Use of a photocatalytically rendered superhydrophilic surface with antifogging properties

Abstract: The invention relates to the use of a composite comprising a substrate and a photocatalytic layer coated thereon, said photocatalytic layer comprising a photocatalytic semiconductor material having a surface which has been rendered hydrophilic by photoexcitation with sunlight, for antifogging said surfaces. Said composite is, for example, a mirror, a lens or a transparent sheet member.

Description: The present invention relates broadly to the art of rendering and maintaining a surface of a substrate highly hydrophilic. More particularly, the present invention relates to the antifogging art wherein the surface of a transparent substrate such as a mirror, lens and sheet glass is made highly hydrophilic to thereby prevent fogging of the substrate or formation of water droplets. It is often experienced that, in the cold seasons, windshields and window-glasses of automobiles and other vehicles, windowpanes of buildings, lenses of eyeglasses, and cover glasses of various instruments are fogged by moisture condensate. Similarly, in a bathroom or lavatory, it is often encountered that mirrors and eyeglass lenses are fogged by steam. Fogging of the surface of an article results from the fact that, when the surface is held at a temperature lower than the dew point of the ambient atmosphere, condensation of moisture being present in the ambient air takes place to form moisture condensate at the surface. If the condensate particles are sufficiently fine and small so that the diameter thereof is on the order of one half of the wavelength of the visible light, the particles cause scattering of light whereby window-glasses and mirrors become apparently opaque thereby giving rise to a loss of visibility. When condensation of moisture further proceeds so that fine condensate particles are merged together to grow into discrete larger droplets, the refraction of light taking place at the interface between the droplets and the surface and between the droplets and the ambient air causes the surface to be blurred, dimmed, mottled, or clouded. As a result, a look-through image through a transparent article such as sheet glass is distorted and a reflective image of a mirror disturbed. Similarly, when windshields and window-glasses of vehicles, windowpanes of buildings, rearview mirrors of vehicles, lenses of eyeglasses, or shields of masks or helmets are subjected to rain or water splash so that discrete waterdroplets are adhered to the surface, their surface is blurred, dimmed, mottled, or clouded to result in the loss of visibility. The term "antifogging" as used herein and in the appended claims is intended to mean broadly the art of preventing occurrence of optical trouble resulting from fogging, growth of condensate droplets or adherent water droplets mentioned above. Obviously, the antifogging art deeply affects the safety as well as the efficiency of various works. For example, the safety of vehicles and traffic will be undermined if the windshields, window-glasses or rearview mirrors of vehicles are fogged or blurred. Fogging of endoscopic lenses and dental mouth mirrors may hinder proper and accurate diagnosis, operation and treatment. If cover glasses of measuring instruments are fogged, a reading of data will become difficult. The windshields of automobiles and other vehicles are normally provided with windshield wipers, defrosting devices and heaters so as to permit views in the cold seasons and under rainy conditions. However, it is not commercially feasible to install this equipment to the side windows and the rearview mirrors arranged outside of the vehicle. Similarly, it is difficult, if possible at all, to mount this antifogging equipment to windowpanes of buildings, lenses of eyeglasses and endoscopes, dental mouth mirrors, shields of masks and helmets, or cover glasses of measuring instruments. As is well-known, a simple and convenient antifogging method conventionally used in the art is to apply onto a surface an antifogging composition containing either a hydrophilic compound such as polyethylene glycol or a hydrophobic or water-repellent compound such as silicone. However, the disadvantage of this method is that the antifogging coating thus formed is only temporary in nature and is readily removed when rubbed or washed with water so that its effectiveness is prematurely lost. Japanese Utility Model Kokai Publication No. 3-129357 (Mitsubishi Rayon) discloses an antifogging method for a mirror wherein the surface of a substrate is provided with a polymer layer and the layer is subjected to irradiation by ultraviolet light, followed by treatment with an aqueous alkaline solution to thereby form acid radicals at a high density whereby the surface of the polymer layer is rendered hydrophilic. Again, it is however believed that; according to this method, the hydrophilic property of the surface is degraded as time elapses because-of adherent contaminants so that the antifogging function is lost sooner or later. Japanese Utility Model Kokai Publication No. 5-68006 (Stanley Electric) discloses an antifogging film made of a graftcopolymer of an acrylic monomer having hydrophilic groups and a monomer having hydrophobic groups. The graftcopolymer is described as having a contact angle with water of about 50°. It is therefore believed that this antifogging film does not exhibit a sufficient antifogging capability. Isao Kaetsu "Antifogging Coating Techniques for Glass", Modern Coating Techniques, pages 237-249, published by Sogo Gijutsu Center (1986), describes various antifogging techniques used in the prior art. The author Mr. Kaetsu nevertheless reports that the prior art antifogging techniques, which consist of rendering a surface hydrophilic, suffer from significant problems which must be overcome in reducing them to practice and that the conventional antifogging coating techniques seemingly come up against a barrier. EP-A-590 477 discloses that a metal-oxide layer exhibiting a photocatalytic activity function is formed on the surface of an inorganic architectural material, such as external wall material, roofing material, internal wall material, flooring material, and ceiling material, including glass, tile, concrete, stone, metal and the like, so as to provide the property of deodorizing a space coming in contact with the architectural material, and antimold, antisoiling properties, and ultraviolet-ray absorbency of the surface of the architectural material, as well as the long-term maintenance of these properties. MASANARI TAKAHASHI ET AL: 'PT-TIO2 THIN FILMS ON GLASS SUBSTRATES AS EFFICIENT PHOTOCATALYSTS' JOURNAL OF MATERIALS SCIENCE, vol. 24, no. 1, January 1989 (1989-01), pages 243-246, XP000046035, discloses that photocatalytic Pt-TiO 2 thin films of various thicknesses (ca. 20 to 100 nm) were prepared on Vycor glass substrates by spray-coating. The photocatalytic activity of the films was examined through the dehydrogenation of methanol in aqueous solution. Accordingly, an obj ect of the invention is to provide an antifogging method which is capable of realizing a high degree of visibility of a transparent substrate such as a mirror, lens and glass. Another obj ect of the invention is to provide an antifogging method wherein the surface of a transparent substrate such as a mirror, lens and glass is maintained highly hydrophilic for a long period of time A still another object of the invention is to provide an antifogging method wherein the surface of a transparent substrate such as a mirror, lens and glass is almost permanently maintained highly hydrophilic. A further object of the invention is to provide an antifogging coating which has an improved durability and abrasion resistance. Another object of the invention is to provide an antifogging coating which can readily be applied onto surface requiring antifogging treatment. Yet another object of the invention is to provide an antifogging transparent substrate such as a mirror, lens and glass, as well as a method of making thereof, wherein the surface thereof is maintained highly hydrophilic for a long period of time to thereby provide a high degree of antifogging property for a long period. In certain apparatus, formation of moisture condensate on a surface thereof often hampers operation of the apparatus when condensate has grown into droplets. In heat exchangers, for example, the heat exchanging efficiency would be lowered if condensate particles adhering to radiator fins have grown into large droplets. Accordingly, another object of the invention is to provide a method for preventing adherent moisture condensate from growing into larger water droplets wherein a surface is made highly hydrophilic to thereby permit adherent moisture condensate to spread into a water film. Disclosure of the Invention The present inventors have discovered for the first time in the world that, upon photoexcitation, a surface of a photocatalyst is rendered highly hydrophilic. Surprisingly, it has been discovered that, upon photoexcitation of photocatalytic titania with ultraviolet light, the surface thereof is rendered highly hydrophilic to the degree that the contact angle with water becomes less than 10°, more particularly less than 5°, and even reached about 0°. Based on the foregoing new discovery; the present invention provides the use of a composite comprising a substrate and a photocatalytic layer coated thereon, said photocatalytic layer comprising a photocatalytic semiconductor material having a surface which has been rendered hydrophilic by photoexcitation with sunlight, for antifogging said surfaces. According to the invention, the surface of the substrate is coated with an abrasion-resistant photocatalytic coating comprised of a photocatalytic semiconductor material. Preferred embodiments of the invention are described in the subclaims. Upon irradiation for a sufficient time with a sufficient intensity of a light having a wavelength which has an energy higher than the bandgap energy of the photocatalytic semiconductor, the surface of the photocatalytic coating is rendered highly hydrophilic to exhibit a super-hydrophilicity. The term "super-hydrophilicity" or "super-hydrophilic" as used herein refers to a highly hydrophilic property (i.e., water wettability) of less than about 10°, preferably less than about 5°, in terms of the contact angle with water. Similarly, the term "superhydrophilification" or "superhydrophilify" refers to rendering a surface highly hydrophilic to the degree that the contact angle with water becomes less than about 10°, more preferably less than about 5°. It is considered, though not predicable with any clarity, that the superhydrophilification process caused by a photocatalyst is a phenomenon somewhat different from photodecomposition of substances resulting from the photocatalytic redox process. However, as described later, it has been observed that superhydrophilification of a surface does not occur unless a light having an energy higher than the band gap energy of the photocatalyst is irradiated. It is considered that, presumably, the surface of a photocatalytic coating is rendered superhydrophilic as a result of water being chemisorbed thereon in the form of hydroxyl groups (OH - ) under the photocatalytic action of the photocatalyst. Once the surface of the photocatalytic coating has been made highly hydrophilic upon photoexitation of the photocatalyst, the hydrophilicity of the surface will be sustained for a certain period of time even if the substrate is placed in the dark. As time elapses, the superhydrophilicity of the surface will be gradually lost because of contaminants adsorbed on the surface hydroxyl groups. However, the superhydrophilicity will be restored when the surface is again subjected to photoexcitation. To initially superhydrophilify the photocatalytic coating, sunlight is used which has a wavelength of an energy higher than the band gap energy of the photo-catalyst, such as titania in which the photoexciting wavelength pertains to the ultraviolet range of the spectrum. Thus, the ultraviolet light contained in the sunlight is advantageously used in such a situation where the sunlight impinges upon the substrate coated by the photocatalytic coating. In the case where the photocatalytic coating is made of silica blended titania as described later, the surface thereof can readily be rendered hydrophilic even by a weak ultraviolet radiation contained in the light emitted from a fluorescent lamp. After the surface of the photocatalytic coating has once been superhydrophilified, the superhydrophilicity may be maintained or renewed by a relatively weak light. In the case of titania, for example, maintenance and restoration of the superhydrophilicity may be accomplished to a satisfactory degree even by a weak ultraviolet light contained in the light of indoor illumination lamps such as fluorescent lamps. The photocatalytic coating exhibits the super-hydrophilicity even if the thickness thereof is made extremely small. It presents a sufficient hardness when made in particular from a photocatalytic semiconductor material comprising a metal oxide. Therefore, the photocatalytic coating presents an adequate durability and abrasion resistivity. Superhydrophilification of a surface may be utilized for various applications. According to the invention, it is used for antifogging a transparent member. According to the invention, a transparent member coated with a photocatalytic coating is prepared, or otherwise, the surface of a transparent member is coated with a photocatalytic coating. The transparent member may include a mirror such as a rearview mirror for a vehicle, bathroom or lavatory mirror, dental mouth mirror, and road mirror; a lens such as an eyeglass lens, optical lens, photographic lens, endoscopic lens, and light projecting lens; a prism; a windowpane for a building or control tower; a windowpane for a vehicle such as an automobile, railway vehicle, aircraft, watercraft, submarine, snowmobile, ropeway gondola, pleasure garden gondola and spacecraft; a windshield for a vehicle such as an automobile, railway vehicle, aircraft, watercraft, submarine, snowmobile, motorcycle, ropeway gondola, pleasure garden gondola and spacecraft; a shield for protective or sporting goggles or mask including diving mask; a shield for a helmet; a show window glass for chilled foods; and a cover glass for a measuring instrument. Upon subjecting the transparent member provided with the photocatalytic coating to irradiation by a light to thereby photoexcite the photocatalyst, the surface of the photocatalytic coating will be superhydrophilified. Thereafter, in the event that moisture in the air or steam undergoes condensation, the condensate will be transformed into a uniform film of water without forming discrete water droplets. As a result, the surface will be free from the formation of a light diffusing fog. Similarly, in the event that a windowpane, a rearview mirror of a vehicle, a windshield of a vehicle, eyeglass lenses, or a helmet shield is subjected to a rainfall or a splash of water, the waterdroplets adhering onto the surface will be quickly spread over into a uniform water film thereby preventing formation of discrete waterdroplets which would otherwise hinder eyesight. Accordingly, a high degree of view and visibility is secured so that the safety of vehicle and traffic is secured and the efficiency of various work and activities improved. Thus, the present invention can be used for an antifogging mirror comprising: a substrate with a reflective coating; and, a substantially transparent layer comprised of a photocatalytic semiconductor material and bonded to the surface of said substrate; said photocatalytic material operating upon photoexcitation thereof to render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer to thereby prevent the substrate from being fogged or blurred with adherent moisture condensate and/or water droplets. Preferably, upon photoexitation, the surface of said layer presents a water-wettability of less than about 10°, more preferred of less than about 5°, in terms of the contact angle with water. Preferably, said photocatalytic material comprises an oxide selected from the group consisting of TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO 3 , Bi 2 O 3 and Fe 2 O 3 . More preferably, said photocatalytic material comprises the anatase form of tirania and may, further, comprise SiO 2 SnO 2 . In the antifogging mirror, said layer may comprise a coating, wherein particles of said photocatalytic material are uniformly dispersed. Preferably, said coating is made of silicone and the surface of said coating is formed of a derivative of silicone in which the organic groups bonded to the silicon atoms of the silicone molecules have been substituted upon photoexcitation at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material. In the antifogging mirror, said substrate may be made of glass containing alkaline network-modifier ions and a thin film for preventing said ions from diffusing from said substrate into said layer may be interleaved between said substrate and said layer. Preferably, said thin film comprises a thin film of silica. In the antifogging mirror, the thickness of said layer is preferably less than about 0.2 micrometers. In the antifogging mirror, said layer may further comprise a metal selected from the group consisting of Ag, Cu and Zn, and/or a metal selected from the group consisting of Pt, Pd, Rh, Ru, Os and Ir. The antifogging mirror is, for example, a bathroom or lavatory mirror, a rearviev mirror for a vehicle or a dental mouth mirror. The present invention can further be used for an antifogging lens comprising: a transparent lens-forming body; and, a substantially transparent layer