Z-scheme photocatalyst systems employing Rh- and Ir-doped metal oxide materials for water splitting under visible light irradiation

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 Fe^3+/2+ one. The combination of SrTiO₃:Ir with SrTiO₃:Rh gave an activity when the [Co(bpy)₃]^3+/2+ and Fe^3+/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 Fe^3+/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.