JP7539780B2 - Titania nanoparticles carrying metal nanoparticles and photocatalysts using the same - Google Patents

Description

The present invention relates to titania nanoparticles carrying metal nanoparticles and photocatalysts using the same.

Visible light responsive titania (titanium oxide) is used as a photocatalytic material. In general, titania powder is dispersed in a solvent.

J. Phys. Chem. B 2003, 107, 14336-14341

Visible light responsive photocatalysts are mainly tungsten oxide-based photocatalysts and titania-based photocatalysts, both of which tend to be highly crystalline. The higher the crystallinity, the higher the refractive index of the oxide, which means that many of these have a very strong whiteness and poor transparency. In recent years, products using visible light responsive photocatalysts include paints that utilize the photocatalytic properties to have antibacterial and antiviral effects, but transparency is an issue from the standpoint of design.

In addition, because the surface is not modified, it is difficult to disperse in water or alcohol, making it difficult to apply evenly and resulting in a lack of transparency. Even if it were possible to apply it, it would shrink significantly when dried, making it prone to cracking and peeling. Furthermore, even if an organic dispersant were used, there would be the problem that the surface activity of titania or tungsten oxide would be impaired.

Conventional methods for synthesizing visible light-responsive photocatalysts include introducing elements such as nitrogen into the crystal structure of titania or tungsten oxide, and supporting metal compounds on oxides. However, these methods have problems such as extremely low photocatalytic activity compared to ultraviolet light-responsive photocatalysts, because the excitation wavelength is visible light with low energy compared to ultraviolet light, and excited electrons in oxides are easily deactivated by recombination.

Therefore, the present invention aims to obtain visible light responsive titania microparticles that have coatability, visible light catalytic activity, and transparency that could not be achieved with existing visible light responsive photocatalysts.

In view of the above object, the present inventors conducted intensive research and found that all of the above problems can be solved by using titania nanoparticles carrying metal nanoparticles, in which acyloxy groups are bonded to at least some of the titanium atoms present on the surface, and in which the mass loss at 200°C or higher is 5% or more when heated to 600°C using a simultaneous differential thermal and thermogravimetric analyzer, and in which metal elements are carried in an amount of 90% or less by mass relative to the remaining solid content of the titania sol when heated at 200°C. Then, after further research, the present invention was completed. That is, the present invention includes the following configurations.

Item 1. A metal nanoparticle-supported titania nanoparticle in which metal nanoparticles are supported on the surface of titania nanoparticles,
The titania nanoparticles have acyloxy groups bonded to at least some of the titanium atoms present on the surface,
When the titania nanoparticles are heated to 600°C using a thermogravimetric and differential thermal analyzer, the mass loss at 200°C or higher is 5% by mass or more; and
Titania nanoparticles carrying metal nanoparticles, the metal nanoparticles being carried on the surface thereof in an amount of 90 mass % or less relative to titanium oxide in the titania nanoparticles.
Item 2. The metal nanoparticle-supporting titania nanoparticle according to Item 1, wherein the acyloxy group is bonded to a titanium atom via a group represented by -OCOR (wherein R represents a hydrogen atom, an alkyl group having 1 to 3 carbon atoms, or a hydroxyalkyl group having 1 to 2 carbon atoms).
Item 3. The metal nanoparticle-supporting titania nanoparticle according to Item 1, wherein the acyloxy group is an acyloxy group derived from at least one organic acid selected from the group consisting of monocarboxylic acids having 1 to 4 carbon atoms and hydroxycarboxylic acids having 2 to 3 carbon atoms.
Item 4. The metal nanoparticle-supporting titania nanoparticles according to Item 3, wherein the organic acid is acetic acid.
Item 5. The metal nanoparticle-supporting titania nanoparticles according to any one of Items 1 to 4, wherein the specific surface area of the titania nanoparticles is 150 to 500 m ² /g.
Item 6. The metal nanoparticle-supporting titania nanoparticles according to any one of Items 1 to 5, wherein the titania nanoparticles do not contain any crystal form other than the anatase type.
Item 7. The metal nanoparticle-supporting titania nanoparticle according to any one of Items 1 to 6, wherein the metal constituting the metal nanoparticle is at least one metal selected from the group consisting of platinum group elements, iron group elements, and Group 11 elements of the periodic table.
Item 8. A photocatalyst comprising the metal nanoparticle-supporting titania nanoparticles according to any one of items 1 to 7.
Item 9. A method for producing the metal nanoparticle-supporting titania nanoparticles according to any one of items 1 to 7 or the photocatalyst according to item 8, comprising:
(A) mixing a titanium-containing substance, an organic acid, and water to obtain a dispersion;
(B) heating the dispersion obtained in step (A) at a temperature higher than 80° C. for 1 hour or more; and (C) mixing the dispersion obtained in step (B) with a metal compound and irradiating the resulting dispersion with ultraviolet light, or (C') mixing the dispersion obtained in step (B) with metal nanoparticles.
Item 10. The method according to Item 9, wherein in the step (C), ultraviolet light of 10 μW/m ² or more is irradiated for 5 minutes or more.
Item 11. The method according to item 9 or 10, wherein in the step (A), the mixing ratio of the titanium-containing substance and the organic acid is such that the amount of acyloxy groups in the organic acid is 1.5 moles or more per mole of titanium in the titanium-containing substance.
Item 12. The method according to any one of Items 9 to 11, wherein in the step (C), the amount of the metal compound used is 90 mass % or less based on the solid content when the dispersion obtained in the step (B) is heated at 200° C., in terms of the mass of the metal element in the metal compound.
Item 13. The method according to any one of Items 9 to 12, wherein the ultraviolet light irradiation in the step (C) is carried out at room temperature (20° C.) or higher.
Item 14. A dispersion of metal nanoparticles-supporting titania nanoparticles, comprising 50 mass% or more of water and the metal nanoparticle-supporting titania nanoparticles according to any one of items 1 to 7 or the photocatalyst according to item 8.
Item 15. The dispersion of metal nanoparticles-supporting titania nanoparticles according to Item 14, which does not contain any organic dispersant other than the organic acid.

According to the present invention, it is possible to obtain visible light responsive metal nanoparticle-supported titania nanoparticles that have the coatability, photocatalytic activity, and transparency that could not be achieved with existing visible light responsive tungsten oxide photocatalysts and titania photocatalysts.

