Method of forming titanium oxide film and titanium oxide film

The present invention relates to a method for forming a titanium oxide film having visible light responsiveness and high photocatalytic activity, and a titanium oxide film.

Titanium oxide (TiO₂) Is attracting attention as a so-called photocatalyst as an environment-friendly environmental purification material that has strong oxidative decomposition power and can safely and easily detoxify various harmful substances just by using light.

The crystal structure of titanium oxide is classified into rutile type, anatase type, and brookite type. Of these, anatase type titanium oxide is mostly used as a photocatalyst.

Generally, light having a wavelength of about 390 to 780 nm is called visible light, and light having a wavelength of less than 390 nm is called ultraviolet light. The bandgap energy between the valence band and the conduction band of anatase-type titanium oxide in bulk crystals is 3.20 eV, which corresponds to light having a wavelength of about 387 nm. That is, anatase-type titanium oxide absorbs only ultraviolet rays having a wavelength shorter than about 387 nm and hardly absorbs visible light, so that it has almost photocatalytic activity against artificial light such as fluorescent lamps and visible light that occupies most of sunlight. Not shown.

Therefore, in recent years, titanium oxide having photocatalytic activity against visible light has been actively developed. In order for titanium oxide to have photocatalytic activity with respect to visible light, it is necessary to "absorb visible light and generate electrons and holes" and "the generated electrons and holes do not recombine due to defects in the crystal". is necessary. For example, Patent Document 1 discloses anatase-type titanium oxide having improved visible light absorption by chemically doping metal ions such as vanadium, chromium, and manganese. It is disclosed that a photocatalyst that operates efficiently with visible light can be formed by substituting a part of oxygen sites in titanium oxide crystals by nitrogen plasma treatment or heat treatment in an ammonia atmosphere with nitrogen.

On the other hand, the bandgap energy between the valence band and the conduction band of rutile-type titanium oxide in bulk crystals is 3.03 eV, which corresponds to light having a wavelength of about 409 nm. Therefore, rutile-type titanium oxide can absorb not only ultraviolet rays but also a part of visible light. Therefore, in principle, rutile-type titanium oxide has visible light responsiveness. However, rutile-type titanium oxide is more likely to have crystal defects than anatase-type titanium oxide, and electrons and holes generated by photoexcitation are recombined due to defects in the crystal, so the photocatalytic activity is compared with that of anatase-type titanium oxide. It is often much lower and has rarely been used as a photocatalyst.

As a method for forming rutile-type titanium oxide with few crystal defects, Patent Document 3 states that titanium oxide is epitaxially deposited on a sapphire single crystal substrate by a laser ablation film forming method, and the titanium oxide film further heat-treated has defects in the crystal. It is disclosed that the amount of sapphire is small and the responsiveness to visible light is exhibited.

By the way, when a titanium oxide film is formed by a sputtering method, a titanium oxide film having a uniform film thickness can be formed over a large area. When the sputtering method is used, various conditions such as the type of gas used for sputtering, the total pressure of the sputtering atmosphere (hereinafter referred to as "sputtering pressure"), and the substrate temperature can be changed. It has the advantage of being able to form a morphological film.

For example, Non-Patent Document 1 discloses a titanium oxide film formed by a reactive RF magnetron sputtering method of oxygen and argon, targeting metallic titanium, and a method for forming the same. In Non-Patent Document 1, rutile-type titanium oxide is likely to be formed when the sputtering pressure is low (0.5 Pa or less) (the proportion of anatase-type titanium oxide is small), and the proportion of anatase-type titanium oxide increases as the sputtering pressure increases. It has become. In sputtering, the lower the sputtering pressure, the higher the energy of the flying particles, and the higher the probability that impurities will be mixed into the titanium oxide film. Therefore, it is considered that rutile-type titanium oxide formed at a sputtering pressure of 0.5 Pa or less has many defects in the crystal.

<patcit num="1"><text>Japanese Unexamined Patent Publication No. 9-262482</text></patcit><patcit num="2"><text>JP-A-2002-355562</text></patcit><patcit num="3"><text>Japanese Unexamined Patent Publication No. 2003-126700</text></patcit><nplcit num="1"><text>Makiko Yamagishi and 3 others, "Thin film TiO2 photocatalyst captured by reactive magnetron sputtering", "Thin Solid Films", 2003 Year, Vol442, P227-231</text></nplcit>

<p> However, in the anatase-type titanium oxide chemically doped with a metal cation such as vanadium of Patent Document 1, although the photocatalytic activity in visible light is recognized, the photocatalytic activity in ultraviolet rays originally possessed by the photocatalyst before doping is lowered. Is often seen.</p><p> Further, it has been pointed out that the nitrogen-doped rutile-type titanium oxide of Patent Document 2 has low chemical stability, although photocatalytic activity in visible light is observed. Further, Patent Document 3 is academically important but poorly practical because a titanium oxide film having few crystal defects cannot be formed unless it is an expensive single crystal sapphire substrate. Further, although the rutile-type and anatase-type titanium oxide can be produced separately by the method for forming the titanium oxide film of Non-Patent Document 1, it is difficult to form the rutile-type titanium oxide having few crystal defects.</p><p> Therefore, a method of forming rutile-type titanium oxide with few crystal defects on the surface of not only a single crystal substrate but also an inexpensive general-purpose substrate is desired.</p><p> In order to solve the above-mentioned problems, the present invention provides a method for forming a titanium oxide film containing rutile-type titanium oxide having few crystal defects and having visible light responsiveness and high photocatalytic activity on the surface of the substrate. The purpose is to do.</p>

