WO2006027106A1

WO2006027106A1 - Method for depositing photocatalytic titanium oxide layers

Info

Publication number: WO2006027106A1
Application number: PCT/EP2005/009129
Authority: WO
Inventors: Bert Scheffel; Christoph Metzner; Olaf Zywitzki; Volker Kirchhoff; Thomas Modes
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2004-09-03
Filing date: 2005-08-24
Publication date: 2006-03-16
Also published as: JP4847957B2; DE102004042650A1; DE102004042650B4; KR20070048723A; CN101018882A; JP2008511434A

Abstract

The invention relates to a method for depositing a photocatalytic titanium oxide layer on at least one object, by means of high-speed electron beam evaporation in a vacuum chamber. According to the invention, an atmosphere containing oxygen is created in said vacuum chamber, a material comprising, predominately, Ti-components is evaporated by means of an electron beam, the deposition is supported by a plasma, the latter being generated on the surface of the material which is to be evaporated as a cathode by means of a diffuse arc discharge, the coating rate is at least 20 nm/s, the object temperature is maintained at between 100 °C and 500 °C during the deposition process and the titanium oxide layer is deposited as crystalline and, predominately, as an anatase-phase.

Description

Process for depositing photocatalytic titanium oxide layers

The invention relates to a method for depositing photocatalytic titanium oxide layers on objects. The photocatalytic effect of titanium oxide layers has been known for years and is exploited in the industry to equip surfaces of objects, for example, with self-cleaning properties. In the case of UV irradiation, photocatalytic titanium oxide has u.v. in the presence of oxygen and water. a. the property of forming OH radicals that contribute to the decomposition of organic soil constituents.

Under UV irradiation is in titanium oxide layers in addition to the decomposition of organic.

Ingredients to observe another effect. As the UV irradiation of a titanium oxide layer increases, it also changes its wettability property with water toward a very hydrophilic surface. The decomposition of organic constituents, on the one hand, and their sub-rinsing and washing away due to the hydrophilicity, on the other hand, thus impart self-cleaning properties to an object surface consisting of photocatalytic titanium oxide.

In the literature, the scope of the term photocatalysis in connection with titanium oxide layers is described differently. In some writings, the term photocatalysis covers only the decomposition of organic components, in other authors, however, the hydrophilic effects are described by the term photocatalysis. It should therefore be noted at this point that the terms "photocatalytic" or "Photo¬ catalysis" in the sense of the invention include both the decomposition of organic constituents and the hydrophilic effects.

In order to show usable photocatalytic effects, a titanium oxide layer may not be arbitrarily formed. For this purpose, it is necessary that the titanium oxide layer is crystalline and is formed as anatase and / or rutile phase.

However, the photocatalytic effects of titanium oxide layers are inseparably coupled with UV irradiation, ie it is only a certain UV irradiation time or UV irradiation dose required until a titanium oxide layer shows sensibly utilizable photocatalytic effects. Titanium oxide layers can maintain these effects only for a certain period of time (relaxation time) after completion of UV irradiation. It is therefore desirable to form photocatalytic titanium oxide layers in such a way that, on the one hand, they require the shortest possible UV irradiation time in order to exhibit photocatalytic effects and, on the other hand, have a long relaxation time.

Various methods for depositing photocatalytic titanium oxide layers are known. A first group includes the so-called sol-gel method (DE 199 62 055 A1, DE 102 35 803 A1). In this case, a liquid or dispersion comprising titanium oxide constituents is first applied to an object to be coated. The application of the liquid / dispersion can be carried out, for example, by spraying, dipping or brushing. Subsequently, the liquid layer is dried and additionally hardened if necessary. Sol-gel processes can achieve high coating rates. Disadvantages arise from the fact that layers produced in this way have, in addition to titanium oxide, other constituents originating from the liquid / dispersion. The loss of concentration of titanium oxide constituents is also associated with limited photocatalytic properties of the deposited titanium oxide layer.

Another known process group for producing photocatalytic titanium oxide layers form CVD processes. From WO 98/06675 a three-step CVD method for coating glass objects is known. In a first step, a gas mixture is prepared, which in addition to titanium tetrachloride comprises an oxygen-containing organic component. Subsequently, this gas mixture is heated to a temperature which is below the threshold temperature at which the titanium from the titanium tetrachloride combines with the oxygen from the organic component to titanium oxide. In a coating chamber, this gas mixture is heated above the threshold temperature, with titanium oxide depositing on the glass object to be coated.

