Photocatalyst thin film, method for forming photocatalyst thin film, and photocatalyst thin film coated product

[Technical field] [0001] The present invention relates to a photocatalytic thin film and a product having a photocatalytic thin film. [Background technology] [0002] The photocatalyst has a photocatalytic action of decomposing a substance by irradiation with ultraviolet rays and a photoinduced superhydrophilic action of making the surface more compatible with water by light. Photocatalytic self-cleaning products that apply these two actions are widely used as exterior wall materials for buildings and exterior building materials such as glass. [0003] In other words, a photocatalyst self-cleaning product with a photocatalyst coated on the surface can decompose the dirt on the surface by the ultraviolet light contained in sunlight, and when it rains, the dirt can be lifted up and washed away by the superhydrophilicity of the surface. Therefore, it is possible to maintain a clean appearance at all times (self-cleaning function). [0004] However, among the photocatalyst self-cleaning products, the photocatalyst self-cleaning glass has various problems described below. Photocatalytic self-cleaning glass is generally titanium oxide (TiO).₂) Is manufactured by coating the glass surface with particles. Since the contact area between the titanium oxide particles and the glass surface is small, the coating layer is easily peeled off. After forming the coating layer, firing at a high temperature of several hundred ° C. can improve the adhesion between the coating layer and the glass, but in this case, the photocatalytic activity is lowered. That is, since soda lime glass widely used for window glass and the like contains a large amount of sodium ions, these sodium ions diffuse to the surface during firing, and a compound of titanium oxide and sodium (sodium titanate). Etc.) and lose the photocatalytic activity. [0005] In order to avoid the problem of deterioration of photocatalyst performance due to alkali diffusion, two methods are adopted in the production of photocatalyst self-cleaning glass. One is called the room temperature curing method, in which fine particles of titanium oxide photocatalyst are mixed with a coating liquid that contains sol-gel components and solidifies at a relatively low temperature, coated with this, and then at about 150 ° C. This is a method of solidifying (see, for example, Patent Document 1). According to this method, as shown in FIG. 2, a photocatalytic thin film 105 in which titanium oxide fine particles 103 are dispersed in a sol-gel film 101 is formed on the surface of a glass substrate 107. [0006] The other is a method called a two-layer coating method, and as shown in FIG. 3, silica (SiO) is used to prevent alkali diffusion from the glass substrate 201 due to firing.₂) And the like, and then the surface of the glass substrate 201 is formed with a photocatalyst coating to form a photocatalyst thin film 205, which is then fired (see, for example, Patent Document 2). [0007] In addition to glass products, glaze-coated ceramic products and enameled metal products also have similar problems. Glazes and enamel are glass liquids that are melted or melted in a medium and contain a large amount of sodium to lower the melting point. Therefore, in order to form a photocatalytic thin film on the surface of these products, a room temperature curing method is used. Alternatively, it was necessary to use the two-layer coating method. Patent Document 1: Japanese Unexamined Patent Publication No. 2001-150586 Patent Document 2: Japanese Patent Application Laid-Open No. 10-53439 Disclosure of invention Problems to be solved by the invention [0008] However, with the room temperature curing method, the adhesion between the glass and the coating film is not high, so the wear resistance of the film is low, and there are very limited applications such as areas where there is almost no physical contact (for example, window glass of high-rise buildings). There is a problem that it can only be applied to. [0009] Further, in the two-layer coating method, the coating must be performed twice, and in some cases, the base film must also be fired. In that case, two firing steps are required, which is a complicated process. There is a problem that high cost is required. [0010] The present invention has been made in view of these problems, and provides a method for forming a photocatalyst thin film capable of forming a photocatalyst thin film having high adhesion to a substrate and high durability in a small number of steps, and a product having the photocatalyst thin film and the photocatalyst thin film. The purpose is. Means to solve problems [0011] In