Particles, aqueous dispersion and film of titanium oxide, and preparation thereof

Background of the invention

1. Field of the invention

The present invention relates to an aqueous dispersed titanium oxide sol, a titanium oxide film formed on a ceramic substrate, a synthetic resin or the like thereof, specific titanium oxide particles and a process for producing an aqueous dispersed titanium oxide sol. The titanium oxide film according to the invention is transparent and has excellent photocatalytic activity and adhesion to a substrate.

Second Description of the relevant state of the art

It is known that titanium dioxide (hereinafter simply referred to as "titanium oxide") has three crystal phases, the anatase, the brookite and the rutile type. When titanium oxide is produced by burning titanium tetrachloride with oxygen in a vapor-phase deposition process, anatase-type titanium oxide is formed, and it is stable at the lowest temperature. When the thus-formed anatase type titanium oxide is heat-treated and the temperature is raised, brookite type titanium oxide is formed at a temperature of 816 to 1040 ° C, and rutile type titanium oxide is formed at a temperature higher than 1040 ° C.

Regarding a process in liquid, the crystal phases of titanium oxide formed by hydrolysis of titanium tetrachloride are described in detail by Kouemon Funaki in "Kogyo Kagaku (Industrial Chemistry)", Vol. 59, No. 11, page 1295. This report indicates that rutile type titanium oxide is mainly formed from a highly concentrated solution and anatase type titanium oxide is formed from a low concentrated solution. It was reported that the formation of fine particles of brookite type titanium oxide was impossible in a liquid phase process.

As apparent from the above, it was difficult to stably produce brookite type titanium oxide in a liquid phase process. When the titanium oxide formed by heating in a process is further heat treated at a high temperature, brookite type titanium oxide can be obtained, but the titanium oxide particles obtained are grown by the heat treatment. It was therefore difficult to obtain fine crystal particles of brookite type titanium oxide.

In the process for producing titanium oxide sol, it is generally the case that crystalline or amorphous titanium oxide particles are dispersed in a dispersion medium or a titanium oxide precursor such as methane alkoxide, titanium sulfate or titanium tetrachloride is mixed in a dispersion medium, after which the precursor is neutralized or hydrolyzed to a titania sol form.

A titania sol is used to make titania particles or to form a titanium oxide film by coating the sol on glass or ceramic.

It is known that titania sol is a photo-semiconductor and has transparency and increased photocatalytic activity when its particle size is small. The photocatalytic activity of titanium oxide has recently been extensively studied. The uses for the photocatalytic activity include the removal of interfering materials for cleaning, odor removal, such as ammonia, for deodorization and sterilization of microorganisms. Titanium oxide is used in various forms, such as mass, particle, film or sol, depending on the application. When the photocatalytic activity is to be combined with the transparency, titanium oxide is often formed as a film. Therefore, titanium oxide is often used as a sol for forming a film.

It is known that the photocatalytic activity of the titanium oxide is higher in the rutile type than in the anatase type. The reason for this is a difference in energy jump of about 0.2 eV between the two types, since the energy jump of the rutile type is 3.02 eV and that of the anatase type is 3.23 eV (see Ceramics 31 (1996), No 10, page 817). Because of this energy jump, anatase type titanium oxide is preferably used as the photosemiconductor.

As for the brookite type titanium oxide, pure material of the brookite type titanium oxide was not obtained, and it was difficult to obtain pure particles of the brookite type titanium oxide having such a high specific surface area that they could be used as the photosemiconductor because of the titanium oxide particles of the Brookittyps be made at such a high temperature that they sinter.

It has been proposed that, when a titanium oxide film is formed on a lighting device such as a glass tube of a fluorescent lamp or a shell thereof by coating with a titanium oxide sol, organic materials such as oil adhered thereto are decomposed by the photocatalytic activity of the titanium oxide.

However, the sols prepared by the methods hitherto known hardly result in a titanium oxide film having high transparency, and thus far, no lighting body having a brookite type titanium oxide film as a photocatalyst has been known.

When a titanium oxide film coated on a substrate of glass, plastic or other material is used as the photocatalyst, the titanium oxide film is required to have high photocatalytic activity. Since the photocatalytic action is based on a reaction at the surface of the particles, the particles should be fine particles having a high specific surface area and having excellent crystallinity for attaining a high photocatalytic activity. It is also necessary that the film be transparent when applied to a lighting fixture. In order to improve the transparency, it is desirable that the particles are fine and monodisperse, as in the case of improving the photocatalytic activity. It is common to use anatase type titanium oxide and finely divided to solve the above problems.