comprised of a photocatalytic semiconductor material and bonded to the surface of said lens-forming body; said photocatalytic material operating upon photoexcitation thereof to render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer to thereby prevent the lens-forming body from being fogged or blurred with adherent moisture condensate and/or water droplets. Preferably, upon photoexitation, the surface of said layer presents a water-wettability of less than about 10°, more preferred of less than about 5°, in terms of the contact angle with water. Preferably, said photocatalytic material comprises an oxide selected from the group consisting of TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO 3 , Bi 2 O 3 and Fe 2 O 3 . More preferably, said photocatalytic material comprises the anatase form of titania and may , further, comprise SiO 2 SnO 2 . In the antifogging lens, said layer may comprise a coating, wherein particles of said photocatalytic material are uniformly dispersed. Preferably, said coating is made of silicone and the surface of said coating is formed of a derivative of silicone in which the organic groups bonded to the silicon atoms of the silicone molecules have been substituted upon photoexcitation at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material. In the antifogging lens, said lens-forming body may be made of glass containing alkaline network-modifier ions and a thin film for preventing said ions from diffusing from said lens-forming body into said layer may be interleaved between said lens-forming body and said layer. Preferably, said thin film comprises a thin film of silica. In the antifogging lens, the thickness of said layer is preferably less than about 0.2 micrometers. In the antifogging lens, said layer may further comprise a metal selected from the group consisting of Ag, Cu and Zn, and/or a metal selected from the group consisting of Pt, Pd, Rh, Ru, Os and Ir. The antifogging lens is, for example, an eyeglass lens, an optical lens, a photographic lens, an endoscopic lens or a prism.. The present invention may further be used for an antifogging transparent sheet member comprising: a transparent substrate; and, a substantially transparent layer comprised of a photocatalytic semiconductor material and bonded to the surface of said substrate; said photocatalytic material operating upon photoexcitation thereof to render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer to thereby prevent the substrate from being fogged or blurred with adherent moisture condensate and/or water droplets. Preferably, upon photoexitation, the surface of said layer presents a water-wettability of less than about 10°, more preferred of less than about 5°, in terms of the contact angle with water. Preferably, said photocatalytic material comprises an oxide selected from the group consisting of TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO 3 , Bi 2 O 3 and Fe 2 O 3 and may, further, comprise SiO 2 or SnO 2 . More preferably, said photocatalytic material comprises the anatase form of titania. In the antifogging transparent sheet member, said layer may comprise a coating, wherein particles of said photocatalytic material are uniformly dispersed. Preferably, said coating is made of silicone and the surface of said coating is formed of a derivative of silicone in which the organic groups bonded to the silicon atoms of the silicone molecules have been substituted upon photoexcitation at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material. In the antifogging transparent sheet member, said substrate may be made of glass containing alkaline network-modifier ions and a thin film for preventing said ions from diffusing from said substrate into said layer may be interleaved between said substrate and said layer. Preferably, said thin film comprises a thin film of silica. In the antifogging transparent sheet member, the thickness of said layer is preferably less than about 0.2 micrometers. In the antifogging transparent sheet member, said layer may further comprise a metal selected from the group consisting of Ag, Cu and Zn, and/or a metal selected from thegroup consisting of Pt, Pd, Rh, Ru, Os and Ir. In the antifogging transparent sheet member the substrate is, for example, a windowpane, which may be made of glass or plastics. Preferably, said windowpane is a windowpane for a vehicle selected from the group consisting of automobile, railway vehicle, aircraft, watercraft, submarine, snowmobile, ropeway gondola, pleasure garden gondola and spacecraft. In the antifogging transparent sheet member the substrate is, for example, a windshield for a vehicle, which may be made of glass or plastics. Preferably, said windshield is a windshield for a vehicle selected from the group consisting of automobile, railway vehicle, aircraft, watercraft, submarine, snowmobile, motorcycle, ropeway gondola, pleasure garden gondola and spacecraft. In the antifogging transparent sheet member the substrate is, for example, a shield of goggles or mask, a shield of a helmet or a cover glass for a measuring instrument. The invention further prevents growth of condensate droplets adhering to a substrate or for causing adherent water droplets to spread over into a uniform water film. To this end, the surface of the substrate is coated with a photocatalytic coating. Once the surface of the substrate has been super-hydrophilified upon photoexcitation of the photocatalytic coating, moisture condensate or waterdroplets that have come to adhere to the surface will be spread over the surface to form a uniform film of water. By applying this method, for example, to radiator fins of a heat exchanger, it is possible to prevent fluid passages for a heat exchange medium from being clogged by condensate whereby the heat exchange efficiency is enhanced. When otherwise this method is applied to a mirror, lens, windowpane, windshield, or pavement, it is possible to promote drying of the surface after wetting with water. Said substrate may further be made of a material, selected from the group consisting of metal, ceramics, glass, plastics, wood, stone, cement, concrete, a combination thereof, and a laminate thereof, and wherein, for prevention of growth of water droplets, said layer operates to cause adherent moisture condensate and/or water droplets to spread over the surface of said layer. In this case, the substrate is, for example, a radiator fin for a heat exchanger and said layer permits adherent moisture condensate and/or water droplets to spread into a water film to thereby increase the efficiency of the heat exchanger. The substrate may further, for example, be a surface of an article selected from the group consisting of a mirror, a lens, a sheet glass, a windshield or a pavement. In the composite used in the present invention, said photocatalytic material preferably comprises an oxide selected from the group consisting of TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO 3 , Bi 2 O 3 and Fe 2 O 3 and may, further, comprise SiO 2 or SnO 2 . More preferably, said photocatalytic material comprises the anatase form of titania or the rutile form of titania. In the composite used in the present invention, said layer may comprise a coating, wherein particles of said photocatalytic material are uniformly dispersed. Preferably,said coating is made of silicone and wherein the surface of said coating is formed of a derivative of silicone in which the organic groups bonded to the silicon atoms of the silicone molecules have been substituted upon photoexcitation at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material. In the composite used in the present invention, said substrate may contain alkaline metal ions and/or alkaline-earth metal ions and a thin film for preventing said ions from diffusing from said substrate into said layer may be interleaved between said substrate and said layer. Preferably, said thin film comprises a thin film of silica. In the composite used in the present invention, the thickness of said layer is preferably less than about 0.2 micrometers. In the composite used in the present invention, said layer may further comprise a metal selected from the group consisting of Ag, Cu and Zn, and/or a metal selected from the group consisting of Pt, Pd, Rh, Ru, Os and Ir. According to the present invention (1) a mirror coated with a substantially transparent layer comprised of a photocatalytic semiconductor material; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by subjecting said photocatalytic material to photoexcitation to thereby render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to the present invention (2) a mirror coated with a substantially transparent layer of silicone in which particles of a photocatalytic semiconductor material are uniformly dispersed; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by (a) subjecting said photocatalytic material of said layer to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material to thereby render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to the present invention (3) a mirror coated with a substantially transparent layer of silicone in which particles of a photocatalytic semiconductor material are uniformly dispersed; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by (a) subjecting said photocatalytic material of said layer to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material whereby the surface of said layer is rendered hydrophilic; and, (b) subjecting said photocatalytic material to photoexcitation to thereby keep the surface of said layer hydrophilic under the photocatalytic action of said photocatalytic material whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to the present invention (4), a mirror prepared by coating the surface of said mirror with a substantially transparent layer comprised of a photocatalytic semiconductor material; and, is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by subjecting said photocatalytic material to photoexcitation to thereby render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to (4), said step of coating preferably comprises the steps of: (a) applying onto said surface a coating composition comprising particles of photocatalytic semiconductor material and a film-forming element of uncured or partially cured silicone or a precursor thereof; (b) curing said film-forming element; and, (c) subjecting said photocatalytic material to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material. According to the present invention (5) a lens coated with a substantially transparent layer comprised of a photocatalytic semiconductor material; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by subjecting said photocatalytic material to photoexcitation to thereby render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to the present invention (6) a lens coated with a substantially transparent layer of silicone in which particles of a photocatalytic semiconductor material are uniformly dispersed; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by (a) subjecting said photocatalytic material of said layer to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material to thereby render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to the present invention (7) preparing a lens coated with a substantially transparent layer of silicone in which particles of a photocatalytic semiconductor material are uniformly dispersed; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by (a) subjecting said photocatalytic material of said layer to photoexcitation so that theorganic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material whereby the surface of said layer is rendered hydrophilic; and, (b) subjecting said photocatalytic material to photoexcitation to thereby keep the surface of said layer hydrophilic under the photocatalytic action of said photocatalytic material whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to the present invention (8), a lens prepared by coating the surface of said lens with a substantially transparent layer comprised of a photocatalytic semiconductor material; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by subjecting said photocatalytic material to photoexcitation to thereby render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to (8), said step of coating preferably comprises the steps of: (a) applying onto said surface a coating composition comprising particles of photocatalytic semiconductor material and a film-forming element of uncured or partially cured silicone or a precursor thereof; (b) curing said film-forming element; and, (c) subjecting said photocatalytic material to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material. According to the present invention (9) a transparent sheet member coated with a substantially transparent layer comprised of a photocatalytic semiconductor material; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by subjecting said photocatalytic material to photoexcitation to thereby render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to the present invention (10) a transparent sheet member coated with a substantially transparent layer of silicone in which particles of a photocatalytic semiconductor material are uniformly dispersed; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by (a) subjecting said photocatalytic material of said layer to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material to thereby render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to the present invention (11) a transparent sheet member coated with a substantially transparent layer of silicone in which particles of a photocatalytic semiconductor material are uniformly dispersed; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by (a) subjecting said photocatalytic material of said layer to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material whereby the surface of said layer is rendered hydrophilic; and, (b) subjecting said photocatalytic material to photoexcitation to thereby keep the surface.of said layer hydrophilic under the photocatalytic action of said photocatalytic material whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to the present invention (12), a transparent sheet member prepared by coating the surface of said transparent sheet member with a substantially transparent layer comprised of a photocatalytic semiconductor material; is prevented from being fogged or blurred with adherent moisture condensate and/or water droplets by subjecting said photocatalytic material to photoexcitation to thereby render the surface of said layer hydrophilic whereby adherent moisture condensate and/or water droplets are caused to spread over the surface of said layer. According to (12), said step of coating preferably comprises the steps of: (a) applying onto said surface a coating composition comprising particles of photocatalytic semiconductor material and a film-forming element of uncured or partially cured silicone or a precursor thereof; (b) curing said film-forming element; and, (c) subjecting said photocatalytic material to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material. According to the present invention (13) growth of water droplets adhering on a substrate coated with a layer comprised of a photocatalytic semiconductor material; is prevented by subjecting said photocatalytic material to photoexcitation to thereby render the surface of said layer hydrophilic; and, causing adherent moisture condensate and/or water droplets to spread over the surface of said layer. According to the present invention (14) growth of water droplets adhering on a substrate, coated with a layer of silicone in which particles of a photocatalytic semiconductor material are uniformly dispersed; is prevented by (a) subj ecting said photocatalytic material of said layer to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material whereby the surface of said layer is rendered hydrophilic; and, (b) causing adherent moisture condensate and/or water droplets to spread over the surface of said layer. According to the present invention (15) growth of water droplets adhering on a substrate coated with a layer of silicone in which particles of a photocatalytic semiconductor material are uniformly dispersed; is prevented by (a) subjecting said photocatalytic material of said layer to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material whereby the surface of said layer is rendered hydrophilic; (b) subjecting said photocatalytic material to photoexcitation to thereby keep the surface of said layer hydrophilic under the photocatalytic action of said photocatalytic material; and, (c) causing adherent moisture condensate and/or water droplets to spread over the surface of said layer. According to the present invention (16) growth of water droplets adhering on a substrate is prevented by coating the surface of said substrate with a layer comprised of a photocatalytic semiconductor material; subjecting said photocatalytic material to photoexcitation to thereby render the surface of said layer hydrophilic; and, causing adherent moisture condensate and/or water droplets to spread over the surface of said layer. According to (16), said step of coating preferably comprises the steps of: (a) applying onto said surface a coating composition comprising particles of photocatalytic semiconductor material and a film-forming element of uncured or partially cured silicone or a precursor thereof; (b) curing said film-forming element; and, (c) subjecting said photocatalytic material to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material. In any of applications (1) to (16), the step of subjecting said photocatalytic material to photoexcitation is carried out with the sunlight. In applications (1) to (12) and (16), the step of subjecting said photocatalytic material