Method for deposition of titanium oxide by a plasma source

The present invention relates to a process for depositing a coating based on titanium oxide with photocatalytic properties on a substrate, in particular transparent. It also targets the substrate thus obtained. Glass-based, ceramic or glass-ceramic substrates are known, more particularly glass, in particular transparent, which are provided with coatings with photocatalytic properties, with a view to manufacturing glazing of various applications, such as utility glazing, glazing for vehicles. or for buildings. The photocatalytic properties imparted to the substrate due to the coating based on titanium oxide give the latter an anti-fouling function. More particularly, the substrate retains appearance and surface properties over time, which in particular make it possible to space cleaning times and / or to improve visibility, by succeeding in eliminating progressively the soiling gradually deposited at the surface of the substrate, in particular soiling of organic origin such as fingerprints or volatile organic products present in the atmosphere, or even soiling of the fogging type. It will be recalled that this cleaning results from the fact that certain semiconductor materials, based on metal oxide, such as for example titanium oxide, are capable, under the effect of radiation of adequate wavelength (in the visible and / or in the ultraviolet), to initiate radical reactions causing the oxidation of organic products: we generally speak of "photocatalytic" or even "photo-reactive" materials. Among the processes commonly used for depositing such a coating, numerous techniques are known: • by decomposition of metal oxide precursors such as for example titanium (pyrolysis techniques: liquid pyrolysis, powder pyrolysis, pyrolysis in value phase called CVD (Chemical Vapor Deposition), techniques associated with sol-gel: soaking or dipping, cell-coating, etc.), • by a vacuum technique (reactive or non-reactive sputtering). The first pyrolysis decomposition technique, which is economical, offers satisfactory results but suffers from a lack of flexibility. Indeed, the decomposition of the precursors takes place directly at the level of the float line, and any modification as to the nature of the substrate (composition, property) on which the coating is deposited necessarily requires a slow adaptation of the loading conditions for raw material. float. In addition, it may be noted under the drawbacks of this deposition technique, a certain difficulty in controlling the surface temperatures and the temperatures of the means for injecting the precursors (nozzle), high temperatures being one of the most important parameters. more fundamental to obtaining a titanium oxide coating having optimal photocatalytic properties. In addition, due to the heat treatment, the deposition can only be carried out on substrates that are not very sensitive to temperature (for example plastic). The second technique mentioned above, called vacuum deposition (using a magnetron line for example), necessarily requires a heat treatment phase of the titanium oxide coating previously deposited under vacuum in order to allow obtaining 'an adequate crystallographic phase. This heating is difficult to implement directly within the magnetron which is under vacuum and it is then necessary to carry out a recovery operation outside the deposit enclosure. Indeed, for the latter to have photocatalytic properties, the titanium oxide should be in an anatase crystallized form, in rutile form or in the form of a mixture of anatase, rutile, brookite with a rate crystallization of at least 25%, especially around 30 to 80%, especially near the surface, (the property being rather a surface property). (The amount by weight of crystallized T O2 relative to the total weight amount of TIO2 in the coating is understood by crystallization rate). The object of the invention is therefore to perfect a process for depositing photo-catalytic coatings on the substrate, which have a marked “anti-fouling” effect on the substrate and which can be manufactured from industrially, which does not have the drawbacks of the previously mentioned techniques. Quite surprisingly, the inventors have discovered that it is possible to use the technique known as PECVD for (Plasma Enhanced Chemical Vapor Deposition) to deposit a photocatalytic coating on a glass substrate or not. This particular technique is known for depositing titanium oxide for waveguide applications (US5295220), or for fission product trapping applications (FR2695507). The subject of the invention is therefore a method of depositing on a substrate a coating based on semiconductor materials based on metal oxides, in particular titanium oxide, which are capable, under the effect of radiation of adequate wavelength, initiating radical reactions causing the oxidation of organic products so as to confer photocatalytic properties on said coating, which is characterized in that the coating with