Enhanced water splitting through two-step photoexcitation by sunlight using tantalum/nitrogen-codoped rutile titania as a water oxidation photocatalyst

Rutile TiO₂ codoped with tantalum and nitrogen (TiO₂:Ta,N) was assessed as a water oxidation photocatalyst for Z-scheme water splitting driven by visible light. This material was prepared by thermal ammonolysis of TiO₂:Ta with dry NH₃ at 773 K, while samples of the oxide precursor were synthesized using a microwave-assisted solvothermal technique, applying various conditions. The photocatalytic activity of the TiO₂:Ta,N during water oxidation to O₂ from an aqueous FeCl₃ solution was found to be greatly affected by the synthesis parameters. The rate of O₂ evolution was increased upon increasing the level of Ta doping in conjunction with a highly-crystalized TiO₂:Ta precursor resulting from applying a higher temperature during synthesis of the oxide. IrO₂ loading of the TiO₂:Ta,N photocatalyst also improved the O₂ evolution activity. The optimized IrO₂/TiO₂:Ta,N photocatalyst was applied to a Z-scheme water splitting system in combination with Ru/SrTiO₃:Rh and in the presence of redox mediators (Fe^3+/2+ or [Co(bpy)₃]^3+/2+). Under AM1.5G simulated sunlight, this system exhibited a maximum solar-to-hydrogen energy conversion efficiency of 0.039%, which was nearly twice as high as the previously reported system with the use of RuO₂/TiO₂:Ta,N (0.021%).