Self-cleaning titanium dioxide and preparation method thereof

Abstract

The invention discloses self-cleaning titanium dioxide and a preparation method, the titanium dioxide sequentially comprises a titanium dioxide substrate, an inorganic coating layer and an organic coating layer from inside to outside; the inorganic film coating layer comprises an Al-doped hydroxyapatite film layer; the organic coating layer is obtained by performing organic modification by adopting organic acid. According to the preparation method, the Al-doped hydroxyapatite inorganic coating and the organic acid organic coating are comprehensively adopted for modification, so that the surface energy of the titanium dioxide is remarkably reduced, the contact angle is increased, and the self-cleaning capability of the titanium dioxide is improved. Meanwhile, the titanium dioxide provided by the invention also has excellent weather resistance, dispersibility and other pigment properties, and can meet the needs of self-cleaning products.

Description

Technical Field

[0001] This invention belongs to the field of titanium dioxide preparation technology, specifically relating to a self-cleaning titanium dioxide and its preparation method. Background Technology

[0002] Self-cleaning coatings are surface coatings that can automatically remove dirt and contaminants. They can reduce cleaning and maintenance costs and improve the durability and aesthetics of materials. Research on self-cleaning coatings has made some progress, especially in the fields of hydrophobic coatings, photocatalytic coatings, and antibacterial coatings. Hydrophobic coatings employ nanoscale structures or special surface chemical compositions to give them extremely high contact angles, causing water droplets to form spherical shapes on their surface, thereby carrying away contaminants along with the water droplets.

[0003] Existing technologies apply self-cleaning surfaces to various substrate surfaces in different ways. For example, PVDF solution is sprayed onto glass substrates to create self-cleaning surfaces; ternary complexes are sprayed onto substrates to form high-performance self-cleaning surfaces; a novel self-healing polyurethane coating is prepared by introducing lignin-based healing agents and dynamic disulfide bonds; a ZnO coating with self-cleaning, UV-resistant, and antibacterial adhesion properties is prepared using a sol-gel spin coating method; a photocatalytic self-cleaning coating with durability in water and long-term natural light photocatalytic effect is prepared by spraying TiO2 particles modified with 1H,2H,2H-perfluorodecyltriethoxysilane onto a semi-cured lithium silicate adhesive; and an environmentally friendly road coating with catalytic hydrophobic function is prepared using hydrophobically modified titanium dioxide as a catalyst and epoxy-optimized silicone resin as the main film-forming material.

[0004] Therefore, current research mainly focuses on how to form self-cleaning surfaces, without addressing pigment-based self-cleaning titanium dioxide. Pigment-based titanium dioxide is widely used in coatings, plastics, papermaking, and other products, making the development of pigment-based self-cleaning titanium dioxide crucial.

[0005] Compared to self-cleaning coatings, firstly, self-cleaning titanium dioxide has extremely high chemical stability, is not prone to aging, and does not experience physical wear, while coatings gradually wear down under conditions such as wind and sun exposure, and are susceptible to aging caused by ultraviolet rays and humid heat, leading to the failure of its self-cleaning function; secondly, self-cleaning titanium dioxide forms a uniform water film rather than water droplets, possessing both anti-fogging and easy-to-wash properties, while self-cleaning coatings easily cause water mist to condense into microbeads, affecting visibility; thirdly, self-cleaning titanium dioxide can be directly mixed into the formula as a functional filler, achieving product upgrades without changing the existing production process, while self-film formation requires a special formula and special construction process, resulting in higher costs and barriers to entry.

[0006] Therefore, this application aims to provide a pigment-type self-cleaning titanium dioxide and its preparation method. Summary of the Invention

[0007] The purpose of this invention is to provide a self-cleaning titanium dioxide and its preparation method to overcome the shortcomings of the prior art.

[0008] The objective of this invention is achieved through the following technical solution: A self-cleaning titanium dioxide comprises, from the inside out, a titanium dioxide substrate, an inorganic coating layer, and an organic coating layer. The inorganic coating layer includes an Al-doped hydroxyapatite film layer; The organic coating layer is obtained by organic modification with organic acids.