photocatalytic property is deposited by chemical deposition in the gas phase, in particular from a mixture of gas comprising at least one organometallic precursor and / or a metal halide of said metal oxide, the deposition being assisted by a plasma source. Thanks to these provisions and in particular to the use of a plasma source to dissociate the metal oxide precursor, it is possible to obtain a photocatalytic coating which does not necessarily require heat treatment at high temperature (beyond 300-350 ° C) to reveal the desired properties, during or after deposition and which is also very flexible, optimal deposition conditions no longer depend on the presence of a nearby heat source (float). In . preferred embodiments of the invention, it is optionally possible to have recourse in addition to one and / or the other of the following arrangements: at least one carrier gas or a mixture of carrier gases chosen from air, nitrogen, helium, argon. An oxidizing agent or a mixture of oxidizing agents is incorporated into the gas mixture. A reducing agent or a mixture of reducing agents is incorporated into the gas mixture. The reaction and deposition phase is carried out at reduced pressure. the reaction and deposition phase is carried out at atmospheric pressure. At least one undercoat is deposited prior to the coating with photocatalylic property, making it possible to provide another functionality to said coating with photocatalytic property and / or to reinforce said properties of said coating. - At least one other type of mineral material is incorporated into the gas mixture comprising at least the organometallic precursor and / or a metal halide of said metal oxide, especially in the form of an amorphous or partially crystallized oxide, for example a silicon oxide ( or mixture of oxides), titanium, tin, zirconium, vanadium, antimony, zinc, tungsten, cobalt, nickel, aluminum, these oxides can be mixed or doped. The photocatalytic coating is deposited on the substrate within the plasma discharge. The photocatalytic property coating is deposited on the substrate outside the plasma discharge. According to another aspect of the invention, it also relates to a substrate obtained according to the previously mentioned method, as well as its variants. This glass-based, ceramic or vitro-ceramic, or plastic substrate provided on at least part of at least one of its faces with a coating with photocatalytic property comprising at least partially crystallized titanium oxide, is characterized in that the crystallized titanium oxide is in anatase form, in rutile form, in brookite or in the form of a mixture of anatase, rutile, brookite. In preferred embodiments of the invention, it is optionally possible to have recourse to one and / or the other of the following arrangements: the crystallized titanium oxide is in the form of crystallites of average size between 0 , 5 and 60 nm, preferably 1 to 50. the coating also comprises an inorganic material, in particular in the form of an oxide or mixture of amorphous or partially crystallized oxides of the silicon oxide, titanium oxide, tin oxide, zirconium oxide, aluminum oxide, oxide vanadium, antimony, zinc, tungsten, cobalt, nickel, these oxides can be mixed or doped. the coating comprises additives capable of extending the photocatalytic phenomenon due to titanium oxide, in particular by increasing the absorption band of the coating and / or by increasing the number of charge carriers by doping the crystal lattice of the oxide or by surface doping of the coating and / or by increasing the yield and kinetics of the photocatalytic reactions by covering at least part of the coating with a catalyst. the crystal lattice of the titanium oxide is doped, in particular by at least one of the metallic or non-metallic elements, - the thickness of the coating is between 5 nm and 1 micron, preferably from 5 to 100 nm the photocatalytic activity of the coating is at least 5.10-³ cm-¹ min ^~1 measured using the TAS test, the RMS roughness of the photocatalytic coating is between 2 and 20 nm, in particular between 5 and 20 nm. the light reflection of the photocatalytic coating is less than 30%, preferably less than or equal to 20%, with a neutral color. the absorption of the photocatalytic coating is less than 10%, preferably less than 5%, it is arranged under the coating with photocatalytic property at least one thin layer with anti-static, thermal, optical function or which acts as a barrier to the migration of alkalis of the substrate. the thin layer with anti-static function, optionally with controlled polarization, and / or thermal and / or optical is based on conductive material of the metal type or of the doped metal oxide type such as ITO, SnO: F, SnO₂ : Sb, ZnO: ln, ZnO: F, ZnO: AI, ZnO: Sn or sub-stoichiometric oxygen metal oxide such as Snθ2-χ θu Znθ2-x with x <2. - the thin layer with an optical function is based on an oxide or a mixture of oxides whose refractive index is intermediate between that of the coating