|As is well- known that, when light of the same or greater energy as the forbidden bandwidths contacts a semiconductor photocatalyst such as TiO2, charge separation of electrons and electron holes occurs. The electrons disperse on the surface of the photocatalyst and react with external substances, causing reductions and oxidations. As shown in Fig. 2, oxygen and water exist throughout the atmosphere and types of active oxygen such as OH radicals form, so normally only oxidation reactions occur. The oxidizing power of the OH radical is greater than that of chlorine and ozone, and a large portion of the multifarious environmental purification capabilities of photocatalysts come from this type of active oxygen.|
|Fig. 2 : Mechanism of photocatalysts|
The OH radical plays a very important role in atmospheric chemistry. Its atmospheric concentration is markedly low at 106 - 105 mol cm-3, but because it has something to do with the reaction process of all airborne substances, its speed of reaction with pollutants is well studied (Table 1). These values serve as an index for determining whether a photocatalyst can treat pollutants or not as well as for knowing relatively how fast it can do so.