In this specification, "containing" is a concept that encompasses all of "comprise," "consist essentially of," and "consist only of." In this specification, when a numerical range is expressed as A to B, it means A or more and B or less.

In this specification, "titanium oxide" or "titania" does not _only refer to titanium dioxide ( _TiO2 ), but also includes titanium trioxide ( _Ti2O3 ); titanium monoxide (TiO); and titanium dioxide with oxygen deficiency, such as _{Ti4O7 and Ti5O9} . Titanium oxide may also contain groups other than Ti- _O -Ti, such as terminal OH groups, which are partially derived from the synthesis of titanium oxide. Titanium oxide having an organic acid or the like bonded to the terminal OH group is also included.

1. Titania nanoparticles carrying metal nanoparticles and photocatalyst The titania nanoparticles carrying metal nanoparticles of the present invention are titania nanoparticles carrying metal nanoparticles on the surface of the titania nanoparticles, in which acyloxy groups are bonded to at least some of the titanium atoms present on the surface of the titania nanoparticles, the metal nanoparticles are carried on the surface in an amount of 90% by mass or less relative to the titanium oxide in the titania nanoparticles, and the mass loss at 200°C or higher when heated to 600°C using a thermogravimetry and differential thermal analyzer is 5% by mass or more.

By adopting such a configuration, the metal nanoparticle-supported titania nanoparticles of the present invention can adjust the average particle size and specific surface area of both the titania nanoparticles and the metal nanoparticles, and the titania nanoparticles and the metal nanoparticles are firmly attached to each other, and are highly dispersible and difficult to aggregate, so that they have excellent coatability and transparency without the use of additional additives such as dispersants. In addition, the metal nanoparticle-supported titania nanoparticles and photocatalyst of the present invention have sufficient coatability and transparency without the use of additional dispersants, so that the visible light catalytic activity is not impaired by additives such as dispersants, and they can have excellent visible light catalytic activity. For this reason, the metal nanoparticle-supported titania nanoparticles of the present invention are useful as photocatalysts (especially visible light responsive photocatalysts).

(1-1) Titania nanoparticles Usually, water, inorganic acids, free organic acids, etc. are almost all volatilized at 200 ° C. or less. On the other hand, the titania nanoparticles constituting the metal nanoparticle-supported titania nanoparticles of the present invention are gradually desorbed in the range of 200 to 600 ° C. because an acyloxy group is bonded to at least a part of the titanium atoms present on the surface. For example, in the case of an acetoxy group, it gradually desorbs in the range of 200 to 600 ° C. with a peak at about 260 ° C. Thus, the titania nanoparticles constituting the metal nanoparticle-supported titania nanoparticles of the present invention are bonded to at least a part of the titanium atoms present on the surface, so that the aggregation of the titania nanoparticles during drying or baking is suppressed, so that cracks, peeling, etc. are unlikely to occur, and the coating properties and transparency are particularly excellent, and cracks, peeling, etc. can be suppressed, and furthermore, the metal nanoparticles described later can be easily supported firmly, resulting in excellent visible light catalytic activity. It is generally known in the art that the presence of acyloxy groups on the surface normally reduces visible light catalytic activity. However, in the present invention, as described above, the aggregation of titania nanoparticles during drying or calcination can be suppressed, and therefore the effect of suppressing cracks, peeling, etc. is particularly excellent. In addition, since the metal nanoparticles described below can be easily firmly supported, the visible light catalytic activity can be improved despite the presence of acyloxy groups.

In addition, the titania nanoparticles preferably have a large amount of acyloxy groups bonded to titanium atoms present on the surface. When acyloxy groups are present on at least some of the titanium atoms present on the surface, as described above, they are gradually removed in the range of 200 to 600°C, and therefore, when heated using a thermogravimetry and differential thermal analyzer (TG-DTA), the mass loss at 200°C or higher is large. In other words, in the present invention, when heated using a thermogravimetry and differential thermal analyzer (TG-DTA), the mass loss at 200°C or higher means an index of the number of acetoxy groups bonded to titanium atoms present on the surface. For this reason, when heated to 600°C using a thermogravimetry and differential thermal analyzer (TG-DTA), the mass loss at 200°C or higher is 5% by mass or more, preferably 7 to 20% by mass. In this case, the detailed conditions for the thermogravimetry and differential thermal analyzer (TG-DTA) are: atmosphere: air, heating rate: 3°C/min.

As described above, the titania nanoparticles have acyloxy groups bonded to at least some of the titanium atoms present on the surface, and it is preferable that the acyloxy groups are bonded to the titanium atoms via groups represented by -OCOR (wherein R represents a hydrogen atom, an alkyl group having 1 to 3 carbon atoms, or a hydroxyalkyl group having 1 to 2 carbon atoms). In other words, it is preferable that the acyloxy groups are acyloxy groups derived from organic acids such as monocarboxylic acids having 1 to 4 carbon atoms or hydroxycarboxylic acids having 2 to 3 carbon atoms.

In the above R, examples of the alkyl group include a methyl group, an ethyl group, and an n-propyl group, and examples of the hydroxyalkyl group include a hydroxymethyl group, a 1-hydroxyethyl group, and a 2-hydroxyethyl group. In other words, examples of the monocarboxylic acid include formic acid, acetic acid, propionic acid, and butyric acid, and examples of the hydroxycarboxylic acid include glycolic acid and lactic acid.

From the viewpoints of volatility, harmfulness, and decomposability, R is preferably a hydrogen atom, a methyl group, a hydroxymethyl group, a 1-hydroxyethyl group, a 2-hydroxyethyl group, or the like, and from the viewpoints of water solubility and odor, a methyl group is preferred. That is, from the viewpoints of volatility, harmfulness, and decomposability, formic acid, acetic acid, and the like are preferred as monocarboxylic acids, and glycolic acid, lactic acid, and the like are preferred as hydroxycarboxylic acids. Also, acetic acid is particularly preferred from the viewpoints of water solubility and odor. These organic acids can be used alone or in combination of two or more kinds.

The average particle size of the titania nanoparticles is preferably 1 to 5 nm, and more preferably 2 to 4 nm. By setting the average particle size of the titania nanoparticles in this range, the metal nanoparticles can be supported appropriately and more firmly, and a film with higher visible light catalytic activity and higher transparency can be formed. In addition, when the average particle size is small, the shrinkage during heating is large, and cracks and peeling from the substrate tend to occur easily. However, the metal nanoparticle-supported titania nanoparticles of the present invention are a material with excellent coatability despite the use of titania nanoparticles with a small average particle size. In the present invention, the average particle size of the titania nanoparticles is measured by observation with a transmission electron microscope (TEM).

