Method for the photocatalytically active coating of surfaces

Abstract

A method for the photocatalytically active coating of surfaces is presented and described, as well as an article (1) photocatalytically actively coated according to this method. The object of providing a method for the photocatalytically active coating of, in particular, metallic surfaces, whereby a permanently stable coating is produced without negatively affecting the photocatalytic activity of the layer, is achieved by a method, in which a substrate article is prepared which has a surface, a metallic adhesion-promoting layer is applied to the surface of the substrate article, a photocatalytically active layer consisting of one or more metal oxides is applied to the adhesion-promoting layer, wherein the metallic adhesion-promoting layer and the surface of the substrate article consist of a different material and the adhesion-promoting layer is selected such that it is not oxidized or reduced by the photocatalytically active layer.

Claims

1. Method for the photocatalytically active coating of surfaces, in which: i) a substrate article (5) which has a surface (3) is prepared, ii) a titanium-containing metallic adhesion-promoting layer (7) is applied to the surface (3) of the substrate article (5), iii) a photocatalytically active layer (9) consisting of one or more titanium metal oxides is applied to the metallic adhesion-promoting layer (7) by cold gas spraying, wherein the metallic adhesion-promoting layer (7) and the surface (3) of the substrate article (5) consist of different material.

2. Method according to claim 1, wherein the photocatalytically active layer (9) is formed from a metal oxide selected from the group consisting of TiO.sub.2 and SrTiO.sub.3.

3. Method according to claim 2, wherein the photocatalytically active layer (9) is formed from titanium dioxide.

4. Method according to claim 3, wherein the photocatalytically active layer (9) is formed from anatase.

5. Method according to claim 1, wherein the substrate article (5) is selected from the series of sanitary, kitchen or medical articles, handholds, light switches, door handles, food belts, beverage filling plants, control elements, keyboards, and bedsteads.

6. Method according to claim 1, wherein the cold spraying is conducted with a spray material which has a particle size with a cross-section of between 5 and 150 m.

7. Method according to claim 1, wherein the pressure for the cold spraying is 20 to 100 bar.

8. Method according to claim 1, wherein the temperature for the cold spraying is 200 to 1200 C.

9. Photocatalytically actively coated article (1), which is obtained by a method according to claim 1.

10. Method according to claim 1, wherein the surface (3) of the substrate article (5) is a metallic surface.

11. Method according to claim 10, wherein the adhesion-promoting layer (7) is applied to the surface (3) by means of a thermal spraying process selected from the series: cold gas spraying, HVOF spraying, plasma spraying, and suspension spraying; or by means of a galvanic process.

12. Method according to claim 10, wherein the surface (3) of the substrate article (5) is formed from high-grade steel, aluminum or copper.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 a partial cross-section of the photocatalytically active coated metallic surface of an embodiment example of the article according to the invention.

(2) In FIG. 1 an embodiment example of a photocatalytically actively coated article according to the invention is represented. The article 1 comprises a substrate article 5 which, in the present embodiment example, is formed from high-grade steel, but in principle can also be formed from copper, aluminium or other, including non-metallic, materials and has a metallic surface 3. In this embodiment example, the metallic surface 3 is provided also made of high-grade steel.

(3) An adhesion-promoting layer 7 made of titanium, which has been applied by means of a thermal spraying process, in the present case using the cold gas spraying technique, adheres to the metallic surface 3. Finally, a photocatalytically active layer 9 made of titanium dioxide in the form of anatase, which is also applied by means of a thermal spraying process, in this case by means of the cold gas spraying technique, adheres to the adhesion-promoting layer 7.

(4) The two layers 7, 9 are applied such that the adhesion-promoting layer 7 physically isolates the photocatalytically active layer 9 from the metallic surface 3, with the result that no chemical reactions can occur between the metallic surface of the substrate material 3 and the photocatalytically active layer 9.

