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Title:
PREPARATION OF TIO2/SIO2 SOLS AND USE THEREOF FOR DEPOSITION OF SELF-CLEANING ANTI- FOGGING COATINGS
Document Type and Number:
WIPO Patent Application WO/2010/053459
Kind Code:
A1
Abstract:
The invention relates to a process for the low temperature production of TiO2/SiO2 sols and to the use thereof for the deposition of thin, optically transparent coatings having self-cleaning and anti-fogging properties. The preparation process according to the invention comprises the preparation of an acidic aqueous sol containing photochemically active TiO2 nanoparticles, the addition of a SiO2 precursor followed by hydrolysis/condensation of SiO2, the addition of amorphous TiO2 hydrolysate, the addition of colloidal SiO2, the dilution of the prepared sol with water and/or organic solvents, the deposition of the prepared sol onto the support, the evaporation of the solvent, and condensation reactions at ambient temperature, and the obtaining of a thin uniform TiO2/SiO2 layer having self-cleaning properties.

Inventors:
CERNIGOJ URH (SI)
LAVRENCIC STRANGAR URSKA (SI)
Application Number:
PCT/SI2009/000052
Publication Date:
May 14, 2010
Filing Date:
October 15, 2009
Export Citation:
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Assignee:
UNIVERZA V NOVI GORICI (SI)
CERNIGOJ URH (SI)
LAVRENCIC STRANGAR URSKA (SI)
International Classes:
C01G23/053; B01J35/00; C09C1/36; C09D151/08; C09D183/04; C09D183/10; C23C18/12
Domestic Patent References:
WO2004005577A22004-01-15
Foreign References:
EP1544269A12005-06-22
US5755867A1998-05-26
EP1728819A22006-12-06
Attorney, Agent or Firm:
ITEM D.O.O. (1000 Ljubljana, SI)
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Claims:
WHAT WE CLAIM IS:

1. A process for preparing Tiθ2/Siθ2 sols applicable for the deposition of thin, optically transparent coatings having self-cleaning and anti-fogging properties, characterized in that the process comprises:

- the preparation of an acidic aqueous sol containing photochemically active TiO2 nanoparticles,

- the addition of a SiO2 precursor followed by hydrolysis/condensation of SiO2,

- the addition of epoxy silanes followed by polymerization into polyethers,

- the addition of amorphous TiO2 hydrolysate acting as a binding agent,

- the addition of colloidal SiO2,

- the dilution of the prepared sol with water and/or organic solvents,

- the deposition of the prepared sol onto the support,

- the evaporation of the solvent, and condensation reactions at temperatures lower than 100 0C,

- the obtaining of a thin uniform TiO2/SiO2 layer having self-cleaning properties.

2. The process according to claim 1 , wherein the acid HCIO4 is used for the preparation of the acidic aqueous sol.

3. The process according to claim 1 , wherein TEOS is used as a SiO2 precursor.

4. The process according to claim 1 , wherein TEOS is added to the cooled, undiluted stock sol.

5. The process according to claim 1 , wherein the SiO2 binding agent is prepared in advance from TEOS and colloidal SiO2 under hydrolysis of TEOS.

6. The process according to claim 1 , wherein GPMS is used as epoxysilane.

7. The process according to claim 1 , wherein the mixed TiO2/SiO2 sol is diluted with water or with various mixtures of the said organic solvents: '

8. TiO2/SiO2 sols prepared according to the previous claims.

9. The use of TiO2ZSiO2 sols according to claim 7, wherein the prepared sol is applied to the surface of the support by means of a cloth-piece, a brush, a spray applicator, and by immersion of a support into the sol.

10. The use of TiO2/SiO2 sols according to claim 7, wherein the utilized supports are glass, concrete, ceramics, and various polymer materials.

11. The use of TiO2/SiO2 sols according to claim 7, wherein no thermal or chemical post-treatments of the formed coating are required.

Description:
Preparation of TiO 2 /Siθ 2 sols and use thereof for deposition of self-cleaning anti- fogging coatings

The invention relates to a process for the low temperature production of Tiθ 2 /Siθ 2 sols and to the use thereof for deposition of thin, optically transparent coatings having self- cleaning and anti-fogging properties. More specifically, the invention encompasses the preparation of optically transparent and stable, thin coatings having photocatalytic and hydrophilic properties starting from TiO 2 ZSiO 2 colloidal solutions, namely sols.

In the course of time, surfaces exposed to the atmosphere and air pollution due to human activity adsorb all kinds of macroscopic particles and individual molecules of organic or inorganic origin. The dirt accumulated on the surface poses an esthetic problem, and in certain cases, also a security problem. The dirt is most troublesome on supports that are transparent for visible light, namely on various glasses and transparent organic polymers. The cleaning of such surfaces is often a time- consuming and risky task. In addition, the utilized chemical agents represent a financial and environmental burden.

One of the solutions, how to keep surfaces clean for a long time, is the use of thin protective coatings deposited on a support (substrate). There exist two possibilities: either hydrophobic or hydrophilic coatings. Characteristic of hydrophobic surfaces is the very high contact angle between the deposited water droplet and the support (>150 °). Consequently, the adhesion of the dirty spherical water droplet to the support is prevented. There have been known several methods for the preparation of hydrophobic surfaces by means of coating with specific polymers or waxes. Physical methods are used, such as the pressurization of polymer drops, ionic etching; or chemical methods, such as chemical vapor deposition (CVD). Although the obtained surfaces have good self-cleaning properties, they are not widely applicable ' owing to disadvantages in the manufacture and stability of the produced surfaces, such as the costly coating of the supports, the frequent fogginess of the coatings, and the premature loss of stability.

