B.

[0053] (3) Add 200 μL of suspension B to solution A and mix evenly. Sonicate for 2 h and stir for 6 h to obtain a mixed solution of hollow TiO2 nanospheres. The mass ratio of hollow TiO2 nanospheres to PVDF is 0.08:1. Then sonicate the mixed solution of hollow TiO2 nanospheres for 2 h and stir for 12 h. Then pour the mixed solution onto a flat quartz glass plate and dry it in a constant temperature oven at 40 °C for 3 h. Then transfer the quartz glass plate to a vacuum oven at 120 °C and dry it for 8 h to completely evaporate the solvent. Finally, peel the film off the substrate to obtain the PVDF film.

[0054] Example 2

[0055] A method for preparing hollow TiO2 nanospheres for PVDF films includes the following steps:

[0056] (1) Preparation of SiO2 nanospheres: First, add 300 mL of aqueous ethanol, 100 mL of deionized water and 12.5 mL of ammonia water to a 500 mL beaker and stir to mix them evenly to obtain a mixed solution with an ammonia water concentration of 28%; then add 12.5 mL of tetraethyl orthosilicate to the mixed solution and stir for 10 h. Finally, collect the white precipitate by centrifugation, and then wash it with deionized water and anhydrous ethanol until it is neutral. Finally, dry it in vacuum at 60 °C to obtain spherical SiO2 nanospheres.

[0057] (2) Preparation of TiO2-coated SiO2: 1 g of SiO2 nanospheres were dispersed in 100 mL of anhydrous ethanol and sonicated for 1 h to obtain a SiO2 nanosphere suspension with a concentration of 10 mg / mL; then 1 mL of ammonia water was added and stirred for 30 min with a mass concentration of 28%; then 3.8 mL of tetrabutyl titanate was added dropwise at a rate of 4.6 mL / h and stirred for 12 h. After the reaction, the product was centrifuged, washed, and dried in a vacuum oven at 80 °C for 12 h to obtain TiO2-coated SiO2.

[0058] (3) Preparation of hollow TiO2 nanospheres: TiO2 coated SiO2 was ultrasonically dispersed in 55% hydrofluoric acid for etching, and then heated in an oil bath at 85°C for 3 hours. The resulting product was then centrifuged, washed, and dried at 80°C. Finally, it was calcined in a tube furnace under an argon atmosphere at a temperature of 350°C, a heating rate of 3°C / min, and a calcination time of 3 hours to obtain hollow TiO2 nanospheres.

[0059] A method for preparing a PVDF thin film includes the following steps:

[0060] (1) Dissolve 1g of PVDF powder in 20mL of DMF solvent and stir for 2h to obtain a uniform and transparent solution A. The concentration of solution A is 50mg / mL.

[0061] (2) Disperse 80 mg of hollow TiO2 nanospheres in a mixed solvent of DMF and acetone at a volume ratio of 1:1, with a volume of 20 mL. Sonicate for 2 h and stir for 0.5 h to obtain a uniform suspension B.

[0062] (3) Add 200 μL of suspension B to solution A and mix evenly. Sonicate for 2 h and stir for 6 h to obtain a mixed solution of hollow TiO2 nanospheres. The mass ratio of hollow TiO2 nanospheres to PVDF is 0.08:1. Then sonicate the mixed solution of hollow TiO2 nanospheres for 2 h and stir for 12 h. Then pour the mixed solution onto a flat quartz glass plate and dry it in a constant temperature oven at 40 °C for 3 h. Then transfer the quartz glass plate to a vacuum oven at 120 °C and dry it for 8 h to completely evaporate the solvent. Finally, peel the film off the substrate to obtain the PVDF film.

[0063] Example 3

[0064] The difference from Example 1 is that in the preparation of hollow TiO2 nanospheres, the rate of adding tetrabutyl titanate in step (1) is controlled at 3 mL / h, while the other steps are the same as in Example 1.

[0065] Example 4

[0066] The difference from Example 1 is that in the preparation process of hollow TiO2 nanospheres, in step (2), TiO2 coated SiO2 is ultrasonically dispersed in a hot sodium hydroxide solution with a concentration of 10 mol / L for etching. The temperature of the hot sodium hydroxide solution is 85°C. All other steps are the same as in Example 1.

[0067] Comparative Example 1

[0068] The difference from Example 1 is that in the preparation process of PVDF film, in step (3), SiO2 nanospheres of equal mass are used instead of hollow TiO2 nanospheres to react with PVDF powder to prepare PVDF film, and the other steps are the same as in Example 1.

[0069] Comparative Example 2

[0070] The difference from Example 1 is that in the preparation process of PVDF film, in step (3), TiO2 is coated on the surface of SiO2 nanospheres of equal mass instead of hollow TiO2 nanospheres to react with PVDF powder to prepare PVDF film. All other steps are the same as in Example 1.

[0071] Comparative Example 3

[0072] The difference from Example 1 is that in the preparation process of PVDF film, in step (3), 20 μL of suspension B is added to solution A and mixed evenly, ultrasonicated for 2 h, and stirred for 6 h to obtain a mixed solution of hollow TiO2 nanospheres. The mass ratio of hollow TiO2 nanospheres to PVDF is 0.008:1. All other steps are the same as in Example 1.

[0073] Comparative Example 4

[0074] The difference from Example 1 is that in the preparation process of PVDF film, in step (3), 2000uL of suspension B is added to solution A and mixed evenly, ultrasonicated for 2h, and stirred for 6h to obtain a mixed solution of hollow TiO2 nanospheres. The mass ratio of hollow TiO2 nanospheres to PVDF is 0.8:1. All other steps are the same as in Example 1.

