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

Abstract

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.