Z-scheme water splitting utilizing CuLi_1/3Ti_2/3O₂ as a hydrogen-evolving photocatalyst with photo-response up to 600 nm

CuLi1/3Ti2/3O2 (CLTO) is a visible-responsive photocatalyst, whose photo-response reaches up to 600 nm, for H-2 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 H-2-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 O-2-evolving photocatalyst, and a Co(bpy)(3)(3+/2+) redox shuttle under visible light. Among the examined samples, Cr2O3/Ru/CLTO produced both H-2 and O-2 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)(3)(3+). 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.