JP7767211B2 - Titania nanoparticles and their manufacturing method - Google Patents

Description

The present invention relates to titania nanoparticles and a method for producing the same.

In recent years, with increased awareness of hygiene, photocatalysts have been put to practical use to make indoor building materials, daily necessities, etc. pathogen-resistant, and paint containing photocatalysts is applied to the surfaces of indoor building materials, daily necessities, etc.

When photocatalysts are irradiated with light, they generate reactive oxygen species that lead to antimicrobial activity, for example from oxygen and moisture on the catalyst surface. Because the photocatalyst itself remains unchanged before and after the generation of antimicrobial active species, it is used as a highly sustainable antimicrobial material. Photocatalysts generally refer to catalysts that absorb ultraviolet light and exhibit photocatalytic activity.

In addition, photocatalysts are designed for use in environments where ultraviolet rays are difficult to reach, such as indoors.
Visible light catalysts that exhibit photocatalytic activity in response to light in the visible light region have been actively developed. The most commonly used visible light catalysts are metal-containing titanium oxide powders or metal-containing titanium oxide sols, which are titanium oxide powders or titanium oxide sols that contain metals such as iron, platinum, gold, copper, or silver.

In addition, as photocatalysts are used indoors or in everyday items, there are environments where light is not present, such as in the shade or at night. Therefore, metal-containing titanium oxide powders and metal-containing titanium oxide sols containing antimicrobial metal species such as silver and copper have also been developed to exhibit antimicrobial properties even in dark places.

However, photocatalytic coatings used on indoor building materials and everyday items are required to be easily applied as a one-component system, to be highly transparent without compromising the design freedom of the substrate, and to have adhesion and crack resistance so that they do not slip or crack when touched.

However, when a suspension of titanium oxide powder containing metals (product names: Lumilesh (Showa Denko K.K.) or Cellmuse (Daicel Miraize Co., Ltd.) dispersed in water is applied to a glass substrate or a highly transparent resin substrate, the white color of the titanium oxide powder becomes prominent, and the transparency of the glass substrate can be impaired. Furthermore, when the coating film formed in this way is rubbed with a finger, the powder can slide off. Thus, when metal-containing titanium oxide powder is used as a paint, there are issues with transparency and substrate adhesion when used alone.

One solution is to use a polymeric organic compound or porous inorganic compound as a binder, fix the metal-containing titanium oxide powder in the binder, and form a coating. However, polymeric organic compound binders coat the surface of the metal-supported titanium oxide, which can reduce photocatalytic activity. Porous inorganic compound binders, while having more voids than polymeric organic compound binders and therefore less likely to reduce activity, are hard and brittle, lacking in crack resistance, which can promote catalyst slippage and reduce service life. Using a binder in this way with photocatalysts can impair the inherent durability and antimicrobial activity of metal-supported titanium oxide.

For example, Patent Document 1 discloses a virus inactivating agent containing a photocatalytic substance having a surface supported with a mixture containing a monovalent copper compound and a divalent copper compound. The examples in Patent Document 1 exemplify an example using _Cu2O powder and an example using _Cu2O powder mixed with a silica-based binder, and disclose an example in which these were applied to a glass plate and brought into contact with viruses under the same conditions for 30 minutes. When _Cu2O was used alone, the virus infectivity decreased by about ¹⁰⁶ in 30 minutes, whereas when a binder was used, the virus infectivity decreased by only about ¹⁰⁵ , resulting in an approximately 10-fold decrease in activity when a material carrier was used in combination with a binder.

In response, a method has been proposed in which an antimicrobial metal is loaded onto titanium oxide sol, which can be applied in a one-component system without using a binder, and a coating film of the antimicrobial metal-loaded titanium oxide sol is formed by applying the one-component system. However, titanium oxide sol has problems such as cloudiness and cracks in the coating film, making it difficult to achieve a coating film that is both highly transparent and durable.

In fact, when a water-soluble titanium oxide sol with an X-ray particle diameter of just 7 nm (product name: CSB, Sakai Chemical Industry Co., Ltd.) was applied to a glass substrate, the coating became cloudy. Furthermore, when titanium oxide synthesized using the method described in Non-Patent Document 1 was applied, cracks occurred.

As such, with conventional technology, it is currently difficult to create a binderless (without using a binder) paint that is highly transparent, crack-resistant, and has excellent adhesion when using metal-containing titanium oxide powder and titanium oxide sol.

As mentioned above, metal-supported titanium oxide powders and sols for indoor coatings have problems such as poor substrate adhesion and crack resistance when used alone. While these can be reinforced by adding binders, there are problems such as the binder coating the catalyst surface or causing the catalyst to slide off. As things stand, this impairs the inherent antimicrobial activity and sustained activity of metal-containing titanium oxide.