<p> The method for forming a titanium oxide film of the present invention is a method for forming a titanium oxide film on the surface of a substrate, and the substrate temperature and the substrate temperature and By sputtering a target containing titanium in a plasma containing oxygen while controlling the sputtering pressure, an upper layer film containing rutile-type titanium oxide is formed on the surface of the lower layer film.</p><p> According to such a method for forming a titanium oxide film, titanium oxide grows semi-matchedly on a lower layer film made of crystal-oriented rutile-type titanium oxide by a sputtering method. Therefore, the upper layer film formed is the lower layer film. Contains rutile-type titanium oxide crystal-oriented on the same crystal plane as. The crystal orientation of the titanium oxide film can be evaluated by the X-ray diffraction method (XRD). In the present specification, "crystal orientation" means that a specific crystal plane is oriented in a certain direction, and in the case of titanium oxide, specifically, the diffraction angle by Cu-Kα rays corresponding to titanium oxide. In the X-ray diffraction pattern in which the diffraction line intensity with respect to 2θ is plotted, it means that the diffraction line intensity at the main peak is 5 times or more the diffraction line intensity of the diffraction peaks other than the main peak. Further, "quasi-matched growth" means that the crystal grows while having the same crystal orientation relationship as the base while being strongly influenced by the atomic arrangement of the underlying polycrystalline layer.</p><p> Examples of the sputtering include ion beam sputtering, RF magnetron sputtering, DC magnetron sputtering, and the like. In particular, RF magnetron sputtering has many advantages such as high sputtering efficiency and the ability to use an insulator target.</p><p> The sputtering target may be a target containing titanium. Other than metallic titanium, titanium oxide, titanium nitride and the like can be mentioned. Titanium metal is particularly preferable because it has high sputtering efficiency. The sputtering efficiency is the efficiency at which the atoms constituting the target are emitted from the target by the sputtering phenomenon.</p><p> Here, the substrate temperature at the time of sputtering is preferably 350 to 600 ° C, more preferably 400 to 500 ° C. If the substrate temperature is lower than 350 ° C, the crystallinity of the upper layer film deteriorates. Further, if the substrate temperature is higher than 600 ° C, the substrate and the titanium oxide film may react, or the titanium oxide film may peel off due to the difference in the coefficient of thermal expansion. When the substrate temperature is 400 to 500 ° C., an upper layer film containing highly crystalline rutile-type titanium oxide can be formed with good reproducibility.</p><p> The sputtering pressure at the time of sputtering is preferably 2.5 to 10.0 Pa, more preferably 3.5 to 5.0 Pa. If the sputtering pressure is less than 2.5 Pa, the film formation rate of the formed upper layer film is too fast, and the proportion of rutile-type titanium oxide having few crystal defects is low, which is not preferable. When the sputtering pressure is 2.5 Pa or more, the proportion of rutile-type titanium oxide with few crystal defects contained in the upper layer film increases, and when it is 3.5 Pa or more, most of the upper layer film becomes rutile-type titanium oxide with few crystal defects, which is high. It is preferable because it exhibits photocatalytic activity. On the other hand, when the sputtering pressure exceeds 5.0 Pa, the film forming speed drops remarkably, and when it exceeds 10.0 Pa, the film forming speed is too slow and the productivity is poor, which is not practical. Therefore, the suitable sputtering pressure is 2.5 to 10.0 Pa, and the optimum sputtering pressure is in the range of 3.5 to 5.0 Pa.</p><p> Further, the film thickness of the upper layer film is preferably formed to be 50 nm or more, and more preferably 100 nm or more. If the film thickness of the upper layer film is smaller than 50 nm, the light absorption rate is low and light cannot be sufficiently absorbed, which is not preferable. When the film thickness of the upper layer film is 50 nm or more, the light absorption rate increases and the photocatalytic activity is sufficient for practical use. Especially when the film thickness is 100 nm or more, it is equal to or higher than that of the anatase-type titanium oxide film. Has photocatalytic activity.</p><p> The underlayer film is preferably formed by sputtering a target containing titanium in a plasma containing oxygen. With such a method, a lower layer film made of rutile-type titanium oxide crystal-oriented can be formed with good reproducibility even on a substrate having low crystallinity, and sputtering can be performed in the same apparatus as the upper layer film. Therefore, it is efficient.</p><p> The sputtering pressure when the underlayer film is formed by sputtering is preferably 0.1 to 1.0 Pa. If the sputtering pressure is less than 0.1 Pa, the film formation rate is too fast and the crystallinity of the formed rutile-type titanium oxide is significantly lowered, which is not preferable. If it is larger than 1.0 Pa, the formed titanium oxide film is of the rutile-type. It is not preferable because it is a mixed phase of anatase-type titanium oxide.</p><p> Further, when the lower layer film is formed by the above-mentioned sputtering method, it is desirable that the film thickness of the lower layer film is 25 nm or more. If the film thickness of the underlayer film formed by this method is less than 25 nm, the crystallinity of titanium oxide formed as the underlayer film is low, which is not preferable. The crystallinity of titanium oxide can be evaluated by the X-ray diffraction method. Specifically, in the X-ray diffraction pattern in which the diffraction line intensity with respect to the diffraction angle 2θ by Cu-Kα rays is plotted, the diffraction showing the strongest diffraction line intensity. It can be expressed by the half price range at the peak (hereinafter referred to as "main peak"). The smaller the full width at half maximum at the main peak in the X-ray diffraction pattern, the higher the crystallinity. As used herein, "low crystallinity" means that the full width at half maximum at the main peak is greater than 1.0 degree. When the film thickness of the lower layer film is 25 nm or more, an upper layer film containing anatase-type titanium oxide and highly crystalline rutile-type titanium oxide can be formed. In particular, when the thickness of the lower layer film is 150 nm or more, the crystal orientation is improved, so that the ratio of anatase-type titanium oxide is small and the upper layer film in which highly crystalline rutile-type titanium occupies most of the film can be formed.</p><p> The titanium oxide film of the present invention has a lower layer film made of crystal-oriented rutyl-type titanium oxide formed on the surface of a substrate and a crystal plane formed on the lower layer film and having the same crystal surface as rutile-type titanium oxide forming the lower layer film. It is composed of an upper layer film containing crystal-oriented rutyl-type titanium oxide, and is characterized in that the band gap energy between the valence band and the conduction band is 3.15 eV or less. When this bandgap energy is 3.15 eV or less, light having a wavelength of 394 nm or less can be absorbed.</p><p> Furthermore, this titanium oxide film has a half-value width of 0.6 degrees or less at the main peak corresponding to rutile-type titanium oxide in the X-ray diffraction pattern in which the diffraction line intensity with respect to the diffraction angle 2θ by Cu-Kα rays is plotted. desirable. When the full width at half maximum is 0.6 degrees or less, the crystallinity is high and crystal defects are reduced, so that the titanium oxide film exhibits high photocatalytic activity.</p>