Another possible solution is represented by hydrophilic coatings, mostly various semiconducting oxides of transition metals, such ZnO, Zrθ 2 , TiO 2 , WO 3 , and the like. The presence of UV radiation and water molecules cause chemical changes on their surface. Consequently, the surface becomes highly hydrophilic; the contact angle between the surface and the water droplet is lower than 5°. Such high hydrophilicity enhances the adhesion of water, and slightly adsorbed inorganic particles, such as sand, are easily washed away by water from the surface.

Hydrophilicity is, however, not the sole cause for the self-cleaning properties of semiconductive oxides. Another important self-cleaning property is the photocatalytic efficiency. When a semiconductor particle absorbs an electromagnetic radiation photon of suitable energy, mostly a photon of the UV region of the spectrum, the valence electron is transferred into the conduction band. The remaining positive hole is a strong oxidant; the electron, however, acts as a reductant. Both may react with molecules adsorbed on the particle surface, which causes the oxidation of the molecules on the positive hole side, and the reduction of the molecules on the electron side. In this way, a complete mineralization of organic molecules into CO 2 , H 2 O and inorganic acids takes place gradually. Anatase TiO 2 has proven to be the most suitable photocatalyst of all studied oxides, because it is non-toxic, chemically and physically stable, inexpensive, and the like. Only nanocrystalline TiO 2 is applicable for window coating, as only nanocrystalline coatings can be manufactured sufficiently light transparent for commercial use. The customary manufacture of anatase TiO 2 requires the burning of the pulverized products produced from sols or thin layers at temperatures over 350 0 C. At lower temperatures the formation of the anatase crystalline phase does not occur, which is of crucial importance for the photocatalytic efficiency. The obligatory high temperature is a limitation to the applicability of the mentioned method. Such methods can be used only in specialized plants, such as glass manufacturing plants. A commercial product obtained by thermal treatment, at

••■ high temperatures, is Pilkington Activ™, which is a glass coated with a 20-30 nm thick layer of nanocrystalline anatase TiO 2 . The layer is mechanically stable, photocatalytically active and has an extremely high transparency in the visible region of the spectrum. The coating is produced by means of chemical vapor deposition (CVD) onto float glass.

For a wider applicability of self-cleaning TiO 2 , a TiO 2 sol has to be prepared for deposition onto surfaces at normal atmospheric conditions that does not require additional thermal treatment. It yields optically translucent and strong coatings. The article lchinose, H.; Terasaki, M.; Katsuki, H. J Sol-Gel Sci Technol 2001 , 22, 33-40 describes the preparation of a peroxo modified anatase sol, which is applicable for the low-temperature manufacture of a photocatalytic coating. Glasses provided with a coating of the obtained sol exhibit photocatalytic activity, with satisfactory adhesion of the coatings to the support. There is nothing reported regarding the optical quality of the manufactured coatings. Another drawback is the rather exacting preparation of the sol, where quite a number of chemicals are used. The article Matsuda, A.; Matoda, T.; Kotanim Y.; Kogure, T.; Tatsumisago, M.; Minami, T. J Sol-Gel Sci Technol 2003, 26, 517-521 describes the preparation of nanocrystalline coatings on glass and on organic, light transparent polymers, wherein the crystallization of TiO 2 into anatase is achieved by hydrothermal treatment of the coatings. For improved strength of the coatings the sols also contain SiO 2 precursors. The coatings exhibit photocatalytic activity; the adhesion to the support surface and the strength of the coating are satisfactory. The disadvantage of the said manufacture is the commercially not feasible hydrothermal treatment of the coatings.

U.S. Pat. No. 5,149,519 discloses the preparation of a low-temperature sol containing crystalline anatase TiO 2 . The obtained sol is, however, not used for preparing thin TiO 2 layers.

The article Yun, Y. J.; Chung, J. S.; Kim, S.; Hahn, S. H.; Kim, E. J. Mater Lett 2004, 58, 3703-3706 describes the preparation of a nanocrystalline TiO 2 coating on commercial sodium glass at low temperatures. Yun et al. prepared the sol starting from titanium tetraisopropoxide in acidic aqueous solutions, and keeping it for several hours under reflux to attain the crystallization of the amorphous TiO 2 . The utilized method of manufacturing the sol by means of commercial chemicals, and a technically - non- exacting procedure, as well as the easily operated deposition of the resulting sol on various supports are the advantages of the studied system. Regarding its concept, it is the nearest prior art relating to the present invention; the latter, however, comprises the additional SiO 2 component to enhance the mechanical and hydrophilic properties of the coatings.

European Patent EP No. 0 913 447 discloses the preparation of self-cleaning liquids for various supports (substrates) with the aid of commercially available TiO 2 and SiO 2 sols. The preparation of self-cleaning liquids also comprises various additives, such as surfactants, organic solvents, and silicone. The coatings are photocatalytically active. In certain cases, the adhesion to the surface of the support and the hardness of the coatings are good. The weak point of the said invention is the use of already commercially deliverable TiO 2 and SiO 2 sols. European Patent EP No. 1 544 269 discloses an invention starting from commercial TiO 2 nanocrystalline particles and SiO 2 colloidal solutions, which are supplemented with a binding agent made of hydrolyzed titanium alkoxide. The coatings thus obtained exhibit good mechanical and hydrophilic properties even in the darkness.

