Deposition of photocatalytic coating on substrate, for anti-stain and/or anti-mist glazing applications, e.g. for building windows, involves plasma-assisted chemical deposition in vapor phase

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 relates to 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 for various applications, such as utility glazing, glazing for vehicles. or for buildings.

The photocatalytic properties imparted to the substrate due to the titanium oxide-based coating give the latter an anti-fouling function. More particularly, the substrate retains appearance and surface properties over time, which make it possible in particular to space out the cleaning operations and / or to improve visibility, by managing to gradually eliminate the soiling that gradually settles down. 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 mist 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 suitable, under the effect of radiation of adequate wavelength (in the visible and / or in the ultraviolet), to initiate radicaiaires reactions causing the oxidation of organic products: we generally speak of “photocatalytic” or “photo-reactive” materials.

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Among the methods currently used for depositing such a coating, many techniques are known:

• by decomposition of metal oxide precursors such as titanium for example (pyrolysis techniques: liquid pyrolysis, powder pyrolysis, value phase pyrolysis known as CVD (Chemical Vapor Deposition), techniques associated with sol-gel: dipping or dipping, cellcoating , ...), • by a vacuum technique (reactive cathodic sputtering or not).

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 imposes a slow adaptation of the loading conditions with raw material. of the float. Moreover, one can note under the disadvantages of this deposition technique, a certain difficulty as regards the control of the surface temperatures and the temperatures of the means of injection of the precursors (nozzle), the high temperatures being one of the parameters the most. more fundamental to obtaining a titanium oxide coating having optimal photocatalytic properties.

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 'a suitable crystallographic phase. This heating is difficult to implement directly within the magnetron which is under vacuum and it is then necessary to perform a recovery operation outside the deposition chamber. In fact, for the latter to have photocatalytic properties, the titanium oxide should be present in an anatase crystalline form, in rutile form or in the form of a mixture of anatase, rutile, brookite with a rate crystallization of at least 25%, especially about 30 to 80%, especially near the surface, (the property being more of a surface property). (The crystallization rate is understood to mean the amount by weight of TiO2 crystallized relative to the total amount by weight of TiO2 in the coating).

The aim of the invention is therefore to develop a process for depositing photocatalytic coatings on a substrate, which exhibit a marked “antifouling” effect with respect to the substrate and which can be manufactured industrially. , which does not have the drawbacks of the previously mentioned techniques.

Completely surprisingly, the inventors have discovered that it was possible to use the technique known as PECVD (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 process for depositing on a substrate a coating based on semiconductor materials based on metal oxides, in particular titanium oxide, which are suitable, under the effect of a radiation of adequate wavelength, to initiate 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 arrangements 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 during or subsequently to the deposition in view. obtaining the desired properties and which, moreover, is very flexible, the optimum deposition conditions no longer being dependent on the presence of a nearby heat source (float).

In preferred embodiments of the invention, one can optionally also have recourse to one and / or the other of the following arrangements:

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- Is injected, parallel to the mixture containing the precursor, at least one carrier gas or a mixture of carrier gases selected 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 sublayer is deposited prior to the coating with photocatalytic property, making it possible to provide another functionality to said coating with photocatalytic property and / or to reinforce said properties of said coating.

Is incorporated into the gas mixture comprising at least the organometallic precursor and / or a metal halide of said metal 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, vanadium, antimony, zinc, tungsten, cobalt, nickel, aluminum, these oxides possibly being mixed or doped.

The coating with photocatalytic property is deposited on the substrate itself within the plasma discharge.

The coating with photocatalytic property is deposited on the substrate outside of the plasma discharge.

According to another aspect of the invention, it also relates to a substrate obtained according to the aforementioned method, as well as its variants.

This glass-based, ceramic or glass-ceramic or plastic substrate provided on at least part of at least one of its faces with a coating with photo-catalytic property comprising at least partially crystallized titanium oxide, is characterized in that the crystallized titanium oxide is in anatase form, in rutile form, brookite or in the form of a mixture of anatase, rutile, brookite.

In preferred embodiments of the invention, one can optionally also have recourse to one and / or the other of the following arrangements:

the crystallized titanium oxide is in the form of crystallites with an average size of between 0.5 and 60 nm, preferably 1 to 50. the coating also comprises an inorganic material, in particular in the form of an amorphous oxide or mixture of oxides or partially crystallized of the silicon oxide, titanium oxide, tin oxide, zirconium oxide, aluminum oxide, vanadium, antimony, zinc, tungsten, cobalt or nickel oxide, these oxides possibly be mixed or doped.

the coating comprises additives capable of extending the photocatalytic phenomenon due to the 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 titanium oxide is doped, in particular with at least one of the metallic or non-metallic elements,

- the coating thickness is between 5 nm and 1 micron, preferably 5 to 100 nm

- the photocatalytic activity of the coating is at least 5.10-³ cm¹ 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 placed under the coating with photocatalytic property at least one thin layer with an anti-static, thermal or optical function, or forming a barrier to the migration of alkalis originating from the substrate.

