Photocatalytic water splitting is a promising approach to direct conversion of solar energy into H₂ and O₂. As water and sunlight are freely available, this would lead to a sustainable and renewable energy source. To this end Kazunari Domen, University of Tokyo, Japan, and his collaborators have been investigating the construction of a Z-scheme water-splitting system consisting of Ru/SrTiO₃:Rh and Ta₃N₅ without a redox mediator.

A Z-scheme photocatalysis system usually consists of a two-step photoexcitation system with two photocatalysts—one producing H₂ (in this case Ru/SrTiO₃:Rh) and the other O₂ (in this case modified Ta₃N₅) that mimics natural photosynthesis. Initially they were able to show that well-crystallized Ta₃N₅ contributed to efficient water oxidation. Modification of Ta₃N₅ with nanoparticulate CoO_x enhanced the water-oxidation activity, but also suppressed the N₂ evolution. Finally, further modification of the O₂ catalyst with metallic Ir, which acts as an electron sink, lead to a redox-mediator-free Z-scheme water-splitting system capable of working under simulated sunlight.

• A Redox-Mediator-Free Solar-Driven Z-Scheme Water-Splitting System Consisting of Modified Ta₃N₅ as Oxygen-Evolution Photocatalyst,
  S. S. K. Ma, K. Maeda, T. Hisatomi, M. Tabata, A. Kudo, K. Domen,
  Chem. Eur. J. 2013.
  DOI: 10.1002/chem.201300579