Visible-Light-Driven Water Oxidation Using Anatase Titania Modified with First-Row Transition-Metal-Oxide Nanoclusters

Composites comprising nanocrystalline anatase TiO2 and nano-clusters of first-row transition-metal oxides (MOx; M = Mn, Fe, Co, or Ni), which can potentially function as water oxidation catalysts, were applied as photocatalysts for water oxidation under visible light (480 < lambda < 900 nm). Although TiO2 showed no visible light absorption, each of the MOx/TiO2 composites was capable of absorbing visible light. Activities during visible-light-driven water oxidation were examined using these materials in the presence of Ag+ as an electron acceptor. Co and Ni were found to be effective modifiers and promoted water oxidation with TiO2, whereas CoOx/TiO2 exhibited the highest activity. The catalytic activities of the MOx series during dark water oxidation were also investigated, employing an established photochemical water oxidation scheme with a Ru(II) trisdiimine complex as a redox photosensitizer in conjunction with weak visible light illumination. Under these conditions, the effect of light absorption by MOx/TiO2 on the reaction could be neglected. This investigation revealed that CoOx/TiO2 had by far the highest activity among the MOx/TiO2 composites. This work thus highlights the important role of the dark water oxidation process that occurs on the MOx surface in determining the overall efficiency of MOx/TiO2 during photocatalytic water oxidation.