Ho-Loaded ZIF-L/COF Heterojunctions as Efficient Photocatalysts for H2O2 Production.

Abstract

Covalent organic frameworks (COFs) exhibit considerable potential as photocatalysts for H2O2 production through the oxygen reduction reaction (ORR) in water. However, their catalytic efficiency is often hindered by sluggish charge separation and a scarcity of sites for O2 activation. Here, a heterojunction of Ho-loaded ZIF-L/triazine-based COF (Ho-ZIF-L/TTA-COF) is designed and synthesized through the electrostatically induced π-π assembly for efficient photocatalytic H2O2 production. The optimized Ho-ZIF-L/TTA-COF heterojunction achieves an impressive H2O2 production rate of 7.19 mmol g-1 h-1 without any sacrificial agents under visible-light irradiation, representing a 7.6-fold enhancement compared to individual TTA-COF. Based on the results of in situ X-ray photoelectron spectroscopy and Kelvin probe force microscopy, the enhanced performance is mainly ascribed to the favorable electrons transfer from TTA-COF to ZIF-L then to the loaded Ho species for greatly enhancing charge separation. Noteworthily, Ho-ZIF-L possesses fully exposed Ho active sites and a large specific surface area, accounting for the efficient 2e- ORR dominated by the produced •O2 - radicals for H2O2 formation.