Overall water splitting under visible light through a two-step photoexcitation between TaON and WO₃ in the presence of an iodate-iodide shuttle redox mediator

A two-step, photocatalytic water splitting system consisting of Pt-loaded TaON (a H₂ evolution photocatalyst), Pt-loaded WO₃ (an O₂ evolution photocatalyst), and an iodate-iodide (IO ₃^-/I^-) shuttle redox mediator is investigated under visible light irradiation. Photocatalytic oxidation of water to O ₂ and reduction of IO₃^- to I^- proceeded with good selectivity over the Pt-WO₃ photocatalyst, even in the presence of a considerable amount of I^- anions in the solution. The key difference between the adsorption properties of IO ₃^- and I^- anions on WO₃ strongly suggested that the photoexcited electrons could react efficiently with IO ₃^- adsorbed on WO₃, whereas the photogenerated holes selectively reacted with water molecules owing to the low adsorptivity of I^- on WO₃. Photocatalytic H₂ evolution on Pt-TaON proceeded efficiently, accompanied by I^- oxidation to IO ₃^- due to a substantial amount of adsorption of I ^- anions on the surface, whereas H₂ evolution was significantly inhibited by the competitive adsorption of IO₃ ^-, which consumes photoexcited electrons. It was also found that WO₃ photocatalysts loaded with platinum oxide (PtO) showed a much higher activity for O₂ evolution in the presence of the electron acceptor IO₃^-, compared to those loaded with Pt metal. Overall water splitting at a steady rate was demonstrated using a combination of Pt-TaON and Pt(PtO)-WO₃ in an aqueous NaI solution with neutral or weakly acidic pH values, where the concentration of NaI significantly affected the efficiency.