<p> According to the method for forming a titanium oxide film of the present invention, a titanium oxide multi-layer film containing rutyl-type titanium oxide having few crystal defects as a recombination center can be obtained from not only a single crystal substrate but also an amorphous body or a polycrystalline body. It can be easily formed on the substrate with good reproducibility. Since the formed titanium oxide film contains rutile-type titanium oxide having few crystal defects, it has visible light responsiveness and functions as a highly active photocatalyst.</p>

As an embodiment of the present invention, crystal orientation is formed on the (110) plane formed on a soda glass substrate by a reactive RF magnetron sputtering method using metallic titanium as a target and a mixed gas of oxygen and argon as an introduction gas. A titanium oxide film formed by forming a lower layer film composed of a rutile-type titanium oxide film and forming an upper layer film containing a rutile-type titanium oxide film on the surface of the lower layer film by a reactive RF magnetron sputtering method. The method will be described.

FIG. 1 shows a schematic cross-sectional view of the titanium oxide film formed by the method for forming the titanium oxide film according to the embodiment of the present invention. In the method for forming a titanium oxide film according to the embodiment of the present invention, oxygen is applied to the lower layer film 3 made of crystal-oriented rutile-type titanium oxide formed on the surface of the substrate 2 in a state where the substrate temperature and the sputtering pressure are controlled. By sputtering metallic titanium as a target containing titanium in the containing plasma, a titanium oxide film 1 in which an upper layer film 4 containing rutile-type titanium oxide having few crystal defects is formed on the surface of the lower layer film 3 is obtained.

The lower film 3 is formed by sputtering a target containing titanium in a plasma containing oxygen in the same manner as the upper film 4. With this method, a lower layer film made of crystal-oriented rutile-type titanium oxide can be formed with good reproducibility even on a substrate having low crystallinity, and sputtering can be performed in the same device as the upper layer film 4. There are advantages. The underlayer film 3 can also be formed by any known method such as various vapor phase synthesis methods or sol-gel methods.

In the embodiment, the lower layer film means a titanium oxide film formed directly on the substrate, and the upper layer film means a titanium oxide film formed on the lower layer film. The upper layer film does not have to be one layer, and may be two or more layers. The titanium oxide film of the embodiment of the present invention composed of the lower layer film and the upper layer film is hereinafter referred to as "titanium oxide multi-layer film" or simply "multi-layer film" to distinguish between the lower layer film and the upper layer film. .. In the following description, the notation "titanium oxide film" is used as a concept including all of the titanium oxide multi-layer film, the lower layer film and the upper layer film constituting the titanium oxide multi-layer film, and the titanium oxide single layer film. To do.

FIG. 2 is a schematic configuration diagram of a main part showing one embodiment of a reactive RF magnetron sputtering apparatus suitable for forming a titanium oxide film in the embodiment of the present invention. The basic configuration is the same as that of a known RF magnetron sputtering apparatus.

