Rational Regulation of the Exciton Effect of Acrylonitrile-Linked Covalent Organic Framework toward Boosting Visible-Light-Driven Hydrogen Evolution

Abstract

Covalent organic frameworks (COFs) exhibit relatively inefficient exciton dissociation and free charge generation, which arise from their high exciton binding energy. Here, a series of crystalline, acrylonitrile-linked COFs are fabricated to regulate exciton effects by strategies of shortening the linker length (TP-PN COF), introducing nitrogen atoms (TP-BPyN COF), and post-protonation (TP-BPyN PCOF). The temperature-dependent photoluminescence (PL) spectra and Nyquist plots reveal a decrease in exciton binding energy and charge transfer resistance of TP-PN COF, TP-BPyN COF, and TP-BPyN PCOF. Thus, an increase in free carrier generation and an extension of carrier lifetime are achieved, as demonstrated by transient photocurrents response and time-resolved fluorescence spectra (TRFS). DFT calculation reveals that low exciton binding energy and charge transfer resistance could relate to higher planarity structure. Besides, light absorption performance was improved by shortening the linker length, while the distribution density of Pt nanoparticles as H 2 evolution reaction (HER) sites was significantly improved by introducing nitrogen atoms as anchor points. As a result, the optimized TP-PN COF and TP-BPyN COF show efficient photocatalytic HER rates of 10,890 and 6457 μmol g −1 h −1 , respectively, improved by 13.92 and 8.26 times compared to that without modification (TP-BPN COF, 782 μmol g −1 h −1 ). Through a simple postprotonation strategy, the charge is redistributed and the structural distortion is reduced. Consequently, the HER rate of TP-BPyN PCOF significantly increased to 12,276 μmol g −1 h −1 . Meanwhile, the HER rate of TP-BPyN PCOF was further boosted to 15,929 and 22,438 μmol g −1 h −1 by optimizing the volume fractions of the sacrificial agent and the pH of the reaction system, respectively. This work could pave the way for developing efficient organic photocatalysts via rational regulation of the exciton effect.

Insights into Substituent Effects on the Fundamental Photocatalytic Processes of Covalent Organic Frameworks toward H2 Evolution and H2O2 Production Reactions

“…The integrated PL intensity of BTT-COFs decreased with the temperature increasing from 180 to 300 K, and the exciton binding energy ( E b ) was obtained by fitting these data based on the Arrhenius eq (Figure a, details are given in the Supporting Information). , It revealed that BTT–OH had the lowest E b (65.5 meV) compared with those of BTT–H (114.7 meV) and BTT–Me (110.3 meV), which meant the excitons were more easily dissociated into free electrons and holes in BTT–OH.…”

Progress of Covalent Organic Framework Photocatalysts: From Crystallinity–Stability Dilemma to Photocatalytic Performance Improvement

“…Excitons are unique electron–hole pairs produced during the photophysics process. ,, Only those excitons that are further dissociated into free charge carriers can be involved in the following photocatalytic redox reactions. However, the inherent low dielectric constants and strong Coulombic force between electrons and holes lead to large exciton binding energy, ,− which is unfavorable to the generation of free charge carriers in organic semiconductors .…”

“…Another major concern is how to improve the photocatalytic performance of the COFs. In the initial steps of heterogeneous photocatalysis, a photocatalyst is excited by incident light with sufficient energy to generate excitons, that is, the bound electron–hole pairs in the crystal lattice of the photocatalyst through Coulomb interaction. , Only when the excitons are dissociated into free charge carriers (i.e., separated electrons and holes), they are available for the subsequent redox reactions; accordingly, the exciton dissociation process is crucial. − It is acknowledged that the binding energy ( E b ) of exciton in a semiconductor is inversely proportional to its dielectric constant ( E b = q 2 /4πε o ε r r o , where q, ε o , ε r , and r o are the elementary charge, the absolute dielectric permittivity, the relative dielectric constant, and the distance between the electron and hole, respectively). , Inorganic photocatalysts usually hold high dielectric constants above 10 thus low exciton binding energy of 10 meV, which is equal to or less than 26 meV (i.e., the thermal disturbance energy k B T, where k B is the Boltzmann constant and T is the room temperature). , Therefore, the excitons of inorganic photocatalysts are substantially dissociated into free charge carriers. Conversely, organic photocatalysts have low dielectric constants of 3–5 and high exciton binding energy from hundreds of meV to 1 eV, thus causing large energy barriers toward exciton dissociation.…”

Constructing Symmetry-Mismatched RuxFe3–xO4 Heterointerface-Supported Ru Clusters for Efficient Hydrogen Evolution and Oxidation Reactions

“…Amazingly, the overpotential of Ru/Ru x Fe 3– x O 4 is only 17.6 mV at a current density of 10 mA cm –2 (η 10 ), lower than that of commercial Pt/C (η 10 = 30.5 mV), far better than the reported previously (Figure 14 and Table S3). , In addition, the Tafel slope for Ru/Ru x Fe 3– x O 4 is far lower than that of Pt/C, indicating the fastest reaction kinetics toward the HER (Figure S15). As shown in Figure b, the mass activity of Ru/Ru x Fe 3– x O 4 is 1576.9 mA mg Ru –1 at 50 mV, ∼10 times higher than that of commercial Pt/C (166.3 mA mg Pt –1 ).…”