Furthermore, while many indoor building materials and everyday items require design in terms of color, metal-containing titanium oxide powder is a cloudy white powder, which means that the range of design possibilities is significantly limited.

Therefore, there is a demand for a highly transparent visible light catalyst that is binderless, adheres to the substrate, and has high physical resistance as a coating film.

Another problem with metal-containing titanium oxide powder and titanium oxide sol is the instability of the metal components contained separately from titanium oxide. For example, as disclosed in Patent Document 2, when a silver complex is added to an aqueous solution in which titanium oxide is dispersed, the color of the solution gradually changes, and eventually silver precipitation is observed. This phenomenon of precipitation due to the instability of the metal component is often seen with elements with low ionization tendency, such as silver and copper.

In response to this, a method has been proposed (Patent Document 2) in which silver and copper are combined and quaternary ammonium is incorporated into titanium oxide sol, thereby stabilizing the metal components with a stabilizer. However, the method disclosed in Patent Document 2 raises safety concerns because it contains quaternary ammonium ions, which are highly toxic to the human body.

Methods such as coating the surface of silver with thiol-based ligands or titanium oxide (Non-Patent Document 2) have also been proposed. However, the method of coating the surface of silver with thiol-based ligands or titanium oxide disclosed in Non-Patent Document 2 has the problem that, if the surface is completely coated, the elution of silver, which is an antimicrobial metal, is prevented, resulting in a loss of antimicrobial activity. On the other hand, if the surface is only partially coated, the antimicrobial metal silver repeatedly elutes and re-precipitates, causing discoloration and precipitation.

Furthermore, when silver salts are used to support silver in photocatalysts, issues such as safety, coloration, and corrosiveness arise. Most inorganic silver salts are highly toxic. For example, the most commonly used silver salt is nitrate, which is a highly toxic substance that reacts with light and deposits black compounds when it comes into contact with hands. Furthermore, when nitrate decomposes and generates acid, it can lead to corrosion of metals, etc. Furthermore, silver salts have poor storage stability, so they must be shielded from light and should be stored in low-oxygen environments at low temperatures. If they change, they cannot be used to synthesize silver nanoparticles, so they require a lot of effort to manage.

When supporting silver on a photocatalyst, it is expected that the smaller the particle size, the higher the activity. On the other hand, silver is more likely to form aggregates than platinum and other metals. Platinum particles of 5 nm or less can generally be produced relatively easily. Compared to platinum, silver is more difficult to form into independent particles of 20 nm or less.

As mentioned above, there is a need for a method for easily and safely producing a visible light catalyst that allows antimicrobial metal components with low ionization tendency, such as silver and copper, to exist stably in a titanium oxide solution without causing precipitation and that does not prevent the elution of these metals.

JP 2013-166705 A JP 2008-260684 A

Journal of the Society of Inorganic Materials, Japan 11.313 (2004): 481-488. Journal of the Japan Color Materials Association, 2014, Vol. 87, No. 2, 43-49

The present invention aims to provide a highly transparent photocatalyst that does not use silver salts, but instead supports fine silver nanoparticles on titanium oxide, which exists stably in water without causing precipitation.

With the above objective in mind, the inventors conducted extensive research and discovered that uniform, highly transparent silver-loaded titanium oxide can be synthesized simply by stirring metallic silver with a dispersion of titania nanoparticles in the presence of visible or ultraviolet light. The dispersion has titania nanoparticles in which acetoxy groups are bonded to at least some of the titanium atoms present on the surface and which exhibit a mass loss of 5% by mass or more at temperatures above 200°C when heated to 600°C using a simultaneous differential thermal and thermogravimetric analyzer.

The inventor then conducted further research and completed the present invention.

In other words, the present invention includes the following configurations:

Item 1.
A method for producing titania nanoparticles, comprising:
a step of adding silver particles to an aqueous dispersion of titania nanoparticles and stirring the mixture to support silver on the titania nanoparticles;
The titania nanoparticles have acetoxy groups bonded to at least some of the titanium atoms present on the surface, and when 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.
Manufacturing method.

Item 2.
A method for producing titania nanoparticles, comprising:
(A) mixing a titanium-containing substance, an organic acid, and water to obtain an aqueous dispersion;
(B) heating the aqueous dispersion obtained in step (A) at a temperature higher than 80°C; and (C) after step (B), mixing silver (metallic silver, silver particles) with the aqueous dispersion and irradiating the silver-containing aqueous dispersion with visible light or ultraviolet light to support silver (metallic silver, silver particles) on titanium (titania nanoparticles).

Item 3.
3. The method according to Item 2, wherein the aqueous dispersion in step (A) contains 50% by mass or more of water.

Item 4.
4. The method according to any one of items 1 to 3, wherein the pH of the aqueous dispersion in step (A) is 2 or more and 5 or less.

