Z-scheme water splitting utilizing CuLi1/3Ti2/3O2 as a hydrogen-evolving photocatalyst with photo-response up to 600 nm

Abstract

CuLi_1/3Ti_2/3O₂ (CLTO) is a visible-responsive photocatalyst, whose photo-response reaches up to 600 nm, for H₂ evolution using sacrificial electron donors such as methanol and S²⁻. In this study, utilization of CLTO in Z-scheme water splitting (Z-WS) was investigated. The photocatalytic performance of Cr₂O₃/M/CLTO as a H₂-evolving photocatalyst, which was prepared by sequential photodeposition of cocatalysts (M: Ru, Rh, Pd and Pt) and Cr₂O₃, was evaluated for Z-WS using BiVO₄, an O₂-evolving photocatalyst, and a Co(bpy)₃^3+/2+ redox shuttle under visible light. Among the examined samples, Cr₂O₃/Ru/CLTO produced both H₂ and O₂ with meaningful rates. Thus, CLTO was first utilized in a visible responsive Z-scheme system for water splitting. The Cr₂O₃ layer played a significant role in the suppression of backward reactions, such as reduction of Co(bpy)₃³⁺. The activity of Cr₂O₃/Ru/CLTO for Z-WS was remarkably affected by the deposition conditions of the Ru cocatalyst. The activity for Z-WS was remarkably improved when the photodeposition of the Ru cocatalyst was conducted in a methanol solution of RuCl₃. Unusually large plate Ru species with 100–200 nm sizes and about 30 nm thickness were present in the highly active sample. Characterization using X-ray photoelectron spectroscopy and X-ray absorption spectroscopy indicated that the Ru cocatalyst was deposited as mainly the oxyhydroxide of Ru. Z-WS also proceeded in the absence of Co(bpy)₃^3+/2+ (the system based on interparticle electron transfer), however, the Z-scheme system using the Co(bpy)₃^3+/2+ electron shuttle showed 10 times higher activity than the interparticle electron transfer system. The external quantum yield and efficiency of solar energy conversion to hydrogen were determined to be 0.5% at 430 nm and 0.029%, respectively.