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Light-Reactive Coating Cleans Surfaces

Photonics Spectra
Dec 2001
Paul Mortensen

If you shine ultraviolet light on a surface coated with titanium dioxide, a common white pigment in paint, it self-sterilizes and defogs. Until now, however, if you used visible light, only approximately 5 percent of which is in the UV region, it would not.

The addition of nitrogen to titanium oxide extends the optical absorption of the film into the visible range, enabling its use on self-cleaning and -sterilizing surfaces. After one minute of exposure to a tungsten lamp, the organic constituents of fingerprints -- perspiration and oil -- are breaking down on the surface coated with the photocatalyst. Courtesy of Ryoji Asahi, Toyota Central R&D Laboratories Inc.

Researchers at Toyota Central R&D Laboratories Inc. in Nagakute, Japan, have discovered that adding nitrogen ions to titanium dioxide yields a photocatalytic coating that displays these effects under visible light. The possible applications include antifogging mirrors and glass, self-sterilizing bathroom tiles and self-cleaning computer touch-displays.

To produce the coatings, the researchers sputter titanium dioxide in a mixture of nitrogen and argon gas. They perform postannealing in nitrogen at 550 °C for four hours. The resulting 300-nm-thick films are yellowish, transparent and crystalline.

Nitrogen-doped titanium dioxide is active up to approximately 500 nm. Upon exposure to this range of visible light, the photocatalytic coating destroys any organic molecules it contacts by exciting electron/hole pairs in the conduction band at the surface. If oxygen or water is present, superoxides and hydroxyl radicals form, which also attack any organic molecules.

Moreover, these hydroxyl radicals strongly attract any other water on the surface, breaking its surface tension and flattening the contact angle to 6°. (A typical contact angle for water on a glass surface is 20° to 50°.) Because the water thus cannot form droplets, the surfaces treated with the film do not cloud or fog.

The researchers have determined that the doped films work in everyday lighting situations. They tested the photodegradation of acetaldehyde gas on a table coated with nitrogen-doped titanium oxide at an illumination of approximately 300 lx across the visible spectrum, which is typical of fluorescent lighting in a Japanese living room. They found that the decomposition rate of the gas by a nitrogen-doped coating was 10 times faster than that by titanium dioxide alone.

Combining films with InGaN LEDs

The scientists are studying the use of the doped coating with InGaN LEDs, which have a peak emission of 390 to 420 nm. The coating can use the full range of the LEDs' output, and they believe that this combination can be developed into compact and inexpensive photocatalysis units, such as air or water purifiers. Another possible application is self-cleaning computer touch-displays, on which the organic constituents of fingerprints – perspiration and oil – can be broken down. They also are looking at other deposition techniques to optimize the photocatalytic performance for the desired application according to cost, coating area and any temperature restrictions of the substrate.

Although the researchers hope to market the technique, the breakdown of organic molecules is dependent on other factors, including surface defects and the matching of the conduction/valence band to the reduction-oxidation level of the targeted molecules. "It is safe to say no photocatalyst can decompose all organic molecules," said Ryoji Asahi, a researcher at the lab and member of the team. "We must test each molecule to be decomposed for each application."

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