European Patent EP No. O 826 633 discloses the preparation of an aqueous dispersion and a thin transparent TiO 2 coating via a low-temperature route starting from TiCU, which is a substantially more economical raw material than any titanium alkoxide. The process, however, requires the removal of chloride ions by means of electrodialysis. International Publication WO 2004/060555 describes an invention starting from titanium peroxide sols and nano-anatase particles, with the addition of urethane acrylic polymers for improved wettability of the support surface, and for reducing the yellow tinge of the thin layers, contributed by the peroxide process. Besides their positive characteristics, the incorporated non-volatile organic additives also have negative ones: namely, the required additional chemicals for the synthesis and the associated additional expense; the questionable long-term integrity of such thin layers owing to degradation of organic matter in the course of photocatalysis. More numerous are documents, such as International Publications WO 2004/108846 and WO 2004/00557 relating to the manufacture of photocatalytically active hybrid organic- inorganic thin layers on the base of siloxanes and TiO 2 dispersions and sols. The technical problem that has not been satisfactory solved as yet, is the provision of thin, optically transparent coatings having self-cleaning and anti-fogging properties obtainable by a non-exacting process for the preparation of sols from available and not overexpensive chemicals, by means of a simple mode of applying the sol onto the support without additional hardening of the coating by thermal treatment; the said coatings having an optimum ratio between mechanical strength and photocatalytic activity, high optical transparency in the complete visible region of the spectrum, and high hydrophilicity under UV radiation.

The object of this invention is a process for the low temperature production of Tiθ 2 /Siθ 2 sols and the use thereof for deposition of thin, optically transparent coatings having self-cleaning and anti-fogging properties, based on a non-exacting process for the preparation of sols from available and not overexpensive chemicals, by means of a simple mode of applying the sol onto the support without additional hardening of the coating by thermal treatment; the said coatings having an optimum ratio between mechanical strength and photocatalytic activity, high optical transparency in the complete visible region of the spectrum, and high hydrophilicity in the presence of UV radiation.

According to the invention, the problem is solved by a process for the preparation of TiO 2 /SiO 2 colloidal sols, and their use in the deposition of thin coatings as claimed in the independent claim.

The invention is illustrated with the aid of Examples and FIGs. representing:

FIG. 1 : X-ray diffraction pattern of the sample prepared from thin layers of TiCVSiO 2 in accordance with the process described in Example 7.

FIG. 2: Evidence for photocatalytic activity of the prepared self-cleaning layers.

FIG: 3: UV-Vis spectrum of the TiO 2 /SiO 2 thin layer on glass in comparison with blank

(uncoated) glass. According to this invention, there is at first performed the process of preparing the anatase TiO 2 sol, followed by the preparation of the photocatalytically active solution, and the deposition thereof onto various surfaces.

The first object of this invention is a modification of the process for the preparation of the anatase TΪO 2 sol produced in acidic aqueous media at temperatures up to 100 0 C. The employed fundamental process was the preparation of low-temperature TiO 2 sol, as published in 2004 by Yun et al. By modifying their process, we succeeded in the obtaining of a stable sol having slightly aggregated TiO 2 particles, wherein their anatase crystalline phase was detected by means of X-ray diffraction. The produced TiO 2 is quite easily dispersed in water and in mixtures of water with certain organic solvents, thus yielding stable sols.

(i) As source of TiO 2 may be employed titanium tetrachloride (TiCU), titanium oxysulfate (TiOSO-O ar) d various titanium alkoxides. Preferably is used titanium tetraisopropoxide TTIP. TTIP (from 5 to 45 ml_) is added under stirring at room temperature to absolute ethanol (from 1 to 10 mL). Immediately thereafter is added drop-wise the aqueous solution of the acid to the obtained liquid mixture at ambient temperature. In the presence of the acid takes place the hydrolysis of titanium compounds, whereby the feebly soluble, amorphous titanium oxide is precipitated from the reaction mixture. The employed inorganic acids are cone. HNO 3 , cone. HCIO 4 , cone. HCI and cone. H 2 SO 4 . The employed organic acids are formic acid, ethanoic acid and propanoic acid. The aqueous solution of the acid is prepared by mixing water (from 30 to 100 mL) with the concentrated acid (from 0.1 to 10 mL). The optimum acid concentration is within the range of 1 to 3 mL acid in 90 mL of water. Too low or a too high acid concentrations lead to the aggregation of the TiO 2 particles, which is an undesirable effect, under the provision that the production of coatings, as transparent as possible, is desired. HCIO 4 has proven to be the most suitable acid.

