JP7694950B2 - Method for producing anatase titanium oxide film - Google Patents

Description

This disclosure relates to a method for producing an anatase-type titanium oxide film.

Titanium oxide continues to attract attention in various technical fields such as photocatalysis and n-type semiconductors. For example, it is known that anatase-type titanium oxide exhibits higher activity in photocatalysis than rutile-type titanium oxide. Since the properties required of titanium oxide differ depending on the application, methods for producing titanium oxide suitable for the intended application are being investigated.

Since anatase titanium oxide irreversibly changes to rutile titanium oxide at high temperatures, most single crystals of anatase titanium oxide are produced using epitaxial growth. For example, the following Patent Document 1 discloses a method for producing a single crystal thin film of titanium dioxide with an anatase crystal structure on a single crystal substrate by a laser ablation film formation method.

Titanium oxide may be produced using a liquid phase method such as hydrothermal synthesis. Hydrothermal synthesis is a material synthesis method classified as a solvothermal method. For example, Patent Document 2 below discloses a method for synthesizing anatase-type titanium oxide particles. The method for synthesizing titanium oxide particles disclosed in Patent Document 2 below includes a step of reacting titanium alkoxide with a hydroxyl group-containing amine to obtain a titanium oxide source, and a step of adding an organic acid amine salt obtained by reacting an organic acid having 8 or more carbon atoms with an amine to an aqueous solution in which the obtained titanium oxide source is dissolved, thereby carrying out hydrothermal synthesis.

Hydrothermal synthesis is also used to manufacture composite titanium oxide films. For example, the following Patent Document 3 discloses a method for forming a composite titanium oxide film on a substrate by hydrothermal synthesis. In the following Patent Document 3, the composite titanium oxide film is selectively formed only on the patterned Ti metal.

JP 2002-030417 A JP 2015-059053 A Japanese Patent Application Publication No. 10-182154

Gas-phase methods such as laser ablation have problems such as high manufacturing costs and high technical requirements. For example, gas-phase methods require expensive deposition equipment. Not only is the deposition equipment expensive, but the operation of the deposition equipment also requires a high level of understanding and skill. In addition to the above problems, gas-phase methods are not suitable for producing large-area, uniform titanium oxide films.

Liquid-phase methods such as the solvothermal method may be able to solve the problems of the gas-phase method. However, the application of conventional solvothermal methods to the production of titanium oxide films does not allow the growth of sufficiently highly oriented anatase-type titanium oxide films.

The objective of one embodiment of the present disclosure is to provide a method for producing a highly oriented anatase-type titanium oxide film by a solvothermal method.

The present disclosure includes the following aspects.
<1> A method for producing an anatase type titanium oxide film, comprising: preparing a titanium-containing liquid containing water, an alcohol, a diketone, an acid, a titanium alkoxide, and a fluorine atom-containing titanium compound; and immersing a substrate in the titanium-containing liquid to form an anatase type titanium oxide film on the substrate by a solvothermal method.
<2> The method for producing an anatase titanium oxide film according to <1>, wherein preparing the titanium-containing liquid includes preparing a mixture containing the water, the alcohol, the diketone, and the acid, and mixing the mixture with the titanium alkoxide and the fluorine atom-containing titanium compound.
<3> The method for producing an anatase type titanium oxide film according to <1> or <2>, wherein the alcohol includes ethanol.
<4> The method for producing an anatase type titanium oxide film according to any one of <1> to <3>, wherein the diketone includes a β-diketone.
<5> The method for producing an anatase type titanium oxide film according to any one of <1> to <3>, wherein the diketone contains acetylacetone.
<6> The method for producing an anatase type titanium oxide film according to any one of <1> to <5>, wherein the acid contains a hydrogen halide.
<7> The method for producing an anatase type titanium oxide film according to any one of <1> to <5>, wherein the acid contains hydrogen chloride.
<8> The method for producing an anatase titanium oxide film according to any one of <1> to <7>, wherein the alkoxy group in the titanium alkoxide has 1 to 8 carbon atoms.
<9> The method for producing an anatase type titanium oxide film according to any one of <1> to <8>, wherein the titanium alkoxide includes titanium tetraisopropoxide.
<10> The method for producing an anatase type titanium oxide film according to any one of <1> to <9>, wherein the fluorine atom-containing titanium compound includes a salt containing TiF ₆ ^2−.
<11> The method for producing an anatase type titanium oxide film according to any one of <1> to <9>, wherein the fluorine atom-containing titanium compound includes ammonium hexafluorotitanate.
<12> The method for producing an anatase type titanium oxide film according to any one of <1> to <11>, wherein the substrate is a _LaAlO3 substrate or a _SrTiO3 substrate.
<13> The method for producing an anatase type titanium oxide film according to any one of <1> to <12>, wherein the heating temperature in the solvothermal method is 110° C. to 170° C.

