What chemical makes self-cleaning windows possible?
Titanium dioxide. Cleaning windows is not one of life’s great pleasures. So how about a window that cleans itself? Sounds like science fiction? Well, it isn’t! Self-cleaning windows are here, thanks to recent developments in “photocatalysis” and “superhydrophilicity.” Tongue-twisting terms, but well worth exploring. Titanium dioxide is a naturally occurring pigment long used to impart a white colour to paints, inks, plastics, paper, ceramics, cosmetics and even food. When used as a pigment titanium dioxide is inert, but when ground into extremely fine particles, it exhibits photocatalysis. This simply means that when exposed to light, in this case specifically to ultraviolet light, it is capable of acting as a catalyst for various chemical reactions. On exposure to ultraviolet light, titanium dioxide releases energetic electrons, which in turn react with oxygen and moisture in the surrounding air to form extremely reactive superoxide and hydroxyl free radicals. These immediately engage any organic compounds that happen to be around, breaking them down to carbon dioxide and water. Grime on windows is mostly a mix of organic compounds that can decompose in response to the photocatalytic activity of a thin layer of titanium dioxide applied to the glass. But light does something else to titanium dioxide as well. It makes it hydrophilic, or “water loving.” Normally glass is hydrophobic, or “water hating,” which is why water tends to form beads on its surface. When the beads drip, they leave tell-tale streaks. However, when titanium dioxide is applied to the surface, water, instead of beading, spreads out into an even this sheet. This means that when rain hits the window it spreads perfectly across the surface, and drains down evenly without leaving streaks. Presto, any dirt that has resisted the photocatalytic effect of titanium dioxide is rinsed away!
It isn’t only glass that can benefit from the free radical generating capability of titanium dioxide. Cement can be kept from graying as well! Perhaps the best example of this technology is the Jubilee Church in Rome, completed in 2003. Made of self-cleaning concrete, it is designed to last for a thousand years, hopefully maintaining its white luster. Air-borne substances can be decomposed as well when they contact a light activated titanium dioxide surface. Titanium dioxide coated tiles in toilets and operating rooms can destroy germs on contact and help fight the spread of disease. In Japan, coated pavement stones have been to reduce the effects of air pollution. In one study, photocatalytic paving decomposed about 15% of the nitrous oxide released by cars travelling on a roadway. Paints formulated with photocatalytic titanium dioxide can even battle indoor air pollutants such as smoke, formaldehyde and benzene. The shelf life of fruit can be increased by eliminating ethylene gas which is responsible for fruit ripening and powdered titanium dioxide has been used to treat polluted water. Those inventive Japanese have even come up with photocatalytic deodorant pantyhose! But the most significant application of this technology may turn out to be in cancer treatment. Injection of titanium dioxide into a tumour, followed by exposure to directed ultraviolet light generates free radicals capable of destroying the tumour without affecting surrounding tissue. It all sounds good, perhaps too good. What happens when the titanium dioxide treated products end up being discarded? Might the titanium dioxide find its way into the ecosystem and affect beneficial microbes? Nobody really knows. Indeed, we may see more clearly through self-cleaning windows, but the long term consequences of the photocatalytic technology are harder to see. And one more point. Only the outside of the self-cleaning windows is coated with the photocatalyst. So you will still need some vinegar and a dose of elbow grease for the inside.