First, the substrate 6 is placed at a predetermined position in the chamber 51, and the inside of the chamber 51 is evacuated by a vacuum exhaust system (not shown). Next, the mixed gas of argon and oxygen is introduced into the chamber 51 via the flow rate controllers 7 and 8. Here, the mixing ratio of oxygen and argon can be accurately controlled by the flow meters 7 and 8. The sputtering pressure (in this case, the sum of the oxygen partial pressure and the argon partial pressure in the chamber) is controlled by the flow rate controllers 7 and 8 to control the flow rates of oxygen and argon, and the vacuum pump is connected to the exhaust port 53 (Fig. The exhaust speed of (not shown) can be adjusted accurately by controlling it with a flow rate adjusting valve (not shown).

The arranged substrate 6 is heated to a predetermined temperature by the halogen lamp 9. The substrate temperature is measured by a thermocouple 10 arranged near the back surface of the substrate 6. Next, RF power is supplied to the cathode 11 to generate plasma 12. Here, the supply of RF power to the cathode 11 is controlled by the RF power supply 19.

By controlling the mixing ratio of oxygen and argon, the sputtering pressure, the RF power, and the substrate temperature (hereinafter referred to as "deposition conditions") to predetermined values, the argon and oxygen accelerated and ionized by the plasma 12 are released. By colliding with the target 13 made of metallic titanium, the repelled target substance (titanium atom) reacts with oxygen to form titanium oxide on the surface of the substrate 6. A shutter 14 that can be opened and closed is installed near the surface of the substrate 6, and when the plasma is unstable such as at the initial stage of plasma generation or when the film formation conditions are changed, the shutter 14 is closed and the plasma is stable under the predetermined film formation conditions. Then, by opening the shutter 14, a titanium oxide film is formed on the surface of the substrate 6.

As the substrate 6, not only a single crystal of titanium oxide but also a polycrystalline and amorphous substrate of titanium oxide and other substances can be used. For example, various metal plates, ceramics, glass and the like can be used. In particular, transparent soda glass is particularly preferably used because it is inexpensive and can be irradiated with light from the substrate side when the titanium oxide film is formed. In short, it suffices as long as it does not deform or chemically change under the conditions of sputtering.

The method of forming the underlayer film will be described. When the film is formed for a predetermined time under the following film forming conditions, a rutile-type titanium oxide film crystallized on the (110) plane to be the lower layer film is formed on the surface of the soda glass substrate.

When the mixing ratio of oxygen and argon is 80 to 95% by volume of oxygen with respect to 5 to 20% by volume of argon, a titanium oxide film is formed from a target containing titanium as reactive sputtering. By controlling the oxygen flow rate and the argon flow rate with the flow rate controllers 7 and 8, the mixing ratio of oxygen and argon can be accurately controlled.

The sputtering pressure (sum of oxygen partial pressure and argon partial pressure) is preferably 0.1 to 1.0 Pa, and particularly preferably 0.1 to 0.4 Pa. When the sputtering pressure is larger than 1.0 Pa, the formed titanium oxide film becomes a mixed phase of rutile type and anatase type titanium oxide. In particular, when it is 0.4 Pa or less, anatase-type titanium oxide is not formed, which is particularly preferable. On the other hand, if the sputtering pressure is less than 0.1 Pa, the crystallinity is remarkably lowered, which is not preferable. The sputtering pressure can be accurately measured by using an ionization vacuum gauge.

RF power is defined by the unit area of the target and is 2.0 to 3.0 W / cm.²If so, it is possible to form a plasma that is stable and has sufficient energy.

The substrate temperature is preferably 600 ° C or lower, and in the embodiment, it is 500 ° C. This is because if the temperature is higher than 600 ° C, the substrate and the titanium oxide film may react, or the titanium oxide film may peel off due to the difference in the coefficient of thermal expansion.

The film thickness of the underlayer film formed under the above film forming conditions is preferably 25 nm or more, and particularly preferably 150 nm or more. If the film thickness is smaller than 25 nm, the crystallinity and crystal orientation of the lower layer film become extremely poor, and it becomes difficult to form an upper layer film containing rutile-type titanium oxide by quasi-matched growth on the lower layer film. When the thickness of the lower layer film is 25 nm or more, the crystal orientation of rutile-type titanium oxide is observed in the lower layer film, and the rutile-type titanium oxide oriented in the same crystal plane as the lower layer film is obtained by the method for forming the upper layer film described later. An upper layer film containing can be formed. In particular, when the thickness of the lower layer film is 150 nm or more, most of the lower layer film becomes rutile-type titanium oxide crystal-oriented on the same crystal plane as the lower layer film, and an upper layer film having particularly few crystal defects is formed on the lower layer film. Can be formed.

The film thickness of the sample can be measured by, for example, a stylus type surface shape measuring device. This device can detect the vertical movement of the probe while scanning the surface by bringing the probe into vertical contact with the part of the sample surface where the film is formed, and can detect the unevenness of the surface. By scanning the probe so as to vertically cross the boundary of the substrate, the film thickness can be measured as a step.

Next, a method for forming the upper layer film of the titanium oxide multilayer film in the embodiment of the present invention will be described.

The upper film to be formed depends on the film formation conditions such as RF power at the time of sputtering, substrate temperature, mixing ratio of oxygen and argon, and sputtering pressure. Hereinafter, the film forming conditions for easily forming the upper layer film containing rutile-type titanium oxide with few crystal defects with good reproducibility will be shown. The mixing ratio of oxygen and argon and the RF power are the same as in the case of forming the underlayer film described above, and thus are omitted.

