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 having a self-purifying function and a product having a photocatalytic thin film. [Background technology] [0002] Conventionally, there are photocatalysts that have the functions of photocatalyst that decomposes substances by irradiation with ultraviolet rays and photocatalytic superhydrophilicization that makes the surface more compatible with water by light. In particular, it is widely used as an exterior wall material for buildings and as an exterior building material such as glass. [0003] In other words, photocatalytic self-cleaning products decompose dirt by the ultraviolet light contained in sunlight, and when it rains, the super-hydrophilicity of the surface allows the dirt to be lifted and washed away, so the appearance is always kept clean (self-cleaning). Cleaning function) is possible. [0004] However, the self-cleaning glass coated with a photocatalyst has some problems. Currently, titanium oxide (TiO) is the only material for photocatalysts.₂) Is used, but in order to firmly adhere the base material and the coating film to obtain a highly durable film, it is necessary to bake at a high temperature of several hundred ° C. [0005] However, since soda lime glass, which is widely used for window glass, contains a large amount of sodium ions, these sodium ions diffuse to the surface during firing, and titanium oxide such as sodium titanate and sodium are combined. It forms a compound and loses its photocatalytic activity. [0006] In order to avoid the problem of deterioration of photocatalyst performance due to alkali diffusion, two methods are used for manufacturing photocatalyst self-cleaning glass. [0007] One is called the room temperature curing method, which is a method in which fine particles of titanium oxide photocatalyst are mixed with a coating liquid that solidifies at a relatively low temperature, coated with the coating liquid, and then solidified at about 150 ° C (for example). See Patent Document 1). [0008] The other is a method called the two-layer coating method, which is silica (SiO) to prevent alkali diffusion from the substrate due to firing.₂) And the like, and then a photocatalytic coating is applied and fired (see, for example, Patent Document 2). 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 [0009] 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. [0010] Further, in the two-layer coating method, although sufficient coating film durability can be obtained, coating must be performed twice, and in some cases, the base film also needs to be fired, so that two firing steps are required. There is a problem that a complicated process and a high cost are required. [0011] Similar problems also exist in glazed ceramic products and enameled metal products. Glaze and enamel are glass liquids that are melted or dissolved in a medium, and by applying this to the surface of ceramics or metal and solidifying and firing, a vitreous film containing silicic acid as the main component can be formed. .. Since it is usually necessary to add a large amount of sodium component to lower the melting point of glaze or enamel, a large amount of sodium ions are present in the formed vitreous film as in soda lime glass. Therefore, in order to form a photocatalytic thin film on the surface of these products, a room temperature curing method or a two-layer coating method must be used. [0012] The present invention has been made in view of these problems, and provides a photocatalyst thin film forming method, a photocatalyst thin film, and a photocatalyst thin film capable of forming a photocatalyst thin film having high adhesion to a substrate, high durability, and self-purifying action in a small number of steps. The purpose is to provide a product that has. Means to solve problems [0013] The method for forming a photocatalyst thin film according to the first aspect of the present invention, which has been made to solve such a problem, is to form a layer containing niobium on the surface of a substrate containing an alkali metal and fire it at a temperature of 500 ° C. or higher. It is characterized by forming a photocatalyst thin film containing a niobium-alkali metal composite oxide. [0014] According to such a method for forming a photocatalyst thin film, it is possible to form a photocatalyst thin film having high adhesion to a substrate, high durability, and a self-purifying action in a small number of steps. This will be described below. [0015] In conventional photocatalysts such as titanium oxide, when a photocatalyst film is formed on the surface of a substrate containing an alkali metal, for example, soda lime glass, glaze-coated ceramic products, or hollow products containing sodium, the sodium of the substrate is formed. The photocatalytic performance of titanium oxide deteriorates due to ions. Therefore, conventionally, in order to suppress the diffusion of sodium ions, an undercoat film has been required between the substrate and the titanium oxide thin film. [0016] However, according to the method for forming the photocatalytic thin film according to