Environmental for improving the photocatalytic efficiency of TiO2

Environmental pollution is one of the major global challenges of today. Semiconductorphotocatalysis is the most efficient method for decomposing organic pollutants in aqueousmedia 1. Among various semiconductor photocatalysts studied, TiO2 has attracted muchattention due to its outstanding properties such as low cost, strong oxidizing power,nontoxicity, photostability, and chemical inertness 2. However, practical application of TiO2in photocatalytic reactions is obstructed by two essential drawbacks: wide energy gap (3–3.2eV) that limits its application to ultraviolet region and fast recombination of electron–holepairs, which are generated after photon absorption when the TiO2 is irradiated with energyequal to or higher than its band gap 3. Various strategies have been adopted for improvingthe photocatalytic efficiency of TiO2. They can be summarized as either changing the energystructure of TiO2, i.e. to extend optical absorption range from ultraviolet to visible region ordecreasing the electron/hole recombination rate 4.An effective method for improving the photocatalytic efficiency of TiO2 is its couplingwith wide band gap semiconductors such as WO3, V2O5, SnO2, CdS, CdSe, etc. 5. CdS hasrelatively low band gap energy (~2.3 eV) and its mixing with TiO2 enhances thephotocatalytic activity of TiO2/CdS system not only because of promotion of visible lightabsorption, 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 ofphotoexcited electrons from conduction band of CdS into the low-lying conduction band ofTiO2. On the other hand, the holes generated in CdS valence band cannot be transferred tovalence band of TiO2 because CdS valence band is more cathodic than that of TiO2. Therecombination between photogenerated electrons and holes is suppressed as a result of theseparation effect and overall photocatalytic activity of TiO2/CdS system is improved.