A photocatalyst is a substance that uses light energy to facilitate a chemical reaction. Titanium dioxide (TiO2) is a photocatalyst. When exposed to enough ultraviolet (UV) light, titanium dioxide can induce the transformation of harmful substances and toxic compounds into benign constituents such carbon dioxide (CO2) and water (H2O).
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Picture Source: Jagiellonian University
When photocatalyst titanium dioxide absorbs UV radiation from sunlight or an 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 promotes the electron to the conduction band of titanium dioxide thereby creating the negative-electron (e-) and positive-hole (h+) pair. This stage is referred to as the semiconductor’s ‘photo-excitation’ state. The energy difference between the valence band and the conduction band is known as the ‘ Band Gap’.
The positive-hole of titanium dioxide breaks apart the water molecule to form hydrogen gas and hydroxyl radical (-OH). The negative-electron reacts with oxygen molecule to form a superoxide anion (-O2). These elements oxidise volatile organic compounds (VOC’s) such as formaldehyde, carbon monoxide, nitrous oxide, ammonia and septic system odours adsorbed on the catalyst surface. They will also eliminate and decompose airborne pathogens like bacteria, viruses and mould.