2D Covalent Organic Frameworks Toward Efficient Photocatalytic Hydrogen Evolution

Abstract

Efficiently producing clean energy is of great importance for sustainable development of the environment. Solar‐driven water splitting for H2 evolution has an important role among the renewable energy technologies. Developing high‐performance and cost‐effective photocatalysts is still a critical task before practical application. 2D Covalent organic frameworks (COFs) as photocatalysts have recently attracted widespread interest thanks to their tunable optical bandgaps, tailor‐made functionality, excellent crystallinity, high specific surface area, and good photo‐ and chemical stability. This Review focuses on the representative progress and remaining challenges in 2D COF‐based photocatalysts for hydrogen evolution.

In search of Covalent Organic Frameworkphotocatalysts: A DFT-based screening approach

“…Indeed, β-ketoenamine-linked COFs usually display good photocatalytic properties for HER, with increased stability with respect to their imine counterparts. 8,56 However, the same COFs, except for a few, present higher values of the computed charge recombination descriptor (Figure 7b). It has been suggested that introducing donor-acceptor units to COFs with this linkage can enhance their photocatalytic activity by stabilizing excited charge transfer processes.…”

“…To aid the evaluation of structure-property relationships for our descriptors, we determined a set of 26 substructures (see SI for the complete list) based on chemical intuition and on their occurrence as building blocks (linkages and linkers) and functional groups in COFs. 8,30,32 Searching for the presence of those substructures in the CURATED COFs allows for methodical pattern-seeking when it comes to the structure-property relationships. Such a pattern has been shown previously, indicating benzene-based functional groups with C 3v symmetry and nitrogen-based functional groups as potential catalytic active sites for OER and HER, respectively.…”

“…Due to the above-mentioned advantages, there is an increasing interest in using COFs and their composites as photocatalysts in overall water splitting, H 2 generation, CO 2 reduction, and degradation of organic pollutants. 7,8 Interestingly, a simple literature search in the Web of Science database (filtered for the keywords "Covalent organic framework" or "COF" and "photocatalysis" or "photocatalyst") shows that to this moment, about 90% of the experimental COFs originally queried for this work remain unexplored for photocatalysis. Given a large number of already synthesized COFs, 9 computational screening approaches can aid the exploration of properties and structure-property relationships by quickly surveying many variables, thus redirecting experimental efforts.…”

Structural Regulation of Thiophene-Based Two-Dimensional Covalent Organic Frameworks toward Highly Efficient Photocatalytic Hydrogen Generation

“…Hydrogen production from photocatalytic water splitting provides a road map for renewable and clean energy strategies. − The photoactive catalyst is the central element in the whole process of photocatalysis. − A substantial amount of effort has been spent to create varieties of competent photocatalysts. − A wide range of materials have been utilized in photocatalytic hydrogen production applications, from inorganic to organic materials. ,− Among these materials, covalent organic frameworks (COFs) with crystalline porous and highly conjugated structures have gained much attention. − The most attractive characteristic of COF materials is the molecular tunability in the structures. − Using the “toolbox” of organic chemistry, diversified structures of the COF materials can be designed and prepared. − This distinguishes COFs from other inorganic photocatalytic materials. As we know, it is normally very difficult to clarify the structure–activity relationship of the inorganic photocatalyst at the atomic level since their structures are basically fixed.…”

Donor–Acceptor Covalent–Organic Frameworks Based on Phthalimide as an Electron-Deficient Unit for Efficient Visible-Light Catalytic Hydrogen Evolution

“…Covalent–organic frameworks (COFs) are a class of porous crystalline materials integrated into well-defined framework networks that have recently become a research hotspot in photocatalytic hydrogen evolution from water due to their high crystallinity, strong intermolecular interactions, intrinsic porosity, and large specific surface area, − Moreover, COFs with D–A structures not only narrow the band gaps and extend photon absorption but also promote charge transport and charge separation . Therefore, the classic constructing units, such as benzothiadiazole, benzobis­(thiadiazole), benzodifuran, and isoindigo, utilized in D–A linear conjugated polymers for organic photovoltaics and organic thin-film transistors have been incorporated to design of D–A COFs, and some of them exhibit unprecedentedly high photocatalytic performances. − To date, the relationships between the structure and property are still ambiguous, and the development of COFs with diverse A units is essential.…”