and that of the substrate, in particular chosen from the following oxides: AI2O3 , Snθ2, ln2θ3, or based on oxycarbide or silicon oxynitride, or possibly based on a mixture of a material with high refractive index with a material with low refractive index (AI2O3 / TiO₂, HAVE₂θ3 / SiO₂, AI2O3 / SnO, SnO₂/ TiO ....) the thin or multilayer layer with an alkali barrier function is for example based on silicon oxide, nitride, oxynitride or oxycarbide, AbOβ: F or Snθ2: F of aluminum nitride, silicon nitride. the substrate is transparent, flat or curved, the substrate is a glass substrate. the substrate is based on a polymer-based substrate, in particular PMMA, polycarbonate, PEN. According to another aspect of the invention, it relates to a glazing “anti-fouling and / or anti-fogging”, monolithic, multiple of the double type. -glazing or laminated incorporating the substrate as previously described for the production of "a self-cleaning glazing", anti-fog and / or anti-fouling, of the organic and / or mineral soiling type, in particular glazing for the building of the double-glazing type, glazing for vehicles of the windshield, rear or side window type of automobile, trains, airplanes, or utility glazing such as aquarium glasses, display cases, greenhouse, interior furnishings, street furniture, or mirrors, television screens, electrically controlled variable absorption glazing ... Other characteristics and advantages of the invention will appear during the following description of one of its embodiments, given by way of nonlimiting example. According to a preferred embodiment, it is proposed to deposit on a transparent substrate based on glass, ceramic or glass-ceramic, in particular glass, or a polymer-based substrate, provided on at least part of at least one of its faces. a coating with photocatalytic property comprising for example titanium oxide at least partially crystallized. Titanium oxide is preferably crystallized "in situ" during the formation of the coating on the substrate. According to the invention, there is further provided under this coating at least one thin layer forming a barrier to the migration of alkalis from the substrate. Titanium oxide is in fact part of the semiconductors which, under the action of light in the visible range or ultraviolet, degrade organic products which deposit on their surface. Choosing titanium oxide to manufacture glazing with an “anti-fouling” effect is therefore particularly suitable, and all the more so since this oxide has good mechanical and chemical resistance: to be effective for a long time, it is obviously important that the coating retains its integrity, even when it is directly exposed to numerous attacks, in particular when mounting the glazing on site (building) or on production line (vehicle), which involves repeated handling by mechanical or pneumatic gripping means, and also once the glazing is in place, with risks of abrasion (wiper, abrasive cloth) and contact with aggressive chemicals (atmospheric pollutants such as SO2, cleaning product, etc.). It has also been observed, in particular in the case of crystallization in anatase form, that the orientation of the growing T O2 crystals on the substrate had an influence on the photo-catalytic performances of the oxide: there is a preferred orientation (1, 1, 0) which clearly promotes photocatalysis. The coating is produced in such a way that the crystallized titanium oxide which it contains is in the form of “crystallites”, at least near the surface, that is to say of single crystals, having a size average between 0.5 and 100 nm, preferably 1 to 50 nm. It is indeed in this dimension range that titanium oxide seems to have an optimal photocatalytic effect, probably because the crystallites of this size develop a large active surface. The coating can also comprise, in addition to the crystallized titanium oxide, at least one other type of mineral material, in particular in the form of an amorphous or partially crystallized oxide, for example a silicon oxide (or mixture of oxides), titanium, tin, zirconium or aluminum. This mineral material can also participate in the photocatalytic effect of crystallized titanium oxide, by itself presenting a certain photocatalytic effect, even weak compared to that of crystallized Tiθ2, which is the case of tin or amorphous titanium oxide. A “mixed” oxide layer thus combining at least partially crystallized titanium oxide with at least one other oxide may be advantageous from the optical point of view, especially if the other or the other oxides are chosen with a lower index. to that of TiO₂ : by lowering the “overall” refractive index of the coating, one can play on the light reflection of the substrate provided with the coating, in particular lowering this reflection. This is the case if, for example, a layer of T.O2 / AI2O3 is chosen, one method of obtaining which is described in patent EP-0 465 309, or in Tiθ2 / Siθ2. It is necessary, of course, that the coating however contains a content of