It is also required that the titanium oxide film has high adhesiveness, and peeling of the titanium oxide film when applied on a substrate should be prevented.

In the conventional method of hydrolysis of titanium tetrachloride, it was difficult to obtain a titanium oxide sol having a very small particle size and excellent crystallinity from the titanium oxide particles in the sol and to achieve high transparency when it was formed into a film.

In the process for the hydrolysis of titanium alkoxide, the particles of the obtained titanium oxide sol have excellent powder properties, including very small particle size, but the sol contains alcohol, causing problems in terms of safety, since an explosion can be caused when the sol is used to form a sol Titanium oxide film is heated. In order to prevent an explosion, a device with large dimensions is required, which leads to economic disadvantages. In addition, titanium alkoxide is much more expensive than titanium tetrachloride.

It is therefore an object of the invention to provide a titanium oxide sol which can form on a substrate a titanium oxide film excellent in photocatalytic activity and transparency and adhesiveness to the substrate, and to make accessible fine particles of brookite type titanium oxide.

SUMMARY OF THE INVENTION

As a result of studies of titanium oxide films formed from titanium oxide sols, the inventors have found that brookite-type titanium oxide having a large energy jump is particularly excellent in photocatalytic activity, and that a titania sol of a certain concentration of chloride ion becomes one Titanium oxide film with improved transparency and improved adhesion leads.

In the present invention, an aqueous dispersed titanium oxide sol comprising water-dispersed particles of brookite-type titanium oxide is provided, wherein the titanium oxide particles have an average particle size of not more than 0.5 μm and have a specific surface area of ​​not less than 20 m² / g ,

Preferably, this sol contains chloride ions in an amount of 50 to 10,000 ppm by weight as elemental chlorine.

The titanium oxide particles are contained in the sol preferably in an amount of 0.05 to 10 mol / l.

According to a preferred embodiment of the dispersed aqueous titanium oxide sol of this invention, it may contain a water-soluble polymer in an amount of 10 to 10,000 ppm by weight. According to another preferred embodiment, the brookite type titanium oxide in the sol has an energy jump of not less than 3.23 eV.

The aqueous dispersed titanium oxide sol of the present invention may further contain alkyl silicate as an adhesive in an amount of 1 to 50% by weight, in terms of silica, based on the weight of the titanium oxide.

According to the invention, the following are also made available:

Brookite type titanium oxide particles having an average particle size of not more than 0.5 μm and a specific surface area of ​​not less than 20 m² / g, and preferably the brookite type titanium oxide has an energy gap of not less than 2.23 eV,

and a titanium oxide film formed on a substrate using the above-defined aqueous dispersed titanium oxide sol.

The substrate on which the titanium oxide film is formed is preferably made of a heat-resistant material such as glass, ceramics and a metal, or is selected from synthetic resins, paper and wood.

The above-defined titanium oxide film may be a sintered film. The invention also provides a process for preparing an aqueous dispersed titanium oxide sol comprising the steps of:

Adding titanium tetrachloride to hot water at a temperature of 75 to 100 ° C and

Hydrolysis of the titanium tetrachloride at a temperature in the range of 75 ° C to the boiling point of the mixture to form an aqueous sol of particles of brookite type titanium oxide.

In this process, the hydrolysis is preferably carried out in a reactor equipped with a reflux condenser.

The process of the present invention may further comprise a step of adding alkyl silicate to the sol in an amount of 1 to 50% by weight as silica, based on the weight of the titanium oxide. According to the present invention, there is also provided a process for producing brookite type titanium oxide particles by filtering and drying this aqueous sol of brookite type titanium oxide particles prepared by the above-described process to produce brookite type titanium oxide particles.

Short description of the drawing

Fig. 1 shows a reactor equipped with a reflux condenser used to prepare a titania sol in an example of the invention.

Description of the Preferred Embodiments

Since the titanium oxide particles in the aqueous dispersed titanium oxide sol are finer, the photocatalytic activity and the transparency of the titanium oxide film are improved. For the photocatalytic activity, it is preferable that the titanium oxide particles are crystalline. However, if the particle size of the titanium oxide particles is too small, such particles are difficult to produce. Therefore, the average particle size of the titanium oxide particles in the sol is preferably in the range of 0.01 to 0.1 μm.

