Mixed-metal Zr/Ti MIL-173 porphyrinic metal–organic frameworks as efficient photocatalysts towards solar-driven overall water splitting

Abstract

Hydrolytically stable MOFs combining Ti(IV) ions and porphyrin ligands are expected to display outstanding photophysical properties, making them ideal photocatalysts for the overall water splitting reaction under solar light. We report here the one-pot synthesis of a microporous MOF built up from a phenolate-based porphyrin ligand and a mixture of Zr(IV) and Ti(IV) ions. A broad set of characterization tools (PXRD, SEM-EDX, TEM, STEM-EDX, solid state UV-vis spectroscopy, XPS, nitrogen sorption) along with DFT calculations were used to confirm that Ti(IV) ions are uniformly inserted in the MIL-173 structure and located in the inorganic building unit. MIL-173(Zr/Ti)-40, containing 40 at% of Ti(IV), exhibits exceptional photocatalytic activity for the overall water splitting into H₂ and O₂ under simulated sunlight irradiation, without the use of any co-catalyst or sacrificial agent. This material operates efficiently under a photoinduced charge separation mechanism as revealed by photocurrent measurements and photoluminescence spectroscopy. This study provides insights for the development of efficient MOF-based materials as photocatalysts for the solar-driven overall water splitting.