| Abstract
| - The role of electron acceptors in semiconductor photocatalysis is critical in determining the overallphotoefficiencies, kinetics, and mechanisms. Polyoxometalates (POMs) are efficient electron acceptors andcarriers in TiO2 photocatalysis. The electron transfer mediating behaviors of a POM (PW12O403-/PW12O404-)couple were investigated photoelectrochemically in the UV-illuminated TiO2 or Pt/TiO2 suspensions andcompared with those of an Fe3+/Fe2+ couple. POMs successfully transfer CB (conduction band) electrons onTiO2 particles to an inert collector electrode with generating photocurrents under UV illumination. Themagnitude of photocurrent is directly proportional to the reduced POM concentration ([POM-]) and markedlydecreases in the presence of dissolved O2 due to the rapid reoxidation of the reduced POMs by O2. Electrondonors are essential in inducing photocurrents, which are negligibly small in the absence of them. Thephotocurrent generation also depends on the kind of electron donors. Whereas formate as an electron donoris more efficient than acetate in generating photocurrents in the POM + TiO2 suspension, it is far less efficientin the Fe3+ + TiO2 system on the contrary. On the other hand, the platinization of the TiO2 surface significantlyaffects the current collection efficiency. The POM-mediated electron transfer to the collector electrode isreduced in the Pt/TiO2 suspension as compared with the naked TiO2 suspension, whereas the Fe3+-mediatedcurrent is higher in the Pt/TiO2 than in the naked TiO2 suspension. The reoxidation rate of the reduced POMis accelerated on Pt surface with reducing the efficiency of current collection. Although POMs are less efficientthan Fe3+ ions in mediating the photocurrent generation in general, they exhibit unique behaviors that aredifferent from those of Fe3+.
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