According to one embodiment of the present disclosure, a method for producing a highly oriented anatase-type titanium oxide film by a solvothermal method is provided.

FIG. 1 is an XRD pattern of the titanium oxide film in Example 1. FIG. 2 is a pole figure of the titanium oxide film in Example 1. FIG. 3 is an XRD pattern of the titanium oxide film in Example 2. FIG. 4 is a pole figure of the titanium oxide film in Example 2.

The following describes in detail the embodiments of the present disclosure. The present disclosure is not limited to the following embodiments. The following embodiments may be modified as appropriate within the scope of the purpose of the present disclosure.

In this disclosure, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower and upper limits.

In the numerical ranges described in stages in this disclosure, the upper limit value described in a certain numerical range may be replaced with the upper limit value of another numerical range described in stages, and the lower limit value described in a certain numerical range may be replaced with the lower limit value of another numerical range described in stages. In the numerical ranges described in stages in this disclosure, the upper limit value or lower limit value described in a certain numerical range may be replaced with a value shown in the examples.

In this disclosure, the term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes as long as the intended purpose is achieved.

In this disclosure, when a composition contains multiple substances corresponding to each component, the amount of each component in the composition means the total amount of the multiple substances present in the composition, unless otherwise specified.

In this disclosure, combinations of preferred aspects are more preferred aspects.

<Method of manufacturing anatase titanium oxide film>
The method for producing an anatase type titanium oxide film according to the present disclosure will be described below. In one embodiment of the present disclosure, the method for producing an anatase type titanium oxide film includes the following steps (1) and (2).
(1) preparing a titanium-containing liquid containing water, an alcohol, a diketone, an acid, a titanium alkoxide, and a fluorine atom-containing titanium compound (hereinafter, sometimes referred to as “step (1)”).
(2) Immersing a substrate in a titanium-containing liquid and forming an anatase-type titanium oxide film on the substrate by a solvothermal method (hereinafter, sometimes referred to as "step (2)").

The above embodiment provides a method for producing a highly oriented anatase-type titanium oxide film by a solvothermal method. It is presumed that the reason why the above-mentioned titanium oxide film is obtained is that the titanium-containing liquid creates an environment suitable for the growth of a highly oriented anatase-type titanium oxide film. For example, the acid catalyzes the dehydration condensation reaction of the reaction intermediate (e.g., the hydrolyzate of titanium alkoxide) in step (2) by adjusting the acidity of the titanium-containing liquid. For example, the diketone can form a titanium-diketone complex in the titanium-containing liquid. The formation of the titanium-diketone complex can maintain the titanium concentration in the liquid without increasing the amount of acid. Therefore, compared to the use of only an acid rather than both a diketone and an acid, the use of a diketone and an acid can achieve the titanium concentration required for the growth of the target anatase-type titanium oxide film in the titanium-containing liquid while preventing or reducing the dissolution of the substrate caused by the acid. Furthermore, the use of a fluorine atom-containing titanium compound contributes to the preferential growth of a highly oriented anatase-type titanium oxide film. Although the detailed mechanism is unclear, it is believed that the action of the fluorine atoms in the fluorine-containing titanium compound affects the crystal structure and orientation of the titanium oxide film. Therefore, an anatase-type titanium oxide film with high orientation is formed.

[Step (1)]
Step (1) is to prepare a titanium-containing liquid containing water, an alcohol, a diketone, an acid, a titanium alkoxide, and a fluorine atom-containing titanium compound. Step (1) may also include producing the titanium-containing liquid.

Water mainly functions as a solvent. In addition, water is involved in the hydrolysis of titanium alkoxide in step (2).

An example of water is purified water. Methods for producing purified water include filtration, distillation, and ion exchange.

