Core–Shell Structured Cocatalysts for Enhancing Redox-Mediated Z-Scheme Photocatalytic Water Splitting and Producing H<sub>2</sub> and O<sub>2</sub> Separately
Hajime Suzuki, Yuya Okada, Suguru Kise, Xinru Sui, Shunsuke Nozawa, Osamu Tomita, Ryu Abe
Abstract
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 H 2 and O 2 . 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 x / M cocatalysts (CrO x represents Cr 2 O 3 · n H 2 O, M = Pt, Rh, and Ni)─where nanoparticulate M metal core species are coated with a thin CrO x shell─can effectively enhance H 2 evolution on HEPs, such as SrTiO 3 and Rh-doped SrTiO 3 (SrTiO 3:Rh), with the redox couple IO 3 – /I –, by suppressing IO 3 – rereduction on the M metal surface. Electrochemical measurements using CrO x -coated and uncoated metal wires strongly support the function of CrO x shells, which are permeable to H + (or water) and H 2 molecules, but not IO 3 –, affording selective H 2 evolution on the CrO x / M cocatalysts. The impact of the CrO x 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 IO 3 – is indispensable for the transfer of IO 3 – anions from the H 2 cell to the O 2 cell containing an OEP. The use of CrO x /Rh/SrTiO 3 and CrO x /Pt/SrTiO 3:Rh coupled with appropriate OEPs affords almost stoichiometric and separate production of H 2 and O 2 under UV- or visible-light irradiation, respectively, from an aqueous solution containing I – and IO 3 – . The effectiveness of the CrO x coating for enhancing H 2 evolution on the SrTiO 3:Rh HEP was confirmed for various redox species [Fe(CN) 6 ] 3–/4–, [Co(bpy) 3 ] 3+/2+, and [SiVW 11 O 40 ] 5–/6– that are operating under nearly neutral conditions, whereas CrO x dissolution under acidic conditions with Fe 3+/2+ and VO 2 + /VO 2+ redox was detrimental. The use of CrO x -based core–shell cocatalysts was proven to enhance the efficiency of visible-light-driven Z-scheme water splitting not only with IO 3 – /I – but also with other redox couples such as [SiVW 11 O 40 ] 5–/6– .