the method for forming a photocatalytic thin film according to the first aspect of the present invention, which has been made to solve such a problem, a layer containing niobia nanosheets is formed on the surface of a substrate containing an alkali metal and fired at a temperature of 450 ° C. or higher. By doing so, a photocatalytic thin film containing a niobium-alkali metal composite oxide is formed. [0012] According to such a method for forming a photocatalyst thin film, a photocatalyst thin film having high adhesion to a substrate and high durability can be formed in a small number of steps. [0013] According to the method for forming a photocatalyst thin film according to the first aspect of the present invention, it can be inferred that the photocatalyst thin film is formed as follows. That is, when a layer containing niobia nanosheets is formed on the surface of a substrate containing an alkali metal and fired at a temperature of 450 ° C. or higher, the alkali metal contained in the substrate is precipitated on the surface of the substrate and the niobia nanosheets. Diffuses into the layer containing. Then, a reaction containing the alkali metal and the niobium nanosheet occurs, and a photocatalytic thin film containing the niobium-alkali metal composite oxide is formed on the surface of the substrate. [0014] For example, as shown in A of FIG. 1, when a layer 5 containing niobia nanosheets 3 is formed on the surface of soda lime glass 1 containing sodium and fired at a temperature of 450 ° C. or higher, it is shown in B of FIG. As described above, the sodium contained in the soda lime glass 1 is precipitated on the surface of the substrate 1 by heating and diffuses into the layer containing the niobia nanosheets. Then, a reaction containing sodium and niobium nanosheets occurs, and a niobium-sodium composite oxide (for example, NaNbO) is formed on the surface of soda lime glass 1.₃) Is formed. [0015] That is, in the method for forming a photocatalyst thin film according to the first aspect of the present invention, the alkali metal contained in the substrate, which has conventionally been a cause of deteriorating the photocatalytic performance, is actively utilized to form a niobium-alkali on the surface of the substrate. A photocatalytic thin film containing a metal composite oxide is formed. This niobium-alkali metal composite oxide exhibits remarkable photocatalytic activity. Further, the photocatalytic thin film containing the niobium-alkali metal composite oxide formed by the above-mentioned forming method has high hardness because it is fired at a temperature of 450 ° C. or higher, and has high adhesion to the substrate, so that it is durable. Excellent in sex. [0016] The high adhesion between the photocatalyst thin film formed by the method for forming the photocatalyst thin film according to the first aspect of the present invention and the substrate is due to the following reasons (i) to (iii). That is, (i) Since niobium nanosheets are used, the niobium oxide particles are in contact with the surface of the substrate over a very large area. (ii) Since the alkaline component in the substrate and niobium oxide directly chemically react with each other by firing, high adhesion is exhibited by integrating the surface of the substrate and the interface of the coating film. (iii) Niobia nanosheets hardly expand or contract in the long axis direction, that is, in the direction parallel to the surface of the substrate even during chemical changes due to firing, so that peeling from the substrate and cracks in the coating film occur. Achieving high adhesion Further, the photocatalyst thin film formed by the method for forming the photocatalyst thin film according to the first aspect of the present invention is formed by using niobia nanosheets and has high smoothness, so that pollutants do not easily adhere to the photocatalyst thin film and the transparency is high. [0017] The niobium nanosheet is niobium oxide having a scaly shape, and its size is preferably in the range of 0.1 to 50 μm, its thickness is preferably in the range of 0.3 to 3 nm, and more preferably 0.5 to 1 nm. The range of is suitable. The aspect ratio of the niobia nanosheet is preferably in the range of 100 to 100,000. [0018] By the way, in the method for forming a photocatalytic thin film according to the first aspect of the present invention, in order to form a layer containing a niobium nanosheet on the surface of a substrate containing an alkali metal, the layered niobium oxide is peeled off with a release agent. Liquid phase coating with a coating liquid such as a dispersion liquid is preferable. [0019] As the liquid phase coating, a coating method generally used for liquid