[0009] Preferably, both the Al-doped hydroxyapatite film and the organic coating layer are formed by a hydrothermal method.

[0010] Preferably, the Al-doped hydroxyapatite film is obtained by adding soluble calcium source, soluble phosphate source and soluble aluminum source to titanium dioxide-based material slurry and carrying out a hydrothermal reaction under pH 9.0~10.0 conditions; The hydrothermal reaction temperature is 200~300℃, and the time is 10~18h.

[0011] Preferably, the organic coating layer is formed by the following steps: The slurry coated with the Al-doped hydroxyapatite film was separated into solid and liquid phases, then re-slurried with water, and organic acid was added for hydrothermal reaction. The hydrothermal reaction temperature is 120~150℃, and the time is 2~4h.

[0012] Preferably, the organic acid is selected from polycarboxylic acids with a decomposition temperature > 150°C; Preferably, the organic acid is at least one selected from citric acid, tartaric acid, fumaric acid, and succinic acid.

[0013] Preferably, the coating amount of the Al-doped hydroxyapatite film, calculated as hydroxyapatite, is 9-20% of the mass of the titanium dioxide substrate; and the Al doping amount, calculated as alumina, is 0.2-0.8% of the mass of the titanium dioxide substrate. The coating amount of the organic coating layer, based on the mass of carboxyl groups, is 0.3 to 0.7% of the mass of the titanium dioxide substrate.

[0014] This application also provides a method for preparing the self-cleaning titanium dioxide as described above, comprising the following steps: S1. Preparation of titanium dioxide-based material slurry; S2. Al-doped hydroxyapatite coating was performed using a hydrothermal method; S3. Organic acid coating is performed using a hydrothermal method.

[0015] Preferably, step S2 further includes: Soluble calcium source, soluble phosphate source and soluble aluminum source were added to titanium dioxide-based material slurry, and a hydrothermal reaction was carried out under pH 9.0~10.0 conditions; The hydrothermal reaction temperature is 200~300℃, and the time is 10~18h; The amount of the soluble calcium source added, calculated as calcium oxide, is 5.6~11.2% of the mass of the titanium dioxide substrate; The amount of the soluble phosphoric acid source added, calculated as phosphorus oxide, is 3.4~8.5% of the mass of the titanium dioxide substrate; The amount of the soluble aluminum source added, calculated as alumina, is 0.2 to 0.8% of the mass of the titanium dioxide substrate.

[0016] Preferably, step S3 further includes: The slurry coated with Al-doped hydroxyapatite was separated into solid and liquid phases, then re-slurried with water, and organic acid was added for hydrothermal reaction. The hydrothermal reaction temperature is 120~150℃, and the time is 2~4h; The amount of organic acid added, based on the mass of carboxyl groups, is 0.3 to 0.7% of the mass of the titanium dioxide substrate.

[0017] Preferably, step S3 is followed by solid-liquid separation, drying and pulverization steps; The drying and pulverizing temperature shall not exceed 120°C.

[0018] This application comprehensively utilizes Al-doped hydroxyapatite inorganic coating and organic acid organic coating modification to significantly reduce the surface energy of titanium dioxide and increase the contact angle, thereby enhancing the self-cleaning ability of titanium dioxide. Simultaneously, the titanium dioxide of this application also possesses excellent pigment properties such as weather resistance and dispersibility, meeting the requirements of self-cleaning products. Detailed Implementation

[0019] This application provides a self-cleaning titanium dioxide, which, from the inside out, comprises a titanium dioxide substrate, an inorganic coating layer, and an organic coating layer. The inorganic coating layer includes an Al-doped hydroxyapatite film layer; The organic coating layer is obtained by organic modification with organic acids.

[0020] The present invention first coats the surface of titanium dioxide with Al-doped hydroxyapatite. The nanosphere structure of hydroxyapatite gives it a low surface energy and a large contact angle. At the same time, due to the low polarity of Al ions, they have low electronegativity and weak polarization ability, making it difficult to form polar bonds. The radius of Al ions is smaller than that of Ca ions, and doping causes lattice shrinkage, further reducing the surface energy and improving hydrophobicity, thereby increasing the self-cleaning ability.

