A substance that helps bring about a light-catalyzed reaction, such as chlorophyll in photosynthesis. Like photosynthesis, the reaction continues throughout the day once begun.

Photocatalytic Oxidation

The most powerful advanced oxidation systems are based on the generation of hydroxyl radicals. The hydroxyl radical is an extremely powerful oxidation agent, second only to Fluorine in power (2.23 in Relative Oxidizing Power). Following is a listing of common chemical oxidants, placed in the order of their oxidizing strength:

Mechanism

When photocatalyst titanium dioxide (TiO2) absorbs Ultraviolet (UV) radiation from sunlight or illuminated light source (fluorescent lamps), it will produce pairs of electrons and holes.

The electron of the valence band of titanium dioxide becomes excited when illuminated by light. The excess energy of this excited electron promoted the electron to the conduction band of titanium dioxide therefore creating the negative-electron (e-) and positive-hole (h+) pair.

This stage is referred as the semiconductor's ' photo-excitation ' state. The energy difference between the valence band and the conduction band is known as the ' Band Gap '.

Super-Hydrophilic

When the surface of photocatalytic film is exposed to light, the contact angle of the phtocatalyst surface with water is reduced gradually. After enough exposure to light, the surface reaches super-hydrophilic. In other words, it does not repel water at all, so water cannot exist in the shape of a drop, but spreads flatly on the surface of the substrate. And the water took the form of a highly uniform thin film, which behaves optically like a clear sheet of glass.

The hydrophilic nature of titanium dioxide, coupled with the gravity, will enable the dust particles to be swept away following the water stream, thus making the product self-cleaning.