to photoexcitation is preferably carried out until the water-wettability of said layer becomes less than about 10°, more preferably less than about 5°, in terms of the contact angle with water. In applications (2), (3), (6), (7), (10), (11), (14) or (15), said step (a) is preferably carried out until the water-wettability of said layer becomes less than about 10°, more preferably less than about 5°, in terms of the contact angle with water. In applications (3), (4), (7), (8), (11), (12), (15) or (16), said step (b) is preferably carried out until the water-wettability of said layer becomes less than about 10°, more preferably less than about 5°, in terms of the contact angle with water. In any of applications (1) to (16) according to the present invention, said photocatalytic material preferably comprises an oxide selected from the group consisting of TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO 3 , Bi 2 O 3 and Fe 2 O 3 and may, further, comprise SiO 2 or SnO 2 . More preferably, said photocatalytic material comprises the anatase form of titania. In any of applications (1) to (4) according to the present invention, the substrate of said mirror may be made of glass containing alkaline network-modifier ions and the surface of said substrate is preferably coated with a thin film for preventing said ions from diffusing from said substrate into said layer. Preferably, said thin film comprises a thin film of silica. In any of applications (5) to (8) according to the present invention, said lens may be made of glass containing alkaline network-modifier ions and the surface of said lens is preferably coated with a thin film for preventing said ions from diffusing from said substrate into said layer. Preferably, said thin film comprises a thin film of silica. In any of applications (9) to (12) according to the present invention, said transparent sheet member may be made of glass containing alkaline network-modifier ions and the surface of said sheet member is preferably coated with a thin film for preventing said ions from diffusing from said substrate into said layer. Preferably, said thin film comprises a thin film of silica. In any of applications (13) to (16) according to the present invention, said substrate may contain alkaline metal ions and/or alkaline-earth metal ions and the surface of said substrate may be coated with a thin film for preventing said ions. from diffusing from said substrate into said layer. Preferably, said thin film comprises a thin film of silica. In any of applications (13) to (16) according to the present invention, the thickness of said layer is preferably less than about 0.2 micrometers. In any of applications (1) to (16) according to the present invention, said layer may further comprise a metal selected from the group consisting of Ag, Cu and Zn, for destroying or inhibiting growth of bacteria and microorganisms adhering to the surface of said layer. In any of applications (1) to (16) according to the present invention, said layer may further comprise a metal selected from the group consisting of Pt, Pd, Rh, Ru, Os and Ir for enhancing the redox action of the photocatalytic material. An antifogging mirror for use according to the present invention can be manufactured by a method (17), comprising the steps of: preparing a substrate with or without a reflective coating; coating the surface of said substrate with a substantially transparent layer comprised of a photocatalytic semiconductor material; and, forming where necessary a reflective coating on the opposite-surface of said substrate prior to or subsequent to or during the course of said step of coating. An antifogging lens for use according to the present invention can be manufactured by a method (18), comprising the steps of: preparing a lens-forming body; and, coating the surface of said body with a substantially transparent photo-reactive layer comprised of a photocatalytic semiconductor material. An antifogging transparent sheet member for use according to the present invention can be manufactured by a method (19), comprising the steps of: preparing a transparent substrate; and, coating the surface of said substrate with a substantially transparent photo-reactive layer comprised of a photocatalytic semiconductor material and presenting upon photoexcitation a water wettability of less than about 10° in terms of the contact angle with water. In methods (17) to (19), the step of subjecting said photocatalytic material to photoexcitation is preferably carried out until the water-wettability of said layer becomes less than about 10°, more preferably less than about 5°, in terms of the contact angle with water. In any of methods (17) to (19), said photocatalytic material preferably comprises an oxide selected from the group consisting of TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO 3 , Bi 2 O 3 and Fe 2 O 3 , and may, further, comprise SiO 2 or SnO 2 . More preferably, said photocatalytic material comprises the anatase form of titania. In any of methods (17) to (19), said step of coating preferably comprises the steps of: (a) applying onto the surface a coating composition comprising particles of photocatalytic semiconductor material and a film-forming element of uncured or partially cured silicone or a precursor thereof; (b) curing said film-forming element; and, (c) subjecting said photocatalytic material to photoexcitation so that the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said layer are substituted at least in part with hydroxyl groups under the photocatalytic action of said photocatalytic material. Step (c) is preferably carried out until the water-wettability of said layer becomes less than about 10°, more preferably less than about 5°, in terms of the contact angle with water. In any of methods (17) to (19), said step of coating preferably comprises the steps of: (a) coating the surface with a thin film of amorphous titania; and, (b) heating said thin film at a temperature less than the softening point of the substrate to transform amorphous titania into crystalline titania. Preferably, prior-to said step of coating, the substrate is coated with a thin film of silica to prevent alkaline network-modifier ions from diffusing from the substrate into said layer. In the above methods (17) to (19), said step (a) is preferably carried out by applying onto the surface a solution of an organic compound of titanium, followed by subjecting said compound to hydrolysis and dehydration polymerization to form said thin film of amorphous titania over the surface. In this embodiment, said organic compound of titanium is preferably selected from the group consisting of an alkoxide, chelate and acetate of titanium. Alternatively, in the above methods (17) to (19), said step (a) is preferably carried out by applying onto the surface a solution of an inorganic compound of titanium, followed by subjecting said compound to hydrolysis and dehydration polymerization to form said thin film of amorphous titania over the surface. In this embodiment, said inorganic compound of titanium is preferably TiCl 4 or Ti(SO 4 ) 2 . Alternatively, in the above methods (17) to (19), said step (a) is preferably carried out by sputtering. In case said layer comprises SiO 2 , said step of coating preferably comprises the steps of: (a) applying onto the surface a suspension comprising particles of crystalline titania and particles of silica; and, (b) heating said substrate at a temperature less than the softening point of the substrate to thereby bond particles to said substrate and to sinter particles with each other. Preferably, prior to said step of coating the substrate is coated with a thin film of silica to prevent alkaline network-modifier ions from diffusing from the substrate into said layer. In case said layer comprises SiO 2 , said step of coating may alternatively comprise the steps of: (a) applying onto the surface a suspension comprising crystalline titania particles dispersed in a precursor of amorphous silica; and, (b) subjecting said precursor to hydrolysis where necessary and to dehydration polymerization to thereby form on said surface a layer of titania particles bound by amorphous silica. In this embodiment, said precursor is preferably tetraalkoxysilane, silanol, polysiloxane having an average molecular weight of less than 3000, or a mixture thereof. In case said layer comprises SiO 2 , said step of coating may alternatively comprise the steps of: (a) applying onto the surface a suspension comprising particles of silica dispersed in a solution of an organic compound of titanium; (b) subjecting said compound to hydrolysis and dehydration polymerization to form a thin film of amorphous titania in which particles of silica are dispersed; and, (c) heating said film at a temperature less than the softening point of the substrate to thereby transform amorphous titania into crystalline titania. In this embodiment, said organic compound of titanium is preferably selected from the group consisting of an alkoxide, chelate and acetate of titanium. In case said layer comprises SiO 2 , said step of coating may alternatively comprise the steps of: (a) applying onto the surface a solution comprising an organic compound of titanium and a precursor of amorphous silica; (b) subjecting said compounds to hydrolysis and dehydration polymerisation to thereby form a thin film comprising amorphous titania and amorphous silica; and, (c) heating said film at a temperature less than the softening point of the substrate to thereby transform amorphous titania into the anatase form of titania. In this embodiment, said organic compound of titanium is preferably selected from the group consisting of an alkoxide, chelate and acetate of titanium. In this embodiment, said precursor is preferably tetraalkoxysilane, silanol, polysiloxane having an average molecular weight of less than 3000, or a mixture thereof. In case said layer comprises SnO 2 , said step of coating preferably comprises the steps of: (a) applying onto the surface a suspension comprising particles of the anatase form of titania and particles of tin oxide; and, (b) heating said substrate at a temperature of less than 900°C to thereby bond particles to said substrate and to sinter particles with each other. In case said layer comprises SnO 2 , said step of coating may alternatively comprise the steps of: (a) applying onto the surface a suspension comprising particles of tin oxide dispersed in a solution of organic compound of titanium; (b) subjecting said compound to hydrolysis and dehydration polymerization to thereby form a thin film of amorphous titania in which particles of tin oxide are dispersed; and, (c) heating said film at a temperature of less than 900°C to thereby transform amorphous titania into crystalline titania. In methods (17) to (19) said step of coating is preferably carried out such that the thickness of said layer is less than about 0.2 micrometers. An antifogging mirror for use according to the present invention is adapted to prevent moisture condensate and/or water droplets adhering on the surface thereof from fogging or blurring the mirror, said mirror comprising: a substrate with a reflective coating; and, a substantially transparent coating of silicone which is bonded to the surface of said substrate and in which particles of a photocatalytic material are uniformly dispersed; the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said coating being substituted upon photoexcitation of said photocatalytic material at least in part with hydroxyl groups whereby the surface of said coating presents a highly hydrophilic property. An antifogging lens for use according to the present invention is adapted to prevent moisture condensate and/or water droplets adhering on the surface thereof from fogging or blurring the lens, said lens comprising: a transparent lens-forming body; and, a substantially transparent coating of silicone which is bonded to the surface of said body and in which particles of a photocatalytic material are uniformly dispersed; the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said coating being substituted upon photoexcitation of said photocatalytic material at least in part with hydroxyl groups whereby the surface of said coating presents a highly hydrophilic property. An antifogging transparent sheet glass for use according to the present invention is adapted to prevent moisture condensate and/or water droplets adhering on the surface thereof from fogging or blurring the sheet glass, said sheet glass comprising: a transparent substrate; and, a substantially transparent coating of silicone which is bonded to the surface of said substrate and in which particles of a photocatalytic material are uniformly dispersed; the organic groups bonded to the silicon atoms of the silicone molecules at the surface of said coating being substituted upon photoexcitation of said photocatalytic material at least in part with hydroxyl groups whereby the surface of said coating presents a highly hydrophilic property. These features and advantages of the invention as well as other features and advantages thereof will become apparent from the following description. Brief Description of the Drawings FIG. 1 shows the energy level of the valance band and the conduction band of various semiconductor photocatalysts usable in the present invention; FIGS. 2A and 2B are schematic cross-sectional views in a microscopically enlarged scale of the photocatalytic coating formed on the surface of a substrate and showing the hydroxyl groups being chemisorbed on the surface upon photoexcitation of the photocatalyst; FIGS. 3-5, 7 and 9 are graphs respectively showing the variation, in response to time, of the contact angle with water of various specimens in the Examples as the specimens are subjected to irradiation of ultraviolet light; FIG. 6 shows Raman spectra of a surface of photocatalytic coating made of silicone; FIGS. 8 and 14 are graphs showing the result of pencil hardness tests; FIGS. 10-13 are graphs showing the contact angle with water versus the molar ratio of silica in silica-blended titania; FIGS. 15 are graphs showing the variation, in response to time, of the contact angle with water when ultraviolet light having different wavelengths is irradiated on the surface of the photocatalytic coating. Best Mode for Carrying Out the Invention A substrate having a surface requiring superhydrophilification is prepared and is coated with a photocatalytic coating. In the case where the substrate is made from a heat resistive material such as metal, ceramics and glass, the photocatalytic coating may be fixed on the surface of the substrate by sintering particles of a photocatalyst as described later. Alternatively, a thin film of the amorphous form of a precursor of the photocatalyst may be first formed on the surface of the substrate and the amorphous photocatalyst precursor may then be transformed into photoactive photocatalyst by heating and crystallization. In the case where the substrate is formed of a non heat-resistive material such as plastic or is coated with a paint, the photocatalytic coating may be formed by applying onto the surface a photooxidation-resistant coating composition containing the photocatalyst and by curing the coating composition, as described later. When an antifogging mirror is to be manufactured, a reflective coating may be first formed on the substrate and the photocatalytic coating may then be formed on the front surface of the mirror. Alternatively, the reflective coating may be formed on the substrate prior to, subsequent to or during the course of the step of coating of the photocatalyst. Photocatalyst The most preferred example of the photocatalyst usable in the photocatalytic coating used according to the invention is titania (TiO 2 ). Titania is harmless, chemically stable and available at a low cost. Furthermore, titania has a high band gap energy and, hence, requires ultraviolet (UV) light for photoexcitation. This means that absorption of the visible light does not occur during the course of photoexcitation so that the coating is free from the problem of coloring which would otherwise occur due to a complementary color component. Accordingly, titania is particularly suitable to coat on a transparent member such as glass, lens and mirror. The TiO 2 is preferably crystalline, more preferably in the form of anatase or rutile. As titania, both anatase and rutile may be used. The advantage of the anatase form of titania is that a sol in which extremely fine particles of anatase are dispersed is readily available on the market so that it is easy to make an extremely thin film. On the other hand, the advantage of the rutile form of titania is that it can be sintered at a high temperature so that a coating excellent in strength and abrasion resistivity can be obtained. Although the rutile form of titania is lower in the conduction band level than the anatase form as shown in FIG. 1, it may be used as well for the purpose of photocatalytic superhydrophilification. It is believed that, when a substrate 10 is coated with a photocatalytic coating 12 of titania and upon photoexcitation of titania by UV light, water is chemisorbed on the surface in the form of hydroxyl groups (OH - ) under the photocatalytic action as shown in FIG. 2A and, as a result, the surface becomes superhydrophilic. Other photocatalysts which can be used in the photocatalytic coating used according to the invention may include a metal oxide such as ZnO, SnO 2 , SrTiO 3 , WO 3 , Bi 2 O 3 , and Fe 2 O 3 , as shown in FIG. 1. It is believed that, similar to titania, these metal oxides are apt to adsorb the surface hydroxyl groups (OH - ) because the metallic element and oxygen are present at the surface. As shown in FIG. 2B, the photocatalytic coating may be formed by blending