[0021] Then, organic acid is used for organic coating modification. During the coating modification process, the carboxyl groups in the organic acid condense with the hydroxyl groups on the surface of titanium dioxide and hydroxyapatite or coordinate with calcium ions, consuming a large number of hydrophilic groups, further reducing the affinity between the surface and water, increasing the contact angle, and thus further improving the self-cleaning ability of titanium dioxide.

[0022] Therefore, this application comprehensively utilizes Al-doped hydroxyapatite inorganic coating and organic acid organic coating modification to significantly reduce the surface energy of titanium dioxide and increase the contact angle, thereby enhancing its self-cleaning ability. Simultaneously, the Al-doped hydroxyapatite coating and organic coating modification of the titanium dioxide in this application mask the photoactivation points of the titanium dioxide, improving its weather resistance. The organic molecules in the organic acid coating modification sterically separate the titanium dioxide particles, preventing particle aggregation and improving dispersibility; moreover, the organic molecules reduce the surface energy at the interface between titanium dioxide and resin, improving formulation compatibility. Therefore, the titanium dioxide in this application possesses excellent pigment properties such as weather resistance and dispersibility, meeting the requirements of self-cleaning products.

[0023] Preferably, the titanium dioxide substrate is rutile titanium dioxide. Compared with anatase titanium dioxide, rutile titanium dioxide has a higher refractive index, more stable crystal form, and better hiding power and weather resistance.

[0024] Preferably, both the Al-doped hydroxyapatite film and the organic coating layer are formed by a hydrothermal method.

[0025] The hydrothermal method essentially utilizes high-temperature, high-pressure solvothermal conditions for inorganic coating. Compared to conventional wet coating, this method produces inorganic materials with higher crystallinity, better uniformity and density, and stronger chemical bonds. For organic modification, the hydrothermal method yields organic films with stronger adhesion, less susceptibility to desorption, and more ordered and uniform self-assembly compared to wet and dry coating. The synergistic effect of the composite hybrid structure of "inorganic coating bottom layer + organic coating surface layer" prepared by the hydrothermal method results in titanium dioxide with superior dispersibility, weather resistance, and self-cleaning ability.

[0026] More preferably, the Al-doped hydroxyapatite film is obtained by adding soluble calcium source, soluble phosphate source and soluble aluminum source to titanium dioxide-based material slurry and carrying out a hydrothermal reaction under pH 9.0~10.0 conditions; The hydrothermal reaction temperature is 200~300℃, and the time is 10~18h.

[0027] Soluble calcium, soluble phosphate, and soluble aluminum sources were uniformly deposited on the surface of titanium dioxide under high-temperature hydrothermal and alkaline conditions, forming an aluminum-doped hydroxyapatite film with a complete nanosphere structure. Simultaneously, the Al source also reacted with the phosphate source and deposited uniformly within the hydroxyapatite film. During the synthesis process, Al… 3+ Due to the smaller ionic radius, Ca is substituted. 2+ The positions of these atoms create lattice vacancies, making the arrangement of surface atoms more "compact," reducing surface dangling bonds, decreasing activity, and lowering surface energy.

[0028] More preferably, the organic coating layer is formed by the following steps: The slurry coated with an Al-doped hydroxyapatite film was separated into solid and liquid phases, then re-slurried with water, and organic acid was added for hydrothermal reaction. The hydrothermal reaction temperature is 120~150℃, and the time is 2~4h. Because the decomposition temperature of organic acids is relatively low, the hydrothermal reaction temperature is lower than that of inorganic coating.

[0029] Inorganic coating introduces a significant amount of salt. If solid-liquid separation is not performed to remove the salt, it will affect the abrasion resistance, salt spray resistance, and storage stability of the application system. If organic modification with organic acids is used before washing, it will be difficult to wash and may result in the loss of loosely bound organic acids during washing. Therefore, this application first performs solid-liquid separation after inorganic coating, then re-pulperses the material before organic acid modification.

