Defect engineering of MIL-125-NH2 nanocrystalline enables piezoelectric–photocatalytic overall water splitting

Abstract

Metal organic frameworks (MOFs) are considered a promising photocatalyst for overall water splitting (OWS) hydrogen production due to their customizable structure. Nevertheless, the inefficient separation rate of photo-generated electron-hole pairs seriously hinders the further improvement of their photocatalytic hydrogen evolution performance in pure water. We report a strategy to optimize the piezo-photocatalytic overall water splitting activity by modulating MIL-125-NH2 nanocrystalline structural defects. By adjusting the amount of NaBH4 added, the defect structure of MIL-125-NH2 can be precisely controlled. NaBH4 is used as a mild reductant to convert NH2-MIL-125(Ti) into defect-engineered D-MIL-X, as indicated by new low-binding-energy Ti 2p/O 1s XPS components characteristic of Ti3+-related defect states and defect-related oxygen species; these defects are expected to tune the electronic structure and promote charge transfer during overall water splitting. Under the synergistic piezoelectric and visible-light irradiation in pure water without any sacrificial reagents, the optimized D-MIL-5 photocatalyst produces H2 and O2 at rates of 357.4 and 178.7 μmol g-1 h-1, respectively, with an H2/O2 ratio close to 2:1, corresponding to a five-fold enhancement over pristine MIL-125-NH2, likely benefiting from the combined effects of defect engineering and the internal piezoelectric field on charge separation. This study provides a green, convenient and low-cost defect generation strategy for MOF, provides more ideas for the synthesis of efficient photocatalysts for overall water splitting, and broadens the vision of defect design and research.