the first phase, a layer containing niobium is formed on the surface of the substrate containing an alkali metal and fired at a temperature of 500 ° C. or higher. Then, the alkali metal contained in the substrate is heated and precipitated on the surface of the substrate, and diffuses into the layer containing niobium on the surface of the substrate. Then, a reaction containing an alkali metal and niobium occurs, and a photocatalytic thin film containing a niobium-alkali metal composite oxide is formed on the surface of the substrate. [0017] For example, as shown in FIG. 1, a layer containing niobium is formed on the surface of soda lime glass containing sodium and fired at a temperature of 500 ° C. or higher. Then, the sodium contained in the soda lime glass is heated and precipitated on the surface of the substrate, and diffuses into the layer containing niobium on the surface of the soda lime glass. Then, a reaction involving soda lime glass and niobium occurs, and a niobium-sodium composite oxide (for example, NaNbO) is formed on the surface of the soda lime glass.₃) Is formed. [0018] That is, in the method for forming the photocatalyst thin film according to the first phase, the alkali metal contained in the substrate, which has conventionally been prevented from reacting with the base film or the like because it deteriorates the photocatalytic performance, is positively used on the surface of the substrate. A photocatalytic thin film containing a niobium-alkali metal composite oxide is formed. [0019] The photocatalytic activity of this niobium-alkali metal composite oxide is shown in "H. Kato et.al., J. Phys. Chem. B, 106, 12441-12447 (2002)." That is, this niobium-alkali metal composite oxide has a self-purifying effect. [0020] Further, since the photocatalyst thin film containing the niobium-alkali metal composite oxide formed by the above forming method is fired at a temperature of 500 ° C. or higher, it has high hardness and high adhesion to the substrate. Has excellent durability. [0021] In addition, in order to optimize the photocatalytic activity, an alkali metal component may be added to the layer containing niobium. [0022] By the way, in order to form a layer containing niobium on the surface of a substrate containing an alkali metal, a vapor phase method such as sputtering or chemical vapor deposition (CVD) may be used, or a solugel solution using niobium alkoxide as a raw material or an organic containing niobium. A liquid phase method using a solution of a metal compound or a coating solution such as a dispersion of fine particle niobium oxide may be used. [0023] In the case of liquid phase coating, all coating methods generally used for liquid phase coating such as spin coating method, dip coating method, spray coating method and roll coating method can be applied. [0024] In the method for forming a photocatalytic thin film according to the first aspect of the present invention, in the liquid phase coating, the layer containing niobium is preferably formed by applying a coating agent containing niobium (the present invention). Second phase). [0025] In this way, the layer can be formed by a simple method of applying a coating agent containing niobium 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. [0026] The method for forming a photocatalytic thin film according to the third aspect of the present invention is to form a layer containing tantalum on the surface of a substrate containing an alkali metal and fire it at a temperature of 500 ° C. or higher to obtain a tantalum-alkali metal composite oxide. It is characterized by forming a photocatalyst thin film containing. [0027] Similar to the method for forming the photocatalyst thin film according to the first phase, the method for forming the photocatalyst thin film according to the third phase also positively utilizes the alkali metal contained in the substrate to form a tantalum-alkali metal composite oxidation on the surface of the substrate. It forms a photocatalytic thin film containing substances. [0028] The photocatalytic activity of tantalum-alkali metal composite oxides is shown in "H. Kato et. Al., J. Am. Chem. Soc., 125, 3082-3089 (2003)." That is, this tantalum-alkali metal composite oxide has a self-cleaning effect. [0029] Further, this photocatalyst thin film containing a tantalum-alkali metal composite oxide has high hardness because it is fired at a temperature of 500 ° C. or higher like the above-mentioned photocatalyst thin film containing a niobium-alkali metal composite oxide. , It has excellent durability because it has high adhesion to the substrate. [0030] In addition, in order to optimize the photocatalytic activity, an alkali metal component may be added to the layer containing tantalum. [0031] To form a layer containing tantalum on the surface of a substrate containing alkali metal, a vapor phase method such as sputtering or chemical vapor deposition (CVD) may be used, or a solugel solution using tantalum alkoxide as a raw material or an organic metal compound containing tantalum. Or a liquid phase method using a coating solution