Item 5.
5. The method according to any one of items 1 to 4, wherein the silver is silver particles having a particle size of 1 μm or more and 1 mm or less.

Item 6.
Titania nanoparticles,
Titania nanoparticles are loaded with silver,
The titania nanoparticles have acetoxy groups bonded to at least some of the titanium atoms present on the surface, and when 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.
Titania nanoparticles.

According to the present invention, it is possible to easily obtain a uniform, highly transparent, and storage-stable dispersion of silver-loaded titanium oxide nanoparticles without using silver salts, which are unsafe and easily altered.

The silver-loaded titanium oxide nanoparticle dispersion obtained by this invention has high substrate adhesion and crack resistance, and can be used to obtain a highly transparent photocatalytic coating film.

1 is a diagram illustrating an embodiment of the present invention. FIG. 1 shows the results of ESCA analysis of the metallic Ag obtained in Example 1. Metallic Ag is generated. 2 is a diagram illustrating an embodiment of the present invention. Fig. 2 shows the silver-containing aqueous dispersion in which silver nanoparticles are supported on titania obtained in Example 1. The silver-containing aqueous dispersion was uniform, and maintained its uniformity and transparency even after 3 months. 3 shows the results of TEM observation of titania nanoparticles supported by silver nanoparticles obtained in Example 1. The primary particles of Ag were 3 nm to 7 nm. 4 shows the results of TEM observation of titania nanoparticles supported by silver nanoparticles obtained in Example 1. Ag was found to be 10 nm to 20 nm in size in the aggregated secondary particles. 5 shows the results of TEM observation of the dispersion liquid carrying metallic Ag obtained in Comparative Example 1. The Ag was in the form of aggregates of Ag nanoparticles with primary particles of 40 nm to 50 nm. 6 is a diagram showing a comparative example. Fig. 6 shows the titania aqueous dispersion obtained in Comparative Example 3. In the titania aqueous dispersion, a supernatant and a precipitate were observed.

The present invention is described in detail below.

In this specification, the terms "comprise" and "containing" are concepts that encompass all of "comprise," "consist essentially of," and "consist only of."

In this specification, when a numerical range is indicated as "A to B," it means "greater than or equal to A and less than or equal to B."

1. Method for Producing Titania Nanoparticles (Silver-Supported Titania Nanoparticles) The method for producing titania nanoparticles of the present invention includes the steps of:
The method includes a step of adding silver (metallic silver, silver particles) to an aqueous dispersion of titanium (titania nanoparticles) and stirring the mixture to support the silver on the titanium,
The titanium has acetoxy groups bonded to at least some of the titanium atoms present on the surface, and when 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.

The method for producing titania nanoparticles of the present invention comprises the steps of:
(A) mixing a titanium-containing substance, an organic acid, and water to obtain an aqueous dispersion;
(B) a step of heating the aqueous dispersion obtained in the step (A) at a temperature higher than 80°C; and (C) after the step (B), a step of mixing silver (metallic silver, silver particles) with the aqueous dispersion and irradiating the silver-containing aqueous dispersion with visible light or ultraviolet light to cause the titanium (titania nanoparticles) to support the silver (metallic silver, silver particles).

In this specification, "titanium oxide" or "titania" does not refer only to titanium dioxide (TiO ₂ ), but also includes titanium dioxide compositions in which oxygen is deficient, such as titanium trioxide (Ti ₂ O ₃ ), titanium monoxide (TiO), Ti ₄ O ₇ , and Ti ₅ O _9. "Titanium oxide" or "titania" may contain groups other than Ti—O—Ti that result from the synthesis of titanium oxide, as represented by terminal OH groups. "Titanium oxide" or "titania" also includes those in which an organic acid or the like is bonded to a terminal OH group.

The titania nanoparticles (silver-supported titania nanoparticles) of the present invention are composites in which silver nanoparticles are supported on the surface of titania nanoparticles, and the titania nanoparticles have acetoxy 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, they experience a mass loss of 5% by mass or more at temperatures above 200°C.

Conventionally, when synthesizing such composites, titanium oxide and silver salts are usually placed together in water, and a reducing agent is added or light is irradiated.

In the method for producing titania nanoparticles (composite) of the present invention, titanium oxide is preferably used as the titanium-containing substance. Titanium oxide preferably has a particle size of 1 nm to 10 nm, a large specific surface area, and acetoxy groups bonded to the surface, and is used in the form of an aqueous dispersion.

In the method for producing titania nanoparticles (composite) of the present invention, silver is not used as a silver salt, silver nanoparticles, etc.; preferably, metallic silver is used, and is stirred in the presence of a titanium-containing substance.

In the method for producing titania nanoparticles of the present invention, minute amounts of silver ions are gradually released from metallic silver, which precipitate as silver nanoparticles on the surface of titanium oxide, which has a large specific surface area. In the method for producing titania nanoparticles of the present invention, finer silver nanoparticles are supported on the surface of titanium oxide compared to conventional techniques.

