[Co(bpy)₃)]^3+/2+ and [Co(phen)₃] ^3+/2+ electron mediators for overall water splitting under sunlight irradiation using Z-scheme photocatalyst system

[Co(bpy)₃]^3+/2+ and [Co(phen)₃] ^3+/2+ redox couples were revealed to play as electron mediators for Z-scheme photocatalyst systems composed of Ru/SrTiO₃:Rh and BiVO ₄ powders for overall water splitting under visible light irradiation. These electron mediators were effective for only the combination of SrTiO₃:Rh with BiVO₄. They did not work when nondoped SrTiO₃ and TiO₂ of H₂-evolving photocatalysts and WO₃ of O₂-evolving photocatalysts were employed. These results indicated that the affinity between photocatalysts and the Co-complex electron mediators was important. The photocatalytic activity depended on pH. Neutral pH conditions gave the highest activity for overall water splitting. Overall water splitting by the present system steadily proceeded for a long time. The Z-scheme photocatalyst system was also confirmed to split water under sunlight irradiation at the rates depending on weather. Moreover, overall water splitting by the Z-scheme photocatalyst system with the Co-complex electron mediator using a reaction cell in which the Ru/SrTiO₃:Rh suspension was divided from BiVO₄ suspension by a membrane filter resulted in H₂ evolution separated from that of O₂.