The specific surface area of the titania nanoparticles is preferably 150 to 500 m ² /g, more preferably 200 to 400 m ² /g. By setting the specific surface area of the titania nanoparticles in this range, the metal nanoparticles can be supported appropriately and more firmly, and the visible light catalytic activity can be easily increased. The specific surface area of the titania nanoparticles is measured by the BET method.

In addition, the titania nanoparticles can have N, Cl and S element concentrations of 0 to 5000 ppm, particularly 0 to 1000 ppm. By setting the N, Cl and S element concentrations of the titania nanoparticles in this range, corrosion of the substrate is easily suppressed. This condition means that there is no impurity derived from an acidic titania precursor such as TiCl ₄ or TiOSO ₄ , or there is only a small amount of impurities. The N, Cl and S element concentrations of the titania nanoparticles are measured by WDX (fluorescent X-ray).

Furthermore, the crystal form of the titania nanoparticles is preferably anatase type. By adopting anatase type, the visible light catalytic activity can be particularly improved. Also, for the same reason, it is preferable that there are no crystal forms other than anatase type, and that the anatase type is 100%.

(1-2) Metal Nanoparticles The metal constituting the metal nanoparticles is not particularly limited, and in addition to platinum group elements such as platinum, palladium, ruthenium, rhodium, iridium, etc., iron group elements such as iron, cobalt, nickel, etc., and elements of Group 11 of the periodic table such as gold, silver, copper, etc. can also be used. Among these, platinum group elements are preferred, and platinum is more preferred, from the viewpoint of being easily supported on the surface of titania nanoparticles and easily improving the visible light catalytic activity.

The average particle size of the metal nanoparticles is preferably 100 nm or less, and more preferably 1 to 10 nm. By setting the average particle size of the supported metal nanoparticles within this range, it is easier to form a film with improved visible light catalytic activity and improved transparency. The average particle size of the metal nanoparticles is measured by observation with an electron microscope.

In the present invention, the amount of metal nanoparticles supported on the surface of titania nanoparticles is 90% by mass or less, preferably 0.001 to 20% by mass, and more preferably 0.01 to 2% by mass, relative to the titanium oxide in the titania nanoparticles. By setting the amount within this range, it is possible to particularly improve the visible light catalytic activity while maintaining the coatability and transparency. Note that if the amount of metal nanoparticles supported exceeds 90% by mass, the metal complex cannot be made into nanoparticles.

2. Method for producing titania nanoparticles carrying metal nanoparticles and photocatalyst As a method for carrying the above-mentioned metal nanoparticles on titania nanoparticles, there are a photoprecipitation method in which a metal compound and titania nanoparticles are mixed and then irradiated with light to carry them, and a method in which metal nanoparticles synthesized by any method are mixed with titania nanoparticles and carried. In the present invention, the photoprecipitation synthesis method is preferred from the viewpoint of visible light catalytic activity.

Specifically, the metal nanoparticle-supporting titania nanoparticles and photocatalyst of the present invention are
(A) mixing a titanium-containing substance, an organic acid, and water to obtain a dispersion;
(B) a step of heating the dispersion obtained in the step (A) at a temperature higher than 80° C. for 1 hour or more, and (C) a step of irradiating the dispersion obtained by mixing the dispersion obtained in the step (B) with a metal compound with ultraviolet light.

From the viewpoint of visible light catalytic activity, the photoprecipitation synthesis method is preferable, but the mixed support method is also adopted.
(A) mixing a titanium-containing substance, an organic acid, and water to obtain a dispersion;
The dispersion liquid obtained in the step (A) can also be produced by a method comprising the steps of: (B) heating the dispersion liquid obtained in the step (A) at a temperature higher than 80° C. for 1 hour or more; and (C') mixing the dispersion liquid obtained in the step (B) with metal nanoparticles.

(2-1) Process (A)
In step (A), a titanium-containing substance, an organic acid and water are mixed to obtain a dispersion.

There are no particular limitations on the titanium-containing substance used, so long as it is a substance that becomes titanium oxide when heated. In other words, titanium oxide and/or titanium oxide precursors are preferred as titanium-containing substances, and specific examples include titanium oxide; titanium hydroxide; titanium alkoxide; titanium halides such as titanium trichloride and titanium tetrachloride (particularly those neutralized with a base); and metallic titanium. These titanium-containing substances can be used alone or in combination of two or more. Among these, titanium alkoxide, titanium hydroxide, or titanium halide (particularly those neutralized with a base) are preferred from the viewpoints of dispersibility, coatability, and visible light catalysis of the resulting titania, and titanium alkoxide is more preferred from the viewpoints of purity, dispersibility, coatability, transparency, and visible light catalysis.

Titanium alkoxides include titanium tetraisopropoxide, titanium tetra n-butoxide, titanium tetra n-propoxide, titanium tetraethoxide, etc., and titanium tetraisopropoxide is preferred from the standpoints of cost, water solubility of by-products, coatability, and visible light catalytic properties.

Depending on the combination of titanium alkoxide and organic acid, the resulting titania may act as a catalyst to liberate an ester compound that is poorly soluble in water, but this does not pose a problem for the titania itself (for example, in the combination of titanium tetra n-butoxide and acetic acid, butyl acetate is produced and liberated when mixed and heated). However, from the perspective of obtaining a uniform dispersion, it is preferable to use a combination of organic acid and titanium alkoxide that produces an organic acid alkoxide with excellent water solubility.

Titanium halides (titanium tetrachloride, titanium trichloride, etc.) are preferably neutralized with a base and the precipitate washed before use from the viewpoints of impurities (halogens), corrosion of the reactor during mass production, crystallinity control, coatability, transparency, and visible light catalysis. In that case, from the viewpoint of dispersibility of the resulting titania, it is preferable to use it without drying.

When using solids such as titanium oxide and titanium metal, the average particle size is preferably 100 nm or less, more preferably 50 nm or less. There is no particular lower limit, but it is usually about 1 nm. If the particle size is large, it can be ground in a dry or wet manner using a planetary ball mill, paint shaker, etc. The average particle size of solids such as titanium oxide and titanium metal is measured by observation with a transmission electron microscope (TEM).