(ii) TiO 2 is photocatalytically active in crystalline form, either as anatase or rutile. The anatase crystalline form exhibits a higher photocatalytic activity than the rutile form. The crystallization of the amorphous TiO 2 into the crystalline form is attained by the heating of the sol. The advantages of this invention are: a) the crystallization is achieved by heating to a temperature even below 100 0 C; b) the crystallization and the dispersion of TiO 2 particles is achieved directly in the aqueous sol, to say without supplementary separation and resuspension of TiO 2 . The suspension obtained in (i) is refluxed from 2 to 100 hours. The optimum refluxing time is from 30 to 60 hours. A shorter refluxing time results in incomplete crystallization of TiO 2 , a longer time, however, does not contribute to a significant enhancement of the activity of the sol and the resulting coatings. The termination of refluxing yields the stock colloidal solution of crystalline TiO 2 (stock TiO 2 sol). The stock TiO 2 sol is stable at room temperature for at least 1 year. The sol contains from 2 to 10 % (w/w) of TiO 2 . The size of TiO 2 particles in the sol, measured by means of the dynamic light scattering method (apparatus 3D DLS SLS) 1 is from 15 to 80 nm.

Another object of this invention relates to the preparation of a photocatalytically active liquid, composed of (i) TiO 2 anatase particles; (ii) a binding agent made by mixing colloidal SiO 2 with hydrolyzed and condensed molecules of silicon alkoxides and organosilanes; (Hi) a binding agent made by hydrolysis and condensation of titanium alkoxide; (iv) an organic solvent, and (v) water. The process for preparing a photocatalytically active liquid is performed as follows.

Separately is prepared the solution or the sol containing SiO 2 , namely the binding agent. The appropriate silicon alkoxide, or a mixture of silicon alkoxides, or silicon alkoxide and suitable organosilanes, are mixed with a commercially available colloidal solution of SiO 2 . Under continuous stirring is added a corresponding volume of an inorganic acid. After ten minutes a determined volume of lower primary alcohols is added to the sol. The thus obtained sol may be utilized after stirring for 24 hours, and has to be used up within 3 weeks. All steps are performed at a temperature of between 15 and 30 0 C.

To a measured volume of the aqueous TiO 2 sol, prepared as described for the first object of this invention, is added a preformed binding agent. Subsequently, water and the organic solvent are added to the prepared sol, and stirred. All steps are carried out at a temperature of between 15 and 30 0 C. (i) As source of anatase Tiθ2 particles is employed aqueous TiO 2 sol, prepared as described for the first object of this invention. TiO 2 is an indispensable component of a photocatalytically active liquid. No self-cleaning coating or layer can be produced without it, since TiO 2 is the component having self-cleaning properties (both super- hydrophilicity and photocatalytic activity). The volume fraction of aqueous TiO 2 sol in the final liquid is from 0.5 to 50%. Too low concentration of TiO 2 results in the formation of optically high-quality and completely transparent coatings that exhibit, however, a too low photocatalytic activity owing to the small concentration of TiO 2 . Too high TiO 2 concentrations in the final liquid result in coatings of high photocatalytic activity. Such coatings, however, do not meet optical standards.

(ii) The main components of the SiO 2 binding agent are the corresponding silicon alkoxide and colloidal SiO 2 . Alkoxy groups in the silicon alkoxide comprise 1 to 4 carbon atoms. Examples of tetraalkoxysilanes are tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetraisopropoxysilane. The most suitable tetraalkoxysilane is tetraethoxysilane (TEOS). The SiO 2 hydrolysate 1) improves the strength and the abrasive resistance of the final coating; 2) enhances and prolongs the superhydrophilic effect of the final coating. As additives for tetraalkoxysilanes may be also employed various organosilanes having bound an organic group instead of one alkoxide group. Such group may be saturated, such as for example a methyl, ethyl, octyl, phenyl group; the corresponding organosilanes are methyl triethoxysilane, ethyl triethoxysilane, octyl triethoxysilane, and phenyl triethoxysilane. Bis(triethoxysilyl)- octane is also included in this group of organic silanes. Its characteristic is the possible crosslinking via the Si-O bonds on both ends of the organic chain. Such organosilanes are employed with the aim to reduce the brittleness of the TiO 2 thin layer. Especially bis(triethoxysilyl)octane is also acting as a crosslinking agent, which enhances the strength of the final layer.

Another possible choice is represented by organosilanes having instead of one alkoxy group an unsaturated carbon-carbon bond. In most cases epoxides or acrylates are employed. The most suitable organic silane is 3-glycidoxypropyl-trimethoxysilane (GPMS). The organo-modified silanes enhance the strength of the prepared thin layer as a result of the polymerization reaction occurring between organic groups: No extreme conditions are required for the initiation of such polymerization, as it may occur even at slightly increased temperature with the aid of catalysts. Alternatively, the reaction may be initiated by means of UV irradiation. With the said organosilanes, the thin layer of the catalyst is further strengthened at low temperatures. If the concentration of the hydrolysate in the final photocatalytically active liquid is too high, the photocatalytic activity of the produced self-cleaning coating is reduced, due to the decreased density of photocatalytically active TiO 2 particles in the final self-cleaning coating. If the concentration of the hydrolysate Siθ2 is too low, the strength of the self- cleaning coatings is decreased. The molar ratio between Ti from the anatase TiC> 2 sol and Si from the Siθ 2 hydrolysate is within the range of 3:1 to 0.5:1. The preferred ratio is within the range of 2:1 to 1 :1.