[0030] Under high temperature and high pressure hydrothermal conditions, the carboxyl groups in organic acids undergo condensation with the hydroxyl groups on the surfaces of titanium dioxide and hydroxyapatite, or coordinate with calcium ions.

[0031] Preferably, the organic acid is selected from organic acids with a decomposition temperature >150°C, to avoid the use of organic acids with lower decomposition temperatures that may decompose or volatilize during processing and storage, detaching from the surface of titanium dioxide and affecting the application effect.

[0032] More preferably, the organic acid is a polycarboxylic acid, which has a stronger binding ability with titanium dioxide and a more stable bond.

[0033] Preferably, the organic acid is at least one selected from citric acid, benzoic acid, tartaric acid, fumaric acid, and succinic acid. All of the above organic acids are polycarboxylic acids. Of course, monocarboxylic acids such as benzoic acid and lactic acid can also be used.

[0034] Preferably, the amount of Al-doped hydroxyapatite film coating, calculated as hydroxyapatite, is 9-20% of the mass of the titanium dioxide substrate; the Al doping amount, calculated as alumina, is 0.2-0.8% of the mass of the titanium dioxide substrate. The coating amount of the organic coating layer, based on the mass of carboxyl groups, is 0.3 to 0.7% of the mass of the titanium dioxide substrate.

[0035] Another aspect of this application provides a method for preparing the self-cleaning titanium dioxide as described above, comprising the following steps: S1. Preparation of titanium dioxide-based material slurry; the slurry can be prepared using conventional methods, with a preferred concentration of 300-400 g / L. This concentration is slightly higher than that of conventional wet coating, especially large-volume inorganic coating, because under hydrothermal high-temperature and high-pressure conditions, it enhances the fluidity and dispersibility of the slurry. Even at a higher concentration, titanium dioxide particles are less prone to agglomeration, and the coating agent can be uniformly deposited on the particle surface through ionic reactions. Simultaneously, the higher concentration increases the reactant concentration per unit volume, accelerates the coating reaction rate, reduces solvent consumption in the reactor, and improves production efficiency, meeting the requirements of efficient preparation of dense films in hydrothermal processes.

[0036] S2. Al-doped hydroxyapatite coating was performed using a hydrothermal method; S3. Organic acid coating is performed using a hydrothermal method.

[0037] Preferably, step S2 further includes: Soluble calcium source, soluble phosphate source and soluble aluminum source were added to titanium dioxide-based material slurry, and a hydrothermal reaction was carried out under pH 9.0~10.0 conditions; The hydrothermal reaction temperature is 200~300℃, and the time is 10~18h; The amount of soluble calcium source added, calculated as calcium oxide, is 5.6~11.2% of the mass of the titanium dioxide substrate; The amount of soluble phosphoric acid source added, calculated as phosphorus oxide, is 3.4~8.5% of the mass of the titanium dioxide substrate; The amount of soluble aluminum source added, calculated as alumina, is 0.2 to 0.8% of the mass of the titanium dioxide substrate.

[0038] Soluble calcium sources can include CaCl2, Ca(NO3)2, etc.; soluble phosphoric acid sources can include H3PO4, Na2HPO4, (NH4)2HPO4, etc.; and soluble aluminum sources can include Al2(SO4)3, AlCl3, etc.

[0039] pH can be adjusted using NaOH solution, ammonia, etc., with ammonia being preferred, at a concentration of 100~200g / L.

[0040] Preferably, step S3 further includes: The slurry coated with Al-doped hydroxyapatite was separated into solid and liquid phases, then re-slurried with water, and organic acid was added for hydrothermal reaction. The hydrothermal reaction temperature is 120~150℃, and the time is 2~4h; The amount of organic acid added, based on the mass of carboxyl groups, is 0.3 to 0.7% of the mass of the titanium dioxide substrate.

[0041] Solid-liquid separation is preferably achieved by pressure filtration, followed by water washing until the filter cake resistivity is reduced. To fully remove inorganic salts introduced during the inorganic coating process.