The substrate temperature at the time of sputtering is preferably 350 to 600 ° C, and most preferably 500 ° C. If the temperature is lower than 350 ° C, the crystallinity of the upper layer film is poor and the photocatalytic activity is reduced. Further, if the temperature is higher than 600 ° C, the substrate and the titanium oxide film may react, or the titanium oxide film may peel off due to the difference in the coefficient of thermal expansion.

The sputtering pressure is preferably 2.5 to 10.0 Pa, particularly preferably 3.0 to 5.0 Pa, and most preferably 3.5 to 4.5 Pa. With such a sputtering pressure, an upper layer film containing rutile-type titanium oxide can be formed with good reproducibility. When sputtering is performed on a substrate on which no underlayer film is formed under the same film forming conditions, the crystal form of the titanium oxide film formed is not a rutile type but an anatase type. In addition, the sputtering pressure can be accurately measured by using an ionization vacuum gauge.

The film thickness of the upper layer film is preferably 50 nm or more, more preferably 100 nm or more. When the film thickness of the upper film is smaller than 50 nm, the light absorption rate is low and the photocatalytic activity is not sufficient. When the film thickness of the upper layer film is 50 nm or more, it has sufficient photocatalytic activity, and when the film thickness is 100 nm or more, it has photocatalytic activity equal to or higher than that of the anatase-type titanium oxide film.

Next, a method for evaluating the titanium oxide multilayer film formed by the forming method according to the embodiment of the present invention will be described.

The crystallinity of the titanium oxide multi-layer film can be evaluated by X-ray diffraction (XRD). When the titanium oxide multi-layer film formed by the forming method of the present invention is evaluated by XRD, the upper layer film has the same crystal orientation as the lower layer film. Therefore, when a rutile-type titanium oxide (110) plane is formed on the lower layer film, When the multi-layer film on which the upper layer film is formed is evaluated by XRD, a peak derived from the rutile (110) plane is detected in the X-ray diffraction pattern. The crystallinity of the multilayer film can be defined by the full width at half maximum of the crystal plane (in this case, the rutile (110) plane) in which the crystals are oriented. The crystallinity of the titanium oxide multilayer film in the embodiment of the present invention is preferably 0.6 degrees or less in half width. When the half width is larger than 0.6 degrees, the crystallinity is low, so that the number of crystal defects increases and sufficient photocatalytic activity is not exhibited. When the half width is 0.6 degrees or less, the titanium oxide multi-layer film exhibits high photocatalytic activity and visible light responsiveness.

The method for evaluating the photocatalytic activity and visible light responsiveness of the titanium oxide film will be described below. Examples of the light source include sunlight for natural light, black light for ultraviolet light, and fluorescent lamp for visible light. Irradiation light from a xenon lamp capable of irradiating light in a wide wavelength range including ultraviolet light and visible light can also be used. The intensity of the irradiated light can be measured using an optical power meter.

The photocatalytic activity of each sample can be evaluated by a conventionally known method. Among them The wet methylene blue decomposition test is preferable because it can be performed with a simple apparatus configuration. The wet methylene blue decomposition test is a method of immersing a sample in a methylene blue aqueous solution and measuring a decrease in the concentration of the methylene blue aqueous solution due to the oxidative decomposition action of titanium oxide caused by light irradiation. Here, the decomposition activity index is the molar concentration of methylene blue that the titanium oxide film decomposes per unit time, and the larger this value is, the higher the photocatalytic activity is. The concentration of the aqueous methylene blue solution can be evaluated using a spectrophotometer.

For the band gap energy (Eg) of each titanium oxide film, the absorption coefficient is calculated from the transmittance / reflectance and film thickness of the titanium oxide film, and the vertical axis plots the absorption coefficient to the 1/2 power and the horizontal axis plots the photon energy. The horizontal axis section obtained by extrapolating the linear region is the Eg value. The bandgap energy is preferably 3.15 eV or less, especially 3.10 eV or less. When the bandgap energy is larger than 3.15 eV, it can hardly absorb visible light and hardly develops photocatalytic activity by visible light. When the band gap energy is 3.15 eV or less, light having a wavelength of 394 nm or less, including visible light, can be absorbed, and when the band gap energy is 3.10 eV or less, light having a wavelength of 400 nm or less can be absorbed.

Examples of the present invention are shown below. An RF magnetron sputtering device (ULVAC, Inc., model number: MUE-ECO-C) was used to form the titanium oxide film. Further, Examples 1 and 2 are titanium oxide multilayer films, Comparative Example 1 is a rutile-type titanium oxide monolayer film (hereinafter, rutile monolayer film), and Comparative Example 2 is an anatase-type titanium oxide monolayer film (hereinafter, anatase monolayer film). Layered membrane) was formed. The rutile-type titanium oxide monolayer film of Comparative Example 1 was formed under the same conditions as the lower layer film of the titanium oxide multi-layer film of Example, and the anatase-type titanium oxide monolayer film of Comparative Example 2 was formed. Is formed under the same conditions as the upper layer film in the titanium oxide multi-layer film. The sputtering pressure was measured using an ionization vacuum gauge (ULVAC, Inc., model number: GI-M2).