TIO2 sufficient to maintain a notable photocatalytic activity and that T O2 remains crystallized. It is therefore considered that it is preferable for the coating to contain at least 40% by weight, in particular at least 50% by weight of TiO 2 relative to the total weight of oxide (s) in the coating. To amplify the photocatalytic effect of titanium oxide of the coating according to the invention, it is first possible to increase the absorption band of the coating, by incorporating into the coating other elements, in particular metallic and based on cadmium, tin, tungsten, zinc, cerium, or zirconium, possibly doped as well as non-metallic elements. It is also possible to increase the number of charge carriers by doping the crystal lattice of the titanium oxide, by inserting therein at least one of the following metallic elements: niobium, tantalum, iron, bismuth, cobalt, nickel, copper, ruthenium, cerium, molybdenum, or non-metallic elements (nitrogen, carbon fluorine) Quite surprisingly, the coating actually presents not one property but two , as soon as it is exposed to adequate radiation such as in the visible and / or ultraviolet range, such as solar radiation: by the presence of photocatalytic titanium oxide, as already seen, it promotes the gradual disappearance, as and when they accumulate, of dirt of organic origin, by causing their degradation by a process of radical oxidation. Mineral soils are not degraded by this process: they therefore remain on the surface, and, apart from certain crystallizations, they are partly easily removed since they no longer have reason to adhere to the surface, the sticky organic agents being degraded by photocatalysis. However, the coating of the invention, which is permanently self-cleaning, preferably also has an outer surface with a pronounced hydrophilic and / or oleophilic character, which has three very advantageous effects: • a hydrophilic character allows perfect wetting of the water that can settle on the coating. When a phenomenon of water condensation occurs, instead of a deposit of water droplets in the form of a mist which impairs visibility, there is in fact a thin continuous film of water which forms on the surface of the coating. and which is completely transparent. This “anti-fog” effect is notably demonstrated by measuring a contact angle with water of less than 5 ° after exposure to light, and,

• after runoff of water, rain in particular, on a surface not treated with a photocatalytic layer, many drops of rain water remain attached to the surface and leave, once evaporated, unsightly and annoying traces of origin mainly mineral. Indeed, a surface exposed to the ambient air quickly becomes covered with a layer of dirt which limits its wetting by water. These soils are added to the other soils, in particular mineral ones (crystallizations, ...) brought by the atmosphere in which the glazing bathes. In the case of a photoreactive surface, these mineral soils are not directly degraded by photocatalysis. In fact, they are largely eliminated thanks to the hydrophilic nature induced by the photocatalytic activity. This hydrophilic nature in fact causes perfect spreading of the raindrops. Evidence of evaporation is therefore no longer present. In addition, the other mineral soils present on the surface are washed, or redissolved in the case of crystallization, by the film of water and therefore largely removed. We obtain an "mineral anti-fouling" effect, in particular induced by rain,

• together with a hydrophilic character, the coating can also have an oleophilic character, allowing the "wetting" of organic dirt which, as for water, then tend to be deposited on the coating in the form of a continuous film less visible than well localized "spots". There is thus obtained an “organic anti-fouling” effect which takes place in two stages: as soon as it is deposited on the coating, the soiling is already hardly visible. Then, gradually, it disappears by radical degradation initiated by photo-catalysis. The coating can be chosen to have a more or less smooth surface. A certain roughness can be sought: • it makes it possible to develop a larger active photocatalytic surface and therefore it induces greater photocatalytic activity, • it has a direct influence on the wetting. The roughness indeed enhances the wetting properties. A smooth hydrophilic surface will be even more hydrophilic when roughened. The term “roughness” is understood here to mean both the surface roughness and the roughness induced by a porosity of the layer or of the sublayer in at least part of its thickness. The above effects will be all the more marked when the coating is porous and rough, hence a superhydrophilic effect of the rough photoreactive surfaces. However, too pronounced, the roughness can be penalizing by favoring the incrustation, the accumulation of dirt and / or by making appear a level of blurring unacceptable optically. It has thus proved to be advantageous to adapt the method of depositing coatings based