[0042] As those skilled in the art will understand, after the organic coating modification in step S3, the process also includes a second solid-liquid separation, drying, and pulverization step to ensure that the particle size of the titanium dioxide product meets the requirements.

[0043] As mentioned earlier, to avoid the decomposition and volatilization of organic acids, the drying and pulverizing temperature should not exceed 120℃. Pulverization can be carried out using equipment such as low-temperature compressed air pulverizers or Raymond mills.

[0044] For the second solid-liquid separation, pressure filtration is preferred. Since the inorganic coating has already been washed with water, the organic coating does not need to be washed again and can be dried directly.

[0045] Example 1 A titanium dioxide-based material slurry was prepared, and the slurry concentration was adjusted to 336 g / L. The slurry was then transferred to a hydrothermal tank. 8.4% CaCl2 solution (calculated as CaO), 5.7% Na2HPO4 solution (calculated as P2O5), and 0.6% AlCl3 solution (calculated as Al2O3) were added to the hydrothermal tank, accounting for 8.4% of the mass fraction of the titanium dioxide substrate. The pH of the slurry was adjusted to 9.8 using ammonia. The temperature was raised to 270℃, and the reaction was carried out for 12 h.

[0046] The slurry in the hydrothermal tank is press-filtered, and then washed with water until the resistivity of the filter cake is [value missing]. The slurry was re-pulverized with water and the concentration was adjusted to 309 g / L. The slurry was transferred to a hydrothermal tank, and citric acid (based on carboxylate mass) at a mass fraction of 0.4% of the titanium dioxide substrate was added. The temperature was raised to 130°C, and the reaction was carried out for 3 hours. The material was then discharged, filtered, dried, and pulverized to obtain the finished product. The drying temperature was 100°C, and the time was 2 hours. The pulverization was carried out using a Raymond mill (Raymond mills do not introduce hot steam, and the pulverization temperature is relatively low).

[0047] Example 2 A titanium dioxide-based material slurry was prepared, and the slurry concentration was adjusted to 384 g / L. The slurry was then transferred to a hydrothermal tank. A CaCl2 solution (calculated as CaO) of 11.2% by mass fraction of the titanium dioxide substrate, an (NH4)2HPO4 solution (calculated as P2O5), and an AlCl3 solution (calculated as Al2O3) of 0.8% by mass fraction were added to the hydrothermal tank. The pH of the slurry was adjusted to 10 using ammonia. The temperature was raised to 300℃, and the reaction was carried out for 10 h.

[0048] The slurry in the hydrothermal tank is press-filtered, and then washed with water until the resistivity of the filter cake is [value missing]. The mixture was slurried with water and the concentration was adjusted to 372 g / L. The slurry was transferred to a hydrothermal tank, and benzoic acid (based on carboxylate mass) at 0.7% of the mass fraction of the titanium dioxide substrate was added. The temperature was raised to 140°C, and the reaction was carried out for 4 hours. The material was then discharged, filtered, dried, and pulverized to obtain the finished product. The drying temperature was 90°C, and the time was 3 hours. The pulverization was carried out using a Raymond mill.

[0049] Example 3 A titanium dioxide-based material slurry was prepared, and the slurry concentration was adjusted to 366 g / L. The slurry was then transferred to a hydrothermal tank. A CaCl2 solution (calculated as CaO), 3.4% H3PO4 solution (calculated as P2O5), and 0.4% AlCl3 solution (calculated as Al2O3) were added to the hydrothermal tank, accounting for 5.6% of the mass fraction of the titanium dioxide substrate. The pH of the slurry was adjusted to 9 using NaOH. The temperature was raised to 200℃, and the reaction was carried out for 18 h.

[0050] The slurry in the hydrothermal tank is press-filtered, and then washed with water until the resistivity of the filter cake is [value missing]. The mixture was slurried with water and the concentration was adjusted to 355 g / L. The slurry was transferred to a hydrothermal tank, and citric acid (based on carboxylate mass) at a mass fraction of 0.3% of the titanium dioxide substrate was added. The temperature was raised to 120°C, and the reaction was carried out for 4 hours. The material was then discharged, filtered, dried, and pulverized to obtain the finished product. The drying temperature was 110°C, and the time was 2 hours. The pulverization was carried out using a Raymond mill.