- the thin film with an anti-static function, possibly with controlled polarization, and / or thermal and / or optical is based on a conductive material of the metal type or of the doped metal oxide type such as ITO, SnO2: F, SnC> 2: Sb, ZnO: ln, ZnO: F, ZnO: Al, ZnO: Sn or metal oxide substoichiometric in oxygen such as SnO2- x or ZnC> 2-x with x <2.

the thin film 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, SnO2, Ιη2θ3, silicon oxycarbon or oxynitride, or possibly based on a mixture of a high refractive index material with a low refractive index material (AI2O3 / T1O2, AI2O3 / S1O2, AI2O3 / SnC> 2, SnO2 / T1O2 ....) the thin or multilayer layer with an alkali barrier function is for example based on silicon oxide, nitride, oxynitride or oxycarbide, AbOs: F or SnCb : F of aluminum nitride, the substrate is transparent, flat or curved.

the substrate is a glass substrate.

the substrate is a substrate based on a polymer, in particular PMMA, of polycarbonate.

According to another aspect of the invention, this relates to an “anti-fouling and / or anti-fog” glazing, monolithic, multiple of the double-glazing or laminated type incorporating the substrate as described above for the manufacture of “ self-cleaning ”glazing, anti-fog and / or anti-dirt, of the organic and / or mineral type, in particular glazing for buildings of the double-glazing type, glazing for vehicles of the windshield type, rear or side windows of automobiles, trains, airplanes, or utility glazing such as aquarium, window, greenhouse, interior furnishings, street furniture, or mirrors, television screens, glazing with controlled variable absorption electrically ...

Other characteristics and advantages of the invention will become apparent from the following description of one of its embodiments, given by way of non-limiting example.

According to a preferred embodiment, it is proposed to deposit on a transparent substrate based on glass, ceramic or vitroceramic, in particular glass, or a substrate based on polymer, provided on at least part of at least one of its faces a coating with a photocatalytic property comprising, for example, at least partially crystallized titanium oxide. The titanium oxide is preferably crystallized “in situ”, during the formation of the coating on the substrate. According to the invention, there is also placed under this coating at least one thin layer forming a barrier to the migration of alkalis originating from the substrate.

Titanium oxide is in fact part of the semiconductors which, under the action of light in the visible range or of ultraviolet rays, degrade organic products which are deposited on their surface. Choosing titanium oxide to manufacture glazing with an “anti-fouling” effect is therefore particularly recommended, especially since this oxide has good mechanical and chemical resistance: to be effective for a long time, it is obviously important for the coating to retain its integrity, even though it is directly exposed to numerous attacks, in particular during the assembly of the glazing on site (building) or on a 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 (windscreen 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 the anatase form, that the orientation of the crystals of T1O2 growing on the substrate had an influence on the photo-catalytic performance of the oxide: there is a preferred orientation ( 1,1,0) which clearly favors photocatalysis.

The production of the coating is carried out in such a way that the crystalline titanium oxide which it contains is in the form of "crystallites", at least close to 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 in fact in this size range that titanium oxide seems to have an optimal photocatalytic effect, probably because crystallites of this size develop a large active surface.

The coating may 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), of titanium, tin, zirconium or aluminum. This material #

mineral can also participate in the photocatalytic effect of crystallized titanium oxide, by itself exhibiting a certain photocatalytic effect, even weak compared to that of crystallized T1O2, which is the case of tin oxide or amorphous titanium oxide.

A “mixed” oxide layer thus combining at least partially crystallized titanium oxide with at least one other oxide can be advantageous from the optical point of view, very particularly if the other or the other oxides are chosen of lower index. to that of T1O2: by lowering the “overall” refractive index of the coating, it is possible to play on the light reflection of the substrate provided with the coating, in particular to lower this reflection. This is the case if, for example, one chooses a layer of Τΐθ2 / Αΐ2θ3, a method of obtaining is described in patent ΕΡ-0 465 309, or of TiC> 2 / SiO2. It is, of course, necessary that the coating contain a sufficient T1O2 content to retain significant photocatalytic activity and that T1O2 remains crystallized. It is thus considered that it is preferable for the coating to contain at least 40% by weight, in particular at least 50% by weight of T1O2 relative to the total weight of oxide (s) in the coating.

To amplify the photocatalytic effect of the titanium oxide of the coating according to the invention, it is first of all possible to increase the absorption band of the coating, by incorporating into the coating other particles, in particular metallic and cadmium-based, of tin, tungsten, zinc, cerium, or zirconium, optionally doped as well as non-metallic particles.

