A Z-scheme photosensitive MOC/g-C3N4 composite catalyst for efficient visible-light driven half and overall water splitting

Abstract

The construction of Z-scheme heterojunction composite catalysts for overall water splitting is promising. Herein, a triangle-shaped prismatic metal–organic cage MOC-Q3 consisting of two photosensitive ligands and three catalytic Pd²⁺ centers is synthesized as an intramolecular photocatalytic molecular device (IPMD) to conduct a photocatalytic light-to-fuel reaction. MOC-Q3 is then coupled with graphitic phase carbon nitride (g-C₃N₄) to form a composite catalyst by intermolecular π–π stacking through quasi-2D/2D contact. And the charge transfers at the MOC-Q3/g-C₃N₄ interface are proven to follow a Z-scheme pathway by radical trapping experiments and theoretical calculations. The optimized 11 wt% MOC-Q3/g-C₃N₄ catalyst exhibits a remarkable photocatalytic H₂ evolution activity of 50.1 mmol h⁻¹ g⁻¹, one of the highest values reported among g-C₃N₄-based Z-scheme systems in recent years. And a high turnover number (TON) of 363 740 based on the MOC moles is achieved in a prolonged photocatalytic experiment lasting 32 h. Furthermore, the overall water splitting reaction was successfully conducted using MOC-Q3/g-C₃N₄ in combination with a classical water oxidation catalyst (WOC) IrO₂, without the addition of any sacrificial agent, yielding H₂ and O₂ amounts of 400.7 and 200.5 μmol g⁻¹ within 5 h, respectively. This study presents an inspiring approach of combining IPMDs and g-C₃N₄ to prepare highly efficient composite catalysts for visible light-driven H₂ evolution and overall water splitting.