[0051] Comparative Example 1 (Conventional Silicon-Aluminum Coating) A titanium dioxide-based material slurry was prepared by adjusting the slurry concentration to 286 g / L, starting stirring, and heating to 55 °C. 3% Na₂SiO₃ was added over 10 min, followed by pH adjustment to 8 using H₂SO₄ over 120 min, and then homogenization for 30 min. 1.8% NaAlO₂ and 1.2% Al₂(SO₄)₃ were added concurrently, maintaining a pH of 8 over 60 min, followed by homogenization for 30 min. The slurry pH was then adjusted to 6.4 using H₂SO₄ over 60 min, and then homogenized for 2 h. The slurry was washed with water, dried, and then pulverized to obtain the product.

[0052] Comparative Example 2 (compared to Example 3, only hydroxyapatite coating) A titanium dioxide-based material slurry was prepared, and the concentration of the titanium dioxide slurry was adjusted to 366 g / L. The slurry was then transferred to a hydrothermal tank. A CaCl2 solution (calculated as CaO), 3.4% H3PO4 solution (calculated as P2O5), and 0.4% AlCl3 solution (calculated as Al2O3) were added to the hydrothermal tank, accounting for 5.6% of the mass fraction of the titanium dioxide substrate. The pH of the slurry was adjusted to 9 using NaOH. The temperature was raised to 200℃, and the reaction was carried out for 18 h.

[0053] The slurry in the hydrothermal tank is press-filtered and washed with water until the resistivity of the filter cake is [value missing]. The product is obtained by drying and pulverizing.

[0054] Comparative Example 3 (Conventional Silicon Coating + Organic Acid Coating) To prepare a titanium dioxide-based material slurry, the slurry concentration was adjusted to 286 g / L, stirring was started, and the temperature was raised to 55℃. 3% Na2SiO3 was added over a period of 10 min. Then, the pH was adjusted to 8 using H2SO4 over a period of 120 min, followed by homogenization for 30 min.

[0055] The slurry is pressed and filtered, then washed with water until the resistivity of the filter cake is [value missing]. The filter cake was re-slurried with water, the slurry concentration was adjusted to 355 g / L and transferred to a hydrothermal tank. Citric acid (based on carboxylate mass) at a mass fraction of 0.3% of the titanium dioxide substrate was added. The temperature was raised to 120℃ and the reaction was carried out for 4 hours. The material was then discharged, filtered, dried by pressure, and pulverized to obtain the finished product. The drying temperature was 110℃ and the time was 2 hours. The pulverization was carried out using a Raymond mill.

[0056] The titanium dioxide provided in the examples and comparative examples was prepared into coatings using conventional methods. The coatings were then applied to a stone slab with a thickness of 125 μm and placed on a constant temperature and humidity test bench. After the coatings were completely dry, oil and dust were thoroughly cleaned with alcohol. The stone slab was then placed horizontally at the test position of an optical contact angle / interfacial tension meter for water contact angle testing. Dispersion and weathering resistance tests were conducted using commonly used methods for testing titanium dioxide dispersibility and weathering resistance. The results are shown in Table 1.

[0057] Table 1

[0058] As shown in Table 1, the water contact angle of the titanium dioxide provided in this application is significantly higher than that of the comparative example, proving that the comparative example provided in this application has excellent self-cleaning ability. Simultaneously, the titanium dioxide provided in this application also possesses excellent pigment properties such as dispersibility and weather resistance. Its weather resistance is comparable to that of titanium dioxide with a silicon-aluminum coating (Comparative Example 1), and its dispersibility is superior to that of titanium dioxide with conventional silicon-aluminum coating. While Comparative Example 3 uses a conventional silicon coating instead of the aluminum-doped hydroxyapatite coating of this application, although its weather resistance and dispersibility are comparable to this application, the silicon oxide coating results in hydrogen bonds between the four hydroxyl groups in the silicon film layer and the H in the water, increasing the thixotropy of the filter cake after water washing (thixotropy means that after drying the filter cake surface, it becomes watery or viscous after external force or storage for a period of time), which is detrimental to water washing.

