Core–ShellStructured Cocatalysts for EnhancingRedox-Mediated Z‑Scheme Photocatalytic Water Splitting andProducing H₂ and O₂ Separately

The Z-scheme water splitting system with a redox couple (oxidant/reductant) that transfers electrons from an oxygen-evolving photocatalyst (OEP) to a hydrogen-evolving photocatalyst (HEP) is promising for achieving high solar-to-hydrogen conversion efficiency by harvesting a wide range of visible light and for enabling separate production of H₂ and O₂. However, a preference for backward electron transfer often hampers the demonstration of aforementioned advantages of the Z-scheme system. Here, we show that the loading of CrO_<i>x</i>/<i>M</i> cocatalysts (CrO_<i>x</i> represents Cr₂O₃·<i>n</i>H₂O, M = Pt, Rh, and Ni)where nanoparticulate <i>M</i> metal core species are coated with a thin CrO_<i>x</i> shellcan effectively enhance H₂ evolution on HEPs, such as SrTiO₃ and Rh-doped SrTiO₃ (SrTiO₃:Rh), with the redox couple IO₃^–/I^–, by suppressing IO₃^– rereduction on the <i>M</i> metal surface. Electrochemical measurements using CrO_<i>x</i>-coated and uncoated metal wires strongly support the function of CrO_<i>x</i> shells, which are permeable to H⁺ (or water) and H₂ molecules, but not IO₃^–, affording selective H₂ evolution on the CrO_<i>x</i>/<i>M</i> cocatalysts. The impact of the CrO_<i>x</i> shell on suppressing backward electron transfer becomes more prominent for reactions in a two-component cell with a membrane filter, wherein the accumulation of a relatively high concentration of IO₃^– is indispensable for the transfer of IO₃^– anions from the H₂ cell to the O₂ cell containing an OEP. The use of CrO_<i>x</i>/Rh/SrTiO₃ and CrO_<i>x</i>/Pt/SrTiO₃:Rh coupled with appropriate OEPs affords almost stoichiometric and separate production of H₂ and O₂ under UV- or visible-light irradiation, respectively, from an aqueous solution containing I^– and IO₃^–. The effectiveness of the CrO_<i>x</i> coating for enhancing H₂ evolution on the SrTiO₃:Rh HEP was confirmed for various redox species [Fe(CN)₆]^3–/4–, [Co(bpy)₃]^3+/2+, and [SiVW₁₁O₄₀]^5–/6– that are operating under nearly neutral conditions, whereas CrO_<i>x</i> dissolution under acidic conditions with Fe^3+/2+ and VO₂⁺/VO²⁺ redox was detrimental. The use of CrO_<i>x</i>-based core–shell cocatalysts was proven to enhance the efficiency of visible-light-driven Z-scheme water splitting not only with IO₃^–/I^– but also with other redox couples such as [SiVW₁₁O₄₀]^5–/6–.