Efficient nonsacrificial water splitting through two-step photoexcitation by visible light using a modified oxynitride as a hydrogen evolution photocatalyst.

A two-step photocatalytic water splitting (Z-scheme) system consisting of a modified ZrO(2)/TaON species (H(2) evolution photocatalyst), an O(2) evolution photocatalyst, and a reversible donor/acceptor pair (i.e., redox mediator) was investigated. Among the O(2) evolution photocatalysts and redox mediators examined, Pt-loaded WO(3) (Pt/WO(3)) and the IO(3)(-)/I(-) pair were respectively found to be the most active components. Combining these two components with Pt-loaded ZrO(2)/TaON achieved stoichiometric water splitting into H(2) and O(2) under visible light, achieving an apparent quantum yield of 6.3% under irradiation by 420.5 nm monochromatic light under optimal conditions, 6 times greater than the yield achieved using a TaON analogue. To the best of our knowledge, this is the highest reported value to date for a nonsacrificial visible-light-driven water splitting system. The high activity of this system is due to the efficient reaction of electron donors (I(-) ions) and acceptors (IO(3)(-) ions) on the Pt/ZrO(2)/TaON and Pt/WO(3) photocatalysts, respectively, which suppresses undesirable reverse reactions involving the redox couple that would otherwise occur on the photocatalysts. Photoluminescence and photoelectrochemical measurements indicated that the high activity of this Z-scheme system results from the moderated n-type semiconducting character of ZrO(2)/TaON, which results in a lower probability of undesirable electron-hole recombination in ZrO(2)/TaON than in TaON.