particles 14 of photocatalyst in a layer 16 of metal oxide. In particular, the surface can be hydrophilified to a high degree when silica or tin oxide is blended in the photocatalyst as described later. Thickness of Photocatalytic Coating In the case that the substrate is made of a transparent material as in the case of glass, a lens and a mirror, it is preferable that the thickness of the photocatalytic coating is not greater than 0.2 µm. With such a thickness, coloring of the photocatalytic coating due to the interference of light can be avoided. Moreover, the thinner the photocatalytic coating is, the more transparent the substrate can be. In addition, the abrasion resistance of the photocatalytic coating is increased with decreasing thickness. The surface of the photocatalytic coating may be covered further by an abrasion-resistant or corrosion-resistant protective layer or other functional film which is susceptible to hydrophilification. Formation of Photocatalytic Layer by Calcination of Amorphous Titania In the case that the substrate is made of a heat resistive material such as metal, ceramics and glass, one of the preferred methods for forming an abrasion resistant photocatalytic coating which exhibits the superhydrophilicity of such a degree that the contact angle with water becomes as small as 0° is to first form a coating of the amorphous form of titania on the surface of the substrate and to then calcine the substrate to thereby transform by phase transition amorphous titania into crystalline titania (i.e., anatase or rutile). Formation of amorphous titania may be carried out by one of the following methods. (1) Hydrolysis and Dehydration Polymerization of Organic Titanium Compound Alkoxide of titanium, such as tetraethoxytitanium, tetraisopropoxytitanium, tetra-n-propoxytitanium, tetrabuthoxytitanium, and tetramethoxytitanium, is used to which is added a hydrolysis inhibitor such as hydrochloric acid and ethylamine, the mixture being diluted by alcohol such as ethanol and propanol. While subjected to partial or complete hydrolysis, the mixture is applied on the surface of the substrate by spray coating, flow coating, spin coating, dip coating, roll coating or any other suitable coating method, followed by drying at a temperature ranging from the ambient temperature to 200°C. Upon drying, hydrolysis of titanium alkoxide will be completed to result in the formation of titanium hydroxide which then undergoes dehydration polymerization whereby a layer of amorphous titania is formed on the surface of the substrate. In lieu of titanium alkoxide, other organic compounds of titanium such as chelate of titanium and acetate of titanium may be employed. (2) Formation of Amorphous Titania from Inorganic Titanium Compound Acidic aqueous solution of inorganic compound of titanium such as TiCl 4 and Ti(SO 4 ) 2 is applied on the surface of the substrate by spray coating, flow coating, spin coating, dip coating, or roll coating. The substrate is then dried at a temperature of 100-200 °C to subject the inorganic compound of titanium to hydrolysis and dehydration polymerization to form a layer of amorphous titania on the surface of the substrate. Alternatively, amorphous titania may be formed on the surface of the substrate by chemical vapor deposition of TiCl 4 . (3) Formation of Amorphous Titania by Sputtering Amorphous titania may be deposited on the surface of the substrate by electron beam bombardment of a target of metallic titanium in an oxidizing atmosphere. (4) Calcination Temperature Calcination of amorphous titania may be carried out at a temperature at least higher than the crystallization temperature of anatase. Upon calcination at a temperature of 400-500 °C or more, amorphous titania may be transformed into the anatase form of titania. Upon calcination at a temperature of 600-700 °C or more, amorphous titania may be transformed into the rutile form of titania. (5) Formation of Diffusion Prevention Layer In the case that the substrate is made of glass or glazed tile which contains alkaline network-modifier ions such as sodium, it is preferable that an intermediate layer of silica and the like is formed between the substrate and the layer of amorphous titania prior to calcination. This arrangement prevents alkaline network-modifier ions from being diffused from the substrate into the photocatalytic coating during calcination of amorphous titania. As a result, superhydrophilification is accomplished to the degree that the contact angle with water becomes as small as 0°. Photocatalytic Layer of Silica-Blended Titania Another preferred method of forming an abrasion resistant photocatalytic coating which exhibits the superhydrophilicity of such a degree that the contact angle with water is equal to 0° is to form on the surface of the substrate a photocatalytic coating comprised of a mixture of titania and silica. The rate of silica to the sum of titania and silica may be 5-90 % by mol, preferably 10-70 % by mol, more preferably 10-50 % by mol. Formation of photocatalytic coating comprised of silica-blended titania may be carried out by one of the following methods. (1) A suspension containing particles of the anatase form or rutile form of titania and particles of silica is applied on the surface of the substrate, followed by sintering at a temperature less than the softening point of the substrate. (2) A mixture of a precursor of amorphous silica (e.g., tetraalkoxysilane such as tetraethoxysilane, tetraisopropoxysilane, tetra-n-propoxysilane, tetrabuthoxysilane, and tetramethoxysilane; silanol formed by hydrolysis of tetraalkoxysilane; or polysiloxane having a mean molecular weight of less than 3000) and a crystalline titania sol is applied on the surface of the substrate and is subjected to hydrolysis where desired to form silanol, followed by heating at a temperature higher than about 100°C to subject silanol to dehydration polymerization to thereby form a photocatalytic coating wherein titania particles are bound by amorphous silica. In this regard, if dehydration polymerization of silanol is carried out at a temperature higher than about 200°C, polymerization of silanol is accomplished to a high degree so that the alkali resistance of the photocatalytic coating is enhanced. (3) A suspension wherein particles of silica are dispersed in a solution of a precursor of amorphous titania (e.g., organic compound of titanium such as alkoxide, chelate or acetate of titanium; or inorganic compound of titanium such as TiCl 4 and Ti(SO 4 ) 2 ) is applied on the surface of the substrate and then the compound of titanium is subjected to hydrolysis and dehydration polymerization at a temperature ranging from the ambient temperature to 200°C to thereby form a thin film of amorphous titania wherein particles of silica are dispersed. Then, the thin film is heated at a temperature higher than the crystallization temperature of titania but lower than the softening point of the substrate to thereby transform amorphous titania into crystalline titania by phase transition. (4) Added to a solution of a precursor of amorphous titania (organic compound of titanium such as alkoxide, chelate or acetate of titanium; or inorganic compound of titanium such as TiCl 4 and Ti(SO 4 ) 2 ) is a precursor of amorphous silica (e.g., tetraalkoxysilane such as tetraethoxysilane, tetraisopropoxysilane, tetra-n-propoxysilane, tetrabuthoxysilane, and tetramethoxysilane; hydrolyzate thereof, i.e., silanol; or polysiloxane having a mean molecular weight of less than 3000) and the mixture is applied on the surface of the substrate. Then, these precursors are subjected to hydrolysis and dehydration polymerization to form a thin film made of a mixture of amorphous titania and amorphous silica. Thereafter, the thin film is heated at a temperature higher than the crystallization temperature of titania but lower than the softening point of the substrate to thereby transform amorphous titania into crystalline titania by phase transition. Photocatalytic Layer of Tin Oxide-Blended Titania Still another preferred method of forming an abrasion resistant photocatalytic coating which exhibits the superhydrophilicity of such a degree that the contact angle with water is equal to 0° is to form on the surface of the substrate a photocatalytic coating comprised of a mixture of titania and tin oxide. The rate of tin oxide to the sum of titania and tin oxide may be 1-95 % by weight, preferably 1-50 % by weight. Formation of a photocatalytic coating comprised of tin oxide-blended titania may be carried out by one of the following methods. (1) A suspension containing particles of the anatase form or rutile form of titania and particles of tin oxide is applied on the surface of the substrate, followed by sintering at a temperature less than the softening point of the substrate. (2) A suspension wherein particles of tin oxide are dispersed in a solution of a precursor of amorphous titania (e.g., organic compound of titanium such as alkoxide, chelate or acetate of titanium; or inorganic compound of titanium such as TiCl 4 and Ti(SO 4 ) 2 ) is applied on the surface of the substrate and then the compound of titanium is subjected to hydrolysis and dehydration polymerization at a temperature ranging-from the ambient temperature to 200°C to thereby form a thin film of amorphous titania wherein particles of tin oxide are dispersed. Then, the thin film is heated at a temperature higher than the crystallization temperature of titania but lower than the softening point of the substrate to thereby transform amorphous titania into crystalline titania by phase transition. Silicone Paint Containing Photocatalyst A further preferred method of forming a photocatalytic coating which exhibits the superhydrophilicity of such a degree that the contact angle with water is equal to 0° is to use a coating composition wherein particles of a photocatalyst are dispersed in a film forming element of uncured or partially cured silicone (organopolysiloxane) or a precursor thereof. The coating composition is applied on the surface of the substrate and the film forming element is then subjected to curing. Upon photoexcitation of the photocatalyst, the organic groups bonded to the silicon atoms of the silicone molecules are substituted with hydroxyl groups under the photocatalytic action of the photocatalyst, as described later with reference to Examples 13 and 14, whereby the surface of the photocatalytic coating is superhydrophilified. This method provides several advantages. Since the photocatalyst-containingsilicone paint can be cured at ambient temperature or at a relatively low temperature, this method may be applied to a substrate formed of a non-heat-resistant material such as plastics. The coating composition containing the photocatalyst may be applied whenever desired by way of brush painting, spray coating, roll coating and the like on any existing substrate requiring superhydrophilification of the surface. Superhydrophilification by photoexcitation of the photocatalyst may be readily carried out even by the sunlight as a light source. Furthermore, in the event that the coating film is formed on a plastically deformable substrate such as a steel sheet, it is possible to readily subject the steel sheet to plastic working as desired after curing of the coating film and prior to photoexcitation. Prior to photoexcitation, the organic groups are bonded to the silicon atoms of the silicone molecules so that the coating film has an adequate flexibility. Accordingly, the steel sheet may be readily deformed without damaging the coating film. After plastic deformation, the photocatalyst may be subjected to photoexcitation whereupon the organic groups bonded to the silicon atoms of the silicone molecules will be substituted with hydroxyl groups under the action of photocatalyst to thereby render the surface of the coating film superhydrophilic. The photocatalyst-containing silicone paint has a sufficient resistance against photooxidation action of the photocatalyst since it is composed of the siloxane bond. Another advantage of the photocatalytic coating made of photocatalyst-containing silicone paint is that, once the surface has been rendered superhydrophilic, the superhydrophilicity is maintained for a long period of time even if the coating is kept in the dark and that the superhydrophilicity can be restored even by the light of an indoor illumination lamp such as fluorescent lamp. Examples of the film forming element usable in the invention include methyltrichlorosilane, methyltribromosilane, methyltrimethoxysilane, methyltriethoxysilane, methyltriisopropoxysilane, methyltri-t-buthoxysilane; ethyltrichlorosilane, ethyltribromosilane, ethyltrimethoxysilane, ethyltriethoxysilane, ethyltriisopropoxysilane, ethyltri-t-buthoxysilane; n-propyltrichlorosilane,n-propyltribromosilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, n-propyltriisopropoxysilane, n-propyltri-t-buthoxysilane; n-hexyltrichlorosilane,n-hexyltribromosilane, n-hexyltrimethoxysilane, n-hexyltriethoxysilane, n-hexyltriisopropoxysilane, n-hexyltri-t-buthoxysilane; n-decyltrichlorosilane,n-decyltribromosilane, n-decyltrimethoxysilane,n-decyltriethoxysilane, n-decyltriisopropoxysilane, n-decyltri-t-buthoxysilane; n-octadecyltrichlorosilane, n-octadecyltribromosilane, n-octadecyltrimethoxysilane, n-octadecyltriethoxysilane, n-octadecyltriisopropoxysilane, n-octadecyltri-t-buthoxysilane; phenyltrichlorosilane, phenyltribromosilane, phenyltrimethoxysilane, phenyltriethoxysilane, phenyltriisopropoxysilane, phenyltri-t-buthoxysilane; tetrachlorosilane, tetrabromosilane, tetramethoxysilane, tetraethoxysilane, tetrabuthoxysilane, dimethoxydiethoxysilane; dimethyldichlorosilane, dimethyldibromosilane, dimethyldimethoxysilane, dimethyldiethoxysilane; diphenyldichlorosilane, diphenyldibromosilane, diphenyldimethoxysilane, diphenyldiethoxysilane; phenylmethyldichlorosilane, phenylmethyldibromosilane, phenylmethyldimethoxysilane, phenylmethyldiethoxysilane; trichlorohydrosilane, tribromohydrosilane, trimethoxyhydrosilane, triethoxyhydrosilane, triisopropoxyhydrosilane, tri-t-buthoxyhydrosilane; vinyltrichlorosilane, vinyltribromosilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriisopropoxysilane, vinyltri-t-buthoxysilane; trifluoropropyltrichlorosilane, trifluoropropyltribromosilane, trifluoropropyltrimethoxysilane, trifluoropropyltriethoxysilane, trifluoropropyltriisopropoxysilane, trifluoropropyltri-t-buthoxysilane; gamma-glycidoxypropylmethyldimethoxysilane, gamma-glycidoxypropylmethyldiethoxysilane, gamma-glycidoxypropyltrimethoxysilane, gamma-glycidoxypropyltriethoxysilane, gamma-glycidoxypropyltriisopropoxysilane, gamma-glycidoxypropyltri-t-buthoxysilane; gamma-nethacryloxypropylmethyldimethoxysilane, gamma-methacryloxypropylmethyldiethoxysilane, gamma-methacryloxypropyltrimethoxysilane, gamma-methacryloxypropyltriethoxysilane, gamma-methacryloxypropyltriisopropoxy silane, gamma-methacryloxypropyltri-t-buthoxysilane;gamma-aminopropylmethyldimethoxysilane, gamma-aminopropylmethyldiethoxysilane, gamma-aminopropyltrimethoxysilane,gamma-aminopropyltriethoxysilane, gamma-aminopropyltriisopropoxy silane, gamma-aminopropyltri-t-buthoxysilane; gamma-mercaptopropylmethyldimethoxysilane, gamma-mercaptopropylmethyldiethoxysilane, gamma-mercaptopropyltrimethoxysilane, gamma-mercaptopropyltriethoxysilane, gamma-mercaptopropyltriisopropoxysilane, gamma-mercaptopropyltri-t-buthoxysilane; β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltriethoxysilane; partial hydrolizate thereof; and mixtures thereof. To ensure that the silicone coating exhibits a satisfactory hardness and smoothness, it is preferable that the coating contains more than 10% by mol of a three-dimensionally cross-linking siloxane. In addition, to provide an adequate flexibility of the coating film yet assuring a satisfactory hardness and smoothness, it is preferred that the coating contains less than 60% by mol of a two-dimensionally cross-linking siloxane. Furthermore, to enhance the speed that the organic groups bonded to the silicon atoms of the silicone molecules are substituted with hydroxyl groups upon photoexcitation, it is desirable to use a silicone wherein the organic groups bonded to the silicon atoms of the silicone molecules are n-propyl or phenyl groups. In place of silicone having the siloxane bond, organopolysilazane composed of a silazane bond may be used. Addition of Antibacterial Enhancer The photocatalytic coating may be doped with a metal such as Ag, Cu and Zn. Doping of the photocatalyst with Ag, Cu or Zn may be carried out by adding a soluble salt of such metal to a suspension containing particles of the photocatalyst, the resultant solution being used to form the photocatalytic coating. Alternatively, after forming the photocatalytic coating, a soluble salt of such metal may be applied thereon and may be subjected to irradiation of light to deposit metal by photoreduction. The photocatalytic coating doped with Ag, Cu or Zn is capable of killing bacteria adhered to the surface. Moreover, such photocatalytic coating inhibits growth of microorganisms such as mold, alga and moss. As a result, the surface of a building, machine, apparatus, household, article and the like can be maintained clean for a long period. Addition of Photoactivity Enhancer The photocatalytic coating may additionally be doped with a metal of the platinum group such as Pt, Pd, Rh, Ru, Os and Ir. These metals may be similarly doped to the photocatalyst by photoreduction deposition or by addition of a soluble salt. A photocatalyst doped with a metal of the platinum group develops an enhanced photocatalytic redox activity so that decomposition of contaminants adhering on the surface will be promoted. Photoexcitation and UV Irradiation For the antifogging purpose of a transparent member