A Direct Z-Scheme Quasi-2D/2D Heterojunction Constructed by Loading Photosensitive Metal–Organic Nanorings with Pd Single Atoms on Graphitic–C₃N₄ for Superior Visible Light-Driven H₂ Production

Rational design of Z-scheme composite photocatalysts with 2D/2D heterojunction is a promising method to convert solar energy into chemical fuels. Herein, a Pd₂L₂-type metal–organic nanoring (MAC-1), which can serve as a photochemical molecular device (PMD), is assembled by catalytic Pd²⁺ centers and photosensitive ligands (M-7). Then MAC-1 is combined with graphite-phase carbon nitride (g-C₃N₄) to construct a direct Z-scheme quasi-2D/2D composite material MAC-1/g-C₃N₄ with Pd single atoms. The optimized 3.5% MAC-1/g-C₃N₄ single-atom catalyst (SAC) shows better photocatalytic performance than g-C₃N₄, MAC-1, Pd/g-C₃N₄, M-7/Pd/g-C₃N₄, and 3.5% MAC-1/g-C₃N₄-mixed and also displays good durability for H₂ production. The H₂ yield rate of 22.3 mmol g⁻¹ h⁻¹ and the corresponding turnover number based on Pd or MAC-1 amount (TON_Pd/TON_MAC-1) of 119 172/238 344 within 50 h are achieved for 3.5% MAC-1/g-C₃N₄, which is one of the highest records of all visible light-driven g-C₃N₄-based photocatalysts reported. Such remarkably enhanced activity can result from the enhanced charge carrier mobility, improved light absorption ability, enlarged heterojunction contact interface, and highly monodispersed Pd active sites. The Z-scheme SAC composed of metal–organic nanorings and g-C₃N₄ nanosheets therefore has great potential as an efficient and sustainable photocatalyst for solar-driven H₂ production.