(1) Titania Nanoparticles Generally, water, inorganic acids, free organic acids, etc. volatilize almost completely at temperatures below 200°C.

On the other hand, the titania nanoparticles that make up the metal-loaded titania nanoparticles of the present invention have acetoxy groups bonded to at least some of the titanium atoms present on the surface, and therefore the acetoxy groups bonded to the titanium atoms gradually detach in the range of 200°C to 600°C.

The titania nanoparticles that make up the metal-supported titania nanoparticles of the present invention have acetoxy groups bonded to at least some of the titanium atoms present on the surface, which prevents the titania nanoparticles from aggregating together during drying or firing. This makes them less susceptible to cracking and peeling, and they are particularly excellent in terms of coatability and transparency.

Titania nanoparticles can suppress cracking and peeling, and are also easy to firmly support the metals described below, resulting in excellent visible light catalytic activity.

Conventionally, the presence of acetoxy groups on the surface reduces visible light catalytic activity.

As explained above, titania nanoparticles are particularly effective at preventing cracking and peeling because they can prevent aggregation of titania nanoparticles during drying or calcination. Furthermore, because they are easy to firmly support metals (described below), they can also improve visible light catalytic activity despite the presence of acetoxy groups.

Titania nanoparticles preferably have a large number of acetoxy groups bonded to titanium atoms present on the surface. If acyloxy groups are present on at least some of the titanium atoms present on the surface, as explained above, the acyloxy groups present on the titanium atoms gradually leave the range of 200°C to 600°C. Therefore, when the temperature is increased using a thermogravimetric differential thermal analyzer (TG-DTA), there is a large loss in mass above 200°C.

In the present invention, when titania nanoparticles are heated using a thermogravimetry-differential thermal analyzer (TG-DTA), the mass loss at 200°C or higher represents an index of the number of acetoxy groups bonded to titanium atoms present on the surface. Therefore, when titania nanoparticles are heated to 600°C using a thermogravimetry-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. The detailed conditions for the thermogravimetry-differential thermal analyzer (TG-DTA) are: atmosphere: air, heating rate: 3°C/min.

As explained above, titania nanoparticles have acetoxy groups bonded to at least some of the titanium atoms present on the surface. Titania nanoparticles may also contain other acyloxy groups. Examples of other acyloxy groups include monocarboxylic acids such as formic acid, acetic acid, propionic acid, and butyric acid, and hydroxycarboxylic acids such as glycolic acid, lactic acid, and 3-hydroxybutyric acid. Transparency can be further enhanced by using or post-adding other acyloxy groups, such as lactic acid, to titania nanoparticles.

The average particle size of the titania nanoparticles is preferably 1 nm to 10 nm, and more preferably 2 nm to 6 nm. The average particle size of the titania nanoparticles is measured by observation with a transmission electron microscope (TEM).

In the present invention, by setting the average particle size of the titania nanoparticles within this range, it becomes easier to support the metals (silver and copper) appropriately and more firmly, and it becomes easier to form a film with higher visible light catalytic activity and greater transparency.

Titania nanoparticles generally tend to shrink significantly when heated when their average particle size is small, resulting in cracks and peeling from the substrate.

The metal-supported titania nanoparticles of the present invention are a material that exhibits excellent coatability even when titania nanoparticles with a small average particle size are used.

The specific surface area of the titania nanoparticles is preferably 150 m ² /g to 500 m ² /g, more preferably 200 m ² /g to 400 m ² /g. The specific surface area of the titania nanoparticles is measured by the BET method.

In the present invention, by setting the specific surface area of the titania nanoparticles within this range, it becomes easier to support the metals (silver and copper) appropriately and more firmly, making it easier to increase visible light catalytic activity.

The concentrations of N, Cl, and S elements in titania nanoparticles can be preferably 0 ppm to 5000 ppm, and more preferably 0 ppm to 1000 ppm. The concentrations of N, Cl, and S elements in titania nanoparticles are measured using WDX (fluorescent X-ray).

In the present invention, by setting the concentrations of N, Cl, and S elements in the titania nanoparticles within this range, corrosion of the substrate can be easily suppressed. This condition means that impurities derived from acidic titania precursors such as TiCl ₄ and TiOSO ₄ are absent or present in only small amounts.

The crystalline form of titania nanoparticles is preferably anatase. In the present invention, the use of anatase can particularly improve visible light catalytic activity. For the same reason, it is preferable that the crystalline form of titania nanoparticles is 100% anatase, with no other crystalline forms being present.

(2) Method for producing metal-supported titania nanoparticles and photocatalyst In the present invention, the method for supporting silver on titania nanoparticles involves mixing metallic silver with titania nanoparticles in the presence of visible light or ultraviolet light.