such as glass, a lens and a mirror, it is preferable that the photocatalytic coating is formed of such a photocatalyst like titania that has a high band gap energy and can be photoexcited only by UV light. In that case, the photocatalytic coating does not absorb the visible light so that glass, a lens or a mirror would not be colored by a complementary color component. The anatase form of titania may be photoexcited by a UV light having a wavelength less than 387 nm, with the rutile form of titania by a UV light having a wavelength less than 413 nm, with tin oxide by a UV light having a wavelength less than 344 nm, with zinc oxide by a UV light having a wavelength less than 387 nm. As a source of UV light, a fluorescent lamp, incandescent lamp, metal halide lamp, mercury lamp or other type of indoor illumination lamp may be used. As the antifogging glass, lens or mirror is exposed to UV light, the surface thereof will be superhydrophilified by photoexcitation of the photocatalyst. In a situation where the photocatalytic coating is exposed to the sunlight as in the case of a rearview mirror of a vehicle, the photocatalyst will advantageously be photoexcited spontaneously by the UV light contained in the sunlight. Photoexcitation may be carried out, or caused to be carried out, until the contact angle, with water, of the surface becomes less than about 10°, preferably less than about 5°, particularly equal to about 0°. Generally, by photoexciting at a UV intensity of 0.001 mW/cm 2 , the photocatalytic coating will be superhydrophilified within several days to the degree that the contact angle with water becomes about 0°. Since the intensity of the UV light contained in the sunlight impinging upon the earth's surface is about 0.1-1 mW/cm 2 , the surface will be superhydrophilified in a shorter time when exposed to the sunlight. In the case that the surface of the substrate is to be self-cleaned by rainfall or to be prevented from adhesion of contaminants, the photocatalytic coating may be formed of a photocatalyst which can be photoexcited by UV light or visible light. The articles covered by the photocatalytic coating are disposed outdoors and are subjected to irradiation of the sunlight and to rainfall. When the photocatalytic coating is made of titania-containing silicone, it is preferable to photoexcite the photocatalyst at such an intensity to ensure that a sufficient amount of the surface organic groups bonded to the silicon atoms of the silicone molecules are substituted with hydroxyl groups. The most convenient method therefor is to use the sunlight. Once the surface has been made highly hydrophilic, the hydrophilicity is sustained even during the night. Upon exposure again to the sunlight, the hydrophilicity will be restored and maintained. It is preferable that the photocatalytic coating is superhydrophilified in advance before the substrate coated by the photocatalytic coating according to the invention is offered for use to the user. Examples The following Examples illustrate the industrial applicability of the invention from various aspects. Example 1 Antifogging Mirror - Antifogging Photocatalytic Coating with Interleaved Silica Layer 6 parts by weight of tetraethoxysilane Si(OC 2 H 5 ) 4 (Wako JunYaku, Osaka), 6 parts by weight of pure water, and 2 parts by weight of 36% hydrochloric acid as a hydrolysis inhibitor were added to 86 parts by weight of ethanol as a solvent and the mixture was stirred to obtain a silica coating solution. The solution was allowed to cool for about 1 hour since the solution evolved heat upon mixing. The solution was then applied on the surface of a soda-lime glass plate of 10cm square in size by the flow coating method and was dried at a temperature of 80°C. As drying proceeds, tetraethoxysilane was hydrolyzed to first form silanol Si(OH) 4 which was then underwent dehydration polymerization to form a thin film of amorphous silica on the surface of the glass plate. Then a titania coating solution was prepared by adding 0.1 parts by weight of 36% hydrochloric acid as a hydrolysis inhibitor to a mixture of 1 part by weight of tetraethoxytitanium Ti(OC 2 H 5 ) 4 (Merck) and 9 parts by weight of ethanol, and the solution was applied to the surface of the above-mentioned glass plate by the flow coating method in dry air. The amount of coating was 45 µg/cm 2 in terms of titania. As the speed of hydrolysis of tetraethoxytitanium was so high, hydrolysis of tetraethoxytitanium partially commenced during the course of coating so that formation of titanium hydroxide Ti(OH) 4 started. Then the glass plate was held at a temperature of about 150°C for 1-10 minutes to permit completion of the hydrolysis of tetraethoxy-titaniumand to subject the resultant titanium hydroxide to dehydration polymerization whereby amorphous titania was formed. In this manner, a glass plate was obtained having a coating of amorphous titania overlying the coating of amorphous silica. This specimen was then fired or calcined at a temperature of 500°C in order to transform amorphous titania into the anatase form of titania. It is considered that, due to the presence of the coating of amorphous silica underlying the coating of amorphous titania, alkaline network-modifier ions such as sodium ions being present in the glass plate were prevented from diffusing from the glass substrate into the titania coating during calcination. Then a reflective coating of aluminum was formed by vacuum evaporation deposition on the back of the glass plate to prepare a mirror to thereby obtain #1 specimen. After the #1 specimen was kept in the dark for several days, a UV light was irradiated on the surface of the specimen for about one hour at the UV intensity of 0.5 mW/cm 2 (the intensity of UV light having an energy higher than the band gap energy of the anatase form of titania, i.e., the intensity of UV light having a wavelength shorter than 387 nm) by using a 20W blue-light-black (BLB) fluorescent lamp (Sankyo Electric, FL20BLB) to obtain #2 specimen. For the purposes of comparison, a reflective coating of aluminum was formed by vacuum evaporation deposition on the back of a glass plate provided neither with silica nor titania coating, the product being placed in the dark for several days to obtain #3 specimen. The contact angle, with water, of the #2 and #3 specimens was measured by a contact angle meter (Kyowa Kaimen Kagaku K.K. of Asaka, Saitama, Model CA-X150). The resolving power at the small angle side of this contact angle meter was 1°. The contact angle was measured 30 seconds after a water droplet was dripped from a micro-syringe onto the surface of the respective specimens. In the #2 specimen, the reading of the contact angle meter, indicating the contact angle with water of the surface, was 0° so that the surface exhibited superhydrophilicity. In contrast, the contact angle with water of the #3 specimen was 30-40°. Then the #2 and #3 specimens were tested for the antifogging capability as well as to see how adherent waterdroplets would spread over the surface. Assessment of the antifogging capability was done by filling a 500 ml beaker with 300 ml of hot water of about 80°C, by thereafter placing on the beaker each specimen for about 10 seconds with the front surface of the mirror directed downwards, and by inspecting immediately thereafter the presence or absence of a fog on the surface of the specimen and inspecting how the face of the tester reflected. With the #3 specimen, the surface of the mirror was fogged by steam so that the image of the observer's face was not reflected well. However, with the #2 specimen, no fogging was observed at all and the face of the tester was clearly reflected. Assessment of the manner of adherent water droplets to spread was carried out by dripping several water droplets from a pipette onto the surface of the mirror inclined at an angle of 45°, rotating the mirror into a vertical position, and thereafter inspecting how the droplets adhered and how the face of the observer reflected. With the #3 specimen, dispersed discrete waterdroplets which were obstructive to the eye adhered on the mirror surface. As a result, the reflected image was disturbed by the refraction of light due to adherent droplets so that it was difficult to observe the reflected image with clarity. In contrast, with the #2 specimen, water droplets adhered onto the mirror surface were allowed to spread over the surface to form a uniform water film without forming discrete waterdroplets. Although a slight distortion of the reflected image due to the presence of the water film was observed, it was possible to recognize the reflected image of the tester's face with a sufficient clarity. Example 2 Antifogging Mirror - Photocatalytic Coating Comprising Silica-Blended Titania A thin film of amorphous silica was formed on the surface of a mirror (made by Nihon Flat Glass, MFL3) in a manner similar to Example 1. Then a coating solution was prepared by admixing 0.69g of tetraethoxysilane (Wako JunYaku), 1.07g of a sol of the anatase form of titania (Nissan Chemical Ind., TA-15, mean particle size of 0.01 µm), 29.88g of ethanol, and 0.36g of pure water. The coating solution was applied on the surface of the mirror by spray coating process. The mirror was held at a temperature of about 150°C for about 20 minutes to subject tetraethoxysilane to hydrolysis and dehydration polymerization to thereby form on the mirror surface a coating wherein particles of the anatase form of titania were bound by a binder of amorphous silica. The ratio by weight of titania to silica was 1. After the mirror was kept in the dark for several days, a UV light was irradiated by the BLB fluorescent lamp for about one hour at the UV intensity of 0.5 mW/cm 2 to obtain #1 specimen. As the contact angle with water of the surface of the mirror was measured by the same contact angle meter as used in Example 1, the reading of the contact angle meter was 0°. Then, in the manner similar to Example 1, the antifogging capability and the manner of adherent water droplets to spread were assessed with respect to the #1 specimen as well as to the "MFL3" mirror not provided with the photocatalytic coating. In the test for antifogging property, with the #1 specimen, no fog was observed at all and the tester's face was clearly reflected, in contrast to the "MFL3" mirror wherein a fog was observed on the surface of the mirror so that the image of the tester's face was not clearly reflected. In the inspection for the manner of adherent water droplets to spread, with the "MFL3" mirror, water droplets dispersed on the surface caused refraction of light to thereby disturb the reflected image, so that it was difficult to clearly observe the reflected image. With the #1 specimen, in contrast, water droplets adhered to the surface of the mirror were spread over the surface to form a uniform water film and, although a slight distortion was observed in the reflected image due to the presence of the water film, it was possible to recognize the reflected image of the tester's face with a sufficient clarity. Example 3 Antifogging Eyeglass Lens First, a thin film of amorphous silica was formed in a manner similar to Example 1 on both sides of an eyeglass lens commercially available on the market. Then, the coating solution similar to that of Example 2 was spray coated on both sides of the lens and the lens was held at a temperature of about 150°C for about 20 minutes to subject tetraethoxysilane to hydrolysis and dehydration polymerization to thereby form on each side of the lens a coating wherein particles of the anatase form of titania were bound by a binder of amorphous silica. After the lens was kept in the dark for several days, a UV light was irradiated by the BLB fluorescent lamp for about one hour at the UV intensity of 0.5 mW/cm 2 . When the contact angle with water of the surface of the lens was measured by the same contact angle meter as used in Example 1, the reading of the contact angle meter was 0°. This lens was mounted to the right-hand frame of eyeglasses, with an ordinary lens being mounted for the purposes of comparison to the left-hand frame. When, several hours later, the tester wore the glasses and took a bath for about 5 minutes, the ordinary lens on the left was fogged with steam so that the eyesight was lost. However, formation of fog was not observed at all on the right-hand lens coated with the photocatalytic coating that had been subjected to UV irradiation. As the tester then intentionally directed a shower on the glasses, obstructive waterdroplets adhered on the left-hand ordinary lens so that a view was interrupted. However, waterdroplets adhering on the right-hand lens promptly spread into water film so that a sufficient view was secured. Example 4 Antifogging Glass - 7 nm Thick Titania Coating A solution containing chelate of titanium was applied on the surface of a soda-lime glass plate of 10cm square in size and titanium chelate was subjected to hydrolysis and dehydration polymerization to form amorphous titania on the surface of the glass plate. The plate was then calcined at a temperature of 500° C to form a surface layer of crystals of the anatase form of titania. The thickness of the surface layer was 7 nm. The surface of the thus obtained specimen was first subjected to irradiation by a UV light for about one hour at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp. As the contact angle with water of the surface of this specimen was measured by a contact angle meter (made by ERMA, Model G-I-1000, the resolving power at the small angle side being 3°), the reading of the contact angle meter was less than 3°. Then, while irradiating by a UV light at the UV intensity of 0.01 mW/cm 2 by using a 20W white fluorescent lamp (Toshiba, FL20SW), the variation, in response to time, of the contact angle was measured. The results are plotted in the graph of FIG. 3. It will be noted from the graph that the surface of the specimen was maintained highly hydrophilic even by a weak UV light emitted from the white fluorescent lamp. This Example illustrates that the surface of the photocatalytic titania coating can be maintained highly hydrophilic even though the thickness thereof is made as extremely small as 7 nm. This is very important in preserving the transparency of a substrate such as a windowpane. Example 5 Antifogging Glass - 20 nm Thick Titania Coating A surface layer of anatase-form titania crystals was formed on the surface of a soda-lime glass plate in a manner similar to Example 4. The thickness of the surface layer was 20 nm. Similar to Example 4, the surface of the thus obtained specimen was first subjected to irradiation by a UV light for about one hour at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp, and then the variation in response to time of the contact angle was measured while subjecting to irradiation by a UV light at the UV intensity of 0.01 mW/cm 2 by using a white fluorescent lamp. The results are shown in the graph of FIG. 4. In this Example, too, the surface of the specimen was maintained highly hydrophilic by a weak UV light emitted from a white fluorescent lamp. Example 6 Antifogging Glass - Effect of Calcination Temperature of Amorphous Titania In a manner similar to Example 1, a thin film of amorphous silica was first formed on the surface of soda-lime glass plates of 10cm square in size and then a thin film of amorphous titania was coated thereon to obtain a plurality of specimens. These glass plates were then calcined at a temperature of 450°C, 475°C, 500°C, and 525°C, respectively. Upon inspection by the powder X-ray diffraction method, the presence of crystalline titania of the anatase form was detected in the specimens calcined at 475°C, 500°C, and 525°C so that transformation of amorphous titania into the anatase form crystalline titania was confirmed in these specimens. However, in the specimen calcined at 450°C, the anatase form of titania was not detected. The surface of the thus obtained specimens was first subjected to irradiation by a UV light for about three hours at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp, and then the variation in response to time of the contact angle was measured by the contact angle meter (CA-X150) while subjected to irradiation by a UV light at the UV intensity of 0.02 mW/cm 2 by using a white fluorescent lamp. The results are shown in Table 1. Contact Angle (*) Calcination Temp (°C) immed. art BLB irradn 3 days later 9 days later 14 days later 450 10 13 15 23 475 0 0 0 0 500 0 0 0 0 525 0 0 0 0 As will be apparent from Table 1, it was found that, in the specimens which were calcined at a temperature of 475°C, 500°C, and 525°C and in which the formation of anatase crystals were confirmed, the contact angle was maintained at 0° and the surface of the glass plate maintained superhydrophilic as long as irradiation of the UV light by a white fluorescent lamp was continued. In contrast, it was observed that the coating of amorphous titania of the specimen calcined at 450°C did not exhibit photocatalytic activity so that the contact angle increased as time elapsed. When a blow of breath was blown upon the specimens calcined at a temperature of 475°C, 500°C, and 525°C, no formation of fog was observed on the specimen surface. Example 7 Antifogging Glass - Effect of Alkaline Network Modifier Ion Diffusion A titania coating solution similar to Example 1 was prepared and was applied by the flow coating method on the surface of a 10cm square soda-lime glass plate. Similar to Example 1, the amount of coating was 45 µg/cm 2 in terms of titania. The glass plate was similarly held at a temperature of about 150°C for 1-10 minutes to form amorphous titania on the surface of the glass plate. The specimen was then calcined at a temperature of 500°C to transform amorphous titania into the