It is also possible to increase the number of charge carriers by doping the crystal lattice of titanium oxide, by inserting therein at least one of the following metallic elements: niobium, tantalum, iron, bismuth, cobalt, nickel, copper, ruthenium, cerium , molybdenum.

Completely surprisingly, the coating in fact exhibits not one property but two, as soon as it is exposed to adequate radiation such as in the visible region and / or ultraviolet rays, such as solar radiation: by the presence of photocatalytic titanium oxide, as already seen, it promotes the gradual disappearance, as they accumulate, of soiling of organic origin, causing their degradation by a radical oxidation process. Mineral stains 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 any 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, also preferably has an outer surface with a pronounced hydrophilic and / or oleophilic nature, which induces three very advantageous effects:

• a hydrophilic nature allows perfect wetting of the water which can be deposited on the coating. When a water condensation phenomenon occurs, instead of a deposit of water droplets in the form of mist obstructing visibility, there is in fact a continuous thin film of water which forms on the surface of the coating. and that is completely transparent. This "anti-fog" effect is demonstrated in particular by measuring a contact angle with water of less than 5 ° after exposure to light, and, • after runoff of water, especially rain, on a surface not treated with a photocatalytic layer, many drops of rainwater remain attached to the surface and, once evaporated, leave unsightly and annoying traces, mainly of mineral origin. Indeed, a surface exposed to ambient air is quickly covered with a layer of dirt which limits its wetting by water. This soiling is added to other soiling, in particular mineral (crystallization, etc.) 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 hydrophilicity induced by the photocatalytic activity. This hydrophilic character indeed causes a perfect spreading of the raindrops. Evaporation traces are therefore no longer present. In addition, the other mineral soiling present on the surface is washed, or redissolved in the case of crystallization, by the water film and therefore largely removed. A “mineral anti-fouling” effect is obtained, in particular induced by rain, • together with a hydrophilic character, the coating can also have an oleophilic character, allowing the “wetting” of organic soiling which, as with water, then tends to be deposited on the coating in the form of a continuous film less visible than well localized "spots". An “organic anti-fouling” effect is thus obtained which takes place in two stages: as soon as it is deposited on the coating, the dirt is already barely visible. Then, gradually, it disappears by radical degradation initiated by photo-catalysis.

The coating can be chosen with 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 a greater photocatalytic activity, • it has a direct influence on the wetting. The roughness indeed enhances the wetting properties. A hydrophilic smooth 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 porosity of the layer or of the sub-layer in at least part of its thickness.

The above effects will be all the more marked as the coating is porous and rough, hence a superhydrophilic effect on the rough photoreactive surfaces. However, being too pronounced, the roughness can be detrimental by promoting encrustation, the accumulation of dirt and / or by causing an optically unacceptable level of blurring to appear.

It has thus proved to be advantageous to adapt the method of deposition of the T1O2-based coatings so that they exhibit a roughness of about 2 to 20 nm, preferably 5 to 15 nm, this roughness being evaluated. by atomic force microscopy, by measuring the value of the root mean square deviation (called Root Mean Square or RMS in English) on a surface of 1 square micrometer. With such roughness, the coatings exhibit a hydrophilic character resulting in a contact angle with water which may 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 T1O2, it preferably has a porosity of the order of 65 to 99%, in particular 70 to 90%, the porosity being defined here indirectly by the percentage of the density. theoretical T1O2, 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 the thickness can depend on various parameters, in particular on the envisaged application of the substrate of the glazing type, or on the size of the crystallites of T1O2 in the coating or on the presence of alkalis 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 supplies, low-emissivity, anti-solar, etc.) can in particular be chosen based on a conductive material of the metal type, such as '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 SnC> 2: F, or with antimony SnC> 2: Sb, or zinc oxide doped with indium ZnO: ln, fluorine ZnO: F, aluminum ZnO: Al or tin ZnO: Sn. They can also be metal oxides substoichiometric in oxygen, such as SnC> 2-x or ZnO2x with x <2.

The anti-static function layer preferably has a square resistance value of 20 to 1000 ohms / square. Provision can be made to provide it with current inputs in order to polarize it (supply voltages for example between 5 and 100V). This controlled polarization makes it possible in particular to fight against the deposition of dust of the order of a millimeter liable to deposit on the coating, in particular dry dust adhering only by electro-static effect: by suddenly reversing the polarization of the layer, we "Ejects" this dust.

The thin layer with an optical function can be chosen in order to reduce the light reflection and / or to make the color in reflection of the substrate more neutral. In this case, it preferably has a refractive index intermediate between that of the coating and that of the substrate and an appropriate optical thickness, and may consist of an oxide or of a mixture of oxides of the aluminum oxide type. AI2O3, tin oxide SnO2, indium oxide Ιη2θ3, oxycarbon or silicon oxynitride. To obtain maximum attenuation of the color in reflection, it is preferable that this thin layer has a refractive index close to the square root of the product of the squares of the refractive indices 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, in particular about 500 to 550 nm.