Specifically, the metal-supported titania nanoparticles and photocatalyst of the present invention are
(A) mixing a titanium-containing substance, an organic acid, and water to obtain an aqueous dispersion;
(B) heating the aqueous dispersion obtained in the step (A) at a temperature higher than 80°C;
(C) It can be produced by a method comprising a step of mixing the aqueous dispersion obtained in the step (B) with silver in the presence of visible light or ultraviolet light.

In step (C), heating can improve the loading efficiency. Higher heating temperatures tend to remove the hydration water from the titania nanoparticles, resulting in increased viscosity and reduced dispersibility. The reaction temperature in step (C) is typically 0°C to 100°C, preferably 10°C to 50°C, and more preferably 15°C to 40°C.

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

There are no particular restrictions on the titanium-containing substance used, as long as it becomes titanium oxide when heated. That is, the titanium-containing substance is preferably titanium oxide and/or a titanium oxide precursor. Specific examples of titanium-containing substances include titanium oxide; titanium hydroxide; titanium alkoxides; titanium halides such as titanium trichloride and titanium tetrachloride (especially 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-containing substances, titanium alkoxide, titanium hydroxide, or titanium halide (especially those neutralized with a base) is preferred from the viewpoints of the dispersibility, coatability, and visible light photocatalytic properties of the resulting titania, and titanium alkoxide is even more preferred from the viewpoints of purity, dispersibility, coatability, transparency, and visible light photocatalytic properties.

Preferred examples of titanium alkoxides include titanium tetraisopropoxide, titanium tetra n-butoxide, titanium tetra n-propoxide, and titanium tetraethoxide. Furthermore, from the viewpoints of cost, water solubility of by-products, coatability, and visible light catalytic properties, titanium tetraisopropoxide is more preferred.

Depending on the combination of titanium alkoxide and organic acid, the resulting titania tends to act as a catalyst to liberate ester compounds that are poorly soluble in water, but this does not pose a problem for the titania itself. For example, when titanium tetra-n-butoxide and acetic acid are combined, butyl acetate tends to be produced and liberated when mixed and heated.

From the perspective of obtaining a uniform dispersion, it is preferable to use a combination of an organic acid and a titanium alkoxide that produces an organic acid alkoxide with excellent water solubility.

When using titanium halides (titanium tetrachloride, titanium trichloride, etc.), from the viewpoints of impurities (halogens), corrosion of the reactor during mass production, crystallinity control, coatability, transparency, and visible light catalytic properties, it is preferable to neutralize the titanium halide with a base and wash the precipitate before use. In this case, from the viewpoint of the dispersibility of the resulting titania, it is preferable to use the washed titanium halide without drying it.

When using a solid such as titanium oxide or metallic titanium as the titanium-containing substance, the average particle size is preferably 10 nm or less, and more preferably 6 nm or less. There is no specific lower limit for the average particle size of the titanium-containing substance, but it is usually around 1 nm. The average particle size of solids such as titanium oxide or metallic titanium is measured by observation with a transmission electron microscope (TEM). If the particle size of the titanium-containing substance is large, it can be ground using a planetary ball mill, paint shaker, or similar device, either dry or wet.

The concentration of the titanium-containing substance in the dispersion produced in step (A) is preferably 0.01 mol/L to 5 mol/L, more preferably 0.05 mol/L 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, preferably a volatile acid, and more preferably acetic acid. For example, acetic acid may be used in combination with formic acid, lactic acid, butyric acid, hydroxybutyric acid, etc.

From the standpoints of dispersibility, coatability, transparency, visible light catalytic properties, and cost, the amount of organic acid used is adjusted so that the number of moles of acetoxy groups per mole of titanium in the titanium-containing substance is preferably 1 mole or more, and more preferably 1.5 moles or more. The more organic acid used, the easier it is to improve stability over time, coatability, transparency, etc. There is no particular upper limit to the amount of organic acid used. The upper limit of the amount of organic acid used is adjusted so that the number of moles of acetoxy groups per mole of titanium in the titanium-containing substance is preferably 10 moles or less.

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

In the present invention, the aqueous dispersion in step (A) preferably contains 50% by mass or more of water. The reaction solvent preferably uses an aqueous solvent such as water as the main component. Specifically, the reaction solvent uses an aqueous solvent such as water in an amount of, for example, 50% by mass or more. The reaction solvent may contain an alcohol or ester during the reaction.

For example, when titanium tetraisopropoxide is used as a raw material, it reacts with organic acids to produce isopropyl alcohol. Heating can also produce isopropyl esters of organic acids.

An alcohol or ester may be added to the dispersion obtained in step (A), or may be generated in the reaction system. This alcohol or ester may be removed by heating in an open system at 100°C or below, by reducing the pressure, or may remain in the reaction solution.