The proportion of water in the titanium-containing liquid is preferably 5% to 45% by volume, and more preferably 15% to 25% by volume. "Proportion of water in the titanium-containing liquid" means the ratio of the total amount of water in the titanium-containing liquid to the total amount of the titanium-containing liquid.

The alcohol mainly functions as a solvent. Furthermore, the alcohol can contribute to promoting the hydrolysis of the titanium alkoxide in step (2). The promotion of the hydrolysis of the titanium alkoxide is believed to be due to the replacement of at least one alkoxy group in the titanium alkoxide with an alkoxy group derived from the alcohol.

Examples of the alcohol include aliphatic alcohols and aromatic alcohols. The aliphatic alcohol may be a saturated aliphatic alcohol or an unsaturated aliphatic alcohol. Examples of the aliphatic alcohol include methanol, ethanol, 1-propanol, 2-propanol (also known as isopropanol), 2-methyl-1-propanol (also known as isobutyl alcohol), 2-methyl-2-propanol (also known as tert-butyl alcohol), 1-butanol, and 2-butanol. Examples of the aromatic alcohol include benzyl alcohol. From the viewpoints of solubility, miscibility, and promotion of hydrolysis of titanium alkoxide in step (2), the alcohol preferably includes an aliphatic alcohol, preferably includes at least one selected from the group consisting of ethanol and 2-propanol, and more preferably includes ethanol.

From the viewpoints of solubility, miscibility, and promotion of hydrolysis of titanium alkoxide in step (2), the number of carbon atoms in the alcohol is preferably 1 to 8, more preferably 1 to 6, and even more preferably 2 to 4.

The titanium-containing liquid may contain one or more types of alcohol.

From the viewpoints of solubility, miscibility, and promotion of hydrolysis of titanium alkoxide in step (2), the proportion of alcohol in the titanium-containing liquid is preferably 5% to 45% by volume, and more preferably 15% to 25% by volume, based on volume. "Proportion of alcohol in the titanium-containing liquid" means the ratio of the total amount of alcohol in the titanium-containing liquid to the total amount of the titanium-containing liquid.

From the viewpoint of solubility and hydrolysis rate control, the ratio of the alcohol content to the titanium alkoxide content in the titanium-containing liquid is preferably 10 to 50, and more preferably 15 to 30, on a volume basis.

The diketone mainly plays a role in forming a titanium-diketone complex in the titanium-containing liquid to maintain the titanium concentration required for the growth of the desired anatase-type titanium oxide film. The diketone also acts as a solvent.

Examples of diketones include β-diketones and γ-diketones. Examples of β-diketones include acetylacetone (also known as 2,4-pentanedione) and methylacetylacetone (also known as 3-methyl-2,4-pentanedione). Examples of γ-diketones include 2,5-hexanedione.

At least one carbonyl group (-CO-) contained in a diketone may be part of an ester bond (-COO-). For example, the term "diketone" in this disclosure includes β-ketoesters. Examples of β-ketoesters include methyl acetoacetate, ethyl acetoacetate, and tert-butyl acetoacetate.

From the viewpoint of the ability to form a titanium-diketone complex, the diketone preferably contains a β-diketone, more preferably contains at least one selected from the group consisting of acetylacetone and ethyl acetoacetate, and even more preferably contains acetylacetone.

From the viewpoint of the formation of a titanium-diketone complex, the number of carbon atoms in the diketone is preferably 5 to 10, more preferably 5 to 8, and even more preferably 5 to 6.

The titanium-containing liquid may contain one or more diketones.

From the viewpoint of preventing or reducing dissolution of the substrate caused by acid while maintaining the titanium concentration necessary for the growth of the target anatase-type titanium oxide film in the titanium-containing liquid, the proportion of diketone in the titanium-containing liquid is preferably 15% by volume to 50% by volume, and more preferably 35% by volume to 50% by volume. "Proportion of diketone in the titanium-containing liquid" means the ratio of the total amount of diketone in the titanium-containing liquid to the total amount of the titanium-containing liquid.

From the viewpoint of the formation of a titanium-diketone complex, the ratio of the diketone content to the titanium alkoxide content in the titanium-containing liquid is preferably 3 to 200, and more preferably 6 to 100, on a substance amount basis.

The acid primarily serves as a catalyst for the dehydration condensation reaction of the reaction intermediate (e.g., the hydrolysate of titanium alkoxide) in step (2) by adjusting the acidity of the titanium-containing liquid.

