Efficient Nonsacrificial Water Splitting through Two-Step Photoexcitation by Visible Light using a Modified Oxynitride as a Hydrogen Evolution Photocatalyst

Abstract

A two-step photocatalytic water splitting (Z-scheme) system consisting of a modified ZrO₂/TaON species (H₂ evolution photocatalyst), an O₂ evolution photocatalyst, and a reversible donor/acceptor pair (i.e., redox mediator) was investigated. Among the O₂ evolution photocatalysts and redox mediators examined, Pt-loaded WO₃ (Pt/WO₃) and the IO₃⁻/I⁻ pair were respectively found to be the most active components. Combining these two components with Pt-loaded ZrO₂/TaON achieved stoichiometric water splitting into H₂ and O₂ 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 (IO₃⁻ ions) on the Pt/ZrO₂/TaON and Pt/WO₃ 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 ZrO₂/TaON, which results in a lower probability of undesirable electron−hole recombination in ZrO₂/TaON than in TaON.