If alcohol is contained in the dispersion, the average particle size of the resulting titania nanoparticles and the silver-supported titania nanoparticles of the present invention tends to be smaller, so alcohol may be intentionally added to control the average particle size.

In the present invention, it is generally preferable not to use inorganic acids (especially strong inorganic acids) such as nitric acid, hydrochloric acid, and sulfuric acid, which are often used in the hydrothermal synthesis reaction of titania nanoparticles. This is because the crystal forms of the resulting titania nanoparticles are a mixture of anatase and brookite, and also because of the storage stability of the resulting dispersion, corrosion of the equipment, impurities, wastewater, etc.

In the present invention, to improve the dispersibility, transparency, uniformity, etc. of the raw materials and make them easier to handle, an inorganic acid can be used supplementarily, for example, in a range of 0.01 mol/L or less, as long as the effect is not impaired. In this case, the concentrations of N, Cl, and S elements in the dispersion obtained in step (A) will all be 0.01 mol/L or less.

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

In step (A), the method for preparing the aqueous dispersion is not particularly limited. In the method for preparing the aqueous dispersion in step (A), the titanium-containing substance, organic acid, and water (solvent) may be mixed simultaneously or sequentially. In the method for preparing the aqueous dispersion in step (A), for example, on a mass production scale, from the viewpoints of preventing aggregation and forming large lumps and facilitating continuous stirring, it is preferable to mix the organic acid and water (solvent) first, and then add the titanium-containing substance while stirring. In the method for preparing the aqueous dispersion in step (A), for example, on a laboratory scale, it is preferable to mix the titanium-containing substance and organic acid first, and then add water while stirring.

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

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

When heating in step (B), stirring is preferably performed to ensure sufficient reaction between the titanium-containing substance, organic acid (acetic acid, etc.), and water. There are no particular restrictions on the stirring method, and conventional methods can be used.

The stirring time (reaction time) is preferably at least 1 hour, more preferably at least 1.5 hours, from the viewpoint of allowing the titanium-containing substance, organic acid (acetic acid, etc.), and water to react sufficiently. There is no particular upper limit to the stirring time, but it is usually 240 hours.

The heating in step (B) is carried out at a temperature above 80°C, preferably at a temperature of 82°C or higher. Setting the heating temperature above 80°C completes the crystallization of the resulting metal-supported titania nanoparticles and the surface functional group reaction, and the reaction solution also has excellent stability over time. On the other hand, if the heating temperature is below 80°C, cracks are likely to occur, the coating properties are poor, and the nanoparticles fall off easily, making it difficult to form a coating film. There is no particular upper limit to the heating temperature, and when the reaction is carried out at normal pressure, it is usually 120°C.

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

(2-3) Process (C)
In step (C), the aqueous dispersion obtained in step (B) is mixed with silver (metallic silver, silver particles) in the presence of ultraviolet light or visible light.

In step (C), the silver is preferably silver particles having a particle size of 1 μm or more and 1 mm or less. When silver particles are used, the particle size is preferably 1 μm to 1 mm from the viewpoint of cost.

The pH of the aqueous dispersion obtained in step (C) is preferably 1 or more and 4 or less, from the viewpoint of preventing corrosion of the equipment.

As the reaction in step (C) progresses, the reaction solution turns yellow, indicating the presence of small silver nanoparticles.

After step (C), titania nanoparticles can be obtained in which silver is supported on the titania nanoparticles, acetoxy groups are bonded to at least some of the titanium atoms present on the surface of the titania nanoparticles, and when 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.

In the metal-loaded titania nanoparticles (titania nanoparticles loaded with silver) of the present invention, the particle size of the silver (metallic silver, silver particles) loaded on the titania nanoparticles is preferably 20 nm or less. Because the particle size of the silver loaded on the titania nanoparticles falls within this range, the metal-loaded titania nanoparticles have transparency and high stability over time. Furthermore, because the particle size of the silver loaded on the titania nanoparticles is extremely small, they exhibit excellent antibacterial properties.

2. Silver nanoparticle-supported titania nanoparticle dispersion The metal-supported titania nanoparticle dispersion of the present invention (particularly a photocatalyst dispersion, and further a visible light responsive photocatalyst dispersion) can be made even more uniform by using the reaction liquid that has undergone the above steps (A), (B), and (C), and adding a dispersion step such as ultrasonic dispersion as necessary.

In the present invention, the metal-loaded titania nanoparticle dispersion is sufficiently dispersed prior to step (C), so strong dispersion or the addition of a dispersant is not necessarily required, and a transparent metal-loaded titania nanoparticle dispersion can be obtained.

The metal-supported titania nanoparticle dispersion of the present invention has good dispersibility, resulting in excellent crack resistance of the coating. The metal-supported titania nanoparticle dispersion of the present invention does not require the addition of a dispersant, making it possible to produce a dense titania coating, and it has excellent coatability and transparency, as well as excellent visible light catalytic activity.

