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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 (electron-hole pairs):
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'.
Wavelength of the light necessary for photo-excitation
is:
1240
(Planck's constant, h) / 3.2 ev (band gap energy)
= 388 nm
 
The positive-hole of
titanium dioxide breaks apart the water molecule to form
hydrogen gas and hydroxyl radical. The
negative-electron reacts with oxygen molecule to form
superoxide anion. This cycle continues when light is
available. The overall mechanism of photocatalytic
reaction of titanium dioxide:
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