The aqueous dispersed titanium oxide sol according to the invention is a sol resulting in a titanium oxide film having improved photocatalytic activity and transparency, and is characterized in that the water-dispersed titanium oxide particles are particles of brookite type titanium oxide having an average particle size of not more than 0.5 μm , preferably 0.01 to 0.1 μm and a specific surface area of ​​not less than 20 m² / g. The brookite type titanium oxide particles have an energy jump of 3.23 eV or more.

From the viewpoint of transparency, it is preferable for the particle size of the titanium oxide particles that the titanium oxide particles are monodisperse and have an average particle size of not more than 0.5 μm, preferably 0.01 to 0.1 μm. Even if the specific surface area of ​​the particles is large, agglomerates of primary particles do not give a transparent film of titanium oxide.

According to the prior art, brookite type titanium oxide can not be produced except by a method of heat treating anatase type titanium oxide. When the brookite type titanium oxide is produced by this heat treatment, the heat treatment causes growth of the brookite type titanium oxide particles to be large in particle size, so that these particles were not used for producing a titanium oxide film.

In a sol in which the above-mentioned brookite type titanium oxide particles are dispersed in water, chloride ions may be contained in an amount of 50 to 10,000 ppm as elemental chlorine, and the sol titanium oxide film not only has excellent photocatalytic activity but also has excellent adhesion to a substrate.

In the aqueous dispersed titanium oxide sol of the present invention, when the concentration of the titanium oxide particles is too high, the particles agglomerate and the sol becomes unstable. If the concentration of titanium oxide particles is too low, problems often occur. For example, the step of forming a titanium oxide film by coating takes a long time. Therefore, the concentration of the titanium oxide particles is preferably in the range of 0.05 to 10 mol / l.

By filtering, washing and drying the aqueous dispersed titanium oxide sol of the present invention, titanium oxide particles can be obtained. The brookite type titanium oxide particles thus obtained have an average particle size of not more than 0.5 μm, preferably 0.01 to 0.1 μm and a specific surface area of ​​not less than 20 m² / g. You have an energy jump of 3.23 eV or more.

When the aqueous dispersed titanium oxide sol is used for producing a titanium oxide film, it is preferable to add a water-soluble polymer in a small amount, for example, from about 10 to 10,000 ppm in order to improve the film-forming ability or the coating formation. Preferred water-soluble polymers include polyvinyl alcohol, methyl cellulose, ethyl cellulose, CMC, starch, etc.

The aqueous dispersed titanium oxide sol of the present invention may be coated on substrates of various materials to form a titanium oxide film on the surface of the substrate. The substrates are not limited and can be made of ceramic, metal, plastic; Wood, paper, etc. exist.

The substrate may be a catalyst support made of alumina, zirconia and the like on which the titania film is provided as a catalyst to produce a catalyst. The substrate may also be a glass tube or a plastic shell of a lighting device, such as a fluorescent lamp, on which the titanium oxide film can be formed. This titanium oxide film is transparent and has photocatalytic activity, so that the film can decompose organic materials such as oil vapor without shielding the light, thereby effectively preventing the accumulation of dirt on a glass tube or plastic sheath. When such a titanium oxide film is formed on a windowpane or a building wall, the deposit of dirt on the disk or the wall can also be prevented. When the film is formed on a windowpane or wall of a high building, the need for cleaning can be avoided or reduced so that it is useful for reducing the cost of maintaining the building.

Methods of applying an aqueous dispersed titanium oxide sol to a substrate include dipping a substrate in a sol, spraying a sol on a substrate, coating a substrate with a sol, and the like. The thickness of the applied sol is about 0.01 to 0.2 mm. After applying the sol to a substrate, the water content of the sol is removed by drying to obtain a titanium oxide film. As mentioned above, this film can be used as a catalyst and the like.

When the substrate is made of a heat-resistant material such as glass, the titanium oxide film formed on the substrate may be heat-treated. By this heat treatment, the film can adhere more firmly to the substrate and can be given higher hardness. The temperature of the heat treatment is preferably not lower than 200 ° C. The upper limit of the heat treatment is not specifically specified and may be determined depending on the heat resistance of the substrate. However, the hardness and the adhesiveness of the film to the substrate are not increased even when the temperature is very high. Therefore, a temperature of not more than 700 ° C is suitable for maintaining the crystal phase of the brookite type titanium oxide.