phase coating such as spin coating method, dip coating method, spray coating method, roll coating method, blade coating method, bar coating method, and flow coating method is applied. It is possible. [0020] Among the above liquid phase coatings, it is preferably formed by applying a coating agent containing niobia nanosheets (second aspect of the present invention). [0021] [0021] In this way, a layer containing the niobia nanosheets can be formed by a simple method of applying a coating agent containing the niobia nanosheets to the surface of the substrate. Therefore, large-scale equipment required for sputtering and the like becomes unnecessary, and a photocatalyst thin film containing a niobium-alkali metal composite oxide can be formed at low cost. The ratio of niobia nanosheets to the coating agent is preferably in the range of 0.05 to 10% by weight. [0022] The niobate nanosheet is KNb, which is a layered niobate compound.₃O₈, K₄Nb₆O₁₇Nb obtained from raw materials such as₃O₈^-, Nb₆O₁₇^-Etc. can be used. These layered niobate compounds are ion-exchanged by acid treatment, and H₃NbO₈, H₄Nb₆O₁₇When a release agent is allowed to act after conversion to a compound such as, the layered structure is peeled off and a nanosheet dispersion can be obtained. Further, as the release agent, quaternary ammonium ions such as tetrabutylammonium ion, tetraethylammonium and tetramethylammonium, and amines such as diethanolamine and triethanolamine can be used. [0023] By the way, there are various substrates containing alkali metals, but soda lime glass (in the case of the third phase of the present invention) and a vitreous film formed on the surface of ceramics by glaze (in the case of the fourth phase of the present invention). Case), or a glassy film formed on the surface of the metal product by enamel (in the case of the fifth aspect of the present invention). Since these substrates contain sodium, which is an alkali metal, when a layer containing niobia nanosheets is formed on the surface thereof and fired, sodium precipitates and diffuses into the layer. Then, a reaction involving sodium and niobium occurs, and a niobium-sodium composite oxide (for example, NaNbO) occurs.₃) A thin film is formed on the surface of the substrate containing sodium. [0024] In this way, soda lime glass, which is mass-produced and used as general glass, glaze, enamel, etc., which are generally used, can be used as a substrate, so that a photocatalyst thin film can be formed at low cost. Can be done. [0025] For example, when the glass substrate on which the photocatalyst thin film is formed is tempered glass or curved glass, the manufacturing process of these glasses originally includes a glass heating process, so coating is applied before the heating process. If implemented, the heating step plays the role of the firing step of the photocatalyst thin film. Therefore, it is not necessary to provide a firing step for the photocatalyst thin film again. [0026] [0027] In any of the above 1st to 5th phases, when the firing temperature is 500 to 550 ° C (in the case of the 6th phase of the present invention), photoinduced superhydrophilicity is remarkably exhibited and remarkable. It also shows a good photocatalytic activity. That is, the amount of sodium ions precipitated at 500 to 550 ° C is optimized, and the hydrophilization activity and photocatalytic activity are further improved. [0028] The photocatalytic thin film according to the seventh aspect of the present invention is a photocatalyst thin film formed by the method for forming the photocatalyst thin film according to any one of the first to sixth aspects of the present invention. [0029] The photocatalytic thin film according to the seventh aspect of the present invention has high hardness and excellent durability. Further, since it forms a niobium-sodium composite oxide, it has both superhydrophilicity and photocatalytic activity, and has a self-cleaning function. In addition, since it is formed using niobia nanosheets, it has high smoothness and is resistant to contaminants. [0030] The photocatalytic thin film according to the eighth aspect of the present invention is a photocatalytic thin film according to the seventh aspect of the present invention, characterized in that the average surface roughness is 5 nm or less. Since this photocatalytic thin film is particularly excellent in surface smoothness, it is particularly effective in preventing contaminants from adhering to it. [0031] The range of the average surface roughness of the photocatalytic thin film according to the eighth aspect of the present invention can be, for example, a range in which the average surface roughness of the substrate itself is the lower limit