anatase form of titania. After keeping the specimen in the dark for several days, a UV light was irradiated on the surface of the specimen for about one hour at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp. Thereafter, the contact angle with water was measured by the contact angle meter (CA-X150), which indicated a contact angle of 3°. It is considered that the reason why in this specimen the contact angle was not reduced down to 0° is that because, contrary to Example 1, the specimen of this Example was not provided with a silica layer interleaved between the glass substrate and the titania layer, the alkaline network-modifier ions such as sodium ions were allowed to diffuse from the glass substrate into the titania coating during calcination at 500°C whereby the photocatalytic activity of titania was hindered. It is therefore believed that, in order to realize the superhydrophilicity of such a degree that the contact angle with water is equal to 0°, it is preferable to provide an intermediate layer of silica as in Example 1. Example 8 Antifogging Glass - Formation of Amorphous Titania By Sputtering A film of metallic titanium was deposited by sputtering on the surface of a 10cm square soda-lime glass plate which was then calcined at a temperature of 500°C. Upon inspection by the powder X-ray diffraction method, formation of the anatase form of titania was observed on the surface of the glass plate. Obviously, metallic titanium was oxidized into anatase by calcination. Soon after calcination, the surface of the specimen was subjected to irradiation by a UV light at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp and the contact angle with water was measured by the contact angle meter (CA-X150) to monitor the variation in response to time of the contact angle. The results are shown in the graph of FIG. 5. As will be apparent from the graph, the contact angle with water was kept less than 3°. This experiment illustrates that, even in the case where the photocatalytic coating is formed by sputtering, the surface of a glass plate is maintained highly hydrophilic upon UV irradiation. Example 9 Antifogging Glass - UV Intensity of 800 Lux A thin film of amorphous silica was formed on the surface of a 10cm square soda-lime glass plate in a manner similar to Example 1. Then the coating solution of Example 2 was applied by spray coating on the surface of the glass plate. The glass plate was then held at a temperature of about 150°C for about 20 minutes whereby a coating in which particles of the anatase form of titania were bound by a binder of amorphous silica was formed on the surface of the glass plate. The ratio by weight of titania to silica was 1. After kept in the dark for several days, the glass plate was subjected to irradiation by a UV light for about one hour at the UV intensity of 0.5 mW/cm 2 by a BLB fluorescent lamp. After UV irradiation, the contact angle with water of the surface of the glass plate was measured by the contact angle meter (CA-X150) and it was found that the contact angle was 0°. Thereafter, the specimen was subjected to irradiation by a UV light for 4 days at the UV intensity of 0.004 mW/cm 2 (800 lux) by using a white fluorescent lamp. While the specimen was under UV irradiation, the contact angle at the surface thereof was maintained less than 2°. When 4 days later a blow of breath was blown upon the specimen, formation of fog was not observed. In this way, it was confirmed that, by a weak UV light available under indoor illumination achieved for example by a white fluorescent lamp, the surface of the glass plate was maintained highly hydrophilic and fogging of the glass plate was prevented. Example 10 Antifogging Glass - Effect of Silica-to-Titania Blending Ratio Next, tetraethoxysilane (Wako JunYaku), a sol of the anatase form of titania (Nissan Chemical Ind., TA-15), ethanol, and pure water were admixed in varying rate to prepare four kinds of coating solutions having different tetraethoxysilane-to-titania sol blending ratio. The rate of tetraethoxysilane to titania sol was so selected that, after tetraethoxysilane was converted into amorphous silica, the rate of silica with respect to the sum of silica plus titania was equal to 10% by mol, 30% by mol, 50% by mol, and 70% by mol, respectively. Each of the coating solutions was applied by spray coating on the surface of a 10cm square soda-lime glass plate which was then held at a temperature of about 150°C for about 20 minutes to subject tetraethoxysilane to hydrolysis and dehydration polymerization whereby a coating in which particles of the anatase form of titania were bound by a binder of amorphous silica was formed on the surface of the glass plate. After being kept in the dark for a week, the specimens were subjected to irradiation by a UV light for about one hour at the UV intensity of 0.3 mW/cm 2 by a BLB fluorescent lamp. After UV irradiation, the contact angle with water of the surface of the respective specimens was measured by the contact angle meter (CA-X150). The contact angle was 0° throughout all the specimens. Thereafter, two specimens with coatings having 30% by mol and 50% by mol of silica, respectively, were subjected to irradiation by a UV light for 3 days at the UV intensity of 0.004 mW/cm 2 by using a white fluorescent lamp. While the specimens were under irradiation, the contact angle at the surface thereof was maintained less than 3°. Example 11 Antifogging Glass - Rutile Form Photocatalytic Coating A titania coating solution was prepared by adding 0.1 part by weight of 36% hydrochloric acid as a hydrolysis inhibitor to a mixture of 1 part by weight of tetraethoxytitaniumTi(OC 2 H 5 ) 4 (Merck) and 9 parts by weight of ethanol. The solution was then applied to the surface of a plurality of quartz glass plates of 10cm square in size by the flow coating method in dry air. The amount of coating was 45 µg/cm 2 in terms of titania. The glass plates were then held at a temperature of about 150°C for 1-10 minutes to subject tetraethoxytitaniumto hydrolysis and dehydration polymerization whereby a coating of amorphous titania was formed on the surface of each glass plate. These specimens were then calcined at temperatures of 650°C and 800°C, respectively, to subject amorphous titania to crystallization. Upon inspection by the powder X-ray diffraction method, it was found that the crystal form of the specimen calcined at 650°C was of the anatase form while the crystal form of the specimen calcined at 800°C was of the rutile form. After keeping the thus obtained specimens in the dark for a week, they were subjected to irradiation by a UV light for 2 days at the UV intensity of 0.3 mW/cm 2 by a BLB fluorescent lamp. After UV irradiation, the contact angle was measured. The contact angle with water of the surface was 0° throughout all the specimens. It will be understood from the foregoing that a surface can be maintained highly hydrophilic not only in the case that the photocatalyst is the anatase form of titania but also in the case that the photocatalyst is the rutile form. For this reason, it seems that the phenomenon of photocatalytic superhydrophilification is not altogether the same as the photocatalytic redox reaction. Example 12 Antifogging Glass - Transmittance Test In a manner similar to Example 1, a thin film of amorphous silica was first formed on the surface of a soda-lime glass plate of 10cm square in size and then a thin film of amorphous titania was coated thereon. The glass plate was then calcined at a temperature of 500°C to transform amorphous titania into the anatase form of titania. The specimen thus obtained was kept in the dark for several days. Then the specimen was placed in a desiccator (24°C in temperature and 45-50% in humidity) housing a BLB fluorescent lamp and was subjected to irradiation by a UV light for one day at the UV intensity of 0.5 mW/cm 2 to obtain #1 specimen. The contact angle with water of the #1 specimen as measured was 0°. Then the #1 specimen was taken out of the desiccator and was promptly positioned above a warm bath held at 60°C and transmittance was measured 15 seconds later. The transmittance as measured was divided by the initial transmittance to calculate a change in transmittance caused by a fog formed by condensation of steam. In a manner similar to Example 7, the surface of a glass plate was coated by the anatase form of titania to obtain #2 specimen. The #2 specimen was placed in the desiccator and was subjected to irradiation by a UV light at the UV intensity of 0.5 mW/cm 2 until the contact angle with water became equal to 3°. The #2 specimen was then placed in a dark place. The #2 specimen was taken out of the dark place at different time points and each time the contact angle with water was measured. In addition, the #2 specimen was first placed each time in the desiccator (24°C in temperature and 45-50% in humidity) until the temperature was equalized whereupon, in a manner similar to the #1 specimen, the #2 specimen was promptly placed above the warm bath held at 60°C and the transmittance was measured 15 seconds later to derive a change in transmittance caused by a fog formed by condensation of steam. For the purposes of comparison, the contact angle with water was measured with respect to commercially marketed flat glass, acrylic resin plate, polyvinylchloride (PCV) plate and polycarbonate (PC) plate, respectively. In addition, each of these materials was placed in the desiccator of the same condition to equalize the temperature and was then promptly placed above the warm bath held at 60°C, the transmittance being similarly measured 15 seconds later whereby a change in transmittance caused by a fog formed by condensation of steam was calculated. The results are shown in Table 2. Specimen Contact Angle with Water (°) Change in Transmittance (%) #1 0 100 #2 (3 hrs later) 5.0 100 #2 (6 hrs later) 7.7 100 #2 (8 hrs later) 8.2 100 #2 (24 hrs later) 17.8 89.8 #2 (48 hrs later) 21.0 88.5 #2 (72 hrs later) 27.9 87.0 Flat Glass 40.6 45.5 Acrylic Resin Plate 64.5 60.6 PVC Plate 75.3 44.7 PC Plate 86.0 49.0 As will be apparent from Table above, it was confirmed that an extremely high antifogging capability could be achieved if the contact angle with water was not greater than 10°. Example 13 (Reference) Photocatalyst-Containing Silicone Coating This Example is related to the discovery that a coating of a certain high molecular weight compound and containing a photocatalyst is rendered highly hydrophilic when subjected to irradiation by a UV light. As substrates, aluminum plates of 10cm square in size were used. Each of the substrates was first coated with a silicone layer to smooth the surface. To this end, a first component "A" (silica sol) and a second component "B" (trimethoxymethylsilane) of the coating composition "Glaska" marketed by Japan Synthetic Rubber Co. (Tokyo) were mixed with each other in such a manner that the ratio by weight of silica to trimethoxymethylsilane was equal to 3. The resultant coating mixture was applied on the aluminum substrates and was subjected to curing at a temperature of 150°C to obtain a plurality of aluminum substrates (#1 specimens) each coated with a base coating of silicone of 3 µm in thickness. Then, the #1 specimens were coated with a high-molecular-weight coating composition containing a photocatalyst. In order to prevent a film forming element of the coating composition from being degraded by photooxidation action of the photocatalyst, silicone was selected as the film forming element. More specifically, a sol of the anatase form of titania (Nissan Chemical Ind., TA-15) and the first component "A" (silica sol) of the above-mentioned "Glaska" were admixed. After dilution by ethanol, the above-mentioned second component "B" of "Glaska" was further added thereto to prepare a titania containing coating composition. The coating composition was comprised of 3 parts by weight of silica, 1 part by weight of trimethoxymethylsilane, and 4 parts by weight of titania. The coating composition was applied onto the surface of the #1 specimen and was cured at a temperature of 150°C to obtain #2 specimen coated with a top coating wherein particles of the anatase form of titania were dispersed throughout a coating film of silicone. Then the #2 specimen was subjected to irradiation by a UV light for 5 days at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp to obtain #3 specimen. When the contact angle with water of the surface of this specimen was measured by the contact angle meter (made by ERMA), surprisingly the reading of the contact angle meter was less than 3°. The contact angle of the #2 specimen measured prior to UV irradiation was 70°. The contact angle of the #1 specimen as measured was 90°. Then, the #1 specimen was subjected further to irradiation by a UV light for 5 days under the same condition as the #2 specimen and the contact angle thereof was measured, the contact angle as measured being 85°. From the foregoing, it has been discovered that, notwithstanding the fact that silicone inherently is substantially hydrophobic, silicone is rendered highly hydrophilic when it contains a photocatalyst and provided that the photocatalyst is photoexcited by irradiation by a UV light. Example 14 (Reference) Raman Spectroscopic Analysis By-using a mercury lamp, the #2 specimen of Example 13 was subjected to irradiation by a UV light for 2 hours at the UV intensity of 22.8 mW/cm 2 to obtain #4 specimen. The #2 specimen prior to UV irradiation and the #4 specimen subsequent to UV irradiation were subjected to Raman spectroscopic analysis. For the purposes of comparison, a UV light was irradiated upon the #1 specimen under the same conditions and the specimen was subjected to Raman spectroscopic analysis prior to and subsequent to UV irradiation. Raman spectra are shown in the graph of FIG. 6. In the graph of FIG. 6, the Raman spectra of the #1 specimen prior to and subsequent to UV irradiation are shown by the single curve #1 because they are identical. Referring to the graph of FIG. 6, in the Raman spectrum of the #2 specimen, a dominant peak is noted at the wavenumber 2910cm -1 corresponding to the symmetrical stretching of the C-H bond of the sp 3 hybrid orbital and a salient peak is observed at the wavenumber 2970cm -1 indicating the inverted symmetrical stretching of the C-H bond of the sp 3 hybrid orbital. It can therefore be concluded that the C-H bonds are present in the #2 specimen. In the Raman spectrum of the #4 specimen, no peak is found at the wavenumbers 2910cm -1 and 2970cm -1 . Instead, a broad absorption band peaking at the wavenumber 3200cm -1 and corresponding to the symmetrical stretching of the O-H bond is observed. It is therefore concluded that, in the #4 specimen, there is no C-H bond but, instead, the O-H bonds are present. In contrast, in the Raman spectrum of the #1 specimen, a dominant peak at the wavenumber 2910cm -1 corresponding to the symmetrical stretching of the C-H bond of the sp 3 hybrid orbital as well as a salient peak at the wavenumber 2970cm -1 corresponding to the inverted symmetrical stretching of the C-H bond of the sp 3 hybrid orbital are noted throughout the points of time prior to and subsequent to UV irradiation. Accordingly, it is confirmed that the C-H bonds are present in the #1 specimen. From the foregoing, it is considered that, when silicone which contains a photocatalyst is subjected to irradiation by a UV light, the organic groups bonded to the silicon atoms of the silicone molecules as represented by the general formula (1) below are substituted with the hydroxyl groups under the action of the photocatalyst so that a derivative of silicone is formed at the surface as shown by the formula (2). where R represents alkyl or aryl group. Example 15 Antifogging Plastic Plate - Antifogging Coating of Photocatalyst-Containing Silicone The surface of a plastic substrate was first coated with a silicone layer to prevent the substrate from being degraded by the photocatalyst. To this end, a coating solution was prepared in a manner similar to Example 13 by admixing the first and second components "A" and "B" of the above-mentioned "Glaska" of Japan Synthetic Rubber Co. such that the ratio by weight of silica to trimethoxymethylsilanewas equal to 3. The coating solution was applied on the surface of 10cm-square acrylic resin plates and was then cured at a temperature of 100°C to obtain a plurality of acrylic resin plates (#1 specimens) each coated with a base coating of silicone of 5 µm in thickness. Next, a sol of the anatase form of titania (Nissan Chemical Ind., TA-15) and the first component "A" of the above-mentioned "Glaska" were admixed and, after diluted by ethanol, the second component "B" of "Glaska" was added thereto to prepare four kinds of coating solutions having different compositions. The compositions of these coating solutions were such that the ratio by weight of titania to the sum of titania plus silica plus trimethoxymethylsilanewas equal to 5%, 10%, 50%, and 80%, respectively. These coating solutions were applied, respectively, onto the surface of the acrylic resin plates coated with the silicone layer and were cured at a temperature of 100°C to obtain #2-#5 specimens each coated with a top coating wherein particles of the anatase form of titania were dispersed throughout a coating film of silicone. Then the #1-#5 specimens were subjected to irradiation by a UV light by a BLB fluorescent lamp for maximum 200 hours at the UV intensity of 0.5 mW/cm 2 and the contact angle with water of the surface of these specimens was measured by the contact angle meter (made by ERMA) at different time points to see the variation in response to time of the contact angle. The results are shown in the graph of FIG. 7. As will be understood from the graph of FIG. 7, in the #1 