Two-step photocatalytic water splitting into H2 and O2 using layered metal oxide KCa2Nb3O10 and its derivatives as O2-evolving photocatalysts with IO3−/I− or Fe3+/Fe2+ redox mediator

Abstract

Two-step photoexcitation (Z-scheme) systems that can split water into H₂ and O₂ under UV light were constructed using a layered potassium calcium niobate (KCa₂Nb₃O₁₀) and its derivatives as O₂-evolving photocatalysts, combined with an appropriate H₂-evolving photocatalyst in the presence of iodate/iodide (IO₃⁻/I⁻) or iron(III)/(II) (Fe³⁺/Fe²⁺) as an electron mediator. The original KCa₂Nb₃O₁₀ showed negligibly low activity for photocatalytic water oxidation to O₂ in the presence of IO₃⁻ as an electron acceptor, since the anionic IO₃⁻ cannot penetrate into the interlayer spaces owing to the strong electrostatic repulsion between IO₃⁻ and the negatively charged niobate layers. The rate of O₂ evolution was significantly increased after the exfoliation-restack process of KCa₂Nb₃O₁₀, certainly due to much facilitated access of IO₃⁻ to the opened reduction sites of nanosheet surfaces. The loading of RuO_x or PtO_x cocatalysts on the samples significantly increased the rate of O₂ evolution. The electrochemical analysis indicated that these cocatalysts effectively decreased the overpotential of IO₃⁻ reduction, which occurs through the 6-electron process. On the other hand, the original layered structure was effective for the photocatalytic O₂ evolution in the presence of Fe³⁺ electron acceptor even without any cocatalysts, suggesting that the interlayer spaces of the layered niobate can work as effective reduction sites for cationic Fe³⁺. Finally, simultaneous evolution of H₂ and O₂ was attempted by using these KCa₂Nb₃O₁₀-based materials as O₂-evolving photocatalysts, combined with an appropriate H₂-evolving photocatalyst. By employing the appropriate combination of KCa₂Nb₃O₁₀-based materials and the redox couple, which was suggested by the result of half O₂-evolution reactions, simultaneous evolution of H₂ and O₂ stably proceeded with higher rates.