Ultrathin covalent organic framework nanosheets for enhanced photocatalytic water oxidation
Enbo Zhou, Xiang Zhang, Lei Zhu, Erchong Chai, Jinsong Chen, Jie Li, Daqiang Yuan, Longtian Kang, Qing‐Fu Sun, Yaobing Wang
Abstract
Photocatalytic water oxidation is a key half-reaction for various solar-to-fuel conversion systems but requires simultaneous water affinity and hole accumulation at the photocatalytic site. Here, we present the rational design and synthesis of an ionic-type covalent organic framework (COF) named tetraphenylporphyrin cobalt and cobalt bipyridine complex (CoTPP-CoBpy 3 ) COF, combining cobalt porphyrin and cobalt bipyridine building blocks as a photocatalyst for water oxidation. The good dispersibility of porous large-size (>2 micrometers) COF nanosheets (≈1.45 nanometers) facilitates local water collection; the ultrafast triplet-state charge transfer (1.8 picoseconds) and prolonged charge separation (1.2 nanoseconds) further contribute to the efficient accumulation of holes in the CoTPP moiety, leading to a photocatalytic dioxygen production rate of 7323 micromoles per gram per hour. Moreover, we have identified an end-on superoxide radical (O 2 · ) intermediate at the active site of the CoTPP moiety and proposed an electron-intermediate cascade mechanism that elucidates the synergistic coupling of electron relay (S 1 -T 1 -T 1 ′) and intermediate evolution during the photocatalytic process.