and the upper limit is 5 nm. The average surface roughness of the glass substrate itself is generally about 0.5 nm, and in this case, the range of the average surface roughness of the photocatalytic thin film according to the eighth aspect of the present invention is substantially in the range of 0.5 to 5 nm. It becomes. The average surface roughness of the photocatalyst thin film according to the eighth aspect of the present invention can be controlled, for example, by changing the peeling state of the niobia nanosheet when the photocatalyst thin film is formed. For example, when a strong release agent (for example, the tetrabutylammonium hydroxy solution (TBAOH) used in Example 2 described later) is used when peeling the niobia nanosheets, the release of the niobia nanosheets proceeds, and as a result, the photocatalytic thin film is peeled off. Surface roughness becomes smaller. On the other hand, when a weak release agent (for example, 3-methoxypropylamine (3-MPA) used in Example 3 described later) is used when peeling the niobia nanosheet, the niobia nanosheet is not completely peeled off and is coated. Since several layers are laminated in the solution, the surface roughness of the photocatalyst thin film becomes slightly large as a result. [0032] The photocatalytic thin film coating product according to the ninth aspect of the present invention includes a substrate containing an alkali metal and a photocatalytic thin film coating according to the seventh or eighth aspect of the present invention provided on the surface of the substrate. It is a product. [0033] In this photocatalyst thin film-coated product, the photocatalyst thin film has high hardness and high adhesion to the substrate, and thus has excellent durability. Further, since this photocatalytic thin film-coated product has a photocatalytic thin film of niobium-sodium composite oxide, it has both superhydrophilicity and photocatalytic activity, and is excellent in self-cleaning function. Furthermore, since the photocatalyst thin film has high smoothness, contaminants are unlikely to adhere to it. [0034] Therefore, when the substrate is glass, it can be made of glass that is hard to be soiled and can maintain transparency for a long period of time. Further, when the substrate is a mirror, it can be a mirror that is hard to get dirty and can maintain the reflectance for a long period of time. Further, when the substrate is a glazed ceramic product, the surface can be kept clean at all times. When the substrate is an enamel product, it can be an enamel product that can always keep the surface clean. [Simple explanation of drawings] [0035] FIG. 1 is a diagram illustrating a reaction involving soda lime glass and niobia nanosheets to form a photocatalytic thin film containing a niobium-sodium oxide compound on the surface of lime soda glass. FIG. 2 is a diagram showing a photocatalytic thin film formed by a room temperature curing method. FIG. 3 is a diagram showing a photocatalytic thin film formed by a two-layer coating method. FIG. 4 is a diagram showing an X-ray diffraction pattern of a photocatalytic thin film. FIG. 5 is a diagram showing an X-ray diffraction pattern of a photocatalytic thin film. FIG. 6 is a diagram showing an X-ray diffraction pattern of a photocatalytic thin film. FIG. 7 is an SPM image showing a photocatalytic thin film formed by the method of the present invention. [Explanation of symbols] [0036] 1, 107, 201 Glass substrate 3 Niobia nanosheet 5, 105, 205 Photocatalytic thin film 103 Titanium oxide fine particles 203 Base film [Best mode for carrying out the invention] [0037] Hereinafter, the present invention will be described based on examples. [Example 1] [0038] (a) Preparation of coating solution Powdered niobium oxide (Nb)₂O₅) 42.61g (160.3 mmol) and potassium carbonate (K)₂CO₃) 7.39 g (53.2 mmol) was dispersed in 150 ml of a mixed solvent (50 ml of ethanol and 100 ml of n-hexane), and mixed with zirconia balls in a ball mill for 24 hours. Then, the solvent was removed by centrifugation, the remaining powder was dried, and then pulverized in a mortar. The crushed powder is transferred to an alumina crucible and 1100.