on T.O2 so that they have a roughness of approximately 2 to 20 nm, preferably 5 to 15 nm, this roughness being evaluated by atomic force microscopy, by measuring the value of the mean square deviation (known as Root Mean Square or RMS in English) over a surface of 1 square micrometer. With such roughness, the coatings have a hydrophilic character resulting in a contact angle with water which can be less than 1 °. It has also been found that it is advantageous to promote a certain porosity in the thickness of the coating. Thus, if the coating consists only of TIO2, it preferably has a porosity of the order of 65 to 99%, in particular from 70 to 90%, the porosity being defined here indirectly by the percentage of the density theoretical of TiO2, which is about 3.8. The thickness of the coating according to the invention is variable, it is preferably between 5 nm and 1 micron, preferably between 5 and 100 nm, in particular between 10 and 80 nm, or between 15 and 50 nm. In fact, the choice of thickness may depend on different parameters, in particular on the envisaged application of the glazing type substrate, or on the size of the UO2 crystallites in the coating or on the presence of alkalies in high proportion in the substrate. It is also possible to envisage a stack of “anti-reflection” layers alternating thin layers with high and low indices, the coating according to the invention constituting the last layer of the stack. In this case, it is preferable that the coating has a relatively low refractive index, which is the case when it is made of a mixed oxide of titanium and silicon. The layer with an anti-static and or thermal function (heating by providing it with current leads, low-emissivity, anti-sun, etc.) can in particular be chosen based on a conductive material of the metal type, such as l silver, or of the metal oxide type doped such as indium oxide doped with tin ITO, tin oxide doped with a halogen of the fluorine type Snθ2: F, or with antimony Snθ2: Sb, or zinc oxide doped with indium ZnO: ln, fluorine ZnO: F, aluminum ZnO: AI or tin ZnO: Sn. It can also be metallic oxides substoichiometric in oxygen, such as Snθ2-x or ZnO∑x with x <2. The layer with anti-static function preferably has a square resistance value of 20 to 1000 ohms / square. Provision may be made to provide it with current leads in order to polarize it (supply voltages for example between 5 and 100V). This controlled polarization makes it possible in particular to combat the deposit of dust of size on the order of a millimeter capable of being deposited on the coating, in particular adherent dry dust only by electro-static effect: by brutally reversing the polarization of the layer, "Ejects" this dust. The thin layer with an optical function can be chosen in order to reduce the light reflection and / or make the color in reflection of the substrate more neutral. In this case, it preferably has an intermediate refractive index between that of the coating and that of the substrate and an appropriate optical thickness, and can be made of an oxide or a mixture of oxides of the aluminum oxide type. AI2O3, tin oxide SnO₂₎ indium oxide ln2θ3, oxycarbide or silicon oxynitride. To obtain maximum attenuation of the color in reflection, it is preferable that this thin layer has an index of refraction close to the square root of the product of the squares of the indices of refraction of the two materials which surround it, that is to say say the substrate and the coating according to the invention. At the same time, it is advantageous to choose its optical thickness (that is to say the product of its geometric thickness and its refractive index) close to lambda / 4, lambda being approximately the average wavelength in the visible range, especially around 500 to 550 nm. The thin layer with an alkali barrier function may in particular be chosen based on silicon oxide, nitride, oxynitride or oxycarbide, in aluminum oxide containing fluorine A OsiF, or in aluminum nitride or based on SnO₂. In fact, it has proven to be useful when the substrate is made of glass, since the migration of sodium ions into the coating according to the invention can, under certain conditions, alter its photocatalytic properties. The nature of the substrate or the sub-layer is also of additional interest: it can promote the crystallization of the photocatalytic layer which is deposited, in particular in the case of CVD deposition assisted by a plasma source, preferably at reduced pressure, or even more preferably at atmospheric pressure (called in English APPECVD (Atmospheric Pressure Plasma Enhanced Chemical Vapor Deposition) All of these optional thin layers can, in known manner, be deposited by vacuum techniques of the sputtering type or by other techniques of the thermal decomposition type such as pyrolysis in solid, liquid or gas phase. Each of the above-mentioned layers can combine several functions, but they can also be superimposed. The subject of the invention is also "anti-fouling" glazing.