The concentration of the titanium-containing substance in the dispersion liquid produced in step (A) is preferably 0.01 to 5 mol/L, more preferably 0.05 to 3 mol/L, from the viewpoints of productivity, viscosity of the reaction liquid, coatability, transparency, and visible light catalytic properties.

The acid used in the reaction is an organic acid, and since a volatile acid is preferable, examples of the acid include a monocarboxylic acid represented by the chemical formula C _n H _2n+1 COOH (n = 0 to 3) (monocarboxylic acid having 1 to 4 carbon atoms) and a hydroxycarboxylic acid having 2 to 3 carbon atoms.

From the viewpoints of volatility, toxicity, and decomposability, formic acid (n=0) and acetic acid (n=1) are preferred as monocarboxylic acids, and glycolic acid, lactic acid, etc. are preferred as hydroxycarboxylic acids, with acetic acid being particularly preferred from the viewpoints of water solubility and odor. These organic acids can be used alone or in combination of two or more kinds.

From the viewpoints of dispersibility, coatability, transparency, visible light catalysis, and cost, it is preferable to adjust the amount of organic acid used so that the number of moles of acyloxy groups per mole of titanium in the titanium-containing substance is 1.5 moles or more, and particularly 2 moles or more. The more organic acid is used, the easier it is to improve stability over time, coatability, transparency, etc. There is no particular upper limit, but it is preferable to adjust the number of moles of acyloxy groups per mole of titanium in the titanium-containing substance so that it is usually 10 moles or less.

The concentration of the organic acid in the dispersion obtained in step (A) is preferably 0.02 to 10 mol/L, more preferably 0.1 to 7 mol/L, from the viewpoints of dispersibility, coatability, transparency, visible light catalytic properties, and cost.

As the reaction solvent, it is preferable to use an aqueous solvent such as water as the main component (specifically, for example, 50% by mass or more), but alcohol or ester may be included during the reaction.

For example, when titanium tetraisopropoxide is used as a raw material, isopropyl alcohol is produced by reaction with an organic acid. Also, isopropyl esters of organic acids may be produced by heating. In other words, alcohol or ester may be added to the dispersion liquid obtained by step (A), or may be generated in the system. This alcohol or ester may be removed by heating in an open system at 100°C or below, or may be removed by reducing pressure, or may remain in the reaction liquid.

When alcohol is contained in the dispersion, the average particle size of the obtained titania nanoparticles and the metal nanoparticle-supported titania nanoparticles of the present invention tends to be small, so alcohol may be intentionally added to control the average particle size.

In the present invention, it is preferable not to use inorganic acids (especially strong inorganic acids) such as nitric acid, hydrochloric acid, and sulfuric acid, which are usually used in the hydrothermal synthesis reaction of titania nanoparticles, not only because the crystal form of the obtained titania nanoparticles is a mixture of anatase type and brookite type, but also from the viewpoint of storage stability of the obtained dispersion, corrosion of the equipment, impurities, drainage, etc. However, when the dispersibility, transparency, uniformity, etc. of the raw material are improved to facilitate handling, they can be used supplementarily within a range that does not impair the effect, for example, in a range of 0.01 mol/L or less. In this case, the concentrations of N, Cl, and S elements in the dispersion obtained in step (A) are all 0.01 mol/L or less.

The pH of the dispersion obtained in step (A) is preferably 2 or more and less than 6, more preferably 2.1 to 5, from the viewpoints of equipment corrosion, handling safety, dispersibility, etc.

In step (A), the method of preparing the dispersion is not particularly limited, and the titanium-containing substance, organic acid, and water (solvent) may be mixed simultaneously or sequentially. In particular, on a mass production scale, it is preferable to mix the organic acid and water (solvent) and then add the titanium-containing substance while stirring, from the viewpoint of being less likely to aggregate and form large lumps, and making it easier to continue stirring. On the other hand, on a laboratory scale, it is preferable to mix the titanium-containing substance and organic acid and then add water while stirring.

(2-2) Process (B)
In step (B), the dispersion obtained in step (A) is heated at a temperature higher than 80° C. for 1 hour or more.

Step (B) may be carried out under normal pressure or under pressure in a closed vessel. From the viewpoint of reducing the average particle size of the titania nanoparticles and the metal nanoparticle-supported titania nanoparticles of the present invention, it is preferable to carry out the reaction under normal pressure, specifically 0.09 to 0.11 MPa. When carrying out the reaction under pressure, it is preferable to carry out the reaction for a short period of time (e.g., about 5 to 30 minutes) at 0.2 MPa or less (0.11 to 0.2 MPa) from the viewpoint of easily forming a film having high visible light catalytic activity and high transparency.

During heating, stirring is preferred from the viewpoint of thoroughly reacting the titanium-containing substance with the organic acid and water. There are no particular limitations on the stirring method, and conventional methods can be followed. Furthermore, from the viewpoint of thoroughly reacting the titanium-containing substance with the organic acid and water, the stirring time is preferably 1 hour or more, and more preferably 1.5 hours or more. There is no particular upper limit on the stirring time, but it is usually 240 hours.

The heating temperature is higher than 80°C, preferably 82°C or higher. If the heating temperature is lower than 80°C, cracks are likely to occur, the coating properties are poor, and the coating falls off easily, making it difficult to form a coating film. There is no particular upper limit to the heating temperature, but when the reaction is carried out at normal pressure, it is usually 120°C.

The pH of the dispersion obtained in step (B) is preferably 2 or more and less than 6, more preferably 2.1 to 5, from the viewpoints of equipment corrosion, handling safety, dispersibility, etc.

(2-3) Process (C)
In the step (C), the dispersion obtained by mixing the dispersion obtained in the step (B) with a metal compound is irradiated with ultraviolet light.

There are no particular limitations on the metal compound used, so long as it is a substance that becomes metal nanoparticles upon reduction. In other words, the metal compound is preferably a metal nanoparticle precursor, and more preferably a metal compound having an anion complex of a negatively charged metal. Specific examples include metal chloride ions (hexachloroplatinate (IV) ion, hexachloropalladate (IV) ion, hexachloroiridate (IV) ion, tetrachloroplatinate (II) ion, tetrachloropalladate (II) ion, tetrachloroaurate (III) ion, tetrachloroiron (III) ion, tetrachlorocopper (II) ion, dichlorocopper (I) ion, etc.), metal thiosulfate ions (bis(thiosulfato)iron (III) ion, etc.), ammine metal ions (dichlorodiammineplatinum (II), etc.), etc. These metal compounds may be hydrates. These metal compounds may be used alone or in combination of two or more.

