Effective strategy for enhancing Z-scheme water splitting with the IO3−/I− redox mediator by using a visible light responsive TaON photocatalyst co-loaded with independently optimized two different cocatalysts

Abstract

Loading an efficient cocatalyst that can selectively enhance favorable reactions is an effective way to improve the efficiency of Z-scheme water splitting with a redox mediator such as IO₃⁻/I⁻. Given a recent finding on a highly active Ru(OH)_xCl_y cocatalyst for IO₃⁻ reduction on the TaON photocatalyst, here we systematically investigate Rh, Co, and Ir-based cocatalysts for enhancing water oxidation thereby improving the total efficiency of O₂ evolution on TaON with the help of the Ru(OH)_xCl_y cocatalyst. The calcination temperature of metal cation (i.e., Rh, Co or Ir) species on TaON, which is co-loaded by Ru(OH)_xCl_y species at a fixed temperature of 200 °C, significantly affected the rate of O₂ evolution from aqueous solution containing IO₃⁻ under visible light irradiation. The rate of O₂ evolution basically increased with increasing temperature at the cocatalyst loading up to 400 °C (for Ir) and 500 °C (for Rh and Co), while calcination at 600 °C drastically lowered the activity due to the detrimental effect on the TaON itself. The highest O₂ evolution rate was obtained by the loading of Rh species at 500 °C, followed by co-loading of Ru(OH)_xCl_y at 200 °C, on TaON. The results of both photocatalytic reactions and electrochemical measurements revealed that the Rh species loaded at 500 °C, which mainly consist of Rh oxides, promote water oxidation more effectively than conventional CoO_x or IrO_x species. The rates of H₂ and O₂ evolution in overall water splitting via the Z-scheme mechanism were significantly increased by the use of the TaON photocatalyst co-loaded with Rh and Ru species as the O₂-evolving photocatalyst, in place of the TaON loaded with only conventional RuO₂ species or previously reported TaON co-loaded with Ru(OH)_xCl_y and Co-based species. These results indicate the effectiveness of co-loading of cocatalysts that are independently optimized for water oxidation and IO₃⁻ reduction for improving the efficiency of Z-scheme water splitting.