The film forming conditions (hereinafter, "film forming condition 1") of the lower layer film and the rutile single layer film in the multi-layer film are shown below. Substrate: Soda glass plate Target: Metallic titanium (purity 99.5% or more, 100 mmφ) Substrate heating temperature: 500 ° C RF power: 2.55 W / cm² Introduced gas: Oxygen (O₂), Flow rate; 9ml / min Argon (Ar), flow rate; 1 ml / min Sputtering pressure (total pressure): 0.3Pa

The film formation conditions (hereinafter, "deposition condition 2") of the upper layer film and the anatase single layer film in the multi-layer film are shown below. Substrate: Soda glass plate (upper layer film) and soda glass (anatase single layer film) on which the lower layer film was formed under film formation condition 1. Target: Metallic titanium (purity 99.5% or more, 100 mmφ) Substrate heating temperature: Room temperature ~ 500 ° C RF power: 2.55 W / cm² Introduced gas: Oxygen (O₂), Flow rate; 9ml / min Argon (Ar), flow rate; 1 ml / min Sputtering pressure (total pressure): 0.3 ~ 4.0 Pa

That is, the following titanium oxide film was formed as an example and a comparative example of the present invention. Example 1: A multi-layer film in which a lower layer film (thickness 300 nm) is formed on a substrate under film formation condition 1 and an upper layer film is formed on a lower layer film under film formation condition 2. Example 2: A multi-layer film in which a lower layer film (thickness 50 nm) is formed on a substrate under film formation condition 1 and an upper layer film is formed on the lower layer film under film formation condition 2. Comparative Example 1: Rutile-type titanium oxide monolayer film formed under film formation condition 1 Comparative Example 2: Anatase-type titanium oxide monolayer film formed under film formation condition 2.

First, a substrate washed with acetone was placed in the apparatus, and after heating the substrate to a predetermined temperature, oxygen and argon flowed into the chamber to generate plasma. Next, the gas flow rate, RF power, and sputtering pressure (total pressure) were set, pre-sputtering was performed for about 20 minutes, and then film formation of the above-mentioned Examples and Comparative Examples was performed. The film thickness of the formed titanium oxide film was measured with a stylus type surface shape measuring instrument (Tokyo Seimitsu Co., Ltd.).

The crystal phase and crystallinity of the titanium oxide films of Examples 1 and 2 and Comparative Examples 1 and 2 were evaluated by an X-ray diffractometer (Shimadzu Corporation, XD-D1; CuKα1.542Å, tube voltage 40 kV, tube current 40 mA). did.

FIG. 3 shows the XRD measurement results of the titanium oxide multi-layer film which is an example of the present invention and the titanium oxide single-layer film which is a comparative example. In Comparative Example 1 (thickness 300 nm) formed under the film forming condition 1, a peak on the rutile (110) plane was detected. That is, it was found that when a titanium oxide film was formed on the substrate under the above film formation condition 1, only rutile-type titanium oxide crystal-oriented on the (110) plane was formed. As a result of forming a rutile single layer film (not shown) having a film thickness of 50 nm under the film forming condition 1 and evaluating the crystallinity, the peak of the rutile (110) plane was not clearly detected. Since the substrate (soda glass) used is amorphous and does not have a specific crystal orientation, it is considered that the crystal structure is disturbed at the initial stage of film formation (thickness of about 50 nm).

Further, in Comparative Example 2 (film thickness 200 nm) formed under the film forming condition 2, only a sharp peak on the anatase (101) surface was detected. That is, it was found that when a titanium oxide film was formed on the substrate under the above film formation condition 2, only anatase-type titanium oxide crystal-oriented on the (101) plane was formed.

On the other hand, in Example 1 in which the film thickness of the lower layer film was 300 nm, almost no peak derived from anatase was detected, and the main peak was the rutile (110) plane. As described above, the film forming condition 2 is a condition in which anatase-type titanium oxide is formed when it is directly formed on a substrate without an underlayer film. Nevertheless, it was found that rutile-type titanium oxide was formed when the film was formed under the film forming condition 2 on the underlayer film formed under the film forming condition 1. That is, from this measurement result, in the method for forming the titanium oxide multi-layer film of the example, the rutile-type structure of the lower layer film crystal-oriented on the rutile (110) plane is inherited, and the upper layer film crystal-oriented on the rutile (110) plane. Was found to have a grown structure.

Further, in Example 2 in which the film thickness of the underlayer film was 50 nm, not only the peak on the rutile (110) plane but also the peak on the anatase (101) plane was clearly detected. From this measurement result, it was found that the upper layer film is titanium oxide in which anatase type and rutile type are mixed. That is, rutile-type titanium oxide is formed as in the case where the lower layer film is 300 nm, but since the crystallinity of the 50 nm rutile film layer, which is the lower layer film, is low, only the rutile-type structure having high crystallinity in the upper layer film is available. It is not formed, and it is considered that the anatase type is also formed. However, considering that only anatase type is formed in the absence of the underlayer film, even when the underlayer film is 50 nm, it has grown by inheriting the structure of rutile type titanium oxide having crystal orientation existing in the underlayer film. It is clear that rutile-type titanium oxide is contained in the upper film.