[0059] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention. Clearly, those skilled in the art can make various alterations and modifications to the invention without departing from its spirit and scope. Thus, if these modifications and modifications of the invention fall within the scope of the claims and their equivalents, the invention is also intended to include these modifications and modifications.

Claims

1. A self-cleaning titanium dioxide, characterized in that, From the inside out, it consists of a titanium dioxide substrate, an inorganic coating layer, and an organic coating layer. The inorganic coating layer includes an Al-doped hydroxyapatite film layer; The organic coating layer is obtained by organic modification with organic acids.

2. The self-cleaning titanium dioxide as described in claim 1, characterized in that, Both the Al-doped hydroxyapatite film and the organic coating layer are formed by a hydrothermal method.

3. The self-cleaning titanium dioxide as described in claim 2, characterized in that, The Al-doped hydroxyapatite film was obtained by adding soluble calcium source, soluble phosphate source and soluble aluminum source to titanium dioxide-based material slurry and carrying out a hydrothermal reaction under pH 9.0~10.0 conditions. The hydrothermal reaction temperature is 200~300℃, and the time is 10~18h.

4. The self-cleaning titanium dioxide as described in claim 2, characterized in that, The organic coating layer is formed through the following steps: The slurry coated with the Al-doped hydroxyapatite film was separated into solid and liquid phases, then re-slurried with water, and organic acid was added for hydrothermal reaction. The hydrothermal reaction temperature is 120~150℃, and the time is 2~4h.

5. The self-cleaning titanium dioxide as described in claim 1, characterized in that, The organic acid is selected from polycarboxylic acids with a decomposition temperature > 150°C; Preferably, the organic acid is at least one selected from citric acid, tartaric acid, fumaric acid, and succinic acid.

6. The self-cleaning titanium dioxide as described in claim 1, characterized in that, The amount of Al-doped hydroxyapatite film coating, calculated as hydroxyapatite, is 9-20% of the mass of the titanium dioxide substrate; the Al doping amount, calculated as alumina, is 0.2-0.8% of the mass of the titanium dioxide substrate. The coating amount of the organic coating layer, based on the mass of carboxyl groups, is 0.3 to 0.7% of the mass of the titanium dioxide substrate.

7. A method for preparing self-cleaning titanium dioxide as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Preparation of titanium dioxide-based material slurry; S2. Al-doped hydroxyapatite coating was performed using a hydrothermal method; S3. Organic acid coating is performed using a hydrothermal method.

8. The method for preparing self-cleaning titanium dioxide as described in claim 7, characterized in that, Step S2 further includes: Soluble calcium source, soluble phosphate source and soluble aluminum source were added to titanium dioxide-based material slurry, and a hydrothermal reaction was carried out under pH 9.0~10.0 conditions; The hydrothermal reaction temperature is 200~300℃, and the time is 10~18h; The amount of the soluble calcium source added, calculated as calcium oxide, is 5.6~11.2% of the mass of the titanium dioxide substrate; The amount of the soluble phosphoric acid source added, calculated as phosphorus oxide, is 3.4~8.5% of the mass of the titanium dioxide substrate; The amount of the soluble aluminum source added, calculated as alumina, is 0.2 to 0.8% of the mass of the titanium dioxide substrate.

9. The method for preparing self-cleaning titanium dioxide as described in claim 7, characterized in that, Step S3 further includes: The slurry coated with Al-doped hydroxyapatite was separated into solid and liquid phases, then re-slurried with water, and organic acid was added for hydrothermal reaction. The hydrothermal reaction temperature is 120~150℃, and the time is 2~4h; The amount of organic acid added, based on the mass of carboxyl groups, is 0.3 to 0.7% of the mass of the titanium dioxide substrate.

10. The method for preparing self-cleaning titanium dioxide as described in claim 7, characterized in that, Step S3 is followed by solid-liquid separation, drying, and pulverization steps; The drying and pulverizing temperature shall not exceed 120°C.

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