(5) Titanium is particularly well-suited as adhesion-promoting layer 7, as it forms a stable surface layer made of titanium dioxide which in turn is itself a very highly photocatalytically active substance and thus promotes the photocatalytic activity of the photocatalytically active layer 9.

EXAMPLES

(6) The chemical stability of the coatings decisively depends on the substrate material. Both in field testing and also in the DCA reactor (an aqueous solution of dichloroacetic acid) significant differences manifest themselves between different substrate materials.

(7) The TiO.sub.2 layers on titanium substrate have the best resistance. In the case of the DCA decomposition above all the TiO.sub.2 layers on copper substrate and in particular the layers on steel substrate exposed to the weather are strongly attacked and peeled off. In field testing the degradation phenomena occurred only in the case of the TiO.sub.2-coated substrates made of aluminium, copper and high-grade steel, which indicates that a chemical interaction takes place between the photo-catalyst and the substrate materials.

(8) Aluminium, as a non-noble metal (1.66 V vs. NHE) is also permanently coated with an oxide layer made of Al.sub.2O.sub.3. However, following the weather influences, in some areas the cold-gas sprayed samples show corrosion cracks, which are not visible on the starting substrates even after 680 days of field testing.

(9) Unlike aluminium and titanium, copper forms no corrosion-resistant oxide ceramic layer on the surface. Copper is corrosion-resistant because in the presence of water it forms a copper (I) oxide Cu.sub.2O layer (red) on the surface as an intermediate product. This layer must be penetrated by electrons and ions in order to allow further corrosion, resulting in extreme slowing down of the further progress of the corrosion as in the case of other passivation layers. Copper (II) oxide CuO is black. Initially, under the influence of TiO.sub.2 both the black-reddish oxides (mixture) are formed and also a greenish covering which however soon disappears again and gives way to the black oxides, i.e. CuO. The peeling off of the TiO.sub.2 coating in the DCA plant can be explained with reference to the DCA reaction: in the presence of oxygen dissolved in water, the Cu.sub.2O layer is no longer stable, but is oxidized to CuO. The high levels of activity of the TiO.sub.2 layers on copper substrates in the DCA decomposition are possibly also favoured by this reaction as, in the DCA decomposition reaction equation, dissolved oxygen must be broken down for the oxidation just as for the copper oxidation.

(10) The most interesting and at the same time most obscure behaviour is exhibited by TiO.sub.2 layers on steel substrate. The 1.4301 steel used here is an austenitic steel (cubic face-centred) and has a composition of iron with <0.08% carbon, 18-20% chromium and 8-10.5% nickel. The corrosion resistance of stainless steels is generally based on a corrosion-resistant chromium oxide layer on the surface of the steel. With a standard electrode potential of Cr.sup.3+/Cr.sub.2O.sub.2-7=+1.33 V vs. NHE, this chromium oxide layer is electrochemically very stable, and should therefore not contribute to the observed peeling off of TiO.sub.2. Nickel, which determines the austenitic structure of the steel, can also not be (solely) responsible for the peeling off of the TiO.sub.2 coating, because it is present dissolved in the iron matrix and a peeling off exclusively as a result of nickel oxidation would be unlikely. Presumably the iron matrix itself plays the decisive role in the delamination: when the steel substrate is coated by cold gas spraying the oxide layer on the surface is very probably broken, with the result that the embedded TiO.sub.2 particles are in direct contact with iron. Iron also forms a passivation layer made of Fe(OH).sub.3 which, with a standard electrode potential of Fe.sup.2+/Fe.sup.3+=+0.77 is less noble than the chromium oxide layer, but nevertheless, due to its poor solubility in water forms a watertight passivation layer. TiO.sub.2 is however able to reduce Fe.sup.3+ to Fe.sup.2+. The better solubility of Fe(OH).sub.2 could have led to a slow leaching of the metallic bond and thus to the observed peeling off of the TiO.sub.2 particles.