Core–Shell Structured Cocatalysts for Enhancing Redox-Mediated Z-Scheme Photocatalytic Water Splitting and Producing 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 CrOx/M cocatalysts (CrOx represents Cr2O3·nH2O, M = Pt, Rh, and Ni)─where nanoparticulate M metal core species are coated with a thin CrOx 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 M metal surface. Electrochemical measurements using CrOx-coated and uncoated metal wires strongly support the function of CrOx shells, which are permeable to H+ (or water) and H2 molecules, but not IO3–, affording selective H2 evolution on the CrOx/M cocatalysts. The impact of the CrOx 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 CrOx/Rh/SrTiO3 and CrOx/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 CrOx 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 CrOx dissolution under acidic conditions with Fe3+/2+ and VO2+/VO2+ redox was detrimental. The use of CrOx-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–.