such as a dispersion of fine tantalum oxide may be used. In the case of liquid phase coating, all coating methods generally used for liquid phase coating such as spin coating method, dip coating method, spray coating method and roll coating method can be applied. [0032] In the method for forming a photocatalytic thin film according to the third aspect of the present invention, in the liquid phase coating, the layer containing tantalum is preferably formed by applying a coating agent containing tantalum. [0033] In this way, the layer can be formed by a simple method of applying a coating agent containing tantalum 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 tantalum-alkali metal composite oxide can be formed at low cost. [0034] By the way, there are various substrates containing the alkali metal, but in any of the inventions of the first to fourth aspects, the substrate containing the alkali metal is soda lime glass (the first aspect of the present invention). (In the case of 5 phases), a glassy film formed on the surface of ceramics by glaze (in the case of the 6th phase of the present invention), or a vitreous film formed on the surface of a metal product by enamel (7th aspect of the present invention). In the case of a situation). Since these substrates contain sodium, which is an alkali metal, when a layer containing niobium or tantalum is formed on the surface thereof and fired, sodium precipitates and diffuses into the layer. [0035] Then, a reaction involving sodium and niobium or tantalum occurs, and a niobium-sodium composite oxide (for example, NaNbO) occurs.₃) Thin films and tantalum-sodium composite oxides (eg NaTaO)₃) A thin film is formed on the surface of the substrate containing sodium. [0036] In this way, soda lime, 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 photocatalytic thin film can be formed at low cost. it can. [0037] [0038] Further, when the temperature in the firing is 500 to 550 ° C in any of the inventions of the first to seventh aspects of the present invention (in the case of the eighth aspect of the present invention), the photoinduced superhydrophilicity is remarkable. It is expressed in and also shows remarkable 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 improved. [0039] The photocatalytic thin film according to the ninth aspect of the present invention is a photocatalytic thin film formed by the method for forming a photocatalytic thin film according to any one of the first to eighth aspects of the present invention, and the alkali metal is sodium. It is a photocatalytic thin film characterized by being. [0040] The photocatalytic thin film formed by this forming method is fired at a temperature sufficient for the substrate and the film to be in close contact with each other, so that the photocatalyst thin film has high hardness and excellent durability. Further, since it forms a niobium-sodium composite oxide or a tantalum-sodium composite oxide, it has both superhydrophilicity and photocatalytic activity, and has a self-cleaning function. [0041] The photocatalytic thin film-coated product according to the tenth aspect of the present invention is a photocatalyst thin film-coated product comprising a substrate containing an alkali metal and a photocatalytic thin film according to the ninth aspect provided on the surface of the substrate. [0042] In this photocatalyst thin film coated product, the hardness of the photocatalyst thin film is high and the adhesion to the substrate is high, so that the durability is excellent. In particular, when the photocatalyst thin film is fired at a high temperature, the adhesion and durability are further excellent. Further, since it has a thin film of niobium-sodium composite oxide or tantalum-sodium composite oxide, it has both superhydrophilicity and photocatalytic activity, and is excellent in self-cleaning function. [0043] 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. [0044] 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] [0045] FIG. 1 is a diagram illustrating how a reaction containing niobium with lime soda glass occurs and a photocatalytic thin film containing a niobium-sodium oxide compound is formed on the surface of the lime soda glass. FIG. 2 is a chart showing the results of analyzing the crystal state of a niobium oxide thin film formed on soda lime glass by X-ray diffraction. [Best mode for carrying out the invention] [0046] Hereinafter, the present invention will be described based on examples. [Example] The present invention will be described with reference to examples. [0047] (a) Manufacture of coating liquid Niobium (V) ethoxide was added dropwise to an appropriate amount of toluene mixed with catechol so that the molar ratio of niobium (V) ethoxydo to catechol was 2: 5. The total amount of solvent was adjusted so that the concentration of niobium (V) ethoxydo was 0.5 M. [0048] Half of the solution was distilled. It was confirmed that the steam temperature reached 100 ° C during distillation, and the volatile components of the mixed solution remaining after distillation were evaporated under reduced pressure of the rotary evaporator. The remaining solid content was further heated to 100 ° C. under vacuum using a vacuum dryer to completely remove the remaining volatile components. Drying was continued until an amount close to the theoretical value (about 98%) of solid matter (niobium catecholate) was obtained. [0049] In this example, a 56.0 g, 98% solid (niobium catecholate) is obtained from a solution of niobium (V) ethoxydo (50.0 g, 0.157 mol), catechol (43.3 g, 0.393 mol) and toluene (320 mL). I was able to. [0050] The obtained solid substance (niobium catecholate) is dissolved in a mixed solution of acetylacetone and toluene at a volume ratio of 1: 4, adjusted so that the concentration of niobium is 0.10 M or 4 wt.