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

Abstract

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 (NH 4 ) 3 RuCl 6 as a precursor followed by calcination in air and tested as photocatalysts for O 2 evolution from an aqueous NaIO 3 solution under visible light (λ > 420 nm). While unloaded TaON showed little photocatalytic activity, Ru loading resulted in observable O 2 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 RuO 2 nanoparticles, which simultaneously promote both the reduction of IO 3 − and oxidation of water. Overall water splitting through two-step photoexcitation (Z-scheme) was also achieved using the optimized RuO 2 /TaON photocatalyst in combination with Pt-loaded ZrO 2 /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 RuO 2 /TaON and the photoreduction of O 2 that can occur on the RuO 2 /TaON surface.