Suppressing the Excitonic Effect in Covalent Organic Frameworks for Metal-Free Hydrogen Generation

Abstract

Photocatalytic hydrogen generation is a promising solution for renewable energy production and plays a role in achieving carbon neutrality. Covalent organic frameworks (COFs) with highly designable backbones and inherent pores have emerged as novel photocatalysts, yet the strong excitonic effect in COFs can impede the promotion of energy conversion efficiency. Here, we propose a facile approach to suppress the excitonic effect in COFs, which is by narrowing the band gap and increasing the dielectric screening via a rational backbone design and chemical modifications. Based on the GW-BSE method, we uncover a linear relationship between the electronic dielectric constant and the inverse square of the optical band gap of COFs of the Lieb lattice. We further demonstrate that both reduced exciton binding energy and enhanced sunlight absorption can be simultaneously realized in COFs with a narrow band gap. Specifically, we show that one of our designed COFs whose exciton binding energy is nearly half that of g -C 3 N 4 is capable of metal-free hydrogen production under near-infrared light irradiation. Our results showcase an effective method to suppress the excitonic effect in COFs and also pave the way for their applications in photocatalytic, photovoltaic, and other related solar energy conversions.

Tuning excited state electronic structure and charge transport in covalent organic frameworks for enhanced photocatalytic performance

“…The development of highly active and low-cost COF photocatalysts is now an international research focus, bridging the fields of chemistry, material science, catalysis, and engineering 9 – 13 . To obtain high-performance COF photocatalysts, researchers are now exploring ways of increasing the visible-light absorption range 14 – 16 , optimizing band structures 17 – 20 , and decreasing the recombination of photogenerated electrons and holes 21 – 23 . Common strategies for achieving these performance-boosting properties include (i) incorporating a photosensitizer into the framework for improving the light-harvesting capability 24 – 27 ; (ii) functionalization of the linkers and tuning of the components to optimize the band gap energy and valence/conduction band potentials 17 , 28 – 31 ; (iii) construction of donor-acceptor moieties to improve charge transfer kinetics and charge carrier separation efficiencies 32 – 35 ; (iv) doping non-metal elements (N, P, S, etc.)…”

“…To obtain high-performance COF photocatalysts, researchers are now exploring ways of increasing the visible-light absorption range 14 – 16 , optimizing band structures 17 – 20 , and decreasing the recombination of photogenerated electrons and holes 21 – 23 . Common strategies for achieving these performance-boosting properties include (i) incorporating a photosensitizer into the framework for improving the light-harvesting capability 24 – 27 ; (ii) functionalization of the linkers and tuning of the components to optimize the band gap energy and valence/conduction band potentials 17 , 28 – 31 ; (iii) construction of donor-acceptor moieties to improve charge transfer kinetics and charge carrier separation efficiencies 32 – 35 ; (iv) doping non-metal elements (N, P, S, etc.) 36 , 37 , single metal sites 38 – 41 , clusters 41 – 43 , or noble metals 29 as a co-catalyst to modulate the photoelectronic properties, thus improving the overall photocatalytic activity.…”

Influence laws of air gap structure manipulation of covalent organic frameworks on dielectric properties and exciton effects for photopolymerization

“…10 It is evident that the ε r is a descriptive factor to quantitatively characterize its relationship with E b along with the photocatalytic activity which is important for the development of efficient photocatalysts. 9,10,19–22…”

“…3,4 Photocatalysts absorb photon energy to generate "hot" electron-hole pairs, which are bound by Coulomb forces that suppress their dissociation to form free carriers, thus reducing the conversion efficiency of light energy. [5][6][7][8][9][10] Consequently, facilitating the dissociation of excitons is critical to improving the photocatalyst activity.…”

“…10 It is evident that the 3 r is a descriptive factor to quantitatively characterize its relationship with E b along with the photocatalytic activity which is important for the development of efficient photocatalysts. 9,10,[19][20][21][22] Pursuant to the International Roadmap for Semiconductors (ITRS), porous materials and "air gap" structures are an effective way to develop low dielectric materials. [23][24][25] Porous materials are primarily designed to increase the porosity in the intrinsic structure of the material in order to effectively reduce the density of the material and thus decrease the number of a Department of Materials-Oriented Chemical Engineering, School of Chemical Engineering, Fuzhou University, Fuzhou 350116, P. R. China.…”