Washing off Dirt by Light

Recent detergent advertisements often mention a strange scientific word, `Photo Catalyst`. Catalyst is a substance that stimulates chemical reactions, increasing reaction speed without changing itself. As the word itself suggests, photo catalyst is a substance, of which catalyst action occurs with light energy.

When photo catalysts receive light, negative (-) electrons and positive (+) fine-holes are made. The core of photo catalysts is the strong redox (reduction-oxidation) power of these electrons and fine-holes.

Phthalocyanine, the photo catalyst used in detergents, creates active oxygen that has good oxidation power in water, breaking down dirt on laundry. However, because the amount contained in detergent is small due to high cost, effects can be seen only if photo catalysts are accumulated in laundry by washing for several times. The laundry, of course, has to be dried in sunlight. The most famous photo catalyst is titanium dioxide (TiO2). Although zinc oxide (ZnO) and cadmium sulfide (CdS) are also in use, titanium dioxide is the most widely used substance, because it is active and does not break down in light. Domestically, several venture enterprises are producing titanium dioxide, which had relied on importing entirely, since last year, and the domestic market scale by 2005 is expected to reach 1 trillion won annually.

Tiles, couches, bags, refrigerators, air-purifiers, vacuum cleaners, and even the interior of tunnels currently use titanium dioxide coating to rid pollution substances and toxic bacteria.

Another advantage of photo catalysis is that it fuses with water well. Car windows may be coated with photo catalyst to make the sight better by diverging rain drops evenly, and later under sunlight, dirt in raindrops may be decomposed. Also, scientists are researching to use the redox effect of photo catalysts to attain hydrogen that can be used in fuel batteries.

puset@donga.com