The thin film with the function of an alkali barrier may in particular be chosen based on silicon oxide, nitride, oxynitride or oxycarbide, aluminum oxide containing fluorine AhC ^ F, or else nitride of silicon. aluminum or even based on SnC> 2. In fact, it has proved to be useful when the substrate is made of glass, since the migration of sodium ions in the coating according to the invention can, under certain conditions, alter its photocatalytic properties.

The nature of the substrate or of the sublayer also has an additional advantage: 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 these optional thin layers can, in a known manner, be deposited by vacuum techniques of the cathode sputtering type or by other techniques of the thermal decomposition type such as pyrolysis in solid, liquid or gas phase. Each of the aforementioned layers can combine several functions, but they can also be superimposed.

A subject of the invention is also “anti-fouling” (organic and / or mineral soiling) and / or “anti-fog” glazing, whether monolithic, multiple insulators of the double-glazing or laminated type, and which incorporate the coated substrates described above.

The invention therefore relates to the manufacture of glass, ceramic or glass-ceramic products, and more particularly the manufacture of “self-cleaning” glazing. These can advantageously be building glazing, such as double glazing (it is then possible to have the coating "on the outside" and / or "on the inside", that is to say on face 1 and / or opposite. 4).

This is particularly advantageous for glazing that is not easily accessible for cleaning and / or which 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 placed on the windshields, side panels or rear windows of a car, in particular on the face of the glazing facing the interior of the passenger compartment. This coating can then prevent the formation of fogging, and / or remove the traces of dirt of the type fingerprints, nicotine or organic material of the volatile plasticizer type "released" by the plastic lining the interior of the passenger compartment, in particular that of the vehicle. dashboard (release sometimes known as “fogging”).

Other vehicles such as airplanes or trains may also find it advantageous to use glazing provided with the coating of the invention.

Many other applications are possible, in particular for aquarium glasses, shop windows, greenhouses, verandas, glasses used in interior furnishings or urban furniture, but also mirrors, television screens, the field of eyewear or any architectural material such as facade material, cladding, roofing 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, ...

Another advantageous application of the coating according to the invention consists in associating it with an electrically controlled variable absorption glazing of the electrochromic, photovoltaic glazing type, liquid crystal glazing optionally with dichroic dye, glazing with suspended particle system, viologen glazing. . Since all these glazing units generally consist of a plurality of transparent substrates between which the “active” elements are arranged, the coating can then advantageously be placed on the outer face of at least one of these substrates.

Particularly in the case of an electrochromic glazing, when the latter is in the colored state, its absorption leads to a certain heating at the surface, which, in fact, is liable to accelerate the photocatalytic decomposition of the carbonaceous substances depositing on it. the coating according to the invention. For more details on the structure of an electrochromic glazing, reference should advantageously be made to patent application EP-A-0 575 207 describing an electrochromic laminated double glazing, the coating according to the invention preferably being able to be arranged in side 1.

This metal oxide coating is therefore produced using the technique known as APPECVD 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 to 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 TiCU, TiPT, Ti ethoxide (butoxide, etc.), Ti diisopropoxide bis (acetylacetonate), Titanium (III) tris (2,2,6,6-tetramethyl-3 , 5-heptanedionate.

This gas mixture can also incorporate at least one oxidant or a mixture of oxidants (air, O2, CO2, N2O, organic: alcohol, ester, etc.) or at least one reducing agent or a mixture of reducing agents (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 / halide) can be used for the metals. For fluorine, for example trifluoroacetic acid (TFA), HF, NF3, etc. will be used. For nitrogen, NH3 or amines (primary, secondary or tertiary) can be used. It is also possible to use precursors containing both titanium and the dopant (for example: Tétrakisdiethylamino titanium, Tétrakisdimethylamino titanium or tetrachlorodiamminotitanium ...)

The reaction gas mixture is then dissociated, by a plasma source, either directly within the plasma, or in a remote, blown manner (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 be advantageous, moreover, to deposit the coating not all at once, but by at least two successive stages, which seems to promote the crystallization of the titanium oxide over the entire thickness of the coating when it is chooses relatively thick.

Likewise, it may be advantageous to subject the coating with photo-catalytic property to a post-deposition heat treatment, once the T1O2 has been formed, in order to improve its crystallization rate. The chosen treatment temperature can also make it possible to better control the crystallization rate and the crystalline nature, anatase and / or rutile, of the oxide.

The deposition process which is the subject of the invention is interesting 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 (eg PMMA, polycarbonate)

However, in the case of a soda-lime glass substrate, multiple and prolonged annealing may promote attenuation of the photocatalytic activity due to too great migration of 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, make it possible to overcome 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 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.