WO2008046588A1

WO2008046588A1 - Method for quantifying the photocatalytic activity of surfaces and use thereof

Info

Publication number: WO2008046588A1
Application number: PCT/EP2007/008973
Authority: WO
Inventors: Thomas Neubert; Frank Neumann; Michael Vergöhl
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2006-10-17
Filing date: 2007-10-16
Publication date: 2008-04-24
Also published as: EP2080011A1; DE102006049009B3; JP5043118B2; JP2010507077A

Abstract

The invention describes a quantifying measurement method for measuring the photocatalytic activity of surfaces. In this case, a thin layer of stearic acid is vapour-deposited onto the photocatalytic surface to be measured. The surface is then irradiated with UV light and the amount of light scattered by the layer of stearic acid (= optical haze) is measured at defined intervals of time. If the surface is photocatalytic, the layer of stearic acid breaks down without a residue, with the result that the optical haze falls to the value of the uncoated surface. A quantitative photocatalytic activity of the surface can then be determined from the time-dependent curve profile of the optical haze.

Description

Method for quantifying the photocatalytic activity of surfaces and their use

The invention describes a quantifying measuring method for measuring the photocatalytic activity of surfaces. In this case, a thin stearic acid layer is vapor-deposited onto the photocatalytic surface to be measured. The surface is then irradiated with UV light and measured at defined intervals of the light scattered by the stearic acid light component (= optical Haze). If the surface is photocatalytic, the layer of stearic acid degrades without residue, so that the optical haze drops to the value of the uncoated surface. From the time-dependent curve of the optical Haze, a quantitative photocatalytic activity of the surface can then be determined.

The measurement of the photocatalytic degradation reaction with an organic test substance is already at a standstill of the technique. The organic test substance can be present as gas, liquid or solid layer on the photocatalytic substrate. Frequently, the degradation reaction is analyzed by quantitative analysis of the chemical degradation products. This requires appropriate know-how for chemical analysis as well as cost-intensive equipment (gas chromatographs, infrared spectrometers, etc.).

In order to be able to measure a photocatalytic degradation reaction in a practical manner, an organic test substance must first be applied to the surface to be measured. The quality of the measurement is closely linked to the homogeneity of the applied test substance layer. In principle, the larger and more homogeneous the test substance can be applied, the more reproducible and more accurate are the measurement results. In the conventional methods, the organic test substance (methyl stearate, stearic acid, etc.) is usually dissolved in an organic solvent, applied to a defined area on the surface to be measured and then evaporated. As a result of drying, larger crystallites and layer thickness inhomogeneities often form in the form of concentric circles around the center, which severely impairs measurement accuracy and reproducibility. In addition, the use of many organic solvents (e.g., hexane) necessary for this purpose is not safe for health reasons, so that suitable suction devices must be present.

DE 10 2004 027 118 A1 discloses a method for the quantitative determination of the photocatalytic degradation of organic dyes on photocatalytically active surfaces by means of fluorescence analysis. The object of the present invention is to specify a simple measuring method which makes possible an efficient, economical and accurate determination of the photocatalytic activity of a substrate.

This object is achieved by the method having the features of patent claim 1. With claim 13 uses of the method are called. The respective dependent claims represent advantageous developments.

According to the invention, a method is provided for quantifying the photocatalytic activity of a surface of a substrate in which a thermally vapor deposited layer in direct contact with the surface of the substrate containing at least one light scattering organic compound is irradiated with short wavelength electromagnetic radiation and by optical measurement methods the time course of the decrease in the amount of the compound caused by photocatalytic degradation is determined.

Such a procedure brings significant advantages:

• The vapor deposition allows the test substance layers (stearic acid) to be applied very homogeneously over a surface area of several square centimeters.

• The layer thickness and the coated surface can be freely selected over a large area. • The application is simple, reproducible and requires only a small technical effort.

• The use of haze thickness measurements allows a non-destructive measurement which is very sensitive to thin stearic acid layers (<300 nm).

• It is not necessary to use partly toxic organic solvents (for example hexane), such as when applying organic test substances by means of solvents.

• The measurement of transparent as well as opaque photocatalytic substrates is made possible by the scattered light can be measured either in transmission or in reflection

In principle, two alternative embodiments of the method are possible. In a first advantageous variant, the layer containing at least one light-scattering organic compound is vapor-deposited directly onto the surface of the substrate.

An alternative embodiment of the method provides that the layer is applied to a further, for electromagnetic radiation in the optical measuring method at least partially transparent substrate and this further substrate is brought into contact with the coated surface of the substrate to be measured. The contact can be non-positive and / or positive. For example, the stearic acid can be evaporated on a UV-transparent quartz glass substrate, wel The coated side is placed on a photocatalytically active surface. The degradation reaction can be measured here by scattered light measurements of the reflected light. As a result, the measuring method can also be used for smooth substrates, which can not be directly coated with stearic acid (built-in photocatalytic product, products with too large dimensions).

