Development of photocatalysts based on solid solutions of oxides and sulfides for water splitting under visible light irradiation

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Various metal tantalates showed activities for water splitting into H_2 and O_2 in stoichiometric amounts under UV irradiation. Among them, LiTaO_3, NaTaO_3, SrTa_2O_6, Sr_2Ta_2O_7, and BaTa_2O_6 were highly active. Especially, NiO(0.05wt%)/NaTaO_3 was the most active and showed 20% of a quantum yield at 270 nm. The activity for water splitting over the NiO/NaTaO_3 photocatalyst was much improved by lanthanide (La, Pr, Nd, Sm, Gd, Tb, and Dy)-doping into NaTaO_3. Lanthanum was the most effective dopant. The quantum yield at 270 nm amounted ca. 50%. Moreover, deactivation which was observed for a non-doped NiO/NaTaO_3 photocatalyst was much suppressed. SEM observation suggested that these effects were mainly due to the decrease in the particle size and characteristic step structure created by the Ln-doping.Scheelite monoclinic BiVO_4 which was synthesized by a soft solution process at room temperature showed a high activity for O_2 evolution from AgNO_<3aq> under visible light irradiation (quantum yield=9% at 450 nm).Zn_<0.999>Ni_<0.001>S photocatalysts (energy gap = 2.3 eV) heat-treated at 773 K in an N_2 flow showed the activity for hydrogen evolution from aqueous solutions containing K_2SO_3 and Na_2S as reducing reagents under visible light irradiation without co-catalysts such as Pt.