The relationship between the substrate temperature under the film forming condition 2 which is the forming condition of the upper layer film of Example 1 and the crystallinity of the formed titanium oxide film was evaluated. FIG. 4 shows the relationship between the half width of the rutile (110) plane and the substrate temperature in the titanium oxide multilayer film of Example 1. Table 1 shows the relationship between the half width of the rutile (110) plane and the substrate temperature in FIG. From FIG. 4, it was found that the diffraction peak on the rutile (110) plane became sharper and stronger as the substrate temperature was increased. It was also found that the half width of the rutile (110) plane became smaller as the substrate temperature became higher. From this measurement result, it can be seen that the upper layer film inherits the crystal structure of the lower layer film and is crystal-oriented even when the substrate temperature is low, and the crystallinity and crystal orientation of the upper layer film are improved as the substrate temperature is higher. It is clear to do. In particular, it was found that a rutile-type titanium oxide film having high crystallinity was formed when the substrate temperature was 400 ° C. or higher.

<tables num="1"><img file="JP2009067648A_D0001.tif" /></tables>

Next, the relationship between the sputtering pressure under the film forming condition 2 which is the forming condition of the upper layer film of Example 1 and the crystallinity of the formed titanium oxide film was evaluated. FIG. 5 shows an X-ray diffraction pattern showing the sputtering pressure dependence of the titanium oxide multilayer film according to the embodiment of the present invention. Table 2 shows the relationship between the half width of the rutile (110) plane and the sputtering pressure in FIG. In the corresponding X-ray diffraction pattern with a sputtering pressure of 3.0 Pa or higher, the diffraction peak on the rutile (110) plane was sharper and stronger. That is, it was found that rutile-type titanium oxide having high crystallinity and high crystal orientation was formed when the sputtering pressure was 3.0 Pa or more. Further, as shown in Table 2, when the sputtering pressure is 3.0 Pa or more, the half width is constant at about 0.4 degrees. That is, it was found that a rutile-type titanium oxide film having high crystallinity was formed when the sputtering pressure was 3.0 Pa or more.

<tables num="2"><img file="JP2009067648A_D0002.tif" /></tables>

Next, as an evaluation of the photocatalytic activity of each sample, a wet methylene blue decomposition test was performed. The wet methylene blue decomposition test is a method of immersing a titanium oxide film in a methylene blue aqueous solution and measuring a decrease in the concentration of the methylene blue aqueous solution due to the oxidative decomposition action of titanium oxide caused by light irradiation. Here, the decomposition activity index is the molar concentration of methylene blue that the titanium oxide film of each of Examples and Comparative Examples decomposes per unit time, and the larger this value is, the higher the photocatalytic activity is. The concentration of the methylene blue aqueous solution was measured by measuring the absorption peak wavelength (about 664 nm) of methylene blue using a spectrophotometer (Shimadzu ultraviolet-visible spectrophotometer, UV160-A).

A titanium oxide film saturated with methylene blue is immersed in a methylene blue aqueous solution (10 μmol / L, 10 mL), and ultraviolet rays (500 μW / cm) with a center wavelength of about 350 nm are used using a black light.²) Was irradiated, and the decomposition activity index was obtained from the time change of the absorbance. Table 3 shows the decomposition activity index of each sample. In addition, when comparing the decomposition activity index, the film thickness of each film thickness was unified at 300 nm on the surface layer. That is, the titanium oxide multi-layer film in Examples 1 and 2 has the film thickness of the upper layer film, and the titanium oxide single-layer film in Comparative Examples 1 and 2 has the total film thickness.

<tables num="3"><img file="JP2009067648A_D0003.tif" /></tables>

The decomposition activity index of methylene blue in the titanium oxide multi-layer film of Example 1 in which the film thickness of the lower layer film is 300 nm is larger than that of the anatase-type titanium oxide monolayer film (Comparative Example 2), which is conventionally considered to have high photocatalytic activity. showed that. That is, it was found that the titanium oxide multilayer film of Example 1 had extremely high photocatalytic activity against ultraviolet rays. The titanium oxide multi-layer film of Example 2 having a film thickness of 50 nm also showed a larger decomposition activity index than that of Comparative Example 2. Example 2 in which the upper layer film is a mixed phase of rutile type and anatase type has a larger decomposition activity index than Comparative Example 2 which is an anatase type titanium oxide monolayer film, and therefore is included in the upper layer film of Example 2. The rutile-type titanium oxide is considered to be more active against ultraviolet rays than the anatase-type titanium oxide. Further, compared with these, the value of the decomposition activity index is clearly smaller in the rutile-type titanium oxide monolayer film of Comparative Example 1. As is clear from the above-mentioned evaluation by XRD, in Comparative Example 1, although it is a rutile single phase, it has low crystallinity and there are many crystal defects which are recombination centers in the crystal, so that sufficient photocatalytic activity is not exhibited. Conceivable.

FIG. 6 shows the relationship between the film thickness and the decomposition activity index of methylene blue by ultraviolet irradiation for the double glazing films of Examples 1 and 2 and the single glazing films of Comparative Examples 1 and 2. The film thickness of the multilayer film of Examples 1 and 2 is the total of the lower layer film and the upper layer film. It was found that the decomposition activity index of the multi-layer film (underlayer film thickness 300 nm) of Example 1 increased sharply as the film thickness increased. Similarly, in the double glazing film (lower layer film thickness 50 nm) of Example 2, the decomposition activity index increases as the film thickness increases. That is, it was found that the photocatalytic activity of the titanium oxide multi-layer film depends on the film thickness of the upper layer film. In particular, when the film thickness of the upper layer film is 50 nm (corresponding to Example 1: 350 nm and Example 2: 100 nm in FIG. 6) or more, anatase-type titanium oxide monolayer film (comparative example) which is conventionally considered to have high photocatalytic activity. 2) Showd a larger value.