Examples of the acid include phosphoric acid ( _H3PO4 ), nitric acid ( _HNO3 ), _sulfuric acid ( _H2SO4 ), acetic acid ( _CH3COOH ) and hydrogen halides. Examples of the hydrogen halides include hydrogen chloride and hydrogen bromide. In the production of _the titanium-containing liquid, the hydrogen halide is preferably added as an aqueous solution of hydrogen halide. For example, an aqueous solution of hydrogen chloride is called hydrochloric acid. From the viewpoint of efficient growth of a highly oriented film, the acid preferably contains hydrogen halide, more preferably contains hydrogen chloride.

The titanium-containing liquid may contain one or more acids.

From the viewpoint of preventing or reducing dissolution of the substrate due to acid while promoting the dehydration condensation reaction of the reaction intermediate in step (2), the pH of the titanium-containing liquid at 25°C is preferably 2 to 4, and more preferably 2 to 2.5. The pH of the titanium-containing liquid is measured using a commercially available pH test paper at 25°C. An example of a commercially available pH test paper is "Stick pH Test Paper pH 0-14.0 (MACHEREY-NAGEL)."

The titanium alkoxide serves as a titanium oxide source. In step (2), the titanium alkoxide undergoes hydrolysis and dehydration condensation to form titanium oxide.

Examples of titanium alkoxides include titanium tetramethoxide, titanium tetraethoxide, titanium tetraisopropoxide, titanium tetra n-butoxide, and titanium tetra tert-butoxide. From the viewpoint of the hydrolysis property of the titanium alkoxide in step (2), it is preferable that the titanium alkoxide contains titanium tetraisopropoxide.

From the viewpoint of the hydrolysis property of the titanium alkoxide in step (2), the number of carbon atoms in the alkoxy group in the titanium alkoxide is preferably 1 to 8, more preferably 2 to 8, and even more preferably 2 to 6. When the titanium alkoxide contains multiple alkoxy groups, the "number of carbon atoms in the alkoxy group" refers to the "number of carbon atoms in each alkoxy group."

The titanium-containing liquid may contain one or more titanium alkoxides.

From the viewpoint of promoting the growth of the titanium oxide film in step (2), the proportion of titanium alkoxide in the titanium-containing liquid is preferably 1.5% by volume to 5% by volume, and more preferably 1.5% by volume to 3% by volume, on a volume basis. "Proportion of titanium alkoxide in the titanium-containing liquid" means the ratio of the total amount of titanium alkoxide in the titanium-containing liquid to the total amount of the titanium-containing liquid.

From the viewpoint of producing an anatase-type titanium oxide film with a sufficient thickness, the ratio of the titanium alkoxide content to the fluorine-containing titanium compound content in the titanium-containing liquid is preferably 2/3 to 2, and more preferably 1 to 2, on a volume basis.

The fluorine atom-containing titanium compound plays a major role in contributing to the preferential growth of a highly oriented anatase-type titanium oxide film.

The fluorine atom-containing titanium compound may, for example, be a salt containing TiF ₆ ^2- . The counter ion of TiF ₆ ^2- may, for example, be a metal ion or an ammonium ion. The salt containing TiF ₆ ^2- may, for example, be sodium hexafluorotitanate (Na ₂ TiF ₆ ), potassium hexafluorotitanate (K ₂ TiF ₆ ), and ammonium hexafluorotitanate ((NH ₄ ) ₂ TiF ₆ ). From the viewpoint of preferential growth of an anatase type titanium oxide film having high orientation, the fluorine atom-containing titanium compound preferably contains a salt containing TiF ₆ ^2- , and more preferably contains ammonium hexafluorotitanate.

The titanium-containing liquid may contain one or more fluorine atom-containing titanium compounds.

From the viewpoint of preferential growth of a highly oriented anatase-type titanium oxide film, the proportion of the fluorine atom-containing titanium compound in the titanium-containing liquid is preferably 0.8% by volume to 3.5% by volume, and more preferably 1% by volume to 2% by volume. "Proportion of the fluorine atom-containing titanium compound in the titanium-containing liquid" means the ratio of the total amount of the fluorine atom-containing titanium compound in the titanium-containing liquid to the total amount of the titanium-containing liquid.