In step (C), the amount of the metal compound used, converted into the mass of the metal element in the metal compound, is preferably 90 mass% or less, more preferably 0.001 to 20 mass%, and even more preferably 0.1 to 2 mass%, based on the solid content when the dispersion obtained in step (B) is heated at 200°C, from the viewpoints of dispersibility, transparency, visible light catalytic activity, and cost.

In step (C), the irradiation intensity of the ultraviolet light is preferably 10 μW/cm ^{2 or more, more preferably 100 μW/cm 2 or} more, and even more preferably 1 mW/cm ² or more, from the viewpoints of ease of supporting the metal nanoparticles on the titania nanoparticles, visible light catalytic activity, reaction rate, productivity, etc. The upper limit of the irradiation intensity of the ultraviolet light is not particularly limited, but is usually 2 W/cm ² .

In step (C), metal nanoparticles are easily generated on the surface of titania nanoparticles by irradiating with ultraviolet light for 5 minutes or more, but from the viewpoint of visible light catalytic activity, irradiation for 10 minutes or more is preferable, and irradiation for 20 minutes or more is more preferable. There is no particular upper limit on the ultraviolet light irradiation time, but it is usually 5 hours.

In step (C), the ultraviolet light may be irradiated in pulses repeatedly at a predetermined time interval, or may be irradiated continuously without interruption, and can be appropriately set according to the required characteristics.

In step (C), the ultraviolet light irradiation is preferably performed at room temperature (20°C) or higher, and more preferably at 50°C or higher, from the viewpoints of ease of supporting the metal nanoparticles on the titania nanoparticles, visible light catalytic activity, reaction rate, productivity, etc. There is no particular upper limit on the temperature, but when the reaction is performed at normal pressure, it is usually 100°C.

In step (C), during irradiation with ultraviolet light, it is preferable to stir the dispersion obtained in step (B) in order to sufficiently react with the metal compound. There are no particular limitations on the stirring method, and conventional methods can be used.

Step (C) may be carried out in an air atmosphere or an anaerobic atmosphere, specifically, in an inert gas atmosphere such as a nitrogen atmosphere or an argon atmosphere.

Then, the titania nanoparticles carrying metal nanoparticles of the present invention can be recovered by precipitating and centrifuging the titania nanoparticles carrying metal nanoparticles in a conventional manner. In other words, titania nanoparticles carrying metal nanoparticles on their surfaces can be obtained, in which a large amount of acyloxy groups are bonded to titanium atoms present on the surface.

The titania nanoparticles present in the dispersion obtained in step (B) are positively charged. The metal compound used in step (C) is usually negatively charged and can be attracted to the titania nanoparticles, and the resulting metal nanoparticle-supported titania nanoparticles can have the metal nanoparticles firmly attached to the titania nanoparticle surface.

(2-4) Process (C')
In the present invention, instead of the above-mentioned step (C), a step (C') can also be adopted in which the dispersion liquid obtained in the step (B) is mixed with metal nanoparticles.

There are no particular limitations on the metal nanoparticles to be used, and the above-mentioned metal nanoparticles can be used. These metal nanoparticles can be metal nanoparticles produced by known methods or commercially available metal nanoparticles.

In step (C'), the amount of metal nanoparticles used is preferably 90% by mass or less, more preferably 0.001 to 20% by mass, and even more preferably 0.1 to 2% by mass, based on the solid content of the dispersion obtained in step (B) when heated at 200°C, from the viewpoints of dispersibility, transparency, visible light catalytic activity, and cost.

In step (C'), the method for mixing the dispersion liquid obtained in step (B) with the metal nanoparticles is not particularly limited and can be performed according to a conventional method. For example, the metal nanoparticles can be added to the dispersion liquid obtained in step (B) and stirred. The stirring method is not particularly limited and can be performed according to a conventional method.

In step (C'), the dispersion obtained in step (B) is mixed with the metal nanoparticles at room temperature (20°C) or higher, and more preferably at 50°C or higher, from the viewpoints of ease of supporting the metal nanoparticles on the titania nanoparticles, visible light catalytic activity, reaction rate, productivity, etc. There is no particular upper limit on the temperature, but when the reaction is carried out at normal pressure, it is usually 100°C.

Step (C') may be carried out in an air atmosphere or an anaerobic atmosphere. Specific examples of an anaerobic atmosphere include an inert gas atmosphere such as a nitrogen atmosphere or an argon atmosphere.

Then, the titania nanoparticles carrying metal nanoparticles of the present invention can be recovered by precipitating and centrifuging the titania nanoparticles carrying metal nanoparticles in a conventional manner. In other words, a large amount of acyloxy groups are bonded to the titanium atoms present on the surface, and the metal nanoparticles are supported on the surface, resulting in titania nanoparticles that are firmly adhered to the surface.

3. Metal nanoparticle-supported titania nanoparticle dispersion The metal nanoparticle-supported titania nanoparticle dispersion of the present invention (particularly the photocatalyst dispersion, and further the visible light responsive photocatalyst dispersion) can be made more uniform by using the reaction liquid that has undergone the above steps (A) to (C) and adding a dispersion step such as ultrasonic dispersion as necessary. At this time, in the conventional visible light responsive photocatalyst dispersion, a uniform dispersion could not be obtained unless a dispersant was used, so in the present invention, a dispersant may be added, but even without adding a dispersant, a dispersion with much better dispersibility than that of a normal visible light responsive photocatalyst can be obtained. As a result of the good dispersibility, the coating also has excellent crack resistance. In addition, as a result of not needing to add a dispersant, a dense titania coating is also possible, and in addition to being excellent in coatability and transparency, the visible light catalytic activity is also excellent.

In this case, in the titania nanoparticle dispersion liquid carrying metal nanoparticles of the present invention, the content of water, which is the main solvent, is preferably 50% by mass or more, particularly 60% by mass or more, from the viewpoints of ease of coating and film properties of the coating, with the total amount of the titania nanoparticle dispersion liquid carrying metal nanoparticles of the present invention being 100% by mass.

It is also possible to take out the titania nanoparticles carrying metal nanoparticles of the present invention from the reaction solution and change the solvent. Water may be removed from the reaction solution by centrifugation, a filtration membrane, or the like, and replaced with an organic solvent. In this case, it is preferable not to dry the titania nanoparticles carrying metal nanoparticles of the present invention from the viewpoints of dispersibility, transparency, etc.

