Titanium dioxide hogs the spotlight

In their unending quest to improve day-to-day life, researchers have processed a common pigment into a form that could lead to new generations of self-sterilizing bathroom tiles and antifog mirrors.

Each year, the United States produces some 1.5 million tons of titanium dioxide, a powder that’s widely used as a sunscreen and a white pigment for paints. In the past few years, chemists have also found that coatings of titanium dioxide can kill bacteria, catalyze the breakdown of dirt and toxic pollutants, and prevent water from building into tiny, fog-forming beads (SN: 3/21/98, p. 186).

But there’s a catch. Materials containing titanium dioxide–whether installed in air ducts, toilets, or operating rooms–behave in these beneficial ways only when exposed to ultraviolet (UV) radiation. Since only about 5 percent of sunlight and typical indoor light falls into the UV range, researchers often use UV lamps to activate these materials, says John T. Yates Jr. of the University of Pittsburgh.

Now, a team of Japanese researchers has discovered that adding nitrogen ions to the titanium dioxide makes coatings that can destroy organic molecules in the presence of visible light or UV wavelengths. Also, a nitrogen-spiked titanium dioxide film holds water so tightly, even in visible light, that the liquid flattens out. In other words, water on the surface doesn’t form the tiny droplets that would cloud a bathroom or car mirror.

“This should allow the main part of the solar spectrum and even poor illumination of interior lighting to be used” with these materials, says Ryoji Asahi of Toyota Central R&D Laboratories in Nagakute. His team reports the results in the July 13 Science.

Previously, other researchers had made related materials by combining titanium dioxide and metals. However, nitrogen-enriched titanium dioxide coatings have benefits over metal-enriched ones, such as better stability and lower cost, the team says.

Nitrogen-enriched coating could replace titanium dioxide materials that have been under development for several years, says Yates. Researchers have experimented with such materials for treating sewage and coating self-cleaning cars and windows, as well as uses in bathrooms and hospitals, says Yates. He and his colleagues now plan to test whether a coating might break down nerve gas on military vehicles as they sit in the sun.