Title : Two-step water splitting under visible light by using Polyoxometalate as shuttle redox mediator Osamu Tomita
Water splitting systems based on two-step photoexcitation, so-called Z-scheme systems, have recently been developed and proven as a promising approach to harvesting a wider range of visible light, because the water splitting reaction is separated into two parts, i.e., H2- and O2-evolving systems. Although the introduction of Z- scheme systems enables us to employ various visible-light responsive photocatalysts, the choice of redox has been limited to simple ion couples such as IO3–/I– and Fe3+/Fe2+. This is mainly due to the problems with mismatching redox potentials and/or irreversibility presented by other materials. The development of redox couples with appropriate redox potentials and sufficient reversibility under mild pH conditions is thus required in order to achieve highly efficient Z-scheme systems.
Here, we have paid attention to the use of transition metal-substituted-polyoxometalates, most of which are known to exhibit reversible redox behavior derived from valence differences between the incorporated transition metals, as effective shuttle redox mediators. We have recently reported a new Z-scheme water splitting system using a polyoxometalate (POM) as a redox mediator; the use of a Mn-substituted silicotungstate or Mo-substituted one (K6[SiW11O39MnII(H2O)] or K4[SiW11O40MoVI], denoted as SiW11Mn and SiW11Mo, respectively) with appropriate photocatalysts enabled a stoichiometric evolution of H2 and O2 under visible light. In the present study, V-substituted silicotungstate (K5[SiW11O40VV], denoted as SiW11V)6 was prepared and employed as redox mediator to pursue the availability of POM. The SiW11VV/SiW11VIV was confirmed to function as electron donor in H2-evolution system or as electron acceptor in O2-evolution system. The SiW11VV/SiW11VIV was revealed function as effective shuttle redox mediator between the two photocatalysts under visible light irradiation.