Magnetron sputtering is also known for depositing photocatalytic titanium oxide layers [O. Zywitzki et al., Surface and Coatings Technology, 180-181 (2004) 538] In contrast to CVD processes and sol-gel processes, magnetron sputtering processes can be used to achieve better photocatalytic properties in titanium oxide layers, but one drawback is the low coating rates, which are below 5 nm / s. All known processes for the preparation of photocatalytic titanium oxide layers have ge that they have too low coating rates and / or realize titanium oxide layers with only limited photocatalytic effects.

The invention is therefore based on the technical problem of providing a method with which photocatalytic titanium oxide layers can be deposited at a deposition rate of at least 20 nm / s. The deposited titanium oxide layers should achieve a better photocatalytic effect compared to the prior art, require a shorter UV irradiation time to trigger the photocatalytic effect and have a longer relaxation time of the photocatalytic effect after UV irradiation.

The solution of the technical problem results from the articles with the features of claim 1. Further advantageous embodiments of the invention will become apparent from the dependent claims.

According to the invention, a photocatalytic titanium oxide layer is deposited on at least one object by means of high-rate electron beam vapor deposition in a vacuum chamber by generating an oxygen-containing atmosphere in the vacuum chamber; a predominantly Ti constituent material is evaporated by means of an electron beam; the deposition is supported by a plasma, the plasma being generated by means of a diffuse arc discharge on the surface of the material to be vaporized connected as a cathode; the coating rate is at least 20 nm / s; Objekt¬ the temperature is maintained during deposition is between 100 ⁰ C and 500 ⁰ C, and the titanium oxide layer crystalline and predominantly anatase phase is deposited as.

Photocatalytic titanium oxide layers in the sense of the invention not only mean pure titanium oxide layers but also titanium oxide layers having doping elements. Objects which can be coated by the method according to the invention consist, for example, of glass (architectural glass panes, displays), metal (façade elements, semi-finished products in strip or plate form), ceramics (tiles, tiles) or plastic (plastic glazings, foils). However, objects to be coated can also consist of other materials and have surface layers of at least one of the materials exemplified above. An essential step of the method according to the invention is the generation of a plasma by means of a diffuse arc discharge. In this case, a high-energy electron beam incident on the surface of the Ti vaporization material is periodically deflected in such a fast and high-frequency manner that at least part of the surface of the Ti material to be vaporized is quasi-uniformly heated and ultimately vaporized. At the same time, the Ti material to be vaporized, which is located, for example, in a crucible, is switched as the cathode of a high-current arc discharge. It forms a so-called diffuse arc, which burns substantially in the area heated by the electron beam surface of the evaporation material. Compared to a cold cathode arc discharge, in which a not even 1 mm ² large base is formed with extremely high current density, a diffuse arc discharge has a diffuse and areal extent on the Evampfungsgut, which essentially corresponds to the quasi-uniformly heated surface of the evaporating material , As a result, a substantial proportion of the generated Ti metal vapor is ionized and thus a high degree of ionization is achieved overall, which contributes to the formation of a photocatalytic titanium oxide layer with improved properties compared to the prior art. The use of the diffuse arc discharge has the further advantage that it does not emit any spatter and is therefore particularly suitable for large-scale plasma-activated vapor deposition.

It is also advantageous if oxygen is introduced into a vacuum chamber in such a way that stoichiometric titanium oxide layers are deposited because these layers have the crystalline phases anatase and / or rutile in high concentrations. For this purpose, an oxygen partial pressure within the vacuum chamber of 5x10 ^"4 mbar to 1x10 ^{" 2} mbar is suitable.

In another embodiment, the deposition of a titanium oxide layer is performed preferred wise at an object temperature in a range from 200 ⁰ C to 300 ⁰ C, because the oxide layer at these temperatures Titan¬ predominantly anatase phase is deposited.

The application of a negative bias voltage in a range of 50 V to 300 V to an object to be coated, through which ionized Ti vapor or oxygen particles are accelerated towards the surface of the object, has an advantageous effect on the layer properties, such as density, Refractive index and chemical resistance of a titanium oxide layer. This negative bias voltage can, for example, with respect to a crucible, in which the Ti material to be evaporated is located, or with respect to an anode be switched. As a bias voltage, a DC voltage or a medium-frequency or high-frequency pulsed voltage can be applied to the object to be coated. The application of Pulsbias has a particularly advantageous effect on the stability of the process control.

In order to obtain a minimum of plasma activation, an arc current of the diffuse arc discharge to the surface of the evaporation material of at least 100 A aus¬ form. While maximum deposition rates of about 5 nm / s can be achieved, for example, by means of magnetron sputtering during the deposition of photocatalytic titanium oxide layers, the method according to the invention enables deposition rates of several hundred nm / s. Very good layer properties are achieved at deposition rates in a range of 30 nm / s to 120 nm / s and at layer thicknesses of 10 nm to 1 μm, preferably 20 nm to 100 nm.

