SCIENCE
Typically, white walls don’t stay white very long. They get dirty. Eventually they yellow and need repainting. But that could be about to change.
A research team from TU Wien — the Vienna University of Technology — and the Università Politecnica delle Marche in Italy has developed special titanium oxide nanoparticles that can be added to standard wall paint to give the paint the capability to clean itself. The nanoparticles use sunlight not only to bind substances from the air, but also to decompose them. The wall actually makes the air cleaner — and cleans itself at the same time.
There are numerous pollutants in indoor air — from residues of cleaning agents and hygiene products to molecules produced during cooking or that are emitted by materials such as leather. In some cases, this can lead to health problems, which, if bad enough, are referred to as “sick building syndrome”.
“For years, people have been trying to use customised wall paints to clean the air," says Prof. Günther Rupprechter from the Institute of Materials Chemistry at TU Wien. “Titanium oxide nanoparticles . . . can bind and break down a wide range of pollutants.”
However, simply adding ordinary titanium oxide nanoparticles to the paint will affect its durability: just as pollutants are degraded by the nanoparticles, they can also make the paint unstable and create cracks. In the worst case, volatile organic compounds can be released, which in turn can be harmful to health. After a certain time, the paint layer becomes grey and tinted. Finally it has to be repainted.
However, the nanoparticles can clean themselves if they’re irradiated with UV light. The UV creates free charge carriers in the particles, decomposing the trapped pollutants from the air. In this way, the pollutants are rendered harmless, but don’t remain permanently attached to the wall paint. The wall colour remains stable long-term.
In practice, however, it would be tedious to repeatedly irradiate the wall with intense UV light to drive the self-cleaning process. “Our goal was therefore to modify these particles in such a way that the photocatalytic effect can also be induced by ordinary sunlight,” explains Rupprechter.
The research team found as much as 96% of the pollutants could be degraded by natural sunlight. The colour itself doesn’t change because the pollutants are not only bound, but also broken down with the help of sunlight.
For such paints to be affordable, it’s important to avoid expensive raw materials. Rather than using precious metals such as platinum or gold, Rupprechter says the team has been using phosphorus, nitrogen and carbon from the dried fallen leaves of olive trees, and the titanium for the titanium oxide nanoparticles has come from scrap metal.