Colloidal SiO 2 is a colloid prepared by dispersion of highly pure SiO 2 in aqueous medium. The average size of the Siθ 2 particles must be within the range of 1 to 200 nm, in order to avoid the scattering of the visible light onto the particles. The advantage of colloidal SiO 2 over SiO 2 hydrolysate resides in higher water stability. The SiO 2 hydrolysate dissolves slowly in water, because the hydrolysis and condensation reactions are not completed. In the opposite case, the colloidal silica is composed of nanoparticles that are completely condensed, consequently having higher water stability. The main functions of colloidal silica in the final photocatalytic solution are: 1) to enhance the abrasive resistance of self-cleaning coatings; 2) to enhance the super- hydrophilicity of self-cleaning coatings. Described is the use of commercially available colloidal silica, namely Levasil 200/30% or 300/30% or 200/30%A (produced by H.C. Starck GmbH), and Snowtex IPA-ST (produced by Nissan Chemical Industries, Ltd.). The molar ratio between Ti from anatase TiO 2 sol (i), and Si from colloidal silica (iv) is within the range of 3:1 to 0.1 :1.

The conventional technique of preparing the binding agent comprises the mixing of an appropriate silicon alkoxide, or a mixture of silicone alkoxides, and a suitable organosilane with a commercially available colloidal SiO 2 solution, such as for example Levasil 200/30%. The molar ratio between Si from the appropriate silicon alkoxide (and/or organosilane), and Si from colloidal SiO 2 is within the range of 2:1 to 1:2. Levasil 200/30% also contains sufficient water that is required for the subsequent step" ^ of hydrolysis of silicon compounds. Under continuous stirring is introduced into the obtained mixture a suitable volume of a strong inorganic acid, chosen from nitric(V) acid, hydrochloric acid, sulfuric(VI) acid, and chloric(VII) acid. The molar ratio between the acid protons and the Si from the appropriate alkoxide (or organosilane) is within the range of 1 :10 to 1 :20. After ten minutes is added to the sol a determined volume of a lower primary alcohol, for example 1-propanol, ethanol, and 1-butanol. The volumetric ratio between the binding agent and the added alcohol is within the range of 1 :1 to 1 :3. The sol prepared in this manner may be utilized after stirring for 24 hours, and has to be used up within 3 weeks. All steps are performed within the range of 15 to 30 0 C. In some cases a suitable silicon alkoxide was stirred directly into the nanocrystalline TiO 2 sol. In this case, the procedure was conducted as follows: to a measured volume of TiO 2 aqueous sol was added the appropriate silicon alkoxide. The stirring is carried out at ambient temperature within a period of 1 to 24 hours, whereby the hydrolysis and the condensation of the SiO 2 precursor are achieved. Subsequently, other components are added to the prepared sol.

(iii) The TiO 2 hydrolysate acts as a binding agent for colloidal SiO 2 and colloidal TiO 2 particles, and as a catalyst for the polymerization of epoxy organosilanes. It is not indispensable that the SiO 2 hydrolysate and the TiO 2 hydrolysate are both present simultaneously in the self-cleaning liquid. As TiO 2 precursors are chosen titanium alkoxides, wherein alkoxy groups comprise 1 to 4 carbon atoms. Examples of employed titanium tetraalkoxides are titanium tetramethoxide, titanium tetraethoxide, titanium tetrapropoxide, titanium tetraisopropoxide, and titanium tetrabutoxide. Owing to its favorable price and sufficiently high stability, the best choice is titanium tetraisopropoxide (TTIP). The TiO 2 hydrolysate is prepared separately, namely by the drop-wise addition of a mixture of titanium alkoxide (10 to 30 ml_) and an organic solvent (5 to 15 mL) to a mixture of water (0.5 to 2 mL), cone. HNO 3 (0.5 to 2 ml_), and an organic solvent (4 to 10 mL). The optimum molar ratio between water and titanium alkoxide has to be within the range of 1 :1 to 3:1 , in any case less than 4:1. A higher molar ratio between water and titanium alkoxide results in complete hydrolysis and condensation of titanium alkoxide. A completely condensed titanium alkoxide is losing its binding ability. The obtained mixture is stirred at ambient temperature for 1-10 hours to ^attain" the hydrolysis and κ the condensation of titaniurrr alkoxide. The prepared hydrolysate contains no crystalline TiO 2 , and whereby it differs from TiO 2 sol (i). Finally, the obtained hydrolysate is diluted with an organic solvent, so that the final mass fraction of TiO 2 is within the range of 1 to 5 %. The organic solvent is chosen from monoalkyl ethers of various glycols having a high wettability of partially hydrophobic surfaces, such as for example glass. Suitable organic solvents are 2- methoxyethanol, propylene glycol butyl ether, and 2-propoxyethanol. The prepared TiO 2 hydrolysate is added to TiO 2 sol (i), so that the molar ratio between Ti in TiO 2 anatase partcles (i) and Ti in TiO 2 hydrolysate (iii) is within the range of 5:1 to 1 :1.

