Rational Design of Crystalline Covalent Organic Frameworks for Efficient CO₂ Photoreduction with H₂O

Abstract

Solar energy‐driven conversion of CO2 into fuels with H2O as a sacrificial agent is a challenging research field in photosynthesis. Herein, a series of crystalline porphyrin‐tetrathiafulvalene covalent organic frameworks (COFs) are synthesized and used as photocatalysts for reducing CO2 with H2O, in the absence of additional photosensitizer, sacrificial agents, and noble metal co‐catalysts. The effective photogenerated electrons transfer from tetrathiafulvalene to porphyrin by covalent bonding, resulting in the separated electrons and holes, respectively, for CO2 reduction and H2O oxidation. By adjusting the band structures of TTCOFs, TTCOF‐Zn achieved the highest photocatalytic CO production of 12.33 μmol with circa 100 % selectivity, along with H2O oxidation to O2. Furthermore, DFT calculations combined with a crystal structure model confirmed the structure–function relationship. Our work provides a new sight for designing more efficient artificial crystalline photocatalysts.

Semiconductor/Covalent‐Organic‐Framework Z‐Scheme Heterojunctions for Artificial Photosynthesis

“…Theoretically, a photocatalyst needs a more negative conduction‐band‐minimum (CBM) potential than the theoretical CO 2 reduction potential to complete the CO 2 RR. As for the H 2 O oxidation reaction, a more positive valence‐band‐maximum (VBM) potential is needed to catalyze the H 2 O/O 2 conversion . The optical band‐structure diagrams of COF‐318, TiO 2 , Bi 2 WO 6 , and α‐Fe 2 O 3 are displayed in Figure f. The results show that COF‐318 is capable of reducing CO 2 to many reductive carbon‐based products (such as CO, CH 4 , or HCOOH), and these semiconductors are capable of oxidizing H 2 O to O 2 .…”

“…With suitable building blocks and functional organic groups, COFs have been shown to have a high visible‐light absorption capacity and fast charge‐carrier mobility . Furthermore, many COF‐based photocatalysts have been successfully used for the light‐driven reduction of CO 2 –. Meanwhile, many inorganic semiconductors (SCs, such as TiO 2 , Bi 2 WO 6 , or α‐Fe 2 O 3 ) have been demonstrated to be outstanding photocatalysts that can achieve efficient H 2 O photooxidation .…”

Direct observation of charge transfer between molecular heterojunctions based on inorganic semiconductor clusters

“…Interestingly, a crystalline molecular heterojunction based on covalent organic frameworks (COFs) has been recently reported (Scheme 1c), representing a diversi-ed combination of organic SBUs with outstanding semiconduction and photoconduction properties. 18,[22][23][24][25][26] Moreover, the systematical assembly of donor and acceptor SBUs allows one to reveal some mechanistic details (e.g. exciton formation and charge separation/migration) for such ordered atomically precise molecular heterojunctions (Scheme 1f).…”

Semiconducting 2D Covalent Organic Frameworks: A New Opportunity for Efficient Solar Fuel Production

“…Among them, Lan et al . reported the first crystalline 2D COF for photocatalytic CO 2 reduction using H 2 O as the electron donor (Figure ) . The electron‐deficient unit Zn‐porphyrin and electron‐rich unit tetrathiafulvalene (TT) were selected to construct the 2D TTCOF‐Zn via covalent imine bonds.…”