Synthesis of core-shell ZIF-67@Co-MOF-74 catalyst with controllable shell thickness and enhanced photocatalytic activity for visible light-driven water oxidation

In this paper, a core-shell ZIF-67@Co-MOF-74 catalyst was synthesized by coating 2,5-dihydroxyterephthalic acid (DHTP) molecules on the surface of ZIF-67 crystals via the ligand exchange method. Notably, the ZIF-67@Co-MOF-74 catalyst with shell thicknesses of 10 nm, 25 nm and 50 nm can be further obtained by adjusting the mass ratio of ZIF-67 and DHTP. Compared to individual ZIF-67 or Co-MOF-74 catalyst, the as-prepared core-shell MOF catalyst exhibited enhanced photocatalytic activities for light-driven water oxidation reaction. Furthermore, the content of oxygen evolution by water splitting increased gradually with the increase in shell thickness. The formation of crystal defects and the uncoordinated hydroxyl and carboxyl groups on the surface of core-shell MOFs facilitated the exposure of the metal catalytic center and the adsorption of water molecules through hydrogen bonding interactions to react with the catalytic active center effectively. In addition, the photogenerated holes and electrons could be excellently separated and rapidly transferred at the interface of ZIF-67 (core) and Co-MOF-74 (shell), resulting in effective increase in the interfacial charge transfer rate. Furthermore, this simple and novel method is also applicable to three other carboxylic acid ligands, which implies that it may be a general method that can be extended to other ligands for fabricating different core-shell ZIF-67@MOF crystals.