Modulating Benzothiadiazole‐Based Covalent Organic Frameworks via Halogenation for Enhanced Photocatalytic Water Splitting

Abstract

Two‐dimensional covalent organic frameworks (2D COFs), an emerging class of crystalline porous polymers, have been recognized as a new platform for efficient solar‐to‐hydrogen energy conversion owing to their pre‐designable structures and tailor‐made functions. Herein, we demonstrate that slight modulation of the chemical structure of a typical photoactive 2D COF (Py‐HTP‐BT‐COF) via chlorination (Py‐ClTP‐BT‐COF) and fluorination (Py‐FTP‐BT‐COF) can lead to dramatically enhanced photocatalytic H2 evolution rates (HER=177.50 μmol h−1 with a high apparent quantum efficiency (AQE) of 8.45 % for Py‐ClTP‐BT‐COF). Halogen modulation at the photoactive benzothiadiazole moiety can efficiently suppress charge recombination and significantly reduce the energy barrier associated with the formation of H intermediate species (H*) on polymer surface. Our findings provide new prospects toward design and synthesis of highly active organic photocatalysts toward solar‐to‐chemical energy conversion.

Thiophene‐Bridged Donor–Acceptor sp2‐Carbon‐Linked 2D Conjugated Polymers as Photocathodes for Water Reduction

“…Under AM 1.5 G irradiation and at 0.3 V versus RHE, the three 2D conjugated polymers on fluorine-doped tin oxide (FTO) electrodes show photocurrents of ≈5.5 µA cm −2 for 2D CCP-Th, ≈2.8 µA cm −2 for 2D CCP-BD, and ≈0.06 µA cm −2 for 2D CN COF-B (Figure 5a,b). Moreover, 2D CCP-Th achieves a photocurrent of ≈7.9 µA cm −2 at 0 V versus RHE, which exceeds those of the reported 2D COFs (0.09-6.0 µA cm −2 ) [8,9,13,[27][28][29][30][31][32] and most carbon nitride materials (g-C 3 N 4 , 0.3-1.2 µA cm −2 ) ( Table S6, Supporting Information). [33][34][35][36][37] As a key parameter to evaluate the intrinsic PEC performance of photoelectrode materials, the incident-photon-to-current efficiency (IPCE) was measured ( Figure S30, Supporting Information).…”

“…Benefitting from the D-A structure, the bithiophene-bridged 2D CCP-Th shows a much narrower optical energy bandgap (2.04 eV), a higher-lying LUMO energy level (−3.41 eV), and higher charge separation and transport than those of biphenyl-bridged 2D CCP-BD and 2D CN COF-B. Therefore, as the cathode material for PEC water reduction, 2D CCP-Th demonstrates a superb saturated photocurrent density up to 5.5 µA cm −2 at 0.3 V and 7.9 µA cm −2 at 0 V versus reversible hydrogen electrode (RHE), which is superior to the reported 2D COFs [8,9,13,[27][28][29][30][31][32] and most carbon nitride (g-C 3 N 4 ) materials (0.09-6.0 µA cm −2 ) [33][34][35][36][37] (Table S6, Supporting Information). Furthermore, density functional theory (DFT) simulations further suggest that the sulfur atoms in the thiophene as well as the cyano groups at the vinylene linkage serve as the active sites for water reduction.…”

Stable Unbiased Photo‐Electrochemical Overall Water Splitting Exceeding 3% Efficiency via Covalent Triazine Framework/Metal Oxide Hybrid Photoelectrodes

“…[47] Transient open-circuit voltage decay (OCVD) measurements were used to further probe the dynamics of photogenerated charge carriers. [48][49][50] As shown in Figure S29 in the Supporting Information, the formation of CTF-BTh layer on Cu 2 O and Mo:BiVO 4 photoelectrodes can clearly increase open-circuit voltages with much slower photovoltage decays. The average lifetimes have substantially increased from ≈40 to ≈121 s and ≈21 to ≈146 s after depositing the CTF-BTh layer on the Cu 2 O photocathode and the Mo:BiVO 4 photoanode, respectively, as calculated from the OCVD plots.…”

Construction of Donor–Acceptor Heterojunctions in Covalent Organic Framework for Enhanced CO2Electroreduction

“…The challenge is to create conductive COFs for highly efficient electrocatalysis of carbon dioxide reduction reaction (CO charge transfer materials when being combined with molecules as electron acceptors. [35][36][37][38][39][40][41][42] Metalloporphyrin has a conjugated electron system and can be used as an excellent electron acceptor or electron transfer carrier. [43][44][45] Moreover, porphyrins generally have small highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO-LUMO) gaps that can easily allow uptake and release of electrons, leading to fast electron transfer to reactant substrates and enhance activities.…”