Multilayer structure and process for producing the same

The present invention relates to a multilayer structure which functions as a photocatalyst and can render its surface hydrophilic upon irradiation with ultraviolet rays or visible rays. The present invention further relates to a process for producing the same.

Members comprising a substrate coated with a titanium oxide film functioning as a photocatalyst are used in various applications. Such members are applicable to various articles so as to take advantage of the hydrophilicity resulting from their photocatalytic function for various purposes such as, e.g., antifogging, cleaning by rain, cleaning by water washing, and antifouling. For example, in the case of articles wherein the substrate is a glass plate, such as, e.g. , windshields and sideview mirrors for vehicles, window glasses for buildings, and mirrors, a technique is used in which the substrate is coated with a photocatalyst film whose surface becomes hydrophilic and is thereby prevented from being clouded with steam or covered with adherent waterdrops.

JP-A-10-278165 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a technique in which a metal target comprising titanium as the main component is used to conduct sputtering in an oxygen-containing atmosphere and the resulting deposit is heated to thereby form on a glass plate a photocatalyst film having anatase crystals and excellent resistance to abrasion and wearing.

However, in order for the technique described above to form a titanium oxide film having photocatalytic activity, it is necessary that a titanium oxide film should be deposited on a substrate heated to 300° C. or higher or that a titanium oxide film which has been deposited on a substrate having room temperature should be heated to a temperature as high as 600° C. or higher. Furthermore, there has been a problem that in the reactive sputtering in which a titanium metal target is used in an oxygen-containing atmosphere, the glow discharge becomes unstable as an electrically insulating oxide accumulates on the titanium metal surface and, as a result, a titanium oxide film cannot be stably obtained.

JP-A-10-310653 discloses a technique in which an anatase titanium oxide film or rutile titanium oxide film is deposited on a glass plate by the sol coating method, organic titanate method, electron beam vapor deposition method, or the like. There is a description therein to the effect that the energy band gap between the conductive band and the valence band in a titanium oxide film functioning as a photocatalyst is regulated by incorporating a metal oxide selected from ruthenium oxide, cobalt oxide, cerium oxide, chromium oxide, rhodium oxide, and vanadium oxide to thereby effectively obtain hydrophilicity.

However, the technique described above has had a problem that a step of heating at a temperature as high as 350° C. or higher is necessary for obtaining a crystalline titanium oxide film having photocatalytic activity, resulting in a complicated production process.

JP-A-1-92176 discloses a technique in which a titanium oxide photocatalyst film is deposited on a glass plate by reactive sputtering using titanium metal as a sputtering target and this titanium oxide film is doped with ions of a metal such as niobium, cobalt, or chromium by ion implantation to improve catalytic activity.

However, the technique described above has had drawbacks that an expensive ion injector is necessary for injecting metal ions in a glass by ion injection, and that it is virtually difficult to deposit a photocatalytically active film on a glass having a relatively large area, such as a window glass for buildings, etc., on an industrial scale.

The present invention has been achieved in order to overcome the problems described above.

One object of the present invention is to provide a multilayer structure comprising a substrate and formed thereon a film having satisfactory photocatalytic activity and practical antifouling properties.

Another object of the present invention is to provide a process for producing a multilayer structure comprising a substrate and formed thereon a film having photocatalytic activity and antifouling properties.

According to the first embodiment of the present invention provides, there is provided a process for producing a multilayer structure comprising a substrate and formed thereon a film mainly comprising titanium oxide and having photocatalytic activity, which comprises sputtering a conductive sinter target obtained by sintering a mixture of particles of titanium oxide and particles of at least one metal oxide selected from the group consisting of niobium oxide, tantalum oxide, vanadium oxide, zirconium oxide, tin oxide, chromium oxide and copper oxide, in an atmosphere capable of having a regulated vacuum to form the film mainly comprising titanium oxide and having photocatalytic activity on the substrate.

The film mainly comprising titanium oxide (hereinafter referred to as “titanium oxide film”) in the present invention is obtained by sputtering, in an atmosphere capable of having a regulated vacuum, a conductive target obtained by sintering a mixture of particles of titanium oxide as the main component and particles of at least one metal oxide, as a minor component, selected from the group consisting of niobium oxide, tantalum oxide, vanadium oxide, zirconium oxide, tin oxide, chromium oxide and copper oxide. The surrounding gas when sputtering that can be used is an inert gas such as argon or a mixed gas comprising an inert gas and oxygen.

Especially when an oxygen-containing atmosphere having a reduced pressure is used in order to sputter a titanium oxide film which does not have a considerable deficiency of oxygen as compared with the stoichiometric amount and is transparent in the visible region, the target surface is inhibited from being covered with an electrically insulating substance because a small oxygen amount is sufficient for the atmosphere.

The target used in the present invention, which is an oxide sinter containing titanium oxide as the main component, is electrically conductive in such a degree that electric current can flow through the surface thereof. It is preferred that a direct current glow discharge be used to bombard the conductive target and thereby deposit a film. This is because a direct current power source, which is relatively inexpensive, can be used to generate a stable glow discharge and deposit a photocatalytically active titanium oxide film on a substrate.

