Covalent organic frameworks (COFs) with highly
designable skeleton and inherent pores have emerged as promising organic
photocatalysts for hydrogen production. However, inefficient solar light harvesting, strong
excitonic effect, and the lack of active sites still pose major challenges to
the rational design of COFs for efficient photocatalytic water splitting and
the structure-property relationship has not been established. In this work, we investigated the
fundamental mechanism of photoelectrochemical conversion in fully conjugated
donor (D)-acceptor (A) COFs in Lieb lattice and proposed a facile strategy to
achieve broad visible and near-infrared absorption, prompt exciton
dissociation, tunable band alignment for overall water splitting, and
metal-free catalysis of hydrogen production. Interestingly, we found that the
exciton binding energy was substantially reduced with the narrowing of optical
band gap and the increase of static dielectric constant. Further, we unraveled
that the hydrogen bond played a vital role in suppressing the overpotential for
hydrogen evolution reaction to enable metal-free catalysis. These
findings not only highlight a novel route to modulating electronic properties
of COFs towards high photocatalytic activity for water splitting, but also
offer tremendous opportunities to design metal-free catalysts for other
chemical transformations.
Covalent organic frameworks in Lieb lattice for tunable photocatalytic water splitting under visible light