Furthermore, it is favorable when the vapor deposition under reduced pressure of between 10 ^-6 mbar and 10 mbar, is preferably carried out between 10 ^-3 mbar and 1 mbar, more preferably between 10 ^-3 mbar and 10 ^"1 mbar.

Likewise, experiments have shown that it is advantageous if the vapor deposition takes place at an elevated temperature, ie, the organic substance is heated during vapor deposition. However, it is essential that the vapor deposition takes place at a temperature which is below the decomposition temperature of the organic compound. For example, for stearic acid temperatures between 300 ⁰ C and 400 ⁰ C are generally used.

The organic compound is preferably selected from the group consisting of vaporizable organic compounds and / or organic fatty acids and their derivatives. According to the invention, derivatives are understood as meaning inorganic and / or organic fatty acid derivatives. These include, for example, esters, ethers and / or salts. In principle, any known organic fatty acid can be used for the process. In particular, the fatty acid is selected from the saturated, monounsaturated and / or polyunsaturated fatty acids. Very particular preference is given to stearic acid and / or methyl stearate. Stearic acid or methyl stearate are biologically completely harmless and very inexpensive.

According to the invention, the layer thickness produced in the process does not play an essential role in carrying out the process, but preferably a coating is produced which has a thickness of from 1 to 1000 nm, preferably from 50 to 300 nm.

Furthermore, it is favorable if the irradiation of the coating takes place over a period of 1 minute to 48 hours, preferably from 10 minutes to 12 hours, particularly preferably from 30 minutes to 6 hours. The irradiation may be continuous, i. throughout the period, but it is also possible to perform the irradiation at intervals. The measurement of the decrease in the amount of the applied compound can be carried out simultaneously with the irradiation, but also after completion of irradiation and / or in the breaks, if the irradiation is carried out at intervals.

UV light is preferably used during the irradiation, in particular radiation having a wavelength between 100 nm and 800 nm, preferably 300 nm and 500 nm, particularly preferably between 360 nm and 420 nm.

According to the invention, it does not matter with what intensity the coating is irradiated on the substrate, the only factor being that the irradiation takes place with a sufficiently high power, so that the most efficient degradation of the organic substance takes place. However, it has proved practicable if the irradiation is applied in an intensity of 0.1 mW / cm ² to 10 mW / cm ² inclusive, preferably from 0.5 mW / cm ² to 5 mW inclusive / cm ² , more preferably from 0.8 mW / cm ² to 1.5 mW / cm ² takes place.

In a further advantageous embodiment, the determination of the residual amount is carried out with optical measurement methods in the visible and / or infrared spectral range. In particular, the optical measurement methods are selected from the group consisting of determination of the absorption, fluorescence, spectral lipometry and / or scattered light measurement. In particular, the scattered light measurement is preferred. Depending on the embodiment, a further light source may be used, but in particular when determining the residual amount via fluorescence, the UV light source used in the irradiation may be the only light source required for carrying out the method, which in this case is another advantageous simplification of the experimental setup leads.

According to the invention uses of the method are also given. In particular, the method is suitable for quantifying the photocatalytic activity of substrates. Preferably, the substrates are selected from the group consisting of support structures which are coated with TiO ₂ , ZnO, SrTiO ₃ , and / or K ₄ NbO ₇ , glass, ceramics, metal, plastics, wood and / or paper. Likewise, the method finds use for the analytical observation of optical changes of organic layers as a result of chemical reactions, eg oxidation and / or crystallization.

The present invention will be described with reference to the following exemplary embodiment with reference to the attached given figure explained without limiting the invention to the specific parameters used there.

embodiment

In the given figure, an apparatus used for the method is described by way of example, at which the execution of the sputtering step of the method is explained in more detail. In this case, a substrate 1, which is fixed on a fixing device 2, introduced into a coating chamber 3, to which a reduced pressure can be applied via a vacuum port 4. Furthermore, the coating chamber 3 has a heating device 5. For example, the heater can be made electrically and heated by a transformer 7. In this case, a crucible 8 with the organic compound used (in this case, stearic acid) is brought into thermal contact with the heating device. After adjusting the negative pressure / vacuum to 10 ^{~ 2} mbar heating of the organic compound (eg stearic acid) takes place, by the applied negative pressure, the boiling point of the organic compound is thereby lowered. Crucible and heater should be such that the heat radiation emitted by this is minimal and the evaporation temperature of the organic test substance can be achieved very quickly. The organic compound is then precipitated on the cool substrate by condensation / resublimation processes as a homogeneous layer with a uniform thickness. Processes that work with dissolved organic substances do not produce such a layer. After coating, irradiation is carried out with a UV lamp having a defined spectrum (as narrow as possible) and known intensity (eg UVA at 366 nm and 1.0 mW / cm ² ). At defined time intervals, the measurement of the layer quantity of the test substance is carried out using suitable optical measuring methods. These can be measurements of the absorption in the visible and in the infrared spectral range as well as measurements of the fluorescence or by means of spectral ellipsometry. In the case of deposited stearic acid, optical scattered light measurements (haze) are suitable since they already show a good correlation with the mass occupation for very thin stearic acid layers. To determine this correlation, appropriate calibration measurements are performed. For this purpose, various amounts of stearic acid are evaporated on glass substrates and the light scattered on these samples is measured. The mass coverage can be determined from the weight difference of the substrates before and after coating with a good analytical balance (measurement accuracy 1 μg). The relationship between optical Haze H and mass occupation μ can be well described by an exponential function (see FIG. 2 and equation 1).