(organic and / or mineral soiling) and / or "anti-fogging", whether they are monolithic, multiple insulators of the double-glazing or laminated type, and which incorporate the coated substrates previously described. The invention therefore relates to the manufacture of glass, ceramic or vitro-ceramic products, and in particular the manufacture of "self-cleaning" glazing. These can advantageously be building glazing, such as double glazing (it is then possible to have the coating “outside side” and / or “inside side”, that is to say on face 1 and / or on face 4). This is particularly interesting for glazing that is difficult to access for cleaning and / or that needs to be cleaned very frequently, such as roof glazing, airport glazing, etc. It can also be glazing for vehicles where maintaining visibility is an essential safety criterion. This coating can thus be arranged on windshields, lateral or rear windows of the car, in particular on the face of the glazing turned towards the interior of the passenger compartment. This coating can then prevent the formation of fogging, and / or remove traces of soiling of the fingerprint type, nicotine or organic material of the volatile plasticizer type "released" by the plastic coating the interior of the passenger compartment, in particular that of the dashboard (release sometimes known under the English term "fogging"). Other vehicles such as planes or trains may also find advantage in using glazing provided with the coating of the invention. Many other applications are possible, especially for aquarium glasses, shop windows, greenhouses, verandas, glasses used in interior or street furniture, but also mirrors, television screens, the field of eyewear or any architectural material of the facade, cladding, roofing material type such as tiles The invention thus makes it possible to functionalize these known products, by giving them anti-ultraviolet, anti-fouling, bactericidal, anti-reflection, anti-static, anti-microorganism, ... properties. Another advantageous application of the coating according to the invention consists in associating it with an absorption glazing. electrically controlled variable of the electrochromic, photovoltaic glazing type, liquid crystal glazing optionally with dichroic dye, glazing with suspended particle system, viologene glazing ... All these glazings being generally made up of a plurality of transparent substrates between which the “active” elements are arranged, it is then advantageously possible to arrange the coating on the external face of at least one of these substrates. In particular in the case of electrochromic glazing, when the latter is in the colored state, its absorption leads to a certain heating on the surface, which, in fact, is capable of accelerating the photocatalytic decomposition of the carbonaceous substances depositing on the coating according to the invention. For more details on the structure of an electrochromic glazing, advantageously reference will be made to patent application EP-A-0 575 207 describing an electrochromic laminated double glazing, the coating according to the invention can preferably be arranged in side 1. This metal oxide coating is therefore produced using the so-called APPECVD technique which consists of chemical deposition in the gas phase, in particular from a mixture of gases comprising at least one organometallic precursor and / or a metal halide of said oxide. metallic (titanium oxide for example in our case), the deposition being assisted by a plasma source. The photocatalytic semiconductor material chosen is titanium oxide. There are others that can be used. Reference may be made to the applicant's patent (FR2738813). The semiconductor material can be doped (N, F, Pt, Pd, Metals ...) to improve its photocatalytic performance or adapt the optical gap and thus be adapted to different wavelengths of the solar spectrum (UV, visible) . The gas mixture used incorporates an organometallic precursor and / or