^{o o}Bake in C for 30 hours. In firing, 1100 over 3 hours and 40 minutes^{o o}The temperature was raised to C, and the cooling was natural cooling. [0039] Obtained potassium niobate (KNb)₃O₈) Was pulverized in a mortar and ion-exchanged for 24 hours with stirring in a 6 M aqueous nitric acid solution. Then, nitric acid was removed with a centrifuge, new nitric acid was added, and ion exchange was carried out in the same manner. Ion exchange with nitric acid was performed a total of 4 times. This ion exchange produced niobium nanosheets composed of niobium oxide. Then, pure water was used instead of nitric acid, and the washing was performed four times by the same method as the above-mentioned ion exchange. Add an appropriate amount of 40 wt% tetrabutylammonium hydroxy solution (TBAOH) and ethanol to the dispersion liquid in which the washed Niobia nanosheet is dispersed in water so that the ratio of water to ethanol is 25:75, and the solid content concentration. A coating solution was obtained in which was 0.25 wt%. (b) Formation of photocatalytic thin film The coating solution prepared above was applied to a soda lime glass substrate (50 mm × 50 mm) by a dip coating method. The pulling speed in the dip coat method was 8 mm / sec. Then, the soda lime glass substrate was calcined in the air for 1 hour to complete a photocatalytic thin film. The temperature control during firing was such that the temperature was raised to a predetermined firing temperature over 1 hour, held at that temperature for 1 hour, and then cooled to room temperature over 1 hour. The above-mentioned predetermined firing temperatures are set every 50 ° C in the range of 300 to 400 ° C and 450 to 600 ° C, and every 10 ° C in the range of 400 to 450 ° C, for a total of 11 types, and the firing temperature is set. Eleven different types of photocatalytic thin films were formed. [Example 2] [0040] (c) Preparation of coating solution Powdered niobium oxide (Nb)₂O₅) And Potassium Nitrate (KNO)₃) And were mixed well. The mixing ratio was 2: 3 (molar ratio). Then, the mixed powder was transferred to an alumina crucible and calcined at 600 ° C. for 2 hours. Next, the powder after calcining was pulverized and mixed, transferred to an alumina crucible, and fired at 900 ° C. for 20 hours. Obtained potassium niobate (KNb)₃O₈) Was pulverized, ion-exchanged for 24 hours with stirring in a 1.0 M aqueous nitric acid solution, and then washed with pure water. This work was done a total of four times. An appropriate amount of 40 wt% tetrabutylammonium hydroxy solution (TBAOH) was added to the dispersion liquid obtained by dispersing the niobia nanosheets in water so that the ratio of water to ethanol was 25:75, and the solid content concentration was increased. A coating solution of 0.25 wt% was obtained. (d) Formation of photocatalytic thin film The coating solution prepared above was applied onto a soda lime glass substrate (50 mm × 50 mm) by a spin coating method. The rotation speed in the spin coating method was 1000 rpm and held for 20 seconds. Then, the soda lime glass substrate was calcined in an air atmosphere for 1 hour. The temperature control during firing was such that the temperature was raised to a predetermined firing temperature over 1 hour, held at that temperature for 1 hour, and then cooled to room temperature over 1 hour. The predetermined firing temperature was 350, 400, 450, 500, 550, 600 ° C., and a total of 6 types of photocatalytic thin films were formed. [Example 3] [0041] (e) Preparation of coating solution Powdered niobium oxide (Nb)₂O₅) And Potassium Nitrate (KNO)₃) And were mixed well. The mixing ratio was 2: 3 (molar ratio). Then, the mixed powder was transferred to an alumina crucible and calcined at 600 ° C. for 2 hours. Next, the powder after calcining was pulverized and mixed, transferred to an alumina crucible, and fired at 900 ° C. for 20 hours. Obtained potassium niobate (KNb)₃O₈) Was pulverized, ion-exchanged for 24 hours with stirring in a 1.0 M aqueous nitric acid solution, and then washed with pure water. This work was done a total of four times. An appropriate amount of 3-methoxypropylamine (3-MPA) was added to the dispersion in which the niobia nanosheet thus obtained was dispersed in water so that the ratio of water to ethanol was 25:75, and the solid content concentration was increased. A coating solution of 0.25 wt% was obtained. (f) Formation of photocatalytic thin film The coating solution prepared in (e) above was applied onto a soda lime glass substrate (50 mm × 50 mm) by a spin coating method. The rotation speed in the spin coating method was 1000 rpm and held for 20 seconds. Then, the soda lime glass substrate was calcined in an air atmosphere for 1 hour. The temperature control during firing was such that the temperature was raised to 500 ° C over 1 hour, held at that temperature for 1 hour, and then cooled to room temperature over 1 hour. [0042] Since the photocatalytic thin film formed in Examples 1, 2 and 3 has the above-mentioned fine structure, it can be made thinner than the structure in which spherical particles are dispersed. [0043] Further, the photocatalytic thin films of Examples 1, 2 and 3 are formed by using niobia nanosheets, because the niobia nanosheets have a large surface