Effects of doping of metal cations on morphology, activity, and visible light response of photocatalysts

Effects of doping of metal cations into wide band gap semiconductor photocatalysts on morphology, visible light response, and photocatalytic performance were studied. Doping of lanthanide and alkaline earth ions improved activity of a NaTaO₃ photocatalyst for water splitting. Lanthanum was the most effective dopant. The NaTaO₃:La with a NiO cocatalyst gave 56% of a quantum yield at 270 nm. This remarkable photocatalytic activity was brought by formations of nano-crystalline particle and surface nano-step structure by the doping. On the other hand, metal cation doping into ZnS, TiO₂, and SrTiO₃ gave visible light responses for H₂ or O₂ evolution from aqueous solutions containing of sacrificial reagents. The visible light responses were due to the electronic transition from donor levels formed with dopants to conduction bands of the host photocatalysts. Codoping was effective to compensate charge unbalance brought by doping of transition metal cations, resulting in the improvement of visible light response for photocatalytic reactions. Among the transition metal-doped photocatalysts, SrTiO₃ doped with Rh (SrTiO₃:Rh) was the novel metal oxide photocatalyst that produced H₂ under visible light irradiation. The SrTiO₃:Rh photocatalyst was employed with O₂ evolution photocatalysts such as BiVO₄ and WO₃ for construction of Z-scheme systems that were active for water splitting into H₂ and O₂ under visible light irradiation.