Constructing Heterogeneous Direct Z-Scheme Photocatalysts Based on Metal–Organic Cages and Graphitic-C₃N₄ for High-Efficiency Photocatalytic Water Splitting

The development of artificial devices that mimic the highly efficient and ingenious photosystems in nature is worthy of in-depth study. A metal-organic cage (MOC) Pd₂(M-4)₄(BF₄)₄, denoted as MOC-Q1, integrating four organic photosensitized ligands M-4 and two Pd²⁺ catalytic centers is designed for a photochemical molecular device (PMD). MOC-Q1 is successfully immobilized on graphitic carbon nitride (g-C₃N₄) by hydrogen bonds to obtain a robust heterogeneous direct Z-scheme g-C₃N₄/MOC-Q1 photocatalyst for H₂ generation under visible light. The optimized g-C₃N₄/MOC-Q1 (2 wt %) system shows high hydrogen evolution activity (4495 μmol g^-1 h^-1 based on the catalyst mass) and exhibits stable performances for 25 h (a turnover number of 19,268 based on MOC-Q1), significantly outperforming pure MOC-Q1, g-C₃N₄, and comparsion materials Pd/g-C₃N₄/M-4, which is the highest one of all reported heterogeneous MOC-based photocatalysts under visible irradiation. This enhancement can be ascribed to the synergistic effects of high-efficient electron transfer, extended visible-light response region, and good protective environment for MOC-Q1 arising from an efficient direct Z-scheme heterostructure of g-C₃N₄/MOC-Q1. This rationally designed and synthesized MOC/g-C₃N₄-based heterogeneous PMD is expected to have great potential in photocatalytic water splitting.