Alternatively, the adhesiveness of the transparent titanium oxide film to the substrate can be increased without heat treatment by adding a suitable adhesive to the aqueous dispersed titanium oxide sol of the present invention. Suitable adhesive includes organic silica-containing compounds such as alkyl silicates. The amount of additive may be (as SiO 2) 1 to 50% by weight of the titanium oxide in the titania sol. When the amount of the adhesive is less than 1% by weight, the desired effect can not be obtained. If the amount of the adhesive is more than 50% by weight, very high adhesion can not be achieved, but the photocatalytic activity of the film is lost since the titanium oxide particles are covered by the adhesive, which is not preferable. The adhesive may be added immediately prior to use (coating or coating) or may be added beforehand in the preparation of the sol, depending on the type of adhesive.

The atmosphere of the heat treatment is not specifically limited and may be air. The time of the heat treatment is not particularly limited and may be, for example, 1 to 60 minutes. The titanium oxide film obtained after the heat treatment has a thickness of about 0.05 to 1.0 μm when the sol is applied in the above-mentioned amount.

The preparation of the aqueous dispersed titanium oxide sol of the present invention will be described below.

The aqueous dispersed titanium oxide sol of the invention is obtained by hydrolysis of titanium tetrachloride under certain conditions.

It is preferred to prevent the hydrogen chloride formed by the hydrolysis from escaping from the reactor and remaining in the sol. When titanium tetrachloride is hydrolyzed while the hydrogen chloride formed escapes, it is difficult to keep the particle size of the titanium oxide particles in the sol small and also the crystallinity of the titanium oxide particles formed is poor.

It is not necessary to completely prevent the hydrogen chloride formed by the hydrolysis from escaping or leaking from the reactor, and it is sufficient that suppression of the escape or leakage is achieved. The method for preventing or suppressing is not restricted. For example, evacuation or depressurization may be used, but the simplest and most effective method is to conduct the hydrolysis in a reactor equipped with a reflux condenser. Fig. 1 shows such a reactor. In Fig. 1, an aqueous solution of titanium tetrachloride 2 is placed in a reactor 1 equipped with a reflux condenser 3. The reactor is also provided with a stirrer 4, a thermometer 5 and a heater 6. When hydrogen chloride and water in vapor form are formed in the hydrolysis, most of the vapor is condensed by the reflux condenser and returned to the reactor so that hydrogen chloride hardly escapes from the reactor.

When the concentration of titanium tetrachloride in the aqueous titanium tetrachloride solution to be hydrolyzed is too low, the productivity is low and the efficiency of imaging a titanium oxide film from the obtained aqueous titanium oxide sol dispersion is low. When the concentration of titanium tetrachloride of the aqueous solution of titanium tetrachloride to be hydrolyzed is too high, the reaction becomes vigorous, so that it is difficult to keep the particle size of the titanium oxide particles small, so that the dispersibility is lowered, which is not a suitable material for forming a transparent Films is received. Thus, a method for producing a sol of high titania concentration by hydrolysis followed by dilution with a large amount of water to adjust the concentration of titanium oxide to 0.05 to 10 mol / l is not preferred. It is desirable that the concentration of titanium oxide in this range be adjusted when the sol is formed. To achieve this, the concentration of titanium tetrachloride in the aqueous solution of titanium tetrachloride to be hydrolyzed is set to be almost equal to the concentration of the titanium oxide to be formed, that is, about 0.05 to 10 mol / l, and if necessary Subsequently, a small amount of water or condensate is added to adjust the concentration of titanium oxide to 0.05 to 10 mol / l.

The temperature of the hydrolysis is preferably in the range of not less than 50 ° C to the boiling point of the aqueous one. Solution of titanium tetrachloride. At a temperature of less than 50 ° C, a long time is required for the hydrolysis. After the temperature is raised to the above temperature, the hydrolysis is carried out at this temperature for about 10 minutes to 12 hours. The time to achieve a certain temperature for the hydrolysis may be shorter when the hydrolysis temperature is lower.

The hydrolysis may be carried out by heating a mixture of water and titanium tetrachloride in a reactor to the predetermined temperature, or alternatively by first heating water in a reactor and adding titanium tetrachloride to the heated water to heat it to the predetermined temperature ,

The specified hydrolysis generally gives brookite type titanium oxide or an anatase type and / or rutile type mixture of brookite type. In order to increase the content of brookite type titanium oxide, a method is suitable in which water is first heated to 75 to 100 ° C, titanium tetrachloride is added to the water, and the hydrolysis is carried out at a temperature of 75 ° C to the boiling point of the solution. According to this method, the content of the brookite type titanium oxide in the total titanium oxide formed can be increased to not less than 70% by weight.

