Research on the water splitting into H
2 and O
2 using two different semiconductor photocatalysts and a redox mediator, mimicking the Z-scheme mechanism of the photosynthesis, is introduced. It was found that the H
2 evolution took place on a Pt-SrTiO
3(Cr-Ta-doped) photocatalyst using an I
− electron donor under the visible light irradiation. And the Pt-WO
3 photocatalyst showed an excellent activity of the O
2 evolution using an IO
3− electron acceptor under visible light. Both H
2 and O
2 evolution was observed with a stoichiometric ratio (H
2/O
2 = 2) for more than 250 h under visible light using a mixture of the Pt-WO
3 and the Pt-SrTiO
3(Cr-Ta-doped) powders suspended in NaI aqueous solution. It is the first finding that the stoichiometric water splitting occurred over oxide semiconductor photocatalysts under the visible light irradiation. We proposed a two-step photo-excitation mechanism using a pair of I
−/IO
3− redox mediator. The quantum efficiency of the stoichiometric water splitting was ca. 0.1% at 420.7 nm.
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