Solar hydrogen production via a Z-scheme water splitting system based solely on perovskite-type tantalum oxynitrides

Abstract

Z-scheme overall water splitting (ZOWS) systems can provide efficient hydrogen production in response to visible light. However, the integration of narrow-bandgap non-oxide photocatalysts as oxygen evolution photocatalysts (OEPs) in such systems remains underexplored. This lack of research arises because existing OEPs and the associated cocatalysts are typically employed to promote oxygen evolution in aqueous silver nitrate solutions and are less effective in ZOWS systems utilizing reversible redox mediators that are prone to backward reactions. The present work synthesized particulate single-crystalline SrTaO₂N with exposed {100} and {110} facets using NaCl as a molten salt. The strategic use of Pt and CoO_x as dual cocatalysts to promote reduction and oxidation reactions, respectively, significantly enhanced the reduction of [Co(bpy)₃]³⁺ ions and improved charge carrier separation. The resulting ZOWS system, employing Pt/CoO_x/SrTaO₂N as the OEP, Cr₂O₃/Pt/BaTaO₂N as the hydrogen evolution photocatalyst and [Co(bpy)₃]^3+/2+ as the redox mediator, achieved stoichiometric H₂ and O₂ evolution. This oxynitride-based system extends the visible light range available for ZOWS to nearly 600 nm, providing a new platform for solar hydrogen production. This work provides important insights into the application of narrow-bandgap oxynitride photocatalysts in ZOWS systems.