When coating substrate materials, such as glass, elements from the substrate may diffuse into the titanium oxide layer, such that the titanium oxide layer is altered such that the photocatalytic properties of the titanium oxide layer are impaired. Therefore, in a further embodiment, at least one additional layer is deposited between an object to be coated and the titanium oxide layer to be applied thereon, which acts as a diffusion barrier. In this way, the diffusion of elements from the substrate (for example potassium in the case of a glass substrate) into the titanium oxide layer can be effectively prevented. Such layers acting as a diffusion barrier advantageously comprise SiO ₂ and have a layer thickness in the range from 10 nm to 200 nm.

The invention will be explained in more detail below with reference to a preferred embodiment.

The single figure shows schematically a device with which the method according to the invention can be carried out. In a vacuum chamber 1, an evaporator crucible 2 is arranged, in which is to be evaporated as the evaporation material 3 titanium. An¬ closed to the vacuum chamber 1 is a high-power axial electron gun 4, which generates an electron beam 5, which is deflected by means of a not shown elektro¬ magnetic deflection on the surface of the evaporator crucible 2 located in the evaporation evaporating material 3 and thus the evaporation material. 3 heated and finally evaporated. Above the vaporizer crucible 3, an electrode 6 is arranged, which encloses the vapor space and can be placed opposite to the vaporizer crucible 3 to a positive voltage. An object 8 made of glass moved over the electrode 6 on a transport device 7 is coated with the evaporated material.

By means of the electron beam gun 4, the high-energy electron beam 5 with a power of about 50 kW is rapidly, high-frequency and periodically deflected in such a way that at least part of the surface of the evaporation material 3 is quasi-uniformly heated and vaporized. A DC voltage of approximately 30 V applied between electrode 6 and evaporator crucible 2 by means of a power supply device 9 causes the formation of a so-called diffuse arc discharge with a current of approximately 300 A, which essentially burns on the surface of the evaporation material 3 which is quasi-uniformly heated by means of electron beam 5. As a result, a high degree of ionization of the vapor is achieved. A bias voltage of -100 V applied to the object 8 by means of a power supply device 10 causes the ionized vapor particles to accelerate to the surface of the object 8.

By introducing oxygen through a gas inlet system 11 into the vacuum chamber 1 during titanium evaporation, a 400 nm stoichiometric TiO ₂ layer is deposited on the object 8 at a steady state deposition rate of about 70 nm / sec. The object 8 is held at a temperature of about 250 ° C.

The TiO ₂ layer deposited on the object 8 using the method according to the invention has markedly improved photocatalytic properties compared to photocatalytic TiO ₂ layers which have been prepared by known processes. This was confirmed metrologically by experimental arrangements.

In a first experimental setup in which hydrophilic properties were investigated, the following three glass samples, each provided with a photocatalytic TiO ₂ cover layer, were compared:

Sample 1 is a commercially available glass sheet having a photocatalytic TiO ₂ layer produced by a CVD method,

Sample 2 a glass sheet with a photocatalytic TiO ₂ layer which was deposited by means of magnetron sputtering, Sample 3 a glass sheet with a abge¬ by the novel abge¬ photocatalytic TiO _r layer.

To characterize the photoinduced hydrophilicity, the contact angle of a water drop, each of which was applied to the samples with a fine cannula, was measured. From the width and height of a water drop, the contact angle with the surface of a sample can be calculated assuming a spherical segment. The smaller a detected contact angle, the better the hydrophilic properties of a sample surface. A contact angle of 0 ° corresponds to a complete wettability of a sample surface, which thus has optimum hydrophilic properties.

Initially, all three samples, which were stored in the dark for several weeks, were thoroughly cleaned. Subsequently, a contact angle measurement was carried out for all three samples and thus an initial value was determined. With a UV-A lamp with a wavelength range of 315-380 nm, the three samples were irradiated under the same conditions. This resulted in an irradiance of 0.5 mW / cm ² on the samples.

After selected time intervals, contact angle measurements were again made on the samples. The measurement results of the contact angles are shown in Tab.

Tab. 1

From Table 1 it can be seen that in the case of TiO ₂ layers which were produced by the process according to the invention, optimum hydrophilic properties were already determined during a measurement after a 15-minute irradiation with UV-A, whereas in the case of Procedure produced sample 1 these optimal properties could only be detected in a measurement after 45 min irradiation. Furthermore, the relaxation behavior of the hydrophilic properties of the glass sample surfaces was investigated. After a uniform UV-A irradiation time, after all three samples had a contact angle of 0 °, the samples were stored in the dark and determined after certain time intervals again an associated contact angle and thus the hydrophilic behavior of the samples. The results of these contact angle measurements are shown in Tab.