The target having conductivity for use in the present invention can be obtained by sufficiently mixing particles, preferably fine particles, of titanium oxide with particles, preferably fine particles, of at least one metal oxide selected from the group consisting of niobium oxide, tantalum oxide, vanadium oxide, tin oxide, chromium oxide and copper oxide, and forming the powder mixture into a given shape by pressure molding. Conventional cold pressing or hot pressing method can be used for this pressure molding.

The conductivity of the target for use in the present invention is preferably such that the surface resistance thereof is 500 Ω/□ or lower, in order to maintain a glow discharge for sputtering the target over a long period of time to thereby form a titanium oxide film in a stable manner. If the surface resistance exceeds 500 Ω/□, it is not preferable in that the glow discharge is apt to be interrupted. From the standpoint of even more stably maintaining a glow discharge over a long period of time, the surface resistance of the target is more preferably 50 Ω/□ or lower.

In the present invention, the formation of a photocatalytically active titanium oxide film is preferably conducted at a substrate temperature of 170° C. or higher. If the substrate temperature is lower than 170° C., it is undesirable in that the titanium oxide film obtained has insufficient resistance to abrasion and wearing and has low photocatalytic activity. From this standpoint, the substrate temperature is more preferably 200° C. or higher.

On the other hand, if the substrate temperature exceeds 270° C. during film formation, the titanium oxide film tends to have a higher degree of anatase crystal orientation and, unexpectedly, the photocatalytic activity thereof decreases gradually. From this standpoint, the substrate temperature during film deposition is preferably 270° C. or lower, more preferably 250° C. or lower.

Preferred metal oxides to be contained in titanium oxide in the present invention are niobium oxide, vanadium oxide and tantalum oxide because these metal oxides are effective not only in relatively easily maintaining a glow discharge necessary for sputtering the sinter target but in obtaining a titanium oxide film having enhanced photocatalytic activity. Of those, niobium oxide is most preferable.

According to the second embodiment of the present invention, there is provided a multilayer structure comprising a substrate and formed thereon a film mainly comprising titanium oxide and having photocatalytic activity (hereinafter referred to as “titanium oxide film”), wherein the film contains as a minor component at least one metal oxide selected from the group consisting of niobium oxide, tantalum oxide, vanadium oxide, zirconium oxide, tin oxide, chromium oxide and copper oxide and is substantially amorphous when analyzed by X-ray diffraction method.

In the case where the titanium oxide film formed is composed of crystals, many grain boundaries are present therein. Electron-hole pairs which have been excited by light in the titanium oxide film are trapped by many lattice defects present on those grain boundaries, so that the number of electron-hole pairs present on the film surface, which contribute to photocatalytic activity, is reduced. Because of this, such a titanium oxide film has reduced photocatalytic activity. In contrast, the multilayer structure of the present invention is reduced in the trapping since it mainly comprises an amorphous matrix. This constitution is thought to bring about a high photoexcitation efficiency. The multilayer structure of the present invention therefore has high photocatalytic activity.

The titanium oxide film is characterized in that it is less apt to have a crystalline structure because the titanium oxide contains a small amount of one or more metal oxides.

In the present invention, the content of the metal oxide(s) as a minor component in the titanium oxide is preferably 1% by weight or higher, more preferably 2.5% by weight or higher. If the content of metal oxides as a minor component in the film is lower than 1% by weight, it is undesirable in that not only stable photocatalytic activity is difficult to obtain but also the target is less apt to retain conductivity (electricity-passing properties) in a degree so as to conduct sputtering in a stable manner, making it difficult to stably form a titanium oxide film.

On the other hand, the content of the metal oxide(s) as a minor component is preferably 10% by weight or lower, more preferably 5% by weight or lower. This is because if the content of the metal oxide(s) exceeds 10% by weight, the content of titanium oxide decreases in relative amount, resulting in lowering photocatalytic activity.

In the present invention, preferred metal oxides as the minor component are niobium oxide, vanadium oxide and tantalum oxide from the standpoints of obtaining high photocatalytic activity and forming a stable glow discharge to form a titanium oxide film with satisfactory reproducibility. Especially preferred from such standpoints is niobium oxide.

Usable substrates include ones made of inorganic materials such as glasses and ceramics and ones made of organic materials such as plastics because in the process of the present invention, film formation on substrates is possible at relatively low temperature. Examples of applications of such substrates include window glasses for buildings and vehicles, exterior and interior materials for buildings, Braun tubes, PDP displays, liquid-crystal display panels, optical recording media, magnetic recording media, and members or housings of domestic electrical appliances or OA apparatus.

In the case where a glass plate, especially one containing alkali components, such as a soda-lime silicate composition, is used as a substrate, it is preferred to dispose, between this glass substrate and a titanium oxide film, a primer film serving to prevent the alkali components of the substrate from dissolving in the titanium oxide film. From the standpoint of further improving antifouling properties, it is preferred to partly or wholly coat the titanium oxide film of the present invention with a hydrophilic film.