The rate of temperature increase is preferably not less than 0.2 ° C / min. more preferably not less than 0.5 ° C / min. because the titanium oxide particles formed become finer as the rate of temperature increase increases.

The preparation of the aqueous dispersion of titanium oxide sol according to the present invention may be carried out in a batch system or a continuous system in which titanium tetrachloride and water are continuously fed to a continuous reactor from whose opposite end the reaction solution is removed and sent to the dechlorination treatment.

The resulting aqueous dispersion of titania sol may then be subjected to a dechlorination treatment or, if appropriate, water is added or removed to adjust the chloride ion concentration, if desired or required, to a value of 50 to 10,000 ppm.

The dechlorination treatment may be a known method such as electrodialysis, treatment with an ion exchange resin or electrolysis. The degree of dechlorination treatment can be determined by means of the pH. When the chloride ion concentration is 50 to 10,000 ppm, the pH of the sol is about 5 to 0.5, and when the chloride ion concentration is in a preferable range of 100 to 4,000 ppm, the pH of the sol is about 4 to 1.

An organic solvent may be added to the aqueous dispersed titanium oxide sol of the present invention to disperse the titanium oxide particles in a mixture of water and an organic solvent.

When a titanium oxide film is formed from the titanium oxide sol aqueous dispersion of the present invention, it is preferred that the aqueous dispersion of titania sol be used directly to form a titanium oxide film. A method in which titanium oxide particles are first formed from the aqueous dispersion of titanium oxide sol, after which the resulting titanium oxide particles are dispersed in water to form a titanium oxide sol, and then the sol thus obtained is used to form a titanium oxide film is not preferred. The reason is that titanium oxide particles have higher surface activity when the particles are finer, but the finer titanium oxide particles are difficult to disperse in water. That is, they form agglomerates which provide a titanium oxide film with impaired transparency or photocatalytic activity.

EXAMPLES

The invention will be described below with reference to examples according to the invention, to which the invention is, of course, not intended to be limited.

Examples 1 to 6

Water was added to titanium tetrachloride (purity 99.9%) to adjust the concentration of titanium tetrachloride of the solution to 0.25 mol / l (as titanium oxide: 2 wt%), while the aqueous solution was cooled by means of a coolant such as ice , was cooled to prevent the temperature of the solution from rising above 50 ° C. One liter of the aqueous solution was placed in a mold as shown in FIG. 1 given reactor equipped with a reflux condenser; and heated to the boiling point of the solution (104 ° C), and the hydrolysis was carried out for 60 minutes while maintaining this temperature. The resulting sol was cooled and then subjected to electrodialysis to remove residual cloride formed in the reaction to the chloride ion concentrations shown in Table 1. Electrodialysis was performed using an electrodialysis machine G3 manufactured by Asahi Kasei Kogyo KK while monitoring the pH of the sol.

Observation of the particles in the brines showed that the average particle sizes of the particles were 0.015 to 0.018 μm.

X-ray diffraction of the particles showed that the particles consisted of crystalline titanium oxide.

Comparative Examples 1 and 2

The procedure of Examples 1 to 6 was repeated except that the chloride ion concentrations were controlled to 30 ppm (Comparative Example 1) and 15,000 ppm (Comparative Example 1).

To the thus formed aqueous dispersions of titania sol having the controlled chloride ion concentrations of Examples 1 to 6 and Comparative Examples 1 and 2 was added a water-soluble polyvinyl alcohol polymer as a film-forming agent in an amount of 1000 ppm based on the weight of the brine. These sols having chloride ion concentrations of 50 to 10,000 ppm were stable and showed no precipitation of titanium oxide particles even after one day (Examples 1 to 6). However, the sol having the chloride ion concentration of 30 ppm showed agglomeration of the titanium oxide particles in the sol and the sol having the chloride ion concentration of 15,000 ppm resulted in a titanium oxide film having a slightly white coloration.

(Comparative Examples 1 and 2)

Using the sols of Examples 1 to 6 and Comparative Examples 1 and 2, titanium oxide films were prepared on glass plates by dip-coating a glass plate with the sol, followed by drying and heat-treating at 500 ° C for one hour. The titanium oxide films obtained were 0.15 μm thick.

