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

Rutile TiO 2 codoped with tantalum and nitrogen (TiO 2 :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 TiO 2 :Ta with dry NH 3 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 TiO 2 :Ta,N during water oxidation to O 2 from an aqueous FeCl 3 solution was found to be greatly affected by the synthesis parameters. The rate of O 2 evolution was increased upon increasing the level of Ta doping in conjunction with a highly-crystalized TiO 2 :Ta precursor resulting from applying a higher temperature during synthesis of the oxide. IrO 2 loading of the TiO 2 :Ta,N photocatalyst also improved the O 2 evolution activity. The optimized IrO 2 /TiO 2 :Ta,N photocatalyst was applied to a Z-scheme water splitting system in combination with Ru/SrTiO 3 :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 RuO 2 /TiO 2 :Ta,N (0.021%). An improved efficiency of Z-scheme water splitting was demonstrated using tantalum/nitrogen-codoped rutile titania as an O 2 evolution photocatalyst.