FIG. 1 is a sectional view showing one embodiment of the multilayer structure according to the present invention.

FIG. 2 is a graphic presentation showing the influence of substrate temperature during titanium oxide film formation on residual triolein amount.

FIG. 3 is a graphic presentation showing the influence of substrate temperature during titanium oxide film formation on the crystalline structure of the film.

Reference Numerals in the drawings are as follows:

1: Multilayer structure of the present invention

2: Glass substrate

3: Photocatalytically active film mainly comprising titanium oxide

4: Primer film

5: Hydrophilic film

Embodiments of the present invention will be explained in detail below.

FIG. 1 is a sectional view showing one embodiment of the multilayer structure according to the present invention. This multilayer structure 1 comprises a glass substrate 2 and, superposed on one side thereof in this order, a primer film 4 comprising silicon dioxide serving to prevent alkali dissolution, a titanium oxide film 3 having photocatalytic activity, and a hydrophilic film 5 . FIG. 2 is a graphic presentation showing the influence of the substrate temperature during titanium oxide film formation on the residual triolein amount. FIG. 3 is a graphic presentation showing the influence of the substrate temperature during titanium oxide film formation on the crystalline structure of the film.

Examples of the glass substrate include plates made of a soda-lime silicate glass, alkali borosilicate glass and aluminoborosilicate glass. Examples of plastic substrates include substrates made of a polycarbonate resin, acrylic resin and polystyrene resin. In the case of using a substrate containing an alkali component, e.g., sodium ions, as in a soda-lime silicate glass, it is preferred to dispose a film of a metal oxide such as silicon dioxide as an alkali dissolution preventive film between the substrate and a titanium oxide film in order to prevent the alkali component, which is apt to move within the glass, from coming into the titanium oxide film from the glass. Silicon dioxide can be film-formed by conventional technique such as sputtering, CVD or a method using an aqueous solution containing hydrosilicofluoric acid supersaturated with silicon dioxide. The thickness of this silicon dioxide film is preferably from 10 to 100 nm.

The sinter target used in forming a photocatalytically active titanium oxide film according to the present invention is produced by, for example, the following manner. Titanium oxide and niobium oxide each are regulated so as to have a particle diameter of from about 0.01 to 50 μm. The niobium oxide is mixed with the titanium oxide in an amount of from 1 to 10% by weight based on the weight of the titanium oxide, and this mixture is molded into a given shape by cold isostatic pressing in water in which the powder mixture is pressed at 3 t/cm 2 . The resulting molded product is degreased at 400 to 500° C., preliminarily burned in an oxygen atmosphere at 1300 to 1500° C. for 4 to 6 hours, and then subjected to hot isostatic pressing at 1,350 to 1,450° C. under a pressure of about 100 MPa in an atmosphere comprising argon and oxygen.

Conductivity is imparted to the sputtering target according to the present invention to such a degree that the target is not charged during sputtering with a direct current glow discharge. From the standpoint of easily obtaining such target surface conductivity, preferred metal oxides for use as the minor component are niobium oxide, vanadium oxide and tantalum oxide. Especially preferred is niobium oxide.

By using the oxide sinter target which is not charged, sputtering can be conducted with a direct current glow discharge. The partial oxygen pressure of the atmosphere used for this sputtering may be lower than that in the reactive sputtering method in which titanium metal is used as a target. Even when the sputtering is conducted over a long period of time, the accumulation of an electrically insulating substance on the target surface can be inhibited. The sputtering atmosphere is regulated so as to have a pressure of from 1 to 5 mTorr in magnetron sputtering (MS) or dual magnetron sputtering (DMS). For maintaining this pressure, the gas to be introduced into the film-formation chamber is preferably regulated so as to have a composition having an oxygen content of from 0.5 to 10% by volume.

The primer film used as an alkali dissolution preventive film in the present invention may be a film of a metal oxide or a film of a nitride. Especially preferred are silicon dioxide, silicon nitride and silicon oxynitride because these compounds show satisfactory adhesiveness to glass plates and titanium oxide films.

In the present invention, both hydrophilicity and antifouling properties can be imparted to the substrate more satisfactorily by coating the titanium oxide film with a hydrophilic film transparent to ultraviolet or visible light. Preferred examples of this hydrophilic film include silicon dioxide films or films containing silicon dioxide as the main component.

Methods for forming the primer film or hydrophilic film are not particularly limited, and conventional methods can be used, such as sputtering, CVD using an organosilane as a starting material, and a method using an aqueous hydrosilicofluoric acid solution supersaturated with silicon dioxide.

The present invention will be explained in more detail below by reference to the following Examples and Comparative Examples, but it should be understood that the invention is not construed as being limited thereto. In each of those Examples and Comparative Examples, the following conditions were used in forming a titanium oxide film.

Target size: 38 cm (length)×13 cm (width)×6 mm (thickness)

Sputtering power: 3 kW

Pressure of sputtering atmosphere: 3 mTorr

Gas introduced: argon/oxygen mixed gas or oxygen gas

Substrate heating: various temperatures according to experiments in the range of from room temperature (about 25° C.) to 350° C.