H (μ) = H _max - (H _max -H _substrate ) 'exp (-B' μ) (Eq. 1)

Hazard measurements on the samples to be examined are now carried out at defined time intervals for the specific measurement. The temporal evolution of the Haze value H (t) can also be described by an exponential function (see FIG. 3 and equation 2).

H (t) = H _max - (H _max - H (t = 0)) ^• exp (R ^• t) (Eq. 2) Defining a decomposition rate r _D as the mass m decomposed during the photocatalytic reaction per area A and time t, this rate of degradation can be determined from the calibration parameter B and the fit parameter R obtained from the decomposition curve by equation 3.

Furthermore, it can define a photocatalytic activity PA as a number N _mo ieküi the destroyed stearic acid molecules per area A ₇ radiation intensity Φ and time t. This can be determined by Equation 4 r from the degradation rate _{D /} of the radiation intensity Φ and the molecular mass m _e i _mo Kuei of stearic acid.

pa = ^Nmolek '"= ^ (Eq.4)

Φ-At m _molecule -Φ

If one _knows the photon energy E _PhOt on and the chemical degradation mechanism or the number of destroyed bonds N _x needed for decomposition, equation 5 can be used to determine a quantum efficiency Q, which is defined as the number of destroyed bonds per incident photon ,

Claims

claims

A method of quantifying the photocatalytic activity of a surface of a substrate, comprising a thermally evaporated layer in direct contact with the surface of the substrate, comprising at least one light scattering organic compound selected from the group consisting of organic fatty acids and their derivatives Derivatives is irradiated with short-wave electromagnetic radiation and by scattered light measurement, the time course of the photocatalytic degradation-related decrease in the amount of the compound is determined.

2. The method according to claim 1, characterized in that the layer is evaporated on the substrate.

3. The method according to claim 1, characterized in that the layer is applied to a further, for e- lektromagnetische radiation in the optical measuring method transparent substrate at least partially and this further substrate is brought into contact with the coated surface of the substrate to be measured.

4. The method according to any one of the preceding claims, characterized in that the vapor deposition under reduced pressure between ICT ⁶ mbar and 10 mbar, preferably between 10 ^'3 mbar and 1 mbar, more preferably between 10 ^{~ 3} mbar and 10 ^"1 mbar.

5. The method according to any one of the preceding claims, characterized in that the vapor deposition takes place at a temperature which is below the decomposition temperature of the organic compound.

6. The method according to any one of the preceding claims, characterized in that the organic compound is selected from the group consisting of vaporizable organic substances.

7. The method according to any one of the preceding claims, characterized in that the organic compound is selected from the group consisting of stearic acid and / or methyl stearate.

8. The method according to any one of the preceding claims, characterized in that a coating tion is generated, which has a thickness of 1 to 1000 nm, preferably from 50 to 300 nm.

9. The method according to any one of the preceding claims, characterized in that the irradiation of the coating over a period of 1 min to 48 hours, preferably 10 minutes to 12 hours, more preferably 30 minutes to 6 hours.

10. The method according to any one of the preceding claims, characterized in that radiation of a wavelength between 100 nm and 800 nm, preferably 300 nm and 500 nm, more preferably between 360 nm and 420 nm is used for the irradiation.

11. The method according to any one of the preceding claims, characterized in that the irradiation in an intensity of 0.1 mW / cm ² to 10 mW inclusive / cm ² , preferably of 0.5 mW / cm ² up to and including 5 mW / cm ² , more preferably from 0.8 mW / cm ² to 1.5 mW / cm ² .

12. The method according to any one of the preceding claims, characterized in that the determination of the residual amount in the visible and / or infrared spectral range.

13. Use of the method according to any one of the preceding claims for the quantification of photocatalytic activity of substrates.

14. Use according to the preceding claim, characterized in that the substrates are selected from the group consisting of support structures which are coated with TiO ₂ , ZnO, SrTiO ₃ , and / or K ₄ NbO ₇ , glass, ceramic, metal, plastics, Wood and / or paper.

Use of the method according to any one of claims 1 to 12 for the analytical observation of optical changes of organic layers due to chemical reactions, e.g. Oxidation and / or crystallization.