Cathodic photoprocesses on titania films and in aqueous suspensions

The interaction and photocatalytic reactivity of 12 metal (or metalloid) ions: Mn2+, Zn2+, Cd2+, Tl+, Ni2+, Pb2+, Cu2+, Se4+, Ag+, Hg2+, Cr6+, and Au3+, with UV-irradiated TiO2 surfaces were compared. Two distinct groups were discerned, the first six ionic species showing essentially no or a very weak tendency to accept photogenerated electrons from TiO2, and the last six, showing a strong tendency to undergo photocatalytic reduction. The selenium species were abnormal, both in their tendency toward strong adsorption on the TiO2 surface in the dark, and in their very high photocatalytic reactivity. Chronopotentiometry measurements on TiO2 films provided complementary data to those obtained from UV-irradiated titania suspensions. Collectively, the photocatalytic reactivity of the 12 species of interest here, could be correlated with the energy location of the corresponding redox couples in solution with respect to the TiO2 conduction band-edge.