a metal halide. For titanium oxide, mention may be made of TiCI, TiPT, Ti ethoxide (butoxide, etc.), Ti diisopropoxide (acetylacetonate), Titanium (III) tris (2,2,6,6-tetramethyl-3 , 5-heptanedionate. This gaseous mixture can also incorporate at least one oxidant or a mixture of oxidants (air, O₂, CO₂, N2O, organic: alcohol, ester ....) or at least one reducing agent or a mixture of reducing agent (H2, hydrocarbons ...) and the carrier gas used is air, nitrogen, helium or argon or a mixture of these gases. Preferably, it will mainly consist of helium and / or nitrogen and / or argon. For dopants or mixed deposits, the same ranges of precursors (organometallic / halides) can be used for metals. For fluorine, we will use for example trifluoroacetic acid (TFA), HF, NF3 ... For nitrogen, we can use NH3 or amines (primary, secondary or tertiary). We can also use precursors containing both titanium and the dopant (for example: Tétrakisdiethylamino titane, Tétrakisdimethylamino titane or tetrachlorodiamminotitanium ...) The reaction gas mixture is then dissociated, by a plasma source, either directly within the plasma, either remotely, blown (indirectly). The metal oxide with photocatalytic property is deposited continuously and uniformly in at least part of at least one of the faces of the substrate. The substrate and the deposition zone incorporating the plasma source having a relative displacement. It may also be advantageous to deposit the coating not at once, but by at least two successive stages, which seems to favor the crystallization of titanium oxide over the entire thickness of the coating when it is chooses relatively thick. Similarly, it may be advantageous to subject the coating with photo-catalytic property, a post-deposition heat treatment or a plasma treatment, once the Tiθ2 has been formed, in order to improve its rate of crystallization. The treatment temperature chosen can also allow better control of the crystallization rate and the crystalline, anatase and / or rutile nature of the oxide. The deposition process which is the subject of the invention is advantageous because the plasma source can be sufficient to provide thermal energy (without having to heat the substrate) sufficient to obtain the desired crystallographic properties at the level of the metal oxide. deposited, substantially less than 300 ° C for a glass substrate, substantially less than 130 ° C for a plastic substrate (for example PMMA, polycarbonate, PEN). In this case, a barrier layer will be interposed between the substrate and the photocatalytic property layer. However, in the case of a soda-lime glass substrate, multiple and prolonged annealing can favor an attenuation of the photocatalytic activity because of too great migration of the alkalis from the substrate to the photoreactive layer. The use of a barrier layer between the substrate, if it is standard glass, and the coating, or the choice of a glass substrate of suitable composition, or the choice of a soda-lime glass whose surface is dealkalized, eliminating this risk. According to a variant of the invention, the coating comprises additives capable of extending the photocatalytic phenomenon due to titanium oxide, by avoiding the recombination of charge carriers in the material. By way of example, a transparent, clear silica-soda-lime glass substrate 4 mm thick is used. It goes without saying that the invention is not limited to this specific type of glass. The glass may also not be flat, but curved. According to a first embodiment, a layer of TiO2 from TiCl4 is deposited by homogeneous discharge operating at atmospheric pressure. The thin layer of TiO2 has been deposited on a clean glass substrate heated to a temperature of 260 ° C. The gas mixture which is introduced consists of helium (He) and oxygen (O₂). The respective flow rates of these gases are 14 slpm and 1 sccm. The organometallic precursor, titanium tetrachloride (TiCU), is poured into a 0.5 I bubbler. This bubbler is heated