Abstract
Various types of Z-scheme systems for water splitting under visible light irradiation were successfully developed by employing Rh- and Ir-doped metal oxide powdered materials with relatively narrow energy gaps (EG): BaTa2O6:Ir,La (EG: 1.9-2.0 eV), NaTaO3:Ir,La (EG: 2.1-2.3 eV), SrTiO3:Ir (EG: 1.6-1.8 eV), NaNbO3:Rh,Ba (EG: 2.5 eV) and TiO2:Rh,Sb (EG: 2.1 eV), with conventional SrTiO3:Rh (an H2-evolving photocatalyst) or BiVO4 (an O2-evolving photocatalyst), and suitable electron mediators. The Z-scheme systems were classified into three groups depending on the combination of H2- and O2-evolving photocatalysts and electron mediator. The Z-scheme systems combining BaTa2O6:Ir,La with BiVO4, and NaTaO3:Ir,La with BiVO4 were active when a [Co(bpy)3]3+/2+ redox couple was used rather than an Fe3+/2+ one. The combination of SrTiO3:Ir with SrTiO3:Rh gave an activity when the [Co(bpy)3]3+/2+ and Fe3+/2+ redox couple ionic mediators were used. The Z-scheme systems combining NaNbO3:Rh,Ba and TiO2:Rh,Sb with SrTiO3:Rh showed activities by using the [Co(bpy)3]3+/2+ and Fe3+/2+ redox couples and also via interparticle electron transfer by just contact with/without reduced graphene oxide (RGO). These suitable combinations can be explained based on the impurity levels of doped Rh3+ and Ir3+ toward the redox potentials of the ionic mediators for the Z-scheme systems employing ionic mediators, and p-/n-type and onset potentials of the photocurrent in the photoelectrochemical properties of those photocatalyst materials for the Z-scheme systems working via interparticle electron transfer.
Highlights
PaperArti cial photosynthesis has attracted attention from the view point of solar energy conversion to storable chemical energy
We have reported that SrTiO3:Rh (EG: 2.3 eV),[20] SrTiO3:Rh,Sb (EG: 2.2–2.4 eV),[21] SrTiO3:Ir (EG: 1.6–1.8 eV),[20,22] BaTa2O6:Ir,La (EG: 1.9–2.0 eV)[23] and NaTaO3:Ir,La (EG: 2.1–2.3 eV)[24] are active for sacri cial H2 evolution in the presence of electron donors such as methanol, while SrTiO3:Rh,Sb,[21] SrTiO3:Ir20 and TiO2:Rh,Sb (EG: 2.1 eV)[25] are active for sacri cial O2 evolution in the presence of electron acceptors such as Ag+
We have successfully developed Z-scheme photocatalyst systems for water splitting under visible light irradiation employing Rh- and Ir-doped metal oxide photocatalysts with longer wavelength responses than conventional SrTiO3:Rh and BiVO4
Summary
PaperArti cial photosynthesis has attracted attention from the view point of solar energy conversion to storable chemical energy. It is crucial to demonstrate a solar water splitting system employing powderbased oxide photocatalysts. A reactor system including a gas separation system of evolved hydrogen from oxygen has been studied in addition to the development of photocatalyst materials aimed towards the practical use of solar water splitting.[10] The separation system can be achieved by the use of a suitable separation membrane. Even if an excellent reactor system with a gas separation membrane system is established, which photocatalyst is employed for it is still a key issue as high efficient photocatalysts for real solar water splitting into H2 and O2 without any sacri cial electron donors and acceptors have not yet been developed. It is essential to develop an efficient photocatalyst for demonstrating a solar water splitting system taking gas separation and safety issues into account
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