Photoelectrochemical Investigation on Electron Transfer mediating Behaviors of Polyoxometalate in UV-Illuminated Suspensions of TiO_2 and Pt/TiO_2

摘要

The role electron acceptors in semiconductor photocatalysis is critical in determining the overall photoefficiencies, kinetics, and mechanisms. Polyoxometalates (POMs) are efficient electron acceptors and carriers in TiO_2 photocatalysis. The electron transfer mediating behaviors of a POM (PW_(12)O_(40)~(3-)/PW_(12)O_(40)~(4-)) couple were investigated photoelectrochemically in the UV-illuminated TiO_2 or Pt/TiO_2 suspensions and compared with those of an Fe~(3+)/Fe~(2+) couple. POMs successfully transfer CB (conduction band) electrons on TiO_2 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 + TiO_2 suspension, it is far less efficient in the Fe~(3+) + TiO_2 system on the contrary. On the other hand, the platinization of the TiO_2 surface significantly affects the current collection efficiency. The POM-mediated electron transfer to the collector electrode is reduced in the Pt/TiO_2 suspension as compared with the naked TiO_2 suspension, whereas the Fe~(3+)-mediated current is higher in the Pt/TiO_2 than in the naked TiO_2 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 Fe~(3+) ions in mediating the photocurrent generation in general, they exhibit unique behaviors that are different from those of Fe~(3+).