%, And applied. It was made into a liquid. [0051] (b) Formation of thin film 0.4 mL of the obtained coating liquid was coated on a 50 × 50 × 3 tmm soda lime glass substrate (SLG) or Pyrex (registered trademark) glass substrate (PYR) by a spin coating method (rotation speed at the time of coating 750 rpm). [0052] The coated glass substrate coated with the coating liquid was fired in an electric furnace to obtain niobium oxide coated glass. The firing temperature was 400,450,500,550,600 ° C, the firing time was 1 hour (heating temperature 10 ° C / min, natural cooling), and firing was performed in the atmosphere. [0053] After firing, a good thin film could be obtained on the coated glass substrate. [0054] (c) Confirmation of film formation state (niobium oxide) The total transmittance, turbidity and color difference of the niobium oxide coated glass thus formed were measured. The total transmittance and turbidity were measured based on JIS K7136 using NDH-500W manufactured by Nippon Denshoku Kogyo Co., Ltd. The color difference was measured based on JIS Z 8722 using SE-2000 of Nippon Denshoku Kogyo Co., Ltd. The results are shown in Table 1 below. [0055] [table 1]<img file="JP5118068B2_D0001.tif" />(d) Photoinduced reduction reaction test of silver nitrate (niobium oxide) The niobium oxide-coated glass obtained in Example (b) was irradiated with artificial sunlight 5 mm below the water surface of a 0.01 M silver nitrate aqueous solution for 1 hour, and the photocatalytic activity of each sample was determined by the presence or absence of precipitation of metallic silver due to a photocatalytic reaction. Was compared and evaluated. The results are shown in Table 2 below. The precipitation of silver (presence or absence of reaction) was visually confirmed. [0056] [Table 2]<img file="JP5118068B2_D0002.tif" />(e) Confirmation of photoinduced superhydrophilicity (niobium oxide) Table 3 below shows the changes in the contact angle of water due to UV irradiation of the niobium oxide-coated glass obtained in Example (b). UV irradiation is 2.4mW / cm using a 40WBLB lamp²The intensity was (intensity when the wavelength of light was 365 nm). [0057] [Table 3]<img file="JP5118068B2_D0003.tif" />(f) Confirmation of crystalline state (niobium oxide) In Example (b), the crystal state of the niobium oxide thin film formed on the soda lime glass was analyzed by X-ray diffraction. The measurement was performed using a thin film X-ray diffractometer (D8 DISCOVER) manufactured by Bruker AXS Co., Ltd. The result is shown in figure 2. [Comparison example] (f) Production of coating liquid (complex of titanium (IV) isopropoxide (TTIP) and catechol) Titanium (IV) isopropoxide (Titanium (IV) isopropoxide) was added dropwise to a mixture of an appropriate amount of toluene and catechol so that the molar ratio of TTIP to catechol was 1: 2. The total amount of solvent was adjusted so that the concentration of TTIP was 0.5M. [0058] [0058] Then half of the solution was distilled. It was confirmed that the steam temperature reached 100 ° C during distillation, and the volatile components of the mixed solution remaining after distillation were evaporated under reduced pressure of the rotary evaporator. The remaining solid content was further heated to 100 ° C. under vacuum using a vacuum dryer to completely remove the remaining volatile components. Drying was continued until an amount close to the theoretical value (about 98%) of solid matter (titanium catecholate) was obtained. [0059] In this comparative example, 38.8 g, 98% solid matter (titanium catecholate) could be obtained from a solution of TTIP (42.6 g, 0.150 mol), catechol (33.0 g, 0.300 mol), and toluene (300 mL). .. [0060] The obtained solid substance (titanium catecholate) is dissolved in a mixed solution of acetylacetone and toluene at a volume ratio of 1: 4, adjusted so that the titanium concentration is 0.10 M or 3 wt.