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 RuOx or PtOx 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 O2stably proceeded with higher rates.
Suzuki, H., Tomita, O., Higashi, M., Abe, R., & Abe, R. (2015). 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. Catalysis Science & Technology, 5(5), 2640-2648. https://doi.org/10.1039/c5cy00128e