Scientists in the USA and Japan are developing a set of smart materials that clean themselves off dirt and stains besides eliminating foul odours and dangerous bacteria.

Exploiting powerful catalytic properties, researchers succeeded in creating tiles, glass, paint, paper and cloth that can keep themselves sparkling clean.

The first item to reach the market, a self-cleaning wall and counter tile, can not only kill bacteria but also eliminate odours and staining associated with smoke from cooking oils and cigarettes, reports the journal Technology Review.

The key to the self-cleansing world of the future is the interaction between titanium dioxide and ultraviolet rays from the sun or fluorescent lights. The special properties of titanium dioxide?a substance used to make paint and tooth-paste white?were first discovered by Tokyo University chemist Akira Fujishima and Associates in 1969. Their research showed that when exposed to solar energy, titanium dioxide has the ability to break down water into hydrogen and oxygen.

After a quarter-century of observation, scientists now understand that the reaction occurs as titanium dioxide absorbs ergy from the UV band of sunlight and reacts with water vapour in the air to produce oxygen molecules. These molecules are energetic enough to break down organic matter into carbon dioxide and trace elements.

?When light shines on the white paint pigment, titanium dioxide, it produces an active form of oxygen that can burn combustible material at room temperature,? says David Ollis, Professor of Chemical Engineering at North Carolina State University. ?It is a fire without a flame.?

Scientists have discovered that titanium dioxide-coated materials can easily remove thin deposits such as bacteria and fingerprints, though they are unable to break down thick splotches of organic materials?such as blood stains?because light and oxygen in the air cannot reach the surface where the reaction occurs.

Fujishima says that when titanium-dioxide tiles were used in the operating rooms and bathrooms of Ako Central Hospital in Ako, Japan, they killed 99.9 per cent of bacteria on their surface. Included among them were penicillin-resistant Staphylococcus and other germs that can cause secondary infections among patients.

The tiles?marketed by Japan?s Toto Corp under the name NeoClean?remain effective even though they are coated with a layer of titanium dioxide only one micron thick, about one-fiftieth the diameter of a human hair. Once the fine layer of compound is permanently affixed?it is commonly sprayed and then baked onto the tile?s surface? the company says it is resistant to the abrasion of ordinary scrubbing that might be needed for thicker stains. Moreover, because titanium dioxide acts only as a catalyst for the photochemical reaction, it theoretically never gets used up.

While cleaning time varies with the thickness of the deposit, Adam Heller, a professor of Chemistry at the University of Texas at Austin, says his experiment shows that titanium dioxide-treated glass removed fingerprints in about two hours. This glass, versions of which both Heller and Fujishima have developed, could be made reactive on both sides, making it ideal for everything from sky-scraper windows to car window glass.

The Japanese have tested other titanium dioxide-treated materials as well. Kazuhito Hashimoto, a chemist at Tokyo University, applied the compound to a porcelain urinal. After a month, the treated urinal looked sparkling clean while an untreated unit was blotched and yellowed. Elsewhere, researchers are experimenting to see if the tiles can keep themselves clean on the walls of heavily polluted car and truck tunnels. And a Japanese paper company is developing windows and partitions for Japanese houses while a camping equipment manufacturer is testing a self-cleaning tent fabric.

But the most promising self-cleaning product is likely to be a wash-itself paint. Both the Texas and the Tokyo laboratories have demonstrated the self-cleaning capacities of paints containing titanium dioxide. While they are not saying exactly how they did it, both claim to have overcome an intrinsic problem in which titanium dioxide breaks down materials that bind pigments in coloured paints.

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