to 10 ° C. and a carrier gas (He) is injected into the bubbler with a flow rate of 140 sccm for transport organometallic vapors. At equilibrium, the total pressure in the reactor is maintained at 1013 mbar ± 50 mbar. The electrodes, covered with an alumina dielectric barrier (0.5 mm ± 0.1 mm), are 5 mm apart and they are supplied with a sinusoidal alternating voltage of 1.1 Volt rms at a frequency of 25 kHz. Under these conditions have obtained a homogeneous deposition of TiO₂. In the above-mentioned deposition conditions, a thin layer of TiO 2 220 nm thick is obtained. This has a photo-catalytic activity. The TAS test (accuracy to within 10%) gives a K = 12.10-³ cm^-1. min-¹ with a 50% decrease in peak air after one hour. This test is described for example in application WO01 / 32578. RAMAN spectrometry analysis shows that TiO₂ is crystallized in anatase form. According to a second embodiment, a layer of TiO₂ from TipT is deposited by homogeneous discharge operating at atmospheric pressure The thin layer of TiO2 was deposited on a clean glass substrate heated to a temperature of 235 ° C. The gas mixture which is introduced consists of helium (He) and oxygen (O2). The respective flow rates of these gases are 11 slpm and 20 sccm. The organometallic precursor, TipT (titanium tetraisopropoxide), is poured into a 0.5 I bubbler. This bubbler is heated to 50 ° C and a carrier gas (He) is injected into the bubbler with a flow rate of 500 sccm to transport the organometallic vapors. At equilibrium, the total pressure in the reactor is maintained at 1013 mbar ± 50 mbar. The electrodes, covered with an alumina dielectric barrier (0.5 mm ± 0.1 mm), are 6 mm apart and they are supplied with a sinusoidal alternating voltage of 1.1 Volt rms at a frequency of 10 kHz. Under these conditions, a homogeneous deposit of Tiθ2 is obtained.

The thin layer thus deposited of ΗO2 has a thickness of 260 nm and has a photo-catalytic activity. The TAS test (accuracy to within 10%) gives a K = 8JO-³ cm-¹. min-¹ with a decrease in peak air of 24% after one hour. Similarly, a RAMAN analysis shows that TiO₂ is crystallized in rutile form. According to a third embodiment, a layer of TiO₂ from TiCI4 is deposited by homogeneous discharge operating at atmospheric pressure. The deposition conditions are identical to those of the first example, the duration is reduced so as to obtain a greater layer thickness. This thin layer of ΗO2 54 nm thick and deposited under the conditions of Example 1 has a photo-catalytic activity. The TAS test (accuracy to within 10%) gives a K = 4.10-³ cm-¹. min^-1 with a decrease in peak air of 16% after one hour. Optical measurements summarized in the table below show that the layer is not absorbent (the absorption of bare glass being between 1.5 and 2)

<img file="WO2005012593A1_D0001.tif" /> It will be noted that the three exemplary embodiments were produced without the interposition of a barrier layer between the Ti couche2 layer and the substrate. According to a fourth embodiment, the Tiθ2 layer was deposited on a clean glass substrate heated to a temperature of 260 ° C. The gas mixture which is introduced consists of nitrogen (N2) and oxygen (O2). The nitrogen flow rate is 10 slpm with 150 ppm of O2. The organometallic precursor, titanium tetrachloride (TiCU), is poured into a 0.5 I bubbler. This bubbler is heated to 10 ° C. and a carrier gas (He) is injected into the bubbler with a flow rate of 140 sccm for transport organometallic vapors. At equilibrium, the total pressure in the reactor is maintained at 1013 mbar ± 50 mbar. The electrodes, covered with a dielectric barrier of alumina (0.5 mm ± 0J mm), are 5 mm apart and they are supplied with a sinusoidal alternating voltage of 1.1 Volt rms at a frequency of 5 kHz. Under these conditions have obtained a homogeneous deposit of Tiθ2. crystallized in anatase form and having a photocatalytic activity of 11.10-³ cm-1.min-1, the thickness of the layer being approximately 150 nm.