area per unit volume and a large contact area with a soda lime glass substrate. , High adhesion to soda lime glass substrate. Therefore, the photocatalytic thin films of Examples 1, 2 and 3 have high adhesion to the soda lime glass substrate. (Comparative example 1) (g) Preparation of coating solution containing titania nanosheets Cesium carbonate and titanium oxide were mixed at a molar ratio of 1: 5.3, and firing was performed twice at 800 ° C. for 20 hours. The produced cesium titanate was repeatedly stirred, filtered, and dried in dilute hydrochloric acid four times to obtain layered titanic acid in which cesium ions were replaced with hydrogen ions. An aqueous solution of tetrabutylammonium hydrochloride was added thereto, and the mixture was stirred for 14 days to prepare titania nanosheets. This titania nanosheet was suspended in a mixed solvent of 75 vol% ethanol and 25 vol% water to obtain a titania nanosheet (TNS) suspension having a solid content concentration of 0.25 wt%. [0044] A mixture of titanium tetraisopropoxide (TTIP) and 17 vol% polyethylene glycol was prepared and added to the TNS suspension so that the molar ratio of TNS to TTIP was 9: 1. It was used as a coating solution. (h) Formation of photocatalytic thin film The coating solution prepared in (g) above was applied to a soda lime glass substrate (50 mm × 50 mm) by a dip coating method. The pulling speed in the dip coat method was 8 mm / sec. Then, the soda lime glass substrate was calcined in the air for 1 hour to complete a photocatalytic thin film. The temperature control during firing was such that the temperature was raised to 500 ° C over 1 hour, held at that temperature for 1 hour, and then cooled to room temperature over 1 hour. (Comparative example 2) (i) Preparation of coating solution Niobium ethoxide and potassium hydroxide were added to ethanol in a molar ratio of 3: 1 and mixed, and KNb₃O₈The coating solution was prepared so as to have a conversion of 1.0 wt%. (j) Formation of photocatalytic thin film The coating solution prepared in (i) above was applied onto a quartz glass substrate (50 mm × 50 mm) by a spin coating method. The rotation speed in the spin coating method was 1000 rpm and held for 20 seconds. Then, the quartz glass substrate was fired in the atmosphere for 1 hour to complete a photocatalytic thin film. The temperature control during firing was such that the temperature was raised to 500 ° C over 1 hour, held at that temperature for 1 hour, and then cooled to room temperature over 1 hour. (Comparative example 3) (k) Preparation of coating solution Niobium ethoxide and potassium hydroxide were added to ethanol in a molar ratio of 3: 2 and mixed, and K was added.₄Nb₆O₁₇The coating solution was prepared so as to have a conversion of 1.0 wt%. (l) Formation of photocatalytic thin film The coating solution prepared in (k) above was applied onto a quartz glass substrate (50 mm × 50 mm) by a spin coating method. The rotation speed in the spin coating method was 1000 rpm and held for 20 seconds. Then, the quartz glass substrate was fired in the atmosphere for 1 hour to complete a photocatalytic thin film. The temperature control during firing was such that the temperature was raised to 500 ° C over 1 hour, held at that temperature for 1 hour, and then cooled to room temperature over 1 hour. (Comparative example 4) (m) Preparation of coating solution 0.0193 g of sodium hydroxide was dissolved in 10 ml of ethanol to obtain a sodium hydroxide solution. Next, the coating solution having a solid content concentration of 0.25 wt% prepared in Example 2 was used as a niobia nanosheet solution to be used in the steps described later. (n) Formation of photocatalytic thin film The sodium hydroxide solution prepared in (m) above was applied on a quartz glass substrate (50 mm × 50 mm) by a spin coating method, and then a niobia nanosheet solution was applied on the quartz glass substrate (50 mm × 50 mm) by a spin coating method. The rotation speed in the spin coating method was 1000 rpm and held for 20 seconds. Then, the quartz glass substrate was fired in the atmosphere for 1 hour to complete a photocatalytic thin film. The temperature control during firing was such that the temperature was raised to 500 ° C over 1 hour, held at that temperature for 1 hour, and then cooled to room temperature over 1 hour. (Comparative example 5) (O) Preparation of coating solution Add 0.14 ml of niobium ethoxide to 10 ml of ethanol and mix to mix and Nb.