Development of new photocatalytic water splitting into H2 and O2 using two different semiconductor photocatalysts and a shuttle redox mediator IO3 -/I-

A new type of photocatalytic reaction that splits water into H-2 and O-2 was designed using a two-step photoexcitation system composed of an iodate/iodide (IO3-/I-) shuttle redox mediator and two different photocatalysts, one for H-2 evolution and the other for O-2 evolution. Photocatalytic oxidation of water to O-2 and reduction Of IO3- to I- selectively proceeded with good efficiencies over TiO2-rutile and Pt-WO3 photocatalysts under UV and visible light irradiations, respectively. The O-2 evolution selectively proceeded even in the presence of a considerable amount of I- in the solutions, although the oxidation of water is thermodynamically less favorable than oxidation of I-. Both the adsorption property of IO3- anions and the oxidation property of the photocatalysts are doubtless responsible for the selective oxidation of water. On the other hand, photocatalytic reduction of water to H) and oxidation of I- to IO3- proceeded over Pt-TiO2-anatase and Pt-SrTiO3:Cr/Ta (codoped with Cr and Ta) photocatalysts under UV and visible light, respectively. The combination of two different photocatalysts results in a stoichiometric evolution of H-2 and O-2 via the redox cycle of IO3- and I-. The photocatalytic water splitting under visible light irradiation (lambda > 420 nm) was demonstrated by using the Pt-SrTiO3:Cr/Ta, Pt-WO3, and IO3-/I- shuttle redox mediator.