Fe/Ru Oxide as a Versatile and Effective Cocatalyst for Boosting Z-Scheme Water-Splitting: Suppressing Undesirable Backward Electron Transfer.

The atificial Z-scheme is a promising and rational strategy for solar-to-chemical energy conversion such as water-splitting. In the Z-scheme, backward redox processes are an essential drawback that should be overcome to increase its efficiency. Here, we demonstrate that the simple co-loading of Fe/Ru oxide, (Fe,Ru)Ox, onto various photocatalysts effectively improves the efficiency of water oxidation by suppressing the undesirable backward oxidation of the redox reagent Fe2+. The (Fe,Ru)Ox co-loading on Bi4TaO8Cl afforded the highest water-splitting activity (apparent quantum efficiency of 1.6% at 420 nm) among the Z-scheme systems employing mixed-anion compounds as O2-evolving photocatalysts. The results of photoelectrochemical and electrochemical measurements along with time-resolved spectroscopy clarified the key roles of Fe/Ru oxide; the Ru oxide component functions as a "collector" of photogenerated carriers and active sites for surface redox reactions, while the Fe oxide component acts as a "blocker" against unfavorable Fe2+ oxidation. The versatile availability of Fe/Ru oxide has been demonstrated for other visible-light-responsive photocatalysts.