Photocatalytic self-cleaning

Photocatalytic self-cleaning encompasses the synergy of two different mechanisms to enable a cleaning assistance effect. The direct oxidation of adsorbing contaminants leads to the decomposition of dirt, which is boosted by the superhydrophilic effect on the active catalytic surfaces in contact with water. A low water contact angle is achieved here by irradiating the surface with UV light. The formation of a thin water film makes contaminants easier to remove (easy-to-clean effect).

Self-cleaning surfaces are being used increasingly today for glass panes, mirrors, roofing tiles, photovoltaic modules and numerous other products. Fraunhofer IST offers a broad portfolio of suitable measuring techniques for the qualification of these products.

 

Determination of photocatalytic activity through the decomposition of methylene blue according to DIN 52980/ISO 10678

UV light in the methylene blue measuring stand.
© Fraunhofer IST
UV light in the methylene blue measuring stand.

For the characterization of active photocatalytic surfaces regarding their activity in the decomposition of organic molecules, a sample is brought into contact with methylene blue in a watery solution and irradiated with UV light. The decrease in the dye content of the solution is continuously determined during measurement using UV/Vis spectroscopy, so that a statement regarding the photocatalytic activity can be made by inference.

Determining the self-cleaning behaviour of coated glass surfaces – dirt test according to DIN EN 1096-5

Aufsprühen des Modellschmutzes im Dirt-Test.
© Fraunhofer IST
Spray application of sample contaminant in the dirt test.

The dirt test is used to determine the self-cleaning effect of coated glass surfaces under artificial weathering. A sample contaminant is applied to the specimen from a spray can. This is followed by alternating UV exposure and spraying with water to simulate sun and rain. Decomposition of the dirt solution is determined using a haze meter during the entire test and is considered an indicator for the surface cleaning effect.

Determining photocatalytic activity by measuring the contact angle according to ISO 27448

Change of the contact angle during UV irradiation and dark storage.
© Fraunhofer IST
Change of the contact angle during UV irradiation and dark storage.

With this method, a fatty acid film is applied to the specimen and decomposed under UV light. This changes the wetting behaviour of the surface. In the course of decomposition, the water contact angle is measured as one of the indices that influences the self-cleaning activity.

Determining the photocatalytic mineralisation performance through fatty acid decomposition

Überprüfung des Streuvermögens mittels Haze-Meter.
© Fraunhofer IST
Determining the diffusion factor using a haze meter.

With this test method, a fatty acid is applied to the active photocatalytic specimen so that a thin, cloudy film is formed. UV(A) radiation decomposes the fatty acid, fully mineralising it into H2O and CO2. The photocatalytic mineralisation performance of the specimen can be subsequently determined using scattered light measurement or directly based on the CO2 resulting from the decomposition reaction.

Determining photocatalytic effectiveness by means of luminescence reduction

Organic luminescence pigments to determine the photocatalytic effectiveness of surfaces.
© Fraunhofer IST, Falko Oldenburg
Organic luminescence pigments to determine the photocatalytic effectiveness of surfaces.

The luminescence reduction of tailor-made organic pigments is a reliable and fast method for evaluating the photocatalytic activity of surfaces and powders. Radicals formed during photocatalytic oxidation destroy the emission centre of the applied luminescence pigment and lead to a decrease of the luminescence intensity. This degradation is used to measure photocatalytic activity. Thus the luminescence reduction process enables the fast quantitative evaluation of photocatalytic materials, both under UV light and visible light.