(iv) Without the addition of an organic solvent to the self-cleaning liquid (otherwise water-based), the deposition of the produced sol onto the support is not feasible, since a spotty, thin TiO 2 layer is left after evaporation of the solvent. Thus, the major function of the organic solvent is to improve the wettability of the self-cleaning liquid on various supports, especially hydrophobic supports (glasses and polymeric supports, such as polyacrylates, polycarbonates, polyethylenes, polypropylenes, polystyrenes..). Because the wettability of the support is improved with the aid of solvents, the properly chosen higher-viscosity solvents enable the deposition of a thicker layer of self- cleaning liquids, which results in enhanced photocatalytic efficiency. The volatility of the employed organic solvents must not be too low, so that they can evaporate at ambient temperature in a sufficiently short time. At the same time, their boiling temperature must not be under 100 0 C, so that they do not evaporate from the surface before the evaporation of water. The utilized organic solvents must be at least partially soluble in water, as the self-cleaning liquid contains a high fraction of water. In addition, the employed solvents must not cause any aggregation of TiO 2 anatase granules. Further provisions are non-toxicity and low volatility of the employed compounds. Suitable choices are monoalkoxy ethers of various glycols, and mixtures of monoalkoxy ethers of glycols and primary alcohols having less than 4 carbon atoms. The best combination proved to be 2-propoxyethanol in combination with 1-propanol, and 2-methoxyethanol in combination with 1-butanol. The volume fraction of the sum of organic solvents in the final photocatalytic liquid must be less than 0.8. If it is higher, the result is the aggregation of TiO 2 . A too low fraction of organic solvents causes problems, how to deposit uniformly a sufficiently thick TiO 2 /SiO 2 layer. The optimum volume fraction of the sum of organic solvents is within the^range of 0.5 and O.8.- (v) Water is added to the final photocatalytic liquid to lower the cost of the product, and to preserve the stability of the produced photocatalytically active liquid. A too high fraction of organic solvents causes the destabilization of the sol, resulting in the aggregation and precipitation of TiO 2 . Such a product is not utilizable. The water volume fraction in the final photocatalytically active liquid must be within the range of 0.7 to 0.2.

The third object of the invention is the deposition of photocatalytically active liquids on various surfaces. The surfaces may be transparent for visible light, such as for example glass, polypropylene, polyethylene, polystyrene, various polyacrylates, polycarbonate... The self-cleaning liquid may be also applied onto surfaces that are not light transparent: concrete, roofing tiles, ceramic tiles, wood, bricks... The surface, on which the self-cleaning liquid is to be applied, has to be clean and dry. Employed are various coating methods: (i) spraying of the liquid by means of sprayers; (ii) applying the liquid by means of a roller; (iii) applying the liquid by means of a brush; (iv) applying the liquid by means of a cloth-piece; (v) immersion of the body into the self- cleaning liquid.

(i) The produced self-cleaning liquid is poured into the sprayer, which may be hand- operated or power-operated, till the formation of a homogeneous liquid film on the surface.

(ii) The roller is dipped into the self-cleaning liquid, whereupon the liquid is applied to the surface. The passage of the roller leaves a homogeneous liquid film drying at ambient temperature.

(iii) The brush is dipped into the self-cleaning liquid, whereupon the liquid is spread onto the surface. The passage of the brush leaves a homogeneous liquid film drying at ambient temperature.

(iv) As the cloth-piece is drawn over the surface it leaves a very thin trace of a fast drying and hardening sol. The formed layer is very thin.

(v) The body to be coated with the self-cleaning layer is immersed into the self- cleaning liquid. The body is then uniformly drawn out of the liquid, whereby the excess liquid flows off. The residual self-cleaning liquid on the body is dried at ambient temperature, and hardened. The application on the support surface leaves in all enumerated Examples a thin liquid film, which dries at ambient temperature forming a solid, partially crystalline, self- cleaning coating, which is transparent for visible light. It has a thickness of less than 100 nm, for the majority of application modes even less than 20 nm. This is, however, in addition to high optical quality also sufficient for good self-cleaning efficiency. The hardening of the coating by heating is not compulsory but optional. The coating is thermally stable (X-ray diffractogramms demonstrate the stability of the anatase phase at a temperature up to 1000 0 C; only then commences the transition into the rutile phase). It has proven self-cleaning properties, and its hydrophilicity is evidenced even in darkness, but is further enhanced under the influence of UV radiation.

Such coatings may also be applied onto surfaces, where mist poses a problem. Accordingly, they prevent the formation of mist on various mirrors (for example car rear mirrors, mirrors in bathrooms, saunas), glasses (for example dioptric glasses).

The invention is illustrated, but in no way limited by the following embodiments.

Example 1

TTIP (15 ml_) is dissolved in absolute ethanol (2.5 ml_). Separately is mixed perchloric acid (70 %; 1 ml_) with water (90 ml_). This solution is added drop-wise with stirring to the solution of TTIP. An exothermal reaction of uncontrolled hydrolysis and condensation of TTIP takes place yielding a white precipitate of hydrated amorphous TiO 2 . The obtained mixture is kept under reflux for 48 hours causing the crystallization and disaggregation of TiO 2 . After the termination of the heating a stable stock sol is obtained.

Example 2

TTIP (15 ml_) is dissolved in absolute ethanol (2.5 ml_). Separately is mixed nitric(V) acid (65 %; 1 ml_) with water (90 ml_). This solution is added drop-wise with stirring to the solution of TTIP. An exothermal reaction of uncontrolled hydrolysis and condensation of TTIP takes place yielding a white precipitate of hydrated amorphous TiO 2 . The obtained mixture is kept under reflux for 48 hours causing the crystallization- and disaggregation of TiO 2 . After the termination of the refluxing the sol is cooled and filtered through a filter paper. The insoluble residue on the filter paper is discarded.

