Self-Cleaning Wall Paint

Research in catalysis has led to the development of a new wall paint that cleans itself when exposed to sunlight and chemically breaks down air pollutants.

Typically, a beautiful white wall paint does not remain beautiful and white forever. Various substances in the air often accumulate on the surface. This can be a desirable effect for a time, as it purifies the air – but over time the color changes and the paint needs to be renewed.

A research team from TU Wien and Università Politecnica delle Marche (Italy) succeeded in developing special titanium oxide nanoparticles that can be added to ordinary, commercially available wall paints to give them self-cleaning properties: these nanoparticles are photocatalytically active; they can use sunlight not only to bind substances in the air, but then to break them down. This way, the wall both purifies the air and cleans itself. Waste materials were used as raw materials for the new wall paint: metal scrap that would normally be discarded and dried fallen leaves. Indoor air contains a wide variety of pollutants, ranging from residues of cleaning agents and hygiene products, to molecules created during cooking, to substances released from materials like leather. In some cases this can lead to health problems; this is called "sick building syndrome."

Prof. Günther Rupprechter from the Institute of Materials Chemistry at TU Wien states: "For years, people have tried to clean the air using specialized wall paints. Titanium oxide nanoparticles are particularly interesting in this context. They can bind and break down a wide variety of pollutants."

However, adding ordinary titanium oxide nanoparticles directly to paint affects the paint's durability: just as they break down pollutants, they can also destabilize the paint itself and cause cracking. In the worst case, volatile organic compounds that could be harmful to health might even be released. After some time, the paint layer begins to look gray and dull, and eventually needs to be renewed.

However, the nanoparticles can clean themselves when irradiated with UV light. Titanium oxide is a photocatalyst that enables chemical reactions when exposed to appropriate light. UV rays create free charge carriers within the particles; these carriers allow pollutants captured from the air to be broken down into small fragments and removed. This way, pollutants are rendered harmless and do not permanently adhere to the wall paint. The paint's color remains stable over the long term.

However, in practice this is not very useful – after all, repeatedly irradiating the wall with intense UV light would be cumbersome. "For this reason, our goal was to modify these particles so that the photocatalytic effect could also be triggered by ordinary sunlight," explains Günther Rupprechter. This was achieved by adding certain atoms such as phosphorus, nitrogen and carbon to the titanium oxide nanoparticles. This changes the light frequencies that the particles can capture, so now not only UV light but also ordinary visible light can trigger photocatalysis.

Qaisar Maqbool, lead author of the study, states: "We have now examined this phenomenon in detail using a wide variety of surface and nanoparticle analysis methods. This allowed us to show exactly how these particles behave before and after being added to wall paint."

The research team mixed the modified titanium oxide nanoparticles with commercially sold ordinary wall paint and rinsed the painted surface with a solution containing pollutants. Subsequently, 96% of the pollutants could be broken down by natural sunlight. The paint's color does not change – because the pollutants are not only bound but also broken down with the help of sunlight.

To achieve commercial success for such paints, it is also important to avoid expensive raw materials. Günther Rupprechter states: "In catalysis, for example, precious metals like platinum or gold are used. But in our case, elements readily available everywhere were sufficient: we obtained phosphorus, nitrogen and carbon from dried leaves collected from olive trees; the titanium needed for the titanium oxide nanoparticles came from metal scrap that would normally end up in the trash."

This new type of wall paint offers multiple advantages at the same time: it removes pollutants from the air, has a longer lifespan compared to other paints, and its production is more resource-efficient because it can be made from recycled materials. Additional experiments are underway and commercialization of the wall paint is being pursued.

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