Role of iron ion electron mediator on photocatalytic overall water splitting under visible light irradiation using Z-scheme systems

A combination system consisting of a H-2 production photocatalyst, Pt/SrTiO3:Rh, and an O-2 production photocatalyst, BiVO4 or WO3, decomposed water into H-2 and O-2 under visible light irradiation in the presence of an Fe3+/Fe2+ redox couple as an electron mediator. O-2 evolution on the BiVO4 photocatalyst was inhibited by Fe2+ ions,because of the oxidation of Fe2+ instead of water. In contrast, H-2 evolution on the Pt/SrTiO3:Rh photocatalyst was enhanced when Fe3+ ions co-existed. It is due to the back-reactions between H-2 and O-2 to form water, and the reduction of Fe3+ by H-2, which easily proceeded on the bare Pt cocatalyst surface, being efficiently suppressed by adsorption of [Fe(SO4)(H2O)(5)](+) and/or [Fe(OH)(H2O)(5)](2+) on the Pt surface. Overall water splitting was achieved with the suppression of the back-reactions even using a Pt cocatalyst. Thus, it clears that iron ions contributed to the present Z-scheme systems not only as an electron mediator but also as an inhibitor of the back-reactions.