Photoelectrochemical investigation on electron transfer mediating behaviors of polyoxometalate in UV-illuminated suspensions of TiO2 and Pt/TiO2

The role of electron acceptors in semiconductor photocatalysis is critical in determining the overall photoefficiencies, kinetics, and mechanisms. Polyoxometalates (POMs) are efficient electron acceptors and carriers in TiO2 photocatalysis. The electron transfer mediating behaviors of a POM (PW12O403-/PW12O40-4) couple were investigated photoelectrochemically in the UV-illuminated TiO2 or Pt/TiO2 suspensions and compared with those of an Fe3+/Fe2+ couple. POMs successfully transfer CB (conduction band) electrons on TiO2 particles to an inert collector electrode with generating photocurrents under UV illumination. The magnitude of photocurrent is directly proportional to the reduced POM concentration ([POM-]) and markedly decreases in the presence of dissolved O-2 due to the rapid reoxidation of the reduced POMs by O-2. Electron donors are essential in inducing photocurrents, which are negligibly small in the absence of them. The photocurrent generation also depends on the kind of electron donors. Whereas formate as an electron donor is more efficient than acetate in generating photocurrents in the POM + TiO2 suspension, it is far less efficient in the Fe3+ + TiO2 system on the contrary. On the other hand, the platinization of the TiO2 surface significantly affects the current collection efficiency. The POM-mediated electron transfer to the collector electrode is reduced in the Pt/TiO2 suspension as compared with the naked TiO2 suspension, whereas the Fe3+-mediated current is higher in the Pt/TiO2 than in the naked TiO2 suspension. The reoxidation rate of the reduced POM is accelerated on Pt surface with reducing the efficiency of current collection. Although POMs are less efficient than Fe3+ ions in mediating the photocurrent generation in general, they exhibit unique behaviors that are different from those of Fe3+.