₂O₅The coating liquid was prepared so as to have a conversion of 1.0 wt%. (p) Formation of photocatalytic thin film The coating solution prepared in (O) above was applied onto a soda lime glass substrate (50 mm × 50 mm) by a spin coating method. The rotation speed in the spin coating method was 1000 rpm and held for 20 seconds. Then, the soda lime glass substrate was calcined in an air atmosphere for 1 hour. The temperature control during firing was such that the temperature was raised to 500 ° C over 1 hour, held at that temperature for 1 hour, and then cooled to room temperature over 1 hour. (Test for confirming the effect of the present invention) (i) Photoinduced hydrophilization test The photocatalytic thin films formed in Examples 1 and 2 and Comparative Examples 1, 2, 3 and 4 were intermittently irradiated with ultraviolet rays using a black light blue fluorescent lamp (BLB). The irradiation intensity of ultraviolet rays was 2.4 mW / cm in Example 1 and Comparative Example 1.²1.0 mW / cm in Example 2 and Comparative Examples 2, 3 and 4²And said. Then, the contact angle of water on the surface of the photocatalyst thin film was repeatedly measured before and during the irradiation with ultraviolet rays. The results are shown in Tables 1 and 2. [0045] [table 1]<img file="JP5118067B2_D0001.tif" />[0046] [Table 2]<img file="JP5118067B2_D0002.tif" /> As shown in Table 1, in each of the photocatalytic thin films formed in Example 1, the contact angle was significantly reduced by irradiation with ultraviolet rays (hydrophilicity was exhibited). In particular, when the firing temperature is 430 ° C or higher, the contact angle drops significantly, the contact angle drops to 20 ° or less after 6 hours of ultraviolet irradiation, and the contact angle drops to around 10 ° after 24 hours of irradiation. lowered. [0047] Similarly, as shown in Table 2, the photocatalytic thin film formed in Example 2 also developed hydrophilicity by irradiation with ultraviolet rays, and this tendency was particularly remarkable at a firing temperature of 450 ° C or higher. [0048] On the other hand, in the photocatalytic thin film formed in Comparative Example 1, the contact angle was only lowered to about 35 ° even after irradiation for 24 hours. Further, in Comparative Examples 2 and 3, the contact angle decreased only to about 27 ° even after irradiation for 6 hours, and in Comparative Example 4, the contact angle decreased only to about 15 ° even after irradiation for 6 hours. [0049] From the above results, the photocatalyst thin film formed using the niobia nanosheets by the methods of Examples 1 and 2 exhibits hydrophilicity without forming a base film between the soda lime glass substrate and the photocatalyst thin film. It was confirmed that it would be done. In addition, it was found that the potassium niobate film formed by the general sol-gel method (Comparative Examples 2 and 3) and the thin film formed by the method of Comparative Example 4 exhibited significantly superior hydrophilicity. It was. (ii) Crystal phase change of the formed thin film The crystal phases of the thin films formed in Example 2 and Comparative Examples 2, 3 and 4 were identified by a thin film X-ray diffractometer (D8 DISCOVER) manufactured by Bruker AXS Co., Ltd. The results of the thin film formed in Example 2 are shown in FIG. 4, the results of the thin films formed in Comparative Examples 2 and 3 are shown in FIG. 5, and the results of the thin film formed in Comparative Example 4 are shown in FIG. [0050] As shown in FIG. 4, in the photocatalytic thin film formed in Example 2, NaNbO was obtained by firing at 450 ° C or higher.₃Diffraction peaks attributed to were observed. [0051] On the other hand, as shown in FIG. 5, in the photocatalytic thin films formed in Comparative Examples 2 and 3, no significant peak was observed in the X-ray diffraction pattern, indicating that potassium niobate having photocatalytic activity was not formed. It was. [0052] Further, as shown in FIG. 6, in the photocatalytic thin film formed in Comparative Example 4, only the peak caused by the layered niobium nanosheet was observed, and it was found that the compound of niobium and sodium was not formed. [0053]If a niobia nanosheet film is formed on a soda lime glass substrate containing an alkali metal and fired as in Example 2 , NaNbO₃Is generated, but as in Comparative Examples 2 and 3, KNb is generated on a quartz glass substrate with less alkali metal.