Paving Out Pollution; February 2002; Scientific American Magazine; by Linda Wang; 1 Page(s)

Buildings, roads and sidewalks have developed an appetite for air pollution. Researchers in Japan and Hong Kong are testing construction materials coated with titanium dioxide-the stuff of white paint and toothpaste-to see how well they can fight pollution.

Better known as a pigment for whiteness, titanium dioxide can clear the air because it is an efficient photocatalyst: it speeds the breakdown of water vapor by ultraviolet light. The results of this reaction are hydroxyl radicals, which attack both inorganic and organic compounds, and turn them into molecules that can be harmlessly washed away with the next rainfall. But it wouldn't work to smear toothpaste on the sidewalk-the titanium dioxide crystals in such applications are too large (about 20 to 250 nanometers wide). The width of the pollution-fighting form is about seven nanometers, offering much more surface area for photocatalysis.