Tab. 2

While sample 1 had no optimum hydrophilic properties even after 3 h and sample 2 after 6 h, a contact angle greater than 0 ° was determined for sample 3, which was produced by the process according to the invention, only after a relaxation time of 10 h.

In a second experimental setup, the deposition of a photocatalytic TiO ₂ layer by means of vacuum evaporation was used to investigate the influence of the diffuse arc discharge on the properties of the deposited TiO ₂ layer. The subject of the investigation was the ability of a TiO ₂ layer to decompose organic substances under UV irradiation and the layer properties of density and refractive index.

In the second test arrangement, the following two glass samples, each provided with a photocatalytic TiO ₂ cover layer, were for comparison: sample 4 a glass pane which was coated by means of vacuum vapor deposition without the aid of a diffuse arc discharge, sample 5 a glass pane which was produced by means of the process according to the invention was coated. Studies on the density of the deposited TiO ₂ layers yielded values of 3.0 g / cm ³ for Sample 4 and 3.85 g / cm ³ for Sample 5. For the refractive index, values of 2.1 and 2.5 of sample 5 were determined for sample.

Furthermore, both samples were each wetted with a drop of a solution having a concentration of 0.01 mmol / L of the substance "methylene blue." Subsequently, both samples were irradiated under the same conditions with a UV-A lamp according to the first experimental set-up The decomposition process of the organic components of the test solution could be followed by the color of the solution with the naked eye.

For sample 4, complete decomposition of the organic components was observed after 72 hours, for sample 5 already after 48 hours. It was therefore possible to demonstrate that plasma deposition based on a diffuse arc discharge has a positive effect on the properties of the deposited TiO ₂ layer when a photocatalytic TiO ₂ layer is deposited by means of vacuum evaporation.

The main advantages of the process according to the invention are thus, on the one hand, the high coating rates resulting from vacuum vapor deposition and, on the other hand, the properties of titanium oxide layers that are improved over the prior art. This applies both to the photocatalytic properties such as the ability of decomposing organic particles and the hydrophilicity and other layer properties such as density, refractive index and chemical resistance.

Claims

claims

1 . Method for depositing a photocatalytic titanium oxide layer on at least one object (8) by means of high-rate electron beam vapor deposition in a vacuum chamber (1), characterized in that

- In the vacuum chamber (1) an oxygen-containing atmosphere is generated;

- A predominantly Ti constituents exhibiting material (3) by means of an electron beam (5) is evaporated;

the deposition of a plasma is assisted, the plasma being generated by means of a diffuse arc discharge on the surface of the material (3) to be vaporized connected as the cathode;

the coating rate is at least 20 nm / s;

- The object temperature during the deposition between 100 ⁰ C and 500 ⁰ C ge keep ge - and the titanium oxide layer is deposited in crystalline and predominantly as anatase phase.

2. The method according to claim 1, characterized in that the Sauerstoff¬ concentration in the vacuum chamber (1) is set such that a stoichiometric titanium oxide layer on the object (8) is deposited.

3. The method according to any one of the preceding claims, characterized in that a negative bias voltage of 50 V to 300 V is applied to the object (8).

4. The method according to claim 3, characterized in that the bias voltage is applied as a DC voltage or as a medium-frequency or high-frequency pulsed voltage an¬

5. The method according to any one of the preceding claims, characterized in that in the vacuum chamber (1) an oxygen partial pressure of 5x10 ^"4 mbar to

1x10 ^"2 mbar is generated.

6. The method according to any one of the preceding claims, characterized in that in the plasma activation, a arc current of at least 100 A is set.

7. The method according to any one of the preceding claims, characterized in that coating rates are formed in a range of 30 nm / s to 120 nm / s.

8. The method according to any one of the preceding claims, characterized in that the object temperature during the deposition is preferably maintained between 200 ⁰ C and 300 ⁰ C.

9. The method according to any one of the preceding claims, characterized in that layer thicknesses of 10 nm to 1 .mu.m and preferably from 20 nm to 100 nm are ab¬ separated.

10. The method according to any one of the preceding claims, characterized in that between the object and the photocatalytic titanium oxide layer, an intermediate layer is deposited as a diffusion barrier.

1 1. A method according to claim 10, characterized in that a SiO ₂ layer is deposited as a diffusion barrier.