From the viewpoint of preferential growth of a highly oriented anatase-type titanium oxide film, the ratio of the content of the fluorine-containing titanium compound to the content of the titanium alkoxide in the titanium-containing liquid is preferably 0.5 to 1.5, and more preferably 0.5 to 1, on a volume basis.

The titanium-containing liquid may contain other components without departing from the spirit of this disclosure. Examples of other components include nitrates and other metal alkoxides.

There are no limitations on the method for producing the titanium-containing liquid, so long as a titanium-containing liquid capable of producing the desired titanium oxide film can be obtained. For example, the titanium-containing liquid is produced by mixing water, alcohol, diketone, acid, titanium alkoxide, and a fluorine atom-containing titanium compound. The mixing method may be selected from known mixing methods. Each raw material may be added at once or in several portions.

The step (1), i.e., preparing a titanium-containing liquid, preferably includes the following steps (1) to (2).
(1) Preparing a mixture containing water, an alcohol, a diketone, and an acid (hereinafter, sometimes referred to as “mixture (A)”).
(2) Mixing the mixture (A) with a titanium alkoxide and a fluorine atom-containing titanium compound.

For example, mixture (A) is produced by mixing water, an alcohol, a diketone, and an acid. The mixing method may be selected from known mixing methods. The addition of each raw material may be carried out all at once or in several portions. It is preferable that the diketone and the acid are added to a mixture containing water and an alcohol. Furthermore, it is preferable that the addition of the acid is carried out after the addition of the diketone.

The method for mixing the mixture (A), the titanium alkoxide, and the fluorine atom-containing titanium compound may be selected from known mixing methods. From the viewpoint of uniformity of the liquid, the mixing time is preferably 5 to 10 hours. The addition of each raw material may be carried out all at once or in several portions. The titanium alkoxide and the fluorine atom-containing titanium compound are preferably added to the mixture (A). Furthermore, the addition of the fluorine atom-containing titanium compound is preferably carried out after the addition of the titanium alkoxide.

[Step (2)]
The step (2) is to immerse a substrate in a titanium-containing liquid and form an anatase type titanium oxide film on the substrate by a solvothermal method.

Examples of the substrate include a _LaAlO3 substrate and a _SrTiO3 substrate. From the viewpoint of the orientation of the titanium oxide film, the substrate is preferably a _LaAlO3 substrate or a _SrTiO3 substrate, more preferably a LaAlO3 substrate, and even more preferably a ( ₀₀₁ )-oriented _LaAlO3 substrate. The surface of the substrate (preferably the surface on which the titanium oxide film is formed) may be mirror-finished.

The reaction apparatus for containing the titanium-containing liquid and the substrate may be, for example, a heat-resistant and pressure-resistant container. The heat-resistant and pressure-resistant container may be, for example, an autoclave. The autoclave may be a known autoclave. The heat-resistant and pressure-resistant container may contain a crucible. For example, a crucible placed in the heat-resistant and pressure-resistant container is used as a container for containing the titanium-containing liquid and the substrate. The components of the crucible may be, for example, a fluororesin. For example, a fluororesin may be polytetrafluoroethylene.

As long as the desired titanium oxide film is obtained, the conditions of the solvothermal method (e.g., heating temperature and heating time) are not limited. From the viewpoint of growing the desired titanium oxide film, the heating temperature in the solvothermal method is preferably 110°C to 170°C, and more preferably 120°C to 160°C. From the viewpoint of growing the desired titanium oxide film, the heating time in the solvothermal method is preferably 15 hours or more, and more preferably 20 hours or more. The upper limit of the heating time may be 48 hours or 30 hours. The heating time may be determined taking into account the thickness of the titanium oxide film.

The anatase titanium oxide film formed by the solvothermal method may be washed. Examples of solvents used for washing include alcohol and water.

The anatase titanium oxide film formed by the solvothermal method may be dried. An example of the drying method is heat drying.

The thickness of the anatase type titanium oxide film is not limited. The thickness of the anatase type titanium oxide film may be determined taking into consideration the application. The thickness of the anatase type titanium oxide film may be 0.2 μm to 3 μm.