Examples of organic solvents used in the dispersion include alcohols. Examples of the alcohol include aliphatic alcohols having 1 to 6 carbon atoms, such as methanol, ethanol, and isopropanol, as well as non-aliphatic alcohols, such as α-terpineol; glycol-based solvents, such as butyl carbitol (diethylene glycol monobutyl ether), hexylene glycol (2-methyl-2,4-pentanediol), ethylene glycol-2-ethylhexyl ether, ethylene glycol monomethyl ether, and propylene glycol monomethyl ether; and diols, such as 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol.

In addition, even if they do not have an OH group, they may have affinity with titania and other solvents (water, alcohol, etc.), and examples of such compounds include diethylene glycol butyl methyl ether, tripropylene glycol dimethyl ether, triethylene glycol dimethyl ether, diethylene glycol dibutyl ether, triethylene glycol butyl methyl ether, tetraethylene glycol dimethyl ether, diethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol diacetate, triethylene glycol diacetate, and tetraethylene glycol diacetate. Among these, diethylene glycol monobutyl ether acetate and tetraethylene glycol dimethyl ether are preferred from the standpoint of boiling point, etc.

The viscosity of the metal nanoparticle-supported titania nanoparticle dispersion of the present invention is adjusted according to the application. For example, it is adjusted to a low viscosity when used for spin coating, dip coating, spraying, etc., and to a higher viscosity when used for brush coating, squeegee method, etc., and it is preferable to adjust the viscosity even higher when used for screen printing to suppress fluidity. The coating film of the present invention obtained in this way is a dense coating as described above.

The present invention will be specifically explained based on examples, but the present invention is not limited to these.

[Example 1]
120g (2mol) of acetic acid was added to 142.1g (0.5mol) of titanium tetraisopropoxide, and the mixture was stirred for 60 minutes, and 538g of water was added. This dispersion had a titanium tetraisopropoxide concentration of 0.625mol/L, an acetic acid concentration of 2.5mol/L, and a pH of 2.2. A large amount of translucent precipitate was generated, but when the mixture was stirred for 60 minutes and then heated at 70°C, all of the precipitate was dissolved. In this dispersion, the concentration of inorganic acid, and the concentrations of N, Cl, and S elements were all 0mol/L.

After that, the mixture was stirred at normal pressure (0.10 MPa) and 85°C for 3 hours, resulting in a uniform, translucent titania dispersion without the use of an organic dispersant. When ultrasonic dispersion was added to this dispersion, the viscosity was reduced and the transparency was increased. This dispersion had a water content of 67% by mass and a pH of 2.3.

The dispersion was dried to obtain titania nanoparticles. The BET specific surface area of the titania nanoparticles was measured to be 250 m ² /g. TEM observation revealed that the average particle size was about 3 nm. X-ray diffraction analysis of the crystallinity of the obtained titania nanoparticles revealed that they were 100% anatase type (no other crystal forms were present).

The dispersion was kept at 200 ° C. using a moisture meter and dried until there was no mass loss. The TG-DTA of the titania nanoparticles was measured by heating up to 600 ° C. at a temperature increase of 3 ° C. / min under an air atmosphere, and the mass loss at 200 ° C. or higher was 10% by mass. This mass loss at 200 ° C. or higher corresponds to the mass loss due to the elimination of acetic acid, which is an organic acid. Since most of the liberated acetic acid volatilizes at 200 ° C. or lower, the fact that the mass loss at 200 ° C. or higher is 10% by mass suggests that a large amount of acetyl groups, which are acyloxy groups, are bonded to titanium atoms in the form of -OCOCH ₃ on the surface of the titania nanoparticles.

In an air atmosphere, 8.3 mg of hexachloroplatinic (IV) acid hexahydrate was added to 30 g (5.2 mass%) of this dispersion, and the mixture was stirred and mixed for 30 minutes at room temperature (20°C) while continuously irradiating with ultraviolet light having an irradiation intensity of 1.5 mW/ ^cm2 . In other words, the platinum content was added so that it was 0.2 mass% relative to the weight of titanium oxide. The water content of this dispersion was about 95 mass% and the pH was 2.3.

Next, this dispersion was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. The transmittance of 400-800 nm (visible light region) was measured using an ultraviolet-visible spectrometer (UV3400, manufactured by Shimadzu Corporation), and the average value was 94.8%. Furthermore, even when the surface of the obtained glass substrate sample was strongly rubbed with a finger, no slipping was observed. An antibacterial test against Escherichia coli (K-12) was conducted using a method based on JIS R1752 (brightness 600 lx, irradiation for 7 hours), and an extremely high antibacterial activity value of 6.6 or more was obtained. Furthermore, the activity value in the dark was 0, which indicates that this antibacterial property is due to the photocatalytic effect.

[Example 2]
Platinum nanoparticles synthesized by polyol reduction, a typical method for synthesizing platinum nanoparticles, were added to 30 g (5.2 mass%) of a dispersion containing titania nanoparticles obtained by the same method as in Example 1 so that the platinum content was 2 mass% relative to the weight of titanium oxide, and the mixture was stirred and mixed for 1 hour to obtain a dispersion. The water content of this dispersion was about 95 mass% and the pH was 2.3.

Next, a dispersion liquid carrying 2 wt% platinum was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. The transmittance of 400 to 800 nm (visible light region) was measured using an ultraviolet-visible spectrometer (UV3400, manufactured by Shimadzu Corporation), and the average value was 94.8%. In addition, even when the surface of the obtained glass substrate sample was strongly rubbed with a finger, no slipping was observed. From this, it can be understood that Example 2 has a high degree of adhesion between the titania nanoparticles and the glass surface. Furthermore, when an antibacterial test against Escherichia coli (K-12) was conducted using a method based on JIS R1752 (brightness 600 lx, irradiation for 7 hours), the antibacterial activity value was 6.6 or more. Since the sample carrying 10 times the amount of platinum of Example 1 showed the same activity value as Example 1, it can be seen that Example 1 has better antibacterial properties when the same amount is carried. Furthermore, the activity value in the dark was 0, indicating that this antibacterial property is due to the photocatalytic effect.