The band gap energy of the titanium oxide films of Examples and Comparative Examples was determined by measuring the transmission spectrum and the reflection spectrum of each sample using a spectrophotometer (JASCO Corporation, V-7100). Table 4 shows the bandgap energy (Eg) values of each sample. The Eg values of rutile-type and anatase-type titanium oxide in bulk crystals are 3.03 eV and 3.20 eV, respectively.

<tables num="4"><img file="JP2009067648A_D0004.tif" /></tables>

Eg is known to change fairly widely with the introduction of impurities and lattice defects. Comparing Example 1 and Comparative Example 1 in which rutile-type titanium oxide is mainly detected in the XRD of FIG. 3, the Eg value of Comparative Example 1 is larger than that of the bulk crystal rutile-type titanium oxide (3.03 eV). On the other hand, it can be seen that the Eg value of Example 1 is close to the Eg value of rutile-type titanium oxide in bulk crystals. That is, it was found that the titanium oxide multilayer film of Example 1 contained a large amount of rutile-type titanium oxide close to bulk crystals and could absorb visible light and ultraviolet rays having a wavelength of 407 nm or less corresponding to Eg = 3.05 eV.

On the other hand, in Comparative Example 1, Eg = 3.18 eV (corresponding to a wavelength of 390 nm), which is large, and it is considered that the visible light absorption is poor. The reason why the Eg value of Comparative Example 1 is large is not clear, but it seems to be related to the low crystallinity of rutile-type titanium oxide of Comparative Example 1. The Eg value of Example 2 was between that of Example 1 and Comparative Example 1. As shown by the XRD of FIG. 3, considering that Example 2 contains anatase-type titanium oxide having a much higher Eg value than rutile-type titanium oxide, the rutile-type titanium oxide contained in Example 2 is contained. Is considered to contain rutile-type titanium oxide having an Eg value close to that of bulk crystals as in Example 1. That is, it is suggested that it absorbs not only visible light and ultraviolet rays having a wavelength of 397 nm or less, which corresponds to Eg = 3.12 eV, but also visible light having a wavelength of 407 nm or less as in Example 1.

Next, the photocatalytic activity of the titanium oxide multilayer film of Example 1 with visible light was evaluated by a wet methylene blue decomposition test. Blue light as a visible light source (light intensity: 1100 μW / cm²) Was used, and light with a wavelength of 420 nm or less cut by a colored glass filter was used so that the light would be in the visible light region. Figure 7 shows the evaluation results of the change over time in the methylene blue concentration due to visible light irradiation in the titanium oxide multilayer film of Example 1.

As shown in FIG. 7, it was found that the concentration of methylene blue decreased with time when irradiated with visible light having a wavelength of 420 nm or less. That is, it was shown that this titanium oxide multi-layer film exhibited photocatalytic properties by irradiation with visible light. It is considered that this titanium oxide multilayer film absorbs visible light and forms holes and electrons to exhibit photocatalytic property as predicted from the above Eg value.

Since the titanium oxide film of the present invention has extremely high photocatalytic activity, it can be suitably used as a purifying material, a deodorizing material, an antifouling material, and a bactericidal material. Furthermore, it can be used as a visible light responsive photocatalyst that activates even indoor lighting such as fluorescent lamps that contain almost no ultraviolet rays.

<figref num="1">It is sectional drawing of the titanium oxide film formed by the method of forming the titanium oxide film in embodiment of this invention.</figref><figref num="2">It is a schematic block diagram of the main part which shows one form of the reactive RF magnetron sputtering apparatus suitable for forming the titanium oxide film in embodiment of this invention.</figref><figref num="3">It is the XRD measurement result of the titanium oxide multi-layer film which is an Example of this invention, and the titanium oxide single-layer film which is a comparative example.</figref><figref num="4">It is an X-ray diffraction pattern which shows the substrate temperature dependence of the titanium oxide multilayer film which is an Example of this invention.</figref><figref num="5">It is an X-ray diffraction pattern which shows the sputtering pressure dependence of the titanium oxide multilayer film which is an Example of this invention.</figref><figref num="6">It is a figure which shows the relationship between the film thickness of the titanium oxide multilayer film which is an Example of this invention, and the decomposition activity index of methylene blue by ultraviolet irradiation.</figref><figref num="7">It is a figure which shows the time-dependent change of the methylene blue concentration by the visible light irradiation in the titanium oxide multilayer film which is Example 1 of this invention.</figref>

Code description

1 Titanium oxide multi-layer film 2,6 boards 3 Underlayer membrane 4 Upper membrane 5 RF magnetron sputtering equipment 51 chamber 52 Introductory port 53 Exhaust port 7,8 Flow controller 9 Halogen lamp 10 thermocouple 11 Cathode 12 plasma 13 Target 14 shutter 15 magnet 16,17 Cooling water 18 Matching box 19,20 power supply 21,22 Earth