%, And applied. It was made into a liquid. [0061] (g) Thin film formation 0.4 mL of the obtained coating liquid was coated on a 50 × 50 × 3 tmm soda lime glass substrate (SLG) or Pyrex (registered trademark) glass substrate (PYR) by a spin coating method (rotation speed at the time of coating 750 rpm). [0062] The coated glass substrate coated with the coating liquid was fired in an electric furnace to obtain a titanium oxide coated glass. The firing temperature was 500 ° C., the firing time was 1 hour (heating temperature 10 ° C / min, natural cooling), and firing was performed in the atmosphere. [0063] After firing, a good thin film could be obtained on the coated glass substrate. [0064] (h) Confirmation of film formation state (titanium oxide) The total transmittance, turbidity and color difference of the titanium oxide coated glass thus formed were measured. The measurement was carried out in the same manner as in (c) of the above-mentioned Example. The measurement results are shown in Table 1 above. [0065] (i) Photoinduced reduction reaction test of silver nitrate (titanium oxide) The titanium oxide-coated glass obtained in Comparative Example (g) was irradiated with artificial sunlight 5 mm below the water surface of a 0.01 M silver nitrate aqueous solution for 1 hour, and the photocatalytic activity of each sample was determined by the presence or absence of precipitation of metallic silver due to a photocatalytic reaction. Was compared and evaluated. The results are shown in Table 2 above. The precipitation of silver (presence or absence of reaction) was visually confirmed. [0066] (j) Confirmation of photoinduced superhydrophilicity (titanium oxide) Table 3 shows the changes in the contact angle of water due to UV irradiation of the titanium oxide coated glass obtained in Comparative Example (g). UV irradiation is 2.4mW / cm using a 40WBLB lamp²The intensity was (intensity when the wavelength of light was 365 nm). [0067] [Comparison result] (Comparison result 1) Table 2 shows the photocatalytic activity of the niobium oxide thin film formed on soda lime glass (normal glass) containing a large amount of sodium ions. For comparison, the photocatalytic activity of the niobium oxide thin film on Pyrex® glass, the titanium oxide thin film on average glass, and the titanium oxide thin film on Pyrex® glass with low sodium ion content is shown. There is. From Table 2, the following three points can be seen. [0068] (1) In the case of niobium oxide thin film / normal glass, photocatalytic activity is exhibited at 500 ° C or higher, but photocatalytic activity is particularly exhibited at a firing temperature of 500-550 ° C. [0069] (2) The expression of photocatalytic activity is inferior in niobium oxide thin film / Pyrex (registered trademark) glass. [0070] (3) In the case of titanium oxide thin film, Pyrex (registered trademark) glass exhibits good photocatalytic activity, but ordinary glass does not exhibit photocatalytic activity. [0071] That is, it can be said that the photocatalytic activity is lost when a large amount of sodium ions are present in the titanium oxide thin film, whereas the photocatalyst activity is better when sodium ions are incorporated in the niobium oxide thin film. [0072] When firing at a low temperature (450 ° C or lower), the amount of sodium diffused is small, and even if film formation is possible, photocatalytic activity does not appear, and sodium ions sufficient for photocatalytic activity to appear at firing at 500 ° C or higher. Is thought to have spread. [0073] FIG. 2 shows an X-ray diffraction image of a niobium oxide thin film formed on zodalime glass. No diffraction peak was observed after firing at 400 ° C for 1 hour, indicating that the state is amorphous. On the other hand, when firing at a temperature of 450 ° C or higher, sodium niobate (NaNbO) indicated by ₃) Is generated, and it can be seen that a composite oxide of niobium and sodium is formed. In particular, large peaks were observed in the films fired at 500 ° C and 550 ° C, suggesting that crystallization progressed and high photocatalytic activity was exhibited. [0074] (Comparison result 2) Table 3 compares the photosuperhydrophilicity of niobium oxide thin films and titanium oxide thin films formed on ordinary glass and Pyrex® glass. From Table 3, the following two points can be seen. [0075] (1) The niobium oxide thin film shows a tendency to become hydrophilic at 400 ° C or higher, but particularly shows superhydrophilicity at a firing temperature of 500 to 550 ° C. [0076] (2) In the case of a titanium oxide thin film, Pyrex (registered trademark) glass exhibits superhydrophilicity, but ordinary glass does not. [0077] In the titanium oxide thin film, the activity is inhibited by sodium ions in superhydrophilicity as in the photocatalytic activity shown in Comparative Result 1, but in the niobium oxide thin film, the superhydrophilicity is lost even in the presence of sodium ions. I can't. It is possible that the reason why the hydrophilic activity is slightly poor after firing at 600 ° C is that it has changed to another crystal system, but there is no confirmation at present. [0078] From the above results, it can be said that the niobium oxide thin film is effective as an effective optical self-cleaning coating for the substrate on which sodium ions are diffused. [0079] 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. [0080] [0080] For example, an alkali metal component such as sodium may be added to the coating liquid composition in order to optimize the photocatalytic activity.