The Rietveld analysis of the powder X-ray diffraction patterns of the titanium oxide showed that the titanium oxide before the heat treatment was a mixture of about 50% by weight anatase type titanium oxide and about 50% by weight brookite type titanium oxide and that the titanium oxide after the heat treatment was 800 ° C or more of rutile titanium oxide was the only type.

(Rating of the film)

The light transmittance, photocatalytic activity and adhesiveness to a quartz glass plate of the titanium oxide films obtained from each of the dispersed aqueous sols of Examples and Comparative Examples were determined.

The light transmittance was measured for a titanium oxide film formed on a quartz glass plate using a spectrophotometer manufactured by Nihon Bunkoh (Japan Spectroscopy) KK while the wavelength was continuously changed from 700 nm to 200 nm. The light transmittance of the film at 550 nm was taken as the light transmittance of the film of the present invention. The results are shown in Table 1.

The photocatalytic activity of the titanium oxide was determined by preparing a reactor using a titanium oxide film-coated quartz glass plate, charging 5 mol / L of oxalic acid into the reactor, irradiating the oxalic acid with a mercury lamp while introducing oxygen into the oxalic acid, and determining the amount of decomposed oxalic acid by redox titration with potassium permanganate measured. The results are shown in Table 1.

The adhesiveness of the film to a substrate was measured by the pencil hardness method and the method of peeling the XY-matrix cut film (JIS K5400). The peel strength in the peeling method of the film cut after the XY matrix is ​​represented by the ratio of the unrelaxed portions to the entirety of the cut portions. Table 1

Examples 7 and 8 and Comparative Examples 3 and 4

The same procedures as in Example 1 and Comparative Example 1 were repeated except that the titanium oxide film was formed on a polyethylene terephthalate-made plastic substrate and the film was dried at 100 ° C but no heat treatment was performed. The evaluations were carried out in the same manner as in Examples and Comparative Examples.

The results are shown in Table 2. Table 2

Example 9

954 Milliliters of distilled water were added to a reactor equipped with a reflux condenser as shown in Fig. 1 and heated to 95 ° C. While stirring at about 200 rpm, 46 ml of an aqueous solution of titanium tetrachloride (elemental titanium content: 16.3% by weight, density: 1.59, purity: 99.9%) was added dropwise at a rate of about. 5 ml / min in the reactor. The temperature of the reaction solution was kept constant during the reaction. As a result, the concentration of titanium tetrachloride changed to 0.25 mol / l (as titanium oxide concentration: 2 wt%).

The reaction solution in the reactor became cloudy soon after the addition of the titanium tetrachloride began, but the temperature of the reaction solution was kept constant and, when addition was completed, heated to near the boiling point (104 ° C) and kept at that temperature for 60 minutes to complete the reaction and to end. After cooling, the formed and residual chlorine was removed by electrodialysis to a pH of 2 (chloride ion: 600 ppm), after which water-soluble polyvinyl alcohol polymer in an amount of 0.1% based on the weight of titanium oxide was added to produce a titania sol.

The sol was filtered and dried in vacuo at 60 ° C to form a powder which was analyzed by X-ray diffraction. As a result, the titanium oxide contained 96.7% by weight of the brookite type, 0.9% by weight of the rutile type and 2.4% by weight of anatase type titanium oxide.

Observation of the powder by means of a transmission electron microscope revealed that the average particle size of the primary particles was 15 nm.

The specific surface area of ​​the powder was 100 m 2 / g as determined by the BET method.

The above sol was spin-coated on a quartz glass plate and dried in a drier at 100 ° C to obtain a transparent film. The light transmittance of the silica glass plate with the film was more than 95% in the visible light range, showing that the plate coated with the film was completely transparent. The film-coated plate showed ultraviolet absorption, which indicated by the absorption end of the light that the energy jump was 3.75 eV. The relationship between the energy jump and the absorption end of the light is given by the following formula (1):

λ = 1239 / Eg (1)

where λ is the end of absorption of the light in the unit nm and Eg stands for the energy jump in the unit eV.

Example 10

The procedure of Example 9 was repeated except that the reaction temperature during the addition of titanium tetrachloride was 75 ° C.

