Environmental faced by human beings. Semiconductor photocatalysis is the

Environmental pollution
is one of the major global challenges faced by human beings. Semiconductor
photocatalysis is the most efficient method for decomposing organic pollutants
in aqueous media 1. Among
various semiconductor photocatalysts that have been studied, TiO2
has attracted much attention as a photocatalyst due to its desirable properties
such as low
cost, strong oxidizing power, nontoxicity, photostability, and chemical
inertness 2. However, practical application of TiO2 in
photocatalytic reactions is
obstructed by two essential drawbacks: wide energy gap (3–3.2 eV) that limits
its application to ultraviolet region and fast recombination of electron–hole
pairs, which are generated after photon absorption when the TiO2 is
irradiated with energy equal to or higher than its band gap 3. One of the approaches
applied to solve these problems is to change the energy structure of TiO2,
i.e. to extend optical absorption range from ultraviolet to visible region and decrease
the electron/hole recombination rate 4.

An effective method for improved photocatalytic efficiency of TiO2
is coupling the TiO2 with wide band gap semiconductors such
as WO3, V2O5, SnO2, CdS, CdSe, etc.
5. CdS has relatively low band gap energy (~2.3 eV) and its mixing with TiO2
enhances the photocatalytic activity of TiO2/CdS system not only
because of promotion of visible light absorption, but it also features better
separation of photogenerated electron–hole pairs 6-8. The position of CdS
conduction and valence band gap edges enables the injection of photoexcited
electrons from conduction band of CdS into the low-lying conduction band of TiO2.
On the other hand, the holes generated in CdS valence band cannot be transferred
to valence band of TiO2 because CdS valence band is more cathodic
than that of TiO2. The recombination between photogenerated
electrons and holes is suppressed as a result of the separation effect and overall
photocatalytic activity TiO2/CdS system is
improved.

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