Importance of Electron Mediator Transparency: Photocatalytic Hydrogen Production from Polyoxometalate using Dye-double-layered Photocatalysts

One-directional electron transfer is crucial for two-step photoexcitation (Z-scheme) water-splitting photocatalysis. We investigated the hydrogen evolution activity of PS-double-layered photocatalysts (X-DSP, X-RuCP6-Zr-RuP6@Pt/KxH4-xNb6O17; X=Zr4+, H+) in the presence of highly charged polyoxometalates-K-6[(SiVW11O40)-W-IV] center dot nH(2)O (V-IV-POM) and K-6[SiW11O39MnII(H2O)] center dot nH(2)O (Mn-II-POM)-as redox-reversible electron donors, to induce effective photocatalyst-donor electrostatic attraction. Surface-phosphonate-comprising H+-DSP completely one-electron oxidized V-IV-POM with 0.39 % apparent quantum yield in the initial hour (iAQY) in both HCl and phosphate buffer aqueous solutions. Conversely, the Zr4+-DSP iAQY decreased to 0.05 % in HCl aq. Considering that the 0.39 % iAQY was retained when replacing V-IV-POM with Mn-II-POM in HCl aq, we supposed that energy transfer deactivation from photoexcited PS* to surface-immobilized V-IV-POM is the plausible origin of the lower iAQY, owing to the stronger visible-light absorptivity of V-IV-POM. This suggests that accumulation of visible-light-transparent electron mediator on the photocatalyst surface is an effective approach for one-directional electron transfer in Z-scheme water-splitting photocatalysis.