[Example 3]
6.3 mg of bis(acetylacetonato)platinum(II) was added to 30 g (5.9% by mass) of a dispersion containing titania nanoparticles obtained in the same manner as in Example 1 except that the amount of water was changed, and the mixture was stirred and mixed for 2 hours and 30 minutes at room temperature (20°C) while continuously irradiating with ultraviolet light having an irradiation intensity of 1.5 mW/ ^cm2 in an air atmosphere (continuous irradiation), to obtain a dispersion. That is, the platinum content was added so that it was 0.2% by mass relative to the weight of titanium oxide. The water content of this dispersion was about 95% by mass and the pH was 2.3. In addition, the irradiation time was about 5 times longer than in Example 1, suggesting that the interaction between the negatively charged complex and the titanium oxide surface was greater, such as with hexachloroplatinum(IV).

Next, this dispersion was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. An antibacterial test against Escherichia coli (K-12) was conducted using a method based on JIS R1752 (brightness 600 lx, exposure for 7 hours), and the antibacterial activity value was 6.5.

[Example 4]
11.1 mg of copper acetate (II) monohydrate was added to 30 g (5.9 mass%) of a dispersion containing titania nanoparticles obtained in the same manner as in Example 1 except that the amount of water was changed, and the mixture was stirred and mixed for 2 hours and 30 minutes at room temperature (20 ° C.) while continuously irradiating with ultraviolet light having an irradiation intensity of 1.5 mW / cm ² in an air atmosphere (continuous irradiation), to obtain a dispersion. In other words, the copper content was added so that the amount of supported copper was 0.2 mass% relative to the weight of titanium oxide. The water content of this dispersion was about 95 mass% and the pH was 2.3.

Then, this dispersion was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. An antibacterial test against Escherichia coli (K-12) was conducted using a method based on JIS R1752 (brightness 600 lx, exposure for 7 hours), and the antibacterial activity value was 5.3.

[Example 5]
0.47 mg of hexachloroplatinic (IV) acid hexahydrate was added to 30 g (5.9% by mass) of a dispersion containing titania nanoparticles obtained in the same manner as in Example 1 except that the amount of water was changed, and the mixture was stirred and mixed for 30 minutes at room temperature (20 ° C.) while continuously irradiating with ultraviolet light having an irradiation intensity of 1.5 mW / cm ² in an air atmosphere (continuous irradiation), to obtain a dispersion. In other words, the platinum content supported was added so that it was 0.01% by mass relative to the weight of titanium oxide. The water content of this dispersion was about 95% by mass and the pH was 2.3.

Next, this dispersion was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. An antibacterial test against Escherichia coli (K-12) was conducted using a method based on JIS R1752 (brightness 600 lx, exposure for 7 hours), and the antibacterial activity value was 4.6.

[Example 6]
5.5 mg of silver acetate (I) was added to 30 g (5.9 mass%) of a dispersion containing titania nanoparticles obtained by the same method as in Example 1 except that the amount of water was changed, and the dispersion was obtained by stirring and mixing for 30 minutes at room temperature (20 ° C.) while continuously irradiating with ultraviolet light having an irradiation intensity of 1.5 mW / cm ² in an air atmosphere (continuous irradiation). In other words, the silver content supported was added so that it was 0.2 mass% relative to the weight of titanium oxide. The water content of this dispersion was about 95 mass% and the pH was 2.3.

Then, this dispersion was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. An antibacterial test against Escherichia coli (K-12) was conducted using a method based on JIS R1752 (brightness 600 lx, exposure for 7 hours), and the antibacterial activity value was 6.6 or higher.

[Example 7]
8.0 mg of iron (II) chloride tetrahydrate was added to 30 g (5.9 mass%) of a dispersion containing titania nanoparticles obtained in the same manner as in Example 1 except that the amount of water was changed, and the mixture was stirred and mixed for 2 hours and 30 minutes at room temperature (20 ° C.) while continuously irradiating with ultraviolet light having an irradiation intensity of 1.5 mW / cm ² in an air atmosphere (continuous irradiation), to obtain a dispersion. In other words, the iron content was added so that the amount of supported iron was 0.2 mass% relative to the weight of titanium oxide. The water content of this dispersion was about 95 mass% and the pH was 2.3.

Then, this dispersion was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. An antibacterial test against Escherichia coli (K-12) was conducted using a method based on JIS R1752 (brightness 600 lx, exposure for 7 hours), and the antibacterial activity value was 6.6 or higher.

[Comparative Example 1]
30 g of acetic acid and 160 g of water were added to 10 g of titania nanoparticles ST-01 (Ishihara Sangyo Kaisha, Ltd., specific surface area 300 ^m2 /g, average particle size calculated from the specific surface area 5 nm, no acyl groups present on the surface), 53.1 mg of hexachloroplatinic acid hexahydrate was added, and the mixture was stirred and mixed for 30 minutes at room temperature (20°C) while being continuously irradiated with ultraviolet light at an irradiation intensity of 1.5 mW/ ^cm2 (continuous irradiation), followed by ultrasonic dispersion, but a homogeneous solution was not obtained. In other words, the amount of platinum supported was added so that it was 0.2 mass% relative to the weight of titanium oxide.

This suspension was also applied to a glass substrate, but the titania film was completely opaque. This shows that the dispersibility of Comparative Example 1 was inferior to that of Example 1. In addition, the adhesion between the titania nanoparticles ST-01 and the glass surface was low, and the titania nanoparticles slid off when lightly rubbed with a finger.

Next, this suspension was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. The transmittance of 400 to 800 nm (visible light region) was measured using an ultraviolet-visible spectrometer (UV3400, manufactured by Shimadzu Corporation), and the average value was 65.7%. When the surface of the obtained glass substrate sample was lightly rubbed with a finger, the titania nanoparticles slid off. From this, it can be seen that the adhesion between the titania nanoparticles ST-01 and the glass surface is lower in Comparative Example 1 than in Example 1. When an antibacterial test against Escherichia coli (K-12) was performed using a method based on JIS R1752 (brightness 600 lx, irradiation for 7 hours), the antibacterial activity value was 2.5. From Example 1 and Comparative Example 1, it can be seen that the visible light catalytic activity of Comparative Example 1 is inferior to that of Example 1.

[Comparative Example 2]
190 g of water was added to 10 g of titania nanoparticles ST-01 (Ishihara Sangyo Kaisha, Ltd., specific surface area 300 ^m2 /g, average particle size calculated from the specific surface area 5 nm, no acyl groups present on the surface), 53.1 mg of hexachloroplatinic acid hexahydrate was added, and the mixture was stirred and mixed for 30 minutes at room temperature (20°C) while being continuously irradiated with ultraviolet light at an irradiation intensity of 1.5 mW/ ^cm2 (continuous irradiation), followed by ultrasonic dispersion, but a homogeneous solution was not obtained. In other words, the amount of platinum supported was added so that it was 0.2 mass% relative to the weight of titanium oxide.