After step (C), an organic solvent may be added to increase the wettability of the metal-loaded titania nanoparticle dispersion, making it easier to coat, and to increase the stability of the silver.

Preferably, the organic solvent used in the dispersion liquid is an alcohol, ether, ketone, or the like. From the viewpoint of affinity with titania, the organic solvent is more preferably an alcohol having four or fewer carbon atoms.

Preferred examples of alcohols include monohydric alcohols such as methanol, ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, sec-butyl alcohol, and tert-butyl alcohol; and polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerin.

Preferred examples of ethers include ethylene glycol monomethyl ether, propylene glycol monomethyl ether, ethylene glycol dimethyl ether, propylene glycol dimethyl ether, 3-methoxy-1,2-propanediol, 1,3-dimethoxy-2-propanol, and trimethoxymethane.

Preferably, the ketone includes, for example, acetylacetone.

These organic solvents can be used alone or in combination of two or more.

Among the organic solvents used in the dispersion, methanol, ethanol, 1-propanol, 2-propanol, 2-butanol, tert-butyl alcohol, etc. are more preferred.

From the perspective of safety when applying the coating liquid of the present invention and affinity with titania, the organic solvent used in the dispersion preferably contains at least ethanol, 2-propanol, 1-propanol, etc. In the present invention, the sum of ethanol, 2-propanol, and 1-propanol is preferably 50% to 100% by mass, and more preferably 60% to 100% by mass, with the total amount of organic solvents being 100% by mass.

The viscosity of the silver-supported titania nanoparticle dispersion of the present invention can be adjusted depending on the application, and it can be made into a paint. When applying the silver-supported titania nanoparticle dispersion by spin coating, dip coating, spraying, etc., the silver-supported titania nanoparticle dispersion is preferably low in viscosity. When applying the silver-supported titania nanoparticle dispersion by brush coating, squeegeeing, etc., it is preferable to adjust the viscosity of the silver-supported titania nanoparticle dispersion to be higher. When applying the silver-supported titania nanoparticle dispersion by screen printing, it is preferable to adjust the viscosity of the silver-supported titania nanoparticle dispersion to be even higher to suppress fluidity. The coating film of the present invention obtained in this way is a dense coating.

Coated products bearing the coating film of the present invention obtained in this manner are not particularly limited. Examples of coated products of the present invention include building materials, building exteriors, building interiors, window frames, window glass, various lenses, structural members, building equipment such as housing, cooking utensils, textile products, furniture, displays, display protective films, plumbing components, vehicle light covers and window glass, exteriors of machinery or goods, dust covers and coatings, display devices and their covers, traffic signs, various display devices, signs such as billboards, sound insulation walls for roads and railways, exteriors and coatings of bridges and guardrails, interiors and coatings of tunnels, insulators, solar cell covers, solar hot water heater heat collector covers, and exterior parts of electronic and electrical equipment used outdoors, particularly transparent members, exteriors of vinyl greenhouses, greenhouses, etc.

The present invention will be specifically explained based on examples.

The present invention is not limited to these.

[Example 1]
To 142.1 g (0.5 mol) of titanium tetraisopropoxide, 120 g (2 mol) of acetic acid was added, and the mixture was stirred for 60 minutes, followed by the addition of 538 g of water. The resulting dispersion had a titanium tetraisopropoxide concentration of 0.625 mol/L, an acetic acid concentration of 2.5 mol/L, and a pH of 2.2.

A large amount of translucent precipitate formed in this dispersion, but after stirring for 60 minutes and heating, the precipitate completely dissolved at 70°C. In addition, the concentrations of inorganic acid, N, Cl, and S elements in this dispersion were all 0 mol/L.

The mixture was then stirred at normal pressure (0.10 MPa) and 99°C for 3 hours, resulting in a translucent, uniform titania dispersion without the use of an organic dispersant.

When this dispersion was subjected to ultrasonic dispersion, the viscosity was reduced and transparency increased. The pH of this dispersion was 2.2.

This dispersion was dried to obtain titania nanoparticles. The BET specific surface area of these titania nanoparticles was measured and found to be 256 m ² /g. TEM observation revealed that the average particle diameter was approximately 3 nm. X-ray diffraction analysis of the resulting titania nanoparticles revealed that they were 100% anatase, with no other crystal forms present.

This dispersion was held at 200°C using a moisture meter and dried until no mass loss was observed. TG-DTA of the titania nanoparticles was measured by increasing the temperature to 600°C at a rate of 3°C/min in an air atmosphere. The mass loss above 200°C was 13% by mass. This mass loss above 200°C is due to the elimination of acetic acid, an organic acid. Since the liberated acetic acid almost completely volatilizes below 200°C, the mass loss of 13% by mass above 200°C suggests that a large number of acetyl groups, which are acyloxy groups, are bonded to titanium atoms in the form of _-OCOCH3 on the surface of the titania nanoparticles.