The obtained titanium oxide powder was analyzed by X-ray diffraction to reveal that the titanium oxide contained 75% by weight of the brookite type and 25% by weight of the rutile type.

Observation of the powder by means of a transmission electron microscope revealed that the average particle size of the primary particles was 10 nm.

The specific surface area of ​​the powder was 120 m² / g, determined by the BET method.

The above sol having a pH of 1 (chloride ion content 3,000 ppm) obtained by the electrodialysis was applied on a silica glass plate and heat-treated at 500 ° C to form a transparent film. The transparent film was also a mixture of brookite type and rutile type titanium oxides as determined by thin film X-ray diffraction. The light transmittance of the film-coated quartz glass plate was more than 95% in the visible light range, showing that the film-coated plate was completely transparent. The film-coated plate showed ultraviolet absorption, which indicated by the absorption end of the light that the energy jump was 3.30 eV.

Comparative Example 5

Particles of anatase type titanium oxide having a primary particle size of 7 nm were dispersed in water by means of an ultrasonic disperser to obtain an aqueous solution of titanium oxide having a concentration of titanium oxide of 2% by weight as in Example 9. Hydrochloric acid was added as a coagulating agent to adjust the pH to 1, followed by the same procedure as in Example 9 to prepare an aqueous dispersed titanium oxide sol. The sol was applied to a glass plate and dried at 100 ° C to form a transparent film.

Comparative Example 6

An aqueous dispersed titanium oxide sol was prepared in the same manner as in Comparative Example 5 except that rutile type titanium oxide particles having a primary particle size of 50 nm were used. The sol showed deposition of titanium oxide particles as in Comparative Example 5 and the particles were redispersed using hydrochloric acid as a coagulant. Since a titanium oxide film formed from the supernatant after deposition showed no photocatalytic activity, the sol was subjected to a dispersion treatment by means of an ultrasonic dispersing machine soon after its preparation, and the resulting sol was allowed to form a titanium oxide film on a glass plate in the same manner as used in Example 9. The photocatalytic activity of the obtained film was determined.

(Rating of the film)

The photocatalytic activities of the titanium oxide films obtained from the sols of Examples 9 to 11 and Comparative Examples 5 and 6 were determined by the method of oxalic acid decomposition. The results are shown in Table 3.

Table 3

Decomposition rate (%) after 4 hours of irradiation

Example 9 55

Example 10 48

Example 11 50

Comparative Example 5 30

Comparative Example 6 Not determined because the substrate was cloudy

In Comparative Example 5, the surface of the glass substrate was not uniform because agglomerates of titanium oxide were formed.

In Comparative Example 6, the photocatalytic activity of the titanium oxide film could not be determined because no transparent titanium oxide film was obtained.

Example 12

In the same manner as in Example 9, 0.25 mol / liter (as titanium oxide: 2 wt%) of an aqueous solution of titanium tetrachloride was subjected to hydrolysis. The resulting reaction solution was concentrated to a titanium oxide concentration of 10% by weight, electrodialysis was performed to remove residual chlorine to a pH of 2 (chloride ion concentration about 600 ppm), and tetramethylorthosilicate Si (OCH₃) ₄. was used as an adhesive in an amount corresponding to 5% by weight of SiO & sub2; to give a titanium oxide sol.

Example 13

The procedure of Example 12 to concentration and electrodialysis was repeated after which it was diluted 5-fold with isopropyl alcohol and tetraethyl orthosilicate Si (OC 2 H 5) 4 was added. as an adhesive in an amount corresponding to 20% by weight of SiO & sub2; was added to form a titania sol.

Example 14

The procedure of Example 12 was repeated except that tetrapropyl orthosilicate Si (OC 3 H 7) 4 was used. instead of tetraethyl orthosilicate was used and in an amount corresponding to 35 wt .-% SiO? was added to obtain a titania sol.

Comparative Example 7

The procedure of Example 14 was repeated except that tetra-propyl orthosilicate was added in an amount of 55% by weight in terms of SiO 2 to obtain a titania sol.

(Rating of the film)

Each of the sols obtained in Examples 12 to 14 and Comparative Example 7 was spin-coated on a quartz glass plate and left to dry to prepare a transparent film. The light transmittance of the film-coated quartz glass plates was more than 95% in the range of visible light, showing that the plates coated with the films were completely transparent.

The pencil hardness test and the adhesion test were carried out using the quartz glass plates coated with the titanium oxide films. The results are shown in Table 4. Table 4