In the X-ray diffraction pattern of an anatase titanium oxide film, the ratio of the intensity of the peak originating from the (211) plane of rutile titanium oxide to the intensity of the peak originating from the (004) plane of anatase titanium oxide is preferably 5% or less, more preferably 4% or less, and even more preferably 3% or less. Furthermore, the above ratio is preferably 2% or less, more preferably 1% or less, and even more preferably 0.5% or less. The above ratio may be 0%.

Applications of anatase titanium oxide films include, for example, photocatalysis and n-type semiconductors.

The present disclosure will be described in detail below with reference to examples. However, the present disclosure is not limited to the following examples. The matters shown in the following examples may be modified as appropriate without departing from the spirit of the present disclosure.

Example 1
[Production of titanium oxide film]
The titanium oxide film was produced according to the following procedure.
(1) 14.5 mL of purified water and 15 mL of ethanol were mixed and stirred for 5 minutes.
(2) 30 mL of acetylacetone was added to the mixture obtained in (1) above, and the resulting mixture was then stirred for 5 minutes.
(3) 51 μL of 12 mol/L hydrochloric acid was added to the mixture obtained in (2) above, and the resulting mixture was then stirred for 5 minutes.
(4) To the mixture obtained in (3) above, 1 mL of titanium tetraisopropoxide was added in five separate 200 μm portions. The resulting mixture was stirred for 5 minutes to obtain a yellow, transparent liquid.
(5) 1 mL of 1 mol/L ammonium hexafluorotitanate aqueous solution was added to the yellow transparent liquid obtained in (4) above. The resulting mixture was stirred for 10 hours to obtain a titanium-containing liquid. The pH of the titanium-containing liquid at 25°C was in the range of 2 to 2.5.
(6) _Two LaAlO3 (001) substrates with a mirror finish on one side were prepared. Each _LaAlO3 substrate was ultrasonically cleaned using each solvent in the following order: acetone, ethanol, and purified water. The cleaning time for each solvent was 15 minutes. The liquid adhering to the mirror-finished surface of the _LaAlO3 substrate was removed with a blower, and the liquid adhering to the non-mirror-finished surface of the _LaAlO3 substrate was wiped off.
(7) The titanium-containing liquid obtained in (5) above was poured into a Teflon crucible having a capacity of 100 mL, and then the two _LaAlO3 substrates after cleaning were immersed in the titanium-containing liquid. Regarding the arrangement of each _LaAlO3 substrate, each _LaAlO3 substrate was leaned against the inner wall surface of the Teflon crucible, and the mirror-finished surface of each _LaAlO3 substrate was facing the bottom of the Teflon crucible. When the mirror-finished surface of each _LaAlO3 substrate was facing the bottom of the Teflon crucible, it was possible to prevent impurities from adhering to or being mixed into the titanium oxide film.
(8) The Teflon crucible was placed in a stainless steel autoclave and heated at 140 ° C for 24 hours to form a titanium oxide film on the mirror-finished surface of the _LaAlO3 substrate. After heating, the autoclave was naturally cooled to room temperature. The _LaAlO3 substrate was removed from the Teflon crucible, rinsed with ethanol and purified water, and dried on a hot plate. The thickness of the obtained titanium oxide film was about 2.5 μm.

[X-ray diffraction method]
The crystal phase, crystallinity, and orientation of the titanium oxide film were evaluated by X-ray diffraction. The measurement device was an X-ray diffraction device (RINT Ultima III) manufactured by Rigaku Corporation. The measurement conditions are shown in Table 1.

Hereinafter, the measurement results of the "θ/2θ scan" are referred to as "XRD pattern" (i.e., X-ray diffraction pattern). In the X-ray diffraction method, CuK _α radiation and CuK _β radiation were used without any special separation operation, so peaks reflecting two types of X-rays appear in the XRD pattern. Therefore, in the XRD pattern, peaks due to CuK _β radiation are clearly indicated as "CuK _β ". On the other hand, peaks in the XRD pattern that are not indicated with any specific notation other than the plane index are peaks due to CuK _α radiation.

The XRD pattern of the titanium oxide film is shown in Figure 1. The pole figure of the titanium oxide film is shown in Figure 2. Figure 1 shows that anatase-type titanium oxide grows preferentially. The ratio of the intensity of the peak derived from the (211) plane of rutile-type titanium oxide to the intensity of the peak derived from the (004) plane of anatase-type titanium oxide was about 1%. Figures 1 and 2 show that anatase-type titanium oxide has good orientation in all three directions, the perpendicular direction and the in-plane direction. As with known examples produced by the vapor phase method, the c-axis of anatase-type titanium oxide is aligned in the perpendicular direction and the a-axis of anatase-type titanium oxide is aligned in the in-plane direction. In other words, the titanium oxide film grows by inheriting the crystal lattice of the substrate, and grows epitaxially.

