Modification of an oxyhalide solid-solution photocatalyst with an efficient O2-evolving cocatalyst and electron mediator for two-step photoexcitation overall water splitting

Two-step photoexcitation overall water splitting based on particulate photocatalysts represents a promising approach for low-cost solar hydrogen production. The performance of an O₂-evolution photocatalyst and electron mediator between two photocatalysts crucially influences the construction of an efficient two-step excitation water-splitting system. Bismuth-tantalum oxyhalides are emerging photocatalysts for O₂ evolution reactions and can be applied in two-step water-splitting systems. In this study, a highly crystalline Bi₄TaO₈Cl_0.9Br_0.1 solid solution with microplatelet morphology was synthesized by the dual flux method. The light absorption intensity and charge transfer efficiency of the Bi₄TaO₈Cl_0.9Br_0.1 solid solution were higher than those of Bi₄TaO₈Cl and Bi₄TaO₈Br; thus, the sacrificial O₂ evolution activity of Bi₄TaO₈Cl_0.9Br_0.1 photocatalyst was obviously enhanced. The two-step excitation water splitting with a solid-state electron mediator was successfully constructed using Bi₄TaO₈Cl_0.9Br_0.1 as the O₂-evolution photocatalyst and Ru/SrTiO₃:Rh as the H₂-evolution photocatalyst. The CoO_x cocatalyst and reduced graphene oxide decorations on the surface of Bi₄TaO₈Cl_0.9Br_0.1 promoted the catalytic O₂ generation process on Bi₄TaO₈Cl_0.9Br_0.1 and electron transfer between CoO_x/Bi₄TaO₈Cl_0.9Br_0.1 and Ru/SrTiO₃:Rh photocatalysts, respectively. As a result, the apparent quantum yield for this overall water-splitting system was 1.26% at 420 nm, which surpassed the present performance of the two-step excitation water-splitting systems consisting of metal oxyhalide photocatalysts. This study demonstrates the validity of high-quality solid-solution photocatalysts with suitable surface modification for efficient solar hydrogen production from water splitting.