Air pollution is often thought of as an outdoor problem, but the air inside buildings can be just as complex – and sometimes more polluted. Dr Dimitrios Kotzias, former official at the European Commission’s Joint Research Centre, has been exploring how innovative building materials containing semiconductor oxides might actively clean the air we breathe, both outdoors and indoors, using nothing more than light. Read More
At the centre of this work is titanium dioxide – a widely used oxide with semiconducting properties which is already found in paints, coatings, and construction products. What makes titanium dioxide special is its ability to act as a photocatalyst. When exposed to light, it triggers chemical reactions on its surface that can break down harmful air pollutants such as nitrogen oxides and volatile organic compounds like benzene and toluene – substances linked to respiratory and cardiovascular problems.
Kotzias’ research looks closely at how these reactions actually work at the microscopic level. When light hits titanium dioxide, it creates charged species, electron-hole pairs on the surface that react with oxygen and water. These reactions produce highly reactive species that can dismantle pollutant molecules, eventually turning them into less harmful substances such as nitrates, carbon dioxide, and water. Importantly, this process happens at room temperature and uses light as its main energy source, making it an attractive low-energy solution.
Early applications focused on outdoor environments, such as coating building façades along busy roads. But since most people spend the vast majority of their time indoors, Kotzias and colleagues also ask a crucial question: can this technology work inside buildings too?
The challenge is that titanium dioxide is mainly activated by ultraviolet light, which makes up only 3 to 5% of outdoor lighting and it is almost completely absent indoors. To overcome this, Kotzias and colleagues explored modifying titanium dioxide by adding tiny amounts of other elements, such as manganese. This process allows the material to respond to visible light, making it effective under indoor conditions. In laboratory studies, this modified titanium dioxide was able to remove nitrogen oxide efficiently using light levels similar to those found indoors.
However, the research also highlights important caveats. When titanium-dioxide-based paints break down air pollutants, they can also interact with the paint’s own ingredients. In some cases, this leads to the formation of unwanted by-products such as formaldehyde, which itself is an indoor air pollutant.
Overall, the research by Kotzias paints a nuanced picture. Photocatalytic coatings have real potential to reduce air pollution passively, using light and chemistry already all around us. But to be truly beneficial, especially indoors, these materials must be carefully engineered to clean the air without creating new problems in the process.