Example 2
[Production of titanium oxide film]
A titanium oxide film was produced in the same manner as in Example 1, except that the heating temperature in the stainless steel autoclave was changed from 140° C. to 160° C. The thickness of the titanium oxide film obtained was about 2.5 μm.

[X-ray diffraction method]
The crystal phase, crystallinity and orientation of the titanium oxide film were evaluated by the X-ray diffraction method described above. The XRD pattern of the titanium oxide film is shown in FIG. 3. The pole figures of the titanium oxide film are shown in FIG. 4. As in FIG. 1 and FIG. 2, FIG. 3 and FIG. 4 show that anatase type titanium oxide grows preferentially and that anatase type titanium oxide has good orientation in all three directions of the perpendicular and in-plane axes. The ratio of the intensity of the peak derived from the (211) plane of rutile type titanium oxide to the intensity of the peak derived from the (004) plane of anatase type titanium oxide was about 1%. No significant difference was observed between the titanium oxide film of Example 1 and the titanium oxide film of Example 2 in the XRD pattern and pole figures.

<Comparative Example 1>
An attempt was made to produce a titanium oxide film by the same procedure as in Example 1, except that the aqueous solution of ammonium hexafluorotitanate was not used, and the volume that was insufficient due to the non-use of the aqueous solution of ammonium hexafluorotitanate was supplemented with purified water. However, only the peak derived from the _LaAlO3 of the substrate was observed in the XRD pattern. This result indicates that a titanium oxide film was not formed.

<Comparative Example 2>
An attempt was made to produce a titanium oxide film by the same procedure as in Example 1, except that hydrochloric acid was not used and the volume that was insufficient due to the non-use of hydrochloric acid was supplemented with purified water. However, only the peak derived from the _LaAlO3 substrate was observed in the XRD pattern. This result indicates that a titanium oxide film was not formed.

Claims (13)

Preparing a titanium-containing liquid containing water, an alcohol, a diketone, an acid, a titanium alkoxide, and a fluorine atom-containing titanium compound;
and forming an anatase type titanium oxide film on the substrate by a solvothermal method by immersing the substrate in the titanium-containing liquid.
A method for producing an anatase type titanium oxide film. The method for producing an anatase type titanium oxide film according to claim 1, wherein preparing the titanium-containing liquid includes preparing a mixture containing the water, the alcohol, the diketone, and the acid, and mixing the mixture with the titanium alkoxide and the fluorine atom-containing titanium compound. The method for producing an anatase-type titanium oxide film according to claim 1 or 2, wherein the alcohol includes ethanol. The method for producing an anatase type titanium oxide film according to any one of claims 1 to 3, wherein the diketone includes a β-diketone. The method for producing an anatase titanium oxide film according to any one of claims 1 to 3, wherein the diketone contains acetylacetone. The method for producing an anatase titanium oxide film according to any one of claims 1 to 5, wherein the acid contains hydrogen halide. The method for producing an anatase titanium oxide film according to any one of claims 1 to 5, wherein the acid contains hydrogen chloride. The method for producing an anatase titanium oxide film according to any one of claims 1 to 7, wherein the number of carbon atoms in the alkoxy group in the titanium alkoxide is 1 to 8. The method for producing an anatase type titanium oxide film according to any one of claims 1 to 8, wherein the titanium alkoxide includes titanium tetraisopropoxide. 10. The method for producing an anatase type titanium oxide film according to claim 1, wherein the fluorine atom-containing titanium compound includes a salt containing TiF ₆ ^2-. The method for producing an anatase type titanium oxide film according to any one of claims 1 to 9, wherein the fluorine atom-containing titanium compound includes ammonium hexafluorotitanate. The method for producing an anatase type titanium oxide film according to any one of claims 1 to 11, wherein the substrate is a _LaAlO3 substrate or a _SrTiO3 substrate. The method for producing an anatase titanium oxide film according to any one of claims 1 to 12, wherein the heating temperature in the solvothermal method is 110°C to 170°C.