In an air atmosphere, an equal amount of water was added to 50 g of this dispersion (5.0% titania by mass) to prepare 100 g of a 2.5% titania dispersion.

0.125 g of silver particles (50 μm) was added to this dispersion and stirred indoors. The reaction to support the silver particles on the titania nanoparticles was carried out using only indoor light (approximately 1000 lx) without irradiation with ultraviolet light. After 3 hours, the dispersion had taken on a yellowish tinge, indicating that the silver nanoparticles had been supported on the titania nanoparticles.

The titania nanoparticles carrying silver nanoparticles were dried under reduced pressure at 80°C, and the resulting powder was analyzed by XRD (X-ray diffraction). Peaks were observed at 2θ = 44-45° and 64-65°, indicating the formation of metallic Ag. Furthermore, semi-quantitative analysis revealed that approximately 0.2% by mass of Ag was carried on _TiO2 .

In addition, ESCA (X-ray photoelectron spectroscopy) analysis revealed a binding energy of 368.1 eV, suggesting that metallic Ag was formed (Figure 1).

After removing the excess Ag, the dispersion was found to be homogeneous, and maintained its homogeneity and transparency even after three months (Figure 2).

TEM observation of the synthesized material revealed that the Ag primary particles were 3-7 nm in size, and even the aggregated secondary particles were at most 10-20 nm in size, consistent with the color of the liquid (silver nanoparticles generally are yellow when they are 30 nm or less, and as the particle size increases, they become redder and turn orange) (Figures 3 and 4).

[Comparative Example 1]
The reaction was carried out in the same manner as in Example 1, except that 0.193 g of silver acetate was used instead of 0.125 g of silver particles, and only under room light (about 1000 lx) without irradiation with ultraviolet light.

In Comparative Example 1, a dispersion containing metallic Ag with a pale blue-purple color was obtained after 8 hours. TEM analysis of Comparative Example 1 showed that the Ag was in the form of aggregates of Ag nanoparticles with primary particles of 40 to 50 nm (Figure 5). In Comparative Example 1, the dispersion was uniform after the reaction, but became turbid after one week, and a slight supernatant was observed after three months.

[Comparative Example 2]
The test was carried out in the same manner as in Example 1, except that a mixture of 2.5 g of commercially available titania (P25: manufactured by Nippon Aerosil) and 47.5 g of water was used instead of 50 g of the dispersion (5.0 mass % titania).

In Comparative Example 2, there was no change in color after 3 hours, and even when stirring was extended to 60 hours, no change occurred. In Comparative Example 2, when the dispersion was left to stand for 4 hours, a supernatant and precipitate were observed.

[Comparative Example 3]
The test was carried out in the same manner as in Example 1, except that a mixture of 2.5 g of commercially available titania (P25: manufactured by Nippon Aerosil), 7.5 g of acetic acid, and 40 g of water was used instead of 50 g of the dispersion (5.0 mass % titania).

In Comparative Example 3, there was no change in color after 3 hours, and even when stirring was extended to 60 hours, no change occurred. In Comparative Example 3, when the dispersion was left to stand for 4 hours, a supernatant and precipitate were observed (Figure 6).

The titania nanoparticle manufacturing method of the present invention makes it possible to easily obtain a uniform, highly transparent, and storage-stable dispersion of silver-loaded titanium oxide nanoparticles without using silver salts, which are unsafe and easily altered.

The silver-loaded titanium oxide nanoparticle dispersion obtained by the titania nanoparticle manufacturing method of the present invention has high substrate adhesion and crack resistance, and can be used to obtain a highly transparent photocatalytic coating film.

Claims (4)

A method for producing titania nanoparticles, comprising:
a step of adding silver particles having a particle size of 1 μm or more and 1 mm or less to an aqueous dispersion of titania nanoparticles and stirring the mixture to support silver on the titania nanoparticles;
The titania nanoparticles have acetoxy groups bonded to at least some of the titanium atoms present on the surface, and when 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.
Manufacturing method. A method for producing titania nanoparticles, comprising:
(A) mixing a titanium-containing substance, an organic acid, and water to obtain an aqueous dispersion;
(B) heating the aqueous dispersion obtained in step (A) at a temperature higher than 80°C; and (C) after step (B), mixing silver particles having a particle size of 1 μm or more and 1 mm or less with the aqueous dispersion and irradiating the silver-containing aqueous dispersion with visible light or ultraviolet light to support silver on titanium. The manufacturing method described in claim 2, wherein the aqueous dispersion in step (A) contains 50% by mass or more of water. The method according to claim 2 or 3 , wherein the pH of the aqueous dispersion in steps (A) and (B) is 2 or more and 5 or less.