Crystal-Facet-Controlled Internal Electric Field in MOF/COF Heterojunction Towards Efficient Photocatalytic Overall Water Splitting.

Covalently integrating two type of crystalline porous materials, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), to form heterostructure photocatalysts inheriting their structural merits has shown inherent advantages in hydrogen evolution reaction. However, how to control the internal electric field in aimed MOF/COF heterojunction to achieve an improved photocatalytic activity is still ambiguous. Herein, for the first time, we report a rational control of the internal electric field in MOF/COF heterojunction by engineering the crystal facet of MOFs to achieve enhanced photocatalytic overall water splitting (OWS) activity. A new type of covalently connected MOF/COF photocatalytic system based on NH2-MIL-125(Ti) and TpBpy-COF was synthesized. As confirmed, the exposed crystal facet of MOFs greatly affected the resultant activity of MOF/COF system. The combination of decahedron NH2-MIL-125(Ti) and TpBpy-COF shows an optimal OWS activity with the H2 and O2 evolution rates of 331.6 and 165.7 μmol g-1 h-1 under visible light, respectively, which is the best performance in COFs or COF-based photocatalyst at present. The great influence of formed anisotropic facets on internal electric field of the S-scheme MOF/COF heterojunction interface is fully confirmed by various characterizations, and the active crystal facet of NH2-MIL-125(Ti) for water oxidation reaction is further proved.