Investigation of cocatalysts on silver-modified Sm2Ti2S2O5 photocatalyst for water reduction and oxidation under visible light irradiation

Surface modification of cocatalysts has been extensively tested efficient to promote photocatalytic water splitting of semiconductor oxides, but its influence on an oxysulfide photocatalyst is limited. In this work, various metals and metal oxides, formed under different preparation conditions and using several deposition methods, were examined as cocatalysts to investigate their effects on the H2 and O2 evolution rates of a 1 wt% Ag-modified Sm2Ti2S2O5 catalyst with an absorption edge of ca. 600 nm. The photocatalytic rate of H2 evolution was found to strongly depend on the species, existing state and dispersion of the deposited metals, with the highest rate of 949 μmol h−1 achieved by using 1 wt% impregnated Rh combined with H2 reduction at 623 K for 1 h. The order of H2 activity of the deposited metals was Rh > Pt > Pd > Ir. IrO2 colloid cocatalysts promoted O2 evolution more strongly than other oxides deposited by impregnation. The pH environment during the synthesis of IrO2 colloids is an important factor in determining the final O2 evolution rate and a suitable pH region (8–9.5) was finally suggested. So far, the optimized apparent quantum efficiencies of H2 and O2 evolution have reached ca. 2.6% and 3.3% at 440 nm, respectively.