[0075] Comparative Example 5

[0076] The difference from Example 1 is that in the preparation of hollow TiO2 nanospheres, the rate of adding tetrabutyl titanate in step (2) is controlled to be 0.5 mL / h, while the other steps are the same as in Example 1.

[0077] Comparative Example 6

[0078] The difference from Example 1 is that in the preparation of hollow TiO2 nanospheres, the rate of adding tetrabutyl titanate in step (2) is controlled at 5.5 mL / h, while the other steps are the same as in Example 1.

[0079] Comparative Example 7

[0080] The difference from Example 1 is that in the preparation of hollow TiO2 nanospheres, the concentration of hydrofluoric acid in step (3) is 5%, while the other steps are the same as in Example 1.

[0081] Comparative Example 8

[0082] The difference from Example 1 is that in the preparation of hollow TiO2 nanospheres, the concentration of hydrofluoric acid in step (3) is 65%, while the other steps are the same as in Example 1.

[0083] experiment

[0084] After depositing circular electrodes on the thin film, the unipolar PE curves of the PVDF thin films prepared in the above examples and comparative examples were tested using a Sawyer-Tower circuit to characterize the energy storage density and energy storage efficiency of the PVDF thin films. The energy storage density of the pure PVDF thin film was 15.2 J / cm². 3 The energy storage efficiency is 50.4%.

[0085] The results are shown in Table 1.

[0086] Table 1

[0087] Example 1 25.3 95.4 Example 2 24.6 93.3 Example 3 22.3 92.7 Example 4 20.8 89.1 Comparative Example 1 16.2 50.6 Comparative Example 2 19.0 54.7 Comparative Example 3 14.2 52.8 Comparative Example 4 8.9 70.0 Comparative Example 5 14.5 60.6 Comparative Example 6 19.2 65.9 Comparative Example 7 16.6 56.3 Comparative Example 8 17.2 60.1

[0088] As shown in Table 1, the PVDF film modified with hollow nanospheres in this invention exhibits an energy storage efficiency of over 90%, significantly higher than the 50% of pure PVDF films. Figure 2 and Figure 3 Scanning electron microscopy images show that pure PVDF films have large grains of 2-3 μm, while PVDF films doped with hollow nanospheres have significantly smaller grains and their spherical structure is disrupted. This indicates that the introduction of hollow nanospheres effectively suppresses the long-range ordered structure of PVDF during growth, introduces more interfaces, and makes the electric dipole moment distribution relatively complex and irregular. This causes PVDF to tend to transform from a ferroelectric to a relaxor ferroelectric, thereby improving the charge-discharge efficiency and discharge energy density of the PVDF film.

[0089] In this invention, in Comparative Examples 1-2, SiO2 nanospheres or TiO2-coated SiO2 were used instead of hollow TiO2 nanospheres. The energy storage density and efficiency of the PVDF films were both lower than in Example 1, indicating that using hollow TiO2 nanospheres as filler to modify PVDF is beneficial for improving the charge-discharge efficiency of the PVDF film. In Comparative Examples 3-4, the doping amount of the hollow TiO2 nanospheres was too high or too low, resulting in lower energy storage density and efficiency of the PVDF films compared to Example 1. This indicates that the doping amount of the hollow TiO2 nanospheres also affects the charge-discharge efficiency of the PVDF film. In Comparative Example 6, the addition rate of tetrabutyl titanate was too fast, making it easy for the titanate ester to form TiO2 nanospheres, which were not easily coated with SiO2 nanospheres. In Comparative Example 8, the concentration of hydrofluoric acid was too high, resulting in a fast etching rate that easily led to significant damage to the hollow TiO2 structure. Therefore, by controlling the rate of titanate addition, the present invention enables TiO2 to uniformly coat SiO2 nanospheres, and by controlling the concentration of hydrofluoric acid solution, the SiO2 nanospheres are completely etched away, resulting in hollow TiO2 nanospheres with very thin walls and extremely high specific surface area, while also possessing porous characteristics, allowing PVDF molecules to enter the interior of the nanospheres and effectively restricting the long-range ordered structure of PVDF.

[0090] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a PVDF thin film, characterized in that, Includes the following steps: (1) Add SiO2 nanospheres to ammonia water and stir evenly, then add tetrabutyl titanate dropwise, controlling the rate of addition of tetrabutyl titanate to be 1.5-4.6 mL / h. After the reaction, centrifuge, wash and dry the product to obtain TiO2 coated SiO2. The mass ratio of SiO2 nanospheres, ammonia water and tetrabutyl titanate is 1:(3-52):(3.3-50). (2) TiO2 coated SiO2 is uniformly dispersed in 8.5%-55% hydrofluoric acid solution for etching. After constant temperature reaction, the obtained product is centrifuged, washed and dried, and finally calcined under an inert atmosphere to obtain hollow TiO2 nanospheres. The hollow TiO2 nanospheres have a particle size of 100-200 nm and a specific surface area of ​​550-1050 m². 2 g -1 ; (3) The hollow TiO2 nanospheres and PVDF prepared in step (2) are added to an organic solvent and mixed evenly to obtain a mixed solution of hollow TiO2 nanospheres. The mass ratio of hollow TiO2 nanospheres to PVDF is (0.01-0.15):

1. (4) The mixed solution is then poured onto the substrate, dried at a constant temperature and then dried in a vacuum. Finally, the film is peeled off the substrate to obtain the PVDF film.

2. The preparation method according to claim 1, characterized in that, The constant temperature drying temperature in step (4) is 35-95℃, and the drying time is 0.5-4h.

3. The method for preparing PVDF thin film as described in claim 1, characterized in that, The vacuum drying temperature in step (4) is 130-145℃, and the drying time is 6.5-24h.

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