Example 3

The stock anatase TiO 2 sol is prepared according to the same procedure as described in Example 1 , except that the proportion of TTIP, HCIO 4 and water is changed. This Example is performed with 15 mL of TTIP, 45 mL of water and 3 ml_ of HCIO 4 .

Example 4

The stock anatase TiO 2 sol was prepared according to the same procedure as described in Example 1 , except that the proportion of TTIP, HCIO 4 and water was changed. This Example was performed with 15 mL of TTIP, 45 mL of water and 1 mL of HCIO 4 .

Example 5

The stock anatase TiO 2 sol was prepared according to the same procedure as described in Example 1 , except that the reflux time was changed. In this Example the sol was refluxed for 24 hours.

Example 6

To a mixture of TEOS (1.11 mL) and colloidal SiO 2 Levasil 200/30% (0.42 mL) is added under stirring HCI (30 μL; 32 %). After stirring for 10 minutes at ambient temperature is added 1-propanol (5 mL) and kept stirring for 24 hours, in order to achieve the final hydrolysis of TEOS. The mass fraction of hydrolyzed and colloidal SiO 2 in the prepared hydrolysate is 8.1 %.

Example 7

To a mixture of TEOS (1.11 mL) and colloidal SiO 2 Levasil 200/30% (1.7 mL) is added under stirring HCI (30 μL; 32 %). After stirring for 10 minutes at ambient temperature is added 1-propanol (5 mL) and kept stirring for 24 hours, in order to achieve the final hydrolysis of TEOS. The mass fraction of hydrolyzed and colloidal SiO 2 in the prepared hydrolysate is 11.5 %. Example 8

To the stock sol of Example 1 (6 ml_) is added the SiO 2 sol of Example 6 (6 ml_). The stirring is performed at ambient temperature, and then are added: water (8 ml_), thereafter 1-propanol (12 ml_), and finally 2-propoxyethanol (39 ml_). The thus obtained sol exhibits a Ti/Si molar ratio of 1 :2.3; the TiO 2 concentration in the sol is approximately 3.3 g/L. The produced sol is stable for at least six months. For the manufacture of a thin layer is chosen a cleaned glass surface in vertical position. The sol is poured into the sprayer. Thereafter the sol is sprayed over a vertically positioned surface, so that it spreads over it completely. The excess liquid is left to flow from the glass surface; the residue dries on the surface. The obtained layer is left for at least one week at temperatures over 20 0 C, so that the final condensation of SiO 2 and the hardening of the thin layer take place.

Example 9

To the stock sol of Example 1 (6 mL) is added the SiO 2 sol of Example 7 (6 ml_). The stirring is performed at ambient temperature, and then are added: water (8 mL), thereafter 1-propanol (12 mL), and finally 2-propoxyethanol (39 mL). The thus obtained sol exhibits a Ti/Si molar ratio of 1 :3.75; the TiO 2 concentration in the sol is approximately 3.3 g/L. The produced sol is stable for at least six months. For the manufacture of a thin layer is chosen a cleaned glass surface in vertical position. The sol is poured into the sprayer. Thereafter the sol is sprayed over a vertically positioned surface, so that it spreads over it completely. The excess liquid is left to flow from the glass surface, the residue dries on the surface. The obtained layer is left for at least one week at temperatures over 20 0 C, so that the final condensation of SiO 2 and the hardening of the thin layer take place.

Example 10

To the stock sol of Example 1 (6 mL) is added the SiO 2 sol of Example 7 (6 mL). The stirring is performed at ambient temperature, and then are added 1-propanol (7.5 mL), and finally 2-propoxyethanol (27 mL). The thus obtained sol exhibits a Ti/Si molar ratio of 1:3.75; the TiO 2 concentration in the sol is approximately 5 g/L. The produced sol is stable for at least six months. For the manufacture of a thin layer is chosen a cleaned glass surface in vertical position. The sol is poured into the sprayer. Thereafter the sol is sprayed over a vertically positioned surface, so that it spreads over it completely. The excess liquid is left to flow from the glass surface, the residue dries on the surface. The obtained layer is left for at least one week at temperatures over 20 0 C, so that the final condensation of SiO 2 and the hardening of the thin layer take place.

Example 11

To the stock sol of Example 1 (5 ml_) is added TEOS (450 μl_). A two-phase system is obtained, owing to the poor solubility of TEOS in aqueous medium. The stirring is kept at ambient temperature resulting in a gradual hydrolysis and condensation of TEOS. After 12 hours TEOS is not present any more as the stock precursor; the sol, however,, retains its stability. To the prepared mixed sol are added: water (40 ml_), 2- methoxyethanol (85 ml_), and 1-butanol (85 ml_), and stirred vigorously. The thus obtained sol exhibits a Ti/Si molar ratio of 1 :1 ; the TiO 2 concentration in the sol is approximately 0.9 g/L. The produced sol is stable for at least six months at ambient temperature.

For the manufacture of a thin layer is chosen a cleaned glass surface. The prepared sol is applied onto the surface of whichever material by means of any of the described methods (a cloth-piece, a sprayer, a brush, by immersion ...).