Furthermore, when this suspension was applied to a glass substrate, the titania film was completely opaque. From this, it can be understood that the dispersibility of Comparative Example 2 was inferior to that of Example 1. In addition, the adhesion between the titania nanoparticles ST-01 and the glass surface was low, and the titania nanoparticles slid off when lightly rubbed with a finger.

Next, this suspension was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. The transmittance of 400 to 800 nm (visible light region) was measured using an ultraviolet-visible spectrometer (UV3400, manufactured by Shimadzu Corporation), and the average value was 87.2%. In addition, when the surface of the obtained glass substrate sample was lightly rubbed with a finger, the titania nanoparticles slid off. From this, it can be seen that the adhesion between the titania nanoparticles ST-01 and the glass surface is lower in Comparative Example 2 than in Example 1. When an antibacterial test against Escherichia coli (K-12) was performed using a method based on JIS R1752 (brightness 600 lx, irradiation for 7 hours), the antibacterial activity value was 1.7. From Example 1 and Comparative Example 2, it can be seen that the visible light catalytic activity of Comparative Example 2 is inferior to that of Example 1. From Comparative Examples 1 and 2, it can be seen that even if free acetic acid is present in the system, it does not affect the visible light catalytic activity, and that the acetic acid modified on the surface affects the visible light catalytic activity.

[Comparative Example 3]
190 g of water was added to 10 g of Lumilesh CT-2 (a commercially available titanium oxide-based visible light responsive catalyst, no acyl groups present on the surface) and ultrasonic dispersion was performed, but a homogeneous solution was not obtained.

When this suspension was applied to a glass substrate, the titania film was completely opaque. This shows that the dispersibility of Comparative Example 3 was inferior to that of Example 1. In addition, the adhesion between the titania nanoparticles and the glass surface was low, and the titania nanoparticles slid off when lightly rubbed with a finger.

Next, this suspension was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. The transmittance of 400 to 800 nm (visible light region) was measured using an ultraviolet-visible spectrometer (UV3400, manufactured by Shimadzu Corporation), and the average value was 59.8%. When the surface of the obtained glass substrate sample was lightly rubbed with a finger, the titania nanoparticles slid off. From this, it can be seen that the adhesion between the titania nanoparticles and the glass surface in Comparative Example 3 is lower than that in Example 1. When an antibacterial test against Escherichia coli (K-12) was performed using a method based on JIS R1752 (brightness 600 lx, irradiation for 7 hours), the antibacterial activity value was 2.0. From Example 1 and Comparative Example 3, it can be seen that the visible light catalytic activity of Comparative Example 3 is inferior to that of Example 1.

[Comparative Example 4]
120g (2mol) of acetic acid was added to 142.1g (0.5mol) of titanium tetraisopropoxide, and the mixture was stirred for 60 minutes, and 538g of water was added. This dispersion had a titanium tetraisopropoxide concentration of 0.625mol/L, an acetic acid concentration of 2.5mol/L, and a pH of 2.2. A large amount of translucent precipitate was generated, but when the mixture was stirred for 60 minutes and then heated at 70°C, all of the precipitate was dissolved. In this dispersion, the concentration of inorganic acid, and the concentrations of N, Cl, and S elements were all 0mol/L.

After that, the mixture was stirred at normal pressure (0.10 MPa) and 85°C for 3 hours, resulting in a uniform, translucent titania dispersion without the use of an organic dispersant. When ultrasonic dispersion was added to this dispersion, the viscosity was reduced and the transparency was increased. This dispersion had a water content of 67% by mass and a pH of 2.3.

Next, this dispersion was applied (spin-coated) to a glass substrate with a thickness of 0.7 mm, and the substrate was dried at 80°C. The transmittance of 400 to 800 nm (visible light region) was measured using an ultraviolet/visible spectrometer (UV3400, manufactured by Shimadzu Corporation), and the average value was 95.0%. An antibacterial test against Escherichia coli (K-12) was performed using a method based on JIS R1752 (brightness 600 lx, irradiation for 7 hours), and the antibacterial activity value was 1.6. From Example 1 and Comparative Example 4, it can be seen that the visible light catalytic activity of Comparative Example 4 is inferior to Example 1. It can also be seen that the high visible light catalytic activity of Example 1 is due to platinum nanoparticles, which are metal nanoparticles that are strongly adhered to the surface.

Claims (6)

A metal nanoparticle-supported titania nanoparticle having metal nanoparticles supported on the surface of the titania nanoparticle,
The titania nanoparticles have acyloxy groups bonded to at least some of the titanium atoms present on the surface,
The titania nanoparticles are of anatase type;
The specific surface area of the titania nanoparticles is 250 to 500 m ² /g;
the metal constituting the metal nanoparticles is at least one metal selected from the group consisting of platinum, silver, and copper;
When the titania nanoparticles are heated to 600°C using a thermogravimetric and differential thermal analyzer, the mass loss at 200°C or higher is 5% by mass or more; and
Titania nanoparticles carrying metal nanoparticles, the metal nanoparticles being carried on the surface thereof in an amount of 90 mass % or less relative to titanium oxide in the titania nanoparticles. The metal nanoparticle-supported titania nanoparticles according to claim 1, wherein the acyloxy group is bonded to the titanium atom via a group represented by -OCOR (wherein R represents a hydrogen atom, an alkyl group having 1 to 3 carbon atoms, or a hydroxyalkyl group having 1 to 2 carbon atoms). The metal nanoparticle-supported titania nanoparticles according to claim 1, wherein the acyloxy group is an acyloxy group derived from at least one organic acid selected from the group consisting of monocarboxylic acids having 1 to 4 carbon atoms and hydroxycarboxylic acids having 2 to 3 carbon atoms. The metal nanoparticle-supported titania nanoparticles of claim 3, wherein the organic acid is acetic acid. The metal nanoparticle-supporting titania nanoparticle according to any one of claims 1 to 4 , wherein the titania nanoparticles are 100% anatase type. A photocatalyst comprising the metal nanoparticle-supporting titania nanoparticles according to any one of claims 1 to 5 .