Role and Function of Ruthenium Species as Promoters with TaON-Based Photocatalysts for Oxygen Evolution in Two-Step Water Splitting under Visible Light

The structure of nanoparticulate ruthenium (Ru) species dispersed on TaON as cocatalysts was characterized by X-ray absorption spectroscopy and scanning electron microscopy. TaON specimens loaded with Ru species were prepared by an impregnation method using (NH4)(3)RuCl6 as a precursor followed by calcination in air and tested as photocatalysts for 02 evolution from an aqueous NaIO3 solution under visible light (lambda > 420 nm). While unloaded TaON showed little photocatalytic activity, Ru loading resulted in observable 02 evolution, and impregnation of TaON with 0.5 wt % Ru followed by calcination at 623 K for 1 h provided the highest photocatalytic activity. Structural analyses and (photo)electrochemical measurements revealed that the activity of this photocatalyst is strongly dependent on the generation of optimally dispersed RuO2 nanoparticles, which simultaneously promote both the reduction of IO3- and oxidation of water. Overall water splitting through two-step photoexcitation (Z-scheme) was also achieved using the optimized RuO2/TaON photocatalyst in combination with Pt-loaded ZrO2/TaON as a H-2 evolution photocatalyst from an aqueous NaI solution. Experimental results suggested that the efficiency of this system is limited by the competitive oxidation of I- with the valence band holes in RuO2/TaON and the photoreduction of O-2 that can occur on the RuO2/TaON surface.