Impact of the Crystallinity of Covalent Organic Frameworks on Photocatalytic Hydrogen Evolution

Abstract

The design and synthesis of crystalline porous materials is a frontier research area, especially in photo(electro)catalysis, due to their fascinating semiconducting properties. In recent years, crystalline metal−organic frameworks (MOFs) and covalent organic frameworks (COFs) have received much research attention in heterogeneous catalysis, ranging from chemical conversion to solar energy production. In addition, COFs with well-ordered framework structures have been extensively studied in the field of heterogeneous photocatalysis for water splitting to produce hydrogen (H 2 ) and oxygen (O 2 ). Due to the synergistic effect of crystallinity, porosity, and conjugation in their frameworks, COFs have been widely explored. In this Review, we aim to discuss the effect of crystallinity of COFs on hydrogen generation via photocatalytic water splitting. We then briefly summarize the basic mechanisms of photocatalytic hydrogen generation, the advantages of crystalline semiconductors over amorphous materials, and the strategic designs of crystalline COFs. In addition, the state-of-the-art developments of crystalline COFs as organic semiconductors for photocatalytic hydrogen evolution have been systematically reviewed. We believe that understanding the structure−property relationship and photocatalytic performance for hydrogen evolution with respect to the long-range structural order of COFs is essential for the further development of innovative crystalline COF-based semiconductors in real-time hydrogen generation applications.

Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Generation

“…Recently, extended framework materials, especially COFs with donor–acceptor systems, have been shown to be efficient photocatalysts for H 2 O 2 generation via the oxygen reduction reaction. , In addition, COFs exhibit properties such as visible light absorption, tunable band gaps, high crystallinity, large surface areas, uniform porosity, tailorable surface functionality, low density, robustness and good thermal stability–essential properties for photocatalytic H 2 O 2 generation. Most importantly, the well-ordered crystalline COF-based semiconductors facilitate the effective charge separation and utilization of photogenerated carriers and inhibit photogenerated electron–hole recombination. , …”

“…One of the beneficial features of COFs is their high crystallinity compared to porous organic polymers, which are amorphous in nature. ,, Crystallinity refers to the extent of long-range structural order in the material, and strongly governs its photocatalytic performance due to its superior charge separation and transport properties. , Generally, the high crystallinity of the COF semiconductor offers an improved stacking of the layer arrangement, which extends the light absorption range and improves the exciton dynamics, thus enhancing the photocatalytic activity. It has been demonstrated that the crystallinity of COFs regulates various essential parameters such as effective charge separation and migration, uniform porosity and reduced electron–hole recombination.…”

Strategic design of covalent organic frameworks (COFs) for photocatalytic hydrogen generation

“…The photocatalytic properties are strongly governed by the crystallinity of photocatalysts. 178 An increase in the crystallite sizes has a strong impact on enhancing the photocatalytic rate, exhibiting low trap density and long diffusion length and thus demonstrating high catalytic performance. Highly crystalline cocatalysts show good photocatalytic performance due to their favorable charge transport properties.…”

Strategies to improve the photocatalytic performance of covalent triazine frameworks

“…Generally, high crystallinity in materials is associated with reduced defect density and trapping centers for photogenerated electron–hole trapping and recombination, which in turn promote efficient charge separation and minimize recombination. 106,107 Improving the crystallinity of CTFs is an effective approach to enhance their photocatalytic performance. 35 Crystalline CTFs offer several advantages over amorphous counterparts, including few defects, 18 tunable bandgaps, 25,108 uniform porosity 109 and regular crystal planes.…”