₃O₈And K₄Nb₆O₁₇From the result that potassium niobate having photocatalytic activity is not produced even if a film having the above composition is formed and fired, the niobate nanosheet film is formed directly on the surface of a substrate containing sodium and fired to obtain a niobate nanosheet. It was confirmed that the sodium ions diffused from the substrate chemically react with each other to form a photocatalytic niobate-sodium compound. [0054] Further, as in Example 2, if a niobia nanosheet film is formed on a soda lime glass substrate containing an alkali metal (sodium) and fired, NaNbO₃However, even if a layer containing sodium is formed by applying a sodium hydroxide solution on quartz glass containing almost no alkali metal as in Comparative Example 4, NaNbO₃From the result that no sodium was produced, it was found that a substrate containing sodium was preferable as a sodium source for forming a compound of niobium and sodium. (iii) Pencil scratching test The hardness of the photocatalytic thin film formed in Example 1 and Comparative Examples 1, 2, 3 and 4 was tested according to the pencil scratch test in JIS-K5400. The results are shown in Tables 3 and 4. [0055] [Table 3]<img file="JP5118067B2_D0003.tif" />[0056] [Table 4]<img file="JP5118067B2_D0004.tif" /> The photocatalytic thin film formed in Example 1 showed a hardness of 9H or more when the firing temperature was 450 ° C or more. The photocatalytic thin film formed in Comparative Example 1 showed a hardness of 9H or more, but the photocatalytic thin films formed in Comparative Examples 2, 3 and 4 had hardnesses of 3H, HB and 6B or less, respectively. [0057] From the above results, the photocatalytic thin film formed using the niobia nanosheet is in contact with the substrate over a wide area, and the substrate and the coat film are directly chemically reacted. It was confirmed that the coating film hardness was significantly higher than that of the gel method film. (iv) Surface roughness and initial stain resistance In order to show that the photocatalytic thin film formed by the method of the present invention has high smoothness and thereby is excellent in initial antifouling property, the photocatalytic thin films formed in Examples 2, 3 and 5 are described below. A test like this was done. The surface roughness was evaluated using a scanning probe microscope (manufactured by JEOL Ltd .: JSPM-5200). The initial antifouling property was evaluated as follows using methylene blue (MB), which is a model substance for stains. The concentration of the methylene blue aqueous solution was adjusted to 0.1 mM, and the glass piece on which the above photocatalyst thin film was formed was immersed in this aqueous solution for 12 hours. After washing the pulled-up sample with water, the absorption spectrum of methylene blue adhering to the coating film was measured with an ultraviolet-visible spectrophotometer (manufactured by Hitachi High-Technologies Corporation: U-3310). Table 5 shows the relationship between the measured arithmetic mean surface roughness and the adsorption area of methylene blue. [0058] [0058] [Table 5]<img file="JP5118067B2_D0005.tif" /> It can be seen that the photocatalyst thin film formed in Examples 2 and 3 has a smaller surface roughness value and is smoother than the photocatalyst thin film formed in Comparative Example 5. Further, the adsorption degree of methylene blue was larger in the photocatalytic thin film having a large surface roughness formed in Comparative Example 5 than in the catalyst thin film having a small optical surface roughness formed in Examples 2 and 3. From this result, it was confirmed that the photocatalyst thin film formed by the present invention has an excellent initial stain property, which is hard to adhere to stains because the surface is smooth. (v) Measurement of surface roughness of Niobia nanosheet thin film The photocatalytic thin film formed on the glass by the method of the present invention has high smoothness, but since the niobia nanosheet itself has a thickness of only about 1 nm, the coat film also has high smoothness. It is considered that the shape is maintained even after firing. Therefore, the surface roughness of the Niobia nanosheet film itself was confirmed. A niobia nanosheet film was formed on a silicon wafer using the same coating liquid as in Example 2, and the average surface roughness Ra was measured using a probe microscope. The drop coat method was used as the coating method. The probe microscope used was SPI3800 / SPA400 manufactured by SII Nanotechnology. The measurement mode is tapping mode, and the cantilever is Si cantilever (20Nm).^-1)It was used. [0059] Figure 7 shows the measured SPM image. The average surface roughness at this time was Ra = 1.2 nm. [0060] As described above, the thin film formed by using the niobia nanosheet originally has very high smoothness, whereby the photocatalytic thin film formed on the glass by the method of the present invention also has high smoothness. It has been shown. [0061] It is needless to say that the present invention is not limited to the above-described embodiment and can be carried out in various modes without departing from the present invention.