Direct Z-scheme photochemical hybrid systems: Loading porphyrin-based metal-organic cages on graphitic-C3N4 to dramatically enhance photocatalytic hydrogen evolution

The rational design of photochemical molecular device (PMD) and its hybrid system has great potential in improving the activity of photocatalytic hydrogen production. A series of Pd6L3 type metal-organic cages, denoted as MOC-Py-M (M = H, Cu, and Zn), are designed for PMDs by combining metalloporphyrin-based ligands with catalytically active Pd2+ centers. These metal-organic cages (MOCs) are first successfully hybridized with graphitic carbon nitride (g-C3N4) to form direct Z-scheme heterogeneous MOC-Py-M/g-C3N4 (M = H, Cu, and Zn) photocatalysts via π–π interactions. Benefiting from its better light absorption ability, the MOC-Py-Zn/g-C3N4 catalyst exhibits high H2 production activity under visible light (10348 μmol g−1 h−1), far superior to MOC-Py-H/g-C3N4 and MOC-Py-Cu/g-C3N4. Moreover, the MOC-Py-Zn/g-C3N4 system obtains an enhanced turn over number (TON) value of 32616 within 100 h, outperforming the homogenous MOC-Py-Zn (TON of 507 within 100 h), which is one of the highest photochemical hybrid systems based on MOC for visible-light-driven hydrogen generation. This confirms the direct Z-scheme heterostructure can promote effective charge transfer, expand the visible light absorption region, and protect the cages from decomposition in MOC-Py-Zn/g-C3N4. This work presents a creative example that direct Z-scheme PMD-based systems for effective and persistent hydrogen generation from water under visible light are obtained by heterogenization approach using homogeneous porphyrin-based MOCs and g-C3N4 semiconductors.