specimen which was not provided with the titania-containing coating, no appreciable change in the contact angle with water was resulted by UV irradiation. In contrast, in the #2-#5 specimens provided with the titania-containing top coating, it will be noted that upon UV irradiation the surface was rendered hydrophilic to the degree that the contact angle with water became less than 10°. In particular, it will be understood that, in the #3-#5 specimens wherein the titania content was greater than 10% by weight, the contact angle with water became less than 3°. Furthermore, it will be noted that in the #4 and #5 specimens having the titania content of 50% by weight and 80% by weight, respectively, the contact angle with water became less than 3° within short time of UV irradiation. When a blow of breath was blown upon the #4 specimen, no formation of fog was observed. After keeping the #4 specimen in the dark for 2 weeks, the contact angle with water was measured by the contact angle meter (CA-X150) and was found to be less than 3°. Example 16 (Reference) Pencil Scratch Test Pencil scratch test was conducted to ascertain the abrasion resistance of the titania-containing top coating. In a manner similar to Example 15, a plurality of 10cm-square acrylic resin plates were first coated with a base coating of silicone of 5 µm in thickness and were then coated with a top coating having varying titania content. The titania content of the top coating was 50% by weight, 60% by weight, and 90% by weight, respectively. According to the method H8602 of the Japanese Industrial Standard (JIS), the surface of the specimens was scratched by various pencil leads to find a hardest pencil lead by which the top coating was peeled off. A similar test was also conducted for a specimen which was coated only with the base coating. The results are shown in the graph of FIG. 8. The top coating having the titania content of 90% by weight was peeled off by a pencil lead of hardness 5B, but the top coating having the titania content of 60% by weight was able to withstand a pencil lead of hardness H and showed an adequate abrasion resistance. Obviously, the abrasion resistance of the top coating increases with decreasing titania content. Example 17 (Reference) Effect of Coating Thickness In a manner similar to Example 13, 10cm-square aluminum plates were first coated with a base coating of silicone of 5 µm in thickness and were then coated with an anatase-containing top coating of varying thickness to obtain a plurality of specimens. The thickness of the. top coating of the #1 specimen was 0.003 µm, the thickness of the top coating of the #2 specimen being 0.1 µm, the thickness of the top coating of the #3 specimen being 0.2 µm, the thickness of the top coating of the #4 specimen being 0.6 µm, and the thickness of the top coating of the #5 specimen being 2.5 µm. While subjecting the respective specimens to irradiation by a UV light at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp, the variation in response to time of the contact angle with water of the surface of the specimens was measured by the contact angle meter (made by ERMA). The results are shown in the graph of FIG. 9. As will be apparent from the graph of FIG. 9, regardless of the thickness of the coating, the surface of the respective specimens was rendered highly hydrophilic within 50 hours of UV irradiation to the degree that the contact angle with water became less than 3°. It will be noted in particular that, even with the titania-containing top coating of the thickness of less than 0.2 µm, a sufficientphotocatalytic activity was achieved to the degree that the top coating surface was rendered highly hydrophilic. In this regard, it is known that a transparent layer is colored due to interference of light when the thickness of the layer exceeds 0.2 µm. This Example illustrates that, by limiting the thickness of the top coating to 0.2 µm or less, the surface of the top coating can be made highly hydrophilic while preventing coloring thereof due to interference of light. Example 18 (Reference) Highly Hydrophilic Photocatalytic Coating of Titania-Containing Silicone In a manner similar to Example 13, a 10cm-square aluminum plate was first coated with a base coating of silicone of 5 µm in thickness. Then, a sol of the anatase form of titania (Nissan Chemical Ind., TA-15) and the second component "B" (trimethoxymethylsilane) of the above-mentioned "Glaska" were admixed with each other and the mixture was diluted by ethanol to prepare a coating composition containing titania. The ratio by weight of trimethoxymethylsilane to titania was equal to 1. The coating composition was applied onto the surface of the aluminum plate and was cured at a temperature of 150°C to form a top coating wherein particles of the anatase form of titania were dispersed throughout a coating film of silicone. The thickness of the coating was 0.1 µm. Then the specimen was subjected to irradiation by a UV light for a day at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp. When the contact angle with water of the surface of this specimen was measured by the contact angle meter (CA-X150), the reading of contact angle was 0°. The specimen was kept in the dark for 3 weeks and the contact angle with water was measured each week. The measured contact angle is shown in Table 3. immed. after irradiation 1 week later 2 weeks later 3 weeks later 0° 2° 1° 3° As will be understood from Table 3, once the surface has been superhydrophilified, superhydrophilicity will be sustained for a substantially long time period even in the absence of photoexcitation. Example 19 (Reference) Antibacterial Enhancer - Ag-Added Photocatalyst In a manner similar to Example 1, a thin film of amorphous silica and a thin film of amorphous titania were formed in sequence on the surface of a 10cm-square soda-lime glass plate and the glass plate was then calcined at a temperature of 500°C to transform amorphous titania into the anatase form titania whereby #1 specimen was obtained. Then an aqueous solution containing 1 weight percent of silver lactate was applied onto the surface of the #1 specimen and the specimen was subjected to irradiation by a UV light for one minute by operating a 20W BLB fluorescent lamp positioned at a distance of 20 cm from the specimen whereby #2 specimen was obtained. Upon UV irradiation, silver lactate underwent photoreduction to form silver deposit and the surface of the specimen was rendered hydrophilic under the photocatalytic action of titania. The #1 specimen was also subjected to UV irradiation under the same conditions. When the contact angle with water of the #1 and #2 specimens was measured by the contact angle meter (made by ERMA), the contact angle in both specimens was less than 3°. When a blow of breath was blown upon these specimens, no formation of fog was observed. For the purposes of comparison the substrate of soda-lime glass as such was tested and it was found that the contact angle with water was 50° and a fog was readily formed upon blowing of breath. Then, the #1 and #2 specimens as well as the soda-lime glass plate as such were tested for antibacterial capability. A liquid culture prepared by shake cultivating colibacillus ( Escherichia coli W3110 stock) for a night was subjected to centrifugal washing and was diluted with sterilized distilled water by 10,000 times to prepare a bacteria containing liquid. 0.15 ml of the bacteria containing liquid (equivalent to 10000-50000 CFU) was dripped on three slide glasses which were then brought into intimate contact with the #1 and #2 specimens and the soda-lime glass plate, respectively, which had previously been sterilized by 70% ethanol. These specimens and plate were then subjected to irradiation of a light of a white fluorescent lamp from in front of the slide glasses for 30 minutes at the intensity of 3500 lux. Thereafter, the bacteria containing liquid of respective specimens was wiped by a sterilized gauze and was recovered in 10 ml of physiological saline and the liquid thus recovered was applied for inoculation on a nutrient agar plate for culture at 37°C for a day. Thereafter, the colonies of colibacillus formed on the culture was counted to calculate the survival rate of colibacillus. The result was that in the #1 specimen and the soda-lime glass plate the survival rate of colibacillus was greater than 70%, but the survival rate was less than 10% in the #2 specimen. This experiment demonstrates that, when the photocatalyst is doped by Ag, the surface of the substrate is not only rendered highly hydrophilic but also is made to exhibit antibacterial function. Example 20 Antibacterial Enhancer - Cu-Added Photocatalyst In a manner similar to Example 1, a thin film of amorphous silica was formed, respectively, on the surface of 10cm-square soda-lime glass plates to obtain a plurality of #1 specimens. Then, similar to Example 1, a thin film of amorphous titania was formed on the surface of the #1 specimen which was then calcined at a temperature of 500°C to transform amorphous titania into the anatase form titania. Then an ethanol solution containing 1 weight percent of copper acetate was applied by spray coating onto the surface of the specimen and, after drying, the specimen was subjected to irradiation by a UV light for one minute by a 20W BLB fluorescent lamp positioned at a distance of 20 cm from the specimen to thereby subject copper acetate to photoreduction deposition to obtain #2 specimen wherein crystals of titania were doped with copper. As inspected by the eye, the #2 specimen presented an adequate light transmittance. A soda-lime glass plate as well as the #2 specimen and the #1 specimen (without titania coating) immediately after fabrication were tested for antifogging capability and the contact angle with water measured. The antifogging test was done by blowing a blow of breath upon the specimen to produce a fog on the specimen surface and by inspecting the presence and absence of particles of moisture condensate by a microscope. The contact angle was measured by the contact angle meter (made by ERMA). The results are shown in Table 4. Immediately After Preparation of Specimen Contact Angle with Water (°) Antifogging Property #2 Specimen 10 no fog #1 Specimen 9 no fog Soda-Lime Glass 50 fogged Further, after being subjected to irradiation by a UV light for a month at the UV intensity of 0.5 mW/cm2 by a BLB fluorescent lamp, the #2 and #1 specimens and the soda-lime glass plate were tested in a similar manner for antifogging capability and contact angle. The results are shown in Table 5. After 1 Month of UV Irradiation contact Angle with Water (°) Antifogging Property #2 Specimen 3 no fog #1 Specimen 49 fogged Soda-Lime Glass 53 fogged Then, the #2 and #1 specimens immediately after preparation and the soda-lime glass plate were tested for antibacterial capability in a manner similar to Example 19. The result was that in the soda-lime glass plate and the #1 specimen the survival rate of colibacillus was greater than 70%, but the survival rate was less than 10% in the #2 specimen. Next, the #2 and #1 specimens immediately after preparation and the soda-lime glass plate were tested for deodorizing performance. The specimens were placed, respectively, in a desiccator of 11 liters in volume made of UV permeable quartz glass and nitrogen gas containing methyl mercaptan was introduced therein in such a manner that the methyl mercaptan concentration equaled 3 ppm. A 4W BLB fluorescent lamp was placed within the desiccator at a distance of 8 cm from the respective specimens to irradiate the specimens at the UV intensity of 0.3 mW/cm 2 . By sampling gas in the desiccator 30 minutes later, the methyl mercaptan concentration was measured by gas chromatography and the removal rate of methyl mercaptan was calculated. With the #1 specimen and the soda-lime glass plate, the removal rate of methyl mercaptan was less than 10%. In contrast, the removal rate of the #2 specimen was more than 90% so that a good deodorizing performance was achieved. Example 21 Antibacterial Enhancer - Cu-Added Photocatalyst The first and second components "A" (silica sol) and "B" (trimethoxymethylsilane)of "Glaska" of Japan Synthetic Rubber Co. were admixed such that the ratio by weight of silica to trimethoxymethylsilane was equal to 3, and the mixture was applied on the surface of a 10cm-square acrylic resin plate, followed by curing at a temperature of 100°C to obtain an acrylic resin plate coated with a base coating of silicone of 3 µm in thickness. Then, a sol of the anatase form of titania (TA-15) and an aqueous solution containing 3 weight percent of copper acetate were mixed and, after adding further the first component "A" (silica sol) of "Glaska" thereto, the mixture was diluted by propanol. Then the second component "B" of "Glaska" was further added to prepare a titania-containing coating composition. The coating composition was comprised of 3 parts by weight of silica, 1 part by weight of trimethoxymethylsilane, 4 parts by weight of titania, and 0.08 parts by weight of copper acetate in terms of metallic copper. The coating composition was applied onto the surface of the acrylic resin plate and was cured at a temperature of 100°C to form a top coating. Then the specimen was subjected to irradiation by a UV light for 5 days at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp to obtain #1 specimen. The #1 specimen and the acrylic resin plate were investigated for antifogging capability, contact angle with water, antibacterial performance and deodorizing function, in a manner similar to Example 20. In the acrylic resin plate, the contact angle with water was 70° and a fog was formed as a blow of breath was blown upon. In the #1 specimen, however, the contact angle with water was 3-9° and formation of fog did not occur. With regard to antibacterial property, in the acrylic resin plate the survival rate of colibacillus was greater than 70%, whereas the survival rate was less than 10% in the #1 specimen. Regarding the deodorizing property, while the removal rate of methyl mercaptan by the acrylic resin plate was less than 10%, the removal rate by the #1 specimen was more than 90%. Example 22 (Reference) Photo-Redox Activity Enhancer - Pt-Added Photocatalyst In a manner similar to Example 1, a thin film of amorphous silica and then a thin film of amorphous titania were formed on the surface of a 10cm-square soda-lime glass plate and the glass plate was then calcined at a temperature of 500°C to transform amorphous titania into the anatase form titania. Then, 1 ml of aqueous solution of chloroplatinic acid 6-hydrate H 2 PtCl 6 ·6H 2 O containing 0.1 weight percent of platinum was applied onto the specimen which was then subjected to irradiation by a UV light for one minute at the UV intensity of 0.5 mW/cm 2 by a BLB fluorescent lamp to thereby form deposit of platinum by photoreduction of chloroplatinic acid hexahydrate to obtain a specimen wherein crystals of titania were doped with platinum. The specimen thus obtained was left as such for a day and was thereafter subjected to irradiation by a UV light for a day at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp. The contact angle measured after UV irradiation was 0° . Example 23 (Reference) The #2 specimen of Example 13 was subjected to irradiation by a UV light for 10 hours at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp to obtain #3 specimen. When the contact angle with water of the surface of this specimen was measured by the contact angle meter (made by ERMA), the reading of the contact angle meter was less than 3°. fouled by oil or fat can be readily cleansed only by soaking it in water without recourse to a detergent, provided that the surface thereof is provided with a photocatalytic coating and if the photocatalyst is photoexcited by UV irradiation. Example 24 Drying of Water Wet Surface The surface of the #1 specimen of Example 13 and #3 specimen of Example 23 were wetted with water and the specimens were left outdoors on a fair day to subject them to natural drying. The ambient temperature was about 25°C. As the #1 specimen was inspected 30 minutes later, water droplets still remained on the surface. In contrast, it was found that the surface of the #3 specimen was completely dried. It is considered that in the #3 specimen provided with the photocatalytic coating, the adherent water droplets were caused to spread into a uniform film of water and for this reason drying was accelerated. This Example illustrates the possibility that an eyeglass lens or automotive windshield wetted with water may be promptly dried. Example 25 Tile with Highly Hydrophilic Surface - Coating of Sintered Titania and Silica A sol of the anatase form of titania (Ishihara Industries of Osaka, STS-11) and a sol of colloidal silica (Nissan Chemical Ind., "Snowtex O") were admixed at a ratio by mol of 88:12 in terms of solid matter and the mixture was applied by spray coating on the surface of a glazed tile (Toto Ltd., AB02E01) of 15cm square in size, followed by sintering for 1 hour at a temperature of 800°C to obtain a specimen covered by a coating comprised of titania and silica. The thickness of the coating was 0.3 µm. The contact angle with water immediately after sintering was 5°. The specimen was kept in the dark for a week but the contact angle measured thereafter was still 5°. As the specimen surface was subjected to irradiation by a UV light for 1 day at the UV intensity of 0.03 mW/cm 2 by using a BLB fluorescent lamp, the contact angle with water became 0°. Example 26 (Reference) Coating of Sintered Titania and Silica - Hydrophilification under Room Light A sol of the anatase form of titania (STS-11) and a sol of colloidal silica (Nissan Chemical Ind., "Snowtex 20") were admixed at a ratio by mol of 80:20 in terms of solid matter and the mixture was applied by spray coating on the surface of a 15cm-square glazed tile (AB02E01), followed by sintering for 1 hour at a temperature of 800°C to obtain a specimen covered by a coating comprised of titania and silica. The thickness of the coating was 0.3 µm. The contact angle with water immediately after sintering was 5°. The contact angle with water as measured after keeping the specimen in the dark for 2 weeks was 14°. As the specimen surface was subjected to irradiation by a UV light for 1 day at the UV intensity of 0.004 mW/cm 2 by a white fluorescent lamp, the contact angle with water became 4°. Accordingly, it was found that the photocatalytic coating was rendered hydrophilic to a satisfactory degree even under indoor illumination. Example 27 (Reference) Coating of Sintered Titania and Silica - Silica Content A sol of the anatase form of titania (STS-11) and a sol of colloidal silica (Nissan Chemical Ind., "Snowtex 20") were admixed at a varying ratio to obtain a plurality of suspensions having a ratio by mol of silica to the solid matter of the suspension of 0%, 5%, 10%, 15%, 20%, 25% and 30%, respectively. 