Example 12

To the stock sol of Example 1 (5 ml_) is added TEOS (400 μl_). A two-phase system is obtained, owing to the poor solubility of TEOS in aqueous medium. The stirring is kept at ambient temperature resulting in a gradual hydrolysis and condensation of TEOS. After 12 hours TEOS is not present any more as the stock precursor; the sol, however, retains its stability. To the prepared mixed sol is added water (40 ml_), and stirred vigorously. The thus obtained sol exhibits a Ti/Si molar ratio of 1.3:1 ; the TiO 2 concentration in the sol is approximately 4.2 g/L. The produced sol is stable for at least six months at ambient temperature.

For the manufacture of a thin layer is chosen a cleaned glass surface. A cotton cloth- piece is dipped into the diluted sol, whereupon it is used to apply a very thin layer onto the glass. After the evaporation of the solvent from the deposited sol, a very thin film of a photocatalytically active coating is left on the glass.

Example 13

To a mixture of water (0.30 mL), cone. HNO 3 (0.84 ml_) and 2-propoxyethanol (4.1 ml.) is added drop-wise a mixture of TTIP (4.7 mL), and 2-propoxyethanol (9.5 mL). The obtained mixture is kept stirring for 4 hours at ambient temperature to attain the hydrolysis and condensation of titanium alkoxide. Finally, the obtained hydrolysate is diluted with 2-propoxyethanol (35 mL). The mass fraction of TiO 2 in the prepared hydrolysate is 2.5 %.

Example 14

To the stock sol (5 mL) of Example 1 is added colloidal SiO 2 sol Levasil 200/30% (0.47 grams), which was premixed with 415 μL of GPMS and 10 mL of 2-methoxyethanol. The stirring is kept on for 2 hours. Thereafter is added partially hydrolyzed TTIP (TiO 2 hydrolysate) (4.1 mL) prepared according to the process described in Example 9. The sol preserves its stability. To the obtained mixed sol are added water (40 mL), 2- methoxyethanol (75 mL), and 1-butanol (85 mL), and stirred vigorously. The yielded sol contains 1.3 g/L of TiO 2 per liter of sol. The molar ratio of anatase Ti/non-crystalline Ti from the hydrolysate/Si from GPMS/Si from the colloid is 1 :0.5:1 :1. The prepared sol is applied onto the surface of whichever material by means of any of the described methods (by a cloth-piece, a sprayer, a brush, by immersion ...). The obtained thin layer is heated for half an hour at 90 0 C.

Characterization

The formation of an anatase crystalline phase was confirmed by X-ray diffraction (FIG. 1). The anatase crystalline phase is already present in the powder sample, which is obtained from thicker coatings with the aid of a sharp scraper, after drying at ambient temperature. Even after the thermal treatment of the sample at 1000 0 C, it still predominates over the rutile phase. Only at higher temperatures is the anatase phase completely converted into the rutile phase. It should be emphasized, with respect to the scientific literature that this is one of the widest, if not the widest temperature window of the thermal stability of the photocatalytically active anatase phase (from ambient temperature to 1000°C). In this manner, the photocatalytic function of the coating is assured even at extremely high temperatures. This may be useful in case of coating an object (for example a brick or a ceramic tile), which has to be subjected to thermal treatment at high temperatures during the finalization step.

FIG. 1 represents X-ray diffractogramms of a sample prepared from thin layers of Tiθ 2 /SiO 2 , which were manufactured according to the process described in Example 11 , namely a thermally non-treated, as well as a thermally treated sample, at different temperatures.

The photocatalytic activity of the produced coatings was proven by a discoloration assay on resazurin. The procedure is described in the article Evans, P.; Mantke, S.; Mills, A.; Robinson, A.; Sheet, D. W. J Photochem Photobiol A Chem 2007, 188, 387- 391. On FIG. 2 the photo on the left side represents the situation at time t=0 min., and the photo on the right side at time t=45 min. of UVA irradiation with a flux of 4 mW/cm 2 . The sample on the left side of each photo represents glass coated with a thin TiO 2 /SiO 2 layer, which was prepared according to the process described in Example 11. The sample on the right side of each photo represents blank glass reference. The color change from blue to pink is a sign of photocatalytic activity on the surface under the dye.

The hydrophilicity of the formed coatings is confirmed by measuring the contact angles. The measured angle between a water droplet and the support surface of Example 9 on a conventional window glass at time t=0 min. is within 30 to 40°. At time t=20 min. of UVA irradiation with a flux of 0.004 W/cm 2 the contact angle decreases from the starting value to 8-9°, which proves the high hydrophilicity of the coating in the presence of UV radiation.

The high transparency of the self-cleaning coating of Example 11 over the whole visible light region is evidenced by the UV-VIS spectrum of the coating deposited on sodium glass (FIG. 3). All types of coatings prepared according to the above described different processes transmit over 95 % of visible light. FIG. 3 represents the UV-Vis spectrum of the sample in comparison with the blank glass support.

The stability of thin self-cleaning coatings was tested in an accelerated weathering exposure chamber with continuous condensation (humidity chamber, produced by Erichsen). The samples were subjected to extreme conditions for 5 weeks. In the humidity chamber were tested the samples of Examples 9 and 10, which were applied to the cleaned glass surface. The analysis of contact angles and photocatalytic activity with the aid of resazurin, prior to the treatment and after the treatment, proved that the layers lost some activity; photovoltaic activity and photoinduced hydrophilicity were, however, still present on the coated glass samples after the treatment.