0.08g of each suspension was uniformly applied by spray coating on the surface of a 15cm-square glazed tile (AB02E01) and each tile was fired for 1 hour at a temperature of 800°C to obtain a plurality of specimens each covered by a coating comprised of titania and silica. The contact angle with water immediately after sintering of the respective specimens was as shown in the graph of FIG. 10. As will be apparent from the graph of FIG. 10, the initial contact angle was lowered by addition of silica. The contact angle with water as measured after keeping the specimen in the dark for 8 days was plotted in the graph of FIG. 11. As will be noted by comparing the graph of FIG. 10 with the graph of FIG. 11, the loss of hydrophilicity resulting from keeping the specimens in the dark is small in the specimens containing more than 10%, in the ratio by mol, of silica. Thereafter, the specimens were subjected to irradiation by a UV light for 2 days at the UV intensity of 0.03 mW/cm 2 by using a BLB fluorescent lamp. The contact angle with water after irradiation is shown in the graph of FIG. 12. It will be noted from the graph that upon UV irradiation hydrophilicity is readily recovered in the case where silica is added to titania. Then the specimens were kept in the dark for further 8 days and the contact angle with water was measured. The results are shown in FIG. 13. It will be noted from the graph that the loss of hydrophilicity resulting from keeping the specimens in the dark after UV irradiation is small in the case where silica is added to titania. A pencil scratch test was carried out to examine the abrasion resistance of the sintered film comprised of titania and silica. The results are shown in the graph of FIG. 14. It will be understood that the abrasion resistivity is increased with increasing silica content. Example 28 (Reference) Coating of Sintered Titania and Tin Oxide - Glazed Tile A sol of the anatase form of titania (STS-11) and a sol of tin oxide (Taki Chemical K.K. of Kakogawa City, Hyogo-Prefecture; mean crystallite size of 3.5 nm) were admixed at various blending ratio (percent by weight of tin oxide to the sum of titania plus tin oxide) shown in Table 11 and the mixtures were applied by spray coating on the surface of 15cm-square glazed tiles (AB02E01), followed by sintering for 10 minutes at a temperature either of 750°C or 800°C to obtain #1-#6 specimens. After sintering, the #2, #4, #5 and #6 specimens were further doped with silver by applying thereon an aqueous solution containing 1 weight percent of silver nitrate and by subjecting silver nitrate to photoreduction deposition. In addition, #7-#9 specimens were further prepared by applying onto the glazed tiles only a sol of tin oxide or a sol of titania and by sintering. After sintering, the #7 and #9 specimens were further doped with silver. Each specimen was kept in the dark for a week and was thereafter subjected to irradiation by a UV light for 3 days at the UV intensity of 0.3 mW/cm 2 by using a BLB fluorescent lamp whereupon the contact angle with water was measured. The results are shown in Table 6. Specimen SnO 2 Ratio (wt %) Sintering Temp. (°C) Ag Contact Angle (°) #1 1 800 None 0 #2 5 800 Added 0 #3 15 800 None 0 #4 15 750 Added 0 #5 50 750 Added 0 #6 95 800 Added 5 #7 100 750 Added 8 #8 0 800 None 11 #9 0 800 Added 14 As will be apparent from Table 6, in the #8 and #9 specimens which were coated only with titania, the contact angle with water exceeded 10°. This is because the alkaline network-modifier ions such as sodium ions diffused from the glaze into the titania coating during sintering whereby the photocatalytic activity of anatase was hindered. In contrast, it will be noted that, in the #1-#6 specimens wherein SnO 2 were blended, the surface was hydrophilifiedto a high degree. As shown by the #7 specimen, tin oxide which is a semiconductor photocatalyst is effective in rendering the surface hydrophilic in a manner similar to titania. Although the reason therefor is not clear, this Example illustrates that the effect of diffusion of the alkaline network-modifier ions can be overcome by adding tin oxide to titania. Example 29 (Reference) Sintered Titania Coating and Diffusion Prevention Layer - Glazed Tile Tetraethoxysilane (marketed by Colcoat, "Ethyl 28") was applied by spray coating on the surface of a 15cm-square glazed tile (AB02E01) which was then held at a temperature of about 150°C for about 20 minutes to subject tetraethoxysilane to hydrolysis and dehydration polymerization whereby a coating of amorphous silica was formed on the surface of the glazed tile. Then, a sol of the anatase form of titania (STS-11) was applied by spray coating on the surface of the tile which was then fired for an hour at a temperature of 800°C. The thus obtained specimen, as well as the #8 specimen of Example 28 tested for the purposes of comparison, were kept in the dark for a week and were then subjected to irradiation by a UV light for 1 day at the UV intensity of 0.3 mW/cm 2 by using a BLB fluorescent lamp whereupon the contact angle with water was measured. In contrast to the contact angle with water being 12° in the #8 specimen of Example 28, the specimen provided with the intervening layer of amorphous silica was hydrophilified to the degree that the contact angle with water became less than 3°. It is therefore considered that the layer of amorphous silica is effective in preventing diffusion of the alkaline network-modifier ions being present in the glaze layer. Example 30 (Reference) Amorphous Titania Calcination Coating and Diffusion Prevention Layer - Glazed Tile In a manner similar to Example 1, a thin film of amorphous silica and then a thin film of amorphous titania were formed in sequence on the surface of a 15cm-square glazed tile (AB02E01). The tile was then calcined at a temperature of 500°C to transform amorphous titania into the anatase form titania. The specimen thus obtained was kept in the dark for several days and was then subjected to irradiation by a UV light for 1 day at the UV intensity of 0.5 mW/cm 2 by using a BLB fluorescent lamp. The contact angle with water of the resultant specimen as measured was 0°. Similar to Example 29, it is considered that the layer of amorphous silica is effective in rendering the surface of a tile highly hydrophilic. Example 31 (Reference) Glazed Tile - Antibacterial Enhancer (Ag Doping) A coating comprised of titania and silica was formed on the surface of a 15cm-square glazed tile (AB02E01) in a manner similar to Example 25. Then an aqueous solution containing 1 weight percent of silver lactate was applied onto the surface of the tile which was then subjected to irradiation by a UV light of a BLB fluorescent lamp to thereby subject silver lactate to photoreduction to form a silver deposit whereby a specimen coated with silver doped'titania was obtained. The contact angle with water as measured was 0°. When the tile was then tested for the antibacterial function in a manner similar to Example 19, the survival rate of colibacillus was less than 10%. Example 32 (Reference) Glazed Tile - Antibacterial Enhancer (Cu Doping) A coating comprised of titania and silica was formed on the surface of a 15cm-square glazed tile (AB02E01) in a manner similar to Example 25. Then an aqueous solution containing 1 weight percent of copper acetate monohydrate was applied onto the surface of the tile which was then subjected to irradiation by a UV light of a BLB fluorescent lamp to thereby subject copper acetate monohydrate to photoreduction to form a copper deposit whereby a specimen coated with copper-doped titania was obtained. The contact angle with water as measured was less than 3°. As the tile was then tested for the antibacterial function in a manner similar to Example 19, the survival rate of colibacillus was less than 10%. Example 33 (Reference) Glazed Tile - Photo-Redox Activity Enhancer A coating comprised of titania and silica was formed on the surface of a 15cm-square glazed tile (AB02E01) in a manner similar to Example 25. Then, the surface of the specimen was doped with platinum in a manner similar to Example 22. The contact angle with water as measured was 0°. The removal rate of methyl mercaptan as measured in a manner similar to Example 20 was 98%. Example 34 (Reference) Effect of Photoexciting Wavelength After being kept in the dark for 10 days, the #8 specimen of Example 28 and, for the purposes of comparison, the glazed tile (AB02E01) without titania coating were subjected to irradiation by a UV light by using a Hg-Xe lamp under the conditions shown in Table 7 and on doing so the variation in response to time of the contact angle with water was measured. UV Wavelength (nm) UV Intensity (mW/cm 2 ) Photon Density (photon/sec/cm 2 ) 313 10.6 1.66 X 10 16 365 18 3.31 X 10 16 405 6 1.22 X 10 16 The results of measurement were shown in FIGS. 15A-15C, wherein the value plotted by white dots represents the contact angle with water of the #8 specimen of Example 28 and the value plotted by black dots indicates the contact angle with water of the glazed tile which was not provided with the titania coating. As will be understood from FIG. 15C, hydrophilification did not occur in the case that a UV light having an energy lower than that of a wavelength of 387 nm corresponding to the bandgap energy of the anatase form of titania (i.e., a UV light having a wavelength longer than 387 nm) was irradiated. In contrast, as will be apparent from FIGS. 15A and 15B, the surface was rendered hydrophilic upon irradiation by a UV light having an energy higher than. the bandgap energy of anatase. From the foregoing, it was confirmed that hydrophilification of a surface is closely related to photoexcitation of the photo-semiconductor. Example 35 Plastic Plate Coated by Photocatalyst-Containing Silicone A titania-containing coating composition similar to that of Example 18 was applied on a polyethyleneterephthalate(PET) film (Fuji Xerox, monochromatic PPC film for OHP, JF-001) and was cured at a temperature of 110°C to obtain #1 specimen coated with titania-containing silicone. Further, an aqueous polyester paint (made by Takamatsu Resin, A-124S) was applied on another PET film (JF-001) and was cured at 110°C to form a primer coating. A titania-containing coating composition similar to that of Example 18 was then applied on the primer coating and was cured at a temperature of 110°C to obtain #2 specimen. Also, a titania-containing coating composition similar to that of Example 18 was applied on a polycarbonate (PC) plate and was cured at a temperature of 110°C to obtain #3 specimen. Furthermore, an aqueous polyester paint (A-124S) was applied on another polycarbonate plate, followed by curing at a temperature of 110°C to form a primer coating, and a titania-containing coating composition similar to that of Example 18 was thereafter applied thereon followed by curing at a temperature of 110°C to obtain #4 specimen. The #1-#4 specimens as well as the PET film (JF-001) and polycarbonate plate as such were subjected to irradiation by a UV light at the UV intensity of 0.6 mW/cm 2 by using a BLB fluorescent lamp and on doing so the variation in response to time of the contact angle with water of the specimen surface was measured. The results are shown in Table 8. Specimen Before Irradiat. 1 day later 2 days later 3 days later 10 days later #1 71° 44° 32° 7° 2° #2 73° 35° 16° 3° 2° #3 66° 55° 27° 9° 3° #4 65° 53° 36° 18° 2° PET 70° 72° 74° 73° 60° PC 90° 86° 88° 87° 89° As will be apparent from Table 8, the surface of the specimens under question was hydrophilified as UV irradiation was continued and about 3 days later the surface is rendered superhydrophilic. As described hereinbefore with reference to Example 14, it is considered that this is due to the fact that the organic groups bonded to the silicon atoms of the silicone molecules of the titania-containing silicone layer were substituted with the hydroxyl groups under the photocatalytic action caused by photoexcitation. As is well-known, a UV intensity of 0.6 mW/cm 2 is roughly equal to the intensity of the UV light contained in the sunlight impinging upon the earth's surface. It will be noted, accordingly, that superhydrophilification can be achieved simply by exposing the titania-containing silicone coating to the sunlight. Example 36 (Reference) Weathering Test of Photocatalyst-Containing Silicone The #1 specimen (aluminum substrate coated with silicone) and the #2 specimen (aluminum substrate coated with titania-containing silicone) of Example 13 were subjected to a weathering test by using a weathering testing machine (made by Suga Testing Instruments, Model "WEL-SUN-HC") while irradiating a light from a carbon arc lamp and spraying rain for 12 minutes per hour and at a temperature of 40°C. The weather resistivity was assessed by the glossiness retention rate (percentage of the glossiness after testing to the initial glossiness). The results are shown in Table 9. Specimen 500 hrs 1000 hrs 3000 hrs #1 91 95 90 #2 99 100 98 As will be apparent from Table 9, the glossiness retention rate remained roughly the same regardless of the presence or absence of titania. This indicates that the siloxane bonds forming the main chain of the silicone molecule were not broken by the photocatalytic action of titania. It is therefore considered that the weather resistivity of silicone is not affected even after the organic groups bonded to the silicon atoms of the silicone molecules are substituted with the hydroxyl groups.

Claims: Use of a composite comprising a substrate and a photocatalytic layer coated thereon, said photocatalytic layer comprising a photocatalytic semiconductor material having a surface which has been rendered hydrophilic by photoexcitation with sunlight, for antifogging said surfaces. Use according to claim 1, wherein said hydrophilic surface has a water wettability of less than 10° in terms of the contact angle with water. Use according to claim 1, wherein said hydrophilic surface has a water wettability of less than 5° in terms of the contact angle with water. Use according to claim 1, wherein said hydrophilic surface has a water wettability of 0° in terms of the contact angle with water. Use according to any of claims 1 to 4, wherein said photocatalytic layer further comprises SiO 2 or silicone. Use according to any of claims 1 to 5, wherein said photocatalytic layer is further coated with a protective layer which is susceptible to being made hydrophilic. Use according to any of claims 1 to 6, wherein said photocatalytic layer further comprises a metal selected from the group consisting of Ag, Cu and Zn. Use according to any of claims 1 to 7, wherein said photocatalytic layer further comprises a metal selected from the group consisting of Pt, Pd, Rh, Ru, Os and Ir. Use according to any of claims 1 to 8, wherein said substrate is transparent. Use according to any of claims 1 to 9, wherein the substrate is a mirror, a lens or a transparent sheet member. Use according to any of claims 1 to 10, wherein the substrate is made of glass containing alkaline network-modifier ions and wherein a thin film for preventing said ions from diffusing from said substrate into said photocatalytic layer is interleaved between said substrate and said layer. Use according to claim 11, wherein said thin film for preventing said ions from diffusing from said substrate into said photocatalytic layer is a layer of silica. Use according to any of claims 1 to 12, wherein said photocatalytic semiconductor material is selected from the group consisting of TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO 3 , Bi 2 O 3 and Fe 2 O 3 . Use according to claim 13, wherein said TiO 2 is in the form of anatase. Use according to any of claims 1 to 14, wherein said photocatalytic layer consists of TiO 2 in the form of anatase. Use according to any of claims 1 to 15, wherein said photocatalytic layer has a thickness of less than 0.2 µm.

Inventors: CHIKUNI MAKOTO, FUJISHIMA AKIRA, HASHIMOTO KAZUHITO, HAYAKAWA MAKOTO, KITAMURA ATSUSHI, KOJIMA EIICHI, MACHIDA MITSUYOSHI, NORIMOTO KEIICHIRO, WATANABE TOSHIYA, CHIKUNI, MAKOTO, FUJISHIMA, AKIRA, HASHIMOTO, KAZUHITO, HAYAKAWA, MAKOTO, KITAMURA, ATSUSHI, KOJIMA, EIICHI, MACHIDA, MITSUYOSHI, NORIMOTO, KEIICHIRO, WATANABE, TOSHIYA, Hayakawa, Makoto, c/o Toto Ltd., KOJIMA, EIICHI, C/O TOTO LTD.
Applicants: TOTO LTD, TOTO LTD.
Application year: 1996
Main classification: C09K3
IPC codes: A47G1, C04B41, B32B27, B08B3, B05D5, A61F9, C23C14, C03C17, G02C7, B01J23, C09D183, E06B7, B08B17, B32B9, B01J19, B01J21, F28F13, B01J37, B60J1, C09K3, A61C19, C08J7, C09D5, G02B1, F21V3, A61L2, B01J35, C09D1, G02B5